Introduction: Waterborne Polyurethane—an “all-rounder” that balances environmental friendliness and performance
Polyurethane (PU) is the most comprehensive in performance and most widely used resin system in the coatings industry—from automotive clearcoats to wood finishes, from leather finishing to industrial anticorrosion—PU is everywhere. However, traditional solvent-based PU contains large amounts of organic solvents (VOC >300–600 g/L), placing enormous pressure under environmental regulations. Waterborne polyurethane—dispersing PU resin in water as nanoscale particles (<0.1–0.5 μm)—has VOC 20–40 MPa / elongation >200–600%)—it is the system in waterborne coatings that is “closest to solvent-based in performance”—and also the fastest-growing category of waterborne industrial coatings (annual growth >15%).
Waterborne polyurethane coatings are divided into two main routes: one-component PUD (waterborne polyurethane dispersion—high-molecular-weight PU particles pre-polymerized and dispersed in water—physically dried to form film—easy application but moderate resistance) and two-component waterborne PU (waterborne polyol + hydrophilic-modified isocyanate—chemical crosslinking—resistance close to solvent-based PU—Pot Life 2-4h). Core raw materials: polyol (polyester—adhesion/polyether—water resistance/polycarbonate—weather resistance), isocyanate (HDI/IPDI—aliphatic—non-yellowing) and chain extender (DMPA—introduces hydrophilic carboxyl groups).
I. Comparison between one-component PUD and two-component waterborne PU
| Dimension | One-component PUD | Two-component Waterborne PU |
|---|---|---|
| Curing mechanism | Physical drying (water evaporation — particle coalescence — no crosslinking) | Physical drying + chemical crosslinking (NCO+OH) |
| Application simplicity | Ready to use — no Pot Life — “foolproof” | Requires on-site mixing — Pot Life 2-4h — needs control |
| Water resistance | Medium-Good (no crosslinking — re-emulsifiable) | Excellent (chemical crosslinking — irreversible) |
| Chemical resistance (ethanol wiping) | Poor (>50 wipes to bare substrate) | Excellent (>200 wipes) |
| Hardness | HB-2H | H-4H |
| Typical applications | Wood furniture/leather/DIY — “convenient — single — component” | Automotive interior/industrial anti-corrosion/flooring — “high performance” |
FAQ
Q1: Why can PUD stably disperse in water—without “oil-water separation”?Hydrophilic groups are introduced on the surface of PUD particles—most commonly DMPA (dimethylolpropionic acid—containing -COOH—neutralized with triethylamine to -COO⁻N⁺HEt₃—ionic hydrophilic). These -COO⁻ negative charges form an electric double layer on the particle surface—particles are stably dispersed due to electrostatic repulsion “each—PU—particle—is—a—microsphere—wrapped—by—negative—charge—they—repel—each—other” so—stable—no—stratification”.
Q2: Why does two-component waterborne PU have a Pot Life of only 2-4h?The side reaction between water and NCO is the biggest killer—water + H2O → amine + CO2—consumes NCO—produces bubbles. The NCO/OH ratio of waterborne PU is usually set at 1.5-2.5 (much higher than the solvent-based 1.05-1.15) “extra—NCO—=—compensate—for—water—consumption—of” but—too—much—NCO—also—means—faster—reaction “Pot—Life—shorter”.
Q3: Why is water-based PU gradually replacing solvent-based NC/PU in wood coatings? Eco-friendly (VOC<50 vs NC>600), safe (odorless—for children’s rooms/bedrooms), performance (PUD’s abrasion/water/yellowing resistance is no inferior to solvent-based), application (one-component PUD—ready to use—DIY friendly)—”Water-based PU = the future of wood coatings”.
Related Reading
Summary
Waterborne polyurethane—PUD (one-component/physical drying/easy to use) and two-component waterborne PU (chemical crosslinking/high performance)—is the system in waterborne coatings whose performance is closest to solvent-based coatings. Kexin New Materials provides a full range of waterborne PU products and formulation support.