Chinese classification number: TE624.82 Literature identification code: A 0 Introduction Lubricating oil products are usually used at very low ambient temperatures, especially in winter and cold car conditions, so they are required to be used Lubricating oil has good low-temperature flow properties, which not only facilitates the normal start-up operation of the machine, but also reduces the wear of the machine during the start-up process.
The lubricants currently used are mainly prepared with mineral oil as the base oil. Mineral base oil is the base oil obtained by refining petroleum fractions. Petroleum usually contains a certain amount of wax. In our country, Most of the petroleum is waxy petroleum, and the wax content of the lubricating oil fraction in some crude oil exceeds 40%. At low temperatures, the wax in the oil will precipitate to form crystals and form a loose and messy three-dimensional crystal network. The oil is wrapped in this loose and messy three-dimensional crystal network, which hinders its flow or even "solidifies".
Reducing the solidification temperature of lubricating oil can expand the operating temperature range of the oil and improve the performance and economy of the oil. Dewaxing oil and using pour point depressants are effective ways to reduce the oil's solidification temperature. From a process perspective, when a certain dewaxing temperature is reached, in order to obtain oil with a lower freezing point, the dewaxing temperature must be continuously lowered, which will bring high costs. The use of pour point depressants is an appropriate way to reduce the It is an effective and economical means to determine the freezing point of oil at a certain dewaxing depth. The author of this article introduces a little understanding of the pour point depressants used in lubricating oils.
1 Solidification of lubricating oil
Many types of lubricating oil are used in low-temperature environments, so they all have low-temperature flow performance requirements, and some use low-temperature dynamic viscosity as the technology Indicators, some use pour point as technical indicators, which are related to the "solidification" of lubricating oil. Pour point is no longer used as a technical indicator in lubricating oil technical specifications. Correspondingly, pour point is used as a technical indicator of low-temperature flow performance of oil products, and the test method is GB/T 3536. The pour point test method is more scientific and reasonable than the pour point test method GB/T 510, but in terms of understanding the low-temperature flow and solidification of oil products, the pour point is easier to understand. The pour point is the highest temperature at which oil loses fluidity under test conditions, and the pour point is the lowest temperature at which oil can flow under test conditions. The two are closely related. In order to be consistent with the Chinese technical terminology of pour point depressant, this article uses the term "solidification" to discuss oil products.
If no obvious liquid level movement is seen under the specified test conditions, the oil at this time is called solidification. The solidification of the oil is related to two situations. The first situation is that due to the structure formed by wax crystals, the lubricating oil cannot flow obviously under test conditions. This situation is called structural solidification, and the pour point depressant only works in this situation. Another situation is that the viscosity of the oil is too high at low temperatures and no obvious flow can be seen under the specified conditions. This kind of solidification is called viscosity-temperature solidification. The pour point depressant not only has no effect on this situation, but also increases the oil content. yield stress.
2 Composition of the pour point depressant
The earliest pour point depressant used in lubricating oils was long-chain alkylnaphthalene with the trade name Paraflow. Later, long-chain alkanes appeared one after another. There are three types: polyphenol, polymethacrylate and poly-alpha-olefin.
Except for poly-alpha-olefin, the three pour point depressants are all surfactants, that is, their molecular structures are composed of two parts, one part is an active group such as an aromatic group, a phenol group and an ester. Etc., the other part is a long-chain alkyl group with the same structure as paraffin. The length of the alkyl group must be sufficient. On the one hand, only the length of the chain alkyl group is sufficient. Substances with active groups and chain alkyl groups can act as surfactants and play a directional adsorption role in the system; on the other hand, the length of the alkyl group must be sufficient. On the one hand, the length of the chain alkyl group should be the same as the length of the paraffin contained in the lubricating oil in order to function better. Similarity and mutual solubility are the basic theorem of solution interaction. The more similar the two are, the easier it is to produce adsorption and crystallization. effect. Since the wax in the lubricating oil is not a pure compound, it is also a complex mixture. Therefore, the molecular weight of the long-chain alkyl group in the pour point depressant is required to be similar to the average molecular weight of the wax contained in the lubricating oil in order to better exert its pour point reducing effect.
3 The pour point depressant mechanism of the pour point depressant
3.1 Directional adsorption
After the pour point depressant itself is a surfactant and added to the lubricating oil, the polar group They will attract each other, and the non-polar groups will also attract each other, and the pour point depressants will be arranged in an orderly manner in the lubricating oil system in space. Polyalphaolefins are not surface-active substances and their ordering is not based on this theory.
Regardless of whether organic matter or inorganic matter, substances with the same molecule have very different properties due to different molecular structures or the arrangement of atoms that make up the molecules. When the atoms that make up the molecules are arranged irregularly or have no obvious rules, When ordered, such substances have an amorphous structure, that is, a non-crystalline structure, and their physical parameters usually have no definite values. When the atoms that make up the molecules are arranged in a regular order, such substances have a fixed structure, that is, a crystal structure. The molecules of poly-alpha-olefin have this kind of regular and ordered crystal structure, and they also rely on this structure to orderly arrange themselves in the lubricating oil system.
When the pour point depressant molecules are ordered and distributed in the lubricating oil system, based on the principle of similar mutual solubility, the wax molecules in the lubricating oil will be adsorbed or distributed in the pour point depressant in a directional and orderly manner. Alkyl moiety.
3.2 Crystal Center
When the temperature of waxy lubricating oil drops to the critical precipitation temperature of wax, the long-chain alkyl groups in the pour point depressant precipitate together to act as the crystal center or crystal nucleus. As a result, the wax in the oil is adsorbed on the crystal nuclei and precipitates, forming large and dense crystals. This large and dense crystal does not wrap or is less wrapped and mixed with oil. There is enough oil and oil flowing outside the wax crystal. space, the freezing point of the oil will decrease.
If there is no pour point depressant in the lubricating oil, there will be no such crystal center. Since the wax in it exists in a chaotic and disordered state, when the temperature of the lubricating oil drops to the critical precipitation temperature of the wax, , a large number of small crystal nuclei will be produced in the oil system. Because there are too many small crystal nuclei, the volume of each crystal nucleus will be reduced, and the sizes of wax crystals are also different, especially their structures are very loose, in which the wrapping and Not only that, these small crystal nuclei are connected to each other in the oil system, both in space and breadth, forming a disordered sheet and network structure in the entire system, making the oil in it Loss of fluidity results in structural solidification.
3.3 Crystallization and precipitation
The pour point depressant and the wax in the oil precipitate at the same time at low temperature, and crystallize with the wax. Due to the orderly arrangement and directional adsorption of the pour point depressant molecules in the lubricating oil and the role of the crystal center, the crystals produced by the pour point depressant and wax can only be oriented and ordered in the oil, thereby avoiding chaos. Disordered sheet-like and network-like structures.
4 Commonly used pour point depressants
Commonly used pour point depressants in lubricating oils include alkyl naphthalenes, polyesters and polyolefins. Polyesters are mainly polymethacrylates, and polyolefins are mainly polyalpha-olefins.
4.1 Alkyl Naphthalene
Alkyl naphthalene is the earliest used lubricating oil pour point depressant. It was put into industrial production by my country Petroleum Dalian Petrochemical Branch in 1954 and is the first in my country. A variety of oil additives produced. The alkyl naphthalene used as a pour point depressant is produced using the chlorinated paraffin process, and the product code is T801. It is still a widely used pour point depressant.
As the main pour point depressant, alkyl naphthalene has good economic efficiency. When used in combination with other types of pour point depressants, better pour point reducing effects can be achieved. Alkyl naphthalenes, which serve as pour point depressants, also act as antioxidants in lubricating oils, a role that has been overlooked. The reason why alkyl naphthalenes act as antioxidants is because in a mixture of hydrocarbons, polycyclic aromatic hydrocarbons will oxidize themselves, protecting other hydrocarbons and slowing down the oxidation rate of the entire oil system, thereby acting as antioxidants. role. In addition, its oxidation products can also play the role of antioxidants, because alkylnaphthalenes are easily oxidized to alkylphenols, and phenols are well-known antioxidants, and the oxidation products of this alkylnaphthalenes still have good Therefore, the freezing point of lubricating oil containing alkyl naphthalene pour point depressant will not increase significantly even if it is deeply oxidized.
4.2 Polymethacrylate
Polymethacrylate is a multi-effect lubricant additive. As a pour point depressant, it has good performance in various lubricants. Polymethacrylate also acts as a viscosity index improver and anti-foaming agent in oil products.
Polymethacrylate is an acrylic non-silicon anti-foaming agent. The effect of this type of anti-foaming agent is greatly affected by the lubricating oil system. Therefore, when using polymethacrylate as a pour point depressant Be sure to coordinate with anti-foaming agents.
4.3 Poly-α-olefin
Poly-α-olefin is a pour point depressant developed in my country. It is a non-surfactant type of pour point depressant, so poly-α-olefin is used as a pour point depressant. The pour point "rebound" phenomenon of surfactant-type pour point depressants, which occurs when the oil pour point increases as the pour point depressant content increases, does not occur.
Since the polyα-olefin pour point depressant molecule does not contain polar groups, it is best to use it in combination with an alkyl naphthalene pour point depressant or an acrylic ester pour point depressant, which can control wax crystals. The arrangement and distribution are more orderly and achieve better condensation reducing effect.
4.4 Other types of pour point depressants
In addition to the three commonly used ones mentioned above, there are dozens of reported pour point depressants, which are sold as commercial products. There are more than ten kinds of pour point depressants, and these kinds of pour point depressants are basically surfactants.
After understanding the mechanism of pour point depressants, it is not difficult to find that many surfactants with similar molecular structures can reduce the freezing point of lubricating oil. However, lubricating oil is a relatively complex product. System, lubricating oil not only has good low-temperature fluidity, but also has many performance requirements. Other performance requirements for lubricating oil are basically achieved through functional additives, and these functional additives are basically surface-level additives. Active substances all have polar functional groups, so the surfactant used as a pour point depressant must interact with the functional additives in the oil. Especially as the polarity of the polar groups in the surfactant increases, it will affect the lubricating oil. The system has an impact and affects the performance of the oil.
After long-term testing and use, it has been proved that the three types of pour point depressants introduced above will basically not have a negative impact on the performance of the oil system. Other functions of oils play an enhancing role to a certain extent. However, for other pour point depressants, especially some surfactants whose pour point depressant effect is improved due to the increase in the polarity of the polar groups in the molecules, the evaluation cannot only be based on the reduction of the pour point. The entire lubricating oil formula system and lubrication should be considered. Only by examining the various technical properties of the oil can we make evaluations and judgments.
5 Differences between pour point depressants in lubricating oils and other oils
In addition to lubricating oils, pour point depressants are also often used in diesel and crude oil to improve their The low-temperature or normal-temperature transportation capability. The pour point depressants used in diesel and crude oil are also surfactants. The pour point depressants used in these two types of oils can only consider the pour point reducing effect on the oil, that is, on the oil. When examining the impact of rheological properties at temperature, there is basically no need to consider the impact on other components and other properties of the oil. This is the main difference in the use of pour point depressants for diesel and crude oil and lubricating oil pour point depressants. In addition, there are many colloids and asphaltenes in crude oil. When selecting a pour point depressant, you must consider giving full play to their polar effects. When the pour point depressant and paraffin crystallize, the polar groups of the surfactant and the colloids should be used. , asphaltene interaction, and the pour point depressant in lubricating oil should be avoided as much as possible to interact with the polar groups of additives in the oil, unless it has a synergistic effect with other functional additives in improving oil performance. Therefore, whether the varieties used in diesel and crude oil can be used in lubricants must be strictly inspected and evaluated.
6 Conclusion
The use of lubricating oil pour point depressants can effectively reduce the solidification temperature of mineral lubricating oils, improve the low-temperature flow properties of oils, and significantly improve the performance and economy of oils. benefit.
Through in-depth research on the three currently commonly used types of lubricating oil pour point depressants, their performance can be explored and brought into play. There are also molecular structures between the same types of these three types of lubricating oil pour point depressants. and molecular size differences, which all affect its matching and performance with lubricant base oils and formulas, and should be considered when researching oil products.
In addition, there are many patent reports on lubricating oil pour point depressants at home and abroad. The reported pour point depressants have excellent performance in the reported oil systems and have a large pour point reduction. However, these Patented products are not widely used, which also shows the importance of matching the pour point depressant with the lubricating oil base oil and formula. Therefore, it is necessary to select the appropriate pour point depressant in a certain base oil and formula system. If possible, it should be based on the basic oil and formula. The oil and formula properties are used to synthesize the pour point depressant, which can achieve better results and economic benefits.
Date of receipt: 2009-10-10.
About the author: Deng Guangyong (1964-), male, senior engineer, graduated from the former Fushun Petroleum Institute majoring in applied chemistry in 1983. He has been engaged in oil analysis, lubricant research and technical services, and has been published publicly. Many articles.