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Several theories have been proposed in order to explain the action of plasticisers. Some involve detailed analysis of polarity, solubility and interaction parameters as well as the thermodynamics of polymer behaviour. Others treat plasticisation as a simple lubrication of chains of polymer, similar to the lubrication of metal parts by oil. Whilst each theory is not exhaustive, an understanding of the plasticisation process can be gained by combining ideas from each theory.
The steps involved in the incorporation of a plasticiser into a PVC product can be divided into five distinct stages:
1. The plasticiser is mixed with PVC resin.
2. The plasticiser penetrates and swells the resin particles.
3. Polar groups in the PVC resin are freed from each other.
4. Plasticiser polar groups interact with the polar groups on the resin.
5. The structure of the resin is re-established, with full retention of the plasticiser being used.
Stages 1-2 can be described as physical plasticisation. The precise details of how this is carried out depend on the technology involved: suspension or emulsion (paste) PVC. The rate at which penetration occurs depends on the physical properties of the plasticiser including viscosity, resin porosity and particle size.
Stages 3-4 however, can be described as chemical plasticisation since the rate at which these processes occur depend on chemical properties such as molecular polarity, molecular volume and molecular weight. An overall mechanism of plasticiser action must give adequate explanations for this as well as the physical plasticisation steps.
The importance of part 5 cannot be stressed enough. No matter how rapidly and easily parts 1-4 occur, if the plasticiser is not retained in the final product this will be rendered useless and commercially unviable.