Synthesis of a novel anti-fog and high-transparent coating with high wear resistance inspired by dry rice fields

Graphical abstract

Abstract

During the outbreak of COVID-19, the fogging of goggles was a fatal problem for doctors. At present, there are many ways to prevent fogging by adjusting surface wettability. However, the mechanical properties of most super-hydrophilic antifogging coatings are poor, easy to lose their antifogging properties when encountering fingers or cloth friction. To address this issue, the Konjac Glucomannan was cross-linked with water-soluble silicone fluid to form a binder, then being combined with the modified Ecokimera to prepare an eco-friendly super-hydrophilic coating that possessed excellent super-hydrophilicity, and the water contact angle (WCA) was 2.51 ± 1°. In addition, the WCA is still about 5° after 180 times of antifogging tests. The friction resistance of the coating was as high as 24 m. Moreover, the light transmittance was only reduced by 3%. Besides, they also had the excellent self-cleaning property. After being stored in the laboratory environment for 90 days, it can still maintain the hydrophilic property (WCA is about 5°). In general, the method proposed in this study is low-cost and eco-friendly, and can be widely used in the preparation of antifogging coatings.

Nomenclature

Abbreviation

WCA

water contact angle

KGM

konjac glucomannan

HEA

hydroxyethyl acrylate

TEOS

tetraethyl orthosilicate

PVA

poly vinyl alcohol

PAA

poly acrylic acid

ECO

ecokimera

DC193

water-soluble silicone fluid

TEOA/TEA

triethanolamine

DI

distilled water

KOT

KGM-DC193-TEOA

m-ECO

modified ecokimera

E-KOT

modified ecokimera-KOT

Roman symbols

$r_s^d$

component of dispersion force

$r_s^h$

polar force component

$\theta$

contact angle of water

$r_{\mathit{lv}}$

surface tension of water

1. Introduction

Medical staff needed to take protective measures to avoid virus infection during COVID-19, and goggles were one of the necessary tools (Lemarteleur et al., 2020, Oldfield and Malwal, 2020). However, the fogging of goggles was a fatal problem in medical work (Bai et al., 2020, Feng et al., 2020, Hu et al., 2020, Sun et al., 2020). Besides, from a security standpoint, the antifogging property was also indispensable in glasses, windshields, etc. (Durán and Laroche, 2019a, Liu and Locklin, 2018, Sun et al., 2020, Tao et al., 2018, Yu et al., 2020). In this case, various antifogging strategies have been proposed by adjusting the surface wettability. For example, the superhydrophobic anti-fog coating could be prepared by airless spray and crystal growth methods (Zhang et al., 2017). Due to the low adhesion of the superhydrophobic surfaces, this strategy requires complex surface structures to be implemented (Liu and Kim, 2014, Wen et al., 2014). Furthermore, it is also technically challenging to produce transparent superhydrophobic coatings (He et al., 2018).

So far, constructing super-hydrophilic surfaces seems to be the most promising antifogging strategy (Nam et al., 2017, Walker et al., 2019, Wang et al., 2019a). When the substrate is in the super-hydrophilic state, water droplets spread out rapidly on the surface to form a liquid film layer (Feng et al., 2020, Hsu et al., 2018). As the liquid film has a large surface area and can quickly evaporate, an antifogging effect is produced (Durán and Laroche, 2019b). At present, the raw materials used to prepare the super-hydrophilic coating, such as oxide (Duan et al., 2018, Hikku et al., 2018, Zhong et al., 2017) and the high molecular polymer (Bakshi et al., 2019), could be adopted in various base materials. Zou et al. (2021) found that konjac glucomannan (KGM) and high amylose corn starch combined to improve the mechanical properties of the films. Zhang et al. (2019) combined the modified TiO₂ and hydroxyethyl acrylate (HEA) to prepare a super-hydrophilic coating with the light transmittance of more than 90%. Apart from that, the coating retains its hydrophilic properties even after 330 days in an external environment and can also rub 16 m on 600 mesh sandpaper. Joshi et al. (2019) used tetraethyl orthosilicate (TEOS), MgF₂, and HNO₃ as raw materials and etched the corresponding microstructures on the glass substrate by HF produced by the reaction. Then, it combined with SiO₂-MgF₂ sol–gel, so that the glass has super-hydrophilic coating by in-situ generation. As for the transparency of the coating, it is as high as 98% (in the visible light region) and can withstand UV for 100 h. Wang et al. (2019) developed self-healing poly (vinyl alcohol) (PVA) / poly (acrylic acid) (PAA) / Ag films with antifogging and antibacterial properties via a simple one-pot strategy. Besides, the films demonstrated the highest 5A adhesion grade and 8H pencil hardness. Although the coatings prepared with these methods are featured with good antifogging properties, they are limited in practical application due to poor mechanical properties. Therefore, the mechanical properties of the super-hydrophilic antifog surface are the core problem of its practical application.

In this study, KGM was combined with water-soluble silicone fluid to form a binder, and modified nanoparticle Ecokimera (ECO) was dispersed among them to prepare the super-hydrophilic coating. The unique double-layer structure of the coating has excellent abrasion resistance and higher light transmittance. Therefore, they were expected to be used in goggles, windshields, glasses, anti-fog glass and so on.

2. Experimental section

2.1. Materials

Water-soluble silicone fluid (DC193, >99%) was purchased from Dow Corning. Konjac Glucomannan (KGM, ≥95%) was purchased from Hefei Bomei biotechnology Co., Ltd. Triethanolamine (TEOA, ≥85%) was purchased from Chengdu Cologne Chemicals Co., Ltd. Sodium hexametaphosphate ((NaPO₃)₆, 65–70%) was purchased from Chengdu Cologne Chemicals Co., Ltd. Ecokimera (ECO) was purchased from Dongguan Jinji Environmental Protection Technology Co., Ltd. Glass slide was purchased from Cologne Chemicals Co., Ltd. Distilled water (DI) came from laboratory. Besides, it should be mentioned that all chemicals were used without further purification.

2.2. Fabrication of super-hydrophilic solution and glass

The specific experimental process is shown in Fig. 1. Firstly, the ECO was modified by TEOA (Zhang et al., 2019), named as m-ECO. Secondly, DC193, TEOA and DI were mixed at a mass ratio of 1:0. 3:30 at low speed and then marked as solution A. After that, 0.65 wt% KGM solution was prepared at room temperature, stirred for 30 min at 50 ℃ and then labeled as solution B. According to the mass ratio of 10:1:0.05, A, B and (NaPO₃)₆ were added in turn. After being stirred for 15 min, they were moved to a water bath at 40 ℃ for 15 min to generate a cross-linking reaction for obtaining the KGM-DC193-TEOA solution, abbreviated as KOT. Finally, 1 wt% m-ECO was added. The final solution was acquired by ultrasonic processing for 40 min and got the m-ECO-KOT solution, abbreviated as E-KOT.

Fig. 1.

Flow diagram of of superhydrophilic coating fabrication.

The glass slides were ultrasonically cleaned in ethanol and deionized water, respectively. Being dried in air, the solution was spin-coated on the substrate at 500 rpm for 10 s and 3000 rpm for 20 s. Then, the sample was naturally air-dried for 12 h.The obtained coating was also deemed as E-KOT.

2.3. Characterization

The water contact angle (WCA) was measured using an optical contact angle tester (JCY, FangRui Instrument Co., Ltd, Shanghai, China). 3 μL water drops were deposited on the surfaces for WCA measurements, and at least three different positions were measured on the same sample. The morphologies of the surfaces were observed via a scanning electron microscope (SEM, FEI-INSPECT F50). The surface chemical compositions of the samples were analyzed using an X-ray photoelectron spectrometer (Thermo ESCALAB 250XI) and a Fourier-transform-infrared-spectrometer (FT-IR) instrument (Magna-IR 750, Nicolet 5700, USA). The transmittance of the samples was carried out via the UV–visible spectrophotometer (UV-3000) at normal incidence in the wavelength between 300 and 800 nm. The number of contaminants on the substrate surface was observed by the metallographic microscope (NMM-820TRF).

3. Results and discussion

3.1. Characterization of coatings

As shown in Fig. 2 a, the wide peak at 3384.7 cm⁻¹ was attributed to –OH in m-ECO (Dilamian and Noroozi, 2021). The peaks at 2962.9 cm⁻¹, 2885.9 cm⁻¹ and 2821.1 cm⁻¹ were assigned to the stretching vibration of C-H in CH₂ (Rekha et al., 2017). The group originally belonging to TEOA was grafted into ECO, showing the success of ECO modification. In Fig. 2 b, the peak at 3365.9 cm⁻¹ disappeared, when the vibration band of –OH was down-shifted to 3138.8 cm⁻¹, which is caused by hydrogen bonding between-OH on m-ECO and –OH in KOT solution (Liu et al., 2020). The peaks at 2965.6 cm⁻¹ and 2867.4 cm⁻¹ were respectively assigned to the stretching vibration of C-H in CH₃ and CH₂ (Santos et al., 2020, Ye et al., 2016, Yin et al., 2020), which could be extrapolated m-ECO combined with KOT solution. The absorption band at 955.5 cm⁻¹ resulted from the Si-OH tensile vibration of the DC193 (Yu et al., 2018).

Fig. 2.

The FT-IR spectra of m-ECO (a) and E-KOT (b).

The XPS survey reveals that Si, C, O and Ti are the main elements of the coating ( Fig. 3 a), of which C and O were jointly determined by DC193, KGM, TEOA and m-ECO. Ti was determined by m-ECO, and Si was determined by DC193 and substrates.

Fig. 3.

XPS survey spectra of E-KOT (a), high-resolution O 1 s (b), C 1 s (c) and Si 2p (d) of E-KOT.

As m-ECO was wrapped in the E-KOT solution, the Ti peak could not be detected ( Fig. 3a). The strong oxygen peak appeared at 532.41 eV ( Fig. 3b), that is to say, the solution contained a lot of oxygen content. One part belonged to -OH and the other part belonged to C-O ( Fig. 3b) (Wang et al., 2019), which means that the solution contains a large number of-OH mainly from m-ECO, conducive to enhance enhancing the intrinsic hydrophilia of the coating. The C1s peak corresponds to C-C at 284.9 eV, C-O at 286.4 eV and C = O at 287.8 eV ( Fig. 3c) (Hu et al., 2016, Songkeaw et al., 2019), or in other words, KGM and DC193 are crosslinked. The Si2p peak corresponds to C-Si at 102.5 eV and SiO₂ at 103.4 eV. Among them, SiO₂ came from the glass substrate ( Fig. 3d) (Aghaei et al., 2018), thus confirming the bonding of the E-KOT to the glass surfaces.

The surface morphology of the substrate before and after treatment was observed by the SEM (Fig. 4). The surface of the original glass was relatively smooth ( Fig. 4 a). However, the sample had a dense granular rough structure ( Fig. 4 b). Then, further magnification revealed a large number of irregular cracks on the coating surface, similar to the structure of a dried rice field ( Fig. 4 c, 4d), which could increase the solid–liquid contact area and promote the liquid absorption of the coating, so as to improve the super-hydrophilic property. Furthermore, it can be seen that there were a large number of prickly pear-like structures on the coating surface with cluster particles overlapping on the surface. The structure is very different from m-ECO, proving that substances in the KOT solution reacted with m-ECO so that it was wrapped (Ao et al., 2017, Kim et al., 2017). ECO, a regular hexagon layer (García-Glez et al., 2017), as a kind of unique titanium dioxide, with strong adsorption and the characteristics of nanoparticles is potential transfer of high-energy translocation reaction (Kim and Chung, 2001), which may generate a redox reaction under the condition of no light and produce strong oxidation of hydrogen and oxygen-free radicals (Kőrösi and Dékány, 2006). Not only can they absorb harmful gas, but also the solution is featured with excellent hydrophilicity. Therefore, ECO can be adopted to construct the rough structure of the bionic super-hydrophilic coating. Besides, as the ECO was covered by TEOA, the morphology of ECO was not observed, which was consistent with the XPS results.

Fig. 4.

SEM images of the bare glass surfaces (a), SEM images of KOT surface (b), (c) and (d) partial enlarged detail in (b), the illustration shows a dried rice field.

3.2. Super-hydrophilicity and durability of the E-KOT coating

The WCA of clean glass and sample glass was measured at room temperature, while the WCA of bare glass was about 30° ( Fig. 5 a). It was hydrophilic due to the existence of some hydroxyl groups on the surface, but could not reach the super-hydrophilicity. By contrast, the WCA of glass treated with E-KOT solution decreased to less than 3° within 1 s ( Fig. 5 b), that is to say, the E-KOT coating possessed excellent super-hydrophilicity. The samples were placed in a petri dish, and the WCA was measured every 15 days to determine the durability of super-hydrophilicity. Fig. 5 c showed that the WCA of the samples was close to 5° after 90 days.

Fig. 5.

Digital images of the WCA for the bare glass (a) and the E-KOT coating (b); change of static WCAs of the E-KOT coating with time (c); these illustrations are WCA images before and after placing the E-KOT coating for 90 days E-KOT coating.

The surface energy is equal to the sum of the components of the dispersion and polar forces (Owens and Wendt, 1969). Water has high polaritywith both the dispersion force component and the polar force component. Therefore, the relationship between the contact angle and surface energy can be expressed as (Owens and Wendt, 1969):

$$1+\cos \theta =2\sqrt{{\gamma }_s^d}\left(\frac{\sqrt{{\gamma }_l^d}}{{\gamma }_{\mathit{lv}}}\right)+2\sqrt{{\gamma }_s^h}\left(\frac{\sqrt{{\gamma }_l^h}}{{\gamma }_{\mathit{lv}}}\right)$$ (1)

where $r_s^d$ is the component of dispersion force; $r_s^h$refers to the polar force component; $\theta$ indicates the contact angle of water, while $r_{\mathit{lv}}$means the surface tension of water. From formula (1), it can be inferred that $\theta$ decreases as $r_s^h$ is large. In other words, super hydrophilic materials can be prepared by increasing the polar force component of solid surface. By combining the interpretation of FT-IR, SEM and XPS results, it can be seen that the reason for the super-hydrophilicity of the E-KOT coating may be as follows: firstly, there are much –OH on the surface of the E-KOT coating, which comes from DC193 and increases the polar force component, thus producing affinity to water molecules. Therefore, the inherent hydrophilicity of the coating is enhanced (Zorba et al., 2010). Secondly, wettability is determined by surface chemical composition and microstructure. When the surface has the same elemental composition, the wettability is enhanced by changing the surface roughness. The ECO NPs provide a rough structure on the nanoscale, increasing the surface roughness. High surface roughness facilitates the establishment of ideal conditions for extreme super-hydrophilic behavior (Liu and He, 2008, Zhang et al., 2005). Thirdly, there are cracks similar to a dried rice field in the coating, which can increase the contact area between solid and liquid, thus further promoting the absorption of the coating to liquid and then increasing the hydrophilicity of the glass surface. In this case, it can be found that the synergy of the above mentioned factors resulted in the outstanding super-hydrophilicity of the E-KOT coating. As the inherent characteristics of the coating enable it to be stable and not to be damaged by the external environment, the super-hydrophilicity can stay in the air for a long time.

3.3. Transmittance of the E-KOT coating

Different m-ECO content of the coating was synthesized on the glass bases, of which the quality scores of m-ECO were 0, 0.5 wt%, 1 wt%, 2 wt%, and 3 wt%, respectively. As shown in Fig. 6 a, the transparency of the samples remained basically unchanged. The transmittance was measured by using UV–VIS spectrophotometer. Then, it can be found that the transmittance of samples in the visible range (400–760 nm) only decreased by about 3% after adding the m-ECO ( Fig. 6 b), showing that the distribution of m-ECO NPs was uniform in the KOT solution, while that at 303 cm⁻¹ decreased significantly with the increase of m-ECO content ( Fig. 6 b), indicating that the E-KOT coating has good absorption ability under UV light.

Fig. 6.

Visual effect of the coatings with different m-ECO purities on glass bases (a); the transmittance of the E-KOT coating with different mass fractions (b).

According to our experimental results, it can be known that when the m-ECO is less than 1 wt%, the mechanical properties of the coating are poor. Generally speaking, the mechanical properties of the coating increase with the increase of m-ECO content. Nevertheless, the increase in the content of nanoparticles will affect the transparency of the coating. Therefore, the best m-ECO content is 1 wt% that is used in all experiments unless otherwise specified.

3.4. Antifogging and self-cleaning properties of the E-KOT coating

The antifogging properties of coated glass and bare glass were shown in Fig. 7 a. Half of the clean glass was coated with the E-KOT solution, while the other half was not treated at all. After that, the sample was put in the refrigerator at 4 ℃ for 1 h and then taken it out to see if the surface fogs. In this case, it was observed that countless small droplets on the untreated glass gathered together, which caused light scattering and resulted in being unable to see the words below clearly. However, the treated surface made the droplets disperse quickly on the surface and then form a film. Therefore, light was unable to scatter, thus achieving the antifogging effect, and the words behind could be seen very clearly. Then, the sample was placed back to the refrigerator for the next test. After 180 cycles, it still has excellent antifogging performance ( Fig. 7 b). In order to determine whether the prepared coating can be applied to other substrates, the coating was rubbed onto the goggles, and the anti-fogging test was carried out in the same way. As shown in Fig. 7 c, it could be seen that the anti-fogging condition was similar to that of the glass substrate. Therefore, the coating can also be adopted on goggles.

Fig. 7.

Antifogging test of uncoated (left) and E-KOT coated (right) glass bases (a); WCAs with different days (b), these illustrations are WCA of sample before and after antifogging experiments for 180 times; visual effect of the coatings with different m-ECO purities on goggles (c).

The surface of glass slides is always easy to be polluted by dust, which affects the transparency and service life. Therefore, the coating is required to be self-cleaning to quickly remove contaminants from the glass surface. In general, super-hydrophilic surfaces have excellent self-cleaning properties. For that reason, the self-cleaning properties of the prepared coating were measured. As was shown in Fig. 8 a and b, the treated and untreated glass was polluted with toner. After the toner was laid flat on the glass slide, it was washed with running water, and the samples after cleaning were observed through the metallographic microscope. Then, it could be clearly seen that the surface of the treated glass had significantly less residual toner than the surface of the untreated glass ( Fig. 8 c, d). The reasons may be as follows: on the one hand, the double-layer structure on the sample surface reduced the contact area between the contaminant and the sample, while on the other hand, it increased the contact area with water and the absorbent capacity (Adachi et al., 2018, Banerjee et al., 2015, Wang et al., 1997). In other words, when the contaminant came into contact with the sample surface, the prickly pear-like micro-nano structure protected the sample from contact with the contaminant. At the same time, the micro-nano structure on the sample surface could absorb more water than the clean glass, thus forming a water film between the sample surface and the pollutants. Under the action of external forces such as gravity or wind force, the water droplets slid out of and carried pollutants away from the surface ( Fig. 8 e), which as two synergistic effects gave the coating excellent self-cleaning properties.

Fig. 8.

Metallographic microscope images of toner on the sample surface and bare glass surface: fouling processes (a) and (b), bare glass (c) and super-hydrophilic glass after rinsing with water (d); the self-cleaning mechanism of water on the surface of the super-hydrophilic coating (e).

3.5. Mechanical properties of the coating

Sandpaper wear tests were performed to determine the mechanical properties of E-KOT coatings. To be specific, the glass was placed on sandpaper (600 mesh) with a weight of 100 g. Then, it was dragged along the ruler of 20 cm and rotated 90°, followed by being again moved for 20 cm to form a cycle (Jin et al., 2017) ( Fig. 9 a, b), which can ensure the transverse and longitudinal wear of the coating surface. It could be seen from Fig. 9 c that the WCA of the glass increased slightly with the increase of wear cycles. After 60 cycles, the WCA of the sample was about 6°. For one thing, the excellent mechanical stability of the sample came from the special structure similar to the combination of dried paddy field and prickly pear, While foranother, the adhesion of KGM provided good adhesion for the coating.

Fig. 9.

The friction testing of (a) transverse and (b) longitudinal; (c) friction times-dependent variations of the static WCA of the E-KOT coating. SEM images of the same sample with the same multiple at different friction times (4 m, 8 m, 12 m, 16 m, 20 m and 24 m, respectively) (d-i).

To further investigate the causes of the E-KOT coating’s super-hydrophilicity loss, the surface morphology change of the coating after every 10 cycles of wear was observed ( Fig. 9 d-i). With the increase of wear times, the surface of the coating gradually became very smooth. Meanwhile, the hydrophilic effect was worse, indicating that the structure similar to the prickly pear determines the mechanical strength of the coating.

3.6. The high wear resistance mechanism of the coating

Based on the above test results, the mechanism concerning high wear resistance of the E-KOT coating is propose in Fig. 10 . (1) A large number of hydroxyl groups are connected on the surface of the m-ECO and DC193, and some of them combine with the hydroxyl groups on the glass to form hydrogen bonds, thus enhancing the bonding performance of the coating (Liu et al., 2020). (2) KGM also contains a certain amount of hydroxyl groups. Part of the coating is bound to the hydroxyl group in solution (DC193, m-ECO, etc.), so that the nanoparticles are closely linked to it, which can not only improve the roughness of the coating, but also enhance the superhydrophilicity. In addition to that, some hydroxyl groups bind to the glass surface of the hydroxyl group to form hydrogen bonds, so that the coating’s mechanical properties will be further improved. At the same time, as the KGM itself is characterized with high viscosity, the mechanical wear resistance of the coating is enhanced to a great extent (Fang, 2021). Therefore, the above three reasons together caused the excellent wear resistance of E-KOT coatings.

Fig. 10.

Mechanism of the coating.

4. Conclusions

The high wear-resistant super-hydrophilic coating was successfully prepared by the simple and low-cost method. The friction resistance of the coating was systematically studied. With the increase of friction times, m-ECO NPs and KOT were erased in sequence. Such double-layer structure led to excellent friction resistance. At the same time, the coating had excellent antifogging and super-hydrophilicity properties. The super-hydrophilic coating has a broad application prospect in goggles, automobile windshields, glasses and so on. Furthermore, the coating can also be adopted in anti-ultraviolet materials.

CRediT authorship contribution statement

Juan Xiang: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Methodology, Validation, Data curation, Formal analysis. Xiaoying Liu: Discussion on experimental factors. Yan Liu: Validation. Lilin Wang: Project administration. Yan He: Project administration. Ling Luo: Data curation. Gang Yang: Resources, Supervision. Xiaohong Zhang: Funding acquisition, Resources. Chengyi Huang: Resources, Project administration. Yanzong Zhang: Methodology, Writing - review & editing, Resources, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.