1985 Volume 2

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.

The principle of reactive sizing is discussed, and the variety of sizes which have been considered is reviewed.

The reactivity of tetradecyl ketene dieter towards water and model compounds for cellulose has been shown to be very low and the results suggest that very little ß -keto ester formation would be expected under normal papermaking conditions.

A study has also been made of the retention of C-14 labelled tetradecyl ketene dimer in laboratory handsheets, and of the degree of sizing which is induced. The results show that the dieter is present in the sheet predominantly in its unreacted form, but that a small percentage is present in a form which is not amenable to extraction by chloroform. This residual size exerts a considerable sizing effect, and the amount present in the sheet is related to the curing conditions used. The nature of the unextractable material has been studied by mild hydrolysis and the results are compatible with a low level of ß -keto ester formation. Comparisons have been made with C-14 labelled hexadecyl hexadecanoate, a compound of similar structure and physical characteristics but which is unable to undergo reaction with cellulose.

A study has also been made of the influence of aluminium sulphate on alkyl ketene dieter sizing. The retention of size in sheets made in the presence of aluminium sulphate is reduced, but not to a sufficiently low level to account for the loss in sizing which is observed. An explanation in terms of the inhibition of reaction of the dieter with cellulose as a result of adsorbed aluminium species is proposed.

According to Smoluchowski’s and Komagata’s theory, the electroosmotic and electrophoretic velocities vary with the depth of cell such that the distribution of velocities must be represented by a symmetrical parabola. However, in practical measurements of the electrophoretic velocities of sedimentary particles such as pulp fines and fillers, the distribution curve of velocities was found to be asymmetrical about the cell center. Thus use of the conventional method often gives rise to a serious error. The reasons for this are discussed and an improved method is proposed.

The time-dependent changes in zeta potentials (ZP) of clay and monodispersed polystyrene latex with added polyethyleneimine (PEI) or cationic polyacrylamide were investigated by the improved method. For instance, the ZP of clay, which was -42 mV originally, became +9 mV just after addition of PEI at a level of 0.1 % on clay, decreased rapidly at an initial stage, became -13 mV after
1 hr and finally reached -17mV. The rate of change in ZP varied widely with the degree of agitation, the addition level of polymer, the molecular weight and so on. The highest difference between initial and final ZP we have obtained hitherto is over 50mV. These phenomena must be closely related to the conformational change of polymers sorbed on solid surface and will offer important and fundamental information on how to use polymer additives in papermaking.

It has long been known that certain simple chemicals can either accelerate or retard the rate of refining of pulps. Based on surface adsorption and osmotic pressure considerations, a hypothesis is proposed to provide a rationale for the behavior of these chemicals. The validity of this theory is demonstrated by the prediction of the effectiveness of a colorless, photostable chemical as a new beating aid and the verification of its performance as an accelerator for the refining of both chemical and mechanical pulp as well as secondary fibers.