Friction in ultra-thin conjunction of valve seals of pressurised metered dose inhalers

Abstract

In many drug dispensing devices, such as syringes and inhalers, a rubber disk is used as a seal. During device actuation the seal is subjected to friction which in turn causes its deformation. This can lead to suboptimal performance of the device and consequent variability in delivered dose. Seal friction is complex, arising from adhesion of rubber in contact with the moving interface, viscous action of a thin film of fluid and deformation of seal asperities. Therefore, the first step in understanding the conjunctional behaviour of rubber seals is the fundamental study of mechanisms of friction generation. A developed model can then be validated against measurements. The validated model can then be used to predict product performance, robustness and variability due to manufacturing tolerances. A friction model, based on the aforementioned mechanisms, for prediction of seal friction has been developed and validated against measured friction tests performed on both nano and component level scales. Pressure changes in the metering chamber have been taken into account in the model. Friction data are presented for nitrile rubber, using a silicon nitride AFM tip for nano-scale interactions and polybutylene terephthalate (PBT) for asperity interactions at a component level, where a traditional friction test apparatus is utilised. Reasonable agreement is found between measurements and model predictions for the nano-scale coefficient of friction of rubber against silicon nitride. Similarly, good agreement has been obtained for the mean coefficient of friction of rubber against PBT. It was found that the model was capable of predicting static friction coefficient reasonably well and the contribution to the coefficient of friction was mostly due to adhesive friction. The inputs of viscous and ploughing friction were negligible.