Engineering Phosphorus‐Modified Flame‐Retardant Thermoplastic Polyurethane via Reactive Melt Processing

ABSTRACT

The development of thermoplastic polyurethanes (TPUs) that combine improved fire behavior with satisfactory mechanical properties remains challenging. In this study, phosphorus‐modified TPUs were prepared by one‐step, solvent‐free reactive melt processing using the phosphorus‐containing diols Exolit OP550 and OP560, 1,4‐butanediol (BDO) as a chain extender, 4,4′‐methylenedicyclohexyl diisocyanate (MDI), and polyethylene glycol (PEG) or polypropylene glycol (PPG) as the conventional soft‐segment polyol. OP550 is an oligomeric phosphate polyol, whereas the lower estimated molecular weight of the phosphonate diol OP560 gives it a chain‐extender‐like character. FTIR spectroscopy supported the consumption of isocyanate groups but did not provide direct molecular‐level evidence of covalent phosphorus incorporation. The effects of phosphorus‐polyol chemistry, nominal phosphorus content (1.5 or 3 wt.%), and conventional soft‐segment chemistry were investigated using SEM–EDX, DSC, TGA, DMTA, mass‐loss cone calorimetry, UL‐94 testing, and tensile testing. The phosphorus‐containing formulations showed formulation‐dependent reductions in peak heat release rate (pHRR) and, in some cases, total heat release (THR). The largest pHRR reduction was approximately 54% for an OP550/PPG‐based formulation. However, the PPG‐based formulations tested by UL‐94 were classified as V‐2 and exhibited flaming melt dripping. Bubble defects prevented reliable UL‐94 and tensile testing of the PEG‐based formulations; therefore, conclusions concerning the balance between tensile properties and fire performance are restricted to the PPG‐based series. The materials are consequently described as non‐optimized phosphorus‐modified TPUs with improved heat‐release behavior rather than as inherently flame‐retardant TPUs.