Dimensional Stability

A measure of the extent to which a paper will resist a change in size as the result of a change in moisture content or the application of a compressing force, as during printing. The cellulose fibers comprising a sheet or web of paper have an affinity for water, which means that they readily absorb water from the atmosphere (or lose water to the atmosphere, depending on the relative humidity and the equilibrium moisture content of the paper). When paper fibers absorb water, they expand primarily in width, but only slightly in length. (Similarly, when a paper loses moisture to the atmosphere, the fibers will shrink primarily in width, but only slightly in length.) Therefore, when a paper undergoes a dimensional change, it will primarily be in the cross-grain direction. (See Grain and Moisture Content.) The percent change in size, either by expansion or reduction, is called the paper's hygroexpansivity. The addition of fillers to the papermaking furnish helps increase a paper's dimensional stability, as fillers do not absorb or lose moisture. The extent to which a paper's fibers have been refined (i.e., how short and how closely bonded the fibers are in the paper) also affects its dimensional stability; the less refined the fibers are the greater the dimensional stability. (But less refining also compromises other paper qualities.) Paper has the highest degree of dimensional stability in relative humidities of 35:50%.

Paper and printing defects such as waffling result from paper's being stretched past its ability to retain its original dimensions. (A paper's ability to return to its original dimensions after a stretching force is removed is called its visoelasticity; stretching beyond a certain point, however, destroys the paper's ability to "snap back" to its original size.) Paper is generally stronger in its grain direction, and stretching that occurs in long-grain paper in the cross-grain direction usually can be compensatzed for by other press procedures.

Some papers, depending on their end use, don't require a high degree of dimensional stability. Those that do include papers to be used for multicolor offset work, where proper registration is important, and forms that are going to be collated into a bound or padded set. Dimensional stability of a paper can be measured with an expansimeter.

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