Method for making water-swellable composite sheet

Claim 1

A method of preparing a tear-resistant composite sheet material having hydrophilic, absorptive particles enmeshed in a network of interlaced microfibrous polytetrafluoroethylene forming a fibril matrix, said hydrophilic absorptive particles being present in said matrix in the range of 1.0 to 10 parts particles per part polytetrafluoroethylene by weight, and said hydrophilic absorptive particles having absorptive capacity greater than 1.0 gram of water per gram of dry particle, said method comprising: (1) dry blending one or more hydrophilic, absorptive particulate materials; (2) admixing water to particulate in a weight ratio in the range of 1:2 to 10:1, to form a damp mixture, the water being present in a quantity insufficient to exceed the absorptive capacity of the particulate; (3) adding gradually, with stirring, to said mixture an equal weight of an aqueous emulsion of polytetrafluoroethylene having in the range of 15 to 25 weight percent solids so that the resulting mass has a cohesive consistency; (4) mixing said mass in an intensive mixer at a temperature between 50° C. and 100° C. for a time sufficient to cause initial fibrillation of said polytetrafluoroethylene particles; (5) biaxially calendering said mass between calendering rolls maintained at about 50° C. to about 100° C. to cause additional fibrillation of said polytetrafluoroethylene particles to form a self-supporting sheet, while closing the gap between the calendering rolls with each successive calendering operation, for a time sufficient to produce a tear-resistant sheet having a tensile strength of at least 0.5 megapascal; and (6) drying the resultant sheet to remove water.

Claim 2

A method of preparing a tear-resistant composite sheet material having hydrophilic, absorptive particles enmeshed in a network of interlaced microfibrous polytetrafluoroethylene forming a fibril matrix, said hydrophilic absorptive particles being present in said matrix in the range of 1.0 to 10 parts particles per part polytetrafluoroethylene by weight, and said hydrophilic absorptive particles having absorptive capacity greater than 1.0 gram of water per gram of dry particle, said method comprising: (1) dry blending one or more hydrophilic, absorptive particulate materials; (2) admixing water to particulate in a weight ratio in the range of 1:2 to 10:1, to form a damp mixture, the water being present in a quantity insufficient to exceed the absorptive capacity of the particulate; (3) adding gradually, with stirring, to said mixture an equal weight of an aqueous emulsion of polytetrafluoroethylene having in the range of 15 to 25 weight percent solids so that the resulting mass has a cohesive consistency; (4) mixing said mass in an intensive mixer at a temperature between 50° C. and 100° C. for a time sufficient to cause initial fibrillation of said polytetrafluoroethylene particles; (5) biaxially calendering said mass between calendering rolls maintained at about 50° C. to about 100° C. to cause additional fibrillation of said polytetrafluoroethylene particles to form a self-supporting sheet, while closing the gap between the calendering rolls with each successive calendering operation, for a time sufficient to produce a tear-resistant sheet having a tensile strength of at least 0.5 megapascal; and (6) drying the resultant sheet to remove water.