Silicone rubber sealing rings are widely used in the automotive, electronics, and medical fields due to their excellent heat resistance, cold resistance, and chemical stability. However, during long-term use, they inevitably age due to factors such as heat and oxygen, ultraviolet radiation, ozone, mechanical stress, and chemical media, leading to a decline in sealing performance. To accurately assess changes in sealing performance after aging, it is necessary to analyze changes in their physical properties, chemical structure, and sealing effect through systematic testing methods.
Visual inspection is an important means of initially judging the degree of aging of the sealing ring. Unaged silicone rubber sealing rings have a smooth surface, uniform color, and are elastic; after aging, they may show discoloration, hardening, cracks, or gaps. For example, silicone rubber may turn yellow and become brittle at high temperatures, and its surface may turn white and lose its luster after long-term contact with strong acids; after aging, moisture leakage marks are easily seen around the lid of an electric pressure cooker sealing ring, and inspection reveals numerous fine cracks on the surface. These signs of visual degradation directly reflect irreversible damage to the sealing ring, requiring further testing to confirm its sealing performance.
Physical performance testing is the core step in quantitatively assessing changes in sealing performance. Aging leads to increased hardness and decreased elasticity in silicone rubber. Hardness changes can be measured using a Shore hardness tester; an increase in hardness indicates material hardening and reduced sealing reliability. Tensile strength and elongation at break tests reflect the material's toughness and strength degradation: after aging, the tensile strength retention rate should typically be no less than 80% of the original value, and the elongation at break retention rate should be no less than 50%. If the rate of change exceeds this range, the seal ring is prone to breakage under stress, leading to seal failure. The compression set test assesses the seal ring's ability to recover its original shape after continuous compression; a lower value is better. If the compression set rate exceeds the standard upper limit after aging (e.g., 25% at standard temperature, 50% at high temperature), the sealing surface will not adhere properly, easily causing leakage.
Chemical stability testing addresses the media environment in which the seal ring comes into contact. Silicone rubber sealing rings may be exposed to oils, acids, alkalis, and other chemicals for extended periods, resulting in decreased corrosion resistance after aging. Liquid resistance tests are conducted by immersing the sealing ring in a specific medium and measuring its volume change, weight change, and performance retention. If the volume expansion rate is excessively high (e.g., nitrile rubber can expand by up to 300% after contact with kerosene), or if the tensile strength and elongation at break decrease significantly, it indicates that the material has been corroded, and the sealing performance cannot be guaranteed. Furthermore, chemical aging may also induce changes in the material's molecular structure. Fourier transform infrared spectroscopy (FTIR) analysis of chemical bond changes is necessary to confirm whether decrosslinking or degradation reactions have occurred, further verifying the chemical stability of the sealing ring.
Microstructural analysis can reveal the deeper impact of aging on sealing performance. Scanning electron microscopy (SEM) can observe the microscopic morphology of the sealing ring's surface and cross-section: before aging, the surface is smooth and the structure is dense; after aging, pores, cracks, or delamination may appear. For example, after high-temperature aging, the surface main chain and side chains of foamed silicone rubber are mainly degraded, while the internal main chain is mainly crosslinked and the side chains are mainly broken. This structural change leads to a decrease in tear strength and an increase in mass loss rate, directly affecting sealing performance. Microstructural analysis clarifies the aging mechanism, providing a basis for optimizing material formulations.
Actual sealing performance testing is the final step in verifying the performance of the sealing ring. By simulating actual operating conditions, the aged sealing ring is installed in equipment for pressure testing or leak detection. If the equipment operates at insufficient pressure, the float valve fails to rise properly (e.g., in an electric pressure cooker), or gas/liquid leakage is detected, it indicates that the sealing ring has failed. Furthermore, long-term operational testing can observe the performance degradation trend of the sealing ring under dynamic operating conditions, providing a reference for determining maintenance cycles.
After aging, silicone rubber sealing rings require a comprehensive evaluation of their sealing performance changes through a systematic approach including visual inspection, physical performance testing, chemical stability testing, microstructural analysis, and actual sealing performance testing. These testing methods not only identify potential failure modes of the sealing ring but also provide data support for material improvement, process optimization, and maintenance strategy development, thereby ensuring the safe operation and service life of equipment.
