Expandable graphite as an intumescent flame retardant in polyisocyanurate-polyurethane foams

Flame retarded polyisocyanurate-polyurethane (FR PIR-PUR) foams have been synthesized by the use of a new flame retardant, expandable graphite (EG foams), never used on industrial scale in polyurethane rigid foams, and a mixture of expandable graphite and triethylphosphate (EG-TEP foams). FR PIR-PUR foams, blown with n-pentane, have been prepared with a constant NCO index (250) in order to achieve greater thermal stability and better fire behaviour than polyurethane foams, without significantly decreasing physical-mechanical properties. It was then investigated, as the presence of the flame retardants influences both physical-mechanical properties and fire performance. In particular, fire behaviour of the foams has been studied through cone calorimeter analysis, by the oxygen index and by the DIN 4102-B2 test. The results show that an increase in expandable graphite amount in EG foams or in triethylphosphate content in EG-TEP foams does not worsen mechanical properties. Increasing the amount of triethylphosphate does not influence the thermal conductivity, but an increase in the quantity of expandable graphite causes a worsening of insulating properties, probably due to the bigger dimensions of the foam cells. The presence of expandable graphite alone or in mixture with triethylphosphate brings an overall improvement in fire behaviour. In particular, the oxygen index (OI) increases in a linear way and the highest OI values are obtained for EG-TEP foams. The results from the cone calorimeter are in agreement with those of OI; EG and EG-TEP filled foams show a considerable decrease of rate of heat release (RHR) with respect to unfilled foams. In particular, for EG-TEP foams, the higher the triethylphosphate content the higher the RHR decrease. The only hazard observed is an increase of CO/CO2 weight ratio in the presence of very high content (25%) of expandable graphite; this effect is not shown when increasing the TEP amount.