The oil seal material directly affects the above three […]
The oil seal material directly affects the above three parameters, because the material changes with time and temperature, and various key parameters also change, such as: with the increase of temperature, the modulus of the material decreases, and the radial force of the oil seal material changes. Thermal expansion, material swelling caused by the sealing medium, softness and hardness of the rubber compound, etc. all affect the radial force and the amount of interference.
Based on the above reasons, the following characteristics should be considered when selecting the oil seal material: the oil seal sealing material is compatible with the sealing medium, and the oil seal sealing material does not swell and harden due to the sealing medium; the oil seal sealing material has good heat resistance and wear resistance. ; The oil seal material has moderate elasticity and can adapt to changes in the roughness and eccentricity of the shaft.
Due to the many changes in the formulation of rubber sealing materials, new sealing materials are constantly emerging, and existing sealing materials are constantly being improved. The following is only a brief description of the most commonly used nitrile rubber, polyacrylate rubber, silicone rubber, fluorine rubber and polytetrafluoroethylene in the manufacture of oil seals.
1. Nitrile rubber
In the manufacture of seals, the amount of nitrile rubber may be greater than the sum of other elastomers. The chemical composition of nitrile rubber is a copolymer of butadiene and propylene, and the propylene content is between 18% and 40%. The oil resistance of nitrile rubber increases with the increase of propylene content, but at the same time, the low temperature flexural flexibility decreases. In order to obtain good low temperature performance, the performance of some high temperature fuels and oils is often sacrificed. Nitrile rubber has good physical properties, and its cold flow resistance, tear resistance and abrasion resistance are better than most other rubbers, but nitrile rubber is not resistant to ozone, weather and sunlight, and this performance can be improved through formulation design. Nitrile rubber is suitable for use in petroleum base oils, fuel oils, water, silicone oils and mixtures of silicone esters and glycols. However, nitrile rubber is not suitable for contact with EP oil, halogenated hydrocarbons, nitrocarbides, phosphate ester fluids, ketones or strong acids, and some automotive brake fluids.
2. Polyacrylate rubber
Polyacrylate (ACM) rubber is an emulsion co-slurry with alkyl acrylate as the main body and other unsaturated monomers. Commonly employed alkyl acrylates are vinyl ethyl and butyl acrylate. The performance of polyacrylate rubber is between that of nitrile rubber and fluororubber. Because its molecular main chain does not contain double bonds, it has high heat resistance, ozone resistance and weather resistance. The functional group containing chlorine (Cl) or (CM) on the side chain makes it have excellent oil resistance and can be used in hot oil at 170℃～180℃. One of the main features of polyacrylate rubber is its resistance to mineral oil, hyperbolic oil and butter at 178°C. It has good resistance to aging and bending cracks, and is suitable for oil seal materials. The main disadvantages of polyacrylate rubber are poor processing performance, sticking to rollers during mixing, poor low temperature performance, poor resistance to water and steam, poor resistance to ethylene glycol and high aromatic oils, large compression set, and poor resistance to water and steam. Metal molds and shafts corrode more. Elasticity, wear resistance and electrical insulation are poor. In addition, due to the high saturation of polyacrylate rubber, the vulcanization rate is slow. Abrasion resistance can be significantly improved with proper formulation, but it is still not as good as nitrile rubber.
3. Silicone rubber
Silicone rubber can maintain its mechanical properties in a wide temperature range, remain flexible at -65°C, and can work for a long time at 230°C. Although the mechanical properties of silicone rubber can be improved through special coordination, the strength, tear resistance and wear resistance of silicone rubber are basically poor. The performance of alkali, weak acid and ozone is generally good; but the oil resistance of silicone rubber is moderate. The chemical properties of silicone rubber can be improved with compounding agents, such as better oil resistance and fuel resistance through compounding agents. But in general, silicone rubber is not suitable for hydrocarbons, such as gasoline, paraffin, light mineral oil, which will cause expansion and softening of silicone rubber in the above-mentioned media. The main advantage of silicone rubber is that it remains elastic at very low temperatures. And silicone rubber can not harden at high temperature for a long time, so compared with other rubbers, it can be used for high temperature and low temperature seals in a wider range. Silicone rubber is used as a rotary seal, and the temperature of silicone rubber is higher than that of ordinary rubber. But silicone rubber is more expensive than most other rubbers.
Fluorosilicone rubber is a more expensive rubber, and its working performance is basically the same as that of silicone rubber, but the use of fluorosilicone rubber is narrower. The main advantage of fluorosilicone rubber is good oil resistance, which is comparable to or close to nitrile rubber. Therefore, fluorosilicone rubber can not only be used outside the operating temperature limit of nitrile rubber, but also has the oil resistance that silicone rubber lacks.
Fluorine rubber is a saturated polymer containing fluorine atoms on the carbon atoms of the main chain or side chain, and has unique and excellent properties. The characteristics of fluororubber are high temperature resistance, oil resistance, strong corrosion resistance, solvent resistance, weathering resistance, ozone resistance, low air permeability, good physical properties, and can work continuously at 200 ℃ ~ 250 ℃. The disadvantage of fluororubber is that the low temperature performance is not very good, and the compression set is large. In order to improve the compression set of fluororubber, a lot of research work has been done at home and abroad.
Plastics tend to be semi-rigid materials and are generally not used as seals. The only exception is polytetrafluoroethylene, a fluorocarbon with unique properties, the most notable feature of polytetrafluoroethylene is chemical resistance to service temperature range; low coefficient of friction in contact with metal, but if not filled Strong, but the mechanical strength of PTFE is not high. A useful aspect of polytetrafluoroethylene is as a seal for composite structures, for example, machined or molded polytetrafluoroethylene can be used as a low friction surface and a chemical resistant cladding surface.