Wood fibers with natural blowing agents for extrusion and injection molding of polypropylene

This paper presents a new two-step approach of foaming polypropylene (PP) containing wood fibers (WF) with tri-sodium citrate dihydrate as the low-temperature chemical blowing agent (CBA) and sucrose as the high-temperature CBA. The first CBA was intentionally selected to thermally degrade during extrusion (lower temperature range), whereas the second CBA during the injection molding (higher temperature range). The CBAs were dissolved in water, and WF were soaked in these solutions, afterward dried, ground, and then mixed with PP and extruded. The extruded granules were then injection molded to produce dumbbell-shaped samples. The porosity, thermal degradation, PP crystallization, mechanical and thermomechanical properties of the wood-polymer composites (WPCs), as well as their susceptibility to acoustic damping, were determined. It was shown that sucrose can be an effective CBA for injection-molded WPC samples. However, the effectiveness of this additive is improved by the addition of tri-sodium citrate dihydrate, which decomposed during extrusion.