Determining Low Temperature Ratings for Rubber Seals
Temperature ratings are typically published for industrial seals so you know where they can and cannot be used.
Many sealing products have testing standards, allowing manufacturers to publish results for easy product comparison.
However, there is no standard that tells you how to determine minimum and maximum temperatures, so how do companies come up with their ratings? Furthermore, if two suppliers offer the same type of material with vastly different temperature ratings, how do you know who is correct, or which number to trust?
Rubber isn’t just made of rubber
Rubber is made up of more than just the raw polymer (nitrile, neoprene, fluoroelastomer, etc.).
On its own, the raw polymer has very poor properties and is difficult to work with, so a variety of materials are added to improve performance and allow for easy processing.
This rubber compound is made of up certain ingredients and processing steps to achieve a particular outcome. So, while the rubber seal is not made of pure raw polymer, it is still called by the polymer name.
This is why there are thousands of neoprene rubber compounds. The raw neoprene polymer is used in combination with other ingredients to make a product with unique performance characteristics.
Think of it like chocolate chip cookies. There are many different recipes you can use to make chocolate chip cookies, it all depends on what you are looking for as an end result. The same thing applies to rubber recipes – you can modify the recipe to better handle higher temperatures, be tougher, stretch more, etc.
Carbon black is one of the most common ingredients added to polymers to make them workable, but it also alters the end performance, as the more carbon black you add the harder the rubber will be.
Unfortunately, there is no single magic rubber recipe that is the best at everything.
Recipes are a compromise, as the ingredient that you add to improve resistance to cold temperatures can make your flexibility worse.
Something else to keep in mind when comparing products is an insidious practice used to lower the cost of rubber products by filling them with sand, clay or even cheaper polymers.
Though it may still be called neoprene, it may only contain 5% of it. With a finished rubber part, you cannot visually tell what the ingredients are, so you are relying on the integrity and knowledge of your supplier to publish honest and accurate information.
Are temperature ratings subjective?
Another complicating factor with developing temperature ratings is considering how the product will be used – is it a static or dynamic seal?
A gasket would be a good example of a static seal, where the gasket is compressed between two flanges and stays in place until it is ready for replacement.
Of course, the gasket moves slightly as the connecting pipelines move and flex, it will adjust and respond. This is considered a static seal because the movements are very minor.
Examples of dynamic seals would be a diaphragm or an expansion joint, where the products are designed to move. While the diaphragm is flexing back and forth, it also must resist the force of the media pressing against it.
How the product will be used is a definite factor when determining the temperature rating of a product. Dynamic seals often receive more conservative temperature ratings due to their use in more strenuous applications.
While the behaviour of the material does not change, the added stressors of movement complicate things, and that is where some subjectivity comes into play.
Tests to determine a minimum temperature rating
Rubber is unique in that at high temperatures, it does not melt, it simply starts to degrade. However, at low temperatures, the material stiffens and becomes brittle. While there is no standard method to determine minimum temperature ratings for industrial seals, there are standardised procedures you can use to test the materials. Below are some common tests:
- ASTM D1329 temperature retraction test – The sample is elongated and frozen, then released and allowed to retract on its own as the temperature rises. This test shows the materials tendency to crystalise at low temperatures, and is most useful when employed in conjunction with other low-temperature tests.
- ASTM D746 and D2137 brittleness point test – The sample is subjected to cold temperatures, then hit with a blunt object and checked for cracks. If there are cracks, additional tests are run at higher temperature intervals until it can be hit without cracking. This test shows the temperature at which the material is considered to be brittle.
- ISO 1432 Gehman Torsional modulus test – Four wires are attached to the corners of the sample, which are then subjected to cold temperatures and twisted. This test shows the relative stiffness of the material over a range of temperatures. Garlock does not currently have this test.
- Dynamic mechanical analysis (DMA) Test – A sample is deformed over a range of temperatures, however, the amount of deformation is related to the material’s stiffness at that specific temperature (sinusoidal deformation). This test gives detailed information about the glass transition temperature (Tg), which is where the material becomes glass-like.
Turning test results into a temperature rating
When testing for brittleness, the method used was determining the temperature at which 50% of the samples fail (fracture or crack). This test can also be used to determine the lowest temperature at which the rubber will not fracture.
However, as a more conservative approach, it is important to find the average temperature when fracture begins.
Using this technique, five samples are tested well below what it is assumed the material can handle, expecting all of them to fail. Then, by raising the temperature 10 degrees Fahrenheit, test five more samples are tested. This process is continued until 50% or more (three or more samples) pass, and this is considered the Brittleness Point. In this example a neoprene rubber used for diaphragms was tested and the Brittleness Point was at -34°C (-30°F).
When rubber gets cold, it begins to form a leather-like texture, where the material is tough but still flexible.
As the temperature continues to drop, the material will lose its flexibility and harden until it reaches a glass-like consistency.
The beginning, or onset, of the glass-like consistency is known as the Glass Transition Temperature (Tg).
While Tg is generally discussed as a specific temperature, the transition actually takes place over a temperature range. The DMA temperature sweep will highlight three key pieces of information related to the low temperature performance of the material:
- The storage modulus E’ onset: The lowest of the three highlighted temperatures, this value is typically a good indicator of the overall effect of temperature on the stiffness of the material, and thus its load bearing capabilities.
- The loss modulus E’’ peak: The middle temperature where the material undergoes a large amount of structural change related to molecular mobility.
- The tan and peak: The highest of the three temperatures is the point at which the material has the highest ratio of viscous response to elastic response under deformation that is the leather-like midpoint between the rubber and glass states.
Using the DMA and brittleness test data, one would obviously want to rate the material higher than the lowest temperature rating, the storage modulus. The question is, what rating should actually be given to the material?
- -36.81°C The storage modulus E’ onset
- -34°C The brittleness point
- -30.86°C The loss modulus E’’ peak
- -21.08°C The tan and peak or leather-like midpoint
To determine this, engineers will need to go back to the product application and look at how this product will be used.
This neoprene rubber is used as a diaphragm, which is a highly dynamic movement and can be used in a wide range of applications, so firms need to make sure that the temperature rating given is acceptable for all situations. Another factor to consider is whether the material will be run consistently at this ultra-low temperature, or if it will only see the ultra-low temperature for short periods of time.
Safety comes first
Garlock has a history of being very conservative with its ratings because safety is its number one priority as a company.
This applies to customers as well. Given the parameters of this product’s use, the firm would typically rate it to -23.3°C (-10°F), which limits the material to just within the beginning of the “leathery” range.
Diaphragms come in various sizes and shapes, and operate at different speeds. Therefore, it is important to include a note alongside temperature ratings to emphasise that these values serve as references only. It is important to assess each application individually to determine product suitability.
Consider the source
Most suppliers use different rubber compounds to make their products, so there is no one rating that can be used for all rubber types, and each supplier should perform their own internal testing.
Determining temperature ratings for industrial seals is a subjective process, so it’s important to use products from a supplier that you trust to perform actual product testing and publish honest, safe results.
The original article was written by Sarah Young, Product Engineer, and Brett Yoder, Applications Engineer at Garlock Sealing Technologies.
Gallagher Fluid Seals is an authorized distributor and partner of Garlcok Sealing Technologies. Contact us today for engineering help or an RFQ.