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Treatment of polyethylene fibre for improved fibre to resin adhesion in composite applications Wood, Geoffrey Michael 1988

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TREATMENT OF POLYETHYLENE FIBRE FOR IMPROVED FIBRE TO RESIN ADHESION IN COMPOSITE APPLICATIONS By GEOFFREY MICHAEL WOOD B.Sc , Cornell University, 1980 A THESIS SUBMITTED IN P A R T I A L F U L F I L L M E N T OF T H E REQUIREMENTS FOR T H E D E G R E E OF M A S T E R OF APPLIED SCIENCE in THE F A C U L T Y OF G R A D U A T E STUDIES Department of Metals and Materials Engineering We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH C O L U M B I A October 1988 © Geoffrey Michael Wood, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of METALS + ^Atsnx/^t,t 6r/v/*, The University of British Columbia Vancouver, Canada Date / 7 &£-T0ac-n. , 19 V\ DE-6 (2/88) A B S T R A C T Tensile properties of polyethylene fibres are shown to be very good in comparison to properties of other advanced composite reinforcing fibres. Nevertheless, the use of polyethylene fibres in polymeric matrix composites suffers due to a poor fibre to resin adhesion. However, its ballistic properties are excellent because of the poor adhesion and also fibre ductility. Applications involving structural use of polyethylene fibres are limited by, among others, the low compressive and shear strengths. These are affected strongly by the degree of adhesion. Improvements in bonding are expected to result in greater commercial appeal for the fibres as the property limitations are reduced. Ultra Violet radiation has been shown previously, in laboratory scale batch studies, to induce graft co-polymerization of monomers to polyethylene films. Improvements in wettability and adhesion result when the grafted polymer is compatible to the bonding medium. In this study the technique was adapted to bench scale, continuous fibre treatment, whereby the monomer was surface grafted to the polyethylene substrate. Acrylic acid monomer was used for this due to its relative safety, small molecular size, and high reactivity. Reaction initiation was provided by use of a benzophenone photosensitizer due to the stability of polyethylene to U V radiation. The reaction was performed by pre-coating the fibres with reactants, then exposure to U V radiation. Results of the continuous process for fibre treatment indicate that the monomer concentration and temperature of the preliminary soakings are key variables. Adhesion improvement was measured by single fibre pullout tests and interlaminar shear strength (ILSS) tests. Of these, the ILSS appeared to be more sensitive for judging small improvements. Tensile tests were used to judge property deterioration due to treatment, and flexural property tests gave a preliminary indication of material behavior. The ILSS showed - i i -marked improvement from 1.5 ksi for untreated material to over 5.2 ksi for the better treatments. A competing treatment, plasma, shows ILSS values around 3 ksi. The flexural test indicated that failure of UV-grafted polyethylene was in tension, whereas failure of plasma and untreated material was in compression. The study has proven successful in improving the adhesion of polyethylene fibres to an epoxy resin matrix. Commercial viability is currently being developed through decreased process residence times and irradiation exposures. - i i i -Table of Contents A B S T R A C T i i Table of Contents iv Table of Tables vi Table of Figures vii List of Symbols vii i Acknowledgements ix 1 INTRODUCTION 1 1.1 A D V A N C E D P O L Y M E R COMPOSITES 1 1.2 A D V A N C E D COMPOSITE PROPERTIES 3 1.3 IMPORTANCE OF INTERFACE TO COMPOSITE STRESS STATES ... 6 1.3.1 TENSION 7 1.3.2 COMPRESSION 8 1.3.3 SHEAR 10 1.3.4 IMPACT 11 1.4 D E V E L O P M E N T OF ADHESION 11 1.5 G E N E R A L 13 2 L I T E R A T U R E R E L E V A N T TO P O L Y E T H Y L E N E ADHESION 14 2.1 HIGH M O D U L U S P O L Y E T H Y L E N E FIBRES 14 2.2 P O L Y E T H Y L E N E ADHESION 15 2.2.1 COMMERCIAL TREATMENTS 15 2.2.2 SPECIALIZED TREATMENTS 16 2.3 T H E INTERFACE A N D ITS EFFECT O N PROPERTIES 21 2.4 C H A R A C T E R I Z A T I O N OF INTERFACE EFFECTS 23 3 E X P E R I M E N T A L METHODS 25 3.1 PHOTOGRAFTING 25 3.1.1 PHOTOGRAFTING APPARATUS AND PROCESS 25 3.1.2 CONFIGURATION VARIATIONS 30 3.2 G E N E R A L \. 31 3.3 FIBRE H A N D L I N G 32 3.3.1 FIBRE WASHING ; 32 -iv-3.3.2 SINGLE FIBRE LAYUPS 33 3.3.3 LAMINATE PROCESSING 35 3.4 TEST METHODS 38 3.4.1 TESTING DEVICES AND DATA REDUCTION 38 3.4.2 SINGLE FIBRE TENSILE TEST 39 3.4.3 SINGLE FIBRE PULLOUT TEST 41 3.4.4 INTERLAMINAR SHEAR STRENGTH (ILSS) TEST 44 3.4.5 FLEXURAL TEST 49 4 RESULTS OF TESTING FIBRES A N D L A M I N A T E S 53 4.1 SINGLE FIBRE TENSILE TEST 53 4.2 SINGLE FIBRE P U L L O U T TEST 55 4.3 I N T E R L A M I N A R SHEAR STRENGTH (ILSS) TEST 57 4.4 TEST OF F L E X U R A L PROPERTIES 59 5 RESULTS OF FIBRE T R E A T M E N T 63 5.1 SELECTION OF U V - G R A F T PROCESS 63 5.2 EFFECTS OF PROCESS V A R I A B L E S O N UV-GRAFTING 65 5.2.1 MISCELLANEOUS PROCESS VARIABLES 65 5.2.2 UV RADIATION EXPOSURE TIME 66 5.2.3 UV RADIATION INTENSITY 69 5.2.4 PRE-SOAKXNG SOLUTION CONCENTRATION 70 5.2.5 PRE-SOAKING SOLUTION TEMPERATURE AND TIME 70 5.2.6 DIP SOLUTION MONOMER CONCENTRATION 73 6 S U M M A R Y A N D RECOMMENDATIONS 75 REFERENCES 78 -v-Table of Tables 1.1 Typical Properties of Advanced Composite Fibres 5 3.1 ILSS vs Span-to-Depth Ratio 47 4.1 Effect of Soaking Variables on Fibre Tensile Properties 53 4.2 Tensile Property Change for Irradiated Spectra 1000 54 4.3 Repeatability of UV-Graft Treatment and ILSS Results 58 5.1 Results of Multiple Irradiation Treatments 68 5.2 Effect of U V Lamp Intensity on ILSS 69 -vi-Table of Figures 1.1 Expanded View of Typical Composite Laminate 4 1.2 Representation of Material Specific Properties 6 1.3 Crack Behavior Under Tension 8 1.4 Compression Induced Failure Modes 9 1.5 Crack Development in Shear 10 3.1 Photografting Apparatus for the PRE Configuration 26 3.2 Fibrillation of Fibres due to Overtreatment 34 3.3 Single Fibre Tensile Sample 34 3.4 Diagram of Single Fibre Pullout Test Layup 35 3.5 Grip Arrangement for Tensile Test 40 3.6 Typical Plots of Single Fibre Pullout Test Results 42 3.7 Edge View Photograph of ILSS Specimen 44 3.8 Punchout Effect Seen in ILSS Test at Small L/d 46 3.9 ILSS vs Span-to-Depth Ratio 48 3.10 Effect of Fibre Volume Fraction on ILSS 49 3.11 Beam Deflection Contributions for Three-point Bend Test 51 3.12 Effect of L/d on Flexural Modulus 52 4.1 ILSS vs. Pullout Debond Load - Different Treatments 56 4.2 ILSS vs. Pullout Load - UV-Graft Treatments 57 4.3 Summary of ILSS Results - Spectra 1000 59 4.4 Unidirectional Flexure of Treated Spectra 1000 60 4.5 Failure in Flexure of Spectra 1000 62 5.1 Preliminary Comparison of PRE, SIM, and POST Treatments '.. 64 5.2 Effect of U V Exposure Time on ILSS 66 5.3 Effect of U V Exposure Time on Pullout Load 67 5.4 ILSS vs U V Time for Different Presoak Temperatures 70 5.5 Pullout vs U V Time for Different Presoak Temperatures 71 5.6 Effect of Soaking Time on ILSS 72 5.7 Effect of Dip Solution Monomer Concentration 73 -vii-List of Symbols M b Monomer B m Polymerized Monomer PE Polyethylene PI Photosensitizer 1 Shear Stress a Normal Stress e Strain 5 Deflection E Young's Modulus G Shear Modulus P Load L Span b Width of rectangular beam d Depth of rectangular beam -viii-Acknowledgements Firstly, I must thank the University of British Columbia and the Department of Metals and Materials Engineering for making my research and education in this field possible. As in any group project, a report discussing the information and results represents an agglomeration of the groups' contribution. Ideas are born through discussion, work is performed through cooperation, and the results are hence those of all involved. Ms. Golnar Riahi is due special credit for her dedication and major contribution to the adhesion improvement study. I would also like to thank especially Dr. Anoush Poursartip for his support, technical assistance and guidance. My thanks also extend to Dr. Ed Teghtsoonian, to Ms. Debbie Yee for her invaluable assistance, and to Mr. Roger Bennett for his advice, work and ability in keeping the mechanical aspects of the project functioning. Thanks are also due to the other members of the composites group, notably Ms. Dorothy Remmer, Mr. Dennis Chinatambi, and Mr. Mark Dorosh. I am also indebted to the Allied Signal Corporation for their funding and support of this project. Lastly, I thank my wife Deborah for photography work and immeasurable other support. -ix-1 I N T R O D U C T I O N This thesis describes work done to improve the effective mechanical properties of polyethylene fibre reinforced composite materials. Polyethylene fibres, produced by spinning and hot drawing to high draw ratios, exhibit excellent physical properties as compared to other commercially important fibres. However, the inert nature of polyethylene results in poor wetting and adhesion when used with the common resins. This restricts development of the fibres potential mechanical properties in polymeric composites. Improving the degree of fibre to resin adhesion is therefore of primary importance. Of the several available fibre treatments, photografting of monomer to the polyethylene surface was selected for study. Reasons included the following: suitability for continuous processing, an ability to control the degree of bonding, the permanence of the treatment, its inherent safety and the uniqueness of the process. This chapter will introduce advanced composite materials in general and compare polyethylene fibre with other fibres. The development of material properties with respect to the fibre/resin interface will be discussed. Finally the topic of adhesion in composites will be introduced. 1.1 ADVANCED POLYMER COMPOSITES Materials typically defined as belonging to the family of advanced polymer composites are boron, carbon, aramid and s-glass fibres, and epoxy, vinylester and polyimide resins. Their properties are substantially better than many other commercially important composite materials. Applications tend to be in high performance structures and the driving force behind their development has been the military and aerospace industries. Historically, advanced composites have proven very important in two areas, applications involving specific properties and/or impact properties. The above mentioned fibres demonstrate excellent specific properties and aramids also show excellent impact properties. -1-Good specific strength and modulus leads to lower weight structures and components, equivalent to fuel savings in the transportation market or performance increase in the military market. Good impact properties lead to applications such as aircraft wing leading edges and military armour where the advantages of light weight can be maximized. The introduction of glass fibre reinforced plastics (FRP) in the 1940s led to commercial marine and transportation usage, taking advantage of the moderate strength, lightweight materials. However, it was the military's development of FRP sandwich panels for fuselages and wings that utilized their specific properties to the fullest. This provided the impetus for the discovery of boron (early 1960s) and soon after carbon fibres as high performance reinforcing materials, given the label advanced composites. The introduction of S-glass, a glass fibre reinforcement specifically tailored for high strength and stiffness, circa 1960, completed the family of high performance, brittle fibres that are currently the mainstay of military aircraft application. Organic fibres, nylon and polypropylene, had been in service throughout the 1960s for impact resistant composites. They were usually used in combination with glass to allow for moderate stiffness. Commercialization of Du Pont's Kevlar aramid, in 1971, marked the first organic fibre that was considered for advanced composites. Its tensile strength and modulus were equal to or greater than carbon and S-glass respectively, the density was lower, and impact properties better than previous materials. Kevlar, S-glass, and carbon became the backbone of the advanced composites industry, accounting for 99% of world consumption in 1984 [1]. The characteristics, price and availability of advanced composite materials have improved significantly since 1971. Specific modulus, specific tensile strength and ultimate strain of carbon has increased steadily over the years. Glass and Kevlar have remained reasonably -2-consistent in properties but production quality and reliability have improved markedly. The first major performance increase since Kevlar was marketed, came in the mid-1970s with the announcement of highly drawn, melt spun polyethylene [2]. Polyethylene has a very low density compared to the other reinforcing materials and the manufacturing route results in quite good tensile properties. Together, these develop excellent specific properties. As an organic material, it also develops excellent impact properties. Commercialization of polyethylene fibre came in the early 1980s by Allied, Celanese, D S M and Mitsui. Polyethylene's market has been mainly ballistic, with components made of Allied's Spectra 900 fibre developing up to 30% weight savings for comparable performance to the previous state of the art materials [3]. As the fibre/resin compatibility improves, allowing full realization of other properties", its market is expected to expand into aerospace and other fields [1]. In 1985, the industrial use of advanced polymer composite materials was predicted to grow 16% a year through the next decade [1]. Important areas of new application were expected to occur due to introduction of specialized material systems and tailored applications. It is envisioned that the growth will occur mainly utilizing carbon, glass, aramids and polyethylene. Carbon and glass will remain the main materials for stiffness and strength critical designs, whereas aramid and polyethylene will be utilized where impact resistance is required. The properties of these materials overlap extensively however, and the individual fibre's application must be determined when optimizing each design. 1.2 ADVANCED COMPOSITE PROPERTIES Composites, as used here, combine reinforcing fibres with a resin matrix to develop material properties. The filaments of high modulus, high strength, low density material are the load carrying medium. However, they have too little material volume individually to be of -3-significance. Even in the large quantity found in a typical composite, their apparent strength and stiffness is useless unless stress continuity can develop between the discreet filaments. The resin binder, a hardened polymeric material, acts to provide a stress transfer medium into and between the fibres. This binder also serves to physically protect the fibres and conforms to and holds the fibres in the desired shape. Typically individual layers of aligned fibres in resin (lamina), will be combined in several orientations with respect to a common load axis to make a laminate with the desired properties [figure 1.1]. A load applied to the laminate resin must be transferred through the matrix, across the fibre/resin interface, and into the fibre. The matrix and fibre may be individually brittle or tough and the interface weak or strong. Optimum properties are developed through a thorough understanding of these factors and knowing when and how to use them to complement a design. Interactions of matrix and fibre will be discussed in the next section. matrix Figure 1.1 Expanded View of a Composite Half Laminate. Density Strength Modulus Specific Specific (gm/cc) (MPa) (GPa) Strength Modulus (km) (Mm) S-Glass 2.49 4590 90 184 3.6 Kevlar-49 1.44 2760 131 192 9.1 Carbon-HS 1.80 3600 250 200 13.9 Carbon-HM 1.81 2410 380 133 21.0 Polyethylene* 0.97 2990 130 308 13.4 Table 1.1 Typical Properties of Advanced Composite Reinforcing Fibres *Spectra 1000 Table 1.1 gives a brief summary of the properties of some fibre types. Note the high specific strength and modulus of polyethylene due mainly to its low density. Figure 1.2 plots the specific values of cross-ply composite laminates and includes two metals for comparison. It was noted earlier that the main application of aramids and polyethylene is in impact service, yet the values of Table 1.1 and Figure 1.2 indicate potentially useful specific tensile strength and modulii for these fibres. The limitation in structural application may occur due to several factors. These are low compressive strength, problems with fibre/resin adhesion, thermal limitations and potential creep problems. The former two limit the ability to develop high shear strength and flexural strength, and will both be discussed further. A market does still exist however, for items such as pressure vessels and some structures where weight is the critical factor, especially if hybrids may be used to improve the properties. -5-(A k_ 0) <5 E CO o I HI X c a> 55 o "o © a. C/3 100 90 80 70 60 50 40 30 20 10 0 -• Spectra 1000 • Carbon-HS -• S-glass Kevlar-49 • Carbon-HM • Aluminum, 2024-T4 1 mild steel i i i i i i i 0 2 4 6 8 1 Specific Modulus (x E-06 meters) Figure 1.2 Graphical Representation of Material Specific Properties 1.3 IMPORTANCE OF INTERFACE TO COMPOSITE STRESS  STATES The process of load sharing between fibres and resin involves transfer of stress across an interface. This varies in importance with the stress condition and may even be the priority design aspect. The various interactions are discussed below as an introduction to work on polyethylene adhesion. -6-1.3.1 TENSION In pure tension, the interface between fibre and resin is generally of secondary importance in advanced composites. Vinylester resin will shrink between 4 and 8 percent on curing, epoxy resins about 3 percent [4]. This places compressive forces on the fibres and results in frictional resistance. Additionally, most advanced composites are cured at elevated temperature. On cooling to operating temperature, the resin's higher coefficient of thermal expansion relative to the fibre leads to compressive stress. The resultant mechanical bond, combined with electrostatic and possibly chemical bonding, usually allows easy shear transfer of load between components. However, the degree of bonding can affect the type of tensile fracture that will occur [5]. A strong bond and brittle resin will generally allow self-similar crack propagation. As a crack tip in the resin intersects the fibre/resin interface, the stress concentration is maintained by the interface and acts primarily on the fibre. This causes potential fibre failure at or near that site and a crack propagating at approximately 90 degrees to the applied stress [Figure 1.3a]. With a weak interface, some yielding may occur and lower the stress concentration, or the interface may preferentially fail. This causes a crack deviation parallel to the applied stress and essentially blunts the crack [Figure 1.3b]. For a typical load bearing system, a combination of these two mechanisms is desirable. This will allow full development of the composite strength with a reasonable degree of fracture toughness [Figure 1.3c]. A typical composite consists of many laminae oriented at various positions to the primary (tensile) axis [Figure 1.1]. This is due to the highly anisotropic nature of the fibres and the necessity to absorb complex loadings. In this situation, it has been shown that tensile loadings can lead to normal and shear stresses near free edges, for example [6], the magnitude and sign depending on lamina orientation. Edge delamination as a result of these -7-• I i III • a) brittle crack type exhibited with strong bond. fibre resin m 1 • I I • I I II II 1 b) crack deviation and blunting with weak bond. c) combination crack blunting and brittle failure, optimum bonding exhibited. Figure 1.3 Crack Behavior Under Tension forces is a major problem in industry. Any cutout, free edge or ply interruption is prone to crack initiation sites by mechanical and environmental means. Interface adhesion and matrix strength form the primary resistance to stresses at these sites. Additionally, inclusion of off-axis plies introduces components of shear and transverse tensile stress, further increasing the requirement of good adhesion. 1.3.2 COMPRESSION Compressive failure in composites is easily recognized but poorly understood and hard to model. Matrix yielding and fibre buckling have been the primary tools on which predictions have been based [7]. These have generally led to unacceptably high results and testing has been the only way to generate data [8]; testing however is difficult and is widely accepted as subject to many sources of error. The importance of the interface can be recognized by qualitative examination but is difficult to quantify. -8-Failure is typically accompanied by micro-buckling and/or kink zone formation in the fibre [Figure 1.4]. A weak interface will separate and promote failure by these mechanisms. This is compounded as the diameter of the fibres is very small and they are prone to misalignment and hence failure initiation sites. Apparent compressive failure is greatly affected by residual (curing) stresses, fibre volume fraction and distribution, voids and experimental technique [8]. -9-Low compressive strength may result in premature failure in bending situations. A beam or plate in bending wil l have one portion in compression and one in tension, depending on the neutral axis location. Early compressive failure will cause a shift in the neutral axis toward the.tensile face and necessitate an increase in the stress level [9]. This precipitates further compressive failure or tensile failure on the opposite face. It is primarily a problem with organic fibres that tend to have large ratios of tensile to compressive strength. 1.3.3 SHEAR In shear loading, as in transverse tension, it is possible for a composite to fail without fracture of the fibres. This property is governed by matrix strength and interfacial adhesion. The relationship of interfacial strength to matrix strength controls the crack propagation path. A very brittle matrix may allow failure without interference from the fibres, Figure 1.5a. Weak adhesion will lead to interface separations and cracks through the matrix at the resulting stress concentrations. The material failure would typically appear as in Figure 1.5b. A stronger adhesion favours crack propagations seen in Figure 1.5a or Figure 1.5c. In shear, failure often occurs when delamination is initiated at an edge site and grows into the laminate, separating the plies. This problem also arises with other loading modes as mentioned previously. a) failure thru resin b) interface separation c) fibre shear failure Figure 1.5 Composite Crack Development in Shear -10-1.3.4 I M P A C T Composites have proven very important in impact applications. Armour plate, bullet proof vests, helmets, and racing canoes, to name a few, rely heavily on the light, impact resistant organic fibre composite materials. Development of impact properties is at direct expense to shear properties or degree of bonding with poor fibre/resin adhesion allowing good impact strength. The ductile fibres (organics are used for this reason) can absorb the energy of impact by slipping against the matrix and deforming without severe matrix constraint. With strong adhesion, slippage, and hence energy absorbed by frictional resistance, is inhibited. This in turn lowers the effective length of fibre over which the impact has caused induced stress, leads to earlier failure, and utilizes less material volume for energy absorption. Impact properties are very often tailored by selection of resin types and modifiers, for example addition of rubberizers to increase resin ductility. Reinforcement usually occurs in a sandwich panel type design to allow for stiffness and further energy distribution and absorption. Materials must normally allow for creep, shear, tensile and compressive loadings and some optimum degree of bonding is required. 1.4 DEVELOPMENT OF ADHESION The basis of composites are two or more dissimilar materials contributing their desirable properties such that their undesirable characteristics are minimized. Fibre reinforced composites, as shown, require a certain degree of fibre/resin adhesion to accomplish this. Characteristics of adhesion depend on the type of resin, fibre type and surface attributes, additives or treatments for surfaces, and processing details. None of the four main types of fibre that have been identified as advanced composite materials, carbon, aramid, S-glass, and polyethylene, bond well to the common resins. Good bonding requires good wettability, which in turn depends on the surface energies. The -11-substrate surface energy must be higher than the wetting medium for complete wetting to occur. Carbon and glass possess large enough surface energies in their virgin state to be completely wet by the resins. However the effects of surface contamination and adsorbed water lead to substantial reduction of the surface energy, hence incomplete wetting [10]. In their virgin form these fibres are also quite smooth and tend to resist good mechanical bonds [11]. Aramid and polyethylene have very low surface energies and smooth surfaces. This results in poor wettability and low frictional resistance so mechanical bonding is usually not adequate. Treatment of some form is a necessity for all the above fibre types. The resins used in advanced composites vary in their ability to bond to the treated fibres. Epoxy resins are generally considered the best for adhesion although vinylesters do provide quite reasonable bonding characteristics. Polyimides generally exhibit good bonding. In all systems, the bond is a strong function of the polymer backbone, additives, and curing agents used [4]. Fibre treatment is generally designed for compatibility to a given resin system. Treatment may be in the form a protective coating that exhibits good compatibility between the fibre and resin and protects the fibre during handling. This coat is sometimes removed after fibre processing and a different, typically thinner coating applied. Glass fibres will often utilize a silane finish of which one end bonds to the hydroxyl groups on the fibre. The free end will adhere to the resin by either chemical bonding or by mechanical means (interdiffusion or entanglement). Carbon fibres utilize an oxidative treatment on their quite reactive surface. This leaves functional groups available to bond to the resin, with improvement in shear strength of 200-300% being reported [12]. It is believed that the aramids are mechanically scoured and may be surface coated with epoxy or polyimide. Oxidation and plasma treatment have been attempted on aramids [13], but, since little has been published, the state of -12-commercial treatment is unclear. Treatments of polyethylene involve surface modification for increased wettability, roughness and/or addition of active groups, and wil l be discussed further. 1.5 GENERAL Announcement of any of the advanced composite fibres has been followed by a period of development where treatment to the fibre and available resins have been modified to make optimum use of the fibre. The preceding introduction to advanced composites, concentrating on the potential development of properties through micromechanics, has indicated the importance of the interface to performance. The newest fibre to the family, polyethylene, is now at the stage where there is considerable interest in developing its structural properties through improvements in interface adhesion. -13-2 LITERATURE RELEVANT TO POLYETHYLENE ADHESION The review of literature pertinent to adhesion of polyethylene and its demonstration through mechanical properties will encompass four areas. These are: discovery and application of high modulus polyethylene fibre; work performed on the adhesion of polyethylene; fibre/resin interface and its effect in advanced composites; and mechanical characterization of interface effects, especially in systems relating to polyethylene. 2.1 HIGH MODULUS POLYETHYLENE FIBRES Introduction of the high modulus versions of polyethylene fibre came in 1973 [2] when it was discovered that hot drawing of extruded fibres improved the modulus. High speed industrial processes were introduced in the mid 1970s by Capaccio, Ward and Smith [14] and by Smith and Lemstra [15]. These involved either a melt spinning or solution (gel) spinning of the fibre followed by hot drawing. The polyethylene fibre modulus attained depends on the manufacturing process and the draw ratio [15,16]. Commercial marketing of polyethylene fibre, based on the good specific properties, was hindered initially by the lack of fibre/resin adhesion. Early studies were therefore limited to ballistic applications, where adhesion was of secondary importance. Most of the published user oriented data was derived from Allied's Spectra fibre [3,17,18,19]. Extensive basic work on composites application, using the fibre alone or in hybrids with carbon and glass, has been reported by Ladizesky and Ward [20,21], Poursartip et. al. [22,23], and Adams et. al. [24,25]. Typically, the work is based on surface treated polyethylene fibre, utilizing mainly plasma, corona or, rarely, chromic acid treatment. The major thrust of recent work has been to develop a data base on impact, flexural, tensile and shear properties. Repeated exposure for industrial users and hence their increased confidence in this new material has also been of key importance. -14-2.2 POLYETHYLENE ADHESION Polyethylene is one of the major commercial bulk plastics in use today. Its widespread application has necessitated much research into surface properties, especially with regard to coating, printing or dyeing. Most of the work has involved use of films which differ from high modulus fibres in several ways. Aside from the obvious differences in surface area to volume ratios, commercial films will typically have lower degrees of crystallinity, randomly interspersed with amorphous regions. This is compared to the fibre's lengthwise highly oriented crystallinity. Films may also exhibit more of a weak, low molecular weight surface layer [26] than the highly drawn fibres. These affect adhesion studies and influence the applicability of past research work to current studies on polyethylene fibres. 2.2.1 C O M M E R C I A L TREATMENTS Polyethylene -(CH2-CH2)- n is an inherently inert material and exhibits a smooth surface with lack of functional groups or reactive sites [20]. Improved wetting and adhesion of its fibres requires that a surface treatment be applied. This must disrupt the C-H bonds and replace H with an active group. Treatment will allow industrial operations such as: bonding, printing, coating, use as a substrate or coating, and heat-sealing. The most common treatments have been corona discharge for thin sections, flame application for thicker sections such as bottles, and chromic acid prior to working with metals. Very thorough reviews of these methods are provided by Brewis and Briggs [26] and Briggs [27]. It is widely accepted that corona treatment in air or nitrogen will leave functional groups on the polyethylene surface for subsequent bonding. These are typically carbonyl, hydroxyl and nitrate. The surface groups will deteriorate over time and dissipate [28], leading to a limited shelf life. Flame treatment can be considered a variation of corona treatment, where temperature is less critical, and -15-tends to produce more functional groups. Chromic acid treatment is generally a surface oxidation and etching. The acid may remove a weak surface layer thought to be responsible for premature shear failure [26]. Ability to develop adhesion is an integral part of all discussions of commercial treatment. The wetting and spreading of inks, binders, etc. over a surface are very important in determining the degree of adhesion. For good wetting, the substrate surface energy should be much higher than that of the wetting medium. Polyethylene has a naturally low surface energy, hence a goal of all the above is to increase it through oxidation. Increased wetting allows stronger mechanical interactions, more intimate contact for chemical and electrostatic bonding, and less voids remaining on the interface [26]. The main controversies arising in adhesion of polyethylene are the relative influence of surface energy, surface topography, weak boundary layers, and specific interactions. These arguments will be compounded when applied to adhesion of fibres, as the surface area to volume ratio is so much larger than films. 2.2.2 SPECIALIZED TREATMENTS PLASMA Plasma treatment has been applied to high modulus polyethylene fibres typically on a laboratory scale, but also to small commercial batches [29]. It has not found extensive use in the bulk material and film industry due to the difficulty in continuous processing. As used here, plasma represents a low pressure, low temperature gas dissociated into free radicals, ions, electrons and molecular intermediates. This occurs through excitation with an energy source, usually radio frequency [30]. The plasma serves two purposes. It donates energy to form polyethylene free radicals, and acts as a source of active groups. These groups combine with the radicals to provide surface functionality. -16-In plasma treatment, the sample is normally loaded into a vacuum chamber. After evacuation, the plasma process gas is introduced and then excited with RF. Conditions of time, temperature and pressure determine the treatment, which will stop very soon after the energy source is switched off. Batch reactors may be retrofitted with internal spools for treating fibres or cloths, but even with this semi-continuous modification, the inherent cycling of reaction conditions leads to very high associated costs. RADIATION INDUCED GRAFTING Surface modification by addition of polymers is the other major specialized treatment applied to polyethylene. Grafting of a polymer with desirable bonding characteristics onto the relatively inert surface allows a host of new properties to be exhibited. Allen [31] provided a general review of adhesion, concentrating on the beneficial effects gained due to enhanced surface roughness and wettability, and also interdiffusion and entanglement of the grafted polymers with the resin polymer. Most applications have employed some form of radiation for the graft energy source, along with an intermediate to enhance the reaction rate. The two primary radiation sources investigated are y and far ultra-violet (UV). y irradiation tends to penetrate and affect internal structure more than U V , which mainly exhibits a surface effect [32,33]. A proposed benefit of y irradiation is cross-linking of the material layers adjacent to the surface [34]. As mentioned previously, this would improve the adhesion by suppressing failure by shear in this weak area. High intensity U V , provided by lasers, may also exhibit a similar effect [35]. The highly aligned crystalline nature of polyethylene fibres may result in weak shear response, as slip may occur between the adjacent crystalline planes. Kinstle [36] and Ballantine [37] have provided thorough reviews of photografting to polyethylene using U V . The pioneering work by Oster et. al. [38,39] in 1957, described the ability of various monomers to photograft to polyethylene and the mechanisms for this grafting. The two accepted routes are as follows: firstly, hydrogen abstraction from the base -17-polymer to leave carbon free radicals. These are available to initiate reaction and polymerization within the monomer. Secondly, after hydrogen abstraction, the available free radicals may react with oxygen to form peroxides. Subsequently, the peroxides react with the monomer to form grafts. The reaction route favored by experimenters is the former - direct combination, due to a faster reaction rate, higher graft yield, and freedom from competing oxygen reactions. A summary of the direct combination graft reactions is as follows: PEn e n e r g y PEn-+H- (1.1) MB m ^ MB- (1.2) MB+PEn- ->-> PEnBM (1.3) MB-+PEn- PEnBM (1.4) MB- + MB- ->-> BM (1.5) Reaction 1.1 indicates the hydrogen abstraction, with energy being supplied by the photosensitizer. Reaction 1.2 displays a direct monomer scission. 1.3 and 1.4 display monomer combination to form graft co-polymers, and 1.5 indicates the homopolymerization reaction. Polyethylene is quite stable to U V radiation and will not enter a chemically reactive excited state on its own. A photosensitizer must be provided in order to form the free radicals. The photosensitizer selectively absorbs radiation in the wavelength provided and becomes excited. The excited species acts as an energy donor, transferring energy to the acceptor (in this case polyethylene). The acceptor produces initiator radicals by H-abstraction [40], see Reaction 1.1, which can then be used to form grafts. The photosensitizer reactions are shown below. -18-absorption.... (1.6) intersystemcrossing... .(1.7) singlet decay ....(1.8) h - abstraction ....(1.9) tripletdecay ....(1.10) <7uinc/tmg....(l.ll) Competing reactions to H-abstraction include initiator recombination, oxygen addition, and termination. Oxygen participates in peroxide reactions and acts in competing reactions with both the surface free radicals and the initiator radicals [41,42]. Photografting is hence normally carried out in inert atmospheres such as nitrogen blanket or low oxygen content liquid. This has been reasonably easy to control for batch system studies, but becomes more difficult to control in continuous treatments. Requirements of the photosensitizer and monomer used are high reactivity, low tendency to homopolymerize (i.e. preferential grafting to polyethylene), low vapor pressures, and reasonably low viscosity and toxicity. Of the available chemicals, acrylic acid (AA) was chosen as monomer and benzophenone (BP) as sensitizer for this study on fibre treatment [43]. Much literature exists on both materials and their effect on polyethylene. Benzophenone was used in Oster's original work [39], reducing the irradiation time by three orders of magnitude. From the work on A A/BP on polyethylene [42,44-50], the following general conclusions may be drawn. These are condensed due to the overlapping nature of much of the research. Pl + hv lPI* lPI* 3PI*+PE 3PI* 3PI* + Q Vv* 3PI* PI PIH • +PE PI PI -19-• surface grafting is inhibited by oxygen. The competing reactions mentioned above result in peroxide driven grafting being thermodynamically less favorable than direct combination polymerization. • polymerization rate is proportional to the square root of both the radiation intensity and sensitizer concentration. This results from the overall reaction being a combination of two steps, production of primary radicals and initiation of chain radicals. • homopolymerization is reduced by vapor phase grafting. This is due to lower rates of contact of the monomer radicals. • monomer diffusion and radical mobility increases with temperature. This is beneficial to bulk or internal grafting but detrimental to surface grafting as the radical quenching rate increases. • surface grafting is promoted by high crystallinity, high rates of monomer consumption, low temperature, low component solubility, and non-swelling solvents. The above effects are all related to the diffusion of radicals in the material. By inhibiting mobility of monomer and substrate radicals, surface reactions are favored over internal reactions. The process of photografting to polyethylene has been studied, in batch form, in many different configurations. The current goal of commercial feasibility makes a continuous process desirable. Toward this end, the system designed was able to evaluate some of the batch variables but adapted them to a fibre line treatment. One promising technique that was not attempted was vapor phase photografting. Vapor phase photografting has been reported to have quite good yields and little interference with homopolymerization [45,50], allowing very active monomers to be used. Batch liquid systems attempted have included the following: The soaking of polyethylene film in a monomer and sensitizer solution and then -20-/ U V irradiation; simultaneous exposure of film to monomer, sensitizer and radiation; and irradiation in the presence of photosensitizer followed by exposure to monomer solution [37,48]. A l l three have provided reasonably good grafting. The best uniformity of graft distribution was shown by monomer exposure after the irradiation stage. The highest graft yields and also highest homopolymerization were produced by irradiating the monomer and substrate together. The discussion of results will include adaptations of the above to continuous processing. 2.3 THE INTERFACE AND ITS EFFECT ON PROPERTIES The effect of the interface on mechanical properties has been subject to much research throughout the history of composites. Specific interactions were discussed briefly in the Introduction. Generally accepted conclusions and recent literature to demonstrate these will be presented here. The majority of the research work has been on carbon and glass fibres with only very recent publications on aramid and polyethylene fibres. Carbon adhesion improvement is developed via oxidation treatments. These result in increased surface area, attached functional groups, enhanced wetting, and removal of a weak outer layer [51,52]. The relative contribution of these is still debated [10,53], with some indication that mechanical interaction may be of key importance. Substantial increase in all shear affected properties has been well documented [12,54-57]. These include 0 and 90 degree compressive strength,, transverse tensile strength, interlaminar and in-plane shear, and the onset strain of resin cracking under tension. Of special importance is the improvement in compressive strength [54,58], both in unidirectional and cross-ply laminates. This and the increased interlaminar shear strength are very significant to industrial applications and are responsible in part for carbon fibre's widespread use [10]. -21-Glass is very similar to carbon as regards adhesion improvement, as would be expected due to the similarities in fibres [59]. However, glass fibre adhesion improvement is developed by surface coupling agents, typically silanes, rather than oxidative treatments [11,12]. The glass bonding mechanism has not been completely defined and discrepancies usually occur with respect to importance of the interphase (a layer of matrix of intermediate composition between pure resin and coupling agent). The various researchers place different emphasis on the chemical or physical behavior of this interphase. A thorough review of the roles of physisorbed and chemisorbed silane coupling agents is given by Ishida [60]. Limited data has been published on aramids due to Du Pont's market control with Kevlar. Researchers have shown increases in adhesion using plasma treatment and surface coupling agents [13,58]. Interlaminar shear improvement has been demonstrated but compressive properties tend not to be affected much as the fibre compressive strength is small to begin with. Aramids fail easily with microbuckling and kinking and hence the matrix is exposed to many failure initiation sites [10]. Polyethylene fibre, as expected, behaves in a similar manner to aramids. Comparing plasma treated polyethylene with untreated fibres, shear and flexural properties are seen to improve, while tensile and compressive properties remain relatively unchanged [14,20,23,30]. Impact energy absorption and flexural resin cracking are also reduced with improved adhesion [61], Resin optimization in conjunction with treatment is reported to be important in developing mechanical properties [61,62], as the combined system is quite sensitive to adhesion improvement. Interestingly, Kaplan et. al. in [30] noted an increase in tensile modulus that could be attributed to some cross-linking within the individual fibres during treatment. Others have also observed an increase in flexural modulus and used FTIR to demonstrate cross-linking [63]. It is accepted that an optimum treatment level exists for these fibres where the fibre will fail in shear prior to the bond failure. -22-2.4 CHARACTERIZATION OF INTERFACE EFFECTS Chua and Piggott [64], Mandell et. al. [65], and Chamis [66] all provide thorough reviews of the testing procedures used in interface characterization. Of the direct tests on interface properties, single fibre pullout from a resin block, disc or bar is the most widely used. The test sample and examples of results will be presented in the next chapter. The single fibre pullout test is a good comparative test and some authors suggest it may also provide data on the interface shear strength and frictional force [64]. However the very small embedded length of fibre, small fibre diameter, inherent variations in fibres, fibre/resin interactions such as local deformations, and difficulty in observing the actual surface during debonding all suggest that the test may have limitations when generating applicable data. Other methods for obtaining data on interface bond strength embed a fibre or small bundle of fibres in a resin block. These methods rely on interface forces between the components due to Poissons ratio difference when under tension or compression. The forces may either debond the interface or break the fibres into small critical lengths, which can then be related to bond strength. Very precise fibre alignment is required for these specimens, and sample preparation is very difficult. The microdebond test has also been proposed as a measure of adhesion [65]. This test relies on a plunger forcing individual fibres to debond. As a compressive force on the fibre must be translated to interface shear force, low compressive strength materials are not good candidates. Tests for laminate shear properties also provide a measure of the interfacial adhesion [67]. The most common is ILSS or interlaminar shear strength, which is a three point bend test with small span to thickness ratios. The ratio is chosen so that shear failure occurs before tensile or compressive failure. The test is quite widely recognized as good for quality control and comparison, but not useful quantitatively. The test suffers with low compressive strength materials [13,68] due to early compressive failure near the load points. Other tests, off-axis -23-tensile, thin-walled torsion tube, and Iosipescu all provide good shear data but are more difficult to prepare and considerably more expensive in time and materials. ILSS testing was chosen for this study as the comparative measure of adhesion. By utilizing a fixed fibre and resin combination, test limitations could be avoided and it was possible to judge various treatments effectively. -24-3 EXPERIMENTAL METHODS Treatment and testing of polyethylene fibres represented the majority of experimental work. Treatments included three variations of the continuous photografting process, with one being selected for optimization. Demonstration of adhesion improvement required mechanical testing of single fibres and unidirectional laminates. The information presented here will include a discussion of the UV-grafting process, techniques for preparation and testing of the fibres and laminates, and a discussion and interpretation of the factors affecting test results. 3.1 PHOTOGRAFTING The following section will be divided into two parts. Firstly, a description of the apparatus and process used for UV-grafting to polyethylene fibre will be presented. Secondly, attempted variations of the main process wil l be briefly discussed. (Results and reasoning behind selection of the primary process will be given in chapter 4.) 3.1.1 PHOTOGRAFTING APPARATUS AND PROCESS The apparatus to be described is based on the process condition labelled "PRE". This indicates a presoak in a solution of monomer, sensitizer and solvent, followed by exposure to U V radiation. This process accounted for the majority of the work performed as it gave the most favorable preliminary results and is the most feasible process for commercial application. Figure 3.1 demonstrates the photografting apparatus as used for the PRE runs. A process description is as follows: Untreated fibre is passed through a coiled tube containing the solution of monomer, photosensitizer and solvent. The purpose of fibre soaking is to infuse benzophenone photosensitizer and acrylic acid monomer into the bulk fibre material. It was felt that a greater population of free radicals may be formed in this manner than with only a surface coat, and that potential grafts extending below the fibre skin (essentially anchored in -25-Untreated Acetone Monomer/Photo- UV-Irradiation Water Fibre Fibre Spool Wash Sensitizer Dip Chamber Wash Wrapping Figure 3.1 Photografting Apparatus as used for the PRE Configuration. the fibre) may also occur. Drawbacks to this are possible fibre damage by the pre-soak solution and a possible weakening of the fibre in response to shear with embedded graft polymers. Both temperature and residence time control are desirable in this pre-soak stage, as material diffusion and fibre crystallinity may be affected. Temperature control is provided by immersing the tube in a water bath set at the required temperature. Residence time of the pre-soaking stage is governed by the line speed and the length of the tube containing solution. Following the pre-soaking stage, the fibres are washed in acetone to remove pre-soak solvent. This is performed as xylene, the pre-soak solvent, wil l absorb U V radiation and interfere with the process. Acetone, the wash and dip solvents, will not interfere with U V absorption [46], and will most likely evaporate prior to U V exposure. After acetone washing, the fibres are passed through a dip bath at ambient temperature. This bath is immediately prior to U V irradiation and serves to surface coat the fibres with monomer and photosensitizer dissolved in acetone solvent. Thorough coating is desirable as this will determine the distribution and yield of surface grafts. With this treatment mechanism, no provision is made for replenishing acrylic acid monomer or benzophenone photosensitizer consumed in the irradiation stage. As shown, the effectiveness of surface grafting is dependent on available photosensitizer to drive the reaction with monomer. An optimum amount of pre-coat material should therefore exist for both substances to provide optimum surface graft efficiency. The coated fibres pass through a central quartz tube that runs the length of the U V irradiation chamber. Counter-current nitrogen flow is provided in this tube and serves to cool the fibres directly and to partially eliminate oxygen from the system. An outer concentric quartz tube allows a water jacket around the central tube for infra-red radiation absorption and further heat removal. U V lamps are positioned above and below these quartz tubes and parabolic reflectors focus radiation on the fibres. A forced air cooling system removes heat from the -27-lamps and cold tap water piped through copper tubing removes heat from near the chamber walls. Following exposure to U V radiation, the fibres are left with a coat of surface grafted material, free homopolymer, remaining unused acrylic acid and remaining benzophenone photosensitizer. The loose material is removed from the fibres by first water washing with a strong spray of hot tap water, then later by at least two acetone washings. At this point the fibres are ready for incorporation into a composite laminate. PROCESS EQUIPMENT DETAILS Initial trials were run with the untreated fibre pulled directly off its feed spool. Tension build-up in the system was unacceptably high and the fibre feed spool had to be outfitted with a DC gearmotor. A freely suspended 12 gram weight was added to provide controlled fibre pre-tensioning prior to the soak stage. The fibre soak tube was a coiled 4 millimeter internal diameter Teflon tube. Temperature control was provided for the soak solution in this tube by a water bath heated with a Corning PC-351 hot plate/stirrer. The typical time allowed for equilibrium was between 30 minutes and an hour. Water temperature was monitored with a dial gauge thermometer and could be controlled to ±1°C. The solution temperature in the tube was periodically checked and found to agree closely with the bath temperature. Soak solution quantity was checked visually during the run and fresh solution added to keep the level constant. This required approximately 10 millilitres solution per two hour run, or about 15% make-up. The acetone wash was changed at the end of each run due to contamination with pre-soak solvent. The dip solution of monomer, sensitizer and acetone required approximately 200 millilitres of solution with depletion of 25 millilitres or less per run. A l l contact surfaces in these process steps were changed to incorporate Teflon coating, as it was suggested that fibre damage may occur as a result of the fibres passing over metal rollers [69]. -28-The U V sources were two Canrad Hanovia U V Curing Systems. Each operated a lamp that drew 15 amperes at low load and 30 amperes at full load. Only these two load settings were specified at time of purchasing, a shortsight that was regretted later in the project. Nitrogen flow was approximately 5 litres per minute (normal conditions) and cooling water and air flow were not directly measured. Air flow was piped to the enclosed lamp reflector unit so as not to affect the high temperature bulb. Air was provided by a Dayton 4C108 blower with a 105/g inch diameter wheel, driven by a G E C one horsepower motor at 2450 rpm. Operating temperature was typically 45°C for the low power setting and 80 to 85°C for the high setting. Entrance and exit fibre rollers, which served as barriers to U V radiation, were ceramic. The hot water spray wash was approximately 45°C and utilized Teflon rollers for fibre contact. Fibres were wound immediately after this stage onto paper covered take-up spools. These were driven by Dayton 0.067 ' horsepower DC gearmotors with Dayton speed controllers, and the winding speed was controlled to within 1% of the required speed. Further acetone washing was performed separately on the drum winding unit prior to panel fabrication. PROCESS SAFETY Safety concerns in the UV-grafting program stem from several sources. Chemical skin burns, both surface and subsurface occurred as a result of contact with the concentrated acrylic acid monomer. The use of acetone and xylene solvents tended to carry acrylic acid into the tissue. The problem was initially recognized upon handling the treated fibre prior to water washing. Very minor injuries early in the program resulted in use of gloves and circumvented any further problems. High levels of U V radiation and high temperatures existed in the radiation chamber. A well sealed enclosure with specially designed entrance and exit ports for the fibres provided radiation protection. U V filtering goggles were available for added safety. Temperature control was provided as noted previously, but the enclosure temperature of -29-85°C when operating at full power was too great, considering the uninsulated metal wall. An added hazard existed when operating in the "SIM" mode, see below, as liquid pumparound systems were adjacent to the U V power source. Splash shields were provided to prevent any problems. 3.1.2 CONFIGURATION VARIATIONS Two major adaptations to the above process were attempted, "POST" -exposure and "SIM" -exposure. Hummel [50] recently described a batch version of the POST process using vapor phase photografting with mainly y radiation but also U V radiation. Ballantine [37] and Angier [48] also discuss this process whereby the raw material is coated with photosensitizer only prior to radiation exposure. After radiation has generated free radicals, the material is exposed to monomer for graft formation. A l l authors noted that temperature and exposure to air were important process variables and vacuum was frequently used. The continuous POST process attempted was similar to the above. Following a dip in acetone/benzophenone photosensitizer solution and exposure to U V radiation, the fibres were passed through one or more U-tubes containing monomer solution. A water bath was employed to control the monomer solution temperature. Control of exposure to air between irradiation and dip stages and temperature control during U V irradiation, both noted as important by Ballantine, would have been very difficult with this system. Most of the pioneering work in the field of radiation induced grafting addresses simultaneous exposure to monomer, sensitizer and radiation, the SIM process. As these studies were all batch work, they differ somewhat from the current system. In the continuous system, flow of graft solution was provided to the central quartz tube of the U V irradiation chamber. A fully submersible pump providing a maximum flow of about 3 litres per minute was used to pump solution. The solution travelled cocurrently with the fibres down the tube and was collected -30-in a glass pot. The pump was housed in this collection vessel and was protected by a copper screen from build-up of homopolymerized material. Manual flow control was provided with an in-line valve and no temperature control source was used for the prelirninary trials. 3.2 GENERAL FIBRE The polyethylene fibres used were supplied by the Allied Signal Corporation. These were their Spectra 1000 fibres, a stabilized, low shrink, low creep material. The fibres were either untreated and unsized, or had an applied plasma treatment. RESIN A standard DGEBA formulation, Shell Epon 828, was the epoxy resin used typically. The resin was cured with Ancamine K54 hardener at 7 phr. Also used, but for the initial screening trials only, was a vinylester resin, Dow Derekane 411, containing 45 weight percent styrene. CHEMICALS The monomer was an inhibited acrylic acid and was distilled to remove impurities prior to use. The photosensitizer was 99+ % pure benzophenone. Xylene and acetone used for solvents in the pre-soak and dip stages were analytical reagent grade material. All chemicals were supplied by the Aldrich Chemical Company. -31-3.3 FIBRE HANDLING This section wil l provide a description of the methods used to clean the fibres after treatment, to prepare specimens for single fibre tests, and to manufacture laminates for testing. 3.3.1 FIBRE WASHING Fibre washing was required for removal of excess solvent, remaining photosensitizer and monomer, and deposited loose homopolymer. Oster [39] noted that grafted material could not be removed by water washing, but made no mention of solvent washing. Both water and solvent washing were deemed necessary as remaining foreign material would change the bond effectiveness and the resin matrix properties near the interface. This change would be caused by interference to resin contact, inconsistent wetting characteristics of the fibre surface and by variation in composition of the resin due to contamination. It is important to note that ILSS values may not automatically decrease with contaminant effects, as Drzal [51] showed that a brittle interphase adjacent to the fibre surface can provide improved shear transfer. In the current system, the effect of washing was to reduce the ILSS values attainable. This was demonstrated as washed fibre panel R4-PRE56 showed an ILSS of 4.05 ksi and unwashed fibre panel R4-PRE57 an ILSS of 4.34 ksi. The panels were otherwise identical. Removal of the remaining photosensitizer is especially important, as it degrades both the polyethylene fibre and epoxy resin material upon exposure to natural sources of U V light. The hot water wash was required prior to wrapping the fibres. Flushing of contaminants, especially homopolymer, was necessary in order to prevent sticking of the fibre bundles when wrapped. The fibres were generally given two or more additional washings prior to use for testing. These utilized an acetone dip bath due to the complexity and safety problems associated with setting up a continuous acetone wash system. The number of washings was -32-subjective and depended on the severity of treatment and amount of apparent discoloration of the acetone on washing. The drum winder/prepregger machine was used to pass the fibres through the bath. The fibre tow was spread by rollers and submerged bars allowed intimate contact with the acetone. Exposure time was approximately 3 seconds per pass. The fibres were air dried and ventilation for the entire system was provided by fume hoods. 3.3.2 SINGLE FIBRE LAYUPS Samples of individual fibres were required for two purposes, to determine tensile properties and to perform the pullout tests. For these, single fibres had to be carefully separated from the tow, consisting of 120 fibres. Separation of the treated fibres was in some cases quite easy. Static charge buildup would cause individual fibres to repel and the bundle would divide naturally. For other treated fibres however, the separation could be very difficult due to two apparent causes. Firstly, damage from overtreatment resulted in fibrillation of the fibres and subsequent entanglement, Figure 3.2, and secondly, certain treatments produced a very stiff, almost rod-like tow with the fibres apparently bonded together. Multiple U V passes with a build-up of grafted material and possible crosslinking of surface grafts were primary examples of the latter. TENSILE SAMPLES Each end of the individual fibre destined for tensile testing was wrapped three to four times around a tab that had double sided adhesive tape on one face. These tabs were spaced across a measured distance of 18.4 centimetres to provide a constant gauge length. A second tab, also with adhesive tape, was placed against the first. This sandwiched several wraps of the fibre between adhesive tape. The tabs were then compressed in a vice and the samples were ready for testing, Figure 3.3. The sample differed from suggested A S T M practice as shall be discussed. -33-0 0 7 5 9 4 5 . 0 K V X £ 5 0 l £ 0 u m Figure 3.2 Fibrillation of Spectra 1000 Fibres due to Apparent Overtreatment. adhesive backed tabs single filament Figure 3.3 Single Fibre Tensile Sample. PULLOUT SAMPLES The typical pullout layup is shown in Figure 3.4. No standard geometry exists for this test and the UBC/All ied method had worked well previously for untreated and plasma treated -34-fibre samples. A flexible R T V silicone rubber mold with incisions cut for the individual fibres and a channel designed for the resin bar was used to prepare samples. Tensioning of fibres across the resin channel allowed correct fibre orientation - perpendicular to the resin bar. Eleven fibres were typically embedded in bars 15.5 cm long, 25 mm wide and 3 mm thick. The fibres' free ends were tabbed in a similar manner to the tensile samples. Resin was added to the mold after the fibres were in place, and once gelation occurred the unit was placed in an oven at 65°C for subsequent cure. 4 p Figure 3.4 Diagram of Single Fibre Pullout Test Layup. 3.3.3 L A M I N A T E PROCESSING Unidirectional laminates were constructed for the ILSS and flexural tests. Unfortunately the resin used is a high viscosity system and did not lend itself well to normal U B C pre-preg practices. A simple modification to standard pre-preg fabrication was used in order to -35-generate high quality panels. Typical pre-preg manufacture consists of laying resin coated fibre tows parallel to each other. This is accomplished through use of a drum winder operating at a large ratio of winding speed to cross-head speed. The fibres are drawn through a resin bath and utilize some type of resin quantity control. The resulting ply is generally B-staged and stored at low temperature, with panel fabrication and final cure performed later. The short working time and high viscosity of the resin system used did not allow the above, as gelation would occur in the resin bath before the required length of fibre could be wrapped. Instead, the dry fibre tow was wrapped onto backing paper and resin applied by brush to the surface. The wet ply was divided into final size sections, excess resin was removed, and the plies immediately stacked in the desired laminate configuration. At this point the panel was placed in cold storage until ready for curing. This process was able to utilize very warm resin with a low viscosity. Resin and hardener were mixed immediately before spreading and the material brushed on while still warm. Good flow and fibre wetout was achieved, and the spreading of epoxy into a thin layer ensured rapid enough temperature decline so as not to initiate reaction. The curing schedule was adapted to meet the requirements of this resin and also the fibre. Polyethylene has an upper temperature limit of about 150°C but the fibre properties are affected at temperatures lower than this. Maximum curing temperature used for this study was 60°C. • The high resin viscosity and rapid gelation coupled with low allowable cure temperature would have lead to excessive void formation if standard cure procedures had been followed. Typically, a panel's temperature is raised slowly and the resin viscosity begins to decrease. When resin flow is optimum, vacuum and possibly slight pressure are applied to consolidate the plies, begin removing excess resin, and most importandy, to remove air pockets. As resin cure begins to advance and crosslinking occurs, the resin -36-viscosity stops decreasing and begins increasing. Pressure is increased during this stage with the goal of obtaining full consolidation with a degree of resin removal consistent with the final design volume fraction. The Epon828/K54 system tended to gel quite rapidly as the temperature was raised from ambient, with little effective resin flow or void removal due to its high viscosity. Essentially, by the time a temperature for good flow could be reached, gelation would have begun to cause an increase in viscosity. In order to obtain good quality laminates it was hence necessary to quickly raise the temperature and consolidate the plies before onset of gelation. This was performed in a hydraulic press with pre-heated plates at 45 to 50°C. The panel, already prepared for its autoclave cure with peel and bleeder plies, would be placed between the platens and pressed between stops to a final, pre-determined thickness. This was done in discreet steps, with pressure applied slowly and relaxed to allow uniform resin flow and good void removal. Pressures up to 100 psi were commonly used for this process. Panels were removed, prepared with new bleeder and breather cloths as necessary, and autoclave cured at 50°C, 15 inches mercury vacuum and 35 psig pressure. A very small amount of resin would sometimes flow out in this final cure. Much trial and error determination led to the cure schedule described, and, as shall be seen in the results, fibre volume fraction varied widely. PANEL CHARACTERISTICS The UV-graft treated fibres in general performed quite differently from the untreated fibres. It was easy to prepare panels of the UV-grafted material up to 70% by volume fibres, fully consolidated and with resin rich faces. The wettability of these fibres by the resin was excellent and few problems existed in layup. In contrast, untreated fibres did not wet well, needed to be fairly heavily worked to get full resin penetration of the tows, and would exhibit many dry fibres on the surfaces of panels that were above 60% by volume of fibres. -37-UV-graft treated panels exhibited little or no fraying on edges cut by band saw, indicating good constraint by the matrix and hence good bonding. The untreated fibres, or weakly treated ones such as corona, tend to fray extensively and need a sacrificial backing when cut by this method. 3.4 TEST METHODS The test methods used are derived from standard tests adopted by the American Society of Testing and Materials, A S T M . The notable exception to this is the single fibre pullout test. This has been used by researchers in various forms, generally being adapted to best suit their needs. Regarding test methods, it should be noted that polyethylene fibre is quite different from currently available fibres and this application is not development of property data but comparison of fibre response to various treatments. It was hence necessary to judge the applicability of standardized tests as well as performance of the material within these tests. 3.4.1 TESTING DEVICES AND DATA REDUCTION The testing machines used were either small or large frame constant displacement rate Instrons. Pullout and tensile work was done with an Instron Type A Tensile Load Cell of 500 gram full scale. ILSS and flexure tests used an Instron GR Load Cell for tension and compression with a 20,000 pound full scale. Dead weight checks were performed in the testing range used and ensured accurate results. A l l testing was performed in a controlled environment room in accord with A S T M guidelines. The deflections for tensile tests were determined by correlating the crosshead and chart speeds and were independently checked with a linear variable deflection transducer (LVDT). ILSS, pullout and tensile test load were determined from the Instron chart recorder. Manual -38-transcription was performed as the tests were run, allowing the values to be cross-checked and errors to be noted with time to correct them. Further analysis and presentation was all accomplished by personal computer. The flexural test generated data from a three point bend test, with accurate determination of the deflection obtained by L V D T . Voltage outputs, from the Instron load cell and the L V D T , were gathered and converted to load and deflection data using an Orion data logger. A Hewlett Packard plotter recorded the load-deflection curves as the test progressed. Approximate moduli and failure stress and strain were calculated to check for reasonable and consistent results. Downloading to a personal computer allowed data reduction to determine stress/strain curves, correction factors, and accurate moduli and ultimate tensile strength. 3.4.2 SINGLE FIBRE TENSILE TEST Tensile testing was performed according to A S T M Standard Test D-3379, Test for Tensile Strength and Young's Modulus for High Modulus Single Filament Materials. The test is valid for single filaments of materials with modulus over 21 GPa and gauge length to diameter ratio at least 2000:1. The Spectra 1000 fibres used had a modulus of approximately 130 GPa and a gauge length to diameter ratio of approximately 6800. The major deviations from the published test method occur for sample mounting and fibre cross sectional area measurement. Fibre diameter was taken to be constant at 0.027 mm and not checked for each filament. The sample mounting technique, described previously, was developed due to difficulty in bonding polyethylene to most common adhesives. By wrapping the fibre around a tab and compressing between adhesive faces, slippage could be avoided. Loading these specimens in the Instron was accomplished by gripping the lower tabs in a jaw and suspending the upper tabbed end from a " U " hook attached to the load cell, Figure 3.5. Problems arose with correct alignment, which was carefully, but only visually, determined, -39-and with oscillation of the freely suspended hook and fibre. Figure 3.5 Grip Arrangement for Tensile Test. The tensile test was considered a preliminary screening test for identifying potential large differences in tensile properties. One goal of the standard test is to identify external stiffness effects on results by utilizing a preliminary test of varying gage lengths. This was not performed as the machine and grip system compliance should not be a significant factor in determining relative modulus changes at a fixed gage length. The standard deviation within results was reasonable enough to accept the values and to allow general statements on change in modulus and strength. -40-3.4.3 SINGLE FIBRE PULLOUT TEST The single fibre pullout test most commonly used [64-66] relies on fibres mounted independently in resin discs. The major drawback to this method is the extensive time involved in preparing the samples and mounting each in the test machine. The authors using these tests have typically been concerned with measurement of the interfacial shear strength of a given material. The method does allow for very good alignment in the test machine, easy measurement of the embedded fibre length, and little distortion of the resin on loading. For the purpose of determining debond characteristics for many different fibre samples, without necessarily measuring the interfacial shear strength, the above was adapted to a multiple fibre layout. Rapid testing and convenient sample preparation were the main benefits, the drawbacks were resin bar deformation on loading and inability to use thin bars to reduce interfacial area. The prepared sample set, described previously, was loaded in an Instron machine. The resin bar was clamped in a specially designed fixture that allowed alignment of individual fibres with the load cell. Clamping was near the ends of the bar. The tabbed end was suspended from the hook fixture shown above for the tensile test. Constant crosshead rate of 0.05 inches per minute was used, and the resulting load/time data plotted on a chart recorder. Typical plots of load versus time are shown in Figure 3.6. The first plot identifies several regions of interest when examining the adhesion characteristics. A debond load has been defined as the first sharp stress decrease noted and indicates initial bond breaking. Typically, the fibre would still be restricted at this point and the load would further increase. At some pullout maximum load, the fibre would break free of the resin and a large drop in load would be seen. The fibre then pulls through the resin at a fluctuating, relatively low load, governed by friction. Figure 3.6b shows another typical -41-a: Low Debond Load w.r.t. Strength. b: High Debond Load w.r.t. Strength. maximum or sometimes TIME TIME Figure 3.6 Typical Plots of Single Fibre Pullout Test. pullout test result. This result was seen with fibres having debond loads near to their ultimate tensile strengths. A series of debonds are noted with a rise to a maximum load followed by a drop to almost zero load. The pullout debond load is defined as the first load peak exhibited. The pullout maximum load is defined as the highest load attained. Two conditions may be contributing to this type of behavior. Firstly, at high loads, Poisson's effect would be reducing the fibre diameter and changing the stress condition at the interface. Debonding thus takes place by shear and tensile force components and is a more catastrophic process. Secondly, resin bar deformation is a strong possibility. Local resin deformation around the fibre, or meniscussing, would aid in debonding due to preferred initiation at the edge sites. -42-Also, macroscopic bending of the resin would induce tensile interface stress on the surface closer to the load and compressive interface stresses on the opposite surface. Overcoming frictional and interfacial resistance would cause the fibre to slip and the resin bar to rebound. Attempts to resolve the above problems by reducing the interfacial contact area with a thinner bar were not successful. The thin bars were too fragile and tended to shatter in the molds. Employing the resin disc or using small individual molds would have been required. The clamping method was changed to incorporate a longer section of the bar to reduce possible bending. -43-3.4.4 INTERLAMINAR SHEAR STRENGTH (ILSS) TEST ILSS tests were conducted according to A S T M D-2344. However, some latitude is allowed in the test method and sample preparation, and was determined to have a major impact on the results. The test is not useful for design purposes, but usually specified for quality control and R & D programs concerned with interply strength. Flat test specimens were used in three point bending, with a span to depth ratio in the range 4 to 5. This is small enough to ensure failure by shear at or near the mid-plane prior to tensile or compressive failure. Specimen shear failure was always noted and is demonstrated by an edge view photograph in Figure 3.7. Figure 3.7 Photograph Showing Edge View of ILSS Specimen. Note failure planes at or adjacent to mid-plane. The requirement of failure by shear rather than tensile or compressive failure is as follows. Shear stress on the mid-plane Ts is given by: -44-3P ( 3 ' 1 ) and the maximum stress on the outer surfaces by: 3PL ° - = (3-2) hence the ratio of strengths is given by: O m ax Z \ L J Noting [18] that the tensile strength of a Spectra 1000 composite is 174 ksi, compressive strength is 10.5 ksi and shear strength about 2.5 ksi, failure by shear should occur at or below an L/d of 2.1. This is in order for compressive failure not to be dominant. As noted above, shear failure was exhibited at higher span-to-depth ratios, and indeed, D-2344 suggests 4 as a minimum L/d. Several factors may explain the above. Firstly, o m a x is generated only in the outer surface layer of the material. Due to the statistical strength distribution of fibrous materials, the likelihood of failure in a limited material volume such as at the surface is smaller than for the test samples. This would indicate a larger applied stress required for failure. Secondly, compressive failure is inhibited by the improved adhesion of the current treatment, as will be shown, and the failure mode is typically tensile. Finally, compressive yielding followed by plastic behavior has been demonstrated for aramids [69] and polyethylene would be expected to behave similarly. This leads to lack of a clearly defined failure zone as the neutral axis shifts to accommodate stress redistribution. The above is supported by Fischer et.al. [68] who demonstrated the dependence of aramid ILSS on loading span-to-depth ratio. A relationship for apparent ILSS versus true ILSS was shown to depend on r, the ratio of ultimate tensile strength to ultimate compressive strength. At the highest allowable span-to-depth ratio for shear failure, the minimum ratio of ILSS is -45-shown to be: tpp l + r 2r (3.4) Note that this gives values of apparent ILSS approximately 50% of the maximum for materials with high tensile to compressive strength ratios. Fischer also showed that the minimum span-to-depth ratio for Kevlar occurred at 1.5-2. Experimental evidence could be extrapolated to show that for Kevlar aramid fibre with an L/d of 4 and r of 5.5, the apparent ILSS was 90% of the theoretical, hence showing test limitations of A S T M D-2344. Similar limitations are expected to occur with polyethylene as r is close to 16.5. P -46-Panels produced in the early part of the adhesion program were generally 16 to 20 plies of unidirectional Spectra 1000/epoxy. This gave a thickness in the range of 1.5 to 2.2 mm. ILSS testing was performed on beams cut from these panels approximately 6 mm wide and at a span to depth ratio of 5. Smaller ratios on these thin panels generally led to a punchout type of effect, where the sample was extruded through the supports, Figure 3.8. The effect of varying L/d was demonstrated in the test results shown in Table 3.1. This panel treatment was identical to earlier panels that had given ILSS results of 4.49, 4.50 and 4.46 ksi at span-to-depths of 5. S P A N L/d ILSS Comments (mm) (ksi) 7 2.3 5.97 punch-out failure 9 2.9 5.11 punch-out and shear failure 11 3.6 4.96 shear failure 12 4 4.83 shear failure 15 5 4.46 shear failure Table 3.1 ILSS vs Span-to-Depth Ratio for Typical UV-Grafted Fibre. Figure 3.9 shows the span-to-depth ratio effect very clearly, and led to modification of the test procedure and panel manufacture. ILSS values are now determined at 4:1 span-to-depth, the minimum allowable by A S T M D2344, with panel thickness' of 2.7 to 3.2 mm. The number of plies increased to 32 to accommodate this change. As mentioned previously, this may still not represent the true ILSS of the material, as even with thick beams compression -47-damage can be identified. (A in CO Span-to-Depth Ratio Figure 3.9 ILSS vs Span-to-Depth Ratio. Also determined was the effect of volume fraction variations on the ILSS. Within one treatment, the number of plies of fibre was altered while the panel thickness was held constant. Effective layup with low void content required using excess resin and allowing considerable bleed-out quantities, especially for low volume fractions. Results are shown in figure 3.10. An increase in ILSS with low volume fractions is demonstrated, which is due to the increasing contribution of the matrix. -48-- very thick panel W (f) - ! 3 Treatment R4-PRE93 30 40 50 60 Volume Fraction Fibres (%) Figure 3.10 Effect of Fibre Volume Fraction on ILSS. 70 3.4.5 FLEXURAL TEST The test for flexural properties is a measure of the combined tensile and compressive response of the material and was of interest for two important reasons. Firstly, improvement of the bond at the fibre/resin interface should lead to increased stability of the composite under compressive load and secondly, it is an easy procedure to identify possible changes in the mode of failure due to treating the material. -49-Zweben [70] presented a very thorough discussion of the flexural test based on the A S T M D-790 method used here. The three point bend test is used at relatively large span-to-depth (L/d) ratios. This assures a large enough ratio of normal to shear load so that shear failure is not exhibited. The modulus, stress and strain are calculated with the following relationships: 6d\ L 3PL J, 2b d\ Slope x 1 + 6 - 4 ( L 3 ^ yAbd j ,where Slope = V °" J initial (3.5) (3.6) (3.7) The bracketed stress correction factor in equation 3.6 is due to an end force effect for large beam deflections. This end force increases the moment, but only becomes significant above about 1% strain (for the current geometry). The modulus Slope was taken as initial slope of the load deflection curve up to 0.1% strain. This value was used as it gave a true linear response for the polyethylene fibre stress-strain curve. Typically, strain up to 0.2% is used for other materials. When applied to polyethylene fibre, this showed a reduction in modulus of up to 30% as compared to 0.1% strain. Zweben demonstrated that for aramid fibres, modulus determination is dependent on L/d due to the contribution of shear deflection which is not accounted for in the test method. Figure 3.11 is a schematic of the components of shear and bending deflection. The shear correction would have to apply to the deflection 8 shown in equations above. For a negligible shear component, it is necessary for: 8, « 1 (3.8) or, using the relationships of Figure 3.11, -50-, rectangular beam (3.9) Taking the Spectra 1000 polyethylene fibre tensile modulus to be approximately 130 GPa and shear modulus to be near 4 GPa, an L/d of 16, which is the minimum specified, would indicate a shear deflection of 15% of the bending deflection. At L/d of 40, this drops to 2.5%. The effect of L/d on modulus for treated Spectra 1000 is shown in Figure 3.12. The data show a similar trend to Zweben's results and flexural testing in general for this material should be performed at L/d greater than 50. -51-10 20 30 Span-to-Depth Ratio 40 50 Figure 3.12 Effect of Loading Span-to-Depth Ratio on Flexural Modulus -52-4 RESULTS OF TESTING FIBRES AND LAMINATES Data may be classified according to the test method used or to the nature of the treatment. This chapter will present the combined results of UV-graft treatment and work done on plasma treatment. These results will be related to the test methods as overall measures of adhesion, and not separated into groups based on individual process parameters. 4.1 SINGLE FIBRE TENSILE TEST Tensile properties of treated Spectra 1000 are presented in Table 4.1. Data were developed to determine the effect of soak solution residence time and temperature on fibres. Break Load (std. dev.) - grams -Modulus (std. dev.) - GPa -Treatment Temp. 50°C 40°C 50°C 40°C Soak Time (t/t0) 0 216 (17) 216 (17) 130 (09) 130 (09) 0.10 207 (19) 216 (16) 127 (11) 117 (09) 0.25 209 (17) 198 (23) 125 (09) 118 (13) 0.50 181 (38) 203 (20) 104 (29) 121 (14) 1.00 176 (37) 230 (18) 98 (22) 133 (04) Table 4.1 Effect of Soaking Time and Temperature on Fibre Tensile Properties. Measured at room temperature. At 40°C, there is no apparent loss in fibre tensile properties. Tensile strength and modulus show a definite decrease after 0.5ro and 1.0f„ soak times at 50 °C. The reduction in strength of 15-20% and modulus of 20-25% is substantial, but still leaves the fibre specific properties -53-competitive with other advanced composite fibre properties. For example, specific tensile strength and modulus are greater than those of Kevlar-49 and S-Glass, even considering a respective 20 and 25% reduction in polyethylene fibre values. Data for tensile strength loss of U V irradiated fibres is presented in Table 4.2. The data is based on the normalized exposed energy, as this allows effects due to different power settings and exposure times to be compared. Data show a decrease in tensile strength with increasing energy, becoming significant between 0.1Po and 0.24fo. Most of the treatments performed in this study used between 0.05Po and 0.24Po. A tensile strength loss of up to 16% may hence be expected due to irradiation. Normalized U V Energy (P/P0) Tensile Strength (grams) 0 208.3 + 30.4 0.10 205.6 ± 12.0 . 0.24 174.0 ± 21.0 0.30 •• 127.0 ± 27.3 0.50 129.5 ± 9.0 0.72 88.0 ± 6.5 1.00 67.4 ± 15.1 Table 4.2 Tensile Property Change for Irradiated Spectra 1000. The presented results indicate that it is desirable to perform the soak operation of fibre treatment below approximately 50° C. However, it is also desirable to use the highest soak -54-temperature allowable. This maximizes the diffusion rate of solvent, photosensitizer, and monomer into the polyethylene fibre. Initial fibre properties are excellent, and hence the trade-off of improved adhesion versus less fibre damage may be towards the former. The result of varying pre-soak bath temperature on ILSS will be discussed later. 4.2 SINGLE FIBRE PULLOUT TEST The pullout test was performed on samples from most of the treatments. As explained previously, large scatter is seen in the values of both the debond load and the maximum load. A general trend is apparent when comparing data of untreated, plasma treated and UV-grafted fibres. The untreated initial debond load was approximately 26 grams, plasma treated varied from 50 to 100 grams, and UV-grafted varied between 80 and 130 grams. Standard deviations were from 25% to 35% of mean values. Pullout data hence indicate better adhesion for UV-grafted material than for plasma or untreated material. The relationship of pullout debond load to ILSS is plotted in Figure 4.1. Figure 4.1 indicates consistency in the two measures of adhesion. However, Figure 4.2, which graphs ILSS versus pullout load for all UV-graft treated fibres, does not show this consistency. The discrepancy between Figures 4.1 and 4.2 may be explained by reference to earlier discussion on the pullout curves. The adhesion improvement seen with UV-grafting is substantial and results in high pullout values for the test geometry used. This gives a test curve similar to Figure 3.6b. As seen, external effects, such as resin bar deformation, become important at this condition and the test is no longer a true test of interfacial adhesion. Figure 4.1 has a wide spread of pullout values with the lower range showing better correlation with ILSS than the upper range. Figure 4.2 shows all U V treatments and the spread of debond to maximum load. The inherently good treatment masks any correlation to ILSS. -55-0 20 40 60 80 100 120 140 160 Pullout Debond Load (grams) Figure 4.1 ILSS vs. Pullout Debond Load - Different Treatments. From these results, it appears that the pullout test is useful as a comparative test of fibre/resin adhesion. However, it does not appear accurate enough, with this geometry, to identify improvements due to UV-grafting. Development of an improved test sample, for this fibre and resin, should make the test more sensitive. -56-3 CO CO UNTREATED i i i i i i J I I I L _ L 20 40 60 80 100 120 140 Pullout Load (grams) 160 180 200 Figure 4.2 ILSS vs. Pullout Load - UV-Graft Treatments. 4.3 INTERLAMINAR SHEAR STRENGTH (ILSS) TEST ILSS test results were used as the primary method of judging adhesion changes. The test sensitivity has proven very good, with standard deviations ranging from 1.1% to a maximum of 7.6% of mean values. Average standard deviation was approximately 2.5% of ILSS value. This is a good indication that manufacturing quality is consistent for any given laminate. The low variability indicates reasonably uniform fibre volume fraction and voids distribution. -57-TEST V O L U M E FRACTION ILSS (ksi) Std. Dev.(ksi) R4-PRE12 0.597 4.46 0.34 R4-PRE59 0.607 4.49 0.07 Table 4.3 Repeatability of UV-graft Treatment and ILSS Results. Repeatability of UV-graft treatment and panel manufacture was demonstrated by panels R4-PRE12 and R4-PRE59. These were repeats of an identical fibre treatment having similar lay-ups, curing schedules and test span-to-depth ratio. The manufacture (and testing) of these panels was over 4 months apart. Results of the ILSS were as shown in Table 4.3. Other panels with identical fibre treatments were made. However, they had different volume fractions or thickness' and hence were not directly comparable. The results were still in the range of 4.5 to 4.8 ksi, close to the above values. ILSS results for all samples indicate a very strong trend in favor of UV-graft treatment. Figure 4.3 plots all ILSS results, as well as the results of untreated and plasma treated fibres. The UV-grafted polyethylene shows better adhesion than the other materials. An almost 4 ksi improvement over untreated fibres and 0.5 to 2.5 ksi improvement over plasma treated fibres is shown. The results are plotted against panel fibre volume fraction. Plasma data demonstrate the trend of increasing ILSS for decreasing volume fraction. -58-6 -5 -— 4 </> CO CO + Untreated Fibre ° Plasma Treated Fibre • UV-Grafted Fibre 20 40 Volume Fraction Fibre (%) 60 Figure 4.3 Summary of ILSS Results for Spectra-1000 4.4 TEST OF FLEXURAL PROPERTIES Flexural property test required laminates containing approximately 1000 feet of treated polyethylene fibre. At the low line speeds used for the experimental set-up, this represented well over a day's continuous operation. For this reason, only two flex panels were made from UV-grafted fibre. One of these did not cure properly and hence only one flexural result is available. Data for all samples cut from this panel show very similar behavior to the curve presented. -59-300 0 0.02 0.04 Strain (mm/mm) Figure 4.4 Flexural Stress-Strain Behavior of Spectra-1000 Results of the flexural test are shown in Figure 4.4. When comparing the three fibre types, it is seen that the material response and type of failure is markedly different. Plasma treated fibres show the same failure mode as the untreated material. This is failure dominated by compression with a region of severely buckled fibre and resin on the upper face. Plasma treated Spectra 1000 exhibits higher failure stress and strain than untreated. This is expected due to the increase in adhesion providing improved behavior in compression. The UV-grafted fibres demonstrates three major departures from the above. The stress-strain response keels over earlier in the test, the ultimate strength is much higher, and the failure -60-mode is primarily tensile. The former is quite important, as it shows lower flexural properties in the strain range typically used for designing composite structural materials. Offsetting this is a much increased margin of safety in response to strain. Figure 4.5 shows the failure of plasma treated and UV-grafted Spectra 1000 composite specimens. The UV treated fibre composite panels typically failed in tension, Figure 4.5a. They exhibited a very widespread region of compressive damage visible on the surface. These tensile failures were similar to those of brittle fibre composites such as glass or carbon. UV-grafted sample failures were in marked contrast to the compression dominated plasma specimen failure, Figure 4.5b. Failure of UV treated material occurred at very high deflections, well above that possible on a three point bend test. The sample of Figure 4.5a was broken by hand. £ t — i Figure 4.5a Failure of UV-Graft Material. -61-Figure 4.5b Failure of Plasma Treated Specimen. Figure 4.5 Photographs showing failure in three point bend for samples of plasma and UV-graft treated Spectra 1000. -62-5 RESULTS OF FIBRE TREATMENT 5.1 SELECTION OF UV-GRAFT PROCESS The U V grafting process alternatives are described in the Literature Review and the Experimental Methods chapters. Selection of a primary treatment for optimization was the first step in the project and was performed via a screening check of the options. Results were judged on the laminate ILSS values and the applicability of the technology to continuous processing. Figure 5.1 shows the ILSS results obtained initially. The POST treatment gave low ILSS values, 2.5 to 3 ksi. This is in agreement with Ballantine [37] who showed that the graft yield, although uniform, was lowest of the three systems. Problems in POST treatment arise firstly due to the inhibiting effect of oxygen and secondly, due to temperature variations seen by the fibre. A controlled temperature nitrogen blanket would have been required to study this system. Industrial problems may be foreseen because the irradiation and soak stages probably cannot be separated, due to limited free radical life. The POST treatment was not pursued for the above reasons. The SIM treatment, as expected from Ballantine's [37] and Angier's [48] work, exhibited a large degree of homopolymerization. This was initially developed on the tube wall and would quickly wash off. (Recall that the SIM process used monomer solution flowing through the central tube and exposed to radiation). A rapidly increasing rate of homopolymerization was observed qualitatively. An initial period of trouble free operation existed, about 20-25 minutes long, after which the pump, connecting lines and especially the irradiation tube would become restricted due to material buildup. Control was attempted through filtration, -63-chemical inhibition, and variation of the monomer solution concentration. These were unsuccessful and extended runs were not practical. Inconsistent results were obtained and the method was abandoned. Fibre Treatment Figure 5.1 Preliminary Comparison of PRE, SIM, and POST Results. The PRE treatment exhibited very good results from the start. Problems were minor and did not hinder operation. As seen in Figure 5.1, ILSS values were quite good. One major concern of the other treatments was interaction of the soaking and irradiation steps. These units are reasonably independent for the PRE set-up, although a potential problem may be evaporation of acrylic acid monomer and benzophenone sensitizer between the stages. PRE-treatment was adopted for further study based on good ILSS results and commercial promise. -64-5.2 EFFECTS OF PROCESS VARIABLES ON UV-GRAFTING Once the preferred treatment method was identified, optimization studies concentrated on the following major process variables: time, temperature, and composition of the pre-soak solution; intensity and residence time of U V irradiation; and dip solution concentration. Goals were twofold: firstly, improvement in ILSS of the treated material and secondly, increasing efficiency of the system with respect to commercial application. 5.2.1 MISCELLANEOUS PROCESS VARIABLES Water addition to the dip solution was examined. Ogiwara [45] had reported an increase in graft yield with water as solvent. For the polyethylene fibre treatment, addition of 7% water changed the ILSS value from 4.78 ksi to 4.75 ksi. Addition of 4.3% water changed the ILSS value from 4.25 ksi to 4.16 ksi. Three possible reasons for lack of noticeable effect of water are: water has no effect on the continuous PRE system (due potentially to different reaction kinetics); nitrogen gas flow evaporates the water; or monomer is fully consumed so no effect can be demonstrated. Airflow to the central quartz U V irradiation tube was provided in place of nitrogen. Oxygen may either form peroxide intermediates which result in grafting, or it may inhibit free radical formation. ILSS values of 4.28 ksi in air compared to 4.25 ksi in nitrogen were obtained. This is a favorable result, as it suggests elimination of the costly nitrogen supply is possible. It is expected that the effect of oxygen would be dependent on the photosensitizer concentration and irradiation dosage. Hence this result is very preliminary and would have to be repeated on further optimization of the UV-graft system. -65-5.2.2 UV RADIATION EXPOSURE TIME Figures 5.2 and 5.3 indicate the effect of exposure to U V radiation for various pre-treatment conditions. Since the exposure time and pre-soak time were directly related (both having constant length tubes), the only true comparison is with no pre-soaking. A reasonably constant response to irradiation at times over 0.25?o is demonstrated. This indicates a possible saturation level at which acrylic acid monomer may be fully consumed. Values do not change for exposure times up to 1.0fo; it therefore seems reasonable to conclude that little or no damage to the fibres has occurred. </> V f=38% untreated J L I L + No Pre-soaking • Soak Time = f(Exposure Time) j i i I 1 1 1 i i i i i 0.25 0.5 0.75 UV Exposure Time (t/LJ 1.0 Figure 5.2 Effect of U V Exposure Time on ILSS. -66-Data are also shown where the pre-soak time was a fixed multiple of the exposure, supporting the no pre-soak system results. A slight decrease in ILSS and pullout load appears when irradiation times are less than 0.25fo. Note that the ILSS value obtained at 0.125fo exposure is for very low volume fraction. A normal volume fraction of fibre, 50 to 60%, would tend to decrease this ILSS result. As expected, a sharp drop in both ILSS and pullout values is demonstrated at zero exposure. w E _^ T3 (0 o 3 O 130 120 110 h 100 90 80 70 60 50 40 J L J L + No Presoaking " Soak Time = f(Exposure Time) J L J I L J L 0.25 0.5 UV Exposure Time (t/y 0.75 1.0 Figure 5.3 Effect of U V Exposure Time on Pullout Load. -67-Also studied was the effect of repeated irradiation of the polyethylene fibre. A single, independent pre-soak step was performed on a spool of fibres. These fibres were subsequently dipped in monomer/sensitizer and irradiated for 0.125fo. Further dip and irradiation cycles made up the various treatments, results of which are presented in Table 5.1. Time (r/O ILSS (ksi) 1 x 0.125 3.78 ± 0.07 2 x 0.125 4.44 ± 0.08 3 x 0.125 4.94 ±0 .11 0.250 3.83 ± 0.08 0.375 4.78 ±0 .11 0.500 4.83 + 0.10 Table 5.1 Results of Multiple Irradiation Treatments as Compared to Single Pass Exposure. The results indicate improvement at the lower exposure times, but no clear difference is noticeable at 0.375fo. Benefit may be gained by replenishing the grafting chemicals and hence yielding a greater graft density. Further study in conjunction with varying dip solution concentration should be undertaken. Fibres treated using the multiple pass technique cohered more than if treated in a single pass. However, the wettability of the fibres, as judged qualitatively on their panel lay-up characteristics, appeared very similar. Fibres coherence is not necessarily a drawback. It improves the handling characteristics for weaving material and eliminates the necessity for -68-twisting the tows. The multiple pass technique seems very promising for commercial application especially i f tailoring the adhesion characteristics is desirable for combined impact and structural service. 5.2.3 UV RADIATION INTENSITY Table 5.2 shows the three trials performed to determine effect of U V lamp intensity on ILSS. Slight or no adhesion improvement occurred at the higher power setting. Large nitrogen flow rates were needed to prevent melting of the fibres and visible fibre damage was almost always present. The local temperature of the radiation chamber was close to 85°C, dangerously high for exposed metal surfaces. The above combination of problems restricted any further work at high power settings. ILSS (ksi) Lamp Power Panel # 15 Amps 30 Amps PRE 13/14 4.55 4.73 PRE20/30 3.77 3.85 PRE22/26 3.65 3.60 Table 5.2 Effect of U V Lamp Intensity on ILSS Work was not practical at power settings lower than 15 amps. This would have required either a major change to the lamp power source, or installation of filters and associated intensity monitoring devices. Considering the results presented so far, it seems reasonable to conclude that lower power irradiation may not be detrimental to the grafting process. -69-5.2.4 PRE-SOAKING SOLUTION CONCENTRATION Two conditions were studied for the pre-soak solution concentration, one with and one without acrylic acid monomer. These tests were performed to determine the effect of an acrylic acid soak. ILSS values of 5.21 ksi were generated with no acrylic acid monomer in solution, as compared to 4.78 ksi with monomer present. The tests were repeated and confirmed the negative effect of soaking with acrylic acid. 5.2.5 PRE-SOAKING SOLUTION T E M P E R A T U R E AND TIME Pre-soaking solution temperature was varied between 40 and 50°C. The effect on ILSS is shown in Figure 5.4. 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4 3.9 3.8 3.7 i i i o 40 C + 45 C • 50 C I I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 U.V. Exposure Time (t/U) 0.8 0.9 1.0 Figure 5.4 ILSS vs U V Time for Different Presoak Temperatures. -70-The best ILSS results were obtained for 50°C, with no difference discernable between ILSS values at 40°C and 45°C. No influence of panel fibre volume fraction on these results was noticeable. No trend was observed with temperature effect on the pullouts, Figure 5.5, as the data were scattered quite randomly. 180 170 160 150 E re 140 s T3 re o 130 _ i *«* lou 120 3 Q. 110 100 90 80 J I I L + O J L J L o 40 C + 45 C • 50 C 0.1 0.2 0.3 0.4 0.5 0.6 0.7 U.V. Exposure Time (t/t«) 0.8 4 J L 0.9 1.0 Figure 5.5 Pullout Load vs U V Time for Different Presoak Temperatures The effect of pre-soaking solution time was not examined in detail. Soaking time for a given U V exposure time was difficult to vary independently, as the tube set-up did not lend itself to adjusting the soak region length. Figure 5.6 shows the three results obtained. Experiments used either no pre-soak tube, a single tube, or two in series. The data, although minimal, indicate a sharp rise in effect of pre-soaking time followed by no further improvement. -71-Considering that acrylic acid in pre-soak solution reduces the ILSS, it may be inferred that the increase is due to benzophenone absorbed into the fibre. Pappas [40] states that 90% of the U V radiation is absorbed within 3|im of the surface. This indicates that only limited penetration of benzophenone into the fibre is required. Weedon and Tarn [72] show that Spectra 1000 polyethylene fibres can withstand brief exposure to temperatures near 130"C with only minor property loss. The above suggest that the soak cycle may be further optimized into a high temperature, short duration exposure to photosensitizer solution. 5.0 4.5 -v> CO 4.0 CO 3.5 3.0 Pre-soak with Acrylic Acid Monomer • 15 A M P S + 30 A M P S J L J I ' ' 0.25 0.5 0.75 1:0 1.25 Presoaking Time (t/y 1.5 1.75 Figure 5.6 Effect of Soaking Time on ILSS. -72-5.2.6 DIP SOLUTION M O N O M E R CONCENTRATION Figure 5.7 shows the effect on ILSS of varying the dip solution monomer concentration. Fibre volume fractions of the test samples are also shown. Arrows have been added to indicate the direction of applied adjustment necessary to standardize the data at 56% by volume of fibres. CO CO 5.6 5.4 5.2 h 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 0.1 56% 48% 52% 49% 62% 58% thin panel 68% thin panel Numbers indicate fibre volume fraction. 0.2 0.3 Dip Solution Monomer (C/C e Acrylic Acid) 0.4 Figure 5.7 Effect of Dip Solution Monomer Concentration on the ILSS. ILSS values rise to a maximum of 5.2 ksi at about 0.3Co. A very sharp decrease is seen at high concentration and an apparent sharp decrease at low concentration. However, the panels labelled thin were tested at 5:1 span-to-depth ratio. The other panels were all tested at 4:1 -73-ratios. Lower span-to-depth ratios give higher ILSS results, hence the values for the thin panels should in reality be greater. The drop off at high concentration is also mis-leading, as this panel had a high, 62%, fibre volume fraction. The dip solution leaves a coating of monomer and photosensitizer on the fibres. This is performed immediately prior to exposure to U V radiation. It is hence a critical process step as it controls the amount and ratio of fibre surface materials. It appeared in earlier work that monomer consumption may have been a limiting factor in graft yield. The relatively constant maximum values seen in Figures 5.2, 5.3, and 5.6 are consistent with the ILSS values of Figure 5.7. It is suggested that repetitions of earlier tests with different dip solution concentrations would result in a series of curves. These should exhibit ILSS maxima corresponding to the values of Figure 5.7. -74-6 SUMMARY AND RECOMMENDATIONS Highly drawn polyethylene fibre has demonstrated some of the highest specific tensile properties of the family of advanced composite reinforcing fibres. Its organic structure and inert surface properties have resulted in excellent ballistic performance. However, full realization of the fibre's potential has been restricted due to poor adhesion to the common resins. The ability of polyethylene to compete with, and complement, the established materials, carbon, glass and aramids, depends on the development of an industrially feasible surface treatment. Poly ethylenes' main drawbacks are its compressive strength, creep resistance and thermal limitations. Historically, polyethylene surface modification work has served to improve the characteristics of film material. This has been mainly for consumer oriented products, where heat sealing, dyeing, and printing have been primary requirements. Based on work performed on films, several treatments applicable to the fibrous material were identified. These were plasma, corona and flame treatment, chromic acid treatment and surface modification by grafting. Increased surface energy and improved surface functionality are goals of all the treatments. Radiation induced grafting was identified as the potentially most beneficial treatment for polyethylene fibres. U V irradiation exhibited more of a surface effect than y radiation and was chosen for further study. This study concentrated on only continuous treatment as commercial feasibility is of key importance. The presented work has demonstrated several factors with respect to adhesion improvement of polyethylene fibres. Adhesion was measured with the ILSS test and the single fibre pullout test. Regarding mechanical testing, the necessity to standardize samples was quite clear. ILSS results were shown to depend on volume fraction of fibres and test span-to-depth ratio. Sample thickness appears to also have an effect, but this was not investigated in detail. -75-Pullout data was shown to require a low debond and maximum load as compared to the fibre strength. The pullout test also requires a resin block stabilized against deformation. Unfortunately, the resin block geometry, debond load, and ease of sample preparation are all related. In its current form, it appears to be a good test for plasma treated and untreated fibres, but is not applicable to effectively judge UV-graft fibre treatments. The tensile test is quite good for property determination and served its purpose well. A tab setup modified from the A S T M standard was necessary for good results. The flexural property test, even with limited data, was a powerful indication of the effect of adhesion improvement. The very high flexural strength attained, as compared to other polyethylene samples, indicates that the compressive properties have improved substantially. The UV-grafted flexural sample tended to fail in tension, whereas the plasma and untreated material failed in compression. This is a major milestone, as it indicates that the fibres' very high specific tensile properties can be developed. The effect of process variables on adhesion of the polyethylene fibres to epoxy matrix has indicated that the process is relatively insensitive to small deviations. Very good ILSS values as compared to plasma are obtained with any UV-graft treatment. The major improvements are seen with optimization of the pre-soak solution concentration in conjunction with the dip solution concentration. In general, it has appeared that adhesion improvements related to U V exposure rise to a maximum and then stabilize. Lowering the exposure energy seems quite possible, and system optimization concentrating on decreased radiation intensity and reduced exposure times appears prudent. The data also suggest a depletion of monomer in the irradiation stage. Possible improvements to adhesion may occur with addition of a monomer replenishing step. This may be in the form of repeated cycles of dip and irradiation or monomer may possibly be used in low concentration in vapor phase during the irradiation stage. Other improvements suggested include using a high temperature short duration -76-pre-soak, increasing the temperature of the dip solution, and finally, consideration of combining the three major process types, PRE, SIM and POST. Envisioned for optimum efficiency is a pre-dip in monomer/sensitizer solution, simultaneous exposure to U V radiation and monomer vapor in nitrogen, and a post exposure dip in monomer to utilize any remaining reactive sites. Test results have shown an improvement in ILSS from 1.5 ksi for untreated material, up to well over 5 ksi for the UV-grafted fibres. This represents the highest consistent ILSS determined for Spectra 1000 polyethylene fibres. Although treatment exposure times and radiation intensities are high, little tensile property decrease is shown for the typical operating conditions. Also, compression dominated behavior in flexure appeared to have been suppressed. Commercial feasibility will be enhanced by reducing the treatment residence times. Test results have indicated that this may be likely, and have formed a good basis on which to further optimize the polyethylene fibre UV-graft treatment. -77-REFERENCES 1 SAMPE J., 21, No. 3, 51, May/June 1985 2 Cappacio, G. and Ward, I .M., Nature Physical Science, 243. 143, (1973) 3 Lin, L .C. , Bhatnagar, A. , Lang, D.C., and Chang, H.W., 33rd Int'l SAMPE Symposium, 883, March 7-10, 1988 4 Handbook of Composites, ed. G. Lubin, 1982, Van Nostrand Reinhold 5 Drzal, L.T. and Rich, M.J. , Composite Materials Testing and Design, 5th Conf., ASTM STP 864, ed. J.R. Vinson and M . Taya, American Society for Testing and Materials 1985 6 Pipes, R.B. and Pagano, N.J., J. Comp. Mater., 4, 538, (1970) 7 Rosen, B.W., Fibre Composite Materials, Ch. 3, A S M 1964 8 Piggott, M.R. , and Harris, B. , / . Mater. Sci., 15, 2523, (1980) 9 Zweben, C , / . Comp. Mater., 12, 422, (1978) 10 Introduction to Composite Materials, Hull, D., Cambridge Univer. 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