The definition of a plasticiser as being a substance added to another material to make it softer and more flexible is extremely far reaching. There is a need to sub-define them according to their effects when added to a polymer, causing an increase in the flexibility and workability brought about by a decrease in the glass transition temperature (Tg) of the polymer in question.
Polyvinyl chloride (PVC) is by far the most widely plasticised polymer due to its excellent compatibility with plasticisers. Although these substances have been used and remain in use with different types of polymers, the development of plasticisers has always followed that of PVC very closely.
The quantity of plasticisers added to a polymer always depends on the magnitude of the desired effects. For example, a small amount of plasticiser could be added to slightly improve the workability of PVC whilst larger additions can completely transform the properties of the product. For example, rigid PVC, also known as unplasticised PVC (PVC-U), is used in pipes and window profiles. Plasticised PVC, on the other hand, may be used for flexible hoses, cable insulation, sheathing and flooring applications.
The best all-round plasticising performance is achieved by molecules containing both polar and non-polar groups. The former ensure that the plasticiser is locked within the polymer and the latter attenuate the attraction forces of the PVC chains, giving the final product enhanced flexibility. Most plasticisers have the same basic structure but for each individual type, the balance of polar and non-polar groups will determine its plasticising performance and usages. Smaller, more polar plasticisers will be more efficient and ensure ease of processing. Larger, less polar and more linear molecules will confer better performance at extreme temperatures.
Two main methods exist for softening a polymer. Firstly, a rigid polymer may be internally plasticised using chemicals that modify the polymer or monomer so that its flexibility is increased. However, a rigid polymer, such as PVC-U, can be more easily plasticised by the addition of a suitable plasticising agent, i.e. by preparing a blend consisting of a resin and a plasticiser. External plasticisation consists in the addition of a plasticiser to a rigid polymer with the level of flexibility adjusted by both the type and quantity of plasticiser added.
In general terms, external plasticisation is the more preferred and common route. This is mainly due to lower overall costs but also to the fact that the use of external plasticisers gives the manufacturer of the final article a certain degree of freedom in devising different formulations for a range of products.
Despite extensive work and research in this field, several problems persist for the internal plasticisation of PVC. First of all, the affinity of the growing polymer chain for vinyl chloride rather than a co-monomer requires the use of significant pressure to incorporate a co-monomer into the chain. Since the use of recovered monomer in PVC production is a standard practice, contamination of vinyl chloride with co-monomer in this respect also creates additional problems. Finally, the increasing complexity of the process can lead to longer reaction times and therefore to increased costs. External plasticisers are relatively cheap and therefore normally preferred while the resulting flexible article is more easily recycled.
External plasticisers can be divided into three distinct groups.
• Primary plasticiser: They are compatible with the polymer over a broad range of concentrations and will, increase its elongation and softness properties when added to it.
• Secondary plasticiser: They have a limited compatibility with the polymer and if added to the polymer on their own, they will not bring about the required elongation and softness. However, when added to the polymer in combination with a primary plasticiser, secondary plasticisers will enhance overall performance.
• Extenders: They are intrinsically incompatible with the polymer and can only be used in combination with primary plasticisers, in order to decrease the cost of the formulation.
A relatively simple survey of the various theories of plasticisation and an explanation of these interactions is given in the following Encyclopedia articles, both by Cadogan, D.F. and Howick, C.J.
To read more about phthalates and PVC please dowload The bonding between PVC and phthalate esters: fact or fiction? a paper by Dr Chris Howick, Product Stewardship Manager, INEOS Vinyls (UK) Ltd.