Polyurethane (PU) composite is a distinct branch of the FRP family. The reinforcement is still glass fiber, but the resin is a two-component polyurethane system rather than the polyester or vinyl ester thermosets used in conventional FRP. The difference in resin chemistry produces a material with a markedly different property profile — one that has found a specific and growing application as the structural thermal break material in building envelope systems, where conventional aluminum or steel framing creates a thermal bridge that undermines the insulation performance of the wall.
Chemistry and Processing
PU composites are produced by pultrusion, using a two-component polyurethane system — polyol and isocyanate — that reacts to form the polymer matrix. The reaction is fast and does not produce a volatile byproduct (unlike polyester curing, which releases styrene). The glass fiber reinforcement is the same E-glass used in conventional FRP, and the fiber volume fraction is similar — 50–65% for pultruded profiles. The resulting composite has the familiar shape of a pultruded structural profile — square tubes, channels, custom hollow sections — but with a darker color (typically dark brown or black) and a different surface texture, slightly more elastomeric to the touch than a polyester profile.
Thermal Performance: The Defining Advantage
The thermal conductivity of PU composite is approximately 0.22–0.28 W/m·K — substantially lower than polyester FRP (0.30–0.35 W/m·K) and dramatically lower than aluminum (160 W/m·K) or steel (50 W/m·K). This property, combined with the structural strength of the glass fiber reinforcement, is what makes PU composite the material of choice for thermal break profiles. In an aluminum window frame or curtain wall system, a PU composite profile inserted between the interior and exterior aluminum extrusions interrupts the thermal path — the heat that would otherwise flow through the continuous metal frame must now pass through a material with thermal conductivity two orders of magnitude lower. The result is a window or door assembly that meets the U-value requirements of modern building energy codes without resorting to wider frame sections or triple glazing alone.
Mechanical Property Comparison
PU composites have mechanical properties in the same general range as polyester FRP, with some important differences. The tensile strength is typically 300–500 MPa (slightly higher than polyester pultrusions due to the higher reactivity of the PU system allowing better fiber wet-out), and the tensile modulus is 18–25 GPa. The key difference is in toughness: PU composites have higher impact resistance and better fatigue performance than polyester FRP, a consequence of the polyurethane matrix being inherently more ductile than the brittle polyester cross-linked network. This makes PU profiles more resistant to handling damage and vibration in service.
Application Logic
PU composite profiles are specified where thermal performance is the primary design driver and structural loads are modest — window and door frames, curtain wall thermal breaks, and building envelope connection details. They are not typically used for primary structural framing (where polyester or vinyl ester pultrusions with longer field history dominate), nor for highly corrosive chemical environments (where vinyl ester provides the maximum chemical resistance). The material sits in a well-defined niche: structural thermal isolation in the building envelope.
For PU composite profile options, see our PU Composite Profiles product range.