2 Types of HDI-based Aliphatic Polyisocyanate Hardener

18th February 2024

Hexamethylene diisocyanate (HDI), also known as 1,6-diisocyanatohexane, is a colorless, transparent liquid with a slightly pungent odor and is highly flammable. It is insoluble in cold water but soluble in organic solvents such as benzene, toluene, and chlorobenzene. HDI has a melting point of approximately 67°C, a relative density of 1.04, a boiling point range of 130~132°C (at 99.725 kPa), a flash point of 140°C, and a refractive index of 1.4530.

HDI possesses a unique linear saturated chemical structure. The molecule contains highly unsaturated double-bonded isocyanate groups (-NCO), making its chemical properties very reactive. It can react with alcohols, acids, and amines, decompose when exposed to water or alkalis, and polymerize in the presence of copper, iron, and other metal oxides. Unlike other isocyanates, HDI's structure lacks benzene rings directly connected to the -NCO groups, thus exhibiting excellent resistance to yellowing. HDI and its derivatives are used as raw materials in various coatings and adhesives.

HDI is the most important monomer in aliphatic diisocyanates (ADI), accounting for approximately 60% of the total demand for ADI. Most HDI is prepared as HD biuret or HDI trimer. HDI, HD biuret, and HDI trimer exhibit excellent weather resistance and heat resistance, making them essential materials for producing polyurethane coatings and elastomers.

In the manufacturing process of curing agents, due to the small molecular weight and high vapor pressure of HDI monomers, they are generally modified before use due to their toxicity. Common modified products include HDI biuret and HDI trimer.

HDI biuret aliphatic polyisocyanate

HDI biuret, as an aliphatic polyurethane curing agent, exhibits excellent weather resistance and good compatibility with solvents such as polyols or butyl acetate. After curing with polyols, it demonstrates good solvent resistance properties, making it widely used in high-end coatings, adhesives, and elastomers.

HDI biuret refers to a polyisocyanate prepolymer with a biuret structure (NCO-NH-CO-NH) synthesized from HDI and another component. It is one of the curing agents widely used in aliphatic polyurethane coatings. Biuret is formed by the reaction of hexamethylene diisocyanate (HDI) with water. There are mainly two steps: the first step involves the reaction of isocyanate with water to form an amine, followed by its reaction with HDI to form ureido diisocyanate. The second step involves the reaction of ureido diisocyanate with HDI to form the biuret structure of triisocyanate (referred to as biuret).

The synthesis of HDI biuret typically involves the gradual addition of water, ensuring that the ratio of n(HDI):n(H2O) in each region of the reactor far exceeds 3, which favors the production of products close to the ideal structure. The addition of water is commonly performed using a method involving the dropwise addition of fine water droplets. The reaction temperature is divided into two stages: the first stage is controlled at around 97°C, a temperature close to the boiling point, which helps in the rapid dispersion of the water droplets into finer droplets; the second stage is maintained between 128~130°C, which is the optimal reaction temperature for HDI biuret formation.

DOXU HDX175-MPX is an aliphatic polyisocyanate curing agents/crosslinkers based on HDI biuret. It can be used as the hardener component for lightfast and weather-stable two-component polyurethane coating or adhesive systems.

Product Specification HDX-175MPX
Type HDI Biuret
Appearance Colorless to pale yellow transparent viscous liquid
Color (Hazen)# <1
NCO% 16.5±0.5
Free Monomer% HDI≤0.5
Solid content% 75±1
Viscosity/ /25℃  (CPS) 130~300

HDI trimer aliphatic polyisocyanate

Under catalysis, HDI can undergo trimerization reactions. The structure of the isocyanurate ring in HDI trimer is stable, giving it advantages such as good thermal stability, abrasion resistance, and corrosion resistance. It is commonly used as a curing agent for polyurethanes in various fields such as furniture, automotive, aerospace, and sports equipment industries.

Under the action of catalysts, HDI self-polymerizes to form HDI trimer, or it can copolymerize with TDI to form TDI-HDI mixed trimers. Trimerization is irreversible. There are no active hydrogen atoms on the isocyanurate ring of HDI, so it does not form hydrogen bonds, resulting in low viscosity. The strong electron-withdrawing effect of the isocyanurate ring increases the reactivity of -NCO in the molecule. Using HDI trimer as a curing agent can formulate fast-drying coatings. The isocyanurate ring of the trimer has high resistance to light and heat, making it less prone to decomposition at high temperatures and allowing for long-term storage. HDI trimer curing agents exhibit excellent weather resistance and heat resistance, surpassing other HDI-derived curing agents, and comply with today's environmental requirements for high solid content and low viscosity characteristics.

DOXU HDX-190 & HDX-100 are aliphatic polyisocyanate curing agents/crosslinkers based on HDI Trimer.

Product Specification HDX-190
Type HDI Trimer
Appearance Colorless to pale yellow transparent viscous liquid
Color (Hazen) ≤60
NCO% 19.8±0.5
NCO equivalent weight Ca. 212
Free Monomer% HDI≤0.5
Solid content% 90±1
Viscosity/ /25℃  (CPS) 450~650


Product Specification HDX-100
Type HDI Trimer
Appearance Colorless to pale yellow transparent viscous liquid
Color (Hazen) ≤60
NCO% 22±0.5
NCO equivalent weight Ca. 191
Free Monomer% HDI≤0.5
Solid content% 100
Viscosity/ /25℃  (CPS) 2500-4000