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

Abstract

The propensity of (natural) polyols dispersed in heavy metal-and zinc-free PVC sheets to improve the initial colour, e.g. the colour during the first minutes of exposure to high processing temperatures (short-term stability), has been investigated. It is shown using W(Lab) values that the initial colour improves upon addition of polyols containing primary hydroxyl groups. The polyols act as HCl scavengers, presumably via an acid-catalysed S N 2 substitution of the primary hydroxyl groups by chloride ions. In contrast, polyols with only secondary or tertiary hydroxyl groups accelerate the thermal degradation of PVC. Notwithstanding, the efficacy of the (natural) polyols containing primary hydroxyl groups will be reduced if the polyol is susceptible to competitive acid-catalysed intramolecular cyclodehydration reactions under the processing conditions. This is substantiated by a comparison of the behaviour of mannitol and 3,4-di-O-methyl-D-mannitol. The methylated derivative, which is less prone to undergo intramolecular cyclodehydration, improves the initial colour of heavy metal-and zinc-free PVC sheets more significantly than mannitol itself.

The compatibility of (natural) polyols with heavy metal- and zinc-free poly(vinyl chloride): Their effect on rheology and implications for plate-out

“…4 ] À0.7 , was also tested in formulation 2, and gave results highly comparable to those obtained with Alcamizer 1 Ò (rheometer curves not shown).…”

“…Although polyols are already being used in leadand zinc-stabilised PVC formulations, they are often thought to be the cause of poor early (initial) colour [3], and the formation of 'plate-out' or 'die build-up'. As we have shown previously, the use of natural polyols, such as sorbitol, does not necessarily lead to poor early colour [4]. In a heavy metalfree PVC formulation various types of natural polyols significantly improved both early colour as well as long-term stability.…”

“…Previously we have shown that, in contrast to general consensus, (natural) polyols when added to lead-and zinc-free rigid PVC compounds do not necessarily lead to a bad early (initial) colour [2,4]. Now, a second potential drawback of polyols, increased plate-out, was investigated.…”

“…Most alditols can undergo acid-catalysed intramolecular cyclodehydration reactions yielding cyclic ethers and water (cf. Scheme 1) [2,4]. In the PVC matrix HCl can act as catalyst in this reaction.…”

Study on lanthanum-pentaerythritol alkoxide as a thermal stabilizer for rigid polyvinyl chloride

“…However, at the processing temperature, unstabilized PVC releases gaseous hydrochloric acid (HCl), which gives rise to the formation of conjugate double bonds or polyene sequences -(-CH5CH-) n -, thereby causing a color change [3,4]. The color of PVC changes rapidly from white to yellow to brown to black [5].…”