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
Because of the characteristics of calcium/zinc stabilised materials they are widely used in many flexible and rigid PVC applications. This type of stabilising system can give products which have a high degree of clarity, good mechanical and electrical properties, excellent organoleptic properties and good outdoor weatherability. As a result, calcium/zinc stabilisers have been incorporated and established in a wide range of applications as shown in table two. These include toys, healthcare products, such as blood bags, semi-rigid and flexible foil for food packaging, bottles for potable water, as well as for potable water pipe.
The performance of more recent developments in calcium/zinc stabilisers also makes them potential technical alternatives to most other stabilising systems, including lead and barium/zinc. As a result of the European PVC industry’s commitment to progressively phase out heat stabilisers based on lead compounds, considerable development work has been carried out on new, improved systems for applications including cable covering, pipe and window profile and many of these are have been in commercial use for several years. These newer forms of calcium/zinc stabiliser 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 these 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. So far, the penetration of calcium/zinc stabilisers in these three sectors is complete in drinking water pipes and cables but not yet in other pipes and profiles. Sales of formulated Ca based stabilizers in Western Europe and Turkey , including Ca/Zn, have increased from 18 kt in 2000 to 80 kt in 2011. Further growth is expected as a result of the progressive phasing out of lead-based systems. 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, again, there are technical issues which need to be overcome.
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.