The importance of sealing technology and the performance indicators for evaluating the quality of sealing materials:

Thread Content

The sealing of fluids (gases, liquids) is a universal technology essential in various industrial fields. Not only industries such as construction, petrochemicals, shipbuilding, machinery manufacturing, energy, transportation, and environmental protection rely on sealing technology, but also cutting-edge industries such as aviation and aerospace are closely related to it. Sealing technology is applied in a wide range of fields; wherever there are devices involved in the storage, transportation, or conversion of fluids, sealing issues arise. The importance of sealing technology: The consequences of sealing failure are extremely serious. In mild cases, it leads to waste of energy and resources through leaks; in severe cases, it can result in loss of control over the system, and even cause fires, explosions, environmental pollution, and other hazards that threaten human safety! According to preliminary statistics, more than one-third of quality incidents involving mechanical equipment and weapons are caused by seal failures; among these, those with serious consequences include the destruction of the Space Shuttle Challenger and loss of life due to a failure in its O-rings ； Fuel tank leak on Russian Soyuz spacecraft kills several astronauts ； After China’s Dongfanghong-3 satellite was launched, pipeline leaks prevented it from functioning properly. This is why mechanical manufacturing companies today spare no expense in choosing high-quality sealing products. Development of sealing materials: With the advancement of science and technology, the operating conditions for sealing structures have become more demanding. Due to the significantly increased temperature, pressure, and corrosivity of the sealed fluid, traditional sealing materials such as felt, hemp fiber, asbestos fibers, and putty can no longer meet the requirements, and they are gradually being replaced by rubber and other synthetic materials. Synthetic materials such as rubber are generally polymers, with functional groups of various types (such as chlorine, fluorine, cyano, vinyl, isocyanate, hydroxyl, carboxyl, alkoxy, etc.) present on their macromolecular chains, which serve as active cross-linking sites. Under the action of catalysts, vulcanizing agents, or high temperatures and high-energy rays, macromolecules change from linear or branched structures to three-dimensional network structures; this process is known as vulcanization. Rubber or other synthetic materials after vulcanization, in which the macromolecules lose their original fluidity, are known as elastomers with high elastic deformation. Commonly used rubbers and synthetic materials include: natural rubber, styrene-butadiene rubber, neoprene, nitrile rubber, ethylene-propylene rubber, butyl rubber, polyurethane rubber, acrylate rubber, fluororubber, silicone rubber, etc. Performance indicators for assessing the quality of sealing materials: 1. Tensile properties. Tensile properties are the first aspects to consider when evaluating sealing materials, including tensile strength, stress at a fixed elongation, elongation at break, and permanent set at break. Tensile strength is the maximum stress to which a specimen can be stretched before it breaks. The stress at a fixed strain (modulus of elasticity at a fixed strain) is the stress achieved at a specified strain. Elongation is the deformation of a specimen under a specified tensile force, expressed as the ratio of the increase in length to the original length. The tear elongation is the elongation of the specimen at the time of rupture. Permanent set is the residual deformation between the marks after the specimen breaks under tension. 2. Hardness: Hardness indicates the ability of a sealing material to resist being pressed in by external forces, and it is also one of the basic properties of such materials. The hardness of a material is related to other properties to a certain extent: the higher the hardness, the greater the strength relatively, the lower the elongation rate, the better the wear resistance, and the worse the low-temperature resistance. 3. Compression performance: Rubber seals are usually under compression, and due to the viscoelastic properties of rubber materials, the pressure decreases over time under compression, resulting in compression stress relaxation ； It cannot return to its original shape after the pressure is removed, showing compressive set. This phenomenon is more pronounced in high-temperature and oily media, and this property is directly related to the durability of the sealing ability of the sealing products. 4. Low-temperature performance: An indicator used to measure the low-temperature characteristics of rubber seals. Two methods for testing low-temperature performance are introduced below: 1) Low-temperature shrinkage temperature: The sealing material is stretched to a certain length and then fixed; it is quickly cooled below the freezing point. Once equilibrium is reached, the sample is released and heated at a constant rate. The temperatures at which the sample shrinks by 10%, 30%, 50%, and 70% are denoted as TR10, TR30, TR50, and TR70 respectively. The material standard is based on TR10, which is related to the brittleness temperature of the rubber. Low-temperature flexibility: After freezing the specimen at a specified low temperature for a set period of time, it is bent back and forth at a specified angle to assess the sealing performance of the seal under repeated dynamic loads at low temperatures. 5. Oil or medium resistance: In addition to coming into contact with petroleum-based, diester, and silicate-based oils, sealing materials in the chemical industry may also be exposed to corrosive agents such as acids and bases. In these media, in addition to corrosion, high temperatures can also cause expansion and a decrease in strength, as well as a reduction in hardness ； At the same time, the plasticizers and soluble substances in the sealing material are extracted, resulting in a reduction in weight and volume and causing leaks. Generally, at a certain temperature, after immersing the material in the medium for a certain period of time, changes in its mass, volume, strength, elongation, and hardness are measured in order to assess the quality of the sealing material’s resistance to oils or other media. 6. Aging resistance: Sealing materials experience a deterioration in their properties when exposed to oxygen, ozone, heat, light, moisture, and mechanical stress; this phenomenon is known as the aging of sealing materials. Ageing resistance (also known as weather resistance) can be expressed by the changes in strength, elongation, and hardness of the material after ageing; the smaller the rate of change, the better the ageing resistance. Note: Weather resistance refers to a series of aging phenomena that occur in plastic products as a result of exposure to external conditions such as sunlight, temperature changes, wind, and rain, including fading, discoloration, cracking, powdering, and reduced strength. Among these, ultraviolet radiation is the key factor that accelerates the aging of plastics.