Application of TMA for Rapid Evaluation of Low-Temperature Properties of Elastomer Vulcanizates

Abstract

Abstract Static thermomechanical analysis has been investigated as a method for determining the low-temperature properties of vulcanizates. Dimensional changes of a sample in free expansion, indentation, or tension are monitored as the sample is heated at a known and reproducible rate. The first derivative of the dimensional change is simultaneously recorded. TMA/DTMA data obtained in free expansion gives the linear thermal expansion coefficient at any temperature during the experiment. The glass transition is observed as a change in slope as in dilatometric determinations. In the indentation mode, the thermogram is the result of competition between thermal expansion and indentation. For tension measurements, the thermogram records the additive sum of thermal expansion and tensional strain development. For indentation and tension, the glass transition is obtained by a standard extrapolation method. A maximum in the DTMA thermogram is also observed which represents the maximum rate of change as the sample is heated through the glass transition range. The temperature of this maximum, Tgd, is characteristic of a vulcanizate. Variation in scan rate between 2 and 20°C/min has little effect on Tgo and no effect on Tgd. At 50°C/min, thermal gradient effects predominate. For a wide range of vulcanizates in tension or indentation, Tgo or Tgd versus the Gehman rigidity modulus gives a linear relationship. Exact temperature agreement is not observed because of differences in the experimental techniques. In blends, several Tgd values are observed. Plasticizer effects can either be observed by a shift in the Tgo or by a change in the thermogram profile, the extent of indentation and temperature range over which it occurs. The extent of indentation correlates with vulcanizate hardness as expected. The indentation of a rubber sample by a flat probe has been treated mathematically in the literature, and the determination of Young's modulus by TMA/DTMA is now being investigated. In conclusion, the ease of sample preparation and the rapidity of TMA/DTMA measurements offers a quick approach to assessing the low-temperature properties of vulcanizates. At 20°C/min scan rate, runs can be completed in as little as four minutes (80°C total scan) if the sample has a single transition.