Characterization of viscoelastic properties of hollow glass microballoon reinforced polymer composites

Advanced lightweight structural composites are finding applications in aerospace, marine and electronic industries. The operating environment of these applications requires the composites to be subjected not only to room temperature static loadings but also to various loading frequencies and temperatures. In this regard, one such structural composite is syntactic foam, which are closed cell composites foams synthesized by dispersing hollow fillers into a matrix. In the present study hollow glass microballoons consisting of three different densities (220, 320 and 460 kg/m³) in two different volume fractions, namely 0.3 and 0.6, are used to fabricate six types of syntactic foams. These syntactic foams are tested in the temperature range of-75 to 195δC at 1 Hz frequency using a dynamic mechanical analyzer. The viscoelastic properties such as the storage modulus, loss modulus and damping parameter, Tanδ, along with the glass transition temperature (T _g) are obtained and related to the microstructural parameters of the syntactic foams, including microballoon volume fraction and wall thickness. The room temperature (30δC) storage modulus of syntactic foams increases with the increase in the wall thickness of the microballoons. Stiffening effect is observed for (a) the increase in the volume fraction of thick walled microballoons and (b) the decrease in the volume fraction of thin walled microballoons. The room temperature storage modulus and Tanδ are found to vary linearly with the density of syntactic foams. In the next step, a combined frequency and temperature loading, consisting of a frequency sweep in the range of 1-100 Hz, is applied at every 5δC over the temperature range of 30-140δC. The time temperature superposition (TTS) principle was utilized to create master curves for the variation of storage modulus over a wide frequency range of 10^-2 to 10⁶ Hz, which is well beyond the tested frequencies. The T_g was found to increase with the increase in the volume fraction of the microballoon. Addition of microballoon to vinyl ester resin helps in retaining the mechanical properties of the syntactic foams above T_g. The William-Landel-Ferry equation is utilized to interpret the results obtained from the TTS.