HDI Biuret vs HDI Trimer: The 2 Types of HDI-based Aliphatic Polyisocyanate Hardeners

For Automotive Refinish, Industrial Coating, and Clear Coat Formulators

If you formulate automotive refinish clear coats, industrial 2K PU coatings, exterior architectural finishes, or any weather-exposed polyurethane system, the choice of HDI-based aliphatic hardener directly determines UV stability, color retention, film hardness, and chemical resistance. HDI-based hardeners — particularly biuret and trimer types — are the industry standard for non-yellowing, weather-stable polyurethane coatings worldwide.

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What Is HDI and Why Aliphatic Matters

Hexamethylene diisocyanate (HDI), also known as 1,6-diisocyanatohexane, is a linear aliphatic diisocyanate — a colorless, transparent liquid with the chemical structure OCN−(CH₂)₆−NCO. Unlike aromatic diisocyanates (TDI, MDI), HDI's structure lacks the benzene ring directly connected to the −NCO groups, which is the root cause of its outstanding weather resistance.

Key Physical Properties of HDI Monomer

• Density: 1.04 g/cm³ at 20°C
• Boiling point: 130–132°C (at 99.7 kPa)
• Flash point: 140°C
• Refractive index: 1.4530
• Solubility: Insoluble in cold water; soluble in benzene, toluene, butyl acetate, chlorobenzene, and most organic coating solvents

Why HDI Is the Workhorse for Non-Yellowing Coatings

HDI accounts for approximately 60% of total aliphatic diisocyanate demand globally. Its dominance in weather-stable coatings comes from three properties:

• ✅ Excellent UV resistance — no benzene-NCO oxidation, so cured films do not yellow under sunlight
• ✅ Outstanding weather and thermal stability — suitable for outdoor and elevated-temperature service
• ✅ Good solvent and polyol compatibility — formulator-friendly in most 2K PU systems

However, raw HDI monomer is never used directly in coatings. The monomer has low molecular weight, high vapor pressure, and significant toxicity. For commercial use, HDI is converted into modified polyisocyanates — most commonly HDI biuret or HDI trimer — which have higher molecular weight, lower volatility, and much safer handling characteristics while retaining HDI's non-yellowing performance.

Type 1: HDI Biuret

HDI biuret is a polyisocyanate prepolymer synthesized by reacting HDI with a small amount of water, forming a characteristic biuret structure (–NH–CO–N–CO–NH–) with three free −NCO groups available for crosslinking with hydroxyl resins.

Key Properties of HDI Biuret

• NCO functionality: 3 (three reactive arms per molecule)
• Excellent weather resistance: Non-yellowing under prolonged UV exposure
• Good solvent compatibility: Compatible with butyl acetate, ethyl acetate, xylene, and most coating solvents
• Polyol compatibility: Excellent with hydroxyl-functional polyacrylics and polyesters
• Film properties: Slightly more flexible than HDI trimer, with very good chemical and solvent resistance after cure
• Free HDI monomer: Typically ≤0.5% (low free-monomer grades for safer handling)

Typical Applications

• Automotive refinish clear coats — the workhorse hardener for body shop 2K clear systems
• Exterior architectural metal coatings — for steel structures, façades, doors, and windows
• Industrial anti-corrosion coatings — top coats over zinc-rich or epoxy primers
• Wood coatings for export markets — where indoor furniture may face some UV exposure
• High-end PU adhesives — for laminating and bonding applications requiring durability
• Plastic and rubber coatings — for substrates with flexibility requirements

Dongsen’s HDI Biuret: HDX-175MPX

HDX-175MPX is Dongsen's HDI biuret aliphatic polyisocyanate hardener for lightfast and weather-stable 2K PU coatings exposed to outdoor environments where non-yellowing is critical.

Property	Specification
Type	HDI Biuret
Appearance	Colorless to pale yellow transparent viscous liquid
Color (Hazen)	< 1
NCO %	16.5 ± 0.5
Free HDI Monomer %	≤ 0.5
Solid Content %	75 ± 1
Viscosity at 25°C (cps)	130 – 300

View full HDX-175MPX TDS and product details →

Type 2: HDI Trimer (Isocyanurate)

HDI trimer is formed by reacting three HDI molecules together under controlled catalysis. The reaction produces a stable, six-membered isocyanurate ring with three external −NCO arms — a rigid, thermally stable backbone that gives HDI trimer its distinctive performance profile.

Key Properties of HDI Trimer

• NCO functionality: 3 (three reactive arms extending from the isocyanurate ring)
• Premium weather resistance: Best-in-class non-yellowing, color retention, and gloss retention under prolonged UV
• Higher film hardness: The rigid isocyanurate ring delivers harder, more chemical-resistant films than HDI biuret
• Excellent thermal stability: The isocyanurate ring resists thermal degradation, suitable for higher-temperature service
• Low viscosity (at 100% solids): Because the isocyanurate ring lacks hydrogen-bonding −NH groups, HDI trimer has unusually low viscosity for its molecular weight, enabling high-solids low-VOC formulations
• High reactivity: The electron-withdrawing isocyanurate ring increases NCO reactivity, supporting fast-drying coating systems

Typical Applications

• Premium automotive OEM and refinish clear coats — where best-in-class gloss and color retention are non-negotiable
• Aerospace coatings — for aircraft exterior finishes
• High-end industrial machinery coatings — equipment used outdoors or in harsh environments
• Sports equipment and recreational vehicle coatings — bicycles, motorcycles, marine
• Wind turbine blade coatings — extreme weather durability requirements
• High-solids low-VOC coatings — leveraging HDI trimer's inherently low viscosity

Dongsen’s HDI Trimer: HDX-100

HDX-100 is Dongsen's HDI trimer hardener at 100% solids, designed for premium clear coats, high-solids coatings, and applications demanding the highest level of weather resistance and film hardness.

Property	Specification
Type	HDI Trimer (Isocyanurate)
Appearance	Colorless to pale yellow transparent liquid
Solid Content %	100
NCO %	21–23%
Free HDI Monomer %	≤ 0.3%
Viscosity at 25°C (cps)	3,500±750
Color (Hazen)	≤ 40

View full HDX-100 TDS and product details →

HDI Biuret vs HDI Trimer: Side-by-Side Comparison

Property	HDI Biuret (HDX-175MPX)	HDI Trimer (HDX-100)
Backbone structure	Biuret (–NH–CO–N–CO–NH–)	Isocyanurate ring
Functionality	3	3
Typical NCO %	16.5 (at 75% solids)	21–23 (at 100% solids)
Typical solid content	75% (in solvent)	100%
Film hardness	High	Higher
Film flexibility	Slightly more flexible	More rigid
Weather resistance	Excellent	Best-in-class
Thermal stability	Very good	Superior
Chemical resistance	Very good	Better
Viscosity	Low (in solvent)	Moderate (at 100% solids)
VOC reduction potential	Standard	Higher (low viscosity allows high-solids formulations)
Cost	Lower	Slightly higher
Best for	Auto refinish, industrial top coats, general 2K PU	Premium clear coats, aerospace, wind turbine, high-solids

How to Choose Between Them

For most automotive refinish, industrial top coat, and exterior 2K PU coating applications, HDX-175MPX (HDI biuret) is the cost-effective workhorse — proven performance, good handling, broad compatibility, and excellent weather resistance.

Choose HDX-100 (HDI trimer) when your application demands:

• Highest level of color retention and gloss retention (premium automotive OEM, aerospace)
• Maximum film hardness and chemical resistance
• High-solids or low-VOC formulation requirements
• Extreme outdoor service (wind turbine, marine, industrial equipment)
• Best-in-class thermal stability

Both products share HDI's defining advantage — non-yellowing under UV exposure — making them suitable for any outdoor or lightfast coating application. For indoor wood and furniture coatings where UV is not a concern, see Dongsen's aromatic polyisocyanate hardeners (TDI adduct, TDI trimer) for lower cost and faster cure.

Beyond Solvent-Based: Waterborne HDI Hardeners

For formulators moving toward low-VOC, water-based 2K PU systems, Dongsen also manufactures hydrophilically modified HDI polyisocyanates — the WD series. These are HDI-based aliphatic hardeners chemically modified to disperse in waterborne resin systems while retaining 2K crosslinking performance, ideal for:

• Waterborne automotive refinish
• Waterborne industrial floor coatings
• Waterborne furniture finishes for VOC-restricted markets
• Eco-friendly architectural coatings

Explore Dongsen's waterborne HDI hardener range →

Which HDI Hardener Fits Your Coating System?

If you're formulating an automotive refinish, industrial top coat, exterior 2K PU, waterborne system, or any non-yellowing coating — we can help you select the right HDI-based hardener based on your resin chemistry, solid content target, performance requirements, and budget.

➡ Automotive Refinish Applications
➡ HDX-175MPX (HDI Biuret) Product Page
➡ HDX-100 (HDI Trimer) Product Page
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