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 )
![[CoOL]](/icons/sm_cool.gif){kind=icon}