Fluorine-Free and Robust Photothermal Superhydrophobic Coating Based on Biochar for Anti-/De-Icing

Abstract

Environmental pollution can be caused by the improper disposal of agricultural waste and the use of fluorinated chemicals. Icing is a natural phenomenon, but the accumulation of ice on the surface of electrical equipment can damage the equipment and reduce power generation efficiency. Preparation of biochar anti-icing coatings with a fluorine-free process promotes resource utilization and environmental protection. In this study, superhydrophobic coatings with photothermal effect prepared based on biochar as a filler, which was blended with multi-walled carbon nanotubes (MWCNTs) and polyurea adhesive. The coating exhibits remarkable durability, as well as anti-icing, antifrosting, and self-cleaning characteristics. Utilizing fluorine-free chemicals enhances the environmentally friendly nature of the coating. The coating exhibits a contact angle of 155°, and the temperature can increase to 47.6 °C within a duration of 10 min. It can complete ice detachment in 128 s and defrosting in 210 s. The coating demonstrated exceptional durability when exposed to mechanical abrasion using sandpaper and steel brushes, water jet impact, acid and alkali corrosion, and tape-peeling tests. This study streamlines the procedure for creating photothermal superhydrophobic coatings, which contributes to environmental conservation and sustainable development. Additionally, it broadens the possibilities for recycling and reusing rejected crops.

1. Introduction

In recent decades, in response to the depletion of fossil fuels, wind power has become a green and renewable form of energy production. However, in high, cold mountainous and coastal areas, wind turbine blades are prone to icing under low-temperature conditions, seriously affecting wind power generation capacity and equipment safety [1,2,3]. Currently, common de-icing methods are divided into active de-icing methods such as mechanical vibration [4] and electric heating [5], which require a significant amount of energy consumption. Inspired by lotus leaves [6], water striders [7], and mosquito compound eyes [8], bio-mimetic superhydrophobic surfaces are widely applied in the field of anti-icing as a passive means due to their excellent water-repelling properties [9,10,11]. Although superhydrophobic coatings can prolong the icing time, the de-icing efficiency of a single superhydrophobic coating is low. Hence, an external energy source needs to be supplied to compensate for the energy required for de-icing. An exemplary demonstration of success is the integration of photothermal surfaces with superhydrophobic surfaces [12,13,14,15]. Utilizing photothermal superhydrophobic coatings for blade anti-icing is essential. The heat generated by sunlight can enhance anti-icing and de-icing efficiency. This is achieved by endowing hydrophobic materials with photothermal conversion properties, thus eliminating the need for additional energy consumption.

In order to alleviate the problem of energy shortage, the recycling of crop straw has become the focus of current attention. Researchers have turned straw into biochar for use as energy fuel [16,17], adsorbents [18,19], and functional material fillers [20,21]. Wang et al. [22] prepared a biochar-based superhydrophobic coating with photothermal properties by directly blending biochar with TiN nanoparticles and modifying it with fluoride-based hydrophobic treatment for anti-icing purposes. However, fluorinated hydrophobic agents are expensive and environmentally polluting. Furthermore, the microstructure of superhydrophobic surfaces is susceptible to wear, resulting in coating failure and restricting the development of industrial applications. Despite the development of superhydrophobic anti-icing coatings based on biomass fillers, there is still a relative lack of research using fluorine-free and environmentally friendly materials to enhance the robustness of these coatings. Therefore, there is an urgent need to design a green, fluorine-free coating that also takes into account mechanical stability and anti-icing efficiency.

The mechanical stability of superhydrophobic coatings is related to the binding force between nanoparticles and the bonding force between nanoparticles and the substrate. The active groups at the end of the silane coupling agent can undergo condensation reactions with the hydroxyl groups on the surface of inorganic nanoparticles to form a siloxane cross-linked structure. This cross-linked structure not only imparts hydrophobicity to the inorganic nanoparticles but also enhances the mechanical performance and durability of the coating [23,24]. Celik et al. [25] grafted dodecyltrichlorosilane onto the surface of silicon dioxide, achieving a water contact angle of 172° and a sliding angle of 1°, demonstrating superhydrophobicity and durability. Zheng et al. [26] fabricated an anti-icing coating by constructing a semi-interpenetrating cross-linked network using sulfhydryl-functionalized polyhedral oligomeric silsesquioxane (POSS) and hydroxyl-terminated polydimethylsiloxane (PDMS), which extended the ice-forming time to three times that of an uncoated surface. In summary, silane coupling agents are a non-fluorinated reagent that not only enhances the hydrophobicity of nanoparticles but also improves their dispersibility and stability to a certain extent. The hydrophobicity and durability of nanoparticles within superhydrophobic coatings can be significantly enhanced. Polyaspartic ester polyurea (PAE) is formed by the reaction of polyisocyanate and polyaspartic ester [27,28], which is a natural amino acid and can be extracted from plants or microorganisms [29] and can be used as an environmentally friendly high-performance coating material [30]. Compared to using fluorides as hydrophobic agents and superhydrophobic coatings, the superhydrophobic coatings prepared with Octyltriethoxysilane (FD-350) and PAE are more environmentally friendly. The unique structure of multi-walled carbon nanotubes (MWCNTs) enables effective absorption and storage of light energy, while also having a wide spectral absorption band [31,32,33,34]. With the addition of MWCNTs, the coating has a photothermal effect while maintaining superhydrophobicity, which improves the anti-icing/de-icing efficiency.

Compared with the work of other researchers, the innovations of our work include simplifying the preparation process of superhydrophobic coatings, utilizing biomass resources and fluorine-free chemicals to reduce the manufacturing cost and promoting environmental protection. Excellent photothermal de-icing performance of the coating was achieved by the addition of MWCNTs. It also overcomes the durability problem of biochar-based superhydrophobic coatings and ensures the longevity of the coating. Crop straw biochar is used as an inorganic nano filler and MWCNTs as a photothermal filler. The fillers were modified with FD-350 fluorine-free chemicals through a simple mixing method to graft long-chain silanes onto MWCNTs of biochar particles to achieve superhydrophobicity and photothermal properties. A photothermal superhydrophobic coating based on biochar material is prepared by mixing it with PAE and a one-step spray method. We evaluated the coatings for wettability, photothermal properties, mechanical and chemical stability, and de-icing and defrosting properties. The results show that the coating prepared by this method is not only environmentally friendly, but also superhydrophobic and photothermal performance is significant, and exhibits excellent de-icing capabilities, enhancing the practical value of the coating in actual applications.

2. Materials and Methods

2.1. Materials

Rice straw was purchased from Lianfeng Agricultural Products Co., Ltd (Lianyungang, China). Hydroxylated carbon nanotubes (MWCNTs-OH, purity ≥ 95%), anhydrous ethanol (EA, purity ≥ 99.7%), ammonia water (Aq, purity ≥ 25%), and butyl acetate (BAC, purity ≥ 99.5%) were all purchased from Shanghai Titan Technology Co., Ltd (Shanghai, China). Octyltriethoxysilane (FD-350, purity ≥ 97%) was purchased from Shanghai Boiling Point Co., Ltd (Shanghai, China). Polyaspartic ester polyurea (PAE) was purchased from Qingdao Marine New Materials Technology Co., Ltd. (Qingdao, China).

Figure 1a illustrates the process of material synthesis, modification, and spraying. Meanwhile, the photothermal effect is also included. Figure 1b demonstrates the chemical reaction principle of the FD-350 modification process. Figure 1c shows the mechanism of mechanochemical functionalization of BC@CNTs by FD-350 and synthesis of PAE.

2.2. Preparation of BC NPs

The rice straw was washed repeatedly 3–5 times with anhydrous ethanol and deionized water to remove surface impurities, then dried in an oven at 70 °C. The dried rice straw was crushed into powder using a grinder and then sieved with a 200-mesh screen. The crucible containing the rice straw powder was placed in a tubular furnace, and nitrogen was input. The rice straw was calcined at 800 °C, with a heating rate of 5 °C/min. After the calcination was complete, it was kept warm for 2 min before being removed. Then, it was washed 3–5 times repeatedly with anhydrous ethanol and deionized water, followed by drying in an oven at 70 °C to obtain biochar particles (BC NPs).

2.3. Modification of BC@CNTs NPs

Take 5 g BC NPs, 0.5 g MWCNTs-OH, and 25 g EA, and mix them before ultrasonic dispersion for 30 min. Then, transfer the mixture to a water bath at 40 °C, add Aq to adjust the pH to 8~8.5, and stir for 15 min. Then, slowly add 6 g FD-350 drop by drop, and continue stirring the water bath for 2 h. After the reaction is complete, filter and dry to obtain the hydrophobic modified BC@CNTs. The mass content of MWCNTs in BC is 0%, 5%, 10%, and 20%, which are named BC@CNTs-0, BC@CNTs-5, BC@CNTs-10, and BC@CNTs-20, respectively, represented by BC@CNTs-X according to the content of MWCNTs.

2.4. Fabrication of BCP Coatings

Take 3 g BC@CNTs and 10 g BAC, and stir magnetically for 10 min; then, slowly add 2 g of component A of PAE drop by drop, and stir for 5 min; then, slowly add 1 g of component B. Pour the mixture into the spray can, set the pressure to 0.45 Mpa, and spray it repeatedly 4–5 times on the 5 cm × 5 cm epoxy resin substrate; then, put it in a 70 °C oven to dry for 24 h. The prepared coatings are named BCP-0, BCP-5, BCP-10, and BCP-20 according to the content of MWCNTs, represented by BCP-X.

2.5. Characterization

Analytical instruments were used to analyze the surface morphology and chemical composition of the prepared BC@CNTS-X and BCP coatings. A Fourier-transform infrared spectrometer (FTIR, Nicolet iS20, Massachusetts, MA, USA) was used to analyze the chemical structure of BC@CNTs-X NPs, a spectrophotometer (UV–vis, Hitachi U4150, Tokyo, Japan) was used to analyze the light absorption capacity of BCP coatings, and a scanning electron microscope (SEM, GeminiSEM 300, Oberkochen, Germany) and transmission electron microscope (TEM, FEI Tecnai F20, Hillsboro, OR, USA) were used to analyze the micromorphology. An optical contact angle meter (OCA25, Filderstadt, Germany) was used to measure the contact angle (CA) and sliding angle (SA) between the BCP-X coating and the water droplet. The volume of the droplet is 3 μL, and the average is taken from 6 repeated measurements at different positions of a sample to avoid randomness.

2.6. Light Absorption and Photothermal Test

Under room temperature (26 °C), a xenon lamp (BBZM-1) was used to simulate sunlight illumination, and an infrared thermal imager (Hikvision-H10, Hangzhou, China) was employed to monitor the surface temperature of the coating. To validate the photothermal equilibrium stability of the coating, warming and cooling tests were conducted for five cycles.

2.7. De-Icing and Defrosting Test

A total of 3 mL of deionized water was injected onto the BCP-X surface, which was then frozen in a refrigerator at −10 °C with a relative humidity of 70% ± 5 for 12 h. After it was completely frozen, the coating was illuminated with a simulated sunlight lamp, and a camera was used to monitor the time it took for the ice to detach from the surface. In addition, in the aforementioned environment, the coating naturally frosts. The coating was then illuminated with a xenon lamp, and a camera was used to monitor the time and area of defrosting to evaluate the defrosting capability.

2.8. Antifouling and Self-Cleaning Test

To evaluate the self-cleaning ability of the coating, cola, tea, and yellow colorant were, respectively, injected onto the surface of the coating, and the contact angle was measured. Secondly, the gravel-covered BCP coating was washed with water flow to test its self-cleaning performance. In addition, the coating was immersed and pulled out from muddy water, and the surface was observed for any dirt or water stains.

2.9. Mechanical Durability Test

The wear resistance of the superhydrophobic anti-icing coating was determined using a linear sandpaper wear test. With a constant load (200 g) applied, the coating was dragged back and forth for 20 cm on a 100-mesh sandpaper of 30 cm × 30 cm, and the contact angle was recorded. The resistance of the coating to water impact was tested using a water drop impact test. The sample was tilted at 45° and exposed to water drop impact, and the contact angle was recorded at regular intervals. The stability of the coating was tested using a tape-peeling test. The tape was adhered to the coating, and a 1 kg weight was used to drag it back and forth 10 times evenly, with the contact angle recorded during repeated adhesion-peeling. Solutions of different pH values were prepared by H₂SO₄, Nacl and NaOH. The corrosion resistance of coatings was tested by soaking in solutions with different pH values for 30 min, with the contact angle recorded.

3. Results

3.1. Characterization Analysis

In the prepared BC@CNTs NPs, Figure 2 shows the FT-IR spectra of BC@CNTs NPs and BC@CNTs-10 NPs. The C-H stretching vibration peaks of octyl (C₈H₁₇) appeared at 2923 cm⁻¹ and 2854 cm⁻¹, indicating the presence of FD-350 in the nanoparticles, which is essential for the nanoparticles to obtain superhydrophobic performance. Since both BC and MWCNTs are carbon materials, C-H bending vibration peaks appear at 1617 cm⁻¹ and 1443 cm⁻¹. The characteristic peak at 3627 cm⁻¹ corresponds to the O-H stretching vibration peak [35].

The unmodified BC@CNTs NPs exist independently among various micro-/nanoparticles, while after modification with a silane coupling agent, the condensation reaction of BC@CNTs NPs with the Si-O groups in FD-350 anchors the micro-/nanoparticles, forming a siloxane cross-linked structure that enhances the bonding force between the micro-/nanoparticles. The surface microstructure is shown in Figure 3a,b. After modification, FD-350 adheres to the surface of BC@CNTs NPs, forming an amorphous rough layer on the surface of the particles, causing mechanical interlinking of the micro-/nanoparticles, as shown in (d), (e), and (f). The prepared BCP-X coating is shown in (c). PAE further tightly adheres the micro-/nanoparticles together and forms protrusions, forming a compact coating on the surface of the epoxy resin plate, implying that it is a holistic superhydrophobic coating.

As shown in Figure 4a, the wetting test of the coatings prepared with different MWCNT content showed that all four coatings had CA > 150° and SA < 10°, demonstrating excellent superhydrophobic performance. This is because BC@CNTs NPs contain a large number of hydrophilic groups. After modification with FD-350, octyl triethoxy silane first undergoes hydrolysis to form octyl silanol and ethanol. The generated octyl silanol further undergoes a condensation reaction with the hydroxyl groups on the surface of BC@CNTs NPs, greatly reducing the number of hydroxyl groups on the particle surface and giving BC@CNTs NPs superhydrophobic performance. To further test the wetting property of the coatings, a droplet was suspended from the needle of a syringe. Moving the syringe horizontally and vertically allowed the droplet to contact the coating, and eventually, the droplet separated from the coating. The coatings exhibited excellent superhydrophobic performance, as shown in Figure 4b,c. It is noteworthy that the content of BC NPs and MWCNTs did not affect the wettability of the coatings.

3.2. Photothermal Performance of BCP Coating

The light absorption capacity of the coating directly affects the photothermal effect of the coating, thereby affecting the de-icing efficiency. Excellent light absorption capacity provides a favorable condition for achieving light–thermal de-icing. The light absorption efficiency of the coating in the wavelength range of 200–800 nm was tested by a spectrophotometer, and the test results are shown in Figure 5a. The light absorption efficiency of the coating with a MWCNT content of 0 is the lowest, and the light absorption efficiency is significantly improved by adding 5%, 10%, and 20%. Figure 5b shows the relationship between the surface temperature of the coating and the time. With the extension of time, the surface temperature of the coating shows an increasing trend. Due to the black coloration of the biochar particles, they have a certain light absorption capacity, achieving a weak photothermal conversion. Even without adding MWCNTs, the surface temperature of the coating still shows a slight increase. However, the trend of increasing surface temperature with the addition of coatings containing different amounts of MWCNTs is more pronounced. This is because, in addition to black biochar absorbing most of the visible light, it has a weak photothermal conversion capacity. MWCNTs have a wide spectral absorption band, and their unique structure can effectively absorb and store light energy, greatly enhancing the light absorption and photothermal conversion capabilities of coating. In addition, we conducted heating and cooling tests to verify the photothermal stability of the coatings. The results show that the coatings can maintain a constant photothermal equilibrium temperature after five heating and cooling cycles, as shown in Figure 5c. Figure 6a–d show the temperature distribution on the surface of the coatings captured by the infrared thermal imager. The results indicate a significant increase in temperature in the illuminated areas. Due to the difference in heat dissipation rates at the edges and the interior, the temperature distribution of the coating surface gradually cools from the center to the edges.

3.3. De-Icing and Defrosting Performance

Superhydrophobic coatings with low surface energy and rough structures can effectively delay the formation of ice. However, in low-temperature and high-humidity environments, when the superhydrophobic coating combines with ice, the mortise and tenon interlocking structure can form. The formation of this structure is mainly due to the micro-/nanostructures on the superhydrophobic surface being able to interlock with the microstructure of ice crystals, thereby forming a stable bond. This makes ice more difficult to melt or slide on the superhydrophobic surface, thereby increasing the ice adhesion and stability [36,37]. However, the photothermal superhydrophobic coating relies on the photothermal effect of the coating to raise the surface temperature, causing the rough microstructure of the coating to first melt in the interlocking structure formed with the ice, thereby reducing the adhesion force, making it easier for the ice to detach, as shown in Figure 7. This means that combining passive and active de-icing capabilities provides a feasible approach for anti-icing.

To evaluate the de-icing performance of the coating, the frozen samples were placed in the same room temperature environment, and the influence of illumination and MWCNT content on the de-icing time was analyzed. The experimental process is depicted in Figure 8a. Figure 8b shows the effects of illumination and MWCNT content on de-icing time. Without illumination, the de-icing time of the BCP-5 coating was as long as 667 s, which was due to the interlocking structure that made it more difficult for the ice to melt or slide on the superhydrophobic surface. Under illumination, the times for BCP-5, BCP-10, and BCP-20 shortened to 176 s, 135 s, and 128 s. This is due to the photothermal effect of MWCNTs, which raises the surface temperature of the coating, causing the ice in the interlocking structure to melt first and reducing the adhesion of ice. In addition, even in the absence of liquid water, frost particles can form on the coating surface in a low-temperature and high-humidity environment, creating conditions for icing when continuously exposed to rain impact and low temperatures. To assess the defrosting performance of the BCP coating, the samples were frozen in the same environment and then exposed to xenon lamp irradiation to observe surface defrosting effects. As shown in Figure 8c, in the absence of illumination, the surface drying time was as long as 960 s. With the extension of the illumination time, the surface drying time shortened as the MWCNT content increased, with the defrosting time of BCP-20 reduced to 210 s. The experiments indicate that the BCP coating exhibits good de-icing and defrosting capabilities.

3.4. Antifouling and Self-Cleaning Performance of BCP Coatings

The antifouling and self-cleaning properties of the coating have an extremely important impact on the anti-icing effect. The accumulation of pollutants in the rough microstructure leads to the failure of the coating’s superhydrophobicity. Figure 9a,b show the wetting behavior of the coating with different pollutants. Cola, tea, and colorant were injected onto the coating surface, and the coating still maintained a state where CA > 150° and SA < 5°. In Figure 9c, gravel and soil mixtures were placed on the coating surface and then subjected to water droplet impact. After rinsing, most of the pollutants had been removed. In Figure 9d, the gravel and soil mixture was made into a turbid liquid, and the coating surface remained unpolluted after repeated immersion and pulling. Because of the high contact angle of the superhydrophobic coating, water droplets roll easily when the surface is tilted. The rolling water droplets pick up dust and contaminants from the surface, resulting in self-cleaning. Superhydrophobic surfaces typically have micron- and nanoscale structures that trap air, forming a cushion of air. This air cushion reduces the contact area of the water droplet with the solid surface, making it easier for the droplet to roll. These tests demonstrate the excellent antifouling and self-cleaning abilities of the coating.

3.5. Mechanical Stability and Chemical Stability of BCP Coatings

The mechanical stability of the coating is a necessary prerequisite for the coating to function effectively over time. The coating is exposed to the impact and corrosion via gravel, soil, and rainwater in the environment. To evaluate the mechanical stability of the coating, this work conducted tests for wear resistance, corrosion resistance, peel resistance and impact resistance, as shown in Figure 10a–f. The contact angles measured are illustrated in Figure 11.

In Figure 10a, the coating surface was subjected to the reciprocating movement of 100-grit sandpaper under a constant load. After 10 cycles, the coating still maintained its hydrophobicity. Similarly, using a steel brush to exert friction on the coating with as much force as possible, the coating maintained its hydrophobicity after 10 cycles, as shown in Figure 10b,c. In the tape-peeling experiment, the coating was repeatedly taped and peeled, as shown in Figure 10d. After 100 cycles, the contact angle of the coating decreased to 148.7°. Water drop impact tests were used to assess the coating’s resistance to water impact, as shown in Figure 10e. The sample was exposed to water droplet impact at a 45° angle, with a water flow rate of approximately 100 drops per minute, and after continuous impact for 3 h, the contact angle of the coating decreased to 145.3°. In Figure 10f, the coating was immersed in solutions with different pH values for 30 min and still maintained a contact angle greater than 150°. These tests confirmed the good mechanical stability of the BCP-X coating.

SEM images of linear abrasion and chemical corrosion are shown in Figure 12. The rough structure of the surface was destroyed after sandpaper abrasion. The wettability of the coatings in strong acid and alkali solutions does not change much. When the superhydrophobic surface is in contact with acid and alkali solutions, the surface forms a protective layer similar to an air cushion due to the presence of rough structures, which slows down the penetration of corrosive media in the solution. In the case of prolonged wetting, the surfaces of the rough micro-/nanostructures are destroyed in strong acid/base solutions, affecting the hydrophobicity of the coating. After immersion in a Nacl solution with a salinity of 32% and drying, Nacl crystals precipitated on the surface of the coating, but the coating was not corroded. This is due to the excellent anti-corrosion ability of PAE.

4. Conclusions

In summary, we exploited the inherent black coloration and light-absorbing properties of rice straw biomass as a filler for superhydrophobic coatings. An extremely simple method was employed to mix biochar particles with MWCNTs for hydrophobic modification, and a one-step spray coating method was used to construct a superhydrophobic coating with photothermal effects, achieving efficient de-icing. The irregular structure of biochar particles provides greater roughness. The contact angles of the prepared coatings all exceeded 155°, with a sliding angle of less than 4°. This work determined the relationship of BC NP and MWCNT content with photothermal performance and de-icing/frosting efficiency, but their content did not affect wettability. The surface temperature and de-icing/frosting efficiency of the coatings were enhanced as the MWCNT content increased. More specifically, at ambient temperature, the BCP-30 coating can reach a temperature of 47.6 °C within 10 min, providing a necessary foundation for achieving photothermal de-icing. It can promote ice detachment within 100 s, complete de-icing within 128 s, and defrost within 210 s. It can be seen that BCP-30 coating has better photothermal properties and de-icing/frosting ability than BCP-10 and BCP-20 coatings. It is worth noting that, after a series of tests, the BCP coating has demonstrated excellent antifouling, self-cleaning, mechanical, and chemical stability, thereby extending its service life. This work utilizes biomass recycling and the use of fluorine-free hydrophobic agents to prepare photothermal superhydrophobic coatings with de-icing and defrosting capabilities, promoting environmental protection and green development and expanding the application scope of waste crop recycling.