Production and evaluation of nanopaper from cotton linter by partial dissolution method

The use of cellulose at nanoscale has been greatly studied in the production of biological compounds due to its high strength, low weight and biodegradability. Nanostructures are synthesized through two mechanisms including top-down and bottom-up approaches. In this study, top-down partial dissolution was used as a simple and fast technique to produce nano cellulose. By controlling dissolution parameters such as time, the solvent interrupts the adjacent nanofibrils linking which is supplied through hydrogen bonds, and solves partially outer chains of nanofibril. During solvent rinsing, the partially dissolved chains re-solidified and welded each other, making consolidated structure in which the main components are undissolved native nanofibrils surrounded by cellulose type II and non-crystalline cellulose. Because of this, the final film was named nanopaper. This study considers the characteristics of fully biocompatible nanopaper directly produced from cotton linter fibers by partial dissolution method.
Refining the cotton linter fibers were done in three steps; pneumatic, washing with hot water followed by treating with sodium hydroxide. The handsheets were made by TAPPI standard method. The partial dissolution of papers with high content of alpha cellulose were done in the solvent N,N-dimethylacetamide/ 9% lithium chloride (DMAc/LiCL) and translucent cellulose nanopaper was obtained through pressing and drying the resulting gel. To evaluate the properties of the nanopaper, field emission scanning electron microscopy, X-ray diffraction, mechanical properties and thermal gravimetric analysis were used.
The diameter of undissolved nanofibrils in nanopaper fell between 60 and 66 nm. . Electron micrographs showed that nanopaper had more uniformity than paper. Visual transparency (back view) of nanopaper was significant due to the liberalization of cellulose nanostructures, the increase of uniformity and density, the loss of surface roughness and the increase of light transmission. The results of tensile properties showed that the nanopaper tensile stress was higher than that of paper. Paper and nanopaper tensile stress were 8.02 and 27.28 MPa, respectively and tensile modulus elasticity were 0.483 and 0.649 GPa, respectively. X-ray diffraction (XRD) of paper matched cellulose type I. During partial dissolution/re-solidification cellulose type II was appeared and non-crystalline phase increased judging from XRD data. The crystallinity degree of paper and nanopaper were measured 84.9 and 54.89%, respectively. The crystallite size of paper and nanopaper obtained 6.44 and 2.55 nm, respectively. The thermal stability of nanopaper was less than that of paper.
In nanopaper structure, undesolved cellulose type Iβ (undesolved nanofibrils) played reinforcing phase role, and cellulose type (II) and the amorphous cellulose formed the matrix phase. Partial dissolution destroyed part of the crystals and after solvent rinsing and re-solidification, some parts of the amorphous chains were rearranged to form crystals of cellulose type II. Finally, a tough translucent nanopaper was produced by creating consolidated nano structures. The reduction of cellulose crystallinity in nanopaper resulted in the loss of thermal stability in nanopaper.