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Summary:
The Forest Product Laboratory's new approach to measuring the in-plane hydroexpansivity of paperboard has been applied to the fine papers used in the production of postage stamps. A vacuum restraint approach apparatus for rapidly inducing large changes in bell jar relative humidity has been used to simulate the entire range of press humidity conditions in a fraction of the time required to assess paper hydrostability by conventional means The technique has been used to evaluate the expansion and contraction characteristics of both gummed and ungummed papers. The relative influence of adhesive and substrate on the hydroexpansivity of the composite has been determined. The method also allowed for the identification of residual strain in the paper and curl effects. The impact of these results on controlling registration and post-printing processing operations such as perforation are discussed.