Battery separator and method of forming same

摘要

The present invention is directed to a sheet product and to a method of forming the sheet product suitable for use as a battery separator. The method comprises forming a mixture of a thermoplastic polymer and fluid having a high vapor pressure, shaping the mixture into a sheet material and subjecting the sheet material to stretching/fluid vaporization at high temperature to form an intermediate material having a ratio of percent fluid to percent polymer crystallinity of between 0.15 and 1 followed by a second stretching/fluid vaporization at a lower temperature while removing a portion of the remainder of the fluid from the sheet. The resultant sheet is annealed and remainder of fluid removed to form a sheet product having a thickness comprising a stratified structure of small and larger pore layered configuration across its thickness.

主权项

1. A method of forming a sheet product comprising:
a) mixing, at a temperature of from 100 to 250° C., a thermoplastic polymer having a degree of crystallinity of at least 30% and a melt flow index (MFI) of less than 0.8 with from about 20 to 80 wt. % of a first fluid having a boiling point of from 135 to 300° C. and a vapor pressure of from 1 to 50 mm Hg at 70° C. to form a substantially uniform mixture; b) shaping the substantially uniform mixture into an initial shaped sheet material having length and breadth dimensions and a thickness sufficient to allow stretching to occur, the initial shaped sheet material being of substantially uniform composition across its thickness; c) contacting the initial shaped sheet material of b) with a second fluid having a temperature up to about 100° C. comprising at least 75 wt % of first fluid and the remainder composed of fluids miscible with said first fluid and having a BP of from 135 to 300° C. and a vapor pressure of from 1 to 50 mm Hg at 70° C. for a time sufficient to cause the sheet material to have a temperature of less than the melt temperature (Tm) of the polymer therein; d) first stretching the sheet material of (c) in at least one first direction at least 225% of the original dimension in said direction while maintaining the sheet at a temperature of from (Tm−70) to (Tm−20)° C. to produce a substantially non-porous sheet product having polymer crystallinity (c) of at least 40% and simultaneously removing first and second fluid from said sheet material by vaporization to cause residual fluid (f) of the first stretched sheet material to be less than about 45 weight percent of the sheet material c) and have a f/c ratio of at least 0.15; e) second stretching the sheet material of d) in at least one second direction which is transverse to the first direction and at least 125% of the original dimension of said second direction while maintaining the sheet at a temperature of at least 20° C. below that of d) to produce a porous sheet product having at least 40% crystallinity and simultaneously removing first and second fluid from said sheet material by vaporization to cause residual fluid of the sheet material to be from about 10 up to about 45 weight percent; f) subjecting the sheet material to a temperature of from (Tm−50) to (Tm−5) ° C. while under machine or transverse tension for a period of time to cause the sheet material to have fixed stretched dimensions; g) removing substantially all first and second fluids from the sheet material by vaporization either during step f) or separately or a combination thereof; h) cooling the sheet material to ambient temperature to form a sheet product.