Extrusion polymerisation processes

Narayan R, Krishnan M, Snook, J B, Gupta A, Dubois Ph (1998), Bulk reactive extrusion polymerisation process producing aliphatic ester polymer compositions . Patent US 5801224. 

In order to understand mixing of rubber one has to have a breadth and depth of knowledge, including polymerisation processes and post-mixing processes such as extrusion and injection moulding. 

Raw PVC polymer has a particulate structure with size and shape determined by the polymerisation conditions. During processes, such as pipe extrusion, sufficient heat and work, as shear mixing, must be applied to the molten polymer to break down and then fully fuse the particle structure. This heat and work must be limited to a level which does not breakdown the polymer structure nor degrade the physical properties. PVC processing must therefore be carefully controlled within optimum conditions with the addition of appropriate additives to protect the polymer chain from thermal and oxidative degradation. 

Process aids are obtained through emulsion polymerisation, a process able to lead to extremely high molecular weights (MW), often above 1 million, when compared with WC MW, which typically range from 50,000 to 65,000 for films and bottles, and from 60,000 to 80,000 for rigid extrusion. 

The term polymer compounding, as it applies to plastics and rubbers, normally describes operations undertaken once the polymer leaves the polymerisation reactor and up to the point where conversion to a finished product takes place, for example, using moulding techniques or die-forming operations based on extrusion. Frequently, compounding is associated with methods for combining polymer with additives, such as fillers, pigments or reinforcements, which may aim to influence its end-performance, appearance, processibility, or simply to reduce overall cost. 

Most silicone fluids and silicone greases are used in the form in which they are supplied and require no further polymerisation or curing by the customer. The other products listed, elastomer- and resin-based materials, usually require further curing in the course of extrusion, moulding, encapsulating or similar processes. They are therefore formulated with appropriate reactive chemical groups, as already described, which can be activated by heat, atmospheric moisture or other means to produce the desired end product. 

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