Plasticizer

A plasticizer (UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture.

Plasticizers are commonly added to polymers and plastics such as PVC, either to facilitate the handling of the raw material during fabrication, or to meet the demands of the end product's application. Plasticizers are especially key to the usability of polyvinyl chloride (PVC), the third most widely used plastic. In the absence of plasticizers, PVC is hard and brittle; with plasticizers, it is suitable for products such as vinyl siding, roofing, vinyl flooring, rain gutters, plumbing, and electric wire insulation/coating.

Plasticizers are also often added to concrete formulations to make them more workable and fluid for pouring, thus allowing the water contents to be reduced. Similarly, they are often added to clays, stucco, solid rocket fuel, and other pastes prior to molding and forming. For these applications, plasticizers largely overlap with dispersants.

Plasticizers for polymers are either solids or low-volatility liquids. According to 2017 data, the total global market for plasticizers was 7.5 million metric tonnes. In North America the 2017 volume was ~1.01 million metric tonnes and in Europe the figure was 1.35 million metric tonnes, split between various end-use applications with a chemical type trend moving to higher molecular weight (HMW) orthophthalates and alternative types following regulatory issues concerning lower molecular weight (LMW) orthophthalates.

Almost 90% of polymer plasticizers, most commonly phthalate esters, are used in PVC, giving this material improved flexibility and durability. Other polymers which can contain high loadings of plasticizers include acrylates and cellulose-type plastics, such as cellulose acetate, nitrocellulose and cellulose acetate butyrate.

The molecules of the plasticizer are immobilized within the matrix formed by the polymer, rather than being part of the polymer. It was commonly thought that plasticizers work by embedding themselves between the chains of polymers, spacing them apart (increasing the "free volume"), or swelling them and thus significantly lowering the glass transition temperature for the plastic and making it softer. It was later shown that the free volume explanation could not account for all of the effects of plasticization. The mobility of a polymer chain is more complex in the presence of plasticizer than what the Flory–Fox equation predicts for a simple polymer chain.

The molecules of plasticizer take control over mobility of the chain - a polymer chain does not show an increase of the free volume around polymer ends. If plasticizer/water creates hydrogen bonds with hydrophilic parts of the polymer, the associated free volume can be decreased. ^{[clarification needed]}

The effect of plasticizers on elastic modulus is dependent on both temperature and plasticizer concentration. Below a certain concentration, referred to as the crossover concentration, a plasticizer can decrease the modulus of a material. The material's glass transition temperature will decrease, however, at all concentrations. In addition to a crossover concentration, a crossover temperature exists. Below the crossover temperature the plasticizer will also increase the modulus.