MULTILAYERING OF FINE PAPER WITH 3-LAYER HEADBOX AND ROLL AND BLADE GAP FORMER
The aim of this study was on the layering of fine paper with a 3-layer headbox and roll blade gap former. It consisted of methodological development and laboratory scale studies, as well as layering studies on the pilot paper machine scale. The potential of layered structures in papermaking and the phenomena affecting layer mixing were also studied. Filler distributions were characterized by two parameters filler distribution shape and symmetry factors. The method developed made it possible to apply a statistical approach to experiments when studying the impact of typical wet end control parameters on the control of filler distributions and the contribution of filler distributions to paper quality parameters. A method by which the center layer fiber distribution may be characterized from the final paper is also presented. The method is based on the use of dyed fibers and a spectrofotometer. In addition, a laboratory device, a multilayer handsheet former for making layered structures, is presented. The process closely resembles a real continuous multilayer process by providing water phase interactions between layers during forming and two-sided dewatering characteristics. In a case study, retention chemical and filler layering with WFC base paper was performed by the multilayer handsheet former. Surface roughness decreased and brightness increased when the filler content in paper surfaces was increased. At constant filler content, paper strength increased as the filler concentration in the paper surfaces increased. The distinction between roll dewatering and blade dewatering in terms of layer mixing as well as differences in the mixing behavior of fibers and fillers in different dewatering phases was studied. Major fiber mixing takes place during the free jet and roll dewatering phase. In blade dewatering, fiber movements are related to formation improvements. The greatest mobility differences between fibers and fillers were found in the blade dewatering phase in roll and blade dewatering. Filler distributions were significantly affected during blade dewatering, with the mobility of the fibers remaining insignificant. The relationship between typical fine paper quality parameters and filler distributions was studied. Formation improvement and densification of the paper were achievable at the same running conditions. Low porosity was achieved by evenly distributed constituents (good formation) and a local rich filler concentration somewhere in the paper structure in the paper thickness direction. Oil absorption two-sidedness was minimized by symmetrical filler distributions. The combination of all these quality factors required the filler distribution to be symmetrical and as uor turned u-shaped as possible. The use of additives layering as a part of paper quality control was also studied. Multilayering was found to be an effective tool for the control of filler distributions. On the other hand it was shown that, in order to get optimized impact of the multilayering on quality properties, the other process variables also need to be controlled.
The yield stress of fibre suspensions
Yield stresses were determined for commercial wood pulp suspensions and synthetic fibre suspensions of low and medium mass concentration. The yield stresses measured represent interfibre failure of the network rather than failure between the suspension and a solid surface. The measurements were carried out in a rotary viscometer at low yield stresses and in a concentric rotary shear tester at yield stresses in excess of 2500 Pa. The experimental results were correlated with the volumetric concentration Cv in equations of the form τy = aCbv where a and b are constant for a given fibre type. It was found that b ≃ 3, in agreement with the predictions of a theoretical model of fibre network strength based upon the interlocking of elastically bent fibres. The dependence of the yield stress on the fibre properties of aspect ratio and modulus of elasticity was not adequately predicted by the model, suggesting that fibre bending alone did not account for the network strength over the concentration range studied.
Motion of pulp fibre suspensions in rotary devices
Suspensions of semi-bleached kraft pulp at low and medium mass concentrations were sheared in a rotary shear tester at angular velocities up to 524 rad/s (5000 rpm). The use of baffled housings, with rotors having lugs, imposed shear within the body of the suspensions and prevented slip at the vessel walls. The torque was measured as a function of angular velocity, and high speed photographs were taken through a transparent plate to aid in interpretation of the findings. The torque vs. angular velocity curves and flow phenomena were found to depend on the mass concentration of the suspension, the gap width between the rotor and housing, and the air content of the suspension. With little gas present, the flow initially involved tangential flow in an active cavity whose radial extent increased with increasing angular velocity. When the flow reached the outer baffles, a marked flow transition occurred, probably explaining what has been called “fluidization” by Gullichsen and Harkonen (1981). As the gas content of the suspension increased, the gas congregated around the rotor, impeding the transfer of momentum to the suspension. The apparent viscosity of a 10% by mass suspension was of the order of 16 Pa · s under the test conditions. 
 
 
 
Des suspensions de pâte a papier kraft semi-decoloree a des concentrations massiques faibles et moyennes ont ete cisaillees dans un appareil d'essai de cisaillement rotatif a des vitesses angulaires allant jusqu'a 524 rad/s (5000 rpm). L'utilisation de systemes a chicanes, avec des rotors munis d'oreilles, impose un cisaillement au sein des suspensions et empeche le glissement aux parois du reservoir. Le couple a ete mesure en fonction de la vitesse angulaire, et des photographies a vitesse rapide ont ete prises a travers un plateau transparent pour faciliter l'interpretation des resultats. On a trouve que les courbes de couple en fonction de la vitesse angulaire et les phenomenes d'ecoulement dependaient de la concentration massique de la suspension, du jeu entre le rotor et l'enveloppe et de la teneur en air de la suspension. Lorsqu'il y a peu de gaz, l'ecoulement entraine initialement un ecoulement tangentiel dans une cavite active dont l'extension radiale augmente avec l'augmentation de la vitesse angulaire. Lorsque l'ecoulement atteint les chicanes exterieures, une transition marquee de l'ecoulement est observee, ce qui correspond probablement au phenomene que Gullichsen et Harkonen (1981) appellent «fluidisation». Lorsque la teneur en gaz de la suspension augmente, le gaz se conglomere autour du rotor, ce qui reduit le transfert de quantite de mouvement vers la suspension. La viscosite apparente d'une suspension a 10% en masse est de l'ordre de 16 Pa-s dans les conditions d'essai.