1985 Volume 2
Oxfordpp 445-482The Origin and Alleviation of the CD Variations in the Physical Properties of PaperAbstractPDF
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.
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.
Oxfordpp 589-611The Dry Strengthening Effect of Cationic Starch Wet-End Addition on Filled PapersAbstractPDF
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.
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.