Explain the impact of wood flour as filler on the tensile strength of a bioplastic composite.

The impact of wood flour as a filler on the tensile strength of a bioplastic composite is complex and depends on several factors. At low concentrations, wood flour can sometimes increase the tensile strength of certain bioplastics. This is because the wood flour particles can act as reinforcing agents, distributing stress more evenly throughout the composite material. The rigid wood flour particles can also hinder crack propagation, preventing the bioplastic from failing prematurely under tensile stress. However, at higher concentrations, wood flour often decreases the tensile strength of bioplastic composites. This reduction in strength is primarily due to poor dispersion and weak interfacial adhesion between the hydrophilic (water-attracting) wood flour and the typically hydrophobic (water-repelling) bioplastic matrix. The wood flour particles tend to agglomerate or clump together, creating stress concentration points that weaken the composite. Moreover, the lack of strong chemical bonding between the wood flour and the bioplastic matrix allows cracks to initiate and propagate more easily along the interface, leading to premature failure. To improve the tensile strength of wood flour-bioplastic composites, surface modification of the wood flour is often necessary. This can involve treating the wood flour with coupling agents, such as silanes or isocyanates, to enhance its compatibility with the bioplastic matrix and promote better interfacial adhesion. For example, adding a small amount of maleated polypropylene (MAPP) can improve the compatibility between wood flour and polypropylene-based bioplastics, leading to higher tensile strength compared to composites without MAPP.