Investigation of the Physical Properties of Elastic Syntactic Foams

Elastic syntactic foam consists of a silicone matrix and polymeric hollow spheres with a diameter in the range of some 10 mu m (so called microspheres). It can be used in the automotive industry as well as a substitute of mineral oil in cable sealing ends. Its use in outdoor applications is discussed at present. For the dimensioning of an insulation system it is necessary to have a precise knowledge about the physical properties of the insulation materials it contains. In addition to the electrical stress, the insulation system is especially exposed to thermal and mechanical stress during its operation. The breakdown mechanism of syntactic foam under AC, DC as well as impulse voltage stress has been introduced in earlier papers. Mechanical, physical and thermal properties of different syntactic foams are investigated in this work. One property, the adhesivity of the syntactic foam, must be sufficient to avoid ablation of the insulation material from the housing. Furthermore, information regarding the compression behavior of different silicones and filling factors is obtained from pressure tests. As the mixing viscosity of the uncured foam is an important characteristic for the encapsulation process, it is determined as well. Finally, the thermal conductivity of this hybrid insulating material is presented. As a main conclusion of this work it can be stated, that the usage of elastic syntactic foam has several advantages for highly stressed systems compared to conventional insulation materials such as epoxy resin. Mechanical stresses due to different materials with various coefficients of expansion and contraction are avoided since elastic syntactic foam features an extensive compressibility, especially for bigger microspheres. Otherwise, such stresses could lead to crack formation and consequently to breakdown. Using higher filling degrees of microspheres improves the compressibility as well as the adhesion to other polymers, but also increases the mixing viscosity and decreases the thermal conductivity.