The compression set of vulcanized rubber refers to the slight deformation retained by the vulcanized rubber material after being subjected to constant pressure under specific conditions, once the pressure is removed. It serves as a key performance indicator reflecting the elastic recovery ability and durability of vulcanized rubber.
Key Influencing Factors
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Rubber type: Different rubber varieties have inherent differences in their compression set performance. For example, silicone rubber and fluororubber exhibit good resistance to high-temperature compression set, making them suitable for high-temperature environments; whereas natural rubber may undergo significant compression set under long-term high-temperature conditions.
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Vulcanization system: The types and dosages of vulcanizing agents and accelerators, as well as the degree of vulcanization (crosslink density), have a significant impact on compression set. An appropriate crosslink density (neither too low nor too high) helps reduce compression set—insufficient crosslinking leads to poor elastic recovery, while over-vulcanization may cause rubber to become brittle and reduce flexibility. Unvulcanized rubber, however, is not affected by this; different types of rubber are all derived from unvulcanized rubber with the addition of fillers and additives.
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Fillers and additives: Reinforcing fillers (such as carbon black and silica), when used properly, can improve the structural stability of rubber, thereby reducing compression set. However, excessive amounts of softeners or plasticizers may migrate over time, leading to increased residual deformation.
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Service environment: Higher temperatures accelerate molecular chain movement and relaxation in rubber, which can easily increase compression set; prolonged pressure or exposure to corrosive media (such as oils, solvents) may also exacerbate permanent deformation.
Testing Methods
Common standards for measuring compression set include ASTM D395 (US) and ISO 815 (international). The general process is as follows:
- Prepare the rubber into standard specimens (e.g., cylindrical or disc-shaped).
- Compress the specimen to a specified ratio (usually 25%) using a fixture, then place it in an oven or medium at a set temperature (e.g., 70°C, 100°C, 150°C) for a fixed duration (e.g., 22 hours, 70 hours).
- After removing the fixture, allow the specimen to recover at room temperature for a specified time, then measure its residual thickness.
- The formula for calculating compression set is: [(Initial compressed thickness - Residual thickness after recovery) / Initial compressed thickness] × 100%.
Conclusion
Compression set is directly related to the service life and reliability of rubber products. For example:
- In sealing components (such as gaskets, O-rings), low compression set ensures long-term effective sealing; excessive deformation may cause leakage due to failure to rebound.
- In shock-absorbing components (such as buffers, bushings), excessive residual deformation reduces shock absorption performance, affecting product safety.
Therefore, controlling compression set through formulation design (e.g., optimizing the vulcanization system, selecting appropriate rubber types) and process adjustments is crucial for ensuring the performance of vulcanized rubber products.