Tin dilaurate stabilizers have excellent PVC compatibility, provide some lubrication, and are useful stabilizers, limited only by their relatively high cost. Dibutyltin dilaurate is now used mainly as an esterification and hydrolysis catalyst, for example, in polyurethane cross-linking and in hydrolytic cross-linking of silanes grafted to olefinic polymers. Usage in PVC has been almost completely superseded by more advanced tin stabilizers.
Replacement of laurate with unsaturated maleate ligands led to greatly increased stabilizer efficiency and, through reduced usage levels, lower cost. Because maleic acid is difunctional, poly mericproducts are generated:
R2SnO + CHCOOH [-R2Sn-O-CO-CH=CH-CO-]n
CHCOOH
These did not provide the (modest) lubrication value of tin laurates; therefore, combination maleate-laurate stabilizers were synthesized:
R2SnO + CHCOOC12 R2Sn-O-CO-CH=CH-CO-OC12
CHCOOH
Today, the tin maleates remain of great value in cases where highly efficient nonsulfur-containing stabilizers are desired. Both maleaes and maleate-laurates are commercially important in flexible PVC applications where maximum clarity and lowest possible odor are needed. In addition, the polymeric maleates ar used to stabilize halogenated flame retardants commonly used in polyolefins.
Various explanations have been proposed for the improvement in efficiency gained in going from the original laurate to maleate ligands. It is clear that whatever mechanism dominates must involve improved coordination to labile chlorine sites, probably through complexing with existing or developing allylic unsaturation.