In the previous article the units of measurement for "Basis Weight", (Pounds per Ream), and "grammage" (grams per square metre ) were described .

The weight of paper per unit area is the most dominant physical property of paper. Most of the other properties both physical and optical relate to it.

The next most obvious property of paper is it's caliper (thickness) which is measured by a micrometer and is expressed in millimetres (previously in thousandths of an inch). This is mostly determined by the paper grammage but it is also affected by the paper finish. Thus, all other things constant, a Smooth Finish paper will be thinner than a Medium Finish paper, and a Medium Finish paper will he thinner than a Rough Finish paper. (See Article No. 9 re Surface Texture)

Caliper is also affected by the pulp furnish and by beating. Thus course pulp fibres will result in a thicker paper than will fine fibres. Also the longer the pulp is beaten and the heavier the loading of the beater roll, the more compact and thinner the paper made from it will become.

Formation also affects caliper. A very "wild" formation will result in a thicker paper than will a very uniform formation.

Density is expressed as grams per cubic centimetre, and depends on the type of fibre used, the degree of beating of the fibre, the grammage of the paper, the degree to which the paper has been consolidated during forming and pressing as well as the degree to which it has been compressed by calendering.

Bulk is the reciprocal of Density and is expressed as cubic centimetres per gram.

Tensile Strength is a measure of the force required to break a strip of paper of a standard length and width held between two clamps under a standard rate of extension. It is expressed as kilonewtons per metre of width. (Formerly pounds/inch of width).

A high Tensile Strength results from such paper properties as relatively high fibre length, a high purity of cellulose in the fibre, (i.e. low lignin content) / a high degree of pulp beating and fibre fibrilation, a high grammage and a high caliper. In general calendering reduces both Tensile and Tear Strengths by crushing and weakening the fibres and the inter-fibre bonds.

If a cellulose pulp is mixed with a wood-containing pulp, (i.e. a mechanical pulp), the higher the proportion of the cellulose pulp the higher will be the Tensile Strength of the Paper.

Tensile Strength is affected by the grain (fibre orientation) of machine made papers, it being higher in the machine direction than in the cross-machine direction.

Tensile Strength and Tear Strength have an inverse relationship.

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 affect by the grain (fibre orientation) of machine made papers it being lower in the machine direction than in the cross-machine direction

Tear Strength and Tensile Strength have an inverse relationship.

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 it's thickness but reduces it's stiffness and other strength properties.

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 airflow 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

As the caliper or thickness of paper is reduced, the smoothness increases and the roughness decreases.

Absolute Brightness is defined as the reflectance of blue light with a specified spectral distribution peaking at 457 nanometres compared to that 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 calcium carbonate in various forms, 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 a 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 a filler in the paper during forming, otherwise it is lost in the white water.

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) Tristimuls Values X, Y and Z, or the CIE L*a*b* co-ordinates, which are determined using a standard reflectometer

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 Length; Breaking Length Metres; Burst; Fold Endurance; Gloss; Grease Resistance; Pick Strength; Softness; Stretch; Tensile Energy Absorption; Water Absorption; and Water Vapour Permeability.

This concludes the CRANBERRY CORNER series of articles. I hope that you, my readers have found them both interesting and beneficial in your particular pursuit of the bookarts. If you have any questions please write to:

Edward "Ted" H. Snider,

RR No.1, Seeleys Bay, ON,

K0H 2N0, Canada

References: