Influence of Aging on the Physical-Mechanical Properties of Acrylic Resin Films Cast from Aqueous Dispersions and Organic Solutions

Abstract

The physical-mechanical properties of the enteric copolymers, poly(methacrylic acid, ethyl acrylate) Eudragit® L100-55, and Eudragit® L30D have been investigated. Free films of the copolymer containing varying levels of glyceryl triacetate (triacetin) and citrate ester (Citroflex®) plasticizers were prepared by both aqueous and solvent casting techniques. Conditioned films were stored at different humidities and temperatures for predetermined time periods prior to testing. Free films with plasticizer concentration ranging from 0 to 30% by weight of the polymer demonstrated that physical aging at room temperature resulted in physical-mechanical changes as the stress-strain curves indicated a decrease in the percent elongation with increases in the tensile strength. Films prepared from the aqueous latex approached a constant state of equilibrium at a faster rate than films prepared from isopropyl alcohol, where the mechanical properties approach a relatively constant value. Free films containing varying amounts of the plasticizer, triethyl citrate, were stored under different humidity and temperature conditions. In addition, a headspace-gas chromatography (HSGC) method was developed to correlate solvent residues with mechanical properties. The results indicated that an increase in temperature and humidity accelerated the rate of solvent evaporation as a function of time. High vacuum and low humidity conditions were demonstrated to be ineffective conditions for solvent removal. Solvent residues were found in all films after 48 hours of exposure to temperatures up to 60°C and/or exposure to relatively high humidity conditions. A direct relationship between plasticizer content and rate of solvent evaporation from the film was established. Higher concentrations of the plasticizer were found to augment the evaporation of the solvents. This enhanced evaporation was due to the stronger molecular interaction forces between the polymer and the plasticizer than between the polymer and the solvent.