Maintaining the Brightness of Mechanical Pulps with Solid-State Perborate Bleaching

Abstract

Since vicinal diols present in carbohydrates may form perborate esters with hydrogen peroxide and borax, such species were loaded onto peroxide bleached and unhleached mechanical pulps. These species were shown to act as slow solid-state oxidizing bleaching agents capahle of partially restoring, in the dark, the brightness losses induced on mechanical pulps during direct outdoor and indoor irradiation. More specifically, bleached TMP handsheets were impregnated at pH 9.5 with a mixture comprised of 2 % borax and 1% hydrogen peroxide. These samples were then irradiated with indoor direct sunlight for 5 hours followed by a dark bleaching period of ahout 7 hours. At the end of this period brightness readings were recorded and the cycle was repeated. thus simulating indoor handling of paper products. For a 33 hour of cumulative direct indoor sunlight irradiation, the percentage stabilization offered was about 70%. When a similar solution was used to impregnate unbleached TMP samples, a brightness gain of about 10 ISO points was obtained without using any heat for the bleaching stage. While repeated irradiation and dark storage cycles, over a cumulative period of 37 hours, resulted in approximately 3 points of brightness losses the initial brightness gains due to bleaching were maintained. The application of perborates possibly offers a potential solution to thermal reversion issues that face most mechanical pulp mills.