Hygroexpansion coefficient and tensile stiffness are important parameters in many paper applications. This study compares several bleached industrial hardwood kraft pulps, comprising five eucalypt pulps from South America, Europe, and Africa as well as an acacia pulp from Asia and a birch pulp from Scandinavia. Refined and unrefined pulps are compared. The results indicate significant differences in hygroexpansion but smaller differences in tensile stiffness index at comparable densities. No single factor offering a reasonable explanation of these differences in hygroexpansion coefficient, such as carbohydrate composition, fibre dimensions, or fibre form, was found. However, correlation between hygroexpansion coefficient and the mechano-sorptive creep stiffness was observed. We suggest that the hygroexpansion coefficient at a given tensile stiffness level can be used to rank pulps in terms of their mechano-sorptive creep properties.
The determination properties of nine commercial multiply board grades, both coated and uncoated, were measured using four testing methods, the Z-Directional Tensile Strength test (ZDTS), the Scott Bond Test (SBT), the Wheel Delamination Test (WDT), and the IGT method. The positions of delamination failure in the thickness direction were then recorded and compared. For the IGT tests, failure profiles and the final failure positions were evaluated. The results showed that the failures in the ZDTS, WDT and SBT tests mainly occur in the middle ply, whereas failure in the IGT tests occurs in the top plies as well as in the middle ply. Some boards presented also more than one failure position for the same type of test. The correlations between the different delamination resistances were also investigated. The WDT and SBT method showed a good correlation, whereas no correlation could be found between the other methods.
Delamination of multiply paperboard during offset printing is caused by the ink tack induced forces applied on the surface of the board. The purpose of this investigation was to analyze the influence of mechanical properties of the plies and interface resistance on the delamination behaviour of the paperboard. Paperboard was modelled as a four-ply sandwich structure. A user-defined interface model described the mechanical behaviour of the interfaces between the plies. The results in terms of critical ink tack length and stress conditions in the interfaces at delamination initiation were defined for different ply structures. The delamination process in multiply paperboard was highly dependent on the stress concentration caused by the ink tack. The delamination always occurred in the interface between top and middle ply. The main stress component causing delamination was tensile stress in the thickness direction, whereas the interlaminar shear was less relevant. Bending stiffness and in-plane tensile stiffness influenced the critical ink tack length, but did not affect the stress situation at the delamination point.
High consistency (HC) defibration followed by secondary stage low consistency refining (LC) is an energy efficient process in mechanical pulping that has been explored for some time. In this study the effects of temperature, pH, specific edge load and specific energy on pulp quality have been investigated for LC refining using a mobile LC refiner rig placed after a primary stage HC refiner In the Braviken Paper mill. The trials showed that the specific energy consumption in production of mechanical pulp can be reduced with at least 15% with preserved pulp properties. High temperature, pH over 7 and low specific edge load were advantageous both for tensile index development and for preserving the fibre length of the pulp. The development of other pulp properties such as shives content and light scattering coefficient, as a function of freeness, were comparable for the second stage LC- and HC-refining.
Model studies on the floatability of an offset ink suspension (without fibres) have been performed using a Hallimond tube and a standard calcium chloride- sodium oleate collector system. The average ink particle size was 1 µm indicating a poor floatability of untreated primary ink particles. This suspension was mixed with deinking chemicals, coating components and dissolved paper chemicals in order to evaluate their effect on flotation performance. The results were combined with data on agglomeration kinetics to determine important factors of the flotation deinking process. The importance of agglomeration was confirmed. The particles remaining after flotation were discrete primary ink particles. An interesting result was that no upper particle size limit for flotation was observed when ink particles was flotated without fibres, although particles larger than 200 µm were present indicating the effect of a fibre network for retention of large particles. Parameters that influence the agglomeration and flotation rate are discussed as well as reasons why some particles do not agglomerate and flotate.
The purpose of this research was to study the effects on print quality when the properties of the flexographic printing plate were changed in a systematic and controlled way. An attempt was made to identify plate parameters that govern good print quality. Coated paperboard was printed in a laboratory printing press using different plates, and print density and dot gain were determined. Essential parameters including mechanical, chemical, and morphological properties of the plate that led to good print quality were identified. The properties of the plates were characterised by means of profilometry, ESCA, contact angle measurements, durometer, and image analysis. The methods used to create printing plates with controlled material properties gave useful information about the ageing and wear of printing plates. The results showed that it was possible to systematically control the properties of the plate by changing the pre-press conditions. Application pressures and dot area influenced print quality. Small-scale roughness and long-scale roughness of the plate governed good print quality with respect to print density and dot gain It appeared that these printing plate properties affect the amount of ink transferred.
Laboratory paper coatings of simple composition, comprising only a single spherical plastic pigment and binder type, were analysed in terms of water-based inkjet print quality. Optical density and gamut of dye colours decreased with increasing pigment size, and followed the binder hierarchy from polyvinyl alcohol (best) to carboxymethyl cellulose to styrene-butadiene latex (worst). For the larger pigment size, density and gamut also decreased with decreasing binder content and increasing coat weight. Colour-colour sharpness was evaluated using four measures of line bleeding, of which mean line width and blurriness were found to be the most useful and well correlated. Both bleeding measures, while following the same binder chemistry hierarchy mentioned above, now worsened with decreasing pigment particle size. This tallies with expectations from capillarity and light scattering, e.g. increasing particle size (in the range studied) increases both liquid penetration rate and opacity, thus resulting in decreased colour richness but increased sharpness.
Strength delivery of softwood chemical pulps has been previously studied extensively without being able to define the underlying morphological factors such as the initiation of weak points within the fibres. In this study an industrial kraft cooking process was simulated with the use of a laboratory batch digester and applying compression and shear forces to the chips during cooking. The results of cooks of pine, to kappa 30 and 45, were significantly affected when subjected to shearing forces during the last 15 minutes of the cook. The typical behaviour of a softwood industrial pulp viz. being more difficult to beat and having lower tear index at comparable tensile index around 90 Nm/g, when compared to a laboratory pulp produced from the same raw material, was obtained. The mechanically treated pulps also had substantially higher dewatering ability reflecting a more collapsed cell structure. Pure compression loading of chips during cooking did not induce these detrimental effects.
Fundamental studies of the deformation behaviour of wood show that combining small amounts of shear along with compression within a roll press reduces the work required for cell wall deformation. This permanent deformation can be increased by: increased number of passes through the nip reduced roll speed increased process temperature. Refining trials showed that the strength properties (except for the tear index), as well as the optical properties of TMP produced from these shear/compression pretreated chips are improved, with the potential of reducing electric energy by approx. 100 kWh/tonne, when compared to untreated chips.
An acoustic technique, with microphone placed near the print nip exit on a sheetfed offset press during trial printing of newsprint, was used to provide information relating to splitting of the ink-fountain solution film. The average acoustic power increased with tack of the ink used and with target optical density. Further, average power decreased during each run, reasonably strongly correlated to increase in fountain solution consumption. This indicates that average power is primarily sensitive to instantaneous tack of the inkfountain solution film, and can be used to monitor tack and indirectly infer ink-water balance in the nip. Laboratory experiments were also performed using the Hydroscope instrument to simultaneously measure tack and average acoustic power of the splitting of inked rollers during fountain solution titration and evaporation. While these two measured parameters were not directly correlated over all conditions of emulsification, both decreased in tandem over intermediate amounts of fountain solution.