Different hardness levels of O-ring materials are designed based on the pressure in the sealing system. High-pressure applications require high-hardness materials, while low-pressure applications require medium- or low-hardness materials.

To improve product quality and lifespan, national standards have standardized the hardness of rubber materials. For instance, nitrile rubber used in hydraulic systems has hardness levels of 60°±5°, 70°±5°, 80°+5°, and 88°±5° (Shore A) to meet various pressure requirements. Typically, hardness levels of 85–90 are suitable for O-rings operating at pressures between 30 and 60 MPa.

In addition to hardness, O-rings used for sealing purposes must exhibit good overall performance with minimal permanent compression. Especially for high-hardness materials, good processing properties are essential.

Designing high-hardness materials often involves a significant amount of carbon black filler. While this enhances the heat resistance of the rubber, excessive carbon black can make the vulcanization process challenging, leading to increased heat generation and potential scorching of the material.

Moreover, a high carbon black content can reduce the elasticity of the rubber and increase permanent compression. Therefore, the selection and coordination of the vulcanization and filler systems are crucial in rubber formulation.

For nitrile rubber, low-sulfur (below 0.5 parts) high accelerators, sulfur-free, and peroxide vulcanization systems are commonly used to improve heat resistance, reduce permanent compression set, and enhance overall performance.

When designing formulations for medium- and low-hardness materials, the principle of simplifying the rubber formulations as much as possible is followed. By using a minimal basic formulation and adjusting the amount of carbon black and plasticizers, formulations for medium and low-hardness levels can be developed to streamline production management while meeting usage requirements.