Influence of aromatic brominated flame retardants on alkane photo-oxidation: A model and polymer study

Photochemical decomposition of decabromodiphenyl ether (I) induced by 300-400 nm light is a radical process leading to the formation of hydrobromic acid (HBr) provided there is a hydrogen donor (RH) in the system. The stepwise debromination of the aromatic ring takes place giving rise to lower brominated diphenyl ethers with a number of by-products. The hydrogen donor is converted to a corresponding R-Br derivative in this process. Free HBr is prone to photolytic decomposition, particularly at more advanced stages of conversion of (I). As shown in the polypropylene case, the radicals thus formed can actively participate in the initiation step of polyolefin chain oxidation. In the presence of HALS the HBr formation through photolysis of (I) is enhanced while the formation of R-Br is totally suppressed. The rate of disappearance of (I) is, however, not influenced by the HALS. The enhancement of HBr formation by HALS is thus attributed to a deeper stepwise debromination of (I) allowing release of more than one bromine radical per molecule of (I). The detrimental effect of brominated diphenyl ethers on the polymer UV resistance thus stems from three main reasons: HBr formation which ruins the stabiliser performance through the formation of inactive aminium salt with HALS; photolysis of HBr as well as of (I) that enhance the photoinitiation of alkane polymer photo-degradation.