Reinforced thermoplastics

Abstract

If a thermoplastic is placed under load, as in a normal tensile tester, the chara­cteristic stress strain curve is of the form given in figure 83. Whether the material is described as tough or brittle depends on the relation between the fracture stress and the yield stress. In many cases the apparently irreversible deformation which takes place on yield is described as flow, but this is seldom, if ever, the case. The extension which takes place consists in the orientation of the tangled molecular network in a manner similar to that which occurs during the extension of rubber, out with characteristic hard thermoplastics the deformation is opposed by viscous Forces so that most of the energy applied is irrecoverably lost as heat. Deformations of this type are largely reversed on heating—as shown in the case of high-density polyethylene (figure 84, plate 23). In order to modify a brittle polymer to make it tougher we need to alter the relation between deformation and fracture so that, at least under short time tests, deformation takes place before fracture so that enough energy can be absorbed to cope with shock conditions. For this purpose the yield stress must be lower than the fracture stress. Naturally the ideal way of achieving this would be to raise the fracture stress, as any reduction in yield stress will normally lead to a softer and less rigid product. The extent and limitations of this problem will now be examined in relation to the styrene plastics.