Peroxide bleaching of thermo-mechanical pulp from stored spruce wood

Although excessive storage of wood is disadvantageous, in some cases it cannot be avoided. This research has investigated the effects of wood storage on thermo-mechanical pulp (TMP) properties and how to overcome the expected disadvantages in brightness by bleaching. Norway spruce logs were stored outside in piles over the summer and compared to fresh wood as a raw material for TMP production. During the summer, the stored wood lost moisture and brightness. After storage, the optical properties did not hold up as well as the process runnability and pulp strength properties. The amount of colored substances in the wood increased both during storage and further in refining, resulting in reduced pulp brightness. This brightness loss can still be recovered by peroxide bleaching, although 8-10 kg/BDt higher peroxide charges are needed to reach, for example, a target brightness of 75%.