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Abstract

A method of making a rubber-containing polyolefin separator entails preparing a pre-mixture (18) that includes polyolefin material (1), silica (2), and processing oil (5) and delivering the pre-mixture to a multi-zone extruder (12) having a sheet die (34). The pre-mixture becomes partly gelled as it advances in the extruder. Rubber powder (6) added at a medial zone (Z4) of the extruder combines with the pre-mixture advancing in the extruder to form a gelled rubber-containing extrudate as it exits the sheet die. The extrudate is processed by extracting a portion of the processing oil to form a separator sheet with dispersed rubber powder in the form of domains of larger average size. The larger rubber domains exhibit a smaller average ratio of surface area to volume and thereby results in slower release by diffusion of the beneficial substance from the rubber domains to the battery electrolyte.

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First Claim