Preparation and Characterization of Multiester Derivative from Plant Oil as Base

Li Xiong; Chen Ligong

(Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education，Chongqing Technology and Business University，Chongqing 400067)

Abstract: As an alternative to petroleum-based lubricants, which are harmful to the environment in excessive amounts, a biodegradable multiester derivative (OFANE) was obtained from plant oil through a chemical modi fication process with four steps as follows: hydrolysis, esteri fication, epoxidation, and ring-opening reaction. The physical and chemical properties of OFANE, such as viscosity, acid value, pour point, evaporation loss, and oxidation induction time were measured. Results showed that OFANE had good low-temperature fluidity, thermal-oxidative stability, and tribological properties. The tribological properties of OFANE with dimeric acid additive were evaluated. The final biodegradation experiment indicated that OFANE had excellent biodegradability. The prepared OFANE showed great potential as substitute for petroleum-based lubricating base oils.

Key words: plant oil, multiester derivative, lubricant base oil, physicochemical property, tribological property

1 Introduction

Approximately 40 million tons of lubricants are annually used worldwide, and 20 million tons are finally discharged into the environment[1]. More than 95% of these waste lubricants are petroleum-based lubricants, which are difficult to degrade naturally and may cause serious environmental pollution[2]. Therefore, waste mineral oil is classi fied as a hazardous waste in many countries.

Plant oil is a renewable resource, which is easily available and inexpensive. It can be used as a raw material for lubricant base oil production. Compared with mineral oil, plant oil not only has good biodegradability[3-6], but also has good lubricity, high flash point, high viscosity index, and good shear resistance[7-10]. Plant oil can be transformed into a base oil of biodegradable green lubricant through proper chemical modifications. The methods for chemically modifying plant oil generally include hydrogenation[11-14], transesterification[15-18], and epoxidation[19-23]. Many researchers used monohydric alcohol to modify plant oil, but the thermal-oxidative stability was unsatisfactory. The reason is that the alcohols they used contain β hydrogen atoms, which facilitated the decomposition and oxidization of prepared esters at high temperatures[24].

In this paper, the dihydric alcohol neopentyl glycol (NPG)was used to modify the plant oil. Given that NPG does not contain β hydrogen atoms, the thermal-oxidative stability of the ester is improved. OFANE was prepared as a lubricant base oil by a method consisting of hydrolysis,esteri fication, epoxidation, and ring-opening reaction, as shown in Figure 1. The physicochemical and tribological properties of OFANE were evaluated. A biodegradable dimeric acid (DA) antiwear additive was added to OFANE to improve its tribological properties. Finally,the degradability of OFANE was evaluated through the biodegradation experiments.

2 Experimental

2.1 Materials

The soybean oil (SO) was purchased from the COFCO Excel Joy (Tianjin) Co., Ltd. The lubricant base oils 150N and DA were provided by the Guangzhou Haozhao Chemical Company and the Guangzhou Shijiu Chemical Technology Company, respectively. All other chemicals and reagents were obtained from the Aladdin Chemical Reagent Company.

Figure 1 Reaction scheme for OFANE formation

2.2 Synthesis

2.2.1 Hydrolysis

A two-step hydrolysis method was used for the hydrolysis of SO. In the first step, 50 g of SO reacted with a 13.83%NaOH solution at 80 °C for 3 h, and the produced glycerol was removed by saturated NaCl solution at 50 °C. In the second step, SO was further hydrolyzed by a 6.92%NaOH solution at the same temperature for 2 h, and glycerol was removed by washing with NaCl solution.Finally, the remaining products reacted with 20 g of 38%HCl at 60 °C for 3 h for fatty acid preparation.

2.2.2 Esteri fication

The fatty acid neopentyl glycol ester (FANE) was prepared by esterification of fatty acid with NPG. Fatty acid (60 g) and NPG (15 g) were added to a threenecked flask equipped with a thermometer, a water distributor, and a condenser tube. Solid acid HND-26 and #120 solvent naphtha were used as catalyst and solvent, respectively. The mixture was heated to 120 °C for 7 h. After the reaction was completed, the catalyst was removed by filtration, and the filtrate was washed with a saturated Na2CO3solution to remove excess NPG.Finally, the obtained FANE was dried under vacuum at 60 ℃ for the removal of residual water and solvent.

2.2.3 Epoxidation

the epoxy fatty acid neopentyl glycol ester (EFANE) was obtained by epoxidation of FANE. The ion exchange resin CD-450 was used as the catalyst. FANE, 30% H2O2and formic acid entered in the reaction at a mass ratio of 1:0.66:0.14. H2O2was slowly added into the flask from a constant pressure funnel for over 2 h. The mixture was heated to 60 °C for 7 h. Then, the catalyst was removed by filtration, and the filtrate was placed in a separatory funnel for removal of the aqueous phase. Finally, residual water and formic acid was removed and EFANE was obtained by placing the upper layer in a vacuum dryer.

2.2.4 Oxirane ring-opening reaction

The EFANE ring-opening reaction was carried out.EFANE (70 g) and acetic acid (14.26 g) were placed in a flask, and tetrabutylammonium bromide was used as the catalyst. The reactants were heated to 100-110 °C for 6 h.Finally, the obtained product was distilled for the removal of residual acetic acid.

2.3 Methods

2.3.1 FT-IR analysis

The FT-IR spectra were recorded on a PerkinElmer spectrometer equipped with a KBr beam splitter. A regular scanning range of 450 cm−1-4 000 cm−1was used for 45 repeated scans at a spectral resolution of 0.5 cm−1.

2.3.2 Viscosity, acid value, and pour point

The kinematic viscosity of each sample was determined according to the ASTM standard D445. The viscosity index can be calculated by the ASTM standard D2270.According to the ASTM standard D974, the acid value of the sample was determined by potassium hydroxide volumetric titration. Pour point was measured according to the ASTM standard D93.

2.3.3 Thermal-oxidative stability test

The oxidation induction time (OIT) of OFANE was determined by pressurized differential scanning calorimetry(PDSC) according to the ASTM standard D5704-2015a.OIT is the time, at which samples begin to undergo an autocatalytic oxidation reaction under high-temperature oxygen conditions, and is used in evaluating the antioxidant properties of the samples. The evaporation loss of the samples was measured using a lubricating oil evaporation loss tester according to the CECL-40-T-87 standard. The samples were heated to 250 °C for 1 h, with the pressure maintained at 196 kPa. In each sample, the evaporation loss was determined according to the difference between the amount of the sample before and after heating.

2.3.4 Friction and wear tests

The tribological properties of OFANE was evaluated using a four-ball friction tester. The maximum nonseizure load (PB), the wear scar diameter (WSD), and the friction coefficient were measured using a MS-10J four-ball friction tester according to the ASTM standard D2266. The friction pair was assembled using a GCr15 steel ball with a diameter of 12.5 mm and a hardness of 59―61 HRC.

2.3.5 Biodegradation test

The biodegradability of the samples was studied through a biodegradability test. The bacterial strain extracted from waste oil was inoculated into a 10 mL enrichment medium. Then, the OD600of the bacterial seed liquid was maintained at 1.20 ± 0.02, and the bacterial seed liquid was continuously inoculated into a 30 mL inorganic salt medium for 7 days. The samples were used as the sole carbon source, and samples without vaccination were used as blank controls. Finally, the remaining sample concentration in the medium was periodically measured with a TOC (total organic carbon) analyzer according to the ASTM standard D2579.

3 Results and Discussion

3.1 Characterization

The FT-IR spectra of FANE, EFANE, and OFANE are shown in Figure 2. The peak at 3 009 cm-1in the FANE spectrum is the characteristic peak of CH in a CH=CH double bond. The peak is not observed in EFANE,and thus the CH=CH double bond is successfully epoxidized. The characteristic peak of the epoxy group at 823 cm-1appears in the infrared spectrum of EFANE but disappears in the infrared spectrum of OFANE,indicating that EFANE undergoes the oxirane ring opening with a carboxyl group. In the infrared spectrum of OFANE, the C=O group (724 cm-1and 1 739 cm-1),CH3group (1 365 cm-1―1 470 cm-1), and OH group(3 442―3 475 cm-1) are clearly visible similar to the C-O-C band in the ester (998―1 100 cm-1).

Figure 2 FT-IR spectra of FANE, EFANE, and OFANE

3.2 Viscosity, acid value, and low-temperature fluidity

The viscosity, acid value, and pour point of FANE, EFANE,or OFANE are shown in Table 1. The viscosity of OFANE is higher than that of FANE or EFANE possibly because acetic acid is introduced at an unsaturated point to increase the molecular weight and viscosity. Moreover, the calculated viscosity index of OFANE decreases. The acid value of OFANE and EFANE is lower than that of FANE. The reason is that FANE contains fatty acids, which increases its acid value. In terms of the low-temperature fluidity,OFANE has the lowest pour point. The introduction of the side chains prevents the base oil from solidifying under low temperature conditions and can enable the base oil to maintain its lubricating property. The side chains can destroy the molecular symmetry and thereby inhibit the stacking of individual molecules at low temperature.

Table 1 Physicochemical properties of FANE, EFANE and OFANE

3.3 Thermal-oxidative stability

The thermal-oxidative stability is an important property for evaluating the oxidation resistance of lubricating oils.

The antioxidation capacity was measured by the PDSC method. The OIT values of FANE, EFANE, and OFANE are provided in Table 1. Owing to the presence of C=C bonds, the antioxidation capacity of FANE is poor.Epoxidation causes the double bond to form a ring, and the reduction in the number of double bonds results in an increase in the antioxidaion capacity of EFANE. OFANE has better antioxidaion capacity. Compared with the OIT of FANE, the OIT of OFANE increases obviously.

Evaporation loss in FANE, EFANE, and OFANE was measured, as shown in Figure 3. Over time,the evaporation loss of each sample increases. The evaporation loss at 80 min tends to be stable, and the evaporation loss in OFANE is the smallest. The evaporation loss in OFANE at 60 min is reduced by 35.7% or 19.6% as compared to the evaporation loss in FANE or EFANE, respectively. The possible reason is the increase in molecular weight and changes in chemical structure.

Figure 3 Evaporation loss of FANE, EFANE, and OFANE

3.4 Tribological properties

The maximum non-seizure loadPBis the maximum load at which a steel ball is not seized up in the lubrication environment at a certain temperature and rotary speed.Average friction coefficient is related to the roughness of the steel-steel contact zone. A four-ball friction tester was used to measurePB(at a rotary speed of 1450 r/min for a test duration of 10 s) and average friction coefficient (at a rotary speed of 1200 r/min for a test duration of 1 h under a load of 392 N) of the samples, as shown in Figure 4.OFANE has the largestPBvalue and the smallest friction coefficient, exhibiting excellent antiwear property because the oil film strength is increased and not easily broken.Compared with the mineral oil 150N, OFANE maintains a better lubricity, with its PBincreasing by 75% and friction coefficient decreasing by 39%.

Figure 4 Average friction coefficient and PB value of 150N mineral oil and test samples

WSD is the diameter of wear-damaged mark formed on the surface of bearing steel balls owing to friction at a certain temperature, speed, load, and duration. The WSD values (at a rotary speed of 1200 r/min over a test duration of 1 h) of the samples vary with load, as shown in Figure 5, denoting that the WSD increases with the load. When the load exceeds 392 N, the WSD of EFANE increases sharply because of possible oil film rupture.Then, the friction surface is in contact and becomes increasingly rough. The WSD of OFANE is the smallest,and OFANE exhibits good wear resistance, which may be resulted from the increased viscosity that is ascribed to its increased molecular weight.

Figure 5 Variation of WSD with different loads

The antiwear property of OFANE was improved by mixing it with a DA additive. The variation in the friction coefficients of OFANE and OFANE with 2% of DA over time are shown in Figure 6. The friction coefficient of OFANE containing DA is significantly lower than that of OFANE, because DA acts as an electron acceptor under extreme conditions, such as high temperatures,and undergoes a tribochemical reaction with Fe atoms of activated friction surface to form a chemical reaction film of ferric dimer. The p-π conjugation effect of the carboxylate anion contained in a ferric dimer molecule increases the stability of the ferric dimer film and reduces the friction.

Figure 6 Variation in friction coefficient with test duration

Figure 7 shows the effect of the DA additive on the WSD values of FANE, EFANE, and OFANE. After the addition of DA, the nonpolar end of DA faces the oil, and the polar end of the carboxyl group faces the metal. The friction surface forms an unstable physical adsorption film[25], which can prevent direct contact between the microprotrusions. With the increase of DA concentration,the chemical reactivity of DA is stronger than that of the monomeric carboxylic acid because DA contains two carboxyl groups. Owing to the high temperature on the friction surface and the catalytic action of the metal,physical adsorption begins to change into chemisorption.DA reacts chemically with the metal matrix element to form a layer of oxygen-containing compound with low shear force, which can prevent direct contact between microprotrusions. Thus DA shows antiwear property and corrosion protection effect[26].

3.5 Biodegradable properties

The variation in TOC over biodegradation time is shown in Figure 8. The decrease in the TOC values of the three samples was not obvious in the first 3 days.The possible reason is the low number of bacteria and delay of growth. Starting from the fourth day, the TOC values of the samples decreased significantly, while the bacteria entered a logarithmic growth period, with the number of bacteria increasing obviously, and the degradation rate increased. On the seventh day, the biodegradation rates of FANE, EFANE, and OFANE reached 92%, 88%, and 86%, respectively, because straight chain esters were more easily degraded than esters with side chains.

Figure 8 Variation of TOC with biodegradation time

4 Conclusions

Figure 7 Variation in WSD with different DA contents

A multiester derivative OFANE was prepared from SO and used as the lubricant base oil, and its physicochemical properties were evaluated. OFANE showed good lowtemperature fluidity and could meet the requirements of lubricating oil at low temperatures. The evaporation loss of OFANE was 7.7% at 250 °C, indicating the excellent thermal-oxidative stability of OFANE. OIT at 150 °C was 22.03 min. The number of C=C bonds was reduced by cyclization and ring-opening reaction, and thus the antioxidation capacity of OFANE was enhanced.Compared with the mineral base oil, OFANE exhibited obvious improvements in its tribological properties, and itsPBincreased by 75%, while its friction coefficient and WSD were reduced. The main reason was that the polar groups of esters in the synthesized products could easily form the adsorption boundary films compared with the mineral oil. Adding DA antiwear additive to OFANE further improved the tribological properties of OFANE possibly because of the formation of ferric dimer films on the surfaces of friction pairs. The biodegradation rate of OFANE was over 85%, indicating its strong biodegradability.

Acknowledgements:The authors gratefully acknowledge the financial support from the Special Project for Scientific and Technological Innovation of Social Undertakings and People’s Livelihood Guarantee of Chongqing Science and Technology Commission. (Project No. cstc2017shms-zdyfX0066).