Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ
Patent
1987-02-18
1988-09-27
Foelak, Morton
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
55 16, 55158, 55159, 264344, 521 64, B29C 2760
Patent
active
047740388
DESCRIPTION:
BRIEF SUMMARY
This invention relates to polyamide membranes and particularly to alcohol-insoluble polyamide membranes and their method of manufacture.
Alcohol-insoluble microporous polyamide membranes are made commercially by two general processes, one a so-called wet process and the other a dry process. In the wet process a membrane forming material is cast as a thin film, either directly into a quenching bath (U.S. Pat. No. 3,876,738) or onto a dry surface for a short time and then into a quenching solution (U.S. Pat. No. 4,340,479). The function of the quenching solution is to remove the non-polyamide ingredients and thereby form the microporous membrane while the material is still in the quench bath. The membrane is removed from the bath, dried and placed on rolls until ready for further handling. Polyhexamethylene adipamide (Nylon 6,6) extrusion grade homopolymer is usually the polyamide of choice because of its superior thermal and chemical stability compared with other types and grades of Nylon. However, it is also known that poly-e-caprolactam (Nylon 6) and polyhexamethylene sebacamide (Nylon 610) can be formed into membranes.
In the dry process (U.S. Pat. No. 4,450,126), the membrane is formed by relatively slow evaporation of the volatile components rather than by rapid quenching. Its advantage lies in the fact that as the membrane reaches the state of nearly complete dryness it can be rolled up as a finished product. In a production line process, the evaporating film is moved through a tunnel, air being blown through the tunnel to assist evaporation.
In U.S. Pat. No. 4,450,126 it is stated that Nylon 6,6 homopolymer did not make a suitable microporous membrane by the dry process, but that a blend of two polymers, one being the usual high molecular weight extrusion grade Nylon 6,6 and the other a Nylon "multipolymer" made a suitable membrane under the experimental conditions disclosed. This multipolymer is a co-polymerized Nylon containing units of nylon 6, 6,6, 610 and 612. It does not possess the thermal or chemical resistance of the extrusion grade Nylon 6,6 and in fact it is even somewhat soluble in alcohols. It had to be used because under the environmental conditions a suitable Nylon 6,6 membrane could not be made. The multipolymer served as a "strengthening and gelation promotor".
On page 3, lines 19 to 25 of U.S. Pat. No. 4,450,126 it is stated that Nylon 6,6 used by itself does undergo gelation, but "it does so after a skin has formed and hence the resultant membrane exhibits high resistance to flow and is of no utility for microfiltration applications." Likewise, homopolymer Nylon 6,6 membranes produced by the dry process are weak and brittle, rather than strong and flexible.
Examples 1 and 2 of U.S. Pat. No. 4,450,126 reiterate these conclusions. On page 7, line 5 it is stated, with reference to Nylon 6,6 membranes made by the dry process, "the homopolymer membranes apparently require a double layer to ensure sterilization efficiency." The one claim is for the dry process preparation of a membrane comprised of two polymers, one extrusion grade Nylon 66 homopolymer and the other the aforesaid multipolymer.
The present invention is concerned with a dry process method that can be used to prepare good quality self-supporting membranes from most, if not all, alcohol insoluble polyamide including Nylon 6,6 alone, in contrast to the prior art methods.
According to the present invention there is provided a method for preparing an alcohol insoluble polyamide membrane, the method comprising evaporating an appropriate membrane-forming casting liquid containing the required polyamide under conditions of high total humidity and an air flow of less than 305 m/min. (1000 ft/min) across the membrane during its formation.
The total humidity is preferably from 0.25 to 0.90 g/mole of air, more desirably from 0.60 to 0.90 g/mole of air.
The air flow is preferably less than 153 m/min (500 ft/min), and more desirably from 46 to 92 m/min (150 to 300 ft/min). The air flow is measured as the linear velocity of air relative
REFERENCES:
patent: 2783894 (1957-03-01), Lovell et al.
patent: 4340479 (1982-07-01), Pall
patent: 4450126 (1984-05-01), Kestring
Ditter Jerome F.
Porter Jeffrey
Domnick Hunter Filters Limited
Foelak Morton
Lewen Bert J.
Sternberg Henry
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