Combustion behaviours of chemically modified polyacrylonitrile polymers containing phosphorylamino groups

Abstract

Acrylic acid-2-[(diethoxyphosphoryl)methylamino] ethyl ester (ADEPMAE) has been synthesized and copolymerized with acrylonitrile (AN) through an aqueous slurry route by employing the redox initiator pair, ammonium persulfate/sodium metabisulfite. The resulting copolymers, having a range of compositions, were purified and subjected to a variety of characterization techniques, such as solution-state NMR, thermogravimetric analyses (TGA), bomb calorimetry and pyrolysis combustion flow calorimetry (PCFC). Chemical modification of polyacrylonitrile (PAN) with P- and N-containing groups leads to significant increases in flame retardance as compared to the unmodified counterpart. Studies of the thermal degradation of the polymers by thermogravimetric analyses, in oxygen, air and in nitrogen, indicate that the mechanism of flame retardance includes a significant condensed-phase component. Generally, the values of heats of combustion (ΔHcomb) and heat release capacity were substantially reduced for the modified polymers as compared with polyacrylonitrile (PAN), pointing towards inhibition of combustion processes by the modifying moieties. This was further established from other parameters obtained from PCFC measurements. It is believed that degradation of the phosphonate ester groups of the ADEPMAE units leads to the production of phosphorus acid species which, in turn, promote cyclization of the pendant nitrile groups of PAN producing, ultimately, aromatic species, contributing to increased production of char.