1. Tensile property of valve stem sealing material

Tensile property is the first property to be considered for sealing materials, including tensile strength, constant tensile stress, elongation at break and permanent deformation at break. The tensile strength is the maximum stress at which the specimen is stretched to fracture. The constant elongation stress (constant elongation modulus) is the stress reached at the specified elongation. Elongation refers to the deformation caused by the specimen subjected to the specified tensile force, which is the ratio of the increment of elongation to the original length. Elongation at break is the elongation at break of the sample. The tensile permanent deformation is the residual deformation between the marks after the tensile fracture of the specimen.

2. Hardness of valve stem sealing material

The hardness of the valve stem sealing material indicates the ability of the sealing material to resist external force and is also one of the basic properties of the sealing material. The hardness of the material is related to other properties to a certain extent. The higher the hardness, the greater the strength, the smaller the elongation, the better the wear resistance, and the worse the low temperature resistance.

3. Compression performance of valve stem sealing material

The rubber seal of the valve is usually in a compressed state. Due to the viscoelasticity of the rubber material, the pressure will decrease with time when it is compressed, which is manifested as the relaxation of the compressive stress; After the pressure is removed, the original shape cannot be restored, which is manifested as compression permanent deformation. This phenomenon is more obvious in high temperature and oil medium, and the performance is directly related to the durability of sealing ability of sealing products.

4. Low temperature performance of valve stem sealing material

As an indicator for measuring the low-temperature characteristics of rubber seals, two methods for testing the low-temperature performance are introduced below: low temperature retraction temperature: stretch the sealing material to a certain length, then fix it, quickly cool it to below the freezing temperature, release the test piece after reaching equilibrium, and raise the temperature at a certain speed. The temperatures at 10%, 30%, 50% and 70% retraction of the recording pattern are respectively represented by TR10, TR30, TR50 and TR70. The material standard is based on TR10, which is related to the brittleness temperature of rubber. Low temperature flexibility: after the sample is frozen at the specified low temperature for the specified time, it is bent back and forth according to the specified angle, and the sealing ability of the seal after the repeated action of dynamic load at the low temperature is investigated.

5. Oil or medium resistance of valve stem sealing material

In addition to contacting petroleum base, diesters and silicic grease oil, valve stem sealing materials sometimes contact acid, alkali and other corrosive media in the chemical industry. In addition to corrosion in these media, it will also lead to expansion, strength reduction and hardness reduction at high temperature; At the same time, plasticizer and soluble substances in the sealing material are extracted, resulting in mass reduction, volume reduction and leakage. Generally, at a certain temperature, after being immersed in the medium for a certain time, the changes of its mass, volume, strength, elongation and hardness are measured to evaluate the oil resistance or medium resistance of the sealing material.

6. Aging resistance of valve stem sealing material

The valve stem sealing material will deteriorate after being affected by oxygen, ozone, heat, light, moisture and mechanical stress, which is called aging of sealing material. Aging resistance (also referred to as weather resistance) can be expressed by the changes in strength, elongation and hardness of the pattern after aging. The smaller the change rate, the better the aging resistance.

Note: weather resistance refers to a series of aging phenomena such as fading, discoloration, cracking, pulverization and strength reduction of plastic products due to the influence of external conditions such as sunlight, temperature change, wind and rain. Among them, ultraviolet radiation is the key factor to promote the aging of plastic.