Forestry, Faculty of
DSpace
UBCV
Dai, Hengliang
Master of Science - MSc
University of British Columbia
This thesis studied the optimization of the pressing parameters for producing oriented strandboard (OSB) panels with a uniform vertical density profile (VDP) for furniture purpose. In order to identify the key parameters and press cycle that produces a uniform VDP, four experiments were developed by two hot pressing methods were considered (conventional hot pressing and steam injection hot pressing). Two main variables (strand thickness and closing position) were considered. The performed tests included internal bond (IB), bending and elastic moduli (MOR/MOE), vertical density profile (VDP), and face and edge screw withdrawal resistance (SWR). Results suggested the optimization of pressing parameters for conventional hot pressing to achieve a uniform VDP is creep closing schedule, which is the press platens stopped at 19.8 mm in 5s during closing segment and then continued closing with a slower rate to target thickness 18 mm in 10s. The total cooking time was 235s, ended with two degas segments (20s respectively) and a 20s opening segment. Compared to traditional hot pressing, the optimized pressing schedule can achieve a more uniform VDP panel. This means it can achieve a higher edge SWR strength which is the key to OSB suitability for upholstered furniture. In addition, examination of the SWR results showed that both face SWR and edge SWR are not affected by the strand thickness.
THE OPTIMIZATION OF THE PRESSING PARAMETERS FOR PRODUCING ORIENTED STRANDBOARD (OSB) PANELS WITH A UNIFORM VERTICAL DENSITY PROFILE (VDP) by Hengliang Dai MSc., Nanjing Forestry University, 2016 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2023 © Hengliang Dai, 2023 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: The optimization of the pressing parameters for producing Oriented strandboard (OSB) panels with a uniform vertical density profile (VDP) submitted by Hengliang Dai in partial fulfilment of the requirements for the degree of Master of Science in Forestry Examining Committee: Dr. Gregory D. Smith, Professor, Department of Wood Science, UBC Supervisor Dr. Scott Renneckar, Professor, Department of Wood Science, UBC Supervisory Committee Member Dr. Gregory Paradis, Assistant Professor, Department of Resources Management, UBC Additional Examiner Additional Supervisory Committee Members: Dr. Philip Evans, Professor, Department of Wood Science, UBC Supervisory Committee Member iii Abstract This thesis studied the optimization of the pressing parameters for producing oriented strandboard (OSB) panels with a uniform vertical density profile (VDP) for furniture purpose. In order to identify the key parameters and press cycle that produces a uniform VDP, four experiments were developed by two hot pressing methods were considered (conventional hot pressing and steam injection hot pressing). Two main variables (strand thickness and closing position) were considered. The performed tests included internal bond (IB), bending and elastic moduli (MOR/MOE), vertical density profile (VDP), and face and edge screw withdrawal resistance (SWR). Results suggested the optimization of pressing parameters for conventional hot pressing to achieve a uniform VDP is creep closing schedule, which is the press platens stopped at 19.8 mm in 5s during closing segment and then continued closing with a slower rate to target thickness 18 mm in 10s. The total cooking time was 235s, ended with two degas segments (20s respectively) and a 20s opening segment. Compared to traditional hot pressing, the optimized pressing schedule can achieve a more uniform VDP panel. This means it can achieve a higher edge SWR strength which is the key to OSB suitability for upholstered furniture. In addition, examination of the SWR results showed that both face SWR and edge SWR are not affected by the strand thickness. iv Lay Summary For Ready To Assemble (RTA) Furniture, edge screw withdrawal resistance (eSWR) is critical because consumers often use electric drills to assemble the furniture, but apply too much torque to the fastener, which results in the screw shredding its embedment material. The low core density of OSB boards made by traditional hot pressing tends to produce lower SWR. This suggests that boards with higher core density and less density variation through the thickness may have improved fastener performance. This thesis studied how to optimize traditional hot pressing methods to achieve a OSB panel with a higher eSWR property. The objective is to optimize develop a uniform vertical density profile (VDP) OSB product that is suitable for replacing plywood, particleboard or solid wood in the manufacture of upholstered furniture. Improved performance on eSWR was achieved with a higher core layer density and a more uniform VDP compared to sheathing grade OSB. v Preface Chapter 2 is based on the work conducted in UBC wood composites lab by Dr. Gregory D. Smith and me. The experiments in Chapter 3 were designed by Dr. Gregory D. Smith, Dr. Chunping Dai and me. All tests in Chapter 4 were completed by Hengliang Dai with the help of Dr. Solace Sam-Brew, Dr. Kate Semple and George Lee. Dr. Gregory D. Smith, Dr. Chunping Dai and Dr. Kate Semple provided suggestions, revisions, and supports throughout the entire thesis. vi Table of Contents Abstract ............................................................................................................................................ iii Lay Summary .................................................................................................................................... iv Preface .............................................................................................................................................. v Table of Contents ……………………………………………………………………………………………………………………………… vi List of Tables ..................................................................................................................................... ix List of Figures ..................................................................................................................................... x Acknowledgments .......................................................................................................................... xiv 1 Introduction ............................................................................................................................................... 1 1.1 Background ......................................................................................................................................... 1 1.2 Research Objectives and Structure of Work ....................................................................................... 5 2 Literature Review ...................................................................................................................................... 7 2.1 Review the manufacturing process for making OSB ........................................................................... 7 2.2 Review the different applications for OSB and the key property for each application ...................... 8 2.2.1 Screw holding strength of wood composite panels as a function of density .............................. 9 2.3 Review the literature on the impact of strand dimensions on board properties ............................. 12 2.4 Review the hot-pressing process and main factors affecting board properties and the development of VDP. .................................................................................................................................................... 12 2.4.1 Furnish moisture content........................................................................................................... 13 2.4.2 Hot press parameters ................................................................................................................ 13 vii 2.5 Review the steam injection process and describe how it differs from hot pressing ........................ 14 2.6 Review on the difference between OSB panels and other wood-based composites ....................... 15 2.6.1 Waferboard vs OSB .................................................................................................................... 16 2.6.2 Particleboard vs OSB .................................................................................................................. 17 2.6.3 Medium density fiberboard vs OSB ........................................................................................... 19 2.6.4 Plywood vs OSB .......................................................................................................................... 20 2.7 Summary and key findings ................................................................................................................ 21 3 Materials and Methods ........................................................................................................................... 22 3.1 Board Fabrication .............................................................................................................................. 22 3.1.1 Materials .................................................................................................................................... 22 3.1.2 Design of Experiment ................................................................................................................. 24 3.2 Methods ............................................................................................................................................ 24 3.2.1 Experimental Design .................................................................................................................. 24 3.2.2 Board manufacture .................................................................................................................... 28 3.2.3 Physical and Mechanical property testing ................................................................................. 28 4 Results and discussion ............................................................................................................................. 30 4.1 Experiment 1: Conventional hot pressing ......................................................................................... 30 4.1.1 VDP analysis ............................................................................................................................... 30 4.1.2 Density and IB ............................................................................................................................ 31 4.1.3 MOE and MOR ........................................................................................................................... 33 viii 4.1.4 SWR ............................................................................................................................................ 36 4.2 Experiment 2: Steam injection hot pressing 1 .................................................................................. 38 4.2.1 VDP analysis ............................................................................................................................... 38 4.2.2 Density and IB ............................................................................................................................ 39 4.2.3 MOE and MOR ........................................................................................................................... 41 4.2.4 SWR ............................................................................................................................................ 43 4.3 Experiment 3: Steam injection hot pressing 2 .................................................................................. 45 4.3.1 VDP analysis ............................................................................................................................... 45 4.3.2 Density and IB ............................................................................................................................ 46 4.3.3 MOE and MOR ........................................................................................................................... 48 4.3.4 SWR ............................................................................................................................................ 51 4.4 Experiment 4: Additional Step closure conventional hot press ........................................................ 52 4.4.1 VDP analysis ............................................................................................................................... 52 4.5 Summary ........................................................................................................................................... 53 5 Conclusions .............................................................................................................................................. 55 References .................................................................................................................................................. 56 ix List of Tables Table 1. Furniture market revenue comparison in billions of US dollars (Furniture - Statista market forecast. 2023) ............................................................................................................................................................. 5 Table 2. Comparison between different literatures based on varying resin content................................. 10 Table 3. Measured values of surface and core densities for the different panel types ............................. 10 Table 4. Mechanical properties of different wood-based composites (Maclntosh and Longworth. 1982) ................................................................................................................................................................... 16 Table 5. Comparison between waferboard and OSB .................................................................................. 17 Table 6. Comparison between Particleboard and OSB ............................................................................... 18 Table 7. Price for MDF and OSB based on thickness (The HomeDepot) .................................................... 19 Table 8. Comparison between MDF and OSB ............................................................................................. 19 Table 9. Comparison between plywood and OSB ....................................................................................... 20 Table 10. Experimental factors of Expt1 ..................................................................................................... 25 Table 11. Experimental factors of Expt2 ..................................................................................................... 25 Table 12. Experimental factors of Expt3 ..................................................................................................... 26 Table 13. Means and standard deviation of physical and mechanical properties of conventional hot pressing (schedule 1 to 6) ........................................................................................................................... 30 Table 14. Means and std Dev of physical and mechanical properties of steam injection hot pressing (cycle 7 to 12) ........................................................................................................................................................ 38 Table 15. Means and std Dev of physical and mechanical properties of steam injection hot pressing 2 (cycle 13 to 16) ...................................................................................................................................................... 45 Table 16. Density difference between face and core layers from Expt. 4 .................................................. 52 x List of Figures Figure 1. Typical sofa frame construction (from Chen, 2003) ...................................................................... 3 Figure 2. Schematic of an upholstered chair showing the internal components (Retrieved from: softdesignlab.com) ........................................................................................................................................ 4 Figure 3. Vertical density profile of OSB, MDF and PB (Wang et al. 2015) ................................................. 11 Figure 4. A photograph of Plywood, MDF, Particleboard and OSB taken by Hengliang Dai ...................... 15 Figure 5. Wood elements, from largest to smallest (Nishimura. 2015)...................................................... 15 Figure 6. Manufacturing process for OSB in Alberta Innovates Technology Futures a) Log selecting; b) Debarking; c) Stranding; d) Strands; e) Drying; f) Screening; g) Blending; h) Formatting; i) Pressing; and j) Sorting. Photos were taken by Hengliang Dai ............................................................................................. 23 Figure 7. Press schedules for (a) Expt 1-Conventional hot pressing, (b) Expt 2-Steam injection pressing 1 SIP1 of boards of different density, and (c) Expt 3-Steam injection pressing 2 using step or continuous closing strategies ......................................................................................................................................... 26 Figure 8. Pressing schedules for Expt 4 ....................................................................................................... 28 Figure 9. Cutting pattern for test specimens .............................................................................................. 29 Figure 10. Expt 1 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and hold position (18 mm, 19.3 mm and 20.7 mm) ................................................................................................................ 31 Figure 11. Internal bonding properties and sample density based on hold position (18 mm, 19.3 mm and 20.7 mm) and strand thickness (0.69 mm and 0.51 mm) a) sample density comparison by hold position b); IB comparison by hold position; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability. The dash line across the cluster is the OSB grade 1 ................... 32 xi Figure 12. Bending properties and sample density based on hold position (18 mm, 19.3 mm and 20.7 mm). a) MOR sample density comparison by hold position; b) MOE strength comparison by hold position; c) MOR strength comparison by hold position. The heavy horizontal ........................................................... 34 Figure 13. Bending properties and sample density based on strand thickness (0.69 mm and 0.51 mm). a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability.......................................................... 35 Figure 14. SWR properties and sample density based on strand thickness (0.69 mm and 0.51 mm) and hold position (18 mm, 19.3 mm and 20.7 mm). a) eSWR strength comparison based on hold position; b) fSWR strength comparison based on hold position; c) eSWR strength comparison based on strand thickness; d) fSWR strength comparison based on strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability ........................................... 37 Figure 15. Expt 2 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and board density (500 kg/m3, 600 kg/m3 and 700 kg/m3) ...................................................................................................... 39 Figure 16. Internal bonding properties and sample density based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3) and strand thickness (0.51 mm and 0.69 mm) a) sample density comparison by target density; b) IB comparison by target density; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability .................................................................................... 40 Figure 17. Bending properties based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3), a) MOR sample density comparison by target density; b) MOE strength comparison by target density; c) MOR xii strength comparison by target density. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability.......................................................... 42 Figure 18. Bending properties based on strand thickness (0.51 mm and 0.69 mm), a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability .................................................................................... 43 Figure 19. SWR properties based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3) and strand thickness (0.51 mm and 0.69 mm) a) eSWR strength comparison by target density; b) fSWR strength comparison by target density; c) eSWR strength comparison by strand thickness; d) fSWR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability .................................................................................... 44 Figure 20. Expt 3 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and steam pressure (10 psi and 25 psi). ...................................................................................................................................... 46 Figure 21. Internal bonding properties and sample density based on steam pressure (10 psi and 25 psi) and strand thickness (0.51 mm and 0.69 mm) a) sample density comparison by steam pressure; b) IB comparison by steam pressure; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability ...................................................................................................... 48 Figure 22. Bending properties based on steam pressure (10 psi and 25 psi), a) MOR sample density comparison by steam pressure; b) MOE strength comparison by steam pressure; c) MOR strength xiii comparison by steam pressure. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability .................................................................................... 49 Figure 23. Bending properties based on strand thickness (0.51 mm and 0.69 mm), a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability .................................................................................... 50 Figure 24. SWR properties a) eSWR strength comparison by steam pressure; b) fSWR strength comparison by steam pressure; c) eSWR strength comparison by strand thickness; d) fSWR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability ...................................................................................................... 51 Figure 25. SWR properties and sample density based on strand thickness (0.69 mm and 0.51 mm) and hold position (18 mm, 19.3 mm and 20.7 mm). Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly differ ............................................................................. 51 xiv Acknowledgments I would like to express the deepest appreciation to my Professor, Dr. Gregory Smith, who continually and persuasively conveyed a spirit of adventure in regard to research and scholarship, and an excitement in regard to teaching. Without his supervision and constant help this work would not have been possible. I would like to thank my committee members, Professor Renneckar, Scott and Professor Evans, Philip, for their constant feedback and support. I am very grateful to Professor Chunping Dai and Dr. Kate Semple for their guidance. Further, I am thankful to George Lee and Tom Chao of the Timber Engineering Group, Brandon Chan, Lief Eriksen and Claudia Ediger of the Center for Advanced Wood Processing, for their technical advice and help in the use of machinery and testing samples. Lastly, I would like to thank the members of my research group, Dr. Solace Sam-Brew and my wife Rongrong Zhang. 1 1 Introduction 1.1 Background Oriented strandboard (OSB) is widely used for sheathing, roofing and subflooring. The main properties for this application are strength and durability. The strength is achieved by manipulating the pressing and material parameters so that the panels have high-density faces and low-density cores which results in good bending properties. The durability is achieved by using durable, structural resins such as Phenol Formaldehyde (PF) or Polymer Methane Diphenol di-Isocyanate (pMDI) resins or a combination of the two. The OSB market is saturated and many OSB producers have tried to add value to and differentiate their OSB products by creating specialty products such as upholstered furniture frames, interior doors and partition walls (Zhan et al., 2014) and the like. OSB is used in North America primarily for single and multi-family construction. OSB producers have tried to export their products to Europe and China, but have had few sales because those markets do not build their houses out of wood; they prefer masonry structures. However, there may be an opportunity for a modified OSB product as China does make a lot of furniture from wood, primarily Particleboard (PB), Medium Density Fiberboard (MDF) and Block Board (BB). These products each have their limitations in terms of a furniture feed stock. PB is an excellent material to which decorative laminates can the bonded, i.e., IKEA furniture, but it has a low-density core in that, when a screw is driven into the core of a panel edge, is easily stripped – an experience that everyone has either had or knows someone who has. MDF is an excellent material for making kitchen panels that have decorative router traces in them because its density through the thickness is fairly uniform, this is referred to as the Vertical Density Profile (VDP), and also paints very well 2 due to the smooth edges or machined surfaces. MDF is most similar to block board which is made from a large number of square sections edge glued together in alternating layers and as such also has a uniform through the thickness density profile. In China, the production of blockboard is largely manual, and thus its production coats are high. Another important difference between these panels is the amount of resin in them and the cost of that resin. PB uses 4-6% by weight of low cost, typically Urea Formaldehyde (UF) resins. MDF uses approximately twice that resin content, 10 -14 % by weight. The resins used for OSB are stronger than UF resin and manufacturers can use much smaller mass fractions of resin, 2 to 5 % by weight depending on resin type. Because of its lower manufacturing cost compared with MDF and BB, OSB could be used for furniture if two of its properties were improved. These are the through the thickness density profile and the smoothness of the edge. If one could make an OSB panel with a uniform through the thickness density profile, one could stop the above mention stripping that occurs when as screw is put into the edge. A uniform density profile would also minimize the voids that are common with the edges of current OSB panels. The next step then is how does one make such a panel, ideally with existing plant equipment. Most OSB plants are equipped with hot presses. Some have a steam injection capability. In either case, those presses are typically used to make thin, 25 mm or thinner, panels. Two questions come to mind. 1. Is there a way to change the press cycle of a HP to produce a panel with a uniform VDP? If this cannot be done, what are the parameters for a steam press that produce and uniform VDP? 2. Can we modify the strand material its self, i.e., make the strands thinner so they heat up more quickly, and get a uniform VDP in this manner? Examination of the sofa frame (Chen, 2003) shows that around half of the components are made of a wood-based product such as solid timber, plywood, particleboard or OSB (Figure 1). It is these hidden 3 framing components and some of the solid wood components that are candidates for replacement with OSB. OSB could more cost effectively be substituted for all hidden wooden components indicated in the Figure 1that need to be fastened together. Figure 1. Typical sofa frame construction (from Chen, 2003) Furniture frames are made up of solid wood, engineered wood products, a variety of polymers and metals, or a mixture of these. A typical frame for a couch or an easy chair consists of solid wood (usually pine or spruce) and plywood pieces that are held together by fasteners (Furniture Repair and Upholstery, 2006). Fasteners used in the upholstered furniture industry are most commonly screws, nails and staples and therefore fastener holding strength is key to OSB suitability for upholstered furniture. The frames of couches, sofas and living room chairs are made from a wood and plywood frame that is wrapped with padding and fabric as shown in Figure 2. Often the visible parts, such as the legs and arm rests, are a carved solid timber. Hidden components, which can be over half of the entire structure, are plywood or other wood composites. It is these components and any hidden solid wood components that are candidates for replacement with OSB. OSB could offer a cost-effective solution to the large number of 4 hidden components such as the back posts, top rail, middle side rail etc. (Figure 1) that need to be fastened together (Chen, 2003). Figure 2. Schematic of an upholstered chair showing the internal components (Retrieved from: softdesignlab.com) The Furniture market is divided into seven segments: The Living Room Furniture segment focuses on furnishings such as sofas, side tables, and media cabinets typically found in the living room. This segment is divided into the following subsegments: Seats & Sofas and Cabinets, TV Stands & Side Tables. The Bedroom Furniture segment covers furnishings and other accessories used for sleeping and the storage of clothing and other personal items. This segment is divided into the following subsegments: Beds, Mattresses, Closets, Nightstands, & Dressers, and Bedding. The Kitchen & Dining Furniture segment covers a diverse range of products that are used to meet people’s eating, food storage and preparation needs. This segment is divided into the following subsegments: Kitchenware and Dining Tables, Kitchen Islands & Pantries. The Outdoor Furniture segment covers products specifically designed to withstand exposure to the elements, such as rain, sun, and wind, and is often made from materials that are weather-resistant. 5 The Home Office Furniture segment focuses on products that ensure a good working environment at home compared to that of a traditional office. This segment is divided into the following subsegments: Desks & Chairs and Bookcases & Filing Cabinets. The Lamps & Lighting segment covers various devices and fixtures that provide illumination in a space. This segment is divided into the following subsegments: Table & Floor Lamps, Ceiling Lights & Chandeliers, and Light Bulbs & Fittings. The Home Décor segment covers a variety of accessories and furnishings designed to make a space more aesthetically appealing. This segment is divided into the following subsegments: Decorations, Carpets & Rugs, Candles, and Curtains & Blinds (Furniture - Statista market forecast. 2023). Table 1 shows the revenue of furniture markets in China, Europe and North America in billion USD (US$) from 2014 to 2027. It shows North America is the biggest market for furniture and China has a huge potential in this area. Table 1. Furniture market revenue comparison in billions of US dollars (Furniture - Statista market forecast. 2023) 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 China 43.07 45.91 46.90 52.87 66.09 68.86 64.87 77.10 82.33 95.58 106.50 118.40 131.30 146.00 Canada 11.44 12.20 11.62 12.62 13.67 14.60 15.06 17.03 17.70 19.22 20.24 21.30 22.44 23.57 US 137.00 151.00 160.50 170.10 187.00 196.70 211.30 230.40 235.20 252.70 262.50 273.10 283.80 295.10 Europe 202.00 182.50 182.90 191.90 204.80 203.30 207.10 232.40 210.70 236.80 244.60 252.80 261.20 269.40 1.2 Research Objectives and Structure of Work The vertical density profile (VDP) of traditional sheathing grade OSB panels is typically U-shaped with higher density faces and a lower density core. Lower core density leads to reduced fastener holding properties such as SWR. Hence, OSB with a uniform density profile can have higher core density and less density variation through the panel thickness, which may be beneficial for its fastener holding performance. The goal of this project is to develop a uniform vertical density profile (VDP) OSB product that is suitable for replacing plywood, particleboard or solid wood in the manufacture of upholstered furniture. 6 The following structure has been used in the thesis: 1. Performing a review on the history of OSB manufacturing and its difference relation to other wood composites panels. 2. Optimization of mat properties including press cycles, core density and SWR properties of uniform VDP OSB. 3. Identification of the key parameters and press cycle that produces a uniform VDP. Goal is to predict VDP from knowledge of material properties and processing parameters during hot-pressing. Key components of the project include: 1. Material characterization – density, wood structure and moisture content, as it affects strand quality outcomes. 2. Testing different closing rates during hot pressing to achieve a uniform vertical density profile strandboard product. 7 2 Literature Review 2.1 Review the manufacturing process for making OSB OSB is an engineered structural panel product composed of thin, longitudinal strands or flakes sliced from whole logs using a disk or ring flaker. In North America the most commonly used species include aspen, southern yellow pines or mixed hardwoods such as spruce and balsam poplar. The strands are sliced parallel to the grain direction of the logs so that the inherent strength of the wood fiber is maintained. After slicing they are dried and blended with a thermosetting adhesive, usually a liquid phenolic resin or polymeric diphenyl methane diisocyanate (pMDI) resin with wax emulsion for moisture resistance. Strands are then machine-formed into a loose mat usually made up of three to five layers and pressed under high heat and pressure into a rigid, dense, structural panel (Structural Board Association, 2007). OSB was developed in the early 1980s to improve the engineering properties of waferboard through better control of strand geometry and orientation of the surface layers. Aligning the strands in each layer perpendicular to adjacent layers gives OSB superior flexural properties to those of randomly oriented waferboard flakes. Early waferboard and OSB products used PF (Phenol formaldehyde) resin, but the industry has shifted in recent decades to pMDI for its greater strength, cost-effective dosage and higher productivity rates (Semple et al., 2006). Manufacturers have switched to using pMDI in the core layer of OSB as its lower curing temperature reduces press times, but avoided using it in surface as it sticks to metallic surfaces. Wood composite board production started in the 1920s with exploded-pulp-based hardboard. The use of waste and recycled wood materials increased during WWII in the US and Europe, with the development of particleboard based on small milled particles from recovered lumber and low-grade logs. Particleboard originated as a cheaper, non- or semi-structural board compared with traditional plywood, but well-suited for low-end furniture frames, cupboard liners and shelves etc. Technologies to produce a stronger and 8 more structural particleboard grades developed in 1970s greatly expanding the fabrication of particleboard for mass-produced furniture and IKEA’s iconic flat-pack self-assemble furniture. The general progression of development of composite panels for furniture include plywood and blockboard, particleboard, NOVOPLY, waferboard, PLYSTRAN, and finally OSB, which has greatly expanded the applications of strand composites into structural sheathing, floor underlayment, as well as basic furniture frames (Zerbe et al., 2015). 2.2 Review the different applications for OSB and the key property for each application OSB panels are now widely used for roof, wall, and floor sheathing in residential and commercial construction. The required aspect ratio (strand length divided by width) for strands should be greater than 3. Generally, the typical strand size is 3 inches in length, 0.5 inch in width and 0.027 to 0.03 inch in thickness (Hiziroglu, 2017). A typical three-layer OSB panel is made by two outer faces with longer strands aligned in the long direction and a core layer that is transversely aligned or laid randomly using a mix of smaller strands and fines. The orientation of strands, particularly in the surface layers, increases OSB bending strength and stiffness in the oriented or aligned direction, also termed ‘machine direction’. Depending on application, the strand size, degree of orientation, and layer configuration are the main factors affecting the mechanical properties of OSB panels (Cai and Ross, 2021). Important mechanical properties of OSB panels include the modulus of elasticity (MOE) and the modulus of rupture (MOR) in bending, tensile strength, internal bond strength (IB) and screw withdrawal resistance (SWR) (Cai and Ross, 2021). IB strength and related properties such as SWR is affected primarily by core density (Garcia et al., 2008). Other strength properties (thickness swelling) are influenced by density (governed by hot press compaction pressure and heat consolidation) and adhesion between strands. Bending strength of OSB is primarily influenced by the high density and degree of strand orientation in the face layers where bending stresses are concentrated. 9 OSB manufacturers aim to increase their product application range outside of traditional construction materials by optimizing the production process, developing and improving product lines for alternative uses such upholstered furniture frames, interior doors and partition walls (Zhan et al., 2014). Initially, OSB was developed and manufactured for use as construction sheathing. However, in the early 1980’s, due to various factors such as higher transportation and shipping costs, a lack of relevant quality and performance standards and nascent experience using the material, OSB products took some time to be adopted on a larger scale. With the evolution of appropriate product quality and construction standards for OSB and improvements in the product, OSB has become more widely accepted and used in a broader range of applications such as interior decoration, the packaging industry, flooring and sheathing (Zhang, 2007). 2.2.1 Screw holding strength of wood composite panels as a function of density Density and strength properties of OSB have been investigated by past studies (Table 2). Neimsuwan (2004) investigated the influence of resin and wax ratio on OSB properties and found that the MOR and MOE, especially in parallel direction, can be improved by increasing the resin ratio, and MOR, MOE, IB and TS will increase by increasing the wax ratio in OSB. Febrianto et al. (2009) fabricated OSB of 600 kg/m3 density from Acacia mangium wood with wood density of 410 kg/m3. Strand dimensions were 60-70 mm in length, 28-30 mm in width and 0.6-0.7 mm in thickness. Three levels of MDI adhesive content (3%, 5% and 7%) were used. The study showed increasing the resin content significantly improved board mechanical properties. 10 Table 2. Comparison between different literatures based on varying resin content Source Resin content (% odw) Density (kg/m3) IB (kPa) MOR (MPa) MOE (GPa) 3.5 612 307 34.9 4.99 Neimsuwan et al (2004) 2.5 644 432 25.7 4.23 3.25 625 484 22.3 3.53 4.00 626 417 22.0 3.39 4.75 631 403 22.8 3.52 5.50 610 377 21.8 3.49 Febrianto et al (2009) 3 570 305 26.0 2.52 5 580 380 32.0 3.08 7 590 463 38.4 3.22 The vertical density profile (VDP) of sheathing grade panels is typically U-shaped with higher density faces and a lower density core. Wang et al. (2015) compared the vertical density profiles of OSB, MDF and PB and found there were no significant differences among the different types of panels (Figure 3). Semple et al. (2015) found the average surface density for OSB made from pure aspen was 980 kg/m3 and the average core density was 650 kg/m3 (Table 3). These values indicate the faces are approximately 30% higher and the core layer 13% lower than the mean density of OSB, which is commonly around 737 kg/m3 (Lam, 2001). Hunt et al. (2017) investigated the VDP of particleboard and found medium density particleboard (750 kg/m3) had a higher surface density of 880 kg/m3 and lower core layer density of 680 kg/m3. Wang et al. (2001) found a steep density profile consisting of a high face density (1100 kg/m3) and low core density (600 kg/m3) in MDF panel. Table 3. Measured values of surface and core densities for the different panel types Panel products Panel density (kg/m3) Density Surface layer (kg/m3) Core layer (kg/m3) MDF 761 1100 600 Particleboard 750 880 680 OSB 737 980 650 11 Figure 3. Vertical density profile of OSB, MDF and PB (Wang et al. 2015) Screws driven into low density wood or wood composite substrates usually fail at lower load levels compared to those of higher density woods. Being threaded, screws ‘lock’ into the substrate and have much greater withdrawal resistance than nails. Screws are much better for squeak-free flooring assemblies, resisting uplift forces that occur in roofs, and holding deck ledgers tight to a building (Nails vs. screws, 2021). However, in some applications such as framing walls and roofs, securing plywood sheathing, and installing siding and roofing, nails are superior for fastening since they are less brittle than screws, which results in the increased shear strength of nailed joints (Truini, 2022). The low core density of OSB boards made by traditional hot pressing tends to produce lower SWR. This suggests that boards with higher core density and less density variation through the thickness may have improved fastener performance (Wang et al., 2007). For Ready To Assemble (RTA) Furniture, eSWR is critical because consumers often use electric drills to assemble the furniture, but apply too much torque to the fastener, which results in the screw shredding its embedment material (Osborn, 2020). The result is a fastener that cannot hold the two pieces together and a likely claim against the manufacturer. 12 2.3 Review the literature on the impact of strand dimensions on board properties OSB is a wood composite panel comprised of strands where there is some degree of orientation with respect to the major board axis. Strength properties of OSB is strongly affected by the strand size, geometry and degree of orientation (Zhuang et al., 2022; Brumbaugh, 1960). To improve the flexural properties of boards the OSB strands need to have an aspect ratio (strand length divided by width) of at least 3 (Barnes, 2001). Strands mostly lie in the range of 3 inches in length, 0.5 inch in width and 0.027 to 0.03 inch. It is usual for strands in the upper and lower faces to be roughly oriented along the major board axis, but in the core small strands and fines are deposited either randomly or in some cases approximately 90° to this direction. Strands in the faces are laid down by forming stations fitted with banks of rotating disks that orient the strands as they fall through. The orientation of strands is affected by the strand size, the spacing between the roller disk and the fall distance the strands from the rotating disk slots during mat forming, which in turn strongly influence the mechanical properties of OSB (Lunguleasa et al., 2021). The smaller core strands are deposited via slots in a transverse rotating drum., Due to the preferential orientation of the large strands in the face layers, the mechanical properties of OSB are often superior to other wood-based panel products (Geimer, 1976; Shaler, 1991). Previous studies on the manufacturing and properties of OSB have shown that the orientation of strands significantly improves the flexural properties (MOR and MOE) of boards but has less influence on their dimensional stability (Suzuki and Sekino, 1982). Previous research has shown that temperature changes and relative humidity are the main factors affecting dimensional stability of boards (Mirski et al., 2013). 2.4 Review the hot-pressing process and main factors affecting board properties and the development of VDP. During hot pressing, the face layers of the mat are subjected rapid heating, steam generation (viscoelastic softening) and compressive deformation and therefore significant densification occurs in these layers. 13 This is a result of the higher temperature of the strands in the face layers compared with the lower temperature in the core which takes longer to heat up via convective heat transfer. When the temperature in the face layers exceeds the core temperature, the strands become compressed and the volume of the material in this layer is reduced, which increases its density (Lenth and Kamke, 2001). Strands moisture content, mat structure and the pressing environment are the three main factors affecting the shape of density profile in wood composite panels (Wang and Winistorfer, 2000). Each of these is discussed in more detail below. 2.4.1 Furnish moisture content The moisture content of strands has a large effect on the formation of the VDP. A higher MC in face layers of mat leads to a higher surface density (Andrew et al., 2001). Heebink (1972) found the VDP of particleboard panels can be effectively reversed by increasing the moisture content in core layer and reducing the MC of the face layers. The furnish MC influenced the extent of densification in the face, intermediate and core layers and strongly affected the location of densification in intermediate layers of the panel. Consequently, the most important effect of furnish MC adjustment was on the intermediate layers of the mat (Andrews et al., 2001). 2.4.2 Hot press parameters During hot pressing, heat and moisture transfer takes place from the face layers in contact with the hot press platens into the core in the form of steam. The outcome is a panel where the density gradient through the board thickness generally consists of high density faces and a low density core (Dixon et al., 2017). Vertical density distribution is strongly affected by press cycle. Traditional press cycles using a slow press closure rate to manipulate the density profile have a slow rate of platen closure until final board thickness or position stop is reached. These have some obvious limitations related to production rates, resin pre-cure and density profile symmetry. 14 The VDP can be manipulated by adjusting the press-closing rate. Wang (2000) investigated changes in the VDP of MDF panels using a short pause (or holding time) during press closing; a strategy termed ‘step closure’. This reduced the difference between the face and core densities from 881 kg/m3 (face) and 705 kg/m3 (core) for continuous closing to 832 kg/m3 (face) and 657 kg/m3 (core) for step closing. The reason given for decreased face density with step closing is that the hold time permits the resin in the face layers to cure and the moisture in it to be driven off which results in an increase in the local glass transition temperature and a decrease in moisture content of the strands and therefore reduces further densification of the face layer. By using a second closing step during the pressing, the densification of the core layers can be increased as the consolidation stress in this layer is increased due to higher heat transmission. 2.5 Review the steam injection process and describe how it differs from hot pressing Steam injection pressing (SIP) can also be used to modify the VDP of wood composites and decrease the hot press time (Jokerst and Geimer, 1994). The press platens feature a grid pattern with perforations ranging from 1 to 2 mm in width. Through these perforations, steam is released, effectively penetrating the void spaces between the strands of the material. Upon contact, the steam condenses on the strands, facilitating the transfer of latent heat of vaporization within a matter of seconds. As a result, the furnish temperature rapidly rises to 100°C (Li et al., 2011). Compared to traditional hot-pressing, all wood elements are heated simultaneously and densification through the mat thickness is similar resulting in an almost uniform vertical density profile. Boards with less density variation have the benefit of higher fastener holding capacities compared to those with a more defined density gradient through the thickness of the board (Wang et al., 2007). 15 2.6 Review on the difference between OSB panels and other wood-based composites This section reviews articles and research papers that highlight the differences between OSB and other wood composites. Low grade logs can be converted into a range of different types of wood based composite panels including medium density fiberboard (MDF), particleboard, plywood/LVL, OSB for use in a wide range of construction sheathing, lining, furniture and other applications. A photograph of these different panels can be seen in Figure 4. Figure 4. A photograph of Plywood, MDF, Particleboard and OSB taken by Hengliang Dai Figure 5 shows the basic wood elements used in wood composite panels from largest to smallest. Figure 5. Wood elements, from largest to smallest (Nishimura. 2015) 16 The mechanical properties (typical static bending and internal bond strength values) of different wood-based composites are compared in Table 4 (Maclntosh and Longworth. 1982). Table 4. Mechanical properties of different wood-based composites (Maclntosh and Longworth. 1982) Static bending properties Specific gravity Modulus of elasticity Modulus of rupture Internal bond Panel products (g/cm3) GPa (×106 psi) MPa (psi) MPa (psi) Hardboard 0.9–1.0 3.1–5.5 31.0–56.5 1.2-3.5 MDF 0.7–0.9 3.6 35.9 0.8-1.9 Particleboard 0.6–0.8 2.8–4.1 15.2–24.1 0.3-1.2 Waferboard 0.6–0.7 3.6 19.3 0.5 OSB 0.5–0.8 4.4–6.3 21.8–34.7 0.3-0.5 Plywood 0.4–0.6 7.0–8.6 33.7–42.6 1.3-2.7 Internal bond property is the tensile strength that is measured by pulling the sample apart in tension transverse to the plane of the sheet. Flexural properties such as modulus of rupture and apparent modulus of elasticity are measured to characterize its bending properties. Various other panel types that can be substituted for OSB in furniture frames, including waferboard, particleboard and MDF, and plywood. The main differences between each of these and OSB are briefly described below. 2.6.1 Waferboard vs OSB Waferboard (WB) and OSB are two types of re-constituted wood panel products collectively known as ‘flakeboards’ (Wood Products Industry, 2002). They are made from thin sliced wood wafers or strands cut from whole debarked logs. Waferboard is made from short flakes (3 to 6 inch in length, 1.5 inch in width and 0.028 inch in thickness) called wafers (Waferboard/Oriented Strandboard Manufacturing, 2002). The wafers are randomly oriented in the mat and glued with a water-resistant phenol formaldehyde resin by heat and pressure. This product was firstly developed in the 1950s and patented by Armin Elmendorf in 1965. The original waferboard was developed without any intentional orientation of flakes. The OSB industry evolved from waferboard industry in the late 1970s and by 1989 most waferboard factories were producing oriented waferboard (OWB) and OSB, whereby the wafers were cut to be longer in the grain 17 direction then in width, and in the case of OSB machine oriented at least in the surface layers. Greater flexural properties can be achieved in OSB panels since the strands are aligned in each layer which are perpendicular to adjacent layers. OWB and OSB were developed as lower cost substitutes for softwood plywood including wall and roof sheathing, and floor underlayment in light-frame construction. Table 5 lists the main differences between waferboard and OSB (Fisette, 2017). Table 5. Comparison between waferboard and OSB Waferboard OSB • Homogeneous wafers, random formation • Lower strength and stiffness • Aligned long strands • Higher strength and stiffness OSB being a purpose-engineered product rapidly increased its market share in the construction sheathing market, as it was able to make use of lower cost species like aspen logs to produce a panel with similar performance and function as plywood. This makes OSB a lower cost but more sustainable solution. In addition, the aligned strands in OSB panels provides better mechanical properties than waferboard, making it better suited for construction purpose. 2.6.2 Particleboard vs OSB Particleboard is typically comprised of two face layers made up of fine wood particles (< 1–2 mm) and a thicker core layer with coarser particles (> 2 mm) (Sackey et al., 2007). Particleboard is used for case products and furniture frames which are typically upholstered (Cai and Ross, 2021). There are some differences (Table 6) between Particleboard and OSB. 18 Table 6. Comparison between Particleboard and OSB Particleboard OSB Advantages • Lower cost • Useful for making ready to assemble furniture • Wood waste can be used • Higher bending strength • Good toughness • Strong nail holding power • More uniform (crisscross structure) • Eco-friendly (less formaldehyde) Disadvantages • absorbs moisture easily Interior grade particleboard is bonded with urea-formaldehyde (UF), melamineurea-formaldehyde (MUF), phenol-formaldehyde (PF) and polymeric diphenyl-methane diisocyanate (pMDI) (Ruffing et al. 2010b) and is more susceptible to moisture-related deterioration. In addition, the material cannot be securely connected with regular screws which makes it unsuitable for exterior use or most construction grade sheathing applications (Nair, 2010). Particleboard is commonly used as a subflooring substrate for carpeting, while OSB is chosen as a base subfloor for its stability and price. OSB is not typically used as a primary material under carpeting. Carpeting generally requires a smooth and even surface for proper installation and performance. OSB, on the other hand, has a rough texture and is primarily used as a structural material for subflooring, roof decking, and sheathing in construction. If considering using OSB as a subfloor before installing carpeting, it's important to note that additional installation steps would be required such as sanding and filling. You would typically need to install a layer of underlayment or plywood over the OSB to create a smoother surface suitable for carpet installation. Compared with OSB, particleboard has a low core density because of the coarser material used in core layer and lower compression ratio, as a result, its edge screw holding capacity is lower than OSB. Semple et al. (2005a,b) and Dettmer (2015) have investigated the eSWR of commercial particleboard and found some products failed to meet the voluntary ANSI A208.1 (1999) and A208.1 (2009) standards (900 N for M2 grade particleboard), with the numbers ranged from 597 N to 777 N. Miljković (2007) found OSB has greater eSWR (56-73% greater than conventional particleboard) and that OSB can be designed and manufactured to have a higher edge screw withdrawal capacity suitable for use in the upholstered furniture industry. 19 2.6.3 Medium density fiberboard vs OSB Medium density fiberboard (MDF) is made from refined softwood fibers that are combined with wax and a synthetic resin such as urea formaldehyde (UF) as binder. The panel is formed at high temperature and pressure with a distinct VDP. In the furniture industry MDF is primarily used for shelving and profiled cabinet doors, speaker boxes and flooring. Table 7 shows the current prices for OSB and MDF panels based on different thickness (The Home Depot). The price of MDF is nearly double than that of OSB panel. One reason is the market demand and supply. The demand for MDF is often higher than that of OSB in certain applications, such as furniture manufacturing, cabinetry, and interior trim. The higher demand for MDF suggests that higher prices for OSB is possible if some of its properties can be tailored to furniture application. Table 7. Price for MDF and OSB based on thickness (The HomeDepot) Product Thickness (in) MDF price (4 ft * 8 ft) OSB price (4 ft * 8 ft) 3/4 $57.98 $22.23 19/32 N/A $17.82 1/2 $47.75 $20.79 7/16 N/A $11.5 There are some differences (Table 8) between MDF and OSB. Table 8. Comparison between MDF and OSB MDF OSB Advantages • Smooth surface (easy to be laminated) • Versatile • Higher MOR • Cost effective • Good mechanical strength and rigidity • Higher MOE • Higher water resistance • Sustainable (longer lifecycle) • Eco-friendly Disadvantages • Cutting blades dull faster • Lower core density • Challenging to paint • Higher edge screw withdrawal (eSWR) For furniture frames, edge SWR (eSWR) is critical since it affects the assembly ability and cyclical stress-strain on the joints over time. In any wood or wood-based panel, a higher core density generally leads to higher eSWR (Farajollah et al., 2022). Wang and Winistorfer (2001) compared the VDPs between OSB and 20 MDF panels and found a greater difference between the face and core density of MDF panels. The density of surface layer (1130 kg/m3) in MDF samples was 80% higher than that in core layer (633 kg/m3). For the eSWR of MDF panels, an average result of 400N found by Hong et al (2017), compared to an average eSWR of 1000N in OSB panels. Higher eSWR can be achieved in OSB panels, which makes OSB more suitable for furniture frames instead of MDF since the corner joint design is an important factor in furniture structure design. 2.6.4 Plywood vs OSB Studies have been made investigating the properties needed for OSB panels as a substitute for plywood or solid wood in upholstered furniture manufacturing and interior applications (Xu et al., 2014). Lower thickness tolerance, better dimensional stability, and good screwing holding capacity at edges were found in a specialized OSB panel produced for interior decoration and furniture making purposes. Meanwhile, the size of the OSB panels could be adapted to reduce the amount of people required in furniture manufacturing and reduce waste. As shown in Table 9, OSB has a lower cost but properties similar to plywood. While OSB is widely used in residential construction, certain properties such as lower bending strength, higher linear expansion and thickness swelling can still limit its use for certain high-performance applications compared with plywood (Wang et al., 2007). Table 9. Comparison between plywood and OSB Plywood OSB Advantages • Lighter • Lower cost • Dimensionally stable (lower shrinkage/swelling) • Better withdrawal and nail head pull-through performance • Stiffer Disadvantages • A tendency to delaminate • Retains water longer • Larger-diameter trees required • Significantly more swelling 21 Plywood’s strength comes from the perpendicular orientation of wood grain in each successive layer of veneer. It also allows the use of cheaper, more plentiful woods like Douglas fir and pine to be used in products that formerly called for softwoods. The efficiency of plywood manufacture was improved in the late 1970’s to compete with the introduction of OSB. Instead of solid sheets of veneer, OSB uses small wood strands glued together in cross-laminated layers (Pixelsmith, 2001). Poo (1988) investigated the direct withdrawal and head pull-through performance of nails and staples in structural wood-based panels and found OSB had equal or better performance than plywood. Raymond (1990) also showed that screw withdrawal strengths in OSB and plywood are very similar. Plywood is more expensive to manufacture and requires large diameter and better-quality logs than OSB manufacture. The above presents a strong case for substitution of solid woods, plywood and other composite panels such as MDF in furniture frames for OSB, which forms the rationale for this study. 2.7 Summary and key findings Parameters that affect OSB panel properties have been studied extensively. Strand size, moisture content, panel density and press closing rate are parameters that significantly affect panel properties. Variations in properties between OSB and other wood-based composites underlies their potential suitability for use for screw or nail fastened furniture frames. The eSWR of OSB is critical for its application in furniture frames and this property increases with increasing core density. Panels with higher core density and less density variation in the vertical direction are better suited for screw holding performance than standard sheathing grade OSB. 22 3 Materials and Methods 3.1 Board Fabrication 3.1.1 Materials A mixture of Canadian hardwood strands: Aspen (85%) and Balsam poplar (15%) in two different thicknesses (0.51 mm and 0.69 mm) were produced at Alberta Innovates Technology Futures in Edmonton, Alberta. The strand thicknesses were representative of those commonly used in OSB manufacturing. The average strand length and width were 11.5 cm and 5.3 cm. The dry strands were mechanically screened through a sieve with a 3/16 inch square mesh. Those strands collected on the sieve were used for board production and the smaller pieces passing through the sieve were classified as fines and discarded. The moisture content of strands after drying 24h prior to board manufacture was 2.95%. A commercial grade polymeric diphenyl methane diisocyanate resin (pMDI), Rubinate®1840, supplied by Huntsman, was used to blend the strands and for hot pressing the boards. This resin was selected because it is representative of pMDI resin used by many producers of OSB panels (Tolko, West Fraser). Figure 6 shows the manufacturing processes for the panels. 23 Figure 6. Manufacturing process for OSB in Alberta Innovates Technology Futures a) Log selecting; b) Debarking; c) Stranding; d) Strands; e) Drying; f) Screening; g) Blending; h) Formatting; i) Pressing; and j) Sorting. Photos were taken by Hengliang Dai 24 3.1.2 Design of Experiment Two hot pressing methods were considered (conventional hot pressing and steam injection hot pressing). For Expt 1 - conventional hot pressing 1, two strand thickness (0.51 mm and 0.69 mm) and three holding positions (18 mm, 19.3 mm and 20.7 mm) were chosen as parameters. For Expt 2 - the first steam injection hot pressing experiment, two strand thickness (0.51 mm and 0.69 mm) and three panel densities (500 kg/m3 600 kg/m3 and 700 kg/m3) were chosen as parameters. For Expt 3 - the second steam injection hot pressing experiment, two strand thickness (0.51 mm and 0.69 mm) and two steam pressure (10 psi and 25 psi) were chosen as parameters. For Expt 4 - conventional hot pressing 2, which was done in Wood composites lab, University of British Columbia the goal is to investigate the optimal closing rate and initial mat temperature for a uniform vertical density OSB boards. 3.2 Methods 3.2.1 Experimental Design The two main factors of interest were the effects of strand thickness and board density. Press cycles were varied to evaluate the changes in the VDP of panels pressed with either conventional or steam injection pressing schedules under similar conditions. The differences in press cycles involved adjusting platen closing rates and steps, holding and cooking times, and steam injection time. The objective was to identify the optimal pressing parameters under which high-strength, light-weight and uniform vertical density OSB boards may be manufactured. Board thickness was kept constant at 18 mm. Three experiments (see Figure 7) were made including: 1) conventional hot pressing Expt1 and 2) steam injection pressing 1 Expt2 and 3) steam injection pressing 2 Expt3. Experiments 1 and 2 each tested six different press cycles and Experiment 3 tested four press cycles. Three replicate panels were pressed per press cycle for a total 25 fabrication of 46 panels across all three experiments. The board fabrication method is described in the next section. Expt1: A set of 16 panels were fabricated to a density of 600 kg/m3 using six conventional hot pressing runs labelled CYCLE 1 to 6 (two strand thickness levels by three hold positions. For each strand thickness level there were three holding positions - 100% (18 mm), 107% (19.3 mm) and 115% (20.7 mm) of target thickness, as shown in Table 10. The press closing schedules are shown in Figure 7a. Table 10. Experimental factors of Expt1 Pressing Cycles # Panel Density (kg/m3) Strand thickness (mm) Hold position (mm) Replicates 1 600 0.69 18.0 3 2 600 0.69 19.3 2 3 600 0.69 20.7 2 4 600 0.51 18.0 4 5 600 0.51 19.3 3 6 600 0.51 20.7 2 Expt2: The aim of Experiment 2 was to create more uniform density boards. To achieve this a series of steam injection (SIP) cycles were tested. Cycles 7 to 12 (Figure 7b) were run using two strand thicknesses (0.51 mm and 0.69 mm) at three different OSB densities — 500 kg/m3, 600 kg/m3 and 700 kg/m3. Platen closing was stopped in three different hold positions and then steam injected at 25 psi for a total of 25 seconds, after which press closed to target panel thickness (Table 11). A total of 19 panels were made using the steam injection schedules labelled CYCLE 7 to 12 (Figure 7b). Table 11. Experimental factors of Expt2 Pressing Cycles # Panel Density (kg/m3) Strand thickness (mm) Steam Pressure (psi) Hold position (mm) Replicates 7 500 0.69 25 45 4 8 600 0.69 25 54 3 9 700 0.69 25 63 3 10 500 0.51 25 45 3 11 600 0.51 25 54 3 12 700 0.51 25 63 3 26 Expt3: Experiment 3 continued on with variations on the steam injection cycles to further improve board density profile optimization (Steam injection 2 is shown in Figure 7c). Two press cycle types were tested by varying the steam pressure. For Type 1 (CYCLE 13 and 15), after 10 seconds of closing, steam was introduced at 25 psi for a total of 25 seconds while the press platen continued to close. In Type 2 (CYCLE 14 and 16), after 10 seconds closing time the platen positions were held steady while steam was injected into the mats at 10 psi for a total of 25 seconds, after which pressing to target thickness resumed. These factors are shown in Table 12. Table 12. Experimental factors of Expt3 Pressing Cycles # Panel Density (kg/m3) Strand thickness (mm) Steam Pressure (psi) Hold position (mm) Replicates 13 600 0.69 25 Ramp close 3 14 600 0.69 10 54 2 15 600 0.51 25 Ramp Close 3 16 600 0.51 10 45 3 Figure 7. Press schedules for (a) Expt 1-Conventional hot pressing, (b) Expt 2-Steam injection pressing 1 SIP1 of boards of different density, and (c) Expt 3-Steam injection pressing 2 using step or continuous closing strategies 27 Expt4: Aspen strands were used for a targeting density of 600 kg/m3 OSB panel manufacturing. Board thickness was kept constant at 18 mm. The moisture content of strands was controlled to 3.5%. Phenol formaldehyde resins (PF), ME1021 RC, supplied by Henkel was used to blend the strands. The two main factors of interest were the effects of press closing time and initial mat temperature. Press schedules were varied to evaluate the changes in the VDP of panels. The differences in each schedule involved adjusting platen closing time and initial mat temperature. The objective was to identify the optimal closing time and initial mat temperature for a uniform vertical density OSB boards. Three schedules were designed including: 1) 3 steps closing hot pressing, 2) creep closing hot pressing and 3) Pre-heating hot pressing. Three replicate panels were pressed per press schedule for a total 9 panels. The press closing patterns are shown in Figure 8. Schedule 1: A set of 3 panels were fabricated to a density of 600 kg/m3 using conventional hot pressing. Platen closing was experienced a three different hold positions stop during the closing segments. Firstly, the primary closing segment was 5s to 19.8 mm of mat thickness and then it was hold for 60s. The secondary closing segment was rapidly closed to 18.9 mm in 3s and experienced another 60s holding time. After that, the mat was pressed to its target thickness 18 mm in 3s. The cooking time was 235s and there were another two degas segments with 20s for each. The last segment was a 20s opening segment. Schedule 2: The aim of this was to investigate the effect of closing time on panel VDP. Schedule 2 was run based on the same density and total time as schedule 1. Platen closing was stopped at 19.8 mm in 5s and then continued closing with a slower rate to target thickness 18 mm in 120s. The total cooking time was 235s, ended with two degas segments (20s respectively) and a 20s opening segment. Schedule 3: This is aim to study the influence of mat pre-heating process before pressing on the improvement of board vertical density profile. The mat was heated in oven to 50°C and then pressed by a shorter total time compared to schedule 1 and schedule 2. The platen was closed to 19.8 mm in 10s and 28 then closed to target 18 mm in 60s. 210s cooking time was set and ended with two degas segments (20s respectively) and one opening segment (20s). Figure 8. Pressing schedules for Expt 4 3.2.2 Board manufacture Dry strands were blended with 3.5% pMDI resin (oven dry weight basis) in a tumble blender. After blending, the strands were formed into randomly oriented mats by hand using a forming box measuring 86.36 * 86.36 cm (34 inch by 34 inch). Teflon sheets were used to prevent the strands sticking to the metal caul plates or press platens. Mats were pressed at 215°C according to the press schedules outlined in Figure 7. The cooled boards were edge trimmed to 71.12 * 71.12 cm (28 inch * 28 inch) and conditioned at 20℃ and 65% relative humidity (RH) for at least 2 weeks prior to cutting and testing. 3.2.3 Physical and Mechanical property testing Test specimens were cut to size according to the pattern shown in Figure 9 and tested for standard physical and mechanical properties according to ASTM D1037-06a. Tests included 6 pieces for vertical density profile (VDP) from the internal bond (IB) specimens, 8 IB specimens, 4 specimens for modulus of 29 rupture and elasticity (MOR/MOE), 4 specimens for thickness swell (TS), and 4 specimens each for edge and face screw withdrawal (eSWR/fSWR) per board. Figure 9. Cutting pattern for test specimens The test data were analyzed using an analysis of variance (ANOVA) and the Tukey–Kramer honest significant difference test using the JMP 13 statistics package to detect statistical differences at the 5% significance level. The strength properties (MOR/MOE and IB) were compared with the Canadian Standards Association CSA O437-93 (2006) requirements for Grade 1 OSB as there is currently no recognized Canadian standard for furniture grade OSB. For SWR properties, the standard of M2 particleboard in ANSI A208.1-1999 is used as a benchmark since there is no standard for furniture guide OSB and there are no minimum values for SWR given in the standard for OSB CSA O437. 30 4 Results and discussion In this chapter, the results from the board fabrication and properties tests are presented. The sensitivity of VDP and board strength properties to changes in strand thickness, or press hold position and steam injection pressure are examined. 4.1 Experiment 1: Conventional hot pressing The physical and mechanical properties of conventional hot pressed (cycle 1 to 6) boards are given in Table 13. Table 13. Means and standard deviation of physical and mechanical properties of conventional hot pressing (schedule 1 to 6) Pressing Cycles Density (kg/m3) IB (kPa) Bending SWR MOR (MPa) MOE (GPa) fSWR (N) eSWR (N) 1 598 425.1 (123.6) 29.5 (3.8) 4.2 (0.4) 1505.8 (325.1) 1134.1 (258.7) 2 594 457.2 (142.1) 32.2 (3.7) 4.2 (0.4) 1527.3 (270.8) 1177.3 (176.6) 3 611 477.5 (108.0) 31.9 (3.1) 4.2 (0.3) 1549.0 (242.4) 1145.4 (296.4) 4 617 275.9 (153.3) 34.8 (8.9) 5.0 (0.1) 1476.6 (263.2) 964.2 (392.4) 5 604 329.5 (160.5) 29.9 (7.4) 4.7 (0.2) 1636.5 (375.0) 1083.1 (339.1) 6 612 315.1 (166.4) 39.9 (6.6) 5.3 (0.6) 1644.5 (273.8) 1119.1 (152.6) Note that the parentheses indicate the standard deviation of the means. 4.1.1 VDP analysis Examination of the VDPs in Figure 10 shows that the curves for OSB made with thin or thick strands at the 18.0 and 19.3 mm hold positions are very similar, which both have typical VDPs with peak densities at the faces and lower density in the core. The narrower hold position at 18 mm produced the highest face densities at 870 kg/m3 where as the 19.3 mm and 20.7 mm hold positions produced similar face densities at 750 kg/m3. The VDP for the 20.7 mm hold position differs with a small second, inner high-density 31 shoulder between 1.5 to 3.0 mm from each face. The highest density was about 770 kg/m3 which was located about 0.5 mm inward from each face and a second peak of 720 kg/m3 approximately 3 mm inward from each face for the 20.7 mm press openings. Higher bending properties were measured for these samples likely due to this shoulder. Figure 10. Expt 1 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and hold position (18 mm, 19.3 mm and 20.7 mm) 4.1.2 Density and IB The density of each sample based on hold position in Expt 1 are compared in Figure 11a. The samples are presented in 3 clusters with the left cluster corresponding to the samples made with an 18 mm hold position. The center and right clusters were made using the 19.3 mm and 20.7 mm hold positions, respectively. The heavy line across each cluster is the computed mean density of all the samples in that cluster. The thinner lines above and below the heavy line correspond to 95% confidence intervals for that mean. Note that adjacent means are significantly different only if the 95% confidence intervals of each mean do not overlap. Examination of the figure shows that all of the means overlap and therefore none of them are 32 significantly different. Based on this result one can conclude that hold position has no effect on density over the range examined. Figure 11. Internal bonding properties and sample density based on hold position (18 mm, 19.3 mm and 20.7 mm) and strand thickness (0.69 mm and 0.51 mm) a) sample density comparison by hold position b); IB comparison by hold position; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability. The dash line across the cluster is the OSB grade 1 Examination of the IB results based on hold position, Figure 11b, also has overlapping 95% confidence intervals, and therefore hold position does not have a significant effect on IB strength. However, there is a trend of increasing IB strength with longer hold time, while not statistically significant over the range of hold times examined here, one cannot rule out that larger open times may have a significant effect on IB. 33 The other variable examined in this experiment was strand thickness. Replotting the data as a function of strand thickness, Figure 11c, shows that the density of the samples is not significantly different. Examination of the IB strength, Figure 11d, does show a significant difference in IB strength with samples made of thicker strands having a significantly higher IB strength (p < 0.0001), about 460 kPa for the thick strands compared with 350 kPa for the thin strands. By using thicker strands, 35.3% increasing of resin content for each individual strand was found, this has been proved by Febrianto et al. (2009). 70.3% resin content increasing for individual strand was achieved by increasing the resin content from 3% to 5% while 143.3% resin content increasing by increasing the resin content from 5% to 7%. The IB strength of OSB made with thick strands exceeded the CSA O437.0-93 IB requirement for O-1 grade of 345 kPa as indicated by the horizontal dash line in Figure 11. OSB made with thin strands did not meet the threshold. The reason for the difference in IB values can be partly explained by the higher resin dosage on each thick strand. The greater number of thin stands per unit mat weight means same resin mass is distributed over a larger surface area of strands in the mat. Depend on the strand thickness, there were 35% more strands in the thin strand board than the thick strand board which means a greater resin dose (35% greater) per strand of the thick strands since the same resin mass is used per mat weight. 4.1.3 MOE and MOR The density of the bending samples based on hold position is compared in Figure 12a. Examination of the figure shows that all of the means overlap and therefore none of them are significantly different. Based on this result one can conclude that hold position has no effect on density over the range examined. Examination of the bending results, Figure 12b, and 12c, also has overlapping 95% confidence intervals, and therefore hold position does not have a significant effect on both MOE and MOR strength. 34 Figure 12. Bending properties and sample density based on hold position (18 mm, 19.3 mm and 20.7 mm). a) MOR sample density comparison by hold position; b) MOE strength comparison by hold position; c) MOR strength comparison by hold position. The heavy horizontal Replotting the data as a function of strand thickness, Figure 13a, shows that the density of the samples is not significantly different. Examination of the bending stiffness, Figure 13b, does show a significant difference with samples made of thinner strands having a significantly higher mean MOE strength (p < 0.0001), about 4.95 MPa for the thin strands compared with 4.18 MPa for the thick strands. For the examination of the MOR results, Figure 13c, it has overlapping 95% confidence intervals, and therefore strand thickness does not have a significant effect on MOR strength. 35 Figure 13. Bending properties and sample density based on strand thickness (0.69 mm and 0.51 mm). a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability All boards met the requirements of CSA-O437 (CSA 1993) for OSB (23.4 MPa for the O-1 Class). From Figure 12 and 13, boards made from thinner strands had higher flexural properties (10.5% higher MOR and 16.1% higher MOE), aided by more uniform surface layer formation resulting in increased surface density of boards. Warmbier et al. (2016) studied the influence of density on the mechanical properties of OSB and found that board density mainly affects MOR and MOE, while the resin content mainly affects the IB. A study carried out by Badejo et al. (1988) showed that flake geometry (length and thickness) influences strength properties including MOR and MOE with thinner flakes producing stronger, stiffer and more dimensionally stable boards. 36 4.1.4 SWR Examination of the SWR results based on hold position, Figure 14, show that both fSWR and eSWR are not affected by strand thickness or board density. All boards exceeded the stipulated requirements for M2 particleboard in ANSI A208.1-1999. Semple et al. (2005) showed the eSWR was not affected by either screw type or machine direction of different sampled commercial particleboards. Two furniture grade particleboards (MS and M2) were tested by these authors, the mean eSWR for each was below 800N for MS and below 1000N for M2. The average of eSWR for tested OSB panels in Table 14 is above 1100N which is higher than the values found in commercial particleboards range from 597N to 777N (Dettmer, 2013). This is proof that the adjusted OSB boards idea works for furniture purpose. 37 Figure 14. SWR properties and sample density based on strand thickness (0.69 mm and 0.51 mm) and hold position (18 mm, 19.3 mm and 20.7 mm). a) eSWR strength comparison based on hold position; b) fSWR strength comparison based on hold position; c) eSWR strength comparison based on strand thickness; d) fSWR strength comparison based on strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 38 4.2 Experiment 2: Steam injection hot pressing 1 Means and standard deviations of physical and mechanical properties of OSB made from steam injection hot pressing 1 (cycles 7 to 12) are given in Table 14. Table 14. Means and std Dev of physical and mechanical properties of steam injection hot pressing (cycle 7 to 12) Pressing Cycles Density (kg/m3) IB (kPa) Bending SWR MOR (MPa) MOE (GPa) fSWR (N) eSWR (N) 7 490 262.113 (75.8) 15.59 (2.7) 2.6 (0.3) 1194.9 (296.6) 1013.8 (322.3) 8 598 414.153 (71.0) 23.01 (3.4) 3.3 (0.4) 1552.0 (248.6) 1440.8 (343.8) 9 678 582.99 (130.5) 33.41 (4.6) 4.4 (0.4) 2116.2 (267.5) 1723.0 (158.7) 10 509 238.21 (78.3) 17.33 (2.9) 3.0 (0.3) 1323.4 (215.2) 1077.4 (238.3) 11 606 333.41 (121.8) 30.19 (3.6) 4.0 (0.4) 1635.2 (344.0) 1410.7 (381.2) 12 633 311.83 (126.9) 30.18 (6.9) 4.0 (0.9) 1748.2 (254.9) 1726.6 (188.9) Note that the parentheses indicate the standard deviation of the value. 4.2.1 VDP analysis Examination of the VDPs of SIP1 boards of Expt 2 shown Figure 15 shows that boards made from thinner strands had a higher average core density than those with thicker strands. Comparing boards with the same strands thickness but made with a different density, the low density boards had a flatter VDP than those with higher density. Compared to the boards with the same density (600 kg/m3) in conventional hot pressing, boards made with thinner strands had a more uniform VDP by steam injection pressing. 39 Figure 15. Expt 2 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and board density (500 kg/m3, 600 kg/m3 and 700 kg/m3) 4.2.2 Density and IB The density of samples from the 500, 600, and 700 kg/m3 target density boards for Expt 2 are compared in Figure 16a. The samples are presented in 3 clusters with the left cluster corresponding to the samples made with 500 kg/m3. The center and right clusters were made with 600 kg/m3 and 700 kg/m3, respectively. Examination of the IB results based on board target density, Figure 16b, shows a significant difference in IB strength with samples from denser boards being significantly higher IB strength (p < 0.0001), about 439.44 kPa for the densest boards (700 kg/m3) compared with 373.78 kPa for 600 kg/m3 boards and 251.12 kPa for 500 kg/m3 boards. Replotting the data as a function of strand thickness, Figure 16c, shows that the density of the samples is not significantly different. Examination of the IB strength, Figure 16d, does show a significant difference in IB strength with samples made of thicker strands being significantly higher IB strength (p < 0.0001), about 293.75 kPa for the thin strands compared with 384.10 kPa for the thick strands. In a similar trend to Experiment 1, the boards made with thicker strands were 25% higher in IB, as expected. Only boards 40 manufactured at the density levels 600 kg/m3 and 700 kg/m3 with the thicker strands met the required minimum IB strength requirement O-1 grade. In keeping with these results, it is recommended to use thicker strands and make boards to 600 kg/m3 and 700 kg/m3 for the SIP cycle experiments. Figure 16. Internal bonding properties and sample density based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3) and strand thickness (0.51 mm and 0.69 mm) a) sample density comparison by target density; b) IB comparison by target density; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 41 4.2.3 MOE and MOR The densities for the bending samples for each target density are compared in Figure 17a. The mean densities showed were found to be significantly different (p < 0.0001). The density of the 700 kg/m3 boards was lower than expected, only 660 kg/m3. Examination of the bending results, Figure 17b, and 17c, shows a significant difference in both MOE and MOR strength with samples from denser boards being significantly higher MOE and MOR strength (p < 0.0001). Compared the same density panels from cycle 8 (23.01MPa, 3.3 GPa) and cycle 11 (30.19 MPa, 4.0 GPa) with the panels in Exp1, panels without steam injection reveals a higher bending strength. The results are in agreement with earlier literature (Chen et al., 2010) who showed that within the panel density range of 449-705 kg/m3 made by 0.69 mm Aspen strands, mechanical properties IB, MOE and MOR were generally improved with increasing panel density. This finding is agreement with the work by Chen who also found that the influence of panel density will decrease when it reached to a certain elevated level. 42 Figure 17. Bending properties based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3), a) MOR sample density comparison by target density; b) MOE strength comparison by target density; c) MOR strength comparison by target density. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability Replotting the data as a function of strand thickness, Figure 18a, shows that the density of the samples is not significantly different. Examination of the bending strength based on strand thickness, Figure 18b and 18c, MOE and MOR were affected by strand thickness (p = 0.02 and p = 0.04, respectively). Both MOR and MOE of panels made by thinner strands are significantly higher than those by thicker strands. 43 Figure 18. Bending properties based on strand thickness (0.51 mm and 0.69 mm), a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 4.2.4 SWR Examination of the SWR results based on board target density, Figure 19, both fSWR and eSWR are not affected by strand thickness or board density. All boards exceeded the stipulated requirements for M2 particleboard in ANSI A208.1-1999. The eSWR of the OSB boards made by steam injection hot pressing cycles significantly exceed the results recorded in furniture grade particleboard by Semple et al. (2005). The eSWR of panels from cycle 8 (1440.8N) was higher than the results for particleboard (average 900N). 44 Figure 19. SWR properties based on target density (500 kg/m3, 600 kg/m3 and 700 kg/m3) and strand thickness (0.51 mm and 0.69 mm) a) eSWR strength comparison by target density; b) fSWR strength comparison by target density; c) eSWR strength comparison by strand thickness; d) fSWR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 45 4.3 Experiment 3: Steam injection hot pressing 2 Means and Standard Deviation (in brackets) of physical and mechanical properties of OSB made by steam injection hot pressing 2 (cycles 13 to 16) Expt 3, are given in Table 15. Cycles with low steam pressure (10 psi) injection when the press were held steady with thicker strands (cycle 14) significantly changed the shape of VDPs, Figure 20, and increased both physical and mechanical properties. Table 15. Means and std Dev of physical and mechanical properties of steam injection hot pressing 2 (cycle 13 to 16) Pressing Cycles Density (kg/m3) IB (kPa) Bending SWR MOR (MPa) MOE (GPa) fSWR (N) eSWR (N) 13 598 344.7 (116.1) 22.8 (7.0) 3.2 (0.9) 1611.5 (180.9) 1359.3 (201.0) 14 594 468.4 (96.8) 25.2 (3.0) 3.3 (0.2) 1620.8 (305.8) 1520.7 (245.2) 15 611 271.4 (91.6) 27.6 (4.8) 4.0 (0.4) 1705.3 (256.0) 1474.3 (255.0) 16 617 381.7 (88.4) 30.3 (5.5) 3.9 (0.4) 1565.6 (229.1) 1360.3 (291.8) Note that the parentheses indicate the standard deviation of the value. 4.3.1 VDP analysis Examination of the VDPs of SIP1 boards in Figure 20 shows Boards made with the thicker 0.69 mm strands pressed at 25psi steam pressure resulted in a slightly higher board density compared to those pressed at 10 psi. Interestingly, this was opposite to the trend observed with the thinner 0.51 mm strands, whereby the boards pressed at 10 psi ended up higher in density. The reason for this interaction is not clear and remains unexplained. 46 Figure 20. Expt 3 - VDPs for boards based on strand thickness (0.69 mm and 0.51 mm) and steam pressure (10 psi and 25 psi). 4.3.2 Density and IB The density of each sample based on steam pressure in Expt 3 are compared in Figure 21a and 21b. The samples are presented in 2 clusters with the left cluster corresponding to the samples made with 10 psi steam pressure and right clusters with 25 psi steam pressure. Examination of the density results based on steam pressure, Figure 21a shows that all of the means overlap and therefore none of them are significantly different. Based on this result one can conclude that steam pressure has no effect on density over the range examined. This has been proved that the density of OSB boards was determined by the adhesive in the board and the compaction pressure on the mat. Examination of the IB results based on steam pressure, Figure 21b, show a significant difference in IB strength with samples made of lower steam pressure being significantly higher IB strength (p < 0.0001) about 416.36 kPa for the low steam pressure compared with 308.81 kPa for the high steam pressure. The reason for the higher IB strength for the 10 psi steam pressure may be due to the resin being forced deeper into the strands with the 25 psi samples. Thus effectively reduces the resin content of the 25 psi samples. 47 Replotting the data as a function of strand thickness, Figure 21c, shows that the density of the samples is not significantly different. Examination of the IB strength, Figure 21d, does show a significant difference in IB strength with samples made of thicker strands being significantly higher IB strength (p=0.0082), about 327.72 kPa for the thin strands compared with 394.14 kPa for the thicker strands. Comparing the press cycles at the different strand thickness (0.69 mm and 0.51 mm) but same steam injection cycles (Figure 21), it can be seen that cycle 14 (0.69 mm strand thickness and 10 psi steam pressure) injection produced a significantly higher IB strength (p = 0.0011 and p = 0.0001) than cycle 16 (0.51 mm strand thickness and 10 psi steam pressure) where the platens were held for 25 seconds during steam. According to Dai (2004), voids between strands is the main factor controlling mat permeability and the permeability of the mat controls the convective heat transfer from the mat surfaces into the core. It is likely that SIP cycle 14 (10 psi/0.69 mm) and cycle 16 (10 psi/0.51 mm) provided more void space for heat transfer during hot pressing since the platens were held for 25 seconds until steam injection ceased, allowing the core temperature to increase faster, which resulted better resin consolidation and improved properties such as IB and bending properties. Compared to void space between strand thickness, generally more furnish need to be added for panels made by thinner thickness, this will result in less void space in the core layer. As a result, panels with thicker strands will achieve a higher density and IB strength. 48 Figure 21. Internal bonding properties and sample density based on steam pressure (10 psi and 25 psi) and strand thickness (0.51 mm and 0.69 mm) a) sample density comparison by steam pressure; b) IB comparison by steam pressure; c) sample density comparison by strand thickness; d) IB comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 4.3.3 MOE and MOR The densities of each bending samples based on steam pressure are compared in Figure 22a. Examination of the figure shows that all of the means overlap and therefore none of them are significantly different. Based on this result one can conclude that steam pressure has no effect on density over the range examined. 49 Examination of the bending results, Figure 22b, and 22c, also has overlapping 95% confidence intervals, and therefore steam pressure does not have a significant effect on both MOE and MOR strength. Figure 22. Bending properties based on steam pressure (10 psi and 25 psi), a) MOR sample density comparison by steam pressure; b) MOE strength comparison by steam pressure; c) MOR strength comparison by steam pressure. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability Replotting the data as a function of strand thickness, Figure 23a, shows that the density of the samples is not significantly different. Examination of the bending strength, Figure 23b and 23c, does show significant differences in both MOE and MOR strength (p < 0.0001 and p = 0.0031, respectively). Boards made with 50 thinner strands gave higher MOE (3.95 MPa) and MOR (28.94 kPa) than those made from thicker strands (3.25 MPa and 23.76 kPa respectively). Zhuang et al. (2022) has shown a face layer with thinner strands provide a higher volume than that of thicker strands, thicker strands result in a lower amount of individual element in the same layer and therefor a lower overlapping face between the strands. Thus, higher numbers of strands in surface layer results higher compression rate during hot-pressing. The results are also in agreement with those of Beck et al. (2009) who showed that thinner strands result in higher MOR and MOE. Figure 23. Bending properties based on strand thickness (0.51 mm and 0.69 mm), a) MOR sample density comparison by strand thickness; b) MOE strength comparison by strand thickness; c) MOR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 51 4.3.4 SWR Examination of the SWR results based on hold position, Figure 24, shows that both fSWR and eSWR are not affected by steam pressure or the strand thickness. The eSWR of all boards significantly exceed the results recorded in furniture grade particleboard by Semple et al. (2005), indeed, the eSWR is 55% higher in this work compared with the earlier M2 grade particleboard (900N). Figure 24. SWR properties a) eSWR strength comparison by steam pressure; b) fSWR strength comparison by steam pressure; c) eSWR strength comparison by strand thickness; d) fSWR strength comparison by strand thickness. The heavy horizontal line denotes the mean and the lighter lines the 95% confidence interval. Error bars represent the upper and lower 95% confidence interval; nonoverlapping means are significantly different at 5% probability 52 4.4 Experiment 4: Additional Step closure conventional hot press 4.4.1 VDP analysis Table 16 shows the density difference between face and core layers. It shows the density profile became flatter by using step closing. These values indicate the average faces density is approx. 4% higher and the density of core layer is 7% lower than its average density. Compared to traditional OSB panels, the density of face layer is 30% higher than its average density, whereas 13% lower in core layer compared with its average density (Semple et al. 2015). In addition, the density difference between face and core layers were decreased from the value 34% for traditional OSB board to 11% for boards tested in Expt 4, which means the panels achieve a more uniform VDP by controlling the closing rate during pressing. Table 16. Density difference between face and core layers from Expt. 4 Schedule ID Sample average density (kg/m3) Density (kg/m3) Difference Face layer 1 Core layer Face layer 2 Face vs Core Average vs Core 1 667.42 700.18 617.06 686.05 11% 8% 2 669.02 697.46 620.26 690.83 10.5% 7% 3 674.71 699.80 624.42 702.88 11% 7% Traditional OSB panel 750 980 650 30% The resulting VDPs of the panels made by Expt 4 schedules (Figure 25) were uniform with less densified and indistinct face zones compared to traditional OSB VDP (Figure 3 and 10). Panels made by schedule 1 had the highest surface density and lowest core density. Panels made by schedule 2 performed a most uniform VDP. Similar density distribution could be found in the panels made by schedule 2 and 3 (Figure 25d). 53 Figure 25. VDPs for Expt 4 a) Schedule 1- Step closing b) Schedule 2- Creep Closing c) Schedule 3- Pre-heating closing and d) Average VDPs for Expt 4 4.5 Summary 1. For conventional hot pressing, results of the panels made by an intermediate holding position 115% (20.7 mm) with thicker strands revealed a most uniform VDP and higher physical and mechanical properties. The average of eSWR for tested OSB panels is above 1100N which is higher than MS particleboard (800N) and M2 particleboard (1000N). 2. For steam injection hot pressing, an intermediate holding position 54 mm for injecting the steam with thicker strands (cycle 8) significantly made the VDP of panel more uniform and increased both physical and mechanical properties. 54 3. For another steam injection method, cycle with low steam pressure (10 psi) injection when the press was held steady by thicker strands significantly changed the shape of OSB and increased both physical and mechanical properties. 4. By controlling the pressing cycles and steam injecting methods, the mechanical properties particularly eSWR was increasing and significantly exceed the results recorded in furniture grade particleboard from previous research. 5. Using step closing hot pressing schedule can narrow the gap of density difference between surface and core layers significantly, which means a more uniform VDP OSB panels can be achieved by these adjusted methods. 55 5 Conclusions The investigations of OSB manufacturing by conventional and steam injection hot pressing showed significant variations in physical and mechanical properties: 1) Several conventional hot pressing cycles were designed to investigate the vertical density profile of OSB, results of the panels made by an intermediate holding position 115% (20.7 mm) with thicker strands (0.69 mm) revealed a most uniform VDP and higher physical and mechanical properties. The average of eSWR for tested OSB panels is above 1100N which is higher than MS particleboard (800 N) and M2 particleboard (1000 N). 2) For steam injection hot pressing, an intermediate holding position 54 mm for injecting the steam with thicker strands (0.69 mm) significantly made the VDP of panel more uniform and increased both physical and mechanical properties. 3) For another steam injection method, cycle with low steam pressure (10 psi) injection when the press was held steady by thicker strands significantly changed the VDP shape to an increased both physical and mechanical properties. 4) By controlling the pressing cycles and steam injecting methods, the mechanical properties particularly eSWR was increasing and significantly exceed the results recorded in furniture grade particleboard from previous research and recommender ANSI values. 5) Creep step closing hot pressing (schedule 2 in experiment 4) was found to be a good way to achieve a uniform VDP OSB panels. 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Thesis/Dissertation
eng
Forestry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Smith, G. D., 1970-
The optimization of the pressing parameters for producing oriented strandboard (OSB) panels with a uniform vertical density profile (VDP)
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