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