Developing hydroxypropyl methylcellulose/hydroxypropyl starch blends for use as capsule materials

Blends of hydroxypropyl methylcellulose (HPMC) with up to 70% hydroxypropyl starch (HPS) were developed for use as hard capsule materials. Polyethylene glycol (PEG) was used as both a plasticizer and a compatibilizer in the blends. In order to prepare hard capsules for pharmaceutical application using the well-established method of dipping stainless steel mold pins into solution then drying at certain temperature, equilibrated solutions with higher solids concentration (20%) were investigated and developed. The solutions, films and capsules of the different HPMC/HPS blends were characterized by viscosity, transparency, tensile testing, water contact angle, SEM, as well as FTIR. The results showed that the blend system is immiscible but compatible in certain degree, especially after adding PEG. The hydroxypropylene groups grafted onto both cellulose and starch improved the compatibility between the HPMC and the modified starch. The higher viscosity of starch at lower temperature improved the viscosity balance of the system, which enlarged the operation window for the dipping-drying technique. The PEG increased the transparency and toughness of the various blends. By optimizing temperature and incubation time to control viscosity, capsules of various blends were successfully developed.