Glass Transition Temperature

In the molten state, the individual molecular chains of a plastic material move freely relative to each other, allowing the material to be moulded within the various

Amorphous polymer Fig. 10.6 Crystallinity in polymers forming processes used for the manufacture of components. As the temperature of melted plastic material is lowered, the freedom of movement of the molecular chains is reduced; gradually the plastic becomes more viscous, until eventually it solidifies at its characteristic melting point temperature. However, even when solid, most plastics remain rubbery or flexible, due to rotations within the individual molecular chains. As the temperature is lowered further, the material will eventually become rigid and brittle, as movement can no longer take place within the individual molecular units. The temperature at which a particular plastic changes from flexible to rigid is defined as its characteristic glass transition temperature. Depending upon the nature of the particular plastic material this may be above or below normal ambient temperatures. Further, the glass transition temperature for a particular plastic can be significantly changed by, for example, the addition of plasticisers, characterised by the differences in physical properties between PVC-U (unplasticised) and PVC (plasticised polyvinyl chloride).

0 0

Post a comment