Battery separator

A sheet product having at least two plies comprising

• (a) at least one firat ply in the form of a microporous sheet having pores of average pore size of from about 0.005 to about 5 microns in diameter and the pores occupy at least about 10 volume percent of the total of the first ply and of a predetermined length and breadth and a thickness of less than 10 mils (0.025 cm) composed of a substantially non-filled polyolefin polymer composition;
• and
• (b) at least one second ply in the form of a microporous sheet of a predetermined length and breadth and of less than 10 mils (0.025 cm) thickness and having pores of average pore size of from 0.005 to about 5 microns in diameter occupying at least about 25 percent of the total volume of said sheet, said sheet composed of a substantially homogeneous admixture of from about 7 to 40 weight percent of a high density polyolefin having a weight average molecular weight of at least 100,000, from 0 to 10 weight percent of a plasticizer for said polyolefin, and from about 50 to 93 weight percent of an inert particulate filler having a particle size of less than about 0.01 cm;
• each of said first and second ply being bonded together to provide a unitary sheet product and the viscosity of the polymer composition of each of said first ply being substnatially equal to or less than that of the polymer composition of said second ply at temperatures ranging from ambient to about 150.degree. C. and, at a transformation temperature of between about 80.degree. and 150.degree. C., being sufficiently decreased to exhibit flow to provide a substantially non-porous sheet while substantially maintaining its predetermined length and breadth dimensions.

In a storage battery having positive electrodes, negative electrodes, an electrolytic composition and separators positioned between adjacent electrodes of opposite polarity, wherein the improvement comprises that the separator is a sheet product having at least two plies composed of

• (a) at least one first ply in the form of a non-filled microporous sheet having pores of average pore size of from about 0.005 to about 5 microns in diameter and the pores occupy at least about 10 volume percent of the total volume of the first ply and of a predetermined length and breadth and a thickness of less than 10 mils composed of a substantially non-filled polyolefin polymer composition;
• said sheet being capable, while a component of the sheet product, of transforming to a substantially non-porous membrane sheet at a transformation temperature between about 80.degree. C. and 150.degree. C. while substantially maintaining the predetermined length and breadth dimensions;
• and
• (b) at least one second ply in the form of a microporous sheet of a predetermined length and breadth and a thickness of less than 10 mils having pores of average pore size of from about 0.005 to about 5 microns in diameter wherein said pores occupy at least about 25 percent of the total volume of said sheet, said second sheet composed of a substantially homogeneous admixture of from about 7 to 40 weight percent of a high density polyolefin having a weight average molecular weight of at least 100,000, from 0 to 10 weight percent of a plasticizer for said polyolefin, and from about 50 to 93 weight percent of an inert particulate filler having a particle size of less than 0.01 cm;
• each of said first and second ply being bonded together into a unitary sheet product and said second ply being capable while contained in a storage battery, of substantially maintaining said microporous structure and said determined length, breadth and thickness at temperatures of from about ambient to at least about 10.degree. C. greater than the transformation temperature of said first ply.

A process of forming a sheet product useful as a battery separator comprising

• (a) forming a first substantially homogeneous admixture of from 5 to 30 weight percent of a first polyolefin, from about 10 to 60 weight percent of a first plasticizer for said polyolefin and from 30 to 75 weight percent of a first particulate filler;
• said first plasticizer being soluble in a material which is substantially a non-solvent with respect to the first polyolefin, the second polyolefin and the second filler;
• said first filler being soluble in a material which is a non-solvent with respect to the first polyolefin, the second polyolefin and the second filler;
• (b) forming a second substantially homogeneous admixture of from about 5 to 30 weight percent of a second polyolefin, from about 10 to 60 weight percent of a second plasticizer for said polyolefin and from about 30 to 75 weight percent of a second particulate filler said second polyolefin being selected from high density polyolefins having a weight average molecular weight of at least 100,000;
• said second plasticizer being soluble in a material which is a substantial non-solvent with respect to the second polyolefin and second filler;
• said second filler having a particle size of less than 0.01 cm, a surface area of from about 10 to 950 square meters per gram and a pore volume of at least about 0.07 cc/gm;
• (c) forming a first sheet of less than 10 mils (0.025 cm) thickness from the first substantially homogeneous admixture;
• (d) forming a second sheet of less than 10 mils (0.025 cm) thickness from the second substantially homogeneous admixture;
• (e) forming a multilayered sheet having at least one layer composed of the first sheet and at least one layer composed of the second sheet, each layer being bonded to, substantially coextensive with and superimposed on the other layers of the multilayer sheet material;
• and
• (f) extracting from the multilayer sheet material substantially all of the first and second plasticizers and substantially all of the first filler therein to provide a sheet product.

A process of forming a sheet product useful as a battery separator comprising

• (a) forming a first sheet comprising an unfilled microporous polyolefin composition of less than 10 mils thickness;
• (b) forming a second substantially homogeneous admixture of from about 5 to 30 weight percent of a second polyolefin, from about 10 to 60 weight percent of a second plasticizer for said polyolefin and from about 30 to 75 weight percent of a second particulate filler which is inert with respect to the components of the battery system in which it is to be used;
• said second polyolefin being selected from high density polyolefins having a weight average molecular weight of at least 100,000;
• said second plasticizer being soluble in a material which is a substantial non solvent with respect to the second polyolefin and said second filler;
• said second filler having a particle size of less than 0.01 cm, and surface area of from about 10 to 950 square meters per gram and a pore volume of at least about 0.07 cc/gm;
• said second composition having a viscosity substantially equal or greater than that of the first composition at temperatures from ambient to about 150.degree. C.,
• (c) superimposing and bonding at least one first sheet onto a second sheet to form a multilayer sheet wherein each layer is bonded to, substantially coextensive with and superimposed on the other layers of the multilayer sheet;
• and
• (d) extracting from the multilayer sheet material substantially all of the second plasticizer from the second sheet material to provide a sheet product.