Composite materials are made according to their application in a variety of ways, such as hand lay-up, pultrusion, resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), Sheet molding compound (SMC), bulk molding compound (BMC), and so on. The bulk molding compound is widely used in transportation, electrical and electronic industries, especially for the mass production of components with a small and complex geometry, due to easy production, high strength to weight ratio, light weight, resistance to the environment conditions, excellent physical properties, good mechanical properties, temperature resistance, dimensional stability, low cost and rapid production process are of great use in the industry. Composites made by the bulk molding compound combination of chopped artificial fibers to improve mechanical properties, mineral fillers for economic benefits and thermosetting resins. Thermoset resins used in these composites include polyester, phenolic, vinyl ester and epoxy and fillers used in this type of composites include clay, talc, carbon black, marble, glass, wood flour, metals and most important and most used calcium carbonate; the fibers used in this type of composite include glass, carbon, Kevlar, boron and basalt; also natural fibers (bamboo, hemp, flax, wood, banana and coconut) as a reinforcement of polymeric materials replacing synthetic fibers has been studied. Glass fiber today is an advanced and cost effective industrial material in many applications. Glass fiber reinforced thermosetting has been significant for many industrial applications due to their mechanical performance and cost. Among thermosetting resins, unsaturated polyester, it has been used for a wide range of applications. The unsaturated polyester resin has the highest application rate among the existing resins in the industry, hence improving the properties of compounds based on unsaturated polyester resins is inevitable. The widespread use of these polymer is due to their relatively low cost, easy processing, good compatibility with a variety of fillers, but, unlike other engineering polymers, unsaturated polyester has less mechanical and thermal properties that limits their applications. These defects can be eliminated by adding various fillers [11-14]. The unsaturated polyester resin is a wide range of applications in the transportation sector, water pipe production, chemical tank, marine application, construction and color industry. 85% of fiber-reinforced polymer products, such as boats, vehicle parts and aircraft, were made using polyester. Therefore, glass composites with unsaturated polyester matrices are of considerable commercial importance.
Research shows that by adding fibers to resin, tensile strength and elastic modulus can be improved in the direction of the fiber, while the toughness and impact strength remain weak. In recent years, the addition of nanoparticles as a second enhancer in the structure of polymer composite fibers has been carried out by many researchers. This group of composites with two types of amplifiers in the micron and nano scales are called multi-scale composites, which have better mechanical properties than fiber composites without filler. In industry, the addition of mineral filler materials to polymer is a common method; these materials not only improve multiple properties, also reduce the cost of composite processes. Today, silica nanoparticles are of great interest to minerals. The advantage of nano silica, small particle size, high specific surface area, commercial availability, industrial synthesis convenience, its competitive cost compared to other nanoparticles and its chemical, thermal and mechanical properties. Silica from the gas phase (fumed silica), the most commonly type of nano silica is known in the market. Fumed silica with a large specific surface area and variable surface properties can easily disperse in liquid and it is able to penetrate many organic liquids by forming a silica network in the liquid structure, which will simultaneously improve electrical, thermal, optical and mechanical properties. The primary particles of silicon dioxide contain siloxane and silanol groups on their surface and because of the interaction of the hydrogen bond between the neighboring silanol groups, the primary particles are interconnected and forming an aggregate, whose dimensions are between 100 to 500 nm.
These aggregate mainly result from agglomerations of nanoparticle, due to the intrinsic forces of the van der waals. Formulations typically require little taste to achieve improvement in properties due to its high surface to weight ratio. The effect of fumed silica on the unsaturated polyester resin matrix has been studied in many papers. For example, Marinković et al studied the mechanical properties of polyester matrix composites reinforced with four types of hydrophobic fumed silica having different specific surfaces area. It was shown that adding hydrophobic fumed silica nanoparticles decreases the tensile strength and tensile modulus, which indicates a poor interaction between the hydrophobic filler particles and the polymer matrix. The mechanical properties of fumed silica-reinforced polyester composites studied by Shokry showed that compressive strength increased steadily with increasing fumed silica content, the tensile strength decreased with increasing fumed silica, the flexural strength and the elastic modulus, there was a rising and descending trend with increasing fumed silica. Mirabedini et al. The mechanical properties of polyester matrix composites reinforced with two types of fumed silica from two companies and two specific surfaces area and two different surface modifications were studied. The tensile strength and elastic modulus for both fumed silica had the same trend, there was increased first, then decreased with increasing fumed silica, and also for both fumed silica with an increase in the percentage of fumed silica, strain until the failure dropped. Khankrua et al studied the mechanical properties of three types of biodegradable polyester matrix compsites reinforced by fumed silica and showed that there was no significant difference in the elastic modulus with increasing fumed silica but in the case of strain to failure and impact strength for all three types of resin, a trend initially followed an increased then a decreased. Rusmirović et al the mechanical properties of polyester matrix composites reinforced with four types of fumed silica, three of which were hydrophobic and a modified hydrophilic silica and two kinds of hardener were studied. By increased the percentage of fumed silica in both the hardener, the tensile strength and strain to failure and the elastic modulus increased. It was also reported that modified hydrophilic fumed silica had better tensile properties than hydrophobic fumed silica.
The mechanical properties of the fumed silica reinforced polyester matrix composites by Martínez et al.  were also studied and showed that the impact strength increased with increasing the percentage of fumed silica and the flexural strength by increasing the percentage of silica, to Steadily decreased. The effect of adding fumed silica to unsaturated polyester with glass fiber was reported by Sequeira et al. It was shown that with the addition of fumed silica, the flexural strength and tensile strength increase steadily. As stated above, unsaturated polyester resin is widely used in fiber reinforced plastic industries, which weak toughness and impact strength, if not reinforced, sensitive to fractures. This problem can be partially solved by combining with elastomers. Styrene Butadiene Rubber (SBR) An artificial elastomer is widely used in tire industry, which has good mechanical properties, excellent abrasion resistance, crack resistance, good elasticity and low heat buildup. Some researchers have investigated the effect of adding SBR rubber to unsaturated polyester resin matrix. Lubi et al conducted a study entitled “Improvement of Non-saturated Polyester Resins: Comparison of Different Types of Elastomer”. The elastomers used in this study included NR , SBR, NBR, CR and IIR . The results indicated that the addition of rubber to an unsaturated polyester resin for tensile strength, elongation at break and impact strength showed an incremental and then a decreasing trend. Cherian et al the mechanical properties of unsaturated isophethalic polyester resin with different types of elastomers were investigated. The elastomers used in this study included NR, SBR, NBR, CR and IIR. The results indicated that the addition of elastomer to unsaturated isophetalic polyester resin could indicate an increasing and decreasing trend in tensile strength, elongation at break, modulus and impact strength. Cherian et al the mechanical properties of unsaturated polyester resin with different types of functionalize elastomers with maleic anhydride were investigated. The elastomers used in this study included NR, SBR, NBR, CR and IIR. The results showed that the addition of functionalize elastomer to unsaturated polyester resin could indicate an increasing and decreasing trend in tensile strength, elongation at break, modulus and impact strength.
So far, no study has been done to investigate the effect of elastomer on unsaturated polyester composites with glass fiber. Moreover, the simultaneous effect of elastomer and silica nanoparticles in these systems has not been studied. In the present study, the effect of adding silica fume and styrene-butadiene rubber (SBR) on the mechanical properties of unsaturated polyester resin with fiber glass reinforcement is investigated. Three-point bending test, tensile test and charpy impact test were used to determine the mechanical properties, namely flexural strength and modulus, deflection at break, tensile strength and modulus, elongation at break and impact strength of the compounds. Scanning electron microscopy (SEM) is also used to observe the dispersion of nanoparticles.