Study on Polypropylene-Based Microporous Wood-Plastic Composites

At present, wood-plastic composites (WPCs) with high hardness, water- and termite-resistance, and reprocessability are widely used in indoor decoration, building renovation, packaging and transportation, and sports equipment. Their relatively low material cost and high consumption help reduce the depletion of forest resources and mitigate the environmental hazards of waste plastics. However, the high material density and poor toughness of WPCs make them prone to brittle fracture under load-bearing applications, limiting their ability to provide deformation warning effects through molecular creep as conventional high polymer materials do.

Given that traditional foaming techniques are insufficient to improve the brittleness of WPCs, researchers at home and abroad have proposed the concept of microporous foaming based on the toughening principle of inorganic nanoparticles. During the load-bearing process of microporous WPCs, impact cracks mainly propagate either directly through the pores or along the pore walls, while the plastic deformation of the pores absorbs a significant amount of energy, thereby blunting the crack tip and inhibiting crack propagation. This technique improves the mechanical performance of WPCs, promotes their high-performance applications, and retains advantages such as lightweight, material efficiency, sound insulation, and thermal insulation.

In the extrusion process of WPCs, the formation of internal microporous structures mainly relies on the sensitivity of gas solubility in the melt to flow and thermal fields, with gas generation achieved through pressure reduction or heating. The microporous structure then forms through the three stages of "nucleation–growth–stabilization." The foaming gas is primarily produced via thermal decomposition of chemical foaming agents or volatilization of physical foaming agents. The generation method not only determines the formation of the internal pore structure but also directly influences pore morphology and size parameters.

Accordingly, polypropylene-based WPCs were used as the study object, employing azodicarbonamide (AC) and sodium bicarbonate (NaHCO3) as chemical foaming agents. Their thermal decomposition behaviors and effects on the pore morphology of WPC samples were comparatively analyzed. Additionally, the effects of composite foaming agents and varying foaming agent contents on the mechanical properties of WPC samples under different gas generation amounts were explored to provide theoretical guidance for subsequent process optimization and product development. The results show:

1. The AC/ zinc oxide composite foaming agent can control the gas-release temperature during thermal decomposition at around 173°C, effectively avoiding the degradation of wood flour caused by high-temperature decomposition. Compared with the AC/ zinc oxide system, the NaHCO3 foaming masterbatch, with lower gas production and a longer decomposition temperature range, easily triggers multiple nucleation events, leading to a bimodal distribution of pore sizes.

2. Compared with the NaHCO3 masterbatch and the composite foaming agent of the two, the AC/ zinc oxide composite system more easily produces a "small and dense" microporous structure, achieving sample performance with impact strength of 6 kJ/m², tensile strength of 8 MPa, and flexural strength of 16.6 MPa.

3. When the AC/ zinc oxide composite is used at a ratio of 1 part, the foaming gas inside the composite material can be completely encapsulated by the melt, preventing gas escape, while also achieving rapid supersaturation to accelerate nucleation rate and improve nucleation quality. This results in a microporous structure with an average pore size of 71 μm and a distribution density of 2.7 × 10⁴ pores/cm³, providing superior load-bearing capacity.