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