Temperature dependence polymer thermal properties, specific heat

It is difficult to calculate thermal conductivity of oriented filled polymers, all the more to ascertain the temperature dependence of thermal conductivity ( ), thermal diffusivity (a), and specific heat (c). The calculation formulae cannot allow for such phenomena as the glass-transition of polymers, the possible lamination of polymer films due to the great discrepancy between the coefficients of linear expansion of the binder and filler, the effect of multiple thermal loading, etc. Therefore, most valuable are the experimental data on thermophysical properties of composite polymers in a wide temperature range (between 10 and 400 K). 

We conclude that thermal conductivity, specific heat and acoustical properties at low temperatures (r < 1 K) can be quantitatively interpreted within the tunneling model for PMMA and PS. At higher temperatures we have measured very different temperature dependence of the internal friction for the two polymers. At the used frequencies and at 0.1 K < 120 K... 

Fluid flow, heating and composition, which change by reaction or by transfer at one interface, represent the specificity of the chemical engineering processes. The response of a system to the applied effects that generate the mentioned cases depends on the nature of the materials involved in the process. All the properties of the materials such as density, viscosity, thermal capacity, conductivity, species diffusivity or others relating the external effects to the process response must be included as variables. The identification of these variables is not always an easy task. A typical case concerns the variation of the properties of the materials, in a nonlinear dependence with the operation variables. For example, when studying the flow of complex non-Newtonian fluids such as melted polymers in an externally heated conduct, their non-classical properties and their state regarding the effect of temperature make it difficult to select the properties of the materials. 

As the measurements have shown, thermal properties of filled polymers depend considerably on filler orientation. Thermal conductivity and specific heat of glass plastics with formaldehyde and epoxy binder increase with increasing temperature, whereas thermal diffusivity falls in inverse proportion with temperature. The direction of the heat flux and orientation of the filler are responsible for the conductance and thermal diffusion in a given direction. Specific heat does not practically depend on the heat flux direction, since it characterizes the scalar value, i.e., energy accumulation. 

In this paper we present a further example of the complexity in the interpretation of the heat release data and an experimental proof of the influence of the relaxation, probably by thermally activated processes, of excited states that contribute to the heat release of the TS at low temperatures. We have studied the long-time heat release of two amorphous polymers with similar low-temperature specific heats and thermal conductivities, cooled under similar conditions. In spite of those similarities we have found a large difference in the absolute value and in the time dependence of the heat release between the two polymers when cooled from temperatures above 15 K. The similarities and differences in the low-temperature properties, their interpretation within the tunneling model, and the influence of thermally activated relaxation are the main scope of this work. Preliminary results were published in Ref [10]. 

The thermal penetration of the cable material was described by the model for Thermally Induced Electrical Failure (THIEF) implemented in FDS. The THIEF model predicts the temperature of the inner cable jacket under the assumption that the cable is a homogeneous cylinder with one-dimensional heat transfer. The thermal properties—conductivity, specific heat, and density—of the assumed cable are independent of the temperature. In reality, both the thermal conductivity and the specific heat of polymers are temperature-dependent. In the analysis, conductivity, specific heat, density, and the depth of the cable insolation were considered as uncertain parameters with relevant influence (see Table 1). 

The central heat source and the guard should have independent power supplies. The cold surface heaters are to be adjusted so that the temperature drops through the two specimens do not differ by more than 1 %. To attain a correct value for properties, the time required should be adjusted - its magnitude depends on the specific apparatus, control system and its operation, the test temperatures, the thermal diffusivity, and thickness of the specimens (Shirtliffe 1974). The conductivity is calculated using the Eq. 10.26. The attainment of equilibrium is important, especially for polymer blends that have low conductivity. The equilibrium times, for example, for cellular materials, are in the order of hours or tens of hours. For this reason, stable over long time period power supplies are necessary. 

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