1993 Volume 1

Virtually all paper and board grades have one or more mechanical property specifications. These are typically fracture properties, but the subfracture mechanical properties are also important. In many situations raw material and papermaking variables impact subfracture and fracture properties in similar and predictable ways.

In this review we discuss the impact of fiber and paper machine variables on the mechanical properties of paper and board up to the point of failure. As with any real material, the physical properties (mechanical, optical, electrical, etc.) are not independent but depend upon the constituents and structure of the material. We discuss these connections where appropriate.

From a historical perspective, those “turning points” that led us to greater understanding of the mechanical properties are pointed out, as are situations where work is needed.

Fibre property, refining requirement, and handsheet strength and optical property interrelations are examined for a eucalypt and several softwood market kraft pulps and blends. Market krafts included in the study are radiata pine pulps of low and medium coarseness, a benchmark pulp from the interior region of British Columbia, and a eucalypt pulp from Brazil. Eucalypt: softwood blends are in proportions of 100:0, 50:50, 80:20, and 0:100, and effects of separate and co-refining are assessed using -a laboratory scale Escher Wyssre finer which is considered to be indicative of commercial scale refining operations.

For the softwood pulps, refining at the low 1 Ws/m specific edge load has minimal effects on fibre shortening, fibre collapse, and wall expansion and delamination. Under these conditions fibres are neither rapidly rewetted nor made flexible. The converse occurs with refining at 3 and 5 Ws/m.

Tensile strengths are relatively high and softwood fibre walls are slow to respond with refining at low specific edge load. Such effects are consistent with the retention of fibre stiffness and length, and the development of high bonding potential. The high bonding potential is presumably developed through selective fibre surface disruption, wetting, and molecular and micro level fibrillation. Light-scattering coefficient/tensile index relations are independent of specific edge load and indicate mutual compensatory responses for these handsheet properties.

For eucalypt: softwood blend proportions of about 80:20, tear/tensile relationships (reinforcement properties) and light-scattering coefficients (optical properties) are roughly the same and independent of the origin or type of softwood used in the investigation. Such results are to be expected since there are only 2-3% by number of softwood fibres included in the 80:20 eucalypt: softwood furnish blends. For 50:50 eucalypt: softwood blends the effects of using softwoods of different fibre quality are also relatively small.

With co-refining reinforcement properties are decreased, and optical properties can be increased depending on specific edge load. It is envisaged with co-refining that the small number of softwood fibres present in the 80:20 eucalypt: softwood softwood blends (<3%) receive disproportionate levels of the refining, and tear strengths decrease forgiven tensile strengths and energy inputs. Also, such an explanation is consistent with the possibility that light scattering coefficients can increase with co-refining. Thus, softwood fibres can be expected to be more refined and hardwood fibres less refined for given energy inputs with co-refining than with separate refining before pulp blending.

In order to produce papers with differing qualities, papermakers use pulps with varying fibre and fines characteristics. Since the early 1900’s, numerous studies have been conducted to investigate the relationships between pulp-fibre and paper properties. However, a common weakness was that previous investigations focused on relationships within restricted ranges of properties of pulp-fibre and paper. Forth is reason,the main goal of this investigation was to examine and to characterize a broad scope of fibre-paper relationships. In order to achieve this goal, a statistical approach was taken. This technique explained much of the total variation in paper properties by investigating a large number of pulp-fibre types, including fines,with a broad range of characteristics. It was discovered that robust and orthogonal non-linear multiple regression models could be developed to predict various paper properties. The multiple curvi-linear regression models reported here (based on the PhD thesis of the first author of this paper) could explain, on the average, 85 ± 10% of the -variance (R2)in the paper properties. Although the models generated are empirical, and thus lack fundamental interpretative meaning, they do clearly rank those fibre properties most important fora given paper property and allow prediction of the “form” of the relationship. These relationships confirm the expectation that there are no universally optimum fibre paper properties. Instead, compromises must be made to achieve an acceptable balance of properties. Such interactions are described in more detail in the paper.

Formation is a critical property for newsprint as well as printing and writing grades of paper. Measurements of the flow and fibre distribution properties in the forming section of an operating paper machine are related to the resulting paper structure. The 3-D M/K Analyzer is used to characterize the final paper structure in terms of the formation index and average floc size. A stochastic model is proposed to provide a basis for interpretation of the final paper formation.

The characterization of surface properties of communication papers reflects both handling and printing requirements . Test methods recently applied in this area range from simple adhesive strength characterization and liquid penetration tests to more sophisticated gas chromatographic and spectroscopic procedures suitable for thermodynamic and chemical constituent analysis . The principles and results of such testing are reviewed .

A cockled paper sheet has lost its planarity because small (5 – 50 mm) randomly spaced areas have bent out of the plane of the paper. The variation in surface height is usually only of the order of one millimeter. Cockling is related to the hygroexpansivity and small-scale inhomogeneity of paper, and is a problem mainly with some lightweight papers and with copying papers.

In this study, cockling is analysed theoretically using the finite element method. The results suggest that cockling is caused by local inhomogeneity in the two-sidedness of paper. Small-scale variation in the fiber orientation angle is especially detrimental in this respect. Large variations in in-plane contraction can also cause paper to buckle. The intensity of cockling increases with decreasing paper thickness. The cockles become more oval as the fibre orientation gets stronger.

The conclusions of the theoretical analysis are supported by experiment. With high basis weight papers, the local fibre orientation of the top side was found to be almost independent of that of the bottom side. Thus, two-sidedness varies considerably on the small length scale, and this allows local curling to take place. At low basis weights the top and bottom sides are no longer independent, which offsets the amplifying effect of smaller thickness.

An experimental examination of the reflection of polarised light (ŝ & p; λ= 633 nm)from surfaces which have a significant specular component is presented. The reflected light intensity is measured using a detector with a narrow acceptance angle (0.50) and it is found, for a number of different surfaces based on coated paper, that as the detector scans through the angular distribution, the ratio of polarised light intensities (R~RS) changes in a systematic way which can be accurately predicted if the surface is modelled as an array of mirror-like facets each with the same refractive index, distributed in and around the mean plane of the coating. The change in the ratio R~RS with detector position is independent of the angular distribution of the facets and depends only on the refractive index of the surface.

A stylus profilometer has been used to evaluate the effect of wet pressing on the collapse of individual fibres from mechanical, chemimechanical and chemical pulps. The chemimechanical and chemical pulp fibres begin to collapse at low pressures and approach complete collapse at high pressures, while the mechanical pulp fibres do not exceed 80% collapse to 5000 kPa. The degree of collapse of southern pine TMP at a given pressure is about the same as that of northeastern spruce/balsam TMP. Since the thick-walled Southern pine fibres are less flexible, it is concluded that transverse collapsibility and flexibility are two independent fibre properties.

On the basis of modelling results and the difference between fibre thickness-measurement from networks pressed in contact with smooth and with rough surfaces, it is suggested that wet pressure transferred locally at fibre contacts within a sheet leads to local collapse forces higher- than expected from nominal wet pressure values. Fibre contacts are initially present in the unpressed sheet.On pressing, free fibre segments will be deflected into contact with other segments above or below them, producing additional fibre crossings as pressing progresses . The effective pressing pressure will be highest at the initial fibre contacts, decreasing to zero at contacts just formed at the end of pressing. Because of the demonstrated effect of wet pressure on fibre collapse, it is the thickness at the initial fibre contacts or those formed early in the pressing process that is important in determining sheet density.

Use of the more appropriate fibre thicknesses, substantially improves the prediction of sheet density by the Interactive Multiplanar Model of sheet structure for a range of pulp types.

Printing on a paper carrier falls into the generic class of’ visual communication processes. This paper aims to provide a systematic overview, based on the literature, on current knowledge concerning the fundamental factors related to paper as an information carrier.

Published work on informational aspects of imaging can be found in diverse fields. Yet it seems that research specifically from the starting point of paper and printing is needed if future advances are to be made in our understanding of how paper properties contribute to visually perceived information.

Print noise and its effect on information capacity and visual image quality are analyzed. Noise in prints originates from the signal (immaterial image information), the printing process and the materials. The frequency bands and orders of magnitude of the noise associated with the different sources are discussed with emphasis on general principles and limits in off set printing.