Kinetics of cellulose dissolution in n-methyl morpholine-N-oxide and evaporative processes of similar solutions

by 1980- Tierney, John William

The lyocell process is an environmentally friendly process for producing regenerated cellulose fibers, but is not entirely understood. The lyocell process uses the hygroscopic solvent N-methyl morpholine N-oxide (NMMO) to dissolve cellulose; the resulting solution is often termed a lyocell solution [1-4]. It is the objective of this study to better understand the process by which cellulose dissolves and the nature of lyocell solutions. By observing the disappearance of cellulose fibers into the solvent, rate data may be obtained from which kinetic parameters may be developed. Additionally an independent method for determining the concentration of cellulose in lyocell solutions was desired so as to better to gauge the effect of concentration on the behavior of the solution. Water affects the behavior of NMMO, making it an important factor in the lyocell process. The water content in lyocell samples may be determined by a number of methods including NMR spectroscopy and Fischer’s method. Unfortunately, these methods each require additional chemicals that add to the cost of the analysis. Therefore a novel method was sought for determining the water content of lyocell samples without the use of additional chemicals. Samples of NMMO, some containing dissolved cellulose, were subjected to thermogravimetric analysis on a Pyris 1 TGA to observe the evaporative process and note any effects of cellulose on that process in an effort to develop a rudimentary approach to determining water content on lyocell samples. Additionally, the dissolution of cellulose into NMMO was observed under a Fourier Transform Infrared Spectrometer and a light microscope. Digital photographs iii with corresponding time measurements were taken of the dissolving cellulose that resulted in dissolution data for single fibers. This was done at several temperatures to extract rate constants for the dissolution process. The results of this project confirmed that cellulose depresses the melting point of NMMO monohydrate and led to a novel method for determining water content in lyocell samples. Detailed mid-infrared spectra were collected for cellulose, NMMO monohydrate, and lyocell samples which were used to develop a predictive model for determining cellulose content in lyocell solutions. Finally, the temperature and surface area dependence for the process of cellulose dissolution in NMMO monohydrate were demonstrated and a rate constant and Arrhenius parameters for the process were obtained. An examination of the phase behavior of NMMO at the onset of cellulose solubility would aid in understanding the dissolution process as would a DSC analysis of NMMO crystallization versus water content. A more detailed multivariate analysis of mid-infrared spectra from lyocell solutions may be performed in the future to improve the predictive model. iv