Strategies to reduce heat-induced darkening for enhanced 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.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering (hsv//eng)

Nyckelord

Bleaching

Brightness

Hydrogen peroxide

Light absorption

Light scattering

Mechanical pulps

Temperature

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