2.Neat Thermoplastic Resins Properties 2.1 Introduction Plastics are commonly classified into two classes.thermoplastics or thermosets, depending on their behavior when heated (4.5.6).A thermoset polymer undergoes various degrees of cross-linking when cured by heat (or other means)[6l.The cross-linking reactions lead to the formation of an insoluble or infusible rigid product,a"set"product,in which chains are joined together to form a three-dimensional structure (5,7].In contrast,thermoplastic polymers do not undergo chemical changes during consolidation:changes are substantially physical [5,6].Generally.thermoplastics are melt fusible and can be consolidated by the application of heat and pressure only.They can be repeatedly sottened by heating and hardened by cooling.There are however some polymers categorized as thermosetting ther- moplastics or pscudo-thermoplastics [1.6,8].They are considercd as thermoplastic as they possess true thermoplastic properties but they are produced essentially like thermosets:they undergo some reaction chemistry during processing cycles.These materials require both curing and heat treatment for effective consolidation (6]. Thermoplastic polymers are not new:they have been known for a long time.It is only recently that the newer so-called high temperature or high performance thermoplastics have been introduced.The early thermoplastic polymers had predominantly aliphatic carbon back- bones in which flexible carbon chains could be extended and rotated into many configurations with relative ease [4,9.10].Rigidity was obtained by restricting the movement of the backbone chain elther by crystallinity such as in polyethylene and polypropylene or by the introduction of side groups as in polystyrene or polymethylmethacrylate.The major limitations with these early thermoplastics which are still on the market are their low elastic modulus,low glass transition temperature (Tg)and poor solvent resistance.In the past few years,a range of thermoplastics based on aromatic polymers have been developed which surmount these limitations.The introduction of rigid aromatic rings instead of aliphatic chains increases the intermolecular forces,thus restricting the movement of the backbone chain [4,10].Hence, mechanical properties,high temperature capability and solvent resistance are greatly improved and can be often equivalent or even better than the best thermosets.For ease of processing.groups such as ether.carbonyl,thioether.amide,methylene,ester.isopropylidine and sulfone are incorporated between the aromatic rings to render the polymer chain more flexible [1,10]. This section presents data on a number of these high performance thermoplastic resins which have the potential to be used as matrix material in fibre reinforced composites aimed at aircraft structural applications.The chemical structure,trade name and producers of these resins as well as their thermal and mechanical properties and solvent resistance are presented. A brief description of each polymer follows which highlights their important characteristics. 32. Neat Thermoplastic Resins Properties 2.1 Introduction Plastics are commonly classified into two classes, thermoplastics or thermosets, depending on their behavior when heated 14, 5, S]. A thermoset polymer undergoes various degrees of cross-linking when cured by heat (or other means] IS]. The cross-linking reactions lead to the formation of an insoluble or infusible rigid product, a “set” product, in which chains are joined together to form a three-dimensional structure 15, 71. In contrast, thermoplastic polymers do not undergo chemical changes during consolidation: changes are substantially physical 15, 61. Generally, thermoplastics are melt fusible and can be consolidated by the application of heat and pressure only. They can be repeatedly softened by heating and hardened by cooling. There are however some polymers categorized as thermosetting ther￾moplastics or pseudo-thermoplastics [ 1, 6. 81. They are considered as thermoplastic as they possess true thermoplastic properties but they are produced essentially like thermosets: they undergo some reaction chemistry during processing cycles. These materials require both curing and heat treatment for effective consolidation 161. Thermoplastic polymers are not new: they have been known for a long time. It is only recently that the newer so-called high temperature or high performance thermoplastics have been introduced. The early thermoplastic polymers had predominantly aliphatic carbon back￾bones in which flexible carbon chains could be extended and rotated into many configurations with relative ease [4. 9. 101. Rigidity was obtained by restricting the movement of the backbone chain either by crystallinity such as in polyethylene and polypropylene or by the introduction of side groups as in polystyrene or polymethylmethacrylate. The major limitations with these early thermoplastics which are still on the market are their low elastic modulus, low glass transition temperature (Tg) and poor solvent resistance. In the past few years, a range of thermoplastics based on aromatic polymers have been developed which surmount these limitations. The introduction of rigid aromatic rings instead of aliphatic chains increases the intermolecular forces, thus restricting the movement of the backbone chain [4, lo]. Hence, mechanical properties, high temperature capability and solvent resistance are greatly improved and can be often equivalent or even better than the best thermosets. For ease of processing, groups such as ether, carbonyl, thioether. amide, methylene. ester, isopropylidine and sulfone are incorporated between the aromatic rings to render the polymer chain more flexible 11. 101. This section presents data on a number of these high performance thermoplastic resins which have the potential to be used as matrix material in fibre reinforced composites aimed at aircraft structural applications. The chemical structure, trade name and producers of these resins as well as their thermal and mechanical properties and solvent resistance are presented. A brief description of each polymer follows which highlights their important characteristics. 3