THE MORPHOLOGY OF BLENDS OF ETHYLCELLULOSE WITH HYDROXYPROPYL METHYLCELLULOSE AS USED IN FILM COATING

Near-equilibrium morphologies of thin films (< 0.2 mu m) of ethylcellulose-rich incompatible blends of ethylcellulose (EC) with hydroxypropyl methylcellulose (HPMC) consist of HPMC-rich macroscopic polydisperse domains (3-approx. 20 mu m) dispersed in an EC-rich matrix implying complete leaching of the water soluble phase. However, for thick films (> 100 mu m) complete leaching was observed only for systems with above 60% w/w HPMC, whereas all other systems showed some HPMC retention. The observed HPMC leaching behaviour is attributed to deviations of the phase decomposition from equilibrium during film formation arising from the rate of viscosity increase and changes in the solvent system composition.