Development a simple method to create the superhydrophobic composite coatings

Introduction

In recent years, superhydrophobic surfaces (water contact angle greater than 150°) have attracted enormous contribution for practical applications such as biomedical devices, liquid separation, antibiofouling surfaces, reduction of ice and snow adhesion, self-cleaning surfaces, stain-resistant textiles, corrosion prevention and turbulence reduction in water-bearing pipes.^1–3 The wettability of solid surfaces is governed by the chemical composition and the geometrical microstructure of the surface. ⁴ It has been shown that on a flat surface the maximum contact angle achievable by lowering the surface energy is 120°. However, the addition of roughness to the surface can increase the contact angle of water without altering surface chemistry. Superhydrophobic surfaces can be achieved by a combination of low surface energy materials, and surface micro- and nanostructures. The most attractive methods to create superhydrophobic surfaces are self-assembly, layer-by-layer methods, plasma treatments, chemical vapor deposition, sol–gel methods, lithography, templates and electrochemical methods.^5–10 However, most of these methods need an extensive use of materials and long fabrication time, require strict conditions and expensive facilities and possibly raise environmental concerns. The present study investigates a very simple; one-step and low-cost superhydrophobic coating prepared by spray coating of CaCO₃ particles, stearic acid and polymer latex suspensions. This technique allows fast, easy and economic deposition of composite superhydrophobic films on the large surfaces. Also, CaCO₃ is a very cheap and widely used material in the construction and oil refining industry, and in the fabrication of paint, ceramic, cement, glass and steel.

Several factors such as CaCO₃ to polymer latex weight ratio, spraying distance and percentage of stearic acid can affect on the water repellency properties of the resulting coating. In most of the studies, influential parameters are investigated separately and kept constant one at a time, without attempting to show their synergistic or antagonistic effects. ¹¹ However, by employing the tabulated basic orthogonal arrays introduced by Taguchi, ¹² it is possible to obtain an inexpensive and easy-to-operate experimental design to study the influence of each parameter through a small number of experiments. In this study, the spray coating parameters were optimized using the Taguchi method and the Minitab software.

Experimental section

Polished aluminum alloy 6061 coupons were used as the substrate. The coupons were ultrasonically cleaned in acetone and water for 5 min each. One gram of stearic acid was first dissolved in 35 mL of acetone. Then, the solution was mixed with a magnetic stirrer at 600 r/min for 20 min at 30℃. An appropriate quantity (Table 1) of CaCO₃ particles (diameter ∼5 µm, Alfa Aesar) was dissolved in 15 mL of double distilled water and then, the suspension was mixed with polymer latex (Acronal® NX4787X, copolymer of styrene and acrylate in water, BASF Corporation), with 10 min agitation at 600 r/min. Finally, the suspension of CaCO₃ and polymer latex was mixed with the stearic acid solution and agitated for 10 min at 600 r/min at 30℃. The slurry was sprayed on aluminum substrates using a spray gun. The sample surface morphology was examined using a LEO field emission scanning electron microscope (FESEM). Water contact angle measurements were carried out using a Kruss DSA 100 goniometer. The contact angles were acquired by fitting the symmetric water drops (∼4 µL) using the Laplace–Young method, which is theoretically considered to be the most accurate because it takes into account the distorted drop shape due to liquid weight. FTIR spectra were measured on a Fourier transformation infrared spectrometer (Perkin-Elmer, Spectrum one). The reflected beam was collected for 24 scans at a resolution of 4 cm⁻¹.

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Table 1.

Orthogonal array L18 of the experimental runs and results

Trial	CaCO3/NX4787 (wt%)	Stearic acid (wt%)	Distance (cm)	CA (°)
1	0	0	25	82
2	0	1	30	107
3	0	2	35	117
4	5	0	25	74
5	5	1	30	132
6	5	2	35	131
7	7.5	0	30	74
8	7.5	1	35	138
9	7.5	2	25	116
10	10	0	35	75
11	10	1	25	139
12	10	2	30	145
13	15	0	30	71
14	15	1	35	133
15	15	2	25	155
16	20	0	35	74
17	20	1	25	142
18	20	2	30	152

Results and discussion

Three controlling parameters were considered to affect the coating water repellency properties: spraying distance and percentage of stearic acid, each one at three levels, and CaCO₃/polymer latex weight ratio at six levels. Testing for these conditions in a full factorial experiment would require 6 × 3 × 3 = 54 trials, meaning a considerable expenditure both cost and timewise. On the other hand, Taguchi’s factorial experiment approach reduces the number of experiments. Table 1 shows the chosen design matrix based on a Taguchi L18 orthogonal array consisting of 18 sets of experimental conditions and the corresponding results for the contact angle measurement.

In the Taguchi method, the term ‘signal’ (S) represents the desirable value (mean) for the output characteristic and the term ‘noise’ (N) represents the undesirable value for the output characteristic. ¹³ In this method, signal-to-noise ratio (S/N) is used to represent a response or quality characteristic and the highest S/N is required. ¹³ There are three types of quality characteristics, i.e. nominal-the-best, larger-the-better and smaller-the-better. In order to obtain the maximum contact angle, the larger-the-better quality characteristic type was selected.

The graphical representation of the S/N ratios is shown in Figure 1. The factor levels with the largest S/N ratios are the optimum levels as they minimize the sensitivity over the range of noises. Therefore, the optimal condition for the spray coating to obtain a superhydrophobic film according to their high S/N ratios correspond to a distance of 25 cm between substrate and spray gun, a percentage of 2% stearic acid and a weight percentage ratio of CaCO₃/NX7487 at 20%. Also, the difference between the highest and lowest values of S/N ratio is a decisive indicator with regard to the influence of a parameter. It can be observed that the percentage of stearic acid presents the most significant differences of S/N ratio at different levels and consequently plays the most important role as to overall water repellency performance. These results show that hydrophobicity is increased by increasing the stearic acid content of the coating. It was shown that stearic acid reacts with CaCO₃ particles and covers them with a monolayer of calcium stearate bicarbonate which creates a hydrophobic tail oriented toward the air.^14,15 As the concentration of fatty acid is increased, the quantity of hydrophobic groups covering the particles increases, resulting in an increase in the water repellency properties of the obtained coating. The CaCO₃/NX7487 ratio also provides a quite large contribution to the hydrophobicity of the resulting coating. This increase in hydrophobicity with higher percentages of CaCO₃/NX4787 could be mainly attributed to the increase in surface roughness due to the increase in CaCO₃ particles. On the other hand, the distance between substrate and spray gun has a minor effect on the water repellency of the coating. However, at higher distances a decrease in water repellency was observed, which can be explained by the decrease in surface micro/nano roughness as the distance from the substrate is increased. OPEN IN VIEWER

An additional experiment with the optimal conditions was carried out to confirm the influence of the optimal combination of the control factors and their levels on performance characteristics.

The SEM images of Figure 2(a) to (d), display a superhydrophobic coating deposited at the optimal conditions. They show the formation of a solid polymer foam-like structure where air can be trapped in the rough surface cavities. This surface morphology reduces the contact area between water and the surface, resulting in increased water repellency. The higher magnification images (Figure 2(c) and (d)) show the presence of micro/nanostructures on the superhydrophobic coating. OPEN IN VIEWER

As we mentioned above, the wetting behavior of superhydrophobic surfaces is governed by chemical composition of surface and roughness of surface. The presence of the hydrophobic groups in our coating was analyzed with FTIR spectra (Figure 3). This figure shows the absorption infrared spectrum of the coating without stearic acid (line a), and without CaCO₃ particles (line b), as well as the superhydrophobic film obtained at optimal conditions (line c). The typical spectrum of the CaCO₃ bands can be observed at 2506, 1792 and 871 cm⁻¹ (line a). ¹⁶ Both the asymmetric (υ_as (CH₂)) and symmetric (υ_s (CH₂)) methylene stretching peaks clearly appear at 2910 and 2840 cm⁻¹ on the film spectra (lines b, c) of the stearic acid modified coatings. ⁹ The presence of this hydrophobic group on a surface can reduce its surface energy and increase its water repellent properties. OPEN IN VIEWER

A polished aluminum surface showed a water contact angle of about 88°, as seen in Figure 4(a). The contact angle obtained at the optimal conditions resulted in greatly improved water repellency with the contact angle increasing to 158° (Figure 4(b)). OPEN IN VIEWER

Therefore, these results show that the application of spray coating and Taguchi method was successful in creating the ultra-water repellent micro/nanostructured composite coating on the aluminum surface.

Conclusion

A simple and low cost technique was proposed for elaborating superhydrophobic coatings by spray coating a mixture of CaCO₃ particles, stearic acid and polymer latex. The Taguchi method was used to optimize the fabrication process parameters in order to obtain the maximum contact angle. The results showed that the percentage of stearic acid has the most important influence on the water repellency of coating. The wettability results obtained by using the optimal conditions proposed by this method showed that the resulting superhydrophobic surface had a very high contact angle of about 158°. The presence of CH₂ groups and rough micro-nanostructure on the surface, contributing to its superhydrophobicity, were displayed by FTIR and SEM analyses, respectively.