Equilibrium Relative Humidity

The relative humidity of the atmosphere at a particular temperature at which paper will neither gain nor lose moisture. At equilibrium relative humidity, paper can be exposed to air without the environment effecting a change in its length and/or width. The primary constituents of paper, fibers of cellulose, have a strong affinity for water, and will gain (or lose) it readily, depending on the amount of moisture in the air. This hygroscopic characteristic of paper makes it dimensionally unstable, as the length and/or width of a paper can change depending on how much water the paper has gained or lost. (See Moisture Content and Dimensional Stability.)

The equilibrium relative humidity of a paper depends on its percentage of fillers—inorganic materials added to paper during papermaking—which neither gain nor lose moisture. Coatings also affect the moisture content of paper. Fillers and coatings increase dimensional stability, which improves printability in many printing processes. A related measurement is equilibrium moisture content.

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