1985 Volume 2

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.

The effects of cationic starch wet-end addition on the mechanical and optical properties of clay loaded papers are discussed. It is shown that massive strength improvements can be achieved at high filler loadings with high starch additions. The properties of super-filled paper structures with filler loadings up to 90% are also reported. Cationic starch wet-end addition is superior to starch impregnation applications (e.g. size press) on highly filled structures. This behaviour is understood from the effect of wet-end starch addition on sheet consolidation (sheet density improvement). Wet-end starch addition increases the drying stress built up during sheet consolidation. This increase is generally much higher for filled papers than for papers of zero filler content.

The effects of fillers and wet-end starch addition on the intensity of stress concentrations in paper structures have also been investigated. It was found that an increase in filler level increases the stress concentration, whereas starch addition leads to a decrease. Addition of starch may actually bring the stress concentration intensity in a highly loaded sheet down to the level of a sheet with no filler at all. On the basis of the results, a mechanism of lubrication by which wet-end cationic starch addition improves the strength properties of filled papers is proposed .

A lumen-loaded pulp is one containing filler which is confined to the lumen surfaces of the fibres. Prerequisites for obtaining such a pulp are filler particles which are small enough to pass through the pit apertures and chemical conditions favourable to a good bond between the particles and the lumen surface.

The first stage in the preparation of lumen-loaded pulp is agitation of the fibres in a concentrated suspension of filler. At high levels of agitation, entry of the filler particles into the lumens appears to be very rapid and the rate of the uptake of particles by the lumen surface is predictable from a Langmuir-type, adsorption-desorption mechanism. At prolonged times of agitation, a plateau level of loading is achieved which approximates to single particle coverage of those sites on the lumen surface capable of holding particles in the applied turbulent field. The second stage in the preparation is that of a wash which removes all filler particles not bound to lumen surfaces.

If subsequent to washing, the loaded fibres are subjected to high turbulence in water, some filler is lost relatively rapidly but the residual filler is much more resistant to removal. The more weakly-held filler can be kept to a minimum by the use of high levels of shear throughout preparation.

A reliable and reproducible testing procedure for the development, product improvement and production control of pressure sensitive copy papers (PSP) is described.

The crucial aspect of the testing procedure (FM-test) is the alignment of many small individual areas so close to each other that a full tone area results from the contact between and partial overlapping of the individual areas. The full tone area surface copies of practically any size, characterized by a very uniform appearance of intensity, can be measured very simply by means of commercially available remission photometers or densitometers. Another important aspect of the FM test is very good reproducibility and adjustment of the writing force, which is the key to reliable conclusions concerning changes of quality and their causes.

Mathematical and statistical evaluations point out that the quality of PSP can be described by a “characteristic curve” (CC), showing the change of image intensity as a function of writing force. A three-parameter equation for the CC allows an easy calculation of several quality criteria characterising the PSP, without the need of performing a multitude of different tests. In an example, discussed in more detail, it is shown, which paper qualities affect the performance of a PSP and how quickly and reliably the correlations can be evaluated by the FM-test.