where A is a constant vo is the initial fiber volume fraction in the fiber network(before compaction) Vr is the fiber volume fraction at any instant during compaction Va is the maximum possible fiber volume fraction The constant A in Equation 5.11 depends on the fiber stiffness and the fiber waviness,and is a measure of the deformability of the fiber network.Since the fiber volume fraction,ve,increases with increasing compaction,Equation 5.11 predicts that o also increases with increasing compaction,that is,the fiber network begins to take up an increasing amount of the applied pressure.On the other hand,the average pressure on the resin decreases with increasing compaction,which can lead to void formation. 5.1.5 GEL-TIME TEST The curing characteristics of a resin-catalyst combination are frequently deter- mined by the gel-time test.In this test,a measured amount(10 g)of a thoroughly mixed resin-catalyst combination is poured into a standard test tube.The temperature rise in the material is monitored as a function of time by means of a thermocouple while the test tube is suspended in a 82C(180F)water bath. A typical temperature-time curve (also known as exotherm curve)obtained in a gel-time test is illustrated in Figure 5.7.On this curve,point A indicates the time required for the resin-catalyst mixture to attain the bath temperature.The begin- ning of temperature rise indicates the initiation of the curing reaction.As the curing reaction begins,the liquid mix begins to transform into a gel-like mass. Heat generated by the exothermic curing reaction increases the mix temperature, which in turn causes the catalyst to decompose at a faster rate and the reaction to proceed at a progressively increasing speed.Since the rate of heat generation is higher than the rate of heat loss to the surrounding medium,the temperature rises rapidly to high values.As the curing reaction nears completion,the rate of heat generation is reduced and a decrease in temperature follows.The exothermic peak temperature observed in a gel-time test is a function of the resin chemistry (level of unsaturation)and the resin-catalyst ratio.The slope of the exotherm curve is a measure of cure rate,which depends primarily on the catalyst reactivity. Shortly after the curing reaction begins at point A,the resin viscosity increases very rapidly owing to the increasing number of cross-links formed by the curing reaction.The time at which a rapid increase in viscosity ensues is called the gel time and is indicated by point B in Figure 5.7.According to one standard,the time at which the exotherm temperature increases by 5.5C(10F) above the bath temperature is considered the gel time.It is sometimes measured by probing the surface of the reacting mass with a clean wooden applicator stick every 15 s until the reacting material no longer adheres to the end of a clean stick. 2007 by Taylor Francis Group.LLC
where A is a constant vo is the initial fiber volume fraction in the fiber network (before compaction) vf is the fiber volume fraction at any instant during compaction va is the maximum possible fiber volume fraction The constant A in Equation 5.11 depends on the fiber stiffness and the fiber waviness, and is a measure of the deformability of the fiber network. Since the fiber volume fraction, vf, increases with increasing compaction, Equation 5.11 predicts that s also increases with increasing compaction, that is, the fiber network begins to take up an increasing amount of the applied pressure. On the other hand, the average pressure on the resin decreases with increasing compaction, which can lead to void formation. 5.1.5 GEL-TIME TEST The curing characteristics of a resin–catalyst combination are frequently determined by the gel-time test. In this test, a measured amount (10 g) of a thoroughly mixed resin–catalyst combination is poured into a standard test tube. The temperature rise in the material is monitored as a function of time by means of a thermocouple while the test tube is suspended in a 828C (1808F) water bath. A typical temperature–time curve (also known as exotherm curve) obtained in a gel-time test is illustrated in Figure 5.7. On this curve, point A indicates the time required for the resin–catalyst mixture to attain the bath temperature. The beginning of temperature rise indicates the initiation of the curing reaction. As the curing reaction begins, the liquid mix begins to transform into a gel-like mass. Heat generated by the exothermic curing reaction increases the mix temperature, which in turn causes the catalyst to decompose at a faster rate and the reaction to proceed at a progressively increasing speed. Since the rate of heat generation is higher than the rate of heat loss to the surrounding medium, the temperature rises rapidly to high values. As the curing reaction nears completion, the rate of heat generation isreduced and a decrease in temperature follows. The exothermic peak temperature observed in a gel-time test is a function of the resin chemistry (level of unsaturation) and the resin–catalyst ratio. The slope of the exotherm curve is a measure of cure rate, which depends primarily on the catalyst reactivity. Shortly after the curing reaction begins at point A, the resin viscosity increases very rapidly owing to the increasing number of cross-links formed by the curing reaction. The time at which a rapid increase in viscosity ensues is called the gel time and is indicated by point B in Figure 5.7. According to one standard, the time at which the exotherm temperature increases by 5.58C (108F) above the bath temperature is considered the gel time. It is sometimes measured by probing the surface of the reacting mass with a clean wooden applicator stick every 15 s until the reacting material no longer adheres to the end of a clean stick. 2007 by Taylor & Francis Group, LLC
400 Peak exotherm temperature =385℉ 350 300 E 250 10F 200 B 180 Bath temp. A 150 3.1 min 4.2 min 6.1 min 100 50 ⊙ 8 10 12 Time (min) FIGURE 5.7 Typical temperature-time curve obtained in a gel-time test. 5.1.6 SHRINKAGE Shrinkage is the reduction in volume or linear dimensions caused by curing as well as thermal contraction.Curing shrinkage occurs because of the rearrange- ment of polymer molecules into a more compact mass as the curing reaction proceeds.The thermal shrinkage occurs during the cooling period that follows the curing reaction and may take place both inside and outside the mold. The volumetric shrinkage for cast-epoxy resins is of the order of 1%-5% and that for polyester and vinyl ester resins may range from 5%to 12%.The addition of fibers or fillers reduces the volumetric shrinkage of a resin.How- ever,in the case of unidirectional fibers,the reduction in shrinkage in the longitudinal direction is higher than in the transverse direction. High shrinkage in polyester or vinyl ester resins can be reduced significantly by the addition of low shrink additives (also called low-profile agents),which are thermoplastic polymers,such as polyethylene,polymethyl acrylate,polyvi- nyl acetate,and polycaprolactone (see Chapter 2).These thermoplastic addi- tives are usually mixed in styrene monomer during blending with the liquid 2007 by Taylor Francis Group,LLC
5.1.6 SHRINKAGE Shrinkage is the reduction in volume or linear dimensions caused by curing as well as thermal contraction. Curing shrinkage occurs because of the rearrangement of polymer molecules into a more compact mass as the curing reaction proceeds. The thermal shrinkage occurs during the cooling period that follows the curing reaction and may take place both inside and outside the mold. The volumetric shrinkage for cast-epoxy resins is of the order of 1%–5% and that for polyester and vinyl ester resins may range from 5% to 12%. The addition of fibers or fillers reduces the volumetric shrinkage of a resin. However, in the case of unidirectional fibers, the reduction in shrinkage in the longitudinal direction is higher than in the transverse direction. High shrinkage in polyester or vinyl ester resins can be reduced significantly by the addition of low shrink additives (also called low-profile agents), which are thermoplastic polymers, such as polyethylene, polymethyl acrylate, polyvinyl acetate, and polycaprolactone (see Chapter 2). These thermoplastic additives are usually mixed in styrene monomer during blending with the liquid Peak exotherm temperature = 3858F 400 350 250 300 200 180 150 100 50 2 4 6 8 10 12 Time (min) 3.1 min 4.2 min 6.1 min A B Bath temp. 10F Temperature (F) FIGURE 5.7 Typical temperature–time curve obtained in a gel-time test. 2007 by Taylor & Francis Group, LLC.
resin.On curing,the thermoplastic polymer becomes incompatible with the cross-linked resin and forms a dispersed second phase in the cured resin.High resin shrinkage is desirable for easy release of the part from the mold surface; however,at the same time,high resin shrinkage can contribute to many molding defects,such as warpage and sink marks.These defects are described in Section 5.3. 5.1.7Vo1Ds Among the various defects produced during the molding of a composite lamin- ate,the presence of voids is considered the most critical defect in influencing its mechanical properties.The most common cause for void formation is the inability of the resin to displace air from the fiber surface during the time fibers are coated with the liquid resin.The rate at which the fibers are pulled through the liquid resin,the resin viscosity,the relative values of fiber and resin surface energies,and the mechanical manipulation of fibers in the liquid resin affect air entrapment at the fiber-resin interface.Voids may also be caused by air bubbles and volatiles entrapped in the liquid resin.Solvents used for resin viscosity control,moisture,and chemical contaminants in the resin,as well as styrene monomer,may remain dissolved in the resin mix and volatilize during elevated temperature curing.In addition,air is also entrapped between various layers during the lamination process. Much of the air or volatiles entrapped at the premolding stages can be removed by(1)degassing the liquid resin,(2)applying vacuum during the molding process,and (3)allowing the resin mix to flow freely in the mold,which helps in carrying the air and volatiles out through the vents in the mold.The various process parameters controlling the resin flow are described in later sections. The presence of large volume fractions of voids in a composite laminate can significantly reduce its tensile,compressive,and flexural strengths.Large reduc- tions in interlaminar shear strength are observed even if the void content is only 2%-3%by volume (Figure 5.8).The presence of voids generally increases the rate and amount of moisture absorption in a humid environment,which in turn increases the physical dimensions of the part and reduces its matrix-dominated properties. 5.2 BAG-MOLDING PROCESS The bag-molding process is used predominantly in the aerospace industry where high production rate is not an important consideration.The starting material for bag-molding processes is a prepreg that contains fibers in a partially cured(B-staged)epoxy resin.Typically,a prepreg contains 42 wt% of resin.If this prepreg is allowed to cure without any resin loss,the cured laminate would contain 50 vol%of fibers.Since nearly 10 wt%of resin flows out during the molding process,the actual fiber content in the cured laminate is 2007 by Taylor Francis Group.LLC
resin. On curing, the thermoplastic polymer becomes incompatible with the cross-linked resin and forms a dispersed second phase in the cured resin. High resin shrinkage is desirable for easy release of the part from the mold surface; however, at the same time, high resin shrinkage can contribute to many molding defects, such as warpage and sink marks. These defects are described in Section 5.3. 5.1.7 VOIDS Among the various defects produced during the molding of a composite laminate, the presence of voids is considered the most critical defect in influencing its mechanical properties. The most common cause for void formation is the inability of the resin to displace air from the fiber surface during the time fibers are coated with the liquid resin. The rate at which the fibers are pulled through the liquid resin, the resin viscosity, the relative values of fiber and resin surface energies, and the mechanical manipulation of fibers in the liquid resin affect air entrapment at the fiber–resin interface. Voids may also be caused by air bubbles and volatiles entrapped in the liquid resin. Solvents used for resin viscosity control, moisture, and chemical contaminants in the resin, as well as styrene monomer, may remain dissolved in the resin mix and volatilize during elevated temperature curing. In addition, air is also entrapped between various layers during the lamination process. Much of the air or volatiles entrapped at the premolding stages can be removed by (1) degassing the liquid resin, (2) applying vacuum during the molding process, and (3) allowing the resin mix to flow freely in the mold, which helps in carrying the air and volatiles out through the vents in the mold. The various process parameters controlling the resin flow are described in later sections. The presence of large volume fractions of voids in a composite laminate can significantly reduce its tensile, compressive, and flexural strengths. Large reductions in interlaminar shear strength are observed even if the void content is only 2%–3% by volume (Figure 5.8). The presence of voids generally increases the rate and amount of moisture absorption in a humid environment, which in turn increases the physical dimensions of the part and reduces its matrix-dominated properties. 5.2 BAG-MOLDING PROCESS The bag-molding process is used predominantly in the aerospace industry where high production rate is not an important consideration. The starting material for bag-molding processes is a prepreg that contains fibers in a partially cured (B-staged) epoxy resin. Typically, a prepreg contains 42 wt% of resin. If this prepreg is allowed to cure without any resin loss, the cured laminate would contain 50 vol% of fibers. Since nearly 10 wt% of resin flows out during the molding process, the actual fiber content in the cured laminate is 2007 by Taylor & Francis Group, LLC
20 AS carbon fiber-epoxy (12-Ply-unidirectional) 18 0 Autoclave 16 pressure: o50 psi △100psi 14 ▣150psi Fiber volume fraction 60%-70% △ 10 0 2 4 6 8 10 Void content(%) FIGURE 5.8 Effect of void volume fraction on the interlaminar shear strength of a composite laminate.(After Yokota,M.J.,SAMPE J.,11,1978.) 60 vol%which is considered an industry standard for aerospace applications. The excess resin flowing out from the prepreg removes the entrapped air and residual solvents,which in turn reduces the void content in the laminate. However,the recent trend is to employ a near-net resin content,typically 34 wt%,and to allow only 1-2 wt%resin loss during molding. Figure 5.9 shows the schematic of a bag-molding process.The mold surface is covered with a Teflon-coated glass fabric separator (used for preventing sticking in the mold)on which the prepreg plies are laid up in the desired fiber orientation angle as well as in the desired sequence.Plies are trimmed from the prepreg roll into the desired shape,size,and orientation by means of a cutting device,which may simply be a mat knife.Laser beams,high-speed water jets,or trimming dies are also used.The layer-by-layer stacking operation can be performed either manually (by hand)or by numerically controlled automatic tape-laying machines.Before laying up the prepreg,the backup release film is peeled off from each ply.Slight compaction pressure is applied to adhere the prepreg to the Teflon-coated glass fabric or to the preceding ply in the layup. After the layup operation is complete,a porous release cloth and a few layers of bleeder papers are placed on top of the prepreg stack.The bleeder papers are used to absorb the excess resin in the prepreg as it flows out during the molding process.The complete layup is covered with another sheet of 2007 by Taylor Francis Group,LLC
60 vol% which is considered an industry standard for aerospace applications. The excess resin flowing out from the prepreg removes the entrapped air and residual solvents, which in turn reduces the void content in the laminate. However, the recent trend is to employ a near-net resin content, typically 34 wt%, and to allow only 1–2 wt% resin loss during molding. Figure 5.9 shows the schematic of a bag-molding process. The mold surface is covered with a Teflon-coated glass fabric separator (used for preventing sticking in the mold) on which the prepreg plies are laid up in the desired fiber orientation angle as well as in the desired sequence. Plies are trimmed from the prepreg roll into the desired shape, size, and orientation by means of a cutting device, which may simply be a mat knife. Laser beams, high-speed water jets, or trimming dies are also used. The layer-by-layer stacking operation can be performed either manually (by hand) or by numerically controlled automatic tape-laying machines. Before laying up the prepreg, the backup release film is peeled off from each ply. Slight compaction pressure is applied to adhere the prepreg to the Teflon-coated glass fabric or to the preceding ply in the layup. After the layup operation is complete, a porous release cloth and a few layers of bleeder papers are placed on top of the prepreg stack. The bleeder papers are used to absorb the excess resin in the prepreg as it flows out during the molding process. The complete layup is covered with another sheet of AS carbon fiber–epoxy (12-Ply–unidirectional) Autoclave pressure: 20 18 16 14 Interlaminar shear strength (ksi) 12 10 8 024 Void content (%) 6 8 10 Fiber volume fraction 60%–70% 50 psi 100 psi 150 psi FIGURE 5.8 Effect of void volume fraction on the interlaminar shear strength of a composite laminate. (After Yokota, M.J., SAMPE J., 11, 1978.) 2007 by Taylor & Francis Group, LLC
Vacuum bag Plastic breather Nonporous teflon Aluminum caul plate Nonporous teflon Bleeder Porous teflon Laminate Porous teflon Bleeder Nonporous teflon Dam Aluminum tool plate FIGURE 5.9 Schematic of a bag-molding process. Teflon-coated glass fabric separator,a caul plate,and then a thin heat-resistant vacuum bag,which is closed around its periphery by a sealant.The entire assembly is placed inside an autoclave where a combination of external pres- sure,vacuum,and heat is applied to consolidate and densify separate plies into a solid laminate.The vacuum is applied to remove air and volatiles,while the pressure is required to consolidate individual layers into a laminate. As the prepreg is heated in the autoclave,the resin viscosity in the B-staged prepreg plies first decreases,attains a minimum,and then increases rapidly (gels)as the curing(cross-linking)reaction begins and proceeds toward com- pletion.Figure 5.10 shows a typical two-stage cure cycle for a carbon fiber- epoxy prepreg.The first stage in this cure cycle consists of increasing the temperature at a controlled rate (say,2C/min)up to 130C and dwelling at this temperature for nearly 60 min when the minimum resin viscosity is reached. 200， Cure 2 h/175C Temparature 150 Vacuum 81 0 02 6 Dwell 100 0.4 4 Pressure 0.6 (sJeq)ensseld 50 2 0.8 0 0 60 120 180 240 300 Time(min) FIGURE 5.10 Typical two-stage cure cycle for a carbon fiber-epoxy prepreg. 2007 by Taylor Francis Group.LLC
Teflon-coated glass fabric separator, a caul plate, and then a thin heat-resistant vacuum bag, which is closed around its periphery by a sealant. The entire assembly is placed inside an autoclave where a combination of external pressure, vacuum, and heat is applied to consolidate and densify separate plies into a solid laminate. The vacuum is applied to remove air and volatiles, while the pressure is required to consolidate individual layers into a laminate. As the prepreg is heated in the autoclave, the resin viscosity in the B-staged prepreg plies first decreases, attains a minimum, and then increases rapidly (gels) as the curing (cross-linking) reaction begins and proceeds toward completion. Figure 5.10 shows a typical two-stage cure cycle for a carbon fiber– epoxy prepreg. The first stage in this cure cycle consists of increasing the temperature at a controlled rate (say, 28C=min) up to 1308C and dwelling at this temperature for nearly 60 min when the minimum resin viscosity is reached. Vacuum bag Plastic breather Nonporous teflon Aluminum caul plate Nonporous teflon Bleeder Porous teflon Laminate Porous teflon Bleeder Dam Aluminum tool plate Nonporous teflon FIGURE 5.9 Schematic of a bag-molding process. 200 150 100 0 0 0 8 2 0 4 0.2 6 0.6 0.8 0.4 120 Time (min) Pressure Dwell Temparature Vacuum Cure 2 h/175C Temperature ( Vacuum (bars) Pressure (bars) C) 60 180 240 300 50 FIGURE 5.10 Typical two-stage cure cycle for a carbon fiber–epoxy prepreg. 2007 by Taylor & Francis Group, LLC.