Gradual replacement of the traditional raw materials
in polyurethane
(PU) synthesis by bio-based ones and introduction of environmentally
friendly physical blowing agents (PBAs) motivate the analysis of thermal
aging of bio-based foams in order to optimize the chemical structure
of the PU matrix so that its permeability to PBA is minimized. With
the aim of elucidating the effect of the PU chemical structure on
thermal aging of foams, rigid low-density closed-cell PU foams were
produced solely from bio-based polyols. The effective diffusivities
of PBA and atmospheric gasses were evaluated based on the measured
thermal conductivity aging of foams and validated by gas chromatography
measurements of gas composition in foam cells. Gas permeability of
the PU polymer was estimated based on effective diffusivity in foams
and foam morphology and found to decrease with increasing crosslink
density, apparently due to reduction in the fractional free volume
in the polymers.