3 Types of Polyurethane Curing Agents
Updated: 2026
Polyisocyanate Curing Agents: 3 Types, Selection Guide and Dongsen Grades
Need a PU Curing Agent for Your Coating System?
If you are formulating wood coatings, waterborne PU coatings, automotive refinish, can and coil coatings, flexible packaging adhesives, or selected PU floor coating systems, the curing agent is not just an additive. It decides crosslink density, pot life, drying speed, hardness, flexibility, chemical resistance, and long-term appearance.
Dongsen supplies polyisocyanate raw materials and crosslinkers for formulators. Send your resin type, solvent or waterborne system, baking temperature, target performance, and estimated monthly demand. Our team can help narrow the suitable grade direction.
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What Is a Polyurethane Curing Agent?
A polyurethane curing agent is usually an isocyanate-functional crosslinker used with hydroxyl-functional resins or other active-hydrogen components. In a PU coating or adhesive formulation, the NCO groups react with the resin side to build a crosslinked polyurethane network.
For buyers, the practical question is not only which curing agent is stronger. The right grade depends on the coating system, resin compatibility, solvent family, drying or baking process, pot life, yellowing resistance, film hardness, flexibility, and final use environment.
3 Main Types of Polyurethane Curing Agents
1. Solvent-Based Polyisocyanates for 2K PU Systems
Solvent-based polyisocyanates are the most common 2K PU crosslinkers. The hardener is mixed with a hydroxyl-functional resin before application, then the NCO groups react with the resin -OH during drying to build the polyurethane network.
The first split is aromatic versus aliphatic. Aromatic TDI-based grades dry fast and give high hardness at low cost, which suits indoor wood and furniture coatings. Aliphatic HDI/IPDI-based grades hold gloss and resist yellowing, so they are the choice for automotive refinish, outdoor furniture, and durable industrial topcoats.
The second split is biuret versus trimer, and this is a structural difference, not just a number on the TDS. An HDI biuret has longer, more linear urea-linked chains, so the cured film is more flexible and adheres better — useful for clear coats and substrates that flex. An HDI trimer (isocyanurate) carries a rigid ring core and a higher effective functionality, which raises crosslink density: harder film, faster cure, and stronger weathering. The resin side also matters — a hydroxyl acrylic pushes toward a harder, faster-curing film, while a polyester-OH leans toward flexibility and adhesion, so the biuret/trimer choice is usually made together with the resin, not in isolation.
- TDI trimer / adduct: indoor wood coating where drying speed, hardness, and cost-performance lead.
- HDI biuret: when flexibility, adhesion, and a balanced clear coat matter more than peak hardness.
- HDI trimer: when higher hardness, faster cure, and weathering resistance are the priority.
2. Water-Dispersible Polyisocyanates for 2K Waterborne Systems
Waterborne PU systems need curing agents that can disperse into the waterborne resin phase. These grades are usually hydrophilically modified HDI-based polyisocyanates. They are used when the formulator wants lower VOC, better waterborne process compatibility, and good film performance in wood, industrial, floor, or adhesive systems.
Waterborne selection should be tested together with the resin dispersion. Key checks include mixing stability, pot life, gloss, film transparency, water resistance, chemical resistance, and final hardness or flexibility.
3. Blocked Polyisocyanates for 1K Baking Systems
Blocked polyisocyanates keep NCO groups temporarily blocked at room temperature. During baking, the blocking group releases and the isocyanate reacts with hydroxyl-functional resin. This makes them suitable for one-component storage-stable systems where the film is cured by heat.
In real selection, deblocking temperature is one of the most important buyer-side filters. For lower baking windows, use grades such as DXB-3370G at 80–100°C. For medium baking systems, DXB-3375D works at 130–140°C. For higher baking windows around 150°C, DXB-3375E or corresponding water-based blocked grades are more relevant.
Dongsen Grade Comparison for Curing Agent Selection
The following table uses typical values for quick screening. All values are typical and subject to the current TDS/COA.
| Direction | Dongsen Grade | Chemistry / Type | Solid % | NCO / Valid NCO % | Viscosity (cps) | Selection Logic |
|---|---|---|---|---|---|---|
| Indoor 2K wood coating | HDX-50BA | TDI Trimer | 51±2 | 7.8±0.5 | 1000±500 | Fast drying, matte wood coating, high-throughput indoor furniture systems. |
| Indoor 2K wood coating | HDX-960E | TDI Trimer | 51±2 | 7.2±0.5 | 110±50 | Low viscosity and high compatibility for acrylic-based wood coating systems. |
| Non-yellowing 2K PU | HDX-175MPX | HDI Biuret | 75±1 | 16.5±0.5 | 210±90 | Balanced flexibility, adhesion, and clear-coat performance. |
| High-hardness aliphatic 2K PU | HDX-190 | HDI Trimer | 90±1 | 19.8±0.5 | 525±75 | Fast drying and higher hardness for aliphatic PU systems. |
| High-solid aliphatic 2K PU | HDX-100 | HDI Trimer | 100 | 22.0±0.5 | 3250±750 | High NCO and high solid build when formulation viscosity can be managed. |
| Waterborne 2K PU | WD-8780/8980 | Water-dispersible HDI | 80.0±2.0 | 15.0±1 | 300±200 | Waterborne coating and adhesive performance improvement. |
| Low-temperature 1K baking | DXB-3370G | Blocked HDI Trimer | 70.0±2.0 | 7.5±0.5 | 2000±1000 | 80–100°C deblocking, PMA carrier; for lower baking window systems. |
| Medium-temperature 1K baking | DXB-3375D | Blocked HDI Trimer | 75.0±2.0 | 11.2±0.5 | 2500±1000 | 130–140°C deblocking, Solvent Naphtha 100 carrier; for can, coil, and baking systems. |
| Water-based blocked baking | BI-70D | Anionic blocked HDI trimer | 70 | 7.2±0.5 | 4000±1500 | 130–140°C deblocking, PMA carrier. Anionic route for waterborne baking systems. |
| Water-based blocked baking | HBY-70D | Cationic blocked HDI trimer | 70 | 7.8±0.5 | 2000±500 | 130–140°C deblocking, PMA carrier. Cationic route with lower viscosity for easier handling. |
Application Selection Guide
For Wood and Furniture Coating Manufacturers
For fast drying and high hardness in an indoor solventborne 2K PU wood coating, start from the TDI-based aromatic grades. HDX-50BA gives faster through-dry and suits matte furniture lines, while HDX-960E is the pick when low viscosity and acrylic compatibility matter more.
Relevant pages: PU wood and furniture coating applications, TDI trimer hardener selection guide.
For Automotive Refinish, Outdoor Furniture, or Non-Yellowing Systems
When yellowing resistance is required, move from aromatic TDI to aliphatic HDI/IPDI grades. HDX-175MPX is a balanced HDI biuret for flexibility and adhesion. Step up to HDX-190 or HDX-100 when you need higher hardness, faster drying, or higher solid content.
For Waterborne 2K PU Coatings
Use a water-dispersible HDI-based polyisocyanate rather than a standard solventborne hardener. WD-8780/8980 fits waterborne 2K PU systems, with 80.0±2.0% solids and 15.0±1% NCO. Test dispersion stability, pot life, and film appearance before scale-up.
Related article: Waterborne polyisocyanate hardener for 2K PU coatings.
For Can, Coil, Flexible Packaging Adhesive, and Baking Systems
For 1K baking systems, match the grade to your actual baking window. DXB-3370G deblocks at 80–100°C, DXB-3375D at 130–140°C, and DXB-3375E at around 150°C. The carrier also has to fit the line: DXB-3370G is supplied in PMA, while DXB-3375D/E use Solvent Naphtha 100.
Relevant pages: can and coil coatings, flexible packaging adhesives.
For PU Floor Coating Systems
Only review the PU floor coating direction when the target system is polyurethane-based. Do not treat epoxy floor coatings as a match. For moisture-curing or PU floor-related systems, review the moisture curing polyisocyanate direction and confirm substrate, solvent system, curing condition, and abrasion-resistance target.
Hybrid and Related Dongsen Solutions
Some buyers search for polyurethane curing agent but actually need a narrower raw material category. Use this map to avoid choosing the wrong direction:
- Isocyanate hardener category: overview of Dongsen polyisocyanate crosslinker options.
- Aromatic polyisocyanates: indoor wood and furniture coating speed, hardness, and cost-performance.
- Aliphatic polyisocyanates: non-yellowing and weather-resistant 2K coating direction.
- Solvent-based blocked polyisocyanates: 1K baking coating and heat-activated crosslinking direction.
- Water-based blocked polyisocyanates: waterborne or water-dispersible baking systems.
- Waterbased polyisocyanates: 2K waterborne coating and adhesive performance improvement.
- Moisture curing polyisocyanates: one-component PU systems that cure with atmospheric moisture.
- SA series polyisocyanates: shoe-sole adhesive hardener direction.
How to Ask Dongsen for a Suitable Grade
To reduce back-and-forth, send the following information when requesting a recommendation:
- Coating or adhesive type: wood coating, waterborne coating, automotive refinish, can and coil, flexible packaging adhesive, PU floor coating, or footwear adhesive.
- System type: solventborne 2K, waterborne 2K, 1K baking, water-based blocked, or moisture-curing PU.
- Resin side: hydroxyl acrylic, polyester-OH, PUD, or other resin type used in your formulation.
- Process: room-temperature dry, forced dry, baking temperature and time, spray/roller/curtain/coating line condition.
- Target result: fast drying, high hardness, flexibility, non-yellowing, gloss, chemical resistance, lower VOC, or improved pot life.
- Current TDS or benchmark requirements, monthly demand, destination country, and preferred packaging.
Request TDS Request Sample Request Quotation Go to Inquiry Form
Send Your Requirement
Share your application and current formulation direction. Dongsen can help you review a suitable polyisocyanate curing agent route.
- ✅ Request TDS and typical parameters
- ✅ Ask for sample evaluation
- ✅ Get a quotation based on quantity and destination
- ✅ Compare solventborne, waterborne, and blocked curing agent routes
- ✅ Review grade direction before scale-up
FAQ
What are the three main types of polyurethane curing agents?
For PU coating formulators, the three common directions are solvent-based polyisocyanates for 2K systems, water-dispersible polyisocyanates for 2K waterborne systems, and blocked polyisocyanates for 1K baking systems.
How do I choose between TDI and HDI curing agents?
TDI-based aromatic grades are usually selected for indoor wood and furniture coatings where fast drying and hardness are important. HDI-based aliphatic grades are preferred when better yellowing resistance and outdoor or automotive durability are required.
Which Dongsen grade is suitable for fast-drying indoor wood coatings?
HDX-50BA can be reviewed for fast-drying indoor 2K PU wood and furniture coatings. It is a TDI trimer with 51±2% solids, 7.8±0.5% NCO, and 1000±500 cps viscosity.
Which grade should I review for waterborne 2K PU coatings?
WD-8780/8980 can be reviewed for waterborne 2K PU coating systems. It is an HDI-based water-dispersible polyisocyanate with 80.0±2.0% solids, 15.0±1% NCO, and 300±200 cps viscosity.
When should I use blocked polyisocyanates?
Blocked polyisocyanates are used when the formulation needs one-component storage stability and heat activation during baking. For example, DXB-3370G is designed for 80–100°C deblocking, while DXB-3375D is designed for 130–140°C baking systems.
Does Dongsen sell finished paints or epoxy coatings?
No. Dongsen supplies polyisocyanate raw materials and crosslinkers for formulators. Dongsen does not sell finished paints, epoxy coatings, wall paint, nitrocellulose, or polyester resin.
Can Dongsen recommend a grade from my current formulation?
Yes. Share your coating type, resin system, solvent or waterborne system, baking temperature, target hardness or flexibility, estimated quantity, and current TDS. Dongsen can help narrow the suitable curing agent direction.