Introduction: Epoxy can also be “water-based,” but at the cost of complex performance trade-offs.
Epoxy resin is inherently hydrophobic (water contact angle >80°) and completely incompatible with water. The chemical essence of “water-based” epoxy resin—(1) Type I—using external emulsifiers (surfactants) to disperse epoxy resin in water as micron-sized droplets (<1-10μm)—like "oil-in-water salad dressing"; (2) Type II—chemically grafting hydrophilic groups (such as polyethylene glycol PEG segments/carboxyl -COOH/amino -NH₂) onto the epoxy resin molecular chain—enabling the epoxy itself to have self-emulsifying ability without external emulsifiers—dispersion particle size <1μm (finer and more stable). The core difference between the two routes is emulsifier residue (Type I/hydrophilic emulsifier remains in the coating—reducing coating water resistance and anti-corrosion performance) vs chemical modification (Type II/no free emulsifier/better coating water resistance—but higher cost + more complex process).
Waterborne epoxy coating is a two-component waterborne anti-corrosion coating system that uses water as the dispersion medium, disperses epoxy resin in water in the form of fine particles through an externally added emulsifier (Type I) or chemical self-emulsification (Type II), and undergoes crosslinking curing after water evaporation when combined with a waterborne amine curing agent.
I. Comparison between Type I and Type II Waterborne Epoxy
| Dimension | Type I (External Emulsification) | Type II (Self-Emulsification) |
|---|---|---|
| Epoxy Particle Size (μm) | 1-10 | 0.2-1 (Finer) |
| Emulsifier | Added externally (1-5%/hydrophilic/residual) | Chemically grafted (no free/not residual) |
| Storage Stability | Medium (6-12 months/layering tendency) | Excellent (>12 months/stable) |
| Coating Water Resistance | Medium-Good (emulsifier absorbs water/blistering) | Excellent (no emulsifier/dense) |
| Cost Index | 1 (baseline) | 1.5-3 |
| Application Scenario | General anti-corrosion (C3-C4) | Heavy anti-corrosion (C5/recommended) |
II. Three Stages of Film Formation — Water Evaporation → Particle Coalescence → Crosslinking
2.1 Stage 1 — Water Evaporation (Wet Film → Particle Contact)
Waterborne epoxy coating after spraying — water begins to evaporate — coating viscosity increases — epoxy and curing agent particles gradually from free movement → mutual contact The water evaporation rate at this stage determines film formation quality — too fast (high temperature / low humidity / strong wind) → coating surface “skins over” first, internal water is sealed in — subsequent evaporation is hindered — coating develops microbubbles / pinholes; too slow (low temperature / high humidity / no wind) — water evaporation is insufficient — particle contact is not tight — coating density is poor. Optimal conditions — temperature 15-30°C / RH 50-70% / light breeze (0.5-1 m/s).
2.2 Stage 2 — Particle Coalescence (Capillary Pressure Driven)
Further water evaporation—water film thickness between particles decreases to <100nmCapillary pressure (P=2γ/r) rises sharply (>10MPa)—pressing adjacent particles togetherParticle surface softens (T>MFFT)—interfacial fusion between particles forms a continuous “honeycomb-like” polymer network—at this point the coating has acquired certain strength and adhesion.
2.3 Stage 3 — Crosslinking and Curing
After interfacial fusion between particles—epoxy groups and amine curing agent begin to diffuse and react across the original particle interfaces to form a covalent crosslinked network—the coating’s final Tg/adhesion/chemical resistance are fully established at this stage. The “full cure” time of waterborne epoxy (23°C)—7–14 days—is longer than that of solventborne epoxy (5–7 days)—because in waterborne systems the epoxy particles require more time for the curing agent molecules to penetrate into the particle interior to complete uniform crosslinking.
FAQ
Q1: Why is water-based epoxy more prone to “flash rust” than solvent-based epoxy?Water-based epoxy coatings on bare steel substrates——water contacts steel for >30min before evaporation——water + O₂ → rapid rusting on steel surface (flash rust / yellow FeOOH)Solvent-based epoxy evaporates quickly after spraying (5-10min)——no prolonged water contact——flash rust risk much lower than water-based. Prevention——(1) Add flash rust inhibitor (organic zinc salt / 0.3%-0.5% / NaNO₂); (2) Spray within <4h after sandblasting; (3) Spraying environment RH<60%.
Q2: What is the negative mechanism of the “emulsifier” in Type I waterborne epoxy on the coating’s anti-corrosion performance?The emulsifier (surfactant/nonionic alkylphenol ethoxylate APEO or fatty alcohol ethoxylate)——(1) The coating’s hydrophilic sites: water molecules preferentially “reside” at the hydrophilic segments of the emulsifier within the coating——reducing the coating’s water barrier efficiency——the blistering density of Type I epoxy in salt spray testing is 2-3 grades higher than that of Type II; (2) Penetration channels for corrosion products: the hydrophilic segments of the emulsifier form “molecular-level water channels” within the coating, through which Cl⁻ penetrates——accelerating substrate corrosion.
Q3: Why is the “Pot Life” of waterborne epoxy longer than that of solvent-based? In waterborne epoxy—the epoxy and curing agent are dispersed in particle form, and the reaction occurs only at the particle interfaces (not throughout the entire system)—therefore the actual effective concentration of reactants is much lower than in solvent-based (where epoxy and amine are in full contact in solution)—the reaction rate is reduced—Pot Life is extended from 1-2h for solvent-based to 2-4h—this is both an advantage (longer application window)—and a challenge (viscosity increase within Pot Life is not obvious—hard to judge by “feel” whether it has expired).
Q4: The decisive impact of MFFT (Minimum Film Forming Temperature) on waterborne epoxy application?MFFT is the minimum temperature at which polymer particles can deform + coalesce into a continuous film—below MFFT—particles remain rigid—unable to coalesce—the coating forms a powdery/discontinuous “sand film” with no protective function. The MFFT of waterborne epoxy is typically >10-15°C—therefore low-temperature (20°C) + substrate (>15°C); (2) adding film-forming aids (e.g., Texanol / lowering MFFT); (3) using special low-MFFT resins.
Q5: Why does water-based epoxy coating sometimes exhibit “Blushing”?During film formation, water evaporates — the coating surface temperature drops (evaporative cooling effect) — surface temperature falls below the dew point — moisture in the air condenses on the coating surface — forming micro water droplets. Micro water droplets scatter light → coating turns white (blushing). Blushing not only affects appearance — trace contaminants in the water droplets and CO₂ in the air react with the amine curing agent — forming “amine bloom” on the coating surface, reducing intercoat adhesion of subsequent topcoats.
Q6: How does EIS (Electrochemical Impedance Spectroscopy) evaluate the anticorrosive performance of waterborne epoxy coatings?EIS measures low-frequency (0.01Hz) impedance |Z|——>10⁹Ω·cm² = excellent (dense coating / very few micropores)——waterborne type II epoxy (self-emulsifying) usually reaches this level; 10⁷-10⁹Ω·cm² = good——waterborne type I epoxy (external emulsification / residual emulsifier makes coating density inferior to type II) usually falls in this range; <10⁷Ω·cm² = significant coating aging——maintenance should be planned. Continuous EIS monitoring (Bode plot / impedance-frequency curve) can track the coating’s complete degradation process from “initially intact” → “water penetration” → “substrate corrosion”.
Q7: Why is “freeze-thaw stability” of waterborne epoxy coatings a major issue for winter transportation?Waterborne epoxy dispersions freeze below <0°C—ice crystals destroy the protective layer of emulsifier/self-emulsifying resin—epoxy particles coalesce—cannot be redispersed after thawing—coating permanently gelled and scrapped. Storage and transportation of waterborne epoxy must be maintained at 5-35°CWinter transport requires insulated trucks (>5°C)—cost is 1.5-2 times that of normal-temperature transport. Freeze-thaw (-5°C/24h→room temperature thawing) cycles <5 times—is a basic quality control test for waterborne epoxy.
Q8: Why is the interlayer adhesion between water-based epoxy and water-based PU topcoat a “weak point”? After curing, the surface of the water-based epoxy coating is hydrophilic (residual emulsifier/carboxyl/hydroxyl). The water-based PU topcoat (also a water-based system) cannot “bite into” the epoxy primer surface like solvent-based PU does (the solvent of solvent-based PU can partially swell the resin on the epoxy surface—creating chemical anchoring). The interlayer adhesion of water-based epoxy/water-based PU (>3-5MPa/pull-off method) is lower than that of solvent-based epoxy/solvent-based PU (>6-8MPa)—it is necessary to apply the PU topcoat before the epoxy primer is fully cured (curing degree 70-80%/6-16h after surface dry of the coating/still has residual active groups) to ensure interlayer chemical bonding.
Q9: What is the risk of “internal moisture escape” when waterborne epoxy coatings are applied in thick coats (>200μm/coat)?Waterborne epoxy single-coat thick application——long internal moisture escape path (>200μm)——(1) surface skins over first——internal moisture is sealed in——forming vapor pressure that breaks through the surface→pinholes; (2) moisture forms micron-scale bubbles inside the coating, which burst after curing→microscopic pores→reduced anti-corrosion performance. The single-coat DFT upper limit for waterborne epoxy is 100-150μm (far below solvent-based epoxy >200μm)——thick films require multiple coats (80-100μm per coat/30-60min flash-off between coats).
Q10: Is the “zero VOC” claim of waterborne epoxy truly real?Waterborne epoxy coating——water >50%/epoxy resin and curing agent each about 20-30%/film-forming aid 3-5%——the film-forming aid (Texanol/Dowanol DPnB/dipropylene glycol butyl ether) is a volatile organic compoundaccording to the test method of GB/T 23985——the VOC of waterborne epoxy coating is usually 50-100g/Lmuch lower than solvent-based epoxy (300-500g/L)——but not zero“Zero VOC” is a marketing expression——the accurate expression is “ultra-low VOC (<100g/L)". The film-forming aid is the main source of VOC in waterborne epoxy——choosing a high-boiling-point low-volatility film-forming aid (e.g., Texanol/boiling point 255°C) can further reduce VOC to <30g/L.
Related Reading
Summary
The two technical routes of waterborne epoxy—Type I external emulsification (emulsifier residue/low cost/general anticorrosion) and Type II self-emulsification (chemical modification/superior water resistance/heavy-duty anticorrosion)—each have suitable applications in terms of anticorrosive performance, cost, and workability. The three-stage film formation (water evaporation → particle coalescence → crosslinking) is highly sensitive to temperature and humidity conditions. MFFT (>10–15°C) is the lower limit for low-temperature application of waterborne epoxy. Kexin New Materials provides customers with a full range of waterborne epoxy coatings (including Type I and Type II) and application technical support.