Clwyd wants review of silicone rubber specs for aerospace

Nuremberg, Germany – Clwyd Compounders is calling for the aerospace industry to update its decades-old specifications for silicone rubber, to better reflect the performance of modern grades.

Aerospace industry specifications for silicone rubber DTD  5531 and 5582 came into use over 40 years ago. In 1999 these were replaced with identical specifications BS F152 and BS F153.

“These specifications were geared around the polymer technology at the time,” said Clwyd, noting that silicone polymer technology has advanced significantly since then.

Compounders can readily meet these specifications using older grades or grades with poorer ‘initial’ results, research by the Wrexham, UK-based compounder has shown.

However, it added, “they are unable to take full advantage of modern superior modern grades without having to excessively post-cure the compound to lower these initial results.”

The UK compounder is, therefore, urging both the rubber and aerospace industries to address these issues promptly.

The issue, said Clwyd, has added urgency given doubts over the continued, long-term availability of older silicone rubber grades that meet the current specifications.

Future work

Rather than drawing definitive conclusions from its own research, Clwyd wants to see future work and investigations to be carried out.

Setting out its vision for the way forward on this issue, the compounder stated:

“It makes little sense to essentially lose the excellent properties of a polymer based on specifications created over 40 years ago.

“Would it make more sense to have an absolute minimum value for the tensile strength and elongation-at-break after heat-ageing, instead of the current percentage drop off limits?

“This would allow, for example, a polymer with a high initial tensile strength and/or elongation at break to drop to values often higher than some polymers’ initial results and still pass – whereas, under the current limits, this could fail due to the percentage drop-off being outside these limits.

“Something we have found from this work is that the results are often on the cusp of the specification limits and due to the required post-cure conditions, they are often unreproducible.

“The heat-ageing tests at 336 hours are in themselves a lengthy procedure, so to have to optimise post-cure conditions for compounds can take a considerable amount of time and hence increase lead times that we can offer customers.

“This has also presented us with the need to understand how to better stabilise these polymers with higher physical properties so that they  remain within the current percentage drop-off limits.

“This is something we feel needs much more investigation within the industry and will become key if the specifications were to remain unchanged.

“Finally, we feel that this is a topic of high importance to both the rubber and aerospace industry and needs addressing promptly, especially as we don’t know how long the older grades that meet the specifications more easily will be available for.”

Article based on a Clwyd report presented in the ‘poster’ sessions at the recent DKT 2018 in Nuremberg, Germany.