Copyrighted Materials Copyright 2009 DEStech Publications Retrieved from www.knovel.com CHAPTER 8 Long Fiber Thermoplastic Matrix Composites 1. INTRODUCTION The techniques of autoclave, hand laminating, filament winding, pultrusion and liquid composite molding have been developed mainly using thermoset matrix composites. This does not mean that these tech- niques cannot be used for thermoplastic composites. An autoclave can be used to process thermoplastic composites provided that the temperature and pressure are high enough. Filament winding (or rather fiber place- ment process) can also be used and pultruded thermoplastic composites parts have also been made. Liquid composite molding can also be used provided that the viscosity of the resin is small enough. Recent develop- ment of low viscosity thermoplastic composites [1] facilitates this pro- cess. The reason for a special chapter dedicated to discussion of thermoplas- tic composites is due to the high viscosity of the thermoplastic resin. This high viscosity demands the use of high temperature and high pressure to enable the resin to flow to the surface of the fibers, to wet and bond with them. This significance can be illustrated via the use of Darcy's law as: K dp (8.1) u dx For the same fiber network, the permeability is the same, whether thermoset matrix or thermoplastic matrix is used. However, the viscosity of thermoset resin is about 500 cP at 20C (Table 2.1, Chapter 2), whereas that of thermoplastic resin such as PEEK is 1,000,000 cp at 400C. If the distance of flow dx is the same in both cases, then the pres- 289
CHAPTER 8 1. INTRODUCTION The techniques of autoclave, hand laminating, filament winding, pultrusion and liquid composite molding have been developed mainly using thermoset matrix composites. This does not mean that these techniques cannot be used for thermoplastic composites. An autoclave can be used to process thermoplastic composites provided that the temperature and pressure are high enough. Filament winding (or rather fiber placement process) can also be used and pultruded thermoplastic composites parts have also been made. Liquid composite molding can also be used provided that the viscosity of the resin is small enough. Recent development of low viscosity thermoplastic composites [1] facilitates this process. The reason for a special chapter dedicated to discussion of thermoplastic composites is due to the high viscosity of the thermoplastic resin. This high viscosity demands the use of high temperature and high pressure to enable the resin to flow to the surface of the fibers, to wet and bond with them. This significance can be illustrated via the use of Darcy’s law as: u K dp dx = µ (8.1) For the same fiber network, the permeability is the same, whether thermoset matrix or thermoplastic matrix is used. However, the viscosity of thermoset resin is about 500 cP at 20°C (Table 2.1, Chapter 2), whereas that of thermoplastic resin such as PEEK is 1,000,000 cp at 400°C. If the distance of flow dx is the same in both cases, then the pres- 289
290 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES sure required for processing PEEK at 400C would be 2000 times more than that of the case of epoxy at 20C.If a pressure of 689 kPa is required for the processing of epoxy at 20C,then it would take a pressure of 1378 MPa to process PEEK at 400C for the same processing time.This is not practical. In the face of the above difficulties,it is desirable to find ways to manu- facture thermoplastic matrix composites because they can offer many ad- vantages,such as the following. a.Thermoplastic resin does not have the constraint of shelf life.To put it another way,thermoplastic composite preforms have infinite shelf life.This is because thermoplastic matrix solidifies upon cooling and there is no chemical reaction taking place to change its liquid state to solid state. b.Thermoplastic resins are more ductile than thermoset resins.Com- posites made of thermoplastic matrix composites therefore can have larger fracture toughness than those made of thermoset matrix composites.The strain energy release rate for carbon/epoxy is about 100 J/m while that of carbon/PEEK is about 1000 J/m.This means that the thermoplastic matrix resin has large plastic deforma- tion whereas thermoset matrix material does not.This is very im- portant for structures that need to resist impact and fracture load. c.Thermoplastic resins can be recycled,i.e.,they can be reheated to take a different form.This is also due to the same reason indicated above in which the material can be heated up to change the solid state to the melt state.At temperature close to the melt point,differ- ent shapes can be formed to make a new product.This behavior can also be used to heal the defects that may exist in the structure. d.The processing time can be fast.The heating and cooling of a ther- moplastic composite material can take place within the order of minutes.This is in comparison with the order of hours for thermoset matrix material.The reason for the long processing time for the thermoset matrix resin is because of the time required for all the chemical bonds to take place.In the case of thermoplastic resin, cooling will solidify the material and this can take place very quickly. e.Thermoplastic matrix resin is weldable.This means that solid parts made of thermoplastic matrix composites may be welded together. For thermoset resins,because of the chemical bonding required for solidification,once the bond is formed,it cannot be used again to form another bond.The weldability of the thermoplastic resin en-
sure required for processing PEEK at 400°C would be 2000 times more than that of the case of epoxy at 20°C. If a pressure of 689 kPa is required for the processing of epoxy at 20°C, then it would take a pressure of 1378 MPa to process PEEK at 400°C for the same processing time. This is not practical. In the face of the above difficulties, it is desirable to find ways to manufacture thermoplastic matrix composites because they can offer many advantages, such as the following. a. Thermoplastic resin does not have the constraint of shelf life. To put it another way, thermoplastic composite preforms have infinite shelf life. This is because thermoplastic matrix solidifies upon cooling and there is no chemical reaction taking place to change its liquid state to solid state. b. Thermoplastic resins are more ductile than thermoset resins. Composites made of thermoplastic matrix composites therefore can have larger fracture toughness than those made of thermoset matrix composites. The strain energy release rate for carbon/epoxy is about 100 J/m2 while that of carbon/PEEK is about 1000 J/m2 . This means that the thermoplastic matrix resin has large plastic deformation whereas thermoset matrix material does not. This is very important for structures that need to resist impact and fracture load. c. Thermoplastic resins can be recycled, i.e., they can be reheated to take a different form. This is also due to the same reason indicated above in which the material can be heated up to change the solid state to the melt state. At temperature close to the melt point, different shapes can be formed to make a new product. This behavior can also be used to heal the defects that may exist in the structure. d. The processing time can be fast. The heating and cooling of a thermoplastic composite material can take place within the order of minutes. This is in comparison with the order of hours for thermoset matrix material. The reason for the long processing time for the thermoset matrix resin is because of the time required for all the chemical bonds to take place. In the case of thermoplastic resin, cooling will solidify the material and this can take place very quickly. e. Thermoplastic matrix resin is weldable. This means that solid parts made of thermoplastic matrix composites may be welded together. For thermoset resins, because of the chemical bonding required for solidification, once the bond is formed, it cannot be used again to form another bond. The weldability of the thermoplastic resin en- 290 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES
Materials 291 ables thermoplastic matrix composites to exhibit some degree of healing upon heating. For the above reasons,special techniques need to be developed for thermoplastic composites manufacturing.The main focus of different strategies to handle thermoplastic matrix composites is to enhance the availability of the resin to the fiber surface.This can be seen in various approaches such as the formation of tapes,the use of fiber commingling, coating fibers with thermoplastic powder,and using fabrics with resin film sandwich.These techniques will be discussed in the subsequent sec- tions of this chapter. 2.MATERIALS As the name implies,thermoplastic matrix composites are materi- als made by the combination of fiber materials and thermoplastic ma- trix materials.Fibers are usually made of carbon,glass or Kevlar. Matrix materials can be made of engineering thermoplastic resins such as nylon,polypropylene or high performance thermoplastics such as polyetheretherketone (PEEK),polyetherketoneketone(PEKK), polyethermide(PED,polyphenylenesulfide (PPS)or polyethersulfone (PES).Table 8.1(repeated from Table 2.6)shows the common high per- formance thermoplastic matrices along with their properties.A more comprehensive list of thermoplastic matrix that can be used to make composites can be found in Reference [2]. Among the semi-crystalline thermoplastic resins,PPS is the least ex- pensive but has low fracture toughness.Both PEEK and PEKK have higher fracture toughness but have higher processing temperatures than PPS.PEEK has been used more than PEKK and has a larger database. PEKK has a lower processing temperature than PEEK and yet PEKK has a higher T and lower cost. Cogswell [3]gave comprehensive information about the physical properties of PEEK.Table 8.2 shows the values. Due to the high viscosity problem,the manufacturing strategies for thermoplastic matrix composites are different from those of thermosets. The objective of these strategies is mainly to compensate for this large viscosity and attempts to get the resin to the surface of the fibers and to wet the fibers. The strategies of manufacturing using thermoplastic composites can be considered to consist of two stages.In the first stage,the preliminary material form is done,and in the final phase,the final product is made.At
ables thermoplastic matrix composites to exhibit some degree of healing upon heating. For the above reasons, special techniques need to be developed for thermoplastic composites manufacturing. The main focus of different strategies to handle thermoplastic matrix composites is to enhance the availability of the resin to the fiber surface. This can be seen in various approaches such as the formation of tapes, the use of fiber commingling, coating fibers with thermoplastic powder, and using fabrics with resin film sandwich. These techniques will be discussed in the subsequent sections of this chapter. 2. MATERIALS As the name implies, thermoplastic matrix composites are materials made by the combination of fiber materials and thermoplastic matrix materials. Fibers are usually made of carbon, glass or Kevlar. Matrix materials can be made of engineering thermoplastic resins such as nylon, polypropylene or high performance thermoplastics such as polyetheretherketone (PEEK), polyetherketoneketone(PEKK), polyethermide (PEI), polyphenylenesulfide (PPS) or polyethersulfone (PES). Table 8.1 (repeated from Table 2.6) shows the common high performance thermoplastic matrices along with their properties. A more comprehensive list of thermoplastic matrix that can be used to make composites can be found in Reference [2]. Among the semi-crystalline thermoplastic resins, PPS is the least expensive but has low fracture toughness. Both PEEK and PEKK have higher fracture toughness but have higher processing temperatures than PPS. PEEK has been used more than PEKK and has a larger database. PEKK has a lower processing temperature than PEEK and yet PEKK has a higher Tg and lower cost. Cogswell [3] gave comprehensive information about the physical properties of PEEK. Table 8.2 shows the values. Due to the high viscosity problem, the manufacturing strategies for thermoplastic matrix composites are different from those of thermosets. The objective of these strategies is mainly to compensate for this large viscosity and attempts to get the resin to the surface of the fibers and to wet the fibers. The strategies of manufacturing using thermoplastic composites can be considered to consist of two stages. In the first stage, the preliminary material form is done, and in the final phase, the final product is made. At Materials 291
总 TABLE 8.1 Commonly Used high Performance Thermoplastic Matrices(courtesy of CYTEC Engineered Materials). PEI PPS PEEK PEKK(DS) Morphology Amorphous Semi-crystalline Semi-crystalline Semi-crystalline T(C) 217 90 143 156 Process Temp (C) 330 325 390 340 Comments:Pros √五ghTg Excellent Extensive database Excellent Moderate processing environmental Excellent environmental temperature resistance environmental resistance Moderate processing resistance √/High toughness temperature High toughness Lower process temperature than PEEK Bonding and painting Comments:Cons Environmental √Low Ts High process Limited database in resistance Low toughness temperature composite form Low paint adhesion High cost
TABLE 8.1 Commonly Used high Performance Thermoplastic Matrices (courtesy of CYTEC Engineered Materials). PEI PPS PEEK PEKK (DS) Morphology Amorphous Semi-crystalline Semi-crystalline Semi-crystalline Tg(°C) 217 90 143 156 Process Temp (°C) 330 325 390 340 Comments: Pros ✓High Tg ✓Moderate processing temperature ✓Excellent environmental resistance ✓Moderate processing temperature ✓Extensive database ✓Excellent environmental resistance ✓High toughness ✓Excellent environmental resistance ✓High toughness ✓Lower process temperature than PEEK ✓Bonding and painting Comments: Cons ✓Environmental resistance ✓Low Tg ✓Low toughness ✓Low paint adhesion ✓High process temperature ✓High cost ✓Limited database in composite form 292
Preliminary Material Combinations (PMCs) 293 TABLE 8.2 Physical Properties of PEEK [3]. Resin in Resin Composite Composite Cooling:crystallization temperature (C) 300 294 Cooling:latent heat (kJ/kg) 49 14 43 Heating:melting temperature(C) 343 342 Heating:Latent heat(kJ/kg) 44 12 39 Heat content at 400C relative to 20C 559 (kJ/kg) Coefficient of thermal diffusivity across 3×10-3 the fiber direction(cm2/s) Coefficient of thermal diffusivity along the 20×103 fiber direction(cm2/s) Thermal expansion along fiber direction 0.5(23-143C), (10-6/C) 1.0(143-343C) Thermal expansion across fiber direction 30(23-143C). (10-6/C) 5(143-343C) Thermal expansion-quasi isotropic 29(23-143C) (10-6/C) the end of the preliminary phase,a preliminary combination of fiber and matrix is made.This combination can be in the form of a tape,fibers with clinging powder,reinforcing fibers commingled with filaments made from thermoplastic matrix,or fabric/film sandwich.Figure 8.1 shows the different preliminary material combinations(PMCs). In the second(and final)stage,the preliminary material combinations (PMCs)are transformed into the final composite product.This transfor- mation is usually done using either compression molding or by fiber placement process.Figure 8.2 shows the two processes as applicable to different material combinations. 3.PRELIMINARY MATERIAL COMBINATIONS (PMCs) In the PMC,it is essential for the matrix to be in the vicinity of the fi- bers.This is important to reduce the time and pressure required to get the matrix to get to the fibers during the final product fabrication.Some of the approaches used will be discussed below. 3.1.Tape A tape consists of unidirectional filaments bonded together by the ma-
the end of the preliminary phase, a preliminary combination of fiber and matrix is made. This combination can be in the form of a tape, fibers with clinging powder, reinforcing fibers commingled with filaments made from thermoplastic matrix, or fabric/film sandwich. Figure 8.1 shows the different preliminary material combinations (PMCs). In the second (and final) stage, the preliminary material combinations (PMCs) are transformed into the final composite product. This transformation is usually done using either compression molding or by fiber placement process. Figure 8.2 shows the two processes as applicable to different material combinations. 3. PRELIMINARY MATERIAL COMBINATIONS (PMCs) In the PMC, it is essential for the matrix to be in the vicinity of the fibers. This is important to reduce the time and pressure required to get the matrix to get to the fibers during the final product fabrication. Some of the approaches used will be discussed below. 3.1. Tape A tape consists of unidirectional filaments bonded together by the maPreliminary Material Combinations (PMCs) 293 TABLE 8.2 Physical Properties of PEEK [3]. Resin Composite Resin in Composite Cooling: crystallization temperature (°C) 300 294 Cooling: latent heat (kJ/kg) 49 14 43 Heating: melting temperature (°C) 343 342 Heating: Latent heat (kJ/kg) 44 12 39 Heat content at 400°C relative to 20°C (kJ/kg) 559 Coefficient of thermal diffusivity across the fiber direction (cm2/s) 3 × 10−3 Coefficient of thermal diffusivity along the fiber direction (cm2/s) 20 × 10−3 Thermal expansion along fiber direction (10−6/°C) 0.5 (23–143°C), 1.0 (143–343°C) Thermal expansion across fiber direction (10−6/°C) 30 (23–143°C), 5 (143–343°C) Thermal expansion—quasi isotropic (10−6/°C) 29 (23–143°C)
Hot melt Tape Solvent solution Fibers with clinging powders Commingled fibers Film stacking FIGURE 8.I Preliminary material combinations(PMCs). Compression molding of tape Compression molding of aligned fiber with clinging powders Compression molding of aligned commingled fibers Compression molding of stack of thermoplastic film and fabric Fiber placement(tape) FIGURE 8.2 Processing the simple form into the final part. 294
FIGURE 8.2 Processing the simple form into the final part. FIGURE 8.1 Preliminary material combinations (PMCs). 294
Preliminary Material Combinations(PMCs) 295 trix resin.Resin needs to exist in liquid form to wet the fibers.This can be done by heating and melting the resin and running the fibers through the bath of liquid resin.Hot melt processes are probably most common.In the hot melt process,the matrix is heated until melting.Its viscosity should become low enough such that flow to the surface of the fiber is possible and wetting can take place.Figure 8.3 shows a schematic of the fibers running through a melt of resin.This can also be done by dissolv- ing the matrix in a solvent and running the fibers through a bath of the so- lution.Solution processes are well established for thermosetting prepolymers.This process is used by Dupont to produce prepreg of Avimid K-III,a thermoplastic polyimide.The prepreg contains a sub- stantial amount of residual solvent and must be cured.Therefore the pro- duction of Avimid K-III composite structures must be conducted in a manner similar to thermosetting composites.The complication in this technique is that solvent needs to be subsequently evaporated,which may give rise to voids and residual solvents. Unidirectional tape is the most common form of thermoplastic ply.By convention,the tape is 0.127-0.152 mm (5-6 mils)and 7.62-30.48 cm (3-12 in)wide.To produce a 30.48 cm wide tape requires approximately 24 tows with 12,000 filaments each of 8 um diameter to the combining process.Conversely,wide tape can be filament wound from a single tow using a large diameter mandrel.This latter approach is convenient for ex- perimental ply production but may not be appropriate for low cost fabri- cation.Conversely,filament winding towpreg directly to produce a consolidated structure is potentially low in cost.This is due to process in- tegration.Fabric plies are difficult to produce from thermoplastic towpreg due to the stiffness of most towpregs.Consolidated towpreg, typically from slit tape,can be braided into two dimensional fabrics,but Fiber Pressure Hot melt of resin FIGURE 8.3 Impregnation of fibers by running fibers through a bath of melted resin
trix resin. Resin needs to exist in liquid form to wet the fibers. This can be done by heating and melting the resin and running the fibers through the bath of liquid resin. Hot melt processes are probably most common. In the hot melt process, the matrix is heated until melting. Its viscosity should become low enough such that flow to the surface of the fiber is possible and wetting can take place. Figure 8.3 shows a schematic of the fibers running through a melt of resin. This can also be done by dissolving the matrix in a solvent and running the fibers through a bath of the solution. Solution processes are well established for thermosetting prepolymers. This process is used by Dupont to produce prepreg of Avimid K-III, a thermoplastic polyimide. The prepreg contains a substantial amount of residual solvent and must be cured. Therefore the production of Avimid K-III composite structures must be conducted in a manner similar to thermosetting composites. The complication in this technique is that solvent needs to be subsequently evaporated, which may give rise to voids and residual solvents. Unidirectional tape is the most common form of thermoplastic ply. By convention, the tape is 0.127–0.152 mm (5–6 mils) and 7.62–30.48 cm (3–12 in) wide. To produce a 30.48 cm wide tape requires approximately 24 tows with 12,000 filaments each of 8 µm diameter to the combining process. Conversely, wide tape can be filament wound from a single tow using a large diameter mandrel. This latter approach is convenient for experimental ply production but may not be appropriate for low cost fabrication. Conversely, filament winding towpreg directly to produce a consolidated structure is potentially low in cost. This is due to process integration. Fabric plies are difficult to produce from thermoplastic towpreg due to the stiffness of most towpregs. Consolidated towpreg, typically from slit tape, can be braided into two dimensional fabrics, but Preliminary Material Combinations (PMCs) 295 FIGURE 8.3 Impregnation of fibers by running fibers through a bath of melted resin
296 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.4 Photo of a roll of unidirectional tape made of carbon/PEKK. three dimensional fabrics are difficult to produce.Figure 8.4 shows a photograph of a roll of tape. The speed of production of the hot melt process(meters per minute) depends on the viscosity of the melt,the thickness of the prepregs to be made and the applied pressure.Darcy's law can be used to estimate the rate of production as illustrated in the example below. Example 8.1 A hot melt process is used to produce prepregs for carbon/PEEK 0.2 mm thick.The temperature of the process is 380C giving rise to the viscosity of the resin of 1000 Pa(sec.A pressure of 1 MPa is applied to induce the flow across the thickness of the prepreg.Determine the maximum rate of production,if the length of the die is 50 cm and the permeability of the fiber preform is assumed to be 10-12 m2. Solution The fastest rate of production occurs when the resin has sufficient time to flow across the thickness of the layer.Using Darcy's law,one has: u=-K4 μAx where, u=the flow velocity across the thickness of the prepreg K=the permeability of the fiber preform Ap the pressure gradient across the thickness of the prepreg Ax=thickness of the laminate
three dimensional fabrics are difficult to produce. Figure 8.4 shows a photograph of a roll of tape. The speed of production of the hot melt process (meters per minute) depends on the viscosity of the melt, the thickness of the prepregs to be made and the applied pressure. Darcy’s law can be used to estimate the rate of production as illustrated in the example below. 296 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.4 Photo of a roll of unidirectional tape made of carbon/PEKK. Example 8.1 A hot melt process is used to produce prepregs for carbon/PEEK 0.2 mm thick. The temperature of the process is 380°C giving rise to the viscosity of the resin of 1000 Pa(sec. A pressure of 1 MPa is applied to induce the flow across the thickness of the prepreg. Determine the maximum rate of production, if the length of the die is 50 cm and the permeability of the fiber preform is assumed to be 10−12 m2. Solution The fastest rate of production occurs when the resin has sufficient time to flow across the thickness of the layer. Using Darcy’s law, one has: u K p x = − µ ∆ ∆ where, u = the flow velocity across the thickness of the prepreg K = the permeability of the fiber preform ∆p = the pressure gradient across the thickness of the prepreg ∆x = thickness of the laminate
Preliminary Material Combinations(PMCs) 297 Substituting in the values yields: 10-2m21MP 1000 Pa-sec 0.2 mm -=5x103 mm/sec Time required to traverse the thickness of the preform: h 0.2mm 1=-= -=40sec. u5×10-3mm/sec For the length of the die of 50 cm,the maximum rate of production R would be: R='=50cm or 1.25 cm/sec I 40 sec =1.25 cm sec 3.2.Fibers with Clinging Powders In the powder clinging process,the matrix powder is made to stick to the surface of the fibers.Figure 8.5 shows a schematic of the process. First,the dry fiber tow is fed from a creel to an air-conditioned spreader. The tow is spread to expose the fiber and grounded in order to pick up charge powder.By spreading a tow to expose virtually every fiber,it is easier to get the liquid resin to the surface of every fiber and it takes less pressure to force a polymer melt through a fiber bed.The fiber tow then enters into a heated chamber where matrix powder is electrified such that it carries an electrical charge,then it is fluidized.The powder is deposited on the band of fibers due to static electricity.At the next station of the Lean Phase 两州州 Air Dense Diffuser Plate Phase Electric Potential Let-off Air Comb Oven Take-up Winder Spreader Winder Air Tension Fluidized Pull Control Bed Rollers FIGURE 8.5 Process to get matrix powders to cling to fibers (reproduced from"The processing science of thermoplastic composites,"by J.D.Muzzy and J.S.Colton,in Ad- vanced Composites Manufacturing,T.G.Gutowski,ed.,with permission from Wiley Interscience)
3.2. Fibers with Clinging Powders In the powder clinging process, the matrix powder is made to stick to the surface of the fibers. Figure 8.5 shows a schematic of the process. First, the dry fiber tow is fed from a creel to an air-conditioned spreader. The tow is spread to expose the fiber and grounded in order to pick up charge powder. By spreading a tow to expose virtually every fiber, it is easier to get the liquid resin to the surface of every fiber and it takes less pressure to force a polymer melt through a fiber bed. The fiber tow then enters into a heated chamber where matrix powder is electrified such that it carries an electrical charge, then it is fluidized. The powder is deposited on the band of fibers due to static electricity. At the next station of the Preliminary Material Combinations (PMCs) 297 Substituting in the values yields: u = = × − 10 1 − 5 10 12 m 3 1000 Pa - sec MPa 0.2 mm mm / sec 2 Time required to traverse the thickness of the preform: t h u = = × = − 0 2 40 . mm 5 10 mm / sec sec. 3 For the length of the die of 50 cm, the maximum rate of production R would be: R L t == = 5 4 1 25 1 25 0 cm 0 sec . cm / sec or . cm / sec FIGURE 8.5 Process to get matrix powders to cling to fibers (reproduced from “The processing science of thermoplastic composites,” by J. D. Muzzy and J. S. Colton, in Advanced Composites Manufacturing, T. G. Gutowski, ed., with permission from Wiley Interscience)
298 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES reinforcement fiber clinging powder FIGURE 8.6 Fibers with clinging powders. process,the material is heated.The resin melts,flows and then wets the fibers.After cooling,the powder is fused into the fibers.After passing through a fluidized bed,the tows enter a tunnel oven to melt the polymer onto the fiber.After cooling,the towpreg is wound onto a take-up roll. There are advantages to the powder mixing process.By avoiding sol- vent or water in the combining operation,there is no need to remove volatiles.The extent of mixing between fiber and powder depends upon the extent to which the tow is spread.It is possible to spread the tow to ex- pose virtually every fiber,thereby achieving good mixing.Spreading the tow and not collapsing it when the polymer is molten leads to a flexible tow that can be braided or woven.The coated tow can be heated and cooled rapidly to minimize polymer degradation.The tow is not exposed to high stress,which minimizes fiber damage.Since powder coating can be accomplished quickly and continuously,dry powder combining pro- cess is potentially inexpensive. The production of towpregs using the electrostatic fluidized bed pro- cess has been demonstrated using numerous thermoplastics and thermosets as well as carbon,glass and aramid fibers.Good fiber wetting was obtained even when the particle size of the powder was substantially greater than the 8 um fiber diameter.In a commercial scale version of this process a line speed of 40 cm/s has been achieved while attaining 40%vol polymer content.Figure 8.6 shows a schematic of the fibers with cling- ing powders. 3.2.1.Slurry and Foam The above approaches appear to work particularly well for fine pow- ders below 25 um.Electrostatic cloud coating has worked successfully for powder well over 100 um.Since many polymers are difficult to grind, the ability to accommodate large particles is a definite benefit.Slurries and foams are being explored as alternative combining methods
process, the material is heated. The resin melts, flows and then wets the fibers. After cooling, the powder is fused into the fibers. After passing through a fluidized bed, the tows enter a tunnel oven to melt the polymer onto the fiber. After cooling, the towpreg is wound onto a take-up roll. There are advantages to the powder mixing process. By avoiding solvent or water in the combining operation, there is no need to remove volatiles. The extent of mixing between fiber and powder depends upon the extent to which the tow is spread. It is possible to spread the tow to expose virtually every fiber, thereby achieving good mixing. Spreading the tow and not collapsing it when the polymer is molten leads to a flexible tow that can be braided or woven. The coated tow can be heated and cooled rapidly to minimize polymer degradation. The tow is not exposed to high stress, which minimizes fiber damage. Since powder coating can be accomplished quickly and continuously, dry powder combining process is potentially inexpensive. The production of towpregs using the electrostatic fluidized bed process has been demonstrated using numerous thermoplastics and thermosets as well as carbon, glass and aramid fibers. Good fiber wetting was obtained even when the particle size of the powder was substantially greater than the 8 µm fiber diameter. In a commercial scale version of this process a line speed of 40 cm/s has been achieved while attaining 40%vol polymer content. Figure 8.6 shows a schematic of the fibers with clinging powders. 3.2.1. Slurry and Foam The above approaches appear to work particularly well for fine powders below 25 µm. Electrostatic cloud coating has worked successfully for powder well over 100 µm. Since many polymers are difficult to grind, the ability to accommodate large particles is a definite benefit. Slurries and foams are being explored as alternative combining methods. 298 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES FIGURE 8.6 Fibers with clinging powders
