Xiang, H, C Sun, D Jiang, Q Zhang, C Dong and L Liu. 2010. Flame retardation and thermal degradation of intumescent flame-retarded polypropylene composites containing spirophosphoryldicyandiamide and ammonium polyphosphate. Journal of Vinyl and Additive Technology 16:161-169.
Synopsis by Evan Beach, Sep 10, 2010
Polypropylene plastic (PP) was less flammable yet remained strong when mixed with two chemicals considered safer than those currently used as flame retardants, report Chinese researchers. The chemical blend achieved the highest flame retardancy rating in standardized tests without significantly impacting the strength of the plastic.
PP, which is coded as number 5 in plastic recycling, is used in numerous consumer products including carpets and thermal underwear.
The new flame retardant is a step forward in finding an alternative to traditional systems that are based on halogen-containing chemicals and antimony trioxide, say the researchers who developed the chemical blend. Alternatives are desired to prevent toxic, corrosive gases from forming during fires. Also, several classes of halogen-based flame retardants – like polybrominated diphenyl ethers – are raising concerns about persistence in the environment, toxicity and accumulation in animals and humans.
One of the additives, ammonium polyphosphate, is well known as a flame retardant, but by itself it cannot protect PP. The scientists invented a second additive – spirophosphoryldicyandiamide (SPDC) – that created synergy in the polymer blend. In the presence of a flame, the two additives formed a protective char layer that shielded the inside of the plastic from heat and prevented flaming drips. The combination also reduced heat, carbon monoxide gas and smoke.
Cost might limit the practicality of the new system. However, perfomance was adequate when the more expensive SPDC chemical was limited to just 25 percent of the flame retardant combination. The total amount of additives in the experiments was 30 percent of the plastic by weight.
The researchers showed that the new additives are safer compared to halogenated chemicals during fires. But, the overall green benefits of their technology are still not calculated. Further testing of the blends’ leaching, persistence and toxicity is necessary. Also, several highly toxic starting materials – like dicyandiamide and phosphorus oxychloride – are needed to make SPDC. These would be dangerous to workers and surrounding communities in the event of an accident.