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High Performance Lubricant Base Stocks from Biorenewable Gallic Acid: Systematically Study on Their Physicochemical and Tribological Properties Ming-Jin Fan, chaoyang zhang, Ping Wen, Wenjing Sun, Rui Dong, De-Suo Yang, Weimin Li, Feng Zhou, and Weimin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01452 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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Industrial & Engineering Chemistry Research

High Performance Lubricant Base Stocks from Biorenewable Gallic Acid: Systematically Study on Their Physicochemical and Tribological Properties Mingjin Fan,a* Chaoyang Zhang,a Ping Wen,a Wenjing Sun,a Rui Dong,a Desuo Yang,a* Weimin Li,b Feng Zhou,b Weimin Liub a

Shaanxi Key Laboratory of Phytochemistry, College of Chemistry & Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China.

b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China.

KEYWORDS: Gallic acid, Gallate, Lubricant, Synthetic ester, Tribological property ABSTRACT: Synthesis, characterization, and evaluation of a series of gallate derivatives as lubricant base stocks are described. The gallate derivatives were synthesized via sequential esterification and alkylation reactions from biorenewable raw material gallic acid. All the synthesized gallate derivatives were structurally characterized by using NMR, IR and HRMS spectral data. Their physicochemical and lubricating properties including density, viscosity, viscosity index, pour point, flash point, volatility, corrosion level, thermal and oxidation stability, friction reducing and anti-wear properties were systematically investigated. Based on the physicochemical and lubricating data, it can be concluded that gallate derivatives have good

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properties in terms of viscosity-temperature characteristic, corrosion ability, volatility and low temperature flow ability. More important, they have predominant thermal and oxidation stability, as well as high temperature friction reducing and anti-wear performances compared with the existing synthetic ester oils. These favorable technical performances together with their biological available resources recommend gallate derivatives as excellent base stocks for the development of high performance green lubricant products. 1. Introduction The gradual depletion of world fossil fuel resources and global demand of environmental concerns have prompted the use of renewable biomass-based resources toward sustainability.1-3 Biomass-based materials, such as biodiesel, biolubricants, bioplastics, biosurfactants, etc. are capable of substituting fossil-based products and, hence, have broad and promising future market.4-10 In view of developing new biolubricant base stocks, vegetable oils and their derivatives are proposed to be the most attractive alternatives to fossil-based lubricants due to their excellent biodegradability and renewability.11-16 However, some of their drawbacks, such as limited performances, poor oxidation stability and low temperature fluidity, restrained their applications in many areas. They cannot compete with fossil-based lubricants and also cannot fulfill the performance requirements of modern lubricant base stocks. Progress in the design and operation of equipment continues to prompt us extending the performance of biomass-based lubricants. A lot of efforts have been made and a wide variety of high-performance synthetic base fluids has been developed.17 These include polyalphaolefins, synthetic esters, phosphate esters, alkylated aromatics, polyalkylene glycols, and perfluoroalkylpolyethers. Among them, synthetic esters are one of the most used types of synthetic base stocks and have many


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outstanding properties such as high biodegradability, low toxicity, good lubricating properties and low temperature flow ability. Also, synthetic esters are superior in dissolving additives and contaminants than other synthetic base stocks, such as polyalphaolefins. These properties have made them found applications in a wide range of industrial area including engine oils, two-stroke cycle oils, compressor oils, hydraulic fluids, greases and aviation oils.18 However, with the development of modern technology, some disadvantages of the existing synthetic esters are appearing, such as poor high temperature and oxidation stability. Many researchers are devoting to investigate the oxidation and decomposition mechanism of synthetic esters in order to develop new synthetic ester products.19-21 Actually, few new synthetic ester skeletons other than plant oil and their derivatives were reported, let alone synthetic esters from renewable raw materials. Gallic acid (3, 4, 5-trihydroxybenzoic acid) and its derivatives are industrially important chemicals used widely in food, pharmaceuticals, cosmetics and pigments industries (Figure 1). Especially, its ester derivatives (propyl, octyl and lauryl gallates) are often used in cosmetics, processed food and food packing materials to prevent oxidative rancidity and spoilage.22-24 Furthermore, they were proved to have many significant biological activities, such anti-cancer, anti-virus, anti-inflammatory, anti-mutagenic and anti-free radical abilities.25 Gallic acid and some of its ester derivatives is widely present in the plant kingdom and food sources like nuts, tea, gallnuts, oak bark, sumac, honey, fruits and vegetables. As a type of biorenewable raw materials, gallic acid and its esters are proved to be the most important and representative phenolic-type anti-oxidants with low biological toxicity and good biodegradability. The outstanding properties of these products have pushed forward the investigation on the application of these materials in various regions. Owing to its rigid benzene ring, three phenolic hydroxyl groups and one carboxylic group, gallic acid shows great potential to enhance the


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physicomechanical properties of materials by modification of the phenolic hydroxyl and carboxylic groups. In our continuing search for high performance green lubricants and lubricant additives, gallate derivatives, a new type of synthetic esters based on the modification of phenolic hydroxyl and carboxylic groups on gallic acid have been synthesized and presented in this work, which are proved to possess outstanding oxidation stability, high thermal stability and good lubricating property especially at elevated temperature. O

OH

HO

OH OH

Figure 1. Chemical structure of gallic acid. 2. Experimental 2.1 Chemicals Gallic acid (Energy Chemical, 98%), methyl gallate (J&K, 99%), propyl gallate (J&K, 98%), isopentyl gallate (TCI, 98%), 2-ethyl-1-hexanol (Aladdin, 99%), 1-bromobutane (J&K, 99%), 1bromooctane (J&K, 98%), 2-ethylhexyl bromide (J&K, 95%), 1-bromododecane (J&K, 98%), ptoluenesulfonic acid (Energy Chemical, 98%), tris(2-ethylhexyl) trimellitate (Phe-3isoC8, Amethyst Chemicals, 99.5%), bis(2-ethylhexyl) sebacate (1088, Amethyst Chemicals, 97%) and pentaerythritol ester (5750, NYCO Company) were used as received. All the other chemicals used in the synthesis were of AR (Analytical Reagent) grade. 2.2 Apparatus and procedure 2.2.1 Synthesis and structural characterization of gallate derivatives


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During the gallate derivatives synthesis, methyl, propyl, isopentyl, octyl gallates were purchased and isooctyl gallates was synthesized in our lab. After that, the final gallate derivatives were synthesized mainly according to the method reported by Cheng and co-workers with slight modifications.26 The detail synthesizing procedures were presented in the Supplementary Information. The structures and purities of the gallate derivatives were characterized by proton nuclear magnetic resonance spectroscopy (1H NMR, 400 MHz), carbon nuclear magnetic resonance spectroscopy (13C NMR, 100 MHz) carried on an Agilent 400 MHz nuclear magnetic resonance spectrometer. The fourier transform infrared (FT-IR) spectra recorded on a Nicolet Nexus 670 spectrometer (Thermo Fisher, USA) and high resolution mass spectra (HRMS) recorded on a Bruker Dalton micrOTOF-Q II instrument were given as further conformations. All the above characterization results were presented in the Supportary Information. 2.2.2 Kinematic viscosity, viscosity index and densities The kinematic viscosities of gallate derivatives and the reference samples (5750, 1088 and Phe-3isoC8) were measured at 40 °C and 100 °C using a SVM 3000 Stabinger viscometer and the corresponding viscosity indexes were automatically calculated based on the ASTM D70422012 method. At the same time, the densities (at 20 °C) of the samples were given on the viscometer. The kinematic viscosities of gallate derivatives at -40 °C were measured on a SYP1003-7D low temperature kinematic viscometer to evaluate their flow ability at low temperature. In all these tests, at least two measurements were conducted for each sample to ensure the accuracy of the data. 2.2.3 Pour point Pour points of the synthesized gallate derivatives and the reference samples were measured according to the ASTM D97-09 method with an accuracy of ±3 °C using a pour point test


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apparatus manufactured by Lawler Manufacturing (DR4-22 L). The temperature was monitored to decrease by decrements of 3 °C until the sample stopped pouring. All runs were carried out in duplicate. 2.2.4 Flash point Flash points of gallate derivatives were measured on a Stanhope-seta flash point tester (820000, U.K.) following the ASTM D3828-09 method. The temperature was monitored to increase by increments of 2 °C until flash point was obtained. 2.2.5 Copper strip corrosion test Copper strip corrosion tests of gallate derivatives and the reference samples were performed according to the method of ASTM D 130. The specific operation is: a polished standard Cu strip (size=75 mm×12.5 mm×3 mm) was put into a testing sample ensuring it was totally immersed, which was then placed in an oven and maintained at 150 °C for 3 h. After completion of the test, the Cu strip was taken out, washed with acetone and photos were then taken. 2.2.6 Thermal stability The thermal stabilities of all gallate derivatives and the reference samples were determined according to the following two methods: increasing and constant temperature thermogravimetric (TG) analysis on a Netzsch STA 449 F3 synchronous thermal analyzer system under nitrogen atmosphere using an alumina crucible. During the increasing temperature TG analysis, the temperature was monitored to increase from room temperature (RT) to 600 °C at a heating rate of 10 °C/min. The percentage of weight loss with rise in temperature was calculated to evaluate the thermal stability of all samples and onset decomposition temperatures were given. During the constant temperature TG analysis, the system was firstly equilibrated at 50 °C and then monitored to increase from 50 to 200 °C at a heating rate of 20 °C/min, 200 °C to 230 °C at a


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heating rate of 5 °C/min and 230 °C to 250 °C at a heating rate of 2 °C/min (based on SH/T 0731-2004 method). After that, the sample was maintained at 250 °C for about 2 h. The percentage of weight loss with passage of time was monitored to evaluate the thermal stability of all samples. 2.2.7 Oxidation stability Rotating bomb oxidation (RBO) test: the RBO tests of gallate derivatives and the reference samples were conducted on a Stanhope-seta rotating bomb oxidation tester (15200-5, U.K.) according to the ASTM D 2272-09 method. The experiments were carried at 150 °C by using 50 g samples, 5.0 mL distilled water and copper catalyst. The vessel was sealed, charged with oxygen to 620±5 kPa pressure and then immersed into the oil bath. The pressure in the bomb is continuously recorded until it has dropped by 175 kPa and the duration (rotating bomb oxidation time) was recorded to evaluate oxidation stability of the sample. Pressurized differential scanning calorimetry (PDSC) tests: the PDSC tests were done using a DSC 204 HP differential scanning calorimeter (NETZSCH) according to ASTM D6186-08(2013) method. Typically about 2.0 mg of a sample was placed in an open aluminium pan and oxidized under a static oxygen pressure of 3.5±0.2 MPa. The temperature was monitored to increase from RT to 350 °C at a heating rate of 10 °C/min. The initial oxidation temperature was calculated from the corresponding exotherm. 2.8 Hydrolytic stability The beverage-bottle test is a standard test method for hydrolytic stability of ester oil (GB/SH/T 0301-93). 75 g ester sample, 25 g water and a polished copper were placed in a standard pressure type beverage bottle. The bottle is then placed in an rotated oven and the temperature was set at 93 °C. The test duration was 48 h and the rotate speed was 5 r/min. At the end of the test, the


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hydrolysis stability was determined by measurement of the viscosity, acid value (AV) of the ester sample and the weight loss of the tested copper piece. 2.2.9 NOACK volatility The NOACK volatilities of gallate derivatives and the reference samples were determined on a lubricating oil evaporation loss tester manufactured by North Dalian Analytical Instrument Co., Ltd according to SH-T 0059-1996 method. The tests were conducted by analyzing weight loss of a sample along with volatization and the percentage of weight lost was reported. 2.2.10 Friction and wear test The tribological properties of gallate derivatives and the reference samples were investigated by evaluating their application on steel/steel contacts with an Optimol SRV-V oscillating reciprocating friction and wear tester at both room and elevated (200 °C) temperatures. A ballon-flat geometry was chosen using an AISI 52100 steel ball (diameter=10 mm, hardness=61±2 HRC, and mean roughness=20 nm) against a static lower flat (AISI 52100 steel, ø 24 mm×7.9 mm, hardness=61±2 HRC). The tests were conducted under the conditions of load=100 N, frequency=25 Hz, amplitude=1 mm and duration=2 h. All interacting surfaces were cleaned with alcohol and air dried before testing. The wear volume of the lower disk was measured using a Bruker NPFLEX surface mapping microscope profile meter. At least two measurements were conducted for each sample and an average wear volume value was reported. 3. Results and discussion 3.1 Structure characterization The structures of gallate derivatives and the reference samples were presented in Figure 2. The structures and purities of gallate derivatives were finely confirmed by 1H NMR, 13C NMR, FTIR and HRMS spectroscopic data. The detail data are presented the Supplementary Information.


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R O

O R

O

O O

O

R

O R

O

R O

O

O R

O

O

1088: R=Isooctyl

R'O

O

R'O

OR

R'O

Gallate derivatives

O O

R R

O

O

R 5750: R=C7-C8 alkyl

O

Phe-3isoC8: R=Isooctyl

3C4-isopentyl: R=Isopentyl, R'=Butyl 3C8-methyl: R=Methyl, R'=Octyl 3C8-propyl: R=Propyl, R'=Octyl 3C8-isopentyl: R=Isopentyl, R'=Octyl 3C8-octyl: R=Octyl, R'=Octyl 3C8-isooctyl: R=Isooctyl, R'=Octyl 3isoC8-propyl: R=Propyl, R=Isooctyl 3isoC8-isopentyl: R=Isopentyl, R'=Isooctyl 3isoC8-isooctyl: R=Isooctyl, R'=Isooctyl 3C12-propyl: R=Propyl, R'=Dodecyl 3C12-isopentyl: R=Isopentyl, R'=Dodecyl

Figure 2. Chemical structures of gallate derivatives and the reference samples. 3.2 Physicochemical property 3.2.1 Kinematic viscosity and viscosity index One of the most important properties of lubricant oil is kinematic viscosity and viscosity index. The viscosity of lubricant oil is its tendency to resist flow, which is directly related to film formation during lubrication. No matter at room or high temperature, it must be high enough to keep good oil film formation between the moving parts, but no too high to give effective lubrication.27,28 Viscosity index indicates the extent of viscosity change of a lubricant along with change in temperature. Higher viscosity index values indicate that a fluid is more resistant to viscosity loss with increasing temperature. Good lubricating base oils are all expected to have high viscosity indexes in order to meet the requirements of practical application, because oil with high viscosity index can resist excessive thickening when the machine is cold and, consequently,


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promotes rapid starting and prompt circulation. On contrary, it can resists excessive thinning when the machine is hot and thus provides full lubrication and prevents excessive oil consumption. The kinematic viscosity (KV) and viscosity index (VI) of gallate derivatives and the reference samples at 40 °C and 100 °C were measured and the results were shown in Table 1. From Table 1, it can be seen that gallate derivatives exhibited kinematic viscosities in the range of 23-91 mm2/s at 40 °C and 4-11 mm2/s at 100 °C. The kinematic viscosities both at 40 °C and 100 °C are found to increase greatly along with the growing of the alkoxy carbon chains, e.g. KV40 (3C4-isopentyl, 23.313)

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