2005 Volume 1

All conventional paper machines exhibit a profile of cross-machine direction (CD) shrinkage which is developed during the drying stage of production. This paper suggests a new approach
to the understanding of these profiles by suggesting that CD shrinkage at a point in the paper depends on the distance of that point from both edges of the sheet and also on the length of the unsupported draws in the dryer section. The nature of the function is unimportant as long as it can be made to fit data for one combination of machine width and effective draw length; substituting other values will then change the shape appropriately. In this paper, a simple exponential decay of shrinkage with distance from the edge is used successfully.

This approach is initially demonstrated for laboratory results from the literature, where it successfully predicts changes produced by varying the length, width and by reversing the MD/CD orientation of samples. It is then shown to be consistent for changes of paper machine furnish, press section draw and sheet grammage produced in a series of trials on M-real, New Thames
PM6. It is finally shown that theses ideas explain the effect on CD shrinkage results from the literature for splitting the sheet at the press section of a newsprint machine and for reduction of dryer section restraint by deactivation of the blow-boxes.

A web is a material whose length is much in excess of the width and the width is much larger than the thickness. Various grades of paper, plastic films, metal foils and laminates can all be broadly categorized as webs. Web media are often stored for periods of time in the form of a wound roll. This form of storage is chosen because it is simply the most convenient and often the only form for storing vast lengths of web with minimal damage and loss. Winding can damage the web and has become a topic which has received considerable attention in the literature. Amongst operations in a web process line, the winder and the unwinder are often sites at which web defects appear that may result in a loss of quality or may lead to a break or burst of the web resulting in lost productivity. The purpose of this paper is to review the state of the science of winding and unwinding rolls. The threads of pertinent information in the literature focus on: the stresses that are wound into rolls, how those stresses are affected by the type of winder employed, how those stresses vary during roll storage, how the stresses predicted by models can be used to predict wound roll defects, and finally the measurements that are available to verify modeling efforts or help solve production problems. Each thread will be examined in this review from the perspective of what references exist but also giving enough detail that an appreciation for the significance of a topic can be developed.

This paper will consider the low consistency refining kinetics of a pulp suspension at a given level of applied specific energy. The mechanical treatment on fibres caught in the confined zones of gap clearance (bar crossings) reveals some heterogeneity. It can be experimentally verified that the physical effects on fibres are not the same whether the confined zones are close to the internal radius (input side) or close to the external radius (output side). One must also account for the dynamic sliding motion of the confined zones through the rotation of the rotor plate (or cone) in front of the stator. Hence, each bar crossing has its own sliding velocity.

In order to predict the radial variability of the cutting effect on fibres, through engineering parameters that can be easily determined (radial coordinate; angular velocity; width of bars, width of grooves, average bar angle, both for rotor and stator patterns), a theoretical understanding of the radial distribution of the pressure must be undertaken. The pressure is locally applied on
the pulp pads in confined zones of the gap clearance.

The best homogeneous results are obtained either with a cylindrical or a conical refiner where the bars are parallel to the rotation axis. The degree of variability on the cutting effect on
fibres is increased with an increase of the bar inclination versus the radial direction and a decrease of the ratio between the internal and external radius.

The role of refining intensity and specific energy in refining of softwood kraft fibre fractions was studied. Several paper properties can be improved by selective refining of fractions. The tensile strength-dewatering resistance relationship benefits from low-intensity refining of the long-fibre fraction. The specific energy input determines the increase in fibre swelling which contributes to a higher sheet density and improved tensile strength. The apparent density-roughness relationship benefits from mild refining of the short-fibre fraction. Refining intensity has a strong effect on the magnitude of the gap between bar surfaces, on fibre shortening, and on the coarseness of fibres with high cell wall thickness. For the short-fibre fraction, which appeared to floccu-
late less, the maximum intensity causing “pad collapse” and more severe fibre shortening was lower than for the long-fibre fraction and feed pulp. The fraction-specific intensity and gap behaviour are believed to relate to the compressibility of flocs under the stress applied by bar surfaces – a phenomenon discussed in recent studies concerning the forces acting on fibre flocs.

The properties of paper are largely dependent on the bonds between the fibres. This is, of course, primarily true of those strength properties that are directly related to the number of bonds in the paper. Other properties are also dependent on such bonds, properties such as the opacity of the paper, its smoothness, porosity, dimensional stability, pore size distribution, linting
propensity, density, stiffness, formation, and compressibility to mention a few.

The normal way of affecting the number of bonds in a paper is through the choice of fibre material and through a correct beating of the pulp. It is true that properties of paper may be manipulated through the choice of beater type, its specific edge load etc to expand the property or process space in paper manufacture. There are still many limitations as to what can be achieved by beating and other process tools, so the practical paper-maker is continuously looking for ways to expand property and process space to be able to manufacture new products or boost paper machine productivity.

In this review the terms “bonding” and “joint strength” will be used interchangeably. “Joint strength” includes both the adhesion zone (2D zone of bonding) and the cohesion zone (3D zone of bonding).

Despite massive efforts over the years, our understanding of the molecular mechanisms of bonding is still in its infancy. There is still the fundamental argument as to the relative contribution of hydrogen bonds, ionic bonds, dipolar interactions, induced polar interactions, long-range van der Waals forces, and covalent forces (for wet strength resins) in various situations. Taken to the extreme, it was once believed that lignin contributed little to bonding in lignin-rich pulps, because they were assumed to be poor hydrogen bonding agents. Not anymore, as it has been realised that strong bonding can be created between mechanically liberated pulp fibres. Though critical experiments still need to be formulated to examine such matters, this review will not focus on them.

It is acknowledged, that hydrogen bond theories have been formulated by Corte and Shashek (1955), Nissan and Sternstein (1964) and others, but it has not been possible to further expand our knowledge from the initial formulations.

This review will instead focus on the use of various dry and wet strength additives to improve bond strength. The authors have made efforts to relate the discussion to the historical and current context of dry and wet strength resins, and to discuss more recent developments in understanding adhesive and cohesive failure.

Hence, after some general considerations and introduction to the concepts of process and property space in paper manufacture, a brief discussion of current paper strength theories will be made. A more detailed account of adhesive and cohesive failure mechanisms will follow, after which dry and wet strength resins will be reviewed. As far as wet strength agents are concerned,
traditional wet strengthening will be given less emphasis; the focus of this later part will instead be on potential chemistries to alleviate tensile creep and compression creep under moist conditions.

Traditional micromechanical theories for the tensile strength of paper do not account for the tensile stiffness of paper, even though in practice tensile strength is closely coupled with tensile
stiffness. Another problem is with the micromechanical input parameters, few of which have a precise meaning in real paper. Especially the interpretation of inter-fiber bonding is ambiguous.
None of the existing theories connects tensile strength with an independently measurable value of bonding degree or bond strength. As a result, the conventional interpretation of tensile strength data is unreliable.

In this paper we will present a “macromechanical” study that connects tensile strength with independently measured values of tensile stiffness and z-directional fracture energy (alternatively
Scott bond or z-directional tensile index). The model expression agrees well with many – but not all – of the experimental datasets that we had available. The disagreements demonstrate that z-directional measurements do not capture some aspects of inter-fiber bonding that contribute to in-plane tensile strength.

Fiber deformations and damage have a considerable influence on both fiber strength and network properties. Through their influence on the fiber network, they can also affect the way paper behaves during converting. Another aspect is their influence on end-use properties, again via their effect on the fiber network.

Separating the effects of fiber deformations and damage is often difficult. We prepared pulps in the laboratory and subjected them to different treatments that change the two relatively
independently in a controlled manner. We found that wet/dry zero-span fiber strength is not dependent on the deformation method or on the extent of the deformations. Neither pulp sheet
density nor Scott bond bonding was greatly affected by the type or method of deformation. In the case of deformed pulps, the pulp sheet tensile properties were dependent on the extent of fiber deformation via fiber segment activation. The pulp that was damaged instead of deformed had fiber deformations to about the same extent as the deformed pulp but lower single fiber strength measured with wet/dry zero-span. It also had lower bonding ability and sheet density. The tear index for the unbleached damaged pulp was 20–25% higher than that of the reference pulps. Fiber deformations (curl and kinks) affect fiber network properties via the lack of fiber segment activation. However, they do not significantly influence fiber shrinkage potential (WRV) or fiber strength.

Digital image correlation was used to measure the full-field deformation of paperboard and handsheet tensile specimens. The correlation technique was able to accurately measure strain in
regions 0.6 by 0.6 mm. Results showed the variation of strain to be much larger than has been previously reported. For machine-made paperboard tested in the cross-direction, the variation of strain increased throughout the tensile test and became erratic near failure, indicating many local failures. The measured strain distribution can be characterized by a Weibull function in agreement with weak-link failure theories. The analysis of a handsheet tensile specimen with a low-grammage region, approximately 4mm wide, showed large negative strains near the region’s edge.

We have exposed pulp handsheets to two different degradative treatments, and compared their effects on strength properties. In accordance with earlier research, tensile stiffness and the shape of the stress-strain curve were independent of cellulose chain length and the fiber defects caused by the degradative treatments. When evaluated at the same viscosity, we found acid vapor treatment to be more detrimental to axial fiber strength than ageing treatment at elevated temperature and humidity. At the same mean fiber strength, acid vapor-treated handsheets show higher tensile strength. This is because acid vapor treatment is more heterogeneous than ageing treatment. The fiber network is able to compensate for the local defects, but not for the general degradation in fiber strength. In both treatments the mechanism for cellulose cleavage is assumed to be acid hydrolysis, the difference in the effect on fibers coming from the treatment conditions. Acid vapor treatment induces a fast reaction at defect sites and discontinuities in a fiber, while ageing treatment induces a slow, more homogeneous hydrolysis in fibers. Unlike axial strength, the Z-directional strength of softwood decreases only after harsh degradation when viscosity has dropped below 400 ml/g. The Z-directional strength of hardwood is already compromised at a viscosity of around 700 ml/g. The differences probably arise from differences in fiber ultrastructure.