Proceeding Articles

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.

Some recent advances in understanding the dynamic interactions of liquids with solid surfaces are reviewed. Specifically, acid-base theory is applied to wetting and adhesion, the meaning of precursor films for spreading is examined, the role of pore morphology in capillary penetration is explored theoretically and experimentally, adsorption and spreading of surfactant solutions is compared to that of aqueous alcohol solutions, three dimensional pressure penetration models are described along with new methods for characterizing penetration and pore size distribution, and means of reducing fiber swelling and the hygroreactivity of paper and board is discussed.

The tortuosity factor is given as the ratio of the theoretical absorption coefficient calculated from the cylindrical model to the observed absorption coefficient by the Bristow method. A new formula to give mean pore radius of paper is derived by taking account of the liquid absorption into every pore size according to the Lucas-Washburn equation.

The liquid absorption model is proposed to allow calculation of transferred liquid volume to paper in an arbitrary time from initial absorption through saturation. The results of the transfer tests of ethyl alcohol to four kinds of papers by the Bristow method show good agreement between the observed and calculated liquid volumes transferred to the papers.

For other non-aqueous liquids which show the same V(t) vs. (7L tlrqY/2 relationship in the liquid transfer tests to a particular paper as ethyl alcohol, it is assumed that the liquid volume transferred to the paper in an arbitrary time can be estimated by means of the model.