Influence of the chemical structure of polyester polyols on the properties and fire resistance of polyisocyanurate foams

Polyisocyanurate foams are high-performance thermal insulation materials widely used in the building industry. They combine low thermal conductivity with good fire resistance, thanks to the presence of isocyanurate structures. The impact of the chemical structure of their main building block, polyester polyols, has however rarely been the topic of a systematic investigation. In this work, a series of polyester polyols was synthesized from phthalic anhydride and various diols, resulting in polyols having a constant hydroxyl index but varying levels of aromaticity. Their viscosity and glass transition temperature were found to increase with the aromaticity. The polyols were then used to synthesize rigid polyisocyanurate foams for thermal insulation, to evidence the impact of aromaticity on the properties and fire resistance of the foams. Increasing the polyol aromaticity limits the molecular mobility, which results in slower foaming reaction times. The differences in reactivity between the polyols also lead to foams with slightly different isocyanurate contents, although they were all synthesized with a constant NCO/OH ratio. The main foam properties (morphology, mechanical properties or thermal conductivity) were mostly unaffected by the polyol aromaticity. The aromaticity was not found to improve the fire behavior of the foams, as evidenced by mass loss cone experiments. However, other factors, such as the isocyanurate content of the foams and the oxygen content of the polyols were found to positively influence the fire resistance. It should be related to a higher propensity to form a dense char layer on top of the material, limiting its degradation during the combustion.