Non-woven sheet containing fibers with sheath/core construction

Abstract

A non-woven sheet contains sheath/core polymer fibers with the polymer in the sheath having a melting point at least 15 degrees centigrade higher than the melting point of the polymer of the core. The fibers have an average diameter greater than 7 microns and the sheet has a normalized air resistance greater than 0.25/(g/m 2 ).

1 . A non-woven sheet comprising:
 a network of substantially-continuous thermoplastic polymer filaments, the polymer filaments each individually comprising a plurality of polymers including at least a first polymer and a second polymer, the melting point of the first polymer being at least 15 degrees C. higher than the melting point of the second polymer, the individual polymer filaments further characterized in that   (1) the first polymer comprises from 10 to 70 weight percent of the total weight of the first and second polymer,   (2) the second polymer comprises from 30 to 90 weight percent of the total weight of the first and second polymer and,   
       the network of filaments being consolidated and fused such that
 (a) the first polymer forms a continuous phase in the fused consolidated network and 
 (b) the second polymer forms a disperse phase in the fused consolidated network 
 
       wherein the consolidated network has a porosity such that the normalized air resistance of the network is at least 0.2 s/(g/m 2 ). 
     
     
         2 . The non-woven sheet of  claim 1  wherein the first polymer is selected from the group comprising polyarylene sulfide, polyimide, liquid crystalline polyester, fluoropolymer and mixtures thereof. 
     
     
         3 . The non-woven sheet of  claim 2  wherein the polyarylene sulfide is polyphenylene sulfide. 
     
     
         4 . The non-woven sheet of  claim 1  wherein the second polymer is selected from the group comprising polyolefin, polyester, polyamide and mixtures thereof. 
     
     
         5 . The non-woven sheet of  claim 2  wherein the polyester is polyethylene terephthalate. 
     
     
         6 . An electrical insulation component for an electrical device comprising the non-woven sheet of  claim 1 . 
     
     
         7 . Insulation useful for an electrical device comprising a polymeric film positioned adjacent to, and attached to, one or more non-woven sheets said non-woven sheets comprising:
 a network of substantially-continuous thermoplastic polymer filaments, the polymer filaments each individually comprising a plurality of polymers including at least a first polymer and a second polymer, the melting point of the first polymer being at least 15 degrees C. higher than the melting point of the second polymer, the individual polymer filaments further characterized in that   (1) the first polymer comprises from 10 to 70 weight percent of the total weight of the first and second polymer,   (2) the second polymer comprises from 30 to 90 weight percent of the total weight of the first and second polymer and,   
       the network of filaments being consolidated and fused such that
 (a) the first polymer forms a continuous phase in the fused consolidated network and 
 (b) the second polymer forms a disperse phase in the fused consolidated network 
 
       wherein the consolidated network has a porosity such that the normalized air resistance of the network is greater than 0.2 s/(g/m 2 ). 
     
     
         8 . The insulation of  claim 7  wherein, the polymeric film is a polyester film. 
     
     
         9 . The insulation of  claim 7  having the form of a slot liner, a closure, a wedge or a stick. 
     
     
         10 . An electrical device comprising the insulation of  claim 7 . 
     
     
         11 . A method for producing a non-woven sheet of multicomponent polymeric fiber comprising the steps of:
 (i) melt spinning at between 3500 to 5000 m/min in the presence of an attenuating force provided by a rectangular slot jet a fiber having an average fiber diameter greater than 7 microns, said fiber further comprising an amorphous sheath component of from 10 to 70 weight percent of a first polymer based on the total weight of polymer in the core and sheath and a core component of from 30 to 90 weight percent of a fibrous second polymer based on the total weight of polymer in the core and sheath, wherein the melting point of the first polymer is at least 15 degrees C. higher than the melting point of the second polymer,   (ii) forming a non-woven web of fibers on a forming belt,   (iii) passing the non-woven web of fibers through heated bonding rolls to fuse the fibers and thereafter,   (iv) smooth calendering said fused fiber web to convert the amorphous sheath of first polymer material into a substantially crystalline continuous phase and further compact and density the non-woven web to embed the fibrous second polymer into the continuous phase of the first polymer and to achieve a web having a normalized air resistance greater than 0.2 s/(g/m 2 ).   
     
     
         12 . The method of  claim 11  wherein the first polymer is selected from the group comprising polyarylene sulfide, polyimide, liquid crystalline polyester, fluoropolymer and mixtures thereof. 
     
     
         13 . The method of  claim 12  wherein the polyarylene sulfide is polyphenylene sulfide. 
     
     
         14 . The method of  claim 11  wherein the second polymer is selected from the group comprising polyolefin, polyester, polyamide and mixtures thereof. 
     
     
         15 . The non-woven sheet of  claim 14  wherein the polyester is polyethylene terephthalate.