Study and analysis of compression set of EPDM rubber with different hardness
Ethylene-propylene-diene monomer (EPDM) rubber has been widely used in automobiles, construction, electrical and other fields due to its excellent weather resistance, ozone resistance, heat resistance, and good electrical insulation properties. The hardness of EPDM rubber is an essential factor in determining its performance, and EPDM rubber of different hardness is different in terms of mechanical properties, aging resistance, chemical corrosion resistance, etc., especially in compression set (CS), EPDM rubber of different hardness shows obvious differences. Therefore, it is of great significance to study the compression set characteristics of EPDM rubber with different hardnesses to optimize the formulation design and improve the service life of the material.
Definition and influencing factors of compression set
Compression set (CS) refers to the deformation of a rubber material that cannot be fully recovered after unloading under certain conditions of temperature, time, and compression ratio, after being subjected to a certain pressure, usually expressed as a percentage. The CS of EPDM rubber is mainly affected by the following factors:
Formulation Factors
Molecular structure of EPDM: ethylene/propylene ratio (E/P ratio) and diene monomer content have a direct impact on the crosslinking density and elastic recovery ability of EPDM. In general, EPDMs with high ethylene content have stronger rigidity and lower CS after crosslinking, while EPDMs with high propylene content have better flexibility but higher CS.
Cross-linked system: sulfur vulcanization system and peroxide vulcanization system have a greater impact on CS. Sulfur-sulfurized EPDMs generally have higher elasticity but larger CS, while peroxide-sulfide EPDMs have lower CS due to stable cross-linking bonds.
Filling system: The amount and type of fillers such as carbon black and silica affect the hardness and recovery performance of rubber. Increasing filler content can increase hardness, but may lead to an increase in CS.
Plasticizers: The addition of paraffin oil, naphthenic oil, and ester plasticizers will reduce the hardness of rubber and improve the flexibility of the material, but excessive addition will reduce the cross-linking density and lead to the increase of CS.
Physical factors
Hardness (Shore A): The hardness of rubber directly affects its compression recovery ability. In general, EPDM rubbers with lower hardness are difficult to recover after compression set, with larger CS, while higher hardness EPDM rubbers have smaller CS.
Temperature: High temperature accelerates the relaxation and creep of rubber, causing CS to increase, so CS is a key performance indicator in high-temperature application scenarios.
Time: As the compression time increases, the cross-linking network of the rubber is subjected to greater stress relaxation, resulting in an increase in CS.
Environmental factors: humidity, oxidants, solvents, etc. have an important impact on the CS of EPDM, especially the phenomenon that the CS of peroxide sulfide EPDM may increase in high temperature and humidity environment.
CS characteristics of EPDM rubbers of different hardnesses
Low hardness EPDM (40-50 Shore A)
Low-hardness EPDMs typically contain more plasticizers and have a lower crosslinking density, exhibiting higher flexibility and low modulus. The compression set characteristics of these EPDM rubbers are as follows:
High CS: Low hardness EPDM is prone to plastic deformation under compressive stress, and the recovery ability is poor, so CS is usually 30-50%.
The effect of temperature is significant: above 100°C, the CS of low-hardness EPDM increases dramatically, often by more than 50% or even as high as 70%.
Application scenario: It is suitable for applications with low requirements for compression recovery, such as automotive weatherstrips, low-load gaskets, etc.
Medium Hardness EPDM (55-65 Shore A)
The formula of medium-hardness EPDM rubber is more balanced, with good resilience and compression resistance, and its CS performance is as follows:
CS medium: generally between 20-35%, compared with low hardness EPDM, there is a large improvement.
The cross-linked system has a significant impact: the CS can be reduced to 15-25% for EPDM with peroxide vulcanization, while the CS for EPDM in sulfur vulcanization system may be in the range of 25-35%.
Good temperature stability: below 120°C, the CS changes less and is suitable for automotive sealing systems, industrial seals, and other occasions that require medium compression recovery.
High hardness EPDM (70-85 Shore A)
High-hardness EPDM rubber has a high filler content and high crosslink density, so its CS characteristics are as follows:
Low CS: Typically in the 10-25% range, especially peroxide sulfide EPDMs with high crosslinking densities, can reduce CS to 10-15%.
Excellent high-temperature stability: It can maintain a relatively low CS (<30%) in an environment of 150°C, which is suitable for long-term use at high temperatures.
Applications: It is suitable for high-pressure seals, high-temperature and chemical-resistant industrial sealing systems, etc.
