1973 Volume 2

The important roles of sheet roughness and porosity and of fibre morphology on the way liquids spread on paper are revealed by static and dynamic scanning electron microscopy. Some preliminary results are described that demonstrate the potential value of the technique.

An approach to the basic processes of ink absorption into paper coatings is made by considering the way in which oil is absorbed into kaolin-based layers (50 microns thick), formed on flexible polyester substrates. Oil layers are applied to these layers by use of an IGT printability tester . The change in gloss level with time after application of the oil film is used as a method for determining the time taken for an oil film of known thickness to be absorbed.

The capillary penetration equation of Washburn is used as a basis to explore the relationship between the physical properties of the oils and the structure of the coatings.

The basic structure of one coating is also determined by the method of mercury porosimetry and is related to the oil absorption rate by use of the equation of Millington & Quirk for the permeability of the layer.

An approach to the study of fluid flow through porous materials based on Darcy’s law and the continuity equation is introduced as a possible means of studying fluid uptake by paper. The approach leads to a flow equation that may be put in the form of a diffusion equation with a diffusivity dependent on the fluid content. Hence, with the boundary conditions imposed during one-dimensional fluid uptake by a porous material initially at a uniform fluid content, the uptake is proportional to the square root of time, which is the same result as given by the Green & Ampt equation and the Lucas-Washburn equation, both of which use the analogy of fluid flow through capillary tubes.

In relating the quality of the coated sheet to the properties of the base paper, the  liquid absorbency of the base paper is a most important consideration for all coating processes.

Nor surprisingly, base sheet absorbency has received a great deal of published attention. Recently, there has been a growing appreciation that the Lucas-Washburn equation does not fully explain the penetration of aqueous fluids into paper. The shortcomings of the Lucas-Washburn equation are clearly indicated in the work described on the mechanism of the size press.

A new method of following the penetration of aqueous solutions and suspensions is presented.

Analysis of the results indicates that a wetting time exists and that the Lucas-Washburn equation is inadequate to describe the penetration of aqueous fluids into paper.

A new model for penetration is suggested, which allows for the swelling of the sheet that occurs as penetration takes place. A penetration equation is derived from this model.

The equation for pick-up of surface sizes on the size press recently proposed by Hoyland & Howarth contains three terms. These are the immobilisation term and the absorption term, which are controlled by the sheet and the hydrodynamic term, which is primarily a function of speed. This paper describes pilot plant experiments to establish how the immobilisation and absorption terms are related to the various aspects of the sheet structure and composition . The most important factors were found to be structural differences associated with freeness changes and the presence of very small quantities of resin size. The latter were important at levels well below those associated with normal size addition.

An attempt is made to demonstrate the relationship between paper as a substrate and the different properties of coating colours by photographs of microtome sections. The results are based on the coating technology of industrial and pilot plant experiences.

The first part deals with multi-roll application, mainly size press operation with starches and coating colours and makes obvious the penetration zone of the different aqueous coatings. The effect of different factors of the coatings (viscosity, starch modification, solids, paper smoothness, sizing degree ofthe base paper and machine speed) on the coating weight and the starch penetration have been investigated and demonstrated.

The effect of coating colour application and the use of airknife and blade applications is shown in paper sectioning pictures and the results are discussed.

Finally, the migration problem of the different binders is investigated, especially using intense hot-air drying.

These demonstrations by micrographs of paper sections show the possibility of making visible some parts of the mechanism of paper and liquid interaction during paper coating.

Printability can be satisfactorily related to fundamental paper properties only if the nature and the consequences of the interactions among paper, ink and printing press during and after impression are well understood.

Our present knowledge of these interactions is critically reviewed with particular reference to wetting, spreading, conformability, roughness, ink transfer, ink setting and print-through. The behaviour of low concentrations of oil in paper is considered in some detail.

Print quality criteria are classified and listed from the point of view of paper printability. The theoretical background of such important print quality characteristics as print density and gloss, evenness, contrast and sharpness are presented. The effects of the components of ink transfer-coverage, immobilisation and splitting-on the resulting ink film structure are discussed. The connection between roughness distribution of paper and coverage in ink transfer are examined and an improvement is proposed for the previous coverage theory of Hsu. The use of laboratory test printing and full-scale pressruns are recommended for printability evaluation of paper and the methods used in determining various print quality numbers are reviewed.

The evaluation of printability is usually done on large printed areas; imperfections on a microscopic scale have been mostly ignored, in spite of their obvious importance. The unevenness of the paper surface often originates from non-uniformity in the morphology of the fibrous material. Thick fibres or particles standing out from the paper surface will produce dark spots, while the very thin, small ones will cause ‘valleys’ having lighter tones. With the increased use of hardwoods, which may contain 10-50 per cent by volume or 10 per cent by weight vessel elements of 50-500 μm in diameter, whereas the fibres may be only 15 μm, these problems are even more serious.

A distinction is made between the two sheet materials named in the title and nine examples are quoted covering a wide range of both of them. A brief description is given of the method of producing such materials and surface micrographs at a magnification of 2,000 times are reproduced to show the wide difference in surface texture.

The physical test data for six of the synthetic and plastics papers are quoted in comparison with similar data for three conventional coated papers. In particular, the ink absorbency of plastics paper is far too low to permit the use of conventional printing inks that dry mainly by solvent absorption. The dimensional stability is exceptionally good and increases as the proportion of cellulose fibres in the sheet diminishes. Most plastics and synthetic papers suffer from a relatively low stiffness compared with conventional papers, but there is definite evidence that a ‘second generation’ of plastics papers is being designed to give greater stiffness by means of a multi-ply construction, in which the centure of the sheet has a low density honeycomb structure.

Generally, the plastics papers have a significantly higher resistance to the dissipation of static electricity than do conventional papers. The presence of mineral coatings considerably reduces the resistivity, as shown by some of the plastics papers that have had a surface coating. Contrary to expectations the consistency of quality of plastics papers is not better than that of conventional papers; in fact, it is often less consistent. Values of the coefficient of variation for some of the papers are given in the text.

In an appendix, a description of the physical characteristics of an unidentified Japanese second generation plastics paper are given. The plies had been separated by a manual technique and their varying grammage, thickness and density were determined.