The synthesis and evaluation of inherently fire retardant/low smoke unsaturated polyester resins
A study has been undertaken to attempt to overcome three major problems associated with common unsaturated polyester resins. These problems are the emissions of free styrene, high flammability and the emission of thick black smoke during combustion. A novel cross-linking monomer, containing two allyloxy substituents and one diethylamino substituent, has' been synthesised and purified in a two stage reaction using cyanuric chloride as the starting material. This new monomer contains five nitrogen atoms in an attempt to reduce smoke evolution and has a low volatility to reduce volatile organic emissions. It was found that this monomer was totally miscible with all resin types, based on both aromatic and aliphatic alkyds, and produced a degree of cross-linking in excess of 95% when cured with dicumyl peroxide at 90°C for 16 hrs and then 140°C for 3 hours. Combustion tests showed that the new resins produced considerably less smoke than the styrenated resins, show higher LOI values, i.e. require a higher percentage of oxygen to sustain combustion, and a lower surface spread of flame. These improved combustion properties are a direct result of the improved charring characteristics of the resins caused by the incorporation of the triazine monomer. It has been shown, however, that normal fire retardant phosphorus additives used in unsaturated polyester resins (UPRs) have little or no effect in the triazine resins and do not show any synergism with the nitrogen atoms in the new monomer. As a result of this, at high additive loading levels, the combustion behaviour improvements shown by the triazine resins over the styrenated resins are drastically reduced. It has also been shown that the triazine resins have a very high burning rate. The lack of phosphorus activity in the triazine resins has been shown to be caused by these resins thermally decomposing at low temperatures where the phosphorus is not active. This low decomposition temperature also leads to rapid decomposition and thus a high burning rate. The lack of synergism might be caused by the tertiary amine substituent being too stable towards interaction with the phosphorus additives; a primary amine might be more suitable. Mechanical studies have shown the new resin to be brittle in comparison to the styrenated resins but that it has increased Barcol hardness values, heat distortion temperature and flexural modulus.