Additives make up a small fraction of a formula but solve some of its biggest problems: controlling cure speed, eliminating bubbles, improving adhesion, preventing sag and protecting against sunlight.
A few tenths of a percent of the right catalyst can turn a slow-curing urethane into one that is walkable in hours. A silane adhesion promoter can be the difference between a coating that holds through wet service and one that peels. That is the power of additives.
Catalysts speed specific reactions: carboxylates of bismuth, zinc or tin for the isocyanate–hydroxyl reaction, and tertiary amines for moisture cure and foams. Moisture scavengers such as molecular sieve and PTSI remove water before it can react with isocyanate. Adhesion promoters, usually organofunctional silanes, bridge inorganic surfaces and organic binders.
Light stabilizers protect topcoats: UV absorbers filter incoming UV, while hindered amine light stabilizers (HALS) trap the free radicals that UV creates. Surface and rheology additives such as defoamers, wetting agents and thixotropes control bubbles, flow and sag.
The small-dose ingredients that control cure, moisture, adhesion, weathering and surface quality.
Used forClassic NCO–OH catalyst in polyurethane coatings, sealants and foams.
Organotins are restricted in many EU mixtures under REACH; bismuth and zinc catalysts are common replacements.
Full profileUsed forTin-free gel catalyst for polyurethane coatings and elastomers.
Favors NCO–OH over the water reaction, but can lose activity to hydrolysis in wet B-sides.
Full profileUsed forGel and blow catalyst in polyurethane foam and some coatings.
Often sold as DABCO® and supplied dissolved in a glycol carrier.
Full profileUsed forCatalyst for 1K moisture-cure urethane coatings and sealants.
Promotes the NCO–water reaction while staying stable in prepolymers.
Full profileUsed forDries pigments and solvents before the isocyanate is added.
Reacts rapidly with water, cutting CO₂ gassing and pinholes in urethanes.
Full profileUsed forWater scavenger in polyurethane B-sides, polyols and sealants.
Locks water inside its pores without releasing gas, essential for bubble-free urethanes.
Full profileUsed forBoosts wet adhesion of epoxy and urethane coatings to glass, metal and concrete.
The silane end bonds to mineral surfaces while the epoxy end joins the binder.
Full profileUsed forAdhesion promoter and primer for epoxy, polyurea and phenolic systems.
Its amine end reacts with epoxy or isocyanate; often added to B-sides or primers.
Full profileUsed forGloss and color retention in aliphatic urethane, acrylic and polyaspartic topcoats.
Traps free radicals created by UV. Works best paired with a UV absorber.
Full profileUsed forLong-life UV filtering in clearcoats and topcoats.
Benzotriazole UV-328 was listed under the Stockholm Convention in 2023, pushing formulators toward alternatives like HPTs.
Full profileUsed forDefoaming, flow and slip in many coating types.
A little goes a long way. Overdosing causes craters and recoat adhesion problems.
Full profileCatalysts lower the energy barrier of a reaction without being consumed. Metal catalysts coordinate with the isocyanate and hydroxyl groups and bring them together; tertiary amines activate them through hydrogen bonding. Selectivity matters: a good urethane catalyst speeds the NCO–OH reaction more than the NCO–water reaction that creates bubbles.
Silane adhesion promoters are two-faced molecules. Their alkoxysilane end hydrolyzes and bonds to hydroxyl groups on glass, metal or concrete, while their organic end (epoxy or amine) reacts into the coating.
Light stabilizers work as a team. UV absorbers convert UV energy into harmless heat inside the film; HALS mop up the radicals that still form, regenerating in a cycle that gives long-lasting protection.
UV light → UV absorber → heatUVA · filtersR• radical + HALS → stable productHALS · traps radicals, regeneratesSi–OR + HO–surface → Si–O–surfaceSilane · bonds to substratesMost additives are dosed at a fraction of a percent to a few percent of the formula.
What each class does, and the classic mistake to avoid.
| Class | Examples | What it does | Common pitfall |
|---|---|---|---|
| Urethane catalysts | Bismuth, zinc, tin (DBTDL) | Speed NCO–OH cure | Too much shortens pot life |
| Amine catalysts | TEDA, DMDEE | Moisture cure, foams | Odor; can over-accelerate |
| Moisture scavengers | Molecular sieve, PTSI | Remove water before it reacts | Under-dosing leaves bubbles |
| Adhesion promoters | GPTMS, APTES | Bond to glass, metal, concrete | Needs the right dose and cure |
| Light stabilizers | UV absorbers, HALS | Gloss and color retention | HALS can clash with acid catalysts |
| Defoamers & flow aids | PDMS, polymeric defoamers | Remove bubbles, improve leveling | Overdosing causes craters |
Additives are concentrated actives, so some are far more hazardous in the drum than in the finished coating. Organometallic catalysts, reactive scavengers and amine catalysts need particular care.
Always follow the product’s Safety Data Sheet, your employer’s safety program and local regulations. This page is general reference information, not a substitute for them.
Organotin compounds such as dibutyltin dilaurate are effective but toxic, and their use is restricted in many EU mixtures under REACH. Bismuth, zinc and zirconium catalysts are common tin-free alternatives.
A UV absorber filters UV light and turns it into heat before it damages the binder. HALS do not absorb UV; they trap the free radicals that form and regenerate to protect for a long time. They work best together.
Most bubbles come from moisture reacting with isocyanate to form CO₂. Dry the raw materials, add moisture scavengers such as molecular sieve or PTSI, avoid damp substrates, and use a defoamer for entrained air.
Its silane end bonds chemically to glass, metal or concrete while its organic end reacts with the coating resin, forming a molecular bridge that improves adhesion, especially in wet conditions.