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1、 Basic Definition

Hydrophobicity: Water contact angle>90 °, superhydrophobicity generally>150 °, small rolling angle; Water droplets are spherical and easily roll off.

Hydrophilicity: water contact angle<30 °, superhydrophilicity close to 0 °; Water quickly spreads into a water film on the surface.

2、 Anti pollution principle

✅  Hydrophobic and stain resistant: The “lotus leaf effect” coating has a low surface energy and micro nano rough structure, and does not adhere to water. When rainwater falls, it takes away floating dust, particles, and oil stains like a mop; The pollutants and coatings are separated by an air cushion, resulting in weak adhesion.

Weakness: Ineffective against oily stains, oil will infiltrate hydrophobic surfaces, infiltrate microstructures, and be difficult to wash away after sticking firmly; After long-term outdoor UV aging, low surface energy functional groups degrade, rough structures are destroyed, hydrophobicity rapidly loses, and oil accumulation and blackening are prone to occur in the later stage.

✅  Hydrophilic and stain resistant: “Water film isolation+photodegradation” The coating becomes super hydrophilic under light irradiation, and rainwater evenly spreads into a continuous water film. The water film is sandwiched between pollutants and the coating, and pollutants float on the water film, which is directly washed away by rainwater; Simultaneous photocatalysis can slowly oxidize and decompose organic stains.

Weakness: The adhesion of dry dust is not weak, and when it is not raining, the dry dust directly adheres to the surface of the coating; Without light exposure (shaded or backlit), the photocatalytic activity is very low and the ability to resist pollution is significantly reduced; Partial hydrophilic coating hardness, acid rain resistance, and salt spray resistance require matching design.

3、 Suggestions for outdoor scene selection

1. External walls, color steel, coil materials, areas with high dust and low oil pollution → Priority should be given to areas with high drainage and rainwater, and inorganic dust as the main dust source; Do not use superhydrophobic materials for high oil pollution environments, as their lifespan is very short.

2. Glass curtain walls, photovoltaic cover plates, tunnel inner walls, oil fume surroundings, and areas prone to algae and mold growth → The key to photocatalytic hydrophilicity is to decompose organic matter, prevent oil films, and prevent fog; Carefully select hydrophilic backlight surface.

3. Correction of Misconceptions

· ❌  Not being hydrophobic necessarily means being resistant to pollution: superhydrophobic is most afraid of oil stains and wear, and many projects lose their lotus leaf effect after 3-6 months;

· ❌  Being hydrophilic does not mean being clean all the time: in areas with long-term drought and low rainfall, the accumulation of dust on hydrophilic coatings actually looks dirtier;

Nowadays, there are many composite ideas: the bottom hydrophobic skeleton+a small amount of photocatalytic components on the surface, taking into account both, but the compatibility of the formula is difficult and prone to interlayer, dispersion, and yellowing problems.