Coating processes – Nonuniform coating – Applying superposed diverse coatings or coating a coated base
Reexamination Certificate
2003-02-12
2004-05-18
Seidleck, James J. (Department: 1711)
Coating processes
Nonuniform coating
Applying superposed diverse coatings or coating a coated base
C427S258000, C428S103000, C428S105000, C428S107000, C428S156000, C428S175000
Reexamination Certificate
active
06737111
ABSTRACT:
FIELD OF INVENTION
The present invention relates to membranes. In particular, the present invention relates to membranes including, but not limited to, thin membranes and to methods of making such membranes.
BACKGROUND
Generally membranes can be defined as selective barriers between two phases. Separation is achieved when some species are transported to a greater extent from one phase to the other. The driving force for the movement of molecules includes concentration differences, electric potential differences (charge) and pressure differences. The rate of transport of molecules through membranes is governed by several factors including pore size, thickness of membrane, membrane fouling rates, etc.
A wide variety of different materials have been utilized for producing membranes. Generally microporous membranes can be divided into two main groups: those formed physically and those formed chemically. Membranes can also be controllable formed by careful manipulation of the solubility of polymers in solution. These physically formed membranes can be produced by either diffusion induced phase separation techniques (DIPS) or temperature induced phase separation (TIPS).
Physically formed membranes are useful for many applications including water purification, dialysis and protein separation. However, the techniques for reliably producing membranes of controlled pore size are often complicated, expensive and not easily reproduced in the laboratory.
Chemically produced membranes are made via a series of chemical reactions to form three-dimensional polymer networks. Thin polymer networks are not generally mechanically strong and are often supported in order to make useful products. The support or substrate is generally made from a material that is relatively inert, has good wet strength and not likely to readily bind proteins. Examples of substrates that have been used previously include fiberglass, polyethyleneterapthalate (PET) and woven nylon.
Recently, a need has arisen for membranes having the following characteristics:
controlled pore size
provide rapid separation
good mechanical strength
be free of soluble impurities
defect free
water resistant
Current methods for producing suitable membranes produce relatively thick membranes with a tendency for large numbers of defects. Whilst they tend to have good mechanical strength, their thickness results in some disadvantages. First, they have slower separation times compared to thin membranes. Second, they require more processing (eg more washes) to remove soluble entities from the membrane. In the case of the aqueous system, water soluble entities are removed. It is highly desirable to remove such water soluble entities, for example residual monomer, as they may react with the species being separated, resulting in an impure product and possibly toxic in nature. In the case of the organic systems, organic soluble entities are removed.
Supported membranes have conventionally been formed on a substrate by casting a membrane-forming polymer between two glass plates. A characteristic of membranes formed by this process is that they have a glossy/shiny appearance. This glossy appearance is the result of the membrane having a continuous polymeric layer over the substrate (see FIG.
1
(
a
)). That is, the resultant membrane is thicker than the substrate.
To produce a thinner membrane according to such conventional methods, a thinner substrate is used. In the case of non-woven substrates, as the substrate becomes thinner, the distance between the fibrils in the substrate increases. At a certain distance, the polymeric layer is no longer able to completely fill in the interstitial spaces between the fibrils of the substrate. This results in the formation of holes in the continuous polymeric layer, producing a defect and a non-functional membrane. In the case of woven substrates, as the substrate becomes thinner, the fibre diameter of the substrate decreases, and with it, a reduction in gel holding ability.
We have discovered that, surprisingly, a functional membrane can be achieved by filling the interstitial gaps or spaces in a substrate with a polymer, preferably crosslinked (see FIG.
1
(
b
)), without forming a continuous constant thickness polymeric layer over the substrate as in the case of conventional membranes. Such membranes, because of their unique structure, have a matt or non-glossy appearance on at least one side, in contrast to the glossy appearance of membranes produced by the conventional methodology described above.
In a first aspect, the present invention provides a polymeric membrane system comprising a substrate and a polymeric membrane, wherein the substrate comprises a plurality of interstitial gaps therein and wherein the polymeric membrane comprises polymeric membrane components spanning the interstitial gaps of the support, the polymeric membrane components being thinner than the substrate.
Preferably the polymeric membrane system of the invention has no detectable soluble entities. In the case of aqueous systems, water soluble entities are not detectable. In the case of organic systems, organic soluble entities are not detectable. Most preferably, the system has no detectable residual monomer(s).
An advantage of forming the membrane in the interstitial gaps of the substrate is that the thickness of the support is not governed by the thickness of the membrane. Therefore, the thickness of each membrane component spanning the interstitial gaps can be decreased so that they are effectively below the surface of the substrate. Thus, the design of the membrane is such that a thin membrane can be achieved while using a substrate that is of sufficient thickness to provide the required mechanical strength for the particular application. An advantage of a thinner membrane is that more rapid separation times can be achieved. Moreover, a thinner membrane, requires less processing to remove soluble entities from the membrane.
The polymeric membrane of the first aspect of the invention may be a crosslinked or non-crosslinked polymeric membrane. Preferably, the polymeric membrane is a crosslinked polymer membrane.
Preferably, the thickness of the membrane components making up the membrane is in a range of about 0.01 mm to 0.5 mm.
The polymeric membrane system of the first aspect of the invention has particular (but not exclusive) application to thin membranes. Preferably, in this case, the thickness of the membrane components making up the membrane is in a range of about 0.01 to 0.1 mm, more preferably about 0.03 to 0.09 mm.
In a second aspect, the present invention provides polymeric system according to the first aspect wherein the polymeric membrane is an ultra-thin membrane.
The polymeric membrane of the polymeric membrane system of the present invention may be formed from any crosslinked or non-crosslinked polymer conventionally used to prepare membranes. Preferably, the membrane is a hydrophilic membrane.
The membrane may be any gel-forming polymer. The membrane may be an electrophoretic gel. Examples of suitable polymers include, but are not limited to polyacrylamide gels and poly HEMA with EGDMA.
The substrate is preferably formed from a material that is relatively inert, has good wet strength and does not bind to the substance undergoing separation (eg proteins). The substrate has a plurality of interstitial gaps therein. Preferably the size of the interstitial gaps is no greater than the thickness of the substrate. The substrate may be woven or non-woven. The substrate may be a woven or non-woven material or a textile. The substrate is in the form of a sheet, web, or any other appropriate form.
The substrate may be formed from any material that is conventionally used as a membrane support. Non-limiting examples of suitable materials for use as substrates include, but are not limited to polyvinyl alcohol, polyethyleneteraphthalate (PET), nylon and fiberglass, cellulose, cellulose derivatives, or any other suitable substrates. Preferably the substrate is hydrophilic nature in the case of aqueous solvent systems. In the case of
Caulfield Marcus Julian
Qiao Greg GuangHua
Solomon David Henry
Baker & McKenzie
Gradipore Limited
Seidleck James J.
Tran Thao
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