Segmented MDI/HMDI‐based polyurethanes with lowered flammability

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Abstract
A series of segmented polyurethanes (PUs) based on 4,4′‐diphenylmethane diisocyanate and 1,6‐hexamethylenediisocyanate; polyoxypropylenediol (POPD); and low‐molecular chain extenders, 1,2‐propanediol or 3‐chloro‐1,2‐propanediol was obtained by solution polymerization and characterized by GPC and microscopic methods. Spherical aggregates with diameter of about 200 nm, arranged in a quasi‐linear mode, were observed by SEM technique; further investigations by the TEM method revealed hard‐segment domains of longitudinal shape, with length of about 5–10 nm, showing some features of the arrangement along a “director” axis. The process of thermal decomposition was monitored by thermogravimetric analysis in both dynamic and isothermal mode; to gain a deeper look into the mechanism of decomposition, kinetic analysis was performed. First, isoconversional methods showed a multistep decomposition route, as the value of (apparent) energy of activation changes from about 150 kJ/mol in the first step up to about 350 kJ/mol in the third step, which corresponds to the energy of dissociation of CC bonds and leads to a char (carbonaceous) residue. Further calculations by means of the linear regression method revealed that kinetic model functions describing the degradation process of PU that does not contain chlorine were Bna → An → Fn, whereby for chlorine‐containing polyurethanes the CnB → An → An model was applied as best fit. Volatile products were investigated by pyrolysis–gas chromatography/mass spectrometry method at 770°C; the low molecular weight decomposition products, which were identified, indicate that the depolymerization process, yielding mainly diols and isocyanates through breakage of urethane bonds, prevailed during the thermal treatment under pyrolytic conditions. Finally, the mechanism of thermal degradation of modified polyurethanes with lowered flammability was proposed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3214–3224, 2004

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