instead of polyester mat materials due to the low melting  point of polyester. Polybenzimidazole resin is extremely high temperature  resistant and is used for high-temperature materials. These  resins are available as adhesive and fiber.  Bismaleimide resins have a higher temperature capability  and higher toughness than epoxy resins, and they provide  excellent performance at ambient and elevated temperatures.  The processing of bismaleimide resins is similar to that  for epoxy resins. BMIs are used for aero engines and high  temperature components. BMIs are suitable for standard  autoclave processing, injection molding, resin transfer  molding, and sheet molded compound (SMC) among others. Thermoplastic materials can be softened repeatedly by  an increase of temperature and hardened by a decrease in  temperature. Processing speed is the primary advantage of  thermoplastic materials. Chemical curing of the material  does not take place during processing, and the material can  be shaped by molding or extrusion when it is soft. Semicrystalline thermoplastics possess properties of inherent  flame resistance, superior toughness, good mechanical  properties at elevated temperatures and after impact, and  low moisture absorption. They are used in secondary and  primary aircraft structures. Combined with reinforcing  fibers, they are available in injection molding compounds,  compression-moldable random sheets, unidirectional tapes,  prepregs fabricated from tow (towpreg), and woven prepregs.  Fibers impregnated in semicrystalline thermoplastics  include carbon, nickel-coated carbon, aramid, glass, quartz, and others.  Amorphous thermoplastics are available in several physical  forms, including films, filaments, and powders. Combined  with reinforcing fibers, they are also available in injection  molding compounds, compressive moldable random sheets,  unidirectional tapes, woven prepregs, etc. The fibers used are  primarily carbon, aramid, and glass. The specific advantages  of amorphous thermoplastics depend upon the polymer.  Typically, the resins are noted for their processing ease  and speed, high temperature capability, good mechanical  properties, excellent toughness and impact strength,  and chemical stability. The stability results in unlimited  shelf life, eliminating the cold storage requirements of  thermoset prepregs. Polyether ether ketone, better known as PEEK, is a high- temperature thermoplastic. This aromatic ketone material  offers outstanding thermal and combustion characteristics  and resistance to a wide range of solvents and proprietary  fluids. PEEK can also be reinforced with glass and carbon. Thermosetting resins use a chemical reaction to cure. There  are three curing stages, which are called A, B, and C.  Prepreg material consists of a combination of a matrix and  fiber reinforcement. It is available in unidirectional form  (one direction of reinforcement) and fabric form (several  directions of reinforcement). All five of the major families of  matrix resins can be used to impregnate various fiber forms.  The resin is then no longer in a low-viscosity stage, but has  been advanced to a B stage level of cure for better handling  characteristics. The following products are available in  prepreg form: unidirectional tapes, woven fabrics, continuous  strand rovings, and chopped mat. Prepreg materials must be  stored in a freezer at a temperature below 0 °F to retard the  curing process. Prepreg materials are cured with an elevated  temperature. Many prepreg materials used in aerospace  are impregnated with an epoxy resin and they are cured at  either 250 °F or 350 °F. Prepreg materials are cured with an  autoclave, oven, or heat blanket. They are typically purchased  and stored on a roll in a sealed plastic bag to avoid moisture  contamination.  [Figure 7-11]  Dry fiber materials, such as carbon, glass, and Kevlar$^{®}$ are  used for many aircraft repair procedures. The dry fabric is  impregnated with a resin just before the repair work starts.  This process is often called wet layup. The main advantage  of using the wet layup process is that the fiber and resin can  Dry Fiber Material Amorphous Thermoplastics Pre-Impregnated Products (Prepregs) Semicrystalline Thermoplastics Thermoplastic Resins Bismaleimides (BMI) Polybenzimidazoles (PBI) Polyether Ether Ketone (PEEK)  Curing Stages of Resins •  A stage: The components of the resin (base material  and hardener) have been mixed but the chemical  reaction has not started. The resin is in the A stage  during a wet layup procedure. •  B stage: The components of the resin have been mixed  and the chemical reaction has started. The material has  thickened and is tacky. The resins of prepreg materials  are in the B stage. To prevent further curing the resin  is placed in a freezer at 0 °F. In the frozen state, the  resin of the prepreg material stays in the B stage. The  curing starts when the material is removed from the  freezer and warmed again. •  C stage: The resin is fully cured. Some resins cure  at room temperature and others need an elevated  temperature cure cycle to fully cure. 7-8