Chemical Compatibility & Resistance
Gasket Material Chemical Compatibility & Resistance
Choosing the correct gasket material for your application is critical, especially in an environment where you may be sealing against aggressive chemicals, such as strong acids or alkalis. In this situation you will want to know which chemicals are present; and make sure that you are using a gasket with the right chemical resistance or compatibility.
The purpose of this article is to give some general information about the chemical resistance of gasket materials, and rubbers in particular.
This article covers:: Some of the rubber gasket materials with the best chemical resistance. | How to make sure that pipe or flange gaskets are chemically resistant. | The best materials to seal against common chemicals | Why it’s best to work with a specialist or expert when specifying your gasket or material.
When designing a gasket or seal, chemical resistance is one of the most important steps to consider. For more information on compatibility with specific chemicals please see our chemical compatibility charts.
Rubber Gasket Chemical Compatibility
Nitrile (NBR) Gasket Chemical Resistance
Nitrile (or NBR) rubber is used with fuels and oils. It has very good resistance to these substances, and is able to seal when fluctuations of temperature are present. Nitrile is generally used in automotive and machine environments, or anywhere that fuel and oil are commonly used. Like other rubbers nitrile comes in different grades. A quality Nitrile specification can be fully submerged in oil and maintain good resistance, and will not swell in reaction to the oil but care must be taken with certain additives and biofuels. A common use of nitrile is as a binder to be mixed with aramid fibres (non-asbestos jointing): together they create a strong heat resistant gasket material that is also very durable in a fuel and oil environment. Where there is burning fuel there is often excessive heat in such environments non-asbestos jointing provides an excellent gasket material.
EPDM Gasket Chemical Resistance
EPDM has good resistance to nitrogen, potassium, and sodium. Due to its low taint and taste, and excellent UV resistance, it is often used in water-based external environments. Any water pipe applications that has high volumes of sodium, or nitrogen, are commonly sealed using EPDM. If heat is an added factor a non-asbestos jointing with an EPDM rubber binder and aramid fibre blend gives an excellent seal against both the chemicals and the heat that the material will be subjected too.
Viton Gasket Chemical Resistance
Viton® is a branded synthetic rubber used in extreme chemical environments. It has excellent resistance to a wide range of chemicals including chlorinated hydrocarbons, fuels, and many acids. It is not suitable for ketones, such as organic acids and acetone, but has very good heat resistance up to 200?C. Viton is very expensive, but should be used consistently where guarding against failure is absolutely critical, and a top quality product is required. One cost-effective solutions for sealing against aggressive chemicals is to add a PTFE envelope to the bore of a gasket to give extra chemical protection.
Pipe Gasket Chemical Resistance
A cost-effective way of sealing pipe gaskets from strong chemicals is to use a PTFE envelope. A PTFE envelope is a liner that covers the area of the gasket that comes into contact with the contents of the pipe. PTFE is expensive, using a solid gasket is not usually recommended as the gasket can creep away from the pressure over time. Using a PTFE envelope both reduces the cost and doesnt compromise the sealing capability of a rubber or aramid fibre based gasket.
A PTFE envelope, in contrast, is a cheaper and effective way of sealing against aggressive chemicals whilst maintain the flexibility and temperature resistance of a quality pipe gasket. PTFE envelopes can be supplied to fit all standard pipe gaskets, or specifically milled to fit gaskets for non-standard flanges.
Ram Gaskets can mill specific dimensions if required. Modified PTFE gaskets, Blue, White and Fawn are popular chemically resistant gaskets.
Sealing Against Common Chemicals
As a general rule, the following materials will seal against these chemicals. If there is more than one chemical present in your environment, please contact us for expert help and support. Also, there may be a range of materials that would work in your application (each with different cost implications) it is therefore best to contact us to find out what options are available to you.
- Ammonia: EPDM, neoprene
- Anti-freeze: glycol based nitrile, EPDM, neoprene
- Chlorine: viton® with PTFE envelope
- Ethanol: EPDM, neoprene, butyl
- Helium: all polymers will seal against helium
- Hydrogen Gas: nitrile, EPDM, neoprene
- Methanol: viton®, nitrile (grade A), silicone rubber and fluorosilicone.
- Nitrogen Gas: Most rubbers will seal against nitrogen gas (nitrile, natural rubber or EPDM would be the most commonly requested
- Oxygen: neoprene, butyl and viton will seal against oxygen, gaskets should be cleaned before installation. (Oxygen cleaned materials are requested to ensure there is no grease present. Certification can be requested for individually bagged and oxygen cleaned gaskets).
- Sulphuric Acid above 96%: PTFE and expanded PTFE, viton FKM, AFLAS, and EPDM with a PTFE envelope.
Rubber Chemical Resistance
The above recommendations are subject to certain qualifications. When designers and engineers specify a rubber there needs to be an awareness that not all rubbers are equal: even the same type or brand of rubber can be subject to variations that will affect its chemical resistance.
There are many different grades of synthetic rubber, and these rubbers come from many sources worldwide. Rubber is a compound of the original base Polymer such as Nitrile or EPDM and is blended with other rubbers, binder additives and fillers. It is important to have a significant proportion of base polymer to achieve the performance in the environment. It is critical to have a good supplier that understands the product and ensures that gaskets are consistently made with the correctly sourced material. The rubber content, and quality, of the gaskets needs to be both reliable and maintained over the course of your production. RAM has a clear supply chain and stock management system allowing for the best quality materials to be supplied to suit each requirement. .
Additives and Chemical Concentration
Chemicals are often blended, which makes the job of specifying the correct gasket material even more problematic. As already outlined, nitrile rubber is often used within sectors such as oil and gas, and the automotive industry. Nitrile usually has an excellent resistance to fuel and oil. However, when the percentages of additives changes in the fuel, such as an increase in naptha (a common fuel additive) nitrile goes from being an excellent sealant to simply a passable one.
For example, it has been widely publicised that Supermarket fuels containing high levels of additives can wreak havoc on seals over long-term use. If you are using a high quantity (greater than 5%) of a fuel additive over a long period of time, it is advisable to use viton: which has excellent resistance to both nitrile and naphtha.
Another example of this is bio fuel additives, such as ethanol in the U.K. When vehicles are left in storage for long periods of time, the ethanol will separate to the bottom of the tank where the localised high percentage can attack rubbers, fuel lines, and the metal of the fuel tank itself.
Chemical Compatibility: Cost Versus Performance
When specifying a material or gasket to cope with chemical resistance, weighing the cost of the materials against the long-term longevity and performance of the product is critical. Testing different materials is a sensible approach, yet in reality there is often not the time or budget to run tests on multiple different assemblies. We are able to advise, give you the benefits of our experience, and provide test samples should they be required.
Our chemical compatibility charts can give you an idea of which materials to specify on your parts or drawing. Having said this, bear in mind that in certain cases higher quality grades of cheaper materials may well provide you with the sealing that you need, and at a reduced cost.
Our chemical charts have a range of A, B, C etc., to show how well they can be used to seal against certain chemicals. Some materials will swell in use, and while this doesnt necessarily lead to a failure it is often part of a compromise of functionality and cost.