Study on the Correlation of the elongation modulus at elevated temperatures and the temperature dependent flexural modulus from DMA measurements of a PP homopolymer

The thermoforming process can be split into the following steps: heating of the semi-finished product (film or plate), prestretching, forming, cooling and demoulding. The plate is heated up to a temperature, where the material is in the rubber elastic state. Thus the determination of temperature dependent mechanical properties is essential for the choice of the forming window in the thermoforming process. In this work the mechanical properties of a polypropylene homopolymer were investigated by two methods. Stress-strain-curves were recorded for temperatures ranging from room temperature to a temperature near the melting point (160 °C) by a hot tensile test. The curves were evaluated and characterised by the tensile moduli and tensile strengths. The material was also tested by dynamic mechanical analysis (DMA) in the three-point bending mode at 1 Hz. The sample temperature was raised from -50 °C to 160°C. The absolute value of the measured dynamic flexural modulus was set in relation to the static tensile modulus (fig. 1) and the tensile strength. The values of these moduli showed a nearly linear correlation at all tempera-tures except 160 °C. So DMA measurements could be a quite time saving and power-ful tool for mechanical material characterisation be¬cau-se it also allows the analysis the visco-elastic properties by analysis of the relaxation spectrum.