DLTMA Measurements of Incompletely Vulcanized Elastomers

Purpose

The influence of vulcanization on the mechanical behavior of elastomers is shown using a partially vulcanized elastomer as an example. 

Sample

Partially vulcanized EPDM

Conditions

Measuring cell: TMA/SDTA840

Pan: No pan

Sample preparation: Cube 1.3 mm thick and with a base area of 11.7 mm2 cut from the starting material. A quartz disk was placed on the upper face of the sample to distribute the force uniformly.

TMA measurement: Heating from 30 °C to 200 °C at 3 K/min; During heating the force applied to the sample was switched between 0.1 N and 0.9 N every 30 s. The measurement program was as follows: TMA method with a dynamic segment from 30 °C to 31.5 °C at 3 K/min and a force of 0.1 N and a second segment from 31.5 °C to 33 °C at 3 K/min and a force of 0.9 N. There then followed a loop from the second to the first segment with an increment of 3 K to an end temperature of 200 °C. 

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Evaluation

The upper half of the diagram shows the sample thickness as a function of temperature. The periodically changing force compresses the sample to different extents. The sample thickness therefore exhibits a periodic change. The modulus of elasticity (Young’s modulus) is inversely proportional to the amplitude of the sample thickness. The corresponding curve is shown in the lower half of the diagram

Interpretation

The measurement curve shows a downward tendency up until about 120 °C. The sample is compressed more and more. The shape of the sample changes irreversibly. This is due to the relatively low degree of vulcanization. In this region, the amplitude of the measurement signal also increases slightly with temperature, i.e. the sample becomes softer. Between 120 °C and 170 °C, the amplitude decreases significantly and at the same time the irreversible change of shape decreases. In this temperature range, Young’s modulus increases. This is a result of the post-vulcanization reaction. Due to continuous cross-linking, dimensional stability and Young’s modulus increase.

At higher temperatures the amplitude remains almost constant. The modulus of the cross-linked system shows only a small decrease. The slight upward tendency of the measurement curve at this temperature is due to the thermal expansion of the sample.

Conclusions

DLTMA can be used to follow the cross-linking of a sample during postvulcanization. The method reacts very sensitively to changes in the degree of cross-linking and is therefore an inexpensive and efficient technique in quality control and damage analysis.

^{DLTMA Measurements of Incompletely Vulcanized Elastomers | Thermal Analysis Application No. HB441 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}