What Is Polyurethane Coating?

Polyurethane (PU) coating is a polymer-based protective system applied to steel pipes for corrosion protection, abrasion resistance, or thermal insulation. PU coatings come in two main forms: thin-film polyurethane topcoats for atmospheric corrosion protection and thick polyurethane foam (PUF) insulation for temperature-controlled pipelines.

When PU Coating Is Used

Polyurethane coatings serve different functions depending on the application:

• Thin-film PU (0.05-1.0 mm): Applied as a topcoat over epoxy primers on above-ground piping, structural steel, and equipment. Provides UV resistance, abrasion resistance, and color retention that epoxy alone cannot achieve.
• PU foam insulation (25-150 mm): Applied as rigid foam around subsea or onshore pipelines to maintain fluid temperature and prevent hydrate/wax formation. This is the primary use in oil and gas.
• PU elastomer lining (3-12 mm): Applied internally to pipe and equipment for abrasion resistance in slurry and mining applications.

PU foam insulation is critical for deepwater flowlines and risers, where seabed temperatures (2-4°C) can cause wax deposition and gas hydrate formation in hydrocarbon fluids.

PU Coating Type	Thickness	Density	Max Temp	Primary Application
Thin-film PU topcoat	50-250 microns	N/A (solid)	120°C	Above-ground piping, structural steel
Rigid PU foam (PUF)	25-150 mm	80-250 kg/m3	100-130°C	Subsea/onshore thermal insulation
Syntactic PU foam	25-100 mm	450-700 kg/m3	130-150°C	Deepwater insulation (high pressure)
PU elastomer lining	3-12 mm	1000-1200 kg/m3	80°C	Abrasion-resistant internal lining
Spray PU foam	25-100 mm	30-60 kg/m3	90°C	Field-applied insulation

PU Foam Insulation System

A typical subsea PU foam insulation system consists of:

1. Anti-corrosion coating: FBE or 3LPE on the steel pipe surface
2. PU foam layer: Rigid closed-cell polyurethane foam injected into a mold around the pipe
3. Outer jacket: High-density polyethylene (HDPE) sleeve for mechanical protection and water ingress prevention

The foam is formed by mixing polyol and isocyanate components, which react exothermically and expand to fill the annular space between the pipe and the outer jacket. Thermal conductivity values for PU foam range from 0.14-0.20 W/m.K, making it one of the most effective pipe insulation materials available.

For deepwater applications (water depths over 1000 m), syntactic PU foam is used. Glass microspheres or macrospheres are embedded in the PU matrix to resist hydrostatic pressure without collapsing the foam cells.

Pro Tip

When specifying PU foam insulation, always define the required thermal conductivity (k-value) at the expected operating and seabed temperatures, not just at laboratory conditions (typically 10°C mean). The k-value of PU foam increases with temperature: a foam that measures 0.14 W/m.K at 10°C may reach 0.18-0.20 W/m.K at higher mean temperatures, significantly affecting the overall heat transfer coefficient (U-value) used in flow assurance models.

Standards

Key standards include ISO 21809-1 Annex G (PU foam coatings for pipelines), ASTM C591 (rigid PU insulation), and project-specific flow assurance specifications. PU topcoats are typically specified per ISO 12944 or SSPC coating system standards.

For an overview of all pipe coating and lining systems, including multi-layer coatings and internal linings, see the main article.

Read the full guide to pipe coatings