Fracture Properties in Filled Papers

The mechanical properties of paper are impaired by the addition of filler. The beating of kraft pulp and addition of starch are possible remedies for this. However, beating also has negative effects because it reduces opacity and bulk, and starch effects are limited by retention. Optimal use of the kraft pulp and starch is therefore important. We show that in pure kraft sheets beating alone can compensate for most of the adverse effects on mechanical properties caused by kaolin addition. In TMP-based sheets with kaolin, the mechanical properties are fairly insensitive to the kraft content unless very high beating levels are used. The primary role of kraft is to improve tensile stiffness, not tensile strength of paper. Starch and beating both improve inter-fiber bonding but beating also raises fiber segment activation. The latter mechanism contributes to tensile stiffness but reduces damage width. The other mechanical properties of paper appear to be insensitive to fiber segment activation.