Calcium/zinc stabilisers

The use of calcium/zinc stabiliser systems has been common in PVC applications for over 25 years.

These materials are generally based on metal carboxylates and will sometimes incorporate other elements to boost performance such as aluminium or magnesium. Because the heat stability in some applications may require some enhancement when using calcium/zinc, organic co-stabilisers will also often be added to this type of formulation. These materials include polyols, epoxydised soya bean oil, antioxidants and organic phosphites.

Properties and applications

The characteristics of calcium/zinc stabilised materials mean that they are widely used in many flexible and rigid PVC applications. This type of stabilising system can provide excellent organoleptic properties and outdoor weatherability to products which have a high degree of clarity and good mechanical and electrical properties. As a result, calcium/zinc stabilisers have been incorporated and established in a wide range of applications ncluding toys, healthcare products such as blood bags, semi-rigid and flexible foil for food packaging, bottles for drinking water, and drinking water pipes. As a result of the European PVC industry’s commitment to substituting lead-based heat stabilisers, considerable development work has been carried out on new, improved systems for applications such as cable covering, pipe and window profile. Many of these newer forms of calcium/zinc stabiliser have been in commercial use for several years, and are in general more complex and expensive than the traditional soaps mainly because of the specialised co-stabilisers required to meet the specific requirements of certain applications. The rate of introduction of such stabilising systems was slowed down by the extensive evaluation work required and the need for acceptance by the specifier. In the sector of flexible foils where the main stabiliser used is a barium/zinc soap, substitution by calcium/zinc materials is also taking place although there are also technical issues which need to be overcome.

Application table

Stabiliser type

Application

 Pb Pb/Ba/Cd Ba/Cd Ca/Zn Ba/Zn Sn K/Zn (2)
Unplasticised PVC
Pipes ++ ++(1)
Fittings ++ +(1)
Profiles ++ ++ +
Foil ++
Bottles ++ ++
Sheet + (+) ++
Plasticised PVC
Cable covering + ++
Foil and sheet + + ++
Flooring ++ + ++
Wall covering (+) ++ + ++
Medical use ++
Tubes and footwear + ++
Food packaging film ++
Fabric coating + ++ ++

Safety, health and environmental issues

Inherently, calcium/zinc stabiliser systems incorporating the proven range of co-stabilisers have low toxicity or are regarded as non-toxic. Calcium/zinc salts of any of the acids in the EU Additives Directive are approved for food contact use, although specific approval would depend on the co-stabiliser incorporated into the stabilising system. Solid calcium/zinc stabilisers have to meet the general requirements for dust emissions in the workplace although dust-free forms are readily available. Zinc is an essential element to human activity and health and has been the subject of various risk assessments. The conclusion of these has been that there are no risks to human health or the environment arising from the use of zinc stearate in PVC heat stabilisers.