Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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stiffness

A term relating to the ability of a material such as paper or board to resist bending while under stress. Resistance to the bending is called flexural stiffness, and may be defined as the product of the modulus of elasticity and the moment of inertia of the section. Other factors being equal, stiffness of paper and board varies as the cube of its thickness and directly with the modulus of elasticity. Stiffness is improved by increasing the degree of hydration of the pulp used in making the sheet, because a high degree of hydration makes paper dense and hard, and provides cohesive strength. The property is obtained more readily with some pulps than with others. Soda pulp, for example, does not produce particularly stiff paper because the relatively long beating period required to develop the degree of hydration required virtually pulverizes the short soda pulp fibers. Sulfite pulp, on the other hand, can be hydrated to the point of hardness before the fibers are entirely broken down; consequently, long-fibered sulfite pulp is more often used in the production of stiff, hard papers. (17 , 98 , 350 )




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