A universal, multifunctional, high-practicability superhydrophobic paint for waterproofing grass houses

Extreme poverty persists in remote and undeveloped areas where grass houses made of straw and maize leaves are common dwellings. These houses are prone to wetting and roof leakage during rain, compromising living conditions. Superhydrophobic materials offer strong water repellency and are anticipated for preparing nonwetting grass houses. However, for real-world outdoor applications, three major challenges limit broader use: mechanical weakness of micro/nanostructured coatings, chemical corrosion that degrades performance, and UV sensitivity that deteriorates hydrophobicity. Beyond overcoming these, additional functionalities such as large-area coating capability, self-healing, anti-icing, resistance to high and low temperatures, photocatalysis, and self-cleaning are desirable but have not been integrated simultaneously into a single universal coating. This study proposes an inorganic-organic superhydrophobic paint (IOS-PA) combining dual-sized TiO2 nanoparticles, epoxy resin, and PFOS to address these challenges and enable practical waterproofing of grass houses.

Prior work has shown that surface micro/nanostructures impart roughness necessary for superhydrophobicity but are vulnerable to mechanical wear. The flexible "coating + adhesive" strategy enhances mechanical durability and broad applicability. All-organic water-repellent coatings using resin polymers and PTFE nanoparticles have achieved mechanochemical robustness due to composite synergy. UV light is a prominent degradant of nonwetting surfaces; long-term anti-UV performance has been realized by integrating metal oxide photocatalysts with hydrophobic polymers such as PTFE nanoparticles, PDMS, and silicone nanofilaments. Additional functions reported individually in literature include large-area coating, self-healing, anti-icing, high/low temperature resistance, photocatalysis, and self-cleaning. However, the comprehensive integration of all these properties into a single, universal superhydrophobic coating suitable for practical outdoor use has not been reported.

Materials: Epoxy resin (ER) and curing agent (Beijing Oriental Yuhong Waterproof Technology Co. Ltd.); TiO2 nanoparticles: P25 (~21 nm) and anatase TiO2 (~200 nm, 100–300 nm) (Evonik Degussa, Aladdin); PFOS (Innochem); dyes: Nile red (Aladdin), methyl blue (MB) and methyl orange (MO) (Chinese Medicine). Grass houses made of straw and maize leaves were sourced commercially.

Synthesis of IOS-PA: 6 g ER dissolved in 30 ml acetone, stirred 15 min. Separately, 6 g TiO2 (21 nm) and 6 g TiO2 (200 nm) dispersed in 150 ml acetone, stirred 15 min. The ER and TiO2 dispersions were combined, stirred, and sonicated for two 15-min cycles. Then 1.2 g PFOS was added, followed by stirring and eight 15-min sonication cycles. Finally, 0.6 g curing agent and 0.2 g PFOS were added with two 10-min sonication cycles to yield IOS-PA. Spray coating was primarily used; brush and dip coating also applicable. A mass ratio ER:TiO2(21 nm):TiO2(200 nm) = 1:1:1 was adopted to balance superhydrophobicity, photocatalysis, and UV resistance. Twenty-four substrates were coated by spraying; large-area coating demonstrated on a PS plate (0.5 × 1 m²).

Substrates: Coated "0D" particles (MnO2, CuO, SiO2, sand), 2D carbon-based (fabric, wood, A4 paper, filter paper), 2D silicon-based (ceramic wafers, glass slides), 2D metals (Zn, Al plates), 2D polymers (PVC, PE, PS films, culture dishes), 3D shapes (tetrahedral, cylindrical, conical), porous (stainless steel mesh, sponges), and natural leaves (maple, apricot, loquat).

Mechanical robustness tests: Sandpaper abrasion on No. 320 paper with a 200 g load, sliding 10 cm at ~7.5 cm/s per cycle; water contact angle (WCA) measured with 10 μL droplets at five positions after cycles (up to 50). Sand impact: 15 g of sand (0.5–2 mm) dropped from 15 cm onto surfaces, repeated 50 times; WCA measured thereafter.

Chemical corrosion tests: Samples immersed in pH 1 solution for 4 h, pH 7 and pH 14 solutions for 8 h; WCA and rolling-off angle (RA) measured post-immersion.

Self-healing tests: Surfaces treated by O2 plasma (1 min) and boiling water (5 min). Superhydrophobicity recovery induced by heating at 150 °C for 5 min; repeated up to 10 cycles.

Anti-icing tests: 10 μL water droplets placed on fresh or coated surfaces at -10 °C; freezing process observed and time to freeze recorded. Ice mobility qualitatively assessed at room temperature.

Thermal resistance: High temperature: oven at 150 °C for 100 h; WCA and RA recorded every 10 h. Low temperature: 200 ml liquid nitrogen poured onto samples; after drying at room temperature, WCA and RA measured; repeated 10 times.

Photocatalytic activity and UV resistance: Dyes prepared as follows: Nile red in ethanol (10 μg/L); MB and MO in water (each 40 μg/L), then mixed with ethanol (water:ethanol = 2:1 v/v). 0.06 g coating particles added to dye solutions; after 30 min dark adsorption, samples irradiated with 365 nm UV (5.0 ± 0.6 mW/cm²) under stirring (500 rpm). Degradation monitored by color disappearance and UV–vis spectrophotometry; dyes also drop-cast on coated samples and irradiated. Long-term UV resistance tested under 365 nm UV (5.0 ± 0.6 mW/cm²) and under ambient sunlight from Nov. 1, 2018 to Nov. 1, 2019 at China University of Geosciences (Wuhan).

Instrumentation: FESEM (HITACHI SU8010, 20 kV) for surface nanostructures; TEM (Tecnai G2 20 S-TWIN) for ER–TiO2 composite morphology; XPS (ESCA-LAB Xi+, Thermo Fisher, Al Kα) for surface composition; XRD (Bruker D8 Advance, Cu Kα) for TiO2 crystal phases; Contact-angle analyzer (DSA100) for WCA/RA with 10 μL droplets at five positions; droplet impact/bounce tests with 10 μL droplets from 15 mm at 0.25 m/s and higher velocities (up to 3 m/s); UV radiometer (HANDY) for intensity; UV–vis spectrophotometer (SHIMADZU) for dye concentration; ESR/EPR (Bruker A300) to detect superoxide and hydroxyl radicals in ethanol after 10 min UV illumination for ER, PFOS–TiO2 NPs, and PFOS–ER–TiO2 NPs.

• IOS-PA structure and surface chemistry: Dual-sized TiO2 nanoparticles (~21 nm and ~200 nm) embedded and coated by thin ER layers (1–3 nm) confirmed by SEM/TEM. XPS showed PFOS grafted onto both ER and TiO2 (O 1s deconvolution indicating Ti–O–Ti and Ti–O–Si), lowering surface energy. XRD confirmed presence of anatase and rutile TiO2.
• Superhydrophobic performance across substrates: Textured nanostructures plus low surface energy yielded WCA > 150° and RA < 10° on diverse substrates, including 0D/2D/3D/porous materials and natural leaves. Large-area coating (0.5 × 1 m² PS) was achieved. Droplet bouncing and non-wetting under impacts were demonstrated: 10 μL droplets released from 15 mm (0.25 m/s) fully rebounded; surfaces resisted higher impact velocities (0.25–3 m/s) and remained water-repellent even under simulated raindrop velocities (~9 m/s).
• Mechanical robustness: After 50 abrasion cycles on No. 320 sandpaper under 200 g load (10 cm per cycle, ~7.5 cm/s), WCA remained >150°. After 50 sand impact cycles (15 g sand, 0.5–2 mm, drop height 15 cm), WCA remained >150°. Similar durability observed on glass, fabric, PS, wood, and ceramic substrates.
• Chemical resistance: After immersion at pH 1 (4 h), and pH 7 and pH 14 (8 h), coatings maintained high WCA and low RA, indicating anticorrosion performance.
• Self-healing: After O2 plasma (1 min) and boiling water (5 min) treatments, superhydrophobicity was recoverable by heating at 150 °C for 5 min, repeatedly for at least 10 cycles without obvious morphology changes.
• Anti-icing: At -10 °C, freezing time of 10 μL water increased from ~30 s on bare glass to ~120 s on coated glass; similar ice delay observed on coated fabric, PS, wood, and ceramic. Ice cubes rolled off easily at room temperature.
• Thermal stability: Coatings withstood 150 °C oven exposure for 100 h with stable WCA/RA (measured every 10 h). After repeated liquid nitrogen exposures (200 ml pour, 10 repeats), WCA and RA remained nearly unchanged, evidencing resistance down to -196 °C.
• Photocatalysis with UV resistance: Under 365 nm UV (5.0 ± 0.6 mW/cm²), coatings degraded organic dyes (Nile red, methyl blue, methyl orange) in solution and on surfaces, attributed to TiO2-generated superoxide and hydroxyl radicals (supported by ESR/EPR). Despite photocatalytic activity, coatings retained superhydrophobicity after prolonged UV (100 h at 365 nm) and after 1 year of ambient sunlight exposure (8640 h), enabling coexistence of photocatalysis and superhydrophobicity and self-cleaning of particulates and organics.
• Application to grass houses: A grass house coated with IOS-PA exhibited favorable waterproof properties, indicating practical utility for improving living conditions in undeveloped areas during rain.

The IOS-PA integrates dual-sized TiO2 nanoparticles with an epoxy resin matrix and PFOS surface modification, achieving a synergistic balance of micro/nanotexture and low surface energy for superhydrophobicity. ER provides strong adhesion, mechanical robustness, and chemical inertness that preserve roughness and non-wetting behavior even after abrasion and chemical exposure. PFOS grafting onto both ER and TiO2 ensures sustained low surface energy. The dual-scale TiO2 affords robust surface roughness and, under UV illumination, generates reactive oxygen species enabling photocatalytic degradation of organics. Crucially, the ER matrix and surface chemistry allow superhydrophobicity to persist during extended UV exposure, addressing the typical trade-off between photocatalysis and hydrophobic durability. These combined properties directly tackle the key barriers to outdoor deployment—mechanical wear, chemical corrosion, and UV degradation—while providing added benefits (self-healing, thermal extremes resilience, anti-icing, self-cleaning), thereby supporting practical waterproofing of grass houses and other real-world substrates.

This work presents a universal, multifunctional inorganic-organic superhydrophobic paint (IOS-PA) composed of epoxy resin, dual-sized TiO2 nanoparticles, and PFOS. The coating achieves robust superhydrophobicity (WCA > 150°, RA < 10°) across diverse substrates and large areas, with demonstrated resistance to mechanical abrasion and sand impact, chemical corrosion (pH 1–14), thermal extremes (150 °C and -196 °C), and icing. It exhibits self-healing and, uniquely, combines effective photocatalytic degradation of organic contaminants with long-term UV-resistant superhydrophobicity (100 h UV irradiation and 1 year sunlight exposure), enabling comprehensive self-cleaning. The paint effectively waterproofed grass house materials, indicating strong potential for scalable production and practical application in undeveloped regions. Future studies could extend long-term field testing across varied climates and substrates and explore environmental and regulatory considerations of the fluorinated components.