Processing of Composite Laminates The processing of polymer matrix composite laminates has been the subject of considerable research during the last two decades [1-11].Multiple physical and chemical phenomena must occur simultaneously and in the proper sequence to achieve desired laminate properties.There are several routes to achieve full consolidation and minimize void content of a polymeric matrix with a reinforcing fiber in volume fractions(50 to 60%)appropriate for structural applications.The most widely accepted approach is by impregna- tion of unidirectional fibers or textile fabrics to create a thin sheet or tape.If the polymer is a thermoset,it is often advanced in its curing state to the B stage(a state of cure of the matrix that is incomplete,but provides high room temperature viscosity of the prepreg).Known as prepreg in this form, it may be stored at low temperature (below freezing)to greatly reduce the rate of cure and thus increase the storage life.After being warmed to room temperature,these prepreg sheets or tapes may then be assembled into a laminate and subjected to a cure cycle. It is also possible to assemble dry fibers into an appropriate geometric form,and then impregnate the entire laminate in a single step.This approach is known as resin transfer molding (RTM),and there are several variations. The weaving of a fabric from reinforcing fibers is a widely accepted approach to creating the fiber preform,although there are other techniques designed to avoid fiber crimp and develop microstructures typical of that achieved with prepreg tape. For prepreg,heat and pressure are first applied to the laminate to reduce the viscosity of the polymer matrix,and achieve full densification of the laminate and coalescence of the laminae through matrix flow.The application of heat to the laminate is governed by the laws of heat transfer and is therefore a time-dependent phenomenon.Further,the pressure in the lami- nate is shared by the polymeric matrix and the fibers.For thermosetting polymers,the kinetic process to achieve gelation and vitrification is a themo- chemical process that is often exothermic.The decrease in polymer viscosity with temperature and its increase with degree of cure for theromsets requires that the necessary flow be achieved prior to gelation or vitrification.For thermoplastic polymers the process involves both viscosity changes and changes in the polymer morphology(degree of crystallinity).Thermoplastic ©2003 by CRC Press LLC
3 Processing of Composite Laminates The processing of polymer matrix composite laminates has been the subject of considerable research during the last two decades [1–11]. Multiple physical and chemical phenomena must occur simultaneously and in the proper sequence to achieve desired laminate properties. There are several routes to achieve full consolidation and minimize void content of a polymeric matrix with a reinforcing fiber in volume fractions (50 to 60%) appropriate for structural applications. The most widely accepted approach is by impregnation of unidirectional fibers or textile fabrics to create a thin sheet or tape. If the polymer is a thermoset, it is often advanced in its curing state to the B stage (a state of cure of the matrix that is incomplete, but provides high room temperature viscosity of the prepreg). Known as prepreg in this form, it may be stored at low temperature (below freezing) to greatly reduce the rate of cure and thus increase the storage life. After being warmed to room temperature, these prepreg sheets or tapes may then be assembled into a laminate and subjected to a cure cycle. It is also possible to assemble dry fibers into an appropriate geometric form, and then impregnate the entire laminate in a single step. This approach is known as resin transfer molding (RTM), and there are several variations. The weaving of a fabric from reinforcing fibers is a widely accepted approach to creating the fiber preform, although there are other techniques designed to avoid fiber crimp and develop microstructures typical of that achieved with prepreg tape. For prepreg, heat and pressure are first applied to the laminate to reduce the viscosity of the polymer matrix, and achieve full densification of the laminate and coalescence of the laminae through matrix flow. The application of heat to the laminate is governed by the laws of heat transfer and is therefore a time-dependent phenomenon. Further, the pressure in the laminate is shared by the polymeric matrix and the fibers. For thermosetting polymers, the kinetic process to achieve gelation and vitrification is a themochemical process that is often exothermic. The decrease in polymer viscosity with temperature and its increase with degree of cure for theromsets requires that the necessary flow be achieved prior to gelation or vitrification. For thermoplastic polymers the process involves both viscosity changes and changes in the polymer morphology (degree of crystallinity). Thermoplastic TX001_ch03_Frame Page 37 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
crystalline polymers will exhibit varying degrees of crystallinity depending upon their thermal history [5] The instantaneous degree of cure of a thermoset polymer is measured by the fraction of total heat generated at a given time divided by the total heat of reaction.The degree of cure ranges from 0 to 1 and can be measured using differential scanning calorimetry (DSC),which determines the heat of reaction as a function of time.As the reaction progresses and the macro- molecular network forms,the rate-controlling phenomenon changes from kinetic to diffusion because of the reduction in polymer free volume.An accompanying reduction in molecular mobility occurs because of molecular weight increase. Uneven distribution of resin may result from nonuniform flow of the polymer through the fiber reinforcement.This is particularly pronounced for laminates with curvilinear geometry and tapered thickness in which local pressure gradients occur.The velocity of flow of a polymer through a porous medium such as fiber mats has been shown to be proportional to the pressure gradient and inversely proportional to the polymer viscosity [12].The pro- portionality constant is known as the permeability[12]. 3.1 Processing of Thermoset Composites The development of an interlocking network during the cure of a thermoset polymer is illustrated in Figure 3.1.As temperature and time increase,the network interconnectivity grows according to the steps illustrated:(a)the prepolymer and curing agents are interspersed,(b)polymer molecular weight (size)increases,(c)gelation occurs and a continuous network is achieved,and(d)cure is complete(see the time-temperature transformation diagram,Figure 3.2).After the polymer approaches vitrification,i.e,the polymer changes from a rubbery to a glassy state,the rate of conversion decreases significantly.Should vitrification occur before completion of the cure reaction,polymer properties will not be fully achieved and voids may form in the laminate.These phenomena must be considered in the develop- ment of an appropriate cure cycle. Figure 3.3 illustrates the flow and compaction phenomena during the curing and consolidation steps.Initially,the increase in temperature serves to decrease the viscosity of the polymer and the polymer carries the applied pressure.As the laminate is vented and flow begins,the fibers deform and act as an elastic spring in assuming a portion of the applied pressure (Figure 3.3).Volatiles produced in the chemical reaction or trapped gases will then escape from the laminate.Finally,the total pressure is carried by the fully consolidated composite panel. Given that composite laminates are often processed in an autoclave, wherein heat transfer is achieved with a pressurizing medium (normally 2003 by CRC Press LLC
crystalline polymers will exhibit varying degrees of crystallinity depending upon their thermal history [5]. The instantaneous degree of cure of a thermoset polymer is measured by the fraction of total heat generated at a given time divided by the total heat of reaction. The degree of cure ranges from 0 to 1 and can be measured using differential scanning calorimetry (DSC), which determines the heat of reaction as a function of time. As the reaction progresses and the macromolecular network forms, the rate-controlling phenomenon changes from kinetic to diffusion because of the reduction in polymer free volume. An accompanying reduction in molecular mobility occurs because of molecular weight increase. Uneven distribution of resin may result from nonuniform flow of the polymer through the fiber reinforcement. This is particularly pronounced for laminates with curvilinear geometry and tapered thickness in which local pressure gradients occur. The velocity of flow of a polymer through a porous medium such as fiber mats has been shown to be proportional to the pressure gradient and inversely proportional to the polymer viscosity [12]. The proportionality constant is known as the permeability [12]. 3.1 Processing of Thermoset Composites The development of an interlocking network during the cure of a thermoset polymer is illustrated in Figure 3.1. As temperature and time increase, the network interconnectivity grows according to the steps illustrated: (a) the prepolymer and curing agents are interspersed, (b) polymer molecular weight (size) increases, (c) gelation occurs and a continuous network is achieved, and (d) cure is complete (see the time–temperature transformation diagram, Figure 3.2). After the polymer approaches vitrification, i.e., the polymer changes from a rubbery to a glassy state, the rate of conversion decreases significantly. Should vitrification occur before completion of the cure reaction, polymer properties will not be fully achieved and voids may form in the laminate. These phenomena must be considered in the development of an appropriate cure cycle. Figure 3.3 illustrates the flow and compaction phenomena during the curing and consolidation steps. Initially, the increase in temperature serves to decrease the viscosity of the polymer and the polymer carries the applied pressure. As the laminate is vented and flow begins, the fibers deform and act as an elastic spring in assuming a portion of the applied pressure (Figure 3.3). Volatiles produced in the chemical reaction or trapped gases will then escape from the laminate. Finally, the total pressure is carried by the fully consolidated composite panel. Given that composite laminates are often processed in an autoclave, wherein heat transfer is achieved with a pressurizing medium (normally TX001_ch03_Frame Page 38 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
涂城 d FIGURE 3.1 Dynamics of thermoset gelation and vitrification.(From L.A.Berglund and J.M.Kenny,SAMPE J.,27(2),1991.With permission.) Gel rubber rubber Sol/Gel gias Liquid go Sol gloss 22 Log time FIGURE 3.2 Time-temperature transformation diagram.(From L.A.Berglund and J.M.Kenny,SAMPE J., 27(2),1991.With permission.) 100% 100% 0% 0% G Pressurization Flow begins G G 100% P 100% 0% 0% Flow Full compaction FIGURE 3.3 Polymer and perform pressurization and flow.(From P.Hubert,Ph.D.Thesis,University of British Columbia,1996.With permission.) ©2003 by CRC Press LLC
FIGURE 3.1 Dynamics of thermoset gelation and vitrification. (From L.A. Berglund and J.M. Kenny, SAMPE J., 27(2), 1991. With permission.) FIGURE 3.2 Time–temperature transformation diagram. (From L.A. Berglund and J.M. Kenny, SAMPE J., 27(2), 1991. With permission.) FIGURE 3.3 Polymer and perform pressurization and flow. (From P. Hubert, Ph.D. Thesis, University of British Columbia, 1996. With permission.) TX001_ch03_Frame Page 39 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
160 140 Tool Surface Top Surface Center of Panel 120 100 220 260 300340 380 420 Time,minutes FIGURE 3.4 Heat transfer through laminate thickness.(From P.Hubert,University of British Columbia Composites Group Report,1994.With permission.) nitrogen,an inert gas),it is important to recognize that the instantaneous temperature within the laminate may not be equal to that of the autoclave Figure 3.4 illustrates a typical thermal cycle and shows that the temperature of the laminate can differ from top surface to interior(center)to tool surface. Thus,the dynamics of heat transfer must be considered when an appropriate cure cycle is developed. Consider the typical cure cycle shown in Figure 3.5,where internal com- posite temperature lags autoclave temperature.Initially,the autoclave tem- perature is increased at a constant rate of 2 to 3C/min until it reaches 110C, and then it is held constant for approximately 1 h.During this stage the polymer is in the liquid state.Next the autoclave temperature is increased to and held at approximately 180C for 2 h.During this stage the polymer passes through gelation at a degree of cure of 0.46 and then approaches vitrification. Vitrification occurs when the instantaneous glass transition temperature (defined as the temperature at which the polymer passes from the rubbery or gel state to the glassy state)of the polymer reaches the temperature of the laminate.In Figure 3.5,the vitrification point occurs prematurely at Stage I StageⅡ StageⅢ 220 Vitrification Point 0.8 V180 Degree of Cure Degree of cure at gelation 0.6 Autoclave Temp 0.4 60 0.2 2 04080120160200240280320360 Time,minutes FIGURE 3.5 Cure cycle with premature vitrification.(From P.Hubert,University of British Columbia Composites Group Report,1994.With permission.) 2003 by CRC Press LLC
nitrogen, an inert gas), it is important to recognize that the instantaneous temperature within the laminate may not be equal to that of the autoclave. Figure 3.4 illustrates a typical thermal cycle and shows that the temperature of the laminate can differ from top surface to interior (center) to tool surface. Thus, the dynamics of heat transfer must be considered when an appropriate cure cycle is developed. Consider the typical cure cycle shown in Figure 3.5, where internal composite temperature lags autoclave temperature. Initially, the autoclave temperature is increased at a constant rate of 2 to 3°C/min until it reaches 110°C, and then it is held constant for approximately 1 h. During this stage the polymer is in the liquid state. Next the autoclave temperature is increased to and held at approximately 180°C for 2 h. During this stage the polymer passes through gelation at a degree of cure of 0.46 and then approaches vitrification. Vitrification occurs when the instantaneous glass transition temperature (defined as the temperature at which the polymer passes from the rubbery or gel state to the glassy state) of the polymer reaches the temperature of the laminate. In Figure 3.5, the vitrification point occurs prematurely at FIGURE 3.4 Heat transfer through laminate thickness. (From P. Hubert, University of British Columbia Composites Group Report, 1994. With permission.) FIGURE 3.5 Cure cycle with premature vitrification. (From P. Hubert, University of British Columbia Composites Group Report, 1994. With permission.) TX001_ch03_Frame Page 40 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
approximately 190 min into the cycle.Because the rubbery-to-glass transition occurs at vitrification,stresses developed as a result of the shrinkage of the polymer with cure progression may not relax during the remainder of the curing cycle.For the case in which vitrification is delayed until a point much later in the process close to cooling,much of this stress will be eliminated by completion of the cycle.Hence,the cure cycle can be tailored to the specific polymer to minimize residual stresses.Of course,thermal residual stresses develop in the laminate upon cooling because of anisotropic thermal expansion,as discussed in Chapters 10 and 12. 3.1.1 Autoclave Molding Figure 3.6 shows the vacuum bag lay-up sequence for a typical epoxy matrix prepreg composite.Different lay-up sequences can be used for other types of prepregs. 1.Thoroughly clean the aluminum plate(10)using acetone or a deter- gent.Then apply mold-release agent to the top surface of the alumi- num plate twice. 2.Lay one sheet of Teflon film(1)and the peel-ply(2)(nonstick nylon cloth)on the aluminum plate.The Teflon film is used to release the lay-up from the aluminum plate,and the peel-ply is used to achieve the required surface finish on the laminate.Note:There should be no wrinkles or raised regions in the peel-ply,and its dimensions should be identical to those of the laminate. 3.Place the prepreg stack(3)on the plate,being sure to keep it at least 50 mm from each edge.Note:Do not cover up the vacuum connection in the plate. 10 1.Teflon Film 7.Teflon Film (holes every 50 mm) 2.Peel Ply 8.Vent Cloth 3.Laminate (prepreg stack) 9.Corkor Rubber Dam 4.Peel Ply 10.Aluminum Plate 5.Teflon Coated Glass Fabric 11.Release Agent 6.Glass Bleeders(1 per 3.5 plies) FIGURE 3.6 Vacuum bag preparation for autoclave cure of thermoset matrix composite. ©2003 by CRC Press LLC
approximately 190 min into the cycle. Because the rubbery-to-glass transition occurs at vitrification, stresses developed as a result of the shrinkage of the polymer with cure progression may not relax during the remainder of the curing cycle. For the case in which vitrification is delayed until a point much later in the process close to cooling, much of this stress will be eliminated by completion of the cycle. Hence, the cure cycle can be tailored to the specific polymer to minimize residual stresses. Of course, thermal residual stresses develop in the laminate upon cooling because of anisotropic thermal expansion, as discussed in Chapters 10 and 12. 3.1.1 Autoclave Molding Figure 3.6 shows the vacuum bag lay-up sequence for a typical epoxy matrix prepreg composite. Different lay-up sequences can be used for other types of prepregs. 1. Thoroughly clean the aluminum plate (10) using acetone or a detergent. Then apply mold-release agent to the top surface of the aluminum plate twice. 2. Lay one sheet of Teflon film (1) and the peel-ply (2) (nonstick nylon cloth) on the aluminum plate. The Teflon film is used to release the lay-up from the aluminum plate, and the peel-ply is used to achieve the required surface finish on the laminate. Note: There should be no wrinkles or raised regions in the peel-ply, and its dimensions should be identical to those of the laminate. 3. Place the prepreg stack (3) on the plate, being sure to keep it at least 50 mm from each edge. Note: Do not cover up the vacuum connection in the plate. FIGURE 3.6 Vacuum bag preparation for autoclave cure of thermoset matrix composite. TX001_ch03_Frame Page 41 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
4.Place a strip of the cork-rubber material(9)along each edge of the panel,making sure that no gaps exist and a complete dam is formed around the laminate.The dam around the lay-up prevents lateral motion of the panel,and minimizes resin flow parallel to the alu- minum plate and through the edges of the laminate (9). 5.Completely encircle the prepreg stack and dam with bagging adhesive making sure that the adhesive material is adjacent to the dam.The purpose of the adhesive material is to form a vacuum seal. 6.Place a peel-ply(4)and a ply of Teflon-coated glass fabric(5)(with the same dimensions as the panel)on top of the prepreg stack.The purpose of the Teflon-coated glass fabric is to prevent the bleeder sheets(6)from sticking to the laminate. 7.Place the proper number of glass bleeder sheets(6)(e.g.,style 181 glass cloth with the same dimensions as the prepreg stack)over the Teflon-coated fabric(5).The bleeder sheets absorb the excess resin from the laminate. 8.Place a sheet of perforated Teflon film (7)(0.025 mm)over the bleeder material.The Teflon film,perforated on 50 mm centers, prevents excess resin from saturating the vent cloth(8). 9.Place a porous continuous-vent cloth(8)(e.g,style 181 glass cloth)on top of the lay-up.Extend the cloth over the vacuum line attachment. Make sure that the vacuum line is completely covered by the vent cloth.The vent cloth provides a path for volatiles to escape when the vacuum is applied and achieves a uniform distribution of vacuum. 10.Place nylon bagging film over the entire plate,and seal it against the bagging adhesive.Allow enough material so that the film conforms to all contours without being punctured. 11.Place the plate in the autoclave and attach the vacuum line(Figure 3.7). An autoclave is generally a large pressure vessel equipped with a temperature-and pressure-control system.The elevated pressures and temperatures,required for processing of the laminate,are com- monly achieved by electrically heating a pressurized inert gas (nitrogen).The use of an inert gas will reduce oxidizing reactions that otherwise may occur in the resin at elevated temperatures,and will prevent explosion of evolving volatiles. 12.Turn on the vacuum pump and check for leaks.Maintain a vacuum of 650 to 750 mm of mercury for 20 min and check again for leaks. 13.After closing the autoclave door,apply the pressure and initiate the appropriate cure cycle(see example shown in Figure 3.8).As the temperature is increased,the resin viscosity decreases rapidly and the chemical reaction of the resin begins.At the end of the temperature hold,at 127C in Figure 3.8,the resin viscosity is at a minimum and pressure is applied to squeeze out excess resin.The ©2003 by CRC Press LLC
4. Place a strip of the cork–rubber material (9) along each edge of the panel, making sure that no gaps exist and a complete dam is formed around the laminate. The dam around the lay-up prevents lateral motion of the panel, and minimizes resin flow parallel to the aluminum plate and through the edges of the laminate (9). 5. Completely encircle the prepreg stack and dam with bagging adhesive making sure that the adhesive material is adjacent to the dam. The purpose of the adhesive material is to form a vacuum seal. 6. Place a peel-ply (4) and a ply of Teflon-coated glass fabric (5) (with the same dimensions as the panel) on top of the prepreg stack. The purpose of the Teflon-coated glass fabric is to prevent the bleeder sheets (6) from sticking to the laminate. 7. Place the proper number of glass bleeder sheets (6) (e.g., style 181 glass cloth with the same dimensions as the prepreg stack) over the Teflon-coated fabric (5). The bleeder sheets absorb the excess resin from the laminate. 8. Place a sheet of perforated Teflon film (7) (0.025 mm) over the bleeder material. The Teflon film, perforated on 50 mm centers, prevents excess resin from saturating the vent cloth (8). 9. Place a porous continuous-vent cloth (8) (e.g., style 181 glass cloth) on top of the lay-up. Extend the cloth over the vacuum line attachment. Make sure that the vacuum line is completely covered by the vent cloth. The vent cloth provides a path for volatiles to escape when the vacuum is applied and achieves a uniform distribution of vacuum. 10. Place nylon bagging film over the entire plate, and seal it against the bagging adhesive. Allow enough material so that the film conforms to all contours without being punctured. 11. Place the plate in the autoclave and attach the vacuum line (Figure 3.7). An autoclave is generally a large pressure vessel equipped with a temperature- and pressure-control system. The elevated pressures and temperatures, required for processing of the laminate, are commonly achieved by electrically heating a pressurized inert gas (nitrogen). The use of an inert gas will reduce oxidizing reactions that otherwise may occur in the resin at elevated temperatures, and will prevent explosion of evolving volatiles. 12. Turn on the vacuum pump and check for leaks. Maintain a vacuum of 650 to 750 mm of mercury for 20 min and check again for leaks. 13. After closing the autoclave door, apply the pressure and initiate the appropriate cure cycle (see example shown in Figure 3.8). As the temperature is increased, the resin viscosity decreases rapidly and the chemical reaction of the resin begins. At the end of the temperature hold, at 127°C in Figure 3.8, the resin viscosity is at a minimum and pressure is applied to squeeze out excess resin. The TX001_ch03_Frame Page 42 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
FIGURE 3.7 Vacuum bag sequence and tool plate placed in an autoclave. 200 160 Temperature 9 80 2C/min Pressure 88星 0.6 40 0.4 Maintain Vacuum Inside Bag 0.2 0 0 0 100 200 300 Time,min FIGURE 3.8 Typical cure cycle for a carbon/epoxy prepreg. temperature hold controls the rate of the chemical reaction and prevents degradation of the material by the exotherm.The pressure is held constant throughout the cure cycle to consolidate the plies until the resin in the laminate is in its glassy state at the end of the cooling phase.The vacuum should be checked throughout the cure cycle.The vacuum is applied to achieve a uniform pressure on the laminate and draw out volatiles created during the cure.Loss of vacuum will result in a poorly consolidated laminate. 14.After the power is turned off to the autoclave,maintain pressure until the inside temperature has dropped to about 100C. 15.Carefully remove the laminate from the aluminum plate.Gently lift it in a direction parallel to the main principal direction of the laminate. 16.Clean the aluminum plate,and store it for future use. ©2003 by CRC Press LLC
temperature hold controls the rate of the chemical reaction and prevents degradation of the material by the exotherm. The pressure is held constant throughout the cure cycle to consolidate the plies until the resin in the laminate is in its glassy state at the end of the cooling phase. The vacuum should be checked throughout the cure cycle. The vacuum is applied to achieve a uniform pressure on the laminate and draw out volatiles created during the cure. Loss of vacuum will result in a poorly consolidated laminate. 14. After the power is turned off to the autoclave, maintain pressure until the inside temperature has dropped to about 100°C. 15. Carefully remove the laminate from the aluminum plate. Gently lift it in a direction parallel to the main principal direction of the laminate. 16. Clean the aluminum plate, and store it for future use. FIGURE 3.7 Vacuum bag sequence and tool plate placed in an autoclave. FIGURE 3.8 Typical cure cycle for a carbon/epoxy prepreg. TX001_ch03_Frame Page 43 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
3.1.2 Resin Transfer Molding of Thermoset Composites Resin transfer molding(RTM)of composite laminates is a process wherein the dry-fiber preform is infiltrated with a liquid polymeric resin and the polymer is advanced to its final cure after the impregnation process is com- plete.An extensive review of the resin transfer molding process can be found in Reference [12].The process consists of four steps:fiber preform manufac- ture,mold filling,cure,and part removal.In the first step,textile technology is typically utilized to assemble the preform.For example,woven textile fabrics are often assembled into multilayer laminates that conform to the geometry of the tool.Braiding and stitching provide mechanisms for the creation of three-dimensional preform architectures. Typically,a thermosetting polymer of relatively low viscosity is used in the RTM process.There have been applications for thermoplastic polymers,but they are rare.Pressure is applied to the fluid polymer to inject it into a mold containing the fiber preform,and the mold may have been preheated.The flow of the fluid through the fiber preform is governed by Darcy's Law [12], wherein the velocity of the flow is equal to the product of the pressure gradient,the preform permeability,and the inverse of the polymer viscosity. Clearly,the lower the polymer viscosity,the greater the flow rate,and the greater the permeability,the greater the flow rate.Note also that because the fiber preforms typically exhibit different geometries in the three principal directions,permeability is a tensor and exhibits anisotropic characteristics. That is,for a given pressure gradient,the flow rates in three mutually ortho- gonal directions will differ.Flow through the thickness of a fiber preform that contains many layers of unidirectional fibers will be quite different than flow in the planar directions.In addition,the permeability of the preform depends on the fiber volume fraction of the preform.The greater the volume fraction, the lower the permeability.It is important to vent the mold to the atmosphere to remove displaced gases from the fiber preform during the mold filling process.Otherwise trapped gases will lead to voids within the laminate. After the polymer has fully impregnated the fiber preform,the third step occurs:cure.This step will begin immediately upon injection of the polymer into the mold and will occur more rapidly if the mold is at an elevated temperature.As the cure of the polymer advances to the creation of a cross-link network,it passes through a gelation phase wherein the polymer viscosity increases and transforms the polymer into a viscoelastic substance, where it possesses both viscous and elastic properties.As this process pro- ceeds and the cross-link network continues to grow,the instantaneous glass transition temperature of the polymer increases.Finally,vitrification of the polymer occurs when its glass transition temperature exceeds the laminate temperature.Should gelation or vitrification(or both)occur prior to com- pletion of mold filling and preform impregnation,the resulting laminate will not be fully impregnated. The viscosity of most polymers is highly dependent on temperature and polymer cure kinetics are controlled by temperature as well.Therefore,heat 2003 by CRC Press LLC
3.1.2 Resin Transfer Molding of Thermoset Composites Resin transfer molding (RTM) of composite laminates is a process wherein the dry-fiber preform is infiltrated with a liquid polymeric resin and the polymer is advanced to its final cure after the impregnation process is complete. An extensive review of the resin transfer molding process can be found in Reference [12]. The process consists of four steps: fiber preform manufacture, mold filling, cure, and part removal. In the first step, textile technology is typically utilized to assemble the preform. For example, woven textile fabrics are often assembled into multilayer laminates that conform to the geometry of the tool. Braiding and stitching provide mechanisms for the creation of three-dimensional preform architectures. Typically, a thermosetting polymer of relatively low viscosity is used in the RTM process. There have been applications for thermoplastic polymers, but they are rare. Pressure is applied to the fluid polymer to inject it into a mold containing the fiber preform, and the mold may have been preheated. The flow of the fluid through the fiber preform is governed by Darcy’s Law [12], wherein the velocity of the flow is equal to the product of the pressure gradient, the preform permeability, and the inverse of the polymer viscosity. Clearly, the lower the polymer viscosity, the greater the flow rate, and the greater the permeability, the greater the flow rate. Note also that because the fiber preforms typically exhibit different geometries in the three principal directions, permeability is a tensor and exhibits anisotropic characteristics. That is, for a given pressure gradient, the flow rates in three mutually orthogonal directions will differ. Flow through the thickness of a fiber preform that contains many layers of unidirectional fibers will be quite different than flow in the planar directions. In addition, the permeability of the preform depends on the fiber volume fraction of the preform. The greater the volume fraction, the lower the permeability. It is important to vent the mold to the atmosphere to remove displaced gases from the fiber preform during the mold filling process. Otherwise trapped gases will lead to voids within the laminate. After the polymer has fully impregnated the fiber preform, the third step occurs: cure. This step will begin immediately upon injection of the polymer into the mold and will occur more rapidly if the mold is at an elevated temperature. As the cure of the polymer advances to the creation of a cross-link network, it passes through a gelation phase wherein the polymer viscosity increases and transforms the polymer into a viscoelastic substance, where it possesses both viscous and elastic properties. As this process proceeds and the cross-link network continues to grow, the instantaneous glass transition temperature of the polymer increases. Finally, vitrification of the polymer occurs when its glass transition temperature exceeds the laminate temperature. Should gelation or vitrification (or both) occur prior to completion of mold filling and preform impregnation, the resulting laminate will not be fully impregnated. The viscosity of most polymers is highly dependent on temperature and polymer cure kinetics are controlled by temperature as well. Therefore, heat TX001_ch03_Frame Page 44 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
1.Bagging Film 6.Inlet Tubing 2.Distribution Tubing 7.Tool Plate 3.Release Cloth 8.Coated Tool 4.Woven Perform 9.Vacuum Tubing 5.Sealant Tape 10.Distribution Media FIGURE 3.9 VARTM process.(Courtesy of B.Grimsley,NASA Langley Research Center,2001.) transfer phenomena must be managed for successful RTM processes.Heat transfer between the polymer and the fiber preform,and between tool, preform,and polymer,as well as exothermic heat generation during the cure of the polymer,are three such phenomena that influence the process [12]. 3.1.2.1 Vacuum-Assisted Resin Transfer Molding (VARTM)Processing Both open-mold approaches,where one surface is bagged with a flexible film,and closed-mold approaches to resin transfer molding are practiced. An example of open-mold RTM,vacuum-assisted resin transfer molding (VARTM)is a common method employed as an alternative to autoclave use In VARTM,atmospheric pressure is utilized to achieve consolidation and impregnation by vacuum bagging the laminate in the same way as discussed in Section 3.1.1.An inlet for the polymer is located at one or more points in the tool or bag,and vacuum outlets are located some distance away.The vacuum pump creates a pressure gradient of approximately 1 atm within the bag,which is sufficient for the impregnation of laminates large in size and complex in geometry.For processes in which final cure occurs after the mold is filled,completion of the cure can be carried out in an oven while atmospheric pressure is maintained on the impregnated laminate. The VARTM procedure for a representative flat 61.0x 30.5 x0.64 cm panel (Figure 3.9)is described in the following steps: 1.Tool surface.The tool is a flat aluminum plate with planar dimen- sions sufficient to accommodate the proposed composite panel. First,clean the metal tool surface using sandpaper and acetone.On the cleaned surface,create a 71 x 30.5 cm picture frame using masking tape.Apply several coats of release agent to the metal surface inside of the masked frame.Remove the masking tape. ©2003 by CRC Press LLC
transfer phenomena must be managed for successful RTM processes. Heat transfer between the polymer and the fiber preform, and between tool, preform, and polymer, as well as exothermic heat generation during the cure of the polymer, are three such phenomena that influence the process [12]. 3.1.2.1 Vacuum-Assisted Resin Transfer Molding (VARTM) Processing Both open-mold approaches, where one surface is bagged with a flexible film, and closed-mold approaches to resin transfer molding are practiced. An example of open-mold RTM, vacuum-assisted resin transfer molding (VARTM) is a common method employed as an alternative to autoclave use. In VARTM, atmospheric pressure is utilized to achieve consolidation and impregnation by vacuum bagging the laminate in the same way as discussed in Section 3.1.1. An inlet for the polymer is located at one or more points in the tool or bag, and vacuum outlets are located some distance away. The vacuum pump creates a pressure gradient of approximately 1 atm within the bag, which is sufficient for the impregnation of laminates large in size and complex in geometry. For processes in which final cure occurs after the mold is filled, completion of the cure can be carried out in an oven while atmospheric pressure is maintained on the impregnated laminate. The VARTM procedure for a representative flat 61.0 × 30.5 × 0.64 cm panel (Figure 3.9) is described in the following steps: 1. Tool surface. The tool is a flat aluminum plate with planar dimensions sufficient to accommodate the proposed composite panel. First, clean the metal tool surface using sandpaper and acetone. On the cleaned surface, create a 71 × 30.5 cm picture frame using masking tape. Apply several coats of release agent to the metal surface inside of the masked frame. Remove the masking tape. FIGURE 3.9 VARTM process. (Courtesy of B. Grimsley, NASA Langley Research Center, 2001.) TX001_ch03_Frame Page 45 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
2.Bagging tape.In place of the masking tape,apply a 1.3-cm-wide silicone bagging tape to the bare metal surface.The silicone tape should again form a 71 x 30.5 cm frame.Add a strip of the tape, 5 cm in length,to the outer edge of the length of the frame at either end.These two strips will provide an added adhesive sur- face for attachment of the inlet and outlet tubing.Leave the paper backing on the silicone tape to protect it during the remainder of the lay-up procedure. 3.Preform.Place the fiber preform stack on the coated tool,inside the tape frame.A 5.1-cm gap should exist between the silicone tape and both edges of the preform to allow room for tubing.No gap should exist between the silicon tape and fiber preform along the panel width to avoid providing a flow pathway outside the pre- form to the vacuum port. 4.Release cloth.Cut one layer of porous release film to 66 x 30.5 cm, and place it on top of the preform.Place the cloth so that it completely covers the preform and allow 5.1 cm in length to overhang and contact the coated metal surface at the injection side of the lay-up. The release film will allow the composite laminate to release from the distribution media.Cut a second piece of release cloth to 5.1 x 30.5 cm, and place it on the tool surface at the vacuum side of the preform. This patch of cloth provides a clear path for the vacuum. 5.Distribution media.Cut one to six layers of highly permeable distri- bution media,e.g.,biplanar nylon 6 mesh to dimensions of 63.5 x 28.0 cm and stack them above the ArmalonTM release cloth.Place the layers of media so that a 2.5-cm gap exists on the top of the preform at the vacuum end.This gap will force the resin to fill through the thickness rather than be drawn directly into the vacuum port.The length of this gap will vary with the desired thickness of the com- posite panel.A 1.3-cm gap should exist between the media and the sides of the preform.This will help prevent resin flow outside the preform.A 5-cm length of the media will overhang the preform at the resin inlet end of the lay-up. 6.Distribution tubing.Place a 28.0-cm length of distribution tubing across the width of the laminate at points 2.5 cm in front of the preform (inlet)and 2.5 cm away from the preform (vacuum).On the inlet side,place the tubing on top of the distribution media that overhangs the preform.At the vacuum side,place the tubing on the 5x 30.5 cm piece of release cloth.Spiral-wrap,18-mm-diameter conduit is an ideal choice for the distribution tubing because it allows the resin to flow quickly into the distribution media and preform in a continuous line across the width.A plastic tube with holes at 2.5-cm intervals also works well.Attach a 13-mm portion of the spiral tubing to both the inlet-supply tubing and the vacuum 2003 by CRC Press LLC
2. Bagging tape. In place of the masking tape, apply a 1.3-cm-wide silicone bagging tape to the bare metal surface. The silicone tape should again form a 71 × 30.5 cm frame. Add a strip of the tape, 5 cm in length, to the outer edge of the length of the frame at either end. These two strips will provide an added adhesive surface for attachment of the inlet and outlet tubing. Leave the paper backing on the silicone tape to protect it during the remainder of the lay-up procedure. 3. Preform. Place the fiber preform stack on the coated tool, inside the tape frame. A 5.1-cm gap should exist between the silicone tape and both edges of the preform to allow room for tubing. No gap should exist between the silicon tape and fiber preform along the panel width to avoid providing a flow pathway outside the preform to the vacuum port. 4. Release cloth. Cut one layer of porous release film to 66 × 30.5 cm, and place it on top of the preform. Place the cloth so that it completely covers the preform and allow 5.1 cm in length to overhang and contact the coated metal surface at the injection side of the lay-up. The release film will allow the composite laminate to release from the distribution media. Cut a second piece of release cloth to 5.1 × 30.5 cm, and place it on the tool surface at the vacuum side of the preform. This patch of cloth provides a clear path for the vacuum. 5. Distribution media. Cut one to six layers of highly permeable distribution media, e.g., biplanar nylon 6 mesh to dimensions of 63.5 × 28.0 cm and stack them above the Armalon™ release cloth. Place the layers of media so that a 2.5-cm gap exists on the top of the preform at the vacuum end. This gap will force the resin to fill through the thickness rather than be drawn directly into the vacuum port. The length of this gap will vary with the desired thickness of the composite panel. A 1.3-cm gap should exist between the media and the sides of the preform. This will help prevent resin flow outside the preform. A 5-cm length of the media will overhang the preform at the resin inlet end of the lay-up. 6. Distribution tubing. Place a 28.0-cm length of distribution tubing across the width of the laminate at points 2.5 cm in front of the preform (inlet) and 2.5 cm away from the preform (vacuum). On the inlet side, place the tubing on top of the distribution media that overhangs the preform. At the vacuum side, place the tubing on the 5 × 30.5 cm piece of release cloth. Spiral-wrap, 18-mm-diameter conduit is an ideal choice for the distribution tubing because it allows the resin to flow quickly into the distribution media and preform in a continuous line across the width. A plastic tube with holes at 2.5-cm intervals also works well. Attach a 13-mm portion of the spiral tubing to both the inlet–supply tubing and the vacuum TX001_ch03_Frame Page 46 Saturday, September 21, 2002 4:51 AM © 2003 by CRC Press LLC
