Mittuniversitetet

A method to produce representative sheets for determination of optical properties of mechanical pulps has been developed. It reduces the risk of contamination and discoloration and can be used with small pulp quantities. The deviation from the expected linear behaviour of the light scattering coefficient, s, at wavelengths corresponding to strong light absorption, has been studied using the Kubelka-Munk model and the angular resolved DORT2002 radiative transfer solution method. This decrease in s could not be explained by errors introduced in the Kubelka-Munk modelling by anisotropic scattering. Linear extrapolation of s can therefore not be justified as a way to obtain a more correct light absorption coefficient, k. For thepulps studied, the decrease in s at short wavelengths had little effect on k at 457 nm.

Hydrogen peroxide is a widely used bleaching chemical for mechanical pulps and is particularly useful when high brightness levels are required. The objective of this work was to study fines as a limiting factor for reaching higher brightness levels in high consistency hydrogen peroxide bleaching of Norway spruce (Picea abies) thermomechanical (TMP) and stone groundwood (SGW) pulps. The hydrogen peroxide bleaching efficiency (i.e. light absorption coefficient reduction) was studied as a function of temperature, alkali charge and fines content using an experimental design based on MODDE software (Umetrics AB).It is known that all types of fines contain more lignin, hemicelluloses, pectins, metals and less cellulose than long fibres. The light absorption coefficient was greater for unbleached TMP than for unbleached SGW pulp and an increased quantity of fines increased the light absorption coefficient for both pulp types. The increase was, however, most pronounced for the TMP. Furthermore, the data suggests that both the fibre fraction and the fines fraction are more coloured in the thermomechanical pulp. In most cases, increased amounts of fines in the pulp had a negative impact on the bleached pulp brightness in spite of the improved light scattering ability accompanying the addition of fines. A level of fines exceeding 50% was especially detrimental resulting in lower pulp brightness due to a higher light absorption coefficient.Increasing the bleaching temperature did not improve the bleaching efficiency. The optimal bleaching temperature at a hydrogen peroxide charge of 4.5% was found to be 70^°C within the tested interval of 70-110^°C. For a fines content of 25%, the stone groundwood pulp displayed a brightness of 83% whereas the thermomechanical pulp achieved a brightness of 79%.

The objective of this work was to improve the understanding on how thermal treatments of mechanical pulps containing iron influence the unbleached brightness and hydrogen peroxide bleachability at high brightness levels. An unbleached Norway spruce stone groundwood (SGW) pulp was subjected to heat and iron (Fe3+) treatments followed by a chelating stage and a high consistency hydrogen peroxide bleaching stage. The hydrogen peroxide bleachability was studied on pulps as a function of pre-treatment at elevated temperatures and strongly bound iron content prior to bleaching and the alkali charge during bleaching. The evaluation was done using an experimental design based on the MODDE software (Umetrics AB).

Heat treatment of the unbleached SGW pulp at 110 or 140 degrees C did not induce a significant increase in light absorption coefficient. Increasing the heat treatment temperature to 170 degrees C did, however, result in an increased light absorption coefficient corresponding to a brightness loss of 3.5% ISO. The light scattering coefficient was found to be essentially unchanged regardless of the heat-treatment conditions applied. The brightness loss resulting from the heat treatment could only partly be regained by hydrogen peroxide bleaching.

An increased amount of strongly bound iron in the unbleached SGW pulp increased the light absorption coefficient. This increase was slightly higher when heat pre-treatment was performed at 140 and 170 degrees C compared to when heating at 110 degrees C. Hydrogen peroxide bleaching was found to reduce the iron content of the pulps and thus regain some of the brightness that was lost. This effect was most pronounced at the higher alkali charge levels. It is thus important to minimise the processing temperature, the time at high temperature and the iron content of the pulp in order to improve the unbleached brightness and bleachability of mechanical pulps.

Under mechanical pulping conditions, elevated temperature initiates reactions in the wood constituents resulting in the formation of coloured structures. In an attempt to reduce the formation of colour, a number of additives have been examined to assess their ability to inhibit heat-induced darkening of a Norway spruce (Picea abies) stone groundwood (SGW) pulp. SGW pulp was used as a model pulp with the purpose of imitating a thermomechanical pulp (IMP). The heat-treated SGW pulp, both with and without additives present, was subsequently bleached with 4.5% hydrogen peroxide at high pulp consistency and with varying alkali charges in order to examine the hydrogen peroxide bleachability of such pulps. Heat treatment at 170 °C resulted in a loss of brightness of about 4% ISO after heating for 60 seconds. The reduction in brightness could only partly be regained through high consistency hydrogen peroxide bleaching; the SGW pulp displayed a brightness of 83.7% ISO whereas the heat-treated SGW pulp reached a brightness of 80.4% ISO. Heat treatment with a low addition rate (1 %) of alkaline hydrogen peroxide, sodium bisulfite or sodium sulfite, with a subsequent hydrogen peroxide bleaching stage, resulted in an improved bleached brightness of about 81.6% ISO, i.e. the loss of brightness due to heat was still approximately 2% ISO. Sodium borohydride (1 %) did not suppress the thermal darkening but improved the bleached brightness by approximately 2% ISO compared to the heat-treated SGW pulp and resulted in a brightness of 82.6% ISO. Addition of a chelating agent (DTPA, 0.11 and 0.45%) and the radical scavengers phthalic hydrazide (1 %) and 1 -hexanol (1 %) suppressed the heat-induced darkening somewhat but did not improve the hydrogen peroxide bleachability to any noticeable extent. Addition of citric acid (0.6%) did not suppress the thermal darkening and resulted in impaired bleachability.