Flame-retardant system for rigid polyurethane foams based on diethyl bis(2-hydroxyethyl)aminomethylphosphonate and in-situ exfoliated clay

Abstract

The flame-retardant rigid polyurethane foams (FR-RPUFs) with organoclay (OMMT) and/or diethyl bis(2-hydroxyethyl)aminomethylphosphonate (DBHP) were prepared by free foaming. The modification effect of OMMT was studied by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA). The dispersion state of OMMT in RPUF was evaluated by XRD and transmission electron microscopy (TEM). The compressive strength and micro-morphology of neat RPUF (N-RPUF) and FR-RPUFs were investigated by universal testing machine and scan electronic microscopic (SEM). The flame retardancy of FR-RPUFs was explored by limiting oxygen index (LOI), vertical burning test (UL94) and cone calorimeter test (CCT). These results indicated that OMMT was exfoliated or intercalated during the synthesis of FR-RPUFs. The incorporation of OMMT endowed RPUF smaller cell size, higher density and compressive strength. Compared to the flame-retardant effect of OMMT or DBHP alone, the OMMT/DBHP system significantly raised LOI value and UL94 rating, reduced heat release rate and total heat release, and increased char yields. During combustion of FR-RPUFs, DBHP boosted the formation of phosphorus-rich char layer, and the char layer was reinforced by nano-dispersed clay sheets to create a physical barrier that can insulate the underlying polymer from heat and slow oxygen uptake and the escape of volatile ingredients produced by polymer degradation. The results from pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) confirmed that the PO⋅ and PO2· radicals pyrolyzed by DBHP can be released into gaseous phase to terminate free radical chain reaction of combustion. The results from TGA indicated that OMMT and DBHP can increase residues. Also, the kinetics analysis demonstrated that OMMT can improve the thermal stability of RPUF. Therefore, a bi-phase flame-retardant effect of OMMT/DBHP system in RPUF was achieved.