Process for preparing polyamide membrane filter media and products thereof

Claim 2

A process according to claim 1 in which precipitated polyamide resin particles are redissolved or filtered out before spreading the casting solution on a substrate.

Claim 3

A process according to claim 1 in which part of the precipitated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

Claim 4

A process according to one or more of the claims 1 - 3 in which the nonsolvent used to contact and dilute the casting solution is a mixture of solvent and nonsolvent liquids containing a substantial proportion of the solvent liquid, but less than the proportion in the casting solution.

Claim 5

A process according to one or more of the claims 1 - 4 in which the polyamide resin is polyhexamethylene adipamide, poly- E-caprolactam or polyhexamethylene sebacamide.

Claim 6

A process according to claim 1 in which the polyamide resin is polyhexamethylene adipamide, the solvent for the polyamide resin solution is formic acid, and the nonsolvent added for dilution is water.

Claim 7

A process according to one or more of the claims 1 - 6, in which the polyamide resin solution film is contacted with the nonsolvent by immersing the film carried on the substrate in a bath of nonsolvent liquid or a bath comprises both solvent and nonsolvent liquids.

Claim 8

A process according to claim 7, in which the bath comprises an amount within the range from about 20% to about 50% of a solvent for the resin; washing the resulting membrane substantially free of solvent; and drying the membrane.

Claim 9

A process according to claim 6, in which the polyamide resin is polyhexamethylene adipamide, the solvent is formic acid, and the nonsolvent is water, and the polyamide resin concentration in the casting solution is within the range from about 10 to about 18% by weight, and the formic acid concentration within the range from about 63 to about 72% or in which the polyamide resin concentration of the casting solution is within the range from about 12 to about 1896, and the fionsolvent is added at a fixed intensity of mixing.

Claim 10

A process according to one or more of the claims 1 - 9, in which the casting resin is continuously spread onto the substrate, the thin film of casting solution is continuously immersed in a bath of nonsolvent liquid, and the bath is maintained at a substantially constant composition with respect to nonsolvent and solvent by continuous addition of nonsolvent to the bath in a quantity to compensate for solvent diffusion into the bath from the thin film of casting solution.

Claim 11

A process according to claim 10, in which the substrate is a nonporous synthetic polymer film having a surface that is wetted by the casting solution and bath.

Claim 12

A process according to claim 10, in which the substrate is a porous web, preferably a fibrous polyester sheet, the web having an open structure which is wetted and impregnated by the casting solution, thereby forming a membrane film having the porous web incorporated as a part thereof or the substrate is a porous web which is not wetted by the casting solution, thereby forming a membrane film having the porous web attached to one surface thereof.

Claim 13

A process according to one or more or the claims 1 - 12, in which the polyamide resin solution has a viscosity within the range from about 5,000 centipoises to about 50,000 centipoises at the operation temperature.

Claim 14

1 4. A process according to one or more of the claims 1 - 13, in which the casting and precipitating temperatures are within the range from about 10°C to the boiling temperature of the lowest boiling solvent or nonsolvent component present.

Claim 15

A process according to one or more of the claims 1 - 14, in which the casting resin solution is clear, and free from suspended material, before being spread upon the substrate to form a film.

Claim 16

A process according to one or more of the claims 1 - 15, in which the membrane is stripped from the support after washing and before drying or the substrate is not stripped from the polyamide resin membrane before drying, and after drying remains attached toq the polyamide resin membrane.

Claim 17

A process according to one or more of the claims 1 - 16, in which the substrate is of polypropylene resin or polyester resin.

Claim 18

A process for preparing skinless hydrophilic alcohol-insoluble polyhexamethylene adipamide resin membranes having pores that are substantially uniform from surface to surface which comprises preparing a flowable solution of the alcohol-insoluble polyhexamethylene adipamide resin in a concentration within the range from about 10% to about 18% by weight in an aqueous formic acid solution containing from about 63% to about 72% formic acid by weight; inducing nucleation of the resin solution by adding water thereto while controlling resin and formic acid concentration, temperature, rate of addition of water and degree of agitation to obtain a visible precipitate of resin particle thereby forming a casting solution; spreading the casting solution on a polyester resin substrate to form a thin film of resin solution thereon; contacting and diluting the film of casting resin solution with an aqueous solution containing from 37 and 50% formic acid and thereby precipitating the polyamide resin from the casting resin solution as a thin skinless hydrophilic membrane; washing the membrane to remove solvent; and drying the membrane.

Claim 19

A process according to claim 18, in which the membrane is stripped from the support after washing and before drying.

Claim 20

A process according to one or more of the claims 18 - 19, in which precipitated polyamide resin particles are redissolved or filtered out before spreading the casting solution on a substrate.

Claim 21

A process according to one or more of the claims 18-19, in which part of the precipitated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

Claim 22

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin, capable when completely immersed in water of being wetted through within no more than one second, and reverting when heated to a temperature just below the softening temperature of the memgrane to a hydrophobic material which is no longer wetted by water, the sheet preferably having through pores extending from surface to surface that are substantially uniform in shape and size or that are tapered, wider at one surface and narrowing towards the other surface of the membrane sheet.

Claim 23

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 22 having absolute particle removal ratings of 0.10 µM to 5 pM.

Claim 24

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to one or more of the claims 22-23 in which the polyamide resin is polyhexamethylene adipamide; poly-e-caprolactam or polyhexamethylene sebacamide.

Claim 25

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to one or more of the claims 22-24 having two membrane layers with the same or differing porosities adherent to each other and forming a single membrane sheet with particle removal characteristics superior to those of the individual membrane layers.

Claim 26

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 25 in which the membranes have tapered or uniform pores.

Claim 27

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 25 in which the membranes are supported or unsupported.

Claim 28

A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to one or more of the claims 22-27 having a thickness within the range from about 0.025 to about 0.8 mm.

Claim 29

A filter element comprising a hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet which is preferably corrugated according to one or more of the claims 22-28 formed in a tubular configuration with the ends of the tube sealed to end caps of which at least one end cap has a central aperture giving access to the interior of the tube, and with the sides of the sheet lapped and sealed together, all seals being fluidtight.

Claim 30

A filter element according to claim 29 in which at least one of the exterior faces of the sheet is adhered to a porous support layer.

Claim 31

A filter element according to claim 30 in which both the membrane sheet and the support layer are corrugated.

Claim 32

A filter element according to one or more of the claims 29-31 comprising a multilayer membrane sheet, the layers being adhered together as one sheet the layers of membrane being preferably separated by a porous support layer to which each membrane layer is adhered.

Claim 33

A filter element according to one or more of the claims 29-32 in which the end caps are of polyester resin preferably of polybutylene glycol terephthalate or of polyehylene glycol terephthalate and the filter element including the end caps is hydrophilic and rapidly wetted by water.

Claim 34

A process for preparing multilayer skinless hydrophilic alcohol-insoluble polyamide membranes which comprises preparing a solution of an alcohol-insoluble polyamide resin in a polyamide solvent; including nucleation of the solution by controlled addition to the solution of a nonsolvent for the polyamide resin, under controlled conditions of concentration, temperature, addition rate, and degree of agitation to obtain a visible precipitate of polyamide resin particles, thereby forming a casting solution; spreading the casting solution on a substrate to form a thin film thereof on the substrate; contacting and diluting the film of casting solution with a nonsolvent liquid for the polyamide resin, thereby precipitating polyamide resin from the casting solution in the form of a skinless membrane; washing the membrane to remove solvent; placing the washed membrane while still wet in contact with at least one other wet washed membrane; and then drying the juxtaposed membranes while maintaining such contact, thereby obtaining a multilayer membrane in which the separate membranes are integral layers thereof.

Claim 35

A process according to claim 34 in which the separate membranes are stripped from the substrate before drying or the separate membranes are dried while supported on the substrate, which thereby becomes an integral part of the multilayer membrane.

Claim 36

A process according to claim 35 in which the substrate is a porous fibrous web preferably of polyester resin or of polypropylene resin.

Claim 37

A process according to one or more of the claims 34-36 in which the polyamide resin is polyhexamethylene adipamide, poly-ε-caprolactam or polyhexamethylene sebacamide.

Claim 38

A process according to one or more of the claims 34-37 in which the membranes are dried under restraint to limit dimensional change, preferably after they had been corrugated.

Claim 39

A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin, capable when completely immersed in water of being wetted through within no more than Q ne second, and reverting when heated to a temperature just below the softening temperature of the membrane to a hydrophobic material which is no longer wette by water, and having at least two membrane layers integrall adhered together.

Claim 40

A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 39 in which the membrane layers are supported on a substrate, which e.g. is a porous fibrous web preferably of polyester resin of polypropylene resin.

Claim 41

A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 40 in which the polyamide resin is polyhexamethylene adipamide, poly-!-caprolactam or polyhexamethylene sebacamide.

Claim 42

A casting resin solution,which is preferably clear and free from suspended material, for forming from hydrophobic polyamide resin a skinless microporous polyamide membrane having absolute particle removal ratings of 0.10 µM to 5 µM or larger in a solid form that is hydrophilic and remains hydrophilic until heated to a temperature just below its softening point, comprising a nucleated solution of an alcohol-insoluble polyamide resin in a polyamide solvent, the nucleation having been induced by addition of a nonsolvent to the solution under controlled conditions of solvent and nonsolvent and resin concentration, temperature, mixing intensity, addition time and system geometry, such that a visible precipitate of polyamide resin forms during the addition of the non- solvent.

Claim 43

A casting resin solution according to claim 4 in which the precipitated polyamide resin has at least partially redissolved.

Claim 44

A casting resin solution according to one or more of the claims 42-43 from which any undissolved resin has been removed by filtration.

Claim 45

A casting resin solution according to one or more of the claims 42-44 having a viscosity within the range from about 500 centipotses to about 100,000 preferably to about 5000 centipoises ... at the temperature existin at the time it is cast as a film.

Claim 46

A casting resin solution according to one or more of the claims 42-45 in which the polyamide resin is polyhexamethylene adipamide, poly-e-caprclactam or polyhexamethylene sebacamide.

Claim 47

A casting resin solution according to one or mor of the claims 42-45 in which the polyamide resin is polyhexamethylene adipamide, the solvent for the polyamide resin is formic acid, and the nonsolvent is water.

Claim 48

A casting resin solution according to one or more of the claims 42-45 in which the polyamide resin is polyhexamethylene adipamide, the solvent is formic acid, and the nonsolvent is water, the polamide resin concentration is within the range from about 10 to about 18% by weight, preferably from about 12 to about 18% by weight and the formic acid concentration is within the range from about 63 to about 72% by weight.

Claim 49

A process for rating the effectiveness of a resin membrane sheet as a filter medium in terms of effectiveness in the removal of a contaminant of known dimensions, correlating such removal with air flow measure ments through the wetted membrane and the thickness of the membrane as a function of pore size and of thickness expressed by the exponential relationships where T R is the titer reduction for the membrane and is the ratio of contaminant content in the influent to the content thereof in the effluent; T R1 is the titer reduction achieved by a membrane of unit thickness; and t is the thickness of the membrane, which compri:' (1) wetting the membrane with a liquid so that through pores are blocked by liquid; (2) applying air to one surface of the membrane at a pressure differential across the membrane that is le than that at which air flow through the membrane proceeds (3) increasing the pressure differential until. air flow through the membrane begins; (4) continuing to increase the pressure differen until a point is reached at which a small increment in pressure causes a sharp increase in flow; designating the pressure at which this occurs as K L for the membrane; (5) determining the K L values for a plurality of such resin membranes having a range of thickness and a range of pore diameters; (6) challenging these membranes with a suspension of contaminant, and determining T R for each membrane; (7) calculating log T R1 for each membrane from the thickness of each membrane using the formula log ; (8) establishing from the K L and thickness values a relationship between the thickness and K L values of equal pore size membranes; (9) correcting the K L values of the membranes to the K L values which would apply to an equal pore size membrane of a selected thickness, designating these values as K L5 ; (10) plotting log T R1 for each membrane against K L5 for that membrane; and (11) from that curve reading off the titer reduction (T R ) which can be expected to be obtained for an unknown membrane of that resin according to the measured K L and thickness (t) for that membrane by (a) measuring the K L and thickness for that membrane; (b) determining K L5 for that membrane; (c) using K L5 to determine T R1 for that membrane; and 5 (d) calculating T R from the equation .

Claim 50

A process according to claim 49 in which the contaminant is a suspension of Pseudomonas diminutiae, thereby determining the ability of the membrane to deliver bacterially sterile effluent.

Claim 51

A process according to claim 50 in which the wetting liquid is water or an alcohol.

Claim 52

A process according to one or more of the claims 49-51 in which the air is applied to each surface of the membrane, and two sets of data obtained, one for each direction of pressurization, by applying air pressure and measuring flow through the membrane successively in both directions, and from the degree of correspondence in the two sets of data, determining the degree of uniformity of the pore configuration of the membrane in each direction of flow.

Claim 53

A process according to one or more of the claims 49-5 2 in which the resin membrane is of polyamide resin, polyhexamethylene adipamide, poly-ε-caprolactam or polyhexamethylene sebacamide.

Claim 54

A process according to one or more of the claims 1-21 in which the casting solution has a viscosity within the range from about 500 centipoises to about 100,00 centipoises preferably to about 5000 centipoises at the temperature exising at the time it is cast as a film.