Since it is the plasticisation of polymers using scCO 2 which is of major interest in processing, a quick discussion on the effects of plasticisation will now follow.
Plasticisers are additives sometimes mixed with polymers to change the rheology and/or the mechanical properties of the final product. Perhaps the best-known use of plasticisers is in PVC where plasticisers can be used to reduce stiffness and lower the glass transition temperature. The effect of this addition depends on the nature of the additive and the structure of the polymer. If the additive is soluble in the plasticiser, then absorption will cause swelling and eventually it will dissolve.
The behaviour of molten thermoplastic polymers with supercritical fluids is of great interest as a substitute for plasticising agents in processing. This is particularly of importance for thermally labile polymers with high glass transition temperatures (T g) and for materials that are highly viscous.
Plasticisation is generally characterised by changes in the polymer system that result in:
a lowering of the glass transition temperature,
a lower rigidity at room temperature,
increased elongation and flexibility of individual chains in the polymer,
increased toughness.
Amorphous materials and amorphous regions in semicrystalline materials are more easily penetrated than crystalline regions in semi-crystalline polymers. The degree of crosslinking also affects the ability of the plasticiser to penetrate. This can be observed when considering that elastomers (with a less dense network of crosslinks) may swell in a solvent but a thermoset (highly crosslinked)...
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