碳纳米管-聚丙烯酸乙酯复合乳液的制备及其润滑性能研究Abstract：In this study, carbon nanotubes-poly (acrylic acid ester) composite emulsion was prepared by in-situ polymerization method. The effect of different parameters such as concentration of carbon nanotubes, pH value, and heating temperature on the lubrication performance of the emulsion was investigated. The results showed that the addition of carbon nanotubes could significantly improve the lubrication properties of the emulsion, and the optimal concentration was found to be 0.5 wt%. The emulsion exhibited the best lubrication performance at pH 7.0, and the heating temperature had a significant impact on the stability of the emulsion. Our findings demonstrated that the carbon nanotubes-poly (acrylic acid ester) composite emulsion had good performance in improving the lubrication of mechanical equipment.Keywords: carbon nanotubes, poly (acrylic acid ester), composite emulsion, lubricationIntroduction：With the rapid development of modern industry, the demand for high-performance lubricating materials has become increasingly urgent. Carbon nanotubes, as a new type of nanomaterial, have excellent mechanical, thermal, electrical and optical properties, and have been widely studied in various fields. Due to their high aspect ratio and excellent chemical stability, carbon nanotubes have been shown to have good lubrication properties when used as lubricants or additives in lubricants.Poly (acrylic acid ester) is a commonly used polymer emulsion in the field of architecture, coatings, and adhesive industries, which has excellent adhesive properties and water resistance. In this study, a composite emulsion was prepared by introducing carbon nanotubes into poly (acrylic acid ester) emulsion to improve its lubrication performance.Experimental Section：Materials：Carbon nanotubes (CNTs) were purchased from Nanjing XFNANO Materials Tech Co., Ltd. Poly (methyl methacrylate-co-butyl acrylate) emulsion was purchased from Sigma Aldrich, Inc. Sodium dodecyl sulfate (SDS), acrylic acid, and potassium persulfate (KPS) were purchased from Sinopharm Chemical Reagent Co., Ltd.Preparation of CNTs-Poly (Acrylic acid ester) Composite EmulsionFirstly, CNTs were dispersed in deionized water by ultrasonic treatment for 30 minutes. The concentration of CNTs was adjusted to 0.1, 0.3, 0.5, 0.7, and 1.0 wt%, respectively. Then, the poly (acrylic acid ester) emulsion was mixed with the CNTs dispersion under mechanical stirring for 30 minutes. Subsequently, the acrylic acid monomer was added to emulsion and stirred for 20 minutes. After the solution was heated to the desired temperature, KPS initiator solution (0.5 wt%) was added dropwise, and the reaction was conducted for 6 hours. Finally, the composite emulsion was centrifuged at 10,000 rpm for 10 minutes and washed with deionized water to remove any unreacted substances.Characterization：The morphology of CNTs was characterized by field emission scanning electron microscopy (FESEM) (JSM-7600F, JEOL). The zeta potential and particle size of the composite emulsion were measured by dynamic light scattering (DLS) (Zetasizer, Malvern). The concentration of CNTs in emulsion was determined by thermogravimetric analysis (TGA) (STA 449F5, NETZSCH). The lubrication performance of the composite emulsion was evaluated by a tribometer (Q800, Anton Paar).Results and Discussion：Morphology and Characterization of CNTsThe FESEM images of CNTs are shown in Figure 1. The CNTs were uniform in size and had a diameter of about 30 nm. The surface of CNTs was smooth and intact, indicating a good dispersion of CNTs in deionized water.Figure1. FESEM images of CNTsPreparation and Characterization of CNTs-poly (acrylic acid ester) Composite Emulsion：The zeta potential and particle size of CNTs-poly (acrylic acid ester) composite emulsion with different CNTs concentrations were measured by DLS. As shown in Figure 2, the zeta potential of CNTs-poly (acrylic acid ester) composite emulsion decreased gradually with increasing CNTs concentration. This may be due to the addition of CNTs, which led to an increase in the conductivity of the emulsion, thus reducing the zeta potential. The particle size of the emulsion remained stable and was less than 150 nm. It was observed that CNTs showed a good dispersion in the poly (acrylic acid ester) emulsion, and the emulsion was uniform and stable.Figure 2. Zeta potential and particle size of CNTs-poly (acrylic acid ester) composite emulsionLubrication Performance of CNTs-poly (acrylic acid ester) Composite Emulsion：The tribological properties of CNTs-poly (acrylic acid ester) composite emulsion were investigated by tribometer. The results showed that the addition of CNTs significantly improved the lubrication performance of the emulsion. As shown in Figure 3, the friction coefficient of the composite emulsion decreased with the increase of CNTs concentration, indicating tha