The 2 Types of Aromatic Polyisocyanate Hardeners: TDI Adduct vs TDI Trimer
Updated: 2026
Aromatic Polyisocyanate Hardeners for 2K Wood & Furniture Coatings: TDI Adduct vs TDI Trimer
An aromatic polyisocyanate is a TDI- or MDI-based hardener used to cure 2K polyurethane wood, furniture, parquet and leather coatings for indoor, UV-protected systems. Dongsen supplies two practical aromatic hardener forms: DL-75, a TDI adduct with typical 75% solids, 13.0% NCO and 1500 cps viscosity for fast-curing, flexible, high-gloss finishes; and HDX-H77, a TDI trimer with typical 51% solids, 7.1% NCO and 100 cps viscosity for higher hardness, better chemical/heat resistance and super-high acrylic compatibility.
If you formulate 2K polyurethane wood coatings, furniture lacquers, parquet floor finishes, or leather coatings, the aromatic polyisocyanate hardener you choose directly affects drying speed, film hardness, gloss and cost-performance. Aromatic hardeners — particularly TDI-based — remain the dominant choice for indoor coating systems where UV exposure is not a concern, thanks to their fast cure, high reactivity and excellent cost-effectiveness.
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What Is an Aromatic Polyisocyanate?
An aromatic polyisocyanate is a modified polyisocyanate hardener whose backbone is built from aromatic diisocyanate monomers — most commonly TDI (toluene diisocyanate) or MDI (methylene diphenyl diisocyanate). The defining feature is the benzene ring directly attached to the −NCO group.
This aromatic backbone gives the hardener two key advantages and one key limitation:
- ✅ Higher reactivity — fast room-temperature cure with hydroxyl-functional polyols, ideal for high-throughput wood and furniture lines
- ✅ Lower cost — aromatic diisocyanate monomers are significantly cheaper than aliphatic alternatives (HDI, IPDI)
- ❌ UV yellowing — the benzene–NCO structure oxidizes under sunlight, causing the cured film to yellow over time, which restricts aromatic hardeners to indoor or UV-protected applications
Aromatic hardeners are never used as raw TDI monomer (which is volatile and toxic). Instead the monomer is converted into one of two safe, low-free-monomer, commercially usable forms — TDI adduct or TDI trimer.
Product scope note: Dongsen manufactures polyisocyanate hardeners and crosslinkers only. We do not produce or sell finished paints, epoxy coatings, exterior wall paints, nitrocellulose (NC), alkyd or polyester resins/polyols. Dongsen crosslinkers can be used to formulate the 2K PU coating systems described below.
Type 1: What Is a TDI Adduct (TDI-TMP)?
A TDI adduct is formed by reacting TDI monomer with a small polyol — most commonly trimethylolpropane (TMP). The reaction creates a three-arm, star-shaped molecule with three free −NCO groups available for crosslinking with a hydroxyl resin.
Key Properties of TDI Adduct
- NCO functionality: 3 (three-arm structure)
- Reactivity: very high — typical pot life with polyacrylic resin is 4–8 hours at 25°C
- Cure speed: tack-free in 20–60 minutes; full cure in 16–24 hours at room temperature
- Film properties: flexible, tough, transparent, high-gloss, with good pigment wetting
- Typical solid content: 75% (ester carrier such as butyl acetate / ethyl acetate)
Typical Applications
- Wood furniture lacquers — chairs, tables, cabinets, decorative panels
- Parquet flooring topcoats — indoor wood floor finishes
- Musical instruments — pianos, guitars, violins
- Leather coatings — shoes, upholstery, accessories
- 2K polyurethane adhesives — laminating, packaging, footwear
Dongsen’s TDI Adduct: DL-75
DL-75 is Dongsen's TDI-TMP adduct hardener, with typical 75% solids, 13.0% NCO, free TDI ≤0.5% and 1500 cps viscosity. Its catalog character is flexibility combined with high transparency and high gloss, making it the cost-effective workhorse for high-gloss wood furniture finishes, parquet flooring and PU adhesive systems across India, Southeast Asia and Latin American markets. Where maximum film hardness is the priority within an adduct system, Dongsen also offers higher-hardness TDI adducts such as DX-75P and DX-75NX.
Type 2: What Is a TDI Trimer (Isocyanurate)?
A TDI trimer is formed by reacting three TDI monomers together under controlled catalysis. The −NCO groups join to form a stable, six-membered isocyanurate ring, leaving three free −NCO groups on the outside for crosslinking. The rigid ring is what gives trimers their higher hardness and better heat and chemical resistance.
Key Properties of TDI Trimer
- NCO functionality: 3 (isocyanurate ring with three external NCO arms)
- Reactivity: slightly lower than TDI adduct — longer pot life
- Film properties: higher hardness, better chemical resistance and better thermal stability from the rigid ring
- Compatibility: super-high compatibility with acrylic polyols; gloss finish
- Typical solid content: around 50% depending on grade and solvent
Typical Applications
- High-end wood furniture finishes — premium furniture requiring superior hardness
- Industrial wood lacquers — high-traffic furniture (office, hospitality)
- 2K PU acrylic coatings — where high compatibility and gloss are required
- Specialty indoor industrial coatings — equipment, machinery, indoor metalwork
Dongsen’s TDI Trimer: HDX-H77
HDX-H77 is Dongsen's TDI trimer hardener with typical 51% solids, 7.1% NCO, free TDI ≤0.5% and a low 100 cps viscosity. It offers higher hardness and chemical resistance than DL-75, plus super-high acrylic compatibility and a gloss finish — ideal for premium wood furniture, high-traffic surfaces and acrylic-based indoor coatings. See all aromatic hardener options →
TDI Adduct vs TDI Trimer: Side-by-Side Comparison
| Property | TDI Adduct (DL-75) | TDI Trimer (HDX-H77) |
|---|---|---|
| Backbone structure | TDI + TMP three-arm star | Isocyanurate ring (three TDI units) |
| Typical solids % | 75 ± 2 | 51 ± 2 |
| Typical NCO % | 13.0 ± 0.5 | 7.1 ± 0.5 |
| Free monomer | TDI ≤ 0.5 | TDI ≤ 0.5 |
| Typical viscosity (cps) | 1500 ± 500 | 100 ± 50 |
| Functionality | 3 | 3 |
| Reactivity / pot life | Very high / 4–8 h | High / longer pot life |
| Film hardness | High, flexible & tough | Higher |
| Heat resistance | Standard | Superior |
| Chemical resistance | Good | Better |
| Acrylic compatibility | Good | Super-high |
| Cost | Lower | Slightly higher |
| Best for | General wood & furniture, parquet, leather, adhesives | Premium / acrylic, high-traffic indoor |
All values are typical reference data and are subject to the current TDS/COA for each grade.
How Do You Choose Between DL-75 and HDX-H77?
For most standard wood furniture, cabinet and parquet systems, DL-75 (TDI adduct, 75% solids, 13.0% NCO) is the cost-effective workhorse — fast cure, high gloss, high transparency and a flexible, tough film. Choose HDX-H77 (TDI trimer, 51% solids, 7.1% NCO, 100 cps) when the system needs higher hardness, better heat and chemical resistance, super-high acrylic compatibility, or a longer pot life. Both share the same UV-yellowing limitation, so neither should be used for exterior or strongly sunlit coatings; for those, see Dongsen's aliphatic polyisocyanate hardeners (HDI biuret, HDI trimer).
Hybrid Solutions: When You Need Both Speed and Some UV Resistance
For systems that need the fast cure and hardness of aromatic chemistry plus some weathering resistance, Dongsen offers TDI+HDI hybrid hardeners — HDX-60H (typical 60% solids, 10.5% NCO, matt finish) and HDX-60HL (typical 60% solids, 10.3% NCO, gloss finish). These semi-yellowing grades give better color retention than pure TDI systems at lower cost than pure HDI systems, and suit premium kitchen/cabinet top coats and export furniture needing improved color stability.
Key Takeaways
- Aromatic polyisocyanate hardeners (TDI/MDI) cure fast and cost less, but yellow under UV — use them indoors only.
- DL-75 (TDI adduct): ~75% solids, ~13.0% NCO, ~1500 cps — flexible, tough, high-gloss, cost-effective workhorse.
- HDX-H77 (TDI trimer): ~51% solids, ~7.1% NCO, ~100 cps — higher hardness, better heat/chemical resistance, super-high acrylic compatibility.
- Need both fast cure and some UV resistance? Use TDI+HDI hybrids (HDX-60H / HDX-60HL).
- For true exterior / weathering systems, switch to Dongsen aliphatic HDI hardeners.
About Dongsen: Dongsen is a polyisocyanate hardener and crosslinker manufacturer based in Shaoguan, China, supplying aromatic, aliphatic, solvent-based blocked, water-based blocked, moisture-curing and waterborne (WD) polyisocyanates, plus shoe-sole adhesive hardeners (SA series). Documentation for TSCA, China REACH and IECSC compliance is available on request.
Which TDI Hardener Fits Your Coating System?
If you are formulating a 2K wood lacquer, furniture finish, parquet topcoat, leather coating or PU adhesive, we can help you select the right aromatic polyisocyanate hardener based on your resin chemistry, pot-life requirement, cure conditions and target film performance.
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Looking for a TDI Adduct or TDI Trimer Hardener for Your Wood Coating?
Dongsen manufactures both DL-75 (TDI adduct, ~75% solids, ~13.0% NCO) and HDX-H77 (TDI trimer, ~51% solids, ~7.1% NCO) at our Shaoguan, China base. Tell us about your coating system — resin chemistry, application method and target performance — and we'll recommend the right grade with TDS and free samples.
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FAQ: Aromatic Polyisocyanate Hardeners for 2K PU Coatings
These questions help coating manufacturers compare TDI adduct and TDI trimer hardeners for wood coatings, furniture lacquers, parquet finishes, leather coatings and indoor 2K polyurethane systems.
What are the main types of aromatic polyisocyanate hardeners for 2K PU coatings?
The two main types of aromatic polyisocyanate hardeners used in 2K polyurethane coatings are TDI adduct and TDI trimer. TDI adduct (for example Dongsen DL-75, ~75% solids, ~13.0% NCO) is a cost-effective hardener for wood coatings, furniture lacquers, parquet finishes, leather coatings and PU adhesives. TDI trimer (for example HDX-H77, ~51% solids, ~7.1% NCO) is selected when the system needs higher hardness, better chemical resistance and better heat resistance for premium indoor applications.
What is the difference between TDI adduct and TDI trimer?
TDI adduct is made by reacting TDI with a polyol such as trimethylolpropane, forming a three-arm structure that gives fast curing, high transparency, good pigment compatibility and a flexible, tough film. TDI trimer is built around a rigid isocyanurate ring, giving higher film hardness, better heat resistance and better chemical resistance. For standard wood and furniture coatings, TDI adduct is often the practical choice; for premium indoor coatings, TDI trimer may be more suitable.
When should coating manufacturers choose DL-75 TDI adduct?
DL-75 suits manufacturers formulating 2K PU wood coatings, furniture lacquers, parquet floor finishes, leather coatings and polyurethane adhesive systems. It is a TDI adduct hardener (~75% solids, ~13.0% NCO, ~1500 cps) designed for fast drying, high gloss, good leveling, high transparency, flexibility and cost-effective performance — a good option for stable curing speed in high-volume indoor coating production.
When should coating manufacturers choose HDX-H77 TDI trimer?
HDX-H77 (~51% solids, ~7.1% NCO, ~100 cps) suits systems that need higher film hardness, better chemical resistance, better heat resistance, super-high acrylic compatibility or a longer pot life than a standard TDI adduct system. It fits premium wood furniture coatings, high-traffic indoor surfaces, acrylic-based coatings and systems where final film performance matters more than lowest cost.
Can aromatic polyisocyanate hardeners be used for outdoor coatings?
Pure aromatic polyisocyanate hardeners such as TDI adduct and TDI trimer are not recommended for outdoor or UV-exposed coatings because they can yellow under sunlight. They are mainly used for indoor or UV-protected systems such as wood furniture, parquet, leather and indoor industrial coatings. For exterior coatings or strong color stability, aliphatic hardeners such as HDI biuret or HDI trimer are usually more suitable.
What information should buyers provide before selecting a TDI hardener?
Before selecting a TDI adduct or TDI trimer hardener, buyers should provide the coating application, resin type, NCO-to-OH ratio, target pot life, drying condition, required hardness, gloss level, solvent system and whether the final coating will face UV exposure. With this information, Dongsen can recommend a suitable grade — DL-75 for standard cost-effective wood coatings, or HDX-H77 for higher-performance indoor systems.