The effect of blending ratio and crowding number on the yield stress of mixed hardwood and softwood pulp fibre suspensions

To study the impact of blending on the yield stress of mixed hardwood and softwood fibre suspensions, a conventional viscometer, coupled with an ultrasonic Doppler velocimetry technique was used to measure the yield stress of hardwood and softwood blending suspensions at mass concentrations ranging from 0.5% to 1.5%. Over the range of concentrations tested, increasing the percentage of softwood in the blend will result in an exponential increase of crowding number. Moreover, the yield stress of all tested samples increased non-linearly with increasing crowding number and the yield stress was found to depend on a power law of the crowding number. For a given crowding number, the yield stress of suspensions decreased with increasing the softwood share in the blending suspensions. Further, with an increase in the percentage of softwood, the yield stress was found to increase linearly at a certain concentration.