Introduction: Why do “clean white walls” turn “gray” after just a few years?
The “Dirt Pick-up Resistance/DPUR” of architectural exterior wall coatings
——is the performance that owners “perceive most directly”
of the coating——white exterior walls——>after 1-2 years show “streaks/spots/dullness”
——that is dirt pick-up. Sources of dirt on building exterior walls——(1) particulate matter in the atmosphere (PM10/PM2.5/dust/soot)
——settles on the coating surface——the adhesion force (van der Waals force/electrostatic force) between the particles and the coating makes the particles ”stick to” the wall; (2) rainwater——the flow pattern of rainwater on the coating “contracting droplet flow” (hydrophobic——uneven water droplets——dust concentrates at the edges of droplets——forms ”black streaks”)——vs ”spreading water film flow” (hydrophilic——dust is uniformly carried away by the water film——clean)
——this is the most important yet most counterintuitive
point in dirt pick-up resistance of architectural coatings: ”hydrophobic is not necessarily clean——hydrophilic is not necessarily dirty”.
I. Comparison of Hydrophilic vs. Hydrophobic Anti-fouling Strategies
| Strategy | Mechanism | Water Contact Angle (°) | Advantages | Disadvantages | Suitable Climate |
|---|---|---|---|---|---|
| Hydrophilic (Photocatalytic TiO₂ / Superhydrophilic) | TiO₂ photocatalytic decomposition of dirt + uniform spreading and rinsing by rainwater | <10 (Superhydrophilic) | Long-term self-cleaning / good rainwater washing effect / decomposes organic dirt | Requires rainwater (>1000mm/year) — ineffective in arid regions; TiO₂ catalytic durability (>5 years) | Rainy / high humidity (South China / Japan / Southeast Asia — recommended) |
| Hydrophobic (Fluorocarbon / Siloxane / Low surface energy) | Low surface energy (<20mN/m) — dust hardly adheres — water droplets roll off carrying dust away | >110 (Strong hydrophobic) | “Lotus effect” / dust hardly attaches / good appearance | Uneven flow of water droplets — produces “black streaks”; requires periodic cleaning | Low rainfall / high wind and sand (North China / Middle East / Australia — recommended) |
FAQ
Q1: Why does the “water bead rolling off” of a hydrophobic coating produce “black streaks” and instead look unclean?
On a hydrophobic (water contact angle >110°) surface, water beads——(1) Water beads are unevenly distributed on the surface
(varying sizes / random positions)——(2) Water beads roll off under gravity
(non-uniform spreading)——the edges of the water beads (water/air/coating three-phase interface)
——dust and dissolved substances enrich at the water bead edges (capillary effect)
——after water evaporates——leaving “ring-shaped” or “streak-shaped” black deposits
——this is the “black streaks”, the most typical soiling pattern of hydrophobic coatings under “intermittent rain” (light rain / discontinuous——insufficient to wash the entire surface)
. Superhydrophilic (<5°)——water fully spreads into a water film
——no “water bead edges”, dust evenly distributed in the water film——carried away with the water film without any “streaks”
——clean.
Q2: Does photocatalytic TiO₂ in building exterior wall coatings remain “self-cleaning” after a “lifetime” of >5 years?
The photocatalytic activity of TiO₂—after >2-3 years due to accumulation of dirt on the TiO₂ particle surface
—pollutants cover the TiO₂ surface—UV cannot reach TiO₂—photocatalysis “stops”. But rainwater washing
—can partially wash away the dirt layer
—restore the photocatalysis of TiO₂—therefore the self-cleaning effect of TiO₂ in “humid and rainy + strong UV” (e.g., Guangzhou/Singapore) climates—can last >5-8 years
—in “dry and rain-scarce” (Beijing/Dubai)—the dirt layer accumulates—photocatalysis stops after >1-2 years—hydrophobic strategy has more advantages.
Q3: Why can’t the “prepared dust” in GB/T 9780-2013 represent real soiling?
GB/T 9780 uses “prepared dust” (fly ash + coal ash—known composition / particle size <45μm)
—test—sprinkle dust on coating surface—use “washing tester” (water flow)
to rinse—measure “reflectance drop rate” (ΔR=R original−R after rinsing)
—ΔR<15%=excellent—15-25%=good—>25%=poor. But actual soiling—(1) contains oily organic matter (vehicle exhaust / cooking fumes)
—not removable by “water rinsing”—requires chemical decomposition by photocatalytic TiO₂—(2) soiling is “cumulative” over months / years
—not the lab’s “one-time dust sprinkling + rinsing”. GB/T 9780 is QC (quality control)—not a prediction of “actual stain resistance”—actual stain resistance—requires >2 years of outdoor exposure.
Related Reading
Summary
For architectural coatings’ anti-soiling performance, hydrophilic (photocatalytic TiO₂ / self-cleaning / high-rainfall) and hydrophobic (fluorocarbon / siloxane / lotus effect / low-rainfall) systems each have their optimal climate zones. Super-hydrophilic surfaces — where water films spread uniformly — avoid “black runoff streaks” and are the optimal choice for high-rainfall regions. GB/T 9780 is a QC standard and cannot replace actual outdoor exposure. Kexin New Materials provides customers with a full range of hydrophilic/hydrophobic anti-soiling architectural coating products and anti-soiling technical support.