Influence of crystallinity on mechanical properties of Poly (ethylene terephthalate), PET, and PET/AS4 composite

Thermoplastic composites show considerable promise for structural applications. Some of the advantages of these materials compared to current thermoset resin composites are the ease of formability, processing ability, damage repair, and bonding as well as better environmental resistance and reprocessing capabilities. For thermoplastic resin matrices capable of some degree of crystallization there is promise of increased toughness, elastic modulus, reduced moisture degradation, and chemical resistance required for use in many aerospace and automotive applications. These properties are thought to vary with the crystallinity of the semi-crystalline resin.

The degree of crystallinity and size of the crystallites in a semi-crystalline polymer have profound effects on the mechanical properties of the polymer and polymer based composites. Therefore, the major objective of this investigation is to evaluate the relationships between mechanical properties and the crystallinity for a nonreinforced polymer of polyethylene terephthalate (PET) and for carbon PAN based fiber reinforced PET composite, PET AS4. Although PET AS4 composites are not commonly used in structural applications, insight into the effects of bulk crystalline content on thermoplastic- carbon composites will be provided. This investigation of the mechanical properties, crystallization kinetics, and morphological attributes of PET and its composite with PAN- based carbon AS4 fiber, PET AS4, has identified several interesting phenomena.

Basic bulk crystallization studies using DSC analysis and mechanical performance testing concluded that, as expected, the tensile modulus increased. The significance lies in the magnitude of that increase. There is a greater increase in the percent change in tensile modulus for the PET AS4 composite than for the neat polymer. This increase in the percent change in tensile modulus of the composite over the neat polymer suggests improved interface strengths as crystalline content is increased. The improved interface strength will increase the overall strength of the composite until the point that brittle matrix transverse fracture occurs.