Effectiveness of pre-treated wood particles and halogen-free flame retardants used in wood-plastic composites

Flame retardancy of wood-plastic composites (WPC) needs to be improved to widen the applications for these composites. This may be accomplished by protecting both polymer matrix and wood filler with a fire-retardant (FR). In this study, hardly inflammable particleboard converted into wood flour (B1-particleboard flour) was used in combination with several halogen-free FRs. The fire performance and thermal properties of the WPC (matrix: recycled polypropylene) were investigated using limiting oxygen index analysis (LOI), a modified single-flame source test, fire shaft test and thermogravimetric analysis (TGA). The combination of B1-particleboard flour, expandable graphite and red phosphorous led to the highest LOI (38.3). However, at the same time, relatively high water absorption of this formulation was observed which was attributed to the reaction of ammonium polyphosphate in the B1-particleboard flour and red phosphorous which was added during the compounding step, leading to increased hydrophilicity of the composite. The combination of B1-particleboard flour and ammonium polyphosphate (APP) or phosphorized expandable graphite to protect the polymer matrix offered the best overall performance when taking into account LOI (here: 34-35), tensile strength, modulus of elasticity and water absorption values. The WPC-formulation including B1-particleboard flour and APP did not pass the requirements of the German fire shaft test according to DIN 4102-1, however, the duration until smoke gas temperature exceeded 200 °C was more than doubled compared to a commercial WPC-profile based on PP without fire-retardants. The modified single-flame source test showed that all WPC formulations were self-extinguishing.