Beijing, China – Wood–plastic composites are in widespread commercial use, with applications including interior panels and package trays in cars, and in the construction sector, for stairs, handrails and so on. Various different polymers have been used, but polyurethane foam is particularly appropriate, as residues from wood processing can be readily incorporated within it.
In particular, a PU composite using wood flour has potential, as they have low density combined with high dimensional stability, alongside high strength and stiffness. And, of course, as it uses waste from wood processing, it has good sustainability credentials.
However, there is a drawback: its low ignition point, combined with high flammability. Scientists at Beijing Forestry University have found that incorporating ammonium polyphosphate as a flame retardant in foamed PU/wood flour composites has promise. The foam was made using MDI, while the wood flour came from poplar.
They looked at different ways of incorporating the FR, and both addition and impregnation improved combustion behaviour, but impregnation worked best. This was achieved by soaking the wood flour in a solution of the FR for 24 hours and then oven dried before the composite was made.
Impregnation gave lower total heat release, peak heat release rate, and prolonged the time to ignition. It also left more residues, and combustion safety was better. They concluded that, although the FR had some negative effects on the composite’s physical properties, it was effective as a flame retardant for composites made from foamed PU and wood flour, and that how the FR is incorporated is important.
The work has been published in the journal Heliyon.