Investigating Sulfur Uniformity in High-Strength Packaging by Synchrotron Techniques

When producing high-yield pulp, like Chemithermomechanical (CTMP), it is crucial to maintain maximum bulk and avoid unnecessary reductions in softening temperature while enhancing fiber stiffness. However, the key to maintaining maximum fiber stiffness lies in achieving even sulfonation of wood chips and uniform distribution of sulfite (-SO3-) ions throughout the wood chips. This is essential for the process. The challenge of creating lighter, stronger, and stiffer paperboards is significant when wood chips vary in size, density, and quality. Additionally, the inner parts of wood absorb less sodium sulfite (Na2SO3) than the outer parts, leading to higher shives (unseparated fibers) content and differently bonded wood fibers. The X-ray fluorescence (XRF) techniques are developed in-house and validated by APS in the USA and Elettra beamlines in Italy. These techniques determine the distribution of sulfonated lignin, providing a better understanding within and between individual fibers. CTMP sulfur homogeneity inspections usually require a spatial resolution of 10µm-15µm. Our methodology is based on this spatial resolution, which contains information on homogeneity. Our research shows sulfonation at the fiber surface is the optimal process parameter. Therefore, it is essential to understand how sulfonate ions (-SO3-) become integrated into the lignin structure of wood fibers, as this could be crucial for future process and product development.