The Function of PVC Stabilizers
Without the addition of a stabilizer, PVC resins begin to degrade at temperatures of 160°C or more, the main re-action being dehydrochlorination.
The results of measurement of the chlorine in the hydrogen chloride gas produced by the degradation of PVC during heating shows, as can be seen, the addition to the resin of a lead stabilizer reduces the amount of hydrogen chloride produced.
As heat degradation proceeds, the resin gradually shows brown discoloration, eventually becoming black.
At the molecular level, dehydrochlorination causes the formation of double bonds, the cutting of molecular chains, and cross-linking, resulting in a reduction of processability, together with a degradation in mechanical strength and electrical properties.
Since PVC is generally processed at temperatures between 160°C (at which degradation begins) and 210°C, it is only through the addition of a stabilizer that it can be molded.