Polymeric microporous membranes have been prepared previously. Most of the commercialized membranes are symmetric in nature. Symmetric membranes have an approximately uniform pore size distribution throughout the membrane. The production of skinless symmetric microporous membranes are described, for example in U.S. Pat. No. 4,203,848 for polyvinylidene fluoride (PDVF) and in U.S. Pat. No. 4,340,479 for polyamide membranes. These preparations are generally described to consist of the following steps: a) preparation of a specific and well controlled preparation of a polymer solution, b) casting the polymer solution in the form of a thin film onto a temporary substrate, c) coagulating the resulting film of the polymer solution in a nonsolvent and d) removing the temporary substrate and e) drying the microporous membrane.
Membrane manufacturers produce membranes that are robust and reliable for sterile filtration using such methods. Such membranes are primarily single layer symmetric membranes, although other structures have been investigated and used for such membranes.
Another single layered structure is the asymmetric membrane, where the pore size of the membrane varies as a function of location within the thickness of the membrane. The most common asymmetric membrane has a gradient structure, in which pore size increases from one surface to the other. Asymmetric membranes are more prone to damage, since their retention characteristic is concentrated in a thick, dense surface region or skin see U.S. Pat. No. 4,629,563. It has been found, however, that increased productivity results from having the feed stream to be filtered contact the larger pore surface, which acts to prefilter the stream and reduce membrane plugging, see U.S. Pat. No. 4,261,834. Additionally, others have been successful in making asymmetric non-skinned or skinless microporous membranes. One such product is sold as Express™ membranes made by Millipore Corporation of Bedford, Mass.
Practitioners in the art of making microporous membranes, particularly asymmetric membranes, have found that membranes which contain large (relative to membrane pore size) hollow caverous structures have inferior properties compared to membranes made without such hollow structures. These hollow structures are sometimes called “macrovoids”, although other terms are used in the art. Practitioners striving for membranes of very high retention efficiency prefer to make membranes without such hollow structures.
Perhaps the most direct variation of the single layer structure is a multilayered unbonded laminate. While laminates can be made from layers of the same or different membranes, they have drawbacks. Each layer has to be made in a separate manufacturing process, increasing cost and reducing manufacturing efficiency. It is difficult to manufacture and handle very thin membranes, less than say 20 microns, because they deform and wrinkle easily. This adds the inefficiency of producing a final product with thin layers. Unbonded laminates can also come apart during fabrication into a final filter device, such as a pleated filter, which will cause flow and concentration non-uniformities.
Other methods of forming multilayered microporous membrane structures are known. U.S. Pat. No. 5,228,994 describes a method for coating a microporous substrate with a second microporous layer thereby forming a two layer composite microporous membrane. This process requires two separate membrane forming steps and is restricted by the viscosities of the polymer solutions that can be used in the process to prevent penetration of casting solution into the pores of the substrate.
Attempts have been made to produce multilayer microfiltration membranes. U.S. Pat. No. 4,770,777, describes a process with the following steps: casting a first membrane layer, b) embedding a fabric support into this first membrane and c) casting a second membrane layer on top of the embedded fabric to form a kind of membrane/fabric/membrane sandwich. The presence of a nonwoven could however lead to defects and imperfections, which are undesirable. U.S. Pat. No. 5,500,167 describes a method of preparing a supported microporous filtration membrane. This method consists of applying a first casting solution onto the first side of a porous nonwoven support material to form a first casting solution layer having a substantially smooth surface, then applying a second casting solution onto the substantially smooth surface of the first casting solution layer to form a second casting solution layer prior to the complete formation of a microporous membrane from the first casting layer, and forming a continuous microporous membrane having first and second zones from the first and second casting solutions such that the first side of the support material is integral with the first zone while not protruding into the second zone, and the first zone has a pore size at least about 50% greater than the pore size of the second zone. This product requires a nonwoven support which could lead to defects or imperfections. In U.S. Pat. No. 5,620,790, a membrane is described made by pouring out a first layer on a support of polymeric material onto a substrate and subsequently pouring out one or more further layers of a solution of polymeric material onto the first layer prior to the occurrence of turbidity in each immediately successive preceding layer, the viscosity of each immediately successive layer of a solution of polymeric material having been the same or less than that of the preceding layer. In U.S. Pat. No. 5,620,790, improved throughput is achieved by making fairly thick membranes which will tend to lower overall permeability.
In all three multilayer casting processes described above, however, a significant interval of time or a setting time between the separate coating steps is essential to the process of forming a multilayer membrane. Such sequential casting can cause the formation of a dense layer at the interface between the two layers in the membrane. This makes these methods undesirable in terms of robustness, since variability of the process during this setting time can lead to non-uniformity. While U.S. Pat. No. 5,620,790 discusses that a minimum setting time of several seconds to 2 minutes are advantageous, the prior art processes cannot reduce the interval between coating applications to essentially no time.
In the present invention, the inventor has surprisingly found that at essentially no time between the coating applications, one forms membranes in which there is a continuous change of membrane structure without a discontinuity through the junction between layers.
Furthermore, in the prior art a well-defined demarcation line is found between the two layers. This demarcation line signifies a drastic change in pore size going from a more open to a more tight structure. It can also signify a region of dense, skin-like structure. Either of these structural regions can lead to a lower permeability and an undesirably fast accumulation of particles at the interface and consequently a drastic flux decline. A more subtle change in pore size between two adjacent would reduce this effect and be beneficial for the retentive behavior of the overall structure of the membrane.
In a brief study as part of the Doctoral thesis of the inventor (“Membrane Formation by Phase Separation in Multicomponent Polymer Systems”; University of Twente, NL 1998), hereby included as reference, the effect of a second polymer solution layer covering a first polymer solution layer on the demixing or phase separation of each layer was studied. The purpose of this work was to qualitatively determine if a dense (non-porous) separation layer of reduced thickness could be formed by covering a first polymer solution layer with a second polymer solution layer of a different composition and then forming a membrane by immersion methods in the usual way. The polymer content and solvents were chosen to produce dense surface layer membranes, such as pervaporation membranes. The author compared the modes of phase separation for single and two layer membranes by comparing overall membrane structure with scanning electron microscopy photomicrographs. No membrane properties were measured. It was inferred from the photomicrographs that the dense layer thickness could be varied by this procedure.
The inventor found in the development of the present invention that in order to produce microporous membranes of improved properties, it was critical that the structure of the interfacial region between the layers be controlled. Initial work showed that if a dense region formed at the interface, membrane properties suffered. In particular, membrane permeability was reduced and filtration throughput was low for membranes having a distinct dense region at the interface. There was no motivation in the prior art to optimize this region in conjunction with overall membrane structure.
Moreover, it was found during the development of the present invention that throughput is controlled by the structure of the layer with the larger average pore size in combination with the structure of the interface. Whereas the prior art had concentrated on optimizing the structure of the retentive layer, particularly for membranes with a dense retentive layer, (e.g., reverse osmosis and pervaporation), in the present invention, the inventor was faced with the problem of controlling the retentive properties of all layers and the interfacial regions as well.
U.S. Pat. No. 5,620,790 teaches that for sequentially cast membranes, a viscosity restraint is imposed in that the viscosity of the lower layer should be higher than the viscosity of the upper layers.
Accordingly, it would be desirable to provide a process for forming a integral multilayer microporous membrane that is macrovoid free. In addition, it would be desirable to provide a simplified process wherein the layers are cast simultaneously in order to form in a controlled manner regions of intermediate pore size at the junctions of adjoining layers.