X-Ray Tomography for Visualizing the Internal Structure of Paper Products and its Correlations with Mechanical Properties

Mechanical pulp has traditionally been utilized in newsprint and low-value paper products. With emerging opportunities in bio-based products, mechanical pulp is now standing out as a greener alternative to plastics and fossil-based materials. The development of new sustainable products based on mechanical pulp involves exploring and combining different size fractions, creating composites with chemical and semichemical pulps, and forming layered or dispersed fiber structures.<br/>The challenge in advancing the production of high-quality and specialty paper products based on high-volume mechanical pulp lies in elucidating the internal distribution and interaction of fibers within composites made from different kinds of pulp fibers. X-ray micro-computed tomography has proven to be a powerful tool for visualizing the 3D internal structure of materials, and its potential for elucidating paper structures at the fiber level has already been demonstrated. However, visualizing reinforcing pulp fibers or different phases of pulp fiber composites remains a challenge due to the similar chemical composition of fibers and the surrounding matrix.<br/>In this work, we present a labeling protocol employing iron oxide nanoparticles to enhance the X-ray attenuation of added fibers or pulp phases, resulting in higher contrast fibers in X-ray tomographs. Through <i>in-situ</i> synthesis and deposition, iron oxide nanoparticles were deposited on the external and lumen surfaces of long fraction pulp fibers to prepare iron-labeled fibers. These iron-labeled fibers were then incorporated into paper handsheets at various loadings and examined via X-ray tomography. An in-house algorithm for the segmentation of iron-labeled and unlabeled fibers was developed based on tomograph histograms. To optimize the segmentation process, we embedded the handsheets in oil to eliminate the signals associated with air (background signal). After image processing, we successfully visualized the spatially and randomly oriented fibers within the handsheet plane at diverse loadings of iron-labeled fibers. Furthermore, we investigated the impact on mechanical properties of randomly oriented iron-labeled fibers in handsheets in comparison to unlabeled fibers to delineate the limitations of labeling fibers with iron nanoparticles as contrast agent for X-ray tomography.<br/>This study not only provides valuable insights into the role of the large fraction of pulp fibers in reinforcing papers but also sheds light on how different kinds of pulp fibers are distributed in paper products and how the formation of a network impacts mechanical properties. A better understanding of the 3D distribution and network formation of fibers can be leveraged to support the development of high-value products based on high-yield mechanical pulp, aligning with opportunities toward sustainable and bio-based materials for the papermaking industry.