Molecular Simulation And Experimental Study On The Antioxdant Effect Of Natural Phenolic Acid Antioxidants In Ester-base Lubricants

Lubricants have an important role to play in the machinery sector and the normal operation of machinery cannot be achieved without the lubricating effect of lubricants.However,during the long-term use of equipment,lubricants inevitably become oxidised,causing them to lose their original function.The oxidation process of lubricants is very complex,with high temperature oxidative degradation being the main cause.Most of the lubricants that fail after oxidation are simply discarded or incinerated,which undoubtedly leads to the spread of toxic substances in them,causing damage to humans as well as the environment.In recent years,synthetic ester-based lubricants with a very high biodegradation rate have become an alternative to traditional mineral oils.However,ester-based lubricants often suffer from poor cold flow properties,low thermal oxidation onset temperatures and poor thermal-oxidative ageing properties,so a suitable antioxidant needs to be selected for their use together.The traditional synthetic antioxidants have the defects of poor thermal stability and easy decomposition of harmful substances by heat.Therefore,non-toxic and non-polluting natural antioxidants have become the object of research in this paper.The solubility,binding energy,diffusion behaviour and O2 barrier properties of three phenolic acid natural antioxidants,ferulic acid,ethyl ferulate and isooctyl ferulate in base oil pentaerythritol oleate(PETO)were investigated by a combination of molecular modelling and experimental methods.The simulations were validated by pressure differential scanning calorimetry(PDSC),rotary oxygen bombardment(RBOT)and Rancimat oxidation stability analyzer(Rancimat),supplemented by infrared spectroscopy and viscosity testing to investigate the internal structure of oxidation and changes in viscosity of different antioxidant/lubricant systems.The antioxidant mechanism of the antioxidant/lubricant system was investigated by a combination of molecular simulations and experiments.The main studies are as follows:(1)Firstly,the phenolic hydroxyl bond dissociation energies(BDE)of the three different antioxidants were derived by quantum mechanical simulation(QM),and it was found that the BDE values of the three antioxidants were not significantly different and the scavenging effect of free radicals was expected to be the same.Therefore,the results of the quantum mechanical simulation calculations show that the chemical activity of the three is the same,and the differences in the physical factors of the three need to be further investigated.The solubility parameter(δ),diffusivity,free volume fraction(FFV)and binding energy(Ebinding)of different antioxidants in PETO base oils were calculated by molecular dynamics simulation(MD),and the permeation coefficients(P)of different systems for O2 were calculated by Monte Carlo simulation(MC)and MD methods.The results show that the antioxidant isooctyl ferulate has the highest compatibility with PETO,the lowest mobility in PETO and the highest binding energy to PETO.In addition,the PETO/isooctyl ferulate system has the lowest calculated FFV value and the highest resistance to O2 penetration.That is,isooctyl ferulate exhibits the best physical properties and has the best oxidative stability.(2)The simulation results were validated using experimental methods.PETO specimens with equimolar concentrations of antioxidants were prepared and the oxidation onset temperature(OOT)of the lubricant was tested using PDSC,the oxidation induction time(OIT)of the samples were tested by Rancimat and ROBT.The results show that the PETO/isooctyl ferulate system has the highest OOT and the longest OIT,validating the conclusion that the simulations predict the best oxidative stability of isooctyl ferulate.In addition,the dynamic viscosity of the lubricant before and after oxidation was tested in an accelerated oxidation test in a thermal oxygen ageing oven and it was found that the system with PETO/isooctyl ferulate showed the lowest dynamic viscosity growth rate and the best antioxidant effect.Infrared spectroscopy of the oxidised samples also showed the least rise in the C=O peak in the PETO/isooctyl ferulate system,again indicating that isooctyl ferulate showed the best antioxidant properties.Finally,the prepared ethanol solutions of different antioxidants with equal molar concentrations of DPPH were tested by UV spectrophotometry and it was found that the isooctyl ferulate system showed the best scavenging effect of DPPH radicals,which also matched the simulation results.According to simulations and experimental studies,isooctyl ferulate has the best antioxidant effect,followed by ethyl ferulate and ferulic acid,phenolic acids with larger molecular structures and longer alkyl chains opposite the phenolic hydroxyl group will have better antioxidant effects.Antioxidants in lubricants not only exert their antioxidant effect by scavenging free radicals,they also present a barrier effect on O2 penetration,thus slowing down the oxidation process of the lubricant.This study explores the physical and chemical anti-aging mechanisms of antioxidants in lubricants through molecular simulations,providing some theoretical guidance for rational analysis of the anti-aging mechanisms of antioxidants in other types of materials and a new model for the development of antioxidants for lubricants.