Before NBR, SBR and Neoprene there was natural rubber. The original elastomeric material for seals and gaskets, natural rubber literally grows on trees. Although supplanted by modern synthetic rubber materials it remains a good choice for some applications.
The origins of rubber
The South American Pará tree produces a milky sap known as latex. In chemical terms this is composed largely of the polymer polyisoprene which is both elastic and waterproof. In 1839 Charles Goodyear discovered that heating this product with sulfur, (the process we call vulcanization,) made it even more elastic. Within a few years it was being used in countless applications, including for sealing.
Synthetic rubber
By the early 1900’s rubber production was firmly established in places like India, Malaysia and Singapore. The two World Wars saw the supply of natural rubber disrupted, and scientists set to work on synthesizing artificial versions.
The result was Styrene Butadiene rubber or SBR. While this didn’t perform quite as well as natural rubber, for many applications it was good enough. Shortly afterwards came other synthetic rubbers aiming to address the weaknesses of SBR.
Natural rubber pros and cons
With ASTM D-2000 designations of AA and BA, natural rubber (NR) is still widely used as a sealing and gasketing material. It works over a temperature range of -60 to +220°F, which is a lower upper limit than most synthetic rubbers. However, it does have very good compression set resistance and excellent resilience. It’s also highly water-resistant, abrasion resistant, and tolerant to alcohols, acids and alkalis.
Where NR doesn’t perform well is in resistance to petroleum-based products. It is also susceptible to ozone attack, making it unsuitable for use around high voltage electrical equipment. Somewhat more expensive than SBR, like any natural product, it can exhibit some batch-to-batch variation.
When the application demands
When a seal needs water and excellent compression set resistance, a natural rubber gasket may be a good choice. To learn more about how this oldest of elastomeric sealing materials, contact a specialist at Hennig Gasket.
