When we choose rubber seals, the first properties to consider mainly include its tensile strength, constant elongation stress, elongation at break, deformation at break, and stress-strain curve. We collectively call this tensile strength. The so-called tensile strength is the maximum tensile stress when the specimen is stretched to fracture. Constant elongation stress (constant elongation modulus) is the stress (modulus) achieved at specified elongation. Elongation is the deformation of the specimen caused by tensile stress, expressed as a percentage of the ratio of the increment of elongation to the original length. Elongation at break is the elongation of the specimen at break. The tear deformation is the residual deformation of the standard distance part of the specimen after tensile fracture.
Then, we consider the basic properties of rubber seals - hardness. The so-called hardness is the ability of rubber to resist the intrusion 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, relatively speaking, the greater the strength, the smaller the elongation, the better the wear resistance, and the poor 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 state of compression, so we must consider the compression performance of rubber seals. Due to the viscoelasticity of rubber, the compressive stress will decrease with time after the rubber is compressed, which is manifested as compressive stress relaxation. After the pressure is removed, the original shape cannot be restored, which is manifested as compression deformation. In high temperature and oil medium, these phenomena are more significant. They will affect the sealing performance of the seal and are one of the important properties of the sealant used in the seal.
Our most commonly used is the brittleness temperature, which refers to the highest temperature at which the sample breaks when subjected to a certain impact force at low temperature, which can be used to compare the low temperature properties of different rubber materials. However, because the working state of rubber parts is different from the test conditions, the brittle temperature of rubber does not mean the lowest working temperature of rubber parts, especially in the oil medium. Secondly, it is the low temperature retraction temperature, which is to stretch the test piece to a certain length at room temperature, and then fix it, cool it quickly below the freezing temperature, release the test piece after reaching the temperature balance, and warm up at a certain speed, record the temperature of the test piece when it is shrunk by 10%, 30%, 50% and 70%, respectively, expressed in TR10, TR30, TR50 and TR70. In the material standard, TR10 is generally used as an indicator, which is close to the brittleness temperature of rubber. Another way to indicate 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, and then frozen at the specified low temperature, and then the load is removed to restore it at low temperature, and the ratio of the recovery amount and the compression amount is called the compression cold resistance coefficient. The greater the coefficient, the better the cold resistance of the rubber.
The living environment of rubber seals is harsh, most of which are very survival in fuel, lubricating oil, hydraulic oil and other systems, so they often come into contact with various oils, and naturally they need to have oil resistance. Rubber in the oil medium, especially at higher temperatures, will lead to expansion, softening and reduce strength, hardness, while the plasticizer or soluble substance in the rubber may be leached by the oil, resulting in weight reduction, volume reduction, causing leakage. Therefore, the oil resistance of rubber is an important property of the rubber material working in the oil medium. Generally, after soaking in oil at a certain temperature for a number of time, the weight changes, volume changes, and changes in strength, elongation and hardness are measured. Sometimes it can also be expressed by the oil resistance coefficient, that is, the ratio of the strength or elongation after soaking in the medium to the original strength or elongation.
The above is a simple introduction to the main properties that rubber seals should consider.