The fate of silicone oil during heat-curing glass siliconization--changes in molecular parameters analyzed by size...

The siliconization of pharmaceutical glass containers, usually for parenteral formulations, is performed in a so-called heat-curing process using diluted aqueous emulsions of medical grade silicone oils. To do this, the emulsion film is spread on the inner container surface, followed by an application of dry heat at temperatures above 300 degrees C. Water and surfactants are removed by degradation and vaporization, while the thermostable poly(dimethylsiloxane) (PDMS) is left on the surface. In the present study, heat-cured siliconized glass containers of two different types were solvent-extracted to obtain material of heat-treated PDMS. These samples were analyzed by size exclusion chromatography (SEC) and high-temperature gas chromatography (GC) with special respect to low molecular-weight siloxanes (LMWS). By comparison with the untreated starting materials, significant changes in the molecular weight distribution (MWD) of the silicone oil were revealed. Almost all of the LMWS present in untreated materials were not detectable in the heat-cured extract of a 100 cSt. Baysilone silicone oil. Small amounts of PDMS-molecules, with chain lengths of 25 up 45 siloxane units, were traceable. The examination of a second product of higher viscosity yielded unexpected results. The heat-treated extract contained none of the siloxanes that were detected in the starting material. Siloxanes of chain lengths of up to 45 units having molecular weights of over 3000 g/mol could not be found after the siliconization process. This led to the conclusion that not only vaporization effects must be responsible for their absence, but also that silicone suffers from a heat-induced degradation. The results of SEC and GC analysis were supported by each other. The whole molecular weight distribution and four distinct fractions were characterized by SEC, while the GC analysis was capable of a high-resolution view into the LMWS fraction below 3500 g/mol. In conclusion, the benefit of the heat treatment is that no LMWS, a source of toxicological concern, remain in the respective containers. On the other hand, an increase in molecular weight and viscosity of the silicone oil, and thus a possible change of the lubricating properties, is likely to happen through removal of LMWS. But these changes probably have no impact on the hydrophobic surface behavior of silicone-treated glass.