Shixing Cui,Zhi Yun*,Xia Gui

College of Chemistry and Chemical Engineering,Nanjing University of Technology,Nanjing 210009,China

1.Introduction

Most current used lubricants,containing petroleum base stocks,are toxic to environment and difficult to dispose after use.Due to strict environmental regulations and concerning for depletion of world fossil fuel reserves,the demand for environmental friendly lubricants is increasing.Vegetable oils are considered to be potential candidates to substitute conventional mineral oil-based lubricating oils,because of their wholly biodegradable,non-toxic and low volatility.Its usage,however,is restricted for low thermo-oxidative stability and poor cold flow behavior[1–5].

Current researches of vegetable oils are mainly concentrating on how to eliminate or reduce the unsaturated double bonds in molecules and the negative impact of the aspects on the stability of vegetable oill.Liet al.found that the derivatives of waste cooking oil exhibited favorable low temperature fluidity,oxidation stability and tribological performances[6].Jumat Set al.offered a novel synthetic approach for chemical modification of oleic acid derivatives to improve their oxidative stability,low temperature fluidity and other physico chemical properties[7].Hong-sik and co-workers worked on the chemical modification of soy-bean oil to obtain a series of ring-opened intermediate product.At the same time,the correlations between pour point and structural variation in the molecules were also studied[8].Yanget al.reported that the anti-wear ability and extreme-pressure performance of phosphate ester were improved[9].Chenet al.disclosed that the production,which was obtained from the reaction between biodiesel and organic silicon,showed improved viscosity-temperature characteristic, flash point and pour point,but it displayed poor lubricating property and oxidative stability[10].

Silane can improve adhesion and mechanical properties between inorganic filler and an organic polymer.It is widely applied in the development of adhesives,coatings,and filled plastics[11,12].Various silane structures have been tested for synthesis of silicon-based biological lubricating base oil,and the presence of silicon can largely enhance some lubrication performance in vegetable oil[10,13].Trimethyl chlorosilane(TMCS),one of the very important silane coupling agents,is mainly used to synthesize drug intermediates and polymer materials[14,15],but rarely used in the preparation of lube baseoil[16],and little information about the preparation as well as the lubrication performance of reaction product was reported.This study synthesized a new kind of silicon-based biological lubricating base oil by reacting methyl oleate derivatives with TMCS,and the reaction conditions were also systematically investigated.

The new silicon-based biological lubricating base oil was obtained from methyl oleate by three steps:epoxidation,open-ring reaction with formic acid and trimethy lsilylation reaction.Three types of methyl oleate derivatives were separately prepared by these reactions.Me thy loleate and its derivatives were characterized by Fourier transformation infra-red(FTIR)and silicon nuclear magnetic resonance(29Si NMR),and lubrication performances of them were examined.

2.Materials and Methods

2.1.Materials

Methyl oleate(C19H36O2,AR),trimethyl chlorosilane(C3H9SiCl,AR),triethylamine(C6H15N,AR),N,N-diisopropy lethylamine(C8H19N,AR),pyridine(C5H5N,AR),and diethylamine(C4H11N,AR),were purchased from Chemical reagent Ltd.(China).Hydrogen peroxide(H2O2,AR),formic acid(HCOOH,AR),phosphoric acid(H3PO4,AR),sodium chloride(NaCl,AR)and all the other chemicals for analytical purpose were obtained from Aladdin Reagent Co.,Ltd.(China).

2.2.Analytical methods

The lubricating properties of the methyl oleate and its derivative were investigated by using MRS-10A four-ball friction and wear tester,NETZSCH STA 409-PC simultaneous thermal analyzer,SYD-0193 fully automatic lubricating oil oxidation stability tester,BF-03 oil kinematic viscosity tester,SYD-261 close- flash point tester and SYD-510F multifunction ester at low temperature in order to detect friction properties(GB/T 3142-1982)[17],thermostability(SH/T 0731-2004)[18],oxidation stability(SH/T0193-2008)[19],kinematic viscosity(GB/T265-88)[20],close- flash point(GB/T261-2008)[21]and pour point(GB/T3535-2006)[22].

The structure was characterized by FTIR spectra(Nicolet-8700,U.S.)and29Si NMR(AVANCE 400,Bruker).The content of silicon in the product was determined by using X-ray fluorescence(LAB CENTER XRF-1800,SHIMADZU).The base value of produces was tested according to industry standards(SH/T0251-1993)[23]by Petroleum products base number tester.

The yield of silicon-based biological lubricating oilYis calculated as follows:

whereMKOH(56.1)andMSi(28.08)are the atomic weights of KOH and silicon respectively andHvis the initial hydroxyl value of ring-opened product,CSiis the quality percentage of silicon in the product which was determined by using XRF.

Fig.1.Synthesized figure.

2.3.Synthesis

2.3.1.Preparation of epoxy methyl oleate

A certain amount of methyl ole ate(40g)and phosphoric acid(2.4g)were put in 250 ml three-necked flask containing the required amounts of the formic acid(5.6 g).Then H2O2(36 g)was added drop-wise at a rate for 0.5 h at 45 °C.After 0.5 h,raised the temperature to 60 °C for 1.5 h,and the resulting mixture was stirred at 500 r·min-1in the whole process of reaction.The mixture was poured into a separating funnel and divided into two layers.The upper layer was washed with water to remove the free acid and distilled under reduced pressure.According to GB/T 1668-2008[24],the oxirane value of epoxy methyl oleate is 3.74%.

2.3.2.Ring-opening reaction

Formic acid(40 g)and calculated amount of epoxy methyl oleate(7.096 g)were placed into the flask and the mixture was kept at 60°C with stirring for 2 h.The mixture was next washed with saturated salt water to remove the free acid until the solution was neutral.Finally,water and light component were separated from the ring-opening product by reduced pressure distillation at 90°C for 1 h.According to GB/T 12008.3-1989[25],the hydroxyl value of ring-opening product is 145 mg KOH·(g oil)-1.

2.3.3.Trimethylsilylation reaction

Trimethylsilylation was carried out by using the ring-opening product prepared in the second step reaction reacted with TMCS.A known amount of ring-opening product was added to a three-necked flask.The calculated amount of TMCS was added in a drop-wise manner over 0.5 h.At the same time,the calculated amount of acid binding agent(C6H15N,C4H11N,C5H5N or C8H19N)was also added in a drop wise to ensure the pH value of system greater than 7 until it is used up.The reaction was agitated at a certain temperature with stirring for the indicated time.The trimethylsilylation reaction product was purified by reduced pressure distillation at 180°C for 2 h to remove acid binding agent and unreacted TMCS.Finally,before detecting silicon content by using XRF,TMCS was guaranteed to be completely removed by using29Si NMR as an evaluation methodology.Otherwise,measurement result of XRF would be inaccurate.

Trimethylsilylation was performed under the conditions varied inthe following ranges: temperature 35 °C to 55 °C, reaction time 1 h to3.5 h, mole ratio of TMCS-to-hydroxyl from 0.25:1 to 1.5:1, and mole ratio of DIEA-to-hydroxyl from 0.25:1 to 1.5:1. The acid binding agentsfor trimethylsilylation were N,N-diisopropylethylamine, pyridine,diethylamine and triethylamine. The concentration of acid bindingagent was expressed as a mole ratio of acid binding agent-tohydroxyl,from 0.75:1 to 1.75:1 (Fig. 1).

3.Results and Discussion

3.1.Effect of the acid binding agent

Fig.2.Effect of acid binding agent on yield.(The molar ratio of hydroxyl number of ringopening product(30 g)-to-TMCS(8.424 g)was 1:1,The reactions were agitated at 40°C with stirring for 2.5 h and stirring speed=500 r·min-1.)

Considering economic feasibility,the abilities of DIEA,pyridine,diethylamine and triethylamine to serve as acid binding agent were evaluated separately.The influences of the selected acid binding agent on yield of silicon-based biological lubricating base oil were shown in Fig.2.The results reveal that the yield increased as the acid binding agent increased at low concentration(mole rate≤1).At this point,the acid binding agent could neutralize HCL generated by TMCS reacting with hydroxyl,and therefore promoted the trimethylsilylation reaction[26,27].When the ratio of acid binding agent-to-hydroxyl increased above 1.25,super fluous diethylamine,triethylamine or pyridine was meaningless against the trimethylsilylation reaction and hydrolysis reaction of Si–O in the alkaline system,so yield no longer increased.But the super fluous DIEA increased the viscosity of systems,which was unfavorable to trimethylsilylation reaction,so the yield decreased.

The effects of these acid bindingagentdecrease in theorder of DIEA,pyridine,diethylamine,and triethylamine.Moreover,pyridine is much cheaper than DIEA but harmful to the body,and it is not suitable for industry applications.In conclusion,DIEA is chosen as the acid binding agent of the reaction.

3.2.Effect of TCMS/hydroxyl molar ratio

The trend of variation in yield with the change in reaction time and concentration of TMCS were studied.Fig.3 shows that the yield increased as the mole ratio of TMCS/hydroxyl increased and reached their highest values of 34.54%at 1.25.When themole ratio of TMCS/hydroxylwasabove1.25,theyielddecreasedapparently.Thisisduetothe fact that the properly increased concentration of TMCS in the system is in favor of the reaction with hydroxyl,but the super fluous TMCS naturally breaks down into HCL,which leads to the pH value of system relatively decreasing even less than 7,and causes an aggravated hydrolysis reaction of Si–O under the same concentration of DIEA.

Fig.3.Effect of TCMS/hydroxyl molar ratio on yield.(The molar ratio of hydroxyl number of ring-opening product(30 g)and DIEA(10.02 g)was 1:1,the reactions were agitated at 40 °C,stirring speed=500 r·min-1.)

On the other hand,at low concentrations of TMCS(mole ratio＜1),the main reaction taking place in the alkaline system was those between TMCS and hydroxyl,and it finished quickly.Therefore,the impact of reaction time on yield can be neglected.However,for higher TMCS concentrations(＞1),as time progressed(＞1.5 h),the yield decreased.This is due to the cleavage of the silicon–oxygen bond in acidic system.All things considered,the optimum yield was obtained at the molar ratio of 1:1.25 and 1.5 h.

3.3.Effect of DIEA-to-hydroxyl molar ratio

The effect of the DIEA/hydroxyl mole ratio and reaction time on the yield was studied and results were shown in Fig.4.When the DIEA/hydroxyl mole ratio increased from 0.25 to 1.25,it would lead to the increase of the yield.The maximum yield of 30.91%occurred at a mole ratio of 1.25.When the DIEA/hydroxyl mole ratio was greater than 1.25,a lower yield was observed.The increasing concentration of DIEA is beneficial to raise the pH of system,which can inhibit the hydrolysis reaction of silicon–oxygen bond,and then improve yield.But excessive amounts of DIEA increased the viscosity of system even for ming colloid,which would restrain TMCS thoroughly contacts between hydroxyl and undoubtedly leading to lower yield.

Fig.4.Effect of DIEA/hydroxylmolar ratio on yield. (Themolar ratio of hydroxyl number ofring-opening product (30 g) and TMCS (8.424 g) was 1:1, the reactions were agitated at 35 °C and stirring speed=500 r·min-1.)

At low concentration of DIEA(＜1),this system was weak acidic,the stability of silicon–oxygen bond became poor.Longer reaction time would increase the occurrence of hydrolysis reaction and led to a lower yield.Nevertheless,when the mole ratio was increased above 1,Si–O of silicon-based biological lubricating base oil was rarely hydrolyzed.Increasing reaction time was favorable to trimethylsilylation reaction.It took more time to reach the maximum yield.At the ration of 1.25,the system has a higher base value than that of the ration of 1.0,which is conducive to the storage of products and extends useful life of the lubricant oil.The highest yield was obtained at the molar ratio of 1:1.25 and 2.5 h.

3.4.Effect of the reaction temperature

To evaluate the effect of the reaction temperature and reaction time on the yield,the molar ratio of TMCS-to-hydroxyl-to-DIEA was 1:1:1 and several runs were conducted at different temperatures in the range of 35–55 °C.As shown in Fig.5,at lower temperatures(35 and 40°C),it was found that yield increased as the temperature increased.At temperature of 40°C,fasten movement of elements is in favor of the contact between molecules,so the yield achieved the peak value of 30.66%.Under lower temperature(35 and 40°C),trimethylsilylation reaction showed lower rate and more stable silicon–oxygen bond.The optimum yield could be obtained in a longer time.

Fig.5.Effect of reaction temperature on yield.(The molar ratio of TMCS-to-hydroxyl-to-DIEA was 1:1:1(8.424 g:30 g:10.02 g),stirring speed:500 r·min-1.)

However,the yield continued to decline at temperature range of 45–55°C.This is because when the temperature was close to the boiling point of trimethylchlorosilane,it would lead to the gasification of TMCS and reduced content of TMCS,which was not conductive to trimethylsilylation reaction. Increasing temperature not only result inmore rapid trimethylsilylation reaction, but also in higher rate of hydrolysisof silicon-oxygen bond.

3.5.FT-IR spectra of product

Fig.6.FT-IR spectra of methyl oleate and its derivative.(A)Methyl oleate;(B)epoxidized methyl oleate;(C)ring-opening product;(D)trimethylsilylation reaction products.

The methyl oleate,epoxidized methyl oleate,ring-opening product and trimethylsilylation reaction product were characterized by FTIR,and the FTIR spectra were shown in Fig.6.Stretching vibration peak of=C–H can be detected at wavenumber 3005 cm-1and the peak of HC=CH at wavenumber 1654.8 cm-1shows C=C bonding present in methyloleate.While,for the FTIR spectrum of epoxidized me thyloleate,the unsaturation peak of C=C(at 1654.8 cm-1)and stretching vibration peak of=C–H(at 3005 cm-1)disappear.Epoxidized methyl oleate shows the peak at 845.47 cm-1due to the ternary ring cis vibration absorption peak.The bonding peak of–O–H at wavenumber 3467.04 cm-1illustrates–OH bonding present in epoxidized methyl oleate.These results indicate that all the C=C double bonds have been converted to epoxy groups and some epoxy rings have been hydrolyzed.

The peak at 845.47 cm-1(related to epoxy groups)disappears following the ring-opening reaction in the infrared spectra of ringopening product.The hydroxyl value of the product is mg KOH·(g oil)-1.FTIR spectrum of trimethylsilylation reaction product not only shows two stretching vibration peak of C–Si–C at wavenumber 890–690 cm-1and a characteristic absorption peak of Si–O–C at wavenumber 1086.07 cm-1,but also shows that hydroxyl stretching vibration peak(3467.04 cm-1)declines significantly.The results demonstrate that TMCS is efficiently reacted with hydroxyl.

3.6.Silicon nuclear magnetic resonance spectra(29Si NMR)

Fig.7.Silicon nuclear magnetic resonance spectra(29Si NMR)of trimethylsilylation reaction products and TMCS.

Table 1Element varieties and contents of different samples.Sample 1 was obtained at the condition of 35°C,2.5 h and TMCS-to-hydroxyl-to-DIEA(molar ratio)=1:1:1.25.Sample 2 was obtained at reaction temperature 40°C for reaction time 1.5 h and TMCS-to-hydroxyl-to-DIEA(molar ratio)=1.25:1:1.

29Si NMR spectra of TMCS and trimethylsilylation reaction product were determined,the results were shown in Fig.7.Fig.7 shows that,the silicon of TMCS is present in theδ30–31region.TMCS reacting with hydroxyl leads to the change of chemical environment,thus a chemical shift between 15 and 18 is observed in Fig.7,which is the characteristics of silicon atom in silicon-based biological lubricating base oil.The result indicates that the silicon-based biological lubricating base oil is obtained successfully.

Furthermore,the δ 7 region in Fig.7 indicates that trace amounts of trimethy lsilanol exist,which is caused by hydrolysis reaction of TMCS and Si–O in silicon-based biological lubricating base oil.29Si NMR spectra of trimethylsilylation reaction product has no signal between δ 30 and 31,this gives the confirmation that no unreacted TMCS remains in the final product after a reduced pressure distillation at 180°C for 2 h.

3.7.Element analysis and base value

Two samples with higher yield were picked out to characterize by element analysis,the results are shown in Table 1.The yield of samples can be calculated according to the silicon content in Table1 and Formula 1.The yield of samples was 30.91%and 34.54%,respectively.The base value of sample 1 is 0.0579 mg·KOH·g-1and that of sample 2 is 0.0214mgKOH·g-1according to in dustry standards(SH/T0251-1993).

3.8.Lubrication performance characterization

The kinematic viscosity,viscosity index,close- flash point,pour point of methyl oleate and its derivative were tested according to industry standards,the results are shown in Table 2.Compared with epoxidized methyl oleate and ring-opening product,the product of the trimethylsilylation modification shows low pour point(without pour point depressant(PPD)),high viscosity index and close- flash point.Methyl oleate and its derivative display higher pour point than 150SN,which may be attributed to hydrogen bonding of the surplus hydroxyl groups present in the products produced after the ring-opening reaction and trimethylsilylation reaction[8].When 1%PPD was added,except for the ring-opened product,whose pour point is 3°C,all other products show pour points below than 0°C.Trimethylsilylation reaction successfully improves the pour point, flash point and sticky temperature performance.

Table 2Viscosity,low temperature fluidity, flash point of the lubricant from methyl oleate(A),epoxidized methyl oleate(B),ring-opening product(C),trime thy lsilylation reaction products(D),150SN mineral oil(150SN)

The oxidation stability was determined by using rotating bomb method(RBOT)and the results are summarized in Table 3.The amount of time from beginning of the experiment to reaching the required pressure drop is oxidation stability,which is also called oxidation induction period(OIT,min).The result illustrates thatthe OIT of trimethylsilylation reaction products is higher than that of other three substances and 150SN mineral oil,which shows good durable ability of the product.Trime thy lsilylation reaction can enhance the oxidation stability of methyl oleate.

Table 3OIT and WSD of the lubricant from methyl oleate(A),epoxidized methyl oleate(B),ringopening product(C),trimethylsilylation reaction products(D)

Worn scar diameter(WSD)determined by four-ball friction tester is used to characterize the friction properties of methyl oleate and its derivative.By using scanning electron microscope(SEM),their grinding plaque morphology was analyzed.Fig.8 displays the SEM pictures of steel balls dealt using methyl oleate and its derivatives respectively.Both Table 3 and Fig.8 show that the steel ball dealt using trimethylsilylation reaction products,has lower WSD,fewer pan furrows and smoother surface.Trimethylsilylation product can form thick protective absorbent films on the metal surface,that the films play an important role in lubricating the surface and decrease wear rate.So the anti-abrasive property is enhanced through trimethylsilylation reaction.

3.9.Thermogravimetric analysis

Thermogravimetric analysis was performed while heating various materials from 0 to 600 °C and the heating rate was 10 °C·min-1in air atmosphere.The change tendency of quality fractions are detailed in Fig.9.The OOT is the temperature at which a rapid increase in the rate of oxidation is observed.A high OOT suggests a high oxidation stability and of the oil[30,31].The results of methyl oleate and its derivatives are presented in Fig.10.

As shown in Fig.10,A and B suffer an expected low OOT result of 217 °C and 232 °C,respectively,due to the presence of active –C=C–bonds and epoxy group in A and B.The removal of unstable epoxy groupinB by converting the mintohy droxy groupssigni ficantlyenhances the thermal stability of C.Thermal stability of C was further improved by the introduction of stable trimethyl silicon.The OOT of methyl oleate and its derivatives is followed by the order:D(265°C)＞C(250°C)＞B(232 °C)＞ A(217 °C).A high OOT indicated a good weather ability of the trimethylsilylation reaction products.

Fig.8.SEM image of the methyl oleate and its derivatives.(A)methyloleate,(B)epoxidizedmethyl oleate,(C)ring-openingproduct,(D)trimethylsilylation reaction products.A(1),B(1),C(1),D(1):×1000;A(2),B(2),C(2),D(2):×3000.

Fig.9.Thermo-stability of methyl oleate and its derivatives.(A)Methyl oleate,(B)epoxidized methyl oleate,(C)ring-opening product,(D)trimethylsilylation reaction products.

4.Conclusions

A new kind of silicon-based biological lubricating base oil was synthesized by a series of active group reactions:epoxidation with methyl oleate as raw material,followed by open-ring and trimethylsilylation reaction.Out of all the acid binding agent studied,DIEA was found to be more effective in terms of trimethylsilylation reaction.The trimethylsilylation reaction obtained a good yield at 40°C for 1.5 h and the molar ratio of hydroxyl/TMCS/DIEA=1:1.25:1.Under the optimum reaction condition,the highest yield is 34.54%.

Lubrication performance of the final product modified with TMCS,such as close- flash point and viscosity index,is greatly improved by comparing the other derivative and 150SN mineral oil.But trimethylsilylation reaction product does not show very low pour points.When 1%PPD was added to the products,pour points markedly lowered.The morphologies of the tested steel balls were analyzed using a scanning electron microscope(SEM),the results suggest that trimethylsilylation reaction product exhibits superior anti-wear property during the course of friction.In conclusion,synthetic silicon-based biological lubricating base oil has potential application prospects in the field of lubricant base oil.

Fig.10.The OOT results of programmed temperature for methyl oleate and its derivatives.(A)Methyl oleate,(B)epoxidized methyl oleate,(C)ring-opening product,(D)trimethylsilylation reaction products.

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