09. Apr 2018 Determination of the degree of siliconisation on rubber seals

 Anita Thijs, R&D Senior Manager Material Development / Surface Treatments and Simon Kervyn, R&D, Manager Materials and Surface Treatments discuss new research findings on the siliconisation methods used on rubber seals and components used in pharmaceutical packaging.
In drug industry, the demand for efficient seals for pharmaceuticals and biological fluids in vials and syringes is very high. Components for syringes and pre-filled syringes like syringe barrels and plungers have to be siliconised as to ensure a low break loose force and a smooth gliding. A thin film of silicone oil is also important to improve the machinability and to avoid clumping of rubber stoppers, plungers, needle covers, and tip caps during shipment or upon steam sterilization in bags.

The degree of siliconisation is adapted as a function of the type of seal. To limit the generation of sub-visible particles and the interaction with protein-based biological drugs it is crucial to keep the siliconisation level as low as possible. A reliable quantification of silicone oil film thickness is important to ensure the quality level and consistency of rubber pharmaceutical products.

Several techniques to determine the degree of siliconisation are described in the literature. Most methods involve the extraction of the silicone oil from the surface of the rubber component and subsequent quantitative analysis of the extract. At Datwyler we use Atomic Absorption Spectroscopy (AAS), and Fourier Transform Infrared spectroscopy (FTIR). The usage of the last one is preferred because it is a quick and reliable method. It provides several advantages particularly with regard to the RTU (Ready To Use) and RfS (Ready for Sterilization) components.

1. The AAS method
The identification of the silicone level is through the determination of the silicone atom content. The silicone oil is removed from the stopper by a solvent (MIBK: methyl isobutyl ketone) and the solution is then analysed by atomic absorption spectrometry (AAS) technique.

2. The FTIR method
The FTIR method takes advantage of the fact that upon excitation by an infrared beam, molecular bonds vibrate at their specific frequency, absorbing the energy. The silicone content can therefore be determined by the intensity of the absorption peak. The more silicone in the sample, the higher the peak will be. These measurements are performed either directly on the sample or on a solution after extraction of the silicon oil with dichloromethane (DCM). The FTIR surface scan method is not quantitative but it is a good tool to get semi-quantitative information about the siliconisation level. The validated FTIR method using DCM as extraction medium is a quantitative method. The advantage of this quantitative FTIR method (with DCM) compared with the AAS method (with MIBK) is the significant shorter analysis time.