The Development of an Antifoam for Use in Suspension PVC Production

Background:

Existing commercially available antifoams had not been optimised for use in S-PVC production. They were difficult to handle and were not particularly effective. As a result plant blockages occurred that increased production costs.

Objective:

To develop an improved antifoam that did not adversely affect the properties of the PVC.

Analysis of the Problem:

Because of the carcinogenic nature of vinyl chloride monomer (VCM), unreacted gaseous monomer must be removed from S-PVC slurries by stripping. During stripping, the slurry is heated, often injected with steam and subjected to a vacuum. The evolving gas bubbles produce a stable foam which increase production costs by reducing plant capacity and causing plant blockages. Close analysis revealed that the foam was a “dry foam”, formed when bubbles of escaping VCM attach themselves to the hydrophobic surface of the 120µm diameter PVC granules. This reduced the density of the granules which then rose to the surface producing the “dry foam” which looked rather like a mousse.

Solution:

The "dry foam" was destroyed by the addition of a wetting agent. Most conventional surfactant wetting agents adversely affected the properties of S-PVC. The most effective, and certainly the most convenient, was the PVA protective colloid used to stabilize the polymerizing monomer droplets. Although the addition of PVA destroyed the "dry foam" and reduced carryover, some foaming still occurred. The excess water soluble polymer produced its own, more conventional, "wet foam".

Foaming tests were used to identify the most effective antifoams for PVA solutions. Earlier work on protein stabilised foams had shown that the addition of sorbitan and glycerol esters were very effective, and they also proved to be effective with PVA foams. Most esters are solid at room temperature and must be melted or dissolved in a solvent before use. Sorbitan esters are relatively expensive.

Unsaturated glycerol ricinoleate (GMR) is a liquid at room temperature and it is also effective at killing the foam. In addition it was already used as an additive in PVC processing, is food contact approved and did not decrease the electrical resistivity of processed PVC. It was also discovered that GMR could be easily dispersed in PVA solutions to give a stable water-in-oil emulsion concentrate. Prior to use, the concentrate readily dispersed in water, and was suitable for direct addition to the PVC stripping vessel. One disadvantage of GMR is that it retards the polymerisation and so is cannot be used during the reaction.

Conclusion:

Investigating what was really happening during stripping, showed why existing antifoams were ineffective and led directly to the development of a system that tackled the route cause of the problem. Furthermore, the antifoam that was developed consisted of ingredients that were already approved for use in PVC and had no detrimental effect on polymer performance.

Since its development this antifoam has become the preferred antifoam in the manufacture of S-PVC and it is still in use today.

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