Aromatic Polyisocyanate Hardener: TDI Adduct vs TDI Trimer Selection Guide
Updated: October 2026
TDI Adduct vs TDI Trimer: Which Route Fits Your PU Coating?
Quick answer: A TDI adduct is made by reacting TDI with a polyol such as TMP; a TDI trimer contains an isocyanurate ring. Adducts are often screened for flexibility and balanced indoor PU film performance. Trimers are often considered when faster hardness development or compatibility with a particular resin system is the priority. Choose by hardness, flexibility, compatibility, drying speed, finish requirement and application system, then test the specific grades under matched conditions. Neither chemistry is automatically better, and aromatic TDI systems are generally intended for indoor use.
For PU coating formulators, technical buyers, coating manufacturers and distributors, the practical task is to select a hardener that fits the resin and production process. This guide compares the two routes without treating a chemistry name as a performance guarantee. Dongsen manufactures polyisocyanate hardeners and crosslinkers, supplied as formulation raw materials.
For the wider formulation route, see our Solventborne 2K solutions and aromatic polyisocyanate hardener range.
What Is an Aromatic Polyisocyanate?
An aromatic polyisocyanate is a modified isocyanate hardener based on aromatic diisocyanates such as TDI (toluene diisocyanate) or MDI (methylene diphenyl diisocyanate). In TDI, the NCO groups are bonded directly to the aromatic ring. Modified hardeners retain reactive NCO groups while changing the molecular architecture.
For broader chemistry context, read the Isocyanate Hardener Guide. Three points matter in practice:
- Interior 2K PU fit: TDI-based aromatic hardeners are widely used in indoor polyurethane coating systems, including wood and furniture finishes.
- UV-yellowing limitation: aromatic isocyanate chemistry is prone to yellowing under UV exposure, so these hardeners are normally positioned for interior or UV-protected systems rather than weathering-focused coatings.
- Modified commercial forms: coating hardeners are supplied as modified polyisocyanates, such as TDI adducts and TDI trimers, which give formulators practical crosslinker forms with controlled product specifications.
Product scope note: Dongsen supplies polyisocyanate hardeners and crosslinkers as raw materials. It does not supply the finished wood paints or furniture coatings discussed in this article.
Type 1: What Is a TDI Adduct (TDI-TMP)?
A TDI adduct is made by reacting TDI with a multifunctional polyol such as trimethylolpropane (TMP). The resulting modified polyisocyanate retains reactive NCO groups that can crosslink with hydroxyl-functional resins in a 2K polyurethane system.
What Matters When Selecting a TDI Adduct?
- Solids and NCO content: these affect formulation solids, hardener dosage and the stoichiometric calculation.
- Viscosity and dilution tolerance: these influence handling, mixing and solvent adjustment.
- Flexibility direction: a TDI adduct is a useful starting direction when flexibility must be balanced with hardness; confirm the result in the complete coating formulation.
- Formulation-dependent cure: pot life, dry speed and final film performance depend on the complete system — resin, catalyst, solvent, temperature and NCO/OH balance — and should not be read as fixed values from the hardener name alone.
Typical Application Direction
- Interior 2K polyurethane wood coatings
- Furniture lacquers, with the required gloss or matte finish verified in the formulation
- Other formulations only after product-specific technical validation
Dongsen TDI Adduct: DL-75
DL-75 is a Dongsen TDI adduct grade to screen for indoor 2K PU wood and furniture formulations. Current product data lists 75 ± 2% solids, 13.0 ± 0.5% NCO, free TDI ≤ 0.5% and 1500 ± 500 cps viscosity at 25°C. All values are typical and subject to the current TDS/COA.
Its supplied NCO content is higher than that of MF-77 in the comparison below. This affects the hardener dose needed to reach a target NCO/OH ratio; it does not establish a higher-performance coating. If flexibility is the reason for screening an adduct, measure it alongside hardness, compatibility and drying behavior.
For substrate and finish context, see the Polyurethane Wood Coatings Guide.
Type 2: What Is a TDI Trimer (Isocyanurate)?
A TDI trimer is formed by trimerizing TDI into an isocyanurate ring while retaining reactive NCO functionality for crosslinking. The ideal simple trimer has three residual NCO groups, giving higher functionality than the original difunctional TDI monomer. A simple TDI–TMP adduct can also have three residual NCO groups: trimer does not automatically mean higher functionality than every adduct. Commercial grades can contain different oligomer distributions; compare supplier data rather than assuming identical functionality within a family.
What Matters When Selecting a TDI Trimer?
- Hardness development: a TDI trimer is often considered when early hardness is a priority. Confirm both the hardness build and the usable working time in the target system.
- Compatibility: some trimer grades can offer advantages with particular acrylic or nitrocellulose-containing resin packages. The actual resin, solvent and mixing ratio decide whether that advantage is present.
- Viscosity: a lower-viscosity grade may simplify mixing and handling, although final application viscosity still depends on the whole system.
- Gloss direction: select the grade against the required clear, gloss or matte finish; the trimer label alone does not specify the finish or drying speed.
- Working window: pot life and viscosity development must be validated at the actual resin ratio, catalyst level, solvent package and temperature.
Typical Application Direction
- Interior wood and furniture coatings
- Gloss-finish 2K PU systems
- Acrylic-containing wood formulations, subject to grade-specific compatibility testing
- Other end uses only after product-specific validation
Dongsen TDI Trimer: MF-77
MF-77 is a current Dongsen TDI trimer grade to evaluate for indoor wood and furniture formulations. Current product data lists 51 ± 2% solids, 7.1 ± 0.5% NCO, free TDI ≤ 0.5% and 100 ± 50 cps viscosity at 25°C. All values are typical and subject to the current TDS/COA.
MF-77's supplied viscosity is lower than DL-75's, but the two grades also have different supplied solids. Use these values to plan mixing and dosage trials. If acrylic compatibility, a gloss finish or early hardness is the objective, confirm each in the actual resin package. Compare drying speed and hardness at matched cure ages; a lower supplied viscosity does not establish either outcome.
For comparisons within the trimer family, continue to the TDI Trimer Selection Guide.
TDI Adduct vs TDI Trimer: What Do DL-75 and MF-77 Specifications Tell You?
| Selection Variable | DL-75 | MF-77 |
|---|---|---|
| Chemistry | TDI adduct | TDI trimer |
| Typical supplied solids | 75 ± 2% | 51 ± 2% |
| Typical supplied NCO | 13.0 ± 0.5% | 7.1 ± 0.5% |
| Free TDI | ≤ 0.5% | ≤ 0.5% |
| Viscosity at 25°C | 1500 ± 500 cps | 100 ± 50 cps |
| Screening question | Can the adduct route deliver the required flexibility and hardness balance? | Does this trimer grade meet the resin compatibility, hardness and finish targets? |
| Application context | Indoor 2K PU wood and furniture formulation screening | Indoor 2K PU wood and furniture formulation screening |
| Outdoor color retention | Aromatic route: yellowing must be considered | Aromatic route: yellowing must be considered |
All values are typical and subject to the current TDS/COA. Solids and NCO are supplied-product values; coating performance requires formulation testing.
Practical takeaway: DL-75 has 13.0 ± 0.5% supplied NCO at 75 ± 2% solids, while MF-77 has 7.1 ± 0.5% at 51 ± 2% solids. Replacing equal weights would change the NCO/OH balance and solvent contribution. Recalculate the dose and normalize formulation solids before comparing the films.
How to Choose Between TDI Adduct and TDI Trimer?
Use the following directions to decide what to screen first. They are starting points for testing, not guaranteed rankings of DL-75 and MF-77.
| Requirement | Preferred Direction | What to Confirm |
|---|---|---|
| Need flexibility | TDI Adduct | Flexibility, adhesion and hardness together at the required cure age. |
| Need faster hardness development | TDI Trimer | Early hardness, sanding or stacking readiness and usable pot life in the same resin system. |
| Need acrylic compatibility | TDI Trimer direction | Clarity, phase stability and viscosity with the actual acrylic resin and solvent package; not all trimers behave alike. |
| Need balanced indoor PU coating performance | TDI Adduct | Hardness/flexibility balance, drying profile and finish at the target NCO/OH ratio. |
| Need a particular gloss or matte finish | Grade-specific screening | Gloss, clarity and matting response; chemistry family alone does not determine appearance. |
| Need exterior color retention | Aliphatic hardener direction | Weathering and color retention of the complete coating; aromatic TDI is generally an indoor route. |
Selection takeaway: start with the requirement that is hardest to meet, then check the remaining constraints. A faster hardness build is useful only if compatibility, flexibility, finish and working time also meet the process target.
How Do You Choose Between DL-75 and MF-77?
Screen DL-75 as an adduct candidate and MF-77 as a trimer candidate using their current TDS and your resin data. Select on matched formulation results, rather than a "standard versus premium" label. For distributors, confirm the buyer's resin system and finish target before proposing a grade.
If the application demands exterior color retention, evaluate aliphatic polyisocyanate hardeners and test the full coating against its exposure requirements.
What If a Pure TDI Route Does Not Meet the Color-Stability Requirement?
TDI/HDI hybrid hardeners provide another chemistry direction to discuss, but the presence of HDI does not make an aromatic-containing grade automatically non-yellowing. Keep any hybrid evaluation tied to the specific grade and exposure requirement. For exterior or light-color coatings where color retention is critical, an aliphatic route is usually the starting point for screening.
Next Test to Run Before Changing TDI Hardener
Before replacing one aromatic hardener route with another, run the candidates under matched formulation conditions rather than comparing isolated TDS values. Use the same resin, target NCO/OH ratio, catalyst level, solvent package, total solids, dry-film thickness, application method, cure temperature and humidity so the result reflects the hardener change rather than a second variable.
- Set the stoichiometric basis: use the actual resin OH value and target NCO/OH ratio to calculate the hardener level for each candidate.
- Check the mixed system first: record clarity, compatibility and initial mixed viscosity before judging film performance.
- Track the working window: compare viscosity development and the usable pot-life window under the same temperature and catalyst conditions.
- Compare the cured result: record hardness at matched cure ages, flexibility, adhesion, gloss and the actual sanding/recoating/stacking endpoint. A tack-free surface alone does not establish through-cure or handling readiness.
For a defined hardness comparison, ASTM D4366 pendulum damping tests provide a recognized method. Use one agreed method throughout the trial; different hardness methods should not be treated as interchangeable.
Open Variables to Confirm Before Grade Selection
- Resin chemistry and OH value: these determine the stoichiometric starting point and can change compatibility behavior.
- Catalyst, solvent package and total solids: these can shift viscosity development, working window and application behavior.
- Process target: required pot life, drying schedule, gloss target, final film requirements and expected UV exposure should be defined before the grade is locked.
Key Takeaways
- TDI adduct and TDI trimer are not interchangeable labels. They are different modified aromatic polyisocyanate architectures and should be selected against the actual formulation target.
- DL-75: TDI adduct, 75 ± 2% solids, 13.0 ± 0.5% NCO, 1500 ± 500 cps; screen the required flexibility/hardness balance in your formulation.
- MF-77: TDI trimer, 51 ± 2% solids, 7.1 ± 0.5% NCO, 100 ± 50 cps; check compatibility, finish and early hardness in your actual resin system.
- Do not select on headline performance alone. Validate resin compatibility, NCO/OH ratio, pot life, viscosity development, dry profile and final film performance under the same formulation conditions.
- Aromatic TDI grades are interior-focused. For strong UV resistance and exterior color stability, evaluate aliphatic HDI hardeners.
Which TDI Hardener Fits Your Wood Coating System?
If you are evaluating a hardener for an interior 2K wood or furniture coating, send Dongsen your resin type, NCO/OH ratio, solids target, mixed viscosity, required pot life, drying conditions, gloss target, solvent package and UV exposure. These details are more useful for grade selection than a general request for "TDI adduct" or "TDI trimer."
Share your formulation and selection priorities with Dongsen to discuss a grade direction and sample evaluation.
Looking for a TDI Adduct or TDI Trimer Hardener for Your Wood Coating?
Dongsen supplies polyisocyanate hardener and crosslinker raw materials. For a comparison of DL-75 and MF-77, share your resin chemistry, formulation ratio, application method and performance targets so our technical team can discuss a grade direction for testing.
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- 🧪 Samples for qualifying formulation projects
- 💬 Commercial quotation based on grade and requirements
- 🔬 Grade-selection support based on your resin system and process
FAQ: TDI Adduct vs TDI Trimer Hardeners for 2K PU Wood Coatings
The questions below cover how coating formulators can compare aromatic TDI-based hardeners for interior 2K polyurethane wood and furniture systems.
What is the practical difference between a TDI adduct and a TDI trimer?
A TDI adduct is made by reacting TDI with a multifunctional polyol such as TMP, while a TDI trimer contains an isocyanurate ring formed by TDI trimerization. In practice, the architecture shapes the available grade profile, so formulators should compare NCO content, solids, viscosity, compatibility, working behavior and finish direction rather than choosing by chemistry name alone.
When should a formulator consider DL-75?
Consider DL-75 for a TDI adduct screening trial when its solids, NCO and handling profile fit an indoor 2K PU formulation. Confirm flexibility, hardness, drying behavior and finish in the target resin system; the product specifications do not establish those outcomes.
When should a formulator consider MF-77?
Consider MF-77 when evaluating a TDI trimer route and its supplied solids, NCO and viscosity fit the mixing plan. If acrylic compatibility or a particular gloss finish is the reason for the trial, verify those requirements with the actual resin and solvent package.
Is TDI trimer better than TDI adduct?
No. TDI trimer and TDI adduct address different formulation requirements. A trimer is often screened for early hardness development or resin compatibility; an adduct is often screened for a flexibility/hardness balance. Neither route guarantees a better coating, and commercial grades must be compared under matched conditions.
Can I replace a TDI adduct with a TDI trimer at the same mixing ratio?
Do not assume that equal-weight replacement preserves the formulation. Recalculate the dose from the supplied NCO content and resin OH value, adjust the solvent and solids balance, then retest compatibility, pot life, drying and film properties.
Can aromatic TDI-based hardeners be used outdoors?
They are generally positioned for interior or UV-protected systems, because aromatic isocyanate chemistry can yellow under UV exposure. Where long-term color retention and exterior weathering resistance are key requirements, evaluate an aliphatic HDI hardener instead.
What information should buyers provide before selecting a TDI hardener?
Provide the application, resin type, NCO/OH ratio, solids target, mixed viscosity, target pot life, drying or baking conditions, gloss level, solvent system, required film properties and expected UV exposure. This allows the hardener to be screened against the actual formulation rather than chosen by model name alone.