Methyl Oleate-Alkylated Tetralin with Dual-Functional Groups as Base Oil with PAO to Improve the Performance of Lithium-Based Grease

A novel dual-functional group base oil was synthesized by alkylation of methyl oleate from biomass and tetralin from coal chemical industry. Interestingly, the monoalkylated product (MOAT) exhibited low pour point, high flash point, and viscosity index compared with the traditional alkyl aromatic lubricating oil (AN5), owing to the combination of the naphthenic aromatic (tetralin) and the methyl oleate framework. MOAT as co-base oil with PAO8 was studied to prepare lithium-based greases for the first time, and their rheological and tribological properties were evaluated in detail. The microstructure of the MOAT-based grease displayed high entanglement of lithium fiber. The 20% MOAT-based grease markedly reduced friction and wear compared with PAO8-based grease and 20% AN5-based grease. The excellent tribological performances were attributed to the strong interaction between the ester group and the soap fiber that provides more relative elasticity of grease and improves tribological behavior. Our finding clearly demonstrated that MOAT had the potential as a new class of base oil for obtaining high-performance lithium grease.


Introduction
The concern about the scarcity of fossil resource and environmental problems contribute to focus on the alternative resources in order to substitute or partly substitute petroleum-derived products.Renewable resources, 104 Page 2 of 11 especially fats and oils, are the most important raw materials in the field of detergents, cosmetics, and lubricants [1][2][3][4].
Lubricating greases play an important role especially in the bearings and gear system lubrication.Different from lubricant oil, lubricating grease is considered a highly structured suspension in which usually a thickener is dispersed in lubricating base oil (mineral or synthetic oils) [5][6][7].The development of bio-based grease starts with a combination of natural and/or eco-friendly ingredients into traditional mineral or synthetic-based oils.Natural vegetable oils such as rapeseed, palm, coconut, castor, soybean, and jatropha oil have been extensively explored as the base oil in lithium-based grease [3,[8][9][10].Although these oils showed good friction and wear characteristics due to the strong interaction of the steel pair, they have poor cold flow properties and are very prone to thermal and oxidative degradation [11][12][13].Currently, there are two ways in the implementation to overcome these shortcomings to a certain extent.One common method of derivatization involves epoxidation.The lubricating grease based on epoxidized soybean oil and several additives exhibited higher anti-oxidation property, but the wear scar was larger than the commercial mineral-based grease [14].Transesterification was also reported as chemical modification.A work conducted by Panchal et al. [15] displayed the transesterification of Karanja oil by C4-C10 alcohol as the base oil for lithium grease, but there is still a big gap between bio-based grease and commercial grease in terms of the friction coefficient and wear scar.This may be caused by the low structure strength of the grease, but rheological properties were not carried out on these lithium-based greases.Now it remains a challenge to found for all well both in the physicochemical as base oil and the tribology properties as greases.
To address these concerns, we focused on methyl oleate as the feedstock of raw materials due to their wide availability and relatively high purity, where the major challenge revolves around reasonable molecular structure building.First, the chemical structure of base oil should have a low unsaturated bond and some branched side chain to provide both high antioxidative, high viscosity index, and low pour point [16,17].Methyl oleate is the natural macromolecular compound with a middle carbon-carbon double bond in the structure; therefore, the introduction of branches in the chain middle by alkylation with aromatics may enhance their physicochemical properties, such as pour point and viscosity index can be improved [18].Further, the addition of a rigid aromatic ring structure in the grease base oil can effectively increase the structural strength of the grease, especially under low lithium fiber entanglement of PAO8-based grease [19].Accordingly, functionalization can be obtained by C-C-linking reactions with the aromatic alkylation reaction of conjugated methyl oleate.In addition, the requirement of the whole carbon number of the base oil was more than 25 in order to obtain high flash point, thus, offering ten carbons of bicyclic aromatic hydrocarbons is an even better option for base oil.Naphthalene is one of the possible sources of raw materials from coal tar and alkylated naphthalene was reported as synthetic lubricating base oil at the early time [20,21].Considering the controversial safety and environmental issues of naphthalene [22], the tetralin obtained from the partial hydrogenation from naphthalene was a suitable candidate.
Here, we report a new dual-functional group lubricating oil by alkylation of methyl oleate and tetralin in the presence of AlCl 3 .The fundamental physicochemical properties of monoalkylated products (MOAT) were investigated and compared with the commercial alkyl naphthalene lubrication oil AN5.The formulated lithium-based PAO8 grease was used as a reference grease to compare various physiochemical, tribological, and rheological properties of blending MOAT in the PAO8 base oil.It was found that the addition 20% MOAT significantly enhanced the strength of the grease, contributing to the reduction in friction and wear.Our findings will provide a fundamental reference to rationally design and synthesize base oil at the molecular level for high-performance lubricating grease.

Friedel-Crafts Alkylation and Analysis of the Products
The monoalkylated products were prepared in the presence of anhydrous AlCl 3 by the Friedel-Crafts reaction of methyl oleate and tetralin (MOAT) in the solvent of n-decane, in which the molar ratio of tetralin, methyl oleate, and anhydrous AlCl 3 was 8:1:1.1.The mixture was heated to 60 °C and maintained for 3 h with stirring.The resulting mixtures were then cooled to room temperature, and an equal volume of saturated sodium chloride solution was added slowly into the flask.After 30 min vigorously stirring, then the upper organic layer was separated and evaporated under reduced pressure to obtain the alkylated products.Analysis was carried out on a Shimadzu GC-2014C gas chromatograph equipped with a column injector (330 °C), an FID detector (330 °C), and an HP-5 column (30 m, 0.25 mm i.d., 0.25 μm film thickness).The column temperature was initially 50 °C for 2 min, then gradually increased to 200 °C at 15 °C min −1 for 2 min, and increased to 300 °C at 5 °C min −1 , maintained at this level for 26 min.GC-MS analysis was carried out on a Shimadzu GCMS-QP2010 Plus instrument equipped with electrospray ionization (EI) (70 eV) for MS; other conditions were the same as for GC.

Greases Manufacture
The process was performed in a stirred batch reactor (60.0 g) using an anchor impeller geometry and a rotational speed of 60 rpm.Processing details and procedures have been extensively described elsewhere [23].In the first step, 4.7 g of 12-hydroxystearic acid and 1.17 g of stearic acid were to be used in the process, and 74% (40.0 g) of the total amount of the base oil was charged into an open vessel.This blend was pre-heated up to 80 °C, and lithium hydroxide (0.96 g) was then slowly added in the form of an aqueous solution.The saponification reaction occurred until neutralization by stirring for 1.5 h to keep the temperature at 120 °C.Once the saponification reaction was completed, the mixture was heated up to 140 °C, and 0.6 g stearic acid was added to the mixture and stirred for 30 min.When the temperature reached 150 °C, 15% (14 g) of the total amount of the base oil was poured into the mixture.Then the heater is adjusted to the maximum power, in order to induce a phase transition of soap crystallites into a waxy phase, and complete the dehydration process.Afterward, the mixture was cooled down to room temperature.Finally, grease was homogenized by a three-roll mill three times at room temperature.

Characterization of Product and Grease
The kinematic viscosities at 100 and 40 °C (KV100 and KV40) were determined following the ASTM D445 method.The VI was calculated using the KV100 and KV40, following the ASTM D2270 method.The density at 20 °C was determined with the density instrument according to the ASTM D4052.The pour point was measured on the pour point test apparatus by the following method of the ASTM D97.The flash point was measured on a flash point tester using the method of the ASTM D92.The aniline point was measured on the Aniline Point Tester according to the method of the ASTM D611.All the measurements above were tested thrice, and the results were obtained by average.
The dropping point of grease was measured according to the national standard GB/T 3498.Steel mesh sub-oil experiments were conducted according to the national petrochemical industry standard SH/T 0324.The PDSC experiments use the differential scanning calorimetry test from Netzsch DSC 204 (Germany).Typically, a sample was placed in an aluminum pan which was in the 100 mL min −1 oxygen flow rate at the ambient pressure and at the 10 °C min −1 heating rate.The onset temperature (OT) of oxidation is calculated from the exotherm in each case.FTIR spectra were obtained with a Shimadzu IRSpirit (Japan) apparatus.Grease samples were placed into a KBr cell plate.The spectra were obtained in a wavenumber range of 400-4000 cm −1 .
The morphological analysis of the soap fiber after washing the grease three times through scanning electron microscopy (SEM) was performed in a Hitachi microscope (Japan), model SU8010.

Rheological and Tribological Behavior
Rheological measurements of the tested grease were conducted on an Anton Paar MCR 302 rheometer (Austria) at 25 °C using a 1 mm gap for a rough plate-to-plate geometry (25 mm diameter).All tests were performed at least three times on fresh samples.
The tribological properties of the grease were evaluated using an Optimol SRV oscillating friction and wear tester (SRV-V Optimol Instruments, Germany) operated at an amplitude of 1 mm and a frequency of 50 Hz for 30 min, the load of 200 N.The friction experiments were carried out at 85 °C.Each worn surface was examined using a threedimensional noncontact optical surface profiler (Zygo, the United States of America).

Structural Analyses and Physicochemical Properties
Friedel-Crafts alkylation with great significance for organic synthesis was used as a common method for the alkylation of unsaturated fatty acids.AlCl 3 was reported to realize the alkylation process at low temperatures [24].Improvement of the yield of the monoalkylated products (MOAT) by optimizing the reaction condition, temperature, time, catalyst loading, and the reactant molar ratio (Fig. 1).Then the yield of MOAT reached 93% at the optimal reaction condition, 60 °C, 3 h; the molar ratio of tetralin, methyl oleate, and anhydrous AlCl 3 was 8:1:1.1.The product of Friedel-Crafts alkylation was a series of isomers; the diversity structure and relative content of the 104 Page 4 of 11 product were studied by GC-MS.The splitting reactivity of tertiary carbon connected to the tetralin was increased, which meant that two fragments with a conjugated system were simultaneously obtained [25].Although the shift and rearrangement of carbocation occurred in the reaction, the amount of 10-, 9-, 8-, and 7-tetralin methyl stearate made up more than 90 percent of all isomers (detailed in supplementary material).
The physicochemical properties are shown in Table 1.After alkylation, although the pour point of the MOAT was lower than AN5, the value was far below the pour point of methyl oleate itself (− 21 °C [26]).Due to the large straight chain group in the structure, the flash point and viscosity index were exhibited higher than AN5.In the process of measuring the aniline point, since the aniline point of MOAT is lower than that of the aniline itself, the mixed aniline point was measured, which showed stronger polarity than AN5.

Rheological Characterization
As can be seen in Fig. 2, where the viscous flow curves of greases containing 10% MOAT and 20% MOATbased grease have been compared to that of the PAO8based lubricating grease and 20% of AN5.The powerlaw model fits the viscous flow behavior of greases fairly well (R 2 > 0.998), in the experimental range of shear rates studied, where "k" and "n" are the consistency and flow indexes in Eq. (1), respectively.These fitting parameters are presented in Table 2.The consistency index increases with the structural strength and the flow index could reveal structure changes.Under normal conditions, the values of the flow index are extremely low due to the typical yielding behavior of greases.There has been a relationship between the decrease in the flow index and the consistency index increases.In terms of the structure of grease, higher values of k resulted in better structural strength.The AN5 has been reported to enhancing the structural strength of lubricating grease [19] 20% AN5 used in the base oil, the flow index was decreased from 0.076 to 0.024, and the consistency index increased from 212 to 498.20% MOAT was added into the base oil, which showed a lower value of flow index (0.010) and higher consistency (946) than AN5.However,  10% MOAT-based grease revealed the closed consistency index (407) with 20% AN5, and the flow index (0.075) was the same as the PAO8-based grease.An explanation of the high consistency index of 10% MOAT-based grease is likely to be caused by interactions between the soap fibers, which is also seen with other colloidal systems [27].
Figure 3 shows the evolution of the linear viscoelasticity functions with frequency, at different components, PAO8, 10% MOAT, 20% MOAT, and 20% AN5.All the formulations contain the same amount of soap concentration.It can be observed that the values of the storage modulus (Gʹ) are always higher than those of the loss modulus (Gʹʹ) in the frequency range between 10 -1 and 10 2 s −1 .The storage modulus slightly increases with frequency, and the loss modulus displays a minimum.For all four greases studied, this mechanical spectrum corresponds with the traditional description given for the conventional lithium-lubricating greases.As can be observed, it exhibited that 20% MOATbased grease showed significantly higher values of both G' and G'' as compared to the PAO8-based grease.However, the addition of AN5 at the 20% amount in based oil only produces an increase in the values of modulus closed with the 10% MOAT-based grease.On the other hand, the overall value of four greases varies in a very small range, but the loss tangent (tanδ) presents quite unexpected experimental results that can be observed.The loss tangent is defined as Gʹʹ/Gʹ ratio at a constant frequency, indicating the relative elastic characteristics of lubricating greases.It is apparent that the PAO8 and 20% AN5-based grease shows a less relative elastic behavior, higher values of the loss tangent, while the MOAT-based grease conferred a more relative elastic behavior to the grease, especially at high frequencies.This points to the properties of ester base oil in the grease.The relative elasticity of lithium-based grease with ester base oil does not decrease with the increase of viscosity, showing a different effect from that of traditional all hydrocarbon lubricating base oil.This fact was attributed to a higher interaction between the base oil and the thickening agent [28][29][30][31].
The soap content has a dramatic effect on the structure of the grease [32].At the 10% content, MOAT had beneficial effects in the modulus.Then reducing the soap content to 8%, the frequency sweep tests were carried out under the same condition, as shown in Fig. 4. For PAO8-based grease, the Gʹ and Gʹʹ decrease slightly with the soap content.The MOAT-based grease exhibits a very closed module with 10% and 20% concentration.But the tanδ value of the 20% MOAT-based grease was lower than the PAO8-based grease.So, more amount of MOAT added in the base oil was able to achieve the same effect with high soap content.
The G′ at the lowest value of tan δ is the ( G 0 N ), which reflects the entangled density of the colloid network, as well as the structural strength.Moreover, the plateau modulus ( G 0 N ) can be easily obtained through a straightforward method from the frequency sweep data [33].Figure 5 displays the plateau modulus ( G 0 N ) with the MOAT, AN5, and PAO8-based grease under different soap contents.The relationship of the ( G 0 N ) versus soap content is 10% soap content of all grease was higher than the 8% soap content corresponding grease.So, more soap content used in the grease can increase the structural strength.And the influence of the soap content was enlarged by adding MOAT to the base oil.

Dropping Point, Oil Separation, and IR Characterization
The results in Table 3 showed that all the grease had similar dropping points at high temperatures (more than 200 °C).
At the same soap content, the dropping point of MOATbased grease was slightly lower than the PAO8-based grease.This may be due to the viscosity difference between PAO8 The infrared spectra of PAO8, MOAT-based grease at the 10% soap content are shown in Fig. 6.The specific peak at 1560 cm −1 and 1579 cm −1 indicated the presence of lithium soap for the greases [34].And the absorption band at 1746 cm −1 indicated the ester group [35] in the MOAT.And there were -CH 2 asymmetric and symmetric stretching bands at 2959, 2923, and 2849 cm −1 .

Microstructure of Greases
Previous studies indicated that the rheological and tribological properties of grease depended on both the morphology and the entanglement level of soap fibers.Figure 7 depicts the SEM morphological characteristics of different lithium greases at the 10% soap content.The fibers  of PAO-based grease were a little more loosely distributed with porosity.The fibrous structure of 20% AN5-based grease showed similar to PAO8-based grease in terms of fiber morphology, but the entanglement level of the 20% AN5-based grease was higher than the PAO8-based grease.Nevertheless, the soap fibers morphology of MOATbased grease was different from the PAO8-based grease, the 10%MOAT-based grease showed thinner fibers and a number of fibers twisting together formed a coarse fiber.This can be explained by the interaction between soap fibers through the bifunctional groups of base oil, which led soap fibers to group together.The microstructure of 20%MOATbased grease appeared a highly compact flat network structure and the failure to show a better three-dimensional structure, possibly because of a collapsed structure of the thickener fibers after the washing oil pretreatment [36].Due to the absence of base oil in the test sample, the thickener fibers are no longer dispersed in a reticular form, and appear highly concentrated.This effect is especially significant when there is a strong interaction between the base oil and the soap fiber [37].The SEM images still showed a high level of entanglement of the soap fibers.And the results of electron microscope observation are consistent with the data given in the value of G 0 N in the rheology part.

Lubricating Performance
The anti-wear and friction-reducing properties of the MOAT at 10% and 20% concentrations in PAO8 base oil for 10% soap content were evaluated by SRV. Figure 8 depicts the evolution of the friction coefficient over time, as well as the average friction coefficient and wear volume for the four greases.The result indicated that MOAT-based grease reduced the friction coefficient and wear volume, which greatly increased the grease's anti-friction and anti-wear properties.Figure 8a and b presents the friction coefficient curves at applied load 100 N and 200 N.Under the load of 100 N, the friction coefficients of MOAT and AN5-based grease were relatively stable after 200 s.But the PAO8-based grease showed a peak during 900-1000 s.This phenomenon also occurred in repeated experiments, so it was not special.
The friction coefficient of 20% MOAT-based grease showed a downward trend in the follow-up and was lower than the PAO8 and 10% MOAT-based grease, while the friction coefficient of 20% AN5-based grease was much higher than the other grease.Under the load of 200 N, it is seen that the PAO8-based grease, 10% MOAT-and 20% AN5-based greases experienced an obvious peak in the initial stage (around 0-30 s) with very high friction coefficient (> 0.2) (Fig. 8b).However, the 20% MOAT-based grease was displayed using relative stable dynamic friction coefficient during the whole test time.Furthermore, the average friction coefficient is shown in Fig. 8c.On the whole, the friction coefficient decreases with the increase in load, which is the characteristic of boundary lubrication.The wear volume was estimated by a 3D optical surface profiler, as shown in Fig. 8d.The lubricating greases with a compact entangled structure possess a high structure strength, resulting in good load-carrying and anti-wear properties.Compared with the PAO8-based grease, the addition of MOAT in the base oil was able to decrease the wear volume.Under the load of 100N, the 10% MOATbased grease wear volume of 1.1 × 10 5 μm 3 was observed in comparison to 4.8 × 10 5 μm 3 for PAO8-based grease at 100N.And the value was far less than the 20% AN5-based grease.When the load was up to 200N, the wear volume of 10% MOAT-based grease (2.9 × 10 5 μm 3 ) was higher than the 20% AN5-based grease (1.8 × 10 5 μm 3 ).However, these two greases had a closed level of entanglement according to the above mentioned.And this result was the explanation for the different soap fiber structures and the relative elasticity of greases.It can be seen from the previous microscope images that the fiber size of 20% AN5-based grease was thicker than MOAT-based grease, so better structural strength of grease under the higher load.But under the condition of the low load (100 N), the MOAT-based grease had a high relative elasticity resulting in good recoverability under a high shear rate, leading to good lubricating performances.So, even though the amount of the MOAT was up to 20% and the higher-level entanglement than 20%AN5-based grease, the 20%MOAT-based grease still displayed low wear volume under high load.
For further comparative research, Fig. 9 shows 3D images of the wear scars on the ball surface of the friction samples lubricated with different greases.Under the load of 100N, PAO8-based grease displayed very extensive furrows on the steel ball surface and the largest diameter of the wear scar among the four greases.20%AN5-based grease cannot effectively reduce the friction coefficient, but it can reduce the wear volume and scar diameter to a certain extent.While the MOAT-based greases exhibited significantly fewer furrows and a small wear scar on the balls.Under the load of 200 N, the asymmetry wear scars of the PAO8, 10%MOAT, and 20%AN5-based grease were shown on the ball due to the obvious peak in the initial stage in Fig. 8b.And several large spalling pits were observed on the wear surface.This frictional behavior is attributed to the breakdown of the lubrication under the highly applied contact loads and subsequently causes a dramatic increase in wear due to adhesion and micro-welds.This phenomenon was observed clearly on the surface of the 20%AN5-based grease.So, it can be observed that the diameter of the wear scar is small but the wear volume is large.The 20% MOAT-based grease displayed in shallow grooves and the scar was smooth and ordered without pits.
Herein, the synergistic combination of two different types of functional groups enhanced the friction-reducing properties of grease and decreased wear by furnishing the anti-wear properties (Fig. 10).Because the interaction between the thickener and base oil is mostly impacted by grease lubrication performance than their physical characteristic, for example, their consistency.The excellent friction and wear reduction properties could probably be ascribed to the strong interaction of the MOAT and the soap fiber.For AN5-based grease, the compact microstructure is a result of modifying the thickener-oil interaction balance.However, some strong contact points between the friction pairs at the compact microstructure, which led to a decrease in the contact area and an increase in the friction coefficient [38].For MOAT, the increase of its structural strength is partly due to the interaction between MOAT and soap fiber, which showed good relative elasticity in the high frequency according to the rheology, it showed better liquid fluidity than AN5-based grease.Therefore, in the process of friction, the soap fiber was occurred to arrange directionally, its internal structural interaction offsets the external friction, and it shows a low friction coefficient.

Conclusions
A new lubricating oil with dual-functional groups was synthesized by alkylation of alkylation MOAT.First, the reaction of tetralin and methyl oleate in the presence of AlCl 3 produced by the monoalkylated products (MOAT) was examined.And the MOAT displayed excellent physicochemical properties compared to commercial alkylated naphthalene (AN5), such as low pour point, high viscosity index and high flash point.Then, the lubricating greases prepared by adding the MOAT into the PAO8 base oil have high consistency and better relative elasticity, which is determined by the rheology properties and the microstructure of the grease.By comparing with the AN5based grease, the relative elasticity of the MOAT-based grease is relative to the functional group of the ester group of the molecular which led to the stronger interaction of the soap fiber than PAO8-based oil molecular.Finally, the tribology properties were examined under the SRV tester.The MOAT-based grease showed that not only a good effect on decreasing wear volume, but also plays a role in reducing friction.By introducing bipolar functional groups into the base oil structure, the thinner soap fibers are twisted through multiple strands, resulting in an improvement between the structural strength and elasticity of the grease.This approach offers a new idea for grease manufacture to improve the grease rheology and tribology properties.Overall, our finding verifies the feasibility of combining two functional groups in base oil molecules for enhanced performance of grease.

Fig. 3
Fig. 3 Frequency dependence of the storage and loss moduli (left), and loss tangent (right), at 25 °C, in the linear viscoelasticity region, for lubricating greases containing 10% soap content

Fig. 4 Fig. 5
Fig. 4 Frequency dependence of the storage and loss moduli (left), and loss tangent (right), at 25 °C, in the linear viscoelasticity region, for lubricating greases The PAO8-based grease showed the average friction coefficient of 0.157 of 100 N and 0.151 of 200 N, which increase to 0.195 of 100 N and 0.156 of 200 N in the presence of 20% AN5.This result suggested that the alkyl naphthalene exhibits inherently high frictional properties under the boundary lubrication regime.As the amount of MOAT added increases under the load of 100 N, the average friction coefficient showed a less visible difference.However, under the load of 200 N, the average friction coefficient showed a tendency of dropping.A maximum reduction in average friction coefficients was observed at an optimum 20% concentration MOAT at which values of 0.145 and 0.125 were observed in comparison to

Fig. 6 Fig. 7
Fig.6 The Fourier transform infrared analysis spectra of four greases at the 10% soap content

Fig. 8 Fig. 9
Fig. 8 Dynamic friction coefficient curves of the four greased at the 10% soap content at 100N (a) and 200N (b), the average friction coefficient (c), and wear volume (d)

Table 1 The
Fig. 2 Viscous flow curves for selected lubricating greases containing different base oils, at 25 °C

Table 2
Values of the plateau modulus, consistency Index, and flow Index for the lubricating greases studied