PVC is one such polymer that cannot be processed without compounding.Although standardizing formulation and carrying out mixing, it is still observed that we do not get the repeatability in terms of processing as well as properties of PVC products.This happens because the quality of ingredients of the compound, including PVC is variable. Most of the PVC processors rely on supplier’s test reports.
Organizations try to buy additives at slightly cheaper rates to save money, even if most PVC processors use the same standard formulation, each time there is a change in additive, the formulation needs to be adjusted.
This leads to:
- More setting time,
- More rework,
- Inconsistency in properties of the product, and
- Increase in production cost.
It is necessary to understand that there is not a standard formulation, so the process cannot be standardized and the process becomes operator dependent. This defines the requirements of the quality management system.
In order to make the process work as per the standard control plan, without major changes by the operator, we need to modify the compound at laboratory level in such a manner that the process works within the set tolerance limits of the control plan parameters.
How is it possible?
There are some instruments which enable it to avoid losses in terms of generating, handling and processing more rework, and Inconsistent quality. The best technical instrument for this type of test is the torque rheometer, able to generate a graph (as you can see from the image) that represents the parameters you want to achieve. By continuing to change the mixture of additives to the raw material you will get graphs always different, and you will be able to find the best combination to achieve the desired result.
Rheology deals with the viscosity and elasticity of a polymer at processing temperature. These data are then processed and compared on a graph generated by the system. A comparison can be made between the reference graph and the graph for the newly formulated compound, using a new additive or same additive from a different supplier, tested at same conditions, for the same quantity of compound and by the same operator.
The comparison will guide us to decide whether to add or reduce thermal stabilizer or lubricant or both and its level. Once this is done, the modified compound would run on the processing equipment in such a manner that the operator would not detect the change of recipe and it will run as a compound running earlier. Operator has only to fine tune within the tolerance limits of processing parameters.
It is possible to deepen the topic also from the site of Yashodan Kanade, an expert technician of the subject.