Synthesis, Tribological and Hydrolysis Stability Study of Novel Benzotriazole Borate Derivative Xiong Liping1*, He Zhongyi1,3, Qian Liang1, Mu Lin1, Chen Aixi1, Han Sheng2, Qiu Jianwei3, Fu Xisheng3 1 School of Basic Science, East China Jiaotong University, Nanchang, Jiangxi, China, 2 Shanghai Institute of Technology, Shanghai, Shanghai, China, 3 PetroChina Lanzhou Lubricating Oil R&D Institute, Lanzhou, China Abstract Benzotriazole and borate derivatives have long been used as multifunctional additives to lubricants. A novel, environmentally friendly additive borate ester (NHB), which contains boron, ethanolamine, and benzotriazole groups in one molecule, was synthesized by a multi-step reaction, and its tribological properties in rapeseed oil (RSO) were investigated by a four-ball tribometer. The hydrolysis stability of the additive was investigated by half-time and open observation methods, and the mechanism of hydrolysis stability was discussed through Gaussian calculation. The novel compound NHB showed excellent performance under extreme pressure, against wearing, and in reducing friction, and its hydrolysis time is more than 1,220 times, which is better than that of triethyl borate. The mass ratio of NHB is bigger than that of the mixed liquid of triethyl borate and ethanolamine. The lone electron of amino N atoms forms a coordination effect with the B atom to compensate for the shortage of electrons in the B atom and to improve the hydrolysis stability of NHB. The surface morphology and the traces of different elements in the tribofilms formed with 1.0 wt.% NHB in were detected with scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy (EDX)and X-ray photoelectron spectroscopy(XPS). The results shown that the additive caused a tribochemical reaction with the steel ball surface during the lubricating process. A mixed boundary lubrication film that contains organic nitrogen and inorganic salts, such as BN, B2O3, FeOx, Fe–O–B, and FeB, was also formed, and the formation of the lubricating film improved the tribological properties of the base oil. Citation: Liping X, Zhongyi H, Liang Q, Lin M, Aixi C, et al. (2014) Synthesis, Tribological and Hydrolysis Stability Study of Novel Benzotriazole Borate Derivative. PLoS ONE 9(1): e83501. doi:10.1371/journal.pone.0083501 Editor: Jie Zheng, University of Akron, United States of America Received September 2, 2013; Accepted November 4, 2013; Published January 22, 2014 Copyright: ß 2014 Xiong et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The work reported here was sponsored by Chinese Natural Science Foundation (No: 21063007); Jiangxi Provincial Natural Science Foundation (No: 20122BAB203011); Jiangxi Education Department Foundation of China (Grant No: GJJ13358). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript Competing Interests: Authors QJ and FX are employees of PetroChina Lanzhou Lubricating Oil R & D Institute. There are no patents, products in development or marketed products to declare. This does not alter the authors9 adherence to all the PLOS ONE policies on sharing data and materials. * E-mail: [email protected] Introduction in environment-friendly base stock. These organic borate ester compounds are directly dissolved in lubricating oils, but inorganic Pure base stock cannot meet all of the stringent requirements of borate salts, such as OLOA 9750 (potassium borate), are dispersed modern industry. For this reason, some functional additives must as insoluble nanoparticles [6]. The antiwear mechanism of B be added to suit the working condition. Traditional lubricant compounds in extreme pressure conditions is due to the formation additives face enormous challenges with the growing requirements of thin layers of boric oxide (B2O3) on the metal surfaces [7]. Boric of environmental protection for lubricating oil [1]. Some organic oxide is converted to boric acid (H3BO3) upon exposure to humid derivatives that contain tribologically active elements (P, S, N, Cl, air, which is a layered material with a specific structure, whereby Zn, Mo, and B) are admixed to oils as additives at relatively low the atoms are covalently bonded to each other, and the layers are concentrations [2]. The main tribochemical reactions will occur weakly bonded. When the layers are stressed, they shear and slide during contact among surface materials, base stock, and lubricat- over one another easily, providing low friction [8]. Some N- ing oil additives in the boundary. The presence of reactive metals, containing heterocyclic compounds, such as benzotriazole and which are exposed by the wear process, can both catalyze triazine, have been used as lubricating oil additive. These lubricant breakdown and initiate chemical interactions with the compounds possess viable tribological performances. Li Jiusheng molecules of the lubricant additives [3]. et al. [9] designed and synthesized a novel B derivative of Boron-based lubricant additives have recently received signif- benzotriazole by combining N, B, and O atoms in one individual icant attention because of their wear-reducing and frictional compound. Their study showed that both benzotriazole and alkyl properties as well as low pollution [4]. Some studies showed that borate groups may simultaneously react on steel surfaces, and borate ester possesses robust extreme pressure and antiwear these reactions may be activated by shear and/or thermal effects at properties, especially in low viscosity oil [5]. surface asperities, which explains the superior tribological prop- Some simple boron compounds, such as borate salt or borate erties of the additive. ester, have also been used as corrosion inhibitor, antioxidant, However, the low hydrolysis stability of borate salts and borate friction modifier, as well as extreme pressure and antiwear additive esters is the biggest limitation for industrial application. Stability is PLOS ONE | www.plosone.org 1 January 2014 | Volume 9 | Issue 1 | e83501 Tribological, Hydrolysis of Benzotriazole Borate Figure 1. Synthesis route of NHB. doi:10.1371/journal.pone.0083501.g001 easily attacked by water due to the lack electrons of the B atom. 2.2 Analysis methods The attack will cause the loss of the effective additive B 2.2.1 Specimens and testing apparatus. All the steel balls component, which decreases the tribological performances of the (w 12.7 mm) used in the test were made of GCr15 bearing steel, lubricating oil [10] and limits its practical application. Some with hardness of HRc 59 to HRc 61. A microscope was used to studies showed that the addition of nitrogen-containing materials, determine the wear scar diameter (WSD) of the three lower balls such as amine, will improve the hydrolysis stability of borate salts with an accuracy 60.01 mm. and esters. N-containing heterocyclic compounds are reported to The Chinese standard test method GB 3142-82, similar with possess excellent extreme pressure and antiwear properties in ASTM D2783, was used to evaluate the maximum non-seizure lubricating oil. The number of N atoms is the main factor that load at a rotation speed of 1,450 rpm for 10 s at room affects tribological performance. The N-containing heterocyclic temperature. Friction and wear tests were examined on a four- compound contains N atoms, which possesses a lone electron pair ball test machine made in Jinan Testing Machine Factory of in the p orbital that will form a complex with the empty 2p orbital China at a rotation speed of 1,450 rpm at different loads for of the B atom to reduce the possibility of attack by some 30 min. nucleophiles, such as water. 2.2.2 Hydrolysis stability. Hydrolysis stability was investi- The ethanolamine and benzotriazole ethanol groups, which gated by an open method. Borate ester with 10 ml 1.0 wt.% increase the number of N atoms in the borate ester molecule, are concentration in liquid paraffin was added to a 25 ml dry glass, introduced in the synthesis of a novel borate ester (NHB) in the and changes in turbidity were observed. The turbidity of the liquid present work. The effects of both benzotriazole substitute and indicates the generation of insoluble boric acid solid. The time ethanolamine group at the alkyl borate part of the molecule on the spent for turbidity change was used as the hydrolysis stability time. tribological performance of rapeseed oil (RSO) are investigated in Hydrolysis stability was investigated by another method, a four-ball machine. Finally, both roughness and the elemental namely, the half-life method. Approximately 50 ml of water and composition of wear scars on the steel balls after the tribological 5 ml of glycerin were mixed in a glass, added with 3 to 4 drops of tests are analyzed using scanning electron microscopy/energy phenolphthalein solution and 0.1 mol/l NaOH solution and then dispersive X-ray spectroscopy (SEM/EDS) and X-ray photoelec- mixed again. This mixture was then added into a beaker with 0.4– tron spectroscopy (XPS). The hydrolysis stability was tested using 1.2 mmol borate ester and twice the amount of NaOH. The the open method and half-time method, and the Gaussian calculation was used to investigate the hydrolysis mechanism. solution was stirred continuously until the color changed from red to colorless. The time spent for the change to occur