Stabilisers

When PVC is heated to between 170 and 180°C, the chlorine and hydrogen present in the molecules are eliminated, and hydrogen chloride (HCI) is released. Once such decomposition starts, unstable structures are formed in the molecule, which further accelerate HCl elimination and decomposition. As PVC is heated to soften during the extrusion or moulding process, prevention of hydrogen chloride elimination due to heat and subsequent decomposition is required. The presence of a stabiliser will prevent this initial release of hydrogen chloride from PVC.

Stabilisers also increase PVC’s resistance to daylight, weathering and heat ageing and have an important influence on the physical properties and the cost of a formulation. They are invariably supplied in the form of application-specific blends of which the main constituents are metal soaps, metal salts and organometallic compounds. The choice of heat stabiliser depends on a number of factors including the technical requirements of the PVC product, regulatory approval requirements and cost.

Heat stabilisers used in a formulation are usually combined with co-stabilisers that are organic materials such as polyols or epoxidised esters. They provide an additive synergistic effect, and, in the case of certain heat stabilisers, an enhancement of overall stabiliser performance.

Lead-based systems have been voluntarily phased out and replaced with calcium zinc (Ca-Zn) or calcium organic stabilisers within Europe under the VinylPlus commitment of the PVC industry.

The majority of tin (Sn) stabilisers are used for rigid packaging films, roofing and transparent rigid sheets for construction applications. The major metals contained in stabilisers are barium (Ba), calcium (Ca), zinc (Zn) and tin (Sn). Ba-Zn and Ca-Zn stabilisers are used under the form of metallic salts such as stearates, while Sn stabilisers are used under the form of organic tin (dialkyl tin compounds).