Drug
Delivery Device Using Polymers With Improved Tribology
POLYMER COMBINATIONS
When identical unmodified polymers
slide against each other, there is a tendency toward
high friction. In general, it is best to avoid these
pairings. However, in some cases these combinations
are favorable because of other interactions and/or
requirements, i.e. laser marking, suitable
sterilization techniques or mechanical specifications.
For combinations of unmodified POM-on-unmodified
POM the static and dynamic coefficient of friction
shows a large difference resulting in a high risk of
noise.
Changing to a combination of
dissimilar materials may provide a solution. For
example, a POM-on-POM combination may generate noise
whilst a POM-on-Polyester pairing may run silently
(Figure 3). This is due to the relatively small
difference between the static and dynamic coefficients
of friction in the POM-on-Polyester combination.
Figure 3: Sliding behavior of POM/POM vs. POM/Polyester,
dotted lines: dynamic CoF, solid lines: static CoF
THE USE OF EXTERNAL LUBRICATION
On initial examination, the addition of external
lubrication is attractive. Lubricants on the surface
of the part create a film during operation and provide
exceptionally low friction between sliding components,
as illustrated in Figure 4. External lubrication is
often adopted as a cost saving measure to reduce the
overall bill of material in a device, or as a
problem-solving step if a design exhibits unwanted
noise in operation due to stick-slip. Lubrication can
either be achieved through coating parts (postsiliconisation)
or discrete lubrication. The downside of external
lubrication is the management of the secondary
process, potential migration of the lubricant to
undesired locations in the device, and chemical
resistance issues. It is difficult to tell whether or
not lubrication will be consistent from device to
device and whether surface lubricants will remain in
the required place to do their job after protracted
storage.
Figure 4: Effect of external
lubrication (silicone oil), dotted lines: dynamic CoF,
solid lines: static CoF |