Research on the Relationship between Molecular Activity of Additives and Lubricating Performance of Aluminum Rolling Oil

Xia Lei; Sun Jianlin; Zeng Yingfeng; Zhang Min; Duan Qinghua

(1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083; 2. Research Institute of Petroleum Processing, Sinopec Corp., Beijing 100083)

Research on the Relationship between Molecular Activity of Additives and Lubricating Performance of Aluminum Rolling Oil

Xia Lei1; Sun Jianlin1; Zeng Yingfeng2; Zhang Min1; Duan Qinghua2

(1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083; 2. Research Institute of Petroleum Processing, Sinopec Corp., Beijing 100083)

In this paper, an ab initio, local density functional (LDF) method was used to explore the relationship between the molecular properties of additives and the lubricating performance of aluminum rolling oil. The structural properties of butyl stearate, dodecanol, docosanol, and methyl dodecanoate were studied according to the density functional theory. The calculated data showed that the atoms in or around the functional groups might be likely the reacting sites. Because of the different functional groups and structure of ester and alcohol, two types of complex additives, dodecanol and butyl stearate, methyl dodecanoate and butyl stearate, respectively, were chosen for studying their tribological properties and performing aluminum cold rolling experiments. The test results agreed with the calculated results very well. The complex ester, viz. methyl dodecanoate and butyl stearate, had the best lubricating performance with a friction coefficient of 0.084 1 and a permissive-rolling thickness of 0.040 mm as compared with that of dodecanol-butyl stearate-base oil formulation.

aluminum; rolling; molecular properties; lubricating performance; DFT

1 Introduction

Nowadays, aluminum cold rolling process is being developed toward a large thickness reduction and a high rolling speed of 2 500 m/min[1]. However, the aluminum-on-steel couple is difficult to be lubricated even at modest loads[2]. Lubricants are usually needed to lower the friction and wear between the mating parts. The commonly used additives in rolling oil of aluminum are fatty acid, fatty alcohol and ester of fatty acid[3]. Though the use of additives could provide high rolling performance[4], it might be easy to produce oil stain in the surface to degrade the quality of aluminum[5]. Because of the limitation to the nature of single additive, it is difficult to achieve good lubricating performance and fine surface quality. Therefore composite additive is the development trend in aluminum rolling oil application.

Traditionally, scientists develop new additives by compounding existing additives or new substances according to experience. It is laborious, expensive and time-consuming without theoretical basis. The application performance of additives relies on their structure and molecular properties which could be calculated according to DFT (density functional theory). DFT methods have become very popular in the last decade due to their accuracy that is similar to other methods but needs less time and smaller investment. K. F. Khaled[6]researched some benzotriazole derivatives as possible corrosion inhibitors for copper in HNO3using DFT. Results obtained from dry-lab process are in good agreement with those recorded from wet-lab experiments. T. Zolper[7]and Y. Q. Tan[8]studied the relationship between molecular structure and tribology of lubricants. However, their study focus was the tribological properties of base oil or additives of a single kind rather than the lubricating performance of complex additives in the rolling process.

This work was to identify the relationship between molecular properties and lubricating performance of aluminum rolling oil. Geometric structure, the highest occupied molecular orbital energy level (EHOMO), the lowest unoccupied molecular orbital energy level (ELUMO), chargeson atoms, total energy and reactivity of four typical molecules were investigated respectively. The experiments of tribological properties and cold rolling were compared with the calculated data as well.

2 Experimental

The base oil for aluminum rolling was a re fined mineral oil. The additives included butyl stearate, methyl dodecanoate, dodecanol and docosanol.

Friction testing was carried out on an MRS-10A four-ball testing machine. The last non-seizure load (PB), the coefficient of friction (μ) and the wear scar diameter (WSD) were measured according to China National Standard GB/T 12583—1998. The surface condition of worn steel balls was observed by an optical microscope.

A four-high rolling mill measuring Φ95/200×200 mm was used to analyze the permissive-rolling thickness (hp) with different rolling oil. The sample was made of 1060 aluminum, the size of which was 0.52 mm×50 mm×200 mm. The degree of reduction was set as 20% in each rolling pass and the roller speed was 60 r/min.

3 Results and Discussion

3.1 Theoretical analysis

The calculations were performed using the Materials Studio DMol3[9-10], a high quality quantum mechanics computer program. These calculations employed anab initio, local density functional (LDF) method. DMol3 used a Mulliken population analysis[11].

The reactive ability of the additive was considered to be closely related to their frontier molecular orbitals, the HOMO and LUMO[12].EHOMO, as a measure of electron donating ability of a molecule, was explained as the adsorption ability on metallic surfaces by the way of delocalized pair of π electrons. On the contrary,ELUMOwas explained as the tendency of receiving electrons of a molecule. Accordingly, the difference betweenELUMOandEHOMOenergy levels, the electronic charge on atoms, and the total energy of additive molecule (ET) were determined[12-13]. Calculations of the HOMO-LUMO gap (ΔE=ELUMO-EHOMO)[14]were conducted while the global hardness,η, was approximately equal to ΔE/2, and could be de fined under the principle of chemical hardness and soft-ness[15-16]. All parameters could also provide information about the reactive behavior of molecules with the additive properties listed in Table 1.

Table 1 Molecular properties of additives calculated with DFT method

All calculated data of the additives are listed in Table 1. Among them the global hardness,η, was a parameter relating to the reactive behavior of the molecule. In Table 1, fatty alcohols exhibited a larger hardness value than that of esters. The hardness of the two types of fatty alcohols was similar and that of esters was nearly equal. In general, the lower theη, the higher the reactivity of additives would be. Therefore, the reactivity order of the additives decreased according to the following order: butyl stearate＞methyl dodecanoate＞dodecanol ＞docosanol. Experientially, the reason leading to the better lubricating performance of fatty acid ester than that of fatty alcohol was the existence of longer carbon-chain in the molecule of fatty acid ester. However, the calculated results showed that the reactivity of additives depended onηwhich was decided by functional group in the molecule rather than the length of carbon-chain.

In order to study the local reactivity of the additives, the Fukui indices for each atom in the molecules had been calculated. These indices were a measure of the chemical reactivity, which was indicative of the reactive regions and the nucleophilic and electrophilic behavior of the molecule. The Fukui function,→, is de fined as the derivative of the electronic density,with respect to the number of electrons,N, at a constant external potential,

The more complete scheme of the reactivity of the studied molecules could be achieved by analysis of the Fukui indices about the distribution of charges and the global hardness[17]. The optimized geometries of the additives are shown in Figure 1.

Figure 1 Molecular structure of dodecanol (a), docosanol (b), methyl dodecanoate (c), and butyl stearate (d)

An analysis of the Fukui indices of additives is shown in Table 2, in which only the largest values are presented. It can be easily found out that among all atoms of the additives, the oxygen atoms are the most possible ones to carry out electrophilic attacks, because these atoms possess the highest values ofwhich indicates that electrons pertaining to the oxygen atoms are more ready to interact with the aluminum surface. In comparison with the two oxygen atoms which belong to the ester functional group in methyl dodecanoate and butyl stearate, the one in carbonyl radical is more active. As for the sites of nucleophilic and radical attacks, the hydrogen atom bonded to oxygen atom has the highest values ofin molecules of dodecanol and docosanol. However, the carbon atom belonging to the carbonyl radical shows relatively high values ofin molecules of methyl dodecanoate and butyl stearate. As a result, the atoms in the functional groups or within the region around functional groups are the active sites. It can be inferred that they are more active in reaction upon or adsorption on aluminum. According to the above-mentioned and research information, two kinds of complex additives were formulated in order to improve the lubricating performance of aluminum rolling oil, which could hardly be undertaken by a single kind of additive. According to the calculated results, dodecanol, methyl dodecanoate and butyl stearate were chosen. The oil formulation A1 was composed of 2.5% of dodecanol, 2.5% of butyl stearate and 95% of base oil, while the oil formulation A2 consisted of 2.5% of methyl dodecanoate, 2.5% of butyl stearate and 95% of base oil.

Table 2 Calculated Mulliken atomic charges and Fukui indices of additives

3.2 Investigation of tribological properties

To investigate the relevancy of the calculated results and the tribological properties of the additives, friction coefficients were measured by the MRS-10A four-ball testing machine. Figure 2 provides the variation ofμwith time. It had been found out that theμof oil formulations A1 and A2 increased during the first 15 min and decreased duringthe last 15 min in the course of testing. The averageμvalues of A1 (0.090 1) and A2 (0.084 1) were lower than the base oil (0.107 0).

Figure 2 The changes in friction coefficients with time

Due to the presence of additives in the base oil, chemical or electrochemical reactions occurred and led to lower friction coefficients during the friction process[18]. Table 3 presents thePB,μand WSD values of the steel balls lubricated by the base oil and oil formulations. It can be seen from Table 3 that thePBvalue of A1 and A2 was 314 N and 333 N, respectively, while thePBvalue of base oil was 294 N. The WSD value of the bas oil, A1 and A2 was 0.63 mm, 0.56 mm and 0.52 mm, respectively. The reason leading to the smaller WSD value of A1 and A2 might be ascribed to the additives in oil formulation that could react upon or be adsorbed on the iron element inside the steel ball to form a good lubrication film during the rubbing period. The lubrication film could reduce friction and enhance anti-wear ability to a certain degree. The reason why the anti-wear and friction reducing properties of A2 were better than those of A1 might be attributed to the lower global hardness of the additives in the oil formulation A2 which could result in a quicker and steady formation of lubrication film. The outcome was in good agreement with the calculated results as shown above. The smaller difference ofPBvalues of the three oil samples might be attributed to the absence of extreme pressure element such as sulfur in the additives.

Table 3 Comparison ofPB,μand WSD of rolling oils

3.3 Investigation of additives on viability for practical application

In order to illustrate the viability of the additives for practical application, cold rolling experiments using test mills were conducted to compare the effect of rolling lubrication provided by two complex additives contained in aluminum rolling oil. Rolling characteristic curves were obtained as shown in Figure 3. It can be easily found out that the thickness of workpiece reduced with an increasing rolling pass. However, the thickness reduction got smaller and smaller with increase in number of rolling passes. Furthermore, the workpiece lubricated by the base oil was the thickest and that lubricated by A2 was the slimmest in each rolling pass.

Figure 3 The rolling characteristic curve of workpiece lubricated by different oil samples.

The permissive-rolling thickness (hp), which was related to the friction coefficient, was obtained by using the Stone equation as shown in equation 2[19], with the results presented in Table 4.

whereμis the friction coefficient,Dis the diameter of the roller,Eis the elastic modulus of the roller,Kis the deformation resistance of material, andis the average tension per unit.

Table 4 Permissive-rolling thickness achieved by different oil samples

Thehpvalue was 0.050 mm, 0.045 mm when the permissive-rolling thickness of workpiece was lubricated by base oil and A1, respectively. However, it was only 0.040 mm with a reduction of 20% when the permissiverolling thickness of workpiece was lubricated by A2. The decrease inhpvalue was achieved thanks to a lubrication film formed between the surface of the roller and aluminum sheet in the presence of the additives that could further cause reduction of the friction coefficient. The smallerhpvalue of A2 indicated that the formation of lubrication film by A2 was more quick and stable, and A2 thereby exhibited a better lubricating performance.

Upon comparing the results mentioned above, it can be found out that the tribological properties and practical application had a close relation with the calculated data. The tribological properties of additives were better when they had lowerηvalue. Thehpvalue of aluminum was smaller upon lubrication of workpiece by oil doped with these additives. The criteria for additives selection could be worked out from above considerations. A good additive must have strong affinity to the metal atoms to form bonds easily with the aluminum surface. The affinity of additive molecules or functional groups to the metal surface could be calculated by quantum chemical method. It could be applied to choose additives and even provide a guide to design of new molecule. The designed new molecule could be calculated using quantum chemical method to provide the information on the activity of the molecule and the affinity between the molecule and the metal surface.

4 Conclusions

1) According to the calculating results, the expected reactivity of fatty acid ester was higher than that of fatty alcohol. The sites that were most susceptible to electrophilic attacks, nucleophilic and radial attacks were within the region surrounding functional groups. The oxygen atom in carbonyl radical was liable for electrophilic attacks in esters, while the carbon atom bound to that oxygen atom was prone to be confronted with nucleophilic attacks. For fatty alcohol, the oxygen atom in hydroxy radical was liable for electrophilic attacks and the hydrogen atoms surrounding the hydroxy radical were prone to be confronted with nucleophilic attacks. Upon comparing the two oxygen atoms in the fatty acid ester, the one in carbonyl radical with a double bond was more active.

2) The lubricating performance of complex ester was better than that of the compound additive consisting of ester and alcohol, which was consistent with the calculated data. Theμ, WSD and permissive-rolling thickness of complex ester were smaller than that of the compound additive consisting of ester and alcohol.

3) The excellent lubricating performance of esters depended on the functional group. The HOMO-LUMO gap (ΔE) or the global hardness (η) could be the theoretical basis for selection of additives.

Acknowledgments:The authors are grateful for the financial support of this study provided by the National Natural Science Foundation of China (No.51274037) and the Cooperation Program between USTB and SINOPEC (No.112116).

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Recieved date: 2013-03-20; Accepted date: 2013-04-22.

Prof. Sun Jianlin, Telephone: +86-10-62333768; E-mail address: sjl@ustb.edu.cn.