Reactivity and accessibility of cellulose in cold alkali dissolution: Effects of hornifying, acidic, and mercerizing pretreatments

The demand for food, clothing, and consumer good raw materials, combined with the pressures of climate change, underscores the need for sustainable alternatives to fossil-based resources. Cellulose, the most abundant biopolymer in nature, offers an attractive renewable feedstock for novel materials such as man-made cellulosic fibers. However, cellulose conversion is inherently difficult due to its resistance to chemical modification and dissolution, known as recalcitrance. Direct solvent systems, such as cold aqueous sodium hydroxide (NaOH), present a low-cost and industrially available route, but their efficiency is limited by the reactivity and accessibility of the cellulose substrate. This dissertation explores how acid hydrolysis and alkaline pretreatments influence the dissolution behavior of softwood kraft pulp in NaOH-based solvents. The research addresses how structural modifications induce hornification in pulp, and how it is linked to cellulose reactivity, accessibility, and regeneration. The study assesses the impact of acid hydrolysis temperature on pulp dissolution and regeneration, examining hornification effects from acid hydrolysis. The effects of alkaline pre-treatments are investigated on fiber swelling and interfibrillar bonding. Dissolution kinetics are studied using a combination of polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Finally, the combined effect of acid hydrolysis, alkaline pretreatments, and drying conditions are tested. The results show that acid hydrolysis enhances dissolution primarily by reducing the degree of polymerization (DP). Pulp hydrolyzed at higher temperature were easier to dissolve, whereas the hydrolysis temperature had only marginal influence on fiber swelling. In contrast, alkaline pretreatments (mercerization) increased pulp porosity and accessibility but simultaneously reduced reactivity: alkali-treated pulps lost their ability to form strong interfibrillar bonds, and their morphology became kinky and curly, yielding mechanically weak handsheets. Furthermore, structural analysis confirmed the conversion of cellulose I to cellulose II. Increased drying temperature was found to decrease pulp swelling. However, surprisingly, the hornified fibers, showed improved solubility is alkali solvents. A key finding of this dissertation is the distinction between cellulose accessibility and reactivity. While alkaline pretreatment increases accessibility through swelling and porosity, it distinctly reduces dissolution reactivity in cold aqueous NaOH. Conversely, acid hydrolysis improves solubility without significantly altering swelling behavior.