Technical Article

Common O-ring Sealing Materials&Their Hydraulic Application Characteristics (One)-Nitrile rubber

The working environment of the hydraulic system (high pressure, fluid medium, temperature fluctuations, contamination, and possible dynamic movement) imposes strict requirements on sealing materials. Common materials for O-rings include nitrile rubber, fluorine rubber, ethylene propylene diene monomer (EPDM) rubber, chloroprene rubber, and polyurethane rubber. This article mainly introduces nitrile rubber.

Nitrile butadiene rubber NBR (Buna-N) is a synthetic rubber made by emulsion polymerization of acrylonitrile (AN) and butadiene (BD). Due to its excellent oil resistance, wear resistance and chemical corrosion resistance, it has become one of the most widely used oil-resistant rubbers in the industrial field.

1. Main advantages

(1) Excellent resistance to mineral oil and animal and vegetable oils: This is its most core advantage in hydraulic oil systems. It has good tolerance to standard industrial mineral oil-based hydraulic oils (L-HL, L-HM, etc.).

(2) Good wear resistance: Suitable for piston rod sealing and piston sealing in low-speed reciprocating motion.

(3) Good pressure resistance (medium to high compression set) : Capable of withstanding relatively high system pressure.

(4) Excellent air tightness: Good sealing effect on liquids and gases.

(5) Low cost: It offers the highest cost performance and is the most widely used.

(6) Low-temperature performance: Standard type: Can reach -30°C to -20°C; Cold-resistant type: The minimum operating temperature of specially formulated nitrile rubber can reach -40°C or even lower.

2. Main disadvantages:

(1) Limited high-temperature resistance: The upper limit of long-term service temperature is generally 100 to 120°C(up to 130°C in the short term). At high temperatures, it will accelerate hardening, cracking and increase permanent deformation.

(2) Poor resistance to polar solvents/chemicals: Not resistant to ketones, ethers, esters, halogenated hydrocarbons, phosphate esters (fireproof oils), and other media.

(3) Poor ozone resistance/weather resistance: Not suitable for long-term exposure to sunlight or environments with high ozone concentrations.

(4) The wear resistance at high temperatures decreases.

3. Typical hydraulic applications:

(1) Static sealing: Hydraulic valve block oil ports, end covers, flanges, pipe joints (SAE, UNO, etc. flanges), plugs, pump/motor end covers, etc., almost all static sealing points. This is the most mainstream static sealing material.

(2) Low to medium-high pressure dynamic seals: Piston rod seals, piston seals, and low-pressure rotating shaft seals (not the first choice) with working pressure below 20 MPa are often used in conjunction with retaining rings to enhance the anti-extrusion capacity.

(3) Application medium: Standard mineral oil-based hydraulic oil (L-HL, L-HM), water-ethylene glycol (HFC) hydraulic fluid (special formula NBR is required), grease.