Liquid purification or separation – Filter – Material
Reexamination Certificate
2001-05-07
2004-11-30
Walker, W. L. (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500360, C264S046100, C264S290200, C264S288800, C428S304400, C428S315500
Reexamination Certificate
active
06824680
ABSTRACT:
BACKGROUND OF THE INVENTION
In recent years, membrane separations have been used extensively in commercial processes such as gas separation, liquid separation, wastewater treatment, etc. A membrane is a thin semipermeable barrier that is capable of separating components of a chemical solution or particles from a fluid as a function of their chemical and physical properties when a suitable driving force is applied across the membrane. Membranes can control species transfer rates from one region to the other. Microporous flat film membranes based on homopolymers for such processes can be produced via a melt process followed by a stretching step. The process used for producing this type of microporous films is well known (e.g., M. L. Druin, J. T. Loft, and S. G. Plovan, “Novel open-celled microporous film,” U.S. Pat. No. 3,801,404 (1974); H. S. Bierenbaum, R. B. Isaacson, M. D. Druin, and S. G. Plovan, “Microporous polymeric films,”
I
&
EC Prod. Res. Develop
, 13, 2 (1974); H. S. Bierenbaum, L. R. Daley, D. Zimmerman, and I. L. Hay, “Process for preparing a thermoplastic microporous film involving a cold stretching step and multiple hot stretching steps,” U.S. Pat. No. 3,843,761 (1974); and Brazinsky, W. M. Cooper, and A. S. Gould, “Process for preparing a microporous polymer film,” U.S. Pat. No. 4,138,459 (1979)). However, to date, membranes having pore size as small as about 1 nm and ranging up to about 200 nm that can be used in severe chemical and high temperature environments have not been produced by a melt process involving multicomponent, multiphase systems.
It is toward the fabrication of microporous flat film membranes having a pore size as small as 1 nm and having significant chemical resistance and thermal stability based on immiscible polymer blends, made by a melt process and post-film-forming treatments that are environment-friendly and economically viable, that the present invention is directed.
The citation of any reference herein should not be deemed as an admission that such reference is available as prior art to the instant invention.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention extends to a microporous membrane which is prepared by a melt process from an immiscible blend of a major and minor component, optionally further including a compatibilizing block copolymer, and extruded into a precursor film, the precursor film subsequently stretched to provide a stable reticulated or interconnected network of microcracks or crazes throughout the membrane, with a resultant porosity and pore size. The major component of the immiscible blend is preferably a polymer or a copolymer. The major component may be a polyolefin polymer, such as by way of non-limiting examples, polypropylene, polyethylene, and poly(4-methyl-1-pentene). The minor component of the immiscible blend is a component immiscible with the major component and is preferably a polymer, such as an immiscible polyolefin, polystyrene, a polyester or a copolymer, and may be present at about 1 percent to about 40 percent by weight of the total. A compatibilizing block copolymer, selected to increase the compatibility between the immiscible major and minor polymers or copolymers, may optionally be present at about 0.5 percent to about 25 percent of the total. An example of a compatibilizing block copolymer for a blend of polypropylene with polystyrene is hydrogenated styrene isoprene/butadiene block copolymer (SEEPS). Optional additional components may be included in the blend, such as a monomer, oligomer or surfactant, to impart certain characteristics to the surfaces of the microcracks.
In a process for preparing the microporous membrane, melt blending is carried out to uniformly disperse the minor component within the major component, including the optional compatibilizing block copolymer, and a non-porous precursor film is formed therefrom, for example by extrusion. The nonporous precursor film may be about 50 to about 300 micrometers in thickness, and comprises a matrix of major polymer in which inclusions or domains of the minor component are uniformly dispersed. Following extrusion, the precursor film is uniaxially or biaxially stretched in at least two steps to a final dimension by about 100% to about 700% with respect to the original dimension. A first cold-stretching step is performed at a temperature from about 15° C. to about 25° C., increasing the dimension of the precursor film in the stretching direction about 20 percent to about 30 percent, and the film is held under tension thereafter. In a second, hot-stretching step, the cold-stretched film is further stretched at a temperature of about 5° C. to about 15° C. below the glass transition temperature of the minor component, to a total increase in dimension of 100 percent to about 700 percent of the original dimension of the precursor film. After post-film-forming treatment, the film may be about 10 micrometers to about 50 micrometers in thickness. The film produced by the foregoing method comprises a matrix of the major polymer with inclusions or domains of the minor polymer distributed uniformly therein, the major polymer further comprising a three-dimensional reticulated or interconnected network of uniformly distributed microcracks of relatively uniform dimensions. If an optional compatibilizing block copolymer is used, the block copolymer may be found coating the minor component inclusion particles. Other optional components present in the blend and present at the surfaces of the microcracks may impart characteristics to the microcracks such as hydrophilicity. The pore size of the film may be from about 1 to about 200 nanometers, and the porosity about 5 percent to about 30 percent or higher.
In a further aspect of the invention, the invention is drawn to a microporous membrane or film of about 10 to about 50 micrometers in thickness comprising a major component with inclusions or domains of a minor component distributed uniformly therein, the inclusions or domains of the minor component optionally coated by a compatibilizing block copolymer, the major component further comprising a three-dimensional reticulated or interconnected network of uniformly distributed reticulated or interconnected microcracks of relatively uniform dimensions. The pore size may be from about 1 to about 200 nanometers, and the porosity about 5 percent to about 30 percent or higher. The major component is preferably a polymer, such as a polyolefin, examples including but not limited to polypropylene, polyethylene or poly(4-methyl-1-pentene). The minor component is immiscible with the major component and is preferably a polymer, such as an immiscible polyolefin, polystyrene or a polyester, and may be present at about 1 percent to about 40 percent by weight of the total. The optional compatibilizing block copolymer may be present at about 0.5 percent to about 25 percent of the total.
In yet a further aspect, a process for the preparation of a microporous membrane of about 10 to about 50 micrometers in thickness comprising a major component with inclusions or domains of a minor component distributed uniformly therein, the inclusions or domains of the minor component optionally coated with a compatibilizing block copolymer, the major component further comprising a three-dimensional reticulated or interconnected network of uniformly distributed microcracks of uniform dimensions of about 1 to about 200 nanometers, the major component being preferably a polymer, such as a polyolefin, and the minor component immiscible with the major component being present at about 5 to about 25 percent by weight of the total and being preferably a polymer, such as an immiscible polyolefin, polystyrene or a polyester, the compatibilizing block copolymer if present is provided at about 0.5 percent to about 25 percent by weight of the total, the process comprising the steps of:
a) preparing an immiscible blend system comprising the minor component uniformly dispersed in the major component, optionally further containing a compatibilizing block copolymer;
b) forming a
Chandavasu Chaiya
Gogos Costas
Sirkar Kamalesh K.
Xanthos Marino
Klauber & Jackson
Menon Krishnan S
New Jersey Institute of Technology
Walker W. L.
LandOfFree
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