PVC THERMAL STABILIZERS
The main classes of thermal stabilizers in current use are lead salts, metal soaps and organo-tin compounds. Some of these have disadvantages in terms of toxicity, environmental pollution and/or high cost (Lin et al., 2006).
The metal soaps and organo-tin stabilizers are safer than lead salts, but their stabilization effects are usually lower than those of lead salts. In fact, calcium and zinc soaps of some seed oils had been investigated and proved to be save thermal stabilizers of PVC (Folarin, 2008).
As a result of the worldwide increase in environmental awareness, attention is currently being focused on thermal stabilizers that are non-toxic and environmentally friendly (Bao et al., 2008). Thermal stabilizers of PVC possess one or more of the following features in addition to capacity for absorption and neutralization of HCl evolved by PVC during degradation:
1. An ability to replace or displace active, labile substituent groups, such as tertiary and allylic chlorine atoms;
2. A capacity to render pro-degradant substances e.g. heavy metal chlorides, inactive;
3. An ability to modify chain reactions, by interrupting conjugated polyene formation and inhibiting the elimination of HCl.
An ideal stabilizer should possess a number of desirable secondary attributes.
Such stabilizers should be colourless, compatible and non-migrating. It should be relatively inexpensive, non-toxic, odourless and tasteless, and should not affect the polymer`s physical and rheological characteristics.