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Lubricating oil is a technology-intensive product, which is a complex mixture of hydrocarbons, and its real performance is the combined effect of complex physical or chemical changes. The basic properties of lubricating oil include general physical and chemical properties, special physical and chemical properties and simulated bench test.
General physical and chemical properties
Each type of lubricating grease has its common general physical and chemical properties to indicate the intrinsic quality of the product. For lubricants, these general physical and chemical properties are as follows:
(1) Appearance (chroma) The color of the oil can often reflect its refining degree and stability. For base oils, the higher the degree of refining, the cleaner the removal of hydrocarbon oxides and sulfides, and the lighter the color. However, even if the refining conditions are the same, the color and clarity of the base oils produced from crude oils of different oil sources and bases may not be the same. For new finished lubricants, due to the use of additives, color as an indicator of the degree of refining of the base oil has lost its original meaning.
(2) Density Density is the simplest and most commonly used physical performance index of lubricating oil. The density of lubricating oil increases with the increase of the amount of carbon, oxygen and sulfur in its composition. Therefore, under the condition of the same viscosity or the same relative molecular mass, the lubricating oil with more aromatic hydrocarbons, more colloid and asphaltene has the largest density, the lubricating oil with more naphthenes is in the middle, and the lubricating oil with more alkanes is the smallest.
(3) Viscosity Viscosity reflects the internal friction of the oil and is an indicator of the oiliness and fluidity of the oil. Without any functional additives, the greater the viscosity, the higher the oil film strength and the worse the fluidity.
(4) Viscosity Index The viscosity index indicates the degree to which the viscosity of the oil changes with temperature. The higher the viscosity index, the less the oil viscosity is affected by temperature, the better the viscosity-temperature performance, and vice versa.
(5) Flash point Flash point is an indicator of oil evaporation. The lighter the fraction of the oil, the greater the evaporation and the lower the flash point. On the contrary, the heavier the oil fraction, the smaller the evaporation and the higher the flash point. At the same time, the flash point is an indicator of the fire risk of petroleum products. The danger level of oil is divided according to the flash point. Flammable products are below 45 ℃ and combustible products are above 45 ℃. It is strictly prohibited to heat the oil to its flash point temperature during the storage and transportation of oil. In the case of the same viscosity, the higher the flash point, the better. Therefore, the user should choose the lubricating oil according to the operating temperature and the working conditions of the lubricating oil. It is generally believed that the flash point is 20~30 ℃ higher than the use temperature, and it can be safely used.
(6) Condensing point and pour point Condensing point refers to the maximum temperature at which the oil stops flowing under the specified cooling conditions. The solidification of oil products is very different from that of pure compounds. The oil does not have a clear solidification temperature. The so-called "solidification" only loses fluidity as a whole, and not all components become solid. The freezing point of lubricating oil is an important quality index that indicates the low temperature fluidity of lubricating oil. It is important for production, transportation and use. Lubricating oil with high freezing point cannot be used at low temperature. On the contrary, it is not necessary to use lubricating oil with low freezing point in areas with high temperature. Because the lower the freezing point of the lubricating oil, the higher the production cost, resulting in unnecessary waste. Generally speaking, the freezing point of lubricating oil should be 5~7 ℃ lower than the lowest temperature of the use environment. However, it should be mentioned in particular that when selecting low-temperature lubricants, the freezing point, low-temperature viscosity and viscosity-temperature characteristics of the oil should be fully considered. Because of the low freezing point of the oil, its low temperature viscosity and viscosity temperature characteristics may not meet the requirements. Condensation point and pour point are both indicators of low-temperature fluidity of oil products. There is no principle difference between the two, but the measurement method is slightly different. The pour point and pour point of the same oil are not exactly the same, the pour point is generally higher than the pour point 2~3 ℃, but there are exceptions.
(7) Acid Value, Base Value, and Neuter Value The acid value is an index indicating that an acidic substance is contained in a lubricating oil, and the unit is mgKOH/g. Acid value is divided into strong acid value and weak acid value, and the combination of the two is the total acid value (TAN). What we usually call "acid value" actually refers to "total acid value (TAN)". The base number is an indicator of the content of alkaline substances in lubricating oil, and the unit is mgKOH/g. The base value is also divided into strong base value and weak base value, and the combination of the two is the total base value (TBN). What we usually call "base number" actually refers to "total base number (TBN)". The neutralization number actually includes the total acid number and the total base number. However, unless otherwise noted, the term "neutralization value" generally refers to only "total acid number", which is also expressed in units of mg KOH/g.
(8) Moisture Moisture refers to the percentage of water content in lubricating oil, usually by weight. The presence of moisture in the lubricating oil will destroy the oil film formed by the lubricating oil, make the lubrication effect worse, accelerate the corrosion of organic acids on metals, rust equipment, and make the oil prone to sediment. In short, the less water in the lubricating oil, the better.
(9) Mechanical impurities Mechanical impurities refer to precipitates or colloidal suspensions that are insoluble in solvents such as gasoline, ethanol and benzene in lubricating oils. Most of these impurities are sand and iron filings, and some organic metal salts that are hardly soluble in solvents brought by additives. Generally, the mechanical impurities of the lubricating base oil are controlled below 0.005% (the mechanical impurities below 0.005% are considered as none).
(10) Ash and sulphated ash Ash refers to the non-combustible substances left after burning under specified conditions. The composition of ash is generally considered to be some metal elements and their salts. Ash has different concepts for different oil products. For base oil or oil products without additives, ash can be used to judge the refining depth of oil products. For oil (new oil) added with metal salt additives, ash becomes a means of quantitative control of the amount of additives added. Sulfated ash is used in foreign countries instead of ash. The method is to add a small amount of concentrated sulfuric acid before burning and ashing after burning the oil sample to convert the metal elements of the additive into sulfate.
(11) Carbon residue oil under the specified experimental conditions, the coke black residue formed after thermal evaporation and combustion is called carbon residue. Carbon residue is an important quality index of lubricating oil base oil, and it is an item specified for judging the nature and refining depth of lubricating oil. The amount of residual carbon in lubricating oil base oil is not only related to its chemical composition, but also related to the refining depth of oil. The main substances forming residual carbon in lubricating oil are: colloid, asphaltene and polycyclic aromatic hydrocarbons in oil. These substances are subjected to strong thermal decomposition and condensation to form carbon residue under the condition of insufficient air. The deeper the refining depth of the oil, the smaller the residual carbon value. Generally speaking, the smaller the residual carbon value of the blank base oil, the better. Now, many oil products contain metal, sulfur, phosphorus, nitrogen additives, their carbon residue value is very high, so the carbon residue containing additive oil has lost the original meaning of carbon residue determination. Mechanical impurities, moisture, ash and carbon residue are all quality indicators that reflect the purity of the oil and reflect the degree of refining of the lubricating base oil.
special physical and chemical properties
In addition to the above general physical and chemical properties, each lubricating oil should also have special physical and chemical properties that characterize its use. The more high quality requirements, or specificity of the oil, the more prominent its special physical and chemical properties. The test methods reflecting these special physical and chemical properties are briefly introduced as follows:
(1) Oxidation stability Oxidation stability indicates the anti-aging performance of lubricating oils. Some industrial lubricating oils with long service life have this index requirement, thus becoming a special performance required by these types of oils. There are many methods for determining the oxidation stability of oil. Basically, a certain amount of oil is oxidized at a certain temperature for a certain period of time in the presence of air (or oxygen) and metal catalysts, and then the acid value and viscosity of the oil are determined. And the formation of sediment. All lubricants have different automatic oxidation tendencies according to their chemical composition and external conditions. Oxidation occurs with the use of the process, and thus gradually generate some aldehydes, ketones, acids and colloids, asphaltenes and other substances, oxidation stability is the inhibition of the above is not conducive to the use of oil material generation performance.
(2) Thermal stability Thermal stability indicates the high temperature resistance of the oil, that is, the resistance of the lubricating oil to thermal decomposition, that is, the thermal decomposition temperature. Some high-quality anti-wear hydraulic oil, compressor oil, etc. have put forward the requirements of thermal stability. The thermal stability of oil mainly depends on the composition of base oil, many additives with low decomposition temperature often have adverse effects on the stability of oil, and antioxidants can not significantly improve the thermal stability of oil.
(3) Oiliness and extreme pressure oiliness is that the polar substances in the lubricating oil form a solid physical and chemical adsorption film on the metal surface of the friction part, thus playing the role of high load resistance and friction and wear resistance, while the extreme pressure is that the polar substances of the lubricating oil are decomposed by tribochemical action on the metal surface of the friction part under high temperature and high load, and undergo tribochemical reaction with the surface metal, A soft (or plastic) extreme pressure film with low melting point is formed, which plays a lubricating role of impact resistance and high load and high temperature resistance.
(4) corrosion and rust due to the oxidation of oil or additives, often cause corrosion of steel and other non-ferrous metals. Corrosion test is generally to put copper strips into oil, place them at 100 ℃ for 3 hours, and then observe the change of copper. The corrosion test is that the steel surface will rust under the action of water and water vapor. The determination of rust resistance is to add 30ml distilled water or artificial seawater to 300ml of test oil, then place the steel rod in it, stir at 54 ℃ for 24 hours, and then observe whether the steel rod has rust. Oil products should have anti-metal corrosion and anti-rust effects. In industrial lubricating oil standards, these two items are usually mandatory items.
(5) anti foam lubricating oil in the process of operation, due to the presence of air, often produce foam, especially when the oil contains surface active additives, it is more likely to produce foam, and the foam is not easy to disappear. The foam generated during the use of lubricating oil will destroy the oil film, cause the friction surface to sinter or increase wear, and promote the oxidation and deterioration of the lubricating oil. It will also cause the air resistance of the lubricating system and affect the lubricating oil circulation. Therefore, anti-foam is an important quality index of lubricating oil.
(6) Hydrolytic stability Hydrolytic stability characterizes the stability of oil under the action of water and metal (mainly copper). When the oil has a high acid value or contains additives that are easy to decompose into acidic substances when exposed to water, this index is often unqualified. Its determination method is to add a certain amount of water to the test oil, mix and stir in the copper sheet and a certain temperature for a certain period of time, and then measure the acid value of the water layer and the weight loss of the copper sheet.
(7) Anti-emulsifying industrial lubricants are often inevitably mixed with some cooling water during use. If the anti-emulsifying properties of the lubricating oil are not good, it will form an emulsion with the mixed water, so that the water is not easy to circulate from the oil tank. The bottom is released, which may cause poor lubrication. Therefore, anti-emulsification is a very important physical and chemical properties of industrial lubricants. In general, 40ml of test oil and 40ml of distilled water are vigorously stirred at a certain temperature for a certain period of time, and then the separation time of the oil layer-water layer-emulsified layer into 40-37-3ml is observed. Industrial gear oil is mixed with water, stirred for 5 minutes at a certain temperature and 6000 r/min, placed for 5 hours, and then measured the milliliters of oil, water and emulsified layer.
(8) air release value hydraulic oil standard has this requirement, because in the hydraulic system, if the air dissolved in the oil can not be released in time, then it will affect the accuracy and sensitivity of hydraulic transmission, and can not meet the requirements of hydraulic system in severe cases. The method for determining this property is similar to foam resistance, but it is measured by the time it takes for the air (mist) dissolved in the oil to be released.
(9) Rubber sealing In the hydraulic system, most of them use rubber as seals. The oil in the machinery inevitably comes into contact with some seals. The oil with poor rubber sealing can make the rubber swell, shrink, harden, and crack, which affects its sealing performance. Therefore, oil and rubber are required to have better adaptability. The rubber sealing index is required in the hydraulic oil standard, which is measured by the change of a rubber ring of a certain size after a certain period of time.
(10) Shear stability In the process of using oil with viscosity enhancer, due to the effect of mechanical shear, the polymer in the oil is cut off, which reduces the viscosity of the oil and affects the normal lubrication. Therefore, shear stability is the special physical and chemical properties of this kind of oil. There are many methods to determine the shear stability, including ultrasonic shear method, nozzle shear method, Wicks pump shear method, FZG gear turbine shear method, these methods are ultimately the determination of oil viscosity drop rate.
(11) Solubility Solubility is usually expressed by an aniline point. The solubility limit aniline points of different grades of oil to composite additives are different. The limit value of low-ash oil is larger than that of over-alkaline oil, and the limit value of single-grade oil is larger than that of multi-grade oil.
(12) The volatility of volatile base oils is related to fuel consumption, viscosity stability and oxidation stability. These properties are particularly important for multi-grade oils and energy-saving oils.
(13) Anti-rust performance This refers specifically to the special physical and chemical properties that anti-rust grease should have. Its test methods include humidity test, salt spray test, lamination test, water displacement test, in addition to louver box test, long-term storage test, etc.
(14) Electrical performance Electrical performance is the unique performance of insulating oil, mainly media loss angle, dielectric constant, breakdown voltage, pulse voltage, etc. The refining depth, impurities and moisture of the base oil have a great influence on the electrical properties of the oil.
(15) the special physical and chemical properties of grease grease in addition to the general physical and chemical properties, special purpose grease and its special physical and chemical properties. For example, grease with good waterproof property requires water spray test. Low temperature torque shall be measured for low temperature grease; Extreme pressure, abrasion resistance and rust resistance shall be measured for multi-effect grease; Long-life grease shall be tested for bearing life. There are also corresponding test methods for the determination of these properties.
(16) other special physical and chemical properties of each oil in addition to the general performance, should have its own unique special performance. For example, quenching oil should measure the cooling rate; Emulsified oil should measure the emulsion stability; Hydraulic guide rail oil should measure the anti-climbing coefficient; Spray lubricating oil should measure the oil mist diffusivity; Frozen oil should measure the flocculation point; Low temperature gear oil should be measured into ditch points, etc. These characteristics require a special chemical composition of the base oil, or the addition of some special additives to ensure.