Altering pore size distributions

Details Altering pore size distributions Altering pore size distributions

1986-10-28

1989-03-21

Spear, Frank

Liquid purification or separation

Filter

Supported, shaped or superimposed formed mediums

21050025, 21050028, B01D 1300

Patent

active

048140830

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to methods of altering pore size distributions of porous separating devices by placing controlled porosity blockages in unwanted sizes of pores or holes found in such devices.
The invention may be applied to a wide variety of porous separating devices such as porous membranes, tubes and hollow fibres which are used for microfiltration, ultrafiltration, dialysis, electrodialysis, diffusion, pervaporation, osmosis and reverse osmosis. For the sake of brevity, such devices shall be referred to as "porous barriers".
A variety of useful processes are based on separations made by selective permeation through relatively thin porous barriers. Many configurations and shapes of the surfaces of such porous barriers are used and the actual pore size distribution includes leaks and holes that may arise during the assembly of devices housing the barriers.
Although separation is not always determined by pore size alone, the presence of outlying pore sizes or unintended leaks is often detrimental and sometimes renders the barrier to be of little use. The inability to avoid pinholes or to plug them without plugging desired pores cause the rejection of a quantity of manufactured product and thus adds to cost. The preferred pore size distribution is always compromised by economic or uncontrollable factors.
The production of the pores in the barrier is often controlled by statistical factors affecting the aggregation of particles so that an undesirably wide range of pore sizes must be made. In a few cases it is the smaller pores which are undesirable. Complex pore size distributions eg: bimodal distributions which cannot be made directly, are also sometimes required.
Pore size distributions are often measured by the "bubble point" method of ASTM 316-70 (reapproved 1977) of the American Society of Testing Methods. The bubble point gives the pressure needed to move the first bubble of gas through the porous barrier when wetted with a liquid of known surface tension. The first bubble comes from the "largest" pore.
In a porous foam the first bubble comes from the pore exit of that transmembrane path, which possesses in cross section throughout its length, a greater ratio of corresponding surface to minimum perimeter than any other path. This precise understanding is vital to the analysis of pore size distributions and rates of fluid flow in the important case of open celled foams. The location of any bubble gas/liquid interface is seldom at the surface. Mostly it is at the most distorted point of a tortuous branching network of channels or at a leaks.
West German Pat. No. 3,312,729 describes accurate apparatus for bubble point measurement for pore sizes in the range from 0.001 to 10 microns.


BACKGROUND ART

East German Pat. No. 149,845 is the only known prior art which discloses the use the surface tension/pore size relationship to control the pore size of individual holes during pore formation. This is described in relation to single sided etching of radiation tracks in polymer foil. The pores of such a foil are not connected into a foam network; they form a simple sieve. The methods disclosed in the East German Patent allow all pore diameters to be regulated by the boundary surface tension between the etching agent and an inert medium. When the pore diameter has become so large that the pressure difference between these two is sufficient to expel the etching agent from the pore, etching ceases. In theory all the pores are the same size so that the pore size distribution is monodisperse, which is not always desired.
There is a need for a method which will find every incorrectly sized pore and then individually and specifically apply a versatile treatment to it.
The known prior art has no direct relevance to the methods and products of this invention which meet this unfilled need.
The present invention differs from known art in that there is no etching nor any step related to the initial production of the pores. Instead there is alteration of existing pore size distributions in an

REFERENCES:
patent: 2714568 (1955-03-01), Teichmann
patent: 3238056 (1966-03-01), Pall et al.
patent: 3331772 (1967-07-01), Brownscombe et al.
patent: 4214020 (1980-07-01), Ward et al.
patent: 4239714 (1980-12-01), Sparks et al.
patent: 4473476 (1984-09-01), McMillan
patent: 4634531 (1987-01-01), Nakagawa et al.

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