Many composite products are made of thermosetting polymers reinforced with synthetic fibers. Despite the high mechanical properties associated with these fibers they are heavy and expensive compared with natural fibers. The use of natural plant fibres, combinations of natural and synthetic fibers, and wood furnish as reinforcement in polyester matrix for making low cost engineering materials has generated much interest recently. Natural fibers with good specific stiffness and strength, low density, low embodied energy, and good biodegradability have an advantage over synthetic fibers. Despite these benefits they have poor compatibility with the matrix due to their hydrophilic nature. This paper reviews the literature on the effects of chemical treatments on fiber-matrix interfacial adhesion and the wettability of natural fibers by polyester. The efficiency of incorporating glass fiber into the natural fiber for the purpose of reducing water uptake and increasing the stiffness of composite is also discussed. Plant fiber/polyester composites Hemp fiberHemp fiber is the most common natural fiber for reinforcing polyester composites. The composites can be manufactured using hand lay-up, compression molding, or resin transfer molding (RTM) methods. The drawback of using plant fibers as reinforcement is their susceptibility to water adsorption due to their hydrophilic nature. Surface modification of fiber, however, can limit their water uptake. The effects of hemp fiber loading, content, and chemical treatments on energy absorbing ability, and on tensile, flexural, and water absorption properties of polyester composites are discussed in this section.Fiber loading and fiber length effects. The energy absorbing mechanisms of hemp fiber/polyester composites associated with the impact fracture have been investigated in detail by Sanadi et al. 1 Composites were made using hand lay-up method and their Izod impact toughness were measured. Results of this study showed the toughness of composites at volume fraction of 24% to be 15 times higher than the pure polyester resin. Scanning electron microscope (SEM) micrographs of fracture surface indicated that various fracture mechanisms including fiber pull-out, plastic flow of the lignin-hemicellulose matrix, hemp fiber splitting, and crack extension at fibril-fibril interface contributed to energy absorption. In a similar study by Dhakal et al. 2 different hemp fiber volume fractions, up to 26%, were used to make nonwoven hemp fiber/polyester composites. A combination of hand lay-up and compression molding methods was used to manufacture these composites. Results indicated the percentage of moisture uptake increased with fiber volume fraction. The increase was attributed to the additive amount of cellulose content at higher

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Various formulations of wood flour and rigid Poly Vinyl Chloride (PVC) composites were prepared and the effects of layering and composite composition on their physical properties were studied. Beech wood flour, PVC, Maleic Anhydride Polyethylene (MAPE), and beech veneer were used to produce the composites in a two-step conventional hot press system. Physical property tests, including equilibrium moisture content (EMC), long term water absorption and thickness swelling were carried out. The results indicated that, by the increase in PVC content from 20% to 30%, EMC, maximum water absorption and maximum thickness swelling decreased. No significant relationship was observed between PVC content and water diffusion coefficient, whereas layered composites on average had higher water diffusion coefficients in comparison with unlayered composites. Results of laminating revealed that it caused a decrease in the EMC, and an increase in maximum water absorption and maximum thickness swelling.

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Various formulations of wood flour and rigid polyvinyl chloride (PVC) composites were prepared and the effects of three accelerated freeze—thaw cycles on their physical and mechanical properties were studied according to ASTM D7031 specification. Beech (Fagus orientalis) wood flour, PVC, maleic anhydride polyethylene (MAPE), and beech veneer were used to produce the composites in a two-step conventional hot press system. A thickness swelling test was carried out as an indicator of physical property. Mechanical characteristics of composites were evaluated after each cycle using three-point flexural tests, and losses in mechanical properties were studied. Results indicated that thickness swelling was not affected by freeze—thaw cycling while significant relationship was observed between PVC content and thickness swelling and with increase in PVC content, maximum thickness swelling decreased. The values of flexural strength (MOR) and modulus (MOE), were significantly affected by PVC content, layer, and freeze—thaw cycling. Layering of the specimens and higher PVC contents improved MOR and MOE while freeze—thaw cycling decreased them at the first cycle. Repeated freeze—thaw cycling did not exhibit a significant effect on the mechanical properties and the effect of the cycles was confined to the first cycle only. Increase of PVC content had positive effect on both physical and mechanical properties.