Composite cellulose nitrate membrane on polyester support
BACKGROUND OF THE INVENTION 
     The invention concerns a microporous isotropic cellulose nitrate (&#34;CN&#34;) membrane on a polyester support which finds application in the production of sorbent reactants, in particular, test strips for use in analytical chemistry and medicinal diagnostics, and to a process for the manufacture of such a membrane. 
     In the case of composite membranes reinforced by film supports, special challenges arise in regard to their required characteristics. The membrane must be durably adhered to the film support and must not separate therefrom during further processing or in the course of its use. Ample hydrophilicity of the membrane must be maintained throughout both storage and during the course of use. Unless such characteristics are met, reproducibility of analytical and diagnostic results cannot be guaranteed. Furthermore, in order for analyses and diagnostic examinations to proceed rapidly, the supported membrane must exhibit a high lateral migratory velocity for the liquids tested. &#34;Lateral migration velocity&#34; means the speed with which a liquid introduced to the surface of a membrane spreads in a direction generally parallel to the support film, and is expressed in units of cm/min or mm/min. 
     Composite supported membranes used as sorbent test strips in analytic and diagnostic procedures are known. German Patent No. 34 07 359 describes a supported membrane comprising an asymmetric membrane upon a macroscopically smooth polyester film made by coating the polyester support with a polymeric casting solution comprising polyamides, polyether carbonates, polyacrylic nitriles or polyurethanes, whereby the polymeric coating is deposited on the support by phase inversion. A primary drawback of such a membrane lies in the unsatisfactory adhesion of the membrane to the support. Although German Patent No. 40 09 186 discloses a method of achieving a more lasting adhesion between an asymmetric polyurethane membrane and a support, the method comprising the use of a special polymer blend support having a roughened surface treated with an anti-static solution, this approach is relatively complex and costly. Another serious drawback of asymmetric membranes is that the asymmetric structure leads to a lateral migration velocity gradient for liquids introduced to the membrane, whereby liquid moves faster in the zone of the larger diameter pores (those adjacent the support) than in the zone of the smaller diameter pores (those on the opposite side of the membrane). Because of this gradient, use of such a supported membrane as a sorbent test strip often yields blurred, inexact and difficultly reproducible indications for the material being tested. 
     Because of its good adsorption characteristics, particularly with respect to substances containing protein, CN is a preferred membrane material for sorbent reactants. However, no durable adhesion of a pure CN membrane on a polyester support may be expected from a mere casting of the membrane on the polyester support. European Patent No. 0 421 235 describes an approach to securing a CN membrane to a support, namely, a porous laminated CN membrane is glued to a polyester support film by a layer of adhesive. The chief disadvantage of such a membrane is that with adhesives of this type, as storage time increases, a loss in adsorption potential and of the hydrophilicity of the membrane layer can occur, because the adhesive migrates into the membrane and reacts with surfactant which is also present in the membrane. Moreover, the fabrication of such laminated membranes and gluing of the same to the supports is a costly multi-step process. Lamination of CN membranes by merely using heat and pressure is not possible both because no durable adhesion is achieved and because it causes the CN membrane to become partially compacted, resulting in a substantial loss in the membrane&#39;s lateral migration velocity. 
     A principal object of the present invention, therefore, is the provision of a composite microporous CN membrane on a polyester support wherein the CN membrane is bonded durably to the support and which exhibits no lateral migration gradient, all without the use of heat, pressure or adhesives. A further object is to provide an economical procedure for the production of such a composite membrane. These objects and others will become apparent upon consideration of the invention, which is summarized and described in detail below. 
     SUMMARY OF THE INVENTION 
     The essence of the present invention lies in providing an adsorbent membrane comprising a polymeric blend of CN and cellulose acetate (&#34;CA&#34;) theft has an isotropic structure. Such a membrane is produced by a phase inversion process following an evaporative procedure, wherein care is taken to ensure that the membrane is free from filter dust. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b are scanning electron photomicrographs (SEMs) of an exemplary membrane of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, it has been discovered that a durable adhesion of an absorbent isotropic CN membrane to a polyester film support may be achieved by casting the membrane from a polymer blend of CN with small quantities of CA. Rather surprisingly, the affinity of such a CN/CA membrane to the polyester support is so great, that no pretreatment of the support or use of any adhesive or any other means for adhesion is required. Since the membrane is directly bonded to the film support, the danger of migration of foreign material into the composite membrane is excluded from the outset. It has further been discovered that with the small portion of CA employed in the polymer blend, the desirable characteristics of CN for use with sorbent reactants, such as good adsorption ability and rapid lateral migration velocity, remain intact. The preferred portion of CA in the polymer blend relative to CN is from 0.5 to 10 wt %, more preferably from 1 to 5 wt %. Since such composite membranes have an isotropic structure, they exhibit virtually no lateral migration velocity gradients. Liquids brought in contact with such membranes spread with nearly uniform lateral speed, completely penetrating the membrane and moving in a plane that is perpendicular to the face of the membrane. Thus, by use of such membranes as sorbent reactant test strips, sharp and highly reproducible imaging of the material to be tested is obtained. 
     It has also been found that so-called &#34;filter dust&#34; is also partly responsible for the appearance of fuzzy and poorly reproducible imaging on the tested materials, which is generally encountered in the surface areas of the membranes. Filter dust forms during the production of the membrane from non-membrane-forming components of the raw materials used which mainly consist of sub-molecular cellulose derivatives. The tendency toward the formation of filter dust is especially pronounced in the case of membranes with large pore diameters--between 0.45 and 15 μm--which is the pore diameter range preferably employed for sorbent reactants because lateral migration velocity increases with the increase in pore size. Membranes prepared in accordance with the present invention have proven to be free of such filter dust imperfections. 
     In a particularly preferred embodiment, a wetting agent or surfactant is introduced to the membrane, either as a component of the CN/CA membrane casting solution, or by injection into the membrane after casting. The amount of surfactant included in the casting solution is preferably in an amount that, upon contact of the test liquid with color indicators such as test reagents in the membrane, an enhanced or sharpened image appears; this phenomenon is particularly helpful if the test reagent is laid out in the membrane in a regular pattern, such as points, lines or cross-hatching. A preferred class of surfactants are the alkyl sulphonates, having alkyl groups containing from 12 to 18 carbon atoms, most preferably averaging 15 carbon atoms. 
     Depending upon pore size, a membrane of the present invention can reach a lateral migration velocity for water of from 26 mm/5 min (pore diameter 0.45 μm) to 82 mm/5 min (pore diameter 10 μm), measured according to vertical absorption analog DIN 53106. 
     The composite CN membranes of the present invention are preferably made by a phase inversion evaporation process and are thereby precipitated directly onto the support film. To this end, the untreated support film is coated with the casting dope, comprising CN and CA in suitable solvents, such as methyl acetate, ethanol, and butanol with low water content, followed by evaporation of the volatile components. For the production of hydrophilic membranes, surfactant is preferably added to the casting dope. For the provision of filter dust-free membranes, one should follow the treatment set forth in German Patent No. 37 08 946, generally comprising dissolution of the polymer blend, followed by reprecipitation. The support film preferably comprises untreated, optically clear polyethylene terephthalate having a thickness of from 50 to 175 μm, most preferably from 100 to 150 μm. 
     EXAMPLE 1 
     A polymeric blend solution was prepared consisting of 8.8 wt % CN, 0.2 wt % CA, 45.3 wt % methyl acetate, 32.8 wt % ethanol and 12.9 wt % 2-butanol (total 100 wt %). To this polymeric blend solution 9.2 wt % water (relative to the 100 wt %) was added to form a casting dope. Prior to adding the cellulose derivatives to the polymeric blend solution, they were first dissolved in acetone and methylene chloride and reprecipitated by the addition of methanol and water to remove extraneous non-membrane-forming substances which are responsible for the creation of &#34;filter dust.&#34; In a covered tensioning machine, the casting dope was coated on an untreated support film of 800 μm-thick polyethylene terephthalate; the volatiles were then evaporated at temperatures between 15° and 25° C. to bring about phase inversion, causing the formation of a CN membrane of isotropic structure with a pore size of 8 μm. FIGS. 1a and 1b are 500×SEMs of cross sections of the membrane produced. FIG. 1a shows the top side of the membrane, while FIG. 1b shows the underside of the membrane that is in contact with the support film. The support film itself is not visible. The isotropic structure of the membrane is evident in both FIGS. 1a and 1b. 
     EXAMPLES 2-4 
     Example 1 was repeated with the exceptions that 0.01 wt % surfactant (sodium alkyl sulphonate having an average alkyl chain of 15 carbons) and 6.5 wt % water (Example 2) and 10 wt % water (Example 4) were added to the casting dope solution. Membranes having an isotropic structure and pore sizes between 1 and 10 μm and with excellent lateral migration velocity were obtained. The results are summarized in the following table. 
     
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            Average Pore Lateral Migration                                
Water       Diameter     Velocity                                         
Ex. No. (wt %)  (μm)      mm/5 min                                     
                                     mm/2 min                             
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2       6.5     0.45         26      17                                   
3       9.2     8.00         68      45                                   
4       10.0    10.00        82      54                                   
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     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.