# Effects of Matrix Modification on the Mechanical Properties of Wood-Polypropylene Composites
## 1 Introduction  
The use of thermoplastic polymers reinforced with wood fibers (WF) has received considerable attention owing to the low energy consumption and environmentally friendly nature of the materials [1; 2; 3]. Wood flour/plastic composites, which combine favorable properties with a low specific mass, have been widely used for non-structural applications, such as decking materials, automotive components, and packing products [4; 5; 6]. The hydrophilic property of WF, similar to all natural fibers, has an inherently low compatibility with hydrophobic polymers, such as polypropylene (PP) and polyethylene (PE) [7]. This incompatibility is unfavorable for interfacial fiber-matrix adhesion and dispersion of the fibers, resulting in inferior mechanical properties of the composites [8]. Various studies indicate that the fiber-matrix interaction plays a critical role in stress transfer between the matrix and the fibers, thereby affecting the mechanical properties of composites [9; 10]. To improve the interfacial adhesion between WF and plastics, chemical or physical modification of WF or the matrix, and the addition of a coupling agent, can be used [11; 12; 13]. Adding a coupling agent is known to efficiently improve interfacial adhesion by forming a chemical bridge between the fibers and the matrix [14; 15; 16]. Maleated polypropylene (MAPP) is a well-known coupling agent for wood-plastic composites [17]. [PERSON] et al. reported that in jute-PP composites at 50% fiber concentration, the addition of 5% _m_-isopropenyl-\(\alpha\)-\(\alpha\)-dimethylbenzyl-isocyanate (m-TMI)-grafted-polypropylene as a coupling agent led to an increase in tensile and flexure strengths over virgin polypropylene, up to 87% and 95%, respectively [18]. [PERSON] et al. reported that fiber modification with sodium hydroxide alsoimproved the interfacial compatibility and properties of the composites [19]. [PERSON] et al. demonstrated that treatment of WF with glutaraldehyde and 1,3-dimethylol-4,5-dihydroxyphenyleneurea enhanced the interfacial compatibility and tensile strength of the composites [20]. Plastic matrix modification is another potential strategy to improve the mechanical properties of wood-polyppropylene composites [21; 22], which involves fewer solvent-based processes, in comparison with fiber modification. Effective chemical modification of polymer matrices has recently been developed via reactions with polar monomers to enhance interfacial adhesion, particularly in the case of polyolefin based composites. [PERSON] reported that for recycled low-density polyethylene matrix, adding peroxide in small amounts resulted in a remarkable improvement in the mechanical properties of the composites [23]. Grafting of glycidyl methacrylate onto a PP matrix improved the fiber dispersion in the PP matrix and enhanced the interfacial adhesion relative to that of unmodified composites [22]. Similar findings were reported for composites based on PP/PE blends grafted with maleic anhydride (MAH) [24]. With the addition of only 1 wt % MAH, the tensile, flexure, and impact strengths of PP/PE composites increased by 216%, 120%, and 62.7%, respectively, relative to those of the unmodified PP/PE composites. However, high initiator concentrations of peroxides are typically required to achieve moderate grafting efficiency, owing to the poor reactivity of MAH toward polypropylene macroradicals. Degradation of polymer matrix by chain scission occurs with increasing concentration of the initiator, which negatively affects the mechanical properties of PP-based composites. Over the past decades, various strategies have been proposed to control PP degradation in melt free-radical grafting, by changing the radical intermediate states to stabilize PP macroradicals [25; 26]. This strategy involves the use of comonomer styrene, \(p\)-(3-butenyl) styrene, or furan derivatives.