THERMOPHYSICAL PROPERTIES EVALUATION ON POLYMERS AND PHASE CHANGE MATERIALS: A SYSTEMATIC STUDY

The accurate determination of thermophysical properties - such as thermal conductivity, thermal diffusivity, specific heat capacity and density - is crucial for understanding heat transfer phenomena in materials. The accuracy in the determination of these properties is often underestimated and issues in repeatability of the results are common in the scientific community. Thermophysical properties are strongly dependent on temperature, hence their evaluation is affected by testing condition and thermal inertia effects. Moreover, thermophysical properties are often indirectly determined through the measurement of other correlated physical quantities, with the disadvantage of the error propagation that has to be taken into account. Many techniques have been developed for the determination of specific heat capacity, thermal diffusivity and thermal conductivity, however, it is common to obtain results without a clear convergence between the different procedures, in particular if testing parameters are selected without a deep knowledge of the consequences of a wrong selection. This study explores various experimental techniques to determine the thermophysical properties form 0 °C to 90 °C of widely studied thermoplastic polymers, as high-density polyethylene (HDPE) and expanded polystyrene (EPS), in order to compare, if available, the obtained results with those published in literature. This work has been then extended to the characterization of the thermophysical properties both of the solid and liquid phase of phase change materials (PCMs). These materials are increasingly used in various energy applications and, in particular, as suitable materials to be integrated into thermal energy storage (TES) systems. The investigated PCMs are eicosane, an oligomer selected as pure substance, and a fatty acid mixture which can better represent a commercial type of PCM. Differential Scanning Calorimetry (DSC) was used to determine specific heat capacity following ASTM E1269-11 standard, relating the heat flux of the samples with that obtained from a sapphire reference, whose specific heat capacity is known as function of temperature, and the results obtained at different scanning rates from 10 to 0.1 °C/min were compared. A recent alternative to the conventional scanning calorimetry technique was also exploited, a method which overlays ramped temperature with a time series of stochastic temperature pulses of different durations (TOPEM) [1]. Various heating rates and pulse heights were probed, relating the accuracy and the reliability of the obtained results. Thermal diffusivity was measured through a Laser Flash Analyzer (LFA)[2] following ASTM E1461-13 and independently with Hot Disk measurement following ISO 22007. Thermal conductivity was an output of hot disk measurement or indirectly determined as product of diffusivity (from LFA), specific heat capacity (from DSC) and bulk density of the material. A third technique to measure thermal conductivity is the heat flow meter (HFM) method following ISO 8301[3], in which a temperature gradient is imposed to the material. This method could be used only for insulating materials, as in the case of EPS.