5.Processing of Advanced Thermoplastic Composites 5.1 Introduction The lack of experience in processing high performance thermoplastic composites contributes to the inertia in utilizing these new materials.The techniques to process thermoplastic composites are not as well established as those developed for thermoset composite materials due to the novelty of this emerging class of materials.But there is a growing interest in the aerospace industry in demonstrating the feasibility to produce high quality thermoplastic composite parts for structural applications.A variety of innovative processes are currently being researched and developed to surmount the important obstacles to easy processing of thermoplastics (the high melt viscosity.high processing temperature,and the lack of drape and tack of prepreg).In this chapter,the processing of continuous fibre reinforced composites with high performance thermoplastic matrices reported in the open literature are reviewed.The benefits and disadvantages of processing thermoplastic composites compared to thermoset composites are discussed.The basic processing steps including fibre treatment,combination of fibres with thermoplastic matrix and processing techniques to produce laminates and form shaped parts are presented along with an overview of the lay-up procedure.residual stresses,processing models available,machining and reprocessability.Forming techniques for thermoplastic composites have been addressed in a more detailed way in a review by Okine [1961.Techniques to combine fibres and thermoplastic matrix polymer.to fabricate laminates and to form parts have been reviewed in References 1.55.195 and 197.The capability to produce APC-2 composite parts from a variety of processing strategies which are currently being researched have been demonstrated in References 102 and 233. 5.2 Advantages Disadvantages The major benefits in processing thermoplastic composites compared to thermoset composites are the unlimited shelf life.the short processing time,and their ability to be remelted and reprocessed.These advantages make them particularly attractive from an economic point of view.For most thermoplastic composites,the shelf life is unlimited and the processing time is in term of minutes rather than hours as it is for thermoset composites because polymerization has been completed before the combination of fibres with matrix [195]. No time is required for this chemical reaction to occur during processing as in the case of a thermoset.Thermoplastics have also the ability to be processed at various heating and cooling rates due to the absence of the exotherm experienced in the case of thermosets [1l.This may be an important issue in field repair considering the extremes encountered in processing conditions.However.as discussed in Chapter 3.the control of the cooling rate in the case of semi-crystalline thermoplastics is very important in determining the morphological structure and mechanical properties of the final composite. 96
5. Processing of Advanced Thermoplastic Composites 5.1 Introduction The lack of experience in processing high performance thermoplastic composites contributes to the inertia in utilizing these new materials. The techniques to process thermoplastic composites are not as well established as those developed for thermoset composite materials due to the novelty of this emerging class of materials. But there is a growing interest in the aerospace industry in demonstrating the feasibility to produce high quality thermoplastic composite parts for structural applications. A variety of innovative processes are currently being researched and developed to surmount the important obstacles to easy processing of thermoplastics (the high melt viscosity. high processing temperature, and the lack of drape and tack of prepreg). In this chapter, the processing of continuous fibre reinforced composites with high performance thermoplastic matrices reported in the open literature are reviewed. The benefits and disadvantages of processing thermoplastic composites compared to thermosel composites are discussed. The basic processing steps including fibre treatment, combination of fibres with thermoplastic matrix and processing techniques to produce laminates and fomr shaped parts are presented along with an overview of the lay-up procedure, residual stresses. processing models available, machining and reprocessability. Forming techniques for thermoplastic composites have been addressed in a more detailed way in a review by Okine [ 1961. Techniques to combine fibres and thermoplastic matrix polymer, to fabricate laminates and to form parts have been reviewed in References 1.55.195 and 197. The capability to produce ARC-2 composite parts from a variety of processing strategies which are currently being researched have been demonstrated in References 102 and 233. 5.2 Advantages / Disadvantages The major benefits in processing thermoplastic composites compared to thermoset composites are the unlimited shelf life, the short processing time, and their ability to be remelted and reprocessed. These advantages make them particularly attractive from an economic point of view. For most thermoplastic composites, the shelf life is unlimited and the processing time is in term of minutes rather than hours as it is for thermoset composites because polymerization has been completed before the combination of fibres with matrix [195]. No time is required for this chemical reaction to occur during processing as in the case of a thermoset. Thermoplastics have also the ability to be processed at various heating and cooling rates due to the absence of the exotherm experienced in the case of thermosets [l]. This may be an important issue in field repair considering the extremes encountered in processing conditions. However, as discussed in Chapter 3. the control of the cooling rate in the case of semi-crystalline thermoplastics is very important in determining the morphological structure and mechanical properties of the final composite. 96
Processing of Advanced Thermoplastic Composites 97 The capability of thermoplastics to be remelted has led to the development of novel manufacturing technologies.Thermoplastic laminates showing voids or defects can be reconsolidated to eliminate these defects whereas a thermoset would be rejected.Excess or scrap material may be reused.Complex three-dimensional parts can be shaped or formed from a flat consolidated sheet.Composite parts can be thermally joined to form a composite assembly which eliminates the need for adhesive bonds and mechanical fasteners. The main drawbacks with processing advanced thermoplastics compared to processing thermosets are their high melt viscosity and high processing temperatures needed to melt them.The high Tg desired for advanced thermoplastic composites leads to high melt viscosity and high processing temperatures,often close to the decomposition temperatures.At such high temperatures,depending on the thermal stability of the polymer which may vary significantly from one polymer to an other,thermal and oxidative degradation and hydrolysis may occur 11951.In general,most organic linkages in high-performance thermoplastic polymers become thermally unstable about 450C [75].For this reason.minimizing the hold time at peak processing temperature and provision of an oxygen free environment are highly recommended.The high melt viscosity of thermoplastic renders the full impregnation of fibres by the polymer rather difficult.To demonstrate the importance of this problem. Cattanach,Guff and Cogswell (55]presented this example:"to make 10 cc of a totally wetted fibrous composite containing 50%by volume of fine(10 um)fibres it is necessary to spread 5cc of resin over 2 m2 of surface area".In the case of thermosets,it can be considered equivalent to spreading a sticky liquor over the surface of a table but in the case of thermoplastics,the sticky liquor is replaced by a material equivalent to two pieces of chewing gum that has to be spread over the same area.The difficulty is increased by the constraint of not physically damaging the fragile fibres.The lack of tack and drape of most thermoplastc prepregs is another drawback.It is difficult to lay-up prepreg plies against a contoured shape.To overcome the problems of high melt viscosity and the lack of tack and drape of prepregs.some innovative processes of combining fibres and thermoplastic polymers and producing high quality laminates have been developed recently:they are presented in this chapter. 5.3 Treatment of fibres For improved composite properties and water and chemical resistance,a good fibre- resin interfacial adhesion is very important.In the case of thermoplastic composites,it is a major concern that the interfacial adhesion to carbon fibres exhibited by the thermoplastic matrices such as polyphenylene sulfides,polyetherimides and polysulfones is less than that observed for epoxies [1].This might be an important factor contributing to the low compression and shear properties of thermoplastic composites.But it is not well understood why the resin interfacial adheslon to carbon fibres is lower with thermoplastic matrices and how it can be improved.According to Muzzy [195].the fact that most thermoplastics have been
Processing of Advanced Thermoplastic Composites 97 The capability of thermoplastics to be remelted has led to the development of novel manufacturing technologies. Thermoplastic laminates showing voids or defects can be reconsolidated to eliminate these defects whereas a thermoset would be rejected. Excess or scrap material may be reused. Complex three-dimensional parts can be shaped or formed from a flat consolidated sheet. Composite parts can be thermally joined to form a composite assembly which eliminates the need for adhesive bonds and mechanical fasteners. The main drawbacks with processing advanced thermoplastics compared to processing thermosets are their high melt viscosity and high processing temperatures needed to melt them. The high Tg desired for advanced thermoplastic composites leads to high melt viscosity and high processing temperatures, often close to the decomposition temperatures. At such high temperatures, depending on the thermal stability of the polymer which may vary significantly from one polymer to an other, thermal and oxidative degradation and hydrolysis may occur [ 1951. In general, most organic linkages in high-performance thermoplastic polymers become thermally unstable about 450” C [75]. For this reason, minimizing the hold time at peak processing temperature and provision of an oxygen free environment are highly recommended. The high melt viscosity of thermoplastic renders the full impregnation of fibres by the polymer rather difficult. To demonstrate the importance of this problem, Cattanach, Guff and Cogswell [55] presented this example: “to make 10 cc of a totally wetted fibrous composite containing 50% by volume of fine (10 pm] fibres it is necessary to spread 5cc of resin over 2 ms of surface area”. In the case of thermosets, it can be considered equivalent to spreading a sticky liquor over the surface of a table but in the case of thermoplastics, the sticky liquor is replaced by a material equivalent to two pieces of chewing gum that has to be spread over the same area. The difficulty is Increased by the constraint of not physically damaging the fragile fibres. The lack of tack and drape of most thermoplastic prepregs is another drawback. It is difficult to lay-up prepreg plies against a contoured shape. To overcome the problems of high melt viscosity and the lack of tack and drape of prepregs, some innovative processes of combining fibres and thermoplastic polymers and producing high quality laminates have been developed recently: they are presented in this chapter. 5.3 Treatment of flbres For improved composite properties and water and chemical resistance, a good fibreresin interfacial adhesion is very important. In the case of thermoplastic composites, it is a major concern that the interfacial adhesion to carbon ilbres exhibited by the thermoplastic matrices such as polyphenylene sulfides, polyetherimides and polysulfones is less than that observed for epoxies [ 11. This might be an important factor contributing to the low compression and shear properties of thermoplastic composites. But it is not well understood why the resin interfacial adhesion to carbon fibres is lower with thermoplastic matrices and how it can be improved. According to Musay [195]. the fact that most thermoplastics have been
98 High Performance Thermoplastic Resins and Their Composites polymerized before having been combined with fibres and that they are relatively inert renders the achievement of a good adhesion between the matrix and the fibres generally difficult.In contrast.Leeser and Banister [77]affirm that most thermoplastics show an affinity to carbon resulting in good fibre-matrix interfaces without the addition of a coupling agent.However,for weaving operations,sizing is requtred for protecting the fibres during the process;usually a small amount of a matrix polymer is applied to the fibre prior to weaving.These authors [77] also affirm that with glass fibres.a coupling agent is needed since most thermoplastics do not adhere well to glass due to the inert nature of the glass surface. Fibre treatment is a means to promote and enhance adhesion.The extensive work reported in the literature dealing with fibre treatment is almost exclusively on thermosets.In addition,most fibre treatment processes for thermoplastic composites are proprietary [1951. The choice of the proper fibre treatment is complex.It depends both on the type of fibre and on the nature of the thermoplastic involved [1].It may include cleaning,etching and oxidizing of the fibres to provide reactive sites for adequate bonding to the sizing and the application of the sizing itself [1951.These operations are often accomplished at the same time as the fibre or prepreg formation in order to reduce the handling of the fibres.If a sizing has to be applied.it must be non-volatile,easy to apply,compatible with the matrix and thermally stable.A fibre treatment tailored for thermoset composites may not be suitable or optimized for thermoplastic composites [1].The possible degradation of the sizing at the high processing temperature of high performance reinforced thermoplastics is also an important issue.At temperatures close to 400C.none of the epoxy sizings will resist degradation. Turner and Cogswell [169]have evaluated the mechanical properties of PEEK based composites with a variety of fibres and have explored the varying interfacial properties that result from the differing fibre types.The fbres included E,R and S glass fbres,aramid fibres (Kevlar).high strength (HS),high modulus(HM),intermediate modulus (IM)and ultra high modulus (UHM)carbon fibres.Mechanical properties with the R glass and aramid fibres were particularly low.It is believed that in the case of R glass fibre,the manufacturer's size may have degraded while in the case of aramid fibres,degradation of the fibres may have occurred due to the high processing temperature which is close to the decomposition temperature of Kevlar. 5.4 Combination of Fibres with Matrix There are several techniques reported in the literature for combining fibres with thermoplastic matrix [1.8.55.71,77,98,195.197-204].They include hot melt coating. solution processing,in-situ polymerization of monomers or pre-polymers,film stacking. powder coating and fibre hybrldization.Some of these are well established since they are employed with thermosets while others have been recently developed especially to overcome the difficulty of fibre impregnation due to the high melt viscosity of the matrix.Depending on
98 High Performance Thermoplastic Resins and Their Composites polymerized before having been combined with fibres and that they are relatively inert renders the achievement of a good adhesion between the matrix and the fibres generally difficult. In contrast, Leeser and Banister 1771 affirm that most thermoplastics show an affinity to carbon resulting in good fibre-matrix interfaces without the addition of a coupling agent. However, for weaving operations, sizing is required for protecting the fibres during the process: usually a small amount of a matrix polymer is applied to the fibre prior to weaving. These authors [77] also aifirm that with glass fibres, a coupling agent is needed since most thermoplastics do not adhere well to glass due to the inert nature of the glass surface. Fibre treatment is a means to promote and enhance adhesion. The extensive work reported in the literature dealing with fibre treatment is almost exclusively on thermosets. In addition, most iibre treatment processes for thermoplastic composites are proprietary [ 1951. The choice of the proper fibre treatment is complex. It depends both on the type of fibre and on the nature of the thermoplastic involved [ 11. It may include cleaning, etching and oxidizing of the fibres to provide reactive sites for adequate bonding to the sizing and the application of the sizing itself [195]. These operations are often accomplished at the same time as the fibre or prepreg formation in order to reduce the handling of the fibres. If a sizing has to be applied, it must be non-volatile, easy to apply, compatible with the matrix and thermally stable. A fibre treatment tailored for thermoset composites may not be suitable or optimized for thermoplastic composites [ 11. The possible degradation of the sizing at the high processing temperature of high performance reinforced thermoplastics is also an important issue. At temperatures close to 400’ C, none of the epoxy sizings will resist degradation. Turner and Cogswell [ 1691 have evaluated the mechanical properties of PEEK based composites with a variety of fibres and have explored the varying interfacial properties that result from the differing fibre types. The fibres included E, R and S glass fibres, aramid fibres (Kevlar). high strength (HS). high modulus (HM), intermediate modulus (IM) and ultra high modulus (UHM) carbon fibres. Mechanical properties with the R glass and aramid fibres were particularly low. It is believed that in the case of R glass fibre, the manufacturer’s size may have degraded while in the case of aramid fibres, degradation of the fibres may have occurred due to the high processing temperature which is close to the decomposition temperature of Kevlar. 5.4 Combination of Fibres with Matrix There are several techniques reported in the literature for combining tlbres with thermoplastic matrix (1, 8, 55, 71, 77, 98, 195, 197 - 2‘041. They include hot melt coating, solution processing, in-situ polymerization of monomers or pre-polymers, film stacking, powder coating and ilbre hybridization. Some of these are well established since they are employed with thermosets while others have been recently developed especially to overcome the difficulty of fibre impregnation due to the high melt viscosity of the matrix. Depending on
Processing of Advanced Thermoplastic Composites 99 the combining process,impregnation may be accomplished during this step of combining fibres and matrix,or later during "in-situ"fabrication of laminates and parts.Based on APC-2 samples fabricated from pre-impregnated products and from products relying on a post impregnation taking place during laminate consolidation.Cogswell [197]showed that pre- impregnated strategies available today lead to better mechanical performance (see Table 29). He attributed this difference to the "fibre attrition during the post moulding impregnation stage where the forces necessary to squeeze the resin into spaces between the fbres also force the fibres together;by contrast in preimpregnated products each fibre is lubricated with a protective coating of viscous polymer".Techniques for combining fibres and matrix are described below. 5.4.1 Hot Melt Coating Hot melt coating [1.55,195,198]is probably the process that is the most commonly employed to combine fibres and matrix.Figure 41 shows a melt extrusion process used by Chung [16]but there are many possible variations in this process (1981.Initially.fibre tows are unwound from a spool or a creel of spools without twisting and they may go through a comb to achieve a better collimation.Fibres are then spread by a roller or air jet to expose as many fibres as possible to the polymer and reduce the gaps in the prepreg material.The fibres are fed into a die where the molten polymer is either added or furnished as a resin coating on release paper which is later removed.At this stage.pressure is applied on the melt in order to coat the individual fibres and not only the fibre bundles.Considerable pressure may be needed to acheive a total impregnation of the fibres (195].At the die exit,the hot tape is cooled and rolled up. No solvents are needed in this process.In the case of semi-crystalline polymers such as PEEK and PPS for which there are no known solvents in which to prepreg.hot melt coating is currently the impregnation method of choice [1.8].The wet out is generally excellent with a low void content but the prepreg obtained is stilf.boardy and tackless.The hot melt coating method is not appropriate for thermoplastic polymers possessing a very high melt viscosity. There is some danger of thermally degrading the polymer when heating it to lower its viscosity. 5.4.2 Solution Processing Solution processing [1.55.77.1951 is very well established for the thermoset polymers. The technique consists of dissolving the resin in a suitable solvent and wetting the fibres with the solution.As it reduces the viscosity of the thermoplastic polymers,full impregnation of fibres becomes much easier.The complete devolatilization of the prepreg will result in a tackless and boardy thermoplastic tape or fabric.If some of the solvent is left,a certain degree of tack and drape can be obtained.The drawbacks associated with this technique are two fold [8,55,77].First,if the prepreg is not devolatlized,the prepreg must be handled by conventional
Processing of Advanced Thermoplastic Composites 99 the combining process, Impregnation may be accomplished during this step of combining fibres and matrix, or later during “in-situ” fabrication of laminates and parts. Based on APC-2 samples fabricated from pre-impregnated products and from products relying on a post impregnation taking place during laminate consolidation, Cogswell [ 1971 showed that preimpregnated strategies available today lead to belter mechanical performance (see Table 29). He attributed this difference to the “Mbre attrition during the post moulding impregnation stage where the forces necessary to squeeze the resin into spaces between the fibres also force the fibres together; by contrast in preimpregnated products each fibre is lubricated with a protective coating of viscous polymer”. Techniques for combining fibres and matrix are described below. 5.4.1 Hot Melt Coating Hot melt coating [ 1, 55, 195. 1981 is probably the process that is the most commonly employed to combine fibres and matrix. Figure 4 1 shows a melt extrusion process used by Chung [16] but there are many possible variations in this process [198]. Initially. fibre tows are unwound from a spool or a creel of spools without twisting and they may go through a comb to achieve a better collimation. Fibres are then spread by a roller or air jet to expose as many fibres as possible to the polymer and reduce the gaps in the prepreg material. The fibres are fed into a die where the molten polymer is eilher added or furnished as a resin coating on release paper which is later removed. At this stage, pressure is applied on the melt in order to coat the individual fibres and not only the fibre bundles. Considerable pressure may be needed to acheive a total impregnation of the fibres [ 1951. At the die exit, the hot tape is cooled and rolled up. No solvents are needed in lhis process. In the case of semi-crystalline polymers such as PEEK and PPS for which there are no known solvents in which to prepreg. hot melt coating is currently the impregnation method of choice [ 1,8]. The wet out is generally excellent with a low void content but the prepreg obtained is stiff, boardy and tackless. The hot melt coating method is not appropriate for thermoplastic polymers possessing a very high melt viscosity. There is some danger of thermally degrading the polymer when heating it to lower its viscosity. 5.4.2 Solution Processing Solution processing [ 1.55.77.1951 is very well established for the thermoset polymers. The technique consists of dissolving the resin in a suitable solvent and wetting the fibres with the solution. As it reduces the viscosity of the thermoplastic polymers, full impregnation of fibres becomes much easier. The complete devolatilization of the prepreg will result in a tackless and boardy thermoplastic tape or fabric. If some of the solvent is left, a certain degree of tack and drape can be obtained. The drawbacks associated with this technique are two fold I8.55.771. First, if the prepreg is not devolatilized, the prepreg must be handled by conventional
100 High Performance Thermoplastic Resins and Their Composites TABLE 29.Properties of APC-2 as a Function of Impregnation Route [197] Axial Short Impact fexural beam energy strength shear 2mm strength sheet MN/m2 MN/m2 J Preimpregnated Products Cross Plied Uniaxial 907 6 Woven Single Tow 929 6 Impregnated Woven Fabric 1052 80 29 Impregnation After Shaping Cowoven Fibres 782 0 1 Film Stacked 680 67 9 Powder Coated Fabric 545 4 Thermocouple Pressure Transducer Spool Pressure Gauge Speed Controller 0 Alr Quench Device 0 Take-Up Roll Roll Alr Banding Jet Tapo Dle Molten Polymer Inlet From Extruder Balance Bars Tenslon Pin FIGURE 41.Hot Melt Coating Process [195] E 400 Linear Fit to Data 375 Kq (61) 350 325 300 275 G 250 0 2 3 Residual Volatiles in Laminate (% FIGURE 42.Tg as a Function of Residual Volatlles In Laminate [77]
100 High Performance Thermoplastic Resins and Their Composites TABLE 29. Properties of APC-2 as a Function of Impregnation Route [197] Preimpregnated Products Cross Plied Uniaxial Woven Single Tow Impregnated Woven Fabric Impregnation Afier Shaping Axial flexural strength MN/m’ 907 929 1052 Sll0l-t Impact beam energy shear 2mm strength sheet MN/m* J 76 23 68 23 80 29 Cowoven Fibres 782 60 13 Film Stacked 680 67 9 Powder Coated Fabric 545 54 Thermocouple Take-Up Roll Molten Polymer Inlet From Extruder Balance Bars Tenslon Pln FIGURE 41. Hot Melt Coating Process [195] Linear Fii to Data 350 - 325 - 300 - 275 - 1 2 3 Residual Volatiles in Laminate (%) FIGURE 42. Tg as a Function of Residual Volatiles In Laminate [77J
Processing of Advanced Thermoplastic Composites 101 thermoset technology where the solvent is removed later on during the curing process.There may be difficulty in removing all of the solvent from the thermoplastic product.The incomplete removal of solvent may be detrimental to the composite part.Figure 42 shows that an increase of the residual volatiles in the laminate may result in a decrease of the Tg [77].In addition,volatiles released at high temperature increase the risk of void and blisters formation in the composites.Secondly,the thermoplastic has to possess sufficient solubility in organic solvents,hence this method is often used with amorphous polymers [1.8].Prepregs made of Torlon polyamideimide.Ultem polyetherimide and Udel polysulfone are produced by this technique.Usually thermoplastic polymers exhibit limited solubility at high concentration and most high performance thermoplastics cannot be dissolved in low boiling point solvents at room temperature (71.195].However,if the thermoplastic polymer is readily soluble in those solvents.it might be attacked by some solvents later as a composite part in service applications. 5.4.3 In-Situ Polymerization of Monomers or Prepolymers This technique consists of the impregnation of the fibres with monomers or pre- polymers in solution followed by in-situ polymerisation [1,8,55,195].The solvent left in the prepreg confers good tack and drape on the material.During storage,the solvent may evaporate and transform the tacky prepreg into a tackless and boardy material.But solvents may be sprayed over a boardy prepreg to give back its handleability quality.In-situ polymerization is suitable for a limited range of polymers.Certain polyimides called thermnosetting thermnoplastic or pseudothermoplastic(described in Chapter 2)can be processed by this approach.They are produced essentially like a thermoset,they undergo some chemical reaction during the processing cycle,but they possess thermoplastic properties.Dupont's Avimid K and Avimid N polyimides are examples of materials produced by this method [1,1951. The reaction of monomers in solution proceeds with the production of water and must be removed with the solvent during autoclave processing.Pressures and heat-up rates are essentially the same as those employed with epoxy,but the processing temperatures are much higher(343C for Avimid K versus 177C for epoxy).Care must be taken so that the evolution of volatiles will not contribute to formation of voids [159].A major disadvantage of this technique is that curing and post-curing is long,even when compared to crosslinkable thermosets.Consequently the advantage of the fast processing of a thermoplastic is lost. 5.4.4 Film Stacking Film stacking consists of interleaving layers of reinforcement fibres in the form of tape or fabric with layers of thermoplastic polymer flms or powder [1,55.197].Wetting of the fibres is achieved during the consolidation process.To obtain a high quality laminate (low vold and good impregnation of the fibres).the stack has to be consolldated under severe conditions(high pressure and temperature,and /or a protracted molding cycle)
Processing of Advanced Thermoplastic Composites 101 thermoset technology where the solvent is removed later on during the curing process. There may be difficulty in removing all of the solvent from the thermoplastic product. The incomplete removal of solvent may be detrimental to the composite part. Figure 42 shows that an increase of the residual volatiles in the laminate may result in a decrease of the Tg 1771. In addition, volatiles released at high temperature increase the risk of void and blisters formation in the composites. Secondly, the thermoplastic has to possess sufficient solubility in organic solvents, hence this method is often used with amorphous polymers [ 1.81. Prepregs made of Torlon polyamideimide. Ultem polyetherimide and Udel polysulfone are produced by this technique. Usually thermoplastic polymers exhibit limited solubility at high concentration and most high performance thermoplastics cannot be dissolved in low boiling point solvents at room temperature (7 1.1951. However, if the thermoplastic polymer is readily soluble in those solvents, it might be attacked by some solvents later as a composite part in service applications. 5.43 In-Situ Polymerization of Monomers or Prepolymers This technique consists of the impregnation of the fibres with monomers or prepolymers in solution followed by in-situ polymerisation [1,8,55,195]. The solvent left in the prepreg confers good tack and drape on the material. During storage, the solvent may evaporate and transform the tacky prepreg into a tackless and boardy material. But solvents may be sprayed over a boardy prepreg to give back its handleability quality. In-situ polymerization is suitable for a limited range of polymers. Certain polyimides called thermosetting thermoplastic or pseudothermoplastic (described in Chapter 2) can be processed by this approach. They are produced essentially like a thermoset. they undergo some chemical reaction during the processing cycle, but they possess thermoplastic properties. Dupont’s Avimid K and Avimid N polyimides are examples of materials produced by this method [1.195]. The reaction of monomers in solution proceeds with the production of water and must be removed with the solvent during autoclave processing. Pressures and heat-up rates are essentially the same as those employed with epoxy, but the processing temperatures are much higher (343’ C for Avimid K versus 177” C for epoxy). Care must be taken so that the evolution of volatiles will not contribute to formation of voids [ 1591. A major disadvantage of this technique is that curing and post-curing is long, even when compared to crosslinkable thermosets. Consequently the advantage of the fast processing of a thermoplastic is lost. 5.4.4 Film Stacking Film stacking consists of interleaving layers of reinforcement fibres in the form of tape or fabric with layers of thermoplastic polymer films or powder ]1,55.197]. Wetting of the fibres is achieved during the consolidation process. To obtain a high quality laminate (low void and good impregnation of the fibres), the stack has to be consolidated under severe conditions (high pressure and temperature, and /or a protracted molding cycle)
102 High Performance Thermoplastic Resins and Their Composites 5.4.5 Powder Coating Powder coating is an attractive continuous process that overcomes the difficulty of working with thermoplastics with high melt viscosities and poor solubility [1,8,55,71.195. 1991.Figure 43 illustrates the powder coating process being developed at Georgia Institute of Technology [71.195,199).Polymer in the form of fine powder (generally 2 to 50 um in diameter [1],but cven 90,110 and 240 um [199])is charged and fluidized.The powder is electrostatically deposited on the fibres passing through a fluidized bed.The fibres are ground to promote powder pick-up.Liquid suspension is a variant of air fluidization (55].The coated fibres exdting from the fluidized bed can be rolled-up immediately [205]or they can be fed into an oven where the polymer melts on the fibres [71,195,199]before it is cooled and rolled up.The resulting towpreg possesses good drape and sometimes good tack if a tackifier is used.If the coated fibres do not go through the heating and melting process,less severe stresses are imposed on the fibres but care has to be taken not to remove powder from the fibres during further handling.To prevent powder removal.water may be sprayed over the prepreg prior to handling or laying-up [2051. One of the main concerns with powder technology is to obtain a uniform distribution of powder around the fibres [8]but,in general a very high degree of impregnation can be acheived 155.195.1991.The powder coating process has demonstrated the capability to produce high quality fibre reinforced prepreg from a wide variety of thermoplastic powders with no evolution of solvent or by-products [8.199).AS4 carbon fibres combined by this process with PEEK have led to laminates with mechanical properties equivalent to laminates made from commerclal APC-2 prepreg tapes [1991.Powder coating can be used with a wide variety of polymers:they need to have the capability to be ground into fine powder [55]and most can be but not all [205].The cost associated with the preparation of a very fine powder from a tough thermoplastic is high [1]but an economic analysis has shown that the powder coating process is economically attractive [199].Major developments are expected to emerge in this new area of powder coating which would increase the choice of matrix materials.Weaving,pultrusion and filament winding are fabrication techniques being considered with powder coated fibres. 5.4.6 Flbre Hybridization Another approach to combine fibres with thermoplastic matrix material which is undergoing considerable development is fibre hybridization [1,98,125.127,200-204].As with the powder coating process.the hybrid technology is particularly attractive when using polymers having high melt viscosity and poor solubility.The process consists of combining a yarn of fibre reinforcement with yarn spun from a thermoplastic material.The combination can be done by commingling.serving or co-weaving (Figure 44)[200.201].In commingling.the reinforcing fibres and thermoplastic fibres are intimately mixed at the individual fibre level
102 High Performance Thermoplastic Resins and Their Composites 5.4.5 Powder Coating Powder coating is an attractive continuous process that overcomes the diificulty of working with thermoplastics with high melt viscosities and poor solubility [ 1, 8, 55, 71, 195. 1991. Figure 43 illustrates the powder coating process being developed at Georgia Institute of Technology [71. 195. 1991. Polymer in the form of fine powder (generally 2 to 50 um in diameter [l], but even 90. 110 and 240 l.un [lQQ]) is charged and fluidized. The powder is electrostatically deposited on the fibres passing through a fluidized bed. The fibres are ground to promote powder pick-up. Liquid suspension is a variant of air fluidization 1551. The coated fibres exiting from the fluidized bed can be rolled-up immediately [205] or they can be fed into an oven where the polymer melts on the fibres 171, 195, 1991 before it is cooled and rolled up. The resulting towpreg possesses good drape and sometimes good tack if a tacklfier is used. If the coated fibres do not go through the heating and melting process, less severe stresses are imposed on the iibres but care has to be taken not to remove powder from the fibres during further handling. To prevent powder removal, water may be sprayed over the prepreg prior to handling or laying-up [205]. One of the main concerns with powder technology is to obtain a uniform distribution of powder around the fibres [8] but, in general a very high degree of impregnation can be acheived 155. 195. 1991. The powder coating process has demonstrated the capability to produce high quality ilbre reinforced prepreg from a wide variety of thermoplastic powders with no evolution of solvent or by-products (8. 1991. AS4 carbon fibres combined by this process with PEEK have led to laminates with mechanical properties equivalent to laminates made from commercial APC-2 prepreg tapes [ 1991. Powder coating can be used with a wide variety of polymers; they need to have the capability to be ground into fine powder [55] and most can be but not all [205]. The cost associated with the preparation of a very fine powder from a tough thermoplastic is high (11 but an economic analysis has shown that the powder coating process is economically attractive [ 1991. Major developments are expected to emerge in this new area of powder coating which would Increase the choice of matrix materials. Weaving, puhrusion and filament winding are fabrication techniques being considered with powder coated fibres. 5.4.6 Fibre Hybridization Another approach to combine fibres with thermoplastic matrix material which is undergoing considerable development is fibre hybridization [l. 98. 125, 127, 200 - 2041. As with the powder coating process, the hybrid technology is particularly attractive when using polymers having high melt viscosity and poor solubility. The process consists of combining a yam of fibre reinforcement with yam spun from a thermoplastic material. The combination can be done by commingling, serving or co-weaving (Figure 44) [200.201]. In commingling, the reinforcing Ilbres and thermoplastic fibres are intimately mixed at the individual fibre level
☐TO VACUUM BAG SS111111111111183119 VE118108808131881848 TOW SPREADER LET-OFF OVEN TAKE-UP POROUS PLATE IONIZED AIR CHARGING MEDIUM DHY-AIH INPUT FLUIDIZED BED FIGURE 43.Electrostatic Fluidized Bed Powder Coating Process [199] COWOVEN PLIED MATRIX COMMINGLED THERMOPLASTIC THERMOPLASTIC THERMOPLASTIC MATRIX MATRIX MATRIX Processing of Advanced Thermoplastic Composites GRAPHITE FIBER GRAPHITE FIBER GRAPHITE FIBER REINFORCEMENT REINFORCEMENT REINFORCEMENT FIGURE 44.Hybrid Yarn Forms [201]
, F-;-TO VACUUM BAG o- LET-OFF TAKE-UP IONIZED AIR POROUS PLATE CHARGING MEDIUM DRY-AIR INPUTFLUIDIZED BED FIGURE 43. Electrostatic Fluidized Bed Powder Coating Process r1w COMMINGLED THERMOPLASTIC MATRIX GRAPHITE FIBER =- REINFORCEMENT THERMOPLASTIC MATRIX __-- mB __- GRAPHITE FIBER REINFORCEMENT FIGURE 44. Hybrid Yarn Forms [201] THERMOPLASTIC MATRIX - GRAPHITE FIBER REINFORCEMENT
104 High Performance Thermoplastic Resins and Their Composites Serving refers to the wrapping of the reinforcement fibres with the thermoplastic fibres. Weaving of bundles of continuous thermoplastic filaments and reinforcement filaments existing as separate yarns result in a "co-woven fabric"whereas weaving commingled hybrid yarns refers to a "commingled woven fabric".In an experimental study.Silverman and Jones [98]found that commingled woven fabric composites of carbon/PEEK generally exhibited higher physical and mechanical properties than co-woven fabric carbon/PEEK composites due to a better fibre/matrix distribution and adhesion.An improvcd blcnding of thc carbon fbrcs with the matrix is achieved in the case of commingled woven fabric composites. Hybrid yarns can be woven into a wide variety of highly conformable and drapeable fabrics.Three-dimensional fabrics have been recently fabricated with commingled yarns, either by stitching layers of fabrics or as a fully integrated structure [125.127.200.204].Flgure 45 shows a three dimensional fibre architecture which is a fully integrated structure. Preliminary experimental results have shown that 3-D woven fabrics lead to composites possessing better damage tolerance and delamination resistance than composites fabricated from 2D woven fabrics or from prepreg tapc (200,2041.rigurc 4G prcocnto the comprcoolon strength after impact obtained by Hua and Ko [204]for APC-2 laminated composites and PEEK/carbon commingled 3D-braid.150 G PEEK/carbon commingled 3-D braid exhibited higher compression strength than APC-2 laminates for the three impact energy levels. However,the results obtained in Reference 207 and presented in Tables 30a and 30b show that 3. D fabrics exhibit lower t45 tensile strength,compressive strength and fracture strain than 2- D fabric and prepreg tape. Woven structures can be designed to conform to very complex contours without preheating and without yarn separation during consolidation [201].Wetting of the fibres is deferred until the consolidation process.Under heat and pressure.the thermoplastic yarn melts,wetting the reinforcing fbres.To ensure a good wetting of the dense fbre network, longer processing time and/or higher temperature and pressure are required for commingled woven fabrics compared to unidirectional thermoplastic prepregs. Presently,commingled yarns of carbon with either PEEK,PEK,PPS [203]and PEI[163] are proaucea commercialry.in general,tne mecnanical properuies optainea irom thermoplastic laminates manufactured from both co-woven and commingled woven fabrics are lower than those obtained with thermoplastic laminates fabricated from prepreg tapes [98, 207].This characteristic is corroborated by the data in References 98 and 207 generated during studies on carbon/PEEK laminates made from unidirectional prepreg tape and fabric and are shown in Tables 30 and 31.The reduction in mechanical properties is attributed to fibre breakage during the weaving process,poor fibre/matrix distribution and in some cases poor fibre/matrix adhesion.In addition,the greater freedom of the fabric fibres to move results in fibre kinks,bends and misalignment before and during consolidation.Conslderable development of this novel technology is anticipated such as improvements in mechanical
104 High Performance Thermoplastic Resins and Their Composites Serving refers to the wrapping of the reinforcement fibres with the thermoplastic fibres. Weaving of bundles of continuous thermoplastic filaments and reinforcement filaments existing as separate yarns result in a “co-woven fabric” whereas weaving commingled hybrid yarns refers to a “commingled woven fabric”. In an experimental study, Silverman and Jones [98] found that commingled woven fabric composites of carbon/PEEK generally exhibited higher physical and mechanical properties than co-woven fabrie carbon/PEEK composites due to a better fibre/matrix distribution and adhesion. An improved blending of the carbon fibres with the matrix is achieved in the case of commingled woven fabric composites. Hybrid yarns can be woven into a wide variety of highly conformable and drapeable fabrics. Three-dimensional fabrics have been recently fabricated with commingled yams, either by stitching layers of fabrics or as a fully integrated structure [125, 127. 200, 2041. Figure 45 shows a three dimensional fibre architecture which is a fully integrated structure. Preliminary experimental results have shown that 3-D woven fabrics lead to composites possessing better damage tolerance and delamination resistance than composites fabricated from 2D woven fabrics or from prepreg tape [200,204]. Figure 46 presents the compression strength after impact obtained by Hua and Ko [204] for APC-2 laminated composites and PEEK/carbon commingled 3D-braid. 150 G PEEK/carbon commingled 3-D braid exhibited higher compression strength than APC-2 laminates for the three impact energy levels. However. the results obtained in Reference 207 and presented in Tables 30a and 30b show that 3- D fabrics exhibit lower i45’ tensile strength, compressive strength and fracture strain than 2- D fabric and prepreg tape. Woven structures can be designed to conform to very complex contours without preheating and without yam separation during consolidation [201]. Wetting of the flbres is deferred until the consolidation process. Under heat and pressure, the thermoplastic yam melts, wetting the reinforcing fibres. To ensure a good wetting of the dense fibre network, longer processing time and/or higher temperature and pressure are required for commingled woven fabrics compared to unidirectional thermoplastic prepregs. Presently, commingled yarns of carbon with either PEEK, PEK, PPS ]203] and PEI [ 1631 are produced commercially. In general, the mechanical properties obtained from thermoplastic laminates manufactured from both co-woven and commingled woven fabrics are lower than those obtained with thermoplastic laminates fabricated from prepreg tapes 198, 2071. This characteristic is corroborated by the data in References 98 and 207 generated during studies on carbon/PEEK laminates made from unidirectional prepreg tape and fabric and are shown in Tables 30 and 3 1. The reduction in mechanical properties is attributed to fibre breakage during the weaving process, poor fibre/matrix distribution and in some cases poor flbre/matrix adhesion. In addition. the greater freedom of the fabric fibres to move results in fibre kinks, bends and misalignment before and during consolidation. Considerable development of this novel technology is anticipated such as improvements in mechanical
Processing of Advanced Thermoplastic Composites 105 FIGURE 45.3D Braid Fabric [204] 60 57.15 APC-2 380G PEEK/Carbon Commingled 3D Brald (s) 50 150G PEEK/Carbon Commingled 3D Braid 41.15 40 39.10 34.83 35.90 34.32 30 30.67 30.71 23.40 20 10 400 In-Ib 500 in-lb 600 In-lb Impact Energy Level FIGURE 46.CAI Strength Comparison of APC-2,380G PEEK/Carbon Oomminglod OD Drald and 130a PEEK/CaILu Cvlllyleu D Dralu [204]
Processing of Advanced Thermoplastic Composites 105 60 FIGURE 45. 3D Braid Fabric [204] -I t.8: - 67.15 406 in-lb l-l APC-2 I::r, 3806 PEEK/Carbon Commingled 3D Braid 160G PEEK/Carbon Commingled 3D Braid 600 in-lb 600 in-lb Impact Energy Level FIGURE 46. CAI Strength Comparison of APC-2,380G PEEK/Carbon Commingled 3D Braid and 150G PEEK/Carbon Commingled 3D Braid [204]