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The use of radio frequency heating to condition, eliminate decay fungi, and fix chromated copper arsenate… Fang, Fang 1999

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THE USE OF RADIO FREQUENCY HEATING TO CONDITION, ELIMINATE DECAY FUNGI, AND FIX CHROMATED COPPER ARSENATE PRESERVATIVE, IN ROUND WOOD by Fang Fang B.Sc. (1990), Capital Normal University, Beijing, China A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Faulty of Forestry; Department of Wood Science) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1999 © Fang Fang, 1999 ln presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada Date Pec. DE-6 (2/88) ABSTRACT Electromagnetic radiation, such as radio frequency heating has been shown to be able to rapidly dry wood and to fix CCA in treated wood. However, the previous research at the University of British Columbia has focused on the drying of sawnwood using a radio frequency vacuum drier with flat parallel plates. This study examined the efficacy of radio frequency/vacuum heating, to rapidly dry roundwood sections as well as accelerate the fixation of chromated copper arsenate (CCA) in treated pole sections. The ability of radio frequency heating to sterilize wood was also examined. Conditioning of roundwood was carried out in a laboratory radio frequency/vacuum dryer at fixed frequency of 13.65 MHz. During drying, the change in the temperature with time, within each roundwood section, was continuously monitored. Evaluation of all the experimental runs showed that the flat electrode design of radio frequency/vacuum could dry roundwood to ca. 25% moisture content without loss in quality in less than 16 hours. Furthermore, radio frequency/vacuum drying produced poles with an uniform final moisture content. Accelerated fixation of chromated copper arsenic was also conducted using radio frequency heating. In this case ho vacuum was applied. The effect of radio frequency heating on the rate of fixation of CCA treated poles were evaluated by two methods: 1) a qualitative hexavalent chromium (VI) color reaction with chromotropic acid; 2) the ii quantitative analysis of hexavalent chromium (VI) in a leachate produced using sample borings. The fixation time was greatly reduced by heating the pole sections to above 80°C at high humidity. All three wood species used in this study required less than 4.5 hours to achieve complete fixation of the CCA. Eight experimental heating runs were further carried out to evaluate radio frequency heating to sterilize the roundwood sections. Sawdust was inoculated with the decay fungi [(Coniophora puteana, Gloeophyllum trabeum (47D) and Postia placenta (120F and 31094B)] and the fungi allowed to grow on the substrate. The inoculated sawdust to selected pole sections was added at predetermine depths from the surface. Following radio frequency heating, the sawdust was recovered and the viability of the fungi was determined. All decay fungi, as well as moulds present in the pole section, were completely killed after 2 hours of heating above 65°C. This confirmed the potential of the radio frequency kiln process to destroy wood decay fungi present in unseasoned poles. iii T A B L E O F C O N T E N T S Page ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES x LIST OF FIGURES xii LIST OF ABBREVIATIONS xvi ACKNOWLEDGEMENTS xvii C H A P T E R 1 BACKGROUND 1 1.1.Introduction 1 1.1.1. The biodegradation of wood pole 1 1.1.2. The limitation of currently used pole drying strategy 3 1.1.3. Radio frequency/vacuum drying strategy 5 1.1.4. Preservatives used to protect wood poles 6 1.1.5. Accelerated fixation of CCA 9 1.2. Thesis objective 11 C H A P T E R 2 RADIO FREQUENCY CONDITIONING/DRYING OF ROUND WOOD 12 2.1. Introduction 12 2.2. Literature review 14 2.2.1. Nature of water in wood 14 2.2.1.1. Wood moisture and the environment 15 iv 2.2.1.2. Moisture content and strength of wood 17 2.2.1.3. Moisture movement during drying process 17 2.2.2. Drying technology 19 2.2.2.1. Air drying 19 2.2.2.2. Conventional kiln drying 21 2.2.2.3. Dehumidification drying 22 2.2.2.4. Vacuum drying 23 2.2.2.5. Superheated steam/vacuum drying 24 2.2.2.6. Dielectric drying 25 2.3. Materials and Methodologies 35 2.3.1. Species and sizes 35 2.3.2. Measurement of initial moisture content (Mi) 35 2.3.3. Radio frequency/vacuum dryer 39 2.3.3.1. Radio frequency amplifier 40 2.3.3.2. Radio frequency oscillator 41 2.3.4. Drying procedure 41 2.3.4.1. Temperature monitoring 41 2.3.4.2. Dielectric runs 42 2.3.5. Data analysis 43 2.4. Results and Discussion 46 2.4.1. Radio frequency/vacuum heating results 46 2.4.1.1. Series 1—using amplifier 46 v 2.4.1.2. Series 2~drying of low initial moisture content sections using the oscillator 52 2.4.1.3. Series 3~drying of medium initial moisture content sections using the oscillator 56 2.4.1.4. Series 4--drying of high initial moisture content sections using the oscillator 59 2.4.2. The effects of initial moisture content 64 2.4.2.1. The effects of the sapwood initial moisture content on the initial heating rate 64 2.4.2.2. The effects of heartwood/ inner sapwood initial moisture content on the initial heating rate 66 2.4.3. The effects of radio frequency / vacuum drying 67 2.5 Conclusions 70 CHAPTER 3 APPLICATION OF RADIO FREQUENCY TO ACCELERATE THE FIXATION OF CCA IN ROUNDWOOD 71 3.1. Introduction 71 3.2. Literature review 72 3.2.1. CCA fixation mechanism 72 3.2.1.1. The period of momentary initial reaction 73 3.2.1.2. The period of primary precipitation fixation 74 3.2.1.3. The period of conversion reactions 75 3.2.1.4. Variables in fixation process 76 3.2.2. Assessment of CCA fixation 78 vi 3.2.3. Practical application of accelerated fixation 81 3.2.3.1. Hot air heating 81 3.2.3.2. Hot water fixation 82 3.2.3.3. Live steam fixation 83 3.2.3.4. Alternative strategy and radio frequency fixation 84 3.3. Materials and Methodologies 86 3.3.1. Radio frequency heating fixation 86 3.3.1.1. Wood preparation 86 3.3.1.2. CCA pressure treatment 86 3.3.1.3. Radio frequency heating treatment 88 3.3.1.4. Sample analysis 89 3.3.2. Simulation experiment with hot water fixation 93 3.4. Results and Discussion 95 3.4.1. Radio frequency heating treatment runs 95 3.4.2. Moisture change profile 97 3.4.3. Assessment of fixation 100 3.4.3.1. Estimating the extent of fixation 100 3.4.3.2. The effect of stored heat 104 3.4.4 Simulation experiment with hot water heating 107 3.5. Conclusions 108 C H A P T E R 4 VISUAL EVALUATION 109 4.1 Introduction 109 4.2 Methodologies 109 vii 4.3 Results and Discussion 110 4.4 Conclusions 113 C H A P T E R 5 STERILIZATION OF WOOD 114 5.1 Introduction 114 5.2 Literature review 116 5.2.1. Why it is necessary to sterilize wood prior to use? 116 5.2.1.1. Decay of poles 116 5.2.1.2. Pole failure in service 117 5.2.2. Sterilization techniques 117 5.2.2.1. Removal of bark 118 5.2.2.2. Application of chemical as biocides 118 5.2.2.3. Irradiation 119 5.2.2.4. Heat treatment 119 5.3. Materials and Methodologies 123 5.3.1. Fungal cultivation in solid media 123 5.3.2. Wood shaving infection experiments 124 5.3.3. Sterilization treatment 126 5.3.3.1. Preparation of the test pole sections 126 5.3.3.2. Radio frequency/vacuum dryer 126 5.3.3.3. Heating treatment 126 5.3.4. Sample analysis 128 5.3.4.1. Growth conditions 128 5.3.4.2. Shaving recovery 129 viii 5.4. Results and Discussion 130 5.4.1 The temperature figure of radio frequency heating treatment 130 5.4.2 Mortality of the fungi 132 5.5. Conclusions 139 C H A P T E R 6 CONCLUSIONS AND RECOMMENDATIONS 140 6.1. Conclusions 140 6.2. Recommendations 141 Literature cited 142 APPENDIX I 163 APPENDIX II 176 APPENDIX III 190 ix LIST OF TABLES Table 2-1. Drying time and moisture content profile of drying series-1 51 Table 2-2. Drying time and moisture content profile of drying series-2 55 Table 2-3. Drying time and moisture content profile of drying series-3 59 Table 2-4. Drying time and moisture content profile of drying series-4 63 Table 3-1. Approximate distribution of chrome (and of Cu and As chemically reacted with it) between lignin and holocellulose 77 Table 3-2. Moisture content % profile 99 Table 3-3. Chromotropic acid solution fixation observations right after radio frequency heating 100 Table 3-4. Analyses data of radio frequency treating of western red cedar roundwood sections 102 Table 3-5. Analyses data of radio frequency treating of Douglas-fir roundwood sections 103 Table 3-6. Analyses data of radio frequency treating of red pine roundwood sections 104 Table 3-7. Chromotropic acid fixation test observations 105 Table 3-8. The effect of stored heat on the fixation rate of red pine 106 Table 3-9. Analysis data of hot eater fixation of southern yellow pine block 107 Table 5-1. Estimated values for strength losses at early stages of decay 117 Table 5-2. Examples of organisms capable of surviving exposure to elevated temperatures 121 x Table 5-3. Presence of Postia placenta 12OF in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln 137 Table 5-4. Presence of Postia placenta 31094B in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln 137 Table 5-5. Presence of Gloeophyllum trabeum 47D in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln 138 Table 5-6. Presence of Coniophora puteana in pre - and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln 138 xi LIST OF FIGURES Figure 2-1. Attraction of water to cellolose 15 Figure 2-2. Cutting pattern of each pole section for the measurement of the overall initial moisture content 38 Figure 2-3. Cutting pattern of each disk for the measurement of the heartwood and sapwood initial moisture content 38 Figure 2-4. Cross-section schematic diagram of the radio frequency/vacuum dryer 39 Figure 2-5. Location of temperature sensors in pole section 42 Figure 2-6. The description of the sample recovery 45 Figure 2-7. Temperature variations at three locations in cedar pole section #C1 (Run 1) 47 Figure 2-8. Temperature variation with time at three locations in cedar and Douglas-fir pole sections in Runs 2 and 3 48 Figure 2-9. Temperature variation with time at three locations in cedar pole sections #C4 and Douglas-fir pole sections #DF4 (Runs 4 and 5) 49 Figure 2-10. Temperature variation with time at three locations in cedar pole sections #C7 and in Douglas-fir pole sections # DF 5 (Runs 7 and 8) 53 Figure 2-11. Temperature variation with time at two locations in cedar pole sections #C9 and at four locations in Douglas-fir pole section #DF24 (Runs Hand 19) 58 xii Figure 2-12. Temperature variation with time at three locations in Douglas-fir #DF 9 and red pine #RP 6 (Runs 20 and 37) 61 Figure 2-13. The effects of sapwood initial moisture content on the initial heating rate when oscillator as radio frequency generator 66 Figure 2-14. The effects of heartwood/inner sapwood initial moisture content on the initial heating rate when oscillator as radio frequency generator 67 Figure 2-15. The relationship of drying time with the loss of moisture content when oscillator as radio frequency generator 68 Figure 2-16. The difference of initial moisture content and the difference of final moisture content 69 Figure 3-1. Fixation of CCA components in southern pine lumber treated with 1.6% CCA-C at 21 °C- Expressate method 79 Figure 3-2. Schedule of the preservation treatment 87 Figure 3-3. The description of the sample recovery 92 Figure 3-4. Apparatus for vacuum impregnation 94 Figure 3-5a. Temperature changes within cedar during radio frequency heating 96 Figure 3-5b. Temperature changes within Douglas-fir during radio frequency heating 96 Figure 3.5c. Temperature changes within the red pine during radio frequency heating 97 Figure 3-6. Effect of stored heat on fixation of red pine pole section 106 xiii Figure 4-1. The appearance of Douglas-fir after radio frequency/vacuum drying 110 Figure 4-2. The appearance of CCA treated red pine after radio frequency heating (I) 111 Figure 4-3. The appearance of CCA treated red pine after radio frequency heating (II) 111 Figure 4-4. The appearance around some knots of CCA treated red pine after radio frequency heating 112 Figure 4-5. The appearance of air dried cedar and Douglas-fir after 6 months 112 Figure 5-1. Temperature figure of sterilization treatment of Douglas-fir and cedar 131 Figure 5-2. Sterilized treatment result of western red cedar with Gloeophyllum trabeum 47D 133 Figure 5-3. Control samples of western red cedar with Gloeophyllum trabeum AIT) 133 Figure 5-4. Sterilized treatment result of Douglas-fir with Gloeophyllum trabeum AIT) 134 Figure 5-5. Control samples of Douglas-fir with Gloeophyllum trabeum 47D 134 Figure 5-6. Sterilized treatment result of western red cedar with Postia placenta 120F 135 Figure 5-7. Control samples of western red cedar with Postia placenta 120F 135 Figure 5-8. Sterilized treatment result of Douglas-fir with Postia placenta 120F 136 xiv Figure 5-9. Control samples of Douglas-fir with Postia placenta 120F XV LIST OF ABREVIATIONS AND TERMS ACA ammoniacal copper arsenic CCA chromated copper arsenic DRIFT diffuse reflectance Fourier transform infrared spectroscopy ESR electron spin resonance FSP fiber saturation point PCP pentachlorophenol RF/V radio frequency/vacuum XPS X-ray photoelectron spectroscopy STERILIZATION the eradication of fungi inside the wood xvi ACKNOWLEDGEMENTS I would like to thank, first and foremost, Dr. John N.R.Ruddick. His enthusiasm and patience ensured that I was able to complete the whole research program. I felt pleased to have him as my supervisor during my Master's program. I highly appreciated Dr. Stavros Avramidis for his technical support of this project. I also like to thank my committee member Mr. Craig Wilson, Timber Specialties Ltd. as well as Manitoba Hydro for their financial support for this work. My special thanks go to Mr. Ben Lucas, Stella Jones, Timber Specialties Ltd. for providing pole materials and Forintek Canada Corp. for their contribution in treating the sections with CCA preservative. I would like to extend my sincere thanks to Dr. Liping Cai for his help in operating the radio frequency kiln and loading the experimental materials when other technical support was unavailable. I would also like to thank faulty of the Department of Wood Science for their contributions to my studies. To my colleagues of the Wood Preservation Group, I offer my gratitude. Finally, my greatest gratitude goes to my dearest husband—Hongping Yu. His encouragement, patience and unfailing spirited support ensured that I am able to complete my graduate studies at UBC. Extended thanks go to my dearest parents and sister for their care and understanding. xvii C H A P T E R 1 BACKGROUND 1.1. Introduction Wood is considered to be one of the most economically acceptable materials in North America. From raw material to finished products, the energy input is 70 times greater for a ton of aluminum than for a ton of lumber, and 17, 3.1 and 3 times greater for steel, brick and concrete block, respectively, than for wood (CORRIM, 1976). In addition, wood is also an excellent insulator. For example, it is 4, 6, 15 and 1,770 times more efficient as an insulator than cinder block, brick, concrete and aluminum, respectively (ASHRAE, 1989). That is why wood is so widely used for electric power distribution posts and cross-arms in North America (Cassen, et al., 1995). The utility pole market is relatively stable in Canada. In 1992, pole production contributed about a half million cubic meters to the total production of treated wood (Stephens, et al., 1994). In Canada, Manitoba Hydro, Ontario Hydro, Quebec Hydro and B.C Hydro, are the major users of treated wood poles. It was estimated that they each purchase in excess of 15-20,000 new poles annually, worth in excess of $20 million (Mann, 1996). 1.1.1 The biodegradation of wood pole Wood being a naturally biodegradable product is often vulnerable to the attacks of wood destroying organisms (Richardson, 1993). Wood decay can originate within the living tree (Richardson, 1993; Cwielong and Rajchenberg, 1995). It also frequently 1 happens in the felled trees if they are not removed quickly for processing (Branzanti and Govi 1994). Air seasoning in yard for removing moisture from peeled poles prior to preservative treatment can also result in the introduction of internal decay and insects (Cassens et al, 1995). In a recent study, virtually all Douglas-fir poles seasoned for one year in the Pacific Northwest contained at least one fungal species (Przybylowicz et al., 1987). Even treated poles in service may be attacked by wood destroying organisms, due to preservative breakdown or loss, or improper application (Ross et al., 1997). This can lower the wood quality, and more seriously, lead to pole failure in service. The elimination of wood decay is therefore very important for the utility pole industry (Freitag and Morrell, 1998). In general, the basic requirements of deteriorating organisms to grow are oxygen, a suitable temperature (15-30C0), moisture and source of nutrients (Cassens, et al., 1995). If one or more of these factors can be controlled, wood decay can be prevented. Many strategies are currently used to limit occurrence of decay in wood prior to treatment. These include the removal of bark, the use of supplemental chemicals such as fumigants and boron rods and heat treatment. However, the residual fumigant can produce problems when the poles are used in sensitive environments. Heat treatment is one of the simplest approaches to kill organisms residing deep in the wood, without any significant residual effect (Freitag and Morrell, 1998). A temperature of 67°C to 71°C maintained for 60 to 75 minutes has been shown effectively to kill nematodes, insects and fungi present in the wood (Morrell, 1995). However, when wood is heated to high temperatures, it can suffer 2 loss in strength and case-hardening (MacLean, 1952; Nicholas and Siau, 1973; Morrell, 1995; Winandy and Morrell, 1998). 1.1.2 The limitation of currently used pole drying strategy Freshly cut lumber has a moisture content over fiber saturation point (FSP) and must be dried to a moisture content of less than this before being used (Bramhall and Wellwood, 1976). It is well known that dry wood in addition to having a lower potential for degradation has a number of other advantages over green lumber (Rietz, 1971), which include: • lighter, which lowers the transportation and handling costs. • stronger, which improves most mechanical properties of wood. • dimensionally more stable, when used under stable low humidity condition. Since most shrinkage and distortion accompanying the drying procedure have taken place once the wood reach a moisture content of 15%. Moreover, dried wood is necessary for most pressure preservative treatments, since it allows preservative solution to occupy the cell lumen. Low moisture content is also necessary for both finishing and gluing of wood. From the point of view of economics, wood drying can add value to the material. For example, in 1995 the price for unseasoned western hemlock baby squares was US$ 432-445/m3, whereas the kiln dried baby squares had an added value of 15% (Li, 1996). With the rising cost of wood products, more attention is being paid to the aspects related 3 to drying (Milota and Wengert, 1995). Currently, about 5x10 m of Canadian lumber is kiln dried annually (Zhang et al., 1997), which indicates the considerable importance of this process for the forest products sector. For this reason, extensive research has been carried out in order to identify more efficient drying methods and technologies (Pound, 1966; Harris and Lee, 1985; Avramidis and Mackay, 1988; Avramidis and Zwick, 1992; Avramidis and Oliverira, 1993; Avramidis et al., 1994; 1996; Avramidis and Zwick, 1996 and 1997). At present, conventional kiln drying is the most common artificial drying method used by the wood industry. Conventional drying is carried out on the basis of convective and conductive heat transfer mechanisms. During this process, large amounts of heat are released through venting resulting in waste of energy (Rietz, 1971; Zhang et al., 1997; Zhou, 1997). There is also a lot of data supporting the ability of kiln-drying to eliminate fungi, insects, and nematodes inside wood (Dwinell, 1990 a, b and c; Graham and Womack, 1972; Ostaff and Cech 1978; Simpson, 1991; Tomminen and Nourteva, 1992). However, the use of artificial drying had not been commonly used for drying poles. The most important reason is economics; kiln drying is considered to be time- and energy-consuming and expensive. A second difficulty of kiln drying is the non-uniform final moisture content (overheating of the smaller diameter sections), resulting in a hardened wood surface and checking (Trumble and Messina, 1985). In addition, exposure to high temperature can negatively affect wood strength (Freitag and Morrell, 1998). Due tb the difference in the bottom and top diameters, conventional kiln drying is not efficient for poles. Air drying is still commonly practiced for seasoning poles in Canada. The whole 4 drying process may take one to two years, depending on the ambient conditions and wood species (Rietz, 1971). A t any time there are several mi l l ion poles being air-dried in North America and these poles require a large storage space. A n alternative drying strategy would therefore be highly desirable for drying utility poles. 1.1.3 RF/V drying strategy Dielectric heating of wood, due to its unique features o f rapid and uniform internally produced heat, has gained renewed interest in the recent years (Nelson and Kraszewski, 1990; Avramidis and Zwick, 1992, Smith et ah, 1996). One o f the most significant applications of dielectric heating in wood has been the radio frequency/ vacuum drying (RF /V) . It has been recognized that with appropriate schedules R F / V drying could prevent lumber from discoloration and produce timber with minimum surface checking and hardening. R F / V drying could also reduce internal stresses, and the dried timber would have a uniform final moisture content compared to that produced by conventional drying (Avramidis and Zwick, 1992 and 1996 Avramidis et al., 1994). A research and development project administered by the Council o f Forest Industries of B . C . (COFI) revealed that in addition to the above advantages, R F / V drying provided significant reduction in the time required to dry thick timber (larger that 100 mm in thickness). For example, white pine (100 mm thick and 2.4m long) required about 54 hours to dry from a moisture content of 146% down to 8% using the R F / V drying, compared to 6 weeks for the conventional drying to achieve the same result (Harris and Lee, 1985). However, it is necessary to demonstrate that strength losses are not significant when applying R F / V drying. 5 1.1.4 Preservatives used to protect wood poles Preservative chemicals are applied to wood in order to protect it against degradation by fungi and insects (Smith et al., 1996). They are traditionally divided into two categories: oilborne and waterborne. Of the oilborne preservatives, only creosote and pentachlorophenol are widely used in Canada (Cassens, et al., 1995). Creosote is the preservative that has persisted since the earliest days of preservative treatments. It is derived from coal-tar, which is a by-product of high temperature carbonization of bituminous coal. As a complex, creosote is effective against most biological organisms that attack wood. Although, not an environmental friendly preservative, it is still commonly used on heavy timber, poles, and piles (Cassens, et al., 1995; Zheng, 1995) Pentachlorphenol dominated the preservative treatment of wood poles in Canada in the early 1970s (Ruddick et al., 1991). It is effective against many wood-destroying organisms such as mold, stain, decay fungi, and insects. It is produced commercially by the chlorination of phenol, using aluminum trichloride as a catalyst. Because of its toxicity and the presence of dioxin contaminants, it became a restricted-use pesticide since November 1986. Waterborne preservatives are formulations of inorganic chemicals that are dissolved in aqueous solutions. The most common waterborne preservatives include ammonical copper arsenate (ACA) and chromated copper arsenate (CCA). Due to the 6 poor surface appearance of the treated timber and an enhanced leachability of arsenic, ACA has not developed commercially to the extent of CCA (Zheng, 1995). CCA preservative is a mixture of inorganic chemicals (chromate oxide, copper oxide and arsenic oxide dissolving in large amount of water). The first formulation of CCA was invented in 1933 by an Indian researcher—Dr. Sonti Kamesan. It was first specified by American Wood Preservers' Association (AWPA) as CCA-type A in 1953. It was then rapidly developed. By early 1940's, it culminated in the first full-scale commercial installation of CCA- treated poles by Bell Telephone system. During the early 1950's, following extensive laboratory studies by Henry and Fahlstrom, a new CCA formulation known as type-C was approved by AWPA. This formulation contained more suitable arsenic: chromium proportions. Today, it is estimated that CCA-type C dominates with over 90% of the CCA treated wood produced annually worldwide (Cassen, et al., 1995). After over 60 years of trials, CCA demonstrate its potential as an outstanding waterborne preservative to provide maximum protection for wood. Currently, lumber and many other types of wood products including posts, poles and some marine piling are treated with CCA in North America (Cassen, et al, 1995). Treated poles were introduced into the utility pole market of Canada in the mid-1970's (Ruddick, 1978). By the end of 1980's utility companies in Canada replaced PCP 7 with CCA as the preservatives for utility poles (Ruddick, et al, 1991). Companies like Bell Canada, Ontario Hydro and B.C Hydro accepted CCA treatment due to its more desirable properties, such as lower flammability, high fungicidal efficiency and longevity compared to other preservatives. (Trumber and Messina, 1985). However, there has been criticism from linemen focused on the climbability of CCA treated utility poles (Trumber and Messina, 1985; Ruddick, et al, 1991, Lacy, 1991; Brudermann, 1994). The increased surface hardness in pine poles could be the result of reactions of chromium with the lignin and/or with cellulose during the fixation process (Pizzi, 1979; 1981; Ruddick, et al., 1991). Efforts to overcome the problem have produced at least two strategies. The first method include modifying the CCA-C formula by adding polyethylene glycol with an average molecule weight of 1000 (CCA-PEG 1000) (Trumber and Messina 1985). Investigation conducted on the fixation process, the CCA retention and pole climbability with the improved formula, concluded that the use of polyethylene glycol tends to reduce the perceived hardening of the treated pole (Trumble and Messina, 1985; Mclntyre and Fox, 1990; Zahora and Rector, 1990). However, loss of PEG with time has led to consideration of polyethylene glycol with an average molecular weight of 8000 (CCA-PEG+) in place of the PEG 1000 (Cooper et al, 1995b; Gilbert et al, 1997). The other strategy called CCA/oil emulsion system, which involved a special heavy lubrication oil. The demonstration study showed that the CCA/oil poles at all retention levels had enhanced climbability. However, some additional improvement in performance need to be further noted (Engdahl, E.K, et al, 1992) 8 An important aspect of CCA treatment is the fixation of the chemicals so that they will not leach out from the treated wood in service. Like all chemical reactions, the fixation process in wood is time and temperature dependent. However, it is also strongly influenced by the relative humidity (Avramidis and Ruddick, 1989; Cooper arid Ung, 1989; Smith, et al., 1996). A common practice at many treating plants is to let freshly treated wood stand for two days on a drip pad so that any unfixed chemicals can drip from lumber and be collected for reuse. However, it is clear that the fixation reaction takes place much more slowly at ambient temperature (Avramidis and Ruddick, 1989), particularly during the winter conditions when most of the consumer lumber is produced (Smith, et al., 1996). As the fixation rate is a function of temperature, at a certain range of initial temperature, normal reaction rates increase rapidly with temperature about two times per 10°C rises in temperature according to Arrhenius equation. -E K=AeKr (1.1) K= fixation rate where A= constant; E= activation energy, J; R= gas constant; T= temperature K. 1.1.5 Accelerated fixation of CCA With the enhanced focus on the environmental impact of the forest products industries in recent years, CCA treaters could not continue to operate their plants in the same way as they did in the past. New approaches of handling freshly treated wood in preservation industry had to be developed to minimize the chance of ground contamination at treating plants and storage yards as well as to prevent the loss of 9 chemical from wood in service. The Canadian Wood Preservation Standards require CCA treated wood to be tested for fixation, prior to its leaving the treating plant (CSA 080, 1989). Consequently, artificial heating of treated timber, either with live steam or hot water has now become more common, particularly for lumber and poles. However, for large dimensions wood products, extended times are required for all of the treated wood to reach a suitable temperature for rapid fixation, which makes it a very much time and energy consuming process. On the other hand, the use of electromagnetic radiation, either radio frequency (1 to 100MHz) or microwave (300 MHz to 300GHz) could significantly accelerate the process, which is independent of the wood thickness. Thus, the use of radio frequency (RF) technologies to rapidly fix CCA in treated timber and poles could be of great value to the wood preservation industry (Avramidis and Ruddick, 1996; Smith, et al, 1996). 10 1.2. Thesis objectives The objective of this study was to confirm the potential benefits RF technology for the utility pole industry. Three components were identified which could benefit the industry that became the focus of this project. The first component was to demonstrate that RF heating could efficiently dry tapered roundwood quickly and uniformly. This study would be done using the existing RF/V kiln, which had flat parallel plates located in the Wood Drying Laboratory at UBC. It was recognized that the use of curved plates may further enhance the process efficiency. The second component focused on the industry need to produce sterilized (without the present of fungi) treated poles. The eradication of internal decay contributes to pole performance by eliminating costly failures after only a few year of service. The research examined the time- temperature relationship achieved during RF/V conditioning of the roundwood sections. The feasibility of achieving the necessary conditions to kill fungi present in the pole sections, in less than two hours of RF heating, was examined. The third component examined the fixation of the waterborne preservative CCA in western red cedar, Douglas-fir and red pine pole sections. The purpose of this investigation was to prove our hypothesis, that RF heating can rapidly fix (in less than four hours) the CCA in treated roundwood. 11 CHAPTER 2 RADIO FREQUENCY CONDITIONDING/DRYING OF ROUNDWOOD 2.1 Introduction Currently, air seasoning is the most commonly used method in North America for drying poles (Newbill and Morrel, 1991). However, air seasoning is time consuming and can not address the need when large number of poles are needed quickly, such as the replacement of poles damaged during the ice storm in Eastern Canada and the U.S. in January, 1998. Meanwhile, air drying requires pole producers to retain a large inventory, leading to high overhead cost. In addition, during the drying process, it is difficult to prevent the loss in quality due to fungal colonization. This can result in premature pole failure in service (Highley et al., 1994). The drying of timber using RF heating has been demonstrated successfully, which was independence of wood thickness. The results also showed that it minimize checking and produced a uniform final moisture content (Avramidis and Zwick, 1992, Avramidis and Dubois, 1992, Avramidis et al, 1994 and 1996). The primary goal of this part of the thesis was to determine whether RF/V heating could efficiently dry tapered roundwood quickly and uniformly. The hypothesis would be tested using the laboratory RF/V kiln, which had flat parallel electrodes and was located at the Wood Drying Laboratory at The University of British Columbia. During the process, the temperatures reached at various heating times in different locations of each 12 pole section, were measured and recorded. The speed of the drying process was then developed. The efficacy of RF/V drying of pole sections could be obtained by the final moisture content profile and the total drying times. 13 2.2 Literature Review Since the properties of wood are profoundly influenced by the presence of water (Richardson, 1993; Haygreen, 1996), it is important to understand the nature of water in wood and how it is associated with its microstructure and properties. Consequently, a review of the literature devoted to the current wood drying practical aspects was prepared. The literature review then focuses on the RF/V of timber heating and the benefits of using this technique. It was anticipated that RF/V drying could bring a bright future to the utility pole industry. 2.2.1 Nature of water in wood Water is a natural constituent of all parts of a living tree (Haygreen, 1996). It is present in green wood in two forms: (a) as free water contained in the cavities of the cells or pores or as vapor water in lumens, and (b) as bound water absorbed to the substance of the wood cell walls (Pratt, 1974). The free water is subjected to capillary forces (Skaar, 1972), while the water within the cell wall is held by adsorption forces (Jokel et al., 1987; Haygreen, 1996). Since the forces holding free water are appreciably lower than those holding the bound water are, the free water in the cell cavity and pores leaves first when the wood dries. Wood, like all plants, has a cellular structure, in which the various wood cells are composed of cellulose, hemicellulose and lignin (Haygreen, 1996). Hydroxyl (-OH) groups exist throughout their structure, particularly in the cellulose and hemicellulose. 14 These hydroxyl groups attract and hold water molecules on the surface of the crystallites or in the amorphous regions by hydrogen bonding (Figure 2-1). Figure 2-1 Attraction of water to cellulose 2.2.1.1 Wood moisture and the environment Since wood is a hygroscopic material, it has the ability to remove water vapor from the surrounding air until it establishes equilibrium with the air. This moisture content designated as the equilibrium moisture content (EMC) is affected by the ambient relative humidity and temperature (Simpson, 1979; Skaar, 1988). Several studies had been conducted on the influence of sorption hysteresis and temperatures on the equilibrium moisture content (Stamm, 1964; Skaar, 1972; Mitchell, 1981; Turc and 15 Cutter, 1984). It is concluded that the hysteresis effect is considerably different between species. High temperature (over 100°C) had a permanent effect on the wood itself. The moisture content of green wood varies considerably among species, geographical location, age and size of the tree, even between heartwood and sapwood in the same tree (Skaar, 1988; Haygreen, 1996). For softwoods, the average green moisture content tends to decrease as a tree grows older (Koch, 1972; Haygreen, 1996). For example, there could be a 30% difference in the average moisture content of a 45 years old southern pine (Pinus sp) compared to trees under 25 years old. For hardwoods, the overall moisture content does not significantly reduce as they grow older (Haygreen, 1996). Several studies have concluded that there are only small differences in moisture content between sapwood and heartwood of hardwoods, whereas the sapwood moisture content of softwoods usually is three to four times higher than heartwood moisture content (Peck 1953; Haygreen, 1996). In some cases there may be seasonal variations in green moisture content. For example, the moisture content is higher during the spring or midwinter than during the summer and early autumn months (Gibbs, et al., 1958; Henderson and Choong, 1968; Koch, 1972). Wood in its final form, such as furniture, building material and utility poles is subjected to fluctuating atmospheric temperature and humidity. The equilibrium moisture content of wood in use is significantly affected by the atmospheric humidity, sometimes periodically and sometimes sporadically, such as when the wood product is exposed to rain (Skaar, 1988). Wood exposed to direct rain or liquid water in any form during use 16 can be colonized by fungi, whereas wood subjected to lower humidity will have a moisture content below 30%, and so will not decay. However, the moisture content to which wood should be dried depends on its final use (Skaar, 1988). 2.2.1.2 Moisture content and strength of wood It has already been noted that the strength or mechanical properties of wood generally increase with the loss of moisture from the cell walls (that is below fiber saturation), while the loss of capillary or "free water" from the cell cavities has no effect (Stamm, 1971; Skaar, 1988; Haygreen, 1996). It has also been found that a moisture content below 5% or 6% may actually cause the reduction in wood strength (Bodig and Jayne, 1982; Gerhards, 1982; Green, etal, 1986). 2.2.1.3 Moisture movement during drying process The movement of water in wood during drying takes place as mass movement of liquid water and/or diffusion of individual water molecules, while diffusion involves both bound water in the cell wall and vapor in the lumen (Haygreen, 1996). When wood is above the fiber saturation point (FSP), liquid water is present in the cell cavities. In this case capillary forces and related thermodynamic potential gradients operate to cause liquid flow. The flow rate is then determined by the combination of these forces and wood permeability (Kozlik, 1960; Siau, 1984; Cudinov et al, 1985; Skaar, 1988; Vermaas, 1998). 17 Bound water movement through the cell wall is the most important mechanism of water transport in the hygroscopic range. There are many different or even controversial ideas regarding this subject (Skaar, 1988). To simulate moisture movement through wood in the hygroscopic range, Stamm proposed an electrical analog model (1964). However, in Siau's (1984) model, the variable resistance (PM) in Stamm's model was neglected. Nelson (1986 a and b) then proposed his own model for moisture diffusion through wood. He defined the movement of bound water using equation 2.1 based on fluid mechanics. This equation is identical to that based on thermodynamics where the diffusion driving force was defined as the moisture flux per unit transport coefficient. The equation derived from the Nelson model is consistent with that from the model published by Siau (Siau, 1984). where, D s is the diffusion coefficient, m2/s; D 0 s the transport coefficient ; S is the tortuosity factor from 2.0-1.7; E s is the activation energy, J; R is the gas constant; T is the temperature K. By assuming that the driving force for diffusion was the partial pressure of water vapor, Hunter (1992) proposed a diffusion equation for water in wood in terms of temperature and moisture gradient. An analytical expression for the activation energy of diffusion in terms of enthalpy and entropy changes associated with the sorption process was then developed. Malmquist and Soderstrom (1996) further studied the physical nature of sorption of water by wood. They concluded that the physical sorption, responsible for the swelling and shrinkage of the sorbent, was coupled with a chemical (2.1) 18 sorption of water to the free -OH group. They also suggested that a number of structural arrangements in the crystalline part of the cell walls, which provided a steric hindrance for swelling in the high moisture content range and the nature of the chemical sorption, determined the isotherms. The enthalpy of the sorbent itself was affected by the chemical sorption whereas the chemical sorption was considerably changed (Malmquist and Soderstrom, 1996). The rate of wood drying is determined by the rate of water diffusion or the rate of mass movement to the wood surface (Haygreen, 1996). In some species the rate of water movement is governed by their anatomical structure, such as the presence of tyloses, aspirated pits etc. (Kozlik, 1960; Vermaas, 1998). High temperature used during drying could have positive effect on the diffusion of water (Stamm, 1964). For example, it was increased tenfold by raising the temperature from 50°C to 120°C. A variety of treatments, such as freezing lumber (Erickson et al., 1966), application of hygroscopic chemicals (Haygreen, 1996) and prestearning of wood (Harris et al., 1989) to increase the movement of water through wood prior to drying, have been found beneficial in some cases. 2.2.2 Drying technology 2.2.2.1 Air drying Air drying of lumber is still practiced throughout the world, especially in Africa and Asia. However, its application in North America is more limited (Milota and Wengert, 1995). Air seasoning is still commonly used in North America for seasoning 19 peeled utility poles (Newbill and Morrell, 1991). This process is usually time-consuming. For example, six months were required to condition paper birch (Betula papyrifera) in Minnesota while over a year was needed for the thick sapwood species, such as red pine (Pinus resinosa) and southern yellow pine to dry (Larson, 1986). Air-drying of timber can be a viable option when long drying times are acceptable and when the drying specifications are not too rigorous. Since air-drying is slow, it usually requires pole producers to retain a large inventory, leading to high overhead costs. In addition, during drying it is difficult to control degradation by fungi. This has been a particular problem in thick sapwood species such as red pine and southern pine where fungal colonization occurs drying the first few weeks of air seasoning (Panek, 1963). Przybylowicz et ah, (1987) noted that the longer the air drying time, the higher the percentage of poles with Basidiomycete fungi. They also observed that the Basidiomycete population gradually changed from fast-growing sapwood colonizers to slow growing heartwood colonizers. After one year of air drying, 90% of the poles were infected with Basidiomycete fungi. Such colonization and subsequent decay will often results in premature pole failure in service (Highley et al., 1994). Accelerated drying of poles has not been adopted previously due to the additional cost factor. However, the industry now faces the impact of increased pressure to ensure the complete fixation of the CCA preservative in treated poles, and hence to ensure negligible contamination of storage yards. Consequently, the industry is installing heating systems to accelerate the fixation process. Such systems could be made more economical 20 by increasing the return on the cost of installation through pre- and post- treatment conditioning of the poles. Electromagnetic dielectric heating, especially RF/V drying of wood, offers significant potential for the utility pole industry. 2.2.2.2 Conventional kiln drying Most lumber, including hardwood and softwood, is dried in some type of kiln (Haygreen, 1996). Conventional drying, referred to "heat- and -vent" drying (Erickson and Seavey, 1990 and 1992; Avramidis and Zwick, 1992; Avramidis, et al, 1994, Zhou, 1997), is the most common industrial method used to dry lumber (Avramidis and Zwick, 1992). Under conventional kiln, the circulating hot air in the kiln provides heat to lumber while at the same time is removing evaporated moisture (Avramidis, et al., 1994; Li, 1996; Zhou, 1997). Usually a pre-designed drying schedule is employed to dry lumber of a particular species and thickness (Boone, et al., 1988; Ismail, 1996; Simpson and Verrill, 1997; Maun and Moore, 1998; Jorgensen, 1998). Conventional drying is widely considered to require a high energy input (Avramidis, et al., 1994). During drying the creation of internal stresses is difficult to avoid, so that some degree of degrade is inevitable, thus reducing the lumber value (Faust, 1990; Zhou and Smith, 1991 a and b; Teischinger, 1992; Avramidis and Zwick, 1992; Eskelsen et al, 1993). One of the most commonly encountered problems in conventional drying of western hemlock {Tsuga heterophylla) and western red cedar {Thuja plicata) is an uneven 21 final moisture content between, and within, the boards. In addition, due to the presence of "wet pockets" resulting from an extremely high extractive content, the drying process takes a long time to complete, which may lead to collapse of the lumber (Kozlik, 1970; Meyer and Barton, 1971; Avramidis and Mackey, 1988; Li, 1996; Zhou, 1997). Having studied the conventional kiln drying of 105 mm x 105 mm x 4 m hemlock (Tsuga spp.) lumber using two drying schedules (beginning at 105°C and 80°C), Sato and Hoshide (1969) concluded that conventional kiln drying also caused severe shrinkage and twisting. Attempts have been made using high temperature schedules to alleviate the uneven final moisture content problem. Avramidis and Mackay (1988) conducted their study on examining 90.5 x 90.5mm Pacific coast hemlock (hem-fir) squares. Although they were able to narrow the difference between the core and shell in final moisture to only 4% to 6% with very little degrade, the drying time was long. For example, it took about 160 hours from initial moisture content of 71% down to an average final MC of 18%. Other attempts to reduce the large variation of final moisture content, such as pre-steaming and conditioning after conventional drying prolonged the total drying time (Avramidis and Mackay, 1988; Avramidis and Olivera, 1993). These problems were more pronounced when conventional drying was applied to thick western hemlock and western red cedar timbers. It was concluded that this drying technique is not appropriate to produce thick high quality products (Zwick et ah, 1995). 22 2.2.2.3 Dehumidification drying Dehumidification drying was first commercially used in Europe, especially for the drying of hardwoods (Haygreen, 1996). The development of electrically operated heat pumps for drying lumber stimulated use in North America (Milota and Wengert, 1995). The heat transferring thermodynamic characteristics of the basic components of the heat pump, the phase diagrams of different refrigerants, and the advantages and limitations of different compressors for this drying technology, were extensively investigated by Chen and Helmer (1982). Other studies have compared dehumidification drying with conventional drying. For example, Cech and Pfaff (1978) conducted a study on drying of 25mm thick hard maple (Acer saccharum) lumber using both methods. They found that the difference of drying time required to reduce the moisture content to 30% was minimal, the energy consumption at that point was 44-53% lower for dehumidification drying and the wood quality was similar for both technologies. Townsend (1978) examined the cost of both methods and reported that the cost of dehumidification drying was almost 60% lower/ 2.36 m3 than that of conventional drying, though the final moisture content was slightly higher (dehumidification drying, 16% after 7 days compared to conventional drying 10% after 9 days). 2.2.2.4 Vacuum drying Vacuum drying is currently one of the most popular of the "new" drying techniques (Milota and Wengert, 1995). The main interest in using vacuum drying is to 23 dry hygroscopic materials, where high temperatures could cause damage to material properties (Perre, et al; 1995; Skansi et al, 1997). This type of drying is particularly appropriate when a small amount of water has to be removed from hygroscopic materials (Forbert and Heimann, 1989). When the sample temperature is increased during the drying process, the rate of drying increases. The drying time was reduced with decreasing absolute vacuum at a constant temperature in the chamber (Skansi et al, 1997). The main advantage of vacuum kilns is the possibility of drying timbers from a surface-dry state down to low moisture contents more rapidly than kiln drying (Perre, et al, 1995). Several studies have shown that the greater the pressure differences between the vapor pressure in the lumen and the ambient pressure, the more efficient is the drying (Moyne and Martin, 1982; Waananen and Okos, 1989). But drying times are still too long for thick timbers using vacuum drying (Skansi et al, 1997). In addition, after investigating the vacuum drying characteristics of 50 mm and 75 mm thick discs cut from some domestic softwoods, Lee and his colleagues (1996) observed severe checks in sapwood after drying, due to the large difference in moisture content between sapwood and heartwood. 2.2.2.5 Superheated steam/vacuum drying Superheated steam/ vacuum drying technology is used in Europe for difficult-to-dry species and for drying specialty products (Anonymous, 1995; Avramidis, et al, 1996). It is a process in which the lumber under a vacuum of approximately 20 kPa, is dried at temperatures above the boiling point of water. Since the boiling point is relatively 24 low due to the vacuum, the lumber is exposed to lower drying temperatures which reduce the risk of drying degrade and result in faster drying process compared to the conventional drying method (Anonymous, 1995). Edlund (1988) dried sawn timber, 50x150mm of Scots pine (Pinus sylvestris) using superheated steam kiln, conventional kiln and in the open. She observed that it took about 46 hours at 80°C, relative humidity 70-75% in a superheated steam vacuum kiln, compared to the 16 days at 34-45°C, relative humidity 60-20% in a conventional kiln and 8 months outside (Oct.-June). According to a comparative study of drying of lumber, lumber seasoned by superheated steam/vacuum had minimal defects, 3 to 5 times shorter drying times, a maximum 4% dollar value drop in grade and was structurally more sound than conventionally seasoned lumber (Rosen, 1981). As part of a preliminary assessment, Forintek Canada Corp. brought this technology to Canada in 1995 and conducted an extended (more than four-year) project on drying schedules for Canadian species. Comparative studies, included drying tests with problem species and thicker Douglas-fir (Pseudotsuga menziesii) and hem fir products (Anonymous, 1995). The information generated with this research will allow Canadian lumber producers to decide whether this drying technology is suitable for their particular products. 2.2.2.6 Dielectric drying Electromagnetic waves According to Schiffmann (1987), all electromagnetic waves are characterized by their wavelength and frequency, and consist of two components, namely, electric and magnetic fields. The electric and magnetic fields are perpendicular to each other and are 25 both perpendicular to the travelling direction (X). When the velocity of the wave passing through another medium and travelling in the X-direction in air or vacuum, it will be reduced. Since the frequency of electromagnetic wave remains the same, therefore its wavelength decreases. Consequently, the depth of penetration is affected. Since electromagnetic heating is bulk heating, it is important that the energy can penetrate as deeply as possible (Zhou, 1997). The energy content and amplitude, as properties of energy wave, will change if electromagnetic wave travels through a medium. For example, the electric or magnetic component is zero at some point, then it builds up to a maximum value, reduces to zero, and builds up to maximum value again with the opposite polarity before again decreasing to zero. This periodic flip-plopping of the waves causes the stress upon ions, atoms and molecules and thus converted to heat. Under a high frequency alternation electric field, two types of heating processes within a material can be carried out. They are inductive and dielectric heating. In inductive heating, a metal coil is used to surround the material. As a good electrical conductor, the metal coil carries an alternation electric current. Consequently, the material inside the coil will become hot due to the development of eddy currents on the surface of the material. In dielectric heating, the heat is produced within the materials itself, when a dielectric material is placed in a high frequency alternating electric field. The mechanism of heat generation in a dielectric material is mainly based on polarization effects (Tinga and Nelson, 1973, Avramidis, et al, 1996). 26 Dielectric properties of wood Wood is capable of holding a relatively small amount of electric charge (Stamm, 1964; Avramidis et al., 1996). When wood is placed within an electric field, the existing electric charges arising from water molecules and hydroxyl ions can move in response. This creates a net surface charge at the boundaries of the wood pieces (Avramidis, et al., 1996). The dielectric properties of wood can be characterized by three parameters: dielectric constant (s'), loss tangent (tan 8) and loss factor (e"), (Torgovnikow, 1993). It is mathematically described as (Pound, 1973): 6"= e'tan (8) (2.2) The dielectric constant and loss tangent are anisotropic properties (Avramidis et al., 1996). They vary greatly according to moisture content, species, temperature, and the frequency used (Stamm, 1964; Avramidis and Dubois, 1992; Torgovnikow, 1993). Skaar (1948) noted that the variation in the moisture content had an "overwhelming" effect on the dielectric constant and loss tangent of wood. He further indicated that below the fibre saturation point the dielectric constant varied as an exponential ratio with the moisture content, but above the fibre saturation point a linear relationship was held. The same result was received from the work of Venkateswaran and Tiwari (1964). While, the loss tangent displayed the existence of a maximum followed by a minimum in some cases (Hearmon and Burcham, 1954; Venkateswaran and Tiwari, 1964 and James, 1975). 27 Both theoretical and experimental work indicated that the dielectric properties of wood were influenced by wood density (Skaar, 1948; Torgovnikov, 1993). Peyskens et al, (1984) reported a positive linear relationship existed between density and the dielectric constant. This relationship became more pronounced as the moisture content increased. However, the relationship between the loss tangent and density was not so clear (Dai, et al.,). Contrary to the studies mentioned above, Lin (1973) concluded that wood density had little effect on its dielectric properties. He furthered reasoned the variability due to different proportions of cell wall substances, which overshadow any possible effect of density on the dielectric properties. Temperature is also considered as one of the most important variables affecting the dielectric properties of wood. Lin (1967) and Nanassy (1970) observed that the dielectric constant increased with an increase of temperature when the moisture content of wood was high. However, the dependence of loss tangent on the temperature is more complex than that of the dielectric constant (Siau, 1995). The relationship between dielectric properties and frequency was somewhat complex, especially in the case of moist wood (James, 1975 and Torgovnikov, 1993). However, the effects on the dielectric properties of wood by some other factors, such as grain direction, electric field strength, need to be further studied. In summary, the loss factor E " relates to the ability of wood to absorb energy from an electromagnetic field. For a fixed frequency, e' is directly proportional to the moisture content 28 (Torgovnikow, 1993). Thus, wood at a high moisture content will heat up faster than dry wood, under the same radio frequency field (Avramidis and Liu, 1994). Power density The power loss in unit volume of a dielectric material such as wood under the influence of an external high frequency electric fields known as power density. Its value depends on the field strength, frequency, loss factor of wood. Since the loss factor of the wood will be reduced as the moisture content drops during RF/V drying, the amount of heat generated within the wood reduces with time and the moisture transfer becomes slower. Consequently, the drying rate decreases with time. On the other hand, with the constant increases in voltage, power density can be maintained. Thus a nearly constant drying rate was achieved. (Avramidis, et al, 1996). However, there is threshold of power density for each species, below that, the occurrence of internal honeycomb will not present (Avramidis and Zwick, 1996). Preliminary investigations with a laboratory scale RF/V unit indicated that power densities of approximately 5 kW/m3 could be used for drying hem-fir 91 mm in cross section, baby squares, while the base case for Douglas-fir is 4.5 kW/m3 (Liu, et al, 1994). Dielectric heating Dielectric heating of wood was first used in the former Soviet Union in 1934 (Pratt and Dean, 1949; Avramidis and Zwick, 1992). Since then, radio frequency heating and microwave heating, as commercial processes, have played an important role in wood drying (Biryukov 1961; Zhou, 1997), gluing (Jain and Dubey, 1988), and pressing 29 (Biryukov 1961; Zhou, 1997) over the last 50 years. However, microwave drying of wood was considered best suited for drying thin, highly permeable lumber (Antti, 1992). Other applications of this dielectric heating technology include heating and drying of materials that contain large quantities of water, such as agricultural grains, foodstuffs (Kim. et al, 1997, Funebo, 1997, Nijhuis et al, 1998) and paper products (Chen, et al, 1990, Lyons, 1972; Wei, 1985). RF heating can also accelerate the fixation of CCA in treated lumber (Avramidis and Ruddick, 1996; Smith, et al, 1996) as well as elimination of pinewood nematodes (Dwinell, et al, 1994). The basic principle involves placing wood between two electrodes and subjecting it to an electric field (E), oscillating at a high frequency within their range of movement. Being polarized, the water molecules in the wood attempted to align with the applied alternating field, thus generating heat (Haygreen, 1996). Oliveria and Neilson (1993) reported the processing for trembling aspen (Populus tremuloides) and white spruce (Picea glauca) into 6.5 and 8.5 cm veneer. The study examined radio frequency drying and conventional drying in terms of their effects on drying times, product quality and final moisture content distribution. The RF drying was superior over the conventional drying. Radio frequency/vacuum kiln design Research has been carried out on combining radio frequency drying with vacuum drying. This process drives the loss of water in the form of vapor by creating a pressure differential over the whole range of moisture content, so as to produce a rapid drying process (Avramidis and Zwick, 1992; and Hayashi and Nagase, 1995). During RF/V drying, both the pressure and temperature gradients in the wood to be dried point toward 30 the same direction, ie., from inside to outside thus causing the moisture to be driven out from the wood (Avramidis and Zwick, 1992; Zhou, 1997). The RF/V prototype kiln was designed for drying sawn softwood timber (Avramidis et al, 1996). Initial strategies (Miller 1966 and 1973) were to develop a RF/V drying process based on controlling the moisture loss using dry- and wet-bulb and lumber surface temperature. In the early 1990's, the first commercial size stainless steel RF/V kiln was designed by COFI and widely used in their project. It contains three vertical electrode magnetic fields and has 55 m3 in capacity. The maximum output power is up to 350 kW (Avramidis and Zwick, 1997). Recently, the second commercial size stainless steel RF/V kiln was further developed and manufactured by Heatwave company in interior, B.C. is 30 m in capacity. Two aluminum electrode plates are horizontally fixed in the kiln. One plate is placed on the top and the other one are placed at the bottom of the load of lumber. The heating energy is provided by a radio frequency generator that has a controllable power output up to 150 kW. Radio frequency/vacuum drying benefit RF/V drying has several significant advantages over other drying technologies. Firstly, it can accelerate the drying process due to selectively heating wetter area inside the wood and the rapidly produced internal heat (Avramidis, et al, 1996). For example, 50 mm thick white spruce was dried in a quarter of the time by RF/V drying compared to conventional drying (Miller 1966; 1973). Similarly RF/V drying of 25 mm 31 thick, 244 cm long red oak (Quercus rubra) boards was completed in 6% of the time taken by conventional drying (Harris and Taras 1984). Lee and Harris (1984) dried red oak in 60 hours by RF drying compared to 35 days in a dehumidification kiln. Harris and Lee (1985) also reported that drying of white pine (Pinus monticola) from a moisture content of 146% to 8%, required 6 weeks for conventional drying, but only 54 hours for RF/V drying. Kanagawa (1989) studied the drying of Douglas-fir lumber, 120 mm thick, 270 mm wide, and 300 mm long, using RF/V and conventional drying. He concluded that the drying times for the former ranged from about one-half to one-tenth of. those in a conventional kiln drying. Avramidis and Zwick (1996) used commercial scale RF/V kiln to dry Douglas-fir, hem-fir and western red cedar timber which ranged in thickness from 36 mm to 152 mm. Good results were achieved with a suitable RF/V drying schedule, whereas conventional drying could not be used for lumber over 50 mm thick (Avramidis, et al., 1996). The RF/V drying was economical and fast, particularly where the lumber thickness exceeded 8cm (Avramidis and Zwick, 1992). This leads to the second major benefit from radio frequency vacuum drying namely that it allows different sizes to be dried at the same time unlike conventional kiln drying where lumber of similar thickness must be dried together (Avramidis and Zwick, 1997). The third benefit from RF/V drying is that it eliminates the problem of slow heat conduction from the surface to the core of the board that occurs during the conventional drying. Instead, it selectively produces heat in moist wood resulting in a high quality product and a more uniform final moisture content distribution within the timber (Biryukov, 1961; Miller, 1966; 1969; Schiffmann, 1987; Nelson and Kraszewski, 1990; 32 Avramidis and Zwick, 1992). Pound (1966) dried 0.9m3 of green obeche (Triplochiton scleroxylon) to a moisture content of 22% and observed uniform final moisture contents both in the longitudinal direction and throughout the thickness of the boards. During a comparison of physical and mechanical properties between red oak timber (100 mm thick and 1.2 m long) seasoned by RF/V and conventional kiln drying, Harris and coworkers (Harris and Taras, 1984; Lee and Harris 1984; Harris and Lee, 1985) reported that the RF/V drying had a uniform final moisture content, a slightly lower specific gravity and compressive strength. However, no significant differences in static bending or shear strength, hardness or relative density between these two drying methods. The RF drying of different sizes and grades Pacific coast hemlock, western red cedar and Douglas-fir timbers were investigated by Avramidis and Zwick (1992) in a commercial RF/V kiln at a vacuum of 2.67 to 3.33 kPa. Minimal lumber degrade occurred with rapid RF/V drying, except severe honeycomb developed in the cedar. Avramidis and Zwick (1996) also reported that RF/V did not cause lumber staining, reduced surface checking, produced no internal stresses and gave a uniform final moisture content. Liu et al., (1994) examined the influence of variable RF electrode voltages on the drying rates, and concluded that the voltage could be increased without causing additional degrade, compared to the use of a constant electrode voltage. There was very little variation in the longitudinal moisture content by either heating method and internal drying stresses in the longitudinal and transverse directions were also absent. With the rising cost of lumber (Milota and Wengert, 1995), more attention is being paid to the costs of processing and producing quality material. Avramidis and 33 Zwick (1997) estimated the capital and operating costs of a commercial RF/V kiln. As a result of their analysis, they considered that RF/V drying is more competitive compared to conventional kilns, when drying 101mm or even thicker lumber, with at least a 15 to 25% lower cost depending on the wood species and grades being dried. Smith et al., (1996) also presented a case study to analysis the production of red oak dimensional squares from Prime, No.l, No.2 and No3 grade logs. They concluded that radio frequency/vacuum drying offered great possibilities to increase the profitability for manufacturing hardwood dimension timber. 34 2.3 Materials and Methodologies 2.3.1 Species and sizes Tapered western red cedar, Douglas-fir and red pine roundwood sections were dried in a series of experimental runs. All pole sections were approximately 165 mm to 240 mm in diameter. During preliminary experiment, three of the Douglas-fir and three of the western red cedar were slightly shorter than usual with lengths that ranged from 800 mm to 1.5 m. The rest of the wood pieces ranged from 2 m to 2.5 m. The western red cedar pole sections had an average sapwood depth of no more than 25 mm. Most of the Douglas-fir roundwood sections had an average depth of sapwood from 33 to 47 mm. They were obtained from Stella Jones at New Westminster, B.C. The red pine pole sections contained from 45 to 80 mm depth of sapwood, and were provided by Timber Specialties Inc. located in Ontario. All of the experimental pole sections were shipped directly to the Wood Drying Laboratory at The University of British Columbia. 2.3.2 Measurement of initial moisture content (M,) The measurement of initial moisture content was required in order to provide information regarding the degree of drying achieved for each drying run. At the beginning of each experiment, two 30 mm thick disk (D) were cut from each end using a chain saw (Figure 2-2). No disk was cut closer than 100 mm from the original uncut end of the pole section to limit the end effect. After cutting, the disks Di and D2 for Douglas-fir and western red cedar were immediately weighed (Wg) on a digital balance and oven 35 dried at 103 ± 2°C for 24 hours or until these weights were constant (W0). Meanwhile, disk D3 and D4 were chopped into four to six different pieces to produce the heartwood and sapwood samples. They were carefully labeled and weighed (Wg) on the digital balance, before being oven dried at 103 ± 2°C for 24 hours and re-weighed (W0). The initial moisture content of each sample was calculated according to the following formula: (2.3) where, Mi is the initial moisture content of the sample, Wg is the green weight (g) of the sample; and W0 is the oven-dry weight (g) of the sample. The average initial moisture contents (Mj) values for each roundwood section were calculated based on the data from Di and D2. It was calculated according to the following formula: M = V 40, 2 +r/2+r22) (2.4) (2.5) (2.6) X (2.7) 36 where, M is the initial moisture content (%) of the whole experimental sections, MDI is the initial moisture content ( %) of disk Di, MD2 is the initial moisture content (%) of disk D2, V is the volume of the whole experiment section (m ), ri is the radius (mm) of disk Di, tz is the radius (mm) of disk D2, while L stands for the length (mm) of the experimental section, x stands for the longitudinal distance (mm) between the end of disk D2 and any other sections ( Figure 2-2), while rx and M X are the radius (mm) and the moisture content (%) of the section in which the point x located, respectively. Equation 2.4 is deduced based on the assumption that the change of moisture content from one end to another end in the pole section is linear. From the equation, it is also noted that the average initial moisture content of each pole section can be achieved from the average moisture content of disk D\ and D2, if the difference of radius of both ends could be neglected. Then the average initial moisture content of sapwood and hardwood were calculated separately according to Eq. 2.3 and computed based on the data from D3 and D4. Each experimental pole section was weighed before drying treatment (Wj) using an electronic scale. After determining the average initial moisture content for each sample, the oven dry weight of each roundwood section could be calculated based on the following formula: W« = "MTT ( 2- 8 ) 37 where, W 0 is the oven dry weight of the roundwood sections (kg); and Wi is the green weight of the roundwood sections (kg). M stands for the initial moisture content of the pole section. < H ( X u D i D 3 Figure 2-2. Cutting pattern of each pole section for the measurement of the overall initial moisture content Figure 2-3. Cutting pattern of each disk for the measurement of the heartwood and sapwood initial moisture content 3 8 2.3.3 Radio frequency/vacuum dryer All the drying experiments were carried out in the laboratory radio frequency/vacuum dryer located in the Wood Drying Laboratory at The University of British Columbia. A cross-section schematic diagram of the radio frequency/vacuum dryer is illustrated in Figure 2-4. RGF Figure 2-4. Cross-section schematic diagram of the radio frequency /vacuum dryer The drying chamber consists of a 2.75m long carbon steel cylinder, which is 0.76m in diameter. It has a removable bolted door on each end. These doors are designed to facilitate loading and unloading of lumber and are coated with epoxy paint on the 39 interior to improve corrosion resistance. To ensure a good seal between the cylinder and the doors, the opening was fitted with rubber O-rings. Two 300 x 2240 x 12.7mm thick aluminum electrode plates (E) supported by polyethylene bolts (S), are horizontally fixed to the upper and lower walls of the cylinder. The vertical space between the plates can be adjusted by raising or lowering the upper electrode plate controlled using a hydraulic pump in order to accommodate the roundwood sections (R) of different diameters. The ambient pressure inside the dryer is controlled by a liquid-ring-vacuum pump (VP) with an air injector attached to the main input line. The water vapor produced during drying or heating is condensed in a heat exchanger (HE) and collected in a collection tank (CT). Any condensate, which accumulates at the bottom of the dryer is also pumped (P) into the collection tank. During drying or heating, the temperatures at various locations inside the roundwood section were monitored by fiber optic thermocouples. All data were collected and recorded by a computer through a data acquisition system. 2.3.3.1 Radio frequency amplifier A series of five runs were carried out using the radio frequency amplifier to generate the thermal energy. It has an output of 2 kW at a fixed frequency of 6.78MHz. Since no automatic drying schedules existed for roundwood, the operation of the kiln was carried out manually. The matching network circuitry of the amplifier, used to create an uniform electric field, also had to be tuned manually. Because the impedance of the wood 40 changed with time due to the reduction in moisture content, it required frequent adjustment. 2.3.3.2 Radio frequency oscillator Except for the first six drying runs, all of the heating experiments (including fixation treatment and sterilization treatment) were carried out using a radio frequency oscillator to generate the electric magnetic field. The oscillator was operated at a fixed frequency of 13.56 MHz, and had a maximum output of lOkW at a maximum electrode voltage of 5 kV. 2.3.4 Drying procedure 2.3.4.1 Temperature monitoring The temperatures inside the pole sections and at the air inside the kiln during drying, were monitored by fiber optic temperature sensors (1 mm in diameter). These sensors were placed at the mid-point of each pole section. A plastic cap in which a small hole was drilled to allow insertion of the thermocouple, was used to fasten the optic fiber thermocouple inside the wood. Two probes were used in run 13, 14 and 15. They were attached through 10 mm diameter holes, located 30 mm away from the surface, and at the center of the pole section. The drying experiment 4 to 6, 12 and 32-37 used three probes. They were also placed in holes lO mm in diameter with one in the outer shell, one at the mid-point of radius of the cross section and the last at the center of the cross section. In order to monitor the temperature of the air inside the chamber during drying, four temperature probes were used in the remained nineteen runs. In this case, one of the 41 probes was left outside of the pole section, while the other three probes were placed described above. (Figure 2-5). Temperature - probe (the air inside the chamber) Temperature-probe (outer shell) Temperature - probe (mid-point of radius) Temperature -probe (core) Figure 2-5. Location of temperature sensors in pole section 42 2.3.4.2 Dielectric runs Each experimental drying run was conducted on one roundwood section, except for the second and third drying experiments. They used two cedar (sample #C2 and #C3) and two Douglas-fir (sample # DF2 and #DF3) round wood sections, placed end to end between the two electrode plates. A l l o f the drying experiments started from ambient temperature (approximately 20°C). For the first three drying runs, the R F chamber was evacuated during the warm-up period, right after 1 hour of heating. In order to increase the rate o f drying, during the remaining drying experiments the k i ln was evacuated when all o f the temperature sensors inside the pole exceeded 60°C or one of the sensors exceeded 95°C. The drying was then carried out at a pressure o f 2.66 -3.60 kPa. A computer recorded the temperature every thirty minutes during the first twelve runs, and every four minutes for the remaining twenty-five runs through a data acquisition system. 2.3.5 Data analysis Immediately at the end of each drying run, the pole section was removed from the ki ln and the temperature sensors were carefully disconnected. Fol lowing that, the whole section was immediately weighed (Wf) on an electronic scale. The average final moisture content of the whole roundwood section (Mf) was calculated using following equation: (wf-w0) M ^ ^ - ^ x l O O P / o (2.9) 43 where, Mf is the final moisture content of the experimental pole, Wf is the final weight of the pole (kg) after drying; and W0 is the oven-dry weight of each pole section (kg) which calculated according to Eq 2.8. Following the weighing, the samples were recovered for determination of heartwood and sapwood moisture content. Two cores were removed from each section using a standard drill fitted with a 25 mm diameter drill bit. The two sampling locations were chosen randomly to represent the whole section, but taking care to avoid any "end effect". By sectioning each core, two to three sapwood pieces and one to two heartwood pieces were obtained from each location (Figure 2-6). After labeling, they were immediately weighed (wf) using a digital balance and oven dried at 103 ± 2°C for 24 hours. The oven-dried weight (w0) of each block was recorded and the final moisture content (Mf) of each sample was calculated according to the following formula: (wf -w0) Mf=X— ^xl00% (2.10) The average final moisture contents of the, sapwood and hardwood for each section were calculated from the individual sample data. 44 Location for sampling Figure 2-6. The description of the sample recovery 45 2.4 Results and Discussions 2.4.1 Radio frequency/vacuum heating results The results o f the temperature variation with time in this study were grouped according to the energy generator and the initial moisture content into four series. Since the purpose of this project was to examine the possibility of fast and uniform drying of tapered poles using RF7V process, the analysis o f the relationship between the energy consumption and the electrode voltage and their impact on the drying speed was not considered. 2.4.1.1 Series 1—using amplifier Five drying experiments were carried out using an amplifier to generate the electromagnetic field in the k i ln (Figure 2-7, 2-8, 2-9). In the first run (Run #1), the cedar pole section was only 112.5 cm long. In runs #2 and #3, two pole sections were placed end to end between the electrodes to create a total length of 223 cm. Each pole was at a different initial moisture content. The sample used were cedar #C2 and #C3 and Douglas-fir #DF2 and #DF3 respectively. In runs #4 and #5, full length pole sections, 210 cm and 244 cm respectively, were dried. During the drying process, the matching system required frequent adjustment due to its sensitivity to the change in the moisture content in the wood. 46 TV 9.7 cm away from the surface (heartwood) T 2: 6.2 cm away from the surface (heartwood) T 3: 4.3 cm away from the surface (heartwood) T 4: the air inside the chamber Figure 2-7.Temperature variation with time at three locations in cedar pole section #Cl(Run 1) 47 Ti : 7.0 cm away from the surface (heartwood of short log) T 2: 8.1 cm away from the surface (heartwood of long log) T 3: 2.0 cm away from the surface (sapwood of long log) T 4: the air inside the chamber 10 Drying of Douglas-fir #DF2 & 3 x x x x y M y K X A - X 50 1 00 1 50 200 250 300 350 400 450 500 550 600 Time (minutes) Ti : 7.0 cm away from the surface (heartwood of short log) T 2 : 8.1 cm away from the surface (heartwood of long log) T 3: 2.0 cm away from the surface (sapwood of long log) T 4 : the air inside the chamber Figure 2-8 Temperature variation with time at three locations in cedar and Douglas-fir pole sections in Runs 2 and 3 48 Drying of Cedar #C4 90 80 70 o 60 0 E 50 3 2 40 a E a 30 i -20 10 0 - • - T 1 m T I _ 1 —•— 1 £. \ i. SLJ t&»-m—-Vacuum 100 200 300 400 500 600 700 800 900 1000 Time (minutes) It: 7.0cm away from the surface T 2 : 5.0cm away from the surface T 3 : 2.0 cm away from the surface Drying of Douglas-fir #DF4 70 60 _ 50 o D £ 40 3 a E .2 20 10 100 200 300 400 500 Time (minutes) 600 4 - • - T 2 - A - T 3 l Vacuum ^ I I 700 800 Ti; 7.0 cm away from the surface T 2 : 5.0 cm away from the surface T 3 : 2.5 cm away from the surface Figure 2-9. Temperature variation with time at three locations in cedar pole sections #C4 and Douglas-fir pole sections #DF 4 (Runs 4 and 5) 49 It can be seen from the above graphs 2-7, 2-8 and 2-9 that the temperature in the core of the specimen (Ti) increased more quickly than the outer part of the section (T3). During the run 1, 2 and 3, the vacuum was applied at the beginning of warming up period. It took about 1-1.5 hours for the vacuum drop to about 4.00 kPa, compared to less than 15 minutes in the following runs, whose vacuum was applied after the temperature reached high degree. This slow vacuum reduction resulted in a smooth increase of temperature during the initial heating period. In order to keep the power input into the unit constant, the power was then artificially decreased, which resulted in the gradually decreased of temperature after reaching its maximum temperature. The same trend in the change in temperature with time was noted for both probes in mid-point of radius sections and that in the outer shell section. The temperature of the air of the chamber remained constant. During the runs 4 and 5, the introduction of the vacuum phase in the kiln began when the temperature probes inside the pole section reached in excess of 60°C for cedar #C4 and reached 40°C for Douglas-fir #DF4 respectively. This caused the water inside the pole to rapidly evaporate, which immediately cooled the pole section and resulted in an abrupt decrease in the temperature. Once the vacuum had reached an equilibrium, the temperature gradually increased as the wood started to heat up due to the vibration of water molecule. The total drying time, initial and final moisture content profiles for this series are listed in Table 2-1. The cedar sections selected for this drying series had a narrow 5 0 sapwood band, which ranged from 13 mm to 28 mm in thickness. The Douglas-fir sections had a large amount of sapwood, which ranged in thickness from 32 mm to 47 mm. The small amount of sapwood together with the low wood density was responsible for the shorter drying time for cedar, compared to Douglas-fir. Unfortunately, all pole sections had low initial heartwood moisture contents of 30% to 35%. The initial moisture content of sapwood varied from below 60% to greater than 100% for cedar, and from 33% to 48% for Douglas-fir. Following several hours of drying, the final moisture content of the sapwood and heartwood were quite uniform. The average difference between them was approximately 2% in runs 1,2 and 5, and about 8% in run 4. Table 2-1. Drying time and moisture content profile of drying series-1 Drying run Ave. depth Sample Ave. Initial Ave. Final 1) tying Sample ID of sapwood location HHHH M C time (length/diameter) (mm) ( % ) ( % ) (hrs) Run 1 Pole section 56.8 22.1 #C 1 (1.13 m, 17 Sapwood 70.1±18.5(4)* 21.5±0.7(2) 9 195/211 mm) Heartwood 29.6±2.7(6) 19.8±1.1(3) Run 2 Pole section 42.1 25.2 #C2(1.26 m, 13 Sapwood 58.3±27.6(4) 16.5±0.5(2) 10 175/185 mm) Heartwood 27.9±5.4 (6) 14.9±0.8 (3) Run 2 Pole section 53.5 26.7 #C 3 (975 mm, 23 Sapwood 101.2±32.2(4) 26.7±3.1(3) 10 170/180 mm) Heartwood 27.0± 1.2(6) 24.7±4.3 (3) Run 3 Pole section 31.8 24.5 #DF2(1.44 m) 37 Sapwood 39.1±2.9(4) 19.3±0.5(3) 10 160/175 mm) Heartwood 26.5±0.9(6) — Run 3 Pole section 36.4 35.1 #DF 3 (790 mm, 32 Sapwood 48.1±30.7(4) 33.1±2.1(3) 10 150/160 mm) Heartwood 35.8±11.6(6) — Run 4 Pole section 47.8 18.7 #C 4 (2.10 m, 28 Sapwood 96.4+17.3 22.2±2.6(8) 15 180/195 mm) Heartwood 39.6±7.0 13.6+1.5(8) Run 5 Pole section 29.3 17.1 #DF 4 (2. 44 m 47 Sapwood 32.8±14.9(8) 19.2±1.5(6) 12 200/230 mm) Heartwood 24.5±4.5(8) 15.7±0.5(6) *data are recorded as X±SD with number of samples in parentheses 51 2.4.1.2 Series 2— drying of low initial moisture content sections using the oscillator Seven drying runs were carried out in this series. Three employed cedar sections and four were Douglas-fir sections. They were all full length ranging from 2.02 m to 2.47 cm, with diameter of 185 mm to 260 mm. The initial moisture contents of the whole roundwood sections were around 35%, with an initial sapwood moisture content of less than 40%. There were only limited differences of initial moisture content between core and outer shell. Since no big difference of the temperature trend occurred between each run within each species, temperature variation with time in run 8 (cedar #C7) and run 9 (Douglas-fir #DF8) (Figure 2-10) were selected to represent the graph of temperature change with time for each species. It was clear from the temperature graph that within thirty minutes the temperature exceeded 65 °C in run 8, whereas it took more than sixty minutes for all the three temperatures in the Douglas-fir section to reach this temperature. The longer time was attributed to the higher density of Douglas-fir. Consistent with the first drying series, the inner wood temperature increased more rapidly than the outer wood. This was because the outer wood surface cooled by loss of moisture during heating (Smith et al, 1996). The rate of temperature increase in the first 30 minutes were almost 5 times greater than that in the remaining drying time in run 8. In run 9, the vacuum was introduced after all three temperatures had reached 65°C. When the vacuum had decreased from 101.32 kPa to about 3.47 kPa, all the temperature probes inside the pole section had shown a 52 temperature reduction of about 5°C. The temperature then increased slightly to its maximum temperature. Drying of Cedar #C7 & E 0 100 80 r 60 20 100 Jf - • - T 1 -m-T2 - A - T 3 25 50 75 100 Time (minutes) 125 150 T i : 9.0 cm away from the surface T 2 : 5.0 cm away from the surface T 3 : 2.5 cm away from the surface Drying of Douglas-fir #DF5 25 50 75 100 Time (minutes) 125 150 T i : 8.0 cm away from the surface T 2 : 5.0 cm away from the surface T 3 : 3.5 cm away from the surface Figure 2-10. Temperature variation with time at three locations in cedar pole sections #C7 and in Douglas-fir pole sections #DF5 (Runs 8 and 9) 53 The drying regimes in this series ranged from 1.3 hrs to 4 hrs (Table 2-2). The moisture contents of the sapwood and heartwood were all around the fiber saturation point or lower, with differences ranging from 0.3% to 11%. Considering the final moisture content after drying, the difference between sapwood and heartwood was approximately 3%, although there were some specimens, which had a much larger difference. Almost all the final sapwood moisture contents were higher than those of the corresponding heartwood. It could be explained that the radio frequency radiation selectively heated wetter areas of the wood during drying (Avramidis and Zwick, 1992). Also, the inner moisture diffused to the outer surface during drying. It was also clear that the drying times were much shorter in this series than those for series-1, due to the more powerful radio frequency generator used. 54 Table 2-2. Drying time and moisture content profile of drying series-2 Drying Run Sample ID Ave. depth of sapwood Sample location Ave. Initial MC Ave. Final M C Drying Time (length/diameter) (mm) (%) ( % ) (hrs) Run 6 Pole section 31.5 23.1 #C 5 (2.46 m, 25 Sapwood 34.6±9.0(8) * 24.7±1.6(3) 1.3 185/210 mm) Heartwood 27.1±2.2(8) 21.2+2.3(3) Run 7 Pole section 30.1 16.5 #C 6 (2.43 m, 19 Sapwood 35.1±2.3(3) 19.8±2.1(3) 1.6 200/240 mm) Heartwood 32.5+1.2(3) 17.7±3.2(3) Run 8 Pole section 34.0 17.7 #C 7 (2.40 m, 25 Sapwood 29.6±2.3(3) 20.1+2.1(3) 2.0 240/260 mm) Heartwood 35.1+1.2(3) 16.9±1.4(3) Run 9 Pole section 27.0 24.7 #DF 5 (2.26 m, 45 Sapwood 34.5±2.8(3) 26.1±2.3(3) 2.0 215/230 mm) Heartwood 27.5±3.6(3) 24.6±3.1(3) Run 10 Pole section 32.5 23.2 #DF6(2.21m) 47 Sapwood 36.6+2.2(3) 23.6+1.0(3) 2.1 205/230 mm) Heartwood 25.5±1.2(3) 20.3+0.9(3) Run 11 Pole section 31.3 20.5 #DF 7 (2.47 m, 33 Sapwood 30.1±1.8(3) 21.6±2.4(3) 2.5 195/215 mm) Heartwood 30.4±4.6(3) 17.5±2.4(3) Run 12 Pole section 31.1 15.7 #DF 8 (2.20 m) 28 Sapwood 34.1±3.2(3) 17.3±1.0(3) 4.0 185/210 mm) Heartwood 28.9+2.6(3) 12.1±3.4(3) * data are recorded in X±SD with the sample number in parentheses 55 2.4.1.3 Series 3—drving of medium initial moisture content sections using the oscillator Three cedar sections and four Douglas-fir pole sections were dried in this series. Since the drying curves within each species were similar, the graphs of the temperature variation with time for run 14 (cedar #C9) and run 19 (Douglas-fir #DF24) were selected to represent the series and are shown in Figure 2-11. During these drying runs, it is interesting to note that in the plot of temperature versus time, the initial increase in the temperature of the outer shell was more rapid than the inner wood for run 19. This is contrary to the earlier result where the core temperature was usually higher at the center of the cross section than that at either the mid-point of the radius, or near to the surface of the sections. However, it is also noted that the rates of initial temperature increase were similar for all probes, with differences only emerging after an hour or more. For the run 14, the temperature increased in the outer shell more slowly than that in the core. The differences in the temperatures in the core and outer sapwood can be explained by the role of water in the pole section during RF heating. Initially the water in the wood will generate heat at similar rates when the moisture contents are similar. As water evaporates from the pole section it will cool the surface, causing the temperature to fall. This is clearly shown during the drying of Douglas-fir #DF24 in run 19. A second factor, which may influence the rate of heating, is the wood density, which is almost twice as high for Douglas-fir than for cedar. The figure also revealed that in run 14 the core temperature was reduced by almost 20°C when the kiln was evacuated while the temperature in the outer shell was not significantly reduced. This 56 could be the result of the lack of free water in the outer part of the pole at the time of introducing vacuum, since the inflection point corresponded to a critical moisture content (Avramidis, et al, 1994). The temperatures in both areas then reached a plateau at approximately 100°C, which remained until the end of drying. In run 19, when the vacuum was introduced, the temperature decreased by approximately 30°C, 20°C and 25°C in the core, the mid-point of radius, and outer shell of the wood, respectively. The temperature at the center then increased slightly before leveling off at approximately 75°C where remained until the end of the drying. The outer sapwood, which had a high moisture content, remained constant at approximately 50°C following the introduction of the vacuum. In this series, the time required to the dried cedar section (ca. 25% in final moisture content) was less than 6 hours due to their narrow sapwood and low initial moisture content. The drying of Douglas-fir required 6 to 9 hours. The average difference of the final moisture content between the outer shell and the core was 2.4%, while the average difference of initial moisture content was almost 30% (Table 2-3). This phenomenon confirms the hypothesis that radio frequency vacuum dryer selectively heating water in the wood causing the wetter areas of the wood to dry more rapidly. 57 120 100 O o • 1 E o 80 60 40 20 Drying of Cedar #C9 J T1 -— T 2 , 1 — 1 1 — — 1 50 100 150 200 Time (minutes) 250 300 350 TV 9.0 cm away from the edge T2: 3.0 cm away from the edge Drying of Douglas-fir #DF24 50 100 150 200 250 300 Time (minutes) TV 9.5 cm away from the edge T2: 6.0 cm away from the edge T3: 3.0 cm away from the edge T4: the air inside the chamber 350 400 450 Figure 2-11. Temperature variation with time at two locations in cedar pole sections #C9 and at four locations in Douglas-fir pole sections #DF24 (Runs 14 and 19) 58 Table 2-3. Drying time and moisture content profile of drying series-3 Drying Run Sample II) (length/diameter) Ave. depth of sapwood (nun) Sample location Ave. Initial M C (%) A\e. Final MC (%) Dr\ ing time (hrs) Run 13 Pole section 34.4 13.7 #C 8 (2.02 m, 21 Sapwood 49.4±21.8(6)* 15.6+0.3(3) 6.0 180/215 mm) Heartwood 27.8±0.8(6) 12.7±1.24(3) Run 14 Pole section 43.2 8.0 #C 9 (2.02 m, 16 Sapwood 53.6+38.5(6) 9.5+0.9(3) 4.0 180/190 mm) Heartwood 42.0±18.8(6) 7.0±1.0(3) Run 15 Pole section 39.8 12.4 #C 10 (2.02 m, 8 Sapwood 19.2±1.4(6) 12.8±0.8(3) 3.0 195/220 mm) Heartwood 45.0±17.0(6) 11.5+0.4(3) Run 16 Pole section 54.5 24.4 #DF 10(2.03 m, 38 Sapwood 69.8±4.88(6) 29.4±2.3(6) 9.0 235/250 mm) Heartwood 32.7±0.42(6) 26.1±1.8(2) Run 17 Pole section 39.5 11.2 #DF 12 (2.03 m, 28 Sapwood 55.9±10.4(6) 9.7±1.1(5) 6.0 190/198 mm) Heartwood 30.9±0.85(6) 9.7±1.2(2) Run 18 Pole section 49.2 15.5 #DF 14(2.03 m, 45 Sapwood 52.1 ±4.0(6) 15.2+2.2(5) 6.5 190/205 mm) Heartwood 32.0±1.8(6) 16.5±1.3(2) Run 19 Pole section 65.1 24.2 #DF 24(2.10 m, 33 Sapwood 86.0±11.5(6) 26.1+4.4(6) 7.0 175/200 mm) Heartwood 34.4±1.4(6) 20.6±0.7(2) data are recorded as X+SD with number of samples in parentheses 2.4.1.4 Series 4—drying of high initial moisture content sections using the oscillator Eighteen drying runs were carried out in this series. All of the experimental specimens were 2.03 m to 2.20 m Douglas-fir and red pine pole sections with an initial moisture content of sapwood over 100% and an initial heartwood moisture content for Douglas-fir of around the fiber saturation point (Table 2-4). The drying time ranged from 10 hours to 16 hours. Figure 2-12 shows the temperature changes with time in run 20 and 37, which are representative for drying of Douglas-fir and red pine. In run 20, it can be 59 seen that the ambient temperature in the kiln remained constant at approximately 25°C. The temperature in the core of the section before introducing the vacuum was the lowest, at around 60°C. It was also interesting to note that before introducing vacuum, there was a steep temperature rise especially in the probe at the outer shell. After the vacuum was applied, the temperature inside the pole section decreased rapidly from 120°C to about 60°C in the outer shell, and from 80°C to around 65°C at the mid-point of radius. At the same time, there was no significant decrease in the temperature of the inner wood. This phenomenon was similar to run 19 in series 3. Following the initial response to the rapid removal of moisture from the kiln by the vacuum, the temperature at the three locations rapidly stabilized, and then remained constant until the end of the drying. The experimental red pine sections contained over 80% of sapwood. The inner initial sapwood moisture content of red pine was higher than that on the outer shell. It is clearly shown in the temperature graph of #RP 6 in run 37 that the rise of the temperature at the center was more rapid than that on the outer shell (Figure 2-12). The core temperature was reduced rapidly by almost 35°C when the kiln was evacuated, while the temperatures in the mid-point of radius and the outer shell were reduced 20°C and 10°C, respectively. Within about thirty minutes, the temperature in three locations inside the pole increased to their maximum. They then gradually reduced due to the loss of moisture from the wood. 60 Drying of Douglas-fir #DF9 £ 80 3 —•—T1 -m—T2 - * - T 3 - * - T 4 . / — M — Vacuum ) s -If 1 0 100 200 300 400 500 600 700 800 900 Time (minutes) Ti : 10.5 cm away from the edge T 2 : 6.5 cm away from the edge T 3 : 3.5 cm away from the edge T 4 : the air inside the chamber Tj.: 8.5 cm away from the surface T2: 5.5 cm away from the surface T 3 : 3.0 cm away from the surface Figure 2-12. Temperature variation with time at three locations in Douglas-fir #DF 9 and red pine #RP 6 (Runs 20 and 37) 61 The moisture content profile for this series was shown in table 2-4. After 10 to 16 hours of drying, all the pole sections had a final moisture content below 25%, with almost one-third having a final average moisture content of less than 18%. The average difference in final sapwood and heartwood moisture content of Douglas-fir sections was calculated at approximately 5.8%. The difference in final inner sapwood and outer sapwood moisture content of red pine was about 10.8 %. This relatively large moisture content difference following drying was due to the much higher initial moisture content in the inner portion of red pine, which is different from the experimental cedar and Douglas-fir pole sections. Apart from run 23 (Douglas-fir 15), all of the Douglas-fir specimens had a final heartwood moisture content below that of the sapwood. While, all of the red pine pole sections had final inner sapwood moisture contents higher than that of the outer sapwood. 62 Table 2-4 Drying time and moisture content profile of drying series-4 Drjing Run Sample ID (length/diameter) Ave. depth of sapwood (mm) Sample location Ave. Initial M C (%) Ave. Final M C (%) D n ing time (hrs) Run 20 #DF 9 (2.03 m, 240/260 mm) 40 Pole section Sapwood Heartwood 76.3 108.8±12.7(6)* 33.6±0.3(6) 22.3 28.0±1.9(5) 14.2±4.6(2) 13 Run 21 #DF11 (2.03 m, 230/240 mm) 34 Pole section Sapwood Heartwood 69.5 104.7±12.0(6) 32.5+2.3(6) 23.1 29.7±4.8(4) 14.1 ±0.9(2) 11 Run 22 #DF13 (2.03 m, 195/205 mm) 39 Pole section Sapwood Heartwood 71.7 100.4±28.2(6) 33.8±2.47(6) 16.2 15.0±4.0(5) 8.2±0.8(2) 11 Run 23 #DF 15 (2.10 m, 220/225 mm) 28 pole section Sapwood Heartwood 59.5 100.6±14.3(6) 33.2±0.8(6) 10.0 5.8±0.3(6) 10.1±2.6(3) 12 Run 24 #DF 16 (2.10 m, 180/210 mm) 38 Pole section Sapwood Heartwood 86.9 126.8±9.0(6) 32.3±1.3(6) 17.9 24.9±3.7(7) 17.7±1.3(2) 12 Run 25 #DF 17 (2.10 m, 190/200 mm) 33 Pole section Sapwood Heartwood 90.6 140.0±6.8(6) 33.5±2.6(6) 22.6 27.1 ±4.1(6) 24.0±2.5(2) 10 Run 26 #DF 18(2.10 m, 200/225 mm) 33 Pole section Sapwood Heartwood 84.2 114.4±19.4(6) 33.3±0.7(6) 24.2 25.9±3.1(6) 20.1+0.6(2) 12 Run 27 #DF 19 (2.10 m, 180/200 mm) 43 Pole section Sapwood Heartwood 88.3 113.6±9.1(6) 31.3±0.2(6) 17.3 23.8±1.5(6) 18.6±0.1(2) 12 Run 28 #DF 20 (2.10 m, 170/195 mm) 38 Pole section Sapwood Heartwood 75.9 102.6±9.8(6) 33.0±0.6(6) 17.2 24.2±4.6(5) 17.1±1.2(2) 11 Run 29 #DF21 (2.10 m, 175/195 mm) 40 Pole section Sapwood Heartwood 78.0 109.0±11.4(6) 33.6±0.3(6) 16.4 20.6±9.0(6) 13.9±4.4(2) 12 Run 30 #DF 22 (2.10 m, 170/205 mm) 35 Pole section Sapwood Heartwood 79.1 111.5±19.9(6) 33.2±0.9(6) 24.8 26.2±7.0(6) 19.0±8.8(2) 11 Run 31 #DF 23 (2.10 m, 190/210 mm) 43 Pole section Sapwood Heartwood 79.3 111.3+10.6(6) 33.2±0.4(6) 24.2 23.2±12.3(6) 16.8±0.8(2) 11 63 Table 2-4 Drying time and moisture content profile of drying series-4 (con'd) Drying Run Sample 11) (length/diameter) Ave. depth of sapwood (mm) Sample location 1 1 Ave. Initial MC (%) Ave. Final M C (%) Drying lime (hrs) Run 32 #RP1 (2.20 m, 155/170 mm) 80 Pole section Sapwood (outer) Sapwood (inner) 110.5 89.7±9.4(6) 134.4±7.7(6) 20.8 14.7±0.4(3) 24.4±2.7(3) 16 Run 33 #RP 2 (2.20 m, 175/195 mm) 90 Pole section Sapwood (outer) Sapwood (inner) 96.1 83.7±8.1(6) 124.9±2.7(6) 24.3 14.6±1.5(3) 29.4±3.6(3) 15 Run 34 #RP 3 (2.20 m, 180/195 mm) 93 Pole section Sapwood (outer) Sapwood (inner) 97.2 86.1±10.4(6) 128.7±4.9(6) 25.1 15.8±1.4(3) 27.8±1.0(3) 15 Run 35 #RP 4 (2.20 m, 170/185 mm) 87 Pole section Sapwood (outer) Sapwood (inner) 98.8 92.4± 11.4(6) 131.1 ±4.3(6) 22.2 16.7±1.0(3) 27.6± 1.2(3) 15 Run 36 #RP 5 (2.20 m, 170/175 mm) 85 Pole section Sapwood (outer) Sapwood (inner) 107.6 93.7±9.7(6) 142.9±5.4(6) 21.5 18.2±1.1(3) 24.9±1.2(3) 16 Run 37 #RP 6 (2.20 m, 165/175 mm) 82 Pole section Sapwood (outer) Sapwood (inner) 79.7 54.1±13.4(6) 93.9±2.8(6) 24.4 16.1±0.8(3) 27.1±0.6(3) 12 *data are recorded as X+SD with number of samples in parentheses 2.4.2 The effects of initial moisture content 2.4.2.1 The effects of the sapwood initial moisture content on the initial heating rate The initial heating rate refers to the temperature changes with time of each probe calculated from the beginning of the heating to the first temperature peak. Since there were only five runs using an amplifier as radio frequency oscillator, in which there were two species, the data was too few to include in this discussion. Due to equipment limitations, the energy consumption during RF/V drying could not be measured when using the oscillator. Based on previous works of RF drying of lumber conducted using the same kiln, the change of the power could be neglected if the 64 control "paddle" was kept at the same level throughout the whole drying process. Under this assumption, it is clear from Figure 2-13 and 2-14 that the initial heating rate of Douglas-fir inside the sapwood portion increases as the sapwood initial moisture content increases when the oscillator was used as the radio frequency generator. This seems to be consistent with the conclusion of high moisture content heating up faster than that of dry wood under a fixed frequency (Avramidis and Liu, 1994). However, for red pine and cedar, the trend was not so obvious. The moisture in the wood can have two effects on the heating rate. On one hand, a high moisture content may increase the efficiency of converting RF wave energy into heat. On the other hand, the large amount of water inside the wood requires more energy to heat up, which may slow down the heating rate. The relationship between the initial moisture content and the initial heating rate depends on which of the above factors dominates. In the case of Douglas-fir, the first factor is more significant, and in the case of cedar, the second factor may be more distinct. For red pine, these two factors may be equal in their influence in the initial heating. If we neglect the influence of the species, it is clear from the graph that in general the higher the initial moisture content results in a higher initial heating. 65 4.0 -f 3.5 •3 3.0 C | o 2.5 2 2.0 •P. 1.5 2 1.0 0.5 0.0 •Cedar • Douglas-fir A Red pine A A • A A A 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Sapwood initial MC % Figure 2-13. The effects of sapwood initial moisture content on the initial heating rate when using the oscillator as radio frequency generator 2.4.2.2 The effects of heartwood / inner sapwood initial moisture content on the initial heating rate The heartwood initial moisture contents of the experimental cedar sections ranged from 27% to 45%. While, the experimental Douglas-fir pole sections were almost at the same level ranging from 25% to 36%. Since the small difference of heartwood initial moisture content between pole sections, there is no regular pattern for the effects of the moisture content on the heartwood initial heating rate. However, it is apparent from the Figure 2-14 that cedar section has a higher initial heating rate than that of the Douglas-fir. This is consistent with their relative density. Red pine appears to show an initial heating rate which is almost the same for a range of moisture content. Unlike the linear relationship of initial heating rate with sapwood initial moisture content, the relationship of heart wood/inner sapwood initial moisture is too random to be depicted. 66 3.5 3.0 3 £ 2.5 E o o o 20 c 1.5 10 o JC •s 10 0.5 •Cedar •Douglas-fir A Red pine 0.0 -I 1 1 1 . . 1 1 0 20 40 60 80 100 120 140 160 Heart/inner sapwoodmoisture content % Figure 2-14. The effects of heartwood/inner sapwood initial moisture content on the initial heating rate when using the oscillator as radio frequency generator 2.4.3 The effects of radio frequency/vacuum drying Drying times As expected, higher moisture content required longer drying times at a given RF power (Figure 2-15). 67 18 16 14 § 12 o 10 £ O) 8 c c o 6 4 2 0 y t i lift • D u n y = 0.195x • u 0.8843 O A • Cedar EJ x O • Douglas-fir 1 1 —r 1 1 1 1 — • Red pine 10 20 30 40 50 60 70 The loss of IMC % 80 90 100 Figure 2-15. The relationship of drying time with the loss of moisture content when using the oscillator as radio frequency generator The difference of initial moisture content and the difference of final moisture content Three species were examined in this drying experiment. The difference of the initial moisture content between sapwood and heartwood or between outer sapwood and inner sapwood varied greatly from less than 5% to higher than 100%. It is interesting to note that the difference in final moisture content is much less ranging from 3 % to 11% (Figure 2-16). 68 100 90 80 * 7 0 O S 60 S 50 c 9> 2 40 30 20 10 0 Q T h e difference of initial MC • The difference of final MC rfi ffi C-L-o C-M-o DF-L-o D F - M o DF-H-o RP-H-o Figure 2-17. The difference of initial moisture content and the difference of final moisture content between heartwood and sapwood or between inner sapwood and outer sapwood (C : cedar; DF: Douglas-fir; RP: red pine; o: oscillator as radio frequency generator; L: low initial moisture content; M : medium initial moisture content; H: high initial moisture content) 69 2.5 Conclusions Based upon all the results obtained in this study, the following conclusions could be made: (1) The flat electrode design of RF/V dryer was practical for the drying of pole sections. (2) RF/V could dry green Douglas-fir and red pine pole sections in less than 16 hours. (3) RF/V could dry cedar sections faster than Douglas-fir or red pine pole sections. (4) RF/V drying resulted in dried Douglas-fir and western red cedar poles with uniform final moisture content. 70 CHAPTER 3 APPLICATION OF RADIO FREQUENCY TO ACCELERATE THE FIXATION OF CCA IN ROUNDWOOD. 3.1 Introduction Preservative chemicals are applied to wood in order to protect it from degradation by fungi and insects. CCA treated wood products are clean, odorless, and durable (Lee, et al., 1993). They account for almost 50% of the treated poles, all of the consumer lumber and wood foundation components produced in Canada (Stephens et al., 1994) and over 14 million cubic meter of lumber produced in the United States (Smith and Shiau, 1998). An important aspect of CCA treatment is the fixation of the components to wood, so that they will not leach from the treated wood in service. However, the fixation process is strongly dependent on temperature and ambient relative humidity (Avramidis and Ruddick, 1989; Smith et al, 1996). The natural rate of fixation is quite low at ambient temperatures (Avramdis and Ruddick, 1989), particularly in winter conditions (Smith et al, 1996). With the enhanced focus on the environmental impact of the forest products industries in recent years, the wood preservation industry has developed industrial practices to prevent ground contamination at treating plants and storage yards, as well as prevent loss of chemicals from wood in service. Currently, the two most common methods of accelerating CCA fixation of wood involved the use of heat while maintaining a relatively high humidity, (ie; live steam or hot water). Both methods have 71 the drawback of slow heat conduction from the surface to the center of wood (Smith, et al, 1996). Dielectric heating, using either RF or microwave energy is considered to have a great value to the wood preservation industry due to its fast heating process and high efficiency (Smith, et al, 1996). This part of project focused on proving the hypothesis of rapid fixation of CCA in treated roundwood, using the existing horizontal electrode RF/V unit located at the Wood Products Drying of the University of British Columbia. 3.2 Literature review With the increased awareness of the environmental impact of industrial operations today, particularly from treated wood in service, it is important to better understand the fixation of CCA, both in terms of understanding the fixation mechanism and in developing analytical methods for determining the degree of fixation. In order to enable the using of CCA preservative more effectively in practice, a review of the literature of the accelerated fixation of CCA was also emphasized. 3.2.1. CCA Fixation mechanism Considerable research efforts have been made to clarify the reactions of chromium, copper and arsenic in CCA with the components in wood by various authors (Wilson, 1971; Dahlgren and Hartford, 1972 a; b; c; Dahlgren, 1974; Pizzi, 1981; 1982 a; b and c; 1983; Hartford, 1986; Ostmeyer et al., 1988; 1989; Ruddick, 1992; Yamamoto and Ruddick, 1992; Yamamoto et al., 1991; 1993; Cooper, et al., 1995). It is generally 72 argued that the CCA fixation reaction is complex, occurring in three phases: a) initial reaction, b) precipitation fixation and c) conversion, whereby the active ingredients in CCA (copper, chromium and arsenic) are fixed or chemically bound to each other and to the wood. Subsequent insoluble reaction products are formed which protect wood for more than 40 years, even when subjected to severe leaching. 3.2.1.1 The period of momentary initial reaction When CCA treating solutions are injected into wood under pressure, they rapidly absorb on to cellulose and lignin of wood (Anderson, 1990). Significant reactions occur by the time that treated wood is removed from the treating vessel (Cooper et al, 1996). This initial reaction is accompanied by an increase of pH (from about 2.0 to greater than 3.0) in the treating solution expressed from the wood immediately after treatment (Dahlgren and Hartford, 1972 a; b and c; Pizzi, 1981; Anderson, 1990; Cooper et ah, 1996). This pH change represents about 90% drop in hydrogen ion concentration. Meanwhile, the CCA components start to chemically bind to wood. For example, copper is drawn from the treating solution and exchanged with the pH dependent ion on wood (Dahgren and Hartford; 1972 a; b and c). This process involves the displacement of protons from weak acid groups in wood and results in a higher cation exchange capacity as the pH rises (Rennie et al, 1987, Cooper, 1991). A smaller fraction of arsenic is rapidly immobilized, presumably due to formation of chromium copper arsenates. Also, the concentration of Cr (VI) in the CCA solution in the wood drops immediately after treatment by 30-60% which attributed to its temporary adsorbance by the wood (Schema 1) (Cooper etal, 1996). 73 Consumption of H + by adsorption of hexavalent chromium on wood during the initial reaction can be expressed as follow: +H 20 2Cr0 3 + H 2 0 —> Cr 2 0 7 2 " + 2lT > 2HCr0 4 + 2 H + > 2H 2 Cr0 4 Wood +H 2 Cr0 4 > Wood.H 2Cr0 4 (Scheme 1) 3.2.1.2 The period of primary precipitation fixation Following the "initial reactions", there exists an extended pH increasing period from pH 3.2 to about 5.5 while the majority of the CCA components react (Cooper, et al, 1996). This large pH change is attributed to the reduction of chromium to the trivalent state while many of the hydroxyl groups present in cellulose are oxidized to carbonyl groups (Schema 2) (Pizzi, 1979; 1982 c; Ostmeyer, et al, 1988; Cooper, et al, 1996). Further, while lignin might be oxidized, it appears to supply most of the metal bonding sites (Pizzi, 1979; 1982 c). Oxidation of a primary alcohol with Cr (VI) (Scheme 2) 3RCH 2 OH + 16IT + 2Cr 2 0 7 2 > 3RCOOH + 11H20 + 4Cr 3 + i i i 3RH + 3 C 0 2 The redox reaction is also expressed as (Anderson, 1990): 4Cr 2 0 7 2 + C 6 H i 2 0 6 + 32 H+ > 6C0 2 + 22H20 + 8Cr 3 + (Scheme 3) Several in situ techniques provide evidence of the type of reactions occurring during this period. For example, studies (Yamamoto and Rokova, 1991; Kaldas and 74 Cooper, 1993; 1998; Ruddick et al, 1992; Cooper et al, 1996) using XPS showed that a decrease in the C2 (hydroxyl group) attributed to oxidation of -OH groups in hemicellulose and cellulose. However, there is very little increase in the highly oxidized groups C3 (carbon bonded to two oxygens or a carbonyl group). This lack of change in C3 was attributed to the oxidation of hydroxyl groups followed by further oxidation of the C3 formed to C4 (carboxylic carbon: -0-C=0). It then followed by decarboxylation with the release of CO2 (Scheme 2) (Williams and Feist 1984, Yamamoto and Rokova, 1991; Ruddick et al, 1992 and 1993). Although conflicting on the status of the final components present in CCA treated wood existed, there is agreement that chromium in treated woods is present as Cr (III) (Anderson, 1990; Cooper et al, 1996). The reduction of chromium can proceed via intermediate chromium oxidation states, particularly by Cr (V), which be stable and remain in treated wood for prolonged time depending on the fixation conditions (Hughes et al 1992;Yamamoto et al, 1993; Ruddick et al, 1994). 3.2.1.3 The period of conversion reactions Although, the reaction products formed by the end of. the primary fixation period are considered to be highly fixed (Anderson, 1990), reactions would continue for several weeks and perhaps months under certain exposure conditions (Yamamoto and Inoue, 1990). As the pH continues to rise at the end of the primary reaction period, the most probable final reaction products: Wood-Cu complexes, lignin - Cr complexes, CrAs04, Cu(OH)CuAs0 4 , C r ( O H ) 3 were formed (Anderson, 1990; Cooper, 1995a; Dahlgren and Hartford 1972 a;b;c; Dahlgren, 1974; 1975a and b; Pizzi, 1982c; 1990a; b). 75 3.2.1.4 Variables in fixation process Like most of the chemical reactions, the fixation of CCA in wood is condition dependent. Many efforts have been made in determining the effects and interactions of the variables during the fixation reaction (Pizzi, 1983 b; Avramidis and Ruddick, 1989, Lee et al, 1993, Chen et al, 1994, Boone et al, 1995, Copper and Ung, 1989, Anderson, 1990; Copper et al, 1996). It was important to know that most of these variables can be optimized to ensure adequate fixation before CCA treated wood being used (Anderson, 1990). Temperature The rate of fixation increases exponentially with wood temperature (Eq.3-1) (Ermusch, et al.., 1980; Anderson, 1990; Cooper et al.., 1996). It was demonstrated by the work of Pizzi (1982 b) and the experiment results of red pine by Chen, et al, (1994). -E K — A e R T (3.1) There is, however, another important effect due to the treating temperature. The distribution of chemicals between holocellulose and lignin changes as the treating temperature changes. When temperature increases, the chemicals fixed to holocellulose as copper arsenate and chrome arsenate increase while the ones fixed to lignin decrease on the very end of the fixation (Pizzi, 1983 a) (Table 3-1). 76 Table 3-1. Approximate distribution of chrome (and of Cu and As chemically reacted with it) between lignin and holocellulose (Pizza, 1983 a) Wood treating temperature (°C) Holocellulose (%) Lignin (%) 20 35 65 60 40 60 80 50 50 100 52 48 Thus, heat treatment should be beneficial not only for the shortening of the fixation time, but also to improve the effectiveness of the preservative (Peek and Willeitner 1981; Pizzi, 1983 a). Wood moisture content and ambient relative humidity effects CCA in wet wood is essentially ionic. Cations such as copper are attracted into the cell wall and anions such as chromate and arsenate tend to be excluded (Anderson, 1990; Cooper and Roy, 1994). If the wood moisture content drops below fiber saturation point (FSP) before fixation is completed, diffusion of chemicals into the cell walls can be impeded and more products will be deposited in the cell lumens. This results not only in a reduced fixation rate, but also in impaired fixation (Conradies and Pizzi, 1987; Avramidis and Ruddick, 1989; Lee et al, 1993; Bonne, et al, 1995; Kaldas and Cooper, 1996; Ung and Cooper, 1996; Cooper et al, 1996; Lebow et al, 1996). The relative humidity ("wet bulb" temperature) in the fixation chamber also affects the fixation rate in the wood. Without high humidity, the fixation rate will decrease (Avramidis and Ruddick, 1989; Cooper, et al, 1996). 77 Other effects Other variables affect the fixation of CCA in wood, such as the very different behavior for softwoods and hardwoods (Pizzi, 1983 b) and the slightly different fixation rate between CCA oxide and salt formulations, the pH and concentration effects in kinetic experiments. However, these effects are minor and have no practical significance (Anderson, 1990). 3.2.2. Assessment of CCA fixation The Canadian Standard for Wood Preservation (CSA 080, 1989) requires CCA treated wood to be tested for fixation prior to the material leaving the treating plant. It was recognized that the ground contamination by CCA can be minimized by ensuring that fixation reactions between the preservative and wood are complete (Cooper and Ung, 1993). A number of procedures have been developed to monitor the progress of CCA fixation following treatment with the preservative (Dahlgren and Hartford, 1972 a; b and c; Foster, 1988; Christensen, 1990; Evans and Nossen 1991; Cooper et al, 1994; 1996). Among them the measurement or detection of hexavalent chromium has become the most widely accepted. The key points in using chromium reduction to monitor fixation is that: 1. Cr (VI) is considered to be the most hazardous component of all CCA solution components, since it has both acute and chronic toxicity properties and unlike arsenic, is very mobile in the soil and can be significantly absorbed through human skin (Cooper et al, 1996). 78 2. Cr (VI) is the last component of CCA solutions to be immobilized during the fixation process (Figure 3-1) (Cooper et al, 1996). Therefore it is a conservative method. 3. Cr (VI) is very reactive and develops strong color reactions with several reagents and is easy to detect or measure (ASTM, 1987). ] in Expressale (ppm) O Cff-VJ o cr o Cu x As 4 6 6 10 12 14 Fixation Time (days) 16 16 20 Figure 3-1: Fixation of CCA components in southern pine lumber treated with 1.6% CCA-C at 21°C-Expressate method A qualitative procedure involved the reaction with chromotropic acid was described by Foster (1988). This approach was subsequently indicated as the standard method by both the AWPA (1998) and the CSA 080 (1989) for determining when the fixation reaction is essentially complete (Cooper and Ung, 1993). The detection threshold for color development is about 15 to 50ppm Cr (VI) in the wood which corresponds to over 98% chromium reduction or fixation depending on the initial solution concentration (Foster, 1988; Cooper and Ung, 1993; Cooper, et al, 1996). Although, it is easy to 79 perform, chromotropic acid test only developed a "GO/NO GO" procedure that gives no real indication the extent of CCA fixation in wood during incomplete stages (McNamara, 1989 a; b; Cooper et al, 1994). Several quantitative procedures have also been investigated (Cooper and Ung, 1993; Cooper, et al, 1994; Cooper et al, 1996). One approach that has been adopted by several treating plants is to use diphenyl carbazide as color reagent and measure the absorbence at the specified wavelength in a visible light spectrometer. Thus, the concentration of chromium in the leachate can be quantitatively estimated. The advantage of this approach is that it provides an estimate of how far fixation has progressed in incompletely fixed material, so the operator can judge how much longer is required to complete fixation in the treating plant. Chromotropic acid was also assessed as an indicator for hexavalent chromium in a colorimetric assay (Forsyth and Morrell, 1990; Cooper and Ung, 1993). Efforts have also been focused on the monitoring fixation approaches independent of the reduction of chromium. Evans and Nossen (1991) suggested an electrical conductivity approach based on the decrease of the conduct electrical charges due to the CCA components becoming insoluble during fixation. The minimum solubility can be measured from the electrical resistance of the wood. Dahlgren and Hartford (1972 a; b; c) have raised the change in pH in wood as another approach. Unfortunately, the pH fluctuates in the latter stages of fixation. The end point can vary considerably both within and between wood species, so this approach is not very reliable (Cooper, et al, 1996). It 80 is also possible to leach the CCA components from treated wood with water and then measured its concentration using Ion coupled plasma (ICP) spectroscopy (Christensen 1990). 3.2.3 Practical application of accelerated fixation Poor fixation in CCA treated wood usually results in high losses of arsenic, when exposed to simulated rain or leaching (Cooper et al., 1996). This reduces the effectiveness of preservative treatment and results in a short service life (Smith and Williams 1973; Thompson et al., 1991; Cooper, et al., 1996). Industrial practices to prevent contamination of treating plants and storage yards, as well as prevent loss of chemicals from wood in service have been developed (Nichol, 1995; Lathan, 1993). The key factors are temperature and ambient relative humidity (Anderson, 1990; Avramidis and Ruddick, 1989 and 1996; Smith, et al., 1996). Since the fixation reaction requires considerably longer times at ambient temperatures (Nichol, 1995, Avramidis and Ruddick, 1989 and 1996), particularly during the winter conditions (Smith et al, 1996), the use of artificial heating while maintaining a relatively high humidity has great potential, particularly for lumber and poles. 3.2.3.1 Hot air heating The hot air approach is often described as kiln heating. Complete fixation could be achieved using hot air heating with moderate and high temperature under good humidity control and air circulation. High temperatures require a much shorter time (Cooper, et al., 1996). However, other studies have suggested that only a moderately high temperature 81 should be applied in kiln drying of CCA treated lumber. This was because with high temperatures, adverse effects, ie. reduction in strength (Barnes and Winandy, 1986; Mitchell and Barnes, 1986; Winandy et al, 1985; Conradie and Pizzi, 1987; Winandy and Boone, 1988; Anderson, 1990), a slower fixation rate and (Avramidis and Ruddick, 1989; Alexander and Cooper, 1991) poor leach resistance (Boone et al, 1994; Chen et al, 1994; Lee et al, 1993) occur. Special care also must be given in the kiln drying after treatment (KDAT) to insure sufficient moisture present (Lathan, 1993). Because of these reasons, it takes longer time for the process to complete. Meanwhile, the increased handling and the capital cost of kilns have inhibited post treatment fixation process development (Lathan, 1993). 3.2.3.2 Hot water fixation The hot water fixation process has several advantages. Firstly, powerful circulation of hot water is able to penetrate packed lumber and heat the center of the pack. Secondly, it tends to remove the surface residues and produces wood with consistent uniform color. Thirdly, the temperature of this process can be easily controlled to avoid sap bleed problem (Lathan, 1993). In practice, both the MSU (Mississippi State University) fixation process and atmospheric pressure hot water process were considered. During the MSU process, the lumber is treated with empty cell process. Immediately following treatment, hot water is introduced under pressure. Since this strategy extends the in-treatment cylinder processing time, it still needs further refinement before it can be brought to full practical 82 application in lumber treatment (Anderson, 1990). Atmospheric pressure hot water process conducted under full cell process showed no difference in mechanical strength, chemical distribution and leach resistance compared to the MSU process (Wood, et al, 1980; Barnes, et al., 1988). It was described as a process with lower environmental risks and lower operating liability (Zohora et al., 1990). 3.2.3.3 Live steam fixation As early as 1927, a British patent for copper/chrome preservatives mentioned the steam fixation strategy (Gunn, 1927). Subsequent work by various authors either using steam at 100°C or super-heated steam at 110-120°C verified its ability to accelerate fixation (Peek and Willeitner, 1984; Barnes, 1985). It seemed that one hour of steam was sufficient to induce complete fixation and no reduction in efficacy of the products (Peek and Willeitner, 1981 and 1984; Preston, 1983). Efforts were also made to confirm the environmental acceptability of steam technology. It is particularly well developed in Europe (Peek et al, 1987; Peek and Wileitner, 1988). It was concluded that this technology has the same advantages as the hot water methods. (Anderson, 1990). However, using super-heated steam for 6 hours caused a reduction in modulus of rupture (Barnes, 1985). Resin exudation could also occur above 80°C for some species and this technology is sensitive to profile of the lumber and configuration of the pack (Anderson, 1990). Further, the application of a steaming vessel to accelerate the fixation could introduce large costs (Lathan, 1993). It also had a drawback of slow heating for a large volume of treated wood (Avramidis and Ruddick, 1996). 83 3.2.3.4 Alternative strategy and RF fixation Recently alternative technologies of accelerating fixation of CCA have been studied. Cooper and Ung (1992) described the possible accelerated fixation process by studying several softwood species using dehumidification kilns with a steam generator. The study revealed that except for western red cedar, which required significantly longer time, all the other species (Douglas-fir, red pine, jack pine, lodgepole pine and southern yellow pine) could be fixed in 8 to 10 hours at 70°C compared to about 14 days at 20°C. However, among all of the alternative accelerating strategies, dielectric heating using either radio frequency (1 to 100MHz) or microwave (300 MHz to 300GHz) showed most promise (Smith, et al., 1996). Avramidis and Ruddick (1996) demonstrated that CCA treated specimens could be completely fixed in 5 hours at 110°C, while visible adverse effects on the quality of lumber was not detected. The fixation time could have been further decreased to approximately 4 hours if higher power densities had been used (Avramidis and Ruddick, 1996). Smith et al. (1996) also determined that in small samples 99% of the chromium was fixed in 30 minutes using microwaves. The key point of using this technology for accelerated CCA fixation is that heating occurs rapidly independent of the thickness and with very little drying (Smith, et al., 1996). For example only about 15% moisture content was lost during 30 minutes of heating. Another benefit of using this technology is that dielectric heating produced minimal internal stresses and had minimal impact on strength properties (Avramidis and 84 Ruddick, 1996; Smith et al, 1996). Also, following fixation the wood could be quickly dried at high temperatures without loss of strength (Smith, et al, 1996). 85 3.3 Materials and Methodologies 3.3.1 Radio frequency heating experiment 3.3.1.1 Wood preparation Eleven Douglas-fir, ten western red cedar and six red pine pole sections, 165-240 mm in diameter and 20-22 m in length were selected in this study. Preliminary radio frequency/vacuum experiments were performed (see chapter 2) prior to conducting this experiment to ensure that the selected sections had a final moisture content of ca. 25%. These pole sections were free of severe surface checking, although the red pine pole sections contained many knots. 3.3.1.2 CCA pressure treatment The pole sections were then divided into ten groups without considering the species. There were two to three poles in each group for subsequent pressure treatment with a 1.7% CCA-C solution. The CCA treatments were carried out at Forintek Canada Corp. using a full cell process (Figure 3-2). Thirty minutes at full vacuum (75 kPa) was first applied and then the whole system was placed under pressure for 2 hour at 1035 kPa. Finally, a 15 minute final vacuum step at 75kPa was applied at the end of the treatment cycle to remove excess solution. 86 1100 1000 900 800 700 600 500 S» 3 400 It) 8 300 0. 200 100 0 -100 -200 r s 0 20 40 50 60 70 80 90 100 110 120 130 140 156-^ 60^  170 1$0 Time (minutes) Figure 3-2. Schedule of the preservation treatment Following removal from the treating cylinder, the specimens were wrapped and immediately transported to the RF fixation chamber. Four cores were then immediately recovered randomly along the length of each cedar Douglas-fir pole section, weighed on a digital balance and oven dried at 103 ± 2°G for 24 hours. The initial and final weight allowed the moisture content of sapwood and heartwood after CCA treatment to be determined. Shavings using a 1cm bit drilled at 3.5cm and 9cm depths from the red pine pole sections were also recovered for the moisture content analysis. The moisture content was calculated using Eq. 2-3 and recorded separately based on the heartwood and sapwood or inner sapwood and outer sapwood. 87 The delay in applying the fixation schedule was between 0.5-12 hours. Prior to fixation, the pole sections were stored under cold conditions (<10°C). Previous studies have shown that cold temperature could slow down fixation to several weeks (Cooper and Ung, 1992; Smith, et al, 1996), so that up to 12 hours delay would have little impact on the study. 3.3.1.3. Radio frequency heating treatment The heating treatment was carried out in a RF heating unit, which was located at the Wood Drying Laboratory at The University of British Columbia. A detailed description of the RF chamber is given in chapter 2.3.3. Fiber-optic temperature sensors were used to monitor the temperature at three ( core Ti , mid- point radius T2 and outer shell T3 ) two (core Ti and outer shell T2) or one location (Ti outer shell) at the midpoint of each pole section. A plastic cap in which a small hole was drilled to allow insertion of the thermocouple was used to fasten the optic fiber thermocouple inside the wood. This was set up in the same way as the one described in Chapter 2.3.4.1. In order to maintain a relative high humidity and avoid the deposit of chemicals on the wood surface before the completion of the fixation, RF heating processes were carried out without a vacuum. All the experimental heating runs started at about 20 °C. In the first eleven runs, fixation treatments were carried out for a period of 6 hours starting from the time the thermo-sensors reached 80°C. The subsequent sixteen runs were 88 conducted 2.8 -3.5 hours for western red cedar sections, 4.5—5 hours for Douglas-fir sections and 3-4 hours for red pine pole sections. The computer was set to record the temperature every four minutes through a data acquisition system. 3.3.1.4. Sample analysis Preparation of chromotropic acid solution 2.0 g of 4,5-dihydroxynaphthalene-2,7-disulfonic acid was dissolved in 400 ml of 0.5 M sulfuric acid (AWPA, A3-1996). The solution was then saved in a spray bottle and stored away from direct sunlight. Preparation of diphenylcarbazide reagent 50 ml distill water and 50 ml acetone were mixed to make 1:1 acetone/distilled water solution. Then, 2.5 g diphenylcarbazide was dissolved in the above solution. The solution was then stored in a brown bottle and kept away from direct sunlight. Preparation of chromium standard solution Chromium (VI) stock solution (ASTM, 1987)—an aliquot of 0.2828g of potassium dichromate that had been oven dried at 105°C for 1 hour was dissolved in 0.25M sulfuric acid. It was then diluted to IL with 0.25M sulfuric acid (1 ml= 0.10 mg Cr). Chromium solution standard (1 ml=0.01 mg Cr)— 10 ml of chromium stock solution was diluted to 100 ml with 0.25 M sulfuric acid. 89 A series of seven standard solutions containing from 0 ppm to 2 ppm of chromium were prepared by diluting measured volumes of the standard chromium to 100ml with 0.5 N sulfuric acid in separate volumetric flasks. The standards prepared were as follows: 0 ppm, 0.1 ppm, 0.2 ppm, 0.3 ppm, 0.5 ppm, 1.0 ppm, 2.0 ppm. Another series of eight standard solutions containing from 0 ppm to 30 ppm of chromium were also prepared by diluting measured volumes of the standard chromium to 100ml with distilled water in separate volumetric flasks. The standards were: 0 ppm, 0.5 ppm, 1.0 ppm, 2.5 ppm, 5 ppm, 10 ppm, 15 ppm, 30 ppm. Testing of samples After RF heating treatment, four core sections were recovered randomly from two positions along the length of the cedar and Douglas-fir pole section. They were immediately weighed on a digital balance and oven dried at 103 ± 2°C for 24 hours to examine the loss of moisture of sapwood and heartwood after heating treatment. Shavings were recovered from four to five holes at 35 mm, 50 mm and 80 mm depths for the loss of moisture content analysis of red pine. The final moisture content was calculated separately using Eq. 2.10, based on the heartwood, inner treated sapwood and outer treated sapwood. These cores and shavings from treated portions were further ground into 20 mesh sawdust. Each sample was in turn compacted and mounted in a sample holder for X-ray analysis with the standardized and calibrated X-ray Fluorescence Analyzer. The sum of the CrC»3, CuO and A S 2 O 5 retention was used to compute the total retention of CCA in each sample. 90 Immediately following heating, three chips recovered from the outer shell of each pole were placed in a disk and sprayed with chromotropic acid solution in a fume hood. They were then placed on a clean white filter paper. After 15 minutes the chips was removed and any pink color due to the presence of unconverted hexavalent chromium was recorded. Since the temperature inside the wood drop slowly following heating, the fixation of CCA will continue with the stored heat. In run 16 (Douglas-fir #DF24), run 17 (cedar #C9), and run 20 (cedar #C12), chips recovered from the outer shell were tested for hexavalent chromium after the sections had been stored at ambient condition for about 6 hours. Chips recovered from red pine #RP2 (run 23), red pine #RP4 (run 25), red pine #RP 5 (run 26) were sprayed with chromotropic acid to detected chromium after one, two, and three hours following heating. These results were recorded to examine the use of stored heat for the fixation of CCA. All the experimental sections were taken out at the end of heating in the kiln. 2.54 cm cores were recovered from each cedar and Douglas-fir pole at three randomly selected locations (avoiding end effect). Two to three treated sapwood pieces were obtained from each location (Figure 3-3). While, shavings from inner and outer treated sapwood portion were also used for the red pine pole sections. Like the chromotropic acid test, shavings from red pine #RP2 (run 23), red pine #RP4 (run 25), red pine #RP 5 (run 26) were also recovered after one, two, and three hours to examine the influence of stored heat. 91 Locations for sampling Figure 3 - 3 . The description of the sample recovery 92 Immediately after the recovery, the cores from cedar and Douglas-fir were placed separately in beakers and shaken with 25.00 ml 0.25 M H2SO4 for 15 minutes, while shavings from red pine were placed in different beakers and agitated with 25.00 ml distilled water for 1 hours. An aliquot of 0.5 ml diphenylcarbazide reagent was then added to the solutions causing them to become slightly colored. A 10 ml sample of the solution was then placed in cuvettes and measured at the absorbency of 540 nm with a pre-calibrated Bausch and Lomb spectrophotometer. 3.3.2 Simulation experiment with hot water fixation Twenty-eight numbered southern yellow pine small blocks with 19x19x19 mm in dimension were selected. They were free of defects and contained almost 100% sapwood. These blocks had been first oven-dried for 24 hours and recorded the weight. These dried blocks were then sprayed with water and weighed again in a digital balance. They were left on the bench for 12 hours and weighed every two hours, until their final moisture content fell to ca. 25% (calculated using Eq 2.3). The blocks were placed in a large beaker in a desicator, which was then evacuated for 20 minutes. After 20 minutes, a 1.7% CCA solution was introduced by way of a special valve arrangement attached to the top of the desicator (Figure 3-4). The pressure in the desicator was then returned to atmospheric pressure and the blocks allowed soaking for another 20 minutes. 93 (A) vacuum disiccator; (B) glass treatment beaker; (C) test wood blocks; (D) glass or other suitable weight; (E) treating solution; (F) polyethylene tubing; (G) three-way stopcock; (H) flask containing treating solution (I) glass joint with "O" ring leading to either vacuum gage or mercury manometer; (J) glass joint with "O" ring; (K) flask for vacuum trap; (L) stopcock to atmosphere; (M) line of source of vacuum Figure 3-4- Apparatus for vacuum impregnation At the end of the soaking period, the blocks were removed from the treating solution, wiped to remove the excessive liquid, and then immediately placed into a beaker full of over 90°C hot water. In order to make the heating uniform for all of the treating blocks, the hot water was stirred. The blocks remained submerged for 2, 4, 5, 6 hours. At each time interval seven blocks were removed. Four of them were immediately placed into beakers and shaken with 25.00 ml 0.25 M H2SO4 for 15 minutes. Unconverted hexavalent chromium was measured at 540 nm with a pre-calibrated Bausch and Lamb spectrophotometer, using diphenylcarbazide as color reagent. The remaining three blocks were oven-dried at 103±2°C for 24 hours and then ground separately into 20 mesh sawdust and mounted at the X-ray Fluorescence Analyzer for analysis of CCA retention. 94 3.4 Results and Discussion 3.4.1 Radio frequency heating treatment runs The plots of wood temperature with time for the heating of the pole sections showed strong similar trends, so that representative plots for each species are shown in Figure 3-5. The first two runs lasted approximately 3-4 hours after both temperature sensors reached 80°C. Starting from an ambient temperature of about 20°C, the maximum temperature reached at the end of heating was ca. 100°C in both runs. It was apparent that the initial rate of temperature increase in the Douglas-fir roundwood sections was slower than that of western red cedar. This was considered to be a result of the differences in their densities. It was also noted that the outer wood temperature in both species increased more rapidly than the inner wood during the initial heating. This was because RF heating is more efficient in the wetter treated portion than that in the drier untreated core (Avramidis and Zwick, 1992). After the temperature reached about 60°C, the outer wood began to lose heat due to surface cooling, so that the temperature rise of the inner portion began to exceed that of the outer zone. The surface cooling arises from evaporation of moisture from the surface of the pole section. The cooling effect is less obvious in the heating of red pine, due to the larger treated wood volume, which created a much greater heat sink. 95 120 Heat ing of c e d a r #C14 50 100 150 200 250 Time (minutes) 300 350 400 Ti: 8.5 cm away from the edge T 2 : 2.5 cm away from the edge Figure 3-5 a. Temperature changes within cedar during radio frequency heating H e a t i n g D o u g l a s - f i r #DF7 120 n l - - T -400 Time (minutes) T i : 9.0 cm away from the edge T 2: 2.5 cm away from the edge Figure 3-5 b. Temperature changes within Douglas-fir during radio frequency heating 96 Heating of red pine #RP1 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Time (minutes) Ti: 8.5 cm away from the surface T2: 5.5 cm away from the surface T3: 3.0 cm away from the surface Figure 3-5c. Temperature changes within the red pine during radio frequency heating 3.4.2 Moisture change profile The average initial moisture content and final moisture content are shown in Table 3-2. They were calculated by the Eq.2.3 and Eq.2.10. Since the pole sections were not incised, the CCA penetrated mainly the outer sapwood. The moisture content of the heartwood was almost unchanged after fixation, while that of the outer shell had a greater reduction than the corresponding part of the treated wood. This was due to the evaporation of moisture from the surface during heating. Since the fixation treatment was conducted using RF without vacuum, the moisture evaporated very slowly, from 6% to 27% per hour depending on the sapwood 9 7 thickness and the wood density. Due to the slow moisture evaporation, it was not surprising to see that al l the heating treatments were carried out above fiber saturation point (FSP). It is also clear to see from Figure 3-6 and 3-7 that the higher the initial moisture content, the greater the loss of moisture content in the wet treated part o f pole sections. After C C A treatment, the outer treated sapwood had a relatively lower initial moisture content than that o f the inner treated sapwood due to the evaporation o f the moisture content. Thus, it is not surprise to see that the speed o f loss of moisture content in the inner treated sapwood is a bit higher than that of the outer sapwood. This is consistent with the phenomena observed in the drying phase. However, in general no trend could be observed which related the loss o f moisture content to the initial heartwood moisture content, which was free of C C A component (Figure 3-8). Subsequent moisture measurements using an electrical sensor moisture meter showed that the treated cedar and Douglas-fir pole sections quickly dried to a moisture content of approximately 35% after one week of storage outside. This drying was attributed to the stored heat in the pole section. 98 Table 3-2. Moisture content % profile Moisture content % Run No. Sample JD* Outer shell (treated) Inner sapwood (treated) Heartwood (untreated) Initial Final loss Initial Final loss Initial Final loss 1 C4 127 75 52 134 102 32 27 23 4 2 C5 106 66 40 118 97 21 25 18 7 3 C14 180 73 107 189 108 81 - - -4 C6 175 72 103 184 111 73 27 24 3 5 C 15 180 64 116 198 125 73 - - -6 DF 4 170 71 99 184 101 83 24 22 2 7 DF 5 109 69 40 124 98 26 - - -8 DF 6 205 84 121 232 160 72 28 22 6 9 DF 7 118 70 48 126 106 21 - - -10 DF 8 147 78 69 137 97 40 - - -11 DF 25 111 81 30 128 96 32 33 31 2 12 DF 17 123 84 49 141 99 42 28 26 2 13 DF 19 136 96 40 145 101 44 25 24 1 14 DF 20 154 91 63 166 109 57 25 23 2 15 DF 23 148 101 47 183 106 77 22 20 2 16 DF 24 135 97 38 145 100 45 25 21 4 17 C9 181 84 97 203 85 118 20 19 1 18 C 10 177 79 98 189 86 103 22 20 2 19 C 11 159 74 85 176 84 92 26 22 4 20 C 12 196 96 100 230 114 116 30 28 2 21 C 13 175 99 76 189 101 88 29 26 3 22 RP 1 230 156 74 242 174 68 34 33 1 23 RP2 245 175 70 256 171 85 37 35 2 24 RP 3 198 117 81 221 153 68 35 34 1 25 RP 4 207 148 59 211 132 79 33 28 5 26 RP 5 250 175 75 254 166 88 32 30 2 27 RP 6 219 143 76 219 146 73 37 35 2 *Note: sample ID was consistent in all chapters; C: cedar; DF: Douglas-fir; RP: red pine. 99 3.4.3 Assessment of fixation 3.4.3.1 Estimating the extent of fixation In the first eleven radio frequency treatment runs, heating was carried out for 6 hours after both temperature sensors reached 80°C. None of the samples showed any reaction with chromotropic acid, indicating that there was no detectable hexavalent chromium (<15 ppm of Cr (VI)) (MacNamara, 1989a and 1989b). For the subsequent sixteen radio frequency runs, the fixation time was reduced, and varied from 2.8 to 5 hours. Consequently some positive responses with chromotropic acid were observed in some samples immediately after the radio frequency heating (Table 3-3). Table 3-3. Chromotropic acid solution fixation observation right after radio frequency heating Species Sample Numbers A\crage K F heating hours Chromotropic acid fixation test Douglas-fir 11 5.410.6 N(10*), VWP(l) Cedar 10 4.4 ±1.6 N (8), WP (1), VWP (1) Red pine 6 3.5±0.5 N (2), WP (2), VWP (2) Note: N: Negative; WP: weakly positive; VWP: very weak positive; *sample numbers The concentration of the Cr (VI) in the leachate derived from wood cores or shavings was assessed using visible spectrophotometer analysis. The results are shown in Table 3-4, 3-5 and 3-6. The CCA retention measured by X-ray Fluorescence Analyzer is also shown in the tables. The degree of fixation was determined by comparing the 100 concentration of the Cr (VI) in the leachate with the total chromium content in the wood sample recovered from each pole section. Based on the available chromium calculated to be present in the treated wood cores, and the amount of Cr (VI) detected in the leachate, the fixation rate was calculated. It was noted that the CCA fixation was greater than 99.9 % for all treatment covering a fixation duration of 2.8 to 6 hours of treating for the western red cedar and Douglas-fir. Comparing the chromotropic acid data with the spectrophotometer analysis, there are some discrepancies (Table 3-3, 3-4 and 3-5). This may be due to the deposit of the chemicals on the surface, from where most experimental chips were recovered for the chromotropic acid test. A second possibility is the enhanced conversion of Cr (VI) to Cr (III) in the acid leaching solution. The fixation rate of red pine (Table 3-6), determined by the Cr (VI) concentration in distilled water leachate was in good agreement with the result of chromotropic acid test. It is clear that the Cr (VI) concentration in the water leachate is significantly higher than that for the cedar ranging from 5ppm to 16 ppm. The fixation rate based on the above calculation ranged from 91% to 98% (Table 3-5). 101 Table 3-4. Analyses data of radio frequency treating of western red cedar roundwood sections Sample ID Heating time (hrs) Ave. Cr (VI) in solution (ppm) CCA retention (kg/m3) Ave. Cr (VI) in acid leachate (lxNT6g) Cr0 3 % wt/wt based Fixation % C 4 6.0 0.16±0.06(3)* 6.52±0.36 (2) 0.64 0.94±0.06 (2) 99.99 C5 6.0 0.12 6.75 0.37 1.081 99.99 C 14 6.0 0.17±0.06(3) 8.54 0.78 1.196 99.99 C 6 6.0 0.18±0.02(3) 8.13 0.36 1.169 99.99 C 15 6.0 0.1810.05(3) 10.06±0.59 (2) 0.23 1.42±0.11 (2) 99.99 C 9 2.8 0.32±0.01 (2) 9.28±2.45 (2) 4.02 1.25±0.31 (2) 99.97 CIO 3.5 0.06±0.01 (2) 1.73±0.24 (3) 0.95 0.23±0.03 (3) 99.96 C 11 3.5 0.02±0.01 (2) 3.7811.80 (3) 0.25 0.43±0.22 (3) 99.99 C 12 3.0 0.19±0.01 (2) 2.42±0.28 (3) 2.39 0.29±0.03 (3) 99.92 C 13 3.5 0.06±0.04 (3) 7.28±0.91 (3) 0.85 1.00±0.11 (3) 99.99 * data are recorded as X ± SD with sample number in parentheses 102 Table 3-5. Analyses data of radio frequency treating of Douglas-fir roundwood sections Sample ID Heating time (hrs) Ave. Cr (VI) in solution (ppm) CCA retention kg/m3 Ave. Cr (VI) in acid Ieachate(lxl0_6g) Cr0 3 % wt/wt based Fixation % DF 4 6.0 0.1410.05(2)* 12.18 0.74 1.19 99.99 DF 5 6.0 0.1610.09(4) 12.2712.38 (2) 1.39 1.2010.27 (2) 99.99 DF 6 6.0 0.1610.06(3) 9.9713.21 (2) 0.25 1.11 99.99 DF 7 6.0 0.0810.01(2) 11.2210.22 (3) 0.06 1.0810.09 (3) 99.99 DF 8 6.0 0.1010.04(2) 14.2912.64 (3) 0.15 1.4510.27 (3) 99.99 DF 25 6.0 0.1710.01(3) 5.83 0.43 0.53 99.99 DF 17 4.7 0.2110.01(2) 13.0113.38 (3) 3.34 1.2210.33 (3) 99.97 DF19 5.0 0.031 6.1110.32(3) 0.02 0.5710.05 (3) 99.99 DF 20 5.0 0.113 3.5211.10(3) 0.52 0.3110.09 (3) 99.98 DF 23 5.0 0.3610.07 (2) 6.5710.50 (3) 1.44 0.6410.06 (3) 99.98 DF 24 4.5 0.4310.03 (2) 6.6512.90 (3) 10.00 0.6610.30 (3) 99.83 * data are record as X ± SD with sample in parentheses 103 Table 3-6. Analyses data of radio frequency treating of red pine roundwood sections Sample I D Heating time (hrs) Ave. Cr ( V I ) in solution (ppm) CCA retention kg/m3 Ave. Cr ( V I ) in water leachate( lxlO^g) Cr0 3 % wt/wt based Fixation % RP 1 4.0 7.18±3.76(4)* 17.37±0.91 (2) 34.51 2.0110.28 (2) 98.3 RP 2 3.0 16.02±0.71(4) 16.61+0.43 (2) 77.02 1.9010.07 (2) 91.1 RP 3 4.0 5.59±4.53(4) 10.45±1.97 (2) 26.87 1.1810.19(2) 97.4 RP 4 3.5 15.00+1.32(4) 14.0110.20 (3) 72.11 1.6210.05 (2) 94.2 RP 5 3.5 10.36±0.88(4) 14.6410.68(3) 49.79 1.6810.09 (2) 95.9 RP 6 3.5 16.18+1.41(3) 21.4710.06 (2) 77.80 2.3210.08 (2) 95.3 * data are recorded as X ± SD with sample number in parentheses 3.4.3.2 The effect of stored heat The temperature inside the pole sections reached ca.l00°C at the end of radio frequency heating. This heat disperses very slowly during subsequent storage. For example, it took at least 5 hours for the temperature of cedar pole sections to reduce from 100°C to 60°C (See Appendix I-drying of cedar #C8). During this period, the stored heat would convert the unfixed Cr (VI) to Cr (lU), resulting in complete fixation of CCA (Table 3-7, Figure 3-6). The chips recovered from the pole sections were also tested using chromotropic acid at different time intervals (Table 3-7). The results are consistent with that from the spectrophotometer test. Based on research by Cooper (1996), the relationship between the residual Cr (VI) content and fixation time is exponential. Our 104 data showed good agreement with Cooper (1996) and had R over 85%. Generally, studies of up to 3 hours of storage after RF heating, showed that a longer storage time corresponded to a lower Cr (VI) content. Table 3-7. Chromotropic acid fixation test observations Sample ID Time (hours) Chromotropic aeid fixation test (wood chips) Kight after stop heating 1 hours of stored heat 2 hours of stored heat 3-6 hours stored heat DF 17 4.7 N - - -DF 19 5.0 N - - -DF 20 5.0 N - - -DF 23 5.0 N - - -DF 24 4.5 VWP - - N C9 2.8 WP - - N CIO 3.5 N - - -C 11 3.5 N - - -C12 3.0 VWP - - N C 13 3.5 N - - -RP 2 3.0 SP WP VWP N RP 4 3.5 WP VWP N -RP 5 3.5 WP N - -RP 6 3.5 VWP VWP N -Note: WP: weakly positive; VWP: very weak positive; SP: slightly positive; N: Negative 105 Table 3-8. The effect of stored heat on the fixation rate of red pine Sample ID Fixation rate % Right After RF heating 1 hour stored heat 2 hours stored heat over 3 hours stored heat RP 2 91.1 94.4 96.6 98.7 RP 4 94.2 95.5 98.1 -RP 5 95.9 97.5 - -RP 6 95.3 97.3 98.2 -o To 3 '55 © o © CO 2 c © tf> © Q. 120 100 0 1 2 3 4 The Stored heating time (hours) Figure 3-6. Effect of stored heat on fixation of red pine pole section 106 3.4.4 Simulation experiment with hot water heating Since, it is difficult to interrupt the fixation experiments in RF kiln and recover samples for analysis at short time intervals, a laboratory experiment was carried out using hot water to examine the relationship between the fixation time and Cr (VI) concentration in the acid leachate of small blocks. The results shown in Table 3-9 confirm that for the first 4 hours of heating, the shorter the time, the higher the Cr (VI) concentrations in the acid leachate. After 4 hours of heating, there was no Cr detected in the acid leachate. Using the same calculation, it was not surprising to find that the fixation degree of the small blocks, which were agitated in acid for 15 minutes, was over 99% fixed after 2 hours of heating (Table 3-10). Table 3-9. Analysis data of hot water fixation of southern yellow pine block Fixation time (hrs) Ave. Cr (VI) in acid solution (ppm) CCA retention kg/m3 Ave. Cr (VI) in acid leachate (lxHT* g) Cr0 3 % wt/wt based Fixation % 2.0 0.81 ±0.15(4)* 3.37±0.17(3) 8.1 0.28+0.01(3) 99.71 4.0 0.17±0.05(4) 3.64±0.52 (3) 1.7 0.30+0.05 (3) 99.94 5.0 - 3.82±0.26 (3) - 0.31+0.01 (3) 6.0 - 3.57±0.60 (3) - 0.30±0.05 (3) * data are recordec as X ± SD with number of sample in parentheses 107 3.5 Conclusions Based upon all the results obtained in this study, the following conclusion could be made: (1) RF heating accelerates the rate of CCA fixation in Douglas-fir, western red cedar and red pine pole sections. (2) Fixation was over 95% completed in less than 2.8 hours for the western red cedar species and 4.5 hours for the Douglas-fir species and 4 hour for the red pine species. (3) With the use of the stored heat the fixation time could be significantly reduced. For red pine pole section in which the CCA was fixed for only 2 hour, 3 hours of storage were sufficient to complete the fixation process. 108 C H A P T E R 4 VISUAL EVALUATION 4.1 Introduction Due to minimal internal stresses produced during RF/V heating, earlier studies have shown that RF/V drying / RF heating can produce products with uniform color and less surface defects (Avramidis and Ruddick, 1996; Smith et ah, 1996). 4.2 Methodologies The primary goal of this part of experiment was to determine whether RF/V heating and drying of poles could produce a good surface appearance. The result was visually checked after RF/V drying and RF heating of treated pole sections. However, only a few of drying treated Douglas-fir and CCA treated red pine pole sections were photographed. Comparison with air dried pole sections was also made to identify the quality of pole sections dried using RF energy. 109 4 .3 Results and Discuss ion After RF/V drying, the surface and the ends of each pole section were visually checked. Except for run 22 (DF #13) and run 25 (DF #17) in which some sections had slight scorch marks on their surfaces due to contact with the top electrode plate during drying, all the others showed only minor checking and had a clean appearance (Figure 4-1). Sample recovery site Figure 4-1. The appearance of Douglas-fir after radio frequency/vacuum drying Following the RF heating, the surface of each treated pole section showed slight checking (Figure 4-2 and 4-3), whereas the surface checking of air dried sections was more severe (Figure 4-4). The RF fixation resulted in pole sections with clean appearances. Due to the large amount of knots in the red pine sections, some bleeding of CCA and resin was observed around a few knots (Figure 4-5). 110 Figure 4-2. The appearance of C C A treated red pine after radio frequency heating (I) Figure 4-3. The appearance of C C A treated red pine after radio frequency heating (II) 111 Figure 4-4. The appearance around some knots of C C A treated red pine after radio frequency heating Figure 4-5. The appearance of air dried cedar and Douglas fir after 6 months 112 4 . 4 Conclusions RFA'' drying could dry pole sections with minor checking and produce products with good appearance. This is also the case after RF heating of CCA treated pole sections. 113 CHAPTER 5 ELIMINATION OF FUNGI IN WOOD 5.1 Introduction Unless protected by chemical treatment, wood will inevitably be degraded by insects or fungi. Indeed, some degradation already begins in the living tree. (Cwielong and Rajchenberg 1995). With the increased attention being given to the customers' interests and the protection of the environment, the prevention of the degradation by these wood-destroying organisms is essential (Mann, 1996; Freitag and Morrell, 1998). Of the current methods, only heat treatment is recognized as an effective method for killing organisms, which are present in wood (Freitag and Morrell, 1998). There is a substantial body of data supporting the use of conventional kiln-drying to eliminate fungi, insects, and nematodes inside wood (Dwinell, 1990a, b and c; Dwinell, et al, 1994; Graham and Womack, 1972; Ostaff, 1978; Simpson, 1991; Tomminen, 1992). However, for the conditioning of poles, air seasoning is currently still the most widely used method. During air drying, the sapwood is vulnerable to fungal colonization (Newbill and Morrell, 1991). This is especially a problem for thick sapwood species (Panek, 1963). Alternative heating techniques, such as RF/V heating had been used to eradicate the pinewood nematode (Hightower et al, 1974; Dwinell, et al, 1994). The effectiveness of this approach appears to be dependent on temperature and independent of the vacuum (Dwinell, etal, 1994). 114 The research in this phase focused on exploring the potential of RF heating to produce sterilized poles. It examined the feasibility of sterilizing roundwood using a two-hour heating period at 65°C or above using the existing RF/V dryer, located in the Wood Drying Laboratory at The University of British Columbia. 115 5.2 Literature Review 5.2.1 Why it is necessary to sterilize wood prior to use? 5.2.1.1 Decay of poles When degradation occurs in poles, it can be located either at the surface due to fungal colonization from the wet soil adjacent to the pole, or internally where it most likely arises due to pre-infection in the tree. Studies have reported considerable potential for decay arising from the living tree, for example, Cwielong and Rajchenberg (1995) found at least one wood rotting fungus in living trees sampled, while Przybylowicz et al. (1987) confirmed that virtually all Douglas-fir poles exposed for 1 year in the Pacific Northwest, contained at least one Basidiomycete fungus. Currently, air seasoning is still the most commonly used method for drying peeled poles prior to preservative treatment (Newbill and Morrell 1991). During this process, the nutrient-rich sapwood, as well as heartwood, is susceptible to fungal colonization (Przybylowicz et al. 1987; Zahora and Dickinson, 1989; Newbill and Morrell, 1991). This is particularly a problem in thick sapwood species such as red pine and southern pine, where fungal infestation occurs during the first few weeks of air seasoning process (Panek, 1963). The longer the air-drying time, the higher the percentage of poles with Basidiomycetes (Przybylowicz et al. 1987). Surface decay in treated poles most commonly appears either at or below the ground line and arises due to the improper application of the preservative or a reduction in effectiveness with extended service life (Cassens, et al, 1995). 116 5.2.1.2 Pole failure in service Following several incidences of pole breaking during handling or installation, Manitoba Hydro conducted intensive inspections of poles in storage and found that more than 10% of the poles had decay, which affected their structural integrity (Mann, 1996). It should be noted that even early colonization of wood can lead to significant strength loss (Table 5-1) Table 5-1 Estimated values for strength losses at early stages of decay Approx. weight loss (%) Toughness Impact bending Modulus of Rupture Brown rot: Softwoods 2 - 20-50 13-50 4 75 25-55 10 - 85 70 Hardwoods .2 36 31-50 32 6 - 80 61 10 60 70-92 -Note: Values obtained from published experimental results and adjusted to equivalent weight-loss levels (adapted from Wilcox, 1978) 5.2.2 Sterilization techniques Many strategies are being used to reduce the risk of infection of wood by destroying organisms, including removal of bark to reduce attractiveness to insects, application of fungicides and insecticides, fumigation, and heat treatment (Freitag and Morrell, 1998). 117 5.2.2.1 Removal of bark Removing the bark represents a basic strategy to reduce the risk associated with insect attack (Aho and Cahill, 1984; Lowell, et al, 1992; Morrell, 1995), since bark serves to retain moisture and many insects require the presence of bark for attack (Furniss andCarolin, 1977). 5.2.2.2 Application of chemicals as biocides The application of biocides shortly after cutting or after processing logs also limit the introduction of fungi and insects into wood. However, due to environmental concerns over worker exposure, and the effect of the residual chemical on the wood, this strategy is no longer widely used in North America. In addition, the chemicals applied to protect the wood surface against the colonization by fungi and insects do not penetrate into the wood very well and so are incapable of eliminating the organisms deep in the log (Morrell, 1995). The application of diffusible biocides, such as boron or fluoride may provide some control of insects and fungi deeper in the wood (Becker, 1976; de Jonge, 1986; Barnes and Williams, 1988; Morrell etal, 1989; 1991). Fumigants represents another widely used method for eliminating pests from grains, wood chips, and other porous materials (Morrell, 1995). In fumigation, a chemical with a high vapor pressure is introduced into the wood. One fumigant used on wood is methyl bromide (Jones, 1963; 1973; Liese and Ruetze, 1984; 1985; Schmidt, 1983 a; b and c). However the treatment is not particularly effective for sterilizing 118 organisms deep within the wood. Because of this and concern over the environmental impact, the use of methyl bromide is no longer used (Kramer, 1992). 5.2.2.3 Irradiation The use of Gamma radiation to eliminate pests from a variety of materials including wood has a long history (Cornwell, 1966; Shuler, 1971; Hansen, 1972). Although it is considered by public as "contamination", irradiation has a number of advantages to sterilize wood. Similar to heat, gamma radiation penetrates into wood. The treatment has little or no effect on mechanical properties of the wood, nor does it affect its durability (Becker and Burmester, 1962; Scheffer, 1963; Shuler, 1971; Smith and Sharman, 1971; Freitag and Morrell, 1998). However, the cost of using this approach as well as the safety problems associated with the sources, are major concerns (Cornwell, 1966). Electron beam irradiation has been proposed as an alternative approach although this technology is currently incapable of treating large volumes of materials (Morrell, 1995). 5.2.2.4 Heat treatment Of all of the currently permitted methods, only heat treatments are recognized as being capable of rapidly killing organisms deep in the wood economically (Freitag and Morrell, 1998). During heating treatment, the rate of heat transfer through wood depends on the starting temperature, wood density, and wood initial moisture content (Siau, 1984). The time required to achieve specific internal temperatures in a log can even be calculated (Siau, 1984). Currently, some pole plants use a heating rate of 1 hour per 25 119 mm of diameter as a guideline for achieving the sterilization requirements of 67°C for 60 minutes at the pith (Morrell, 1995). This rate appears to be acceptable for poles with a diameter of less than 30 cm, but is inadequate for larger diameter poles (Maclean, 1946; Dost, 1984; Sahle-Demessie et al., 1992). Extensive studies have indicated that heating wood to 52°C for 30 minutes is effective in eliminating pinewood-nematode in logs (Kinn, 1986; Dwinell, 1990 a; b; and c; Smith, 1991). However, it would allow a wide variety of wood destroying fungi to survive as shown in Table 5-2 (Morrell, 1995). According to Chidester (1937; 1939), a minimum temperature of 67°C for 75 minutes was necessary for eliminating most wood-inhabiting fungi from southern pine. Newbill and Morrell (1991) studied the effect of elevated temperatures on the survival of Basidiomycete fungi colonizing untreated Douglas-fir poles and concluded that none of the fungi survived a 75-minute exposure at 65.6°C, despite differences in temperature tolerance of the sapwood-and heartwood-colonizing species. Similar results were obtained from the study of fungi colonizing other wood species (Abooki, 1978; Miric and Willeitner, 1984). Thus, APHIS recommended heating poles at 77.1°C for 75 minutes to achieve sterilization (USDA Forest Service, 1991). 120 TABLE 5-2 — Examples of organisms capable of surviving exposure to elevated temperatures (adapted from Morrell, 1995) Species Taxomonic grouping Reported tolerance or optimum temperatures Source Neolenitius lepideus Basidiomycete 55°C for 20.5hr. 65°Cfor1.0 hr. 75°C for 0.3hr. Chidester, 1939 Chidester, 1937 Gloeophyllum saepiarium Basidiomycete 55°Cfor10.5hr. 65°Cfor1.0hr. Chidester, 1939 Chidester, 1937 Haematostereum sanguinolentum Basidiomycete 57°C for 0.4 hr. Newbill and Morrell, 1991 Pastia placenta Basidiomycete 71°Cfor 0.4 hr. 65°Cfor1.0hr. 57°C for 6.0 hr. Newbill and Morrell, 1991 Paxillus panuoides Basidiomycete 55°Cfor1.0hr. Miric and Willeitner, 1984 Allescheria terrestris Ascomycete 45°C to 50°C optimal for growth Hulme, 1979 Chaetomium thermophilum Ascomycete 45C °to 50°C optimal for growth Hulme, 1979 Bacillus spp. Bacteria 45°C to 50°C optimal for growth Humphrey, 1920 It should be noted that shorter time periods using higher temperatures, can be nearly as effective as heating wood for a longer time period at moderate temperatures (Chidester, 1937; Newbill and Morrell, 1991). Heat treatment can negatively affect wood properties, such as wood strength (MacLean, 1952; Nicholas and Siau, 1973; Freitag and Morrell, 1998). Consideration 121 must be given to the ultimate end product when using this strategy. Kiln drying should maintain wood temperatures at or below 93 °C to prevent excessive reduction in strength, especially when long drying times are involved (Graham and Womack, 1972). A limitation of conventional heat treatment of wood is the requirement to transfer heat from the wood surface to the core by conduction. This is time consuming and can lead to overheating of the wood surface. An alternative process in wood involves the use of RF/V technology (Dwinell, et al, 1994). 122 5.3 Materials and Methodologies 5.3.1 Fungal cultivation in solid media Fungal species The selection of wood destroying fungi was based on those decay organisms that are specified for standard evaluation of wood preservative treatments. They have usually been frequently isolated in North America. Based on their destructiveness to the wood and tolerance of being eliminated in the pole, four decay fungi were used in this project. The fungi were Postia placenta 31094B, Postia placenta 120F, Gloeophyllum trabeum AID and Coniophora puteana. They were selected from "standard" fungal strains, although the strain of the Coniophora puteana. was isolated by the former researcher of NSERC/Industrial Wood Preservation Research group at the University of British Columbia, and identified by the National Identification Service, Agriculture Canada, Ottawa. Preparation of media 2% malt agar was prepared by dissolving 33g malt agar and 10 g bacto agar in 1000ml distilled water. The solution was then autoclaved for 20 minutes at 103 kPa and 120°C, and poured (approx. 20ml per plate) into thirty pre-sterilized petri dishes (100mm xl5mm standard, Fisher) and allowed to solidify under sterile conditions on a laminar flow bench. 123 Inoculation and incubation of fungal cultures Tubes containing the selected fungi were taken from the culture collection, and placed on the sterile laminar flow bench. Using a sterilized metal loop, a piece of the agar that contained some fungal mycelium was removed from the " tubes" and gently placed at the center of the agar in the petri dish. The metal loop was then passed through the flame to sterilize it prior to removing a fresh fungal inoculation to another petri dish. The petri dishes with fungi were then sealed with parafilm and incubated at 25°C for two weeks until the fungi fully covered the whole plates. 5.3.2 Wood shaving infection experiments Shaving preparation 1000 ml (approx. 500 g) shavings were hand drilled from 19x19x200 mm southern yellow pine billets. In addition, ninety dowels, 70 mm in length, and 5 mm in diameter, and ninety dowels, 40 mm in length, and 5 mm in diameter, were also prepared from 19x19x200 mm southern yellow pine billets. The above shavings and dowels, as well as commercial machined dowels (10 mm in length, 5 mm in diameter) were placed in different beakers which were sealed with aluminum foil. They were then autoclaved at 103 kPa and 120°C for 25 minutes to sterilize them. The beakers were placed on a laminar flow bench and sterilized water poured over on the shavings to raise their moisture content to around 50%. All the beakers were then sealed and stored prior to use. 124 Inoculation of shavings Method A: The beakers with sterilized shavings and petri plates with fully grown fungal mycelium were transferred to the laminar flow bench. An aliquot of 20ml sterile shavings was added to each plate and spread to cover the surface. Core (0 3 mm) taken from agar at the edge of the growing colony of Postia placenta 120F were then placed on top of the shavings. The plates was then sealed to prevent moisture loss and incubated at 25°C for another two weeks until the shavings were visibly covered with fungal mycelium. The process was then repeated with each of the other fungi, Postia placenta 31094B, Gloeophyllum trabeum 47D and Coniophora puteana. Method B: A solution of 2% malt agar (prepared as described before) was poured into disposable petri dishes. Once solid, a sterile piece of pre-cut circular sheet of cellophane were prepared and placed over the agar. This procedure was adopted to prevent the wood pieces from contacting the agar surface as moisture would be drawn into the wood from the agar causing water-logging of the test samples (Dubois, 1999). About 20ml of sterile 50% moisture content shavings was then carefully spread on top of the cellophane. Agar cores (0 3 mm) were then taken from the edge of a growing colony of Postia placenta 120F, and placed on top of the cellophane in contact with the shavings, to inoculate the plates. All plates were sealed with parafilm and incubated at 25°C. The procedure was then repeated with each of the other three fungi. 125 5.3.3 Sterilization treatment 5.3.3.1 Preparation of the test pole sections Three Douglas-fir and three western-red-cedar round wood sections (2.2 m in length and 240 mm in diameter) were selected for this experiment. They were obtained from Stella Jones Company in New Westminster, Vancouver and had been radio frequency pre-seasoned (Chapter 2) and CCA treated (Chapter 3) in early phase of the research. All the experimental pole sections had average initial moisture contents of 35% (tested by moisture meter). Immediately before the sterilization treatment, eighteen holes, 10 mm in diameter, were randomly drilled in each pole. Six holes were drilled to depths of 100 mm, 70 mm and 25 mm. All of the experimental holes of the roundwood sections were then vacuumed to remove sawdust and sprayed with ethanol to minimize interference by mold fungi before transferring inoculated shavings 5.3.3.2 Radio frequency/vacuum dryer The drying chamber used was as described in section 2.3.3, with the difference of that no vacuum was employed. 5.3.3.3 Heating treatment Transferring of infected shavings Sterilized tweezers were used to transfer the infected shavings into the holes. The tweezers were sterilized by dipping them in ethanol and passing them through a flame before and after each transfer of sawdust. About 3 ml of the shavings was placed into each hole. After the transfer, pre-sterilized dowels with a suitable length were hammered 126 into the holes to seal them and keep the shavings in place at the desired depth from the pole surface. Each round wood section was inoculated with three replicas, for each of the three fungi, at three different depths. The inoculated shavings were transferred quickly in order.to minimize contamination by mold. This contamination is possible since the environment was un-sterile, and fungal spores are constantly available in the air to provide a source of contamination. Heating runs During the first experiments, control samples, which had been inoculated with Postia placenta 31094B and Coniophora puteana but had not been subjected to any heat treatment, failed to yield the decay fungi during re-isolation. Consequently these experiments were repeated only with Postia placenta 62.5°C at three different depths with three replicas. Altogether, there were eight heating runs: four for the two Douglas-fir sections and four for the two western red cedar pole sections. As described in the Section 2.3.4, two fiber optic thermo sensors were used to monitor the temperatures inside the wood during each heating. The fiber optic-thermo-sensors were placed at the mid-point of the each section. One was in the core of the section, and the other was in the outer shell. All of the sterilization treatments were carried out for 2 hours starting from the time when both thermo-sensors reached 65°C. The variation of temperature with time was recorded. 127 Controls— pole sections A control pole section of each wood species was inoculating with the test fungi and placed under ambient conditions for approx. 2.5 hours, based on the average heating time for each wood species. 5.3.4 Sample analysis 5.3.4.1 Growth conditions In order to keep mould fungi and bacteria from growing on the agar, benomyl and tetracycline were added. Benomyl inhibits the growth of mould, while tetracycline, an antibiotic is added to prevent bacteria growth. 1. Preparation of standard solution Benomyl stock solution (1 mg/ml): an aliquot of 0.1 g of benylate (50% ai) was dissolved in a minimum of 30 ml of distilled water, which was then made up to 100ml with additional distilled water. Tetracycline stock solution (10 mg/ml): an aliquot of 0.5g of tetracycline hydrochloride was added to 10 ml pre-sterilized distilled water, which was and then diluted to 50 ml with additional sterile distilled water 2. Benomyl tetracycline malt agar A 20g sample of malt extract and 30g of agar were weighed and dissolved in 2000ml of distilled water. A 30ml benomyl stock solution was then added (15 ppm) and 128 the bottle containing the solutions placed in an autoclave. It was sterilized using a liquid cycle, which lasted for 20 minutes at 103 kPa and 120°C. The bottle was allowed to cool, and 20ml of sterile tetracycline hydrochloride stock solution was added asceptically to the agar solution (lOOppm). The whole solution was mixed well and dispensed into one hundred and eight pre-sterilized petri dishes on a laminar flow bench. The plates were stored on the laminar flow bench until used in the experiment. 5.3.4.2 Shaving r ecovery The radio frequency units was turned off after 2 hours of heating, timed from the point that both fiber optic thermo sensors reached 65°C. The roundwood section was removed immediately. The plug was removed from each sampling location in turn and the shavings quickly recovered using flame sterilized tweezers. They were spread over the agar in marked plates. The procedure was repeated in turn for each sampling location, and for each fungus, including those control pole sections. The plates were sealed with parafilm, carefully labeled and incubated at 25°C for two weeks. The plates were checked to identify the recovery of the fungi after two weeks and four weeks and the result recorded. 129 5.4 Results and Discussion 5.4.1 The temperature figure of radio frequency heating treatment Since the graphs of temperature versus time showed no significant differences for all of the heating treatments, representative plots for each species are shown in Figure 5-1. From these two graphs. It can be seen that all of the sterilization treatments started at an ambient temperature of around 20°C and were heated for 2 hours after both Ti and T2 exceeded 65°C. The temperature at end of heating was ca. 115°C in all of the eight runs. It was clear that the initial rate of temperature increase in the Douglas-fir pole sections was slower than that of western red cedar due to the differences in their density. It took about 50 minutes for both sensors inside the Douglas-fir round wood species to exceed 65°C. However it only took less than 40 minutes for both of the thermo sensors to exceed 65°C for western red cedar pole sections. It was also noted that the inner wood temperature in Douglas-fir pole sections increased more rapidly than the outer wood one during the initial heating. This is due to the surface cooling effect and more efficient heat convection in the center of the poles resulting from higher inner moisture (Morrell, 1995; Smith, et al, 1996). However, this difference was not so obvious for the western red cedar pole sections. 130 Figure 5-1: Temperature figure of sterilization treatment of Douglas-fir and Cedar 131 5.4.2 Mortality of the fungi Since all of the experimental sections were removed after exposure to over 65 °C for two hours, a time-based and a temperature-based mortality curve for the death of the selected decay fungi could not be obtained. However, no decay fungi could be recovered from shavings, which had been subjected to radio frequency heating after two weeks' incubation (Table 5-3 to 5-6 and Figure 5-2, 5-4, 5-6, 5-8). Furthermore, no mold fungus was present in any of the above samples. This indicated that the heat treatment was sufficient for the eradication of all the fungi. Unfortunately, due to contamination with mold from the un-sterilized control pole sections, only a small amount growth was observed for Postia placenta 31094B and no growth occurred with shavings recovered from the section inoculated with shavings containing Coniophora puteana (Table 5-4, 5-6). Even the plots with the shavings inoculated with Gloeophyllum trabeum 47D after recovering, showed the presence of slight contamination by mold (Table 5-5 and Figure 5-3, 5-5). The repeated experiment with a more vibrant strain Postia placenta 62.5°C showed good recovery from the controlled shavings at ambient temperature with no mold infection (Table 5-3 and Figure 5-7, 5-9). 132 Figure 5-2. Sterilized treatment result of western red cedar with Gloeophyllum trabeum 47D R F sterilized (Core) R F sterilized (mid-point of radius) R F sterilized (outer shell) Gloeophyllum trabeum 47D Douglas-fir Figure 5- 4. Sterilized treatment result of Douglas-fir with Gloeophyllum trabeum 47D Figure 5-5. Control samples of Douglas-fir with Gloeophyllum trabeum 47D 134 Figure 5-6. Sterilized treatment result of western red cedar with Postia placenta 120F Figure 5-7. Control samples of western red cedar with Postia placenta 120F 135 Figure 5-8. Sterilized treatment result of Douglas-fir species with Postia placenta 120F Table 5-3 Presence of Postia placenta 120F in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln Controls Heat-treatment one Heat treatment two* Sample ID Sample No. Inspection Sample ID Sample No. Inspection Sample ID Sample No. Inspection DF 5 6 +++ DF 4 9 0 DF 4 9 0 DF 7 9 0 DF 7 9 0 C5 6 +++ C4 9 0 C4 9 0 C6 9 0 C6 9 0 *the repeat heat treatment 0 No growth Postia placenta 120F +++ Over 90% recovered live Postia placenta 120F DF Douglas-fir C Western red cedar Table 5-4 Presence of Postia placenta 31094B in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln Controls Heat-treatment one Heat treatment* Sample Sample Inspection Sample Sample Inspection Sample Sample Inspection ID No. ID No. ID No. DF 5 6 + DF 4 9 0 DF 4 9 0 DF 7 9 0 DF 7 9 0 C5 6 + C4 9 0 C4 9 0 C6 9 0 C6 9 0 *the repeat heat treatment 0 No growth Postia placenta 31094B + Less than 10% recovered Postia placenta 31094B DF Douglas-fir C Western red cedar 137 Table 5-5 Presence of Gloeophyllum trabeum 47D in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln Controls Heat-treatment one Heat treatment two* Sample Sample Inspection Sample Sample Inspection Sample Sample Inspection ID No. ID No. ID No. DF 5 6 +++,1 DF 4 9 0 DF 4 9 0 DF 7 9 0 DF 7 9 0 C 5 6 +++,1 C 4 9 0 C 4 9 0 C 6 9 0 C 6 9 0 *the repeat heat treatment 0 No growth Gloeophyllum trabeum 47D +++ Over 90% recovered Gloeophyllum trabeum 47D I Infected by mould DF Douglas-fir C Western red cedar Table 5-6 Presence oi Coniophora puteana in pre- and post- heat treatment inspection of Douglas-fir and western red cedar in radio frequency/vacuum kiln Controls Heat-treatment one Heat treatment Sample Sample Inspection Sample Sample Inspection Sample Sample Inspection ID No. ID No. ID No. DF 5 6 0,1 DF 4 9 0 DF 4 9 0 DF 7 9 0 DF 7 9 0 C 5 6 0,1 C 4 9 0 C 4 9 0 C 6 9 0 C 6 9 0 *the repeat heat treatment 0 No growth Coniophora puteana 1 Infected by mould DF Douglas-fir C Western red cedar 138 5.5 C o n c l u s i o n s With the increased need for the wood treating industry to produce preservative treated poles in which the untreated interior is sterile, RF heating represents an interesting option. The study confirmed that heating the wood to above 65°C for 2 hours killed all fungi present in the wood. 139 C H A P T E R 6 CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions The results of this project indicate that completed conversion of Cr (VI) to Cr (III) during fixation of CCA-C in freshly treated Douglas-fir, western red cedar and red pine pole sections can be achieved in less than 5 hours, with a fixed radio frequency of 13.56 MHz. The stored heat after completion of radio frequency heating can further assist the conversion of unfixed Cr (VI) to Cr (III). This study also demonstrated that radio frequency heating, when accompanied by a vacuum, can quickly dry pole sections. It took less than 16 hours at temperatures > 65°C to dry the poles 2.0- 2.4 m in length, and 160-240 mm in diameter, from the initial moisture content > 80 % to 25 % moisture content. This compares favorably to the usual one year required by the current air drying strategy. Dielectric heating of wood to temperature > 65 °C also eradicated the decay fungi : Coniophora puteana, Gloeophyllum trabeum 47D and Postia placenta 12OF, 31094B, when inoculated shavings were randomly placed throughout the pole section. Mould fungi were also eradicated. 140 6.2 Recommendat ions The successful completion of these studies provides a complete basis for the design of a semi- commercial RF/V drying schedule for poles. Profiles of drying rate for different pole species using RF/V kiln drying need to be developed. In addition, future research is needed to examine the influence of the air gap between the chamber and poles as well as between poles when loaded in a manner usual for bulk drying. Other research, which could be benefited, relates to drying of other pole species and the influence of air gaps on the efficacy of this process. While it is known that conventional heating of wood during drying could produce some reduction in strength, the actual losses from RF heating are not known. Studies of RF/V drying of sawn wood have demonstrated its cost effectiveness compared to conventional drying particularly for large dimension timber. An economic analysis of radio frequency drying, CCA fixation and sterilization of treated utility poles should be conducted. This should include consideration to improve using curved electrode plates instead of the current flat plates for roundwood and the possibility of combining the radio frequency chamber with CCA preservative treatment retort. The reduction of fixation time using RF heating by completing the fixation process using stored heat as well as by increasing of power density of the RF unit without causing the loss of strength would also influence the economic analysis. 141 Literature cited: Abooki, R.D.B.1978. 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The Council of Forest Industries, Vancouver. 162 APPENDIX I Complete figures of temperature with time in radio frequency/vacuum drying experiment 163 100 Drying of Cedar #C5 20 40 60 Time (minutes) 80 Tf. 8.5 cm away from the surface (heartwood) T2: 5.0 cm away from the surface (heartwood) T3: 2.5 cm away from the surface (sapwood) 100 Drying of Cedar #C6 100 40 60 80 Time (minutes) TV: 9.0 cm away from the surface (heartwood) T2: 5.0 cm away from the surface (heartwood) T3: 2.0 cm away from the surface (sapwood) 164 Drying of Douglas-fir #DF6 20 3 25 50 75 100 Time (minutes) 125 150 175 T i : 9.0 cm away from the surface (heartwood) T2: 5.0 cm away from the surface (heartwood) T3: 2.5 cm away from the surface (sapwood) Drying of Doug las - f i r #DF7 ^+—• • — _ ^ — • • ^" * • -""^ vacuum T1 T3 0 25 50 75 100 125 150 175 T i m e (minutes) T I : 9.0 cm away from the surface T2: 5.5 cm away from the surface T3: 2.5 cm away from the surface 165 100 Drying of Douglas-fir #DFS 80 o 0) k_ 3 ro i— a a. E 60 40 Vacuum ro V— •Si a. E o 50 100 150 Time (minutes) 200 T l : 8.5 cm away from the surface T2: 5.5 cm away from the surface T3: 2.5 cm away from the surface Drying of Cedar #C8 100 200 300 400 Time (minutes) 500 T l : 9.0 cm away from the surface T2: 3.0 cm away from the surface 250 300 600 700 166 Drying of D o u g l a s - f i r #DF10 100 200 300 Tim e (m inu tes ) 400 TI: 10.5 cm away from the surface T2: 6.5 cm away from the surface T3: 4.0 cm away from the surface T4: the air inside the chamber 500 600 Drying of Cedar #C10 120 20 80 100 120 Time (minutes) TI: 9.0 cm away from the surface T2: 3.0 cm away from the surface 200 167 Drying of Douglas-fir # D F 1 1 i_ 3 ra >— <D a. £ 120 100 100 200 300 400 500 Time (minutes) 600 700 800 Tl: 10.5 cm away from the surface T2: 6.5 cm away from the surface T3: 3.0 cm away from the surface T4: the air inside the chamber Drying of Douglas-fir #DF12 -T1 -T2 T3 -T4 O 80 2 0 Vacuum _wiini iui i inr i t*—•' im""M»> ' 50 100 150 200 250 Time (minutes) 300 Tl: 10.5 cm away from the surface T2: 6.5 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 350 400 1 6 8 Drying of Douglas-fir #DF13 <0 I— D *-> nj i _ <u a E 0) 100 200 300 400 500 Time (minutes) 600 TI: 10.5 cm away from the surface T2: 6.5 cm away from the surface T3: 2.5 cm away from the surface T4: the air inside the chamber 700 800 TI: 10.5 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 169 " 80 (3 60 Drying of Doug las - f i r #DF15 100 200 300 400 500 600 700 800 Tim e (m inu tes) T1; 11.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber Drying of Douglas-fir #DF16 o 13 i _ a a. E o 100 200 300 400 500 600 Time (minutes) TI: 11.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 700 800 170 Drying of Douglas-fir #DF17 100 200 300 400 500 Time (minutes) Tl: 11.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 600 700 Drying of Douglas-fir #DPI 8 o o 0) Q. £ 0) 0 100 200 300 400 500 600 700 800 Time (minutes) Tl: 10.5 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 171 Drying of Douglas-fir # DF19 120 100 200 300 400 500 Time (minutes) 600 120 100 Tl: 9.5 cm away from the surface T2: 5.0 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber Drying of Douglas-fir #DF20 O 80 is 60 Qi Q. E ® 40 20 Vacuum 100 200 300 400 Time (minutes) 500 T l : 9.5 cm away from the surface T2: 5.5 cm away from the surface T3: 3.5 cm away from the surface T4: the air inside the chamber 700 800 -T1 -T2 T3 -T4 600 700 172 Drying of Douglas-fir #DF21 V a c u u m 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 Time (minutes) 6 0 0 7 0 0 8 0 0 TI: 9.0 cm away from the surface T2: 5.0 cm away from the surface T3: 3.0 cm away from the surface T4: the air inside the chamber Drying of Douglas-fir #DF22 120 100 200 300 4 0 0 500 6 0 0 Time (minutes) TI; 9.5 cm away from the surface T2: 5.0 cm away from the surface T3: 3.0 cm away from the surface T4: the air inside the chamber 700 800 173 Drying of Douglas-fir #DF23 120 100 200 300 400 500 Time (minutes) 600 700 800 TI: 10.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.0 cm away from the surface T4: the air inside the chamber Drying of Red pine #RP1 100 200 300 400 500 600 700 800 900 1000 1100 Time (minutes) TI: 8.5 cm away from the surface T2: 5.5 cm away from the surface T3: 3.0 cm away from the surface 174 Tl: 9.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.0 cm away from the surface Drying of Red pine #RP 3 —•—T1 -m—T2 T3 ij I • Vacuum p-1 — — — i 1 1 1 1 1 1 i 1 100 200 300 400 500 600 700 800 900 1000 Time (minutes) Tl: 9.5 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface 175 Drying of Red pine #RP 4 100 2 0 0 3 0 0 4 0 3 5 0 0 6 0 0 700 8 0 0 9 0 0 1000 Time (minutes) TI: 9.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.0 cm away from the surface 120 i 100 o 80 •S 60 01 C L E •2 40 Drying of Red pine #RP 5 -T1 -T2 T3 Vacuum 20 100 200 300 400 500 600 700 800 900 1000 1100 Time (minutes) TI: 8.5 cm away from the surface T2: 5.5 cm away from the surface T3: 3.0 cm away from the surface 176 APPENDIX II Complete figure of temperature with time in radio frequency fixation experiment 177 Heating of c e d a r #C4 120 100 80 + *_ ro C L E i -•s 60 + 40 1 20 — 50 100 150 200 250 Time (minutes) 300 350 400 120 T l : 7.0 cm away from the surface T2: 5.0 cm away from the surface T3: 2.0 cm away from the surface Heating of ceadr #C 5 50 100 150 200 250 Time (minutes) 300 350 T l : 8.5 cm away from the surface T2: 2.5 cm away from the surface 400 178 120 i Heating of cedar #C6 50 100 150 200 250 Time (minutes) 300 350 400 140 120 Tl: 9.0 cm away from the surface T2: 2.5 cm away from the surface Heating of cedar #C15 100 o o 0} ro k_ a> Q. £ 50 100 150 200 250 Time (minutes) 300 350 400 Tl: 7.5 cm away from the surface T2: 2.0 cm away from the surface 179 50 100 150 200 250 300 350 400 450 500 550 Time (minutes) TI: 7.0 cm away from the surface T2: 5.0 cm away from the surface T3: 2.0 cm away from the surface Heating of Douglas-fir DF5 50 100 150 200 250 Time (minutes) 300 TI: 3.5 cm away from the surface 350 400 180 50 100 150 200 Time (minutes) 250 300 350 TI: 8.5 cm away from the surface T2: 4.0 cm away from the surface Heating Douglas-fir #DF8 120 50 100 150 200 250 Time (minutes) 300 350 TI: 8.5 cm away from the surface T2: 2.5 cm away from the surface 400 181 Heating Douglas-fir #DF17 120 -, 250 100 150 200 Time (minutes) Tl: 2.5 cm away from the surface 300 350 Heating of Douglas-fir #DF19 50 100 150 200 250 300 Tim e (m inutes) Tl: 3.5 cm away from the surface 350 400 182 o o <u i-3 -*-» ns i_ <u CL E o 100 90 80 70 -60 50 40 30 20 10 0 50 Heating Douglas-fir #DF20 100 150 200 250 Time (minutes) TI: 3.5 cm away from the surface 300 350 400 Heating of Douglas-fir #DF23 0) 3 u. o c E a 50 100 150 200 250 Time (minutes) 300 350 400 TI: 3.0 cm away from the surface 183 Heat ing of Doug las - f i r #DF24 50 100 150 200 250 Tim e (m inutes) T l : 3.0 cm away from the surface 300 350 120 100 Heating of Douglas-fir #DF25 •T1 -T2 O 80 + 2 60 } ro t_ v o. S 40 20 r 50 100 150 200 250 Time (minutes) 300 T l : 9.0 cm away from the surface T 2 : 2.0 cm away from the surface 350 400 184 120 Heating of cedar #C9 50 100 150 Time (minutes) 200 Tl: 2.5 cm away from the surface 250 Heating of cedar #C10 120 50 100 150 200 Time (minutes) Tl: 2.0 cm away from the surface 185 Heating of cedar #C11 120 100 50 100 150 200 Time (minutes) 250 300 TI: 2.5 cm away from the surface Heating of cedar #C12 120 100 50 100 150 Time (minutes) TI: 2.5 cm away from the surface 200 250 186 Heating of red pine #RP2 CO O -» O O O - * - T 1 -m-12 T3 80 70 -60 50 40 30 20 , 10 -0 - , . . . 1 1 1 . , 0 20 40 60 80 100 120 140 160 180 200 Time (minutes) Tl: 9.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.0 cm away from the surface 187 Heating of red pine #RP3 20 40 60 80 100 120 140 160 180 200 220 240 260 Time (minutes) TI: 9.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.5 cm away from the surface Heating of red pine #RP4 /7* 20 40 60 80 100 120 140 160 180 200 220 240 Time (minutes) TI: 9.0 cm away from the surface T2: 6.0 cm away from the surface T3: 3.0 cm away from the surface 188 Heating of red pine #RP5 20 40 60 80 100 120 140 160 180 200 220 240 Time (minutes) TI; 8.5 cm away from the surface T2; 5.5 cm away from the surface T3: 3.0 cm away from the surface Heating of red pine #RP6 110-, -* -T1 100 -m-T2 90 T3 80 70-60 50 < 40 J 30 f 20- f 10 0 20 40 60 80 100 120 140 160 180 200 220 240 Time (minutes) TI: 8.5 cm away from the surface T2: 5.5 cm away from the surface T3: 3.0 cm away from the surface 189 APPENDIX III Complete figure of temperature with time in sterilization experiment 190 Heating run 4 (Cedar #C 6) 100 Time (minutes) Tl: 9.0 cm away from the surface T2: 2.5 cm away from the surface Heating run2 ( Douglas-fir #DF4) 50 100 Time (minutes) 150 Tl: 9.0 cm away from the surface T2: 2.0 cm away from the surface 200 191 

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