Pentaerythritol esters are commonly used in the lubrication field that can not be replaced by mineral oils due to their excellent lubrication performance, low volatility, thermal stability, as well as their biodegradability and renewable raw materials. This paper reported the esterification reaction of pentaerythritol or dipentaerythritol with two or more mixed fatty acids, including n-heptanoic acid, n-octanoic acid, 2-ethylhexanoic acid, nonanoic acid, 3,5,5-trimethylhexanoic acid, decanoic acid and dodecanoic acid, synthesizing nine kinds of pentaerythritol mixed esters (PE). The esterification reaction was affected by molecular steric hindrance, growth of alkyl chains and increase the proportion of dipentaerythritol would reduce the esterification rate of PE. The infrared spectroscopy analysis results indicated that these PEs were successfully synthesized. By testing the viscosity, pour point, thermogravimetric analysis, evaporation loss, oxidation stability and tribological properties of PEs, the relationship between the properties and structure of PEs were systematically studied. The viscosity test showed that the viscosity index values of all PEs were above 120, indicating excellent viscosity-temperature performance. Moreover, the viscosity and viscosity index of PEs showed an increasing trend with the increase of alkyl chain length. Increasing the proportion of dipentaerythritol could improve the viscosity of PEs with less impact on the viscosity index. The pour point test showed that the pour point of the ester oil could be obviously reduced by branched-chain fatty acid. As the molecular weight of fatty acids and the proportion of dipentaerythritol increased, the pour point of PE showed an upward trend. Thermogravimetric testing showed that the 10% thermal decomposition temperature of all PEs were higher than 310 ℃, indicating that they showed excellent thermal stability. The increase in both molecular weight of fatty acids and proportion of dipentaerythritol would increase the thermal decomposition temperature. All PEs were evaporated for 1 hour at 250 ℃, the results showed that increasing the molecular weight of fatty acids reduced the evaporation loss of PE. However, the evaporation loss of ester oil increased with the increase of the proportion of dipentaerythritol. The oxidation stability test showed that the initial oxidation temperature of PE decreased with the increase of molecular weight of fatty acids, increasing the proportion of dipentaerythritol had no obvious effect on the oxidation stability. All lubricants were tested under experimental conditions of 392 N, 1 200 r/min, 60 min, 75 and 150 ℃, the average friction coefficients below 0.12 and wear scar diameter (WSD) below 0.98, indicating that they showed good tribological properties. The friction experiment results showed that compared with 3,5,5-trimethylhexanoic acid, 2-ethylhexanoic acid showed better lubrication performance. As the length of the straight chain acid increased, the friction coefficient and wear scar diameter of PEs were decrease sequentially at 75 ℃. As the proportion of dipentaerythritol increased, the friction coefficient and wear scar diameter of PEs were no obvious change at 75 ℃. As the length of the straight chain acid increased, the friction coefficient of PEs decreased, the wear scar diameter of PEs first decreased and then increased at 150 ℃. As the proportion of dipentaerythritol increased, the friction coefficient of PEs gradually decreased and the phenomenon of fluctuation in wear scar diameter at 150 ℃.

In summary, the relationship between the molecular structure of PEs and their tribological and physicochemical properties were studied by changing the structure of branched chain acid, the length of straight chain acid and the ratio of dipentaerythritol. It was found that PE showed good tribological properties and excellent high and low temperature properties, providing technical support for the structure adjustment and performance improvement of pentaerythritol esters.