Nanowebs

Abstract

A nonwoven web of fibers that have a number average diameter of less than 1 micron. The web can have a Poisson Ratio of less than about 0.8, a solidity of at least about 20%, a basis weight of at least about 1 gsm, and a thickness of at least 1 micrometer.

1 . A nonwoven nanoweb comprising polymer nanofibers, said web having a Poisson Ratio of less than about 0.8, a solidity of at least about 20%, a basis weight of at least about 1 gsm, and a thickness of at least about 1 μm. 
   
   
       2 . The nonwoven nanoweb of  claim 1  optionally having discrete discontinuous either bonded or unbonded areas and having less than about 15% by area in the plane of the web comprising melted regions and the web being not adhesively bonded. 
   
   
       3 . The nonwoven nanoweb of  claim 2  having less than about 1% by area in the plane of the web comprising melted regions. 
   
   
       4 . The nonwoven nanoweb of  claim 1  having a basis weight of less than about 50 gsm. 
   
   
       5 . The nonwoven nanoweb of  claim 1  in which the Poisson Ratio is measured under tensile stress applied in the machine direction of the web. 
   
   
       6 . The nonwoven nanoweb of  claim 1 , which is a calendered web. 
   
   
       7 . The nonwoven nanoweb of  claim 1 , having a maximum pore size of from about 0.1 μm to about 15 μm and a mean flow pore size of from about 0.01 μm to about 5 μm. 
   
   
       8 . The nonwoven nanoweb of  claim 1 , having an electrical resistance of less than or equal to about 2 ohms-cm 2  in 2 M lithium chloride in methanol electrolyte, and a MacMullin number of from 2 to 15. 
   
   
       9 . The nonwoven nanoweb of  claim 1 , which has less than about 20% necking in the cross direction when a tension of 100 g/cm is applied in the machine direction of the web. 
   
   
       10 . The nonwoven nanoweb of  claim 1 , having a Surface Stability Index of greater than about 17,513 N/m. 
   
   
       11 . The nonwoven nanoweb of  claim 1 , having a coefficient of friction of less than about 0.9. 
   
   
       12 . The nonwoven nanoweb of  claim 1 , which has a tensile modulus in the machine direction of at least about 69 MPa. 
   
   
       13 . The nonwoven nanoweb of  claim 1 , which has a tensile strength at break in the machine direction of at least about 4.1 MPa. 
   
   
       14 . A nonwoven nanoweb that is formed by a process comprising calendering a polymeric nanoweb between a nip between a first roll and a second roll and applying a pressure to the web across the thickness of the web, in which one of the first roll and the second roll is a hard roll, the other roll being a soft roll having a hardness less than Rockwell b 50, and heating the web to a temperature between the T g  of the nanoweb polymer and its T om , wherein the calendered nanoweb has less than about 15% by area in the plane of the web comprising melted regions. 
   
   
       15 . The nonwoven nanoweb of  claim 14 , having a Poisson Ratio of less than about 0.8, a solidity of at least about 20%, a basis weight of at least about 1 gsm, and a thickness of at least about 1 μm. 
   
   
       16 . The nonwoven nanoweb of  claim 14 , having a maximum pore size of from about 0.1 μm to about 15 μm and a mean flow pore size of from about 0.01 μm to about 5 μm. 
   
   
       17 . The nonwoven nanoweb of  claim 14 , which has a tensile modulus in the machine direction of at least about 69 MPa, and a tensile strength at break in the machine direction of at least about 4.1 MPa. 
   
   
       18 . A process for stabilizing the surface of a polymeric nanoweb comprising calendering the nanoweb through a nip between a first roll and a second roll and applying pressure to the web across the thickness of the web, wherein one of the first roll and the second roll is a hard roll, the other roll being a soft roll having a hardness less than Rockwell B 50, and heating the web to a temperature between the T g  of the nanoweb polymer and its T om . 
   
   
       19 . The process of  claim 18 , wherein the hard roll comprises raised regions that form a pattern of bonded regions on the nanoweb. 
   
   
       20 . The process of  claim 18 , wherein the hard roll is un-patterned. 
   
   
       21 . The process of  claim 18 , further comprising stretching the nanoweb at said temperature in the machine direction and/or the cross direction, either before or after calendering. 
   
   
       22 . A nonwoven nanoweb comprising polymer nanofibers having a solidity of at least about 20%, a basis weight of at least about 1 gsm, a thickness of between about 1 μm and 400 μm and a maximum pore size of about 15 micrometers, wherein the nanoweb has less than about 15% by area in the plane of the web comprising melted regions. 
   
   
       23 . The nonwoven nanoweb of  claim 22 , wherein said maximum pore size is between about 0.1 micrometer to about 15 micrometers, said web having a mean flow pore size between about 0.01 micrometer to about 5 micrometers. 
   
   
       24 . The nonwoven nanoweb of  claim 23 , having a ratio of maximum pore size/mean flow pore size between about 1.1 to about 6. 
   
   
       25 . The nonwoven nanoweb of  claim 22 , having a solidity between about 20% and about 80%. 
   
   
       26 . The nonwoven nanoweb of  claim 25 , wherein the solidity is between about 20% to about 40%. 
   
   
       27 . A nonwoven nanoweb comprising polymer nanofibers having a solidity of at least about 20%, a basis weight of at least about 1 gsm, a thickness of between about 1 μm and 400 μm and a tensile strength at break in the machine direction of at least about 4.1 MPa, wherein the nanoweb has less than about 15% by area in the plane of the web comprising melted regions. 
   
   
       28 . The nonwoven nanoweb of  claim 27 , which has a tensile modulus in the machine direction of at least about 69 MPa. 
   
   
       29 . The nonwoven nanoweb of any of  claims 1 ,  14 ,  22 , or  27 , further comprising a second web joined in a face-to-face relationship with the nonwoven web and wherein the second web is selected from the group consisting of one or more nanowebs, a scrim, and any combination of the preceding laminated together. 
   
   
       30 . A filtration media comprising the nonwoven nanoweb of any of  claims 1 ,  14 ,  22 , or  27 . 
   
   
       31 . A separator for an energy storage device comprising the nonwoven nanoweb of any of  claims 1 ,  14 ,  22 , or  27 .