When PVC is heated to 170~180°C, chlorine and hydrogen in the molecules are eliminated and release of hydrogen chloride becomes evident. 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 stabiliser prevents such initial elimination of hydrogen chloride from PVC.
Therefore, use of stabilisers (metal compounds) is essential to prevent the chain reaction of decomposition. They can also impart to the PVC enhanced 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.
The main heat stabilisers in a formulation are usually combined with co-stabilisers which are organic materials such as polyols or epoxidised esters: they provide an additive synergistic effect, especially in the case of some forms of heat stabiliser, an enhancement of overall stabiliser performance. Every stabiliser has typical uses, although a number of different types may be used in the same application sector.
Although lead-based systems are being voluntarily phased out within Europe under the Vinyl 2010 voluntary commitment of the PVC industry in favour of Ca/Zn or Ca/organic stabilisers, they still represent about one half by tonnage of the stabilisers used in Europe at present. They are mainly used for rigid PVC construction applications such as pipes/fittings and window profiles.
The choice of heat stabiliser depends on a number of factors including the technical requirements of the PVC product, regulatory approval requirements and cost.
The majority of tin - Sn - stabilisers are used for rigid wrapping films, roofing and transparent rigid sheets for construction applications. Tin stabilisers are employed for drinking waste applications in some countries where lead has been banned in this application for many years.
The major metals contained 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 stabilisers are used as metallic soaps such as stearates, while Sn stabilisers are used as organic tin (dialkyl tin compounds). Other than metallic soap, Pb stabilisers are used as basic sulphate, basic carbonate, or basic phosphate.
