US 4,726,989 AGranted

Microporous materials incorporating a nucleating agent and methods for making same

AssigneeMinnesota Mining and Manufacturing Company

InventorJames S. Mrozinski

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Abstract

The present invention relates to microporous materials incorporating a nucleating agent and methods for making the same. The method comprises the steps of melt blending a crystallizable thermoplastic polymer with a nucleating agent which is capable of inducing subsequent crystallization of the thermoplastic polymer and with a compound which is miscible with the thermoplastic polymer at the melting temperature of the polymer but phase separates on cooling at or below the crystallization temperature of the polymer, forming a shaped article of the melt blend, cooling the shaped article to a temperature at which the nucleating agent induces the thermoplastic polymer to crystallize so as to cause phase separation to occur between the thermoplastic polymer and the compound, thereby providing an article comprising an aggregate of a first phase comprising particles of crystallized thermoplastic polymer in a second phase of the compound, and stretching the article in at least one direction to provide a network of interconnected micropores throughout. Preferably, the compound is removed either before or after stretching. The use of a nucleating agent in accordance with the present invention provides for the manufacture of a wide variety of microporous materials heretofore not possible having a wide range of porosity and other physical characteristics.

Claims35

Dependent claims

A method of making a microporous article, comprising the steps of:

melt blending to form a mixture comprising about 15 to about 80 parts by weight of crystallizable thermoplastic polymer, sufficient nucleating agent to initiate subsequent crystallization of said thermoplastic polymer at a significantly greater number of crystallization sites as compared to crystallization without said nucleating agent, and about 85 to about 20 parts by weight of a compound with which said thermoplastic polymer is miscible and in which said thermoplastic polymer will dissolve at the melting temperature of said thermoplastic polymer but which will phase separate on cooling to a temperature at or below the crystallization temperature of said thermoplastic polymer;
forming a shaped article of the melt blended mixture;
cooling said shaped article to a temperature at which said nucleating agent initiates said crystallization sites within said thermoplastic polymer so as to cause phase separation to occur between said compound and said polymer, thereby providing an article comprising an aggregate of a first phase comprising particles of crystallized thermoplastic polymer in a second phase comprising said compound with adjacent thermoplastic polymer particles being distinct but having a plurality of zones of continuity, wherein said particles have a size which is significantly reduced as compared to the size said particles would have if no nucleating agent were present;
and
stretching said shaped article in at least one direction to separate adjacent particles of thermoplastic polymer from one another to provide a network of interconnected micropores therebetween and to permanently attenuate the thermoplastic polymer in said zones of continuity to form fibrils.

2 more elements

A microporous material comrising about 15 to about 80 parts by weight of crystallizable thermoplastic polymer, about 0.1 to about 5 parts by weight of a nucleating agent which is capable of inducing crystallization of said thermoplastic polymer, and about 85 to about 20 parts by weight of a compound with which said thermoplastic polymer is miscible and in which said thermoplastic polymer will dissolve at the melting temperature of said thermoplastic polymer but will phase separate on cooling to a temperature at or below the crystallization temperature of said thermoplastic polymer, said microporous material having an internal structure characterized by a multiplicity of spaced, randomly dispersed, non-uniform shaped, equiaxed particles of said thermoplastic polymer coated with said compound, adjacent coated particles throughout said material being separated from one another to provide said material with a network of interconnected micropores and said adjacent thermoplastic polymer particles being connected to each other by a plurality of fibrils consisting of said thermoplastic polymer, said particles containing enough of said nucleating agent such that the size of said particles is reduced over the size said particles would have if no nucleating agent were present.

Dependencies are inferred from the claim text.