How to select compounds for a rubber product – Ultimate Guide
The two main determinants of product quality in case of rubber products are material and the manufacturing process. Provided that the manufacturing is flawless, choosing the right compound for a rubber product is crucial for several reasons. Human error may lead to failure in application – the worst that can happen -, which might be the result of either miscalculation during the design phase or inaccuracy during the installation. If there are any issues with the sealing, it’s usually due to the incorrect choice of rubber. Evidently, price is not the only factor to be taken into consideration when choosing a compound. The application environment and features of the compound must match in order to provide optimal performance. Use our detailed guide to the world of compounds to make sure that you always choose the perfect material!
Case study about how silicone and EPDM behave differently under high temperatures
Let us demonstrate why it is crucial to choose the right material for a part with a short case study. In case of an application where the temperature might vary drastically, it is essential to select a rubber that will not melt, deform or become brittle. Such errors caused by the smallest parts in a larger machine might even lead to having to stop the whole production process. Be aware that dramatic temperature changes are disadvantageous to all rubbers.
In this case, we are comparing two rubber products both in size of 58 mm, one made out of silicone and one of EPDM. Silicone, an inorganic polymer, is popular in a variety of industries due to its resistance to extreme heat. It’s also commonly used in the food industry due to being a sterile elastomer. EPDM, a synthetic rubber, is a frequent material of the automotive industry for window or door sealing. Its greatest benefit is the resistance to ozone, UV and exposure to various weather conditions.
The two materials react very differently to heat. Silicone only starts to degrade at 230°C, whereas EPDM shows signs of degradation already at 130°C, which can be easily reached in a production environment. As for performance at cold temperatures, silicone has a superior performance here, as well, with a brittle point at -60°C, while EPDM reaches its brittle point already at -40°C.
However, these materials do not have to reach their physical limits to show signs of deformation. Already at 100°C EPDM shrinks around 17%, at 125°C 52%. It hasn’t reached its limit but the size of the product shrank to half. During the same test, the silicone part remained the same size and did not show any deformation. Silicone does not only do better under high temperatures, but also provides a longer product life cycle and doesn’t need to be replaced that often. Make sure to take downtime and replacement costs into account when choosing the material.
The questions you have to ask yourself before choosing the material
The following questions can guide a dialogue that will lead to choosing the right compound for your needs. You do not need to have a full understanding of the specifications; our goal is rather to draw attention to the importance of details. If you can answer the following questions, we are sure that we can find the material providing the best possible product quality.
What will the rubber part be used for?
First of all, determine if the part should transmit or absorb energy and provide structural support. Some parts may also be used for transmitting or sealing fluid. Of course, in this case, the specific fluid must be specified, as well.
At what temperature will the part be used?
When thinking about temperature, do not only consider the constant temperature at which the part will be used. Make sure to take into account both the maximum and minimum service temperatures
What requirements does it have to meet physically?
Different environments require quite different physical properties. For example, if abrasion is expected, it affects the choice of compound greatly. Consider what tensile strength, tensile stress and tensile elongation – degree of elasticity without breaking – the product should bear. It’s useful to have a general understanding of hardness and compression set which refers to the resistance under extensive load. To prevent deformation, the flexural modulus must be determined a.k.a. the resistance to deformation. A specific gravity target, viscosity or melt flow requirements also all belong into this category.
Are there any regulations for this application?
In highly flammable environments, all part must be flame retardant. For applications in the food industry, you might need to comply with FDA or NSF standards.
Will the part be used for an indoor or outdoor application?
This question is key because UV exposure may affect properties such as color. Be as specific as possible about the length of exposure to sunlight and expected temperatures.
What chemicals will the part be exposed to?
List all the chemicals that the part is expected to be in direct contact with such as acids, bases, oils and other. Also, indicate the timeframe of the exposure.
Watch our video to find out how compounds are born:
Get to know the most commonly used materials
Having a good grasp of the most frequently used materials and their characteristics could help you a great deal when reaching a point of decision. Should you ever hesitate about which material might be the optimal choice, come back to our compound specifications page where we detailed every piece of information you might need.
Until then here’s a quick intro to the most common materials in the industry:
This generally non-reactive, stable and extreme temperature resistant elastomer (rubber-like material) outperforms conventional rubbers. It is commonly used as an electrical insulator or radiator hose. However, oils and acids can attack the compound.
Natural Rubber aka caoutchouc
Natural rubber is harvested from trees in Asia and it can be used alone or in combination with other materials. It’s greatest advantages are high tear strength and extreme water resistance, but it’s not friends with oils and solvents generally.
As we have seen before, this synthetic rubber is mainly used for sealing surfaces. Its physical properties fall somewhere between those of natural and synthetic rubber. Be aware that it should not be exposed to petroleum based products for long.
Chloroprene aka CR
This is the synthetic rubber that makes up Neoprene, a trademark of DuPont. Should you need resistance to sunlight, this is the right compound for you. Its high temperature rating, flame and oil resistance make it ideal for the automotive industry.
NBR aka nitrile-butadiene
Nipol, Krynac and Europrene might sound more familiar than NBR, but they all refer to the same compound. This synthetic rubber is resistant to any kind of oil and high temperatures. Should you need a compound that does well under abrasion, this might be the right choice for you.
Excellent elongation and tensile strength are what this elastomer is great at. Moreover, it’s also resistant to light, ozone and oxidation and has good abrasion and compression set properties. However, high temperatures and acids should rather be avoided.
Viton aka FPM
DuPont introduced this fluoroelastomer polymer to the market to meet the special needs of the aerospace industry. Since then it has gained wide popularity in several industries due to its high resistance to heat (300°C), fuel and chemicals; not to mention that it also meets FDA standards, hence complies with the requirements of the food industry.
CSM aka chlorosuplhonated rubber
Also trademarked by DuPont, extreme temperature, UV light and chemical resistance make this compound a common choice. Given its excellent weather resistance, it’s often used for roofing materials, cable coating and automotive parts.
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