Plasticization of cellulose ethers used in the film coating of tablets

Plasticizer/polymer interactions have been studied by measuring the intrinsic viscosities of both ethyl cellulose and hydroxypropyl methylcellulose in a series of dialkyl phthalates and in a series of liquid glycols respectively. A correlation was found between the intrinsic viscosity of the polymer/plasticizer solutions and the tensile strength, elongation at rupture and work done in stressing to failure of cast films—the mechanical properties being at a minimum when the intrinsic viscosity was at a maximum. This correlation held only within a homologous series of plasticizers and none was found for plasticizers of different structures. A relationship was found between the lowering of a calculated glass transition temperature of hydroxypropyl methylcellulose in the presence of the plasticizers propylene glycol, polyethylene glycol 200 and glycerol and the intrinsic viscosity of the corresponding solutions—the higher the viscosity the greater the lowering of the transition temperature.