As core components for controlling medium leakage in industrial systems, rubber seals are widely used in fields such as automotive engines, chemical pipelines, and aerospace equipment. Their sealing performance directly determines the operational safety and reliability of systems. When the operating environment temperature exceeds the tolerance threshold of the rubber material, the seals often develop issues such as loss of elasticity, structural cracking, and increased sealing clearance—these problems lead to medium leakage, triggering equipment failures and even safety accidents. According to statistical data on industrial equipment failures from Pexxon Rubber, a customized rubber manufacturer, failures of rubber seals in high-temperature environments account for over 62% of total failures in sealing systems. Therefore, in-depth analysis of their failure mechanisms and influencing factors holds significant practical significance for optimizing seal design and extending service life.
1. Failure Mechanisms Under High-Temperature Environments
The sealing performance of rubber seals relies on the material’s high elasticity and structural integrity. High-temperature environments undermine this sealing foundation by inducing thermochemical aging and physical property deterioration. The specific mechanisms can be categorized into two types:
(a) Seal Failure Caused by Thermochemical Aging
Thermal Oxidative Aging
Elevated temperatures accelerate the reaction between rubber molecular chains and oxygen—this effect is particularly pronounced in rubbers containing double bonds (e.g., nitrile butadiene rubber (NBR), natural rubber (NR)). Double bonds easily break to form free radicals, triggering chain degradation or cross-linking reactions. For example, when NBR is exposed to temperatures above 150°C, the bonding force between cyano groups (-CN) and the main chain weakens, leading to gradual decomposition into small-molecule compounds. This causes increased material hardness and decreased elastic modulus. Meanwhile, antioxidants added to the rubber volatilize rapidly or are consumed by oxidation at high temperatures. When the antioxidant content drops below the critical threshold of 0.5%, the aging rate increases exponentially—cracks gradually form on the seal surface, reducing the adhesion of the sealing interface.
Thermal Degradation Reactions
Different rubber materials exhibit distinct thermal degradation pathways. Although fluororubber (FKM) has excellent high-temperature resistance, its C-F bonds break under thermal excitation at temperatures above 280°C, releasing hydrogen fluoride gas and causing pores and embrittlement in the material. Silicone rubber (VMQ), on the other hand, tends to undergo main chain depolymerization at high temperatures, generating low-molecular-weight siloxanes. This results in a volume shrinkage rate of 5%–8%, widening the sealing clearance. Furthermore, if the system contains media such as oils, acids, or alkalis, high temperatures accelerate the erosion of rubber by these media. For instance, acidic substances formed by the oxidation of engine oil at 180°C react with cross-linking agents in the rubber, destroying its three-dimensional network structure and further exacerbating failure.
(b) Seal Failure Caused by Physical Property Deterioration
Increased Compressive Set
Rubber seals achieve sealing by generating contact pressure through pre-compression. At high temperatures, molecular chain movement intensifies, accelerating stress relaxation and significantly increasing compressive set. For example, when hydrogenated nitrile butadiene rubber (HNBR) operates continuously at 150°C for 1,000 hours, its compressive set rate rises from 15% (at room temperature) to over 45%. At this point, the seal cannot return to its original shape, and the contact pressure of the sealing surface falls below the medium pressure, causing leakage.
Deterioration of Elasticity and Strength
High temperatures shift the glass transition temperature (Tg) of rubber toward higher ranges. However, when the temperature exceeds the material’s thermal deformation temperature, the elastomer transitions to a viscous flow state, leading to a sharp decline in tensile strength and elongation at break. Experimental data shows that the tensile strength of NBR at 120°C decreases by 30% compared to room temperature, and its elongation at break at 180°C is less than 50% of that at room temperature. Under vibration or pressure fluctuations, the seal is prone to tearing and loses its sealing function.
2. Key Factors Influencing High-Temperature Failure
(a) Rubber Material Properties
Different rubbers exhibit significant differences in high-temperature resistance, which directly determines the critical failure temperature of seals:
NBR: Contains acrylonitrile groups, offering excellent oil resistance but a low high-temperature limit (long-term service temperature typically ≤ 120°C).
HNBR: Improves thermal stability by saturating double bonds, with a tolerance range of 150–180°C.
FKM: Relies on the high bond energy of C-F bonds, achieving a long-term service temperature of 200–260°C and making it the material of choice for high-temperature, strongly corrosive environments.
VMQ: Excels in low-temperature resistance but has poor long-term stability at high temperatures, with structural aging likely above 200°C.
Incorrect material selection in practical applications can cause seals to fail in a short period.
(b) Temperature Parameters and Exposure Duration
Temperature Intensity
When the ambient temperature exceeds the rubber’s critical thermal aging temperature, the failure rate follows a temperature coefficient effect—for every 10°C increase in temperature, the aging rate accelerates by 2–3 times. For example, the service life of FKM is approximately 1,000 hours at 260°C, but only about 100 hours at 290°C.
Temperature Fluctuations
Repeated thermal cycling (alternating heating and cooling) exacerbates fatigue aging of rubber. Thermal expansion and contraction create microcracks inside the material, which gradually expand into macroscopic defects, leading to a stepwise decline in sealing performance.
(c) Seal Structure and Installation Process
Compression Amount Design
Rubber expands thermally at high temperatures. If the initial compression exceeds 30%, stress relaxation accelerates, increasing compressive set. If the compression is less than 15%, thermal contraction at high temperatures directly exposes the sealing clearance. A reasonable compression amount—typically 20%–25%—must be calculated based on the rubber’s coefficient of thermal expansion (usually 10⁻⁴–10⁻³/°C) and the operating temperature range.
Installation Precision
Dimensional deviations of the seal groove, or scratches and distortion during installation, cause uneven local stress on the seal. Areas with concentrated stress are prone to aging and cracking at high temperatures. For example, in flange sealing of chemical pipelines, if the depth deviation of the seal groove exceeds 0.5mm, the local compression of the rubber gasket becomes insufficient, leading to preferential leakage from this area at high temperatures.
3. Analysis of Practical Failure Cases
Take the failure of an automotive engine crankshaft oil seal as an example. This seal operates in a complex environment with high temperatures (150–200°C), engine oil media, and high-speed crankshaft rotation (3,000–6,000 rpm), and is typically made of HNBR. After a certain vehicle model had traveled 80,000 kilometers, it developed an engine oil leakage fault. Disassembly revealed hardening, cracking, and a dark brown color on the seal lip.
Failure Cause Analysis
Thermal Oxidative Aging: Long-term high-load operation of the engine raised the oil seal’s operating temperature to 190°C—exceeding HNBR’s long-term service temperature of 180°C. This caused the rubber molecular chains to break, resulting in loss of elasticity in the seal lip.
Medium Erosion: Acidic substances formed by the oxidation of engine oil at high temperatures reacted with antioxidants in the oil seal, accelerating antioxidant consumption and increasing the aging rate by 40%.
Structural Design Flaw: The seal lip adopted a single sealing surface without an elastic compensation structure. When the compressive set reached 50% at high temperatures, the lip could no longer fit tightly against the crankshaft surface, forming a leakage channel.
4. Strategies to Improve the Reliability of Rubber Seals in High-Temperature Environments
(a) Material Optimization and Innovation
Composite Material Modification
High-temperature resistance of rubber is enhanced through processes such as blending and filling:
For example, FKM is compounded with polytetrafluoroethylene (PTFE). Leveraging PTFE’s high-temperature resistance (long-term service temperature of 260°C), the composite improves FKM’s wear resistance while retaining its elasticity.
Adding nano-silica to NBR reduces the thermal oxidative aging rate by 30% and decreases the compressive set rate by 15%.
High-Efficiency Antioxidant Systems
A “primary antioxidant + auxiliary antioxidant” compounding scheme is adopted. For instance, combining hindered phenol-based primary antioxidants with phosphite-based auxiliary antioxidants forms a synergistic antioxidant network inside the rubber. This extends the effective action time of antioxidants at high temperatures, prolonging the service life of NBR at 150°C from 500 hours to 1,000 hours.
(b) Structural and Process Improvements
Multi-Lip Seal Design
A “primary lip + secondary lip” dual-seal structure is used: the primary lip handles main sealing, while the secondary lip provides secondary protection. An elastic support ring is installed at the lip to offset compressive set at high temperatures and ensure stable contact pressure.
Precision Installation Control
Automated seal assembly equipment is used to control the dimensional deviation of the seal groove within ±0.1mm. During installation, a dedicated silicone-based grease (resistant to temperatures above 200°C) is applied to avoid scratches on the lip and reduce local stress concentration.
(c) Operating Environment Regulation
Thermal Insulation Protection
A heat insulation sleeve is installed around the seal, or a cooling system is used to control the temperature of the sealing area within the rubber’s tolerance limit. For example, reducing the temperature of the engine oil seal area from 190°C to 160°C can extend the seal’s service life by 2–3 times.
Medium Optimization
Corrosion inhibitors and antioxidants are added to high-temperature system media to reduce the formation of acidic substances. For instance, adding zinc dialkyldithiophosphate (ZDDP) to engine oil inhibits oil oxidation and reduces the erosion rate of rubber.
In essence, the degradation of the sealing performance of rubber seals in high-temperature environments results from the combined effects of thermochemical aging and physical property deterioration. The degree of failure is influenced by multiple factors, including material properties, temperature parameters, and structural design. In practical applications, a comprehensive strategy—encompassing accurate material selection, optimized structural design, and operating environment regulation—is required to delay failure. With the development of nanomaterials and intelligent sealing technologies, custom rubber manufacturer Pexxon Rubber is further developing high-temperature rubber seals with self-healing capabilities to fundamentally address high-temperature failure issues and ensure the safe and stable operation of industrial systems.