Polyurethane Cell Structure Improvers for Fine-Cell Rigid Insulation Foam Panels: A Comprehensive Overview Introduction Rigid polyurethane (PUR) and polyisocyanurate (PIR) foams are widely used as insulation materials in construction, refrigeration, and transportation due to their excellent thermal insulation properties, high strength-to-weight ratio, and relatively low cost. The thermal conductivity of these foams is strongly dependent...