High-molecular EPDM grade for dynamic applications

A new grade of ethylene-propylene-diene rubber (EPDM) that can compete with natural rubber (NR), which is susceptible to aging, has been developed by Lanxess (Cologne). This new grade is characterized by its exceptionally high molecular weight, which gives the synthetic rubber its extremely high NR-type strength and resilience.

Nonetheless, as a saturated polymer, this grade also still features the heat, weathering and ozone resistance typical of EPDM rubber. Keltan 9565Q is thus not only superior to natural rubber in terms of heat aging but can also offer this benefit in highly dynamic applications according to Lanxess. In addition, this new ultra-high-molecular Keltan grade opens up new potential for more economical EPDM processing compared with NR.

"Customers in the mechanical engineering and automotive industries are demanding more and more types of rubber that withstand high operating temperatures for long periods," explained John Beelen, synthetic rubber expert in the High Performance Elastomers business unit of Lanxess. "Due to its high resilience and tear resistance, natural rubber is still in demand in the industrial sector but can hardly meet the rising temperature requirements."

In rubber components subject to high stresses such as damping elements, natural rubber was to date considered almost indispensable - despite its heat sensitivity. The reasons for this dilemma lie in the typical molecular architecture of the time-honored raw material. Long chain molecules with a highly regular structure ensure outstanding elastic properties. But the unsaturated chemical bonds in the polymer backbone make it susceptible to decomposition through ozone and oxygen and/or gradual post-crosslinking through radicals, UV light and heat. This causes vulcanized natural rubber to form cracks and become brittle in the medium term when exposed to heat. Additives that make this process slower are naturally not a long-term solution.

With Keltan 9565Q, Lanxess chemists are meeting the challenge by way of a different, more sustainable approach: through improved synthesis they have been able to further increase the molecular weight of their new EPDM grade, thus achieving similar levels of dynamic performance as natural rubber - even after heat aging. "We are able to demonstrate this with a series of extensive tests," said Beelen.

With regard to their tan delta values - a measure of dynamic performance - specimens made of the new Keltan grade achieve a similar low level to that of NR. In terms of tear resistance and elongation at break they attain much better results than NR after 168 hours at 100 and 120 °C - this is where the fully saturated nature of the EPDM molecule makes itself felt.

Further, Keltan 9565Q's compression set is superior to that of conventional cured natural rubber and at least on a par with that of heat-stabilized NR (EV crosslinking). Besides strength, resilience and aging resistance, also fatigue life time is very important. Pre-aged samples of Keltan 9565Q have comparable life time as NR samples. However, when the compounds based on Keltan 9565Q are modified, fatigue life is strongly improved and attains the same level as a heat stabilized NR compound.

"Keltan 9565Q opens up the way for EPDM to applications involving high dynamic stresses in which up until now natural rubber's low aging resistance had to be taken into account," said Beelen.
"Keltan 9565Q can also help to prolong maintenance intervals and make engines and units faster and more powerful: they can run at higher temperatures without damping elements becoming subject to rapid fatigue on exposure to heat."

The new material from Lanxess brings benefits even for the process. Compared to natural rubber, EPDM is not susceptible to reversion owing to the lack of double bonds in the molecule. As Beelen explained, "the narrow specifications of the polymer eases manufacture. Processes become more reproducible and consistent." At the same time, the synthetic rubber can be processed at higher temperatures - this can substantially increase productivity.