Selection factors of O-ring rubber seal compound

When we choose a rubber seal, the first properties to consider mainly include its tensile strength, tensile stress, elongation, elongation at break, permanent deformation at break, and stress-strain curve. We collectively call it tensile strength. The so-called tensile strength is the maximum tensile stress when the sample is stretched to fracture. The constant elongation stress (modulus at constant elongation) is the stress (modulus) attained at a specified elongation. Elongation is the deformation of the specimen caused by tensile stress, expressed as a percentage of the ratio of the elongation increment to the original length. The elongation at break is the elongation of the specimen at break. The tear set is the residual deformation of the gauge length portion of the specimen after tensile fracture.

Then, we consider the basic property of rubber seals - hardness. The so-called hardness is the ability of rubber to resist the invasion of external pressure. The hardness of rubber is related to some other properties to some extent. For example, the higher the hardness of the rubber compound, the higher the strength, the lower the elongation, the better the wear resistance, and the poorer the low temperature resistance. High hardness rubber can resist extrusion damage under high pressure. Therefore, the appropriate hardness should be selected according to the working characteristics of the parts.

We know that rubber seals are often in a compressed state, so we must consider the compression performance of rubber seals. Due to the viscoelasticity of rubber, after the rubber is compressed, the compressive stress will decrease with time, which is manifested as compressive stress relaxation; after the pressure is removed, the original shape cannot be restored, which is manifested as compression permanent deformation. These phenomena are more pronounced in high temperature and oil media. They will affect the sealing performance of the seal and are one of the important properties of the rubber compound for the seal.

The most commonly used is the brittleness temperature, which refers to the highest temperature at which the sample ruptures when subjected to a certain impact force at a low temperature, which can be used to compare the low temperature properties of different rubber compounds. However, because the working state of rubber parts is different from the test conditions, the brittleness temperature of rubber does not indicate the minimum working temperature of rubber parts, especially in oil medium. The second is the low temperature retraction temperature, which is to stretch the test piece to a certain length at room temperature, then fix it, quickly cool it to below the freezing temperature, release the test piece after reaching the temperature balance, and heat up at a certain speed, record the return of the test piece. The temperature at 10 percent , 30 percent , 50 percent and 70 percent shrinkage is expressed as TR10, TR30, TR50 and TR70, respectively. In material standards, TR10 is generally used as an index, which is close to the brittleness temperature of rubber. Another way to express the low temperature performance of rubber is to measure its cold resistance coefficient. Generally, the sample is compressed to a certain amount of deformation at room temperature, then frozen at a specified low temperature, and then unloaded to recover at a low temperature. The ratio of the recovery amount to the compression amount is called the compression cold resistance coefficient. The larger the coefficient, the better the cold resistance of the rubber.

The living environment of rubber seals is harsh, and most of them live in systems such as fuel oil, lubricating oil, hydraulic oil, etc., so they often come into contact with various oils, and naturally they need to have oil resistance. Rubber in oily medium, especially at higher temperature, will cause expansion, softening and decrease of strength and hardness, while plasticizers or soluble substances in rubber may be leached by oil, resulting in weight loss, volume reduction, and leakage. Therefore, the oil resistance of rubber is an important property of the rubber compound working in oil medium. Generally, the weight change, volume change, and the change of strength, elongation and hardness are measured after soaking in oil for several times at a certain temperature. Sometimes it can also be expressed by the oil resistance coefficient, that is, the ratio of the strength or elongation after immersion in the medium to the original strength or elongation.