Delignification and Bleaching Response of Earlywood and Latewood

Abstract

The delignification response in cooking and the impact of bleaching on earlywood and latewood were studied. Spruce earlywood and latewood chips were pulped by the kraft process and subsequently treated with one bleaching chemical at a time. In cooking, latewood required a higher alkali charge to reach the same kappa number. No difference in the light absorption coefficient between the different fiber types was observed. After oxygen delignification the earlywood fibers had a higher light absorption coefficient at the same kappa number. The difference in light absorbing material was maintained when bleaching was performed with chlorine dioxide, ozone, and peracetic acid. Hydrogen peroxide decreased the light absorbing structures in the earlywood to the same level as for latewood. The earlywood pulp had a higher brightness at a given kappa number than the latewood. The higher brightness remained through all bleaching operations and was primarily due to a higher light scattering ability.

Keywords:

• Softwood
• latewood
• earlywood
• delignification
• bleaching response

The authors express their thanks to Rose Marie Karlsson for skilful experimental work and to Anna Tubek‐Lindblom and Elisabet Bergnor‐Gidnert for fruitful discussons. They also thank VINNOVA (Swedish Governmental Agency for Innovation Systems) and the project steering group with the participating companies; AssiDomän, Billerud, Fiber Tracker, GL&V, Kemira, Korsnäs, and Metso.