Changes within the modified kraft process aiming to improve TCF bleachability

the investigated process modifications were: presulphonation, postsulphonat ion, polysulphide pretreatment and polysulphide pretreatment combined with a postsulphonation. The influence of the degree of delignification in the c ook, on the selectivity and bleachability, was also investigated. The pulps were produced from softwood chips and were oxygen delignified before bleac hing according to an AZQP sequence. All the investigated process variants h ad better process selectivity (pulp viscosity at a given brightness) and th e pulps produced with a postsulphonation showed better bleachability, than the reference pulp. The bleachability and the process selectivity was impro ved by interrupting the cook at a higher kappa number and instead extending the delignification in the oxygen stage.