We choose the rubber seal, first consider the properties including tensile strength, the tensile stress and elongation, elongation at break, permanent deformation, and the stress-strain curve. We collectively refer to tensile strength. Tensile strength is the maximum tensile stress when the specimen is stretched to fracture. The elongation stress (modulus of elongation) is the stress (modulus) that is reached at the prescribed elongation. Elongation is the deformation caused by tensile stress, and is expressed by the percentage of elongation increment to the ratio of the original length. The elongation is tensile elongation of the specimen. Permanent deformation is the tensile fracture after the gauge part of the residual deformation.
Then, we consider the basic properties of rubber seals - hardness. Hardness refers to the ability of rubber to resist the invasion of external pressure. The hardness of rubber is related to some other properties to a certain extent. For example, the rubber hardness is high, relatively speaking, the strength of large, small elongation, good wear resistance, and low temperature resistant performance is poor. High hardness rubber can resist extrusion failure under high pressure. Therefore, the proper hardness should be selected according to the working characteristics of the parts.
We know that rubber seals are often in compression, so we have to consider the compression properties of rubber seals. Because of the viscoelasticity of rubber, the compressive stress of rubber decreases with time, which shows compression stress relaxation. After removing the pressure, the original shape can not be restored, and the compression permanent deformation appears. These phenomena are more pronounced in high temperature and oil media. They affect the sealing performance of the seal, and it is one of the important properties of the sealing compound.
Brittleness temperature is the most commonly used, which means the highest temperature of rupture occurs when the sample is impacted by low temperature. It can be used to compare the low temperature properties of different rubber compounds. However, because of the different working conditions of rubber parts, the brittleness temperature of rubber does not indicate the minimum working temperature of rubber parts, especially in oil medium. Secondly, low temperature retraction temperature is at room temperature tensile test piece to a certain length, then fixed, rapidly cooled to a temperature below the freezing temperature, achieve the balance after the release of specimen, and to a certain rate of heating, the specimen records 10%, 30%, 50% back and 70% temperature, respectively represented by TR10 TR50, TR30, and TR70. Generally, TR10 is used as an index in material standards, which is close to the brittleness temperature of rubber. Another way to indicate low temperature properties of rubber is to determine its coefficient of cold resistance. Generally, the sample is compressed at room temperature to a certain amount of deformation, and then frozen at the specified low temperature, then unloaded to make it recover at low temperature. The ratio of recovery and compression is called compression cold resistance coefficient. The greater the coefficient, the better the cold resistance of rubber.
Rubber seals have poor living environment, most of them are in the fuel, lubricating oil, hydraulic oil and other systems, so often contact with a variety of oil, naturally it also needs to have oil resistance. Rubber in oil medium, especially at higher temperatures will lead to expansion, softening and reducing strength, hardness, and rubber plasticizer or soluble substances may be oil extraction, lead to weight loss, the volume is reduced, cause leakage. Therefore, the oil resistance of rubber is an important performance of the rubber compound in the oil medium. Generally, the change of weight, volume, strength, elongation and hardness are determined after soaking for several times at a certain temperature. Sometimes it can also be expressed as oil resistance coefficient, that is, the ratio of strength or elongation to original strength or elongation after immersion in the medium.