Lubricant is a technology intensive product and a complex mixture of carbohydrate, whose real using performance is a complex comprehensive effect of physical or chemical changes. The basic properties of lubricant generally consist of general physical and chemical properties, special physical and chemical properties and simulation bench test.

Each type of lubricating oil or grease shares common physical and chemical properties, which indicates the inherent quality of the product. For lubricating oil, these physical and chemical properties are as follows:

(1) Appearance (color)
The color of oil usually reflects its refining degree and stability. For base oil, generally the higher the refining degree is, the more cleanly its hydrocarbon oxides and sulfides will be removed, and the lighter its color will be. However, even with the same refining conditions, the base oil produced by different sources of oil and with different crude oils will vary in color and transparency degree.

For new finished products of lubricant, due to the adding of additives, color has lost its original meaning in identifying the refining degree of the base oil.

(2) Density
Density is the simplest and commonest indicator of physical properties of lubricant. The density of lubricant increases with the increase of the content of carbon, oxygen and sulfur. Therefore, under the same viscosity or same relative molecular mass, the density of lubricant containing much aromatic hydrocarbon and colloid and pitch is the highest, the density of lubricant with much naphthenic hydrocarbon ranks middle, and the density of lubricant with much hydrocarbon alkyl is the lowest.

(3) Viscosity
Viscosity reflects the internal friction of the oil, and is an indicator of oiliness and flowability of the oil. When no functional additive is added, the higher the viscosity is, the higher the strength of oil film will be and the worse its flowability will be.

(4) Viscosity index
Viscosity index indicates the extent to which the oil viscosity changes with the temperature. The higher the viscosity index is, the lower the effect of temperature on oil viscosity will be, the better its viscosity-temperature property will be. Otherwise, it will be poorer.

(5) Flashing point
Flashing point is an indicator of oil evaporability. The lighter fraction of the oil is, the higher its evaporability will be and the lower its flashing point will be. Otherwise, the heavier fraction of the oil is, the lower its evaporability will be and the higher its flashing point will be. Meanwhile, flashing point also indicates the fire hazard of petroleum products. The hazard level of oil is rated based on its flashing point, which is rated as inflammable when its flashing point is below 45℃, and as combustible when its flashing point is above 45℃. In the course of storage and transport of oil, it is strictly prohibited to heat the oil to its flashing point. Under the same viscosity, the higher its flashing point is, the better it will be. Therefore, when selecting lubricant, the users should consider the using temperature and working conditions of the lubricant. Generally, the lubricant can be used safely when its flashing point is 20~30℃ higher than its using temperature.

(6) Condensation point and pour point
Condensation point is the highest temperature when oil stops flowing under the prescribed cooling conditions. The condensation of oil is different from that of pure compounds. Oil has not a fixed condensation temperature. The so-called "condensation" means that the whole loses flowability, but not all components have turned solid.

The condensation point of lubricant is an important quality indicator indicating the flowability of lubricant under a low temperature, which is of important significance for production, transport and use of the oil. Lubricant with a high condensation point cannot be used under a low temperature. And it is not necessary to use lubricant with a low condensation point in a region with a relatively high temperature, because the production cost will rise if the condensation point of lubricant is lower, which will cause a waste. Generally, the condensation point of lubricant should be 5~7℃ lower than the lowest temperature of the using temperature. However, what should be mentioned is, when selecting low-temperature lubricant, the users should comprehensively consider its condensation point, low temperature viscosity and viscosity temperature property, because even with a low condensation point, the oil might fail to meet the requirements in low temperature viscosity and viscosity temperature property.

Condensation point and pour point are both indicators of flowability of oil under a low temperature, which have no difference principally, but are different in testing method. The condensation point and pour point of the same oil are not completely equal. Generally the former is 2~3℃ lower than the latter, of course there is also exceptions.

(7) Acid number, base number and neutral number
Acid number indicates the content of acid substance in the lubricant, with a unit of mg KOH/g. Acid number consists of strong acid number and weak acid number, the combination of two is referred to total acid number (TAN). The "acid number" usually mentioned by us in fact refers to "total acid number (TAN)".

Base number indicates the content of base substance in the lubricant, with a unit of mg KOH/g.

Base number is also divided into strong base number and weak base number, the combination of the two is referred to as total base number (TBN for short). The "base number" usually mentioned by us in fact refers to "total base number (TBN)".

Neutral number in fact covers TAN and TBN. However, unless indicated otherwise, the "neutral number" mentioned usually by us in fact only refers to "TAN", whose unit is also mg KOH/g.

(8) Moisture
Moisture refers to the percentage of water in lubricant, usually a percentage of weight. The existence of moisture in lubricant will disrupt the oil film formed in the lubricant, which will worsen the lubricating effect, accelerate the corrosion of organic acid to metal, rust the equipment and make the oil generate sediment. Generally, the less the moisture contained in lubricant is, the better it will be.

(9) Mechanical impurities
Mechanical impurities refer to the sediments or colloidal suspensions that exist in lubricant and are insoluble in solvents such as gas, ethyl alcohol and benzene. Of these impurities, most are sands and scrap iron and some organic metal salts insoluble in water that come from additives. Usually, the content of mechanical impurities in the base oil of lubricant is controlled below 0.005% (it will be deemed as free of mechanical impurities when the content is below 0.005%).

(10) Ash content and sulfated ash
Ash refers to the non-combustible substances left after the lubricant is burned under prescribed conditions. The composition of ash is generally believed to be some metal elements and their salts. Ash is a different concept for different lubricants. For base oil or oil without additives, ash can be used to identify the refining depth of lubricant. For lubricant added with metal salt additives (new lubricant), ash is an approach for quantitative control of the amount of additives. Foreign countries adopt sulfated ash instead of ash. Its method is: add a little concentrated sulfuric acid before lubricant sample is burned into ash, which will convert the metal elements of additives into sulfate.

(11) Carbon residue
Under the prescribed experimental conditions, the charred black residues of lubricant after heating and evaporation and burned are known as carbon residue. Carbon residue is an important quality indicator of the base oil of lubricant and an item used for identifying the nature and refining depth of lubricant. The amount of carbon residue in base oil of lubricant is not only related with its chemical composition, but also related with its refining depth. The carbon residue mainly comes from substances such as colloid, pitch and PAH in the oil. These substances will generate carbon residue after being decomposed and condensed in a strong heat when the air is not adequate. The higher the refining depth of the lubricant is, the smaller its carbon residue value will be. Generally speaking, the smaller the carbon residue value is, the better the blank base oil will be.

Now, many lubricants contain additives containing elements such as metal, sulfur, phosphorus and nitrogen. Their carbon residue value is very high. Therefore, measurement of carbon residue has lost its original meaning for lubricants with additives. Mechanical impurities, moisture, ash and carbon residue are all quality indicators reflecting the purity of lubricant and the refining degree of base oil.