Degradation of plastics

The degradation of plastics is most frequently attributed to the breakdown of the long molecular chains (Fig. 10.9) or, in the case of PVC, the loss of plasticiser. Polymeric molecular chains may be broken by the effect of either heat, ultraviolet light or ozone, or by a combination of any of these factors, thus reducing their average molecular chain length. Discolouration occurs through the production of molecular units with double bonds, usually causing a yellowing of the plastic. Surface crazing and stress cracks may develop where degradation has caused cross-linking, resulting in embrittlement of the surface.

Where plasticiser is lost by migration from PVC, the glass transition temperature is gradually raised, so eventually the material becomes brittle at ambient temperatures. Typically, high-boiling point oils such as dibutyl phthalate and dioctyl phthalate are incorporated into the original PVC, but these gradually evaporate leaving the surface vulnerable to cracking and shrinkage.

Degradation

Degradation

Breaking of polymer chain into shorter lengths Loss of hydrogen chloride

CI CI CI CI CI X Loss of hydrogen chloride

I III

CI CI CI CI

Production of double bonds - yellow degraded polymer

I III

CI CI CI CI

Production of double bonds - yellow degraded polymer

Fig. 10.9 Degradation of plastics

0 0

Post a comment