4 High Performance Thermoplastic Resins and Their Composites 2.2 Properties of Neat Thermoplastic Resins 2.2.1 Chemical Structure and Some Physical Properties Table 1 lists the high performance thermoplastic polymers that are discussed in the present report.Although this list is not exhaustive,it provides a good indication of the thermoplastics that have been and are being investigated for use as matrix materials for high performance composites.Most of these neat resins are either commercially available or nearly so,in either industrial or developmental quantities.Some of them are provided as a neat resin or filled with short fibres but not yet reinforced with continuous fibres in a prepreg tape or fabric form.Although it is included in the present list,polyphenylquinoxaline (PPQ)is not expected to be avallable in the form of fibre reinforced matrix because of its low modulus,high viscosity and its high cost. Table 2 presents the chemical structure of some of these thermoplastics.The dominant aromattc character in their polymer backhone is clearly shown.Density.Poisson's ratio. Limiting Oxygen Index(L.O.I.)and viscosity are presented in Table 3.Density varles from 1.15 to 1.45 depending on the thermoplastic matrix:the polyamide J-2,a product from E.I.Dupont de Nemours,has the lowest density while N-polymer,a polyimide from Dupont and Eymyd,a polyimide from Ethyl Corporation.have the highest. The melt viscosities of high-molecular weight thermoplastics are much higher than most thermoscts.At processing temperature,thermosets have viscosities less than 1000 poise [2],which is much less than the viscosities presented in Table 3 for thermoplastics.The low viscosity of epoxy formulations results in high melt nlow properties in the uncured state leading to good wetting of the fibres during prepreg manufacture [1l.Figure 1 shows the relationship between solution viscosity.melt viscosity,number average molecular weight and the glass transition temperature (Tg)presented in [9].As shown,the desired high Tg leads inevitably to high melt viscosity.Unfortunately,the high melt viscosity of thermoplastics renders processing difficult as high processing temperatures are required to achieve a low melt viscosity for good consolidation and fibre impregnation:and the viscosity may still be too high for complete impregnation of continuous fibre bundles.Processing becomes difficult at melt viscosities above 5500 poise [9].Melt viscosities of 102 to 104 poise are desirable for the fabrication of composites [1].It is then a question of compromise between processability of thermoplastic composites and their high temperature performance as reflected by Tg. L.O.I.numbers found in Table 3 give an indication of the material's resistance to burning,which may be very important in certain applications.For example,aircraft interiors such as sidewall panels,storage bins.partitions,galley doors and celling panels have to meet certain combustibility requirements to comply to the more and more stringent U.S.Federal Aviation Administration (FAA)cabin safety regulations [57]."L.O.I.is the minimum4 High Performance Thermoplastic Resins and Their Composites 2.2 Properties of Neat Thermoplastic Resins 2.2.1 Chemical Structure and Some Physical Properties Table 1 lists the high performance thermoplastic polymers that are discussed in the present report. Although this list is not exhaustive, it provides a good Indication of the thermoplastics that have been and are being investigated for use as matrix materials for high performance composites. Most of these neat resins are either commercially available or nearly so, in either industrial or developmental quantities. Some of them are provided as a neat resin or filled with short fibres but not yet reinforced with continuous fibres in a prepreg tape or fabric form. Although it is included in the present list, polyphenylquinoxaline (PPQ) is not expected to be available in the form of fibre reinforced matrix because of its low modulus, high viscosity and its high cost. Table 2 presents the chemical structure of some of these thermoplastics. The dominant aromatic character in their polymer backbone is clearly shown. Density, Poisson’s ratio, Limiting Oxygen Index (L.O.I.) and viscosity are presented in Table 3. Density varies from 1.15 to 1.45 depending on the thermoplastic matrix: the polyamide J-2, a product from E.I. DuPont de Nemours, has the lowest density while N-polymer, a polyimide from DuPont and Eymyd, a polyimide from Ethyl Corporation, have the highest. The melt viscosities of high-molecular weight thermoplaslics are much higher than most thermosets. At processing temperature. thermosets have viscosities less than 1000 poise [2]. which is much less than the viscosities presented in Table 3 for thermoplastics. The low viscosity of epoxy formulations results in high melt flow properties in the uncured state leading to good wetting of the fibres during prepreg manufacture 111. Figure 1 shows the relationship between solution viscosity, melt viscosity, number average molecular weight and the glass transition temperature (Tg) presented in 191. As shown, the desired high Tg leads inevitably to high melt viscosity. Unfortunately, the high melt viscosity of thermoplastics renders processing difficult as high processing temperatures are required to achieve a low melt viscosity for good consolidation and fibre impregnation: and the viscosity may still be too high for complete impregnation of continuous fibre bundles. Processing becomes difficult at melt viscosities above 5500 poise [9]. Melt viscosities of 102 to 104 poise are desirable for the fabrication of composites [ 11. It is then a question of compromise between processability of thermoplastic composites and their high temperature performance as reflected by Tg. L.O.I. numbers found in Table 3 give an indication of the material’s resistance to burning, which may be very important in certain applications. For example, aircraft interiors such as sidewall panels, storage bins, partitions. galley doors and ceiling panels have to meet certain combustibility requirements to comply to the more and more stringent U.S. Federal Aviation Administration (FAA) cabin safety regulations [57]. “L.O.I. is the minimum