Cellulose solutions in aqueous hydroxide bases

Licentiate thesis, 2020

Cellulose is a renewable and widely available carbohydrate polymer whose utility for mankind only increases as we move towards a circular bioeconomy. Unlike most plastics or metals, it cannot be melted, and therefore, solution processing is important. The dissolution of cellulose is, however, not trivial, and traditional industrial methods must derivatize it in order to dissolve it adequately. Today, a number of direct solvents have been developed with considerable variation in both the structure of the solvent and the mechanism through which dissolution occurs. Despite this, one solvent that was reported as early as the 1920s still holds interest: cold aqueous NaOH solutions. Since the main processes in the pulp and paper industry are water-based, and NaOH is one of the main chemicals used, it is easy to see the attraction behind this solvent. The dissolution capacity and solution properties are however poor and must be improved. In order to design a solvent, the interactions between cellulose and the solvent as well as the interactions between cellulose molecules needs to be understood.

Therefore, the purpose of this thesis has been to improve understanding for interactions governing cellulose dissolution and behaviour in NaOH(aq) and other aqueous solutions of hydroxide bases. In order to achieve this, cellulose dissolution at low temperatures in an aqueous solution of NaOH, combined with selected quatenary ammonium hydroxide bases, tetramethylammonium hydroxide (TMAH) and benzyltrimethylammonium hydroxide (Triton B), as well as urea, have been inves- tigated. The impact of different solvent combinations on the maximum dissolution capacity of cellulose and the rheology of the solutions were also investigated. The structure of the solvents was studied using spectroscopic and calorimetric methods. It was found that by combining hydroxide bases in an aqueous solution, the dissolution capacity and stability of the solution was improved and the effect was comparable to the addition of urea. The resulting solution properties depended on the base pair employed, probably due to a difference in contribution of hydrogen bonding versus van der Waals interactions, which in turn determines such properties as the temperature stability of the solution.