Advances in establishing the universal mechanism of PVC stabilization

Chemical transformations of alkyltin mercaptoacetate stabilizers in PVC films during PVC processing were studied using ¹¹⁹Sn-NMR spectroscopy for identifying and quantitatively measuring concentrations of tin-containing components in the rigid films. It was experimentally shown that while functioning as a heat stabilizer under high temperature and high shear conditions in the PVC matrix, most but not all of dioctyltin bis(2-ethylhexyl marcaptoacetate), is converted into its corresponding mono-chloro-thioester; it remained in the polymer matrix with some of the excess initial bis(2-ethylhexyl mercaptoacetate) present. Analogously, under the same conditions, mono-octyltin tris(2-etylhexyl mercaptoacetate) was primarily transformed into its mono-chloro thioester with only minor amounts of the dichloro-thioester, and those were present in the polymer with some of the excess initial tris(2-ethylhexyl marcaptoacetate). Neither dioctyltin dichloride nor mono-octyltin trichloride was present in the properly stabilized, compounded and processed PVC. Formation of the alkyltin chlorides (known also as Lewis acids that catalyze dehydrochlorination of PVC) in the films was likely prevented by the fast reaction of those with the initial mono- and dialkyltin thioester components of the stabilizers. The lower conversion of mono-alkyltin tri-thioester (compared to dialkyltin bis-thioester) during compounding can be attributed to possible ligand exchange between the bis-thioester and mono-alkyltin mono-chloro-thioester, that in turn led to the regeneration of mono-alkyltin tri-thioester, as it was postulated as well as determined spectroscopically.

These results and conclusions shed new light on previously unexplained and unknown aspects of PVC stabilization, uncovered apparent similarities in terms of the mechanisms of PVC stabilization between the mono-/di-alkyltin thioesters with the “synergistic interplay” between zinc- and calcium (or barium)-containing components in mixed metal as well as calcium-organic (Ca-Or) stabilizers and allowed to propose for the first time the Universal Mechanism of PVC stabilization that could be applied to most typical heat stabilizers containing at least two synergistic components.