Nitrile Butadiene Rubber (NBR) is a synthetic rubber known for its exceptional resistance to oil, fuel, and other petroleum-based chemicals. It is widely used in products like protective gloves, automotive hoses, seals, and O-rings due to its balance of mechanical strength and resistance to swelling in hydrocarbon fluids. As an organic polymer, its capacity to withstand elevated temperatures is a primary consideration for many applications.
Nitrile’s Thermal Resistance Profile
Nitrile rubber has moderate heat resistance, suitable for continuous operation within a specific temperature window. For most standard NBR compounds, the maximum recommended temperature for continuous service is approximately 120°C (248°F). Exposure above this point risks accelerating the material’s aging process and compromising its integrity.
NBR can tolerate short-term, intermittent temperature spikes, sometimes up to 135°C (275°F). However, prolonged exposure at these higher levels causes the rubber to harden and lose elasticity. This hardening is caused by thermal-oxidative degradation, which reduces the material’s ability to maintain a seal. Thermal decomposition, where the material structure begins to break down, typically starts around 360°C to 430°C.
Structural Chemistry Behind Heat Stability
The thermal performance of NBR is linked to its composition as a copolymer of acrylonitrile (ACN) and butadiene. Butadiene segments form the flexible backbone but contain double bonds susceptible to attack by oxygen at elevated temperatures, leading to thermal aging and hardening.
Acrylonitrile units, comprising 18% to 50% of the compound, introduce polarity and rigidity to the structure. A higher percentage of ACN content increases the glass transition temperature, enhancing thermal stability. The ACN unit exhibits a higher decomposition temperature than the butadiene unit, contributing to overall heat resilience. This molecular architecture limits oxygen diffusion, mitigating the rate of thermo-oxidative degradation.
Thermal Performance Relative to Common Materials
NBR sits in a mid-range position compared to other common elastomers regarding heat resistance. Its maximum continuous service temperature of 120°C is similar to Neoprene, though NBR’s resistance to petroleum oils at these temperatures is superior.
Elastomers like EPDM (Ethylene Propylene Diene Monomer) offer a broader thermal range, often reaching up to 150°C, making them better choices for hotter environments without oils. For significantly higher thermal demands, specialized fluoroelastomers such as Viton (FKM) or Silicone rubber are used. These materials can withstand continuous temperatures up to 204°C and 232°C, respectively. Applications requiring sustained temperatures above 120°C necessitate a switch to these high-performance materials.
Flammability and Fire Exposure
It is important to distinguish between nitrile’s heat resistance and its flammability characteristics. Nitrile rubber is a combustible organic polymer, meaning it will burn if exposed to a direct flame or a sufficiently high heat source. Its flame resistance is considered poor compared to materials engineered for fire safety.
Upon direct exposure to fire, NBR will ignite and sustain a flame, resulting in the emission of dense black smoke. Due to the nitrogen in the acrylonitrile component, thermal decomposition can release toxic byproducts, including hydrogen cyanide, at temperatures around 430°C. NBR is not a self-extinguishing material and should not be used as a fire barrier or in applications where direct flame exposure is a risk.