Radical formation in peroxide-bleached kraft pulp

The stability of hydrogen peroxide bleached kraft pulp against radical formation under different conditions was studied by the electron paramagnetic resonance (EPR) technique. Among the conditions investigated were darkness and light, heating at elevated temperatures and UV radiation. In addition, the effects of Fe and Mg ions and lignin content on relative radical concentrations were examined. Samples which represented commercial softwood and hardwood pulps were taken from the pulping process before final acidification and drying. All EPR spectra were measured at room temperature and consisted of a single line with line width of about 0.8 mT. The middle point of the peak corresponds to the gr value 2.004-2.005. The relative radical concentration was found to increase least on storing in the dark and most on exposure to UV radiation. The relative radical concentration seemed to increase in two phases: a faster initial increase and then a slower reaction. The observed photochromic effect showed a similar two-phase reaction.