The manufacture of high performance polymer matrix composite laminates with continuous fibers usually involves prepreg sheets. Prepreg is a continuous fiber impregnated with polymer or polymer precursor. An example of a polymer precursor is an epoxy resin, which is subsequently cured (usually at high temperature and pressure) to form an epoxy polymer, a hot solid. Figure 1.2 shows the SEM image of the surface of the carbon fiber epoxy prepreg. The fibers are covered with resin, so they cannot be identified. An example of a polymer in a prepreg is nylon, which is a thermoplastic polymer. The softening of the thermoplastic polymer when it is heated above the glass transition temperature makes the prepreg flexible, thus meeting the expected shape. In the case of epoxy resin prepreg, as long as the resin is not cured, the prepreg is soft and sticky. In order to prolong the service life of the epoxy resin prepreg, the prepreg is usually stored in the refrigerator, because the low temperature in the refrigerator helps to slow down the curing process. Even in refrigerators, the life of epoxy prepreg is only a few months.
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