Manufacturing Process of SPI-Pore Membrane Filters
No one does it like SPI does it!
SPI-Pore Polycarbonate Membrane Filters are made in a complex process
involving several different steps, starting with the procurement of the
very highest purity monomer for the special manufacture of the starting
polycarbonate film material. For example, even the slightest relaxation
of the high demands for ultra purity of the starting monomer could very
easily result in contaminant particles ending up in the final membrane
product.
Once the film is made, it is then exposed in a very controlled way to
charged particles in a nuclear reactor. Only a relatively small number of
sites in the world have this kind of capability. At the charged
particles pass through the polycarbonate film, they leave what are called
"sensitized tracks". The number of these "tracks" per unit area depends
primarily on the length of time the film is in the reactor. Since time
can be tightly controlled, so can also be tightly controlled the pore
density. While all SPI Pore Polycarbonate Membrane Filters have the same
pore density, it is possible to produce product of different pore
densities, for the same pore size although for favorable economics, one would have
to be prepared to produce quite a large amount of material.
The next step in the process involves the etching of the "sensitized
tracks" with a strong caustic etchant. By varying the time of exposure
to the etchant, the concentration of the etchant, and also the
temperature, the irradiated film can be etched to a precisely controlled
pore size. The batch to batch reproducibility is quite outstanding, a
feature most important when trying to reproduce results when using
membrane filters from different production runs.
We are often times asked about providing information pertaining to the etching
process by which the pores are produced from the polycarbonate membrane film material.
The reason most often cited is to produce from one batch of starting material, pores
of slightly differing diameters, generally in conjunction with filtration media
research projects.
And indeed, the pores can easily be made bigger with sodium hydroxide but the
key is to know when to stop the process and how to stop it. In the commercial
process, there is used a number of in-line air flow measurements that are used
to determine when to stop the process, otherwise the NaOH will just keep eating
away the membrane filter.
So the experimental challenge when contemplating this kind of pore size enlargement
is to figure out a way to have a very controlled etch of the pore coupled with
the ability to measure the progress of the etching and then to stop it the moment
it has been enlarged to its target diameter.
Another one of the other parameters that must be tightly
controlled at this
point is the geometry of the cross-section going through the etched
"track". For example, is the cross-section uniformly cylindrical, or is
there a constriction at the center slowing down filtration or is it
bowed out at the center slightly (to increase flow rate since the
membrane is more wetable, an obviously desirable feature).
But this is not all. There is more. There is an intrinsic propensity
of polycarbonate film at this point to collect a static charge, that is,
just from rubbing against itself, enough of a charge can be built up the
film would want to stick to itself. Or particles being captured from
air would tend to be repelled by the membrane filter. Consequently, all
SPI-Pore Polycarbonate Membrane Filters are exposed to a special
anti-static treatment during the manufacturing process so that as a
final step before packaging, such charges are completely discharged,
thereby minimizing such effects. Naturally, we are not eliminating
them, but users will find that relative to alternatives, the charge
situation is quite well under control.
Note: For certain situations one should be using the carbon black-filled
plastic membrane filter holders.
Have need for out-of-the-ordinary membrane filter media, for example,
samples of varying pore sizes? To special order such samples would
require quite a substantial investment. However, we might have some
sample material , in strictly limited quantities, that might be either
close to or exactly what is desired. Therefore
let us know your
special needs and let us come up with a suggestion.
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Friday October 10, 2008
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