Lubricating properties of oil-in-water emulsion with low oil ...

SCIENCE CHINA Technological Sciences • RESEARCH PAPER •

February 2013 Vol.56 No.2: 369–375 doi: 10.1007/s11431-012-5092-2

Lubricating properties of oil-in-water emulsion with low oil concentration: Competitive wetting effect LIU ShuHai* & TAN GuiBin College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing, Beijing 102249, China Received October 29, 2012; accepted November 17, 2012; published online December 18, 2012

Oil-in-water (O/W) emulsions are widely used in metal working such as hot rolling and cutting. Three kinds of O/W emulsions with low oil concentration were prepared which include conventional emulsion (CE), miniemulsion (MNE) and microemulsion (ME). The lubricating properties of O/W emulsions with low oil concentration were investigated using the tribological testers and the thin film interferometry based on the relative optical interference intensity method. The tribological test results under boundary lubrication show that the friction coefficient and the total losing weight can be clearly seen: CE < MNE < ME. The lubricating film thicknesses under elastohydrodynamic lubrication and thin film lubrication show that a relationship of the film formation abilities: CE > MNE > ME. Competitive wetting behavior of water and oil on solid surface was confirmed to play an important role in the film formation and tribological behaviors of O/W emulsion. oil-in-water (O/W) emulsion, water-based lubrication, tribological properties, lubricating film thickness, competitive wetting Citation:

Liu S H, Tan G B. Lubricating properties of oil-in-water emulsion with low oil concentration: Competitive wetting effect. Sci China Tech Sci, 2013, 56: 369375, doi: 10.1007/s11431-012-5092-2

1 Introduction Water based emulsions (oil-in-water (O/W) emulsions) are widely used in metal working such as hot rolling and cutting [1, 2]. O/W emulsion is a lubricant composed of oil in the form of droplets suspended in water. Hence there are many works on the tribological properties of O/W emulsion [1–10]. The influence of rolling speed on the elastohydrodynamic film thickness of O/W emulsion was widely studied by some researchers, e.g. Nakahara et al. [2], Baker et al. [4], Zhu et al. [6] and Ma et al. [10]. O/W emulsions include the dispersed phase (oil) and the continuous phase (water). The dispersions are usually unstable, so phase separation may occur as soon as stirring ceases. To make a kinetically stable emulsion, a third com-

ponent, surfactant, or emulsifier is required. Based on the droplet size of oil, O/W emulsion can be divided into three kinds: conventional emulsions in the micrometer range, miniemulsions with submicrometer droplet size, and microemulsions with very small droplet sizes (typically 10 nm) [11]. The conventional emulsions are non-equilibrium systems whose properties depend not only on thermodynamic conditions but also on preparation methods. However microemulsions are equilibrium structures distinctly different from conventional emulsions. Microemulsions can have characteristic properties such as ultralow interfacial tension, large interfacial area and capacity to solubilize both aqueous and oil-soluble compounds [12]. In O/W emulsion with low oil concentration, microemulsions and miniemulsions are very important due to the small droplet size. The studies about O/W emulsion were mainly focused on the conventional emulsion. Little work was reported about

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the lubricating properties of microemulsion and miniemulsion. In the present study, we measured the lubricating properties of O/W emulsions with low oil concentration, which include conventional emulsion, miniemulsion and microemulsion, using the tribological testers and the thin film interferometry based on the relative optical interference intensity (ROII) method.

2 Experimental 2.1

Preparation of O/W emulsions

Polyalphaolefin (PAO) was used as the micro-content oil phase, which was purchased from Chemical Regent Co., Ltd. (Beijing, China) and was used as received. Deionized water was obtained from Milli-Q system. Three kinds of O/W emulsions were prepared with PAO, deionized water and surfactant C16E6 used without further purification. The oil and water emulsions were agitated by a mechanical agitator/mixer. The oil size distributions were obtained from dynamic light scattering (Nano ZS90, Malvern Instruments, UK) at 25°C. 2.2

Film thickness measurements

Under elastohydrodynamic lubrication (EHL) and thin film lubrication (TFL), the measurement of lubricant film thickness was carried out using optical interferometry (NGY6, State Key Laboratory of Tribology, China) shown in Figure 1, which was described in refs. [13, 14]. It comprises four basic units: microscope, lubricant container, instrumental body, and control cabinet. The resolution of the instrument is 0.5 nm in the vertical direction and 1 μm in the horizontal direction. A contact is formed between the flat surface of a rotating glass disc and a reflective steel ball. The glass disc is coated with a thin semi-reflective layer of chromium. The glass disc is rotated and drives the ball in nominally pure rolling. The surface roughness Ra of the glass disc and the steel ball measured by AFM are 3.2 and 5.6 nm separate-

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ly. The load applied to the ball is 30 N which corresponds to a Hertzian pressure of 0.53GPa. A new, 7/8-in. diameter, steel ball was used for each test. The test temperature was maintained at 25 °C for all measurements. The refractive index of the measuring film thickness is a necessary input in the calculation of the film thickness. Refractive index was measured for each aqueous solution tested by using an Abbe refractometer, as shown in Table 1. 2.3

Tribological tests

Under boundary lubrication (BL), the tribological properties of O/W emulsions were investigated using a SRV tribometer (Optimol Instruments Prüftechnik GmbH, Germany) with a reciprocating ball-on-disc configuration. The upper balls used in all experiments were made from GCr15. The lower test specimens were 45# steel. The tests were performed under a load of 200 N; stroke, 2 mm; frequency 20Hz; and room temperature (25˚C). The testing duration was 30 min. Wear was calculated by the mass loss on the steel ball and block. The specimens were ultrasonically cleaned with acetone and dried in oven for 10 min before and after testing, respectively. The frictional force was continuously recorded during testing. The worn surfaces of tribo-couples were examined by scanning electric microscopy (SEM). 2.4

Physicochemical properties of O/W emulsions

Contact angle measurements were conducted using the sessile drop technique performed on a contact angle goniometer (JC2000A, China) at 25±1°C. The samples were ultrasonically cleaned in ethanol and in deionized water successively, for 10 min each. One sample was further placed into a hermetic chamber where the atmosphere was previously saturated with the liquid of measurement. A 3 mL droplet of the liquid was deposited on the surface of the sample, and immediately after stabilization, an image of the droplet was captured. A minimum of 10 values of contact angles was collected per each group of samples. The viscosity was measured using a rheometer with concentric cylinder geometry (Haake RV20). The temperature was controlled using a temperature-controlled water bath surrounding the measuring cup in addition to the standard circulating water inside the cup holder. The test temperature was maintained at 25°C. Table 1

Refractive index and viscosity of O/W emulsions

O/W emulsion Figure 1 Picture of optical interferometry. The interfered light forms an image of monochromic interference fringes graphed by a charge coupled device camera. The optical image is displayed and digitized by a computer.

Concentration (wt%)

Refractive index at 25°C

Viscosity at 25°C (mPa s)

CE

0.5

1.3832

1.035

MNE

0.5

1.3745

1.026

ME

0.5

1.3653

1.028


3 Results 3.1 Physicochemical properties of various O/W emulsions Figure 2 shows the oil size distributions obtained from the three kinds of O/W emulsion samples with 0.5 wt% oil concentration. For No. 1 O/W emulsion, the intensity size distribution with z-average diameters is 19 nm, as shown in


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Figure 2(a). It can be seen from Figure 2(b) that the average oil droplet size of No. 2 O/W emulsion is 210 nm. Figure 2(c) shows the intensity size distribution of No. 3 O/W emulsion. The first peak of average diameter of No. 3 O/W emulsion presents at around 1 µm. The presence of the second peak of No. 3 O/W emulsion has resulted in the average diameter for this sample increasing to 12 µm. The O/W emulsions with oil droplet of micrometer, submicrometer, and nanometer, were designated henceforth as conventional emulsion (CE), miniemulsion (MNE), and microemulsion (ME). According to oil size distribution, hence, the Nos. 1, 2, 3 O/W emulsions belong to ME, MNE and CE, respectively. 3.2 Tribological properties of various O/W emulsions under boundary lubrication

Figure 2 Intensity size distributions obtained from the O/W emulsion samples with 0.5 wt% oil concentration. (a) ME, (b) MNE, (c) CE. Measurements were performed at 25 °C.

Figure 3 shows the friction coefficient as a function oftime of ME with various concentrations. Five kinds of concentrations are presented for discussion, 0.01, 0.05, 0.1, 0.3, and 0.5wt%. Obviously different tribological performances can be found between ME and water. The significant reduction can be found between ME and water. The significant reduction of friction coefficient by ME shows that ME can easily form the lubrication layers, which can protect the rubbing surfaces under boundary lubrication. When the oil concentration of ME is 0.5wt%, the friction coefficient is about 0.1 after the running-in period of 700 s. However, when the oil concentration of ME is 0.01wt%, the friction coefficient is about 0.13 after the running-in period of 2000 s. It can be seen that the friction coefficient and the running-in time of ME increase with the oil concentration decreased. Figure 4 shows the friction coefficient as a function of time of ME, MNE and CE with 0.5wt% oil concentration. After the running-in time (100 s), the friction coefficient of ME is a steady state (~0.12). For MNE, the friction coefficient is about 0.09 after the running-in period (140 s). After the running-in time (160 s), the friction coefficient of CE is reduced to 0.07. It can be seen that the friction coefficient and the running-in time increase with the average oil droplet size decreased. And the friction coefficient of emulsions is higher than one of oil but lower than one of water. Figure 5 shows the losing weight of discs and balls after the wear test under various emulsions lubrication. Figure 6 shows the SEM micrographs of discs (Figures 6(a1)−6(d1)) and balls (Figures 6(a2)−6(d2)) after the wear test, under water or various emulsions lubrication. It is found that the worn surfaces, lubricated by various emulsions, exhibit some micro-grooves (Figures 6(a1)−6(c1), 6(a2)−6(c2)). In the SEM micrograph (Figures 6(a1), 6(a2)) of the worn surface under CE lubrication, solid particles along the sliding directions were observed. However, the rubbing surfaces lubricated by pure water (Figures 6(d1), 6(d2)) are much rougher than the former and many pits or spalls can be observed due to contact fatigue and adhesive

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fatigue [15]. As shown from the results, the friction coefficient and the total losing weight (disc and ball) can be clearly seen: CE ME. Obviously, the result cannot be explained by the viscosity of various O/W emulsions (as shown in Table 1). What is the origin of this increase in the film thickness? As is known to all, oil-in-water emulsion consists of a small volume of oil dispersed in water in the form of small particles. In the two-phase lubrication system, the competitive wetting process will happen when two kinds of liquid with quite different wetting abilities repulse each other in contacting with the solid surface. The next section tries to give a clear discussion about the competitive wetting behavior.

4 Discussion

Figure 4 Friction coefficient versus time of water, oil (PAO) and various O/W emulsions (ME, MNE and CE with 0.5 wt% oil concentration). Temperature: 25 °C; load: 200 N; tester: Optimol SRV tester.

The lubricating film-formation of O/W emulsion can be illuminated by the competitive wetting behaviors in the two-liquid phase system shown in Figure 8. The contact angles of water and PAO oil on chromium layer are 89±2° and 12±2°, respectively, as shown in Figure 9. The wetting ability of PAO oil on chromium layer is stronger than one of water on chromium layer is due to litter contact angle of PAO oil on chromium layer. So PAO oil can form an oil pool in the contact region, which directly determines the film formation ability of emulsion, as shown in Figure 10. In the competitive wetting process of water and PAO, water will separate PAO and the solid surface in the contact region. Based on the Young-Dupré equation, WSO is the work of adhesion to obtain unit new interface by separating oil and the solid surface using water as shown below [16]: WSO   SW   OW   SO ,

(1)

where the interfacial tension between the solid surface (chromium layer) and water SW is 29.3 mN m1 [17], OW is the interfacial tension between oil and water, and SO is the one between the solid surface (chromium layer) and PAO oil. SO can be determined using the Young’s equation [16]:

 SO   S   O cosSO , Figure 5 Losing weight of discs and balls after the wear test under various emulsions lubrication (ME, MNE and CE with 0.5 wt% oil concentration). Temperature: 25 °C; load: 200 N; tester: Optimol SRV tester.

(2)

where the surface energy of chromium layer S is 30.6 mN m1 [17], O is the surface tension of oil, and SO is the static contact angle of PAO oil on chromium layer. The surface energy of the solid can be estimated using the semi-



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Figure 6 SEM micrographs of discs and balls after the wear test under water and various O/W emulsions lubrication. Temperature: 25 °C; load: 200 N; tester: Optimol SRV tester. (a1) Disc (lubricant: ME); (a2) ball (lubricant: ME); (b1) disc (lubricant: MNE); (b2) ball (lubricant: MNE); (c1) disc (lubricant: CE); (c2) ball (lubricant: CE); (d1) disc (lubricant: water); (d2) ball (lubricant: water).

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where  i   id   ip is the decomposition of the surface energy of either the solid or liquid phase into its disperse and polar components, with i representing the index for solid (S) or PAO oil (O).The dispersive component  Sd and polar component  Sp of the surface energy of chromium layer S are 27.5 and 3.1 mN m1, respectively [17]. The polar component  Op of the surface tension of oil O is

Figure 7 Film thickness versus rolling speed of water and various O/W emulsions (ME, MNE and CE with 0.5 wt% oil concentration). Temperature: 25 °C; load: 30 N; tester: optical interferometry.

Figure 8

Lubrication model of O/W emulsion.

0 mN m1. Based on eqs. (3) and (2), O and SO are 28.2 and 3.0 mN m1, respectively. Surfactants have been known to reduce the interfacial tension between oil and water OW. The relationship of surfactant concentration in O/W emulsion is CE < MNE < ME. Hence the relationship of OW is CE > MNE > ME, and the relationship of the calculated work of adhesion on solid surface is CE > MNE > ME. The calculated work of adhesion on solid surface predicts the difficulty level for film forming, which results in different amounts of oil wetting on solid surface. Thus, the relationship can be obtained on the effective viscosity: CE > MNE > ME, and the film formation abilities are CE > MNE > ME. The thicker lubricating film will reduce the friction force. Hence, the friction coefficient and the total losing weight (disc and ball) can be clearly seen: CE ME. Competitive wetting behavior of water and oil on solid surface was confirmed to play an important role in the film formation and tribological behaviors of O/W emulsion, which is also of help to understand the behavior and mechanism of lubricant properties in oil, water and aqueous lubrication [18−21]. This work was supported by the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20100007120010), the Tribology Science Fund of State Key Laboratory of Tribology (Grant No. SKLTKF11A05), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-04-17).


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