Effect of ammonium polyphosphate to aluminum hydroxide mass ratio on the properties of wood- flour/polypropylene composites
In this study, two halogen-free inorganic flame retardants, ammonium polyphosphate (APP) and aluminum hydroxide (ATH) were added to wood-flour/polypropylene composites (WPCs) at different APP to ATH mass ratios and at a constant total flame retardant loading of 30 % by mass. Water soaking tests were carried out to measure water uptake and indicated a lower hygroscopicity and/or solubility of ATH as compared to APP. The mechanical properties of WPCs were assessed by three-point bending and impact strength tests. The flexural property was not significantly affected by the APP to ATH mass ratio. The addition of 30 % by mass of APP and/or ATH to the neat WPCs (no flame retardants) decreased the flexural strength by about 15 % and increased the flexural modulus by about 25 %. The impact strength appeared to increase when the ATH to APP mass ratio increased. Surprisingly, no reduction in impact strength was observed in the WPCs containing 30 % mass of ATH, as compared to the neat WPCs. Cone calorimetry and smoke generation tests were used to characterize the reaction to fire of WPCs. APP appeared to be more effective than ATH in reducing the peak of heat release rate. However, compared to the neat WPCs, smoke generation decreased with the addition of ATH but increased with the addition of APP. Noticably, WPCs containing the combination of 20 % by mass of APP and 10 % by mass of ATH (WPC/APP-20/ATH-10) showed the lowest peak of heat release rate and total heat release values (reduction of about 50 % and 32 %, respectively, compared to the neat WPCs). WPCs combusition residues were analyzed by scanning electron microscopy, laser Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). Thermogravimetric analysis coupled with FTIR spectroscopy was used to identify the organic volatiles produced during the thermal decomposition of WPCs. WPC/APP-20/ATH- 10 showed the most compact carbonaceous residue and the highest degree of graphitization.