Effects and types of additives in Marine lubricating oil

With the improvement of diesel engine strengthening degree and the use of inferior fuel oil, the working condition of lubricating oil is getting worse and worse, and straight chain pure mineral oil can no longer meet the working needs of diesel engine. In recent decades, with the development of chemical industry, many additives have been added into pure mineral oil (base oil), forming a new type of oil composed of base oil and additives. The so-called additive refers to a small amount of substances added to improve and enhance the quality of petroleum products and give new properties to meet the requirements of use and storage performance. There are many kinds of petroleum additives and their uses are very wide. Its main functions are:

(1) Reduce the formation and accumulation of harmful sediments on engine components.

(2) Neutralize acidic substances and reduce their corrosion to equipment.

(3) To prevent corrosion of equipment and parts.

(4) Reduce friction and wear of equipment and parts, and prolong the service life of equipment and parts.

(5) The oxidation and thermal decomposition of lubricant is delayed and its service life is prolonged.

(6) Change the physical properties of lubricant, such as improving its viscosity index and viscosity temperature characteristics; Reduce the pour point, improve the low temperature performance; Reduce foam formation and so on.

Currently commonly used additives are generally divided into the following categories according to their performance:

1. Detergency dispersant (detergency additive)

In the diesel engine because of the bad working conditions, even if the use of thermal oxidation stability is very good lubricating oil will inevitably form carbon deposits, paint film and other deposits on the piston and cylinder. Detergent-dispersants has the effect of two aspects, on the one hand, because it has solubilization and condensation scattered suppression effect, so can prevent the oxidation of lubricating oil and fuel further condensation film, carbon deposition and sludge formation, at the same time can have formation of the carbon deposit, paint film, such as washing down from the workpiece surface, scattered into small particles suspended in the oil, keep the parts clean. On the other hand, because of its alkalinity, it can neutralize the sulfuric acid generated after the combustion of sulfur-containing fuel and the organic acid generated by the oxidation of lubricating oil. Because of its good effect on improving the performance of lubricating oil, the dispersant has been widely used in internal combustion engine lubricating oil. Generally, the higher the performance grade of lubricating oil, the greater the content of the additive. The API classification contains 7-8.5% CD grade additives and 12-14% super CD grade additives. (Cylinder lubricants, which will be introduced later, have a higher additive content, 20 ~ 30% cylinder oil for TBN70.) Most of these additives are clear dispersant additives.

There are many kinds of dispersant additives, which can be divided into two categories: ash and ash free. Such as ash calcium petroleum sulfonate, calcium alkyl phenol, calcium alkyl salicylate, no ash succinimide. All kinds of dispersant additives have their own advantages in various functions (such as solubilization, inhibition, dispersion and neutralization), and they have different degrees of water resistance and demulsification resistance of lubricating oil. In order to obtain more comprehensive dispersant properties, several dispersants should be properly combined and used. Due to the use of sulfur-containing fuel, the amount of high-alkaline dispersants accounts for a large proportion, which is mainly carbonate. This makes the highly alkaline lubricating oil accompanied by a high ash content, and the ash content of the highly alkaline cylinder oil can even reach 5-8%, which is a negative side. At present, one of the research directions of diesel engine lubricant is to develop towards low ash content.

Additive to prevent the formation of paint film at high temperature is called detergent additive; Additives to prevent the formation of sludge deposits at low temperatures are called dispersants. In Our country, it is collectively referred to as detergency dispersant. It has two functions: one is the washing effect, which can make the carbon smoke particles and asphalt resin particles deposited on the components in a dispersed suspension state, reduce carbon deposition and sludge, keep the components clean and prevent system blockage. The other is neutralizing acid. This additive is alkaline, which can control the organic acid formed by oxidation of oil, and neutralize the inorganic acid formed by combustion products into the crankcase, playing the role of corrosion inhibitor. It is an important additive for cylinder oil.

2. Oil agent, extreme pressure agent (antiwear agent)

Oiliness agent and extreme pressure agent can reduce wear under the condition of boundary lubrication. But oily agent can only form physical adsorption film or chemical adsorption film on the metal surface, can only work at lower temperatures. Extreme pressure agent can form reaction film. That is, the metal on the friction surface and the sulfur, phosphorus and other elements contained in the extreme pressure agent can interact with each other at a higher temperature to generate a low melting point alloy, so as to prevent the metal surface scratch, fusion welding (fusion wear) under harsh conditions. Zinc dialkyl dithiophosphate not only has good anti-oxidation and anti-corrosion but also has good extreme pressure wear resistance.

Oiliness agent and extreme pressure agent can reduce wear in boundary condition, but their mechanism is different. Oily agent is active substance with polar groups, it can be directed to the metal surface to form not easy to destroy the boundary adsorption film, in order to reduce wear, commonly used oily agent vulcanized whale oil, vulcanized cotton seed oil, etc.; Extreme pressure agent can decompose and produce active compounds under high temperature and high load, and generate low melting point compounds on the metal surface to form reaction films, which can reduce friction, prevent abrasion, reduce wear and strengthen the bearing capacity of oil film. It is mainly composed of organic compounds containing chlorine, sulfur and phosphorus, such as sulfur-chlorinated paraffins and phosphate esters.

3. Viscosity index improver and viscosity enhancer

Some engine lubricants are added to increase viscosity. The main function of viscosifier is to improve the viscosivity of lubricating oil. The lubricating oil added with viscosifier is called thickener oil. Used in vehicles that work outdoors. It is also used in lifeboat diesel engines. Thickener oil viscosity is not too high at low temperature, so that the diesel engine can still start at low temperature; The viscosity will not be too low at high temperature to ensure reliable lubrication of the cylinder during normal operation. Thickener oil can be used in summer and winter. Save energy.

These additives can improve the viscosity of base oil, improve its viscosity-temperature characteristics and improve its viscosity index. The lubricating oil with such additives is called thickener oil. For the outdoor use of diesel engines (such as lifeboats) winter starting temperature of -30℃, normal operation after the cylinder temperature can reach 200℃. Thickening machine oil at low temperature can make the viscosity increase little, and at high temperature thickening, in order to meet the different needs of diesel engines in winter and summer. Viscosifier can not only improve the hydrodynamic properties of lubricants, but also improve the adsorption capacity of lubricants. Its composition is mostly oil soluble chain polymer organic compounds, such as polymethacrylate and polyisobutene.

4. Defoaming agent

Defoaming agent can reduce the surface tension of foam, inhibit the occurrence of foam and make the bubble burst and disappear, to prevent the formation of stable foam. Such as dimethyl silicone oil.

5. Pour point depressant (pour depressant)

Pour point depressant does not change the temperature of paraffin precipitation, but only the structure of paraffin. It is adsorbed on the crystallization surface of paraffin in the oil, so that it can only generate small crystals, prevent the formation of crystallization net, so as to change the fluidity at low temperature and reduce the freezing point of the oil. Long chain alkyl naphthalene is commonly used.

6. Anti-rust additive and anti-corrosion agent

The role of antirust agent is to rely on its own polarity, adsorption on the interface of metal and oil to form a protective layer, prevent water and metal contact rust.

Anti-corrosive agents protect the surfaces of non-ferrous metals such as bearing alloys from and neutralize acids produced by oil oxidation and gas.