Syntactic foams are low density composites which consist of hollow glass microspheres dispersed in a matrix material such as polymer, ceramic or metal. They are mainly used in thermal insulation and transportation applications due to their low thermal conductivity and light weight.
The main advantage of syntactic foams is their tunable properties. By changing the type and composition of microspheres and the matrix material, it is possible to compose a syntactic foam with desired properties. In general, production of syntactic foams with low density and high mechanical properties is a challenging task. Among thermoset plastics, epoxy is the most common matrix material used in the syntactic foam studies since it has been widely applied in transport vehicles. On the other hand, thermoplastic syntactic foams have their own opportunities such as ease of production, moldability, recyclability and higher mechanical properties. As one of the earlier studies on thermoplastic syntactic foams, J-Z Liang used acrylonitrile–butadiene–styrene copolymer (ABS) as matrix material because of its preferable mechanical properties. Addition of hollow glass microspheres into ABS matrix enhanced both the flexural and tensile strengths due to high interfacial adhesion between the ABS and glass.
Addition of hollow glass microspheres into high-density polyethylene (HDPE) matrix, lowered both the density and thermal conductivity of HDPE and improved its tensile yield strength and modulus. However, low toughness value of these composites led to the necessity of using a compatibilizer, and as a result, higher toughness values were obtained. In syntactic foam studies, several approaches were improved in order to produce foams with low density and high mechanical properties. One of them is selecting the microsphere type and composition which are the main constituents to determine the syntactic foam properties.