2 Types of Aromatic Polyisocyanate Curing Agent

1st February 2024

There are three types of aromatic curing agents: TDI ADDUCT (Toluene Diisocyanate Adduct), TDI TRIMER (Toluene Diisocyanate Trimer).

TDI-TMP Polyisocyanate Curing Agent

One commonly used curing agent for two-component polyurethane coatings is TDI-TMP (Toluene Diisocyanate - Trimethylolpropane) adduct. Due to its excellent performance, polyurethane coatings formulated with this curing agent are widely used.

reaction of TDI-TMP

Reaction of TDI-TMP

The TDI-TMP addition reaction is a typical NCO/OH reaction, characterized by the following features:

  1. The reaction is irreversible and requires heating, resulting in stable products without the formation of small molecular by-products.
  2. The two NCO groups on the TDI molecule exhibit different reactivity due to their positional differences, while the three primary hydroxyl groups on the TMP molecule do not show such differences.
  3. The reaction is highly branched and multifunctional.
  4. It is mostly carried out in solution form.
  5. The quality of diisocyanate with three NCO groups per molecule has a significant impact on its compatibility with paint resins, glossiness, and film strength. Therefore, strict quality control is required in the production process. Even slight fluctuations in process conditions or minor changes in formulations can result in substandard products.
  6. To synthesize high-quality TDI-TMP adducts, it is recommended to control the TDI-to-TMP mass ratio within the range of 3.89 to 3.91. TMP and solvents must be dehydrated, and appropriate solvents should be used. The reaction temperature should be maintained between 75 to 95°C, and the reaction time should be between 2.5 to 4 hours. Adding suitable antioxidants not only yields colorless products but also delays yellowing during the storage of adducts and coatings.


Doxu provides aromatic polyisocyanate based on TDI Adduct, for the formulation of two-component polyurethane coatings.

DL-75 PU hardener

Type TDI adduct
Appearance Colorless to pale yellow transparent viscous liquid
Color  (Fe-Co) ≤1
Solid Content% 75±2
NCO Content% 13.0±0.5
Free Monomer% TDI≤0.5
Viscosity (cps) 1500±500
Solubility ≥3.0


TDI Trimer Polyisocyanate Curing Agent

Phenylene diisocyanate (TDI) trimer serves as a cross-linking curing agent in two-component polyurethane coatings. Compared to TDI monomer, it offers advantages such as low volatility and high functionality. Formed by the polymerization of three molecules of TDI itself, the six-membered ring structure of TDI trimer is rigid and highly polar, rendering it soluble only in some highly polar solvents. When paired with hydroxyl resin, it exhibits higher selectivity and poorer pigment wetting.

reaction of TDI TRIMER

Reaction of TDI Trimer

Under the catalytic action, three molecules of TDI monomer undergo a nucleophilic reaction. The nitrogen and oxygen in the isocyanate group have a large electron cloud density and high electronegativity, while the carbon has a small electron cloud density and electropositivity, making the carbon atom electrophilic and susceptible to nucleophilic attack. Catalyzed by a catalyst, isocyanate groups add to each other, leading to a polymerization reaction. Lewis bases are commonly used as catalysts in this reaction.

  1. The O=C-N-R structural unit on the TDI trimer ring exhibits good thermal stability. Moreover, there are no unstable hydrogen atoms on the isocyanurate ring, resulting in excellent thermal stability of products containing isocyanurate rings. Therefore, TDI trimer has become a widely used product in the coating, foam, and plastic stool industries.
  2. The key to the TDI trimerization reaction lies in selecting the appropriate catalyst. There are many types of catalysts for the TDI trimerization reaction, with commonly used types including alkaline metal carboxylates (sodium oxalate), phosphorus-containing compounds (tributylphosphorus), tertiary amines (DMP-30), and organic metal compounds.

Doxu provides aromatic polyisocyanate based on TDI Trimer, for the formulation of  fats-drying two-component polyurethane coatings.

HDX-50BA PU hardener

Type TDI Trimer
Appearance Transparent Liquid
(Fe-Co) Color# ≤1
NCO% 7.8±0.5
Solid Content% 51±2
Free Monomer% TDI≤0.5
Viscosity (cps) 1000±500
Solubility ≥0.6

HDX-960E PU Hardener

Type TDI trimer
Appearance Colorless or slightly yellowish, transparent
Solid Content% 51±2
NCO% 7.2±0.5
Free Monomer% TDI≤0.5
Viscosity/ /25℃  (CPS) 125±50
Solubility ≥3.0