Each type of lubricating grease has its common general physical and chemical properties to indicate the internal quality of the product. For lubricating oil, these general physical and chemical properties are as follows:

(1) Appearance (chroma)

The color of an oil product can often reflect its refining degree and stability. As 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 the base oil produced by different oil sources and base crude oils may be different.

For the new finished lubricating oil, due to the use of additives, color has lost its original significance as an indicator to judge the refining degree of base oil.

(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 same viscosity or the same relative molecular weight, the lubricating oil with more aromatics, more colloids and asphaltenes has the highest density, the one with more cycloalkanes is in the middle, and the one with more alkanes is the lowest.

(3) Viscosity

Viscosity reflects the internal friction of an oil product and is an indicator of its oiliness and fluidity. Without any functional additives, the higher 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 oil viscosity changes with temperature. The higher the viscosity index, the less the influence of temperature on the oil viscosity, the better the viscosity temperature performance, and vice versa.

(5) Flash point

Flash point is an indicator of oil evaporation. The lighter the fraction, the greater the evaporation and the lower the flash point. On the contrary, the heavier the distillate, 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 products is classified according to the flash point. Flammable products are those whose flash point is below 45 ℃, and flammable products are those whose flash point is above 45 ℃. During the storage and transportation of oil products, it is strictly prohibited to heat the oil products to its flash point temperature. Under the same viscosity, the higher the flash point, the better. Therefore, the user should select the lubricating oil according to the service temperature and the working conditions of the lubricating oil. It is generally believed that the flash point is 20 ~ 30 ℃ higher than the operating temperature, which can be used safely.

(6) Freezing point and pour point

The freezing point refers to the maximum temperature at which the oil stops flowing under the 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 to indicate the low temperature fluidity of lubricating oil. It is of great significance 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 lubricating oil, the higher the production cost, resulting in unnecessary waste. Generally speaking, the freezing point of lubricating oil shall be 5~7 ℃ lower than the minimum temperature of the operating environment. However, it should also be mentioned that when selecting low-temperature lubricating oil, it should be comprehensively considered in combination with the freezing point, low-temperature viscosity and viscosity temperature characteristics of the oil. The low temperature viscosity and viscosity temperature characteristics of low pour point oil may not meet the requirements.

Freezing point and pour point are indicators of low-temperature fluidity of oil products. There is no principle difference between them, but the determination method is slightly different. The pour point and pour point of the same oil are not exactly the same. Generally, the pour point is 2 ~ 3 ℃ higher than the pour point, but there are exceptions.

(7) Acid value, alkali value and neutralization value

Acid value is an indicator indicating that the lubricating oil contains acidic substances, and the unit is mgkoh/g. Acid value can be 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 alkali value is an index indicating the content of alkaline substances in lubricating oil, and the unit is mgkoh/g.

The alkali value can also be divided into strong alkali value and weak alkali value, and the combination of the two is the total alkali value (TBN). What we usually call "base number" actually refers to "total base number (TBN)".

Neutralization value actually includes total acid value and total alkali value. However, unless otherwise noted, "neutralization value" generally refers to "total acid value" and its unit is mgkoh/g.

(8) Moisture

Water content refers to the percentage of water content in lubricating oil, usually by weight. The presence of water in the lubricating oil will damage the oil film formed by the lubricating oil, deteriorate the lubrication effect, 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 in lubricating oil that are insoluble in solvents such as gasoline, ethanol and benzene. Most of these impurities are sand, stone and iron filings, as well as some organic metal salts that are difficult to dissolve in solvents brought by additives. Generally, the mechanical impurities of lubricating oil base oil are controlled below 0.005% (mechanical impurities below 0.005% are considered as none).

(10) Ash and sulphated ash

Ash refers to the non combustible substance 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 products (new oil) with metal salt additives, ash content becomes a means to quantitatively control the amount of additives. Sulfuric acid ash is used to replace ash in foreign countries. The method is as follows: a small amount of concentrated sulfuric acid is added after the oil sample is burned and before burning and ashing to convert the metal elements of the additive into sulfate.

(11) Residual carbon

Under the specified experimental conditions, the burnt black residue formed after heated evaporation and combustion of oil products is called carbon residue. Carbon residue is an important quality index of lubricating oil base oil, and it is an item specified to judge the nature and refining depth of lubricating oil. The amount of carbon residue in lube base oil is not only related to its chemical composition, but also related to the refining depth of the oil. The main substances that form carbon residue in lube oil are gum, asphaltene and polycyclic aromatic hydrocarbons in the oil. Under the condition of insufficient air, these substances undergo strong thermal decomposition and condensation to form residual carbon. The deeper the refining depth, the smaller the carbon residue value. Generally speaking, the smaller the residual carbon value of blank base oil, the better.

At present, many oil products contain additives of metal, sulfur, phosphorus and nitrogen, and their residual carbon value is very high. Therefore, the residual carbon of additive oil has lost its original meaning of residual carbon determination. Mechanical impurities, moisture, ash and carbon residue are all quality indicators reflecting the purity of oil products and the refining degree of lubricating base oil.