Effect of the ratio of softwood kraft pulp to recycled pulp on formation and strength efficiency in twin-wire roll forming

Research has been undertaken to investigate how the tensile strength efficiency and Z-strength efficiency in twin-wire roll forming is affected when changing formation through the softwood kraft content in a mixture with recycled pulp. The effect of a change in formation through the softwood kraft content and the effect of a change in formation through headbox consistency were also compared. Five kraft contents (0, 25, 50, 75 and 100%) were investigated at volumetric headbox flow rates of 111 and 1,77l/sm. Three additional flow rates were used at a kraft content of 50%. Impaired formation through the softwood kraft content reduced tensile strength efficiency and Z-strength efficiency. Both these properties were similarly affected whether the formation was changed by the softwood kraft content or the headbox consistency. The relative impact of formation was larger for Z-strength efficiency compared with tensile strength efficiency. The reduction in tensile strength efficiency was replicated in a smaller relative increase in tensile strength for machine-made paper than for handsheets made of the furnish. Considering the effect on strength efficiency when evaluating furnishes has been demonstrated as an important concern.