Improvement of chemical stability and durability of superhydrophobic wood surface via a film of TiO₂ coated CaCO₃ micro-/nano-composite particles

Abstract

A superhydrophobic wood surface with a water contact angle of 155° and a sliding angle of 4° was made by attaching stearic acid modified micro-/nano-composite particles made of titanium dioxide coated calcium carbonate. According to tests, the as-prepared wood surface had an outstanding chemical stability and durability.

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Introduction

The discovery and development of superhydrophobicity is related to the hydrophobic effect of the lotus leaf, which was discovered by Barthlott and Neinhuis.^1,2

When contact angles between material surfaces and water droplets are higher than 150°, the material surfaces are called superhydrophobic surfaces.^3–11 In the past decade, considering the important potential of such surfaces for numerous scientific and industrial applications,^12–15 researchers have made vast efforts toward the fabrication of bionic superhydrophobic surfaces. In addition, diverse substrate materials have been tested to develop superhydrophobic surfaces, including metals,^16–19 polymers,^20,21 papers,^22,23 and other materials.^24–26 Unlike the aforementioned materials, the surfaces of wood products are soft, uneven, and spotted with pits and debris produced during wood processing.²⁷ Moreover, wood is sensitive to moisture because of the hydrophilicity of its chemical structure, which consists of cellulose, hemicelluloses and lignin.²⁸ All these components provide abundant OH groups.

However, according to the Cassie model, the liquid droplets do not infiltrate into the rough grooves. Instead, they situate on the solid–air–liquid interfaces. In summary, the apparent contact angle (θ_e) is related to two factors: the liquid–solid interfaces and the liquid vapor interfaces. The equation described below explains the relationship between the factors:

cos θ_e = f cos θ + f − 1

where f is the fractional area of the solid surface with the intrinsic contact angle θ. On the basis of this equation, Fig. 1 intuitively shows the relationship between the intrinsic contact angle (θ), the fraction of the water–solid interface area (f) and the apparent contact angle (θ_e).³ It is apparent that the intrinsic contact angle has a positive effect, whereas the fraction of the water–solid interface area has a negative effect on the apparent contact angle. In view of the abovementioned conjecture, any hydrophobic material has the possibility to become superhydrophobic through reduction of the contact area between the liquid and solid interface. Moreover the conjecture also shows that the surface roughness plays an important role in superhydrophobicity at low f levels.

Fig. 1 The relationship between the apparent contact angle (θ_e), the intrinsic contact angle (θ) and the fraction of water–solid interface area (f).

Based on this principle, only a few methods have been utilized to date to achieve superhydrophobic surfaces on wood, such as a sol–gel process,⁵ solution-immersion⁴ and hydrothermal methods.^28–30 Fu et al. tested the superhydrophobicity of a wood surface coated with zinc oxide nanorod arrays via hydrothermal treatment. The prepared wood surface showed good repellency to some polar chemicals.²⁸ In addition, silica particles have been used to synthesize surface roughness in numerous ways. Wang et al. fabricated a bionic and stably superhydrophobic coating on a wood surface; however, the chemical stability was not promising.³¹ Even though the PVA/SiO₂ composite polymer coating designed by Liu et al. had improved mechanical robustness, its chemical stability is not mentioned.¹⁷

In general, these approaches are simple and low-cost in the applications of wood materials; however, the chemical stability and durability of the superhydrophobic surfaces are limited. In this study, we found another simple way of using micro-/nano-composite particles for the fabrication of superhydrophobic surfaces with outstanding chemical stability and durability. As an inorganic material, calcium carbonate is inexpensive and abundant, which makes it suitable for industrial use. Moreover, the particle size and the morphology can be easily controlled. Thus, calcium carbonate was used as the coating material in this experiment to fabricate the micro-/nano-composite particles. In addition, titanium dioxide particles were coated on the calcium carbonate particles, resulting in the micro-/nano-composite particles emulsion. Wood substrates were immersed into the prepared emulsion and modified with stearic acid afterwards. The treated wood surfaces exhibited superhydrophobicity. Furthermore, the water contact angle increased to 155°. The chemical stability and durability of the superhydrophobic wood surfaces were tested in different chemical environments. The results showed a great improvement in resistance under different conditions. The introduced method delivers a promising commercial feasibility for diverse woodworks.

Methodology

Due to the combined effects of the surface roughness and stearic acid (as a layer of low surface energy film), air is trapped in the interspaces and cavities of the superhydrophobic wood surfaces, and the water droplet mostly contacts the trapped air (the contact angle of the water droplet with air is expected to be 180°). Therefore, water droplets cannot infiltrate into the treated samples. The treated samples present the property of superhydrophobicity. The process used to form the superhydrophobic wood surface is shown in Fig. 2.

Fig. 2 Schematic of the forming process of the superhydrophobic wood.

Results and discussion

The anisotropic surface of wood and its microstructure have been well described in previous works.^32–37 Thus, the discussion in this paper will be limited to the surface and structure changes of the wood before and after the treatment. Fig. 3 presents images of a pristine wood surface and the superhydrophobic wood surface. The images below are the 10 times magnified macroscopic images. The thin film is light-transmitting, and there were no distinct differences in the macroscopic appearance of the wood surfaces. Thus, the prepared superhydrophobic wood retains the texture and the colors of the pristine wood.

Fig. 3 Macroscopic images of (a) the pristine wood and (b) the superhydrophobic wood.

Fig. 4 shows scanning electron microscopy (SEM) images of the pristine wood surface and the superhydrophobic wood surface at low and high magnifications, respectively. Fig. 4a obviously reveals that the pristine poplar wood is a heterogeneous and porous material, and the diameters of its pits range from 2 to 3 μm. From the low magnification image of the superhydrophobic wood surface, as shown in Fig. 4b, it can be seen that titanium dioxide coated calcium carbonate micro-/nano-composite particles deposited on the wood surface and filled the pits of the poplar wood. The composite particles adhered to the wood surface through the force of physical absorption.¹⁷ Fig. 4c demonstrates that a large number of composite particles stacked over the wood surface in a random distribution with diameters ranging from 200 to 300 nm in width and 600 to 800 nm in length. Several particles aggregated into bouquet-like shapes. The inset image of Fig. 4c shows the titanium dioxide coated calcium carbonate micro-/nano-composite particles in high magnification. The composite particles have a rough surface with diameters of 200 nm to 300 nm. The micro-/nano-multiscale composite particles were produced by coating the nanometer scale titanium dioxide on the micrometer scale calcium carbonate. Since the micro-/nano-multiscale composite particles form a rough surface, this could effectively improve the roughness of the wood surface when the composite particles adhere to the surface of the wood.

Fig. 4 SEM images of (a) the pristine wood surface and the surface of wood covered with titanium dioxide coated calcium carbonate micro-/nano-composite particles at (b) low and (c) high magnifications.

The arrangement of the composite particles described above creates interspaces and cavities between the particles and bouquet-like microspheres, thus roughening the wood surface with a micro-/nano-multiscale rough structure.

The existence and bonding of the stearic acid molecules on the surface of the micro-/nano-composite particles were characterized by Fourier transform infrared spectroscopy (FT-IR). Fig. 5 shows FT-IR spectra of the pristine wood and the superhydrophobic wood. The peak at 1637 cm⁻¹ and the broad absorption band at 3415 cm⁻¹ are due to –OH groups.^29,38,39 In addition, the two strong adsorption peaks at 2920 cm⁻¹ and 2850 cm⁻¹ can be attributed to –CH₃ and –CH₂ asymmetrical stretching vibrations and symmetrical stretching vibrations, respectively, indicating the existence of long alkyl chains on the composite particles.^27,40–44 Moreover, the broad absorption band at 2922 cm⁻¹ is due to the –CH₃ groups, which are provided by the wood components. The weak enhancement of the peak at 1464 cm⁻¹ is due to the symmetric stretch of –COOH groups in CH₃(CH₂)₁₆COOH, which further verifies the excellent attachment of stearic acid to the composite particles.^5,40,45,46 In Fig. 5b, the peak at 873 cm⁻¹ is related to the calcite-type CaCO₃.^41,47–52 This peak is not very noticeable, owning to the low content of CaCO₃ at the surfaces. Because of the strong absorption peak of the wood components in the region of 400–800 cm⁻¹ and the low content of TiO₂, the peaks of TiO₂ are not discernible in Fig. 5b. It is inferred that the composite particles are physisorped onto the wood surface, because no new peak is observed in Fig. 5b compared to Fig. 5a except the peaks for wood, TiO₂, CaCO₃ and stearic acid. Thus, no bond is formed between the wood and the composite particles. The assignments of the FT-IR absorption bands of the superhydrophobic wood are listed in Table 1.

Fig. 5 FTIR spectra of (a) the pristine wood surface and (b) the superhydrophobic wood surface.

Table 1 Assignments of the FT-IR absorption bands of superhydrophobic wood

Band positions (cm^−1)	Assignments
873 cm^−1	CO3^2− out-of-plane bending vibration
1464 cm^−1	–COOH symmetrical stretching vibrations
2850 cm^−1 and 2920 cm^−1	–CH3 and –CH2 asymmetrical stretching vibrations and symmetrical stretching vibrations
1637 cm^−1 and 3415 cm^−1	–OH groups

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Energy dispersive X-ray spectrometry (EDX) was employed to determine crucial information regarding the chemical compositions of the pristine wood surface and the superhydrophobic wood surface. Fig. 6 shows the EDX spectra of the pristine wood surface and the superhydrophobic wood surface. Fig. 6a shows that only C, O and Au (from the coating layer used for the EDX measurements) could be detected from the EDX spectrum of the pristine wood, showing that there is no compound except wood ingredients in the sample. From Fig. 6b, it is observed that when the micro-/nano-composite particles are deposited on the wood surface, followed by the stearic acid treatment, elements such as C, O, Ca, Ti and Au are found in the EDX spectrum. The Ca and Ti elements are derived from the micro-/nano-composite particles. The C and O elements hail from the stearic acid modifier and the wood components.

Fig. 6 EDX spectra of (a) the pristine wood surface and (b) the superhydrophobic wood surface.

Consequently, FT-IR along with EDX confirmed that the stearic acid molecules, combined with the titanium dioxide coated calcium carbonate micro-/nano-composite particles on the wood surface, contribute to lower the surface energy of the composite particle films.

The wettability was measured by the contact angle of water droplets on the wood surfaces. Fig. 7 shows images of the water droplets on the pristine wood surface, the wood surface decorated with the micro-/nano-composite particles, the wood surface modified with the stearic acid reagent and the treated wood surface. It can obviously be seen that the pristine wood shows hydrophilicity and the water contact angle is 77°. The water droplet can be instantly absorbed by the wood when the wood surface is covered with the titanium dioxide coated calcium carbonate micro-/nano-composite particles and the water contact angle is 7°. Concerning the wood surface modified by stearic acid, the water contact angle reaches about 120° and the wood surface shows hydrophobic properties to some extent. However, after the deposition of the composite particles followed by stearic acid treatment, the wood surface exhibits the property of high superhydrophobicity.

Fig. 7 Images of water droplets on the surfaces of different substrates. (a) The pristine wood surface; (b) the wood surface covered with titanium dioxide coated calcium carbonate micro-/nano-composite particles; (c) the wood surface modified with stearic acid and (d) the wood surface treated with the superhydrophobic film.

During modification with stearic acid, the carboxyl groups react with the hydroxyl groups of the composite particles and thus the hydrophobic alkyl chains were grafted onto the composite particles, as detailed in Fig. 8. This process transformed the wood surface from hydrophilic to superhydrophobic. The water contact angle of the wood surface was about 155°, and the sliding angle was less than 4°.

Fig. 8 The experimental strategy for the modification of titanium dioxide coated calcium carbonate micro-/nano-composite particles on the wood surface.

Moreover, we also evaluated the thermal degradation behaviour of the pristine wood and the superhydrophobic wood. As shown in Fig. 9, the thermogravimetric analysis (TGA) curves indicate that the temperature regions of the main thermal degradation are in the ranges of 227.5–378.8 °C for the pristine wood and 66.6–378.8 °C for the superhydrophobic wood, which shows an obvious increase of the temperature range for the thermal degradation. The initial weight loss (∼2.5%) in the temperature range of 30–70 °C is attributed to the evaporation of the bound water that naturally exists in wood cells. For the superhydrophobic wood, the thermal degradation temperature in the range of 66.6–212.3 °C is for stearic acid, corresponding to the weight percent decrease from 96.51% to 93.11%. Therefore, stearic acid occupies 3.4% of the superhydrophobic wood. The residue at 800 °C for the wood samples increased from 12.65% to 13.65% after superhydrophobic treatment, which means the composite particles occupy 1.0% of the superhydrophobic wood. Therefore, minimal composite particles and stearic acid were attached to the surface of the wood, improving the hydrophobic properties of the wood surface.

Fig. 9 TGA curves of (a) the pristine wood and (b) the superhydrophobic wood in a nitrogen atmosphere.

The chemical stability and durability of the superhydrophobic wood were investigated. Fig. 10 shows the time dependent water contact angle curves of the superhydrophobic wood surfaces in different chemical environments.

Fig. 10 The water contact angle curves of the superhydrophobic wood surface at different (a) storage times, (b) UV exposure times, (c) pH values, (d) temperature conditions, and (e) humidity conditions.

After the superhydrophobic wood was maintained under ambient conditions for 6 months, the value of the water contact angle showed no obvious change, remaining around 155°. The result shown in Fig. 10a indicates that the superhydrophobic wood has excellent stability and durability in ambient conditions. In order to evaluate the superhydrophobicity of the as-prepared wood sample in harsh conditions, we tested the water contact angle of the samples after UV exposure. From Fig. 10b, the water contact angles of the superhydrophobic wood only slightly decreased to 153.5° from 155° after UV exposure for more than 300 min, exhibiting the long-term stability and durability of the resulting surface. Under ambient conditions, calcium carbonate plays an important role in the composite particles; this compound remains stable as conditions change. As for UV exposure, titanium dioxide and calcium carbonate have the same contributions to the anti-chemical environmental changes. Based on the two significant points discussed above, the surface roughness does not change under various chemical conditions, showing the stability and durability of the superhydrophobic wood.

Moreover, we found that the modified wood surface possesses superhydrophobic properties not only for pure water but also for corrosive liquids such as acidic and basic solutions. Fig. 10c shows the relationship between pH values and water contact angles on the superhydrophobic wood surface. This shows that the water contact angles are always larger than 150° at pH values ranging from 2 to 13. The water contact angle decreased only under strongly acidic conditions with pH 1 and strongly basic conditions with pH 14 (to 145° and 144°, respectively). This result is very important for engineering wood surfaces with superhydrophobicity in the wide pH range of corrosive liquids. The main reason for this characteristic relates to two essential factors: titanium dioxide presents weak acidity, and the calcium carbonate particles were coated with titanium dioxide particles, which protects the surface roughness from droplets with pH values from 1 to 6. Therefore, the composite particles will not be influenced by acidic conditions. Calcium carbonate shows weak basicity, and the calcium carbonate particles were not completely covered by the titanium dioxide particles. This feature is the determinant to sustain the surface roughness under basic conditions. Thus, even the composite particles will not be influenced by basic conditions. Because of these synergistic effects, the composite particles will be relatively stable in acidic or basic environments.

Furthermore, the superhydrophobic wood was tested in different temperature and humidity conditions. Fig. 10d shows the water contact angle curve of the superhydrophobic wood under different temperature conditions. The water contact angles did not show an obvious change from 5 °C to 45 °C, illustrating that the structure of the superhydrophobic layer of the prepared wood did not mutate. The superhydrophobicity was not influenced by temperature in this region. The water contact angle of the superhydrophobic wood was 144° when the temperature was below the freezing point. The water droplet froze when it dripped onto the surface of the superhydrophobic wood. The state of the water droplet changed from liquid to solid. The interface altered and the water contact angle reduced. The melting point of stearic acid is 56 °C; as a result, the state of the stearic acid and the superhydrophobic layer changed when the temperature was higher than 55 °C. Moreover, the water contact angles decreased conspicuously. The structure of the superhydrophobic layer was destroyed. In consequence, the water contact angle was 138° when the temperature was 65 °C. Therefore, in the proper temperature range (above the freezing point and below the solution temperature of the stearic acid), the prepared wood surface showed exceptional superhydrophobicity. The water contact angles were evaluated after the superhydrophobic wood was stored under different humidity conditions. As seen in Fig. 10e, the contact angles were maintained at around 155°. Therefore, the superhydrophobic wood shows improved stability and durability under different humidity conditions.

The abovementioned results significantly reveal that the superhydrophobic wood obtained via this simple immersion method possesses outstanding chemical stability and durability under ambient conditions, in ultraviolet light, in corrosive liquids, at different temperatures, and at different humidity levels.

Conclusions

To improve the chemical stability and durability of superhydrophobic wood surfaces, this study explored the effectual fabrication of a film containing titanium dioxide coated calcium carbonate micro-/nano-composite particles followed by modification with stearic acid on wooden substrates. The superhydrophobic wood surface was obtained via a simple immersion method. The simple modification method exhibited the commercial feasibility of this film. The addition of titanium dioxide and calcium carbonate composite particles plays a vital role for improving the chemical stability and durability of the superhydrophobic wood surface. This enhancement of the chemical properties of the wood can be attributed to the composite particles. The independent effects and the synergistic effects of titanium dioxide and calcium carbonate undoubtedly have an influence in counteracting changes in the chemical environment. The chemical stability and durability of the superhydrophobic wood surface have been investigated on several superhydrophobic wood surface samples under ambient storage, after ultraviolet light exposure, and at different pH values, temperatures and humidity conditions. On the basis of our results, the chemical stability and durability of the superhydrophobic wood surface coated with a film of titanium dioxide coated calcium carbonate micro-/nano-composite particles were greatly improved. A chemically stable and durable superhydrophobic wood surface is promising for commercial applications in the wood industry.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (31470584).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04000k

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