Routes to halogen-free flame-retardant polypropylene wood plastic composites

Developing halogen-free flame retardants with reasonably high efficiency, which thus function at limited loadings in polypropylene-based wood/plastic composites (WPC), is still a challenge. Cost-effective flame-retarded WPC have been identified as a way to open the door to an interesting, broader spectrum of application in the building and transportation sectors. This work imparts a systematic comprehensive understanding and assessment of different basic routes to halogen-free flame-retarded WPC, taking into account economic and environmental considerations. Cheap, halogen-free single-component flame retardants and their multicomponent systems are investigated at reasonable filling grades of 20 wt%. The basic routes of promising synergistic multicomponent systems are discussed, and their potential and limits assessed. Optimizing the consistency of fire residue; closing the surface of inorganic-organic residual layers; the thermal stabilization and design of the residue, eg, synergistic combination of ammonium polyphosphate and expandable graphite; and the combination of different flame-retardant mechanisms, eg, intumescence and flame inhibition, are proposed as promising routes to boost the flame-retardant efficiency.