Ester hydrolysis of cellulose acetate and cellulose acetate phthalate in aqueous suspension and solution, and solid state
The increasing usage of aqueous film coating in the pharmaceutical industry has generated interest concerning the stability of polymer film forming agents to ester hydrolysis. Cellulose acetate phthalate (CAP) and cellulose acetate (CA) are examples of polymers which either have or are being considered for film coating applications in the form of aqueous dispersions. This study was undertaken to determine the aqueous stability of these polymers to ester hydrolysis. The stability of CA suspensions to ester hydrolysis was examined over the pH range of 2.15 to 10.0. Regions of maximal and minimal stability were identified, and the pH at which the rate of hydrolysis was minimal was calculated. The effect of the degree of substitution on polymer stability was determined by comparing the rates of hydrolysis for CA suspensions of 32.0, 39.8 and 43.6% acetyl content. The stability of the acetyl and phthalyl ester of CAP to hydrolysis was determined over the pH range of 2.2 to 10.0. Using pseudo-first order rate constants, a pH-rate profile was constructed for each ester. The profile for acetyl hydrolysis was very similar to that observed for CA. Phthalate hydrolysis proved to be a much more complex process, involving intramolecular catalysis. The activation energies for the hydrolysis of each ester moiety was calculated. The rates of phthalyl and/or acetyl ester hydrolysis were determined for CAP and CA powder and film samples. The samples were stored under various conditions of temperature and relative humidity. Acetyl hydrolysis was much greater for CA film than powder samples. The rate of acetyl hydrolysis was greater than that for phthalyl loss for CAP powder samples. Activation energies were calculated, verifying these observations. Solid state nuclear magnetic resonance was used to examine the extent of ester hydrolysis for CA and CAP powder samples.
Peck, Purdue University.
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