Anisotropic polyvinyl formal resin microporous membrane and its preparation

Claim 3

The method set forth in claim 1 wherein said polyvinyl formal resin has a molecular weight (wt. average) in the range of 10,000-44,000, an hydroxyl content of from 5.0-9.0% expressed as percent of polyvinyl alcohol, an acetate content of between 9.5-30% expressed as percent polyvinyl acetate and a formal content of between 68-82% expressed as percent polyvinyl formal.

Claim 4

The method set forth in claim 2 wherein said casting dope comprises between about 10 to about 20% of said resin and about 90 to about 80% of said solvent.

Claim 5

The method set forth in claim 3 wherein said solvent is selected from the group consisting of dimethylsulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide and wherein said leaching liquid comprises water.

Claim 6

The method set forth in claim 4 wherein said casting dope is coated on said support to an initial film thickness of from about 10 to about 40 mils.

Claim 7

The method set forth in claim 5 including the additional and subsequent step of drying said membrane to reduce the pore size thereof.

Claim 8

The method set forth in claim 5 including the additional step of treating said membrane with an alkaline solution to reduce the pore size thereof.

Claim 11

A microporous polymeric membrane prepared according to the process of claim 1.

Claim 12

A microporous polymeric membrane prepared according to the process of claim 6.

Claim 13

A microporous polymeric membrane prepared according to the process of claim 7.

Claim 14

A microporous polymeric membrane prepared according to the process of claim 8.

Claim 15

An amisotropic microporous polymeric membrane consisting essentially of a polyvinyl formal comprising: a. a microporous surface skin, b. a cellular support structure disposed below said skin and consisting of interconnected cell walls forming a first plurality of relatively small cells immediately below said skin surface, a second plurality of substantially larger cells on the side of said membrane opposite said skin and at least a third plurality of intermediate sized cells disposed therebetween, with the cell walls being more closely spaced in the area of said first plurality of cells than in the area of said second plurality of cells, at least said second and third plurality of cells being larger in width at the ends thereof displaced from said skin.

Claim 16

An amisotropic microporous polymeric membrane consisting essentially of a polyvinyl formal comprising: a. a microporous surface skin, b. a cellular support structure disposed below said skin and consisting of interconnected cell walls forming a first plurality of relatively small cells immediately below said skin surface, a second plurality of substantially larger cells on the side of said membrane opposite said skin and at least a third plurality of intermediate sized cells disposed therebetween, with the cell walls being more closely spaced in the area of said first plurality of cells than in the area of said second plurality of cells, at least said second and third plurality of cells being larger in width at the ends thereof displaced from said skin.

Claim 17

An anisotropic, polymeric microporous membrane consisting essentially of a polyvinyl formal resin and comprising: a. a microporous surface skin, and b. a cellular structure disposed beneath said skin body, said cellular structure comprising a plurality of generally elongated cells wherein the relative sizes of individual cells vary in relation to their distance from the surface skin with those cells most remote from said skin being relatively larger, said cells being arranged with their maximum dimension perpendicular to said surfaces and wherein the ends of said cells nearest said surface skin are generally narrower than the opposite ends thereof, and c. said polyvinyl formal resin being selected from those polyvinyl formal resins having a molecular weight (wt. average) in the range of 10,000-40,000, a hydroxyl content of from 5.0-9.0% expressed as percent of polyvinyl alcohol, an acetate content of between 9.5- 30% expressed as percent polyvinyl acetate and a formal content of between 68-82% expressed as percent polyvinyl formal.

Claim 18

An anisotropic, polymeric microporous membrane consisting essentially of a polyvinyl formal resin and comprising: a. a microporous surface skin, and b. a cellular structure disposed beneath said skin body, said cellular structure comprising a plurality of generally elongated cells wherein the relative sizes of individual cells vary in relation to their distance from the surface skin with those cells most remote from said skin being relatively larger, said cells being arranged with their maximum dimension perpendicular to said surfaces and wherein the ends of said cells nearest said surface skin are generally narrower than the opposite ends thereof, and c. said polyvinyl formal resin being selected from those polyvinyl formal resins having a molecular weight (wt. average) in the range of 10,000-40,000, a hydroxyl content of from 5.0-9.0% expressed as percent of polyvinyl alcohol, an acetate content of between 9.5- 30% expressed as percent polyvinyl acetate and a formal content of between 68-82% expressed as percent polyvinyl formal.

Claim 19

The membrane set forth in claim 14 wherein said membrane further includes a fibrous support adjacent the surface of said membrane opposite said surface skin, said support being selected from the group consisting of paper, fabrics and felted materials.

Claim 20

The membrane set forth in claim 14 wherein said cells are at least partially separated from one another by porous walls and wherein the pore size of said walls exceed the average pore size of said surface skin.

Claim 21

The membrane set forth in claim 14 wherein said pores of said surface skin have an average size of between about 70A and about 120A.

Claim 22

A microporous membrane consisting essentially of a polyvinyl formal resin and comprising: a. a microporous surface skin, b. a cellular support structure disposed below said skin and consisting of interconnected cell walls forming a first plurality of relatively small cells immediately below said skin surface, a second plurality of substantially larger cells on the side of said membrane opposite said skin and at least a third plurality of intermediate sized cells disposed therebetween, with the cell walls being more closely spaced in the area of said first plurality of cells than in the area of said second plurality of cells, at least said second and third plurality of cells being larger in width at the ends thereof displaced from said skin, and c. said polyvinyl formal resin being selected from those polyvinyl formal resins having a molecular weight (wt. average) in the range of 10,000-40,000, a hydroxyl content of from 5.0-9.0% expressed as percent of polyvinyl alcohol, an acetate content of between 9.5-30% expressed as percent polyvinyl acetate and a formal content of between 68-82% expressed as percent polyvinyl formal.

Claim 23

A microporous membrane consisting essentially of a polyvinyl formal resin and comprising: a. a microporous surface skin, b. a cellular support structure disposed below said skin and consisting of interconnected cell walls forming a first plurality of relatively small cells immediately below said skin surface, a second plurality of substantially larger cells on the side of said membrane opposite said skin and at least a third plurality of intermediate sized cells disposed therebetween, with the cell walls being more closely spaced in the area of said first plurality of cells than in the area of said second plurality of cells, at least said second and third plurality of cells being larger in width at the ends thereof displaced from said skin, and c. said polyvinyl formal resin being selected from those polyvinyl formal resins having a molecular weight (wt. average) in the range of 10,000-40,000, a hydroxyl content of from 5.0-9.0% expressed as percent of polyvinyl alcohol, an acetate content of between 9.5-30% expressed as percent polyvinyl acetate and a formal content of between 68-82% expressed as percent polyvinyl formal.