Characterization of regenerated cellulose prepared from different pulp grades using a green solvent

The cellulosic polysaccharide shows many potential applications due to its abundance, renewability, biodegradability and biocompatibility. Nowadays, the dissolution of this biopolymer via the alkaline based green solvent system has gained much attention in developing cellulose based composites for pharmaceuticals, waste water treatment or packaging applications. In this work, we aim at dissolving different grades of sustainable pulps in an aqueous NaOH/urea/thiourea system, involving the freeze thawing process, followed by re-precipitation with distilled water. This particular dissolving medium is chosen as it is economic and environment-friendly. Optical microscopy is used to study the extent of cellulose solubility in this system. Dissolving grade pulp (D.G.P.), with a degree of polymerization DP = 678.27, exhibits higher solubility after 3 freeze thawing cycles, compared to bleached hardwood pulp (B.G.P., DP = 990.67) and tissue grade pulp (T.G.P., DP = 1356.54). DP is further correlated with the average fibre length of the three samples obtained from a Morfi fibre analyser. Additionally, fibre identification through the microscopic slide staining procedure reveals that the long flexible softwood fibres present in the T.G.P. sample are responsible for its high DP. The solubility percentage calculated for D.G.P., B.G.P. and T.G.P. is 72.90%, 57.2% and 57%, respectively. The flow behaviour of each dissolved cellulosic sample is of Newtonian type at high shear rates, considering the same cellulose concentration at ambient temperature. X-ray diffraction patterns (XRD) and ATR-FTIR spectra indicate that the ordered crystalline structure of the samples is successfully destroyed and converted to cellulose II during the dissolution and regeneration process. Field emission scanning electron microscopy (FE-SEM) reveals porous structures for all samples, created due to phase separation of the cellulose solutions during regeneration with distilled water. Thus, the key finding of the study establishes that the regeneration via the green solvent can produce more amorphous cellulosic materials, which may be a suitable additive for papermaking, due to their enhanced bonding ability with wood fibres.