TABLE OF CONTENTS As already explained in Section 2.3.1, commercially produced polyvinyl chloride (PVC) is inherently thermally unstable, due to structural defects formed during polymerisation and during processing. The degradation process has already been shown in Section 2.3.1.5 with the labile Cl* atoms being the initiation sites of PVC heat degradation. Examples of structure defects are shown in Figure 3.1. Other examples of initiation sites are the 2,4-dichlorobutyl branches and the presence of peroxy groups, and so on.
Heat stabilisers retard dehydrochlorination and autoxidation and reduce fragmentation. In addition, they cure existing damage. They also help to maintain light stability during use.
The thermal degradation process can be followed by analysis of the colour generated in a static or dynamic environment, usually with reference to a standard. Alternatively, heat stability can be measured on a time basis, based on hydrogen chloride (HCl) evolution.
The role of the heat stabiliser can be summarised as follows:
Substitute structural defects for more stable groups.
Stop the dehydrochlorination zipper effect by substituting the allylic chloride formed during degradation.
Scavenge evolved HCl which has a prodegradant effect.
React with free radicals formed (antioxidant) to avoid discoloration and crosslinking by thermal, high-stress, or photochemical processes.
May provide lubrication.
There are many different types of heat stabilisers based on different metal salts or soaps, acting synergistically, for PVC. These are covered in the following sections.
Lead compounds, usually based on...
