Anodisc™ Inorganic Aluminum
Oxide Membrane Filters
Polypropylene support ring
The 25 and 47 mm diameter Anodisc™ Inorganic Aluminum Oxide Membrane Filters are made with a
polypropylene support ring that runs around the circumference of the
membrane. Because of the high fragility of the membrane itself, this
support ring permits one to pick up the membrane filter with normal
membrane filter tweezers
(which we highly recommend using for these filters) without fear of
damaging the fragile membrane area. Note that the 13 mm diameter membranes come without support ring.
Support ring thickness: 45µm
In most instances, in most holders, the polypropylene support ring is
completely isolated from the flowing liquid. Well, it might not be 100%
isolated, of course, but the point is, that when used even with liquids
normally a solvent for polypropylene, the dissolution rate is so slow that
in most instances it can be dissolved.
Note that the membrane itself is 60µm, but the ring is only 45µm.
This means that in the area where the support ring is in contact with the
membrane there is a greater thickness. Also, the support ring has a
slightly larger diameter than the membrane, so there is an area where
there is only PP and no membrane (but very small).
We have been asked about how one would remove the polypropylene support ring.
Perhaps one might first consider why one would ever want to do this in the
first place! We ourselves are not sure, but in a few instances, the inorganic
membrane was going into a UHV system and it was required that no organics be
in the system. In another case, the membrane was to be heated in a UHV system
and it would have been undesirable to have degrading polypropylene coming off
in the environment of the UHV system.
So there are instances when one would want to remove the polypropylene
support ring.
But we do want to make clear that in use, the main mechanism of membrane
failure, without the support ring, is crack propagation from the edge.
That is why the elastic polypropylene support ring is so important because
it stops this from happening. It also provides a simple and convenient way
and place to pick up the membrane with tweezers. We do know for a fact that
the membrane without the support ring, when picked up with tweezers, cracks
very easily. People who have tried this report a failure rate of 35-50% of
the membrane filters.
So we are in fact sometimes asked about making the membranes without the
support rings, but there just seem to be too many difficulties just in
shipping the product from us to you without its getting broken.
Although we have not tried it ourselves, we have had reports that when
solvents for polypropylene are used to dissolve away the polypropylene,
surface tension forces, if one is not really careful, can cause damage and
break up of the membrane itself. We are eager to hear from anyone who might
have solved this problem of liquid dissolution as the method of removal of
the support ring.
The only way we know of for sure is the use of dry plasma etching, and for
that we recommend our own SPI Plasma Prep™ II plasma etcher. Using
oxygen as the etching gas, the polypropylene support ring can be removed in
about ten minutes or less. Is this something SPI could do for a customer and
then send membranes with the ring already removed? We have considered it,
but then there is the challenge of shipping such a fragile sample and having
it arrive at the customer's laboratory in one piece! We think it might be
possible by using what are called the
SPI Membrane Boxes.
However, we do not ourselves have the requisite experience to be able to
guarantee the arrival of such membranes in this packaging system in one piece.
We hope this information about the support ring material, for those
concerned about it, will be useful. This kind of discuss is a perpetual
"work in progress" so any comments you have have to offer would be greatly
appreciated and we would share them with future readers of this page.
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