The use of polymeric cellular materials that present densities below 40 kg/m3 have experimented an extraordinary increase in the thermal insulation market.
One method to improve the capability of these materials to work as thermal isolators is introducing nanometric particles in the formulation. Sepiolites are natural clays with a needle-like morphology that can be sucessfully combined with several polymers. However, the potential improvements of physical properties are only possible if the dispersion degree of the particles in the polymer is optimum. Nanoparticles can act as nucleation sites upraising the number of cells per unit volume and decreasing the cell size. These changes in the cellular structure should have an impact on several properties such as thermal conductivity. In addition, the introduction of nanoparticles in a neat matrix could deteriorate the capacity of the material to withstand the extensional forces generated by the gasses during the foaming stage. As a consequence of that, some degeneration phenomena like coalescence could increase when the particles are used.
In the current study blends of polystyrene with different types (organo-modified particles or natural particles) and different contents of sepiolites are produced in a twin screw extruder. To evaluate the dispersion degree of the particles shear rheology has been used. In addition, the extensional rheological behavior has been characterized to evaluate the effect of the particles in the strain hardening of the polymer.
Gas dissolution foaming have been used to produce cellular materiales from the nanocomposites. Samples with a thickness of 2 mm have been introduced in a high-pressure vessel in which CO2 gas is introduced at 8MPa and 40⁰C. After 8 hours of saturation time, pressure is released. Finally, materials are foamed in a silicone bath at 120⁰C, obtaining samples that present an expansion ratio around 30 times. The inclusion of only a 2% sepiolites organomodified with a quaternary ammonium salt has produced a reduction of a 82% in the average cell size. In addtion, in some formulations it has been possible to decrease the density when sepiolites have been used. The thermal conductivity of the samples has been measured and it has been possible to find a clear reduction (of 9%) of this property. This is significant reduction that makes sepiolites excellent candidates to be used in the production of PS foams with improved thermal insulation capabilities.