Introduction: The “Waterborne Journey” of Alkyd, the Oldest Coating Resin
Alkyd resin (Alkyd / from Alcohol + Acid) is the oldest synthetic resin in the coatings industry (1920s / General Electric), using vegetable oils (soybean / linseed / castor oil) + polyols (glycerol / pentaerythritol) + polybasic acids (phthalic anhydride) as raw materials — low cost + easy application (brush / roller / spray) + good adhesion + moderate anti-corrosion performance — still accounts for >15% of the global coating resin market (>2 million tons / year) to this day. However, traditional alkyd resins contain >40% organic solvents (200# solvent gasoline / xylene) — VOC >300g/L — facing huge pressure under environmental regulations. Waterborne alkyd — replacing solvent with water — is the core direction for the “greening” of alkyd resins — but faces two major core challenges — (1) driers become “deactivated” in water (Co²⁺ hydrolysis); (2) the presence of water causes “flash rust” on the coating on bare steel.
Waterborne alkyd coating is a single-component waterborne anti-corrosion coating that uses waterborne alkyd resin (externally emulsified or self-emulsified) as the film-forming substance, water as the dispersion medium, and cures via oxidative crosslinking (air-drying) of unsaturated fatty acids in vegetable oils catalyzed by metal driers (cobalt/zirconium/calcium).
I. Deactivation of Driers in Waterborne Alkyd Coatings — Chemical Mechanisms and Solutions
The drier system of traditional solvent-based alkyd Co (primary drier/catalyzes oxidation of fatty acid chains) + Zr (auxiliary drier/”synergist” of Co/accelerates Co activity) + Ca (auxiliary/stabilizes Co) is completely dissolved in solvent — high catalytic efficiency — coating surface dry 1-2h / hard dry 4-6h. In water-based alkyd — Co²⁺ slowly hydrolyzes in water (Cobalt Leaching) Co²⁺+2H₂O→Co(OH)₂↓(pink precipitate)+2H⁺ — hydrolyzed Co(OH)₂ loses catalytic activity — coating curing severely slowed down (surface dry >6h / hard dry >24h) — even in humid environments “never dries” (Permanent Tack / permanently tacky).
Solution(1)Chelated drier—use Co-LS (ligand / e.g. 2-ethylhexanoic acid / acetylacetone) Co²⁺ is “wrapped” by the ligand, hydrolysis is suppressed—catalytic activity remains >80%;(2)Cobalt-free drier—use Fe/Mn/V-based driers (e.g. Borchers Dry 0411 / Nuodex Web Co-free)—no cobalt used at all—avoids cobalt hydrolysis issues + eliminates cobalt’s carcinogenic risk (Co²⁺ is classified by IARC as Group 2B possible carcinogen);(3)Self-emulsifying alkyd—hydrophilic groups grafted onto alkyd molecules coordinate with Co²⁺—reduce free Co²⁺ concentration in the aqueous phase—mitigate hydrolysis.
FAQ
Q1: Why is “flash rust” more severe in water-based alkyd coatings than in water-based acrylic?Water-based alkyd has extremely slow drying speed (surface dry >2h vs water-based acrylic 2h)——flash rust risk is much higher than fast-drying water-based acrylic. Flash rust inhibition for water-based alkyd——(1) add flash rust inhibitor (organic zinc salt/NaNO₂/0.3%-0.5%); (2) substrate sandblasting + phosphating (phosphating film prevents water from contacting bare steel); (3) water-based alkyd primer not recommended for direct use on bare steel should be used with phosphating/passivation pretreatment——or as topcoat/middle coat (applied on already primed substrate).
Q2: Self-emulsifying vs external emulsifying — which type of waterborne alkyd has better anticorrosive performance?Self-emulsifying — no free emulsifier — dense coating — water resistance > external emulsifying — anticorrosive performance (salt spray >500h / vs external emulsifying >300h). External emulsifying — emulsifier residue — hydrophilic sites — preferential water molecule penetration — coating blistering / loss of adhesion — weak anticorrosive performance. However, the cost of self-emulsifying alkyd is 2–3 times that of external emulsifying — most industrial waterborne alkyd coatings are still external emulsifying type (cost-performance driven).
Q3: Waterborne alkyd and high-solids alkyd (VS>70%)—which is the better route for the “greening” of alkyd? Waterborne alkyd—VOC<100g/L (water-based)—but drier deactivation + flash rust + slow drying + weak corrosion resistance. High-solids alkyd—VOC<200g/L (low solvent)—drier system same as solvent-based (no deactivation)—drying speed and corrosion resistance far superior to waterborne alkyd is the “high-performance route” for the “greening” of alkyd Although VOC (200g/L) is higher than waterborne (<100g/L)—but overall performance (drying/corrosion protection/adhesion) is closer to traditional alkyd—in heavy-duty anti-corrosion scenarios (C3-C4) high-solids alkyd is more reliable than waterborne alkyd.
Q4: Why does water-based alkyd coating often experience “skin formation in the can”?The liquid level inside the water-based alkyd bucket — water evaporation — surface resin concentration increases — exposed to O₂ in the air — drier catalyzes oxidationsurface resin oxidative polymerization → skin formation (thickness >1mm / same issue as solvent-based alkyd). Water-based alkyd skin formation is due to water evaporation (slower than solvent) — skinning speed is slower than solvent-based ( >48h vs >12h) — but long-term opening of the lid (>7 days) will still cause skinning — solutions — (1) seal with nitrogen filling in the bucket; (2) add anti-skinning agent (methyl ethyl ketoxime MEKO / 0.1%-0.3%).
Q5: Can water-based alkyd be blended with acrylic/PU emulsion?Alkyd and acrylic/PU have large differences in solubility parameters; direct blending will cause phase separation—the coating film becomes “hazy/loses gloss/reduced adhesion”. However, acrylic/PU-modified alkyd—chemically grafting acrylic or PU segments onto the alkyd molecule (via copolymerization/transesterification/vinyl monomer grafting)—achieves “fusion” at the molecular level, giving the coating film the combined benefits of alkyd’s low cost + acrylic/PU’s fast drying and hardness, which is the most important technical direction for water-based alkyd modification.
Q6: The “yellowing” issue of waterborne alkyd?The fatty acid chains of alkyd resin contain unsaturated double bonds (linolenic acid/linoleic acid/conjugated double bonds) — oxidative reactions during curing and long-term aging generate conjugated oxidation products (yellow to brown) — white/light-colored waterborne alkyd coatings yellow (Δb>5) within months to years — this is the main aesthetic limitation of waterborne alkyd in decorative coatings.
Q7: Positioning of water-based alkyd coatings in industrial anti-corrosion — competition with water-based epoxy/PU?Water-based alkyd — lowest cost (10-20 RMB/kg) — medium anti-corrosion performance (C3) — suitable for “good enough” light anti-corrosion scenarios (indoor steel structures/metal products). Water-based epoxy — medium cost (25-50 RMB/kg) — excellent anti-corrosion performance (C4-C5) — suitable for heavy anti-corrosion. Water-based PU — high cost (40-80 RMB/kg) — excellent weather resistance — suitable for outdoor. Water-based alkyd and water-based epoxy/PU are not “who replaces whom” but rather graded by anti-corrosion level and budget.
Q8: What is the strategic impact of China’s vegetable oil resources (soybean oil/tung oil) on the supply of raw materials for waterborne alkyd?China is the world’s largest soybean importer (>100 million tons/year). Soybean oil is one of the most commonly used vegetable oils for alkyd resins. China’s native tung oil (Tung Oil / containing >80% conjugated trienoic acid — extremely fast drying) is a high-quality alkyd raw material — but production is limited (<100,000 tons/year). The development of China's waterborne alkyd coatings has the advantage of stable raw material supply — but it should be noted — using edible oil (soybean oil) for industrial coatings — creates “land competition” with food security. In the future, more non-edible oils (castor / jatropha) and waste vegetable oils (refined gutter oil) are needed as sustainable raw materials for alkyd.
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Summary
The core technical bottleneck of waterborne alkyd coatings—the hydrolytic deactivation of driers (Co/Zr/Ca) in water—is addressed through chelated driers (Co-LS) and Fe/Mn non-cobalt driers. The “flash rust” risk of waterborne alkyd is higher than that of waterborne acrylic—primers are not recommended for direct use on bare steel. High-solids alkyd (VS >70%/VOC <200 g/L) is a "high-performance alternative" to waterborne alkyd and is more reliable in C3–C4 anti-corrosion scenarios. Kexin New Materials provides customers with full-range waterborne/high-solids alkyd coating products and technical support.