Depth-Resolved Analysis of Dimensional Stability (DS) in Poly(urethane-isocyanurate) Rigid Foams under Humid and Thermal Stress

This study examines the dimensional stability (DS) of polyisocyanurate rigid (PIR) foams under high relative humidity (RH = 90%) and elevated temperature (T = 70 °C), representative of the extreme conditions that may be encountered in roofing applications. Lateral expansion is observed in the plane perpendicular to the rise direction, which is strongly depth-dependent (Z-axis) and decreases from 5.3% for the surface layer (0–5 mm) to around 1.2% for the inner layers (Z ≥ 15 mm). This phenomenon is mainly attributed to the lower isocyanurate content near the surface and is amplified by the higher partial pressure of isopentane (+9%) near the surface. In contrast, contraction occurs in the rise direction (also called the thickness direction or TD) and remains nearly constant at −1% for Z ≥ 15 mm. This anisotropic behavior is attributed to the elongated cell morphology in the TD, inevitably leading to a decrease in mechanical strength in the plane perpendicular to the rise direction. Moisture content, quantified using Karl Fischer titration, is estimated at around 2.2 wt % (under humid conditions) and appears to play a dual effect, both plasticizing the polymer matrix and increasing internal gas pressure. Postcuring at T = 140 °C significantly increased cross-linking and improved DS, reducing foam expansion in the machine direction (MD) and cross-machine direction (CMD) from ΔL/L0 ≈ 5% to about 2% for the surface layer (0–5 mm). All these findings point toward the dominant role of moisture level, isocyanurate content, and cell shape factor of the final products on the long-term dimensional stability of PIR foams in humid, thermally stressed environments.