Understanding the origin of microcompressions in cellulose fibres using 4D X-ray micro-computed tomography

Local defects in cellulose fibres, commonly referred to as microcompressions, increase the apparent strain to fracture in paper, an increasingly important parameter for products demanding stretch. This study presents an investigation into the origin of microcompressions by examining the behaviour of aspen wood under uniaxial compression applied along the grain, a process mimicking the action of a screw press and refiners during mechanical pulping. Using X-ray micro-computed tomography, a series of 10 tomograms with 30 µm of compressive deformation between images were acquired at 1.2 µm voxel size. For each tomogram, image analysis routines were developed to segment and label individual tracheids, vessel and ray elements, and track their motion between sequential frames. We classified the evolution of the axial trajectory of each element as either non-buckled or buckled, with a prominent presence of microcompression with axial deformations of less than 3%. We argue that microcompression precedes buckling. Significantly, we advance that localized deformation events (microcompressions, buckling, telescopic shortening) nucleate near vessel elements, and spread with increasing levels of compression; no particular structure appears to be immune to collapse. Finally, we liberated fibres in the compressed sample through chemical pulping and found that the damage occurring from compression was permanent.