1.3 Processing of Composite Materials 11 Unidirectional composites suffer from poor mechanical properties in the trans- verse direction(i.e.,perpendicular to the direction of pultrusion).Thus,structural composites are usually not unidirectional.Composites with continuous fibers in various orientations are commonly fabricated by hand lay-up of unidirectional fiber tapes or woven fabrics and impregnation with a resin.The molding,called bag molding,is done by placing the tapes or fabrics in a die and introducing high- pressure gases or a vacuum via a bag to force the individual plies together.Bag mold- ing is widely used to fabricate large composite components for the skins of aircraft. The fabrication ofa high-performance polymer-matrix composite laminate con- taining continuous fibers commonly involves prepreg sheets.A prepreg is a sheet of continuous oriented fibers that have been impregnated with a polymer or a polymer precursor.An example ofa polymer precursor is an epoxy resin,which upon subse- quent curing(usually under heat and pressure)forms epoxy polymer,a thermoset. Figure 1.2 shows a scanning electron microscope image of the surface of a carbon fiber epoxy prepreg.The fibers cannot be discerned because they are covered with the resin.An example of a polymer in a prepreg is nylon,which is a thermoplastic polymer.The softening of the thermoplastic polymer upon heating above the glass transition temperature allows the prepreg to be flexible,thus conforming to the desired shape.In the case of an epoxy resin prepreg,the prepreg is flexible and tacky as long as the resin has not been cured.In order to increase the usable period of an epoxy resin prepreg,the prepreg is typically stored in a freezer,as the low temperature in the freezer helps slow down the curing process.Even with storage in a freezer,the usable period of an epoxy resin prepreg is only a few months. Instead of using unidirectional fibers,one can use a woven fiber fabric.The fabric may be impregnated with the resin or the polymer prior to being stacked 100μm200X Figure 1.2.A scanning electron microscope image of the surface of a carbon fiber epoxy prepreg.The fibers cannot be discemed due to their being covered with the resin
1.3 Processing of Composite Materials 11 Unidirectional composites suffer from poor mechanical properties in the transverse direction (i.e., perpendicular to the direction of pultrusion). Thus, structural composites are usually not unidirectional. Composites with continuous fibers in various orientations are commonly fabricated by hand lay-up of unidirectional fiber tapes or woven fabrics and impregnation with a resin. The molding, called bag molding, is done by placing the tapes or fabrics in a die and introducing highpressuregasesoravacuumviaabagtoforcetheindividualpliestogether.Bagmolding is widely used to fabricate large composite components for the skins of aircraft. The fabrication of a high-performance polymer-matrix composite laminate containing continuous fibers commonly involves prepreg sheets. A prepreg is a sheet of continuousorientedfibersthathavebeenimpregnatedwithapolymerorapolymer precursor. An example of a polymer precursor is an epoxy resin, which upon subsequent curing (usually under heat and pressure) forms epoxy polymer, a thermoset. Figure 1.2 shows a scanning electron microscope image of the surface of a carbon fiber epoxy prepreg. The fibers cannot be discerned because they are covered with the resin. An example of a polymer in a prepreg is nylon, which is a thermoplastic polymer. The softening of the thermoplastic polymer upon heating above the glass transition temperature allows the prepreg to be flexible, thus conforming to the desired shape. In the case of an epoxy resin prepreg, the prepreg is flexible and tacky as long as the resin has not been cured. In order to increase the usable period of an epoxy resin prepreg, the prepreg is typically stored in a freezer, as the low temperature in the freezer helps slow down the curing process. Even with storage in a freezer, the usable period of an epoxy resin prepreg is only a few months. Instead of using unidirectional fibers, one can use a woven fiber fabric. The fabric may be impregnated with the resin or the polymer prior to being stacked Figure 1.2. A scanning electron microscope image of the surface of a carbon fiber epoxy prepreg. The fibers cannot be discerned due to their being covered with the resin
12 1 Composite Material Structure and Processing and consolidated to form a composite.Related to weaving is braiding,which is commonly used to form a composite tubing. The fabric may be stacked in the absence ofa resin and then the resin in infiltrated into the stack-a process known as resin transfer molding(RTM).The advantage of using a fabric is that fabrics are easy to handle.RTM is attractive in that it allows the fabrication of composites of intricate shapes.In RTM,a fiber preform (usually prepared by weaving or braiding and held under compression in a mold) is impregnated with a resin.The resin is admitted at one end of the mold and is forced by pressure through the mold and preform.The resin is subsequently cured. This method is limited to resins oflow viscosity,such as epoxy.A problem with this process is the formation of surface voids by the volatilization of dissolved gases in the resin,the partial evaporation of mold releasing agent into the preform,or the mechanical entrapment of gas bubbles. The weaving of fibers to form a fabric results in the local bending of the fibers in the resulting fabric.The bending occurs where a fiber in one direction crosses that in another direction,as illustrated in Fig.1.3.Due to this bending,the me- chanical properties of a composite made from fabric tend to be inferior to those of a composite made from unidirectional prepregs. Most of the composite fabrication methods mentioned above involve the im- pregnation of the fibers with a resin.In the case of a thermoset,the resin is a liquid that has not been polymerized or is partially polymerized.In the case of a thermoplastic,the resin is either the polymer melt or the polymer dissolved in the solvent.After resin application,solid thermoplastic results from solidification in the case of melt impregnation,and from evaporation in the case of solution impregnation.Both amorphous and semicrystalline thermoplastics can be melt processed,but only the amorphous resins can normally be dissolved.Because of the high melt viscosities of semicrystalline thermoplastics (due to their long and rigid macromolecular chains),direct melt impregnation of semicrystalline thermoplastics is difficult.Melt impregnation followed by solidification produces a thermoplastic prepreg that is stiff and lacks tack;solution impregnation usually produces prepregs that are drapeable and tacky,although this character changes as solvent evaporation occurs from the solution.The drapeable and tacky character of thermoplastic prepregs made by solution impregnation is comparable to that of thermoset prepregs.Hence,the main problem with resin impregnation occurs for semicrystalline thermoplastics.Instead of thermoplastic impregnation of fibers by using a melt or a solution of the thermoplastic,solid thermoplastic in the form of powder,fibers,or slurries can be impregnated. Figure 1.3.The bending of a fiber in a woven fabric.The cirde represents a fiber that is perpendicular to the paper.The other paper,which is in the plane of the paper,bends around it
12 1 Composite Material Structure and Processing and consolidated to form a composite. Related to weaving is braiding, which is commonly used to form a composite tubing. Thefabricmaybestackedintheabsenceofaresinandthentheresinininfiltrated into the stack – a process known as resin transfer molding (RTM). The advantage of using a fabric is that fabrics are easy to handle. RTM is attractive in that it allows the fabrication of composites of intricate shapes. In RTM, a fiber preform (usually prepared by weaving or braiding and held under compression in a mold) is impregnated with a resin. The resin is admitted at one end of the mold and is forced by pressure through the mold and preform. The resin is subsequently cured. This method is limited to resins of low viscosity, such as epoxy. A problem with this process is the formation of surface voids by the volatilization of dissolved gases in the resin, the partial evaporation of mold releasing agent into the preform, or the mechanical entrapment of gas bubbles. The weaving of fibers to form a fabric results in the local bending of the fibers in the resulting fabric. The bending occurs where a fiber in one direction crosses that in another direction, as illustrated in Fig. 1.3. Due to this bending, the mechanical properties of a composite made from fabric tend to be inferior to those of a composite made from unidirectional prepregs. Most of the composite fabrication methods mentioned above involve the impregnation of the fibers with a resin. In the case of a thermoset, the resin is a liquid that has not been polymerized or is partially polymerized. In the case of a thermoplastic, the resin is either the polymer melt or the polymer dissolved in the solvent. After resin application, solid thermoplastic results from solidification in the case of melt impregnation, and from evaporation in the case of solution impregnation. Both amorphous and semicrystalline thermoplastics can be melt processed, but only the amorphous resins can normally be dissolved. Because of the high melt viscosities of semicrystalline thermoplastics (due to their long and rigid macromolecular chains), direct melt impregnation of semicrystalline thermoplastics is difficult. Melt impregnation followed by solidification produces a thermoplastic prepreg that is stiff and lacks tack; solution impregnation usually produces prepregs that are drapeable and tacky, although this character changes as solvent evaporation occurs from the solution. The drapeable and tacky character of thermoplastic prepregs made by solution impregnation is comparable to that of thermoset prepregs. Hence, the main problem with resin impregnation occurs for semicrystalline thermoplastics. Instead of thermoplastic impregnation of fibers by using a melt or a solution of the thermoplastic, solid thermoplastic in the form of powder, fibers, or slurries can be impregnated. Figure 1.3. The bending of a fiber in a woven fabric. The circle represents a fiber that is perpendicular to the paper. The other paper, which is in the plane of the paper, bends around it