Comparison Between Buna -N and Natural Rubber use in Sealing in Valves

COMPARISON BETWEEN BUNA-N & NATURAL RUBBER

BUNA –N ( Natural Butile Rubber) commonly called as Nitrile chemically, is a copolymer of butadiene and acrylonitrile. Acrylonitrile content varies in commercial products from 18% to 50%. As the nitrile content increases, resistance to petroleum base oils and hydrocarbon fuels increases, but low temperature flexibility decreases. Due to its excellent resistance to petroleum products, and its ability to be compounded for service over a temperature range of -35°C to +120°C (-30°F to +250°F), nitrile is the most widely used elastomer in the seal industry today. Also many military rubber specifications for fuel and oil resistant O-rings require nitrile based compounds. It should be mentioned that to obtain good resistance to low temperature, it is often necessary to sacrifice some high temperature resistance. Nitrile compounds are superior to most elastomers with regard to compression set, tear, and abrasion resistance. Nitrile compounds do not possess good resistance to ozone, sunlight, or weather. They should not be stored near electric motors or other ozone generating equipment. They should be kept from direct sunlight. However, this can be improved through compounding. NBR is the standard material for hydraulics and pneumatics. NBR resists oil-based hydraulic fluids, fats, animal and vegetable oils, flame retardant liquids (HFA, HFB, HFC), grease, water, and air. Special low-temperature compounds are available for mineral oil-based fluids. By hydrogenation, carboxylic acid addition, or PVC blending, the nitrile polymer can meet a more specified range of physical or chemical requirements.

The quality of Nitrile-compounds depends on the percentage of acrylonitrile in the base polymer. The following table indicates the change of properties as a function of acrylonitrile content.

Natural Gum Rubber
Natural gum rubber sheet, natural color, smooth finish. Excellent tear strength, abrasion resistance, resilient. Used in general gasketing and as sand and shot blast curtain. Meets ASTM D2000.

Elastomer

Color

Available Durometer
(SHORE A)

Typical Tensile
PSI (Minimum)

Ultimate
Elongation
(%Min)

Temperature
Range

Standard Thicknesses

NR (Gum)

Natural

40

3000

600

-20ºF to +140ºF

1/16, 3/32,
1/8, 3/16, 1/4,
3/8, 1/2, 3/4, 1

Nitrile (Buna-N)
NBR sheet, commercial grade, smooth finish. Excellent resistance to oils, solvents and fuel. Meets ASTM D2000-BF.

Elastomer

Color

Available Durometer
(SHORE A)

Typical Tensile
PSI (Minimum)

Ultimate
Elongation
(%Min)

Temperature
Range

Standard Thicknesses

NBR
(butadiene-acrylonitrile)

Black

40-80

1000

300

-20ºF to +170ºF

1/32, 1/16,
3/32, 1/8,
3/16, 1/4, 3/8,
1/2, 3/4, 1

NATURAL RUBBER (NR)
Natural rubber is a product coagulated from the latex of the rubber tree, hevea brasiliensis. Natural rubber features low compression set, high tensile strength, resilience, abrasion and tear resistance, good friction characteristics, excellent bonding capabilities to metal substrate, and good vibration dampening characteristics.	Temperature Range (dry heat)
	low		high
	- 60 °F -51 °C		220 °F 104 °C
	Application Advantages
	» excellence compression set » good resilience and abrasion » good surface friction properties
Primary Uses	Application Disadvantages
O-rings, rubber seals and custom molded rubber components for: » rubber to metal bonded vibration isolators and mounts » automotive diaphragms » FDA applications for food and beverage seals	» poor resistance to attack by petroleum oils » poor ozone, UV resistance
NITRILE (NBR)
Nitrile is the most widely used elastomer in the seal industry. The popularity of nitrile is due to its excellent resistance to petroleum products and its ability to be compounded for service over a temperature range of -22°F to 212°F. Nitrile is a copolymer of butadiene and acrylonitrile. Variation in proportions of these polymers is possible to accommodate specific requirements. An increase in acrylonitrile content increases resistance to heat plus petroleum base oils and fuels but decreases low temperature flexibility. Military AN and MS O ring specifications require nitrile compounds with low acrylonitrile content to insure low temperature performance. Nitrile provides excellent compression set, tear, and abrasion resistance. The major limiting properties of nitrile are its poor ozone and weather resistance and moderate heat resistance, but in many application these are not limiting factors.	Temperature Range (dry heat)
	low	high
	-22 °F -30 °C	212 °F 100 °C
	Application Advantages
	» excellent compression set, » superior tear resistance » abrasion resistance
	Application Disadvantages
	» poor weather resistance » moderate heat resistance
	Modifications
	» acrylonitrile content (ACN) from 18% to 50% » peroxide vs. sulfur donor cure system » XNBR improved wear resistance formulation
Primary Uses	Specialized Applications
O-rings, rubber seals and custom molded rubber components for: » Oil resistant applications » Low temperature applications » Fuel systems, automotive, marine, and aircraft » General Industrial Use	» NBR NSF standard 61 for potable water applications » NBR WRc, KTW water applications » NBR FDA white list compounds