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Q&A With DuPont: Seal Of Approval

Wed, 06/18/2008 - 6:21am

Chem.Info sits down with Russ Schnell, a DuPont Performance Elastomers application engineer, to discuss trends, capabilities and expectations of elastomer seals in facilities.

Many of us have experienced a leaking faucet at home-situations in which the drip-drip-drip of water finally forces us to replace the rubber washer seal. For those applications, the problem is a mere annoyance. However, the importance of selecting the correct elastomeric seal for critical applications cannot be underestimated. In factories and chemical plants around the globe, a leak, or perhaps a major seal failure, can result in downtime, damaged equipment or an environmental incident. The following questions briefly address some of the issues that arise when dealing with elastomer seals.
Q:
Suppose I have a specific chemical ?sealing application. How do I select the appropriate elastomer? ?
A:
There are numerous reference sources available to assist in material selection, both in hard copy and on the web. Unfortunately, you may not find a material recommendation that exactly fits your set of application conditions. Use your references to narrow your set of material choices, and then contact your elastomer part supplier for product advice. Finally, obtain samples of the best candidates, and evaluate them in your specific application environment. DuPont Performance Elastomers has online tools, such as the Chemical Resistance Guide and the Kalrez® Application Guide, to aid in selection at www.dupontelastomers.com.
Q:
After testing these products, is there an easy way to determine which product would work best??
A:
Many of us have labs that are available for test work, but they are not equipped for extensive elastomer testing. However, it is still possible to make a determination of expected performance. On the simplest level, expose the elastomer samples to the process chemicals and temperature used-the longer the exposure, the better. I would generally suggest at least two weeks, but four weeks would be preferred. Visually look for any changes in the sample surface. Also consider weight gain, and if possible, volume swell and hardness change. Typically, the material with the least property change is the one to select. If there is a weight gain of more than 15 percent, then consider a different product for the application. ?
Q:
What is the best way to gain an ?understanding of the differing elastomer compositions?
A:
In general, it is almost impossible to determine the different elastomers by appearance, as most O-rings are black. Specific gravity can be one differentiator, but it can vary greatly depending on the amount and type of fillers that are used in the elastomer. A more definitive way to identify a class of elastomers is by FTIR analysis, or a thermogravimetric analysis can be conducted. Combining these methods provides significant information on O-ring type and composition, including the identification of elastomer class.?
Q:
How should purchasing specify an ? elastomer?
A:
Ideally, the plant engineering group should develop an elastomer specification for every plant process. This specification details the elastomer type (supplier product number) and performance requirements. The purchasing group would then purchase according to this specification. If a new product is offered as a less expensive alternative, the supplier should document that it meets the specification regarding elastomer type and, most importantly, that it meets the specification performance criteria.
Q:
What are some of the more common leakage problems that you encounter? ?A:

It can be a major mistake to assume that the elastomer seal must be the primary cause of a leak. It may often be true, or it could be that the elastomer seal failed because it was installed incorrectly. When an elastomer seal is damaged and has failed, it is imperative to do a root cause failure analysis. Verify the correct elastomer was used and that it is compatible with application fluids. Also, be sure there were no process upsets by checking the groove to be sure it is properly designed, etc. If necessary, don’t be afraid to ask for assistance from your seal supplier to resolve the issue. Next, replace the elastomer seal with the correct material, and be sure to document the incident to prevent a recurrence in the future.
Q:
Elastomers come in different colors—what does this represent from a performance perspective? ?A:

It depends on what materials are used to achieve the color. For example, consider two 75 durometer black nitrile rings. One ring utilizes carbon black filler to achieve its color and durometer. The other uses a non-black clay filler to achieve durometer and a black dye to achieve color. All other ingredients remain the same. The clay filler would impart different physical properties to the elastomer when compared to the carbon black O-ring and may not give the desired performance. Now consider two 75 durometer nitrile materials, one red and one green. If the same formulations are utilized to achieve particular durometer hardness and the only difference is the dye to achieve color, there should not be any difference in performance. Note: When specifying seals for pharmaceutical applications, color may be something that can be extracted out and may therefore introduce unwanted constituents into a product.

Q: What is the typical service life of an elastomer?
A:
Unfortunately, you may not find a material recommendation that precisely fits your set of application conditions. Therefore, trying to estimate the lifetime of the seal in a specific set of conditions is not practical. Utilize the tools available to select the best elastomer; expose the elastomer samples to process conditions; and contact your elastomer part supplier.
Q:
Most elastomer data sheets show short-term performance, but what about long-term specifications??A:

Long-term testing often takes significant time and resources. Short-term testing is beneficial in that it provides a quick indication about materials that are not suitable for some services, and which materials should be researched further. With regard to long-term testing, there are some elastomer part suppliers that believe long-term data is imperative for individuals looking to extend their mean time between repair. Contacting your elastomer part supplier is the best way to determine what information is available.?Q:

What test results should I consider in an elastomer evaluation? ?
A:
It depends on the environment that you are trying to seal. Generally, elastomers that have low chemical volume swell, minimal hardness changes and low compression sets are good choices. If your process involves a high-pressure environment and extrusion resistance is imperative, modulus test results should be your focus. Is there potential for rapid gas decompression? Look for test results that assess material suitability in that type of environment.?Q:

What if I frequently use EPDM and FKM, but I notice different performances when I order the same product? ?A:

Elastomers can be compounded many different ways and utilize many different ingredients. Not all ingredients are identical and interchangeable. Therefore they can impart different performance characteristics to a finished part. As described in question four, it is important that you purchase a material that meets your internal specifications. Reputable elastomer part suppliers document any ingredient changes and assess the impact on final performance before releasing the product for sale. l ?
Russ Schnell is an application engineer with DuPont Performance Elastomers, and has been in the Kalrez® business for 18 years. He currently provides technical support to the chemical process, pharmaceutical and aerospace industries. He has a BSChE from Columbia University and an MBA from the University of Delaware. More information is available by visiting www.dupontelastomers.com. ?
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