Liquid purification or separation – Filter – Material
Patent
1985-01-09
1986-11-18
Czaja, Donald
Liquid purification or separation
Filter
Material
264 41, 264 49, 21050038, 21050039, B29C 3902
Patent
active
046234632
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
Synthetic polymeric membranes are used for separation of species by dialysis, electrodialysis, ultrafiltration, cross flow filtration, reverse osmosis and other similar techniques.
2. Description of the Prior Art
Prior art methods for forming synthetic polymeric membranes are discussed in Australian patent specification No. 505,494 of Unisearch Limited.
One such method which was developed by S. Loeb and S. Sourirajan at the University of California, Los Angeles concerned the formation of a membrane from a solution of a polymer (such as cellulose acetate) in a solvent (such as acetone media). A thin layer of the solution was spread onto a glass plate and the solvent allowed to evaporate. As the surface layer dried faster than the layer underneath, a skin surface layer was formed and beneath that was a semi-solid matrix. The membrane was then quenched in another solvent whereby the remaining polymer was precipitated to form a sponge-like backing of the membrane.
The membrane forming technique disclosed in Unisearch Australian patent specification No. 505,494 is broadly described as being the controlled unidirectional coagulation of the polymeric material from a solution which is coated onto a suitable inert surface. The first step in the process is the preparation of a "dope" by dissolution of a polymer. This is said to be achieved by cutting the hydrogen bonds (which link the molecular chains of the polymer together) with a solvent. After a period of maturation, the dope is then cast onto a glass plate and coagulated by immediate immersion in a coagulation bath which is capable of diluting the solvent and annealing the depolymerization compound which has been used. According to the one example given in the specification, the "dope" consisted of a polyamide dissolved in a solvent which comprised hydrochloric acid and ethanol.
In contradistinction to the polymeric solution teachings of Australian patent specification No. 505,494 and its discussion on the prior art, the present invention is based on the concept that the liquid material out of which the membrane is cast is a colloidal suspension so that the surface pore density is significantly increased over the surface pore density of prior membranes.
DISCLOSURE OF THE INVENTION
According to the present invention, a single, chemically homologous, hydrolysable thermoplastic material having both relatively non-crystalline and relatively crystalline portions is dissolved in a suitable solvent under conditions which cause the relatively non-crystalline portions of the thermoplastic material to dissolve while at least a portion of the relatively crystalline portion does not dissolve but forms a colloidal dispersion in the solvent. The colloidal dispersion and solvent may be then coated onto a surface as a film and thereafter precipitation of the dissolved thermoplastic portion in the film is effected to form a porous membrane.
In the porous membrane so formed, the colloidal relatively crystalline portions of the thermoplastic material are joined together by the precipitated relatively non-crystalline portions, with pores in the membrane being defined by spaces between said relatively crystalline portions. The pore sizes are governed by their proximity to the surface which the film is coated and thus form a graduation of sizes.
The single, chemically homologous, hydrolysable thermoplastic starting material may be selected from any polymeric material having relatively crystalline and relatively non-crystalline portions. The following materials are typical examples of the thermoplastics starting materials which may be used in carrying out the invention: caprolactam, the cyclic amide of 6-aminocaproic acid and polyamide 6,6 or Nylon 6,6 which is formed from hexamethyenediamine and adipic acid. component of polyamide 6,6 is replaced by an aromatic component components are wholly aromatic) such as the polymer of p-phenylenediamine and terephthalic acid,
The solvent is selected having regard to the starting materi
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Anderson Eric W.
Ford Douglas L.
Kopp Clinton V.
Czaja Donald
Memtec Limited
Tentoni Leo B.
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