When PVC is heated to 170~180°C,chlorine and hydrogen in the molecules are eliminated and release of hydrogenchloride becomes evident. Once such decomposition starts, unstable structuresare formed in the molecule, which further accelerate HCl elimination anddecomposition. As PVC is heated to soften during the extrusion or mouldingprocess, prevention of hydrogen chloride elimination due to heat and subsequentdecomposition is required. The stabiliser prevents such initial elimination ofhydrogen chloride from PVC.
Therefore, use of stabilisers (metal compounds) is essential to prevent thechain reaction of decomposition. They can also impart to the PVC enhancedresistance to daylight, weathering and heat ageing and have an importantinfluence on the physical properties and the cost of a formulation. They areinvariably supplied in the form of application – specific blends of which themain constituents are metal soaps, metal salts and organometallic compounds.The choice of heat stabiliser depends on a number of factors including thetechnical requirements of the PVC product, regulatory approval requirements andcost.
The main heat stabilisers in a formulation are usually combined with co-stabilisers which are organic materials such as polyols or epoxidisedesters: they provide an additive synergistic effect, especially in the case ofsome forms of heat stabiliser, an enhancement of overall stabiliserperformance. Every stabiliser has typical uses, although a number of differenttypes may be used in the same application sector.

Lead-based systems are being voluntarily phased out within Europeunder the Vinyl 2010/VinylPlus voluntary commitments of the PVC industry. They are being replaced by Ca/Zn or Ca/organic stabilisers

In the 2000-2011 period, lead stabiliser consumption (in the EU-15) decreasedby 103,972 tonnes (-81.8%), and calcium organic stabilisers (in the EU-15 plusNorway,SwitzerlandandTurkey)increased by 62,108 tonnes. Overall, lead stabiliser consumption decreased by71.4% in the EU-27 compared to 2007. Completion of lead stabilisers’ phase outI scheduled for 2015. i   They are still used for some rigid PVCconstruction applications such as sewer pipes/fittings and window profiles.

The majority of tin – Sn – stabilisers are used for rigid wrapping films,roofing and transparent rigid sheets for construction applications. Tinstabilisers are employed for drinking waste applications in some countrieswhere lead has been banned in this application for many years. The major metalscontained in stabilisers are lead (Pb), barium (Ba), calcium (Ca), and tin(Sn). The stabilisers are classified into Pb stabilisers, Ba-Zn stabilisers,Ca-Zn stabilisers, and Sn stabilisers. Ba-Zn stabilisers and Ca-Zn stabilisersare used as metallic soaps such as stearates, while Sn stabilisers are used asorganic tin (dialkyl tin compounds). Other than metallic soap, Pb stabilisersare used as basic sulphate, basic carbonate, or basic phosphate.