Mid Sweden University

There is a growing demand for the utilization of sustainable materials, such as cellulose-based alternatives, over fossil-based materials. However, the inherent drawbacks of cellulosic materials, such as extremely low wet strength and resistance to moisture, need significant improvements. Moreover, several of the commercially available wet-strength chemicals and hydrophobic agents for cellulosic material treatment are toxic or fossil-based (e.g., epichlorohydrin and fluorocarbons). Herein, we present an eco-friendly, high-yield, industrially relevant, and scalable method inspired by birch bark for fabricating hydrophobic and strong cellulosic materials. This was accomplished by combining simple surface modification of cellulosic fibers in water using colloidal particles of betulin, an abundant triterpene extracted from birch bark, with sustainable chemical engineering (e.g., lignin modification and hot-pressing). This led to a transformative process that not only altered the morphology of the cellulosic materials into a more dense and compact structure but also made them hydrophobic (contact angles of up to >130°) with the betulin particles undergoing polymorphic transformations from prismatic crystals (betulin III) to orthorhombic whiskers (betulin I). Significant synergistic effects are observed, resulting in a remarkable increase in wet strength (>1400%) of the produced hydrophobic cellulosic materials.

The widespread use of synthetic plastics in packaging materials poses significant environmental challenges, prompting the search for biobased, biodegradable, and non-toxic alternatives. This study focuses on improving high-yield pulps (HYPs) as sustainable materials for packaging. Enhancing wet strength and barrier properties of papers from bleached chemi-thermomechanical pulps (BCTMPs) is crucial for their application in water- and air- resistant wrappers. Traditional wet strength agents raise environmental and health concerns; therefore, this research explores the use of lignin, in the form of microparticles (LMPs), as a natural biopolymer that offers a safer alternative. However, the low viscosity of LMPs hampers their dispersion as a coating, requiring thickening agents (such as cationic starch (CS), chitosan (CH) or sodium alginate) for an effective coating formulation. Results demonstrate a synergistic effect of LMP coatings with CH or CS, enhanced by hot-pressing at 260 °C for 30 s, which improves dry and wet mechanical properties and decreases air permeability. The use of LMPs as a water-resistant interlayer between BCTMP paper sheets further improves the wet tensile index to 40 kN·m/kg for CH + LMPs and 23 kN·m/kg for CS + LMPs interlayer, representing 55 and 38 % of their respective dry tensile indices. 

Defibration of wood chips in high-yield pulping such as CTMP production involves sulfonation of wood chips using (Na2SO3). When aiming to improve product properties, one key issue to investigate is the evenness of the sulfonation, i.e., the distribution of the sulfite (SO32-) ions. The challenge is that the inner parts of the wood chips absorb much less sodium sulfite than the outer parts. As a result, less sulfonated wood fibers have different bonding properties. It is likely that the efficiency and evenness of fiber separation in a chip refiner depend greatly on how evenly the chips have been sulfonated. Uneven sulfonation then results in higher shives (unseparated fibers) content which impairs product properties. We suggest a laboratory-scale miniaturized X-ray fluorescence (XRF) scanner for measuring sulfur distribution in the wood chips on-site. By minimizing the differences in sulphonate content between fibers, we can minimize the requirement for sulfite (SO32-) dosage to a certain degree of fiber separation, thereby reducing the total amount of electricity used in chip refining. There has been a significant improvement in commercial XRF microscopy scanners over the last few years, but the spatial resolutions achieved are insufficient. We have developed an XRF scanner optimized for sulfur fluorescence energies [1], and further continued this development by implementing frontier technology polycapillary X-ray optics. We present spatial resolution measurements and discuss the relevance and usability of the proposed measurement methodology to demonstrate its performance.

[1] Rahman, H., et.al, ACS Omega 2022, 7, 51, 48555–48563, DOI: 10.1021/acsomega.2c07086

Broader use of bio-based fibres in packaging becomes possible when the mechanical properties of fibre materials exceed those of conventional paperboard. Hot-pressing provides an efficient method to improve both the wet and dry strength of lignin-containing paper webs. Here we study varied pressing conditions for webs formed with thermomechanical pulp (TMP). The results are compared against similar data for a wide range of other fibre types. In addition to standard strength and structural measurements, we characterise the induced structural changes with X-ray microtomography and scanning electron microscopy. The wet strength generally increases monotonously up to a very high pressing temperature of 270 ◦C. The stronger bonding of wet fibres can be explained by the inter-diffusion of lignin macromolecules with an activation energy around 26 kJ mol−1 after lignin softening. The associated exponential acceleration of diffusion with temperature dominates over other factors such as process dynamics or final material density in setting wet strength. The optimum pressing temperature for dry strength is generally lower, around 200 ◦C, beyond which hemicellulose degradation begins. By varying the solids content prior to hot-pressing for the TMP sheets, the highest wet strength is achieved for the completely dry web, while no strong correlation was observed for the dry strength.

To extend the application of cost-effective high-yield pulps in packaging, strength and barrier properties are improved by advanced-strength additives or by hot-pressing. The aim of this study is to assess the synergic effects between the two approaches by using nanocellulose as a bulk additive, and by hot-pressing technology. Due to the synergic effect, dry strength increases by 118% while individual improvements are 31% by nanocellulose and 92% by hot-pressing. This effect is higher for mechanical fibrillated cellulose. After hot-pressing, all papers retain more than 22% of their dry strength. Hot-pressing greatly increases the paper’s ability to withstand compressive forces applied in short periods of time by 84%, with a further 30% increase due to the synergic effect of the fibrillated nanocellulose. Hot-pressing and the fibrillated cellulose greatly decrease air permeability (80% and 68%, respectively) for refining pretreated samples, due to the increased fiber flexibility, which increase up to 90% using the combined effect. The tear index increases with the addition of nanocellulose, but this effect is lost after hot-pressing. In general, fibrillation degree has a small effect which means that low- cost nanocellulose could be used in hot-pressed papers, providing products with a good strength and barrier capacity. 

In the pulp and paper industry, about 5 Mt/y chemithermomechanical pulp (CTMP) are produced globally from softwood chips for production of carton board grades. For tailor making CTMP for this purpose, wood chips are impregnated with aqueous sodium sulphite for sulphonation of the wood lignin. When lignin is sulphonated, the defibration of wood into pulp becomes more selective, resulting in enhanced pulp properties. On a microscopic fibre scale, however, one could strongly assume that the sulphonation of the wood structure is very uneven due to its macroscale size of wood chips. If this is the case and the sulphonation could be done significantly more evenly, the CTMP process could be more efficient and produce pulp even better suited for carton boards. Therefore, the present study aimed to develop a technique based on X-ray fluorescence microscopy imaging (µXRF) for quantifying the sulphur distribution on CTMP wood fibres. Firstly, the feasibility of µXRF imaging for sulphur homogeneity measurements in wood fibres needs investigation. Therefore, clarification of which spatial and spectral resolution that allows visualization of sulphur impregnation into single wood fibres is needed. Measurements of single fibre imaging were carried out at the Argonne National Laboratory’s Advanced Photon Source (APS) synchrotron facility. With a synchrotron beam using one micrometre scanning step, images of elemental mapping are acquired from CTMP samples diluted with non-sulphonated pulp under specified conditions. Since the measurements show significant differences between sulphonated and non-sulphonated fibres, and a significant peak concentration in the shell of the sulphonated fibres, the proposed technique is found to be feasible. The required spatial resolution of the µXRF imaging for an on-site CTMP sulphur homogeneity measurement setup is about 15 µm, and the homogeneity measured along the fibre shells is suggested to be used as the CTMP sulphonation measurement parameter.

An underestimated problem in the rapidly growing CTMP industry is uneven sulphonation. Optimizing the unit operations before chip refining, chip washing, steaming, impregnation, and preheating improves efficiency, provides smoother fiber properties, and reduces the cost of certain properties in the final product. Impregnation is crucial to the CTMP quality, and a further improvement in its smoothness requires a careful study of the optimization of pulpwood chipping and the chipping process with reduction technology at sawmills. The CTMP system, however, is difficult to optimize due to the lack of rapid measurement methods for determining the smoothness of the impregnation at the fiber level. The ability to study how the processing system can be optimized requires a robust method of measuring the degree of sulphonation at the fiber level. It is possible to study CTMP's degree of sulphonation at the fiber level by measuring the distribution of elemental sulphur and counterions of the sulphonate groups, such as sodium or calcium. Thus, we are developing an XRF (x-ray fluorescence) technology based on scanning imaging and energy-resolved X-ray spectrum from a collimated X-ray source. The measurement technology is developed so that it can be used in pulp industry laboratories.

In this work, high-rate fibre analysis has been used for direct feedback control of pulp quality by application of a new control strategy for a two-stage refining process in the Holmen Hallsta mill, Sweden. The application is based on control of pulp freeness, estimated from the continuous fibre analysis results from a BTG Single Point Morphology ana-lyzer. The goal was to create a robust and simple control strategy. The new strategy includes control of plate gap, con-sistency and the hydraulic force difference between the stages. Expressed as standard deviation, the freeness and av-erage fibre length variations were reduced by 50% and 25% respectively. The small size of the pulp chest in this process also benefits stronger feedback control. Long-term operation suggest that high-rate fibre analysis can be used to reduce faster quality variation.