Peroxide bleaching brightness increase

Figure 1. The effect of storage time (+23°C) on the brightness increase after accelerated photoyellowing of peroxide-bleached groundwood for 1 hour.

Figure 3. The increase in brightness (%) at 457 nm during storage for 24 h (+23°C) after irradiation for a peroxide-bleached groundwood.

Various tests were carried out in an attempt to obtain further information concerning the nature of the chromophoric groups involved. To study the influence of extractives and other low molecular mass compounds, samples of unbleached and peroxide-bleached groundwood were extracted with acetone and then irradiated. These samples showed a brightness increase upon storage after irradiation but not to the same extent as the reference samples and, thus, part of the reactions responsible may occur in the low molecular compounds removed by extraction. 

Figure 5. The ESR signal intensity and the brightness increase of a peroxide-bleached groundwood sample after accelerated irradiation (1 h) as a function of storage time at room temperature.

The effects of dose rate (0,107, 330, 700 and 3000 krad/h) on the pulp ISO brightness, for a constant 2.S Mrad dose for the unbleached TMP and a peroxide bleached TMP (P-TMP) are presented in Figure 2. The ISO brightness of both pulps as shown increases. However, at the highest dose rate available (3000 krad/h) the increase in ISO brightness is much smaller. 

The curve marked by a DOT represents the bleaching effect of alkaline hydrogen peroxide. The difference in the absorption coefficient at 457 nm is about -2.0 m2/kg and associated with an increase of 4.1 in the ISO brightness as the result of peroxide bleaching. Upon y irradiation of an alkaline hydrogen peroxide bleached pulp (P-TMP), one still attacks the phenolic hydroxyl groups or coniferaldehyde groups around 350 nm as indicated by the curved marked with a DIAMOND. 

The brightening effect attained depends on the peroxide charge, as shown in Fig. 8-24. Peroxide bleaching results in a comparatively large brightness increase, sometimes about 25% (ISO), which is much more than can be achieved with dithionite. However, the response of various pulp types to peroxide bleaching varies greatly. 

By this means peripheral, color-inducing functionalities are oxidized and decolorized over a bleaching period of 2-3 hr while the bulk of the lignin macromolecules remain intact (insoluble). Thus a brightness improvement of 8-10 points is possible while the yield loss is kept low. Unfortunately peroxide-based bleaching systems tend to be expensive relative to other alternatives for groundwoods, and hence this method is focused on pulps destined for tissue papers, paper napkins, and some specialty papers, rather than for newsprint applications. However, the increased resistance to color reversion of peroxide-bleached pulps tends to offset the increased cost of this step. 

Oxidative bleaching, mainly with peroxide as the bleaching agent, for brightness increase by fiber lightening. 

Aging of bleached pulps in presence of high humidity results in higher values of peroxide formed than aging at low humidity. The total amount of peroxide increases linearly with the time of aging.. . . Moisture was found to promote not only peroxide formation but, to some extent, also, brightness reversion, particularly in absence of air. Aldehyde groups formed by hydrolysis were found to be involved in peroxide formation. 

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