Article re-posted with permission from Parker Hannifin Sealing & Shielding Team. Original content can be found on Parker’s Blog.
A common question fielded by Parker O-ring Application Engineers is “will a (insert polymer family) O-ring work with (insert chemical mixture).” Not a day goes by where I do not field this question in some way, shape, or form. Which, honestly, makes perfect sense, because chemical compatibility is one of the two most important factors in designing a seal, the other being size. Choosing the right compound can literally make or break your seal and to the general designer, this can be a massive undertaking. There are so many rubber compound families out there and hundreds and hundreds of chemicals, so how can you know whether your seal is going to hold up? Well, today, I hope to give you a simple, and quantitative way to figure that out.
The Parker O-Ring Handbook
Let’s start by showing you what Parker has to offer in the way of chemical compatibility information. The Parker O-Ring Handbook (ORD 5700) has an entire section dedicated to this and within it you will find hundreds and hundreds of chemicals. Section VII, Compatibility Tables for Gases, Fluids, and Solids, lists the specific chemical, the recommended Parker compound to use for that chemical, and then gives a general rating out of 4 for chemical compatibility for all polymer families. As explained on the first page of that section, a rating of 1 indicates “Satisfactory” compatibility, 2 indicates “Fair”, 3 indicates “Doubtful”, and a 4 indicates “Unsatisfactory”. In general, a 1 means the compound can be used in just about any scenario with the given chemical. A rating of 2 indicates that an interaction between the compound and chemical in a static seal is generally ok, but dynamic seals should be avoided. A 3 indicates that even a static seal is doubtful to work during an interaction between a given compound and chemical. Finally, a rating of 4 means you should avoid contact between an elastomer and that particular chemical if at all possible. An “X” indicates that there is not enough data for the interaction between a given chemical and polymer to make a clear rating. This makes finding a recommended compound for a single chemical very easy, but as you all might expect, that is not always the case. It is extremely common for a seal to see a variety of fluids at different times, in combination with one another, and even at different temperatures. Fortunately, there is a way to determine whether a polymer family will work in these more complex environments as well.
An example
Let us take an example where we assume a seal is going to see a mixture of 50% water, 20% ethylene glycol and 30% anhydrous ammonia. How would you determine what compound family would be best for this seal? A good rule of thumb is to use this method with EPDM, Nitrile and FKM. These three represent the general spectrum of chemical compatibility. EPDM is more compatible with water-based chemicals, FKM is more compatible with oil/hydrocarbon based chemicals, and nitriles are somewhere in the middle.
Calculations
Let’s take this approach for this example. First, we want to determine the rating for FKM in each of the listed chemicals. There are many different “types” of water in Section VII of the O-ring handbook (DI Water, boiler feed water, seawater, etc.) but for this example, let’s take regular water. If we find where the “water” row and “FKM” column intersect, you will find that it is given a 2 rating. Record this number; we will use it later. Next, find “ethylene glycol” and determine where that row and FKM meet and you will see this is also given a 1 rating. Finally, let’s look up FKM’s compatibility with anhydrous ammonia. Using the same method, you will find that it is given a rating of 4. As a general rule, if any chemical in the mixture has a rating of 4 or X (for no data) with a given polymer family, you should not use that polymer as a seal for the application, no matter how little of the mixture it makes up. Regardless, let’s proceed with the calculation to determine FKM’s compatibility with the mixture.
The general principle is to multiply the respective decimal values of the chemicals’ % composition in the mixture with the rating given between those chemicals the polymer family. So for FKM the calculation would look as follows:
0.5 * 2 + 0.2 * 1 + 0.3 * 4 = 2.4
All the decimal values should add up to 1 (or 100% of the mixture) and the rating scale is the same as the scale for individual compounds. If every chemical had a 1 rating with a given polymer, the final value would also be 1. If every chemical had a 4, the final value would be a 4. The closer the final value is to 1, the better. In this case, even though 2.4 isn’t a relatively high value and might be acceptable in some cases, we would not recommend FKM because it has a 4 rating with anhydrous ammonia.
Let’s repeat this calculation for Nitrile and EPDM. For nitrile, you can use the same method to find that it has a 1 rating with water, 1 for ethylene glycol, and 2 for anhydrous ammonia. The calculation for nitriles compatibility would be
0.5 * 1 + 0.2 * 1 + 0.3 * 2 = 1.3
A rating of 1.3 is a very good rating and we would expect Nitrile compounds to work well with this mixture. Let us do the same calculation with EPDM to see if it may be even better than nitrile. When you look at the compatibility ratings for EPDM with water, ethylene glycol, and anhydrous ammonia you will find that it is given a rating of 1 in each of those chemicals. This makes the calculation extremely simple
0.5 * 1 + 0.2 * 1 + 0.3 * 1 = 1
We would expect any general purpose EPDM compound to be the best polymer family to use in application with this specific mixture.
There are some cases where other polymer families, such as Butyl, Hydrogenated Nitriles, Neoprene, Polyacrylate, etc may be the “best” for a given application, but these are usually in cases where the seal may come in contact with very specific chemicals or other application considerations. As a general rule, starting with performing the described calculation on the three polymer families above will give you one of them that is “most compatible” and a good starting point.
For any specific questions, or if your calculation shows that none of these polymers are very compatible with the given mixture, please contact our Parker O-Ring Applications Engineers via our live chat service on Parker O-Ring Division's website.
Contributed by Tyler Karnes, Applications Engineer, Parker O-Ring Division.
Source: http://blog.parker.com/chemical-compatibility-a-critical-component-in-seal-design