Superhydrophobic Coatings on Cellulose-Based Materials with Alkyl Ketene Dimer Pickering Emulsion: Fabrication and Properties

Abstract

In this study, a stable alkyl ketene dimer (AKD) Pickering emulsion was obtained using chitosan and titanium dioxide (TiO₂) as effective emulsifiers to disperse AKD. Superhydrophobic filter paper was prepared, using the filter paper as the substrate, by dipping it into AKD Pickering emulsion and then drying the coating papers at different temperature. The contact angle of the treated filter papers dried at 45 °C could reach more than 150°, and these filter papers effectively separated oil–water mixtures with an efficiency of over 93%. It is worth noting that the preparation process of the superhydrophobic material was simple and mild, and all the raw material was green without secondary pollution to the environment, so it has great practical application potential. This experiment could provide a new idea for the preparation of AKD superhydrophobic coatings and broaden their application.

1. Introduction

Superhydrophobic materials are characterized by surfaces that exhibit water contact angles greater than 150° and rolling angles less than 10°. Because of their superior water resistance, superhydrophobic materials have attracted more and more attention. Superhydrophobic materials have a wide range of applications in self-cleaning [1,2,3], anti-corrosion [2,4,5], anti-icing [6,7,8], and water/oil separation. The principle of this technology is based on producing micro/nano rough structures and a surface of low energy [9,10].

As a hydrophobic substance with low cost and low surface energy, wax can be used to prepare superhydrophobic materials. Alkyl ketene dimer (AKD) is an insoluble white waxy solid. AKD is a widely used sizing agent in the paper industry as an inexpensive sizing agent. It is a low-surface-energy wax capable of forming a non-wettable layer on the substrate [11,12,13,14,15]. At present, there is little research on AKD superhydrophobic coatings, and the preparation methods are mainly as follows. Quan [16] et al. used a supercritical CO₂ solution in a quick expansion process to prepare a fractal AKD coating on paper. When AKD’s supercritical CO₂ solution was sprayed onto the substrate, a rapid phase transition from the supercritical state to the gas phase resulted in flake AKD particles, approximately 1 μm–2 μm in size, forming on the matrix. The contact angle of the paper reached 153°. Since then, according to the RESS method (rapid expansion of supercritical solution) described by Quan et al., more researchers have successfully obtained superhydrophobic coatings. Another approach involves using inorganic nanoparticles to create a rough microstructure on a substrate, which is then treated with AKD to reduce its surface energy. Typically, this method requires nanocellulose to act as a binder, strengthening the bond between the coating and substrate. For instance, Arbatan [17] et al. created a superhydrophobic coating on filter paper by immersing the paper in an aqueous suspension of precipitated calcium carbonate (PCC) and nanocellulose. After blotting and drying at 112 °C, the paper was impregnated with an AKD solution in heptane and cured at 105 °C. As a result, the obtained superhydrophobic papers had a contact angle of 160° and sliding angle of 5°. The last method was to disperse AKD directly in the solvent (water, ethanol, etc.) without using surfactant and spray it to the substrate. After the solvent evaporated, AKD formed a certain microstructure on the substrate, thus improving the hydrophobic performance. Esmaeil [18] et al. treated a glass sheet by immersing it in a molten AKD polymethylheptene solution. After curing the AKD, the glass sheet was dipped in ethanol and air-dried. This process resulted in a porous micro/nano structure on the treated glass surface, achieving a contact angle of about 160°.

Usually, AKD needs to be melted or emulsified when used. The particle size range of AKD products, approximately 0.5 μm–2 μm, is advantageous for creating rough microscopic structures on various substrates. In addition, the hydrophobic property of AKD provides favorable conditions for the construction of an AKD superhydrophobic material system. In this study, a facile and low-cost method was utilized to prepare an AKD superhydrophobic coating. Firstly, AKD was made into an emulsion and coated on fiber-based filter paper. Different from the traditional AKD emulsification technology, solid particles (chitosan and TiO₂) were used to emulsify AKD in this experiment to prepare an AKD Pickering emulsion. Solid particles have high emulsifying efficiency, and stable emulsions can be formed with a lower emulsifier dosage. The solid particles used as emulsifiers are mostly natural substances, so the Pickering emulsions are safer and environmentally friendly. In this experiment, the filter paper coated with an AKD Pickering emulsion could reach superhydrophobic state after drying treatment. This manuscript also discussed the properties of the emulsion and the oil–water separation properties of the filter paper. The preparation method adopted in this study was simple and mild, and all the raw materials were easy to obtain. No toxic organic solvents were needed in the preparation process. Through the oil–water separation experiments, the AKD superhydrophobic coating could be used for effective oil–water separation.

2. Materials and Methods

2.1. Materials

Industrial-grade AKD wax was from Kuer Chemical Technology Co., Ltd. (Beijing, China). Chitosan (chemically pure) and titanium dioxide (of 150 nm particle size) were purchased from Shanghai Aladdin Biological Technology Co., Ltd. (Shanghai, China). Analytically pure acetic acid was purchased from Foshan Xilong Chemical Industry Co., Ltd. (Foshan, China). Analytically pure n-hexane was from Tianjin Damao Chemical Reagent Factory (Tianjin, China). The distilled water was homemade in our laboratory.

2.2. Preparation of AKD Pickering Emulsion

(1)

2.0 g of glacial acetic acid and 198.0 g of deionized water were mixed in a beaker to obtain 1.0 wt% acetic acid solution. Then, a certain amount of chitosan was added into the acetic acid solution. After mixing and stirring evenly, the chitosan powder was completely dissolved to prepare an acetic acid solution of chitosan.

(2)

8.0 g of melted AKD was added to the heated chitosan acetic acid solution at 65 °C. This mixture was then emulsified at 10,000 rpm for 10 min using a high-speed homogenizer. During emulsification, a water bath insulated the AKD to prevent solidification, which could disrupt the process.

(3)

A 0.5 wt% dispersion solution of titanium dioxide (TiO₂) was prepared using ultrasonic dispersion for 10 min. A designated amount of heated chitosan acetic acid solution and TiO₂ dispersion was combined with 8.0 g of melted AKD, followed by emulsification at 10,000 rpm for 10 min. Once cooled to room temperature, an AKD emulsion with chitosan/TiO₂ as emulsifiers was formed. The emulsification process of AKD by chitosan was the same as that of chitosan/TiO₂ but without the addition TiO₂ dispersion.

2.3. Preparation of Superhydrophobic AKD Coatings

Fiber filter paper was chosen as the base material in this study. Firstly, the AKD emulsion was diluted with deionized water. The filter papers were immersed in the diluted AKD emulsion for 10 min and then dried at different temperatures (45 °C and 90 °C).

2.4. Characterization

The stability of the AKD emulsion was characterized by the change in the volume fraction of the emulsion phase with the standing time. The higher the ratio of the emulsion phase volume, the better the stability of the emulsion. The particle size of the AKD in the emulsion was obtained using a Malvern automatic laser particle size analyzer (Mastersizer2000E, Malvern Instruments Ltd., Worcestershire, UK). The static water contact angle of the paper sample was measured with a contact angle tester (SDC-100S, Dongguan Shengding Precision Instrument Co., Ltd., Dongguan, China). Then, 5 µL water droplets were dropped on the filter paper surface, and the water contact angle was calculated by manually fitting the ellipse. The contact angle of each sample was measured 3–5 times, and its average value was finally taken. The microstructure of the paper sample was observed using scanning electron microscopy (Regulus8220, Hitachi Ltd., Tokyo, Japan).

The water/oil separation ability of the coated filter paper was carried out using a simple filtration method. Water and n-hexane were mixed at a ratio of 1:9 (mass ratio) and a small amount of methylene blue was dropped into it to dye the water. We used the device as shown in Figure 1 for oil–water separation operation. We placed the coated filter paper in the sand core funnel and poured the oil–water mixture in to achieve separation. When the oil was completely filtered and only blue water phase was left on the filter paper, the separation was completed. We recorded the water quality before and after separation.

3. Results

3.1. The Stability of AKD Pickering Emulsion

A stable, non-layered emulsion can be stored longer and is beneficial for practical applications. In Pickering emulsions, the stability typically increases with the number of solid particles adsorbed to the oil–water interface. This leads to a denser particle arrangement and, subsequently, a reduced emulsion particle size [19].

As shown in Figure 2, three of the prepared emulsions were stratified after standing for 24 h. When the amount of chitosan was 1.0%, 4% water precipitated in the lower layer of the emulsion. The longer the placement time, the more obvious the emulsion stratification phenomenon was. When the chitosan concentration was 1.5%, the emulsion remained stable even after a week, exhibiting no stratification, indicative of high stability. With chitosan and TiO₂ as emulsifiers (the amount of chitosan was fixed at 1%), the emulsion stability was poor when the amount of TiO₂ was 1%, and the emulsion phase volume was only 75.3% after one week. When the amount of TiO₂ increased to 3%, the emulsion did not stratify and was very stable. As emulsifiers, chitosan and TiO₂ adsorbed onto the AKD/water interface, forming a protective barrier. An increase in emulsifier concentration resulted in a denser protective layer and consequently improved the emulsion stability. The stability of the AKD emulsion increased with the increase in the amount of chitosan or TiO₂. Figure 3 intuitively shows the stability of all the emulsions after a week of standing. The more water precipitated, the more unstable the emulsion.

3.2. The Particle Size of AKD Emulsion

As be seen in Figure 4, when chitosan was used as the emulsifier, the average particle size of the emulsion was smaller with 1.5 wt% chitosan (15.5 μm). Using both chitosan and TiO₂ as emulsifiers, the average emulsion particle size diminished as the TiO₂ concentration increased. For instance, an emulsion with 1.0% chitosan and 3% TiO₂ exhibited an average particle size of just 8.5 μm. The change in the average particle size of the emulsion was closely related to the effective adsorption of solid particles on the oil–water interface. During the emulsification process, large oil droplets are dispersed into small ones through shear action, and the adsorption of solid particles on the oil–water interface reduces the free energy of the system, which is increased by the decrease in the oil droplets, thus stabilizing the emulsion. An increase in emulsifier amount will make the emulsion system more stable, thus reducing the size of the dispersed phase of emulsion. The emulsion stability and particle size data both showed that chitosan and titanium dioxide could be well adsorbed at the AKD–water interface, which reduced the average particle size of the emulsion.

The morphology and size of micro/nano structure on the surface of the material affect its wettability. Therefore, the coating of the AKD emulsion may have influence on the water contact angle of the paper surface to a certain degree.

3.3. Hydrophobic Properties of AKD-Coated Filter Paper

Filter paper is mainly made of plant fiber, which is very hydrophilic, and the contact angle of water on filter paper is equal to 0°. In this study, an AKD emulsion was used to impregnate and heat-treat the filter paper, and the hydrophobicity of the filter paper was greatly improved. Figure 5 shows the contact angle change in the paper pattern after treatment.

AKD played a hydrophobic role because of the long-chain alkyl group in its structure, which has a certain promoting effect on the hydrophobicity of the coating. Given AKD’s properties, with a melting point between 45 °C and 50 °C and susceptibility to hydrolysis above 65 °C, different drying temperatures could influence the hydrophobicity of the treated paper. For instance, paper treated at 45 °C exhibited a contact angle of 153.7°, while the same paper dried at 90 °C only reached 139.6° (shown in Figure 5). The surface energy and microstructure were the main factors affecting the surface wettability. For the same emulsion-coated filter paper, the surface composition was the same. The difference in hydrophobicity observed at the two temperatures could be attributed to potential AKD hydrolysis at elevated temperatures. Additionally, it is speculated that AKD particles, which melt and spread at 90 °C, lose their spherical structure, impacting the final hydrophobicity. Although the filter papers could also reach a hydrophobic state, they struggled to reach superhydrophobicity due to the change in microstructure. In order to verify this conjecture, the surface morphology of the filter papers was measured in the experiment (shown in Figure 6 and Figure 7).

3.4. Surface Morphology of AKD Emulsion-Coated Filter Paper

The surface morphology had a very important effect on the wettability of the materials. Figure 6a,b show the untreated filter paper’s structure, composed of interwoven plant fibers. Upon coating with the AKD emulsion and drying, fine structures, likely AKD particles, appeared on the paper surface. When the filter paper was dried at 45 °C for a period of time, as shown in Figure 6c,d, the surface of the filter paper was adsorbed by many microspheres. These microspheres were AKD particles that were wrapped by some pieces, filling most of the surface of the filter paper. It could be concluded that the pieces on the AKD microspheres were chitosan because in the emulsion, chitosan filled the space between the AKD droplets and adsorbed on the AKD droplets to stabilize the emulsion. When the AKD emulsion was loaded on the filter paper and dried, the AKD droplets became waxy microspheres, while chitosan remained on the AKD surface or aggregated between the AKD wax spheres. Through treating via emulsion impregnation and drying, the roughness of the filter paper surface structure increased and the surface energy decreased; the treated filter paper underwent a transformation from a completely hydrophilic surface to a notably hydrophobic one. When the emulsion-dipped filter paper was dried at 90 °C, there were no AKD wax spheres on the filter paper surface, but the AKD was in the form of petal-shaped lamellar wax. The chitosan pieces were embedded in the AKD wax layer and could no longer be clearly seen (Figure 6e,f). Because 90 °C far exceeds the melting temperature of AKD, the AKD microspheres melted into pieces. This also explains why the hydrophobic properties of the coated filter paper were different at two temperatures.

Figure 7 showed the surface morphology of the filter paper impregnated with the AKD emulsion emulsified with 1.0% chitosan and 3.0% TiO₂ (dried at 45 °C). As seen from Figure 7a,b, the AKD microspheres with composite emulsifier adsorbed some TiO₂ nanoparticles and chitosan on the surface, which fully increased the roughness of the paper surface and contributed to an improvement in hydrophobic properties. There were also part some TiO₂ particles and chitosan flakes that populated the AKD wax spheres. Figure 7c,d show the surface morphology of the filter paper after drying at 90 °C. The microstructures supported on the filter paper fibers were no longer spherical AKD, but sheet structures intermingled with granular nano-TiO₂. The chitosan and molten AKD mixed together to form flakes that were difficult to distinguish. It can be seen in Figure 6 and Figure 7 that the emulsion composition and drying temperature affected the surface morphology of the treated filter papers and then affected the hydrophobic performance of the filter papers.

3.5. Oil–Water Separation Properties of AKD-Coated Filter Paper

Superhydrophobic materials have great application potential in the separation of oil and water mixtures because of their low surface tension (hydrophobic and lipophilic). The abundant pore structure of filter papers provided the possibility for filtration and separation. The oil and water separation efficiency of the prepared superhydrophobic filter papers were tested and the results are shown in Figure 8. All the treated paper samples exhibited impressive oil–water separation efficiencies, exceeding 93%, with the best performance recorded at 98.8%. This demonstrates the potential of the developed superhydrophobic filter papers in practical oil–water separation applications. The relationship among separation efficiency, contact angle, and pore structure needs further study.

4. Conclusions

In this work, an AKD Pickering emulsion was prepared by using solid granular chitosan and TiO₂ as emulsifiers. For the AKD emulsion, the compound emulsifier (chitosan/TiO₂) was better than the single emulsifier (chitosan). Superhydrophobic filter papers were prepared by applying an emulsion coating and heat treatment to the filter paper. The contact angle of the filter paper dried at 45 °C could reach 155.7°, and all treated papers effectively separated the oil–water mixture with efficiencies over 93%. This kind of superhydrophobic fiber material has great potential in practice.