Synthesis of Carbon-Rich Hybrid Foam from GAP-Modified Polyurethane

摘要

4,4'-Diisocyanato diphenylmethane(MDI)-based polyur-ethanes melt and start to burn at 150-200°C.Mainly H2O,CO2,CO,HCN,and N2 are formed.The new modified polyurethane shows a different pyrolysis behavior.GAP-diol(glycidyl azide polymer),which was used as a modifying agent,is a well-known energetic binder with a high burning velocity and a very low adiabatic flame temperature.The modified polyurethane starts to burn at approximately 190°C because of the emitted burnable gases,but it does not melt.The PU foam shrinks slightly and a black,solid,carbon-rich hybrid foam remains.TGA and EGA-FTIR revealed a three-step decomposition mechanism of pure GAP-diol,the isocyanate-GAP-diol,and PU-GAP-diol formulations.The first decomposition step is caused by an exothermic reaction of the azido group of the GAP-diol.This decomposition reaction is independent of the oxygen content in the atmosphere.In the range of 190-240°C the azido group spontaneously decomposes to nitrogen and ammonia.This decomposition is assumed to take place partly via the intermediate hydrogen azide that decomposes spontaneously to nitrogen and ammonia in the range of 190-240°C.The second decomposition step was attributed to the depolymerization of the urethane and bisub-stituted urea groups.The third decomposition step in the range of 500-750°C was attributed to the carbonization process of the polymer backbone,which yielded solid,carbon-rich hybrid foams at 900°C.In air,the second and the third decomposition step shifted to lower temperatures while no solid carbon hybrid foam was left.Samples of PU-GAP-diol,which were not heated by a temperature program but ignited by a bunsen burner,formed a similar carbon-rich hybrid foam.It was therefore concluded that the decomposition products of the hydrogen azide,ammonia and mainly nitrogen act as an inert atmosphere.FTIR,solid-state ~(13)C-NMR,XRD,and heat conductivity measurements revealed a high content of sp~2-hybridized,aromatic structures in the hybrid foam.The carbon-rich foam shows a considerable hardness coupled with high temperature resistance and large specific surface area of 2.1 m~2·g~(-1).