1993 Volume 2

In spite of forty years of research, it is still unclear how the mechanical properties of paper, particularly strength, depend on the disordered geometry of the fibrous network. Most of our understanding of the fracture phenomena of paper is based on illustrative microscopic observations. Theoretical models have traditionally been focused on the behaviour of a typical element in the network. However, the failure process seldom starts from, or proceeds through, “typical” elements. Instead, the statistical distribution of local failures is crucial for the strength of paper. With the ever more powerful computers it is now possible to simulate numerically the behaviour of disordered systems such as paper. I believe that computer simulations, in combination with new measurements and effective data analysis will lead to a better understanding and more accurate microscopic characterization of the strength and fracture properties of paper.

The fines of mechanical and chemical pulps have a character very distinct from their respective fibre fractions. The fines can be regarded as a special furnish component and the optimum quality and percentage of fines in the sheet depends on the properties of the fibre fraction used. An addition of suitable fines can improve remarkably the properties of a printing paper.

This paper deals with the structural, optical and strength properties of fibre networks, as influenced by fibre coarseness and the introduction of different types of fines. The response of these networks to calendering is also studied. Fibre fractions of different coarseness and lignin content are used. Confocal laser scanning microscopy (CLSM) is used to determine the changes in the structure and microscopic roughness of the sheet.

Printing papers contain both chemical pulp and mechanical pulp. Chemical pulp fibres are commonly used as reinforcement, while mechanical pulp maintains opacity and printability. Modelling was performed to provide hypothetical interrelationships between strength and bonding capacity of the fibres. It is suggested that the paper strength, within certain limits, is described by the number and type of bonds occurring in the fibre network. The number of possible bonds was determined as the optically active total area of the fibres, i.e. the light scattering coefficient. Actual bonds occur only when potential bonds are accessible, and accessibility should be improved with increased fibre flexibility and compressibility. It implies that paper sheet density has to be introduced into the model along with the light scattering coefficient. In addition, the tensile strength of the paper is supposed to depend on rheological conditions, i.e. shearing speed that is determined as the tensile speed, and viscoelasticity of the paper describing the type of bonds.

In this study, the methods used by both industry and paper physicists for evaluating paper toughness (resistance to breakage) are critically reviewed, and a new method for determining the critical value of the J-integral, J, is presented. Difficulties arising from the use of tensile strength and tensile energy absorption in the evaluation of paper toughness are highlighted. J,, values obtained with the Leibowitz non-linear technique were relatively close to those obtained with a new method developed by the authors when the latter was used in conjunction with key-curve analysis for determining the critical point. This result suggests that the Liebowitz non-linear technique may give a relatively accurate J integral value at the crack initiation point with less experimental effort. Experimental results show that it may also be possible to use notched specimens in the standard tensile testing configuration for J-integral estimation, which would be an attractive method for industry.

For printing paper grades, runnability in the paper machine and in the printing press is partly attributed to the ability of the paper to tolerate flaws and defects in the paper. In these operations the paper fails due to forces applied in the plane of the sheet. It is important for the papermaker to have access to a relevant test method which can characterize those pulp properties applicable to this type of failure. Papermakers knew as early as the 1920’s that the strength of a cracked paper was a unique and useful paper property. This lead to the development of the out-of-plane tear strength test method. The tear test principle has, however, been criticized for many reasons. The most severe criticism lies in the mode in which the paper fails. The mode of fracture in most production and processing operations is seldom an out of-plane tearing, Mode III, but rather an in-plane crack propagation, Mode I.

Thermography has been found to be useful for detecting the local variations in temperature of paper sheet under strain. The changes in temperature images during the course of tensile straining could describe the local deforming and fracturing process of paper. A thermal deformation pattern of well formed paper was locally uneven, but uniform wholly throughout the specimen. On the other hand, the deformation pattern of poorly formed paper was uneven in micro and macro scale at the early stage of its plastic deformation region. However, the deformation pattern was fairly uniform through the specimen at the later stage of its plastic deformation.

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In many cases paper products are of a wound roll format. The wound roll product must have integrity such that it does not slip internally or telescope in printing or other converting operations or in the hands of a consumer if the wound roll is the final format of the product. Winding models which predict internal stresses within wound rolls begun development over twenty’ years ago. The purpose of this paper is to (1) show how the models can be used to insure roll integrity and (2) show how paper properties can affect the integrity of the wound roll and (3) show recent developments in wound roll models.