Institute of Particle Technology > Thermal material properties

Thermal material properties

Thermal properties and cooling behaviour of materials using the example of polypropylene-composites with different filler fraction in a injection molding tool

Compared with metals polymers do not conduct heat very well. Nevertheless, the thermal diffusivity can be raised significantly by adding additives, which are mostly metal or metal-oxide particles. These composites still can be processed by typical plastics processing machines. Common applications of such materials are heat sinks for power electronics, coil sheathings, plain bearings, automotives (e.g. heat dissipation of the dashboard) and many more.

If the thermal diffusivity of the composite are ought to be isotropic, the used filler particles should have a low aspect ratio; thus they are of spherical or irregular shape. The higher the aspect ratio the more anisotropic is thermal diffusivity. High aspect ratios are present in fibres. But of course there also is composites containing spherical particles a certain anisotropy in thermal diffusivity owed to the manufacturing process which may lead to an anisotropic particle distribution. This occurs in the injection molding process.

Nevertheless the increased thermal diffusivity in polymer-particle compounds has an important advantage:
The higher the thermal diffusivity the faster the injection molded material cools down and the faster is the molding cycle.

Cooling behavior of polypropylene-magnetite-composites with varying particle filler contents in a mold.

In the figure above the measured course of temperature versus time during cooling of polypropylene-magnetite composites in a mold can be seen. The less magnetite exists in the compound the higher is the temperature of the compound after a given cooling time related to the injection time t=0. It is shown that the material in the mould cools faster down the more magnetite is in the compound. The reason for this behaviour is the higher conductivity of the composite by increased magnetite concentration. A possibility to measure the thermal diffusivity/conductivity is the Laserflash method.

The thermal diffusivity is not only influenced by the mass of the admixed material but also by the material itself. In the figure below, the change of the cooling behaviour of polypropylene in a mould is shown. In the following figure, the change in cooling-behaviour of polypropylene in a mould is shown for a volume of 30%  in addition of magnetite, barium sulfate, fiber glass, talcum, strontium ferrite and copper, respectively.

Cooling-behaviour of polypropylene-composites with different filler material at a filler content of 30 vol% in a mold

In the figure you can see that the admixing of copper, which has an extremly high thermal conductivity, causes a very fast cooling of the compound in the mold. It seems like the other investigated materials have not very big differences in their cooling behaviour, but this is only due to the application of a logarithmical scale, which does not show the differences.

This application on a logarithmical scale is favourable, because the slope of the cooling curve should be, theoretically, the thermal conductivity of the material. You get this result by solving the one-dimensional heat conduction equation with the aid of many simplifications (cf. publications). In spite of this simplifications you can calculate the measured experimental temperature gradation with this model quite good, what you can see in the following figure.

Comparison of the measured and the theoretical calculated cooling-curve of a composite in a mold

Here, the simplification was made that the thermal diffusivity is independent on the temperature and does not change during the cooling process. But in fact, this assumption does not correspond to the physical facts. In the following figure you can see the variation of the diffusivity of two chosen polypropylene-composites dependent on temperature-changing.

Temperature dependence of the thermal diffusivity of two polypropylene-composites
thermal diffusivity

In the figure the decrease of the diffusivity of a polypropylene-magnetite and a polypropylene-barite particle composite with increasing temperature can be seen. Those measurements can be determined by the Laserflash-method. Likewise it is visible that the thermal diffusivity of magnetite-filled polypropylene is higher than those of the barite-filled one during the whole investigated temperature range.

With the aid of the thermal diffusivity α you can calculate the heat conductivity λ with the formula λ = ρ * α * cp , whereas cp pictures the specific heat capacity.

Therefore, the specific heat capacity was determined via DSC measurement in the same temperature range as in the figure above. In the following figure you can see the rising of the specific heat capacity of both polypropylene composites in the measured temperature range.

temperature dependence of the specific heat capacity of two polypropylene composites
specific heat capacity

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