# A Review on Styrene Substitutes in Thermosets and Their Composites
## 1 Introduction  
The fast growth of chemical engineering has driven a wider industrial utilization of thermosets cured from unsaturated sites, including unsaturated polyester (UPE), vinyl ester (VE), and triglyceride-based resins. UPE is a linear polymer with unsaturated C=C bonds resulted from saturated di-alcohols with saturated or unsaturated di-acids via a condensation polymerization (Figure 1a) [1]. It is welcomed because of its low cost, high strength, excellent chemical corrosion resistance, good thermal and electrical properties [2]. Bisphenol-A VE is prepared from methacrylic acid and bisphenol-A epoxy resin (Figure 1b) [3, 4]. The fast crosslinking process of C=C bonds gives the materials favorable mechanical properties and outstanding chemical and water resistance. In considering the environmental impact and energy consumption, thermosetting resin from vegetable oils is a greener alternative to get rid of using fossil fuels. Its various triglycerides combined with three long-chain fatty acids and glycerol have different length of chains and a range of unsaturation degree, leading to the distinction of curing behavior [5]. As a typical example, acrylated epoxidized soybean oil (AESO) is one of the commercially available vegetable-oil-based resins (Figure 1c) [6]. These commercially available or synthesized resins and their composites are attractive for respective merits and have been widely used in coating, adhesive, automobile parts, turbine blade, etc. [7, 8, 9, 10, 11]. Upon these resins mentioned above, all of them refer to the free-radical polymerization of unsaturated C=C bonds from main molecular chains. The large or long main chain structure leads to a strong intermolecular force, which is significantly harmful to resin processability and efficiency of crosslink. For example, UPE is solid at room temperature, which makes it hard to process and disperse uniformly for wetting fibers well. The flexible long fatty chains in AESO result in a low mechanical property. Therefore, in common cases, introducing small molecular comonomers as RDs into crosslink network is necessary to enhance their performance at service. Currently, styrene is a traditional and preferential RD that has advantages such as low cost, low molecular weight, high reactivity, and excellent polymerizability [12; 13; 14; 15]. With the aid of styrene, improvements occur in resin processability including reduced viscosity and curing temperature as well as increased curing rate and degree, thus leading to better mechanical properties and durability [16]. However, styrene is classified as a volatile organic compound (VOC) for low boiling point and high saturated vapor pressure. Even worse, styrene is not environmental-friendly as a hazardous air pollutant and is harmful to human health [17; 18]. The Environmental Protection Agency of USA has legislated to limit styrene emission content in composite fabricating workshop (<50 ppm) [19]. In addition, the National Institute of Health of USA listed it as a potential carcinogen in 2011 and emphasized it again in 2016 [20]. Therefore, it is highly desired to reduce styrene emission or to replace styrene with other greener monomers during the preparation of thermosets and their composites. In this review, we classify RDs into typically crosslinkable monomers and novel monomers derived from renewable materials. These monomers usually possess a range of characteristics: (1) at least one unsaturated site for participating in crosslinking; (2) good compatibility for making homogeneous resin; (3) low volatility for avoiding the emission of VOC. It can be regarded as low volatile crosslinker in thermosets, and other functions may be bestowed on it as well. The research on greener RDs has practical engineering benefits like improving product performance, satisfying requirements of sustainable industry. Figure 1: Representative structure of (**a**) unsaturated polyester (UPE), (**b**) VE [3] and (**c**) acrylateed epoxidized soybean oil (AESO) [6].