At Kiwa, products for a large number of diffe-rent applications are certified. In a lot of these applications rubber and rubber products are playing an important role. Certification gives a good assurance that the products are suitable for the intended application but normally there is no real lifetime expectation involved. Programs for certification are based on rather standard and mostly average conditions of use and every deviation of that has a great impact on the lifetime. Confidence for the customers Nowadays, a lot of customers want to plan in detail their schedules for maintenance and replacement and for that they need pretty specific information on the lifetime expecta-tion of the products they buy. Therefore questions like ‘How long will it last?’, are asked more and more. Without further investigations the producer of the products will not be able to answer such questions. Also fulfilling the requirements of a standard or evaluation guideline is not telling enough in this respect. Kiwa has the knowledge and the necessary laboratory facilities and equipment to answer such lifetime prediction questions. How does it work? Very important is to know as good as possible the exact conditions that a product will see throughout its entire life. Here all details regarding media, stresses and temperatures are needed to get the full picture. With all that information a test program can be set up in order to accelerate the different processes that might cause deterioration of the material. The results of the different tests are then evaluated with respect to the conditions of the specific application. Normally this has to be done for each property separately because the acceleration factors can be different. After all those calculations the separate outcomes are combined to give a grounded estimation on the lifetime of a specific product for a specific application. Afterwards the estimation can be adjusted in case the circumstances during actual functio-

ning turn out to be different then those expected. Also it is possible to use, at least a lot, of the results for other applications of the same rubber compound. It is remarked that a proper lifetime prediction strongly depends on the test results and the variation in properties at one side but the same importance is given to the data on the application at the other side. Explanation: more details Keeping in mind that roughly thermal oxidation processes accelerate with a factor 2 to 3 with every ten degrees raise in temperature it is obvious that being to conservative in giving the temperatures for the intended use is leading to far to short lifetime expectations.

An example Often the sales departments of manufacturers like to give an as high as possible temperature that can be used. Here no restrictions are given. For instance maximum temperature of use is 150 °C. Without further information then all calculations are made as if the product would be used at a constant temperature of 150 °C. In real practice it might well be that a temperature of 150 °C is only present during 1 % of the lifetime and only at the start of a process, while the rest of the time tempera-tures varying between 40 °C and 130 °C are normal. Without knowing anything more and assuming that all other time a temperature of 130 °C is present in stead of 150 °C it makes a big difference of at least a factor 22 = 4 in lifetime expectation.

Rubber consultancy & lifetime prediction

Date 24 June 2010

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Kiwa Nederland B.V. Sir W. Churchill-laan 273 Postbus 70 2280 AB RIJSWIJK The Netherlands Tel. +31 70 414 44 00 Fax +31 70 414 44 20 www.kiwa.nl

This simple example shows the importance of accurate data of the intended application. Of course it is not always known exactly what will happen with a product after it is designed, produced and taken into use. Then the advantage of using the data for new calculations to adjust the estimation is of importance. Similar reasoning goes for other influences like: media, stresses, environment and so on. For all it is important to know as precise as possible what can be expected in order to make a good estimation of what will happen in future. Kiwa has to knowledge to help you do this and to carry out the necessary measurements and calculations.

A small example from praxis to give some idea of what it is all about. The case has to do with a seal for a box which is supposed to hang in the open air outdoor in Japan. A lifetime of at least 15 years should be assured. Here a silicone rubber was chosen as material. Given the conditions outdoor in Japan the influences that should be taken into account are: oxidation, ozone, UV, water, micro biological attack, permanent defor-mation and stress relaxation. By using silicone as base for the material, the ozone, UV, cold water and micro biological influences can be neglected and also oxidation is probably not relevant. Properties measured are: hardness, compres-sion set and stress relaxation. All as function of time at different elevated temperatures. The results are summarized in the graphs. Hardness (method ISO 48)

For hardness it is clear that predictions are not possible based on the results. Also it is clear that oxidation indeed is no issue for this material.

Results of the hardness measurements

Results of the compression set measurements Compression set (ISO 815) Here smooth lines showing an increasing deformation of the test pieces are obtained. Such results can be used to make an evalua-tion using Arrhenius (ISO 11346).

Leading to a graph of the time needed to reach a certain change versus the reciprocal tempe-rature. From this graph the lifetime at a certain temperature can be drawn and conclusions on a real lifetime expectation can be given. For more information please contact: Jaap Havinga or Hans Naus Senior Rubber Consultants E-mail: rubber@kiwa.nl Phone: +31 70 414 4555

The Arrhenius equation describes the relation-ship between time and temperature of a first order chemical process:

RTE a

eAt ×= with: t = exposition time [s] A = constant Ea = apparent activation energy [J/mol] R = gas constant [8,314 J/mol K] T = absolute temperature [K]