| Cellulose is a material that does not melt at the temperature lower than its degradation temperature. Strong intra- and inter-molecular hydrogen bonds in cellulose prevent its molecules from dissolution in most common solvents. Although there are a number of approaches to produce regenerated cellulose, such as viscose rayon, cuprammonium cellulose, Lyocell fibers, the market is shrinking due to the environmental and economic feasibility concerns of these methods. Some other processes, such as using organosolvents and ionic liquid can also dissolve cellulose, but the high cost and organic solvent recovery problems hinder their further applications in a large scale. For these reasons, cellulosic materials are regarded as un-moldable materials. Because of the un-moldable identity, wood and cotton fibers are difficult to be refabricated as other thermosetting and thermoplastic polymers. Therefore, fundamental understanding of the cellulose dissolution chemistry in aqueous solution is particularly interested by cellulose chemists. If an effective, economic and environmentally friendly cellulose dissolution method can be developed, a new platform for producing moldable cellulosic intermediate materials will be created providing new opportunities for using cellulosic materials as a renewable and sustainable engineering polymers.;Effective cellulose solvents should be able to break down these forces and for this purpose, sodium hydroxide is the most popular solvent. NaOH can cause cellulose to swell and can even dissolve cellulose in a narrow range of the phase diagram. It was found that for cellulose with low to moderate degree of polymerization, the maximal solubility occurs with 8∼10% soda solution. In recent years, researchers found that sodium hydroxide with urea at cold temperature can dissolve cellulose better than sodium hydroxide alone. However, the lack of sufficient understanding of the NaOH and NaOH/urea dissolution process significantly constrains its applications. In order to fully understand the cellulose dissolution in alkali system, there are several aspects of problems that need to be addressed. Our focus in this study is in the interaction of cellulose with alkali solution at low temperatures, the improvement of its solubility, and the effect of hemicellulose and lignin.;Our results from kinetic analysis showed that the dissolution of cellulose in NaOH/urea solution has strong dependence on the temperature of reaction. There is a critical temperature for reaction to take place in the studied solution system. The optimum NaOH concentration in this study is at 6% for high molecular weight cotton linter. The Gibbs free energy of cellulose decrystallization in 6% NaOH with and without urea had a turn point at temperature -15°C. Above and below this temperature, the contribution of adding urea to enhance the solubility of cellulose is opposite.;For solubility improvement, we used enzymatic hydrolysis as a pretreatment to alkali dissolution. The results showed that the enzyme pretreatment greatly enhanced the dissolution degree of cellulose in NaOH/urea solution with much shorter time. Enzyme treatments did not change crystal type and crystal size, slightly increased crystallinity of cellulose and the molecular weight reduced rapidly. High crystallinity did not necessarily result in low solubility, or at least crystallinity alone could not explain the difficulty of cellulose dissolution.;In the study of the effect of hemicellulose and lignin on cellulose dissolution, we found that due to the low lignin content of our raw material, its influence on cellulose dissolution is not significant; the responses of different sugar components in hemicellulose to the dissolution solvents were quite different, in that xylan was quickly removed but mannan were sustained. The bleaching chemicals of sodium chlorite can not only remove lignin, but also degrade cellulose. The solubility increase observed was due to the reduction of cellulose molecular weight rather than the decrease of lignin amount.;In a summary, we have gained new insight into mechanism of the cellulose dissolution in alkali solution with or without urea, especially its dependency on temperature. We also explored a new way to enhance the cellulose solubility and found that a certain component of hemicellulose may have special connection to cellulose that cannot be easily broken by chemical treatments, possibly leading to the difficulty of wood fiber dissolution. |
