Toughness and crystallization enhancement in wood fiber-reinforced polypropylene composite through controlling matrix nucleation

Wood fiber/polypropylene (WF/PP) composites are commonly used to efficiently utilize natural resources and reduce environmental problems, but the low impact toughness has been a major disadvantage of this composite. Most studies are focused on fiber modification or adding coupling agents to overcome this problem, but few of them paid attention to the microstructure of the matrix and its effect on the interface adhesion. Isotactic polypropylene (PP) is polymorphic with different morphological forms: α-, β- and γ-forms. α-PP is predominated form, while β-PP has received considerable attention due to its promising toughness behavior. This study investigated the effect of adding a highly active β-nucleating agent (TMB) for PP on the mechanical and crystallization properties of PP/WF composite. Specifically, the impact strength, the nucleating effect, content of β-crystal, non-isothermal crystallization behavior and mechanical properties of the composite were investigated. Results showed that the content of the β-form crystal of nucleated PP (K value) reached 87% with the addition of 0.3 wt% TMB. Composites with TMB increased the crystallization peak temperature, accelerated the crystallization process and improved its impact strength by 53.12% with the addition of 0.3 wt% TMB nucleating agent, though tensile and flexural properties were dropped by about 10–20%. Also, the spherulite size of nucleated PP in the composite was dramatically decreased.