Cotton powder for infrared transmission measurements

Abstract

Cotton cut in a Wiley mill to pass a 20-mesh screen was ground in a vibratory ball mill similar to one that has been used in Germany and Holland. Contamination of the cotton was practically eliminated by using a chromium-plated hardened steel jar and chromium alloy balls. The jar holder is held by a slip ring instead of being bolted in place. The jar and holder automatically rotate slowly on their axis, and this motion counteracts the tendency of the cotton in the jar to settle and grind nonuniformly. The jar is cooled by blowing air over it and becomes barely warm to the touch. Five grams of dried cotton disintegrated rapidly during the first 15 minutes of milling. The largest particles remaining after 30 minutes of milling were about 10 microns in major dimension, and there was little reduction of the maximum size with longer milling. Cuprammonium fluidity of the cotton increased rapidly in the first Hour of milling and more gradually thereafter. The carboxyl content of the cotton increased very slowly with time of milling and reached only about 0.2 percent after 8 hours. The moisture regain of the cotton at 65-percent relative humidity at 21° C increased rapidly during the first hour of milling, from 7.1 to 12.5 percent, and then remained practically constant. X-ray diffraction measurements showed that the cellulose was converted almost completely to the amorphous form in 30 minutes, and that it underwent little change thereafter. The powdered cotton obtained from the vibratory ball mill was mulled in mineral oil. The resulting paste was injected into rock salt cells for infrared absorption measurements. The powder worked reasonably well in this procedure, distinctly better than the coarse powders obtained by other methods of grinding. No significant differences were found in the infrared absorption of a regenerated cellulose film, the cotton ground in a Wiley mill to 200 mesh, and cotton ground in the vibratory ball mill for periods of time from 15 minutes to 8 hours. It is concluded that the vibratory ball mill, as modified, reduces cotton, rapidly and completely, to a very fine powder, suitable for infrared transmission measurements and other purposes. Milling results in practically negligible oxidation of the cellulose, but in a marked decrease in degree of polymerization, and in almost complete conversion of crystalline to amorphous cellulose.