Mechanisms of TMP peroxide bleaching using Mg-based alkalis.

The driving force for pulp and paper industry is to develop new technologies with less impact to the environment. In this context, the successful approach in the present paper was the substitution of sodium hydroxide during peroxide bleaching of thermomechanical pulp (TMP) by magnesium hydroxide or magnesium carbonate. Looking at brightening development, thioacidolysis showed that Mg-based processes eliminated less coniferaldehydes than NaOH-based bleaching. Moreover, 19F NMR analysis showed slightly more residual quinone in lignin isolated from NaOH-based bleached pulp than lignin extracted after Mg-based bleaching. Replacing NaOH by Mg-based compounds affected the strength properties of the paper: tensile index was 10% lower than conventional bleached paper, probably due to Mg2+ adsorption on the acid groups of fibers. However, the brightness reversion of Mg-based bleached paper decreased by about two ISO points during photoageing under sunlight. X-ray photoelectron spectroscopy supported the conclusion that Mg-based bleaching caused less re-deposition of lignin and extractives onto the fiber surface compared to the conventional process resulting in less light-induced yellowing. [ABSTRACT FROM AUTHOR]