In the automotive industry, seals are key components that ensure the normal operation of core components such as engines, transmissions, and fuel systems. Its performance directly affects the durability, fuel efficiency, and safety of the vehicle. As the core material of seals, high-strength oil resistant rubber compound needs to meet multiple requirements such as oil resistance, mechanical strength, dimensional stability, and environmental adaptability. This article takes the automotive specific rubber compound with a Shore hardness of 70 ± 5 as the starting point, and analyzes how it achieves a precise balance of "oil resistance strength hardness" through molecular structure regulation and process innovation from three dimensions: material formula design, performance optimization mechanism, and typical application scenarios, providing a reliable solution for automotive seals.
1、 Formula design: The "golden balance" between oil resistance and mechanical strength
1.1 Selection of Base Rubber: Oil Resistance Advantages of Acrylic Rubber (ACM)
Automotive seals are exposed to hydrocarbon media such as gasoline, diesel, and lubricants for a long time. Although traditional nitrile rubber (NBR) has excellent oil resistance, it is prone to aging and hardening in high temperature (>150 ℃) or sulfur-containing environments, leading to seal failure. Acrylic rubber (ACM), with its unique molecular structure, serves as the matrix for high-strength oil resistant adhesives
Polar main chain: The ester group (- COO -) in the ACM molecular chain is a strongly polar group that can form hydrogen bonds or dipole dipole interactions with oil molecules, significantly reducing the medium permeation rate. Experimental data shows that after soaking ACM in IRM903 standard oil for 70 hours, the volume change rate is only 8.2%, much lower than the 15.6% of NBR.
Heat stability: ACM has a glass transition temperature (Tg) of -20 ℃ to 0 ℃ and can maintain elasticity within the range of -30 ℃ to 175 ℃. After 168 hours of thermal aging at 150 ℃, the tensile strength retention rate of a certain domestic ACM rubber compound still reached 85%, while NBR was only 62%.
Low temperature performance optimization: By introducing ethyl acrylate (EA) monomer, the rigidity of the molecular chain can be reduced. For example, ACM copolymers with an EA content of 30% have a brittle temperature reduced from -15 ℃ to -25 ℃, meeting the low-temperature starting needs of automobiles in northern regions.
1.2 Reinforcement System: Synergistic Effect of Nano Calcium Carbonate and Carbon Black
To achieve the goal of a Shore hardness of 70 ± 5, it is necessary to regulate the hardness and strength of the rubber material through the use of reinforcing agents
Nano CaCO3: Nano CaCO3 with a particle size of 50-100nm can fill the gaps between rubber molecules and form physical cross-linking points. Experiments have shown that the addition of 30phr nano calcium carbonate to ACM adhesive increases the tensile strength from 8.5MPa to 12.2MPa, the hardness from 62 Shore A to 68 Shore A, and the wear resistance by 40%.
High wear resistant carbon black (N330): The surface active groups of carbon black can form chemical adsorption with rubber molecules, enhancing interfacial adhesion. When the dosage of N330 is 40phr, the hardness of the rubber material reaches 72 Shore A and the tensile strength is 15.8MPa, but the dosage needs to be controlled to avoid exceeding the hardness limit (>75 Shore A causing a decrease in elasticity).
Collaborative formula design: Using a composite system of "nano calcium carbonate+carbon black", precise control of hardness and strength can be achieved. For example, in a certain formula, the ratio of nano calcium carbonate to carbon black is 2:1 (by mass), the hardness of the rubber material is stable at 70 ± 2 Shore A, and the tensile strength reaches 14.5MPa. The comprehensive performance is better than that of a single reinforcement system.
1.3 Oil resistant additives: the "double insurance" of anti-aging agents and plasticizers
Antioxidant 4010NA: Antioxidant 4010NA containing N-isopropyl-N '- phenyl-phenylenediamine structure can effectively capture free radicals generated by oxidation in oil and delay rubber aging. In the 150 ℃ hot oil aging test, the addition of 2phr 4010NA adhesive increased the tensile strength retention rate from 72% to 88%.
Tri-n-octyl trimellitate (TOTM): As an oil resistant plasticizer, the three ester groups in TOTM molecules can form hydrogen bonds with rubber polar groups, reducing oil molecule penetration. The experiment showed that adding 10phr TOTM to the adhesive material and soaking it in IRM903 oil for 70 hours reduced the volume change rate from 12.5% to 6.8%, and the hardness change was only ± 1 Shore A.
2、 Performance Optimization: Process Breakthrough from Laboratory to Mass Production
2.1 Mixing process: precise control of temperature and time of the mixing machine
Mixing is a crucial step in determining the uniformity and performance stability of rubber materials
Segmented feeding method: First, add ACM raw rubber and nano calcium carbonate, and mix at 80 ℃ for 3 minutes; Add carbon black and anti-aging agent, heat up to 110 ℃ and mix for 5 minutes; Add vulcanizing agent and accelerator, and release the rubber at 100 ℃. This process can evenly disperse the reinforcing agent and avoid hardness fluctuations caused by agglomeration.
Mooney viscosity control: By adjusting the mixing time, the Mooney viscosity of the rubber material (ML1+4 @ 100 ℃) is controlled within the range of 50-70. Low Mooney viscosity (<40) can lead to excessive flowability and unstable hardness of the rubber material; If it is too high (>80), it increases the difficulty of processing and is prone to the risk of burning.
2.2 Sulfurization System: Innovation of "Double Sulfurization" of Peroxides and Sulfur
Traditional ACM rubber materials use peroxide vulcanization, but there are problems with slow vulcanization speed and high cost. A certain enterprise achieves a balance between performance and efficiency through a "peroxide+sulfur" composite sulfurization system:
Main vulcanizing agent: Peroxide DCP (dicarbonamide peroxide) provides a cross-linked network skeleton to ensure heat and oil resistance.
Auxiliary vulcanizing agent: Adding 0.5phr sulfur can accelerate the sulfurization reaction, shorten the sulfurization time (T90) from 12 minutes to 8 minutes, and increase the tensile strength by 2.3MPa.
Optimization of vulcanization curve: Through DSC (differential scanning calorimetry) analysis, the vulcanization temperature is determined to be 170 ℃, and the fluctuation range of rubber hardness is controlled within ± 1.5 Shore A.
2.3 Hardness uniformity control: "fine-tuning" technology for molding process
Compression molding is a hurdle that affects the dimensional accuracy and hardness uniformity of seals:
Mold temperature gradient design: The surface temperature of the mold cavity is controlled at 175 ± 2 ℃, and the center temperature is 170 ± 2 ℃ to avoid uneven vulcanization of the rubber material due to temperature differences.
Optimization of holding time: Through experiments, it has been determined that extending the holding time from 5 minutes to 8 minutes can reduce the standard deviation of seal hardness from 2.8 to 1.5 Shore A.
Online detection and sorting: Using a hardness tester to perform 100% testing on finished products, automatically sorting products with hardness exceeding the range of 70 ± 5 to ensure a factory pass rate of ≥ 99.5%.
3、 Typical application: Full scene coverage from engine to fuel system
3.1 Engine Oil Seal: Dual Challenges of High Temperature Oil Resistance and Low Friction
The engine oil seal needs to work for a long time at a high temperature of 150-180 ℃, while also bearing the frictional force generated by the rotation of the crankshaft:
Material scheme: Using ACM rubber compound with Shore hardness of 70, combined with polytetrafluoroethylene (PTFE) coating, the friction coefficient is reduced from 0.8 to 0.2, extending the oil seal life to 200000 kilometers (traditional NBR oil seal is only 100000 kilometers).
Structural optimization: Design a "wavy" lip structure to increase contact area and reduce local stress concentration. In bench testing, a certain model of oil seal operated continuously for 500 hours without leakage, with a hardness change of only ± 1 Shore A.
3.2 Fuel pipe joint sealing ring: balance between ethanol gasoline resistance and dimensional stability
With the popularity of ethanol gasoline (E10/E15), the sealing ring needs to resist the swelling of alcohol media:
Material modification: Introducing 10% fluororubber (FKM) blend into ACM to reduce the ethanol swelling rate from 12% to 3.5%. After soaking in E15 gasoline for 168 hours, the size change rate of a certain type of sealing ring is less than 0.5%, which meets the SAE J200 standard.
Compression deformation control: By optimizing the vulcanization system, the compression deformation rate (175 ℃× 70h) is reduced from 35% to 18%, ensuring that the sealing ring maintains elasticity for a long time.
3.3 Transmission Shaft Seal: Breakthrough in High Pressure Sealing and Dynamic Fatigue
The gearbox shaft seal needs to withstand 0.5-1.0MPa oil pressure while resisting the reciprocating motion of the shaft:
High strength design: Using ACM rubber material with a hardness of 72 Shore A, the tensile strength reaches 16.5MPa and can withstand an instantaneous pressure of 1.2MPa without breaking.
Dynamic fatigue test: Under the conditions of a shaft diameter of 50mm and a speed of 3000rpm, there was no leakage after continuous operation for 1000 hours, and the hardness change was only ± 2 Shore A, far exceeding the industry standard (500 hours).
Conclusion: Industrial upgrading from materials to systems
With the development of new energy vehicles and intelligent driving, automotive seals are evolving from a single function to integration and intelligence. For example, intelligent oil seals with integrated pressure sensors can monitor leakage risks in real time, while self-healing coating technology can automatically fill microcracks below 0.1mm by releasing repair agents through microcapsules. In the future, high-strength oil resistant rubber compounds will further integrate cutting-edge technologies such as nanomaterials and 3D printing to achieve precise manufacturing through "on-demand customization", providing more efficient and reliable sealing solutions for the automotive industry.