Formulation Guides

Step-by-Step Polyurethane Topcoat Formulation for Exterior Steel

Two-pack polyurethane (2K PU) topcoats are the finishing coat of choice for structural steel, industrial equipment, and infrastructure across the GCC. Their combination of excellent UV resistance, gloss retention, chemical resistance, and flexibility makes them ideal for the harsh conditions of Saudi Arabia's NEOM giga-projects, offshore platforms in the Arabian Gulf, and exposed steel in Dubai's coastal environment. This guide walks through the complete raw material selection and formulation process for a high-performance exterior 2K PU topcoat.

Understanding 2K PU Coating Chemistry

A two-pack polyurethane coating consists of two components that are mixed immediately before application:

• Part A (Polyol Component): Contains the hydroxyl-functional resin (acrylic polyol, polyester polyol), pigments, solvents, and additives.
• Part B (Isocyanate Hardener): Contains the polyisocyanate crosslinker (HDI trimer, IPDI trimer, or HDI biuret).

The NCO:OH ratio (isocyanate index) is critical — typically 1.0:1.0 to 1.1:1.0. Excess isocyanate (up to 1.1) can improve crosslink density and chemical resistance. Insufficient isocyanate results in a soft, poorly resistant film.

Raw Material Formulation — Part A (Polyol Component)

Raw Material	Function	% by Weight	Notes
Acrylic Polyol Resin (60% in xylene, OH value ~110)	Primary binder	30–40%	High OH content for good crosslink density
TiO2 Rutile (chloride grade)	White pigment, UV protection	15–20%	Chloride TiO2 for superior durability and chalk resistance
Xylene / Butyl Acetate (1:1 blend)	Solvent, viscosity control	15–20%	Butyl acetate slows evaporation, improves flow in heat
BYK-306 or equivalent (silicone surface additive)	Flow, levelling, substrate wetting	0.3–0.6%	Prevents cratering on metal substrates
UV Absorber (benzotriazole type)	UV stabilisation	1.0–1.5%	Essential for GCC exterior exposure; Tinuvin 1130 or equiv.
HALS (hindered amine light stabiliser)	Free radical scavenging, gloss retention	0.8–1.2%	Works synergistically with UV absorber
Dispersing Agent (phosphate ester type)	TiO2 dispersion, flocculation prevention	1.0–1.5%	Critical for achieving full TiO2 opacity
Anti-settling Agent (modified urea or polyamide wax)	Prevents pigment settling in storage	0.5–1.0%	Must be compatible with polyol system
Drier / Catalyst (DBTL trace)	Accelerates NCO/OH reaction	0.05–0.1%	Use sparingly — excess drastically reduces pot life

Part B — Isocyanate Hardener Selection

For exterior GCC applications, use one of these aliphatic isocyanate hardeners:

• HDI Trimer (e.g. Desmodur N3300): Excellent UV resistance, high NCO content (~21%), hardest film. Best for applications requiring maximum gloss retention over 10+ years.
• IPDI Trimer: Slightly lower UV resistance than HDI but better low-temperature flexibility. Better suited for coastal UAE applications with temperature cycling.
• HDI Biuret (e.g. Desmodur N100): Higher viscosity, faster reactivity. Good for applications where film build per coat is critical.

Mix ratio (by weight) is determined by the NCO content of Part B and OH content of Part A. Calculate: Ratio = (OH value of Part A × molecular weight of NCO) / (NCO content % × molecular weight of OH × 1000).

Application Parameters for GCC Conditions

Parameter	Ideal	GCC Summer Challenge	Solution
Temperature	15–30°C	40–50°C ambient	Apply early morning; use retarder solvents
Relative Humidity	40–75% RH	10–30% RH in desert	Use slower-evaporating solvents; avoid direct sunlight
Pot Life	4–6 hrs (25°C)	1–2 hrs (40°C)	Mix smaller batches; add DBTL catalyst very sparingly
DFT per coat	40–60 μm	Risk of sag in heat	Add anti-sag agent; apply in two coats if >60μm needed

Sourcing PU Topcoat Raw Materials in the GCC

Raykem supplies polyurethane raw materials across the UAE and Saudi Arabia, including TDI 80/20 and MDI for PU foam applications. For topcoat formulation, our team can source aliphatic polyisocyanate hardeners, acrylic polyol resins, UV absorbers, and HALS stabilisers. Our solvent range includes Xylene, Butyl Acetate, Toluene, and specialty co-solvents in drum and IBC quantities. Contact us for technical data sheets and pricing on specific components.