Comparative analysis of silane and maleic anhydride coupling agents in wood polymer composites: Adhesion mechanisms and impact on mechanical properties

Abstract

Wood polymer composites (WPC) have emerged as versatile and sustainable materials with diverse applications. However, achieving optimal interfacial adhesion between hydrophilic wood fibers and hydrophobic polymer matrices remains a key challenge, impacting the overall mechanical performance of WPC. This thesis aimed to address this challenge by investigating the effectiveness of different coupling agents in improving interfacial adhesion and mechanical properties of WPC. Wood fiber polypropylene (PP) composites were produced using two different approaches: by modifying the fiber surface before compounding or through an in-situ compatibilization method in a hot/cold mixer and extruder. Interfacial adhesion between wood fiber and PP was enhanced by the addition of 3-(trimethoxysilyl) propyl methacrylate (MPTMS) and vinyltrimethoxysilane (VTMS) with or without initiator, and the results were compared to those obtained using the commonly used maleic anhydride grafted polypropylene (MAPP), as well as WPC without any coupling agents. Injection-moulded specimens were prepared for mechanical testing, and the fractured surface from tensile testing was characterized by SEM. Water up-take of WPC was measured by immersing the specimens in water. All results showed that the interfacial adhesion between wood fiber and PP matrix is crucial in determining the properties of WPC. SEM images confirmed that the interfacial interaction between wood fiber and PP matrix determines the overall mechanical properties of the composite. Silanes with an initiator resulted in higher tensile strength compared to WPC without coupling agents, confirming the enhanced interfacial adhesion of WPC. Moreover, the incorporation of silane coupling agents with an initiator resulted in decreased water absorption, fulfilling the requirement for water resistance in outdoor applications. Among all the combinations, WPC with MAPP exhibited the highest tensile and flexural strength by improving the fiber-matrix adhesion and changing the failure mode from fiber/matrix debonding to fiber fracture. However, there was no significant difference observed in the modulus among all the samples.