Secondary stabilizers, zinc stearate

Using new methods (7) for following the transformation of metal soaps in the polymer, this chapter studys the synergism obtained from chloride ions between zinc and calcium stearate in the absence or presence of secondary stabilizers during PVC thermomechanical treatments in the Brabender plasticorder or in a rolling mill. 

Synergistic Mixture Zinc Stearate and Organo Compounds as Secondary Stabilizers. From the previous results, it is clear that in order to prevent PVC cross-linking and to increase the action time of stabilizer (previously defined with the Brabender plasticorder (1,3,4,6,7)), it is necessary to impede zinc chloride accumulation in the polymer. 

To study the influence of secondary stabilizers on zinc chloride formation, we have prepared PVC sheets on a rolling mill with the different binary mixtures zinc stearate and a-phenylindole, epoxidized soya-bean oil, or /3-aminocrotonate esters, and we have followed the zinc chloride formation by coulometric titration of chloride ions. In Figure 5 we have combined the percentage of zinc chloride that may be liberated with respect to the initial amount of zinc stearate in the presence of a-phenyl-indole (Curve 1 )> epoxidized soya-bean oil (Curves 2 and 3), and butane-diol- -aminocrotonate (Curve 4) during heating in a stove at 180°C. 

PVC can be protected with a variety of stabilizer systems. The choice of stabilizer depends on the probable decomposition mechanisms involved. Primary and secondary stabilizers are generally used in combination. The primary stabilizers are mainly acid acceptors such as metallic soaps. Typical examples are, metallic soaps such as cadmium, barium, and zinc stearates which also act as lubricants. These chemicals simply react with HCl and are able to reduce further degradation. The mechanism of stabilization and properties of various metal soaps are reviewed (39,40). Calcium and zinc stabilizers are low in toxicity and they are often prepared in the form of a dispersion in epoxidized oil. Barium and cadmium stabilizers are mainly solid products those which are liquid contain phosphite groups. The compatibility of PVC stabilizers containing heavy metals (41) and synergetic effects of metal stabilizers (42) are reviewed. 

Calcium stearate, categorized as a metal soap, is both a lubricant and a secondary stabilizer. In European applications, lead, zinc, cadmium, and calcium stearates provide lubrication and heat stability to PVC lead and cadmium are being phased out. The calcium stearate acts as a nfild fusion promoter in most cases however, at high usage levels, the behavior can change character, and further increases in calcium stearate may actually delay the onset of PVC fusion. 

Polymer compositions containing pentaerythritol are also used as secondary heat-, light-, and weather-resistant stabilizers with calcium, zinc, or barium salts, usually as the stearate, as the prime stabilizer. The polymers may be in plastic or fiber form (87—89). 

When used in PVC organophosphites act as secondary heat stabilizers. In other resins such as PP, PS, HOPE and ABS plastics they function as antioxidants. p-Diketones are well-known co-stabilizers more particularly for zinc carboxylates. The synergistic action of epoxidized fatty acid esters like ESO or butylepoxy stearate as plasticizer, HCl scavenger and stabilizer for labile chlorine atoms in the catalytic presence of Zn and Cd salts is shown in Fig. 7 (see also 5.2.3)... 

Powder stabilizers described as lubricating are based on blends of stearates, originally barium and cadmium, or barium, cadmium, and zinc, but now mostly barium or calcium with zinc. Moderately lubricating powders are based on laurates rather than stearates. Minor amounts of calcium carbonate may be added to aid dispersion, or titanium dioxide for color improvement. At one time, Ba/Cd stabilizers contained a minor fraction of BPA as antioxidant Ca/Zn stabilizers used BHT, or BHT and a thiodipropionate secondary. These have been largely replaced in modern Ba/Zn and Ca/Zn stabilizers with more effective antioxidants (as described above). Powders often contain polyols such as pentaerythritol (PE200 ). These function as HCl... 

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