Engineering validation of a mechanical recycling process for rigid polyurethane and polyisocyanurate foams into functional insulation blocks

• Engineering-scale validation of PU/PIR foam mechanical recycling • Cold-press rebonding without external heat input demonstrated • Functional insulation blocks produced from EoL waste streams • Thermal conductivity retained within insulation-grade range • Repeated recycling effects on mechanical and fire behaviour assessed Rigid polyurethane (PU) and polyisocyanurate (PIR) foams are widely used insulation materials whose thermoset structure limits conventional reprocessing at end-of-life. This work presents the engineering validation of a mechanical recycling process based on shredding and reactive rebonding of rigid PU/PIR particles using an MDI prepolymer combined with a lignin-containing binder system. Rebonded insulation blocks (0.5 × 1.2 × 2.0 m) were produced under cold-press conditions (30 ± 3 kPa) without external heat input. The process was applied to production scrap, end-of-life PU foam, and materials subjected to repeated recycling cycles. Apparent density, compressive strength, thermal conductivity, reaction-to-fire behaviour, water vapour transmission, and microstructure were evaluated to assess functional performance. Recycled blocks retained thermal conductivity within the typical range of polymeric insulation materials (0.033–0.041 W/m·K) and exhibited compressive strength suitable for non-load-bearing insulation applications. Repeated recycling increased density and reduced mechanical performance in PIR-based systems, whereas first-cycle recycling of PU improved compressive properties. Microstructural analysis revealed binder-rich phases and partial cell opening, explaining structure–property relationships. The results demonstrate the technical feasibility of producing functional insulation blocks from rigid PU/PIR waste streams using a scalable mechanical rebonding approach.