Polyester polyols were gradually replaced by polyether polyols owing to their several
advantages such as low cost, ease of handling, and improved hydrolytic stability over the
former. Poly(tetramethylene ether) glycol (PTMG), was introduced by DuPont in 1956 by
polymerizing tetrahydrofuran, as the first commercially available polyether polyol. Later,
in 1957, BASF and Dow Chemical produced polyalkylene glycols. Based on PTMG and
4,4’-diphenylmethane diisocyanate (MDI), and ethylene diamine, a Spandex fibre called
Lycra was produced by Dupont. With the decades, PU graduated from flexible PU foams
(1960) to rigid PU foams (polyisocyanurate foams-1967) as several blowing agents,
polyether polyols, and polymeric isocyanate such as poly methylene diphenyl
diisocyanate (PMDI) became available. These PMDI based PU foams showed good
thermal resistance and flame retardance.