Why Buna N is so popular for use with hydrocarbons

We love a novel material at Martin’s Rubber; something that has been specifically developed to do a very challenging job under a range of unusual conditions. However, our objective is never to reinvent the wheel or overengineer things unnecessarily. There are some fantastic rubbers already on the market that work perfectly for many industrial applications – and Buna N is one that has been around for nearly a century and really pulls its weight as a veteran of materials. But it’s important to understand what a material is good at and why. If you’re looking at a rubber compound suited to contact with hydrocarbons and that will provide great heat resistance and stability, then Buna N is likely to be one of the materials at the top of your list. We take a look below in more detail at the following topics:

• Buna N’s origins
• Why nitrile butadiene rubber is popular
• Why Buna N is resistant to hyrdrocarbons
• Weaknesses of Buna N
• Blends and alternatives
• Availability of nitrile butadiene

Buna N’s origins

Buna N is a trade name for nitrile butadiene rubber (NBR), with the name Buna N owned by BASF during the twentieth century up until the nineteen-eighties. Buna-S was simultaneously developed in the US during the 1930’s, and is fundamentally the same compound, developed as a synthetic alternative to natural rubber. Nitrile rubber, or NBR, as it is most commonly known, went on to become one of the most popular rubbers in circulation (not withstanding natural rubber, used globally for tyre manufacture).

Why is Buna N rubber so popular?

Nitrile rubber (NBR) is valued most widely for its resistance to oils, fuels and chemicals. It is the elastomer of choice for many automotive and mechanical aerospace applications – not only is it an effective rubber grade for fuel resistance, but it also offers good heat stability and will withstand temperatures up to 100˚C, or 120˚C in hot air applications.  As well as aerospace, nitrile rubber is often found in automotive applications and is popular for oil resistant seals and diaphragms, fuel lines and other components that may come into contact with hydrocarbons.

Why is Buna N resistant to hydrocarbons?

Due to the presence of acrylonitrile (ACN) in the polymer backbone, nitrile rubber (NBR) provides resistance to non-polar fluids such as hydraulic oil or petrol/diesel fuels. The level of acrylonitrile can vary in different grades, and the higher the ACN content, the poorer the low temperature resistance and flexibility. However, those grades with the highest ACN content allow the highest resistance to swelling in hydrocarbon fluids.

Weaknesses of Buna N

No rubber can excel at everything; in order to provide a resistance or strength in one area, it is almost inevitable that another property will be compromised due to the careful chemical balance that must be maintained within rubber compounds. Due to its chemical structure, NBR has poor resistance to UV, ozone or weathering. This can be improved by using the saturated version of NBR, hydrogenated nitrile butadiene rubber (HNBR) – though this comes with a higher price tag.

Blends and alternatives

For improved resistance to UV, blends with PVC can be used, however this is at the expense of elastic return so is not recommended for many sealing applications. The introduction of PVC into the NBR structure increases the material’s stiffness, whilst lowering the elongation-to-break. The benefit of this is the additional resistance provided by PVC, allowing for high gloss cosmetic finishes, increased UV and ozone resistance without compromising oil and fuel resistance. PVC also significantly increases the range of applications for use of NBR in cabling as it allows the NBR-PVC material to operate at higher temperatures. However, PVC is becoming increasingly regulated and so now comes with a relatively high administrative burden.

If the level of oil resistance required is fairly modest but the need for UV resistance is high, then neoprene can be a good alternative where mechanical properties are not critical; neoprene has the equivalent of a low level ACN NBR, but HNBR would generally be considered a better alternative.

Availability of nitrile butadiene

Nitrile rubber (NBR) is one of the materials we use most commonly, with over a quarter of our rubber mouldings being manufactured in either NBR or HNBR. Although we saw extreme shortages of rubber compounds during the Covid period, NBR is now readily available with material shortages not a common issue.

Nitrile for rubber moulding

Nitrile rubber (NBR) can be used for all three of our moulding types – compression, injection and transfer – without any difficulties, and is used widely by our aerospace and motorsport customer base.

Nitrile butadiene rubber is one of the most useful general-purpose rubber compounds in circulation for applications where hydrocarbons are in contact. It does have its areas of weakness, and these can be addressed by using HNBR or by considering a more bespoke compound that may offer a more specific range of properties. But, if you select NBR as your material of choice, you’ll be in good company and will be choosing a compound that is great for moulding and will provide you with a product that can be manufactured consistently and reliably.

Author: Jason White, Technical Engineer.

If you would like to understand whether NBR is the right material for your application, or would like advice on suitable materials for your rubber moulding, take a look at our chemical compatibility chart, or contact our team.