Super detailed aging reason of rubber seal!
In the petrochemical industry, sealing is very important. Many leakage accidents are caused by seal failure, which is closely related to the aging of rubber seal. Let's take a look at the causes of seal aging with Xiaobian!
First, look at the aging of seals.
① Soft and sticky seals: thermal oxidation of natural rubber and aging of chlorohydrin rubber.
② The seal becomes hard and brittle: the aging of cis-1,4-polybutadiene rubber, NBR and SBR.
③ Seal crack: ozone aging of unsaturated rubber, photooxidation aging of most rubber, but the crack shape is different.
④ Moldy seal: the rubber is biologically aged.
Rubber seal
As a result of the above reasons, the physical and chemical properties of the seals are directly changed. Changes in physical and chemical properties of seals: changes in specific gravity, thermal conductivity, glass transition temperature, melting point, refractive index, solubility, swelling, rheology, molecular weight, molecular weight distribution, cold resistance, air permeability, water permeability, light transmittance and other properties.
Changes of physical and mechanical properties of seals: tensile strength, elongation, shear strength, fatigue strength, elasticity, wear resistance are decreased.
Change of electrical properties of seals: change of electrical properties such as insulation resistance, dielectric constant, dielectric loss, breakdown voltage, and decrease of electrical insulation.
What are the causes of seal aging?
The function of oxygen: oxygen reacts with rubber molecules in the form of free radical chain, and the molecular chain breaks or over crosslinks, resulting in the change of properties of silicone rubber products. Oxidation is one of the important reasons for rubber aging.
Thermal effect: increasing temperature can cause thermal cracking or thermal crosslinking of silicone rubber products. But the basic function of heat is activation. Increasing oxygen diffusion rate and activating oxidation reaction can accelerate the oxidation reaction rate of rubber, which is a common aging phenomenon - hot oxygen aging.
The function of water: there are two aspects of the function of water: silicone rubber products are easy to be damaged when they are exposed to rain or immersed in water in humid air. This is because the water-soluble substances in rubber and water-soluble groups are extracted and dissolved by water. Especially under the alternating action of water immersion and atmospheric exposure, the damage of rubber seal will be accelerated. However, in some cases, moisture does not damage rubber, and even delay aging.
Function of ozone: the chemical active oxygen of ozone is much higher and more destructive. It also breaks the molecular chain, but the effect of ozone on rubber varies with whether rubber deforms or not. When it is used as deformed rubber (unsaturated rubber), cracks will appear in the direction of stress, which is called "stress cracking". When it is used as non deformed rubber, only oxide film will be formed on the surface without cracking.
The reason of light: the shorter the light wave, the greater the energy. The damage to rubber is the high energy ultraviolet. UV can not only directly cause the fracture and cross-linking of rubber molecular chain, but also produce free radicals due to the absorption of light energy, which can initiate and accelerate the reaction process of oxidation chain. Light also plays a role in heating the seal, but unlike heat, light will make the rubber seal with high rubber content appear network crack, which is called "light external crack".
Mechanical stress effect: due to the viscosity of polymer, the sealing ring is in the deformation cycle, its relaxation process is too late to complete, and then enter the next deformation cycle, so that the internal deformation of rubber sealing ring is increasing. When the stress is large, the molecular chain will break and generate free radicals, which will lead to the oxidation chain reaction of rubber sealing molecules; when the rubber sealing ring is deformed repeatedly, its mechanical stress will weaken the atomic valence force in the molecular chain of rubber sealing elements, thus reducing its activation energy of oxidation reaction. The oxidation cracking reaction of rubber sealing ring was accelerated. The reduction of oxidation activation energy is the end of the process that mechanical energy is converted into chemical energy in the fatigue process; when rubber seal is deformed repeatedly, it will produce hysteresis phenomenon, which will cause internal friction, so that the internal heat of rubber material is generated, thus accelerating the oxidation chain reaction of rubber seal.
Other functions: there are other electrochemical corrosion, radiation and biological effects.
