J. Cent. South Univ. Technol. (2007)02-0255-05

DOI: 10.1007/s11771-007-0051-y                                                                                              

Dynamic wetting of rolling oil on aluminum surfaces

ZHOU Ya-jun(周亚军), ZHOU Hong-hui(周宏慧)

（School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China）

Abstract: Static and dynamic contact angles of stock oil and its solutions with additives(fatty acid, fatty alcohol, fatty methyl ester usually used in rolling aluminum) were measured on aluminum surface (Alloy 1145) by sessile drop technique on an OCA35 dynamic contact angle tester. The effect of additive on the drop spreading was investigated as well. It is shown that the drop spreads very quickly in the first 500 ms after the lubricant contacts with the aluminum surface, and then does slowly later. The dynamic contact angle decreases exponentially with time. In contrast to the stock oil, although addition of polarity additive of long chain alkyl into stock oil is able to decrease the surface tension of solutions, it weakens the wetting dynamic, which results from the adsorption at the expanding solid/liquid interface. Among the same long chain polarity organic compounds used, dynamic wetting decreases in the order of fatty acid, fatty alcohol and fatty ester. The blend of fatty alcohol and fatty methyl ester can improve the oil wetting dynamics and promote the lubricant spreading.

Key words: aluminum; surface; dynamic wetting; spreading; rolling oil; additive            

1 Introduction

During plastic deformation process, the metal material is deformed continuously to generate virgin metal surfaces that differ greatly from initial metal surface in physical and chemical properties. Virgin metal surface has a high chemical activity to easily adhere to the roller surface, which demands that the lubricant must spread quickly on the virgin surface between the working interfaces, and become a layer of uniform lubricating thin film to cover effectively the virgin surface. The lubricant with good wetting ability to the virgin surface has two benefits: one is to prevent surface metal from adhering by reducing the attraction between metal atoms of both surfaces; the another is to penetrate the fine crack of surface to soften the surface metal, reduce the shear strength^[1-2], avoid friction and wear in processing, promote the product quality, and prolong the mould life.

The lubricant wetting ability is often identified by solid-liquid-gas three-phase equilibrium contact angle. According to Young equation, if equilibrium contact angle θ is greater than 90?, the liquid can’t spread on the solid surface; if equilibrium contact angle θ is less than 90?, the liquid can spread on surface. Liquid wettability increases as equilibrium contact angle θ decreases. HUANG et al^[3-4] and YAN et al^[5] investigated the wetting behavior of lubricant on aluminum and cupper surface respectively according to static contact angle. But it is only less than 100-200 ms that aluminum material passes through the deformation zone in industrial processing such as high speed rolling, drawing and cutting. As a result, apart from the fundamental problem of whether a given metal is wetted by the lubricant, much attention is paid to the rates of wetting processes. Particularly, people are often interested to know how fast a lubricant dynamically wets on metal surface.

Fatty acid, fatty alcohol and fatty methyl ester with long chain are usual lubricating additives for aluminum plastic deformation^[6]. Many studies on the mechanism of reducing friction and tribochemistry were carried out^[7-8], but dynamic wetting behavior of  lubricant was less reported. It is known that wetting of lubricant on surface is the foundation and precondition of lubricating, so the wetting dynamics of rolling oil takes key role in metal fabrication. There are many factors that influence the lubricant dynamic spreading. We simplify the model as one of lubricant drops on the aluminum surface. When the lubricant contacts with the surface, the drop will spread spontaneously under surface tension. The contact angle θ changes from 180? to the equilibrium contact angle (static contact angle). By analyzing the contact angle and diameter change of the drop during the process, we can compare different influences on dynamic wetting of lubricant with various additives.

2 Experimental

2.1 Material

The lubricating additives used in this study were long chain fatty acid(FAC), fatty alcohol(FAL) and fatty methyl ester(FME), all with the same alkyl chain length. Fatty acid and fatty alcohol were of analytic purity, and fatty methyl ester was a commercial product. The purity of the three additives was more than 98.5%. The stock oil was commercial rolling oil, whose physicochemical properties are listed in Table 1. The solutions were prepared by adding 5%(mass fraction) additive into the stock oil.

Table 1 Physicochemical properties of used stock oil

The aluminum substrates, cut from cold rolling sheet of aluminum alloy 1145 along the rolling direction into 35 mm×20 mm×1.2 mm(length×width×high) slabs, were cleaned twice or three times with the analytic reagent petroleum aether of distillation range from 60 to 90 ℃ to remove oil trails on their surfaces, then rinsed with alcohol and deionized water and air dried. The roughness was 1.5 μm. To reduce the influence of surface roughness, one aluminum substrate was only used. After testing every time, it was rinsed with analytic pure grade petroleum aether three times and dried by electric hair dryer.

2.2 Wetting measurements and data processing

The wetting measurements were carried out with OCA35 dynamic angle tester (DataPhysics  Germany) equipped with a video camera that can collect up to 50 images per second. The device has some advantages: 1) measuring stage with software to control motorizedly and adjust in x-, y- and z-axes for accurate sample positioning and drop pick up from the dosing needle; 2) high-speed video system with adapters and CCD camera; 3) range of contact angle measurement from 0? to 180? with measuring accuracy of ±0.1? and position  accuracy of ±0.01 mm. In our measurement, aluminum slab was put on the measuring stage, and its long side (namely rolling direction) was vertical to observation direction. Before adding the lubricant drop, the injection system was washed twice with petroleum aether and then the sample. The dynamic wetting of the lubricant on aluminum surface was tested according to sessile drop method. Drops of lubricants of volume about 3 μL were applied to the surface to reduce the influence of gravity. We applied 50 images per second to record the spreading process.

Our measurement was carried out in the open air at room temperature of (24±1)℃.

The SCA20 software of OCA35 gives some dynamic information such as the contact angle, drop spreading diameter from the drop profile. Because contact angle and drop diameter changed with time, the recording intervals of repeated test of the same sample were different. We cannot directly deal with data according to arithmetic mean value method. But we can apply interpolating method to treat the experimental data and then get the mean value.

3 Results and discussion      

3.1 Dynamic spreading of stock oil on aluminum surfaces

The images of the dynamic spreading of stock oil on aluminum surfaces are shown in Fig.1. When a liquid drop was placed in contact with the solid, it spread around rapidly and changed into thin-pie shape 31 ms later. The profile of the drop and its shadow looked like a convex glass. The diameter of the drop increased and the thickness decreased continuously with time evolution, and the drop turned into a thin oil film 3 s later. Obviously, the spreading process was quite rapid.

Fig.1 Images of spreading process of stock drop on aluminum surface

The liquid spreading ability on solid surfaces can be judged by spreading coefficient S:

S=δ_sg-δ_lg-δ_sl                       (1)

where  δ_ij  refers to the interfacial tensions of solid/gas, liquid/gas and solid/liquid, respectively.

Clean aluminum surface is a high-energy surface, and its surface tension is about 1 100 mJ/m. The surface energy of stock oil is only 30 mJ/m. Because there is a tremendous difference from their surface tensions, spreading parameter S is big. Therefore, the stock oil can spread on aluminum surface very quickly.

The measured dynamic contact angle and spreading diameter of drop are shown in Fig.2. In the first 500 ms of the liquid contacting the solid, the diameter increased almost linearly, and dynamic contact angle decreased dramatically. The contact angle relaxed from its initial maximum value of 180? at the moment of contact to 8.0?, and the drop almost transferred into a layer of oil film. After that, the drop diameter increased slowly and dynamic contact angle decreased slowly as well. The curves obtained were fitted by the exponential decay function: y=y_∞+a₁exp(-t/b₁)+a₂exp(-t/b₂) to determine the quasi-equilibrium value y_∞ that no longer depends on time.

Fig.2 Contact angle and base diameter versus time for stock oil on aluminum substrate surface For the contact angle, the fitting equation is  

  (2)

Chi²/DoF=0.004 55, R²=0.999 4

For drop spreading diameter, the fitting equation is

 (3)

Chi²/DoF=1.12×10^-4, R²=0.997 4

where  θ(t) is the dynamic contact angle, D(t) is the drop diameter, and t is spreading time.

When t is infinite, the drop spreading achieves balance. The last two terms of the Eqns.(2) and (3) are equal to zero, so θ equals 2.73 and D(∞) equals 5.90. Specifically, when the drop is at balance, its static contact angle θ₀ is 2.73? and its diameter is 5.90 mm.

3.2 Influence of polarity organic compounds

In practical rolling process, the aluminum rolling oil is the mixture of the stock oil and  4%-8%(mass fraction) additives. Long chain organics such as acid, alcohol and ester always are used as additives and they are able to decrease friction and wear. In this experiment, we added 5% fatty acid, fatty alcohol and fatty methyl ester as additives respectively into the stock oil. In contrast to the stock oil, the surface tension of solution decreased obviously, which is listed in Table 2. The kinematical viscosity increased slightly. This could promote the liquid spreading, but the static contact angle didn’t decrease and inversely increased obviously. The dynamic contact angle of the solution was clearly bigger than that of the stock oil as well. The result shown in Fig.3 indicates that polarity compounds cannot promote the dynamic wetting of rolling base oil. This reflects a paradox relationship between lubricating and wetting.

Fig.3 Dynamic contact angles for four different liquids on aluminum surface

The paradox results from the long chain polarity molecules of additives of fatty acid, fatty alcohol and fatty methyl ester in the solution absorbed by solid surfaces. The molecules are absorbed on the aluminum in the vertical direction and gather very closely for the cohesion with each other. They form the thin film to prevent metal from contacting directly, so the friction and the wear between metals decrease effectively^[9]. The absorption force at the liquid/solid interface makes the concentration of polarity molecules in the solution close to solid surface higher than the average one^[10]. Correspondingly, increasing the viscosity of the solution  close to solid surface will increase the energy dissipation during the drop spreading process and decrease its dynamic wetting. BLAKE and DE CONINCK pointed out that solid-liquid interactions modify both the driving force and the resistance to wetting^[11]. For a liquid meniscus advancing across the surface of a solid, these two effects have opposing consequences. Strong interactions increase both the driving force and the resistance, and the two effects do not simply cancel out. As a result, the maximum rate at which a liquid can wet a solid may exhibit its own maximum at some intermediate level of interaction. Our experimental data are shown to support their findings.

3.3 Influence of polarity functional group on rolling oil’s wetting

Among the polarity compounds with the same alkyl, the alcohol is usually known as the best wetting medium, the ester the worst and the acid the intermediate^[12]. According to wetting dynamics of fatty acid, fatty alcohol and fatty methyl ester on aluminum surfaces shown in Figs.3 and 4, we can find that the contact angle of solution is in the order of fatty acid＜fatty alcohol＜fatty ester, and the drop base diameter of solution is in the order of fatty acid＞fatty alcohol ＞fatty ester at the same time. The static contact angles of the drops are the same as the dynamic contact angle. All those results show that the wettability of fatty methyl ester is the worst, fatty acid is the best and fatty alcohol is the intermediate. Practical application in aluminum rolling producing also confirms this conclusion. To promote the rolling speed and the aluminum foil surface finish, we can add some dodecanoic acid in the rolling oil. This measure has two effects: one is to improve the wettability of the rolling oil, the other is to enhance the lubricating ability of the rolling oil.

Fig.4 Dynamic drop base diameters for four different liquids on aluminum surface

3.4 Influence of mixture of alcohol and ester on dynamic wetting

Although fatty acid has good lubrication and wetting dynamics, it corrodes equipment, and it is easier to form brown oil stain that intensively influences the product quality. So in practical application, the main lubricating additive used in rolling aluminum is not fatty acid, but fatty alcohol and fatty ester. Recently many researches are concentrated on the tribochemistry and lubricating mechanism of fatty alcohol and fatty ester, which show that the blend of the alcohol and the ester can enhance the lubricating ability of the rolling oil, and decrease the friction force and metal wear. We measured the wettability of their mixtures, and the result (Table 2) shows that the static contact angle of mixture of 1% fatty methyl ester, 3% fatty alcohol and the stock oil is 4.0?, which is lower by 1.3? than that of the mixture of 5% fatty alcohol and the stock oil. The static contact angle of mixture of 3% fatty methyl ester, 1% fatty alcohol and the stock oil is 8.3?, which is lower by 0.5? than that of the mixture of 5% fatty methyl ester and the stock oil. All these results confirm that the mixture of fatty methyl ester and fatty alcohol as additives can decrease the static contact angle and enhance the oil lubricating ability. The mixtures with different proportions of fatty methyl ester and fatty alcohol have influence on lubricating dynamic contact angle as well. The result is shown in Fig.5. When the ratio of fatty methyl ester to fatty alcohol is 3?1, the mixture has the smallest dynamic contact angle. When  the ratio of fatty methyl ester to fatty alcohol is 1?3, the mixture has the biggest dynamic contact angle. This indicates that increasing proportion of fatty alcohol in lubricant properly is in favor of promoting the wetting behavior.

Fig.5 Effect of mixture of alcohol and ester on dynamic contact angle

Table 2 Characteristics of solutions used in present study

4 Summary

1) The wetting behavior of stock oil and its solutions of additives was characterized by dynamic wetting measurement, in order to analyze the equilibrium state. All the measurement data obtained were fitted by exponential decay function.

2) The drop of all lubricants used can spontaneously spread very quickly on aluminum surfaces into thin-film oil in 3-4 s. The contact angle decreases dramatically in the first 500 ms after the lubricant contacts with the aluminum, and then becomes slow later.

 3) Addition of some long chain polarity organic compounds such as fatty acid, fatty alcohol and fatty methyl ester into the stock oil can promote the lubrication, but weaken the dynamic wetting. Among additives used, the dynamic wetting decreases in the sequence of fatty acid, fatty alcohol and fatty methyl ester. The mixture of fatty methyl ester and fatty alcohol can improve the wetting behavior in contrast to pure fatty methyl ester or pure fatty alcohol, which is beneficial to the spreading of the lubricant.

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Foundation item: Project (01C26224300195) supported by the National Innovation Fund for Technology Based Firms

Received date: 2006-06-12; Accepted date: 2006-08-20

Corresponding author: ZHOU Ya-jun, Doctoral candidate; Tel: +86-731-8879044; E-mail: zhouyjun@mail.csu.edu.cn

 (Edited by YANG Bing)