Drug Delivery Device Using Polymers With Improved Tribology

*First cycles simulate running-in effects, mainly important in single-use medical devices. Running-in effects reflect breakaway forces, lubricant contamination, asperities and injection molding effects on virgin surfaces. On the other hand, multi-use injectors and inhalers focus on long-term performance, which is analyzed during the second part of the measurement, called “steady contact”.

THE USE OF INTERNAL LUBRICATION

Internal lubrication of polymers is achieved either during production via the use of master batches in the injection molding process, or as a ready-to-use compound directly from the raw material supplier. It is difficult to match the extremely low levels of friction achievable by surface film lubrication. Regardless, when the right lubricant is used for the combination of base polymers involved, excellent results can be achieved. The type and form of the lubricant is critical. Some lubricants have excellent dispersion within the polymer (i.e. waxes) and migrate to the surface of the polymer during injection molding. Others form discrete pockets of lubricant (i.e. PTFE) requiring a running-in phase to get some initial surface wear to be exposed to optimum lubrication potential. Figure 5 gives an overview of the sliding performance of unmodified POM-on-unmodified POM, resulting in the highest coefficients of friction as well as versus POM with different tribological modification (internal lubricants).

Figure 5: The influence of different internal lubrication on coefficient of friction (CoF), dotted lines: dynamic CoF, solid lines: static CoF

Some of the modifiers that improve sliding performance in POM strongly influence the mechanical properties of the material as depicted in Figure 6. While the addition of silicone oil provides excellent tribological performance, the oil can also result in a

Figure 6: The influence of different internal lubricants on mechanical properties