Theories

The lubricity theory

It is based on the assumption that resin rigidity arises from intermolecular friction binding the chains together in a rigid network. When the polymer is heated, these frictional forces are weakened allowing the plasticiser molecules to be inserted between the chains.  Once incorporated into the polymer, plasticiser molecules shield the chains from each other, thus preventing the re-formation of the rigid network. Whilst attractive in its simplicity, this theory does not explain the success of some plasticisers and the failure of others.

The gel theory

This extends the lubricity theory by suggesting that plasticisers act by breaking the resin-resin attachments and interactions, masking these centres of attachment from each other and preventing their reformation. Such a process may be regarded as being necessary but again, by itself, it is insufficient to completely explain a plasticised system. While a certain concentration of plasticiser molecules will provide plasticisation by this process, the remainder will act more in accordance with the lubricity theory. Unattached plasticiser molecules will be swelling the gel and facilitating their movement, thus imparting flexibility.

Molecules acting according to the action described above, on the basis of molecular size measurements, constitute the bulk of plasticiser molecules. If plasticisation took place solely by this method, it would not be possible to explain the ability of PVC resins to accept their own weight in plasticiser without exudation i.e. large amounts of additional space ("free volume") are created which other plasticiser molecules can occupy.

The free volume theory

This extends the lubricity and gel theories and also allows a quantitative assessment of the plasticisation process.

Free Volume of a polymer is described by the equation: Vf = Vt - V0

In which:

• Vf = the free volume of the resin
• Vt = specific volume at a temperature t
• V0 = Specific volume of an arbitrary reference point, usually taken as zero degrees Kelvin

Free volume is a measure of the internal space available in a polymer for the movement of the polymer chain, which imparts flexibility to the resin. A rigid resin such as unplasticised PVC, possesses very little free volume whereas flexible resins have relatively large amounts of free volume. Therefore, plasticisers act so as to increase the free volume of the resin and also to ensure that free volume is maintained as the resin-plasticiser mixture is cooled down after melting.

Combining these ideas with the gel and lubricity theories, it can be seen that plasticiser molecules not interacting with the polymer chain must simply fill free volume created by those molecules that do. These molecules may also be envisaged as providing a screening effect that prevents interaction between neighbouring polymer chains and, thus, preventing the rigid polymer network from reforming upon cooling.

For the plasticised resin, free volume can arise from:

• Motion of the chain ends
• Motion of the side chains
• Motion of the main chain

These motions can be increased in a variety of ways:

• Increasing the number of end groups
• Increasing the length of the side chain
• Increasing the possibility of main group movement by the inclusion of segments of low steric hindrance and low intermolecular attraction
• Introducing of a lower molecular weight compound which imparts the above properties
• Raising the temperature

The introduction of a plasticiser, which is a molecule of lower molecular weight than the resin, has the ability to impart a greater free volume to the material being treated since:

• There is an increase in the proportion of end groups
• It has a glass transition temperature (Tg) lower than that of the resin itself.

A detailed mathematical treatment of this can be carried out to explain the success of some plasticisers and the failure of others. Clearly, there is no ideal plasticiser for all applications and the final choice will be a compromise between the above ideas and physical properties such as volatility, compatibility, high and low temperature performance, viscosity, etc.

Generalised structure theories and antiplasticisation

In their simplest form, these theories attempt to produce a visual representation of the plasticisation mechanism. They are based on the concept that if a small amount of plasticiser is incorporated into the polymer mass it imparts slightly more free volume and facilitates the movement of macro-molecules. Many resins tend to become more ordered and compact as existing "crystallites" grow or new "crystallites" form at the expense of the more fluid parts of the amorphous material.

For small additions of plasticiser, its molecules may be totally immobilised by attachment to the resin by various forces.  These tend to restrict the freedom of small portions of the polymer molecule so necessary for the absorption of mechanical energy. Therefore, it results in a more rigid resin with a higher tensile strength and base modulus than the base polymer itself.  This phenomenon is known as anti-plasticisation.