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

N H Vonk,M G D Geers,J P M Hoefnagels

doi:10.1515/npprj-2020-0071

N H Vonk, M G D Geers + Show 1 more

Open Access

https://doi.org/10.1515/npprj-2020-0071

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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.

Highlights

A recently-developed method, based on Global Digital Height Correlation of surface topographies is here exploited to measure the full-field hygroexpansion of single fibers, i. e. a surface strain tensor map over the full field of view is obtained as function of time
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)
Fiber-width tracking with micro-radiography (Tydeman et al 1965) Fiber end tracking with universal length measuring machine (Meylan 1972) Feature tracking with confocal laser scanning microscopy (Nanko and Wu 1995) Ellipsoid fitting of fiber diameter using confocal scanning microscopy Feature tracking with atomic force microscopy (Lee et al 2010) Full-field correlation of surface height profiles [this work]

Summary

Introduction

The hygro-expansion of single fibers or fibrils has been extensively studied by Tydeman et al (1965), Meylan (1972), Nanko and Wu (1995), Weise and Paulapuro (1995) and Lee et al (2010). Full-field hygro-expansion data of single softwood and hardwood pulp fibers is called for, enabling the direct determination of both longitudinal and transverse hygroexpansion coefficients during absorption and desorption cycles. This data enriched the parameter identification and provides in-depth insight in hygroscopic behavior. This paper addresses the question whether one can determine, from a single experiment, the full-field hygroexpansion (and parameter identification) of single softwood and hardwood pulp fibers during wetting and drying cycles, allowing direct identification of the absolute longitudinal and transverse hygro-expansion magnitudes. A comprehensive data analysis including parameter identification is conducted and the validity of the measurements is discussed, followed by conclusions

Materials and methods

Method

Results and discussion

Conclusion