Paper exhibits anisotropic behavior basically because of two factors: 1) as stated above, the orientation of the fibers in the machine direction causes them, and therefore the sheet, to expand or contract preferentially in the cross direction. In fact, at one time it was thought that this was the major reason for anisotropic behavior in paper; 2) more recently, however, although orientation of the fibers is still thought to play a role in the behavior, the major reason is considered to be built-in strain, also known as dried-in strain. The matting of fibers formed from the wet slurry on the wire of the machine moves into the dry end of the papermaking machine and is dried rapidly in the drier sections. Because it is wet, the web will contract during drying. Because the tension of the web is in the plane of the machine direction, the paper is able to contract considerably in the cross direction but not in the machine direction, which results in a built- or dried-in strain. The drying is actually so rapid that relaxation of the strain is impossible on the machine, nor does it occur after the web of paper leaves the machine as the dried paper does not have sufficient plasticity for this to happen. Consequently, a machine-made paper has an inherent dried-in strain. Actually, there is some dried-in strain in the cross direction also, as the felts of the machine reduce shrinkage somewhat in that direction. This characteristic results in an anisotropic sheet, one which will expand more in the cross direction than in the machine direction when exposed to an increase in the relative humidity of the surrounding atmosphere. If, at some time in the future, the paper is wetted in water, the dried-in strains will be released and, if the sheet is then allowed to dry freely and without restraint, its anisotropic characteristics will be largely lost. Anisotropic characteristics in paper are of great importance in printing, especially in printing processes which involve wetting the paper.
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