Plasticisers: a long history
Early examples of plasticisers include water for softening clay, and oils for waterproofing ancient boats. There are more than 300 different types of plasticisers of which about 50-100 are in commercial use. The most commonly used are phthalates and adipates.
PVC is essentially rigid at normal temperature. This is due to short distances between molecules, that create strong intermolecular forces. When PVC is still just a resin, or powder, heating it causes molecular agitations that become stronger than the intermolecular forces. This widens the distances between the molecules, and results in the softening of the PVC resin When plasticizers are added to PVC at this stage, the plasticiser molecules place themselves between the distanced PVC molecules, preventing the PVC molecules from returning closer together at a normal temperature. The softness of the material is therefore maintained. Such a process is called plasticising.
In Europe, about 30 % of the total PVC resin production is used for flexible PVC products.
The plasticising process
The process of plasticising PVC can make the material harder or softer. Before being made into products, PVC is still just a resin, or powder. At room temperature, this PVC is qualified as rigid: the distances between the PVC’s molecules are short, thus creating strong intermolecular forces. Heating PVC resin causes molecular agitations that become stronger than its intermolecular forces, thus widening the distances between the molecules and resulting in the softening of the resin. Plasticizers are then added to the PVC, placing themselves between the distanced PVC molecules. On returning to room temperature, the PVC molecules are prevented from returning close together: the softness of the material is therefore maintained.
Plasticisers are more than additives
Importantly, plasticisers are not just additives (like pigments or fillers): they are major components that determine the physical properties of polymer products. They are compatible with PVC, have the desired plasticizing efficiency that makes PVC soft and do not easily migrate into air or water.
A wide range of plasticisers
In Western Europe about one million tonnes of phthalates are produced each year, of which approximately 900,000 tonnes are used to plasticize PVC. The most common are diisononyl phthalate (DINP) and diisodecyl phthalate (DIDP). Apart from phthalates, several other kinds of plasticisers are used to meet specific requirements, including adipates for low temperature resistance and trimellitates for heat resistance.