Studies on never‐dried cotton

Abstract

AbstractThe differences in the porous and crystalline structure of cell walls in never‐dried and nature‐dried cottons have been examined by measurements of dye adsorption and desorption, centrifugal liquid retention, and decrystallization due to swelling in sodium hydroxide. Equilibrium dyeings of these two cottons have been carried out at 50 and 60°C with two direct dyes, Chlorazol Sky Blue FF and Chrysophenine G, and the adsorption isotherms obtained. The dye uptake at limiting saturation is found to be quite large for never‐dried cotton as compared to nature‐dried cotton, indicating a larger number of sites available to dye molecules in the former sample. However, dyeing parameters such as affinity and differential heat of dyeing are found to have lower values for never‐dried cotton. This is attributed to the “frozen” structure of a large amount of water held by never‐dried cotton, which retards the adsorption of dye molecules. Studies on retention of liquids (glycerol and water) by cotton by use of centrifugation techniques reveal a larger amount of pore volume in never‐dried cotton than in nature‐dried cotton. X‐Ray studies on decrystallization of cotton by swelling in NaOH indicate that the phase transformation to cellulose II in never‐dried cotton is complete at 25% (w/w) concentration of NaOH, whereas under identical swelling conditions about 10% residual, unconverted cellulose I is found in the case of nature‐dried cotton. A somewhat similar anomaly is found in the dye desorption measurements. Under conditions when the dye can be completely stripped from nature‐dried cotton, the never‐dried cotton has been shown to retain about 50% to 80% of the adsorbed dye. These observations are attributed to irreversible pore closure during drying of never‐dried cotton. Structural collapse occurring during drying of never‐dried cotton, after subjecting it to solvent exchange with a large number of organic liquids, was studied by x‐ray diffraction, optical microscopy, and centrifugal liquid retention techniques. It was demonstrated that the structural collapse is proportional to the polarity of the organic solvent employed in the final exchange of never‐dried cotton, prior to drying. It is concluded that the structural collapse and the development of inaccessible zones in fiber during drying can be reduced if the water in never‐dried cotton is exchanged with a nonpolar solvent.