Proceeding Articles

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.

Lack of surface strength in coated paper can cause significant problems both in offset printing and in different converting operations. Several factors can influence this surface strength. The first part of this paper summarizes previous work concerning the relation between the mechanical properties of the coating film itself and the dry surface strength of coated paper. From studies including both clay and CaC03 pigments, it was concluded that both the in-plane mechanical properties of the coating films and the dry surface strength were to a large degree determined by the choice of pigments and the type of styrene-butadiene (SB) copolymer used as binder. The difference between clay-based and CaCO3-based coatings was of special interest in this study. The second part of this paper describes a laboratory method for measuring the wet strength of coated paper. The method has been applied to paper coated with either clay or CaC03, the coatings being bonded with different amounts of SB. The fracture in the coating in the wet state is shallow and, contrary to the fracture in a dry coating, does not penetrate down to the base paper. The particles removed are also rather small, typically 10-30 4m in diameter. Paper coated with CaC03 had a higher wet surface strength than paper coated with clay. This may be due to a higher degree of adhesion between CaC03 and SB than between clay and SB, which would yield a higher water resistance.

The positive intercept which appears on a graph of K and N ink absorption versus a function of absorption time has previously been interpreted as a measure of surface roughness. In the case of coated surfaces, evidence is offered that it is an indicator of a porous top layer of the coating with distinctly different properties from those of the main part of the coating.

It is shown that the total void volume of this top layer depends on both the capillary suction exerted by the substrate and on the speed of drying of the coating and its pore size is intermediate between that of the substrate and the main coating layer.

Following a brief review of past practices, an equation for the relationship between measured viscosity and volume solids is derived. The derivation of the equation is based on the concept of shear induced anisotropy, which is also presented. Using data developed on polystyrene and titanium dioxide dispersions, the equation is shown to be valid over a wide solids range. The equation allows the calculation of the degree of packing of the dispersed particles through the use of the relative shear volume. The equation and the shear induced anisotropy concept are then applied to the known properties of paper coatings. This discussion gives further insight into the hydrodynamics of blade coating and the nature of pseudoplastic and dilatant systems.

Investigations on the self-sizing of a pure cellulose paper show that surfactant acts to prevent self-sizing by solubilizing fatty acid molecules and forming a physical barrier to chemical bond formation between cellulose and fatty acid molecules. The acceleration of self-sizing by alum has a mechanism similar to that of conventional rosin-alum sizing: the formation and polymerization of aluminum soaps on the fibre surface. It is suggested that the reaction between cellulose hydroxyl groups and aluminum soaps, as proposed in the literature, is not likely. Instead, material that is not solvent-extractable is either extensively polymerized or reacted with residual carboxyl groups on the fibre surface.

Knowledge on the oleophilic, resinous, extractives of wood with their relevance to pulping, paper making and paper properties is summarized.

The main physical and chemical features of the two types of wood resin – the one in resin canals and the one in parenchymatous cells, mainly within the rays – are reviewed in respect to the above mentioned theme. These features are also analyzed with special reference to alkaline deresination and pitch problems.

Difficulties in washing out resin after kraft pulping due to the effects of ionic strength and Ca-ions are discussed as well as deresination in bleaching operations.

Present pitch problems in paper making and control measures are reviewed. Effects of wood resin on paper properties e.g. deposits in laser printers and smell or taste in board for liquid containers are discussed.

Using simple, joint-strength measurements, the bond between printing papers and various thermoplastic-polymer formulations is defined. It is demonstrated that the bond strength to the paper-polymer interface strongly depends on the sizing processes with which the paper has been treated during manufacturing. Increased amounts of rosin sizes and synthetic, cellulose reactive sizing agents both lower the paper-polymer adhesion. However, the effect is more pronounced for synthetic-sizing agents, as also demonstrated by Swanson and others for extrusion-coated papers and board. The decrease in bonding strength correlates with a decrease in paper wettability, as measured by capillary penetration of organic liquids of suitable surface tension, or as defined by surface energetics using the elution gas-chromatography technique.

The mechanism of sizing with soluble rosin soap size differs from the sizing mechanism of dispersed rosin acid size. Cationic fixing agent has to be used with the latter size to assure proper attachment of the size to furnish components. The problem of size attachment is not unlike that for the alkaline AKD size emulsion. The relative extent of attachment is assessed by a high shear stirring test.

Sizing involves a complex array of interactions in which surface reactions play an important role. The large surface area fines and fillers in the furnish reduce sizing efficacy. Size accumulates on fillers/fines to a higher extent than expected from surface area alone. On the basis of experimentally determined adsorptivities, size distribution in a furnish can be estimated. Size on fiber appears to be more efficient than on fillers/fines, compared at equal surface coverage. This requires that we minimize the amount of fine particulates in the furnish. Fillers/fines capture a significant amount of applied size; thus, first pass fillers/fines retention has to be maximized to retain size in the sheet. Size distribution also can be affected by the proper choice of process conditions.