Role of stabilizer in retaining underwater volume resistivity

The main function of heat stabilizers for Polyvinyl chloride is to slow the degradation process of PVC and make processing of polymer easy, like all other ingredients of compounding formulations, PVC heat stabilizers also affect the volume resistivity of the final compound.

During stabilization reaction, many by-products are formed which affect water absorption and dielectric properties of the compounds. These by-products vary depending upon primary and secondary stabilizers used in the stabilizer formulation. For example in conventional Lead (Pb) based stabilizers like Lead Stearate and Tribasic Lead Sulphate after stabilization we get Lead Chloride (PbCl2) as a by-product. Lead Chloride is neither hygroscopic nor dissolves in water. Hence, compounds made using Lead-based stabilizers easily pass IS 694 test.

However, things are a little complex in Calcium Zinc stabilizers. After stabilization, we get CaCl2 and ZnCl2. These salts (especially Calcium Chloride) are hygroscopic in nature. Hence when the insulation is submerged in water, these salts tend to actively absorb moisture which in turn affects the resistivity of the compound. We cannot use any other system in place of Calcium and Zinc stabilizers as they are core stabilizers, i.e. they are responsible for preventive stabilization action.

Secondly to stabilize compounds for high-end applications, either we have to increase the dosage of stabilizers or we have to improve the efficiency of the existing stabilizer so that in a comparatively lower dosage one can achieve good properties.

To solve these issues, at NOVISTA, we approached this problem with a different angle. Instead of only looking at stabilization process as a chemical phenomenon, we also used physical phenomena for improving the efficiency of stabilizers. In new generation Ca/Zn stabilizers we focus on the crystal structure of the stabilizers. A very unique particle morphology allows degradation products and byproducts of stabilization to be trapped in a cage of crystal which doesn’t allow these products to interact with polymer matrix and moisture.

As a result, we found that although there may be many factors responsible for diminished underwater volume resistivity, NOVISTA new generation Ca/Zn stabilizer is not one of them. In conclusion, stabilizers do not improve electrical properties as such but, a good stabilizer is important for retaining electrical properties of the compound. Though it is not the only factor to be considered.