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Introduction of sealing materials

Introduction of sealing materials

2022-09-29

Commonly used sealing engineering plastics


1. Polytetrafluoroethylene (PTFE) and its filling modification


The melting point of PTFE is 327 °C, the density is 2.18~2.3g/cm3, the Shore hardness is D50~65, and it has excellent chemical stability and self-lubrication. Has excellent comprehensive properties, such as: high and low temperature resistance, corrosion resistance, aging resistance, non-adhesion, high insulation, high lubrication, non-toxic. However, PTFE has poor creep resistance, poor wear resistance, and large expansion coefficient of products, so it is usually necessary to overcome these shortcomings by filling and modification. Commonly used fillers are copper powder, carbon fiber, glass fiber, graphite, molybdenum disulfide, polyphenylester, polyether ether ketone, etc. One or more substances can be filled according to different working conditions and uses


1. Pure PTFE products


Pure PTFE can process seals such as retaining rings and gaskets. It has the characteristics of high temperature resistance, corrosion resistance and self-lubrication. However, it is prone to creep under load and causes leakage. It is usually used in the food, pharmaceutical and chemical industries.


2. PTFE filled copper powder modified products


Copper powder is the most commonly used filler for PTFE. The PTFE seals modified by filling with copper powder have great improvements in compression resistance, wear resistance, thermal conductivity and creep resistance. According to different performance requirements, different copper powder composition, particle size and filling ratio can be selected to make its performance meet the actual needs. This product is usually used to make wear-resistant guide rings, wear-resistant belts, oil/cylinder seals, etc. Copper powder may be corroded in strong acid environment, so it is not recommended to use in strong acid service.


3. PTFE filled carbon fiber modified products


The modified polytetrafluoroethylene seals filled with carbon fibers have great improvements in compression resistance, wear resistance, creep resistance and tensile strength, and at the same time increase the hardness of the products, which are suitable for rotary seals. It is used in the production of spring energy storage plug seals, automobile gearbox oil rings, etc. Carbon fiber may be oxidized and decomposed in strong acid environment, so it is not recommended for use in strong acid conditions.


4. PTFE filled glass fiber modified products


The modified PTFE seals filled with glass fibers have great improvements in compression resistance, wear resistance, creep resistance and tensile strength, and at the same time increase the hardness of the products, suitable for rotary seals, usually It is used to make gaskets, spring energy storage plug seals, automobile gearbox oil rings, etc. Glass fibers may decompose in alkaline environments and are therefore not recommended for use in alkaline or water-containing applications. Foshan Damanson Seal


d5. PTFE-filled graphite or molybdenum disulfide modified products


The main purpose of filling graphite or molybdenum disulfide modification is to improve the compression resistance, wear resistance and creep resistance of PTFE seals while maintaining the low friction coefficient and self-lubrication of PTFE to a large extent. Features. It is usually filled in combination with other fillers, or it can be filled alone. It is mainly used to make spring-energy storage pan plug seals, cylinder seals, self-lubricating shaft sleeves, etc. It is suitable for working conditions requiring low friction coefficient or inconvenient to add lubricant.


6. Polytetrafluoroethylene filled polyphenylester or polyether ether ketone modified products


The PTFE seals modified by filled polyphenylester or polyetheretherketone have great improvement in compression resistance, wear resistance, creep resistance and high temperature resistance, especially in high temperature environment. Toughness and hardness have been greatly improved. Suitable for static seals, dynamic seals and compressor self-lubricating piston rings for long-term operation in high temperature environments.


2. Polychlorotrifluoroethylene (PCTFE)


The melting point of polychlorotrifluoroethylene is 210~230°C, the density is 2.1~2.18g/cm3, and the Shore hardness is D75~D90. Polychlorotrifluoroethylene has excellent rigidity and corrosion resistance, and its mechanical properties at room temperature are better than that of polytetrafluoroethylene. It has high compressive strength and small creep. Under the low temperature environment, the compression rebound rate is large, and it has good elastic recovery force. It still maintains a certain elasticity at -265°C. It is one of the known polymer materials with excellent ultra-low temperature comprehensive performance. Polychlorotrifluoroethylene is a crystalline polymer, so its mechanical properties are greatly affected by temperature, and there are certain differences due to the degree of crystallinity and molecular weight. Its tensile strength and hardness will follow the advancement of crystallization. increase, but the elongation will decrease. The grades with large molecular weights are usually formed by molding and cannot be formed by extrusion due to poor melt fluidity. Polychlorotrifluoroethylene is an ideal sealing material under low temperature and high pressure conditions, and can be used for a long time in the range of -200 to 125°C. Polychlorotrifluoroethylene seals have special uses in high-tech fields, and are recommended for use in tube pumps, valves and joints in ultra-low temperature conditions such as liquid oxygen, liquid nitrogen, and liquefied natural gas.


3. Ultra-high molecular weight polyethylene (UHMWPE)


The melting point of high molecular weight polyethylene is 130~136°C, the density is 0.94g/cm3, and the Shore hardness is D40~D60. It combines the superior properties of almost all plastic materials: impact resistance, wear resistance, chemical corrosion resistance, low friction coefficient, good self-lubricating properties, and still has excellent mechanical properties in ultra-low temperature environments, -70°C is still very good. High impact performance, still maintains toughness and strength at -196°C, hardly absorbs water and does not swell, has better wear resistance and creep resistance than PTFE materials, and is proved to be wear-resistant by mortar wear test The resistance can reach 6.6 times that of ordinary carbon steel. Ultra-high molecular weight polyethylene with a molecular weight of less than 5 million can be extruded after technical treatment, and can only be molded with a molecular weight of more than 5 million. The long-term operating temperature range of UHMWPE seals is -180°C to 70°C. It is recommended for conditions with high sliding performance requirements or poor lubrication, high wear resistance requirements, or water-containing media and dry friction prone conditions. It is also commonly used for sealing in ultra-low temperature environments. Its disadvantages are insufficient tensile strength, poor high temperature resistance, low hardness, and low thermal conductivity. Foshan Damanson Seal


4. Polyetheretherketone (PEEK)


The melting point of polyetheretherketone is 334°C, the density is 1.265~1.32g/cm3, the Shore hardness is D82~D90, and the tensile strength is higher than 92MPa. Polyetheretherketone is a special polymer material with high mechanical strength, high temperature resistance, impact resistance, flame retardant, acid and alkali resistance, hydrolysis resistance, wear resistance, fatigue resistance, radiation resistance and good electrical insulation properties. Polyetheretherketone resin has good high temperature performance and high melting point. The continuous use temperature can reach 260°C, and the load heat deflection temperature after adding glass fiber or carbon fiber is up to 316°C. Polyetheretherketone resin has good toughness and rigidity, and it has excellent fatigue resistance to alternating stress comparable to alloy materials. Due to its excellent high temperature resistance, self-lubrication, wear resistance and fatigue resistance, it has become one of the most popular high-performance engineering plastics today. It is mainly used in aerospace, automotive industry, electrical and electronic and medical equipment and other fields. Because of its good mechanical properties, high temperature resistance, wear resistance and high pressure resistance, polyether ether ketone is often used to manufacture compressor valve plates, piston rings, high temperature seals, sliding bearings, etc.


Commonly used sealing rubber


1. Nitrile rubber (NBR)


Nitrile rubber is a synthetic rubber produced by low temperature emulsion polymerization of butadiene and acrylonitrile. It has excellent oil resistance, high wear resistance, water resistance, air tightness and excellent bonding properties. The operating temperature range of nitrile rubber is -30°C to 100°C, and the service life will decrease when used at 120°C. It is widely used in the production of various oil-resistant rubber products, O-rings, gaskets, casings, cable rubber materials, etc. It is also an essential elastic material in the automotive, aviation, petroleum and other industries. The disadvantages of nitrile rubber are poor low temperature resistance, poor ozone resistance, poor insulation performance, and slightly lower elasticity.


2. Fluorine rubber (FKM/FPM)


Fluorine rubber is a synthetic rubber containing fluorine atoms. It has the characteristics of high temperature resistance, oil resistance, medium resistance and resistance to various chemicals. At the same time, it has a high degree of chemical stability. Its operating temperature range is -20°C~200°C, and when used for dynamic sealing, the low temperature resistance range is -20°C~-15°C. Mainly used to make oil seals, O-rings, gaskets, fluororubber strips, fluororubber plates, sealing rings, etc. The disadvantage of fluororubber is that it has poor low temperature resistance and high density.


Three, ethylene propylene diene rubber (EPDM)


EPDM rubber is a terpolymer of ethylene, propylene and non-conjugated dienes. Its main polymer chain is completely saturated, this feature makes EPDM rubber resistant to heat, light, oxygen, especially ozone, so it has very good aging resistance, good weather resistance, excellent electrical insulation properties, and resistance to It has the characteristics of good chemical corrosion and good impact elasticity. It is mainly used in the fields that require aging resistance, water resistance, corrosion resistance, electrical insulation and other fields to make heat-resistant conveyor belts, cables, anti-corrosion linings, sealing gaskets, door and window sealing strips, plastic modification, etc.


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