Abstract
An extensive heterogeneous kinetic study was carried out for acid catalysed propionylation of cotton cellulose. In this study a series of metal chlorides and sulphuric acid as catalysts were used, and the effect of temperature, solvent medium, propionylation solution composition, and catalyst concentration on propionylation were investigated. The degree of propionylation was determined by a saponification method and, where necessary, confirmed by infrared spectroscopy and microanalysis. The reaction process was described on the basis of an initial, first, and second stage. Up to 15% of the completed reaction appeared to take place in the initial stage which is suggested to be diffusion controlled. Whereas the first and second stages were shown to be nondiffusion ones since they obeyed pseudo first-order kinetics with respect to the reacting hydroxyl groups. A mechanism and transition state complex were proposed for the propionylation reaction. Acid catalysed propionylation conditions (catalyst, temperature, solvent or non-solvent and propionylation solution composition) were investigated for the preparation of high molecular weight cellulose tripropionate in the shortest time. Optimum conditions were found for both the solution and fibrous processes, with molecular weights of samples being determined viscometrically in chloroform. Solubility behaviour and solvent power of cellulose tripropionate in a large number of solvents was studied. The results were explained on the basis of possible types of interaction between solvent and polymer. Fractionation by the non-solvent addition method was employed for cellulose tripropionate, and the solvent-precipitant systems of propionic acid-water and acetone-petroleum ether were investigated. A satisfactory fractionation was only obtained in the case of the acetone-petroleum ether system. Number-average molecular weights of fractionated cellulose tripropionate samples were determined osmometrically. Solution viscosities of these samples were measured in chloroform from which the Mark-Houwink constants and molecular conformation parameters were obtained. The cellulose tripropionate chain appeared to be more flexible than that for cellulose triacetate. The applicability of gel permeation chromatography for measuring the molecular weight of cellulose tripropionate was considered using different methods of calibration with standard polystyrene samples. A hydrodynamic rather than Q-factor method of calibration was found to give the best agreement with osmometric molecular weights for sharp cellulose tripropionate fractions.