1985 Volume 1

This paper reviews current knowledge of pulp flocculation over the entire range of consistency found in pulping and papermaking, from dilute suspensions of fibres in water to dry fibres suspended in air. Some of the key factors that govern flocculation in low consistency aqueous suspensions are shown to apply with changing emphasis over the entire consistency range. Several recent findings from our laboratory that fill gaps in current knowledge are also reported.

A new individual wet fibre flexibility test has been devised, and used to investigate the relationship between fines-free sheet apparent density and the wet flexibility of the constituent fibres. Linear relationships were found for a number of pulps, ranging from bleached chemical to thermomechanical, and for several different pulp treatments. It was concluded that changes in the average wet fibre flexibility of a particular pulp, caused by mechanical and/or chemical treatment, can be evaluated by measuring the apparent density of the fines-free sheet, at least over the investigated range (240-760 kg/m³).

The z-direction (ZD) elastic properties of paper have received little attention in the past because of measurement difficulties. This paper describes the effects of wet pressing, refining, and yield on three ZD elastic properties, C33, C44, and C55. The elastic parameters were measured using ultrasonic methods on an unbleached kraft oak pulp. The ZD elastic parameters were very sensitive to wet pressing pressure. Increasing the level of refining or decreasing pulp yield produced increases in C33, C44, or C55, which were greater than would be expected by wet pressing alone to the same density. A plausible explanation for this behavior is that the refining and yield changes also significantly change the ZD stiffness and shear stiffness of the fiber cell wall.

Experimental and theoretical measures of flocculation were studied using image analysis. An experimental study of commercial board samples led to the proposal of three descriptive floc features, namely, size, ‘definition’, and contrast. Numerical values were obtained from an ensemble averaged linear auto-correlation function.

In addition a theoretical model of formation was simulated to compare degrees of flocculation. The theoretical structure was created by using a poisson cluster model in conjunction with a coverage model. This led to the superposition of fibrous micro0-flocs whose flock centre radii, R, and fibre content, N, determine the severity of the formation. The variance and p.t.p correlation of the resulting image textures were computed. These measures were found to have a lower limit which is set by the fibrous structure of the flocs.

The findings from the simulation study were then applied in principle to the variance and size information extracted from the board samples to explain their structure. The versatility of programmable image analysis systems was demonstrated for formation measurement.

The thermal conductivity of paper was measured using a thermoacoustic method based on the propagation of a periodic temperature wave in the medium. Thermal diffusivity and thermal conductivity can be calculated from the resulting phase shift.

The thermal conductivities of sheets prepared from different pulps were measured under standard conditions and at 70°C and 10% RH.

In paper, heat is conducted through both the solid phase and the gaseous phase. In the case of dense paper and at high moisture contents, heat transfer due to diffusion of water vapour makes a major contribution.

The results were used to construct a qualitative physical model for the conduction of heat in paper. In the normal paper density range of 400 – 900 kg/m³ heat conduction can be explained in terms of layers of air and solid phase connected together in different ways.

At higher densities and higher moisture contents the mechanisms of hear conduction change.

The heat conduction characteristics of paper are better explained using thermal diffusivity calculated in terms of basis weight than by using thermal diffusivity and thermal conductivity.

Some fundamental aspects of mechanical actions on wood pulp fibers have been studied. Specifically, the objectives of the study were to produce internal fibrillation in pulp fibers, evaluate its influence on paper properties, and establish its importance relative to external fibrillation and fines in terms of paper property development.

An apparatus was constructed that subjected pulp fibers (in the form of a wet handsheet) to repeated compressive loading cycles. Pulp was also treated in an experimental apparatus designed to promote external fibrillation, and in a Valley beater. Several fiber and paper properties were measured.

The results showed that internal fibrillation could be produced with the repeated compressive action of the apparatus. The effect of internal fibrillation on paper properties caused a threefold increase in breaking length, from 2 km to 6 km. This level was 75% of the highest breaking length achieved with the Valley beater, which was 8 km. The reason for the difference in breaking length between the samples is due to the (calculated) differences in fiber-fiber bond shear strength. By adding fibrillation to internally fibrillated fibers and forming the sheet, no change in breaking length was achieved. However, adding fines to a suspension of internally fibrillated fibers increased the sheet breaking length, almost to the level produced by Valley beaten fibers.

From these results, it was concluded that internal fibrillation in pulp fibers plays the largest part in improving sheet breaking length. Also, fines are a necessary supplement in the fiber network to improve interfiber bonding. The effect of external fibrillation had no apparent influence on improving breaking length, but its importance may lie elsewhere in the sheet forming process. Internal fibrillation and fines can be produced in separate steps, indicating an additive approach to developing sheet properties through refining is possible.