Thermal properties of polymers by non-steady-state techniques

Abstract

The nature of the molecular structure of polymers makes the properties of such materials markedly temperature dependant. Modelling heat transfer under steady-state or transient conditions is of fundamental importance in engineering design as well as for the tailoring of thermal and mechanical behaviour of materials. Thermal conductivity, thermal diffusivity and specific heat, namely the thermal properties, are the three most important physical properties of a material that are needed for heat-transfer calculations. Nowadays, several different techniques for the determination of the thermal diffusivity and thermal conductivity may be found in the literature. Recently, transient techniques have become the preferable way for measuring thermal properties of materials. In this work, two absolute and non-steady-state methods are employed in the experimental determination of thermal properties of some selected polymers: the laser flash technique and the hot-wire technique. In the hot-wire technique, samples were prepared from the extrusion process in the shape of a rectangular parallelepiped, with the molten mass for each sample being approximately 1500 g. Thermal conductivity, thermal diffusivity and specific heat were simultaneously determined from the same experimentally determined thermal transient. In the laser flash technique, disc shaped samples were prepared, either by hot pressing approximately 50 mg of material, or by cutting the discs from long cylindrical bars. In this technique, only the thermal diffusivity is determined from the experimentally determined thermal transient. The thermal conductivity was derived from the thermal diffusivity with the knowledge of the bulk density and the specific heat. Specific heat was experimentally determined using a modulated differential scanning calorimeter, and the density was obtained from the PVT curve at each temperature. Since the sample mass ratio between both techniques is approximately 3×10 4, and the speed of the cooling process may generate different morphologies, this phenomenon may be partially responsible for any discrepancy between both techniques for a specific polymer. Experimental results obtained by both techniques are checked against each other, as well as, when possible, with data found in literature.