Polyurethane (PU) is a polymer family formed by the reaction of polyols with di- or polyisocyanates, commonly MDI (diphenylmethane diisocyanate) or TDI (toluene diisocyanate), combined with catalysts, blowing agents, and additives. In construction, PU is used as sealant, adhesive, rigid and flexible foam, and as a binder in coatings and waterproofing membranes.
Chemical Composition
The urethane linkage (-NH-CO-O-) results from the addition reaction between an isocyanate group (-N=C=O) and a hydroxyl group (-OH). Varying the polyol structure (polyester or polyether) and the isocyanate index allows formulators to adjust hardness, elasticity, and chemical resistance of the finished product. Two-component (2K) systems cure through this reaction on site; one-component (1K) systems cure by reaction with atmospheric moisture.
Applications in Construction
Typical uses include elastic sealants for movement and connection joints, structural and assembly adhesives, liquid-applied waterproofing membranes, rigid foam boards for thermal insulation (PUR/PIR), flexible foam for acoustic and cushioning applications, and protective or decorative coatings for concrete, metal, and wood substrates.
Material Properties
Rigid PUR/PIR insulation boards reach thermal conductivity values of λ = 0.022–0.028 W/(m·K) at densities of approximately 30–45 kg/m³. PU sealants are classified according to EN ISO 11600 into movement capability classes such as 12.5E or 25LM, permitting cyclic joint movement of up to ±25 % of the joint width. Cured elastomeric PU coatings commonly reach Shore A hardness values between 40 and 90, depending on polyol type and crosslink density.
Regulatory Framework
Isocyanates are classified as respiratory sensitizers and are subject to restriction under REACH Regulation (EC) No 1907/2006, Annex XVII, entry 74. Since 24 August 2023, industrial and professional users handling products containing more than 0.1 % w/w monomeric diisocyanate must complete mandatory safety training before use. This obligation applies to on-site processing of two-component PU adhesives, sealants, coatings, and foams.
Standards and Testing
Rigid polyurethane foam products for building insulation are tested and classified under EN 13165, which specifies thermal resistance, compressive strength, and dimensional stability requirements. PU sealants for façade and glazing joints are covered by EN 15651 (parts 1–4), defining classification criteria such as elastic recovery, adhesion/cohesion behavior, and resistance after artificial weathering exposure.
Processing and Handling
Two-component PU systems require precise mixing of resin and hardener components; pot life typically ranges from a few minutes to about 30 minutes, depending on formulation and ambient temperature. Full curing of sealants and coatings generally occurs within 24 to 72 hours at 23 °C and 50 % relative humidity, with complete mechanical performance reached after approximately 7 days.