In the previous Newsletter, 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.

CALIPER, OR PAPER THICKNESS. The next most obvious property of paper is its 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 be thinner than a Rough finish paper. (See the January 2002 issue of Hand Papermaking Newsletter for article on Surface Texture.)

Caliper is also affected by the pulp fibre used and by beating. Thus, coarse 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 paper with a much more variable caliper than will a very uniform formation.

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

TENSILE STRENGTH. 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 and other paper properties will be discussed in the October newsletter.