PVC stabilization

Many PVC stabilizer formulations also contain one or more organic costabilizers that can also absorb hydrogen chloride. Typical of these additives are epoxidized fatty acid esters and organophosphites ... 

The commonly used commercial lead-based PVC stabilizers rely on one or more lead(II) oxide groups bound to the primary bivalent lead salt. 

Commercial Stabilizers. There are six lead salts and soaps that typically are used in the commercial PVC stabilizers. The lead stearate soaps are often combined with the lead salts to provide lubrication and added stabilizer activity. The key to the high activity of these stabilizers is the very high lead content. Table 5 describes six commonly used lead stabilizers. 

By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

Non-Lead PVC Stabilizers Closiag Performance Gap ia Wine," Plastics Technology Maga ne, 35—36 (1993). 

Dibasic Lead Sulfate. Dibasic lead sulfate [12036-76-9] 2PbOPbSO, is a white powder, mol wt 749.70, mp 961°C. The dibasic compound can be prepared by fusion of the two components. It has been sold as a PVC stabilizer in Japan and is sold in Europe in combination with dibasic lead phosphite. 

Tetrabasic Lead Sulfate. Tetrabasic lead sulfate [12065-90-6] 4PbO PbSO, mol wt 1196.12, sp gr 8.15, is made by fusion of stoichiometric quantities of Htharge (PbO) and lead sulfate (PbSO heat of formation, Ai/ = — 1814 kJ/mol (—434.1 kcal/mol). Alternatively, tetrabasic lead sulfate may be prepared by boiling the components in aqueous suspensions. At about 70°C, tribasic hydrate reacts with lead oxide to form tetrabasic sulfate. At 80°C, this transformation is complete in - 20 hours. Tetrabasic lead sulfate is used in limited quantities in Europe as a PVC stabilizer. However, in the United States, lead-acid batteries have been developed by BeU Telephone Laboratories, which contain tetrabasic lead sulfate. Such batteries are used for emergency power at telephone switchboard stations and have an anticipated service life of over 50 years. 

Many organic hahdes, especially alkyl bromides and iodides, react direcdy with tin metal at elevated temperatures (>150° C). Methyl chloride reacts with molten tin metal, giving good yields of dimethyl tin dichloride, which is an important intermediate in the manufacture of dimethyl tin-ha sed PVC stabilizers. The presence of catalytic metallic impurities, eg, copper and zinc, is necessary to achieve optimum yields (108) ... 

Table 7. Typical Commercially Significant Organotin PVC Stabilizers...

Metal carboxyiates have been considered as nucleophilic agents capable of removing aHyUc chlorine and thereby affording stabilization (143). Typical PVC stabilizers, eg, tin, lead, or cadmium esters, actually promote the degradation of VDC polymers. The metal cations in these compounds are much too acidic to be used with VDC polymers. An effective carboxylate stabilizer must contain a metal cation sufftcientiy acidic to interact with aHyUc chlorine and to facihtate its displacement by the carboxylate anion, but at the same time not acidic enough to strip chlorine from the polymer main chain (144). 

In the organic chemicals industry, H2O2 is used in the production of epoxides, propylene oxide, and caprolactones for PVC stabilizers and polyurethanes, in the manufacture of organic peroxy compounds for use as polymerization initiators and curing agents, and in the synthesis of fine chemicals such as hydroquinone, pharmaceuticals (e.g. cephalosporin) and food products (e.g. tartaric acid). 

Starnes and Plitz [51], in their studies of PVC stabilization with di( -butyl)tin bis ( -dodecyl mercaptide) or with mixtures of the mercaptide and di( -butyl)tin dichloride, found that the rate of dehydrochlorination was inversely related to the amount of sulphur content of approximately 0.9%. The following mechanism of allyl chloride substitution was proposed [Eq. (12)]. 

Horst mechanism of PVC stabilization. In fact, based on his observations Naqvi [134] has proposed an alternative model for the degradation and stabilization of PVC based on polar interactions within the polymer matrix, which will be discussed in the following section (Section VI). 

The products are used as intermediates for PVC stabilizers. Further applications of the halogenotin hydrides can readily be envisaged. 

The organotins have a wide range of uses, which are largely specific for the different organotins. Thus, mono-and disubstituted organotin compounds are not suitable as biocides, and trisubstituted organotin compounds are not suitable as PVC stabilizers. 

The production figure for organotin PVC stabilizer in 1996-1998 in Japan was 6983-8649 tonnes per year (Chemical Daily Co., Ltd, 1999, as communicated to the Final Review Board by Dr J. Sekizawa). 

---