Modifying the structure of microcrystalline cellulose by different drying methods and mechanical treatments

Recently, wood-derived cellulosic micro- and nanomaterials have shown significant potential for pulp and paper technology. However, their adoption and commercialization have not progressed as expected. Therefore, there is currently an increasing research interest in using cellulosic micro- and nanomaterials in various industries outside pulp- and papermaking, such as pharmaceuticals, cosmetics and the food industry. Microcrystalline cellulose (MCC) is a purified, partially depolymerized nonfibrous form of cellulose, a crystalline powder composed of porous particles. MCC (holding E-code E460i) as such is safe for oral consumption for both human and animal purposes. Reflecting MCC's multiple current applications and future potential, it is important to further investigate the possibility of producing MCC with different cross-sectional shapes to increase the surface area due to its great importance for specific applications. Microfibrillated cellulose (MFC), that can be produced from MCC, consists of microfibril bundles forming a weblike fiber network and providing its multiple uses as thickeners, emulsifiers or additives in food, paints, and coatings, as well as cosmetics and medical products. In addition, MFC is ideal as a reinforcement in composites and, concurrently, to reduce the utilization of petroleum-based components. The modification of the never-dried form of MCC with novel techniques and thus producing new types of micro-sized cellulose products, are the focus of this doctoral research. Firstly, the study investigated the drying of the never-dried MCC with two different solids contents using three different drying methods: high-velocity cyclone drying, spray drying, and fluidized bed drying (Paper 1). The effects of these drying techniques on the geometrical dimensions and morphology of the dried MCC particles and aggregates were studied. The results revealed that the morphology of the dried MCC was highly dependent on the initial raw material properties and the liquid removal mechanism during drying. Fluidized bed drying best preserved the original MCC morphology, yielding discrete particles with high surface area and less aggregation. Spray drying produced small, circular particles with homogeneous size distributions, while high-velocity cyclone drying resulted in the largest, most heterogeneous, and irregularly shaped particles and aggregates. Secondly, the study focused on the production of MFC-hydrogels from never-dried MCC using high-pressure mechanical treatment (Paper 2) and later with a Masuko laboratory grinder with different refining degrees (Paper 3). The effects of the treatments on the crystalline structure, morphology, geometrical dimensions, specific surface area, and rheological properties of the resulting MFC gel were analyzed. Results indicated that both mechanical processes produced partially detached crystalline areas, increasing surface area and porosity, leading to the formation of more stable MFC hydrogels due to enhanced hydrogen bonding between cellulose particles. Additionally, using never-dried MCC as a raw material in refining resulted in a stronger MFC gel with superior storage and loss moduli compared to the ones produced with pre-dried MCC. Specific energy consumption data from Masuko grinder also indicated that mechanical energy is more effectively transferred to the never-dried MCC structure, suggesting that energy can be saved when producing MFC from never-dried MCC via refining.