Investigation of the post-hardening effect of silica filled NR compounds

Frances van Elburg (Contributor), Fabian Grunert, A. Risthaus (Contributor), Anke Blume (Contributor)

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Rubber products usually are expected to show a property profile that is stable for a certain period of storage after production. However, especially silica filled Natural Rubber (NR) compounds often show an increase in stiffness during storage of the vulcanized products, sometimes up to 10-20 %. This effect is better known as post-hardening which affects the extent to which the product functions well during service. In general, it is known that stiffness and flexibility are inversely related. An increasing stiffness, which happens during post-hardening, reduces the flexibility of a material. This is especially disadvantageous in dynamic applications. To understand and prevent this effect from happening, all ingredients of a model compound formulation were investigated with regards to their probability to cause hardening during storage and were compared to what is already stated in literature. As a results, the most likely explanation seemed to be a change in the crosslinking structure. It was expected that polysulfidic bonds, possessing a lower bond dissociation energy, break and di- and monosulfidic bonds (higher bond dissociation energies) were formed leading to an increase in stiffness. To confirm this change, the relatively new temperature scanning stress relaxation (TSSR) measurement as well as the thiol-amine method to determine cross linking structure were performed on fresh produced rubber samples as well as artificially aged ones (stored at elevated temperatures for different times). It could be shown that indeed the overall crosslinking density was higher within the stored samples. However, surprisingly it also could be seen that the amount of polysulfidic bonds increased as well, which is in contrary to previous expectations. With the support of these findings silica filled NR compounds could be re-designed showing a less pronounced post-hardening effect during storage which would provide more stable in-rubber properties beneficial for the applications.
Original languageEnglish
Publication statusPublished - 11 May 2023
EventIRC RubberCon 2023 - University of Edinburgh, Edinburgh, United Kingdom
Duration: 9 May 202311 May 2023


ConferenceIRC RubberCon 2023
Country/TerritoryUnited Kingdom
Internet address


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