Untersuchungen zum Mechanismus der PVC‐Stabilisierung mit Organozinn‐Verbindungen

Abstract

ZUSAMMENFASSUNG:Die derzeitigen Ansichten uber Startstellen, Mechanismen und Folgeprodukte der thermischen HCI-Abspaltung aus PVC werden kurz erortert.Die Kinetik der thermischen Dehydrochlorierung von unstabilisiertem und rnit 0,05 bis 2,O Gew.-% Di-n-octylzinn-di-thioglykolsaure-2-athylhexylester (I) stabilisiertem PVC The current views on starting positions, mechanisms and products resulting from thermal elimination of HCI from PVC, are briefly discussed.The kinetics of the thermal dehydrochlorination of unstabilized PVC as well as PVC stabilized with 0.05 to 2.0% by wt. of Di-n-octyl-tin-di-(2-ethylhexyl-thioglycolate) (I) have been investigated in detail. The rate of HCI-elimination from all PVC samples, as determined via conductivity measurements, was higher in air than under N2-atmosphere. After an initial period, which is characteristic for the stabilizer content of the respective sample, the rate of HCI elimination becomes constant, since no autocatalysis occurs via the eliminated HCI. In the beginning the velocity of HCI elimination decreases with increasing stabilizer content. The lowest value is reached at ca. 0.5% by wt. of the stabilizer, thereafter it remains constant. A similar tendency is shown in the case of the loss of weight of the samples as determined via thermogravimetry. The activation energy of PVC-degradation as well as the so-called 'maximum degradation temperature' reach their highest value (67 kcal/mol or 303°C respectively) at 0.5% by wt. of the stabilizer in PVC. Accordingly, there is an upper limit of the maximum effective stabilizer concentration in PVC. viz. about 0.5% by wt.Radio-tracer and ESCA-investigations show that the thioglycolic acid ester residues from I are incorporated into thermally stressed PVC, in form of sulfide-, sulfoxideand sulfonic structures, to the extent of 25X-independent of the stabilizer content of the samples. In the case of the n-octyl residue this is I 1 % by wt. of the stabilizer.Radio-GPC investigations show that the thioglycolic acid ester residues are preferentially linked to PVC molecules of shorter chain lengths. Since increasing amounts of stabilizer retard the discolouration of PVC, it is assumed that there is a direct relationship between the rate of incorporation of the ester residue and the PVC-discolouration.The stabilized, thermally stressed PVC absorbs a significantly larger amount of H 3 T 1 compared to pure or stabilized, untreated PVC respectively. Therefore, it can be assumed that HC1-polyen complexes or other complex structures having easily exchangable HCI or chloride ions, exist in stabilized, partially degraded polymers.

Principles of Stabilization

“…In an alcoholic solution, dibutyltin dilaurate reacts with HCl much faster than do metal soap stabilizers. 207 All these findings point to very high reactivity of organotins with HCl. For a model compound, the reactivity of organotins was very high.…”