Robust Superhydrophobic Surface on Polypropylene with Thick Hydrophobic Silica Nanoparticle-Coated Films Prepared by Facile Compression Molding

: Superhydrophobic surfaces have been extensively studied for their unique interfacial interaction between water and the surface, and they can be used for self-cleaning, drag reduction, anti-icing, and other applications. To make the superhydrophobic surfaces, nano/microscale structures and a low surface energy should be realized. The development of a durable superhydrophobic surface was hindered by the vulnerability of the surface to mechanical contact. To improve the robustness of the superhydrophobic surface toward mechanical damage, the hydrophobic polypropylene (PP) surface was coated with a thick layer of hydrophobic silica nanoparticles (SNPs) using a simple compression molding process. The thick layer consists of SNPs and PP, and the roles of SNPs and PP are nano/microscale structures with a low surface energy and binder for nanoparticles, respectively. This revealed improvement in the superhydrophobic tendency, with an apparent contact angle of about 170 ◦ and a sliding angle of less than 5 ◦ . The morphology and the corresponding elemental analysis of the PP/SNPs coated ﬁlms were investigated using ﬁeld emission scanning electron microscopy and energy-dispersive spectrometry. The mechanical durability of the superhydrophobic surface was evaluated by the scotch tape test and scratch test with sandpaper. The coated ﬁlms with SNPs showed the superhydrophobic behavior after 25 tape tests. In addition, the coated ﬁlms with SNPs showed a contact angle greater than 150 ◦ and a sliding angle less than 10 ◦ after a 100-cm scratch test with 1000 grit sandpaper, under a weight of 500 g, on an area of 40 × 40 mm 2 . The chemical stability of PP/SNPs coated ﬁlms was also investigated in acidic, neutral, and alkaline medium solutions. The ﬁlms showed good stability under the acidic and neutral medium solutions even after 24 h, but an alkaline medium could damage the surface. The obtained results demonstrated the robustness of the superhydrophobic coating with SNPs.


Introduction
Superhydrophobic surfaces have been extensively studied for their unique interfacial interaction between water and solid surface, and the superhydrophobic surface was defined by a water contact angle (CA) greater than 150 • , and sliding angle (SA) less than 10 • [1,2] with numerous special properties, such as self-cleaning [3][4][5], anti-icing [2,6], water-proofing [7] and anti-fouling [4,5,7]. These properties may come from the minimum contact area of the water-solid interface as well as the trapped air at the interface, and they can control the material and energy transport at the interface. Researchers have confirmed that the superhydrophobic surface is produced by this combination of nano/microscale surface structures and surface materials with low surface energy [8]. The superhydrophobic surface is an evolutionary adaptation in nature and these surfaces can be easily observed from nature such as lotus leaves and butterfly wings, wherein superior non-wetting is achieved [9]. T. Onda et al. [10] reported the first artificial superhydrophobic surfaces in 1995. Since 1995, many researchers have reported many creative ways to make rough surfaces with low surface energy.
The compression molding is one of commonly used manufacturing processes in industry, and the fabricated micro-nano structures on thermoplastic polymer materials make the surface more durable than a coated surface [18][19][20]. There are several polymer materials used in the compression molding process such as polyvinylidene fluoride [7], polystyrene [21], polypropylene [19], and silicone rubber [18]. Thermoplastic polypropylene (PP) is widely used due to its lightweight and non-toxicity properties. Additionally, the superhydrophobic surface on PP can be easily achieved by softening upon heating and hardening by cooling, or using a solvent due to its hydrophobicity [7,22,23].
Hydrophobic silica/polymer composites were used to fabricate a superhydrophobic coated surface. Several studies have been reported on the preparation of superhydrophobic coated surfaces with polymer materials, such as SiO 2 /polystyrene [13,16] and SiO 2 /polyvinyl chloride [13]. This SiO 2 /polymer composite coating exhibits a strong potential for industrial applications. Silica nanoparticle/polymer composite coated surfaces were prepared by spin-coating [13] and radiation polymerization [16] techniques. In addition, PP and silica nanoparticles (SNPs) have been used for superhydrophobic surfaces. Hydrophobic silica coated superhydrophobic PP membrane fabricated by dip coating was used for oil-water separation [24]. Superhydrophobic PP/silica coating using drop casting of PP and silica solution was used for cell adhesion control [25]. The practical use of superhydrophobic nanoparticle/polymer composite surfaces is severely limited by poor mechanical durability. All-organic coatings have good chemical stability but poor mechanical durability. Recently, researchers have focused on the durability improvement of superhydrophobic surfaces [9]. Some researchers have reported mechanical durability by using scratch tests on sandpaper. Metal polymer composites [26,27], etched metal substrates with low surface energy [28,29], and hydrophobic nanoparticles with polymer [30][31][32] have been studied for robust superhydrophobic surfaces, and they showed good superhydrophobicity after scratch testing. However, the usage of toxic chemicals [26][27][28]32] and relatively long process time [29] were limitations.
In this research, a facile compression molding was used for fabrication of superhydrophobic surfaces on PP with silica nanoparticles in order to overcome the poor durability of superhydrophobic coated surfaces. Then, the surface morphology of the fabricated coated films was evaluated by field emission scanning electron microscopes (FE-SEM), and the coating thickness was analyzed using energy-dispersive spectrometry (EDS) lines. Meanwhile, the durability of the fabricated PP/SNPs coated films by scotch tape test and sandpaper test were investigated. The chemical stability of the fabricated superhydrophobic coated surface was tested with medium solutions having different pH values [33,34]. The results showed robust superhydrophobic coated films with the merits of self-cleaning and water bouncing.

Materials and Methods
For sample preparation and chemical stability test, commercially available homopolymer polypropylene (Hopelen, SJ-170T, Lotte Chemical Corporation, Seoul, Korea), hydrophobic silica nanoparticles (specific surface area 120 m 2 /gm, fumed silica K-D15, OCI Company, Seoul, Korea), and medium solutions (buffer solutions with pH 2, pH 7, and pH 13., Daejung Chemicals & Metals CO.LTD., Siheung, Korea) were purchased and used as received. The pH 2 solution, pH 7 solution, and pH 13 solution were produced by hydrochloric acid, water, and sodium hydroxide, respectively. Superhydrophobic coated films were prepared by a compression molding process with a heating press, and the sample size was 40 × 40 mm. A heating press machine (D1P-25J, Dae Heung Science., Incheon, Korea) was used in the compression molding process to fabricate the superhydrophobic coated surface. A mold with a cavity size of 40 × 40 × 10 (L × W × H) mm was used for molding of the pellet and powder materials.

Fabrication of Superhydrophobic PP/SNPs Coated Films
A schematic diagram of the fabrication processes is shown in Figure 1. The superhydrophobic surface was fabricated by polypropylene and hydrophobic silica using a compression molding process. The flat hydrophobic surface was prepared by PP pellets with an average size of about 2 to 3 mm. The flat PP plate was prepared via a compression molding process using the heating machine. The PP pellet was put into the cavity of the mold. The material was placed in the mold cavity; the mold was installed on the lower platen of the heating press machine at 180 • C for five minutes. Subsequently, the mold was pressed using a hydraulic pump until the required pressure (from 7.5 to 8 MPa) was achieved. The upper and lower platens were kept closed to allow the pellet PP to completely cure. When the compression molding was finished, the mold was taken out and was cooled at ambient temperature for 30 min. The superhydrophobic PP/SNPs films were prepared using the heating press machine under pressure via a compression molding process. The fabricated polypropylene plate was placed inside the mold cavity. The hydrophobic silica powder was put into the cavity on the PP plate; the mold was placed on the lower platen of the heating press machine at 160 • C for five minutes. Subsequently, the two platens were pressed closer together using a hydraulic pump until the desired pressure (from 7.5 to 8 MPa) was reached. This pressure was repeated one time. The platens were kept closed to allow the PP/SNPs films to achieve complete compression. When the cycle was complete (13 min), the platens were opened and allowed to cool at ambient temperature. After 40 min of cooling, the fabricated samples were detached from the mold. Figure 2 shows the fabricated PP/SNPs films and images of original materials. To study the effect of hydrophobic silica nanoparticles, three relative weight ratios of SNPs such as the maximum weight for full mold cavity, a third, and a ninth were used. The detailed process parameters for the compression molding process of the hydrophobic silica nanoparticles are summarized in Table 1.  Relative weight ratio of hydrophobic silica (Maximum weight for molding cavity: 0.53 g) 100% 33% 11%

Characterization of Surface Morphology and Wettability
The surface morphologies of the fabricated superhydrophobic PP/SNPs coated films were evaluated with field emission scanning electron microscopy (JEOL, JSM-6500F, Tokyo, Japan). The chemical composition of the fabricated superhydrophobic coated films was determined by energy-dispersive spectrometry using the same instrument. Three different positions were observed to evaluate the elemental composition of three samples with different weights of SNPs.
The CA and SA of the prepared PP/SNPs coated films were measured with a contact angle meter (SDL200TEZD, FEMTOFAB, Seoul, Korea) at room temperature. The measurement of CA and SA for all samples was carried out five times in order to obtain reliable results. The volume of a water droplet was 10 microliters for the CA and SA.

Mechanical Durability
The scotch tape test and scratch test with sandpaper were carried out for the mechanical durability of the superhydrophobic surface with PP/SNPs coated films. The tape test was conducted along the test method B of ASTM D 3359-09. The scotch tape (Scotch-550, 3M, Seoul, Korea) was attached to the superhydrophobic PP/SNPs coated films, and then the tape was removed. A thin eraser was applied with a pressure of 6.1 kPa (1 kg weight on 40 × 40 mm of area) to ensure good contact was made between the composite coated films and scotch tape [6]. The second mechanical durability test of the fabricated samples was evaluated via a sandpaper abrasion method [35]. The superhydrophobic surface was placed on the 1000 grit sandpaper (CC-1000 Cw, Daesung Abrasive Co. LTD, Seoul, Korea) with a pressure of 3.1 kPa (500 g weight on 40 × 40 mm of area). The fabricated PP/SNPs coated films were moved by 10 cm followed by 90 • rotations, and the procedure was repeated. The CAs and SAs were measured every cycle of the scratch test.

Chemical Stability and Aging Stability
Chemical and aging stabilities were studied on the fabricated superhydrophobic surface after about 300 days. First, the surface wettability of the fabricated PP/SNPs coated films was observed after 300 days storage at ambient condition. Consequently, a chemical stability test was carried out to evaluate the durability of the superhydrophobic PP/SNPs coated films at room temperature using acidic, neutral, and alkaline medium solutions. The prepared superhydrophobic PP/SNPs coated films were placed with the medium solutions, and the CAs and SAs were measured at different time intervals. Each sample was tested with the medium solutions for 0, 1, 3, 6, 9, 12, and 24 h immersion. At the time intervals, the fabricated PP/SNPs coated films were taken out from the medium solutions, the surface was dried at room temperature for one hour, and the CAs and SAs were measured.

Wettability
The wettability of the fabricated PP/SNPs coated films was examined by CA and SA measurements, as shown in Figure 3 (Video S1). The coated films exhibited good superhydrophobicity when the compression pressure was between 7.5 and 8 MPa with a heating temperature from 160 to 180 • C. The CA was about 175 • (Figure 3a) and the SA was about 3 • . The rough surface structures and surface energy determined by chemical compositions are important for the extreme wettability. Herein, the addition of the hydrophobic silica nanoparticles could increase the surface roughness by making nano/microscale structures, and the hydrophobic coating on silica could reduce the surface energy. Consequently, superhydrophobic PP/SNPs coated films with a high CA have been obtained [13].

Surface Morphology
The surface morphology of the coated films was analyzed using FE-SEM. The coated films with SNPs having a 100% weight ratio during the compression molding process were used. As shown in Figure 4, nanoscale silica particles covered the entire surface, and random microscale rough structures were observed. The surface morphology was not changed with different relative weight ratios of SNPs. These nano-micro hierarchical structures with hydrophobicity may make the surface superhydrophobic. Additionally, the cross section of the fabricated superhydrophobic PP/SNPs coated films were observed to evaluate the thickness of the films. To determine the effect of SNP weight difference, the PP/SNPs coated films with different relative weight ratios of SNPs, such as 11%, 33%, and 100%, were evaluated. The result showed that different thicknesses of fabricated superhydrophobic coated films were obtained according to the different relative weight ratio of SNPs, as shown in Figure 5. The results showed that the thickness of the coated film with relative SNP weight ratios of 11%, 33%, and 100% were from 13 to 25 µm, 13 to 29 µm, and 15 to 34 µm, respectively. As the weight of SNPs increased, the overall thickness of the coated film increased. Additionally, the line EDS was used for the chemical composition on the PP/SNPs coated film. As shown in Figure 6, the PP/SNPs coated film with 100% relative weight ratio of SNPs is composed of carbon, oxygen, and silicon elements, and the chemical compositions of the film were clearly different from the base PP layer and the mounting material layer. All the samples with different weight ratios showed similar results.

Mechanical durability
The durability of the superhydrophobic surface is a very crucial feature for practical usage. Mechanical durability is the major challenge for superhydrophobic surfaces. In this study, mechanical durability of the fabricated superhydrophobic PP/SNPs coated films was characterized using the scotch tape test and scratch test with sandpaper.
First, the scotch tape test was carried out with samples with different relative SNP weight ratios of 11%, 33%, and 100%. After the scotch tape test, the CA and SA were observed. The test schematic diagram and five different positions for the CA and SA measurements are shown in Figure 7. The scotch tape test of the fabricated PP/SNPs coated films is shown in Video S2. Adhesion and peeling of tape were repeated for the coating degradation. The first scotch tape test did not affect the CA and SA, and this result showed the film's robustness. The CA and SA results after the tape test for the fabricated superhydrophobic coated films with different relative SNP weight ratios of 11%, 33%, and 100% were summarized in Figure 8. The average values were used, and the error bars in Figure 8 indicate the minimum and maximum values, and a 90 degrees value in SA means that the water droplet was attached on the surface even with tilting of 90 degrees. The scotch tape test results showed that the coated films could resist scotch tape peeling with results of 25 repetitions for 100%, 20 repetitions for 33%, and 10 repetitions for 11% for superhydrophobicity with the average CAs greater than 150 • and average SAs less than 10 • . From the scotch tape-peeling test, the fabricated superhydrophobic surface with the PP/SNPs coated films showed good mechanical durability, and the durability could be increased by increasing the thickness of PP/SNPs film.  Second, a scratch test was performed on the superhydrophobic surface of PP/SNPs coated films. Figure 9 displays the schematic diagram of the scratch test with sandpaper and five positions of CA and SA measurements after the scratch test. The scratch test with sandpaper was performed up to the travel distance of 460 cm, and the surface did not show any sliding angle after the travel distance. The scratch test of the PP/SNPs coated film with a 100% relative weight ratio of SNPs was demonstrated (Video S3), and Figure 10 shows the CA and SA results after the scratch test with a 20-cm travel distance. The average CA was less than 150 • after a travel distance of 380 cm and the average SA became greater than 10 • after a travel distance of 120 cm. Additionally, the scratch test with sandpaper proved that the fabricated superhydrophobic PP/SNPs coated film exhibited good mechanical durability. The change of wettability and the film surface under scratch test was also observed, as shown in Figure 11. The surface of the as-prepared sample was white, and there was almost no damage for the first scratch test cycle (10 cm distance). From the second scratch test cycle (20 cm distance), the color change was clearly observed because of gradual wear of PP/SNPs film. Until 100 cm distance in the scratch test, the sample showed high CA of about 170 • and SA less than 10 • ; even the sample surface showed a clear non-white area. As the distance in scratch tests increased, the CA decreased and the SA increased. Finally, the SA was not measured after 250 cm in the abrasion test because the PP/SNPs film was severely damaged.

Stability in Ambient Air and Chemical Stability
The fabricated superhydrophobic PP/SNPs coated films were stored under ambient condition for 10 months. Then, the surface wettability was evaluated with the CA and SA for stability in ambient air. The results showed that the average CA and SA for five samples after 10 months in ambient air were 176.9 • and 3.2 • , respectively. The CA and SA were not degraded after storage, which indicated that the fabricated PP/SNPs coated films have long-term stability in air.
The pH effect on chemical stability of the fabricated superhydrophobic PP/SNPs coated films was evaluated by immersion in medium solutions with pH 2, pH 7, and pH 13. This experiment was carried out as a function of immersion time such as 0, 1, 3, 6, 12, and 24 h under three different medium solutions: acidic, neutral, and alkaline conditions. Then, the CAs and SAs were measured. Figure 12 showed the average CAs and SAs of the three samples with three different measurement positions for each sample. For all solutions, the CA of the fabricated films gradually decreased with respect to the immersion time. Similarly, the SA gradually increased with respect to the immersion time. The CA decreased in the ranges of 176 ± 1 • and the SA increased in the ranges of 3 ± 1 • to 13 ± 3 • within 1, 3, 6, and 12 h after immersion in acidic and neutral solutions. The results showed a small change of wettability. However, the CA decreased to 43.0 • and there was no SA after 12 h immersion in the alkaline solution. The average CAs of the fabricated superhydrophobic PP/SNPs coated surface of PP/SNPs coated films after immersion in acidic, neutral, and alkaline solutions for 24 h were 166.3 • , 167.6 • , and 74.6 • , respectively. Additionally, the average SAs of the PP/SNPs coated films after immersion in acidic, neutral, and alkaline solutions for 24 h were estimated to be 12.9 • , 12.7 • , and no sliding angle, respectively. These results indicate that the fabricated superhydrophobic PP/SNPs coated films have good chemical stability for acidic and neutral solutions. However, the fabricated surface can be damaged by alkaline solution. The reason why the alkaline solution changed the wettability of the fabricated samples may be attributed to the chemical degradation of hydrophobic coating on silica nanoparticles. The dimethyldichlorosilane or polydimethylsiloxane was used in surface treatment for hydrophobicity of the silica nanoparticles [36]. This chemical structure can be degraded by hydrolysis, and hydrolysis can generate a hydroxyl group which can attack the silicon atom. The presence of hydroxyl groups makes the surface more hydrophilic. The dimethyldichlorosilane or polydimethylsiloxane can be degraded rapidly with high alkaline conditions [37].

Discussion
Hydrophobic silica nanoparticles have been widely used to produce nano-microscale structures on surfaces with low surface energy by various coating processes with binders for the realization of a superhydrophobic surface. The important barrier for the wide usage of these superhydrophobic coatings with SNPs is poor mechanical durability. Mechanical durability is the main issue limiting the application of superhydrophobic coatings [12]. Mechanical contact or abrasion causes the gradual loss of superhydrophobic coating, and a thin coating layer can be easily destroyed, thus limiting the superhydrophobicity function. To overcome this problem, we prepared the superhydrophobic coated films that had a thick coating layer with hydrophobic silica nanoparticles using compression molding. The obtained results from the mechanical durability tests revealed that the presence of thick PP/SNPs coated films successfully resisted adhesion and abrasion. The thick superhydrophobic PP/SNPs coated film with a thickness thicker than 10 µm exhibited good mechanical durability and good resistance in scotch tape and scratch tests using sandpaper. However, most superhydrophobic coatings with thin films are not suitable for resisting mechanical contact or abrasion. Therefore, contact or abrasion with small pressure could damage the surface structures and the surface lost superhydrophobicity [6]. In addition, the fabrication process is simple and uses commercially available compression molding without using toxic chemicals. All materials are commercially available and non-toxic. Moreover, there were no additional pre-or post-treatments. Therefore, the proposed fabrication method can be used widely without new equipment or materials.
To show the superhydrophobic performance, self-cleaning and water droplet bouncing were demonstrated. The self-cleaning performance of the PP/SNPs coated films was demonstrated with the sugar powder, as shown in Figure 13 (Video S4). As water droplets slide on the superhydrophobic coated surfaces, the sugar powders can be trapped by the water droplets [38]. The water droplet bounce on the fabricated superhydrophobic PP/SNPs coated films is demonstrated in Figure 14 (Video S5). The fabricated film showed good superhydrophobicity to repel the water droplets [39,40].

Conclusions
We developed a simple compression molding process for the fabrication of superhydrophobic PP/SNPs coated films. The obtained PP coated films with different SNP weight ratios exhibited superhydrophobic behavior with a high CA of about 170 • and SA less than 5 • . The scotch tape test and scratch test were used to check the mechanical durability of our fabricated PP/SNPs coated films. The results showed that PP/SNPs coated films could maintain the superhydrophobic tendency after several repetitions of tape tests and long-distance scratch tests. Moreover, increasing the SNP weight ratio in the coated films resulted in the overall improvement of mechanical durability. The surface morphology and corresponding EDS linear scan for elemental analysis showed the successful fabrication of thick superhydrophobic PP/SNPs films with nano-microscale structures having hydrophobic SNPs via a facile compression molding process. The chemical stability of the fabricated superhydrophobic coated surface was elucidated for using acidic, neutral, and alkaline solutions. The PP/SNPs coated films showed high chemical stability in acidic and neutral solutions. However, the chemical stability of the fabricated superhydrophobic coated surfaces in the alkaline solution was not sufficient. In this research, nanoscale silica particles covered the entire surface, and random rough nano-microscale structures were formed. These nano-micro hierarchical structures with hydrophobicity could make the surface superhydrophobic. The practical meaning is to confirm the feasibility of the mass-production of robust superhydrophobic surfaces with the widely used facile compression molding process using commercially available materials as purchased without the additional treatment or usage of toxic chemicals. We believe that the proposed fabrication method of superhydrophobic coated surfaces would be effective for realizing a robust superhydrophobic surface in real applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.