Effect of the drying temperature on the structural nonequilibrium of cotton cellulose

Abstract

A method for quantitatively evaluating the least mobile fraction of moisture sorbed by cotton fibres with the kinetic curves of the change in the desorption process parameters is proposed. The hypothesis that the separated form of the most slowly removed water is localized in sections of the surface layer of crystallites decrystallized on drying is substantiated. Based on an analysis of the temperature curve of the amount of difficult-to-remove moisture, the temperature of the relaxation transition (120-125°C) related to the cooperative mobility of the kinetic elements in the amorphous regions of cellulose was determined.The urgency of studying the reaction of cellulose with water is due to the wide practical importance of cellulose materials and the dependence of their properties on the moisture content. The causes of the important effect of the drying conditions on the reactivity and processability of cellulose are still insufficiently clear. In addition to the "keratinization" effect [1], the changes in the supramolecular structure of cellulose materials when their moisture content is varied can also play a certain role in this phenomenon [2,3]. Together with X-ray structural analysis (XPA), important information on the change in the structural state of dried polymers can also be obtained with sorption measurements [4]. We investigated the kinetic features of the final stage of desorption of water from cotton cellulose in the 20-100°C temperature region and established their correlation with the structural rearrangements in the polymer.The sorption measurements were conducted on a TG 209 F1 thermal microbalance from Netzsch. Moistened samples of cotton fibres were placed in a thin-walled platinum crucible with a perforated lid which ensured removal of desorbed water vapors. At the beginning of the experiment, the measurement cell was heated to the given temperature at the rate of 15°/min and then operation of the furnace in the isothermal mode was ensured. The error of measuring the sample weight and temperature was 5 . 10 -3 mg and 0.1°, respectively. The experimental data were processed with software for thermal analysis (Proteus Software).The curves in Fig. 1 illustrate the character of the change in the process parameters of desorption of water from cotton fibres at 60°C. Due to the cooling effect of evaporation of the moisture, the temperature of the moist sample (curve 7) during drying was always lower than the temperature of the heat carrier (argon) which is only attained at the end of drying. Due to this interdependence of mass and heat transfer processes, the temperature curves can more finely reflect all of the features of binding of moisture with the fibre in comparison to the integral sample weight loss curve (TG curve). For this reason, they are frequently used together with the drying rate curves (DTG) to determine the position of the critical points corresponding to the boundaries of the periods of removal of different forms of sorbed water from the investigated polymer materials [5].According t...