Theoretical and experimental investigations into the delamination tendencies of bilayer tablets

Delamination is one major problem in the production of layered tablets, yet there is little knowledge about the physical reasons for this to occur. The aim of this work was to explore the theoretical influence of thermal stresses and strains that can develop during tabletting and to devise an experimental method that can be used to detect delamination tendencies in bilayered tablets. Theoretical considerations have shown that thermal stresses due to development of heat during powder compaction can result in delamination, and this effect is the more pronounced the larger the Young's modulus for the individual layer materials is. Elastic mismatch further enhances delamination tendencies. Experiments on mixed powder beams showed that there is only limited adhesion between particle surfaces of a model drug (acetylsalicylic acid) and model excipient (lactose monohydrate), indicative of limited adhesion between similar interfaces in layered tablets. A three-point bending test was developed to determine the far field stress intensity factor for bilayered compacts. Under the test conditions employed, lactose monohydrate behaved as a brittle material, whereas acetylsalicylic acid demonstrated ductility, which resulted in considerable differences in the far field stress intensity factor values, depending on whether the excipient or the drug formed the downward facing layer during the bending test. Ductile phase toughening was observed when the drug formed the downward facing layer, and hence for bilayer tablets made from these two powders lactose monohydrate must form the downward facing layer during the test. Using the correct test configuration the far field stress intensity factor correctly predicted practically observed delamination between the two material layers. Hence, the proposed fracture mechanics approach could become a formulation tool in the development of bilayered tablets.

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