2K PU primer additives must be screened with the NCO/OH balance, pot life and recoat window visible. CHLUMIAG® 3467 has the strongest public reaction evidence: 100% active, hydroxyl value ≤85 mg KOH/g and an explicit hydroxyl–isocyanate reaction route. CHLUMIFE® 3370 is described as reactive, but its public page gives no hydroxyl value. CHLUMIFE® 3166 is a general surface additive with pinhole, whitening and foam-stabilization warnings.
None should be treated as a drop-in cure-neutral primer fix. First remove contamination, spray, flash, substrate and resin/crosslinker errors; then calculate or verify any reactive contribution and test the complete primer/topcoat system.
2K PU primer decision map
| Evidence at the panel or batch | First action | Additive route |
|---|---|---|
| Craters follow contamination, oil, silicone or poor substrate preparation | Remove the source and remake the blank | No additive rescue |
| Clean blank still needs PU-compatible wetting/leveling | Confirm formulation mode and NCO/OH calculation | 3467 controlled ladder |
| Reactive surface modification is desired but OH data are missing | Obtain current TDS/CoA and equivalent guidance | 3370 only after evidence |
| Primer already has pinholes, whitening or retained foam | Diagnose air/outgassing/compatibility first | Do not lead with 3166 |
| Produto | Published facts | Potential primer role | Critical boundary |
|---|---|---|---|
| 3467 | 100% active; OH value ≤85; solvent dose 0.05–1% total; resin modification 0.5–5% on resin | PU-compatible wetting, leveling and anticrater screen | Maximum OH value is not a batch-specific equivalent; verify NCO/OH impact |
| 3370 | Reactive hydroxyl polyester-modified polysiloxane; waxy solid; ≥50°C initial melt; 0.05–3%; not waterborne | Conditional solvent 2K PU surface-modification route | No public OH value or equivalent; handling in actual resin needs confirmation |
| 3166 | 100% active; 0.05–1%; UV/solvent/water; dry feel, wetting and leveling claims | Later broad surface-balance comparison | Public warnings: pinholes, whitening and foam stabilization |
Diagnose the primer before the additive
Document substrate, pretreatment, sanding, cleaning, humidity, resin and polyisocyanate lots, mix ratio, induction, reducer, spray viscosity, film build, flash and cure. Map when the defect appears. Immediate pullback suggests wetting or contamination; bubbles during flash may be air or outgassing; late gloss or hardness loss may be cure imbalance.
Prepare a fresh blank with clean tools before screening silicone chemistry. An additive that hides contamination can make later adhesion, sanding and topcoat failures harder to diagnose.
Use 3467 only with reaction accounting
3467 is the best documented first candidate because its page supplies a hydroxyl value ceiling and explicitly describes reaction with isocyanate. For solvent coating modification, the public dose is 0.05–1% of total formulation; the separate 0.5–5% of resin route is for resin modification and must not be copied into a finished-primer ladder.
Confirm the current TDS, actual batch hydroxyl information and whether the proposed use is reactive or nonreactive surface control. Include its OH contribution in the NCO/OH calculation when applicable, then recheck pot life, gel profile, hardness and chemical resistance.
Do not infer a 3370 equivalent
3370 is a 100%-solid waxy product with an initial melting point of at least 50°C. Its page describes reaction with isocyanate and a 0.05–3% total-formula range, but does not publish hydroxyl value or equivalent weight. Alkanes and aromatics are listed as dilution options.
Before use, obtain quantitative OH data, dissolution/incorporation guidance and confirmation for the actual primer resin. Do not assume it has the same reaction demand as 3467 or remains uniformly incorporated after cooling.
Keep 3166 out of a defect-rescue shortcut
3166 supports surface tension reduction, dry feel, smoothness and leveling, but its own page warns of pinholes, whitening and foam stabilization. Those risks overlap common primer complaints. It should therefore enter only after air, outgassing and contamination are controlled and only with adhesion and recoat checks.
Build a pot-life trial, not one drawdown
- Prepare blank plus a compact supplier-supported dose ladder.
- Hold resin/crosslinker equivalents, reducer, temperature and mix order.
- Apply at initial, middle and end-of-pot-life intervals.
- Measure viscosity, gel tendency, wetting, craters, sag, pinholes and film build.
- Cure under the same schedule, sand, then topcoat inside several validated recoat intervals.
Measure cure, adhesion and recoat
ASTM D5402 provides a solvent-rub comparison for assessing cure development in organic coatings. ASTM D3359 provides tape-adhesion procedures; confirm method suitability for the substrate, film thickness and project specification.
Also record pot-life viscosity, through-cure, pencil or instrumented hardness, sanding dust/loading, intercoat adhesion, gloss, color, water/chemical resistance and aged recoat. A smooth primer that shortens pot life or weakens the topcoat interface does not pass.
Repeat the critical comparison after warm storage of each component and after the longest permitted mixed-material hold. Look for additive separation, crystallization, viscosity drift and changes in crater response. A candidate that performs only in a freshly opened laboratory sample is not ready for plant approval.
Request a stoichiometry-safe shortlist
Send resin OH value and solids, polyisocyanate NCO content, target index, mix ratio, reducer, pigment/filler package, current additives, pot life, flash/cure, film build, defect timing, sanding and topcoat window. Ask Longchang Chemical for current 3467 and 3370 reaction data before a 2K PU primer trial.