Acetylation of cellulose I and II studied by limiting viscosity and X‐ray diffraction

Abstract

AbstractWhen cellulose triacetates and some hydrolyzed acetates are boiled in 2.5N hydrochloric acid there is no residue. Under the same conditions cellulose is hydrolyzed, and a residue is obtained with a limiting viscosity that is related to the average length of the cellulose crystallites. These findings are combined to develop a method for studying the progress of acetylation through the amorphous portion of cellulose and into the crystallites, and to investigate the relative reactivities of cellulose I and cellulose II. Acetates were made from cotton and wood cellulose by a “fibrous” (heterogeneous) esterification involving sulfoacetic acid or perchloric acid catalyst in acetic acid‐acetic anhydride; the final acetyl contents (10–41%) were attained by stopping the reaction at various points short of the triester (rather than by hydrolyzing a triester). When these acetates were boiled in 2.5N HCI they did not disappear completely, and the residues were cellulose I, indicating that cellulose acetate had been removed. With increasing acetyl the yield of residue decreased, and beyond about 33% acetyl the viscosity and x‐ray measurements showed that the length and width of the crystallites decreased. However, when a nonsolvent such as toluene was added to the acetylation medium, the limiting viscosity did not change over the same acetyl range (up to 40%). Samples of varying acetyl values were taken during a regular acetylation of cotton linters in a mixer with sulfuric acid catalyst. X‐ray studies of the residues obtained by boiling the acetates in 2.5N HCI revealed the presence of unreacted cellulose I even after 40% acetyl had been reached. This explains why the manufacture of cellulose esters from cellulose I requires complete esterification before they are hydrolyzed to the desired acetyl level. It was shown that there is a distinct difference between the acetylation reactivity of cellulose I and cellulose II. This indicates the importance of avoiding cellulose II formation during the refinement of cellulose for the manufacture of cellulose acetate in a process involving activation with acetic acid.