Elastomer materials are employed in almost all technical fields due to their high elasticity. An essential property of elastomer materials is the capability of storing deformation energy and releasing it back to the entire system, when necessary. One measure of this property consists of the material-immanent restoring forces, which – depending on the system – can be generated from the energy stored and can easily amount to 90% or more of the energy stored. This “valuable“ property is restricted, however, to a narrow temperature range that defines the operating and working temperatures for the respective application. For this reason, the temperature behavior of elastomer materials is of central importance.
So-called temperature sweeps are used to record the thermal behavior of elastomer materials, which can generally be parameterized at different heating rates. A high heating rate of 5°C/min, for example, is preferable to a heating rate of 1°C/min since a result is delivered in a shorter time and testing is therefore faster and more cost-effective. However, the question arises as to how to evaluate the results for different heating rates.
This Application Note addresses this question and examines the heating-rate dependence of the DMA GABO Eplexor® series.
Four temperature sweeps on samples of the same rubber compound were carried out from -80°C to 20°C at heating rates of 1, 2, 3 and 5°C/min with the DMA GABO Eplexor® 500 N (figure 1).
The lower operating temperature of elastomer materials is limited by the Température de Transition VitreuseThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature, Tg. The Tg characterizes the temperature at which elastomer materials change from a hard and relatively brittle state to a rubber-like elastic state. In practice, the Tg is defined as the maximum of the loss factor tanδ. The heating-rate dependence of the Tg is depicted in figure 1.
Figure 2 shows that the Tg is shifted to higher temperatures with higher heating rates. For a temperature sweep, the Tg amounts to -42.3°C at a heating rate of 1 °C/min and to -41.4°C at a heating rate of 5 °C/min. This corresponds to a positional change of the Tg of approx. 1°C. The maximum of the loss factor, tanδ, has changed by 0.01 at most. This observation can be illustrated by the poor Conductivité ThermiqueThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity of most plastics. This causes a shift in material-specific transition effects, such as RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation maxima or glass transition temperatures, to higher temperatures (in the case of positive heating rates) or to lower temperatures (in the case of negative cooling rates). A higher heating rate leads to “drag effects” and the sample lags behind the furnace temperature. A heating rate of 1°C/min will therefore correctly reflect the sample-specific effects, while a high heating rate will cause a shift of these effects on the temperature scale.
These minimal shifts of the Tg’s position and the maximum of the loss factor, tanδ, as a result of the different heating rates are due to a very good temperature distribution inside the DMA GABO Eplexor® series, achieved by using a fan in the measuring chamber. A direct consequence of these findings is the reduction of the measuring time needed for temperature sweeps via the use of higher heating rates, for example, 5°C/min instead of, for example, 1°C/min. A prerequisite for this is knowledge of the heating-rate dependence of the Tg of the materials tested.