Preparation of Colorful, Infrared-Reflective and Superhydrophobic

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Surfaces, Interfaces, and Applications

Preparation of Colorful, Infrared-Reflective and Superhydrophobic Polymer Films with Obvious Resistance to Dust Deposition Jing Zhang, Chenxi Zhu, Jian Lv, Weicheng Zhang, and Jie Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12567 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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ACS Applied Materials & Interfaces

Preparation of Colorful, Infrared-Reflective and Superhydrophobic Polymer Films with Obvious Resistance to Dust Deposition Jing Zhang, Chenxi Zhu, Jian Lv, Weicheng Zhang, Jie Feng* College of Materials Science & Engineering, Zhejiang University of Technology, Hangzhou 310014, China *Corresponding authors. Prof. Jie Feng, E-mail: [email protected]

ABSTRACT: In recent years, polymer films containing deep color and near infrared (NIR)-reflective pigments have received much attention for their potential applications in energy saving fields. However, in practical environments, dust present in the air is easily adsorbed and adheres to the surface of these films, thus gradually reducing their NIR-reflectance. In this work, black or deep red infrared-reflective pigments were firstly mixed with melted LDPE and then the resulting composite was thermally pressed on to a metal template possessing micro- and nano surface roughness. After being cooled to a suitable temperature, the LDPE composite film was peeled from the template. Ultraviolet-visible-near infrared spectroscopy and an indoor infrared lamp irradiation test both confirmed that the prepared films exhibit high NIR reflectance and high heat reflectance. Moreover, due to the stretching-controlled

micromolding

process,

the

films

all

exhibited

a

superhydrophobic (SH) property. After incubation in outdoor conditions for one month, the NIR reflectance of the SH films remained almost consistent; however, the films that did not possess SH property showed a marked decrease in their ability to reflect NIR radiation. By a combination of SEM imaging, we conclude that our films are able to resist dust deposition and thus avoid deterioration of their infrared-reflective properties. We believe that these colorful, infrared-reflective, SH and cost-effective films have potential application for reducing energy consumption where minimal solar irradiation is required. KEYWORDS: Dark, infrared-reflective, superhydrophobic, polymer film, Anti-dust deposition 1

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 INTRODUCTION As a renewable energy source, solar energy provides a significant contribution to mitigating energy-supply problems for human society.1 However, excessive solar energy also creates various problems, for example, in summer, undesirable heat generated from solar irradiation seriously increases building air-conditioning energy consumption. 2, 3 In China, the interior temperature of a car under summer sunshine for 2 hours can reach 75 oC. Many methods have been developed to improve the thermal insulation efficiency of buildings or cars for making the interior cool. For example, hollow glass balls have been widely used in the exterior wall of buildings.4 Porous polyurethane pieces have been utilized in the interior of cars. However, these strategies are both passive methods. Compared with heat insulation, the ability to reduce the absorption of solar radiation from the roof or body of a car, i.e., make the surface temperature as low as possible, is more important. Several hundred years ago, people found that clothes and roofs that were either white or a light color were cooler than their black counterparts. This is because white possesses the best solar-reflective performance.3 As a result, roofs possessing a waterproof function are generally covered by an argentate aluminum piece. The tents on ground track field stand are generally designed white. However, white or light-colored roofs or tents are readily contaminated by dust in the air, which gradually abates their NIR-reflectivity. Recently, pigments with deep color and high infrared reflectivity were developed by doping metal oxides with transition metals or rare-earth metals.5-7 Polymer coatings

8, 9

or films

10-14

with high infrared reflectance

were thus created by mixing resin with these colorful pigments. However, in practical applications, dust in the air is easily adsorbed and adheres to their surface, which may lead to a gradual deterioration in NIR-reflectivity.15-17 It is well-known that superhydrophobic (SH) surfaces possess self-cleaning properties, generally realized by the cleaning action of rain water.18 There are a 2


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number of methods that can be employed to make the surface SH, with the most popular being the template method

19

and wet chemical etching

20-22.

With regards to

the polymer, template replica molding may be the most facile and convenient method for obtaining surface superhydrophobicity. In our early studies,23, 24 we successfully prepared polyethylene (PE) SH films by using a stretching-controlled micromolding process, with a PDMS stamp from fresh lotus and wet chemically-etched metal surfaces, as templates. This process may integrate with the melt-flow casting process, a widely employed technique for manufacturing polymer films on a large scale. In this work, we prepared a series of polymer films that possess both an infrared-reflective property and a surface SH property. In brief, dark or deep red infrared-reflective polymer composites were firstly prepared via melt-blending polymers with the corresponding colored infrared-reflective pigments. The composites were then thermally pressed on stainless steel surfaces which were chemically etched in advance. After cooling to a suitable temperature, e.g., lower but near the melting point or free-flowing temperature of each polymer, the composite films were peeled from the template. As expected, the majority of these films simultaneously exhibited excellent infrared-reflective and surface SH properties. Moreover, the SH property provides a surprising new function, e. g., in weather without rain, almost no deterioration in infrared-reflectance was observed after these films were exposed to outdoor conditions over one month. This demonstrates that our films can resist dry dust deposition. We believe that these colorful, infrared-reflective, surface SH and cost-effective films may be widely used in cooling roofs, tents, car bodies and even bicycle seats.

 EXPERIMENTAL SECTION Materials Infrared-reflective pigments ("cool cold" pigment, Black 30C941, Brown 30C888) were purchased from Shepherd Color Company (USA); Low density polyethylene 3



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(LDPE, N220), High density polyethylene (HDPE, SH1200) and Polypropylene (PP, PPH-T03) were all supplied by Sinopec Shanghai Petrochemical Co. Ltd. Polyolefin elastomer (POE, 8440) was purchased from Dupont Dow Company. Stainless steel (GB0Cr18Ni9) was purchased from Shanghai Aofeng Metal Products Co. Ltd. FeCl3· 6H2O and H3PO4 were supplied by Sinopharm Chemical Reagent Company. Hydrochloric acid (HCl) and H2NCSNH2 were bought from Ningbo Chemical Reagent Company. Others pigments and particles were on general sale. All the chemicals were chemical grade and used directly after being received. Preparation of infrared-reflective polymer composites The LDPE and "cool cold" black pigments were blended in a HAAKE torque rheometer (Polylab A, Thermo electron corporation) at 130 oC for 10 min with a rotor speed of 45 rpm. The "cool-cold" black pigment content was 0-5.0 wt% (all compared against the mass of LDPE). Other type of polymers, such as HDPE, PP and POE, and other type of pigments, such as deep-red "cool cold" pigment, white nano TiO2 nanoparticles and carbon black, were also applied according to a similar formulation and process with the only differences being the melting or flowing temperatures. For example, the blending temperatures with HDPE, PP and POE were 160 oC, 180 oC and 85 oC, respectively. Fabrication of Stainless steel Template The stainless-steel templates were fabricated using a similar process to that described in the corresponding reference.24 Briefly, the mechanically polished stainless-steel plates, with an area of 15 cm x 20 cm and thickness of 2 mm, were cleaned ultrasonically in acetone for 0.5 h. Then they were immersed in an etching aqueous solution consisting of 500 g/L FeCl3, 200 g/L HCl，100 g/L H3PO4 and 10 g/L H2NCSNH2 at room temperature for 1 h. Finally, the etched stainless-steel plates were thoroughly washed with water and dried by N2 for use as templates in the following replica micromolding experiment. 4


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Stretching-controlled micromolding process The stretching-controlled micromolding was carried out according to the procedure described in the corresponding reference.24 Briefly, LDPE/"cool cold" black pigment composite was first thermally pressed into a piece with area 15 x 20 cm2 and thickness 1 mm by using two smooth glass plates as templates (150 oC, 2 min). Then the resulting composite piece was thermally pressed on the above stainless-steel template (150 °C, 10 min, 3.5 kPa pressure). After being cooled to 80 oC,

the composite films were quickly peeled off from the templates with the aid of

pinchers. Other types of composite films were molded in the same manner except for a difference in peeling temperatures. For example, the peeling temperatures to HDPE, PP and POE were 100 oC, 120 oC and 60 oC, respectively. Characterization The surface morphology of films was scanned by a field-emission scanning electron microscopy (F-SEM, NovaNano450, FEI). The contact angle (CA) and sliding angle (SA) of the film surfaces were measured by a contact angle meter (OCA35, Dataphysics, German). A 4 μL droplet was used. The CA and SA values are the averaged values of 5 different measuring points on each surface. To confirm whether the LDPE SH films possess self-cleaning ability, we first prepared "dirty water" by mixing 100 mL water with 2 g dust collected from the outdoor windowsill, and then soaked the LDPE SH films (5.0 wt% “cool cold” black pigment) in the "dirty water". After different immersion periods, the films were taken out and their CAs and SAs were measured. We measured the surface mechanical performance of the prepared LDPE films by dragging a piece of white A4 printing paper on films under poises with weight of 10, 50, and 100 g (corresponding the pressure at 1 kPa, 1.9 kPa, and 2.6 kPa, respectively). One single journey (right or left) was calculated as 1 time of abrasion (see supporting information Scheme S1).

5



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The infrared reflectance spectra of the prepared polymer composite films were analyzed using a UV-visible-infrared spectrometer (Lambda750, PE company, USA). To simulate applications, the films were completely covered on a ringent polystyrene (PS) foam box (with width, length and depth at 152015 cm, wall thickness at 2 cm) under 275-Watt infrared lamp radiation (Scheme 1), recording the changes of their surface temperature and the interior temperature of the box. 1. Infrared lamp 2. Polymer film 3. PS foam box 4. Foil paper 5. Thermometer

1 40 cm

2 3 4 5

Scheme 1. Device for testing the infrared-reflective effect of the polymer films

To test the stability of the infrared reflectance, the as-prepared films were placed on an outdoor windowsill for 1 month in which there was no rain, and their infrared reflectance was measured after 7, 14 and 30 days incubation. Changes in the surface morphology of the films after 30 days of outdoor incubation were also checked by F-SEM. The wind speed during outdoor incubation was approximately 1-2 m/s, which is a gentle breeze. The dust on the outdoor windowsill is hydrophilic. To demonstrate that the matter covering the LDPE films was dust, ATR-FTIR (Thermal fisher Nicolet 6700) and energy dispersive spectrometer (EDS, Oxford X-MaxN80) data of the LDPE/"cool cold" black films (2.0 wt%) before and after 30 days of outdoor exposure were collected. As a control, the FTIR of dust collected from the outdoor windowsill was also measured. To thoroughly clarify the mechanisms of deposition and anti-deposition of dust on the SH surface, we selected three types of contaminant models, i. e., hydrophilic nano-silica with diameter of 70 6


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nm, hydrophilic CaCO3 with diameter of 12 μm, and hydrophobic CaCO3 with diameter of 1-2 μm. In a closed and top transparent carton box (55 cm × 30 cm × 35 cm), we subjected them to an air blower with speed of 15 m/s, and then allowed them to homogeneously fall on the LDPE/"cool cold" black films (2.0 wt%) with SH and general surfaces (see supporting information Scheme S2). Finally, after being blown at a speed of 1 m/s for 2 h, changes in weight were recorded over different times using an electronic balance. The real diameters of the trial particles were confirmed by dynamic light scattering (Brookhaven Instruments, NanoBrook Omni).

 RESULTS AND DISCUSSION Morphology and wettability of the LDPE films The SH property of a solid surface can be measured quite simply by dropping water on the surface. Thus, once the LDPE films were peeled off from the stainless-steel templates, they were rinsed with water. The results showed that the LDPE films, irrespective of whether they contained any pigments (including “cool cold” black and deep red pigments, carbon black and white TiO2) or not, all exhibited typical SH properties. Water wettability measurements showed that the CAs of the micromolded films are all larger than 152 and the SA values all less than 4 (Figure 1). LDPE general films prepared using a smooth glass template had a CA of 108 and SA larger than 70. It is well known that the SH property originates from surface micro-roughness and low surface energy. Here, there should be no change in the surface energy after the micromolding process. Thus, the surface roughness contributes predominantly to the SH property. Figure 2 A/B shows the surface morphology of the stainless-steel template used in the micromolding process. It clearly shows that the chemical etching results in a large number of micro-, submicro and even nano structures appearing on the stainless-steel surface. Although the holes on the template surface are far less deep than those produced by replica-molding from fresh lotus leaf,23 the stretching between the LDPE 7



and template microstructures endows the resulting LDPE film surface with homogeneous micro and nano structures with a high aspect ratio (Figure 2 C/D and E/F); The height of the micro-/ submicro structures is approximately 10-20 μm and the aspect ratio is about 5-10 (see supporting information Figure S1). It is these rough structures that provide the SH property. Furthermore, due to their low content and being imbedded within LDPE, pigment filling has no negative influence on the micromolding process and the resulting LDPE films’ superhydrophobicity (Figure 1), even though the pigments are hydrophilic. Although only flat metal templates possessing small areas were used here, the micromolding process can be easily expanded to the steel roller applied in the melt-flow casting process for preparing LDPE films on a large scale. 160

CA () SA ()

155 150

10 8 6

145 4

140

2

135 130

SA ()

CA ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

1

2

3

Pigment contents (wt%)

4

5

0

Figure 1. CA and SA of the micromolded LDPE films with different "cool cold" black pigment content.

Self-cleaning ability and superhydrophobicity stability The infrared-reflective LDPE film should possess a self-cleaning ability, otherwise dust that adheres to the film surface would negatively influence infrared reflectivity. Figure 3 gives the CA and SA of the LDPE SH films soaked in "dirty water" over different periods of time. It can be seen that even after 10 h, the LDPE films still 8


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possessed superhydrophobicity. In another experiment, the white nano SiO2 was dropped homogeneously on to the black LDPE SH films, then the film surface was rinsed with flowing water. The results showed that all the “dust” could be rinsed away (see Supporting Information Movie 1). These results demonstrate that the SH and infrared-reflective LDPE films possess self-cleaning ability, and that the SH property is retained even after prolonged contact with water.

B

A

100 μm

C

10 μm

D

10 μm

1 μm

F

E

10 μm 10

1 μm

μm 9



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Figure 2. SEM images of stainless-steel template (A/B), micromolded pure LDPE film (C/D) and micromolded LDPE composite film containing 5.0 wt% “cool cold” black pigment (E/F). B, D, F are magnified images of A, C, E, respectively. The insets are profiles of 4 L water droplets on the films, and the CAs were all larger than 150.

10


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160

10 CA () SA ()

155

8 6

150 4 145

140

SA ()

CA ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

0

2

4

6

Time (h)

8

10

0

Figure 3. CA and SA of the LDPE SH films soaked in "dirty water" for different periods of time.

The abrasion resistance of the SH surface is important for their practical applications. The abrasion results show that the LDPE film surface with SH property do have limited mechanical performance. Under 1 kPa pressure, the films can resist abrasion 100 times (see supporting information Figure S2). Under 1.9 or 2.6 kPa pressure, the films can resist abrasion 40 times. The films lose their SH properties if these limits are exceeded. This may be caused by the lying down of the micro-/ submicro structures due to abrasion. For SH surfaces prepared by materials with a high modulus, its mechanical performance should be better. Moreover, with regards to the films we prepared here, no frequent wearing occurs during their practical application. NIR-reflective properties of LDPE films As the solar energy is mainly visible and near infrared light, we measured the reflectance of the LDPE films with a wavelength range from 250 to 2500. Figure 4A shows the reflectance of the LDPE SH films. Compared with the LDPE SH film containing 0.5 wt% carbon black (CB) and pure, transparent LDPE film, the LDPE SH films containing "cool cold" black pigments showed much higher reflectivities in 11



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the infrared region, especially in the range between 1000 to 1750 nm. The SH films containing 2 wt% "cool cold" black pigments possess NIR (780-2500 nm) reflectivity of up to 40%. The white LDPE film containing 0.5 wt% TiO2 exhibited the highest reflectance with regards to the whole wavelength range among all LDPE films. However, the color white is easily contaminated by environmental pollution. "Cool cold" pigments with a deep color are commonly used in many fields, such as for cooling roofs, tents, car bodies and even bicycle seats. The UV-visible-infrared transmittance of the LDPE SH films with different “cool cold” black pigment content was also measured (see Supporting Information Figure S3). It can be seen that white LDPE/TiO2 film exhibits the lowest transmittance but the transmittance of pure LDPE film reaches 90%. The SH films containing "cool cold" deep red pigments showed similar reflective properties (see Supporting Information Figure S4). As the amount of the "cool cold" black pigments increased, the infrared reflectance of the LDPE SH films increased correspondingly. Moreover, the infrared reflectivity of LDPE/"cool cold" black film (2.0 wt%) is similar to that of the LDPE/ "cool cold" black film (5.0 wt%), especially between the wavelength range of 1700 nm to 2250 nm, while both are higher than those of the LDPE/ "cool cold" black films (0.5 wt% and 1.0 wt%) across the whole infrared region. This confirms that there is a top-limit with regards to the amount of "cool cold" black pigments in improving the composite’s infrared reflectance. Therefore, in consideration of cost, the LDPE/"cool cold" black film (2.0 wt%) is the best formula. For comparison, we also measured the UV-visible-infrared reflectance of the LDPE/"cool cold" black general films (with smooth surface) to study whether the surface structure influences the reflectance of the LDPE films. As shown in Figure 4B, the reflectance of the LDPE/"cool cold" black general films showed a similar trend to that of the LDPE/"cool cold" black SH films (Figure 4A); A similar reflectance is exhibited for films containing the same "cool cold" black pigments. This demonstrates

12


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that the SH property has no negative influence on the UV-visible-infrared reflectance of the LDPE films.

90

Pure LDPE 0.5 wt% 1.0 wt% 2.0 wt% 5.0 wt% 0.5 wt% (CB) 0.5 wt% (TiO2)

A

80 70 60 50 40 30 20

90


B

80 70 60 50 40 30 20 10

10 0

100

Refletance (%)

100

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

1000

1500

2000

2500

0

500

Wavelength (nm)

1000

1500

2000

2500

Wavelength (nm)

Figure 4. UV-visible-infrared reflectance of the LDPE films with different pigment content. A: SH films; B: general films.

In order to confirm the heat reflective effect of the LDPE/"cool cold" black SH films, the films surface temperature and the interior temperature of the polystyrene (PS) foam box covered with the films under infrared lamp irradiation were measured. As shown in Figure 5A, compared with the LDPE/CB SH film, the LDPE/"cool cold" black SH films exhibited a much lower surface temperature (60 oC). This is because the LDPE/CB film is almost nonreflective and almost all the light energy is adsorbed, thus resulting in a much higher surface temperature (90 oC). On the contrary, LDPE film containing white TiO2 reflects almost all the light energy, thus providing the lowest surface temperature (35 oC). A similar trend is also observed on the surface of the LDPE/"cool cold" black general films (Figure 5B).

13



100

90


80 o

70 60 50 40 30 20 10

B

90


80 o

A

Temperature ( C)

100

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70 60 50 40 30 20

0

5

10

15

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25

30

10

0

5

Time (min)

10

15

20

25

30

Time (min)

Figure 5. Surface temperatures of the LDPE films under infrared radiation. A: SH films; B: general films.

It seems surprising that the surface temperature of pure LDPE film is the lowest among all the films, except for the white TiO2 film; this is normal because pure LDPE is transparent to both visible light and infrared light.11, 25 It adsorbs less solar energy and so its surface temperature is very low. Moreover, the surface temperatures of the LDPE films containing different amounts of "cool cold" black pigment do not correspond to the order of the infrared reflectivities (Figure 4). Among these results, the LDPE film containing 0.5 wt% "cool cold" black pigments possessed the lowest surface temperature. This is because it is highly transparent to visible and infrared light. Although its reflectivity to infrared light is the lowest, its transmittance to infrared light is the highest (see Supporting Information, Figure S3). It adsorbs less solar energy thus its surface temperature is not high. Except for this film, the higher the "cool cold" black pigment content, the lower the film’s surface temperature. Figure 6A shows the interior temperature of the PS foam box covered with LDPE SH films. Combining Figure 4 and Figure S3, it shows that lower UV, visible and infrared reflectance corresponds with a higher interior temperature, irrespective of absorbance. This is because either through absorbance or direct transmission, the radiation eventually enters the box. The absorbed UV, visible and infrared light energy increases the body temperature of the LDPE films, which is conducted to the inside of the box through the air. As a result, the interior temperature of the PS foam 14


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box covered with the LDPE SH film containing 0.5 wt% CB showed the highest value, reaching 64 oC. The LDPE SH film containing 5 wt% "cool cold" black pigments, provides a corresponding temperature of only 52

oC,

showing excellent

heat-insulating performance. Of course, the white LDPE/TiO2 SH film provides the lowest interior temperature (45 oC). A similar trend is observed for the interior of the PS foam box covered with LDPE/"cool cold" black general films (Figure 6B). 70

o

50 40 30

Pure LDPE 1.0 wt% 5.0 wt% 0.5 wt% (TiO2)

20 10

B

60

Temperature ( C)

o

70

A

60

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

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0.5 wt% 2.0 wt% 0.5 wt% (CB)

20

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Pure LDPE 1.0 wt% 5.0 wt% 0.5 wt% (TiO2)

20 10

0

5

Time (min)

10

0.5 wt% 2.0 wt% 0.5 wt% (CB)

15

20

25

30

Time (min)

Figure 6. Interior temperature of PS foam box covered with the LDPE films under infrared radiation. A: SH films; B: general films.

Stability of NIR reflectance of LDPE films It is well-known that SH surfaces possess self-cleaning performance. However, such self-cleaning performance generally depends on rinsing with rain or other sources of water. In arid regions or dry seasons, water is a scarce resource. If the SH film is able to resist dry dust deposition, durable infrared reflectance would be attained. Figure 7 shows the changes in the UV-visible-infrared reflectance of the LDPE film containing 5.0 wt% "cool cold" black pigment over 30 days of outdoor incubation. It can be seen that after this period of time, only a slight change has occurred in the LDPE SH film. However, with regards to the LDPE general film, reflectance decreases significantly after just 7 days of outdoor exposure. After outdoor exposure over 30 days, the NIR reflectance decreased by approximately 30%. Since there was no rainfall during this 30-day period, why is the change in NIR reflectance between the two types of films so different? 15



100 90

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5.0 wt% (SH) 5.0 wt% (SH) (7 d) 5.0 wt% (SH) (14 d) 5.0 wt% (SH) (30 d)

A

80 70

90 80

Reflectance (%)

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

5.0 wt% 5.0 wt% (7 d) 5.0 wt% (14 d) 5.0 wt% (30 d)

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Figure 7 UV-visible-infrared reflectance of the LDPE film containing 5.0 wt% black “cool cold” pigments after being placed outdoors for 30 days without rain. A: SH films; B: general films.

A

B

30 μm

100 μm D

C

100 μm

30 μm

Figure 8. SEM images of LDPE general film (A/B) and SH film (C/D) both containing 5.0 wt% black” cool cold” pigments after been placed outdoors for 30 days without rain. B, D are magnified images of A, C, respectively.

SEM images of the LDPE films, prior to and after outdoor incubation, provide the answer as to why there is such a large discrepancy in the NIR reflectance between 16


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these two types of films. As shown in Figure 8A/B, a large amount of dust falls on to the surface of the LDPE general film, and this dust prevents the "cool cold" black pigments from reflecting infrared radiation, thus resulting in a decrease in reflectance. However, very little dust adheres to the surface of the LDPE SH film (Figure 8C/D) and the film still retains its highly hydrophobic property. The CA decreases to 147.3±1.5 and SA increases to 20.5±2.0. However, after being rinsed with water, the surface recovers its typical SH property. For the LDPE SH film, this may be due to the falling dust being easily removed by a slight wind, thus keeping the surface clean and maintaining its reflective properties (Figure 9). Infrared ray

Wind

Infrared ray

Dust

Dust

Wind SH surface

General surface

Figure 9. Mechanism of how the SH surface resists dry dust deposition and maintains its infrared reflectance level. Wind easily removes falling dust from the SH surface.

To further demonstrate that the matter covering the LDPE films is dust, ATR-FTIR and EDS data of two types of films, before and after 30 days of outdoor exposure, were obtained (see supporting information Figure S5 and Figure S6). From Figure S5, we can conclude that the matter on the surface is indeed dust because the main characteristic adsorption peaks of the films after 30 days of outdoor exposure and the dust are the same (1025 cm-1, 1625 cm-1, 3400 cm-1). EDS results (Figure S6) further confirm this observation, i.e., after 30 days of exposure, the content of Si and Ca from the dust is much higher on the LDPE general film than on the LDPE SH film. Figure 10-12 show that the different resistance of the two types of surfaces to dry dust deposition. We can see that generally, the SH surface possesses significant 17



anti-deposition behavior to dry “dust”. However, resistance of the SH surface to dry dust deposition is different to different types of particles. The SH surface has higher deposition resistance to hydrophilic particles than to hydrophobic particles (Figure 11 and Figure 12). EDS results (see supporting information Figure S7 to S12) also confirmed this conclusion. The reason for this may be that compared to hydrophobic particles, the interactions between hydrophilic particles and the SH surface are lower. Normally, small particles, especially those with a diameter less than the gap widths of the SH surface, may be locked in place by the micro or nanostructures of the SH surface. However, here, the amount of so called SiO2 nanoparticles on the SH surface is low. This is due to the SiO2 nanoparticles aggregating into larger and very light micro-particles (with diameter of 12 m). When these particles fall on the LDPE SH surface, they are easily blown away by a slight wind (Figure S7 to S9). 20 18

LDPE SH film (2.0 wt%) LDPE general film (2.0 wt%)

16 14 2

Weight (g/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12 10 8 6 4 2 0 0

20

40

60

80

100

120

Time (min) Figure 10. Weight changes of hydrophilic nano-SiO2 falling on the LDPE/"cool cold" black films (2.0 wt%) after being blown by 1 m/s wind over different times.

18


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18 LDPE SH film (2.0 wt%) LDPE general film (2.0 wt%)

16 14

2

Weight (g/m )

12 10 8 6 4 2 0

0

20

40

60

80

100

120

Time (min) Figure 11. Weight change of hydrophilic CaCO3 particles (1-2 μm) falling on the LDPE/"cool cold" black films (2.0 wt%) after being blown by 1 m/s wind over different times.

22 LDPE SH film LDPE general film

20 18

Weight (g/m )

16 14

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 10 8 6 4 2 0

0

20

40

60

80

100

120

Time (min)

Figure 12. Weight changes of hydrophobic CaCO3 particles (1-2 μm) falling on the LDPE/"cool cold" black films (2.0 wt%) after being blown by 1 m/s wind over different times.

In addition to SH films, we also investigated the anti-dust deposition performance of epoxy SH coating (the coating was fabricated using the method described in reference 17) and found that the epoxy SH coating also possesses an excellent ability to resist dry dust deposition (see supporting information Figure S13). These results confirm that the anti-dust deposition phenomenon may be universal to SH surfaces if 19



Page 20 of 26

the particles are large enough to be suspended on the micro- or nanostructures of the SH surface and also if the particles are not too hydrophobic. In addition to LDPE, other polymers including HDPE, PP and POE, were also investigated in this study. However, compared with LDPE, HDPE and PP, the micromolding process was unable to provide a POE surface with a SH property. This may be due to the elastic characteristic of the POE, e.g., during the peeling off procedure, the microstructures of the POE are not formed easily during stretching. Furthermore, we believe that the “cool cold” pigments with other deep colors, such as blue, yellow and brown, can also be used in preparing polymer films with a SH surface and durable infrared reflective property. Recently, heat reflective coatings with SH surfaces have been reported. For example, by using epoxy resin modified with epoxysiloxane, Roppolo et al. successfully prepared an infrared-reflective, surface hydrophobic, self-cleaning and UV-cured coating.26 Sriramulu et al. created a surface SH, self-cleaning and infrared-reflective coating by controlling the surface morphology and properties of silica nanoparticles and by aggregation-induced segregation of perylene diimide (PDI).27 However, these studies only used light-colored pigments. More recently, Zhang, et al. manufactured a SH and self-cleaning solar reflective coating with blue-grey color using styrene-acrylic emulsion and functional pigments followed by grinding the coating surfaces using emery papers. 15 Wu, et. al. prepared a SH cool coating using BiOCl1-xIx coated hollow glass microsphere and polystyrene resin.

16

However, they both only measured the self-cleaning property of the coatings and have not investigated their ability to resist the deposition of dry dust. The work we reported herein may be the first time in which the anti-deposition of dry dust on SH surfaces has been reported.

20


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 CONCLUSIONS In summary, dark color, infrared-reflective polymer films with SH surfaces have been successfully prepared through melt-blending and micromolding processes. Due to the introduction of the "cool cold" pigment, the resulting composite films exhibited high infrared and heat reflectance. Furthermore, the micromolding process provides the composite films with SH properties and self-cleaning ability. More surprisingly, the SH surface is able to resist dry dust deposition. This may be the first time that such a phenomenon has been reported. As the micromolding employed here can be easily expanded to the melt-flow casting technique, we believe that the work described in this report can be readily scaled up. The colorful, infrared-reflective, SH and cost-effective films may be widely used in roofs, tents and even automotive bodies allowing for a reduction in surface temperature and lessening the need for air-conditioning in the summer.

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Self-cleaning ability of the LDPE SH films containing 5.0 wt% “cool cold” black pigment (Movie S1); Drawing test for measuring the abrasion resistance of the LDPE/"cool cold" black (2.0 wt%) film with SH surface (Scheme S1); Device for accelerating dust deposition of films (Scheme S2); SEM images of LDPE/ “cool cold” black pigment (5.0 wt%) composite SH film viewed from 45 obliquity and section (Figure S1); Changes of CA and SA of SH LDPE/"cool cold" black film (2.0 wt%) within different abrasion times under 1.0 kPa pressure (Figure S2);

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UV-visible-infrared transmittance of the LDPE SH films with different “cool cold” black pigment content (Figure S3); UV-visible-infrared reflectance of the LDPE SH films containing different “cool cold” deep red pigment content (Figure S4); ATR-FTIR of the LDPE/"cool cold" black general and SH films (2.0 wt%) before and after being placed outdoors for 30 days (Figure S5); EDS results of the LDPE/"cool cold" black general and SH films (2.0 wt%) before and after being placed outdoors for 30 days (Figure S6); EDS results of the nano-SiO2 on LDPE films (A: SH film, B: general film) being blown by 1 m/s wind for 120 min (Figure S7); SEM of the LDPE films being previously deposited with hydrophilic nano-SiO2 (A: SH film, B: general film) and then being blown by 1 m/s wind for 120 min (Figure S8); The real aggregation diameter of the so called nano SiO2 particles measured by dynamic light scattering (Figure S9); EDS results of the hydrophilic CaCO3 with diameter of 1-2 μm on LDPE films (A: SH film, B: general film) being blown by 1 m/s wind for 120 min (Figure S10); SEM of the LDPE films being previously deposited with hydrophilic CaCO3 with diameter of 1-2 μm (A: SH film, B: general film) and then being blown by 1 m/s wind for 120 min (Figure S11); The real diameter of hydrophilic CaCO3 particles measured by dynamic light scattering (Figure S12); Weight changes of nano-SiO2 falling on the surface of the epoxy/"cool cold" black coating after being blown by 1 m/s wind over different times (Figure S13).

 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 22


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Notes The authors declare no competing financial interest.

 AKNOWLEDGEMENTS Financial supports from the National Natural Science Foundation of China (No. 51172206) and from the Public Welfare Foundation from Zhejiang Province (2016C31G2020062) are gratefully acknowledged.

 REFERENCES (1) Timilsina, G. R.; Kurdgelashvili, L.; Narbel, P. A. Solar Wnergy: Markets, Economics and Policies. Renew. Sust. Energ. Rev. 2012, 16, 449465. (2) Vandenbosch, G. A. E.; Ma, Z.; Upper Bounds for the Solar Energy Harvesting Efficiency of Nano-antennas. Nano Energy 2012, 1, 494502. (3) Akbari, H.; Pomerantz, M.; Taha, H. Cool Surfaces and Shade Trees to Reduce Energy Use and Improve Air Quality in Urban Areas. Sol. Energy 2001, 70, 295310. (4) Chung, S. Y.; Han, T. S.; Kim, S. Y.; Kim, J. H. J.; Youm, K. S.; Lim, J. H. Evaluation of Effect of Glass Beads on Thermal Conductivity of Insulating Concrete Using Micro CT Images and Probability Functions. Cement Concrete Comp. 2016, 65, 150162. (5) Ianoş, R.; Muntean, E.; Păcurariu, C.; Lazau, R.; Bandas, C.; Delinescu, G. Combustion Synthesis of a Blue Co-doped Zinc Aluminate Near-infrared Reflective Pigment. Dyes Pigments 2017, 142, 2431. (6) Schildhammer, D.; Fuhrmann, G.; Petschnig, L. Synthesis and Optical Properties of New Highly NIR Reflective Inorganic Pigments Re6Mo2O15, (Re =Tb, Dy, Ho, Er). Dyes Pigments 2017, 140, 2228. (7) Gao, Q.; Wu, X. M.; Fan, Y. M.; Meng, Q. L. Color Performance and Near Infrared Reflectance Property of Novel Yellow Pigment Based on Fe2TiO5, Nanorods Decorated Mica Composites. Dyes Pigments 2017, 146, 537542. (8) Raj, A. K. V.; Rao, P. P.; Sameera, S.; Divya, S. Pigments Based on Terbium-doped Yttrium Cerate with High NIR Reflectance for Cool Roof and Surface Coating Applications. Dyes Pigments 2015, 122, 116125. (9) Meenakshi, P.; Selvaraj, M. Bismuth Titanate as an Infrared Reflective Pigment for Cool Roof Coating. Sol. Energ. Mat. Sol. C. 2018, 174, 530537. (10) Xiang, B.; Yin, X. P.; Zhang, J. A Novel Cool Material: ASA (acrylonitrile-styrene-acrylate) Matrix Composites with Solar Reflective Inorganic Particles. Compos. Sci. Technol. 2017, 145, 149156. (11) Syed, K. H.; Ahmed, M.; Ghany, A. Evaluation of PE Films Having NIR-reflective Additives for Greenhouse Applications in Arid Regions. Adv. Mater. Sci. Eng. 2013, 2013, 18.

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(12) Raj, A. K. V.; Rao, P. P.; Divya, S. Terbium Doped Sr2MO4, [M = Sn and Zr] Yellow Pigments with High Infrared Reflectance for Energy Saving Applications. Powder Technol. 2017, 311, 5258. (13) Xiang, B.; Zhang, J. A New Member of Solar Heat-reflective Pigments: BaTiO3, and its Effect on the Cooling Properties of ASA (acrylonitrile-styrene-acrylate copolymer). Sol. Energ. Mat. Sol. C. 2018, 180, 6775. (14) Soumya, S.; Mohamed, A. P.; Paul, L.; Mohan, K.; Ananthakumar, S. Near IR Reflectance Characteristics of PMMA/ZnO Nanocomposites for Solar Thermal Control Interface Films. Sol. Energ. Mater. Sol. Cells 2014, 125, 102112. (15) Yang, Z.; Xue, X.; Dai, J. G.; Li, Y. W.; Qin, J.; Feng, Y.; Qu, J.; He, Z. Y.; Sun, P. C.; Xu, L. J.; Zhang, T.; Qu, T. J.; Zhang, W. D. Study of a Super-non-wetting Self-cleaning Solar Reflective Blue-grey Paint Coating with Luminescence. Sol. Energy Mater. Sol. Cells 2018, 176, 6980. (16) Gao, Q.; Wu, X. M.; Fan, Y. M.; Meng, Q. L Novel Near Infrared Reflective Pigments based on Hollow Glass Microsphere/BiOCl1-xIx Composites: Optical Property and Superhydrophobicity. Sol. Energy Mater. Sol. Cells 2018, 180, 138147. (17) Zhang, J.; Lin, W. Q.; Zhu, C. X.; Lv, J.; Zhang, W. C.; Feng, J. Dark, Infrared Reflective and Superhydrophobic Coatings by Waterborne Resins. Langmuir 2018, 34, 56005605. (18) Wen, G.; Guo, Z. G.; Liu, W. M. Biomimetic Polymeric Superhydrophobic Surfaces and Nanostructures: from Fabrication to Applications. Nanoscale 2017, 9, 33383366. (19) Feng, J.; Huang, B. Y.; Zhong, M. Fabrication of Superhydrophobic and Heat-Insulating Antimony Doped Tin Oxide/Polyurethane Films by Cast Replica Micromolding. J. Colloid Interface Sci. 2009, 336, 268272. (20) Feng, J.; Qin, Z. Q.; Yao, S. H. Factors Affecting the Spontaneous Motion of Condensate Drops on Superhydrophobic Copper Surfaces. Langmuir 2012, 28, 60676075. (21) Feng, J.; Pan, Y. C.; Qin, Z. Q.; Ma, R. Y.; Yao, S. H. Why Condensate Drops Can Spontaneously Move Away on Some Superhydrophobic Surfaces but Not on Others. ACS Appl. Mater. Interfaces 2012, 4, 66186625. (22) Hao, Q. Y.; Pan, Y. C.; Zhao, Y.; Zhang, J.; Feng, J.; Yao, S. H. Mechanism of Delayed Frost Growth on Superhydrophobic Surfaces with Jumping Condensates: More Than Interdrop Freezing. Langmuir 2014, 30, 1541615422. (23) Feng, J.; Huang, M. D.; Qian, X. Fabrication of Polyethylene Superhydrophobic Surfaces by Stretching-controlled Micromolding. Macromol. Mater. Eng. 2009, 294, 295300. (24) Feng, J.; Lin, F. Y.; Zhong, M. Q. Stretching-controlled Micromolding Process with Etched Metal Surfaces as Templates towards Mass-producing Superhydrophobic Polymer Films. Macromol. Mater. Eng. 2010, 295, 859864. (25) Hsu, P. C.; Liu, C.; Song, A. Y.; Zhang, Z.; Peng, Y. C.; Xie, J.; Liu, K.; Wu, C. L.; Catrysse, P. B.; Cai, L. L.; Zhai, S.; Majumdar, A.; Fan, S. H.; Cui, Y. A Dual-mode Textile for Human Body Radiative Heating and Cooling. Sci. Adv. 2017, 3, e1700895. (26) Roppolo, I.; Shahzad, N.; Sacco, A.; Tresso, E.; Sangermano, M. Multifunctional NIR-Reflective and Self-Cleaning UV-Cured Coating for Solar Cell Applications Based on Cycloaliphatic Epoxy Resin. Prog. Org. Coat. 2014, 77, 458462.

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(27) Sriramulu, D.; Reed, E. L.; Annamalai, M.; Venkatesan, T. V.; Valiyaveettil, S. Synthesis and Characterization of Superhydrophobic, Self-Cleaning NIR-Reflective Silica Nanoparticles. Sci. Rep-UK. 2016, 6, 3599336002.

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For Table of Contents:

100 90 80

A

70 60 50 40 30 20 10 0

100

5.0 wt% (SH) 5.0 wt% (SH) (7 d) 5.0 wt% (SH) (14 d) 5.0 wt% (SH) (30 d)

90 80

Reflectance (%)

Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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B

70 60 50 40 30 20 10

500

1000

1500

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2500

0

500

Wavelength (nm)

Infrared ray

Wind

5.0 wt% 5.0 wt% (7 d) 5.0 wt% (14 d) 5.0 wt% (30 d)

1000

1500

Wavelength (nm)

2000

2500

Infrared ray

Dust

Dust

Wind Superhydrophobic surface

General surface

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Preparation of Colorful, Infrared-Reflective and Superhydrophobic

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