Variables affecting the thermal yellowing of TCF-bleached birch kraft pulps

The purpose of this investigation was to identify the most important variables influencing the thermal yellowing of TCF-bleached kraft pulps. The variables studied were: hexenuronic acid content, dry content, temperature, time, pH, and iron content. All these variables influence the process of colour reversion. It was shown that there is a strong relationship between hexenuronic acid content and the magnitude of the colour reversion. Of the conditions investigated, long time (6 days), high temperature (80 degreesC), slightly acidic conditions (pH 4.5) and a moist environment (92% dry content) caused the greatest brightness reversion. At 99% dry content the discoloration was insignificant. Impregnation of handsheets has shown that ferrous ions are more detrimental than ferric ions to the colour formation. It is suggested that the hydrolysis of hexenuronic acid into the known acidic degradation products, 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid can not be the rate limiting reaction in the colour formation. It is also suggested that new aromatic structures are formed during the thermal yellowing process. Finally it was shown that the discoloration can be partly washed away with water.