Anti-aging Modification Technology and Long-Life Service Guarantee Solution for Rail Track Pads
What are the main aging types and influencing factors of under-rail pads?
The main aging types of under-rail pads include thermal-oxidative aging, ultraviolet aging and fatigue aging. These three types of aging interact with each other and jointly accelerate the performance attenuation of the pads. Thermal-oxidative aging refers to the oxidation reaction between rubber molecular chains and oxygen in high-temperature environments, leading to molecular chain scission or crosslinking, which makes the pads hard and brittle. The track temperature can reach above 60℃ in summer high-temperature periods, which will greatly accelerate the speed of thermal-oxidative aging. Ultraviolet aging is caused by ultraviolet radiation in sunlight. Ultraviolet rays have high energy, which will destroy the chemical bonds of rubber molecules, resulting in chalking and cracking on the pad surface. Especially in open lines without shelter, ultraviolet aging is more serious. Fatigue aging is that micro-cracks are generated inside the pad under the repeated action of train loads, and the continuous expansion of cracks leads to structural damage of the pad. The load impact of heavy-haul lines is large, and the speed of fatigue aging is much faster than that of ordinary-speed lines. Factors affecting aging also include environmental humidity, acid-base media, etc. Humid environments will promote oxidation reactions, and acid-base media such as acid rain will corrode the pad surface and further accelerate the aging process. The combined effect of these factors will shorten the service life of ordinary under-rail pads to 5-8 years.
What are the core additive types and action mechanisms of anti-aging modification for under-rail pads?
The core additive types of anti-aging modification for under-rail pads include antioxidants, ultraviolet stabilizers and anti-aging agents. These three types of additives work synergistically to comprehensively delay the aging process of the pads. The antioxidants mainly select a compound system of hindered phenols and phosphites. Hindered phenolic antioxidants can capture free radicals generated by oxidation reactions, interrupt the oxidation chain reaction, and prevent the scission of rubber molecular chains; phosphite antioxidants can decompose peroxides generated by oxidation reactions. The combined use of the two can increase the thermal-oxidative aging life by more than 3 times. Ultraviolet stabilizers are divided into ultraviolet absorbers and quenchers. Ultraviolet absorbers can absorb the energy of ultraviolet rays and convert it into heat energy for dissipation, avoiding ultraviolet rays from damaging the rubber molecular structure; quenchers can quench the excited state of rubber molecules excited by ultraviolet rays to the ground state, reducing molecular chain damage. The combined use of the two can increase the resistance to ultraviolet aging by 40%. The anti-aging agents select naphthylamine and quinoline products, which can inhibit the fatigue aging of rubber. Their molecules can adsorb on the active sites of rubber molecules, prevent the initiation and expansion of micro-cracks, and significantly improve the fatigue resistance of the pads. The addition amount of additives must be strictly controlled: the addition amount of antioxidants is 1.5%-2.5%, the addition amount of ultraviolet stabilizers is 2%-3%, and the addition amount of anti-aging agents is 1%-1.5%. Excessive addition will lead to abnormal elastic modulus of the pads and affect the vibration reduction effect.
What are the process optimization measures for anti-aging modification of under-rail pads?
The process optimization measures for anti-aging modification of under-rail pads mainly focus on three key links: mixing process, vulcanization process and post-treatment process. By accurately controlling process parameters, uniform dispersion of additives is ensured and the modification effect is improved. The mixing process adopts the segmented mixing method. First, put the rubber base material into the internal mixer and mix at 80-90℃ for 3-5 minutes to fully plasticize the base material; then add antioxidants and anti-aging agents, mix at 100-110℃ for 5-7 minutes to ensure uniform dispersion of additives; finally add ultraviolet stabilizers, mix at 90-100℃ for 2-3 minutes to avoid decomposition and failure of stabilizers due to high temperature. The vulcanization process adopts the gradient vulcanization method. The first-stage vulcanization temperature is 140-145℃ for 8-10 minutes to initially form the pad; the second-stage vulcanization temperature is 120-130℃ for 12-15 hours to further optimize the crosslinking structure of the rubber and improve the uniformity of crosslinking density. The crosslinking density is controlled at about 1.5×10⁻⁴mol/cm³, which can balance elasticity and anti-aging performance. The post-treatment process includes surface spraying of protective layer, spraying a polyurethane protective layer with a thickness of 5-10μm on the pad surface, which can isolate ultraviolet rays and oxygen and further improve weather resistance; at the same time, stress relief treatment is carried out, placing the vulcanized pad in a 50℃ environment for 24 hours to eliminate internal residual stress and prevent cracking caused by stress release during use.
What are the differentiated requirements for anti-aging performance of under-rail pads in different climatic regions?
The environmental differences in different climatic regions are large, and the requirements for anti-aging performance of under-rail pads also show significant differentiated characteristics, with the core being to match the dominant aging factors of the region. The dominant aging factors in high-temperature and arid regions (such as the northwest inland) are thermal-oxidative aging and strong ultraviolet radiation. It is required that the thermal-oxidative aging life of the pad is ≥15 years, and the elastic modulus change rate after ultraviolet aging is ≤10%. It is necessary to focus on strengthening the addition amount of antioxidants and ultraviolet stabilizers, and adopt dark protective coatings to reduce ultraviolet absorption. The dominant aging factors in high-humidity and rainy regions (such as the south China coast) are damp-heat aging and mold erosion. It is required that the damp-heat aging resistance of the pad is ≥1000 hours, and the mold resistance grade reaches grade 0. It is necessary to add mold inhibitors and optimize the vulcanization process to improve crosslinking density and prevent moisture from penetrating into the rubber interior. The dominant aging factors in alpine regions (such as the three northeastern provinces) are low-temperature embrittlement and freeze-thaw cycle aging. It is required that the impact toughness of the pad at -40℃ is ≥15kJ/m², and there is no cracking after 100 freeze-thaw cycles (-40℃～20℃). It is necessary to select cold-resistant rubber base materials and add plasticizers to improve low-temperature elasticity. The dominant aging factors in plateau regions with strong ultraviolet radiation (such as the Qinghai-Tibet Plateau) are strong ultraviolet radiation and low-pressure aging. It is required that the ultraviolet shielding rate of the pad is ≥80%, and the thermal-oxidative aging rate in low-pressure environments is reduced by 50%. It is necessary to adopt high-content ultraviolet stabilizers and dense crosslinking structures to resist the dual effects of strong ultraviolet radiation and low pressure.
What are the detection methods and acceptance standards for anti-aging performance of under-rail pads?
The detection methods for anti-aging performance of under-rail pads include three categories: accelerated aging test, natural exposure test and mechanical property test. The acceptance standards must comply with TB/T 2626-2018 railway under-rail pad standards. The accelerated aging test is the core detection method, including thermal-oxidative accelerated aging and ultraviolet accelerated aging. Thermal-oxidative accelerated aging is carried out in accordance with GB/T 3512 standard, aging in a 100℃ hot air environment for 72 hours, testing the change rate of tensile strength and elongation at break before and after aging, requiring the change rate ≤20%; ultraviolet accelerated aging is carried out in accordance with GB/T 16422.3 standard, aging under ultraviolet lamp irradiation for 1000 hours, requiring no chalking or cracking on the surface, and the elastic modulus change rate ≤15%. The natural exposure test selects exposure sites in typical climatic regions, exposes the pad samples for 2 years, and regularly detects performance changes, requiring the vibration reduction performance retention rate after 2 years ≥80%. Mechanical property tests include hardness test, tensile test and impact test after aging, with Shore hardness change ≤5 degrees, tensile strength retention rate ≥80%, and impact toughness at -40℃ ≥10kJ/m². The acceptance standard stipulates that the qualification rate of anti-aging performance detection must reach 100%, and the sampling ratio is 3 groups of samples per batch, 5 pieces per group. If one piece fails to meet the standard, the anti-aging performance of the batch of pads is judged as unqualified and prohibited from being put into use.