Mid Sweden University

Global environmental campaigns emphasize the need to understand better how renewable raw materials can be utilized effectively. There is a notable trend towards replacing fossil-based materials with fiber-based alternatives across various packaging applications. Solid and lightweight composite packaging structures can be produced in an environmentally friendly and energy-efficient way. In recent years, wood fibers have gained popularity as a packaging material. High-yield pulps, such as CTMP (Chemithermomechanical pulp), which achieves a 95% yield, are increasingly used in packaging. Worldwide, 5-10 Mt/y of CTMP are produced from softwood and hardwood chips for paperboard manufacturing. During tailor-making CTMP, wood chips are impregnated with aqueous sodium sulfite (Na2SO3) to sulfonate the wood's lignin. This sulfonation (-SO3-) softens the wood chips, enabling more selective defibration into the pulp. As a result, the pulp properties, including the bulk and strength characteristics of the final packaging, are enhanced. Several factors influence the quality of wood chips, including the chipping method used for pulpwood, sawmill chipping practices, and the chip screening system. Developing an impregnation technology that ensures an even distribution of sodium sulfite (Na2SO3) can be challenging. It is essential to measure the distribution of sulfonate groups in individual fibers and wood chips at a micro-scale; however, existing processing methods often need to be more robust and complex, making this difficult. If better measurement techniques were available, we could understand how sulfonation operates before defibration, improving the impregnation process. Sulfur impregnation can be studied using spatial and spectral resolutions to investigate the degree of sulfonation at the microscale. Therefore, we propose creating a laboratory-scale miniaturized X-ray fluorescence (XRF) scanner to measure sulfur distribution in wood chips on-site. We aim to minimize differences in sulfonate content between fibers, allowing us to reduce the dosage of sulfite (SO32-) needed for fiber separation and, consequently, lower the overall electrical energy used in chip refining. Research facilities, including APS beamline in the United States, Elettra beamlines in Italy, and Diamond light source in Oxford, United Kingdom, have validated X-ray fluorescence (XRF) techniques developed in-house. These techniques enable the measurement of sulfonated lignin distribution, providing a more detailed understanding of distribution within and between individual fibers. To ensure the homogeneity of sulfur distribution required for CTMP, we typically need a spatial resolution of 10-15 μm. We have developed our methodology based on this spatial resolution, which informs us about homogeneity. Our research has shown that sulfonation at the fiber surface is the most effective process parameter. Therefore, it is crucial to understand how sulfonate ions (-SO3-) are integrated into the structure of lignin in wood fibers, as this knowledge could be vital for developing future products and processes of high-strength packaging.