Lubricating Oil Conventional Indicators

Lubricating oil is a kind of technology intensive product, is a complex mixture of hydrocarbons, and its real performance is the comprehensive effect of complex physical or chemical change process. The basic properties of lubricating oil include general physical and chemical properties, special physical and chemical properties and simulation bench test.

General physical and chemical properties

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

(1) Viscosity

Viscosity reflects the internal friction of an oil, and is an indicator of oil's oiliness and fluidity. Without adding any functional additives, the higher the viscosity, the higher the oil film strength and the worse the fluidity.

(2) Viscosity index

The viscosity index shows the degree to which the viscosity of an oil varies with temperature. The higher the viscosity index is, the less the oil viscosity is affected by temperature, the better its viscosity and temperature performance is, and the worse it is otherwise.

(3) Flash point

Flash point is an index indicating the evaporability of oil products. The lighter the oil fraction, the more evaporative and the lower the flash point. On the contrary, the heavier the oil fraction, the less evaporative, the higher the flash point. At the same time, flash point is an indicator of the fire risk of petroleum products. The danger level of oil is divided according to the flash point. It is strictly forbidden to heat the oil to its flash point temperature during the storage and transportation of oil. Under 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 lubricating oil. It is generally believed that the flash point is 20 ~ 30℃ higher than the use temperature, which can be used safely.

(4) acid value and base value

Acid value is the indicator of the acid substance in the lubricating oil, the unit is mgKOH/g. There are two kinds of acid values, strong acid value and weak acid value, and the combination of the two is the total acid value (TAN). When we say "acid number", we actually mean "total acid number".

The base value is an index indicating the content of basic 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 means "total base number (TBN)".

(5) Freezing point and pour point

Freezing point is the temperature at which oil stops flowing under specified cooling conditions. The solidification of oil is very different from that of pure compounds. Oil does not have a clear solidification temperature, the so-called "solidification" as a whole to see the loss of fluidity, not all the components become solid.

The freezing point of lubricating oil is an important quality index of 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. In contrast, low freezing point lubricants are not necessary in areas with higher temperatures. Because the lower the freezing point of lubricating oil, the higher its production cost, resulting in unnecessary waste. Generally speaking, the freezing point of lubricating oil should be 5 ~ 7℃ lower than the *** temperature of the operating environment. But it is especially important to mention that the freezing point, low temperature viscosity and viscosity-temperature characteristics of the oil should be considered when selecting low temperature lubricating oil. Because of the low freezing point oil, its low temperature viscosity and viscosity-temperature characteristics may not meet the requirements.

The freezing point and pour point are both indexes of low temperature fluidity of oil products. There is no difference in principle between them, but the determination method is slightly different. The freezing point and pour point of the same oil are not completely equal. Generally, the pour point is 2-3 ℃ higher than the freezing point, but there are some exceptions.

(6) Mechanical impurities

Mechanical impurities refer to precipitates or colloidal suspensions in lubricating oils that are insoluble in solvents such as gasoline, ethanol and benzene. Most of these impurities are such as sand and iron filings, as well as some organometallic salts that are insoluble in solvents brought by additives. In general, lubricating base oils have mechanical impurities below 0.005% (mechanical impurities below 0.005% are considered to be free).

(7) Moisture

Moisture is the percentage of water content in the oil, usually by weight. The presence of water in lubricating oil will destroy the oil film formed by lubricating oil, make the lubrication effect worse, accelerate the corrosion effect of organic acids on metals, rust equipment, and make oil easy to produce sediment. In short, the less water in the oil, the better.

(8) Ash and sulphate ash

Ash is the non-combustible material left after burning under prescribed conditions. The composition of ash is generally considered to be some metal elements and their salts. Ash has different concepts for different oils. For base oils or oils without additives, ash can be used to judge the refining depth of an oil. For oil products with metal salt additives (new oil), ash becomes a means to quantitatively control the amount of additives added. Sulphuric acid ash is used instead of ash in foreign countries. The method is to add a small amount of concentrated sulfuric acid before burning the oil sample after burning, so that the metal element of the additive is converted to sulfate.

(9) Carbon residue

Oil in the specified experimental conditions, heat evaporation and combustion after the formation of the char black residue called carbon residue. Carbon residue is an important quality index of lubricating oil base oil. It is a specified item for judging the nature and refining depth of lubricating oil. In lubricating oil base oil, the amount of carbon residue is not only related to its chemical composition, but also related to the refining depth of oil. The main substances forming carbon residue in lubricating oil are: gelatin, asphaltene and polycyclic aromatic hydrocarbons in oil. These substances under the condition of insufficient air, by strong thermal decomposition, condensation and formation of carbon residue. The deeper the refining depth of oil, the smaller the carbon residue value. Generally speaking, the smaller the carbon residue value of blank base oil, the better.

Now, many oil products contain metal, sulfur, phosphorus, nitrogen element 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 of oil purity, reflecting the degree of refining of lubricating base oil.

Special physical and chemical properties

In addition to the above general physical and chemical properties, each lubricating oil shall have special physical and chemical properties that characterize its use. The more high quality requirements, or strong specificity of the oil, its special physical and chemical properties are more prominent. The test methods reflecting these special physical and chemical properties are briefly described as follows:

(1) Thermal stability

Thermal stability indicates the high temperature resistance of oil, that is, the resistance of lubricating oil to thermal decomposition, that is, the thermal decomposition temperature. Some high quality compressor oil and so on 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 tend to have adverse effects on the stability of oil, and antioxidants can not improve the thermal stability of oil obviously.

(2) Oxidation stability

Oxidation stability shows the anti-aging performance of lubricating oil. Air compressor lubricating oil has this index requirement, so it has become a special performance of these kinds of oil requirements. There are many ways to determine the oxidation stability of oil. Basically, a certain amount of oil is oxidized at a certain temperature for a certain time in the presence of air (or oxygen) and metal catalysts, and then the acid value, viscosity change and precipitation formation of oil are measured. All lubricants have different automatic oxidation tendencies depending on their chemical composition and external conditions. With the use of the process of oxidation, and gradually generated some aldehydes, ketones, acids and glial, asphaltene and other substances, oxidative stability is to inhibit the use of the above is not conducive to the production of oil substances.

(3) Corrosion and rust

Corrosion of steel and other non-ferrous metals is often caused by oxidation of oil or additives. Corrosion test is generally put the red copper strip in oil, placed at 100℃ for 3 hours, and then observe the change of copper; In the rust test, corrosion will occur on the steel surface under the action of water and water vapor. To determine the rust resistance, 30ml distilled water or artificial sea water is added to 300ml test oil, and then the steel rod is placed in it, stirred at 54℃ for 24 hours, and then the steel rod is observed whether there is corrosion. The oil should be resistant to metal corrosion and rust, both of which are usually required in industrial lubricating oil standards.

(4) oil and extreme pressure

Oil is polarity of lubricating oil content on friction parts solid physical and chemical adsorption film is formed on the metal surface, which have the effect of high load and friction and wear resistance, and extreme pressure property is the polarity of the lubricating oil content on friction parts of the metal surface, the friction chemistry decomposition, high temperature, high load and metal friction and the surface chemical reaction, Forming low melting point soft (or plastic) extreme pressure film, so as to play the impact resistance, resistance to high load and high temperature lubrication.

(5) Foam resistance

Lubricating oil in the process of operation, because of the existence of air, often produce foam, especially when the oil contains surfactant additives, it is easier to produce foam, and foam is not easy to disappear. The foam produced in the use of lubricating oil will destroy the oil film, make the friction surface sintering or increase wear, and promote the oxidation and deterioration of lubricating oil, but also make the lubrication system air resistance, affect the lubricating oil cycle. Therefore, bubble resistance is an important quality index of lubricating oil.

(6) resistance to emulsification

Industrial lubricating oil is often inevitably mixed with some cooling water in use. If the emulsification resistance of lubricating oil is not good, it will form an emulsion with the water mixed, so that the water is not easy to release from the bottom of the circulating tank, which may cause poor lubrication. Therefore, emulsification resistance is a very important physical and chemical properties of industrial lubricants. General oil is 40ml test oil and 40ml distilled water at a certain temperature for a certain time, and then observe the oil-water-emulsion layer separated into 40-37-3ml time; Industrial gear oil is to test oil and water mixed, at a certain temperature and 6000 RPM stirring for 5 minutes, placed for 5 hours, and then measure the number of ml oil, water, emulsification layer.

(7) hydrolytic stability

Hydrolytic stability characterizes the stability of oil under the action of water and metal (mainly copper). When the acid value of oil is high, or contains additives which are easy to decompose into acidic substances in water, it often makes this index unqualified. Its determination method is to add a certain amount of water to the test oil, mix and stir the copper sheet at a certain temperature for a certain time, and then measure the acid value of the water layer and the weight loss of the copper sheet.

(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 hydraulic transmission accuracy and sensitivity, serious can not meet the use of hydraulic system requirements. This property is measured in a similar way as foam resistance, except that it measures the time at which air (fog) dissolved in the oil is released.

(9) Rubber sealing

In the hydraulic system to rubber seals are in the majority, the oil in the machinery inevitably contact with some seals, rubber sealing is not good oil can make rubber swelling, shrinkage, hardening, cracking, affecting its sealing, so the oil and rubber have a better adaptability. Hydraulic oil standards require rubber sealing index, it is a certain size of rubber ring immersion oil after a certain time to measure the change.

(10) shear stability

In the process of using the oil with viscosifier, due to the action of mechanical shear, the polymer in the oil is cut off, so that the viscosity of the oil decreases, affecting 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, there are ultrasonic shear method, nozzle shear method, Vickers pump shear method, FZG gear machine shear method, these methods are the ultimate determination of oil viscosity decline rate.

(11) Volatility

The volatility of base oils is related to fuel consumption, viscosity stability, and oxidation stability, which are especially important for multigrade and energy efficient oils.

(12) Solubility

Solubility is usually expressed as an aniline point. Different grades of oil have different aniline solubility limits for compound additives. The limit value of low ash oil is larger than that of overalkaline oil, and the limit value of single grade oil is larger than that of multi-grade oil.

(13) Electrical performance

Electrical properties are the unique properties of insulating oil, including dielectric loss Angle, dielectric constant, breakdown voltage, pulse voltage and so on. The refining depth, impurity and moisture of base oil have great influence on the electrical properties of oil.

(14) rust resistance

This refers specifically to the special physical and chemical properties of rust prevention grease should have, its test methods include humidity test, salt spray test, laminated test, water displacement test, in addition to the screen test, long-term storage test, etc.

In the operation process of air compressor, the production of sludge, carbon deposition and other sediments will seriously affect the efficiency of the compressor, lead to the decrease of heat dissipation, gas production efficiency and energy consumption of the compressor. What's more, it will cause the compressor equipment failure, increase the maintenance cost, and even cause serious accidents such as shutdown and explosion.

In the process of long-term continuous operation of air compressor, impurities, moisture and other components in the air are easy to enter the air compressor oil, which is the external pollution source of sediment.

The main components of conventional air compressor oil on the market are hydrocarbons. Air compressors often operate under high temperature and high pressure conditions. Coupled with oxygen, moisture and metal catalysis, air compressor oil will have oxidation reaction, leading to oil deterioration and the generation of intrinsic oxides. The higher the intensity of air compressor oil contact with high temperature and oxygen, the more severe the degree of oxidation reaction. Peroxides and hydroperoxides undergo alcohol-aldehydes condensation to form aggregates or polymers, which are further oxidized to highly hydrocarbon oxides, commonly referred to as oxides. These oxides are usually very viscous and tend to form deposits in the nose, oil lines and shells. Common deposits include paint film, sludge, coking and carbon deposition.

Paint film is A viscous film formed by resin separation and dehydration and/or polymerization on a hot surface. This deposit is not very serious, just a thin film. Paint film is a derivative of lubricant. When the temperature of the air compressor parts is low, the sediment produced is oil sludge. Sludge can be soft or slightly hard in consistency. If the working condition is mild, the resulting sludge may be soft. Such sludge is known as low-temperature sludge, and they occur when the ambient temperature is lower than 95℃. High-temperature sludge occurs when the ambient temperature is higher than 120℃ and its consistency is larger.

When the oxidation of oil intensifies enough, a large number of polymers and gelatinous materials come together to form what is called coking. The wear and tear of the space-time press leads to metal debris entering the oil. These metal ions are the natural catalysts for the oxidation reaction, which accelerates coking. Coking is also a sediment that may contain oil, water, resins (similar to paint film) and solid contaminants, among others.

Different types of air compressor oil will have different tendency to produce coking. After excluding the cause of compressor mechanical failure, the quality of air compressor oil directly determines the coking tendency of air compressor.

Carbon deposition is a more stubborn and harmful substance than coke. Under the repeated action of high temperature and high pressure, the gel produced by oxidation interacts with the oxygen, dust and impurities in the compressed air to become bituminous and further carbonize, forming carbon deposition.

Oxidation of air compressor oil will generate peroxides and free radicals, and further oxidation into sticky deposits. If soft sediment is not removed in time, it is easy to further polymerize into hard carbon deposition and coking under high temperature and high pressure operating environment. The light quality of these harmful substances will cause the air compressor to increase the operating temperature, increase the energy consumption, reduce the gas production efficiency, block the oil circuit, wear and other faults, affect the operation efficiency of the air compressor, increase the maintenance cost; It causes serious safety accidents such as nose lock, compressor shutdown and equipment explosion.