Integral supported separation membranes

Supported separation membranes are disclosed wherein the permselective membrane is integral with a polymeric support of non-woven fibers having a particular air permeability, the integral membranes exhibiting far superior lifetimes and retention capability, especially for microorganisms.

BACKGROUND OF THE INVENTION
 The invention concerns integral porous separation membranes supported on
 nonwoven polymeric supports such as are typically employed as filter media
 in conventional filtration modules for the filtration of fluids.
 The term "porous separation membranes" is generally understood to refer to
 ultrafiltration and microfiltration membranes that have pores with
 diameters of about 0.001 to 0.1 .mu.m and about 0.01 to 30 .mu.m,
 respectively. Such membranes are conventionally fabricated from organic
 polymers, and are often brittle, with little capacity for mechanical
 loads. For increasing mechanical structural strength, porous membranes of
 organic polymers are supported with a wide variety of support materials
 such as woven material, knitted fabrics, non-woven materials, or films.
 The terms "integral" and "nonintegral" are differentiated by the type of
 bonding between the membrane and the support material.
 The term "integral supported membranes," refers to those wherein the still
 fluid permselective membrane material is brought into contact with the
 support material and the membrane is formed by the mutually
 interpenetrating bond formed between the permselective membrane and the
 support. The support then, to a certain extent, penetrates into the
 membrane or may even be entirely encapsulated by the membrane. The
 membrane can be reinforced on one or both sides.
 In contradistinction, in the case of a non-supported membrane, the already
 cast permselective membrane is applied directly onto the support by, for
 instance, lamination or by an adhesive. From the viewpoints of production
 and applications, integral supported separation membranes are preferable.
 The cost of the production of integral supported separation membranes is
 governed by the degree of increase of mechanical strength desired, but
 increased mechanical strength is typically attainable at the cost of loss
 of service life or filtration capacity, often up to some 40% and usually
 accompanied by a decrease in retention capability for the substances to be
 removed, all as compared to non-supported membranes. Such drawbacks are
 unacceptable in industrial applications.
 Thus, the object of the invention is the provisions of integral supported
 membranes having longer service lifetimes, higher filtration capacity and
 improved retention capability. This object and others which will become
 apparent to one of ordinary skill are met by the present invention, which
 is summarized and described in detail below.
 BRIEF SUMMARY OF THE INVENTION
 The invention comprises the provision of integral supported porous
 separation membranes wherein the support for the permselective membrane is
 made of a mat or fleece of compacted non-woven fibers the mat exhibiting a
 basis weight of from about 20 to about 40 g/m.sup.2, a thickness from
 about 90 to about 170 .mu.m and an air permeability of between about 1200
 and about 2400 L/m.sup.2.multidot.s at a pressure of 0.002 bar.
 Rather surprisingly, it has been discovered that the integrated supported
 membranes of the present invention exhibit a number of unexpected
 advantages at the same flux relative to integral supported membranes of
 the prior art. Membranes fabricated in accord with the present invention
 have improved retention capability, particularly for microorganisms, and
 can be monitored with greater safety for integrity, since they possess a
 bubble point some 40% higher and a rate of diffusion reduced by about a
 factor of 10 to 20. These advantages permit filtration equipment equipped
 with such supported membrane to be tested for integrity with greater
 safety, especially in sensitive filtration applications such as
 pharmaceuticals, biotechnicals, and in the food and beverage industries.
 Similar advantages are gained with filtration devices for medical and gene
 technology, where a higher degree of safety is paramount.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In a preferred embodiment of the invention, the non-woven material for the
 support comprises a multi-component or composite fiber, wherein the
 composite includes a first polymer and a second polymer such that the
 second polymer is present on at least a portion of the surface of the
 composite fibers and exhibits a glass transition temperature which is
 lower than the glass transition temperature of the first polymer.
 In a further preferred embodiment, the composite non-woven fiber support is
 made out of a mat of fibers, wherein the core advantageously comprises a
 polymer having a higher glass transition temperature, for instance
 polypropylene, with an enveloping outside layer of a chemically resistant,
 thermoplastic polymer, such as polyethylene.
 The permselective separation membrane itself may be formed from any polymer
 which is suitable for the formation of a porous membrane according phase
 inversion casting techniques, including solvent evaporation, coagulation
 bath and combinations of these processes. Particularly well-adapted to
 such procedures are membranes cast from polysulfones, polyethersulfones,
 polyamides, polyacrylamides, polyacrylnitriles, cellulose hydrates and
 cellulose esters. Particularly preferred are polyethersulfones, polyamides
 and the cellulosic esters cellulose acetate, cellulose nitrate and
 mixtures of the two.
 The integral supported separation membranes of the invention can be
 fabricated into filters which are flat, pleated or tubular, and may even
 be used as a support material for additional membrane layers.

EXAMPLE 1
 A permselective membrane casting dope containing 7.2 wt % cellulose acetate
 was cast by a conventional phase inversion technique onto a compacted
 non-woven fiber support material to a thickness of about 130 .mu.m wherein
 the non-woven fiber support material comprised a mat of fibers, the fibers
 having a polypropylene core with a surrounding layer of polyethylene. The
 thickness of the non-woven mat support was about 130 .mu.m, its basis
 weight about 30 g/m.sup.2, and its air permeability about 1800
 L/m.sup.2.multidot.s at 0.002 bar.
 The so-formed integral supported separation membrane had a bubble point of
 3.2 bar and a diffusion rate of .ltoreq.2.3.times.10.sup.-3 ml/cm.sup.2 at
 a pressure differential of 2 bar or approximately 0.8.times.10.sup.-3
 ml/cm.sup.2 at a pressure differential of 1.5 bar. Its permeability to RO
 water at room temperature was 14.8 ml/cm.sup.2 /min.multidot.bar. The
 membrane quantitatively retained the bacterium Brevibacterium diminutum
 with a logarithmic reduction value (LRV) of &gt;8.
 The service life or filtration capacity of a separation membrane is
 conventionally expressed as flux in the filtration of a 10% aqueous brown
 raw sugar solution through the membrane at a pressure differential of 1
 bar for 10 minutes, as compared to the flux of the same solution through a
 standard (0.45 .mu.m pore polyamide-6.6) membrane under the same
 conditions, and is expressed as a percentage of the capacity of the
 standard membrane filter. When so measured, service life of the integral
 supported membrane of the present invention was 450%.
 Comparative Example
 A second integral supported separation membrane was fabricated in
 substantially the same manner as in Example 1, except that the support was
 a 250 .mu.m-thick mat of different non-woven polymeric fibers having the
 same basis weight as the support of Example 1 and an air permeability of
 about 4000 L/m.sup.2.multidot.s at 0.002 bar.
 This comparative integral supported separation membrane had a bubble point
 of 2.8 bar, a diffusion rate of .gtoreq.13.1.times.10.sup.-3
 ml/cm.multidot.min at a pressure differential of 1.5 bar and permitted
 bacteria to quantitatively pass the same bacterium as in Example 1 with an
 LRV of zero. Its permeability to RO water at room temperature was
 substantially the same as the integral supported membrane of Example 1,
 and its service life was 97% as measured in the same manner as in Example
 1.
 The terms and expressions which have been employed in the foregoing
 specification are used therein as terms of description and not of
 limitation, and there is no intention, in the use of such terms and
 expressions, of excluding equivalents of the features shown and described
 or portions thereof, it being recognized that the scope of the invention
 is defined and limited only by the claims which follow.