290 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES sure required for processing PEEK at 400C would be 2000 times more than that of the case of epoxy at 20C.If a pressure of 689 kPa is required for the processing of epoxy at 20C,then it would take a pressure of 1378 MPa to process PEEK at 400C for the same processing time.This is not practical. In the face of the above difficulties,it is desirable to find ways to manu- facture thermoplastic matrix composites because they can offer many ad- vantages,such as the following. a.Thermoplastic resin does not have the constraint of shelf life.To put it another way,thermoplastic composite preforms have infinite shelf life.This is because thermoplastic matrix solidifies upon cooling and there is no chemical reaction taking place to change its liquid state to solid state. b.Thermoplastic resins are more ductile than thermoset resins.Com- posites made of thermoplastic matrix composites therefore can have larger fracture toughness than those made of thermoset matrix composites.The strain energy release rate for carbon/epoxy is about 100 J/m while that of carbon/PEEK is about 1000 J/m.This means that the thermoplastic matrix resin has large plastic deforma- tion whereas thermoset matrix material does not.This is very im- portant for structures that need to resist impact and fracture load. c.Thermoplastic resins can be recycled,i.e.,they can be reheated to take a different form.This is also due to the same reason indicated above in which the material can be heated up to change the solid state to the melt state.At temperature close to the melt point,differ- ent shapes can be formed to make a new product.This behavior can also be used to heal the defects that may exist in the structure. d.The processing time can be fast.The heating and cooling of a ther- moplastic composite material can take place within the order of minutes.This is in comparison with the order of hours for thermoset matrix material.The reason for the long processing time for the thermoset matrix resin is because of the time required for all the chemical bonds to take place.In the case of thermoplastic resin, cooling will solidify the material and this can take place very quickly. e.Thermoplastic matrix resin is weldable.This means that solid parts made of thermoplastic matrix composites may be welded together. For thermoset resins,because of the chemical bonding required for solidification,once the bond is formed,it cannot be used again to form another bond.The weldability of the thermoplastic resin en-sure required for processing PEEK at 400°C would be 2000 times more than that of the case of epoxy at 20°C. If a pressure of 689 kPa is required for the processing of epoxy at 20°C, then it would take a pressure of 1378 MPa to process PEEK at 400°C for the same processing time. This is not practical. In the face of the above difficulties, it is desirable to find ways to manu￾facture thermoplastic matrix composites because they can offer many ad￾vantages, such as the following. a. Thermoplastic resin does not have the constraint of shelf life. To put it another way, thermoplastic composite preforms have infinite shelf life. This is because thermoplastic matrix solidifies upon cooling and there is no chemical reaction taking place to change its liquid state to solid state. b. Thermoplastic resins are more ductile than thermoset resins. Com￾posites made of thermoplastic matrix composites therefore can have larger fracture toughness than those made of thermoset matrix composites. The strain energy release rate for carbon/epoxy is about 100 J/m2 while that of carbon/PEEK is about 1000 J/m2 . This means that the thermoplastic matrix resin has large plastic deforma￾tion whereas thermoset matrix material does not. This is very im￾portant for structures that need to resist impact and fracture load. c. Thermoplastic resins can be recycled, i.e., they can be reheated to take a different form. This is also due to the same reason indicated above in which the material can be heated up to change the solid state to the melt state. At temperature close to the melt point, differ￾ent shapes can be formed to make a new product. This behavior can also be used to heal the defects that may exist in the structure. d. The processing time can be fast. The heating and cooling of a ther￾moplastic composite material can take place within the order of minutes. This is in comparison with the order of hours for thermoset matrix material. The reason for the long processing time for the thermoset matrix resin is because of the time required for all the chemical bonds to take place. In the case of thermoplastic resin, cooling will solidify the material and this can take place very quickly. e. Thermoplastic matrix resin is weldable. This means that solid parts made of thermoplastic matrix composites may be welded together. For thermoset resins, because of the chemical bonding required for solidification, once the bond is formed, it cannot be used again to form another bond. The weldability of the thermoplastic resin en- 290 LONG FIBER THERMOPLASTIC MATRIX COMPOSITES