Mechanical properties of some pigmented and unpigmented aqueous-based film coating formulations applied to aspirin tablets.

The Brinell hardness and Young's modulus of pigmented and unpigmented films of hydroxypropyl methylcellulose alone, and in combination with either polyethylene glycol 400 (plasticizer) or polyvinyl alcohol, which were applied to aspirin tablets, have been measured. Generally hardness and modulus data showed similar trends. The hardness and modulus of hydroxypropyl methylcellulose fell in the presence of polyethylene glycol 400 as a result of its plasticizing action. On the other hand, the hardness and modulus of the film former rose slightly when polyvinyl alcohol was initially incorporated, probably due to the crystalline phase of the additive, and then decreased when the level of the additive was further raised. Hardness and modulus were higher in films pigmented with talc than in those containing titanium dioxide because of the plate-like shape of talc and its greater interaction with the polymer systems. Some correlation was found between the Young's moduli of the applied films and those of the corresponding free films, with the moduli of the latter two 2-5 times greater. Ageing at 37 degrees C and 75% r.h. was found to cause a decrease in the mechanical properties of the unplasticized film coating systems probably as a result of decreased molecular order and enhanced polymer chain mobility.