In gear oil, thickening agents only play a partial role in increasing the thickness of the elastohydrodynamic lubrication film (EHD). There are three reasons for this. Firstly, in the EHD oil film, polymer molecules can be arranged in sequence in the direction of motion to be tightly packed, and their spatial dimensions tend to shrink; ② Polymer molecules are blocked at the entrance of the contact zone and cannot enter the EHD membrane; ③ Polymer molecules are sheared.

Shear stability is an important indicator of gear oil, especially compared to other oils, because of the shear stress experienced by gear oil as mentioned earlier. In my opinion, the ultrasonic method (SH/T 0505) is not suitable for testing the shear stability of gear oil. Firstly, the severity is not sufficient, and secondly, the repeatability of the ultrasonic test results is not satisfactory; The nozzle shearing method (SH/T 0103) is suitable for dealing with internal combustion engine oil and hydraulic oil, but it is difficult for gear oil with high viscosity, and there is also a problem of repeatability in the results; So, the FZG gear testing machine method (SH/T 0200) is closer to the actual working conditions of gear oil, such as selecting test conditions, 6-level load, 90 ℃ oil temperature, 3000 revolutions per minute, 20 hours, and measuring the viscosity loss before and after the oil. It will have good discrimination for the tested samples, but the disadvantage is that the required sample size for this test is slightly larger. Recently, the 'Determination of Viscosity Shear Stability of Transmission Lubricants - Cone Roller Bearing Testing Machine Method' has been introduced, which provides a good option and reduces the amount of samples used. The testing cost and workload are also relatively small, which is good!

To ensure the shear stability of gear oil, we generally use low molecular weight polymethacrylates and low molecular weight polyisobutene as thickeners. Polyisobutylene with a weight average molecular weight of 2000-4000 can be used in industrial gear oil to ensure the shear stability of the product. However, it should be noted that such polyisobutene should be hydrogenated saturated to limit the saturation of its double bonds, in order to ensure the thermal oxidation stability of the oil product is not a problem. However, it is interesting that when comparing thickened and non thickened industrial gear oils using oxidation experiments (SH/T 0123), the viscosity growth trend before and after thickening of the sample is often smaller? Caused by thermal polymerization of viscosity increasing agent molecules. Furthermore, the low-temperature viscosity of polyisobutene is too high to be used as a viscosity enhancer for low viscosity vehicle gear oils (such as the 75W series). In this case, low molecular weight polymethacrylates are more commonly chosen. In fact, this is also the reason why ethylene propylene copolymer OCP is rarely used in gear oils.

In short, it may be too complicated for us to delve into the structure, mechanism of action, and performance of viscosity enhancers. There are several things we need to know:

1. Whether it is a chain structure such as OCP or a star structure such as CSI (styrene butadiene block copolymer), the thickening ability of the thickening agent depends on the volume it occupies in the oil. The larger the volume occupied, the stronger the thickening ability.

2. Under the same conditions, only PMA with a viscosity temperature coefficient greater than 1 can improve the viscosity temperature performance of the base oil. Therefore, oils containing PMA have a high viscosity index VI and good low-temperature performance.

3. Shear stability directly affects the viscosity grade stability of multiple engine oils and is closely related to their performance. Reducing the molecular weight of polymers can improve shear stability, but with large amounts added, there will be an increase in sediment and a deterioration in cleanliness (due to polymer thermal decomposition). So, the ability to increase viscosity and shear stability should be well balanced.

4. Non thickened base oil is more sensitive to thickeners as its viscosity decreases. PMA has good low-temperature performance and is suitable for preparing low viscosity, large-span internal combustion engine oil (such as 5W/40), low-temperature hydraulic oil ATF、 An indispensable thickening agent for large-span gear oil (such as 75W/90).

5. With the advancement of synthesis and polymerization processes, the introduction of other functional groups such as amine, amide, boronic ester, etc. into the molecules of thickeners can lead to the emergence of more and more varieties of multi effect thickeners. It is believed that in the future, thickeners with anti-oxidation, anti-wear, dispersion and other effects will be a development direction.