FIBER DEFORMATIONS AND STRENGTH POTENTIAL OF KRAFT PULPS

In this paper it is demonstrated that an unbleached mill kraft pulp after discharge from the digester has a lower strength potential (tear index at tensile index 90) than its corresponding laboratory pulp, i.e. it has an incomplete strength delivery, The reduction amounts to 5-20 % and the variation and average level are about the same for Kamyr continuous and for batch digester systems. Changes in fiber properties that may explain the reduction in strength have been studied, It was found that the fiber length was reduced by some 2 % which corresponds to a strength reduction of approximately 2 % as established in this study, The fiber form factor (curl) was also affected, the mill pulps being more curled than the laboratory pulps. Similarly it was established that the mill pulp contained considerably more deformations of other types such as twists, damaged walls, folds and knees as well as compressions and dislocations. These deformations influence the strength negatively but in a quantitatively unknown way. Finally it was found that the mill pulps from batch digester systems had a 4 % higher coarseness than coresponding laboratory pulps which, according to previous experience, should result in a 2 % higher yield and further in a strength reduction of approximately 4 %, The yield difference found in this study is not in line with common ideas, and it is evident that the assumed correlation between coarseness and pulp yield should be studied further