Basic Chemistry of Flame Retardation of Poly(Ethylene Terephthalate)

Thitivat Suebsaeng, Marquette University

Abstract

The purpose of this study was to investigate the chemistry involved in the pyrolysis reactions of poly(ethylene terephthalate), (PET), and the chemical basis of the flame retardancy imparted to this polymer by some putative flame retardants, i.e. red phosphorus, triphenylphosphine, zinc chloride, zinc fluoride, and zinc acetate. The solid products from pyrolysis of PET were characterized by CP/MAS$\sp{13}$C-NMR and FT-IR spectroscopy. Over the temperature range of 300-400$\sp\circ$C, PET was converted to a highly crosslinked polyaromatic char. After thermal cleavage of PET with formation of free carboxyl and vinyl ester groups, there are two competing reaction pathways. These intermediates may be converted to volatile products primarily acetaldehyde, CO, and CO$\sb 2$, or a crosslinked polymer may be formed. The vinyl polymerization of vinyl ester followed by chain stripping and cyclization/crosslinking of polyene are responsible for the formation of polyaromatic char. All additives that have been used in this study change the decomposition pathway of PET. Red phosphorus and zinc chloride also promote the formation of char. Pyrolysis of PET in the presence of red phosphorus yielded an intractable polyaromatic phosphate ester. The formation of phosphate ester may be responsible for char formation. Lewis acid catalysis of polymerization of vinyl ester is the proposed mechanism that accounts for the char enhancement caused by zinc chloride. Char formation was not observed with zinc fluoride, zinc acetate of triphenylphosphine. These zinc salts react with PET to form carboxylate salts in a stoichiometric fashion. Triphenylphosphine reacts with PET to produce triphenylphosphine oxide and ethylene. Triphenylphosphine oxide is a gas phase retardant for PET and consequently triphenylphosphine has some flame retardant effect. The presence of ethylene which is co-produced with the triphenylphosphine oxide renders triphenylphosphine a much less effective flame retardant than triphenylphosphine oxide.