With regard to what makes a rubber comply with the BS 6920 standard there are many factors. A lot of rubbers do not always comply with the BS 6920 odour and flavour and growth of microorganisms tests, although some rubbers “fail” all the tests.

The first thing is that before BS 6920 testing commences the rubber must be at least 4 weeks old from the date of manufacture. The test samples should be stored in open containers, eg paper bags, in a clean atmosphere. This is to allow volatile components to dissipate from the rubber.

Experience has shown that the method of manufacture (moulding extrusion etc), curing and post cure process can affect the performance of the rubber in the BS 6920 tests. The longer the curing time and higher temperature of the curing process can give better results than a short cure time at a lower temperature. A hot-air post cure is generally better than steam curing.

The formulation is also very important; generally silicone rubbers perform better than NBR, which performs better than EPDM. Natural rubber is very unlikely to comply with BS 6920, except possibly as an ebonite.

Some plasticisers and processing oils used in rubbers are known to support microbiological growth. There is a variation with concentration, but it does not appear to be a linear response, naphthenic oils often perform better than paraffinic oils. Generally the harder the rubber the better it does in the BS 6920 tests.

The concentration of carbon black can also effect the performance in the tests, usually the greater the concentration the better the performance.

Waxes used for protection from ozone can migrate to the surface of rubbers and depending on the type support microbiological growth.

Similarly mould release agents can cause failures in the BS 6920 tests.

The BS 6920 odour and flavour test can be affected by the curing system, eg sulfur or peroxide based which can give rise to distinct odours and flavours.

Microbiological growth or chemicals leaching from the rubber can affect the BS 6920 appearance of water of test.

Poor BS 6920 results in the extraction of substances that may be of concern to public health (cytotoxicity) test can be the result of sulfur cure systems.

If a lead based cure system or lead compounds are used as filler, this can lead to a failure in the BS 6920 extraction of metals test.

This is just a guide to what can cause a failure in the BS 6920, there has not been any systematic research into the effect of different formulations / ratios, manufacturing conditions and curing conditions.