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Preparation and anti-icing property of a lotus-leaf-like superhydrophobic low-density polyethylene coating with low sliding angle.

INTRODUCTION

The 1998 ice storms in Eastern Canada and 2008 ice storms in South China caused tremendous damage and large economic losses (1), (2). Thus, developing anti-icing and deicing materials or technologies are perfectly desirable for outdoor equipments to reduce ice formation on their surfaces in cold seasons 13-51. To assure the maximum reliability of the outdoor equipments and decrease the economic loss caused by icing, two main strategies have been developed to reduce or eliminate the ice accumulation on outdoor equipments (6). The two main strategies are (i) active deicing methods such as thermal, electrical or mechanical techniques (7), and (ii) passive methods which protect exposed surfaces with anti-icing coatings to significantly reduce or eliminate ice adhesion strength (6). The active techniques, such as thermal and mechanical methods, are currently used widely but energy hungry and expensive to make and operate. On the contrary, the passive technique such as anti-icing coating is cheaper and environmentally friendly, which does not need external energy to deice or prevent ice accumulation. Thereby, anti-icing coatings and materials have received more and more research attention lately (8). Icing in the atmosphere is usually occurred when a surface contacts with the super-cooled water in the environment with low temperature. The first step of this process, the surface wetted by super-cooled water, plays a crucial role to determine the feasibility of icing, where the preexisting "surface/ air" interface is substituted by a "surface/water" interface (9). Therefore, decreasing wetting or increasing the hydrophobicity of solid surfaces is an efficient way for retarding icing (10). Thus, superhydrophobic surfaces or coatings, on which water droplets remain spherical with a contact angle larger than 150[degrees] (11-15), could be potential anti-icing materials for protecting the outdoor equipments in cold seasons or winter (16), (17). However, up to date, only very limited number of papers have been published to report the investigation of anti-icing property of superhydrophobic surfaces (9), (16). In this work, we prepared a lotus-leaf-like superhydrophobic low-density polyethylene (LDPE) coating with low sliding angle and studied its anti-icing property.

EXPERIMENTAL

Materials

A commercial grade LDPE (obtained from Maoming Petroleum Chemical Corporation of China) was used without further treatment. Xylene (mixture of isomer, obtained from Changsha Huihong Chemical Plant of China) was chosen as the solvent. The analytical grade [NH.sub.4][HCO.sub.3] was obtained from Changsha Huihong Chemical Plant of China. The glass plates (80 mm X 80 mm) were cleaned with detergent, then rinsed in de-ionized water for several times and dried with nitrogen gas.

Preparation of Smooth LDPE Coating

A total of 1 g of LDPE resin was dissolved slowly in 100 mL xylene to form a uniform solution with the concentration of 0.01g [mL.sup.-1] at 120[degrees]C. Some of the LDPE solutions were dropped onto a cleaned glass plates and the solvents were evaporated for 2 h in a dry atmosphere in the oven at 120[degrees]C, then a smooth LDPE coating was obtained.

Preparation of Superhydrophobic LDPE Coating With High Sliding Angle

Dropping some LDPE solutions with the concentration of 0.01g [mL.sup.-1] onto a cleaned glass plates and the solvents were evaporated for 10 h in a dry atmosphere in the oven at 30[degrees]C, then a superhydrophobic LDPE coating with high sliding angle was obtained.

Preparation of Superhydrophohic LDPE Coatings With Low Sliding Angle

Three gram [NH.sub.4][HCO.sub.3] was added into 50 mL LDPE solution with the concentration of 0.01g [mL.sup.-1] to form a mixed solution. After stirring the mixed solution for 50 s, a few drops of the mixture were dipped onto a cleaned glass plates. The solvents were first evaporated for 0.5 h in a dry atmosphere in the oven at 50[degrees]C, and then evaporated for 2 h at 30[degrees]C. As a result, a superhydrophobic LDPE coating with low sliding angle was obtained.

Characterization

The surface morphologies of the LDPE coatings were observed on a scanning electron microscopy (SEM) of HITACHI S-3000N. The wettabilities of the LDPE coatings were measured on a data-physics OCA 20 contact-angle system with about 5 [micro]L droplets. The equilibrium contact angles of 5 id, water droplet were measured by sessile drop method. The sliding angle was measured by tilting the sample stage from 0[degrees] to higher angles and then putting a droplet on the sample using a micro-gauge. When the droplet rolled off the surface, the angle of the sample stage was the sliding angle.

Anti-icing Test

The anti-icing tests were performed in a climatic chamber with a working temperature of -5[degrees]C. The sampies were fixed on the sample stage placed 20 cm above the bottom of the chamber. The super-cooled water droplets with a temperature of near 0[degrees]C were sprayed onto the sample surfaces through sprinklers with the pore diameter of 1 mm. The heights of the sprinklers to the sample surfaces are 20 cm. To evaluate the anti-icing and ice accumulation of the samples, the samples were weighed on an electronic balance every 1 min interval. Before spraying super-cooled water droplets, samples were placed in the system for 30 min at a temperature of -5[degrees]C to make them as cold as the natural outdoor equipments in ice storm.

RESULTS AND DISCUSSION

When LDPE solution was dried at 120[degrees]C, a smooth transparent coating was obtained. Figure la is the SEM image of the smooth LDPE coating obtained at 120[degrees]C. The water contact angle of the smooth coating is only 103 [+ or -] 1.8[degrees] (shown in Fig. 2a). To obtain a superhydrophobic LDPE coating, rough surface microstructures are necessary according to the investigations on the natural and artificial superhydrophobic species (18-26). Thus, we should change the preparation condition to make the LDPE coating rougher. Previous reports showed that lower drying temperature for polymer solution increased the surface roughness of polymer coating, and the corresponding water contact angle was increased (27-29). Similarly, we also decreased the solvent evaporation temperature of the LDPE solutions to obtain a superhydrophobic LDPE coating. Figure lb and c are the SEM images of the LDPE coating prepared by evaporating solvents of LDPE solutions at 30[degrees]C. Compared with Fig. la, we know that the surface of LDPE coating obtained by evaporating solvents of LDPE solutions at 30[degrees]C is rougher. For a rough surface, Wenzel proposed a model (30) shown in Eq. 1 to describe the wettability,

cos[[theta].sub.r] = r cos [theta](1)

Here, r is the roughness factor, and 0 and Or are the equilibrium contact angles of a liquid on a smooth solid surface and a rough solid surface, respectively. According to Eq. 1, if the roughness is increased, the contact angle of a hydrophobic solid surface will increase also. Because LDPE is an intrinsic hydrophobic surface with the water contact angle of 103 [+ or -] 1.8[degrees] (shown in Fig. 2a), thus, according to the Wenzel model, we believe that the rough LDPE coating obtained at 30[degrees]C should be more hydrophobic than the smooth one. We tested the water contact angle of the LDPE coating obtained at 30[degrees]C, the value of the average water contact angle was 152 [+ or -] 1.7[degrees] (shown in Fig. 2b), indicating that a superhydrophobic LDPE coating could be obtained by evaporating the solvent of the LDPE solution at 30[degrees]C. However, the water sliding angle was so high that the water droplet cannot slide off even though the LDPE coating was titled until vertical.

The results were similar to the previous report of Lu et al. (27). In practice, sliding angle is an important criterion for a superhydrophobic surface (12). In some cases, e.g., self-cleaning, the sliding angle is more important than maximum water contact angle since sliding angle is directly related to driving force of a liquid drop (31). To decrease the water sliding angle, Lu et al. (27) added some cyclohexanone into the LDPE solution to adjust the crystallization time and nucleation rate, which resulted in the change of microstructures of LDPE film, and then a superhydrophobic LDPE surface was obtained with sliding angle of only 1.9[degrees]. However, the evaporation velocity of the cyclohexanone is too slow because of the high boiling point (155.7[degrees]C), and the solvent evaporation time to obtain the superhydrophobic LDPE surface is long. From the viewpoint of practical applications, shorter preparation time is more desirable. Herein, we developed a facile method to obtain a superhydrophobic LDPE coating with low sliding angle. That was, adding 3 g [NH.sub.4][HCO.sub.3] into 50 mL LDPE solution with the concentration of 0.01 g [mL.sup.-1] to form a mixed solution. After stirring the mixed solution for 50 s, we dripped the mixture onto a cleaned glass plates. The solvents were first evaporated for 0.5 h in a dry atmosphere in the oven at 50[degrees]C, and then evaporated for 2 h at 30[degrees]C. As a result, a lotus-leaf-like LDPE coating (shown in Fig. 1d and e) was obtained. As we all know, natural lotus leaf has a high water contact angle up to 160[degrees] and a very low sliding angle with the value of 1.9[degrees]. We tested the water contact angle and sliding angle of the lotus-leaf-like LDPE coating, and the value were 156 [+ or -] 1.7[degrees] (shown in Fig. 2c) and 1[degrees], respectively. It indicates that the lotus-leaf-like superhydropho-bic LDPE coating has similar wettability with the natural lotus leaf. It also means that a water droplet can roll easily on this lotus-leaf-like superhydrophobic LDPE coating. Compared with the investigation of Lu, et al. (27), our preparation time is much shorter, and the preparation process is easier to control. From Fig. 1d and e, a lot of pores can be observed on the superhydrophobic LDPE coating. Theoretically, air can be trapped in these pores. Thereby, the lotus-leaf-like LDPE coating can be considered as a composite comprised of the air trapped in the pores and the LDPE, it can be described by the Cassie-Baxter model (32) shown in Eq. 2:

cos [[theta].sub.r] = [f.sub.1] cos [theta] - [f.sub.2] (2)

Here, [f.sub.1] and [f.sub.2] are the fractions of solid surface and air in composite surface, respectively (i.e., [f.sub.1] + [f.sub.2] = 1), while [theta] is the equilibrium contact angle on a flat solid surface. This equation predicts that increasing the fraction of air ([f.sub.2]) will increase the contact angle of the LDPE coating; otherwise the contact angle of the LDPE coating will decrease if [f.sub.1] (the fraction of solid surface) increase, According to the water contact angle data on the smooth LDPE coating and the lotus-leaf-like LDPE coating, [f.sub.2] value calculated by Eq. 2 is about 0.90. This means that air occupies about the 90% of contact areas when the porous lotus-leaf-like LDPE coating contact with the water droplet, which is responsible for the superhydrophobic property. For the low sliding angle of the lotus-leaf-like superhydrophobic LDPE coating, we can explain the possible reason according to the viewpoint of Oner et al. (33), (34) and Miwa et al. (31). Oner et al. (33), (34) argued that the structure (shape, length, continuity of contact, amount of contact) of the three-phase (solid--liquid--air) contact line was important for sliding angle: discontinuous, unstable, and contorted contact line were necessary to form a superhydrophobic surface with low sliding angle. Miwa et al. (31) believed that the surface structures that could trap air were important for the preparation of low sliding angle surfaces. According to the SEM images of the lotus-leaf-like LDPE coating (shown in Fig. 1d and e), the three-phase contact line is discontinuous on the surface and the porous lotus-leaf-like surface structure can trap air, suggesting that the lotus-leaf-like LDPE coating may have a low sliding angle and water droplet can slide easily on the surface.

According to the SEM images shown in Fig. 1 and some phenomena observed in experiments, we can explain the possible formation process of the superhydrophobic LDPE coating as following: In general, the polymer solution becomes thermodynamically unstable when drying the solution under certain conditions. As a result, phase separation will occur to form a polymer rich phase and a polymer poor phase. The concentrated phase solidifies after phase separation and forms the matrix, whereas the polymer poor phase forms the pores (29). When LDPE solution was dried at 120[degrees]C, the temperature was so high that the solvent evaporated quickly and phase separation was forbidden. Therefore, a smooth transparent coating was formed (shown in Fig. la). As the temperature was decreased to 30[degrees]C, the solvent evaporation rate was low, and phase separation occurred and rough surfaces were formed (shown in Fig. lb and c). When the [NH.sub.4][HCO.sub.3] was added into the LDPE solution, the [NH.sub.4][HCO.sub.3] was decomposed as [NH.sub.3], [H.sub.2]O, and [CO.sub.2] in the oven. The decomposed [H.sub.2]0 is a nonsolvent for LDPE and it would act as a precipitator, and the precipitated polymer (LDPE) acted as nuclei and the polymer rich phase preferred to aggregate around the nuclei to decrease surface tension (28). Thus, the phase separation was promoted further. In addition, the continuous volatile [NH.sub.3] and [CO.sub.2] was helpful to prevent the polymer rich phase to aggregate and form more pores. The combined action of volatile [NH.sub.3], [CO.sub.2], and [H.sub.2]O made the surface microstructure of the formed LDPE coating more rougher and complicated, which is helpful to trap more air and obtain a superhydrophobic coating with low sliding angle.

To know the anti-icing of the superhydrophobic LDPE coatings, the as-prepared smooth LDPE coating, superhydrophobic LDPE coating with high sliding angle obtained by evaporating solvents of LDPE solutions at 30[degrees]C, and superhydrophobic LDPE coating with low sliding angle obtained by adding some [NH.sub.4][HCO.sub.3] into the LDPE solution were all investigated in a climatic chamber with a working temperature of -5[degrees]C. We first horizontally fixed the samples on the stage of the climatic chamber for 30 min to make them as cold as the natural outdoor equipments in ice storm, then the ice accumulation on the samples was studied by spraying super-cooled water droplets with diameter about 1 mm via sprinklers above the samples 20 cm. On the smooth LDPE coating, it was quickly covered by separate water droplets at the first minute of spraying, and a whole ice film appeared only after 5 min of spraying. On the superhydrophobic LDPE coating with high sliding angle, the water droplets were pinned on the surface and quickly transformed into ice and other water droplets preferred to accumulate around the original ice crystal, and the superhydrophobic LDPE coating with high sliding angle was completely covered by the ice after 7 min of spraying. Interestingly, the sprayed water droplets quickly rolled off the superhydrophobic LDPE coating with low sliding angle, and no ice crystal or ice cake appeared on the superhydrophobic LDPE coating with low sliding angle even after 60 min of spraying. The increased weights caused by icing on the tested samples are shown in Fig. 3. The weight of smooth LDPE coating and superhydrophobic LDPE coating with high sliding angle shows linear increase within the experimental time. However, the increased weight of the superhydrophobic LDPE coating with low sliding angle is almost zero within experimental error. Thus, in comparison with the smooth LDPE coating and superhydrophobic LDPE coating with high sliding angle, the superhydrophobic LDPE coating with low sliding angle can effectively mitigate the ice accretion. However, compared with the smooth LDPE coating, the superhydrophobic LDPE coating with high sliding angle did not show obvious advantage for anti-icing. It means that the sliding angle is very important for the anti-icing property of superhydrophobic materials or coatings. That is, if the sliding angle is smaller for small water droplets, the anti-icing of the superhydrophobic LDPE coating will be better. These above results are very useful and important for the design and preparation of outdoor engineering materials and equipments with excellent anti-icing property in practical applications.

For the above phenomena and results obtained in the experiment of anti-icing, we can explain them as follows: When super-cooled water droplets are sprayed onto the superhydrophobic LDPE coating with low sliding angle, the water droplets are hard to rest on the coating and quickly roll off the surface due to the low adhesion and limited contact area between water droplets and lotus-leaf-like superhydrophobic LDPE coating with low sliding angle. Thus, ice is hard to form on the lotus-leaf-like superhydrophobic LDPE coating due to the lack of water even though the temperature is low enough. For superhydrophobic LDPE coating with high sliding angle, many sprayed water droplets are pinned on the coating even though some large water droplets may roll off the sample under the gravity, which result in the ice formation on the sample within short time. When super-cooled water droplets are sprayed onto the smooth LDPE coating, the water droplets spread on the sample. Thus, the contact area and adhesion between smooth LDPE coating and water droplets are larger than that of lotus-leaf-like superhydrophobic LDPE coating with low sliding angle. Therefore, the movement of water on the smooth LDPE coating is relatively hard and far slower than that of lotus-leaf-like superhydrophobic LDPE coating with low sliding angle, which make the water quickly transform into ice on the smooth LDPE coating under the low temperature.

CONCLUSIONS

In conclusion, smooth and superhydrophobic coatings with low and high sliding angle were all prepared by simple methods. The ice accumulation on different LDPE coatings was investigated in an artificial climatic chamber. Compared with the smooth LDPE coating, the superhydrophobic LDPE coating with high sliding angle did not show obvious advantage for anti-icing. However, the lotus-leaf-like superhydrophobic LDPE coating with low sliding angle exhibited excellent anti-icing property, which is mainly attributed to the combined action of high contact angle and low sliding angle. It indicates that the ultra-low water sliding angle is necessary for superhydrophobic anti-icing coatings or materials. This work will provide a new way to fabricate anti-icing coating and thus find applications in a variety of fields.

ABBREVIATIONS

LDPE Low-density polyethylene

SEM Scanning electron microscopy

Correspondence to: Dr. Zhiqing Yuan; e-mail: byxy2001@l63.com Contract grant sponsor: Natural Science Foundation of China; contract grant number: 51103036; Contract grant sponsor: Youth Foundation of Hunan Educational Committee; contract grant number: 11B035; Contract grant sponsor: Science Foundation of China; contract grant number: 20090461482; Contract grant sponsor: Natural Science Foundation of Hunan province; contract grant number: 10JJ4033; Contract grant sponsor: Science and Technology Planning Project of Hunan Province; contract grant number: 201ORS4011.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Zhiqing Yuan, (1) Jiping Bin, (1) Xian Wang, (1) Qilong Liu, (1) Dejian Zhao, (1) Hong Chen, (2) Haiyun Jiang (1)

(1.) School of Packaging & Materials Engineering, Hunan University of Technology, Zhuzhou, Hunan 412007, People's Republic of China

(2.) Central South University of Forestry & Technology, Changsha, Hunan 410004, People's Republic of China

DOI 10.1002/pen.23184
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Author:Yuan, Zhiqing; Bin, Jiping; Wang, Xian; Liu, Qilong; Zhao, Dejian; Chen, Hong; Jiang, Haiyun
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Nov 1, 2012
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