Microcalorimetry may offer a more accurate route to assessing the lifetime of rubber materials
Understanding how polymeric materials degrade is an ongoing challenge that faces a range of critical industries. In the nuclear power sector, such materials are used in seals, cable insulation, and gaskets, amongst others. For decades, predicting the service lifetime of rubber components has been done via accelerated thermal ageing coupled to mechanical testing. In these experiments, polymeric materials are aged at significantly higher temperatures than they would experience in real-world applications, with results extrapolated back to the material’s likely service conditions. Though straightforward, this process involves several assumptions that may not be correct. As a result, there is a growing effort to find alternative, more realistic means of analysing the ageing process in – and predicting the lifetime of – polymer components.
A group of Swedish researchers suggest that high sensitivity microcalorimetry (MC) might be the answer. Writing in Polymer Testing [DOI: 10.1016/j.polymertesting.2023.107948], they describe the use of MC to study the degradation of ethylene-propylene-diene-terpolymer (EPDM) in conditions close to the actual service conditions the rubber experiences in nuclear power plants. The choice of EDPM rubber was further motivated by the complexity of its ageing process, with several possible events contributing to it, depending on the temperature. They prepared two versions of EDPM. One made with a small amount – 0.8 parts per hundred rubber – of a commercial antioxidant (EPDMa) and one made without the antioxidant (EPDMn).
In its simplest terms, microcalorimetry involves the real-time monitoring of heat flow rates through a material. At least 1000 times more sensitive than differential scanning calorimetry (DSC), yet capable of analysing much larger samples, MC can detect tiny changes in a material, e.g., those resulting from oxidation. The authors write that, in this study, they wanted “…to determine whether MC can be used to reveal early signs of degradation and the associated activation energy at temperatures close to the actual service conditions”. In addition, they wanted to explore whether MC can be used “to capture thermally induced dynamic events”, which would involve operating it in non-isothermal and quasi-isothermal conditions.
Other techniques were used to support the MC analysis in characterizing unaged and aged material. They included gas chromatography, scanning electron microscopy / energy dispersive X-ray spectroscopy (EDX), DSC, and Fourier transform infrared spectroscopy (FTIR). The roles of moisture and gamma radiation were omitted from all analysis.
The MC tests showed numerous differences between the heat flow behaviour of unaged EPDMa and EPDMn even at low temperatures. The authors also found evidence that, beyond about 90 °C, the antioxidant begins to melt, which aligns with the observed levelling off in exothermal heat flow curves. Together, these results suggest that MC is sufficiently sensitive to detect the melting of a compound present in minute quantities. These changes were too small to be observed by FTIR. However, MC measured similar heat flows in unaged and the aged samples. This suggests that in the current set-up, it is unsuitable for use as a diagnostic tool, i.e., it cannot be used to determine if a material has gone through an ageing/oxidation process.
Chromatography and EDX unveiled the presence of peroxide by-products on the surface of both aged and unaged EPDMa, which they say “probably had a significant role on the temperature dependence of the observed heat flow data.” Calculation of the activation energy of EPDMa found two distinct curves – one in the low temperature region (52–111 °C), and a higher value in the high temperature region (111–160 °C)
The authors conclude that when complemented with other techniques, MC “…. can be a valuable tool in assessing ageing mechanisms and their activation energies at close to real service conditions”, which “…can make the lifetime prediction of polymer products more realistic in several applications.” However, they say, for nuclear power applications “…the effects of gamma radiation and water/steam need to be included.”
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Mohit Pushp, Anders Lonnermark, Peter Vikegard, Xin-Feng Wei, Mikael Hedenqvist. “Ageing tests closer to real service conditions using hyper-sensitive microcalorimetry, a case study on EPDM rubber,” Polymer Testing Volume 120, March 2023, 107948. DOI: 10.1016/j.polymertesting.2023.107948