Re-understanding the In-plane Hygro-expansion of Freely and Restrained Dried Paper Handsheets

The larger hygro-expansivity of freely compared to restrained dried handsheets has been intensively studied during the past decades. To investigate the role of the fibers forming the sheets on this complex phenomenon, in this work, the hygro-expansivity of fibers picked from freely and restrained dried handsheets is characterized. To do so, a versatile, highly accurate, fiber hygro-expansion methodology based on Global Digital Height Correlation is proposed, which enables identification of the transient full-field hygro expansivity of single paper fibers. It was found that the longitudinal, transverse and shear hygro-expansivity of fibers picked from freely dried handsheets is significantly larger than fibers picked from the restrained dried handsheet. Furthermore, a restrained dried fiber can yield the hygroexpansivity of a freely dried fiber after being subjected to a sufficiently long wetting period, implying that the moisture-induced release of dried-in strain drives the hygro-expansivity differences. Finally, the sheet-scale hygro-expansivity is comparable to longitudinal fiber hygro-expansivity for both handsheet types. The presented results are of key importance for understanding the paper hygromechanics and improve their applicability.