Thermal dissociation of urethanes studied by FTIR spectroscopy

Abstract

The thermal dissociation of urethane groups has been studied by FTIR spectroscopy depending on the type of diisocyanate, the structure of the hard segment and the composition of polyurethane. Independent of the composition and the structure, the dissociation started between 150 and 160°C. Consequently, the dissociation sets in at temperatures below the melting temperature of the model hard segments, i. e. already in the solid state. The NCO absorption band reaches its maximum intensity between 180 and 190 "C. This result opposes the references in literature suggesting a four-centre reaction for transurethanization. ZUSAMMENFASSUNG:Mittels FTIR-Untersuchungen wurden der Beginn der thermischen Dissoziation der Urethanbindung sowie das Dissoziationsverhalten allgemein in Abhangigkeit von der Art des Diisocyanats, der Hartsegmentstruktur sowie der Polyurethanzusammensetzung im Temperaturbereich zwischen 25 und 210 "C untersucht. Unabhangig von der Struktur und Zusammensetzung wird bereits ab 150 "C ein Beginn der Urethanspaltung registriert. Somit setzt die Dissoziation unterhalb der Schmelztemperatur der Hartsegmente, also bereits im festen Zustand, ein. Die Intensitat der freien NCO-Bande erreicht zwischen 180 und 190 "C ein Maximum. Diese Ergebnisse widersprechen der Literaturhypothese uber den Mechanismus der Transurethanisierung durch einen Urethanaustausch im Sinne einer Vierzentrenreaktion.

Improving Melt Flow of Polyoxymethylene (“High-Speed POM”): Additive Design, Melt Rheology, and In Situ Composition Gradient Formation

“…Urethanes instead did not show any processing limitations. Joel and Hauser in contrast reported that some aliphatic urethanes show a decompose/reconnect equilibrium with high isocyanate concentration above 180 °C. The isocyanurate urethanes presented in our work however are applicable in the whole processing window of POM.…”

Thermoreversible cross-linking of maleated ethylene/propylene copolymers with diamines and amino-alcohols

“…DielseAlder reactions between furan and maleimide groups [4,5] and between two cyclopentadiene groups [6,7] have been successfully used for thermoreversible crosslinking of polymers. Urethane formation from isocyanate and hydroxyl groups is also thermoreversible [8,9] and has been used for polymer cross-linking [10,11]. Previous work in our group showed that the reverse reaction at elevated temperatures results in the depolymerization of linear polyurethanes and in easy processability, provided that the molecular weights are not too high [12e14].…”

Reactivity of urethanes at high temperature: Transurethanization and side reactions

“…Most reports on the reactions of urethanes at high temperatures are concerned with 4,4′‐methylene diphenyl diisocyanate‐based systems (MDI‐based systems) because TPUs heavily rely on MDI …”

“…Yang et al detected free isocyanate when annealing urethane model compounds based on MDI and 1,4‐butanediol at temperatures above 150 °C. Similarly, Joel et al reported an increasing isocyanate concentration when annealing urethane model compounds based on toluene diisocyanate (TDI), MDI, and 4,4′‐methylene dicyclohexyl diisocyanate (H 12 MDI) above 150 °C. Several studies reported a decrease in isocyanate concentration and an increase of urethane concentration when cooling down such samples, which evidences that the degradation is reversible .…”