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]