Earlier works in literature on the hygro-expansion of paper state that the
larger hygro-expansivity of freely compared to restrained dried handsheets is
due to structural differences between the fibers inside the handsheet. To
unravel this hypothesis, first, the hygro-expansion of freely and restrained
dried, hardwood and softwood handsheets has been characterized. Subsequently,
the transient full-field hygro-expansion (longitudinal, transverse, and shear
strain) of fibers extracted from these handsheets was measured using global
digital height correlation, from which the micro-fibril angle was deduced. The
hygro-expansivity of each individual fiber was tested before and after a
wetting period, during which the fiber's moisture content is maximized, to
analyze if a restrained dried fiber can "transform" into a freely dried fiber.
It was found that the longitudinal hygro-expansion of the freely dried fibers
is significantly larger than the restrained dried fibers, consistent with the
sheet-scale differences. The difference in micro-fibril angle between the
freely and restrained dried fibers is a possible explanation for this
difference, 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 immersion
in water for a day. Various mechanisms have been studied to explain the
observations on freely and restrained dried hardwood and softwood, fiber and
handsheets including analysis of the fibers' lumen and cross-sectional shape.
The presented results and discussion deepens the understanding of the
differences between freely and restrained dried handsheets.Comment: 43 pages, 15 figures, 2 table