Studies on the pyrolysis and potential flame retardancy of low-substituted starch phosphates

Investigations on the pyrolysis and potential flame retardancy imparted by solvent-free and semi-dry phosphorylation of different starches using sodium orthophosphates were conducted. The samples – low-substituted starch phosphates (SP) with degrees of substitution DS_P < 0.5 - were subjected to differential scanning calorimetry, thermogravimetry coupled with evolved gas analysis and pyrolysis – gas chromatography – mass spectrometry. The data obtained as well as features of charring residues examined using microscopic and spectroscopic methods were related to structural aspects of SP – analysed by means of various spectroscopic techniques - and compared with those of native starches. It was found that charring and polyphosphate formation and the thermal resistance of the solid SP residues increased significantly if the DS_P was at least 0.1. Accordingly, the exothermal decomposition, the temperature-induced loss of mass and the decomposition rates of SP decreased distinctly compared to native starch. The activation temperatures of SP and the formation of low-molecular pyrolysis products including aliphatic, cyclic, and aromatic aldehydes and ketones as well as anhydrosugars decreased markedly, even at DS_P < 0.1. The results confirm the potential flame-retardancy of SP achieved by flame-inhibiting effects, despite low phosphorylation degrees, in both the gas and condensed phases.