Two-component polyurethane (2K PU) wood coating is a room-temperature crosslinking furniture finish made of a hydroxyl-bearing resin (hydroxy-acrylic, alkyd or polyester, in solution or dispersion) as Part A and a polyisocyanate prepolymer as the Part B hardener; the two are metered on site and cure through the addition reaction between —NCO and —OH groups. Unlike single-pack evaporative lacquers, its film properties are governed mainly by the mix ratio, the working time after mixing and the cure conditions, not by solvent release alone.
TL;DR: The controlling variable of 2K PU furniture coating is the NCO:OH molar ratio (isocyanate index) — typically 1.05–1.15 for solvent-borne systems. Below 1.0 the network is under-crosslinked and stays soft and tacky; above 1.2 free —NCO remains, favouring CO₂ pinholes and respiratory sensitisation. Pot life after mixing is usually several hours; light and white topcoats should use aliphatic hardeners (HDI, IPDI) to avoid the yellowing of aromatic TDI/MDI. Acceptance references: GB/T 23997-2009, GB 18581-2020, GB/T 23987-2009.
1. How it cures
After mixing, the isocyanate group is attacked by the hydroxyl oxygen to form the urethane link —NH—CO—O—. At 25 °C the rate constant is of the order 10⁻²–10⁻⁴ L/(mol·s), a medium-speed reaction, hence the several-hour working time. Ambient moisture competes for —NCO (H₂O + —NCO → —NH₂ + CO₂, then urea), consuming effective NCO and potentially leaving CO₂ micro-bubbles, so water control runs through formulation, application and cure.
Catalyst selectivity
Organotin catalysts such as DBTDL favour the —NCO/—OH path with few bubbles; tertiary amines push the —NCO/water reaction and amplify CO₂ pinholing, so they are avoided in high-gloss topcoats. Higher humidity shortens the drying and recoat window.
2. Key figures
Drawn from GB/T 23997-2009 (solvent-borne polyurethane wood coatings for indoor use), GB 18581-2020 (harmful substances) and GB/T 23987-2009 (artificial weathering).
- Drying (GB/T 23997-2009): touch dry ≤ 1 h, hard dry ≤ 24 h.
- Mix ratio: solvent-borne NCO:OH 1.05–1.15; waterborne 2K PU 1.2–1.5.
- Pot life: solvent-borne several hours; waterborne measured ≈ 4–6 h, with about 30% of effective —NCO consumed near the limit.
- Adhesion (cross-cut, GB/T 9286): ≤ grade 1.
- Abrasion: mass loss ≤ 0.050 g at 750 g / 500 r (topcoat).
- Yellowing (GB/T 23987-2009, UVA-340, black-panel (60±3) °C, 0.68 W/m², 168 h): ΔE ≤ 3.0 clear / ≤ 6.0 pigmented.
- VOC (GB 18581-2020): ≤ 500 g/L topcoat, ≤ 550 g/L primer.
- Free TDI (GB 18581-2020): ≤ 0.4% clear / ≤ 0.7% pigmented.
- Extractable heavy metals (GB 18581-2020): Pb ≤ 90, Cd ≤ 75, Cr ≤ 60, Hg ≤ 60 mg/kg.
3. The NCO:OH ratio: the master switch of crosslink density
Why a slight NCO excess
Stoichiometry is exact at 1.0, but trace water consumes —NCO, so solvent-borne systems run 1.05–1.15 to cover water loss and add a few urea crosslinks, lifting hardness and chemical resistance. The ratio should be computed from the polyol hydroxyl number (mgKOH/g) and the hardener —NCO content, not by “adding extra hardener makes it tougher”.
What happens off-target
- NCO:OH < 1.0 (OH excess): under-crosslinked, soft and tacky, with weaker water and solvent resistance.
- NCO:OH > 1.2 (large NCO excess): residual free —NCO reacts with air moisture over time, giving CO₂ pinholes and brittle urea zones, plus a sensitisation and compliance risk.
Waterborne systems need a higher apparent ratio because the water load consumes more —NCO; this is the most common formulation error.
4. Pot life and the shop-floor window
Pot life is the time after mixing before viscosity rises beyond application. Furniture lines dose small, frequent batches to stay inside it; past pot life the film loses levelling and shows orange peel and craters. Viscosity and gloss alone are poor indicators — published work finds haze and pendulum hardness track —NCO consumption more closely.
Relation to the recoat window
Recoating should occur after touch-dry but before full crosslinking to secure intercoat adhesion; once fully cured, the layer must be abraded first. The 24 h hard-dry limit of GB/T 23997-2009 is the ceiling for scheduling.
5. Choosing the hardener: aliphatic vs aromatic
| Hardeners | Weathering / yellowing | —NCO (typ.) | Reactivity | Furniture use |
|---|---|---|---|---|
| HDI trimer (aliphatic) | Excellent, non-yellowing | ≈ 21–23% | Medium | White / light topcoats |
| IPDI / H₁₂MDI (alicyclic) | Excellent | ≈ 15–25% (prepolymer) | Lower (steric) | Weather + chemical resistance |
| TDI adduct (aromatic) | Poor, yellows | ≈ 12–14% | High | Dark primers, indoor systems |
| Polymeric MDI (aromatic) | Poor, yellows | ≈ 15–25% (prepolymer) | High | High-activity / pigmented, shielded |
For light and white furniture topcoats, prefer aliphatic (HDI) or alicyclic hardeners; aromatic TDI/MDI yellow under UV and belong on dark faces or on primers hidden by the topcoat.
6. Hardness build and cure schedule
Pencil and pendulum hardness do not peak at hard-dry: the network keeps densifying at room temperature, reaching usable hardness near 24 h and approaching stability around 7 days. Packing, stacking and inspection nodes should allow this post-cure. Mild heating accelerates crosslinking but, done too fast, amplifies moisture and CO₂ pinholing.
7. FAQ
Must the ratio be metered exactly?
Yes — from the supplier's mass ratio, the polyol hydroxyl number and the hardener —NCO content. Overdosing hardener raises residual free —NCO, causing pinholes and sensitisation.
Why does a white topcoat yellow over time?
Usually an aromatic (TDI/MDI) hardener yellowing under light. Switch to HDI or IPDI and accept against the ΔE ≤ 3.0 clear requirement of GB/T 23997-2009.
How long can mixed material sit before spraying?
System and temperature dependent: solvent-borne several hours, waterborne about 4–6 h. Beyond pot life, discard — do not “rescue” it with thinner.
What about workshop humidity?
High humidity drives the —NCO/water reaction, producing CO₂ micro-bubbles and a tacky surface; work within the recommended temperature/humidity band and set the dry and recoat schedule accordingly.
How is environmental compliance checked?
Per GB 18581-2020: VOC ≤ 500 g/L topcoat / ≤ 550 g/L primer, free TDI ≤ 0.4% / ≤ 0.7%, heavy metals Pb ≤ 90 / Cd ≤ 75 / Cr ≤ 60 / Hg ≤ 60 mg/kg. Ask for a type-test report covering GB/T 23997-2009.
Sources
- GB/T 23997-2009, Solvent-thinned polyurethane wood coatings for indoor decorating and refurbishing (in force 2010-02-01).
- GB 18581-2020, Limit of harmful substances of wood coatings.
- GB/T 23987-2009 (artificial weathering / yellowing method); GB/T 9286 (cross-cut adhesion).
- Open literature on waterborne 2K PU formulation and pot-life testing (Shanghai Coatings 2012; Modern Paint & Finishing 2019; Paint & Coatings Industry 2016).
Last updated: 2026-10-16 | Compiled by the technical team of Kexin New Materials (Guangdong) Co., Ltd. Limit values are quoted from GB/T 23997-2009 and GB 18581-2020; mix ratios and process parameters must be verified against the data sheet of the product in use. Supplied on EXW / FOB terms, for B2B customers.