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Basic Properties of Lubricating Oils-General Physical and Chemical Properties

Issuing time:2022-03-08 15:59

Lubricating oil is a technology-intensive product and a complex mixture of hydrocarbons, and its real service performance is the comprehensive effect of complex physical or chemical change processes. 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 kind of lubricating grease has its common general physical and chemical properties to show the intrinsic quality of the product. For lubricating oil, these general physical and chemical properties are as follows:


(1) Appearance (chroma) The color of oil can often reflect its refining degree and stability. For base oil, the higher the refining degree, 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 transparency of base oils produced by different oil sources and base oils may be different. For the new finished lubricating oil, because of the use of additives, color has lost its original meaning as an index to judge the refining degree of base oil.


Implementation standard: GB/T 6540

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(2) Density is a simple and 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 weight, the density of lubricating oil containing more aromatic hydrocarbons, more gum and asphaltene is * * * * *, the density of lubricating oil containing more naphthenes is in the middle, and the density of lubricating oil containing more alkanes is * * * smaller.


Implementation standard: GB/T 1884

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(3) Viscosity, which reflects the internal friction of oil, is an indicator of oiliness and fluidity of oil. Without any functional additives, the higher the viscosity, the higher the oil film strength and the worse the fluidity.


Implementation standard: GB/T 265

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(4) Viscosity index Viscosity index indicates the degree of oil viscosity changing with temperature. The higher the viscosity index, the less the oil viscosity is affected by temperature, the better its viscosity-temperature performance, and the worse it is.


(5) Flash point Flash point is an indicator of oil evaporation. The lighter the oil fraction, 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, flash point is an indicator of the fire risk of petroleum products. The danger grade of oil products is divided according to the flash point. The flash point below 45℃ is flammable, and the flash point above 45℃ is flammable. It is strictly forbidden to heat the oil products to its flash point temperature during the storage and transportation of oil products. At the same viscosity, the higher the flash point, the better. Therefore, users should choose lubricating oil according to the operating temperature and working conditions of lubricating oil. Generally speaking, the flash point is 20 ~ 30℃ higher than the service temperature, so it can be used safely.


Implementation standard: GB/T 3536

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(6) Freezing point and pour point Freezing point refer to the * * * * * temperature at which oil stops flowing under specified cooling conditions. The solidification of oil products is quite different from that of pure compounds. There is no definite solidification temperature for oil products. The so-called "solidification" only loses fluidity as a whole, and not all components become solids. The freezing point of lubricating oil is an important quality index indicating the low temperature fluidity of lubricating oil. It is of great significance for production, transportation and use. Lubricating oil with high condensation 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 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 * * * * * temperature of the service environment. However, in particular, when selecting low-temperature lubricating oil, the freezing point, low-temperature viscosity and viscosity-temperature characteristics of oil should be considered * * * * *. Because of the low freezing point oil, its low-temperature viscosity and viscosity-temperature characteristics may not meet the requirements. Solidification point and pour point are both indicators of low-temperature fluidity of oil products. There is no principle difference between them, but the measuring methods are slightly different. The freezing point and pour point of the same oil product are not completely equal, and generally the pour point is 2 ~ 3℃ higher than the freezing point, but there are some exceptions.


Implementation standard: GB/T 3535, GB/T 510

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(7) Acid value, base value and neutralization value The acid value is an indicator that lubricating oil contains acidic substances, and the unit is mgkoh/g. There are two kinds of acid value: strong acid value and weak acid value, and the combination of them is the total acid value (TAN for short). What we usually call "acid value" actually refers to "total acid value (TAN)". Alkali number is an index indicating the content of alkaline substances in lubricating oil, and its unit is mgkoh/g. Alkali value is also divided into strong alkali value and weak alkali value, and the combination of the two is the total alkali value (TBN for short). What we usually call "base number" actually refers to "total base number (TBN)". The neutralization value actually includes total acid value and total base value. However, unless otherwise indicated, the general term "neutralization value" actually only refers to "total acid value", and its unit is also mg KOH/g.

Implementation standard: GB/T 264


(8) Moisture refers to the percentage of water content in lubricating oil, usually by weight. The existence of water in lubricating oil will destroy the oil film formed by lubricating oil, make the lubricating effect worse, accelerate the corrosion of organic acids to metals, rust equipment and make oil products prone to sediment. In short, the less water in lubricating oil, the better.


Implementation standard: GB/T 260

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(9) Mechanical impurities Mechanical impurities refer to precipitates or colloidal suspended solids that exist in lubricating oil and are insoluble in solvents such as gasoline, ethanol and benzene. Most of these impurities are sandstone and iron filings, and some organic metal salts which are insoluble in solvents brought by additives. Generally, the mechanical impurities of lubricating oil base oil are all controlled below 0.005% (the mechanical impurities below 0.005% are considered as nothing).


Implementation standard: GB/T511

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(10) Ash and sulfuric acid ash refers to the non-combustible substances left after burning under specified conditions. The composition of ash is generally considered as 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 products (new oil) added with metal salt additives, ash becomes a means to quantitatively control the amount of additives added. Use sulfuric acid ash from abroad instead of ash. The method is to add a small amount of concentrated sulfuric acid before burning and ashing the oil sample, so that the metal elements of the additive can be converted into sulfate.

Implementation standard: GB/T 508


(11) Under specified experimental conditions, the charred black residue formed after heated evaporation and combustion of carbon residue oil is called carbon residue. Carbon residue is an important quality index of lubricating oil base oil, and it is a specified item for judging the properties and refining depth of lubricating oil. The amount of carbon residue in lubricating oil is not only related to its chemical composition, but also related to the refining depth of oil. The main substances that form carbon residue in lubricating oil are gum, asphaltene and polycyclic aromatic hydrocarbons. Under the condition of insufficient air, these substances are decomposed and condensed by strong heat to form carbon residue. The deeper the oil refining depth, the smaller the carbon residue value. Generally speaking, the smaller the carbon residue value of blank base oil, the better. At present, many oil products contain additives of metal, sulfur, phosphorus and nitrogen, and their carbon residue value is very high, so the carbon residue of oil containing additives has lost its original meaning of carbon residue determination. Mechanical impurities, moisture, ash and carbon residue are all quality indexes that reflect the purity of oil products and the refining degree of lubricating base oil.


Implementation standard: GB/T 17144

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