Tin Mercaptides

Cost bilizers. In most cases the alkyl tin stabilizets ate particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyl tin mercaptides. Mercaptans are quite effective costabilizets for some of the alkyl tin mercaptides, particularly those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyl tin mercaptide ate currendy the most efficient stabilizers for PVC extmsion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roU mill performance of some methyl tin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. 

The various lubricants formulated into PVC to improve the processing can also enhance the performance of the stabilizet. In pigmented apphcations, calcium soaps, eg, calcium stearate, ate commonly used as internal lubricants to promote PVC fusion and reduce melt viscosity. This additive is also a powerfiil costabilizer for the alkyl tin mercaptide stabilizers at use levels of 0.2 to 0.7 phr. Calcium stearate can significantly improve the eady color and increase the long-term stabiUty at low levels however, as the concentration increases, significant yellowing begins to occur. 

Whereas other metal salts, especially lead stearates and srdfates, or mixtures of Groups 2 and 12 carboxylates (Ba—Cd, Ba—Zn, Ca—Zn) ate also used to stabilize PVC, the tin mercaptides are some of the most efficient materials. This increased efficiency is largely owing to the mercaptans. The principal mechanism of stabilization of PVC, in which all types of stabilizers participate, is the adsorption of HCl, which is released by the PVC during degradation. This is important because the acid is a catalyst for the degradation, thus, without neutralization the process is autocatalytic. 

Unlike other stabilizers, tin mercaptide, or the mercaptan that is formed after the HCl reacts with the mercaptide, can react with the allyhc chlorine to produce a sulfide (47), thus eliminating the labile chlorine groups and stopping the unzipping. 

The selected organic salts of Ba, Cd, Zn, Sn, Pb, and Ca perform with varying degrees of acceptability. Barium-cadmium and barium-cadmium-zinc are the most widely used tin mercaptides and organotin complexes are utilized where ultimate clarity is required lead salts dominate in electrical applications, and calcium-zinc combinations have been traditionally used when FDA approval is necessary. Organic phosphites are also utilized as so-called chelators, complexing the half-chloride salt... 

Barium-cadmium-zinc Tin mercaptide Phosphite chelator... 

The system containing barium-cadmium-zinc stabilizer significantly outperformed the corresponding system based on tin mercaptide. The addition of the phosphite did not alter relative performance in Ba-Cd-Zn systems (Table VII). 

Stannous octoate (esp. for flexible foams) dibutyltin dilaurate (DBTDL) (esp. for rigid foams) tin mercaptides, such as dioctyltin mercaptides... 

The first generation tin mercaptide stabilizers were dialkyltin long-chain mercaptans (1), traditional mercaptoacetate products (2), and mercaptoacetic ester products, having an average tin content of 18% in the stabilizer. When a stabilizer with a high content of a monoalkyltin compound derived from a mercaptoethanol ester (3), was demonstrated to provide outstanding early color and color retention in white pipe, the industry quickly accepted this departure from traditional mercaptoacetate products. 

This results from the fact that the tin mercaptide possess both nucleophilic and electrophilic properties, which permit substitution by the cyclic mechanism shown in Scheme 3.3.3, rather than the elimination which is normally observed with such structures. 

Stabilizer or Minimise or eliminate Interrupts free-radical Tin mercaptides, barium-... 

Gordshtein et al. (18) proposed that the ability of simple tin mercaptides to undergo ligand exchange reactions was caused by dr — pir conjugation in the Sn-S bonds. They proposed also that tin alkoxides form 1 1 complexes with tin chlorides because of the lack of dir — Pit conjugation in the Sn-O bond. No definite conclusions were stated regarding tin carboxylates. Starnes (19) has reported that di-butyltin dimethoxide is not a very efficient PVC stabilizer. 

Among these heat stabilisers, those containing tin (such as tin mercaptides, and tin carboxylates or maleates) are considered to be the most efficient, and can be used in a wide variety of applications. Metallic tin is harmless but there are suspicions that organotin-compounds can be toxic to the central nervous system and the liver. However, tin stabilisers have a low capacity for migrating, and hence they are still considered to be safe. Tin stabilisers such as methyl and octyl tins are used in food contact applications. For PVC, it is believed that tin stabilisers act as HCl scavengers (generating tin chloride) as well as an antioxidant. Thio-tin compounds (preferred for rigid pipe extrusions and profiles (for window frames) of PVC) may develop an odd odour due to sulfur. 

Poly(vinyl chloride) or a chlorinated pol5rvinyl chloride is stabilized with composition comprising either methyl, butyl or octyl tin mercaptide stabilizer and salt of a polymeric polyacid material in the solid form. The last improves performance of tin stabilizer, especially if the moisture level in composition is higher than 0.52 wt%. 

Aluminum chloride is produced during s5mthesis of tin mercaptide thermal stabilizers. It was always believed the ammonium chloride has to be removed from stabilizer because otherwise its performance will suffer. This invention has shown that some quantities can be left in the stabilizing composition giving some benefits for stabilizing performance. 

Traditional substances used as co-stabilisers include polyols, epoxidised soya bean oil, phosphites and certain antioxidants. Arkema markets Stavinor calcium stearate co-stabilisers for use with calcimn-zinc systems. Akcros Chemicals sells P24D, which stands for pyrrolidene-2,4-dione. Siid-Chemie is now marketing a synthetic hydrotalcite, Sorbacid 911, while the PQ Corporation is selling two grades of aluminosilicate (zeolite) imder the name Advera 401. They act as scavengers for acids and metal chlorides, and enable users of tin mercaptides to use less tin, saving on material costs. They also allow stabiUsation to take place with the very low tin levels required by law in food contact applications. 

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