The spontaneous ignition behavior of flame-retarded cotton cellulose has been investigated using differential thermal analysis (DTA). One durable, phosphonium salt-urea-ammonia polycondensate (Proban CC), and one nondurable, ammonium polyphosphate (Amgard TR) commercially flame-retardant-treated fabrics have been studied. The information obtained is compared to that previously reported for untreated cotton cellulose. The onset of spontaneous ignition, temperature Ti, was determined as a function of [O2] in the flowing O2/N2 atmosphere to which samples were exposed in the DTA furnace, whose temperature was raised at a known linear heating rate. The activation energy Ep for the rate-determining pyrolysis reaction was calculated. In the case of the Amgard TR fabric, Ep increased from 145 kJ mol−1 at 21% O2 to 261 kJ mol−1 at 50% O2, whereas that for the Proban fabric increased from 230 to 400 kJ mol−1.
A plot of 1/Ti vs. In [O2] shows two linear regions that interact at about 40% O2, which is just above the limiting oxygen index values for these two flame-retarded samples. This intersection is interpreted as indicating that the combustion mechanism of these flame-retarded fabrics changes as [O2] passes through this 40% level. Below this value, significant amounts of char remained after the DTA experiment, suggesting that volatiles only were ignited, whereas above this value, both volatiles and char were burnt away. This is supported by evidence from the DTA traces. The difference in slopes of the two regions of the 1/Ti vs. [O2] plots is used to obtain a value for Eox, the activation energy for gaseous oxidation. Values for Eox of 270 and 536 kJ mol−1 were obtained for the Amgard TR and Proban CC fabrics, respectively. These are considerably higher than is the value of 215 kJ mol−1 previously reported for untreated cotton. Thus, one of the ways in which these flame retardants reduce the ease of combustion of the cotton is by increasing the activation energy for the oxidation of the evolved organic species.