“Daily Share 4.19” – Knowledge about lubricants

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Codes and meanings of lubricants: Lubricants are liquid or semi-solid substances used in various types of automobiles and machinery to reduce friction and protect mechanical components and workpieces. They primarily serve functions such as lubrication, cooling, rust prevention, cleaning, sealing, and shock absorption. First, we should understand the codes of lubricating oils and their meanings. 　　According to the provisions of GB/T7631.1-1987, the code for lubricating oil consists of a category, a type, and a number, and is written in the format: category + type + number. A category refers to the classification of petroleum products; lubricants are one type of petroleum product, and lubricant materials are denoted by L. 　　　　Types refer to the categorization of lubricating oils. They are grouped according to their application scenarios, with corresponding letters used to represent each type: A – total loss systems ； C——Gear ； D——Compressor ； E — Internal combustion engine ； F——Stator, bearings, clutch ； G — Guide wheel ； H——Hydraulic system ； M——Metal processing ； P — Pneumatic tools ； T——Turbine ； Z – such as steam cylinders – is the initial letter of the category column; in fact, there may be one or more additional letters in that category column to indicate further subcategories of that type. 　　　　The numbers represent the viscosity grades of lubricating oils. These values correspond to the kinematic viscosity at 40°C (in some cases, they are batch numbers; however, it must be specified that otherwise they refer to the viscosity at 40°C). The unit of measurement is mm²/s. According to GB/T 3141-1994, there are 20 viscosity grades in total: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1000, 1500, 2200, and 3200. The Functions of Lubricants and Their Composition I. Functions of Lubricants Lubricants are liquid lubricants used in various types of machinery to reduce friction and protect the machinery as well as the components being processed. Their main functions include lubrication, cooling, rust prevention, cleaning, sealing, and cushioning. Lubricating oils account for 85% of all lubricant materials, with a wide variety of types and grades; the global annual consumption is currently around 38 million tons. The overall requirements for lubricants are: (1) to reduce friction and wear, thereby lowering frictional resistance to save energy, reducing wear to extend the lifespan of machinery, and improving economic efficiency ； 　(2) Cooling: It is necessary to remove the friction heat from the machine at all times ； 　(3) Sealing: leakage prevention, dust protection, and air leakage prevention are required ； 　(4) Corrosion and rust resistance; it is necessary to protect the friction surfaces from oil degradation or external corrosion ； 　(5) Clean rinsing: it is necessary to remove the dirt accumulated on the friction surface ； 　(6) Stress dispersion buffering, load distribution, and impact mitigation as well as shock absorption ； 　(7) Kinetic energy transmission, hydraulic systems, remote-controlled motors, and frictionless continuously variable transmission, etc. II. Composition of lubricating oil Lubricating oil generally consists of two parts: base oil and additives. Base oil is the main component of lubricants and determines their basic properties, while additives can compensate for and improve the shortcomings of the base oil’s performance, endowing it with certain new properties; they are an important part of lubricants. 1. Lubricant base oils: Lubricant base oils are mainly divided into two categories: mineral base oils and synthetic base oils. Mineral base oils are widely used and account for a large proportion (over 95%), but in some applications products formulated with synthetic base oils are necessary, which has led to the rapid development of synthetic base oils. Mineral base oil is derived from crude oil. The main production processes for lubricant base oils include: atmospheric and vacuum distillation, solvent deasphalting, solvent refining, solvent dewaxing, and clay or hydrogenation-based further refining. In 1995, China’s current standards for lubricant base oils were revised, primarily by changing the classification method, and standards for two specialized types of base oils—those with low freezing points and those that have undergone advanced refining—were added. In the production of mineral-based lubricants, the most important thing is to select the best crude oil. The chemical composition of mineral base oils includes high-boiling-point, high-molecular-weight hydrocarbons and non-hydrocarbon mixtures. Its composition generally includes alkanes (linear, branched, highly branched), cycloalkanes (monocyclic, bicyclic, polycyclic), aromatics (monocyclic aromatics, polycyclic aromatics), cycloalkyl aromatics, as well as oxygen-, nitrogen-, and sulfur-containing organic compounds, along with non-hydrocarbon compounds such as resins and asphaltenes. 2. Additives: Additives are the essence of modern high-grade lubricating oils. Proper selection and appropriate addition of additives can improve their physical and chemical properties, impart new special characteristics to the lubricating oil, or enhance certain existing properties, thereby meeting higher requirements. Carefully selecting additives, carefully balancing them, and formulating them appropriately based on the required quality and performance of the lubricant is key to ensuring its quality. Commonly used additives include: viscosity index improvers, pour point depressants, antioxidants, detergents and dispersants, friction modifiers, oiliness agents, extreme pressure agents, antifoam agents, metal passivators, emulsifiers, and corrosion inhibitors. The general physical and chemical properties of lubricating oils: Lubricating oils are technology-intensive products that are mixtures of complex hydrocarbons, and their actual performance in use is the result of a combination of complex physical or chemical processes. The basic properties of lubricating oils include general physical and chemical properties, special physical and chemical properties, and simulated bench tests. Each type of lubricating grease has certain common physical and chemical properties that indicate the inherent quality of the product. For lubricating oils, these general physical and chemical properties are as follows: (1) Appearance (colority) – The color of the oil often reflects its degree of refinement and stability. For base oils, generally the higher the degree of refining, the more thoroughly the hydrocarbon oxides and sulfides are removed, resulting in a lighter color. However, even under the same refining conditions, the base oils produced from crude oils of different sources and parent groups may vary in color and transparency. For new finished lubricating oils, due to the use of additives, color has lost its original significance as an indicator of the degree of refinement of the base oil. 　(2) Density. Density is the simplest and most commonly used physical property indicator for lubricating oils. The density of lubricating oil increases as the amounts of carbon, oxygen, and sulfur in its composition increase. Therefore, at the same viscosity or the same relative molecular mass, lubricating oils with a high content of aromatics, as well as those with high levels of gums and asphalts, have the highest density; those with a high content of naphthenes have a density in between, while those with a high content of alkanes have the lowest density. (3) Viscosity: Viscosity reflects the internal friction of oil products; it is an indicator of the oiliness and fluidity of such products. Without the addition of any functional additives, the higher the viscosity, the greater the strength of the oil film and the poorer its fluidity. (4) Viscosity index: The viscosity index indicates the extent to which the viscosity of an oil changes with temperature. The higher the viscosity index, the less the viscosity of the oil is affected by temperature, indicating better viscosity-temperature performance; conversely, the lower the viscosity index, the worse the performance. (5) Flash point: The flash point is an indicator of the volatility of oils. The lighter the distillate of an oil product, the greater its volatility, and the lower its flash point. Conversely, the heavier the distillate of an oil product, the lower its volatility, and the higher its flash point. At the same time, the flash point is an indicator of the fire hazard of petroleum products. The hazard level of oils is determined based on their flash point: oils with a flash point below 45°C are considered flammable, while those with a flash point above 45°C are considered combustible. It is strictly prohibited to heat oils to their flash point temperature during storage and transportation. At the same viscosity, the higher the flash point, the better. Therefore, when selecting lubricating oil, users should choose based on the operating temperature and the working conditions of the oil. It is generally believed that a flash point 20–30°C higher than the operating temperature ensures safe use. (6) Freezing point and pour point: The freezing point refers to the highest temperature at which the oil stops flowing under specified cooling conditions. The solidification of oils differs greatly from that of pure compounds. There is no definite solidification temperature for oils; the so-called “solidification” simply means that the oil loses its fluidity as a whole, but not all of its components turn into solids. The pour point of lubricating oil is an important quality indicator reflecting its low-temperature fluidity. It is of great significance for production, transportation, and use. Lubricants with a high freezing point cannot be used at low temperatures. On the contrary, in areas with higher temperatures, there is no need to use lubricants with a low freezing point. Because the lower the freezing point of the lubricating oil, the higher its production cost, resulting in unnecessary waste. Generally speaking, the freezing point of lubricating oil should be 5~7°C lower than the lowest temperature of the operating environment. It is particularly important to note that when selecting a lubricant for low temperatures, comprehensive consideration should be given to the oil’s freezing point, viscosity at low temperatures, and viscosity-temperature characteristics. Because oils with low freezing points may also have low-temperature viscosity and viscosity-temperature properties that do not meet the requirements. Both the freezing point and the pour point are indicators of the low-temperature flowability of oils; there is no fundamental difference between them, only slight variations in the methods of measurement. The freezing point and pour point of the same oil product are not exactly equal; generally, the pour point is 2–3°C higher than the freezing point, though there are exceptions. Special physical and chemical properties of lubricating oils: Each type of lubricating oil should also possess specific physical and chemical properties that characterize its usage characteristics. The higher the quality requirements, or the greater the specificity of the oil, the more pronounced its special physical and chemical properties become. The test methods that reflect these special physical and chemical properties are briefly described as follows: (1) Oxidation stability. Oxidation stability indicates the anti-aging performance of lubricants; some industrial lubricants with a long service life are required to meet this criterion, making it a special property specified for such types of oils. There are many methods for determining the oxidation stability of oils. Basically, a certain amount of oil is oxidized at a specific temperature for a set period of time in the presence of air (or oxygen) and metal catalysts, after which the acid value, changes in viscosity, and the formation of precipitates in the oil are measured. All lubricants have different tendencies toward autoxidation, depending on their chemical composition and the external conditions in which they find themselves. Oxidation occurs over time as it is used, gradually producing substances such as aldehydes, ketones, acids, as well as gums and asphaltenes. Oxidation stability is the property that prevents the formation of these substances that are detrimental to the use of oil products. (2) Thermal stability: Thermal stability refers to a lubricant’s ability to withstand high temperatures, that is, its resistance to thermal decomposition, which is expressed by the thermal decomposition temperature. Requirements for thermal stability have been established for some high-quality anti-wear hydraulic oils, compressor oils, and similar products. The thermal stability of oils depends primarily on the composition of the base oil; many additives with low decomposition temperatures tend to have an adverse effect on the stability of the oils ； Antioxidants also cannot significantly improve the thermal stability of oils. (3) Oiliness and extreme pressure properties: Oiliness arises from the polar substances in lubricants forming a strong physicochemical adsorption film on the metal surfaces at the points of friction, thereby providing resistance to high loads and frictional wear. Extreme pressure properties, on the other hand, result from these polar substances decomposing due to frictional chemical reactions under high temperatures and heavy loads on the metal surfaces at the points of friction; these reactions lead to the formation of a low-melting-point, soft (or plastic) extreme pressure film, which provides protection against impacts as well as high loads and high temperatures. (4) Corrosion and rusting: Oxidation of oils or the effect of additives often leads to corrosion of steel and other non-ferrous metals. In corrosion tests, copper strips are generally placed in oil and left at 100°C for 3 hours, after which the changes in the copper are observed ； The corrosion test involves the formation of rust on the steel surface under the influence of water and moisture. To determine the rust resistance, 30 ml of distilled water or artificial seawater is added to 300 ml of test oil, after which a steel rod is placed in it and stirred at 54°C for 24 hours; thereafter, it is checked whether rust has formed on the steel rod. Oils should have resistance to metal corrosion and rust prevention; in industrial lubricant standards, these two properties are usually required to be tested. (5) Antifoam property: During operation, lubricants often form foam due to the presence of air. This is especially true when the oil contains surfactant additives, as foam is more likely to form and it is also difficult for such foam to disappear. Foam formation during the use of lubricant can damage the oil film, cause sintering of the friction surfaces or increase wear, accelerate the oxidation and deterioration of the lubricant, and also cause air blockages in the lubrication system, affecting the circulation of the lubricant. Therefore, antifoamability is an important quality indicator for lubricants and similar products. (6) Hydrolytic stability: Hydrolytic stability indicates the stability of oil products under the action of water and metals (mainly copper). When the acid number of an oil product is high, or when it contains additives that readily decompose into acidic substances upon contact with water, this indicator often fails to meet the required standards. Its determination method involves adding the test oil to a certain amount of water, mixing and stirring it with a copper sheet at a specified temperature for a certain period of time, and then measuring the acid value of the water layer and the weight loss of the copper sheet. (7) Emulsification resistance: Industrial lubricants often inevitably become mixed with some cooling water during use. If the lubricant lacks good emulsification resistance, it will form an emulsion with the water that has mixed in, making it difficult to remove the water from the bottom of the circulation tank – which can lead to poor lubrication. Therefore, anti-emulsibility is a very important physicochemical property of industrial lubricants. For ordinary oils, 40 ml of the oil sample is mixed vigorously with 40 ml of distilled water at a certain temperature for a specified period of time, after which the time it takes for the oil layer, water layer, and emulsion layer to separate into 40-37-3 ml is observed ； Industrial gear oil is prepared by mixing the oil with water, stirring at a certain temperature and at 6,000 revolutions per minute for 5 minutes, allowing it to stand for 5 hours, and then measuring the volumes of oil, water, and the emulsion layer. (8) Air release value: This requirement is specified in the hydraulic oil standards, because in hydraulic systems, if the air dissolved in the oil is not released in a timely manner, it will affect the accuracy and sensitivity of hydraulic transmission; in severe cases, it will fail to meet the requirements for using such hydraulic systems. The method for measuring this property is similar to that for antifoaming capacity, except that it determines the time it takes for the air (mist) dissolved within the oil to be released. (9) Rubber sealing performance: In hydraulic systems, rubber is commonly used as a sealing material. In mechanical devices, oils inevitably come into contact with various sealing components. Oils with poor sealing properties can cause the rubber to swell, shrink, harden, or crack, thereby affecting its sealing ability; therefore, it is necessary for oils to have good compatibility with rubber. Hydraulic oil standards require a rubber sealing index, which is measured by the change that occurs in a rubber ring of a certain size after being immersed in oil for a specified period of time. (10) Shear stability: In oils to which thickeners have been added, mechanical shear during use causes the high-molecular polymers in the oil to be broken down, resulting in a decrease in the oil’s viscosity and affecting proper lubrication. Therefore, shear stability is a specific physical and chemical property that must be measured for such oils. There are many methods for measuring shear stability, including the ultrasonic shear method, the nozzle shear method, the Wicks pump shear method, and the FZG gearbox shear method; all of these methods ultimately aim to determine the rate of viscosity decrease in the oil. (11) Solubility The solubility is usually expressed by the aniline point. The solubility limit aniline point of composite additives varies with different grades of oils; the limit value for oils with low ash content is higher than that for highly basic oils, and the limit value for single-grade oils is higher than that for multi-grade oils. (12) Volatility: The volatility of the base oil is related to fuel consumption, viscosity stability, and oxidation stability. These properties are particularly important for multi-grade oils and energy-saving oils. (13) Rust resistance: This refers specifically to the special physical and chemical properties that rust-proof greases should possess. The testing methods include humidity tests, salt spray tests, laminating tests, and water displacement tests; in addition, there are also greenhouse tests and long-term storage tests. (14) Electrical properties: Electrical properties are unique characteristics of insulating oils, including the dielectric loss angle, dielectric constant, breakdown voltage, pulse voltage, etc. The degree of refinement of the base oil, as well as impurities and moisture, all have a significant impact on the electrical properties of the oil. (15) Special physical and chemical properties of greases Besides their general physical and chemical properties, greases designed for specific applications possess special physical and chemical characteristics as well. Greases with good water resistance are required to undergo a water test ； For low-temperature grease, low-temperature torque needs to be measured ； Multi-effect lubricants need to be tested for extreme pressure and wear resistance as well as rust prevention properties ； Long-life lubricants require bearing life tests and similar procedures. There are also corresponding test methods for measuring these properties. (16) Other special physical and chemical properties: In addition to their general properties, each type of oil should have its own unique special properties. For example, the cooling rate of quenching oil needs to be determined ； The emulsification stability of the emulsified oil needs to be determined ； The anti-creep coefficient of hydraulic guide rail oil needs to be measured ； The dispersion of oil mist needs to be measured for spray lubricants ； The freezing point and flocculation point of refrigeration oil need to be measured ； Low-temperature gear oil needs to be tested for pitting, etc. All these properties require a special chemical composition of the base oil, or the addition of certain special additives to ensure them.