Floc structure and flow properties of pulp fiber suspensions

The rheological properties of pulp and PET fiber suspensions were measured with a parallel-plates type rheometer. A high speed CCD camera was used to observe the changes in the floc structures that were produced during the rheological measurement. The flow curves of the pulp fiber suspensions showed Newtonian flow in the low shear rate range. With increasing shear rate, the shear stress increased and then became unstable, namely, jumps in the flow curves were observed, which are due to the formation of fiber flocs. At a higher shear rate, the flocs disappeared and the systems showed Newtonian flow again. The flow curves of the polyethylene terephthalate (PET) fiber suspensions showed non-Newtonian flow even in the low shear rate range. We found that the uniform distribution of fibers became uneven and then the flocs began to form at the critical shear rate; and over the critical shear rate, the fibers began to uniformly disperse again, i.e., the flocs disappeared. The two-dimensional fast Fourier transform (FFT) technique and Guinier approximation were used to obtain the radius of gyration, R_g, of the flocs. It is considered that the R_g of the flocs is useful to characterize the mechanism of fiber flocculation.