Preparation of water-soluble nanographite and its application in water-based cutting fluid
© Chen et al; licensee Springer. 2013
Received: 8 November 2012
Accepted: 18 January 2013
Published: 26 January 2013
Water-soluble nanographite was prepared by in situ emulsion polymerization using methacrylate as polymeric monomer. The dispersion stability and dispersion state of graphite particles were evaluated by UV-visible spectrophotometry and scanning electron microscopy, respectively. The water-soluble nanographite was then added into the water-based cutting fluid as lubricant additive. The lubrication performance of water-based cutting fluid with the nanographite additive was studied on four-ball friction tester and surface tensiometer. Results indicate that the modification method of in situ emulsion polymerization realizes the uniform and stabilized dispersion of nanographite in aqueous environment. The optimal polymerization condition is 70°C (polymerization temperature) and 5 h (polymerization time). The addition of nanographite decreases the friction coefficient and wear scar diameter by 44% and 49%. Meanwhile, the maximum non-seizure load (P B ) increases from 784 to 883 N, and the value of surface tension (32.76 × 10−3 N/m) is at low level. Nanographite additive improves apparently the lubrication performance of water-based cutting fluid.
KeywordsNanographite Emulsion polymerization Lubrication performance Water-based cutting fluid
In recent years, nanographite has received considerable attention due to its natural features . On one hand, nanographite possesses the special properties of nanomaterials such as the quantum-size effect, the small-size effect, and the surface or interface effect . On the other hand, it has the advantages of natural graphite flakes such as the self-lubrication and boundary-lubrication abilities. Therefore, nanographite exhibits great superiority in the lubrication field, especially under harsh circumstances like high-temperature or extreme-pressure conditions [3, 4]. However, nanographite is difficult to apply in water-based fluid because of its hydrophobicity [5–7].
Cutting fluid plays an important role in the manufacturing industry as lubricant . It can be mainly classified into two categories: oil-based and water-based cutting fluid. The primary functions of cutting fluid include lubrication, cooling, cleaning, and antirust. At present, the lubrication performance of oil-based cutting fluid is outstanding, but its cooling property is inferior. On the contrary, water-based cutting fluid shows powerful ability in cooling, cleaning, and antirust, but it is relatively weak in lubrication . Nowadays, increasingly strict environmental regulations result in higher operating costs for metal cutting. Water-based cutting fluid is utilized more and more popularly, owing to its low-cost and less-waste emissions than oil-based cutting fluid . However, the water-based cutting fluid is not ideal due to its inferior lubrication ability . Consequently, it is necessary to find a way to enhance the lubrication property of water-based cutting fluid. Up to now, a great deal of research has been done on this subject [9–11]. One simple approach is putting additives into regular lubricants to reduce friction and wear, which has been widely applied in lubrication engineering .
Many researchers [12–14] have reported that nanoadditives are effective in improving the properties of lubricants. They applied different kinds of nanoparticles made of polymer, metal, organic, or inorganic materials to the fabrication of nanolubricants. In order to make the sufficient exertion of the lubricating advantage of nanographite, this research aims to improve the lubrication performance of water-based cutting fluid by adding nanographite as an additive . In this study, commercially available nanographite and water-based cutting fluid were used as materials. Graphite nanoparticles were firstly modified through in situ emulsion polymerization to obtain the water-soluble nanographite [16–19]. UV-visible (vis) spectrophotometry was used to evaluate dispersion stability and determine the optimal polymerization condition. Afterwards, water-soluble nanographite was added into water-based cutting fluid as lubrication additive. The dispersion state of nanographite  in aqueous environment was characterized by scanning electron microscopy (SEM), and the lubrication performance of water-based cutting fluid with nanographite additive was tested by some tribological experiments.
pH: immerse pH test strip into the test solution, and then contrast it with the standard strip.
Foam volume: pour the test solution (70 mL) into a 100-mL cylinder with a stopper. After shaking (1 min) and stewing (10 min), observe the volume of the remaining foam.
Surface tension: test using an interface tensiometer.
Antirust ability: measure by cast iron (two categories, single or lamination). GB/T3142 is the Chinese National Standard test methods of lubricants (determination of load-carrying capacity). Both maximum non-seizure load (P B ) and weld load (P D ) are tested on a four-ball friction tester.
Basic properties of QDW618 water-based cutting fluid
Foam volume V(ml) (≯)
Surface tension σ(mN/m) (≯)
Antirust ability t(h)
Abrasion resistance f(N) (≮)
8 ~ 10
Preparation of water-soluble nanographite
The hydrophobicity of graphite nanoparticles is the major impediment in using nanographite as an additive in water-based fluid to improve the lubrication performance. In order to take the lubrication advantage of nanographite to water-based fluid, surface modification is necessitated to obtain water-soluble nanographite. In this study, water-soluble nanographite was prepared through in situ emulsion polymerization using methacrylate as polymeric monomer. Prior to polymerization reaction, graphite nanoparticles were pretreated by ultrasonic dispersion. The nanographite (1.0 wt.%) was added into a water solution with sodium dodecyl benzene sulfonate (SDBS). As surfactant, SDBS could favor the dispersion of graphite nanoparticles during the ultrasonic process. Ultrasonic pretreatment was carried on an ultrasonic treatment device (Shanghai Ultrasonic Device Co., Shanghai, China; FS-250) for 10 min. The effects of ultrasonic dispersion were observed by SEM. Methacrylate was refined by vacuum distillation before being used as polymeric monomer. The refined methacrylate and the pretreated nanographite were mixed into a four-necked flask. Three of the four necks were used to connect the thermometer, stirring device, and nitrogen, respectively. The other one was left for sampling. A spot of sodium bicarbonate (0.1 wt.%) was also added into the mixture to adjust the pH. Potassium persulfate was employed as the initiator of polymerization. The reaction temperatures were set as 60°C, 70°C, and 80°C. Under each reaction temperature, the sampling time was 4, 5, and 6 h. The entire experiment was conducted under nitrogen atmosphere. The final samples were separated by centrifugation (3,000 rpm, 30 min), and the supernatants were collected.
Absorbance of different supernatants was measured by UV–vis spectrophotometry (Shimadzu Co., Nakagyo-ku, Kyoto, Japan; UV-2450) to evaluate the dispersion stability. The spectral region is 700 to approximately 250 nm. In the experiment, one of the colorimetric wares was enclosed by the supernatant with nanographite as testing sample, and the other one was enclosed by the supernatant without nanographite as reference sample. The dispersion state of graphite particles in aqueous environment was characterized by SEM (Hitachi High-Tech, Minato-ku, Tokyo, Japan; S-4800). SEM images under different magnifications displayed the micromorphology of graphite emulsion.
The supernatant (obtained under optimal polymerization condition) was added into QDW618 water-based cutting fluid with the ratio of 2.0 wt.%. This mixture was named as nanographite fluid. The QDW618 water-based cutting fluid had been diluted by deionized water with the ratio of 1:10. The diluted QDW618 was named as base fluid to make contrast with the nanographite fluid. A series of tribological parameters were obtained by the four-ball friction tester (Jinan Co., Jinan, China; MR-10A) to evaluate the lubrication performance of the nanographite fluid and base fluid. Conditions of the four-ball wear tests are 600 rpm (spindle speed), 392 N (loads), and 1 h (testing time). Also, the frictional materials in the tests were GCr15 standard steel balls. The maximum non-seizure load (P B ) was measured according to GB3142-82 (Chinese National Standard: spindle speed 1,400 to approximately 1,500 rpm, testing time 10 s). In addition, the surface tension was tested on a surface tensiometer (Kruss Co., DKSH Hong Kong Limited, Shanghai, China; K-12) to investigate the wettability.
Results and discussion
Effects of ultrasonic dispersion
Tribological parameters of base fluid and nanographite fluid
Mean friction coefficient (μ)
WSD D (mm)
Maximum non-seizure load P B (N)
Surface tension σ (×10−3 N/m)
The cutting fluid owes its lubrication ability from the lubricating film between the cutter and workpiece. Nanographite particles possess the features of high-temperature resistance and self-lubrication ability which favor the formation and strengthening of the lubricating film. Therefore, the nanographite additive improves apparently the lubrication performance of the water-based cutting fluid.
In this study, water-soluble nanographite was prepared through in situ emulsion polymerization. The graphite particles could disperse uniformly and steadily in aqueous environment after surface modification. The nanographite additive improved the friction-reducing and antiwear properties of the water-based cutting fluid. The mean friction coefficient and WSD reduced by 44% (from 0.106 to 0.059) and 49% (from 1.27 to 0.65 mm), respectively. The P B value increased from 784 to 883 N. Meanwhile, the small surface tension indicated the enhancement of wettability. In general, nanographite additive made up the defect of current water-based cutting fluid whose lubrication ability was not ideal.
QC, XW, YL, and TY are graduate students, and ZW is a professor at the College of Science in China University of Petroleum (East China).
This work was supported by the Gold-idea Program of China University of Petroleum (grant no. JD1112-13) and the National University Student Innovation Program (grant no. 091042514).
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