The Yielding And Thixotropic Flow Behavior In Pulp Fiber Suspensions

The pulp and paper industry consists of various process streams where flow is complex, from the cooking section to sheet formation, involving the flow of pulp fiber suspensions at different mass concentrations. Pulp fiber suspensions are multiphase heterogeneous mixtures, within which fibers can be considered as solid phase. To achieve optimal and effective functionality of pulp and paper manufacturing and operations, stablize the down-stream processing of the product for various grades, knowledge of pulp suspension rheology is of great importance.Based on the brief discussion on the yielding, thixotropic and viscoelastic properties of pulp fiber suspensions, this paper firstly reviewed past studies on the origin and measurements of yield stress of pulp fiber suspensions and its impact factors, moreover, details about the applications of yield stress in pulp and paper industry were presented as well. Secondly, three yield stress measuring methods, ie., shear stress ramp method, extrapolation of the flow curve method and ultrasonic Doppler velocimetry (UDV) technique, were used to study the yield stress of chemical pulp fiber suspensions, the resulting yield stress from three measuring technique were then compared with each other. In addition, considering the paper manufacturing process, this paper studied the influences of refining and blending operations on the yield stress of chemical pulp suspensions. Futhermore, three rheological tests including hysteresis-loops, creep tests, and step-wise experiments were used to investigate the thixotropic rheology of chemical pulp fiber suspensions. In the end, based on the usage of non-wood fiber raw materials in Chinese paper industry, the thixotropic flow behavior of two typical non-wood fiber suspensions were also examined.The main conclusions of this paper are identified as follows:(1) Pulp fiber suspensions are multiphase heterogeneous mixtures, within which fibers can be considered as solid phase. Above a critical mass concentration, pulp fiber suspensions present yielding, thixotropic and viscoelastic properties, which are govered by flow behavior of the flocs within pulp suspensions. Among these rheological properties, yielding and viscoelastic properties have been mostly investigated. For the yield stress of pulp suspensions, studies can be divided into three aspects:the effect of fiber morphological properties and external factors such as temperature, pH and shear history on the yield stress; the rheological technique and devices for yield stress measurments; applications of yield stress of pulp suspensions in relevant fields of papermaking industry.(2) For a certain pulp suspension, different measuring methods result in different yield stress values under a given mass concentration. For bleached hardwood pulp suspensions, the yield stress values induced from shear stress ramp method and extrapolation of the flow curve method were 175%～507% and 32%～161% larger than that obtained from UDV technique; while for the bleached softwood pulp suspension, when compared to UDV, the yield stress obtained from shear stress ramp method and extrapolation of the flow curve method resulted in 4%～121% and -29.4%～40.3% higher values.(3) Under steady-state shear conditions, over the range of concentrations tested, the yield stress τy was found to depend on the concentration Cm via a power law relationship τy=aCmb.(4) At mass concentrations ranging from 0.5 to 1.5%, as the freeness decreased, the yield stress of hardwood suspensions increased to a maximum value then decreased, this variation in yield stress was also observed in softwood suspensions with mass concentrations above 1%. However, when the concentration was lower than 0.75%, the yield stress of softwood suspensions increased with decreasing freeness. This behaviour can be understood from underlying fiber properties of fibrillation, curl and stiffness which are impacted by PFI mill, suggesting that freeness alone did not account for the yield stress over the concentration range studied.(5) Over the range of concentrations tested, increasing the percentage of softwood in the mixed hardwood and softwood pulp suspensions will result in an exponential increase of crowding number, and the crowding number values N was found to depend on the blending ratio x via a formula N=j+kex/74.6; moreover, with an increase in the percentage of softwood, the yield stress was found to increase linearly at a certain concentration and the yield stress τy was found to depend on the blending ratio x via a formula τy=τ+γ·x.(6) The yield stress of mixed hardwood and softwood pulp suspensions obtained through UDV technique increased non-linearly with increasing crowding number, and the yield stress τy was found to depend on the crowding number N and fiber average aspect ratio A via a power-law function τy= 1.81(N/A)2.65.(7) Above a critical mass concentration, pulp fiber suspensions present yielding and thixotropic properties, wood and non-wood pulp suspension systems exhibited strong mechanical similarities. The yield stress of the pulp suspensions were highly dependent on the time of rest prior to the measurement. As the relaxion time increased from 120s to 600s, the yield stress increased 2.79% and 4.71%, respectively, for bleached hardwood and softwood pulp suspensions under the same crowding number of 120, as the relaxion time exceeded 600s, the yield stress values increased slightly and tended to be constant; while for bleached bamboo and unbleached corn stover pulp at a given crowding number of 120, as the relaxion time increased from 120s to 300s, the yield stress increased 4.65% and 3.15%, as the relaxion time exceeded 300s, the yield stress values increased slightly and tended to be constant.(8) Pulp suspensions exhibited a plateau in their hysteresis flow curves where a slight change in the shear stress generated a jump in the corresponding shear rate. Moreover, the yield stress τy was found to depend on the crowding number N and fiber average fiber length l via a formula τy= 9.28 × 10-11(N/l)2.27; during creep experiments under controlled stress, a viscosity bifurcation was observed, where the suspensions stopped flowing to reach a saturated state below a critical stress and reached a steady state of flow at higher values;a simple exponential model μapp(t)=a·exp(-t/τ)+μ∞ can be used to quantify the thixotropic response of pulp to stepwise changes in the shear rate and the characteristic time t reflects the changes in pulp suspension microstructure.(9) As the temperature increased from 20℃ to 60℃, the flow curves of bleached bamboo and unbleached corn stover shifted downward, and the yield stress decreased 7.6% and 10.6%, respectively, for bleached bamboo and unbleached corn stover pulp suspensions; as the pH increased from 3.0 to 11.0, the flow curves and yield stress values remain unchanged for these two kinds of non-wood pulp suspensions.