Polyurethane foam expands and sets in the panel mould through the reaction of two components (polyol and isocyanate); at a density of 40 kg/m³ it forms a closed-cell structure that does not absorb water. Its thermal conductivity is 0.020–0.024 W/mK: noticeably better than stone wool and EPS, meaning a thinner panel for the same insulation. Because the sheet and the foam are one piece, the panel both insulates and carries load.
PIR (polyisocyanurate) is the version of the same chemistry with a higher isocyanate ratio and greater heat resistance. In a fire it forms a charred layer on the surface that slows the spread of flame; it reaches EN 13501 class B-s1,d0. PIR panels are demanded for large cold storage warehouses, logistics facilities and projects where insurance conditions require it; the insulation value is almost the same as PUR.
As the foam ages (the cell gas is exchanged with air), the insulation value falls somewhat over the years; the tightness of the sheet slows this. Quality indicators: homogeneous foam (no voids), density above 40 kg/m³, no delamination between sheet and foam, clean joints at the edges. Sonkar checks its panels against these criteria in its own production.