HNBR

Definition and Distinction from NBR

HNBR stands for hydrogenated acrylonitrile-butadiene rubber. It is produced from NBR (acrylonitrile-butadiene rubber) by hydrogenating the polymer. Hydrogenation here means that double bonds in the polymer chain are largely saturated. With NBR, these double bonds are typical attack points for reactions with oxygen or ozone.

For sealing technology, HNBR is mainly relevant when a seal is exposed for longer periods to heat, atmospheric oxygen, and ozone and NBR ages too quickly under these conditions. HNBR is therefore frequently classified as a high-performance elastomer, because it often combines NBR’s behavior in oily media with a noticeably higher aging reserve. An elastomer is a rubber-elastic material that deforms under load and largely returns to its original shape afterward.

Why Hydrogenation Improves Aging and Ozone Resistance

Ozone and oxygen preferentially attack reactive double bonds. Through hydrogenation, the chain becomes more saturated and therefore chemically less reactive. In practice, this shows up as slower thermo-oxidative aging (aging caused by heat and oxygen) and as better ozone resistance than with NBR. This matters in particular when seals are operated not only in oil but also exposed to air, or when temperature cycles and standstill periods dominate.

Properties of HNBR in Sealing Technology (Relevant for Design and Service Life)

HNBR is frequently used in seals because of its high mechanical strength. This includes tear strength, which matters at dynamic sealing points, because edge loading and local elongations occur there. Abrasion resistance is equally relevant, because friction contact — for example at a piston rod or on rotating shafts — can promote material loss and therefore leakage.

A second core point is the retention of properties under prolonged exposure to heat and oily media. For many applications, the initial hardness is not decisive. What matters is whether the seal still remains sufficiently elastic after months or years and continues to maintain its sealing force. Concrete characteristic values, however, depend strongly on the compound — that is, on the cross-linking system, fillers, plasticizers, and target hardness.

Compression Set: Significance for Tightness

The compression set describes how strongly an elastomer remains permanently deformed after prolonged compression. It is therefore a measure of the recovery capability after pressure loading. In a static seal — for example an O-ring in a groove — a low compression set typically means that the contact pressure is better preserved over time. As a result, the risk that gaps reopen and a leak develops decreases, especially at elevated temperature.

Application Areas: Media Resistance and Temperature Range (Hydraulics/Pneumatics Focus)

HNBR is in many cases used in hydraulic and pneumatic systems, because it often harmonizes well with mineral-oil-based hydraulic oils and offers an increased reserve against heat and aging. Frequently cited temperature ranges lie roughly at approx. $-25\hspace{0.17em}°C$ to $+150\hspace{0.17em}°C$, although this classification is suitable only as a rough orientation. In practice, the actual application limit is determined by the compound, sealing geometry, pressure, type of motion (static or dynamic), and the specific fluid.

With water-glycol hydraulic fluids, the assessment is typically more demanding. Here, temperature, additive packages, and water content can influence elastomer behavior considerably. Therefore, approvals are often confirmed via compatibility tables and application-specific tests rather than via generic material lists.

Limits and Typical Exclusion Criteria (in Brief)

Blanket statements often fail because of additives, temperature peaks, and frictional heat in dynamic applications. In addition, compounds within HNBR itself differ noticeably — for example through different cross-linking or filler systems. For special fluids and critical operating conditions, a specific material approval is therefore usually more useful than a general material recommendation.

Material Selection and Typical Sealing Forms

HNBR is often an obvious choice when NBR still fits in terms of media but the required temperature and aging resistance is no longer sufficient. By contrast, FKM (fluoroelastomer) is often selected when very high temperatures or specific chemicals dominate, while EPDM is frequently used for hot water/steam or glycol-based brake fluids. Material selection therefore mostly follows the question: which medium is present, which temperature acts continuously, and how much aging reserve is needed for the required service life?

In sealing technology, HNBR is typically found in:

• O-rings for static and moderate dynamic applications,
• Sealing lips (e.g., radial shaft seals) and wipers (also called scrapers) in hydraulics,
• Diaphragms and elastomer parts of composite seals.

Typical Components: O-Ring, Sealing Lip, Wiper / Scraper (Overview)

O-rings often benefit from HNBR where compression and temperature act together over a long time, because the compression set strongly influences the sealing force. Sealing lips and wipers (or scrapers) use the combination of mechanical robustness and abrasion resistance, since motion, pressure cycles, and dirt ingress continuously load the edge. In many applications, the interplay between material, surface, and lubrication ultimately decides — because friction not only causes wear but also raises the local temperature.

A brief note in closing: at limit temperatures, with special fluids, or at strongly dynamic sealing points, a targeted material and compound specification based on approvals and test data is usually worth the effort.