Experimental multi-scale approach to determine the local mechanical properties of foam base material in polyisocyanurate metal panels

Effect of surface curing temperature on depth-specific physical properties of poly(urethane-isocyanurate) foam panels in relation with local chemical composition and morphology

“…The higher mold temperature seems to promote the formation of isocyanurate, which may be explained by the higher activation energy of isocyanurate with respect to urethane formation. Similar urethane/isocyanurate gradients have been reported recently for industrial PIR foam insulation panels with metal facing [19] and flexible facing [25]. isocyanurate content increased between t~1 h and t~4500 h. However, this "supposed" increase in isocyanurate content is not visible in Figure 3 where the values of h1410/h1595 peak height ratio even have the tendency to be slightly lower between 40°C and 80°C.…”

“…Interestingly, the evolution of the isocyanurate/phenyl ratio as a function of depth for T~70°C and reported in Figure 9 compares well with the results obtained for commercial products taken in the European market [25]. PIR/PUR gradients were also reported by Gahlen et al [19] on commercial polyisocyanurate-based metal-foam composite elements (obtained from Covestro) of varying thickness (40, 60 and 100 mm). However, these gradients were limited to the first 10-15 mm (instead of 20-30 mm in our case) on each face while the metal sheets were pre-heated to about 60°C.…”

“…Even if the Shore hardness of the foam is not determined solely by its elastic modulus, but also depends on the compressive strength and on the viscoelastic behavior of the foam (ASTM D2240-05), it is reasonable to say that the decrease in Shore hardness (at the surface) with the increase of Tmold means that the Young's modulus of the foam under the facing becomes smaller. This outcome is just the opposite of our expectations since it was hoped that the increase in isocyanurate content would increase the Young's modulus of the foam base material [19], thereby increasing the stiffness of the foam under the facing. However, this is not the only study that follows that direction.…”

“…with the increase of Tmold is in line with the data reported in the literature. It could be interesting to compare the effect of the increase in the polymer matrix modulus from the surface (Tsurface~60°C) to the core (Tcore~150°C) obtained from Gahlen et al [19] with the effect of the decrease in the foam density (surface layer) observed in our case for Tmold=60°C and 150°C. Making the hypothesis that the variation of the density of the foam base material (Esolid) with Tmold is negligeable (∆ρsolid variations should be below 1%), it can be written from equation 7 that…”

“…As a result, the stiffness of the foam base material should not depend strongly on the crosslink density. A recent study on polyisocyanurate-based metal-foam composite elements brought up, however, new aspects that are somewhat encouraging [19]. These authors investigated the relation between isocyanurate content (estimated through ATR-FTIR spectroscopy measurements at different depths of the foam panel) and elastic properties of the polymeric matrix by performing nanoindentation tests on tiny foam fragments (at the scale of cell struts) embedded in a thermoset epoxy resin.…”