Wednesday  |  Conference Center B  |  02:50 PM–03:10 PM

#13528, Unravelling the Physics behind the Hygro-expansion of Paper: A Multi-scale Study

The extreme sensitivity of paper-based products to moisture content variations is one of their major concerns during application. As generally known, an increase in moisture content greatly affects the mechanical and geometrical properties of the material at all length scales, down to the single fiber level.

Therefore, a novel micro-mechanical characterization methodology is recently developed in which optical profilometry and a dedicated Global Digital Height Correlation (GDHC) algorithm is used to characterize the continuous full-field 3D moisture-induced dimensional change (hygro-expansion) of single (natural) fibers during wetting and drying with great precision. It was found that paper fibers tend to swell 20-30 times more in transverse compared to longitudinal direction.

The proposed methodology is naturally extended to allow identification of the full-field hygro-expansion of paper inter-fiber bonds. Characterization of orthogonally bonded inter-fiber bonds revealed novel insights in the inter-fiber interactions, i.e., (i) the transverse hygro-expansion of both fibers strongly reduces when approaching the bonded area, due to the significantly lower longitudinal hygro-expansion of the bonded fiber. (ii) The transverse hygro-expansion of one fiber matches the longitudinal hygro-expansion of the crossing fiber reasonably well in the bonded area, indicating the transferal of the large transverse strain upon the crossing fiber. (iii) Bending of the bonded region occurs during moisture uptake or release, attributed to the large difference in longitudinal and transverse hygro-expansion of the orthogonally bonded fibers.

An analytical bi-layered laminate model, containing the hygro-mechanical properties of the tested paper fibers forming the inter-fiber bond, is proposed and can describe the found bending behavior. The laminate model is extended to predict the sheet-scale hygro-expansion while only requiring the single fiber hygro-mechanical properties. The characteristics of the already-tested single fibers were used to determine the sheet-scale hygro-expansion, and a reasonably good match was found with the sheet-scale hygro-expansion characteristics of paper sheets formed from the same fibers.

In conclusion, the novel experimental data on inter-fiber bonds allowed derivation and validation of a bi-layered laminate model, which in turn allowed coupling of the single fiber characteristics towards the sheet scale.

Niels Vonk   Eindhoven University of Technology
Ron Peerlings   Eindhoven University of Technology
Marc Geers   Eindhoven University of Technology
Johan Hoefnagels   Eindhoven University of Technology