Proceeding Articles

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.

It is shown that many physical properties of paper vary significantly in the CD, as much as 10% or more. These variations are believed to be due to the great dependence of sheet properties on the jet minus wire speed differential, and on the large random variations in the headbox discharge velocity, especially from older, air-padded headboxes run at speeds greatly exceeding their design capacity. These jet velocity variations arise both from partially plugged tube bundles feeding the headbox, and from eddy currents created by the slice rectifier roll.

Instead of replacing such headboxes to overcome these problems, it is proposed that two elements of slow ( 100 m/min), early 20th century Foundriniers missing from modern high speed machines be re-introduced . One is a “stilling” Zone on the first part of the wire, once supplied by an apron; this can now be provided by a wide non-dewatering forming board. The second element is a unique Formation Shower which generates CD shear-inducing (like that due to a shake) repetitive ridges, and keeps the stock dispersed throughout the forming zone. The excellent results obtained in the first commercial installation employing these concepts at 650 m/min on light weight papers are presented.

The penetration of fluids into paper is discussed with particular reference to the phenomenon of wetting. An experimental programme is described in which the distribution of penetration over the area of the sheet is measured.

The results indicate that considerable non-uniformity exists and, furthermore, an increase in the degree of sizing of the sheet leads to an increase in the non-uniformity of its wetting by aqueous fluids.

The moisture sensitivity of compression and tensile strength is compared for a range of packaging papers. It is shown that compression strength falls off more rapidly with increasing moisture content than tensile strength. This is especially true in the range up to 10% moisture content, where there is little effect of tensile strength. Results were obtained using the STFI short span compression test and tensile test carried out in silicone oil. Also included are Concora Medium Tests (CMT) for fluting. Concora Liner Tests (CLT) for liner and Ring Crush Tests (RCT) for compression.

This difference in moisture sensitivity is also very evident for papers which have been given different wet strengthening treatments. For example, after 60 min. of water immersion, such wet strength papers can retain a wet tensile strength which amounts to 30 – 40% of the 50% RH value. The corresponding wet compression strength retention is only 15% to 25%. It is also shown that the tensile stiffness is more moisture sensitive than the tensile strength.

The results are discussed with reference to the glass transition that cellulose and hemicelluloses at 20 0C pass through at a given moisture content corresponding to about 10% moisture content for a kraft paper. This transition particularly affects the moduli of the paper, while for tensile strength thermal softening apparently also has some positive effect, by reducing stress concentration.

This paper describes some studies of the effects of drying restraints and sheet density on the in-plane and out-of-plane hygroexpansivity of paper. It is shown that drying restraints in the RTE range, i.e. between 20% and 0% moisture content, greatly influence hygroexpansivity, with in-plane hygroexpansivity being lower the lower the RH to which the paper has been dried under restraint. For sheets of high density, the volume expansivity is not of affected by the drying restraint, and the reduction in in-plane hygroexpansivity is compensated by an increase in out-of-plane hygroexpansivity.

For freely dried sheets both the in-plane and the out-of-plane hygroexpansivity increase with increasing density. The volume hygroexpansivity at various densities is similar to that for wood of the same density. For sheets dried under restraint the density has only a slight influence on the in-plane hygroexpansivity. The out-of-plane hygroexpansivity is higher than for freely dried sheets but includes changes which are probably irreversible, particularly at low densities.

The effects of preconditioning, moisture content and relative humidity during adsorption and desorption on the compression strength of paper were evaluated for a kraft liner and an NSSC-fluting over a range of moisture content from 1-23%. The method used was the STFI Short Span test.

In general, the results show that compression strength decreases with increasing moisture content. More specifically, if compression strength is evaluated as a function of moisture content, the data points fall on a single curve for both adsorption and desorption . This result is independent of the moisture history of preconditioning of the sample.

If compression strength is evaluated as a function of the relative humidity of the test environment, the moisture history and preconditioning both exert a large influence on the test result. This indicates that samples of unknown moisture history should be preconditioned in much drier atmospheres than previously recommended.

The apparent light scattering coefficient of a filler can be shown to be dependent on pulp type, freeness and sheet forming conditions . This is because a large part of this apparent light scattering is in fact due to the effect filter has on pulp. In this paper an attempt is made to provide a mechanistic model for the interaction and provide a quantitative expression relating light scattering to the effect of filler on the fibre as represented by the change in sheet strength. Examples are given of the use of this model to explain the effects of changes in wet pressing pressure and filler aggregation on the apparent light scattering of the filler.

The distribution of filler particles in printing papers increases in importance with increasing filler content. In this paper it is shown that conclusions about the chalk distribution can be made from formation measurements on sheets before and after removing the chalk respectively. Chalk can be chemically removed using an HC1-Propanol solution.

Measurements of filler distribution have been made on fine paper containing 35-70% chalk. The sheets were formed in a twin wire, roll former laboratory machine at 500 m/min. A three component dry strength,/retention aid system was used, including starch, one anionic polymer component, and one cationic polymer component. Both when using a strategy for maximum retention and for optimum formation it was found that the local filler grammage was proportional to the local fiber grammage.

Measurements of formation on base paper and roll coated paper respectively revealed that there is a strong negative correlation between local filler grammage and local fiber grammage, which results in very even formation of the coated sheet.