CH688612A5 - polyvinylidene fluoride membrane. - Google Patents

Description

       

  
 



  The present invention relates to a porous membrane comprising polyvinylidene fluoride, as well as to methods of preparing and using such a membrane. The present inventive membrane has been found to be particularly useful in the filtration of pharmacological and other solutions, more particularly in the removal of viruses from such solutions.



  When preparing solutions which are intended for administration to animal or human hosts, such as pharmacological and life support solutions, it is important that such solutions are as free as possible from substances which may cause negative reactions in humans. host. One such contaminant that is particularly concerned are viruses. Viruses are the cause of many feared diseases around the world, such as, for example, polio, hepatitis, and AIDS.



  Many physical and non-physical methods are currently used to either eliminate or inactivate viruses. Non-physical methods used to inactivate viruses include, for example, heat pasteurization and chemical treatment. However, these methods do not affect all viruses equally. In addition, if biotherapeutic agents are present, such agents may also become inactivated. In addition, the chemicals used in chemical treatments can have a detrimental effect on a host.



  An alternative approach involves physical separation treatment. Such methods use filtration membranes, for example, symmetrical or asymmetric microporous or ultrafiltration membranes, to remove viruses from a solution. Such membranes remove viruses by either adsorption, sieving, or a combination of adsorption and sieving. Screening is generally preferred over adsorption because of the greater control that can be exercised over the screening process and because screening is less likely to allow the virus to accidentally pass through the filtration membrane.



  When filtering viruses, the size of the target viruses must be taken into account in order to select the appropriate filtration medium. While not all viruses are the same size, viruses are typically characterized as either "large" or "small" viruses. Large viruses include viruses about 0.08 µm effective diameter and larger, for example, adenoviruses, rheoviruses, and herpes viruses. Small viruses include viruses that have an effective diameter of about 0.025 to 0.028 µm, for example, hepatitis viruses, polio viruses, and parvoviruses.



  The effective screening of viruses is now limited by the available filtration membranes. While both microporous and ultrafiltration membranes have been proposed to screen viruses, each of these membranes is inadequate in various aspects.



  Microporous membranes are characterized as being isopropic and skinless. In other words, they have a consistent pore structure, and their ability to remove particles, as measured by, for example, titer reduction, depends on the size of the pores and the thickness of the membrane. The smallest average pore size commonly available in this type of membrane, however, is only about 0.04 µm, for example, Ultipor N66-NDP (Pall Corporation, Glen Cove, New York). While such membranes are capable of removing relatively large viruses using membranes of reasonable thickness, they generally cannot eliminate those that fall into the smaller size category. Attempts to prepare a microporous membrane with smaller pores have been unsuccessful to date.



  Ultrafiltration membranes are characterized as being asymmetrical, that is to say that they have a non-uniform pore size through their thicknesses. Specifically, such membranes typically consist of an integral bilayer, in which one layer is thin skin which shows what have been called crack-like cracks, while the other layer is a thick substructure containing a concentration high macrovides or finger-like intrusions. The thin layer has a relatively small pore size, while the thick sub-structure has a relatively larger pore size. It is the skin, which is integral with the balance of the membrane, which provides the membrane with its filtration graduation.

  Ultrafiltration membranes are generally available in a pore diameter range of 0.001 to 0.02 µm.



  Ideally, the entire skin completely covers the macrovides of the thick substrate. In practice, however, the skin above the macrovides almost always contains significant defects, such as cracks, pinholes, and other defects and imperfections, which either breach the skin layer or cause a failure during use. Thus, there can be no certainty regarding the integrity of the membrane and its rate of removal.



  Thus, although the ultrafiltration membranes are used in practice, the membranes are used on a statistical basis in view of the integrity defects. That is, since only a small portion of the filtered liquid will pass through one of the defects, and since only a portion of all the filtered liquid contains the undesirable material that is sought to be eliminated, the probability is that only a small amount of such material will pass through the membrane. While this may be acceptable for certain applications, this is unacceptable for other applications, particularly in those situations in which the filtered liquid is intended to be administered to a human or animal and any virus or the like which has passed to through the membrane can create a serious health problem for the recipient.



   In addition, the manufacture of acceptable ultrafiltration membranes is difficult in view of their structure and the defects which invariably accompany this structure. To date, no ultrafiltration membrane has been manufactured which is in fact free from defects. In addition, due to the extremely thin layer (of the order of a few microns thick), which is responsible for all the filtration characteristics of an ultrafiltration membrane, it is quite difficult to reproduce on a consistent basis of ultrafiltration membranes which have the same level of defects, pore size and pore size distribution.

  In addition, the integrity and other properties of such membranes cannot even be easily tested after manufacture and before actual use due to typical test procedures, for example, "bubble point" and KL tests, which would require test pressures so excessively high that they would break or otherwise damage the membranes.



  Thus, an important need remains for a filtration membrane which is capable of providing efficient and predictable removal of small particles, such as viruses, from a fluid. Such a membrane should preferably show minimal adsorption properties so as to prevent backflow and other undesirable filtration effects. In addition, the filtration membrane must be easily reproducible and testable for its integrity before actual use. A commercially adaptable method for preparing such membranes would also be desirable. The present invention provides such a filtration membrane, as well as related methods of preparing and using such a membrane.

  These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention given here.



  The present invention provides an isotropic, skin-free, porous polyvinylidene fluoride membrane having a KUF of at least 15 psi (103 kPa), and preferably less than 50 psi (345 kPa), when tested using the skin liquids with an interfacial tension of approximately 4 dynes / cm (4 mN / m). The present membrane according to the invention preferably has a titer reduction of at least 10 <8> against a T1 bacteriophage, more preferably also against a coliphage PR772, and even more preferably also against a PP7 bacteriophage. The present membrane may have a thickness of about 20 thousandths (500 mu m) or less and even as little as about 5 thousandths (125 mu m) or less.



  Thus, the subject of the invention is a polyvinylidene fluoride, isotropic, skinless, porous membrane having (a) a KUF of at least 103 kPa when tested using pairs of liquids having an interfacial tension of approximately 4 mN / m and / or (b) a reduction in titer of at least 10 <8> against a T1 bacteriophage.



  According to a characteristic of the membrane of the invention, this membrane has a KUF of 103 kPa to 345 kPa.



  According to another characteristic of the membrane of the invention, said membrane has a reduction in titer of at least 10 <8> against a T1 bacteriophage.



  In this case, said membrane also has a titer reduction of at least 10 <8> against coliphage PR772 and / or bacteriophage PP7.



  More specifically, the membrane of the invention has a titer reduction of 10 <2> or less against bacteriophage PP7.



  An additional feature of the inventive membrane is that said membrane has a thickness of about 500 µm or less.



  The membrane of the invention may further comprise a surface coating and of a polymer making said membrane hydrophilic and less susceptible to protein adsorption.



  In this case, said polymer comprises one or more acrylic or methacrylic monomers having hydroxyl functional groups.



  Said polymer may have been grafted by irradiation on said membrane.



  The preferred irradiation is electron beam irradiation.



  The present invention also provides a method of preparing such a membrane by providing a molding solution comprising a polyvinylidene fluoride and a solvent therefor, by heating the molding solution to a uniform temperature of about 57 DEG C to about 60 DEG C, by distributing the molding solution on a substrate to form a film, by soaking the film in a quench bath so as to form a porous membrane, and by washing and drying the porous membrane.



  Thus, a method according to the invention for the preparation of porous membranes is characterized in that it comprises the drying of a wet molded membrane by exposure of said membrane to microwave irradiation under conditions sufficient to effect removal liquid from said membrane.



  According to a characteristic of the method of the invention, said membrane comprises polyvinylidene fluoride.



  More specifically, a method according to the invention for preparing a membrane comprises providing a molding solution comprising polyvinylidene fluoride and a solvent therefor, heating said molding solution to a uniform temperature of about 57 DEG C to about 60 DEG C, distributing said molding solution on a substrate to form a film, quenching said film in a quench bath to form a porous membrane, and washing and drying said porous membrane.



  According to a characteristic of this method, said membrane is at least partially dried by exposure of said membrane to microwave irradiation.



  According to an additional characteristic of the method of the invention, said membrane is treated to provide said membrane with a surface coating of a polymer making said membrane hydrophilic and less susceptible to the adsorption of proteins.



  In this case, said treatment comprises the binding to the surface of said membrane of a polymer comprising one or more acrylic or methacrylic monomers having hydroxyl functional groups.



   Said polymer is then grafted by irradiation onto said membrane.



  A preferred polymer irradiation is electron beam irradiation.



  The member prepared according to the method of the invention is also an object of the present invention.



  The invention also relates to a method of filtering a fluid comprising passing a fluid through the membrane according to the invention or obtained by the manufacturing method according to the invention.



  This filtration method according to the invention is further characterized in that said fluid comprises viruses before having passed through said membrane and contains less virus after having passed through said membrane.



  The invention also relates to a filter element comprising the membrane of the invention or obtained by the manufacturing process of the invention having sides which have been overlapped and sealed to form a tubular configuration having an outer surface, an interior, and two ends, and end caps sealed at the ends of the tube, where at least one of said end caps has a central opening providing access to the interior of the tube and where all of said seals are sealed to the fluid.



  According to a characteristic of the filter element of the invention, said membrane is ridged.



  According to another characteristic of the filter element of the invention, at least one of the sides of said membrane is adhered to a porous support layer.



  In this case, the filter element according to the invention can comprise multiple membranes adhered together.



  In addition, said membranes can be separated by a porous support layer to which each membrane is adhered.



  The invention will be better understood during the description which follows and which is made with reference to the accompanying figures in which:
 
   - fig. 1A and 1B are scanning electron micrographs of a polyvinylidene fluoride membrane of the present invention taken at an enlargement of 500 (fig. 1A) and at an enlargement of 5.0K (fig. 1B);
   - fig. 2A and 2B are scanning electron micrographs of the top (Fig. 2A) and the bottom (Fig. 2B) of a polyvinylidene fluoride membrane of the present invention taken at 10.1K magnification;
   - fig. 3 is a graph representing a curve showing the relationship between the temperature of the molding solution (DEG C) and the KUF resulting from the membrane (psi and kPa);

   
   - fig. 4 is a graph representing a curve showing the relationship between the pressure drop (DELTA P) across the membrane divided by the thickness of the membrane (inch of Hg / thousandth and cm of Hg / mu m; logarithmic scale) and the membrane KUF (psi and kPa).
 



  The present invention provides a new isotropic, skinless, porous membrane having a pore graduation which is less than what has been previously achieved with such membranes. The pore size characteristics of the present inventive membrane can be characterized in terms of KUF as well as titer reduction.



  Specifically, the present invention provides an isotropic, skinless, porous polyvinylidene fluoride membrane having a KUF of at least about 15 psi (103 kPa), preferably at least about 17 psi (117 kPa), and more preferably at least about 20 psi (138 kPa), when tested using pairs of liquids with an interfacial tension of about 4 dynes / cm (4 mN / m). The present inventive method will typically have a KUF of less than about 50 psi (345 kPa), for example, from about 15 psi (103 kPa) to about 50 psi (345 kPa); and more generally will have a KUF of less than 40 psi (276 kPa), when tested using pairs of liquids with an interfacial tension of approximately 4 dynes / cm (4 mN / m).

  Even more generally, the present inventive membrane will have a KUF of less than about 30 psi (207 kPa), for example, from about 18 psi (124 kPa) to about 30 psi (207 kPa), when tested using pairs of liquids with an interfacial tension of approximately 4 dynes / cm (4 mN / m).



  The present inventive membrane can also be characterized by its reduction in titer against various phages. The present inventive membrane preferably has a titer reduction of at least about 10 <8> against the bacteriophage T1, more preferably also against the coliphage PR772 smaller, and most preferably against the bacteriophage PP7 even smaller also. The reduction in titer of a particular membrane of the present invention is quite predictable based on the KUF and the thickness of the membrane. In addition, the titer reduction can be tailored to be within fairly strict limits, highlighting a narrow distribution of pore sizes.

  For example, the present inventive membrane may have a titer reduction of at least about 10 <8> against a T1 bacteriophage, or even against a coliphage PR772, while having a titer reduction of about 10 <2> at least against PP7.



  The present inventive membrane can be prepared to be of any suitable thickness and can be formed in layers so as to achieve a desired thickness. Generally, the present inventive membrane will have a thickness of about 20 thousandths (500 mu m) or less, preferably 10 thousandths (250 mu m) or less, and more preferably about 5 thousandths (125 mu m) or less. For most applications, the present inventive membrane can have a thickness of approximately 3 thousandths (75 μm) to approximately 5 thousandths (125 μm). The present inventive membrane may be of these varied thicknesses and will be further characterized by the above values of KUF and / or of reduction in titer.

  Thus, although the membrane of the present invention is prepared to be quite thin, for example, about 1 to 5 thousandths (25 to 125 µm thick) or even as thin as 1 to 3 thousandths (25 to 75 mu m) thick, the present inventive membrane can still provide an excellent reduction in titer against viruses.



   Since the present inventive membrane is isotropic, it has a substantially uniform and symmetrical pore structure. Representative membranes of the present invention are shown on the scanning electron micrographs of Figs. 1A (X 500) and 1B (X 5.00 K), which show the structure of fine and uniform pores of the membrane, and in the scanning electron micrographs of FIGS. 2A and 2B (both X 10.1K), which show the bottom and top views of the same membrane. In addition, the isotropic nature of the present inventive medium is illustrated by the straight line drawing in the graph of FIG. 4. This figure describes a plot of the pressure drop (DELTA P) of the membrane independent of the thickness of the membrane (inch of Hg / thousandth and cm of Hg / mil) on a logarithmic scale as a function of the KUF of the membrane (psi and kPa).

  The resulting curve and straight line is indicative of an isotropic membrane.


 Determination of KUF
 



  Test methods known as "bubble point" (ASTM F316-86) and KL test methods have been used in the past to evaluate the pore size characteristics of microporous membranes. Although these test methods, particularly the KL test method, can be used to assess the present inventive membrane, such tests require high pressures in conjunction with very small pore membranes, which can cause reliability problems. Thus, the present inventive membrane is preferably characterized by the use of the KUF test method which has been developed by Pall Corporation to provide a means to more reliably assess the pore size and the integrity of the membranes with very small pore graduations.



  The KUF test method is described in US patent application series No. 07/882 473 filed May 13, 1992. According to the KUF test method, the membrane to be tested is first well wetted with a wetting liquid which is able to completely wet the membrane. A displacement liquid, which is immiscible with the wetting liquid used to wet the membrane which has a low, stable interfacial tension, is placed in contact with the upstream side of the wet membrane. Pressure is then applied in increments to the displacement fluid, and the flow of displacement fluid through the membrane is measured as a function of the pressure applied. The displacement liquid must be stable but immiscible with the wetting liquid, and the interfacial tension between the two liquids must be approximately 10 dynes / cm (10 mN / m) or less.

  Controlling the interfacial tension at less than 10 dynes / cm (10 mN / m) allows the displacement of the fluid to be achieved at much lower pressures than in a similar test carried out normally with a water / air interface (i.e. i.e., in bubble point or KL test methods). In addition, it is important that the interfacial tension between the two liquids remains constant during the test procedure. Plotting the flow velocity of the displacement liquid, per unit area of the membrane, through the membrane as a function of the applied pressure can be made, and a straight line can be drawn through the steep part of the resulting curve, using a regression analysis that will intersect the horizontal axis at a given pressure value.

  This point of intersection is considered to be the value of KUF and is directly related to the size of the pores of the membrane. Since there is no flow in diffusion through a membrane which is free from defects, the speed of flow of the displacement liquid through the membrane before the value of KUF is zero, i.e. - say, a flat line in the typical plot of the flow velocity as a function of pressure.



  The KUF values quoted here were determined using pairs of liquids having an interfacial tension of approximately 4 dynes / cm (4 mN / m). In particular, the above-mentioned KUF values were determined using n-pentanol, saturated with water, as the wetting liquid, and water, saturated with n-pentanol, as the displacement liquid. . The immiscible phases are mutually saturated to ensure that the interfacial tension between the liquids which is approximately 4.4 dynes / vm (4.4 mN / m) at room temperature, does not change due to dissolution of one phase in the other. Other factors such as temperature must also remain relatively constant during the test procedure in order to avoid significant changes in the interfacial tension between immiscible liquids during the test.

  While other pairs of liquids can be used to determine KUF, such as, for example, n-butanol and water, n-pentanol and water were used here because the values of KUF thus obtained were within a practical range for the measurement and because the mutual high solubilities of n-pentanol and water ensured that there was a selective tension adsorption of one of the components by the membrane, which then a such adsorption would have little or no effect on the value of KUF obtained. Other alcohol / water systems, for example, n-octanol / water and n-hexanol / water, and other pairs of non-alcoholic liquids, could, of course, be used similarly in the determination of KUF .



  The interfacial tensions for several organic liquids which form a phase boundary with water are reported in the book Interfacial Phenomena, 2nd ed., By J. T. Davies and E.K. Rideal (1963), are described below, with the solubilities of the various components in water as reported in the Chemical Rubber Handbook (CRC), 1970 edition.
 <tb> <TABLE> Columns = 4
 <tb> Head Col 1: Compound
 <tb> Head Col 2: Surface tension
 interfacial
 (dynes / cm or mN / m)
 <tb> Head Col 3: Temp.
 (DEG C)
 <tb> Head Col 4:

  Solubility
 (g / 100 g H2O)
 <tb> <SEP> ethyl ether <SEP> 10.7 <SEP> 20 <SEP> 7.5
 <tb> <SEP> n-octanol <SEP> 8.5 <CEL AL = L> 20 <CEL AL = L> 0.054
 <tb> <SEP> n-hexanol <SEP> 6.8 <SEP> 25
 <tb> <SEP> 20 <SEP> 0.6
 <tb> <SEP> aniline <CEL AL = L> 5.85 <SEP> 20
 <tb> <SEP> n-pentanol <SEP> 4.4 <SEP> 25
 <tb> <SEP> 22 <SEP> 2.7
 <tb> <SEP> ethyl acetate <SEP> 2.9 <SEP> 30
 <tb> <SEP> 15 <SEP> 8.5
 <tb> <SEP> isobutanol <SEP> 2.1 <SEP> 20
 <tb> <CEL CB = 3 AL = L> 15 <SEP> 10.0
 <tb> <SEP> n-butanol <SEP> 1.8 <SEP> 25
 <tb> <SEP> 1.6 <SEP> 20
 <tb> <CEL CB = 3 AL = L> 15 <CEL AL = L> 9.0
 <tb> </TABLE>



   Although only body fluids and water are described in the table immediately above, the KUF test method, as previously indicated, can be performed using any pair of immiscible liquids.



  According to the KUF test method, the wetting liquid can be a single liquid compound, such as n-octanol, and the displacement liquid can also be a single compound, such as water, if it is substantially insoluble in n-octanol. Alternatively, the wetting liquid can be an equilibrium mixture comprising a first liquid compound, such as n-pentanol, which is saturated with a second liquid compound, such as water. The second liquid compound, saturated with the first is then used as the displacement liquid. With regard to the two embodiments, it is important that the interfacial tension between the two liquids remains relatively constant while the test is carried out.

  It is thus recommended that the phases be stable in composition, that is to say, that when the phases are in contact no net flow of fluid occurs through the interface. Thus, there is no substantial variation in the solubility of the displacement liquid in the wetting liquid, which, if present, would affect the results.



  In practice, the KUF test is usually carried out with each of the immiscible phases saturated with the fluid in which it is in intimate contact. For example, the solubility of n-pentanol in water is 2.7 g per 100 g of water at 22 DEG C. Since the n-pentanol will dissolve in water it is preferred that the water phase is saturated with n-pentanol. Similarly, with the n-pentanol phase, it is preferred that it be saturated with water. Mutually saturated phases are easily achieved by shaking a mixture containing sufficient amounts of each of the fluids together in a container or separating funnel. In the tests and examples described here, the organic phase was in each case used to wet the membrane.

  This is an obvious extension of the method of inverting fluids, i.e., wetting the membrane with the aqueous phase and pressurizing the upstream side of the membrane with the organic phase.



  Absolute KUF values, of course, will vary depending on the particular alcohol / water system, although values obtained using other alcohol / water systems can generally be correlated to the KUF values of the n-pentanol / water system by use of the ratio of their respective interfacial tensions. For example, a KUF value of approximately 45 psi (310 kPa) in the n-pentanol / water system is equivalent to a KUF value of approximately 18 psi (124 kPa) in the n-butanol / water system (c that is, 45 psi (or 310 kPa) x 1.8 / 4.4).


 Title reduction
 



  Titer reduction refers to the ability of a particular membrane to remove a given particle from a fluid. As such, titer reduction is a standard measure of the membrane's ability to eliminate biological organisms, such as bacteria and viruses. While any suitable particle can be used in determining the titer reduction, the titer reduction of the present inventive membrane was evaluated by competing the membrane with the bacteriophages T1 and PP7 (generally a 50:50 mixture of the two bacteriophages at a level of 10 <9> to 10 <1> <0> bacteriophages / ml) in phosphate gel buffer. For the purposes of the evaluations reported here, E. coli ATCC N DEG 11303 was the source of phage T1, and P. aeruginosa N DEG ATCC 15612 was the source of phage PP7.

  In addition to the bacterisphages T1 and PP7, the present inventive membrane has also been tested against the coliphage PR772. The phage source PR772 for the purposes of the evaluations reported here was Professor H. W. Ackerman, Department of Microbiology, Faculty of Medicine, University of Laval, Quebec, Canada.



  The reduction in the titer of a membrane is defined as the ratio of the phage contained in the influent to that obtained in the effluent. Since the size of phage T1 is about 0.078 mu m, the size of phage PR772 is about 0.053 mu m, and the size of phage PP7 is about 0.027 mu m, these phages provide excellent models for attesting efficiency of membrane removal for larger, intermediate, and smaller viruses. A membrane is generally considered to have an "absolute" elimination capacity compared to a particular particle, for example, phage T1 as representative of larger viruses, when it has at least a titer reduction of 10 <8>, and preferably at least 10 <1> <0>, against this particle.

  Of course, an absolute elimination capacity of a membrane compared to the phages PR772 or PP7 would confirm the absolute elimination capacity of this membrane to larger viruses.



  Since these biological organisms are capable of rapid replication, they allow easy detection of the smallest amounts in the filtrate of a test solution. Thus, the inability to detect a quantity of a particular element such as a model biological organism in the filtrate of a test solution is excellent confirmation of the fact that the particular membrane actually prevents all biological organisms in the competing liquid. to pass through the membrane.

  In addition, since the amount of viruses found as contaminants in most commercial processes rarely exceeds about 10 <4> / ml, the ability of the present inventive membrane to have a titer reduction of 10 <8> or more can provide virtually absolute assurance of the elimination of all viruses from a wide variety of fluids, particularly those involved in commercial processing, for example, pharmaceutical production.



  The reduction in titer is a function of the KUF value of a membrane and the thickness of the membrane. Since the pressure drop across a membrane is affected exponentially by the KUF of a membrane, while the pressure drop across a membrane is only affected linearly by the thickness of the membrane, small improvements in reduction The titer of a particular membrane can generally be accomplished in a more economical manner by increasing the thickness of the membrane, for example, by providing multiple layers of the same membrane.


 Pressure drop
 



   The pressure drop across a membrane is quite important in the use of such membranes for filtration purposes. The present inventive membrane advantageously provides the desired titer reduction against a certain particulate material with a satisfactory pressure drop (DELTA P) through the membrane. The pressure drops referenced here have been calculated using conventional techniques, such as those described in US Pat. No. 4,340,479, and all the pressure drop values reported here (inch of Hg (cm of Hg) or psi (kPa )) were determined at a constant air flow speed of feet / min (61 cm / min).


 Method of preparation
 



  The membranes of the present invention are prepared from polyvinylidene fluoride (PVDF) using the wet molding procedure described in U.S. Patent 4,340,479 in conjunction with the specific temperature constraints discussed herein. Suitable polyvinylidene fluoride can be used, such as Kynar resins <TM> 761 and PVDF 761. Polyvinylidene fluoride will typically have a molecular weight of at least about 5000 daltons, preferably a molecular weight of at least about 10,000 daltons.



  The present inventive method for preparing the membranes described here includes providing a molding solution comprising polyvinylidene fluoride and a solvent therefor, heating a molding solution to a uniform temperature of about 57 DEG C at about 60 DEG C, distributing the molding solution on a substrate to form a film, quenching the film in a quench bath to form a porous membrane, and washing and drying the porous membrane. The temperature of the molding solution is directly related to the KUF of the resulting membrane, as illustrated in the graph in fig. 3 which contains a curve plotted of the temperature of the molding solution (DEG C) as a function of the KUF of the resulting membrane (psi and kPa).

  For example, a molding solution temperature of approximately 58 DEG C will result and the formation of a membrane having a KUF of approximately 31 psi (214 kPa), while a molding solution temperature of approximately 60 DEG This will result in the formation of a membrane having a KUF of approximately 17 psi (117 kPa).



  It has surprisingly been found that the temperature at which the polyvinylidene fluoride molding solution is maintained is quite critical for the preparation of the present inventive membrane. Great care should be exercised to ensure that the temperature of the molding solution is uniform, i.e., at a particular temperature +/- 0.01 DEG C, so as to ensure substantial uniformity of the pore structure in the membrane.

  In addition, while not wishing to be bound by a particular theory underlying the present invention, it appears that the molding solution has at least a short-term memory, so that it is difficult to prepare the present inventive membrane by a desired KUF value if the temperature of the molding solution has significantly exceeded about 60 DEG C at any time during the processing of this molding solution, even if the temperature is then lowered to below about 60 DEG vs.

  One possible explanation for this apparent effect is that while a single temperature can be reported for the molding solution and the temperature is actually the average of a range (or distribution) of temperatures in the molding solution so that a significant portion of the molding solution may actually be well above the value of the reported temperature.

  This may explain the particular success in the preparation of suitable membranes by the use of the preferred techniques of the present inventive method, which involve approaching the desired temperature of the molding solution with the heaters with increasing accuracy. (which not only effectively provides a uniform temperature for the molding solution but also significantly reduces the possibility that a portion of the molding solution will significantly exceed about 60 DEG C, and more preferably the desired final temperature of the molding solution, at any given step in the heating process).



  While the present inventive method can be carried out in a variety of suitable ways, a preferred embodiment of the present inventive method begins with the formation of a solution consisting of polyvinylidene fluoride in powder form, a solvent for the resin, advantageously dimethylacetamide, and a nonsolvent, advantageously isopropanol. The solution comprises approximately 10 to approximately 20% by weight, and advantageously approximately 15 to 17% by weight of polyvinylidene fluoride. The balance of the solution comprises the solvent and non-solvent in a weight ratio in the range of from about 90:10 to about 70:30, preferably about 80:20.



  The temperature of the polymer solution is then increased to the temperature of the desired molding solution. Small amounts of polymers can be efficiently handled at uniform temperatures in a one-step process, for example a few hundred grams of polymer in a liter of solution can be uniformly heated in a jacketed kettle with pale stirring at high rpm. With larger quantities, particularly quantities of commercial production, it is impractical to use a one-step heating process such as a jacketed kettle due to the significant temperature variations in the molding solution which must be avoided to satisfactorily prepare the present inventive membrane.



  With regard to larger quantities, therefore, the temperature of the molding solution is preferably increased in stages so as to carefully control temperature uniformity while minimizing the time required to increase the temperature. At each successive step, the temperature is increased and brought closer to the molding temperature necessary to prepare a membrane of the desired KUF, but in a manner which ensures greater uniformity (i.e., a distribution of narrower temperature) so as to ensure that the molding solution does not exceed the desired molding temperature.



   In particular, a larger amount of polyvinylidene fluoride can be placed in a thermostatically controlled tank and dispersed in an appropriate mixture of solvent and non-solvent. During the mixing of the molding solution, the temperature of the tank is maintained at a temperature which allows the contents inside to reach a temperature of about 47 DEG C to about 51 DEG C, which is well below the desired molding solution temperature. This average temperature is chosen so as to ensure that no significant portion of the solution exceeds the desired molding solution temperature between about 57 DEG C and about 60 DEG C.

  The components should remain in the tank until the polyvinylidene fluoride has dissolved, and the resulting solution heated uniformly with this equipment, for example, for 16 hours or so.



  The molding solution is then preferably transported through a heat exchanger to raise its temperature to around 52 DEG C and then passed through an in-line mixer (or other suitable high precision heating device) which increases the temperature of the molding solution to the desired uniform molding solution between about 57 DEG C and about 60 DEG C +/- 0.01 DEG C. Temperature uniformity is very important for the uniformity of the pore structure in the resulting membrane.



  After the molding solution has been heated in the in-line mixer, and before the molding solution is distributed (i.e., molded) on a substrate, the viscosity of the molding solution will typically be increased by passing the molding solution through a viscosity leveler, for example, another heat exchanger which lowers the temperature of the molding solution to about 35 DEG C or thereabouts. The molding solution is then distributed on a suitable substrate, for example, mylar, quenched by exposure to a suitable quench bath, for example, an aqueous solution of dimethylacetamide and isopropanol, and washed, for example, with l deionized water, by conventional techniques so as to form the present inventive membrane.



  After washing is complete, the wet membrane is collected and dried. While drying can be undertaken by other suitable means, for example, by heating in an oven, it has been found that drying by direct application of heat, such as in an oven, results in an undesirable increase in the size of the pores of the resulting membrane. This was overcome, however, by the application of microwave energy to the membrane in order to effect drying of the membrane. Microwaves having a frequency of about 24 MHz are preferably used, although any frequency can be used as long as the pore size or the KUF value is not unduly affected.


 Heat treatment
 



  The resulting polyvinylidene fluoride membrane can be heat treated or baked, if desired, to improve its properties. In particular, the present inventive membrane can be heated under conditions to improve the resistance and the subsequent hydrophilization of the membrane.



  Preferably, the present inventive membrane is heated to a temperature of at least about 80 DEG C for a time sufficient to reach a state so that, when subsequently hydrophilized, the resulting hydrophilized membrane has substantially uniform hydrophilic properties. Of course, the present inventive membrane should not be heated to a temperature so high that the membrane becomes soft and deforms, either under its own weight or due to the tension from any mechanical means by which the membrane is supported in the heating process. Typically, the upper temperature limit will be approximately 160 DEG C. The amount of heating time will vary with the heating temperature and the nature of the heated membrane.

  For example, small pieces of flat sheet membrane that are in direct contact with a high temperature surface may require only a brief exposure, for example, less than a minute, to heat, while a rolled membrane of several hundred linear feet may require many hours of low temperature heating for the membrane to reach an appropriate equilibrium temperature. More preferably, the present inventive membrane is heated to a temperature of about 120 DEG C for about 24-72 hours, especially for about 48 hours.



  The heat treatment of the present inventive membrane can be carried out without the membrane being restricted; however, the membrane is preferably dimensionally restricted during the heat treatment so as to minimize or avoid dimensional changes in the membrane, for example, shrinking. Any suitable means can be used to dimensionally restrict the membrane. For example, the membrane may be placed in a frame or may be wound on a core or roll, preferably with interlayer material, such as fibrous nonwoven material, to prevent layer-to-layer contact of the membrane. . More preferably, the present inventive membrane is heat treated in the form of rolls interleaved with a fibrous polyester nonwoven material.



  The heat treatment of polyvinylidene fluoride membranes is more fully described in US Pat. Nos. 5,196,508 and 5,198,505. These patents also describe the improvements in surface modifications which can be obtained by heat treatment of polyvinylidene fluoride membranes.


 Surface modification
 



  The resulting polyvinylidene fluoride membrane is hydrophobic and shows a significant tendency to adsorb proteins and the like, which may be present in the filtered liquid. These characteristics are undesirable in that they contribute to a higher pressure drop across the membrane and may ultimately result in premature discharge of the membrane and / or, in some cases, the formation of a screen layer. secondary on the surface of the membrane. As a result, the present inventive membrane is preferably surface modified to make it hydrophilic (i.e., having a critical wetting surface tension (CWST) of at least about 72 dynes / cm (72 mN / m) as determined by the CWST test described in US Patent 4,880,548) and less susceptible to protein adsorption and repression.



   Such a surface modification of the present inventive membrane can be carried out in any suitable manner and is preferably accomplished by graft polymerization of a suitable monomer on the surface of the membrane. Preferred examples of such monomers include acrylic or methacrylic monomers having alcohol functional groups, such as, for example, hydroxyethylacrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl-methacrylate and combinations thereof , particularly hydroxypropylacrylate and / or hydroxyethylmethacrylate. These monomers can be combined with small amounts of acrylic monomers having no alcohol functional groups, such as methyl methacrylate, as described in U.S. Patent 5,019,260.



  Any suitable means can be used to polymerize the appropriate monomers on the membranes of the present invention. Radiation grafting is the preferred technique for achieving this result. The irradiation source can come from radioactive isotopes such as Cobalt 60, Strontium 90, and Cesium 137, or from machines such as X-ray machines, electron accelerators, and ultraviolet equipment. Preferably, however, the irradiation is in the form of electron beam irradiation. It has been found that, using this form of irradiation, a very uniform distribution of irradiation can be provided. This in turn results in a final product which is grafted more uniformly compared to those membranes which are grafted using other sources of irradiation, for example, Cobalt 60.



  The grafting will typically be achieved by either irradiating the membrane and then exposing it to an appropriate solution of the monomer or by irradiating the membrane while it is exposed to an appropriate solution of the monomer. Whichever procedure is used, grafting should be carried out in the absence of oxygen, for example, under a nitrogen atmosphere, since the oxygen will react with the creative sites created by the radiation exposure, thereby lowering the number of sites available for the desired binding of the polymer. If the membrane is irradiated before immersion in the monomer solution, the membrane should contact the monomer solution as quickly as possible to avoid undesirable reactions resulting from the loss of reactive sites for binding of the polymer to the surface of the membrane.

  The monomer solution can comprise any appropriate concentration of the monomer to be polymerized by grafting, typically 1 to 10% by volume of monomer in a solvent system, generally water itself or with a suitable alcohol such as alcohol t- butyl. The preferred monomer solution in the context of the present invention is 4% by volume of hydroxypropylacrylate, 25% by volume of t-butyl alcohol, and 71% by volume of deionized water. The details and parameters of the grafting of membrane polymers is well known in the art.



  While graft polymerization can be carried out in the absence of crosslinking agents, it is preferred that such crosslinking agents be used, particularly when the above mentioned acrylate monomers are polymerized by grafting on the surface of the membrane. Any suitable crosslinking agent can be used in the context of the present invention.

  Suitable crosslinking agents include said polyacrylate and methacrylate groups of diols and polyols, particularly linear or branched aliphatic diols such as ethylene glycol, 1,2-propylene glycol, diethylene glycol, dipropylene glycol, dipentylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene oxide glycol, and poly (ethylene oxide-copropylene oxide) glycol, as well as triol acrylates such as trimethylolpropane triacrylate. Examples of other crosslinking monomers which can be used in the present invention include allyls, maleimides, unsaturated dicarboxylic acids, vinyl aromatics, polybutadienes, and trimellitic acid esters.

  Other suitable crosslinking agents are described in U.S. Patents 4,440,896, 4,753,938, 4,788,055 and 4,801,766.



  Polyethylene glycol dimethacrylates in which the molecular weight of polyethylene glycol is from about 200 to about 600 are preferred crosslinking agents in the context of the present invention. Polyethylene glycol 600 dimethacrylate, especially in conjunction with hydroxypropylacrylate grafting by irradiation on the surface of the membrane, is the most preferred crosslinking agent.



  The crosslinking agent can be used in any suitable amount. Typically, the crosslinking agent will be added to the grafting solution in an amount of from about 0.025% by weight to about 5% by weight, more typically in an amount from about 0.05% by volume to about 2% by weight. volume. Thus, for example, a solution of monomers containing 4% by volume of hydroxypropylacrylate in water and t-butyl alcohol preferably contains about 0.5% by volume of polyethylene glycol 600 dimethacrylate as an agent. crosslinking.


 Illustrative uses
 



  The present inventive membrane can be used in any suitable application, including many applications in which ultrafilation membranes are commonly used. In view of the excellent reduction in titer of the membrane against viruses and particles of similar sizes, the present inventive membrane has particular utility in the filtration of pharmacological liquids and the like, although the present inventive membrane can be used to filter any suitable fluid.



  Therefore, the present invention provides a method of filtering a fluid comprising passing a fluid through the present inventive membrane, particularly an isotropic, skinless, porous membrane of polyvinylidene fluoride having a KUF of at least 15 psi (103 kPa) when tested using pairs of liquids with an interfacial tension of approximately 4 dynes / cm (4 mN / m) and / or having a titer reduction of at least 10 <8> against a T1 bacteriophage. The fluid to pass through the present inventive membrane comprises viruses, for example, in excess of 10 <2> / ml or even 10 <4> / ml, before passing through the membrane, which viruses can be removed from the fluid so that the fluid comprises less than 10 <2> / ml or even no virus after passing through the membrane.

   Thus, the present inventive membrane can be used to treat fluids to reduce or eliminate viruses therefrom, and can also be used to recover and concentrate viruses from fluids for viral identification, testing, and the like.



  The ability of this inventive membrane to be tested for integrity with relative ease, and to be consistently prepared on a commercial basis, allows the inventive membrane to provide a predictable removal rate for given substances . In addition, the excellent removal characteristics of the present inventive membrane are obtained at a reasonable pressure drop across the membrane. Thus, since the present inventive membrane can be used in applications in which ultrafiltration membranes are commonly used, the present inventive membrane will prove to be much more desirable than, and have better performance, ultrafiltation membranes in these same applications.



  The present inventive membrane can be used alone or can be laid with a suitable support structure. Similarly, the present inventive membrane can be used in suitable filters, filter cartridges, and the like. Of course, in view of the highly uniform nature of the pore structure in the present inventive membrane, as well as the low susceptibility of protein adsorption of the grafted embodiments of the membrane, the present inventive membrane can be used in dead end filtration applications, as well as in tangential or cross flow filtration applications.



  The present inventive membrane is expected to be especially useful in filter elements, such as filter cartridges, which are generally described in US Patent 4,340,479. Preferred filter elements using the present inventive membrane include the present inventive membrane under the sheet form, where the sides of the membrane have been overlapped and sealed to form a tubular configuration having an outer surface, an interior, and two ends, and end caps sealed at the ends of the tube, where at least the one of the end caps has a central opening providing access to the interior of the tube, and where all the seals are fluid tight. The present inventive membrane is preferably striated in such a filter element so as to provide a large membrane surface for the volume of the filter element.

  At least one side of the membrane will typically be bonded to a porous support layer, and, in such a situation, the membrane and the porous support layer will generally both be striated. The filter element may comprise a single membrane of the present invention, or, more preferably, comprises a multiplicity of such membranes bonded together. When there are multiple membranes in the filter element, the membranes are preferably separated by a porous support layer to which each membrane is bonded. The other aspects of the filter element may be of any other suitable construction and prepared from any suitable material.

  For example, the end caps can be prepared from a suitable polymeric material, such as polyester, particularly, polybutylene glycol terephthalate or polyethylene glycol terephthalate. The filter element can be constructed using techniques which are well known in the art.



  The following examples further illustrate the present invention and, of course, should not be taken as limiting in any way its scope.


 Example 1
 



  This example illustrates the preparation of several filtration membranes according to the present invention. The various filtration membranes were prepared using different temperatures of molding solution to demonstrate the effect of the temperature of the molding solution on the KUF of the resulting filtration membrane.



  A molding solution was prepared from 17.0% by weight of polyvinylidene fluoride resin, 66.4% by weight of dimethyl acetamide (solvent), and 16.6% of isopropanol (non-solvent). The molding solution was stirred in a closed container to dissolve the polyvinylidene fluoride resin in the 80:20 w / w solvent / non-solvent mixture, and the temperature of the molding solution was increased to 50.9 DEG C and maintained at this temperature.



  Four samples of molding solutions were then passed through an in-line mixer, and each of the samples of molding solutions was brought to a different temperature. Each of the solutions was then cooled to increase the viscosity, molded as a film on a substrate, and subjected to a quench bath comprising 42% by weight of water, 51% by weight of dimethyl acetamide, and 7% by weight. isopropanol. The quench bath was maintained at 30 DEG C. The molding film generally remained in contact with the quench bath for less than one minute. The resulting membrane was then washed with water to remove the solvent, and the membrane was microwave dried under restriction to prevent shrinkage. The membranes were thus prepared with each of the four samples of molding solutions.



  The temperatures of each of the molding solution samples and the KUF values of each of the resulting membranes are described below.
 <tb> <TABLE> Columns = 3
 <tb> Head Col 1: Sample
 <tb> Head Col 2: Temperature
 (DEG C +/- 0.01 DEG C)
 <tb> Head Col 3: KUF
 (psi) [kPa]
 <tb> <SEP> 1A <SEP> 58.22 <SEP> 30 [207]
 <tb> <SEP> 1B <SEP> 58.97 <SEP> 23 [159]
 <tb> <CEL AL = L> 1C <CEL AL = L> 59.77 <SEP> 18 [124]
 <tb> <SEP> 1D <SEP> 60.17 <SEP> 17 [117]
 <tb> </TABLE>



  The resulting data is plotted as the temperature of the molding solution (DEG C) as a function of KUF (psi and kPa) as a graph in fig. 3. As is apparent from the data, an increase in the temperature of the molding solution in the range of about 57 DEG C to about 60 DEG C results in a corresponding decrease in the KUF of the prepared filtration membrane from the molding solution.


 Example 2
 



   This example illustrates the preparation of the polyvinylidene fluoride membranes of the present invention which have been provided with a graft polymerized coating so as to make the membrane hydrophilic and less susceptible to protein binding. The properties of such membranes both before and after grafting have been evaluated to demonstrate that the grafting process does not adversely affect the gradation of the membrane pores and contributes only a modest increase in the pressure drop across the filtration membrane.



  Several membranes with different KUF values were prepared according to the procedure described in Example 1. A portion of each membrane was grafted using an electron beam grafting procedure. In particular, the membranes were passed under an electron beam generator (with settings at 175 kV and 3 mA) at a speed of 20 feet / min so as to reach the total irradiation rate of 2.4 Mrad . The membranes were then passed through a grafting solution of 4% by volume of hydroxypropyl acrylate, 25% by volume of t-butyl alcohol, and 71% by volume of deionized water, rolled up under a nitrogen atmosphere. (that is to say, protected from oxygen, and stored for several hours before washing the examples of ungrafted monomer. The grafted membranes were dried on a frame at 100 ° C. for 10 minutes.



  The KUF, the thickness, and the pressure drop (DELTA P) through each membrane, in non-grafted form, have been determined, the results are described here after.
 <tb> <TABLE> Columns = 7
 <tb> Head Col 1: Sample
 <tb> Head Col 2 to 4 AL = L: Ungrafted membrane
 <tb> Head Col 5 to 7 AL = L: Grafted membrane
 <tb> Head Col 2 AL = L: thickness
 (thousandth +/- 0.2 mil) (mu m +/- 5 mu m)
 <tb> Head Col 2: KUF
 (psi)
 [kPa]
 <tb> Head Col 3: DELTA P
 (inch Hg) [cm Hg]
 <tb> Head Col 4:

  KUF
 (psi)
 [kPa]
 <tb> Head Col 5: DELTA P
 (inch Hg) [cm Hg]
 <tb> Head Col 6: wettable by water
 <tb> <SEP> 2A <SEP> 1.6 <SEP> 17 <SEP> 5.5 <SEP> 18 <SEP> 5.7 <CEL AL = L> yes
 <tb> <SEP> [41] <SEP> [117] <SEP> [14.0] <SEP> [124] <SEP> [14.5]
 <tb> <SEP> 2B <SEP> 1.6 <CEL AL = L> 24 <SEP> 10.2 <SEP> 25 <SEP> 13.8 <SEP> yes
 <tb> <SEP> [41] <SEP> [165] <SEP> [25.9] <CEL AL = L> [172] <SEP> [35.1]
 <tb> <SEP> 2C <SEP> 1.9 <SEP> 21 <SEP> 16.7 <SEP> 24 <SEP> 28.7 <SEP> yes
 <tb> <CEL CB = 2 AL = L> [48] <SEP> [145] <SEP> [42.4] <SEP> [165] <SEP> [72.9]
 <tb> <SEP> 2D <SEP> 2.0 <SEP> 24 <CEL AL = L> 16.6 <SEP> 25 <SEP> 28.7 <SEP> yes
 <tb> <SEP> [51] <SEP> [165] <SEP> [42.2] <SEP> [172] <CEL AL = L> [72.9]
 <tb> <SEP> 2E <SEP> 1.9 <SEP> 17 <SEP> 8.0 <SEP> 16 <SEP> 6.8 <SEP> yes
 <tb> <CEL CB = 2 AL = L> [48] <CEL AL = L> [117] <SEP> [20.3] <SEP> [110] <SEP> [17.3]
 <tb> </TABLE>



  As is apparent from the resulting data, the grafting of the present inventive membranes makes its membranes hydrophilically desirable, i.e., wettable with water, while adversely affecting the characteristics of KUF and diaphragm pressure drop only modestly.


 Example 3
 



  This example illustrates the excellent titer reductions against various viruses which are characteristic of the present inventive membrane.



  Various membranes (142 mm discs of approximately 1.5-2.0 thousandths (38.50 μm) in thickness) were prepared according to the procedure described in Example 1 and were grafted according to the procedure described in Example 2. The grafted membranes were put in competition with a 50:50 mixture of bacteriophages T1 and PP7 (at a level of approximately 10 <1> <0> bacteriophages / ml) in phosphate gel buffer. As previously discussed, the size of phage T1 is about 0.078 µm, while the size of phage PP7 is about 0.027 µm. Thus, these bacteriophages are quite representative of larger or smaller viruses, respectively. The titer reductions of each membrane, alone or in multiple layers were determined as the ratio of the particular phage contained in the influent to that present in the effluent.

  The KUF of the non-grafted membrane, the number of layers of membranes tested, and the reduction in the titer (TR) for each phage are given below.
 <tb> <TABLE> Columns = 5
 <tb> Head Col 1: Sample
 <tb> Head Col 2: KUF
 (psi) [kPa]
 <tb> Head Col 3: Number of
 layers
 <tb> Head Col 4: TR
 (T1 phage)
 <tb> Head Col 5:

  TR
 (PP7 phage)
 <tb> <SEP> 3A <SEP> 23 [159] <SEP> 3 (SEP) <> 1.3 x 10 <1> <0> (SEP) <> 4.4 x 10 <9>
 <tb> <SEP> 3B <CEL AL = L> 21 [145] <SEP> 3 (SEP) <> 1.3 x 10 <1> <0> <SEP> 0.0 x 10 <6>
 <tb> <SEP> 3C <SEP> 24 [165] <SEP> 1 (SEP) <> 9.1 x 10 <9> <SEP> 5.0 x 10 <1>
 <tb> <SEP> 3D <SEP> 24 [165] <SEP> 2 (SEP) <> 9.1 x 10 <9> <SEP> 4.0 x 10 <2>
 <tb> <CEL AL = L> 3E <SEP> 24 [165] <SEP> 3 (SEP) <> 1.0 x 10 <9> <SEP> 5.0 x 10 <4>
 <tb> <SEP> 3F <SEP> 24 [165] <SEP> 3 <CEL AL = L >> 1.0 x 10 <1> <0> <SEP> 1.0 x 10 <5>
 <tb> <SEP> 3G <SEP> 23 [159] <SEP> 3 (SEP) <> 1.0 x 10 <1> <0> <SEP> 9.0 x 10 <5>
 <tb> <SEP> 3H <SEP> 17 [117] <SEP> 3 (SEP) <> 1.0 x 10 <1> <0> <SEP> 5.7
 <tb> </TABLE>



  The resulting data shows that the filtration membrane of the present invention can have a very high titer reduction and can be capable of "absolute" removal of viruses, as is particularly apparent with sample 3A. In addition, this titer reduction capacity can be accomplished with remarkably thin membranes, as exemplified by Example 3C. In addition, the resulting data demonstrates that the filtration membrane of the present invention has a very uniform pore structure. For example, the 3H sample is capable of eliminating all the T1 bacteriophages while essentially letting all the PP7 bacteriophages pass. Thus, the 3H sample membrane has a pore size between about 0.078 µm and about 0.027 µm, which is a quite narrow pore distribution.


 Example 4
 



  This example further illustrates the excellent reduction in titer of viruses which characterize the present inventive membrane.



  The grafted filtration membrane of Example 3 which was designated sample 3F was put in competition with a mixture of coliphages PR772 (at a level of 5.2 × 10 <8> phages / ml) and PP7 bacteriophages (at a level of 1.7 x 10 <9> phages / ml) in phosphate gel buffer. As previously described, the size of phage PR772 is approximately 0.053 μm, while the size of phage PP7 is approximately 0.027 μm. Thus, these phages are quite representative of the intermediate-sized and smaller viruses, respectively. The titer reductions of each virus membrane in multiple layers was determined as the ratio of the particular phage contained in the influent to that present in the effluent.

   The KUF of the ungrafted membrane, the number of layers of membranes tested and the titer reduction (TR) for each phage are described below.
 <tb> <TABLE> Columns = 5
 <tb> Head Col 1: Sample
 <tb> Head Col 2: KUF
 (psi) [kPa]
 <tb> Head Col 3: Number of
 layers
 <tb> Head Col 4: TR
 (PR772 phage)
 <tb> Head Col 5: TR
 (PP7 phage)
 <tb> <SEP> 3G <SEP> 23 [159] <SEP> 3 (SEP) <> 5.2 x 10 <8> <SEP> 2.2 x 10 <6>
 <tb> </TABLE>



  The results determined confirm the excellent reduction in titer of the present inventive membrane against viruses of intermediate size. In addition, in view of the moderate removal efficiency of this particular membrane sample against the much smaller phage PP7, the pore size of the particular membrane sample is shown to be quite small, that is, i.e., below about 0.053 mu m, while the pore size distribution of the sample is also shown to be quite narrow, i.e. somewhat less than about 0.027 mu m to below about 0.053 mu m.


 Example 5
 



  This example illustrates the approximate lower operating limit in terms of pore sizes of the present inventive membrane with respect to satisfactory titer reduction against larger viruses.



  A membrane 1.8 thousandths (46 μm) thick was prepared according to the procedure described in Example 1 and evaluated with regard to KUF, the pressure drop (DELTA P), and the reduction in the titer ( TR) against bacteriophage T1 and PP7 as described in Example 3. The resulting data are described below.
 <tb> <TABLE> Columns = 6
 <tb> Head Col 1: Sample
 <tb> Head Col 2: KUF
 (psi) [kPa]
 <tb> Head Col 3: DELTA P
 (inch Hg) [cm Hg]
 <tb> Head Col 4: Number of layers
 <tb> Head Col 5: TR
 (phage)
 <tb> Head Col 6: (T1 TR
 (PP7 phage)
 <tb> <SEP> 5A <SEP> 17 [117] <SEP> 6.8 [17.3] <SEP> 1 <SEP> 7x10 <6> <SEP> <10
 <tb> <CEL AL = L> 5B <CEL AL = L> 17 [117] <SEP> 6.8 [17.3] <SEP> 2 (SEP) <> 9 x 10 <8> <SEP> <10
 <tb> </TABLE>



  The resulting data demonstrate that the present inventive membrane having a KUF value of about 17 psi (117 kPa) and a thickness of at least about 3.6 thousandths (92 mu m) will show a titer reduction of more than 10 <8> against larger viruses. The fact that the present inventive membrane of this example had an "absolute" elimination capacity with respect to the largest phage T1 while essentially having no elimination capacity with respect to the smaller phage PP7 demonstrates that the present medium inventive not only has a pore size of between about 0.078 mu m and about 0.027 mu m, but that the pore size distribution is quite narrow, i.e., below 0.078 mu m to above about 0.027 mu m.


 Example 6
 



  This example illustrates the low protein adsorption characteristics of the grafted filtration membrane of the present invention.



  An immersion charge binding test was carried out on samples of grafted filtration membranes prepared according to the procedure of Example 2 (samples 6A-6D), as well as on non-grafted controls (6E and 6F). Each membrane was immersed in an IgG solution containing 125 I goat IgG and 200 μg / ml of unlabeled goat IgG for 60 minutes. Each membrane was washed with phosphate buffered saline (PBS) and evaluated for the adsorbed IgG. The membranes were then washed with a 1% aqueous SDS solution in 2M urea and evaluated for the adsorbed IgG component. The membranes were then washed with an aqueous solution of 1% SDS in 2M urea and evaluated again for the adsorbed IgG.

  The results of these assessments are described below.
 <tb> <TABLE> Columns = 6
 <tb> Head Col 1: Sample
 <tb> Head Col 2: Substrate
 <tb> Head Col 3: Graft solution
 <tb> Head Col 4: Thickness
 (mil)
 [mu m]
 <tb> Head Col 5: Adsorption
 Post-PBS
 (mu g / cm <2>)
 <tb> Head Col 6: Adsorption
 Post-SDS
 (mu g / cm <2>)
 <tb> <SEP> 6A <SEP> PVDF <SEP> 12% HEMA <SEP> 1.9 [48] <SEP> 25.0 <SEP> 19.5
 <tb> <SEP> 6B <CEL AL = L> PVDF <SEP> 9% HEMA <SEP> 1.8 [46] <SEP> 28.8 <SEP> 23.9
 <tb> <SEP> 6C <SEP> PVDF <SEP> 9% HEMA <CEL AL = L> 1.7 [43] <SEP> 22.4 <SEP> 19.6
 <tb> <SEP> 6D <SEP> PVDF <SEP> 4% HPA <SEP> 1.7 [43] <SEP> 12.4 <CEL AL = L> 8.8
 <tb> <SEP> 6E <SEP> PVDF <SEP> none <SEP> 1.6 [41] <SEP> - <SEP> 86.6
 <tb> <SEP> 6F <CEL AL = L> PVDF <SEP> none <SEP> 1.5 [38] <SEP> - <SEP> 82.6
 <tb> </TABLE>



  The resulting data demonstrates that a filtration membrane of the present invention which has been appropriately graft polymerized will have a low level of protein adsorption. The present inventive membranes grafted with hydroxyethyl methacrylate (HEMA) showed a much reduced level of protein adsorption compared to the non-grafted control. In addition, the present inventive membrane grafted with hydroxypropylacrylate (HPA) adsorbed only about half of proteins compared to the HEMA grafted membranes of the present invention.


 Example 7
 



  This example illustrates that microwave drying of the present inventive membrane has no significant opposite effect on the filtration characteristics of the membrane.



  Two membrane samples were prepared according to the procedure described in Example 1. One of the membranes was dried with a microwave dryer (designated 7A), while the other membrane was dried on a steam drum drier (designated 7B). The KUF values of the two membranes were determined both before and after drying, the results are described below.
 <tb> <TABLE> Columns = 3
 <tb> Head Col 1: Sample
 <tb> Head Col 2: KUF (as molded)
 (psi) [kPa]
 <tb> Head Col 3: KUF (dried)
 (psi) [kPa]
 <tb> <SEP> 7A <SEP> 22 [152] <SEP> 21 [145]
 <tb> <SEP> 7B <SEP> 22 [152] <SEP> 17 [117]
 <tb> </TABLE>



  These results demonstrate that microwave drying of the membranes, in contrast to conventional drying, does not substantially affect the pore size of the present inventive membrane.


 Example 8
 



  This example illustrates the isotropic nature, i.e., the symmetrical pore structure, of the present inventive membrane.



   Several membranes of different KUF values were prepared according to the procedure described in Example 1. The KUF and the pressure drop (DELTA P) divided by the thickness (inches of Hg / mil and cm of Hg / mu m) of each membrane has been determined, and the results are described below.
 <tb> <TABLE> Columns = 3
 <tb> Head Col 1: Sample
 <tb> Head Col 2:

  KUF (psi) [kPa]
 <tb> Head Col 3: DELTA P / mil (inch Hg / mil) [cm Hg / mu m]
 <tb> <SEP> 8A <SEP> 12 [83] <SEP> 1.50 [0.15]
 <tb> <CEL AL = L> 8B <CEL AL = L> 12 [83] <SEP> 1.32 [0.13]
 <tb> <SEP> 8C <SEP> 13 [90] <SEP> 1.50 [0.15]
 <tb> <SEP> 8D <SEP> 16 [110] <SEP> 3.27 [0.33]
 <tb> <SEP> 8E <SEP> 17 [117] <SEP> 2.84 [0.28]
 <tb> <SEP> 8F <SEP> 17 [117] <SEP> 2.93 [0.29]
 <tb> <CEL AL = L> 8G <SEP> 17 [117] <SEP> 2.63 [0.26]
 <tb> <SEP> 8H <SEP> 17 [117] <SEP> 4.27 [0.43]
 <tb> <SEP> 8I <CEL AL = L> 18 [124] <SEP> 2.65 [0.27]
 <tb> <SEP> 8J <SEP> 18 [124] <SEP> 4.06 [0.41]
 <tb> <SEP> 8K <SEP> 19 [131] <CEL AL = L> 3.90 [0.39]
 <tb> <SEP> 8L <SEP> 21 [145] <SEP> 11.20 [1.12]
 <tb> <SEP> 8M <SEP> 21 [145] <SEP> 5.33 [0.53]
 <tb> <CEL AL = L> 8N <SEP> 22 [152] <SEP> 20.75 [2.08]
 <tb> <SEP> 8O <SEP> 22 [152] <SEP> 7.85 [0.79]
 <tb> <SEP> 8P <CEL AL = L> 23 [159] <SEP> 8.00 [0.80]
 <tb> <SEP> 8Q <SEP> 23 [159] <SEP> 14.40 [1.44]
 <tb> <SEP> 8R <SEP> 23 [159] <CEL AL = L> 11.00 [1.10]
 <tb> <SEP> 8S <SEP> 24 [165] <SEP> 12.08

   [1.21]
 <tb> <SEP> 8T <SEP> 24 [165] <SEP> 14.96 [1.50]
 <tb> <SEP> 8U <SEP> 24 [165] <SEP> 14.07 [1.41]
 <tb> <SEP> 8V <SEP> 24 [165] <SEP> 11.93 [1.19]
 <tb> <CEL AL = L> 8W <SEP> 24 [165] <SEP> 14.52 [1.45]
 <tb> <SEP> 8X <SEP> 24 [165] <SEP> 9.70 [0.97]
 <tb> <SEP> 8Y <CEL AL = L> 25 [172] <SEP> 22.27 [2.23]
 <tb> </TABLE>



  The resulting data are plotted as the pressure drop (DELTA P) across the membrane divided by the thickness of the membrane (inch Hg / thousandth and cm Hg / mu m) on a logarithmic scale as a function of the KUF of the membrane (psi and kPa) as a graph in FIG. 4. The curve plotted is the result of a least squares adjustment and at a correlation factor of 0.87. As is apparent from the data, an increase in the KUF results in a logarithmic increase in pressure drop as a function of the thickness of the membrane thickness. This relationship is characteristic of an isotropic filtration membrane, and confirms that the present inventive filtration medium is isotropic in nature.



   All the references cited are incorporated here in their entirety by reference.



  While this invention has been described with an emphasis on preferred embodiments, it will be apparent to those of ordinary skill in the art that variations of preferred products and preferred methods can be used and it is understood that The invention can be practiced other than specifically described here. Therefore, this invention includes all modifications encompassed by the spirit and scope of the invention as defined by the following claims.


    

Claims (22)

      1. Polyvinylidene fluoride membrane, isotropic, skinless, porous, having (a) a KUF of at least 103 kPa when tested using pairs of liquids having an interfacial tension of approximately 4 mN / m and / or ( b) a reduction in the titer of at least 10 <8> against a T1 bacteriophage. 2. Membrane according to claim 1, characterized in that said membrane has a KUF of 103 kPa to 345 kPa. 3. Membrane according to claim 1 or 2, characterized in that said membrane has a reduction in titer of at least 10 <8> against a bacteriophage T1. 4. Membrane according to claim 3, characterized in that said membrane has a reduction in titer of at least 10 <8> against a coliphage PR772 and / or a bacteriophage PP7. 5. Membrane according to any one of Claims 1 to 3, characterized in that the said membrane has a reduction in titer of 10 <2> or less against a bacteriophage PP7. 6. Membrane according to any one of claims 1 to 5, characterized in that said membrane has a thickness of approximately 500 μm or less. 7. Membrane according to any one of claims 1 to 6, characterized in that said membrane comprises a surface coating of a polymer making said membrane hydrophilic and less susceptible to protein adsorption. 8. Membrane according to claim 7, characterized in that said polymer comprises one or more acrylic and methacrylic monomers having hydroxyl functional groups. 9. A method of preparing a membrane according to any one of claims 1 to 8, characterized in that it comprises providing a molding solution comprising polyvinylidene fluoride and a solvent therefor, heating of said molding solution at a uniform temperature of about 57 DEG C to about 60 DEG C, distributing said molding solution on a substrate to form a film, quenching said film in a quench bath so as to form a porous membrane, and washing and drying said porous membrane. 10. Method according to claim 9, characterized in that said membrane is at least partially dried by exposure of said membrane to microwave irradiation. 11. Method according to claim 9 or 10, characterized in that said membrane is treated to provide said membrane with a surface coating of a polymer making said membrane hydrophilic and less susceptible to protein adsorption. 12. Method according to claim 11, characterized in that said treatment comprises the binding to the surface of said membrane of a polymer comprising one or more acrylic or methacrylic monomers having hydroxyl functional groups. 13. Method according to claim 11 or 12, characterized in that said polymer is grafted by irradiation on said membrane. 14. Method according to claim 13, characterized in that said irradiation is an irradiation by electron beams. 15. Membrane prepared by the method according to any one of claims 9 to 14. 16. Use of a membrane according to any one of claims 1 to 8 for the filtration of a fluid, characterized in that it comprises the passage of a fluid through said membrane. 17. Use according to claim 16, characterized in that said fluid comprises viruses before having passed through said membrane and contains less virus after having passed through said membrane. 18. Use of a membrane according to any of claims 1 to 8 in the form of a filter element, said membrane having sides which have been overlapped and sealed to form a tubular configuration to form an exterior surface, an interior, and two ends, and end caps sealed at the ends of the tube, where at least one of said end caps has a central opening providing access to the interior of the tube, and all of said seals are fluid tight. 19. Use according to claim 18, characterized in that said membrane is ridged. 20. Use according to claim 18 or 19, characterized in that at least one of the sides of said membrane is adhered to a porous support layer. 21.  Use according to claim 20, characterized in that said filter element comprises multiple membranes adhered together. 22. Use according to claim 21, characterized in that said membranes are separated by a porous support layer to which each membrane is adhered.