Polyol (-OH) crosslinks with polyisocyanate (-NCO) to form urethane bonds. Aliphatic HDI does not yellow, ideal for outdoor weathering topcoats and automotive soft-touch texture paints.
2K PU: Component A (polyol + pigment) + Component B (HDI trimer) mixed on-site:
Excess -NCO continues to react with atmospheric moisture to form urea:
Aromatic MDI/TDI UV-induced chain scission yellowing, primer only; Aliphatic HDI trimer QUV 2000h retengloss rate ≥85%.
| Curing Agent | NCO% | Yellowing Resistance | Application |
|---|---|---|---|
| TDI Trimer | ~8% | Poor | Indoor primer, flooring |
| MDI Prepolymer | ~16% | medium | anti-corrosion, Elastomer |
| HDI Trimer | ~22% | Excellent | Auto, outdoortopcoat |
| IPDI Trimer | ~12% | Excellent | High-end soft-touch, 3C |
| H12MDI | ~23% | Excellent | Yellowing resistant special type |
| Parameter | Typical Value |
|---|---|
| NCO/OH equivalent ratio | 1.05-1.1 (slight excess) |
| Pot Life | 2-4 h @25℃ |
| Surface Dry | 0.5-1 h |
| Full Cure | 7 d @25℃ |
| Application humidity | ≤75% (except moisture-cure type) |
| Defect | Cause | Solution |
|---|---|---|
| Yellowing | Used aromatic curing agent | Outdoor must use HDI/IPDI |
| Blistering | Humidity too high | Humidity >80% Stop application |
| Loss of gloss | Curing agent excess, moisture | Strict ratio, sealed pail |
| Tacky, not dry | Insufficient NCO ratio | Recalculate equivalent ratio |
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