Full-field hygro-expansion characterization of single softwood and hardwood pulp fibers

Abstract

The dimensional stability of paper products is a well-known problem, affecting multiple engineering applications. The macroscopic response of paper to moisture variations is governed by complex mechanisms originating in the material at all length-scales down to the fiber-level. Therefore, a recently-developed method, based on Global Digital Height Correlation of surface topographies is here exploited to measure the full-field hygro-expansion of single fibers, i. e. a surface strain tensor map over the full field of view is obtained as function of time. From the strain field, the longitudinal and transverse hygro-expansion and principle strains can be calculated. Long- and intermediate-duration dynamic tests are conducted on softwood and hardwood fibers. A large spread in the softwood fiber’s transverse and longitudinal hygro-expansion coefficient ratio was found, while hardwood fibers behave more consistently. Computing the principle strain ratios reduces this spread, as it takes into account the variations of the deformation direction, which is directly affected by the micro-fibril angle (MFA). Furthermore, long-duration tests allow identification of the half-times at which the fibers equilibrate. Finally, the determined major strain angles for all fibers are consistent with the MFA ranges reported in the literature.

Transient hygro- and hydro-expansion of freely and restrained dried paper: the fiber-network coupling

“…In contrast, for FD handsheets, the cellulose molecules in the "dislocated regions" contract during drying, retaining its (more or less) amorphous configuration, resulting in a larger hygro-expansion of FD fibers. The lower hygro-expansivity of RD compared to FD fibers (between 30 and 90% RH) was confirmed in (Vonk et al, 2023c) and (Vonk et al, 2023a), and it was indirectly shown (for hardwood fibers) that indeed the hygro-expansion along the micro-fibrils was lower for RD fibers. Furthermore, the alignment of the "dislocated cellulose regions" due to an external force was confirmed using molecular dynamics simulations in (Khodayari et al, 2020).…”

“…Paajanen et al (2022) showed that during single fiber hygro-expansion up to a RH of 90%, the hemi-cellulose inside the fibers softens between 40 and 60% RH, whereas the cellulose micro-fibrils swell between 55 to 95%. In (Vonk et al, 2021(Vonk et al, , 2023c the transient dimensional changes accompanying these phenomena during hygro-expansion (between 30 and 95% RH) was characterized, revealing curves which are similar to conventional sorption isotherms (Fellers, 2007;Paajanen et al, 2022). However, characterization of paper above M C levels associated with 95% RH remains undiscovered, which according to Salmén et al (1987) is the relevant M C range during which phenomena occur that strongly affect the hygro-expansivity of the paper sheet.…”

“…As generally known, restrained dried (RD) paper yields a significantly lower hygro-expansion than freely dried (FD) paper (Salmén et al, 1987;Uesaka et al, 1992;Larsson and Wågberg, 2008;Urstöger et al, 2020;Vonk et al, 2023c). Salmén et al (1987) stated that during the paper formation process when the paper is fully saturated, the so-called "dislocated regions", which make up ∼ 50% of the cellulose micro-fibrils, are soft and stretched through the restrained drying procedure, but subsequently hardened during drying from a "wet" to a 90% RH state (Agarwal et al, 2013).…”

“…Moreover, the geometry is better preserved during hygro-expansion, as inter-fiber bonds remain stable and out-of-plane deformations are less likely to occur due a more homogeneous M C distribution enabling easier quantification of the in-plane expansion. Hence, hygro-expansion is usually applied for studying the (continuous) transient dimensional change of paper sheets (Salmén et al, 1985;Uesaka et al, 1992;Niskanen et al, 1997;Fellers, 2007;Larsson and Wågberg, 2008;Vonk et al, 2023c), and its micro-constituents, including fibers (Meylan, 1972;Joffre et al, 2016;Vonk et al, 2021). In contrast, the time-dependent spectrum of the transient hydro-expansion of fibers has not yet been reported in the literature.…”

Experimental characterisation of the local mechanical behaviour of cellulose fibres: an in-situ micro-profilometry approach

“…Despite the 10% filtering threshold, it was a posteriori noticed that various pixels surrounding the eliminated data were still too noisy, causing convergence problems in the GDHC procedure. In order to overcome this problem, the regions with missing data values were uniformly expanded in the height profiles by eliminating the one layer of pixels located directly along their circumference, which further reduced the influence of local noisy data on the correlation analysis (Vonk et al 2020(Vonk et al , 2021. Subsequently, the average stress in an ROI was calculated from the applied force and the average cross-section of the ROI.…”