Microporous sheet product and methods for making and using the same

#The present invention relates generally to microporous sheet products and relates more particularly to a novel microporous sheet product and to methods of making and using the same.

#Microporous sheet products are well-known and commonly used articles found in items as diverse as, for example, storage batteries, food packaging materials, and ultrafiltration devices. For example, in storage batteries, microporous sheet products are commonly used as battery separators. Typically, a storage battery includes at least one pair of electrodes of opposite polarity and, in many cases, includes a series of electrode pairs of alternating polarity. The current flow between the electrodes of each pair is maintained by an electrolyte. Depending on the nature of the battery system, the electrolyte may be acidic, alkaline, or substantially neutral. For example, in alkaline storage batteries, which include, but are not limited to, primary, secondary, nickel, zinc and silver cells, the electrolyte is generally an aqueous solution of potassium hydroxide. By contrast, in lead acid batteries, the electrolyte is typically a sulfuric acid solution, and, in lithium rechargeables, the electrolyte is typically a lithium salt solution in an aprotic organic solvent or solvent blend.

#A battery separator is typically provided in a storage battery between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates since such direct contact would result in a short circuit of the battery. In general, it is highly desirable for the separator to possess one or more of the following qualities: (i) to be thin and lightweight to aid in providing a battery of high energy density and specific energy; (ii) to have a structure that inhibits dendrite formation between the electrode plates; (iii) to have the ability to enhance the uptake of the electrolytic composition over the electrode plates and, in so doing, to promote a substantially uniform distribution of the electrolytic composition over the electrode plates (an effect generally referred to as wicking); (iv) to provide the property of freely permitting electrolytic conduction; and (v) to have a dimensionally stable structure even during thermal excursions (internal or external heating). It is further highly desirable for the separator to be made in an economical and environmentally safe manner while being substantially free of defects, such as pinholes and the like.

#The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts:

#FIG. 1 is a scanning electron microscope (SEM) image, taken as a side view, of a microporous sheet product suitable for use as, for example, a battery separator, the microporous sheet product being prepared according to the present invention;

#FIG. 2 is a schematic side view of a microporous sheet product suitable for use as, for example, a battery separator, the microporous sheet product being prepared according to the present invention and being shown prior to being imbibed with a polar or ion-containing liquid;

#The present invention is directed at a novel microporous sheet product, as well as to methods of making and using the same. The present invention is based, in part, on the surprising discovery that a microporous sheet product having desirable properties can be prepared by melt-extruding an extrusion mixture to produce a sheet material, the extrusion mixture comprising a thermoplastic polymer, a superabsorbent polymer, and a compatibilizing agent. The compatibilizing agent promotes mixing between the thermoplastic polymer and the superabsorbent polymer and, in addition, creates (i.e., by phase-separation) micropores in the sheet material, the compatibilizing agent substantially filling the micropores of the sheet material to produce a “wet” porous structure. In certain instances, for example, where the microporous sheet material is used as a food packaging material, the thus-produced microporous sheet material may be used without any further processing. In other instances, for example, where the microporous sheet material is used as a battery separator, the microporous sheet material may thereafter be processed to extract the compatibilizing agent from the sheet material, thereby producing a “dry” porous structure. In any event, whether the porous structure is “wet” or “dry,” the resultant porous structure comprises an open-celled matrix of thermoplastic polymer in which superabsorbent polymer particles are dispersed. When the microporous sheet product of the present invention is thereafter exposed to a polar or ion-containing solvent, the superabsorbent polymer particles absorb the solvent and swell. Such swelling causes a reduction in the pore size of the microporous sheet product, as well as an increase in the electrolytic-conductivity of the microporous sheet product. Surprisingly, the absorption of solvent by the superabsorbent polymer particles also causes some of the superabsorbent polymer particles to migrate irreversibly from within the matrix to the exterior of the matrix whereas other superabsorbent polymer particles remain within the matrix. Those superabsorbent particles that migrate from within the matrix to the exterior of the matrix tend to increase the hydrophilicity of the exterior surfaces of the sheet product.

#For purposes of clarity, some of the terms used herein and in the appended claims to describe the subject invention are explained further below:

#The term “sheet material” is intended to refer to a unitary article having two large surfaces with respect to its length and breadth dimensions and having a thickness between said surfaces. In general, the term is used to describe structures achieved during the initial extrusion or shaping of material into a sheet-like form and of structures produced during subsequent processing of the sheet material.