Calcium zinc stabilizer is one of the most successful developments in recent years. It has been the progress made in solid Ca Zn Stabilizer formulations to replace lead on a cost-effective basis. It has also been important that the components of these systems have a satisfactory low-toxicity profile.
Ca Zn formulations are a complex blend consisting primarily of:
• Calcium soap (stearate or laurate)
• Zinc soap (stearate or laurate)
• Acid scavengers such as a hydrotalcite, zeolite, and metal oxides/hydroxides
• Organic co-stabilisers such as the diketone stearoyl benzoylmethane
• Antioxidant
• Polyols, and so on.
Calcium stearate acts as an acid acceptor in addition to providing lubrication. Zinc stearate is used to improve initial and early colour, in combination with the co-stabiliser. This functions by the co-stabiliser complexing the zinc stearate and so preventing the early formation of zinc chloride, which is a prodegradant. It should also be noted that PVC resins may react differently in the presence of zinc (zinc sensitive) and this has an influence on stabiliser formulations. Synthetic hydrotalcites and zeolites (already used in other industries) have been developed for use in PVC stabilisation. These form addition complexes at degrading sites. Such sites tend to be deactivated and the catalytic and highly mobile HCl captured before elimination of further HCl. Hydrotalcites have a layer structure based on magnesium aluminium hydroxycarbonates containing exchangeable anions. Zeolites are crystalline sodium aluminosilicates with a cubic crystal structure of zeolite.
They are microporous with uniform pores and a high internal surface area. Antioxidants are included at very low levels to inhibit the oxidation of the polymer matrix arising not only from thermal processing but also from subsequent photochemical and environmental influences.
Antioxidants are included at very low levels to inhibit the oxidation of the polymer matrix arising not only from thermal processing but also from subsequent photochemical and environmental influences. Solid Ca Zn systems have also been developed for plasticised-PVC (PVC-P) applications as replacements for liquid Ba Zn stabilisers where volatility and emissions may be an issue at the processing and end-use stages. Specific Ca/Zn systems are also available for food contact and medical use meeting the strict regulations that these materials have to satisfy.
