An Environmental Approach to PVC Stabilisation
We were interested in investigating the stabilisation behaviour of 6-aminouracils, a new class of heavy metal free thermo-stabiliser for PVC. We have therefore investigated the performance and mechanism of stabilisation for 1,3-dibutyl-6-aminouracil, both alone, and in combination with a sodium perchlorate co-stabiliser. For the performance assessment of this stabiliser, we have utitlised a combination of dehydrochlorination rate data, UV-spectra data, and weight average molecular weight data. For mechanistic investigations, we have measured the average polyene sequence lengths and average number of sequences at different levels of dehydrochlorination. Further, we have investigated the stabilisation behaviour of the stabiliser with pre-formed polyene sequences (pre-degraded PVC), as well the extent of reaction of stabiliser with both virgin and pre-degraded samples during stabilisation. Thus, the stabilisation of PVC with 1.0 mol% 1,3-dibutyl-6-aminouracil has been shown to proceed primarily via a labile chlorine substitution reaction in growing sequences. Unfortunately, a random HCl elimination reaction, catalysed by the basic amine in the enaminone segment of the 6-aminouracil, results in the rapid formation of new labile sites. The stabiliser alone therefore offers excellent short term colour stability, but no long term stability. Addition of the sodium perchlorate co-stabiliser has been shown to markedly reduce the rate of random HCl elimination, therefore significantly lowering the number of new labile sites formed. A second, but less pronounced, inhibiting effect of co-stabiliser is the rate of labile chlorine substitution. However, the random reaction of stabiliser with double bonds in polyene sequences, known as 'colour improvement', which is only a minor stabilising reaction in the uracil only system, is dramatically accelerated by the addition of the sodium perchlorate co-stabiliser. A mechanism for 'colour improvement' involving the reaction of an iminium perchlorate salt intermediate is proposed.
We were also concerned with the preparation of polymer-bound stabilisers, where the stabiliser, as part of a larger co-monomer, is introduced during the polymerisation stage. Depending upon the stabilisers mechanism of action during PVC processing, we would like a non-chain-grafting stabiliser to be permanently bound, and a chain-grafting stabiliser to become unbound during the processing-stage. We have therefore investigated thermal intrasequence lactonisation in several vinyl chloride-alkyl/aryl methacrylate copolymers. Here, a five membered lactone ring is formed in the polymer backbone and the ester substituent is split off. We found that the rate of lactonisation was controlled by the nature of the methacrylate substituent, being most rapid for tertiary and secondary substrates, and most hindered for primary substrates di-substituted at the C2 position. Lactonisation was not observed for phenyl methacrylate. Thus, 1,1-dimethylalkylmethacrylate was chosen as the structure most suitable regarding the polymerisable 'head' of a chain-grafting additive co-monomer, and phenyl methacrylate was chosen as the structure most suitable regarding the polymerisable 'head' of a non-chain-grafting additive co-monomer. A mechanism for lactonisation is proposed.
base catalysed dehydrochlorination
labile chlorine substitution
neighbouring group participation