Large‐Scale, Abrasion‐Resistant, and Solvent‐Free Superhydrophobic Objects Fabricated by a Selective Laser Sintering 3D Printing Strategy

Abstract Manufacturing abrasion‐resistant superhydrophobic matters is challenging due to the fragile feature of the introduced micro‐/nanoscale surface roughness. Besides the long‐term durability, large scale at meter level, and 3D complex structures are of great importance for the superhydrophobic objects used across diverse industries. Here it is shown that abrasion‐resistant, half‐a‐meter scaled superhydrophobic objects can be one‐step realized by the selective laser sintering (SLS) 3D printing technology using hydrophobic‐fumed‐silica (HFS)/polymer composite grains. The HFS grains serve as the hydrophobic guests while the sintered polymeric network provides the mechanical strength, leading to low‐adhesion, intrinsic superhydrophobic objects with desired 3D structures. It is found that as‐printed structures remained anti‐wetting capabilities even after undergoing different abrasion tests, including knife cutting test, rude file grinding test, 1000 cycles of sandpaper friction test, tape test and quicksand impacting test, illustrating their abrasion‐resistant superhydrophobic stability. This strategy is applied to manufacture a shell of the unmanned aerial vehicle and an abrasion‐resistant superhydrophobic shoe, showing the industrial customization of large‐scale superhydrophobic objects. The findings thus provide insight for designing intrinsic superhydrophobic objects via the SLS 3D printing strategy that might find use in drag‐reduce, anti‐fouling, or other industrial fields in harsh operating environments.


Content Experimental Section
Figure S1 Characterization of the PP and HFS grains and their size distribution.                            Note. S1 Details of the 3D printed superhydrophobic wing shell and transmedia vehicles.
Note. S2 Details of the printed superhydrophobic shoe and the commercial shoe with superhydrophobic coating. the HFS content.

Table S2
Comparison of sliding angles and sizes of the printed superhydrophobic objects in this study and existing reports. Movie S10 Fracture surface test.
Movie S11 Oily residual test.
Movie S12 Half meter scaled superhydrophobic printed object.
Movie S13 Anti-wetting property of the printed plane shell.
Movie S14 The printed superhydrophobic/normal transmedia vehicles flying out of the water.
Movie S15 Mechanical wear and water droplet test of the printed superhydrophobic shoe and the commercial shoe with superhydrophobic coating.

Experimental Section
Preparation of the composite grains: Hydrophobic-fumed-silica (HFS) and compounding polypropylene (PP) particles were purchased from Evonik Industries AG and Wanhua Chemical Group Co., Ltd., respectively. Polyether block amide (PEBA) particles, polyethylene (PE) particles, polystyrene (PS) particles and polymethyl methacrylate (PMMA) particles were bought from Evonik Specialty Chemicals (Shanghai) Co., Ltd, Dongguan Suzhiyuan Plastic Group Co., Ltd., Guangdong Silver Age Sci. & Tech. Co., Ltd. and Evonik Industries AG, respectively. The as-received PP (D50 = 77 μm) has a bulk density of 0.32 g/cm 3 , a melting point of 149 ℃ and a molecular weight of ~80000, which can meet the powder spreading effect and the design of printing layer thickness. The PP/HFS composite grains with different HFS weight ratio of 0, 1, 2, 3, 4 and 5 wt% were obtained by homogeneously mixing the PP and HFS grains in a ball mill mixer with a rate of 500 rpm for 10 minutes. Other composite grains (including 4 wt% HFS) for polymer universality were prepared in the same way. Before the selective laser sintering (SLS) 3D printing, all the composite grains were sifted through 80 mesh to remove bulk flocs.
SLS 3D printing to fabricate superhydrophobic objects: All the three-dimensional models were designed using 3D max software. The 3D printing machine (HUAKE 3D S320) with a 50 W CO 2 laser was employed to print the digital models. The PP/HFS composite grains with 4 wt% HFS was chosen as the printing materials for printing a variety of complex models due to its balanced superhydrophobicity and mechanical strength. The optimal parameters for 3D printing process were as follows: the preheating temperature of 140 ℃, laser power of 29 W, laser scan velocity of 4000 mm/s and the layer thickness of 0.1 mm. In terms of other PP/HFS composite grains with different HFS weight ratio, the printing parameters except the laser power remained. The specific laser energy density can be found in Table S1 below. The SLS processing parameters for polymer universality, including PEBA, PE, PS and PMMA, were shown in Table S3. The mass fraction of all polymers was 96 wt% and the mass fraction of HFS was 4 wt%. Self-cleaning testing: Self-cleaning of dirts on the printed superhydrophobic surfaces were tested as follows. Clay, grit, sawdust or concrete debris were placed onto the printed superhydrophobic surfaces, respectively. Then, several dyed water droplets were dropped on the printed superhydrophobic surfaces to test the self-cleaning property ( Figure S8).
Specifically, the superhydrophobic sample was first held at an angle; then the viscous liquids were then dropped onto the superhydrophobic surface. The viscous liquids slipped off and there was no residue on the superhydrophobic surface, indicating that the superhydrophobic surface has a good repulsion to the viscous liquids.  Figure S16 and Movie S8), tape adhesion ( Figure S17 and Movie S9), and fracture surface ( Figure S18 and Movie S10).
1. Figure 3b shows the knife cutting test. A cutter knife (Deli 2043) was used to scratch the printed sample surface, and then dyed water droplets were dropped to verify its superhydrophobicity. After quicksand impacting, several dyed water droplets were applied to the impacted surface to verify its superhydrophobicity.

Resistance of Organic Coatings by Falling Abrasive
3. Figure S15  Then, press the file hard on the sample with the other hand and grind it back and forth for tens of cycles. Finally, drop dyed water droplets on the damaged surfaces to demonstrate its superhydrophobic stability. Figure S16 shows that the abrasion-resistant ability of the printed sample was tested by a sandpaper abrasion method. The printed sample was horizontally put onto the flocking sandpaper with different roughnesses, respectively (Flocking sandpaper, Grit No. 60, 240 and 1000 Gold Cattle). The sample was abraded 12 cm by the sandpaper under a weight of 200 g for 1000 times ( Figure S16a,b). Water contact angles were measured after the 50 th , 100 th , 200 th , 400 th , 600 th , 800 th and 1000 th abrasion tests, respectively ( Figure S16c).

The tape test was performed in reference to Standard Test Methods for Measuring
Adhesion by Tape Test (D3359-09). First, place the printed flat sample horizontally on the table. Then, stick the tape (PVC electrical insulation tape, Gongniu Group Co., Ltd.) on the sample surface and roll a weight of 500 g back and forth to make the tape fully bond with the sample (Figure S17a,b). Further, remove the tape by grabbing the free end of the tape and quickly pull it off (not jerked) at an angle of as close to 180° as possible ( Figure   S17c). Finally, a few dyed water droplets were added to the taped sample to verify the superhydrophobic effect ( Figure S17d). 7. The damaged fracture surfaces by hands ( Figure S18) were performed to demonstrate the intrinsic superhydrophobicity of the printed sample. The printed sample with 4 mm thickness was broken off forcibly, and the dyed water droplets were dropped on the fracture surfaces. The rapid traceless sliding of water droplets indicated the superhydrophobicity of the fracture surfaces.
Oily residual test: Oily residual was performed by dropping the molten paraffin with ceresin (the melting point of 52-54 ℃, Aladdin industrial corporation) on the printed sample surface, and cooling naturally for a few minutes to cure ( Figure S19a). A few dyed water droplets were dropped to test the superhydrophobicity after removing the oily residual ( Figure S19b,c).
Aging resistance test: Aging resistance test was performed by a sunlight weather-conditions meter (FY3600 + , Wenzhou Fangyuan instrument Co., Ltd), and the insolation test environment was temperature of 35℃, humidity of 40%, irradiance of 42 W/m 2 , irradiation range of 300-400 nm.                              Table S3.
Note S1. Details of the 3D printed superhydrophobic wing shell and transmedia vehicles.
The printed superhydrophobic wing shell and transmedia vehicle contain 4 wt% HFS and 96 wt% PP, and the processing parameters were the preheating temperature of 140 ℃, the laser power of 29 W, the laser scan velocity of 4000 mm/s and the layer thickness of 0.1 mm.
In order to better contrast the superhydrophobic effect, the normal wing shell and transmedia vehicle containing 1 wt% HFS and 99 wt% were printed. Their processing parameters were the preheating temperature of 140 ℃, the laser power of 14 W, the laser scan velocity of 4000 mm/s and the layer thickness of 0.1 mm.
The printed superhydrophobic wing shell and the printed normal wing shell were fixed on the left and right wings of the airplane, respectively. Then, we used a sprayer to spray the aerosolized water onto the surface of the airplane. The results showed that the superhydrophobic wing on the left had almost no water residue left, while the normal wing on the right was covered with water droplets (Figure 4c and Movie S13).
The printed transmedia vehicles were magnetically controlled to fly out of the water at a speed of ~1 m/s. The superhydrophobic transmedia vehicle kept clean during the process of flying out of the water, while the normal transmedia vehicle had water film on the fuselage during flying out of the water, and kept water residue after leaving the water ( Figure S23 and Movie S14).
Note S2. Details of the printed superhydrophobic shoe and the commercial shoe with superhydrophobic coating.
The printed superhydrophobic shoe contains 4 wt% HFS and 96 wt% PEBA, and the processing parameters were the preheating temperature of 120 ℃, the laser power of 15 W, the laser scan velocity of 2000 mm/s and the layer thickness of 0.12 mm. The commercial shoe with superhydrophobic coating was prepared by spraying superhydrophobic coatings on the commercial shoe. The superhydrophobic coating solution was obtained by ultrasonic dispersion of 1 g of HFS in 100 mL ethanol.
In order to illustrate the mechanical stability of the printed superhydrophobic shoe, a home-made shoe polishing device including of a motor, caterpillar band and three grinding units adhered on the caterpillar band was prepared ( Figure S24). The superhydrophobic effects of the printed superhydrophobic shoe remained even after 1000 th wear with sandpaper, brush and grit (Figure 4d). However, the commercial shoe with superhydrophobic coating lost its superhydrophobic effect after dozens of sanding (Figure 4e and Movie S15).