Wood plastic composite (WPC), an environmentally friendly material, has drawn increasing interest in recent years, owing to the distinct combination of high strength and elasticity derived from wood fiber or natural fibers, as well as durability and fatigue resistance derived from its polymer matrix [1
]. WPC products have many applications in the construction, interior decorating, household appliance, and transportation industries [4
]. With growing awareness of the global environmental energy crisis and resource constraints, extensive research has been conducted into the use of bio-based and biodegradable plastics, such as poly(lactic acid) (PLA) [5
], poly(butylene succinate) [7
], and poly(butylene adipate-co
], in WPC to replace non-degradable plastics, such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene, which are derived from petroleum resources. The use of biodegradable polymers to produce environmentally-friendly bio-composites is a promising trend [9
PLA is a non-toxic and biodegradable bio-based thermoplastic polyester with numerous desirable properties, such as high strength and high stiffness [11
]. It is a polyester resulting from the polymerization of lactic acid, a bio-based monomer produced by the fermentation of biomass feedstocks. However, extensive application of PLA has not yet been achieved because of certain limitations, such as brittleness, slow crystallization, and relatively high cost [13
]. To overcome these disadvantages, low-cost natural fibers with higher modulus and rigidity, such as wood flour [14
], have been considered as a reinforcement for PLA. Wood flour/PLA bio-composites can not only reduce the overall cost of the material and enhance the brittleness of PLA, but are also potentially completely biodegradable [15
However, the incompatibility between PLA and wood fiber (hydrophobicity versus hydrophilicity) leads to poor wettability and interfacial adhesion between the two materials [17
]. Therefore, interfacial modification between wood fiber and PLA matrix is the key to improving the mechanical properties of wood flour/PLA bio-composites. To improve the compatibility of these two phases in the bio-composites, many modification methods have been developed, including alkali treatment [18
], silane modification [20
], acetylation [22
], addition of compatibilizers [24
], etc. Recently, one of the more promising interfacial modification approaches was adopted for use in binary/ternary incompatible PLA blends or composites, which included grafting a reactive moiety, such as maleic anhydride (MAH), onto the polymer matrix and then using this graft copolymer functioning as a compatibilizer in immiscible systems [26
]. MAH grafted PLA has been used as a compatibilizer in PLA blends with starch [28
], clay [30
], and natural fibers [31
] to improve the interfacial adhesion.
Graft copolymers were also used in WPC, and relatively higher grafting degrees of the graft copolymer were found to be beneficial to improving the compatibility between the wood flour and the PLA matrix [16
]. The addition of styrene (St) was shown to be an effective way of increasing the grafting degree of MAH onto a polymer matrix, and could activate MAH, as well as restrain the degradation of the polymer matrix [35
]. The presence of St as a comonomer donates electrons, which promote the activity of MAH, helping to render its unsymmetrical structure and its π radical-anion bonds [36
]. Although styrene has been reported to be a comonomer for polyolefin-g
-St/MAH systems, less focus has been directed toward St-assisted MAH-grafted polyesters, such as PLA. Moreover, no papers have reported on the preparation of wood flour/PLA bio-composites with styrene-assisted maleic anhydride-grafted poly(lactic acid) (PLA-g
-St/MAH) as a compatibilizer. The goal of this study was to explore the effect of PLA-g
-St/MAH on the interfacial properties of wood flour/PLA bio-composites.
In the current study, a novel PLA-g-St/MAH was synthesized using St as a comonomer and dicumyl peroxide (DCP) as an initiator by melting graft reactions. This is the first attempt to incorporate PLA-g-St/MAH into wood flour/PLA bio-composites to improve interfacial adhesion between the wood flour and the PLA matrix. In addition to the effects of PLA-g-St/MAH on mechanical properties and morphology, the rheological properties of the composites with the addition of PLA-g-St/MAH were examined. The correlation between the rheological behavior and the mechanical properties was further clarified, as well.
Styrene-assisted free-radical melt grafting of MAH onto PLA (PLA-g-St/MAH) was achieved. PLA-g-St/MAH was used as an efficient compatibilizer for wood flour/PLA bio-composites. The effects of the loading rate of PLA-g-St/MAH on rheological and mechanical properties, as well as the morphology of the composites, were investigated. Wood flour/PLA composites using PLA-g-St/MAH as a compatibilizer exhibited higher storage modulus, complex viscosity, equilibrium torque, and shear heat, indicating that better compatibility was achieved with the addition of PLA-g-St/MAH. When the content of PLA-g-St/MAH was 3 wt %, the mechanical properties of the composite reached their maximum values, and were higher than those for the composite with 3 wt % PLA-g-MAH. The maximum values included increases of 63.06, 15.09, 19.02 and 64.00% in tensile strength, flexural strength, impact strength, and elongation at break, respectively, compared with the uncompatibilized composites. Compatibility of the wood flour/PLA composites was improved significantly after the addition of PLA-g-St/MAH because of the good interfacial adhesion between the wood flour and the PLA matrix. PLA-g-St/MAH proved to be an effective compatibilizer in wood flour/PLA bio-composites, and a suitable content of PLA-g-St/MAH could optimize the mechanical properties of wood flour/PLA bio-composites.