Long-term heat stabilisation by (natural) polyols in heavy metal- and zinc-free poly(vinyl chloride)

Abstract

The long-term heat stabilisation efficiency of (natural) polyol additives in heavy metal-and zinc-free poly(vinyl chloride) (PVC) has been investigated. It is shown that polyols, such as sorbitol and xylitol, markedly reduce the dehydrochlorination rate and improve Congo Red values. Extraction experiments on unprocessed and ground-processed PVC-sorbitol (1.0 phr) mixtures after thermal degradation at 200 C revealed that especially in the ground-processed PVC-sorbitol system, sorbitol is partly converted into its mono-and dianhydro-derivatives 1,4-sorbitan and isosorbide, respectively. Apparently, the HCl released during thermal degradation acts as the catalyst. Similar intramolecular cyclodehydration reactions also occur with the natural polyols, erythritol and xylitol, under these conditions. Scrutiny of the measured dehydrochlorination rates and the Congo Red values for groundprocessed heavy metal-and zinc-free PVCepolyol mixtures show that in particular polyols containing primary hydroxyl groups exert long-term heat stabilisation and that they act as efficient HCl scavengers.

The effect of (natural) polyols on the initial colour of heavy metal- and zinc-free poly(vinyl chloride)

“…These results confirm the initial colour W(Lab) data and substantiate that both long-term [8] as well as shortterm heat stability appear to depend on the presence and availability of primary hydroxyl groups. Striking in this context is the observation that the tertiary hydroxyl group of 1-adamantanol exerts a distinct destabilising effect.…”

“…Strikingly, 1,6-hexanediol outperforms sorbitol, which contains two vicinal diol configurations. As shown previously [8], acid-catalysed intramolecular cyclodehydration of sorbitol (to 1,4-sorbitan, and finally isosorbide, Scheme 3) is a competitive side reaction leading to a considerable reduction of the available primary hydroxyl groups that can act as HCl scavengers (Fig. 5 and cf.…”

“…In addition, for a,u-diols with chain lengths longer than three carbon atoms, acid-catalysed intramolecular cyclodehydration can become a competitive side reaction under the applied reaction conditions. Such a competitive side reaction will effectively reduce the amount of available primary hydroxyl groups [8]. Whereas 1,4-butanediol can be converted into the thermodynamically stable intramolecular cyclodehydration product tetrahydrofuran, for a,u-diols with chain lengths longer than five carbon atoms this side reaction will be less favourable.…”

“…The significant difference in performance between the pentitols and the tetritols cannot be explained on the basis of competitive intramolecular cyclodehydration [15], as tetritols will give cyclodehydration products with only inactive secondary hydroxyl groups (see also Ref. [8]). …”

“…Unfortunately, however, in these PVC formulations the polyol additives exert a detrimental effect on the initial colour [5,7]. We recently reported that for heavy metal-and zinc-free PVC formulations (natural) polyols, especially those containing primary hydroxyl groups, act as HCl scavengers by substitution of the primary hydroxyl moieties by chloride ions [8]. These results prompted us to study in more detail the effect of (natural) polyols on initial colour in the absence of heavy metal-or zinc-containing heat-stabilising additives.…”