Most of existing commercial unsaturated polyester (UPE) resins are typically composed of a UPE and a high amount of highly volatile, flammable, and toxic styrene. The emission of hazardous styrene in the handling of the UPE resins and the preparation and use of fiber-reinforced UPE composites poses severe hazards to people's health and the working environment. In this study, two non-hazardous chemicals have been investigated for the replacement of styrene in the UPE resins. The first chemical is acrylated epoxidized soybean oil (AESO) that is derived from soybean oil and is not toxic and not volatile. The second chemical is methyl cinnamate that is extensively used as a food additive. Styrene-free UPE resins were prepared through combinations of these styrene replacements and three different UPEs. The styrene-free UPE resins were cured without the reinforcement of fibers and then evaluated and characterized for their properties. Kenaf fibers and glass fibers have been investigated as reinforcing fibers for the styrene-free UPE resins. An efficient process of making the fiber-reinforced UPE composites was developed. The viscosity and the pot life of the styrene-free UPE resins under different temperature were studied. The curing behavior of the resins under heat was investigated. The viscoelastic properties of the cured UPE resins and the fiber-reinforced UPE composites were characterized with dynamic mechanical analyses. Mechanical properties such as flexural and tensile properties of the composites were measured. It was found that these styrene replacements were comparable or even superior to styrene in terms of the mechanical properties of the composites. The weight ratio between a styrene replacement and a UPE was studied in detail for maximizing mechanical properties. The curing mechanisms of these styrene-free UPE resins are discussed in detail.
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