Thermal and photo-chemical degradation of nylon 6,6 polymer: Part 1—Influence of amine-carboxyl end group balance on luminescent species

Abstract

Nylon 6,6 polymers of different degrees of polymerisation have been characterised by ultra-violet (uv) absorption, fluorescence and phosphorescence emission spectroscopy, viscosity number and amine end-group balance and the data related to their subsequent thermal and photo-chemical stabilities. The latter, determined by the percentage change in viscosity number, clearly shows that in both cases polymer stability is markedly influenced by the final stages of polymerisation under both steam and nitrogen atmospheres, the latter being the most detrimental. During both of these processes the molecular weight increases, as indicated by an increase in viscosity number, as does the concentration of fluorescent and phosphorescent impurities and products absorbing uv at 294 nm. Extraction of some of these species by 2-propanol results in a marked improvement in polymer photo-stability, implicating the involvement of these chromophores in initiation of photo-oxidation. Thermal degradation at 120°C confirms the presence of two types of fluorescent species. The first is reactive, rapidly diffuses out of the polymer and is associated with cyclic α, β-unsaturated carbonyl compounds; the second is non-volatile in-chain in type, increases in concentration and is associated with α-ketoimide structures. However, whilst correlations between the presence of light absorbing/luminescent carbonyl species is evident to some extent another, more important, factor based on the concentration of amine end-groups appears to be dominant. In this case a higher concentration of amine end-groups at lower degrees of polymerisation results in an increase in polymer stability, both thermally and photo-chemically and is associated with the radical/oxygen scavenging ability of the amine end-group, not unlike that of the well-known hindered piperidine light stabilisers. This is confirmed by the observation that treatment of nylon 6,6 films with sulphur dioxide gas, which reacts with the amine groups, thus generating sulphonamides, sensitises the photo-chemical oxidation of the polymer, an effect which contrasts with that found earlier for polyolefins.