Tham khảo tài liệu 'tribology lubricants and lubrication 2012 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 192 Tribology - Lubricants and Lubrication hydraulic fluids boat engines 2 stroke engines tractors agriculture equipments cut fluids cooling fluids etc Erhan Asadauskas 2000 . Esters have been used as lubricants since the beginning of the 19th Century in the form of natural esters in pig fat and whale oil Whitby 1998 . During World War II a large number of synthetic fluids were developed such as alcohol and long chain acids esters that presented excellent low temperature properties. Nowadays the esters represent only of the world lubricants market. However while the global consumption of lubricants has been stagnant the consumption of synthetic oils has grown approximately 10 per year. This growing esters consumption is due to performance reasons and also to changes on the environmental laws of several European Community countries mainly Germany. Esters have a low environmental impact and its metabolization consists of the following steps ester hydrolysis beta-oxidation of long chain hydrocarbons and oxygenases attack to aromatic nucleus. The main characteristics that reduce the microbial metabolization or degradability are Branching position and degree that reduce the beta-oxidation Molecule saturation degree Ester molecular weight increase. The strongest effect of the ester group on the lubricant physical properties is a decrease in its volatility and increase in its flash point. This is due to the strong dipole moment London forces that keeps the ester molecules together. The ester group affects other properties too such as thermal and hydrolytic stabilities solvency lubricity and biodegradability. Besides esters mainly from polyalcohols as trimethylolpropane TMP produce a unimolecular layer on the metal surface protecting it against wear. This layer is produced by the oxygen atoms which are presents in the ester molecules. The ester s most important physical-chemistry properties are viscosity viscosity index VI pour point lubricity thermal and hydrolytic .
