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Earlier works in literature on the hygro-expansion of paper state that thelarger hygro-expansivity of freely compared to restrained dried handsheets isdue to structural differences between the fibers inside the handsheet. Tounravel this hypothesis, first, the hygro-expansion of freely and restraineddried, hardwood and softwood handsheets has been characterized. Subsequently,the transient full-field hygro-expansion (longitudinal, transverse, and shearstrain) of fibers extracted from these handsheets was measured using globaldigital height correlation, from which the micro-fibril angle was deduced. Thehygro-expansivity of each individual fiber was tested before and after awetting period, during which the fiber's moisture content is maximized, toanalyze if a restrained dried fiber can "transform" into a freely dried fiber.It was found that the longitudinal hygro-expansion of the freely dried fibersis significantly larger than the restrained dried fibers, consistent with thesheet-scale differences. The difference in micro-fibril angle between thefreely and restrained dried fibers is a possible explanation for thisdifference, but merely for the hardwood fibers, which are able to "transform"to freely dried fibers after being soaked in water. In contrast, this"transformation" does not happen in softwood fibers, even after full immersionin water for a day. Various mechanisms have been studied to explain theobservations on freely and restrained dried hardwood and softwood, fiber andhandsheets including analysis of the fibers' lumen and cross-sectional shape.The presented results and discussion deepens the understanding of thedifferences between freely and restrained dried handsheets.

Effect of restrained versus free drying on hygro-expansion of hardwood and softwood fibers and paper handsheet