CHARACTERIZATION OF UNBLEACHED KRAFT PULPS BY ENZYMATIC TREATMENT, POTENTIOMETRIC TITRATION AND POLYELECTROLYTE ADSORPTION

The surface properties of unbleached softwood (Pinus sylvestris) and h ardwood (Betula verrucosa) kraft pulps were modified by treating the f ibre surfaces with specific xylanase and mannanase (Trichoderma reesei ). The amounts and accessibilities of surface carboxyl groups in untre ated and enzyme-treated fibres were analyzed using adsorption of catio nic polyelectrolytes and high precision potentiometric titration. Acco rding to the potentiometric titration both softwood and hardwood pulps contained two types of acidic groups, one with pK(1) approximate to 3 .3 (uronic acid in xylan) and one with pK(2) approximate to 5.5 (proba bly carboxylic group in lignin), The relative amount of the weaker aci d in softwood and hardwood pulp was 10% and 20% of the total acid cont ent, respectively. The ratio of the two acids in pulps was not changed bq mannanase treatment, while xylanase treatment decreased the relati ve amount of the stronger acid. This indicates that the major part of the carboxylic groups are bound to xylan, Xylanase treatment removed a bout 30% of the xylan in both birch and pine pulps and, consequently 2 2% of the acid groups in hardwood pulp but only 8% of the acid groups in softwood pulp. Hence, in softwood kraft pulp fibres the content of uronic acid side-groups in accessible xylan on the surfaces of the fib res is lower than that on the hardwood fibres.