Synthesis,Performance And Mechanism Of Boron Containing Lubricant Additives Derived From Salicylaldehyde Schiff Base

Thanks to the advantages of antiwear,antifriction,antioxidation,non-toxic and odorless,organic borate esters have become a very important research topic as multifunctional lubricant additives,however,they are quite limited by their fairly easy hydrolysis,which leads to the research of the antihydrolytic and antiwear performance of borate esters to balance the antihydrolytic,tribological and antioxidant performance.In addition,the most recent ILSAC GF-6 specification demands higher temperature antioxidation and antiwear performance of lubricant additives.Therefore,it is our greatest interest to design boron containing multifunctional additives bearing higher temperature antioxidation and antiwear performance with excellent hydrolysis stability.In this dissertation,the salicylaldehyde Schiff base ligands were chelated by boron(III)to prepare new organic borate esters with N,O-coordinated boron-containing six-membered ring structure.These new borated esters not only demonstrated excellent hydrolysis stability,but also effectively showed excellent antioxidant and tribological properties.In addition,the relationship between the structure and the performance of the new borate ester additives was further investigated,and the mechanisms of tribology and antioxidation were explored.The main research contents are as follows:Firstly,four different types of salicylaldehyde Schiff base antioxidants(SSPDs)were designed and synthesized.The combination of SSPD antioxidant with zinc dialkyldithiophosphate(ZDDP)could significantly improve its antioxidant,antiwear and extreme pressure properties,but meanwhile could not reduce its corrosion resistance property.The results of scanning electron microscopy(SEM),3D laser confocal and X-ray photoelectron spectroscopy(XPS)not only confirmed the contribution of SSPDs and ZDDP to the tribological properties,but also found that the tribofilms were composed of metal oxides,metal phosphates,metal sulfates,metal sulfides,nitrogen metal complexes and decomposition products of SSPDs.At last,the synergistic mechanism between ZDDP and SSPDs at different temperatures was studied by ¹H and ³¹P NMR.And the results showed that SSPDs preferentially decompose the cage structure of ZDDP to form a new complex through the coordination of nitrogen in Schiff base imine structure and zinc atoms in ZDDP.Secondly,the salicylaldehyde Schiff base antioxidant was reacted with BF₃ to prepare a new metal and phosphorus free borate ester BFSSb1 by introducing B and F elements into the salicylaldehyde Schiff base antioxidant with N,O-coordination structure.BFSSb1 showed excellent hydrolysis resistance,and can maintain 1224hours stability at 60 ^oC and relative humidity of 75%.Compared with base oil,traditional antioxidant diphenylamine(DPA)and ZDDP,the oxidation induction period(OIT)of BFSSb1 increased by 2.25,1.33 and 4.76 times respectively,showing excellent antioxidant capacity.In addition,BFSSb1 has also showed excellent tribological performance,increasing the P_B of base oil by 57.6%.Moreover,the average wear volume(WV)of steel ball lubricated by BFSSb1 is 64.4%lower than that lubricated by ZDDP.SEM and XPS confirmed that BFSSb1 is the key for the improvement of the tribological performance.In conclusion,BFSSb1 achieved a highly efficient balance of antioxidant,antiwear and hydrolysis resistance.The relationship between structure and properties showed that the improvement of antiwear properties of the additive is mainly attributed to B and F atoms in the compound,and,the diphenylamine functional group in BFSSb1 is the key source of the antioxidant properties.Thirdly,four more borate esters(BNSb,BFNSb,BSPD and BFSPD)based on salicylaldehyde Schiff base were prepared with different boron sources.The borate ester additives showed excellent high temperature stability and hydrolytic resistance.In addition,the additives also showed much better antioxidant effect than DPA,2,6-di-tert-butyl-methy phenol and ZDDP,even though the antioxidant ability of BFNSb,and BFSPD is slightly weaker than that of BNSb and BSPD.At the same time,BFNSb showed excellent antiwear properties,but BNSb exhibited antagonistic effect when the load is more than 392 N.The effect of B and F atom on the antioxidant and antiwear properties was carefully examed.The introduction of B atom can improve the antioxidant,but the presence of F weakens the antioxidant performance.In terms of antiwear performance,due to the large steric resistance of the phenylboronic acid functional group in BNSb,which prohibits the formation of good adsorption membrane on the steel surfacte,resulting in poor antiwear performance;on the other hand,the F atom in BFNSb promotes the molecular adsorption to the steel surface,and the formation of B₂O₃and Fe F₃ through the tribochemical reaction further enhances the antiwear performance of the additive.Finally,in order to improve the oil solubility of the boron containing lubricant additives of salicylaldehyde Schiff base,a new type of borated polyisobutylene succinimide ashless dispersant,PIBSI-BSb,was designed and synthesized.PIBSI-BSb shows excellent antiwear performance.Compared with polyisobutylene succinimide dispersant(PIBSI),commercial borated polyisobutylene succinimide dispersant 154B and multifunctional additive ZDDP,the WV of PIBSI-BSb is reduced by 79.1%,61.8%and 60.3%,respectively.Furthermore,the morphology and chemical composition of the worn surface were analyzed by SEM,EDS and XPS.The XPS results showed that the tribofilm was composed of B₂O_3,Fe O,N-containing organic compounds and iron fluoride.The mechanism of competitive adsorption and successive antiwear of additives was revealed.In addition,PIBSI-BSb shows better antioxidant performance than the commercial products DPA,PIBSI and 154B,the OIT of PAO with PIBSI-BSb increased by at least 2.3 times.And the synergistic antioxidant mechanism of borated salicylaldehyde Schiff base and PIBSI was proposed.Finally,PIBSI-BSb shows similar dispersibility as PIBSI,and is stable for more than 1050 hours at 50 ^oC and relative humidity of 95%,showing excellent hydrolysis resistance.In conclusion,as an environmental friendly borated ashless dispersant,PIBSI-BSb achieved the balance and efficient combination of the dispersibility,antihydrolysis,antioxidant and antiwear properties,and has the potential of multifunctional lubricant additive.