Eco-friendly cold-pressed self-forming high-strength polyurethane/wood-fiber composite with flame retardance

Abstract

Wood-plastic composites (WPCs), which are widely used in construction and furniture, need to be treated with fire protection. In this work, polyurethane (PU) was applied to derive simultaneously flame-retardancy-improved and mechanically strengthened wood-polyurethane composites (WPUCs). It was constructed with ammonium polyphosphate (APP) and PU by an eco-friendly way of pressing at ambient temperature. The combination of APP and PU improves the flame retardancy and smoke suppression of WPCs. The results indicated that the flame retardancy of WPUCs is the best when the mass of APP is 18% of PU, making LOI reach 31.2%. In the combustion test, the peak of heat release rate and total smoke production for WPUCs were respectively decreased by 42.1% and 89.7% in the presence of the above ratio of APP and PU. In addition, the results of the functional group test show that PU contains highly reactive -NCO which is bonded to the -OH and moisture in the wood fiber, resulting in improvement of physical and mechanical properties. The mechanism for the excellent flame retardancy of WPUCs revealed that polyphosphoric acid produced by APP pyrolysis catalyzed PU into the char, and PU was arched by the resulting gases such as NH3 to form the tiny spherical structure. Meanwhile, the spherical structure increased the height and density of the carbon layer, which worked in blocking heat and the exchange of substances. This work provides a simple and economic strategy for the preparation of flame-retardant WPCs.