Standards that give shelf lives for elastomers, group different types into categories. BSI SO2230 puts silicone into Group C which has the lowest sensitivity to ageing effects and gives silicone an initial ten years shelf life.
After this it needs to be tested in some way to prove that it is still “fit for purpose”. Fit for purpose usually means that the material still meets the manufacturers original specification although it can be very difficult to test an extruded or moulded part to original specification nevertheless things can be done. Thereafter it needs testing every 5 years.
Another British Standard Aerospace Series BS 4F 68 puts silicone into group X, shelf life for Group X and none specified. Both of these standards are quoted on the Silicone Engineering delivery notes.
Silicone Rubber – Life in Service (LIS):
Once again and as is often the case with these questions it all depends on many factors. When we get asked about shelf life of silicone it often relates to the environment of where it is being kept. If the product is packaged and delivered and placed on a shelf in perfect cool conditions then shelf life would be an awful long time.
In our laboratory, we can perform heat age testing where we prematurely age the product by exposing it to elevated temperatures, this has the effect of increasing the chemical reaction. This is what decomposition is, a chemical reaction, this is why the product needs to be kept in cool conditions. At intervals during the aging period, Silicone Engineering removes a sample and performs various tests to determine how much it has degraded. After each time period the product becomes worse. As a general rule, when a product’s physical properties degrade to approximately 50% of its original values then the stuff is getting to the end of its useful life and is probably no longer fit for purpose.
Why would any designer specify 400% elongation and keep the part in service when the elongation was now much lower at say 100%?
You will notice words are commonly used like probably, general rule and approximately, this is because with these discussions nothing falls off the edge of a cliff, it is a gradual thing that is happening as the silicone degrades and someone needs to “make a call”. This general rule is not the same for every customer or application, indeed the silicone may still work well down to as far as 10% of its original properties but realistically they would want that percentage to be a bit higher. Consideration needs to be given to the various stages within the life of the silicone.
– Used brand new = standard life in service (whatever that is for the application and conditions)
– Been on the shelf for 10 years = standard life in service, minus some
– Been on the shelf for 20 years = standard life in service, minus some more
How do Silicone Engineering quantify the “minus some” and the “minus some more” because the rubber has aged. There is another rule of thumb that is generally accepted in regard to performing ageing testing to try to predict the life of an elastomer. An increase in temperature of 10°C will effectively halve the life in service. Therefore, if a seal operating at 50°C would last 6 years in service, then an increase of service temperature to 60°C would halve the life in service to 3 years.
This is of course a rough estimate and should not be taken too literally but it gives a broad answer.
Of course the opposite works too in regard to shelf life, keep it cool and it will last longer however, this does not mean in freezing temperatures as this could also have a negative effect on shelf life.
Life in Service example;
They test a silicone sheet as found (new) and record the results. In service the sheet will operate at 150°C. Silicone Engineering then heat age a similar sheet in a lab oven at a higher temperature say 200°C (but it could be higher or lower) until the physical properties are approximately 50% of the original values. For this particular application and for purposes of this example, we can say that product has reached the end of a useful life in service. Using the above ‘ready reckoner’ formula, and using it in reverse, we can then work out an approximate life in service at 150°C.
LIS Continuous @ 200°C = 6 weeks
LIS Continuous @ 190°C = 12 weeks
LIS Continuous @ 180°C = 24 weeks and so on
I have worked out that LIS Continuous @ 150°C would be around 190 weeks.
If the product was seeing 150°C for only 8 hours a day then the LIS would be approximately three times this.
This is a hypothetical example to show you how operating temperature can affect the life in service.
To conclude, storage, packaging, temperature, humidity, mechanical/abrasion, pressure from stacked items are all important factors that can affect the shelf life of silicone rubber. Under ideal conditions silicone even after a considerable time will arguably still be in the same condition it was on the day of manufacture and will be fit for the purpose for which it was made. If stored in hot conditions with other adverse influences then the shelf life can be reduced somewhat. Shelf life and life in service are linked as one will affect the other. During adverse storage the shelf life and so the life in service is potentially eroded however, it is important to remember it is a gradual process of degradation which happens naturally.
After a period of 10 years, parts need to be inspected to ensure the physical properties are not reduced. The truth is though, that it is when the parts are in service that significant degradation will take place and it is then that Silicone Engineering cannot legislate for the uncertainty of, and the variation in, the service conditions. Life in service will be very different to any ageing conditions undertaken in the lab, so they cannot say how long a product will last in service, only try to back up and support other information that may suggest a LIS estimate. Silicone Engineering would also guess that in the days of Just In Time etc. there will be few cases of companies buying silicone then stock piling them for 10 or 20 years.