Determination of primary adsorbed water in cotton fibers by drying techniques

Abstract

AbstractA number of techniques for the determination of nonsolvent, or hydrate, water upon the cellulose molecule have been advanced. The results have been expressed as both maximum and stoichiometric amounts of water as hydrate. The values determined have been limited, however, to a system in which the fiber was dried prior to reaction and determination of the adsorbed water. The existence of hydrate has been attributed to chemical combination of water at the hydroxyl groups of the cellulose. It is well known that during the drying of many materials two rates of drying are encountered, a constantrate period and a falling‐rate period. The constant‐rate period is devoted to the evaporation of free water and the falling‐rate period to the removal of “bound” or capillary water. Since the moist fiber may contain two types of “bound” water, it was theorized that a second falling‐rate period should be in evidence during the drying of a fibrous material. Experimentation showed a definite second falling‐rate period, which was attributed to the evaporation of primary adsorbed or hydrate water. The method utilized in this work was a drying technique employing a convection type oven over which was mounted a torsion balance making it possible to take continual weight change readings while maintaining the sample in the conditioning atmosphere. The determinations were made at a temperature of 105°C. Six fiber–water systems were investigated which included combinations of drying and mercerizing of alcohol–benzene extracted cotton fibers. The data obtained were plotted as drying rate versus per cent dry fiber and the results were expressed as the molar ratio of H2O: C6H10O5 at the point of intersection of the two falling‐rate lines. The molar ratio found for the several systems investigated were: extracted cotton 1.65; extracted, dried, cotton 0.52; extracted, mercerized cotton 2.12; extracted, mercerized, dried cotton 1.08; extracted, dried, mercerized cotton 1.04; and extracted, dried, mercerized, dried cotton 0.89. The values obtained agree closely with the known quantities advanced by other authors employing different techniques. From the values obtained it appears that drying greatly reduces the ability of cellulose to sorb water. Mercerization results in increased primary adsorbed water uptake. However, drying prior to mercerization reduces the number of hydroxyl groups available for hydrate formation over that of a fiber which is undried before mercerization. It seems that regardless of subsequent treatment in a fiber–water or fiber–water‐NaOH system, prior drying reduces the water taken up as hydrate over that taken up by a fiber which has not been dried. The results indicate that this method permits a quantitative determination of primary adsorbed water in fiber–water systems which was impossible to determine by methods previously employed.