Anisotropy of the stiffness and strength of rigid low-density closed-cell polyisocyanurate foams

Abstract

The cells of polymer foams are usually extended in the foam rise direction, causing a geometrical anisotropy, the degree of which, characterized by the cell aspect ratio R, depends on foam density and production method. Such elongated cell shape translates into anisotropy of the mechanical properties of foams. Rigid low-density closed-cell polyisocyanurate foams of apparent density ranging from ca. 30 to 75kg/m3, containing polyols derived from renewable resources, have been produced and tested for the stiffness and strength in the foam rise and transverse directions in order to experimentally characterize their mechanical anisotropy. Analytical relations for foams with rectangular parallelepiped and tetrakaidecahedral (Kelvin) cells were considered for predicting the mechanical characteristics of the polyisocyanurate foams in terms of their apparent density, geometrical anisotropy, and characteristics of the base polymer. Open-cell models were found to produce conservative estimates of foam stiffness and strength, albeit very close for the former when a tapering strut geometry was allowed for in the Kelvin cell model. Extending the rectangular parallelepiped cell model to the closed-cell case allowed a reasonably good description of variations of both the stiffness and strength with foam density.