This information is reprinted from the Cranberry Corner column of Hand Papermaking Newsletter #60 (October, 2002).
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Physical and Optical Properties, Part 2
TEAR STRENGTH. Tear Strength is a measure of the force, applied perpendicularly to the plane of the paper, that is required to tear one or more sheets of paper clamped between two sets of jaws through a specified distance after the tear has been started, using a standard tearing tester. The measured result is used to calculate the tearing resistance of a single sheet of paper. The Tear Index is the Tear Strength corrected for grammage and is expressed as millinewtons per grams/square metre.
A high Tear Strength results from such paper properties as a high fibre length, a low degree of beating, a high grammage, and a high caliper. Tear Strength is affected by the grain (fibre orientation) of machine made papers, it being lower in the machine direction than in the cross-machine direction.
STIFFNESS. Stiffness is defined as the bending moment in gram centimetres required to bend a clamped strip of paper through a specified angle. The higher the paper grammage, and the thicker the paper caliper, and thus the lower the density, the stiffer the paper will be, all other properties being equal.
Stiffness is also affected by the degree of beating of the pulp. A highly beaten pulp will have a higher degree of interfibre bonding and thus will produce a stiffer paper than one that is beaten lightly. Calendering paper not only reduces its thickness but reduces its stiffness and other strength properties.
ROUGHNESS/SMOOTHNESS. These two paper properties are inversely related and are tested on differently designed instruments. Roughness is determined by measuring the rate of flow of air under standard pressure between the paper surface and two concentric, annular metal rings applied to the paper (Sheffield method). The rougher the paper is, the higher the rate of air flow and the higher the reading. Smoothness is determined by measuring the time required for a standard volume of air to pass between the paper surface and a smooth, annular metal disc applied to the paper (Gurley method). The smoother the paper is, the longer the time required and the higher the reading. At constant grammage, as the caliper or thickness of paper is reduced, the smoothness increases and the roughness decreases.
BRIGHTNESS. Absolute Brightness is defined as the reflectance of paper illuminated by blue light with a specified, spectral distribution peaking at 457 nonometres; compared to the reflectance of a perfectly reflecting, perfectly diffusing surface. It is expressed as a percentage.
Brightness of paper is affected by the brightness of the pulps used in the furnish. Higher brightness paper can also be obtained by adding proportions of fillers such as powdered calcium carbonate, talc, and titanium dioxide. The addition of fillers to paper furnish has a negative effect on interfibre bonding and so reduces both Tensile and Tear Strengths.
PRINTING OPACITY.4 The Printing Opacity of paper is defined as the ratio of the reflectance of a single sheet of paper backed by a black body, divided by the reflectance of the same sheet backed by an opaque pad of similar paper, using a standard reflectance meter. High Printing Opacity of a paper results from a high grammage, a high caliper (i.e., low density), and from using pulp that has a high fibre length (i.e., is lightly beaten).
Fillers are also added to the furnish of some papers to increase opacity. However, retention aids must be used to retain the filler in the paper during forming, otherwise it is lost in the white water.
COLOUR. The measurement of colour is a complex science. The colour of paper depends in a complicated way on the interaction of the characteristics of the observer, and a number of physical factors; such as the spectral energy distribution of the illuminant, the geometry of illuminating and viewing, and the nature and extent of the background surrounding the paper, as well as the optical properties of the paper itself. The colour of a sample of paper can be characterized by means of three colour coordinates such as the CIE (Commission Internationale d’Eclairage) Tristimulus Values X, Y, and Z; or the CIE L*a*b* coordinates, which are determined using a standard reflectometer.
OTHER PAPER PROPERTIES. There are many other paper properties which are more or less important depending upon the end use of the paper. Some of these other paper properties are as follows: Abrasion; Air Permeability; Bending Strength; Breaking Length Metres; Burst; Fold Endurance; Gloss; Grease Resistance; Pick Strength; and Water Vapour Permeability.
CONCLUSION. This concludes the CRANBERRY CORNER series of articles. I hope that you have found them both interesting and beneficial in your particular pursuit of the book arts. If you have any questions please write to: Edward “Ted” H. Snider, RR No. 1, Seeleys Bay, Ontario K0H 2N0, Canada.
References: 1Standard Testing Methods, Pulp
and Paper Technical Association of Canada.
2TAPPI Test Methods, Technical Association of the Pulp
and Paper Industry (U.S.A.) 3G. A.
Smook, Handbook for Pulp & Paper Technologists, Joint
Textbook Committee of the Paper Industry, 1986.
4K.W. Britt, Handbook of Pulp and Paper
Technology, Van Nostrand, 1970.