Long-term storage stability studies of vinyl ester resins using commercial antioxidant polymerisation inhibitors

The use of resins with carbon-carbon terminal functional groups, often referred to as vinyl ester resins, have found a wide commercial use, and their production is in the millions of tonnes per year, as either unsaturated polyesters (estimated 3.0 million tonnes in 2018, world-wide), acrylate or methacrylate monomers and oligomers (estimated 150 thousand tonnes in 2018, world-wide). In all cases the resins are synthesised, stored and transported for further processing into the finished article, either as a coating or a resin often at a second site. The use of antioxidants as polymerisation inhibitors is widely known and understood, but the relative efficiency of each of these compounds in different resin types is not widely published. This study has looked at a range of amine, epoxy and urethane acrylate and methacrylate oligomers. These oligomers were synthesised to control the antioxidant concentrations and comparative stability testing was undertaken to obtain representative data in terms of long term stability at various temperatures and concentrations (100 to 1000ppm), with testing at 20°C taking between 3 to 5 years to complete for each sample set. The resins were evaluated in glass vials so that the head space conditions (air or nitrogen) could be controlled and to mimic industrial practice. The stability data showed that for a given concentration, the stable radical compound (4-hydroxy-2,2,6,6-tetramethyl-1-piperdinyloxy) was the most efficient, although it was the most expensive and discoloured the resin.

The antioxidants evaluated were a mixture of commercially available compounds and which have use within various segments of the polymer industry, with both phenolic antioxidants and stable radical compounds examined. Rheology was used to evaluate the degree of polymerisation and resultant gel formation due to free radicals formed by peroxides. The measurement of the dissolved oxygen content of monomers was investigated to determine the amount of oxygen present. A number of antioxidants were used to create blocked isocyanates to evaluate their potential for use in high temperature resin systems with limited success due to their high unblocking temperatures.

Since the bulk of resins supplied are subsequently cured to form coatings or resins, work was also undertaken to see what effect the addition of antioxidants have upon the curing by free radical sources either peroxide or Ultra-Violet active initiators at 1% loading, with resins both fresh and 12 months old. It was found that at these concentrations of free radical sources, the rate of polymerisation was not significantly affected at the highest concentrations of the antioxidants used.