If you browse the market, in terms of gear oil, the biodegradable industrial gear oil varieties on the market, such as Jiashiduo's Biotrans 220, Total CarterBio 220, Shell's Shell Naturelle 220, etc., all use synthetic esters as base oils. From the current development trend, synthetic esters are considered the most ideal base oil for biodegradable lubricants. This is because it has the following advantages: ① excellent lubricity; ② Excellent high temperature stability and volatilization stability; ③ Good viscosity temperature performance; ④ Good biodegradability; ⑤ Good oxidation stability; ⑥ Low toxicity, etc.

In addition, synthetic esters also have the advantages of being miscible with mineral oils and most synthetic oils and additives. However, the price of synthetic ester oil is relatively high, usually 4-12 times that of mineral oil, and its anti emulsification performance is poor.

According to the number and position of ester groups in the molecule, synthetic esters are divided into monoesters, diesters, polyol esters, and diesters. The base oils for biodegradable lubricants often include diesters, polyol esters, and diesters, which have a biodegradation rate of over 70%. Polyol esters are esterification products of polyols and monobasic acids, typically obtained by esterification of neopentyl polyols, trimethylolpropane, pentaerythritol, and long-chain carboxylic acids (usually C6-C13 or oleic acid).

Compound ester is a low molecular weight polymer formed by polyols (neopentyl glycol, trimethylolpropane, pentaerythritol) and diacids, with a monoacid or monoalcohol at the end.

1. Ester oils have excellent viscosity temperature and low-temperature properties. Chain length increases, viscosity and viscosity index increase, and pour point increases; Adding side chains increases viscosity, decreases pour point, and increases viscosity index.

2. The viscosity of diesters is relatively low, but the viscosity index is high, generally exceeding 120, and the pour point is generally below -60 ℃, which is difficult to achieve with mineral oils of the same viscosity. ② The viscosity index of polyol esters is lower than that of diesters, but higher than mineral oils of the same viscosity, and the pour point is also much lower than that of mineral oils. ③ Compound esters have high viscosity but low pour point and high viscosity index, and are generally used as blending components to increase the viscosity of oil products.

3. Ester oil has good thermal stability. The thermal decomposition temperature of diesters is 280 ℃, much higher than mineral oils of the same viscosity, while the thermal decomposition temperature of polyol esters is above 310 ℃.  

4. Synthetic esters have good biodegradability. The biodegradability of synthetic esters is closely related to their chemical structure, and the introduction of branched and aromatic rings usually reduces the biodegradability of synthetic esters.