SUMMARY:Tannins are polyphenolic compounds found mainly in bark. When reacting with iron, they form strongly coloured complexes, which through contamination from the bark may induce a brightness decrease of mechanical pulps. Wood itself contains phenolic compounds, which can form coloured complexes with iron. We have investigated gallotannin as a model for metal-binding sites in the pulp. The behaviour of tannin-iron complexes in solution and in pulp has been studied. In aqueous solution, the tannin-iron complexes can be decolourised by the addition of diethylenetriaminepentaacetic acid (DTPA). The colour of the tannin-iron complexes was very pHdependent. Thus in solution, these were decolourised at low pH and at high pH the spectral characteristics were changed substantially. We have studied the effects on brightness and heatinduced brightness loss of the impregnation of thermomechanical pulp (TMP) with 30 parts per million iron (ppm i.e. mg/kg) either as iron or tannin-iron as well as the possibility to decrease such effects by using various solvent extractions. The tanniniron impregnation causes a decrease in ISO-brightness of approximately 3% and an increase in the light absorption coefficient (k) by approximately 2 m 2 /kg at the tannin-iron absorbance maximum, 565 nm. These effects are approximately ten times higher than those observed for the pulp only impregnated with iron. Extraction with 1% by weight of DTPA provides a way to reduce the brightness decrease induced by the tanniniron complexes and the observed decrease can be attributed to removal of iron from the pulp. Acid extraction was the most efficient way to reduce the iron content in the pulps and to decoulorise the tannin-iron impregnated pulp. However, after acid extraction of iron impregnated pulps, new chromophores were evidently formed. Addition of the reducing agent, sulphite, to extractions had no effect on the iron removal or the brightness of the impregnated pulps. The heat-induced brightness loss is not influenced by the addition of tannin-iron or iron. The brightness loss caused by heat was lower for pulps extracted with DTPA. ADDRESSES OF THE AUTHORS:Linda Friman (linda.friman@mh.se), Hans Höglund (hans.hoglund@mh.se) and Hans-

This paper is published in the open archive of Mid Sweden University DIVA http://miun.diva-portal.org with permission of the publisher Citation for the peer-reviewed published paper:Logenius L, Friman L, Agnemo R. The influence of temperature and moisture on the optical properties of cellulose in the presence of metal chlorides and glucuronic acid.

This paper is published in the open archive of Mid Sweden University DIVA http://miun.diva-portal.org with permission of the publisher Citation for the peer-reviewed published paper:Logenius L, Friman L, Agnemo R. The influence of temperature and moisture on the physical properties of cellulose in the presence of metal chlorides and glucuronic acid.

SUMMARY:Tannins are polymeric, phenolic constituents found in the bark of pine and spruce. When reacting with iron ions, tannins form strongly coloured complexes. Thus, the presence of bark in the mechanical pulping process leads to decreased brightness of the pulp. In order to evaluate the effects of the presence of iron on the properties of pulp, we have impregnated thermomechanical pulp (TMP) with 30 parts per million (ppm i.e. mg/kg) iron either as Fe 3+ or as tannin-iron complexes and studied how such treatments affect bleachability and heat-induced brightness reversion. The bleaching agents studied are hydrogen peroxide and sodium dithionite. Treatment of the tannin-iron impregnated pulp with 1% by weight of diethylenetriaminepentaacetic acid (DTPA) before bleaching with 4% hydrogen peroxide almost eliminated the brightness loss caused by the impregnation. Such a treatment also removed all of the added iron from both the tannin-iron and FeCl 3 impregnated pulps. Approximately 5% more of the added peroxide was required for oxidation of the tannins in the tanniniron impregnated pulp. Contrary to what was observed with peroxide bleaching, dithionite bleaching did not reduce the amount of iron in the pulps. Instead, the added iron and tanniniron negatively affected the dithionite bleaching, even if the pulps were extracted with DTPA before bleaching. It should therefore be advantageous to first bleach with peroxide, which removes most of the iron, and then with dithionite. Compared with dithionite, peroxide yields a more efficient bleaching. The reason for this is that the former reduces the light absorption coefficient, the k-value, more efficiently in the whole visible spectrum, whereas dithionite reduces it mainly at shorter wavelengths. In our experiments, the addition of tannin-iron or FeCl 3 to the untreated pulp did not increase heat-induced brightness reversion. This is supported by the fact that although extraction of the samples with DTPA before bleaching lowered the iron content slightly, it did not affect the brightness reversion. The initial brightness reversion of the dithionite bleached pulps was larger than that observed for the peroxide bleached pulps. ADDRESSES OF THE AUTHORS:Linda Friman (frimanlinda@hotmail.com), Hans Höglund (hans.hoglund@mh.se) and Hans-Erik Högberg