Heat-sealable films

Abstract

A multilayer polyolefin film of the type suitable for packaging application in which heat seals are formed, and in its preparation the multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110° C. The surface layer has a thickness which is less than the thickness of the substrate layer. The heat-seal layer can be formed of syndiotactic polypropylene polymerized in the presence of a syndiospecific metallocene catalyst and having a melt flow index of less than 2 grams/10 minutes. The multilayer film can take the form of a biaxially-oriented film. In the production of the multilayer film incorporating a substrate layer and a heat-sealable surface layer, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer is extruded to form a surface layer that is bonded to the interface of the substrate layer at a temperature within the range of 150-260° C.

1 - 20 . (canceled)  
     
     
         21 . A process for the production of a multilayer film having a substrate layer and a surface layer comprising: 
 (a) providing a first crystalline thermoplastic polymer;    (b) extruding the propylene polymer and forming the polymer into a flexible substrate layer comprising an interface surface;    (c) providing a second polymer comprising a syndiotactic propylene polymer comprising a melt flow index of less than 2 grams/10 minutes produced by the polymerization of propylene in the presence of a syndiospecific metallocene catalyst effective to form a surface layer, the surface layer capable of producing a heat seal with itself at a seal temperature less than 110° C.;    (d) extruding the syndiotactic propylene polymer to form a surface layer; and    (e) bonding the surface layer to the interface surface of the substrate layer to form a multilayer film having a surface layer of syndiotactic propylene polymer which has a thickness that is less than the thickness of the substrate layer.    
     
     
         22 . The process of  claim 21 , wherein the first polymer is an isotactic propylene polymer.  
     
     
         23 . The method of  claim 21 , wherein the substrate layer film is formed by orienting the substrate layer form in at least one direction and thereafter forming the surface layer by extrusion-coating the syndiotactic polypropylene on to the oriented substrate layer film.  
     
     
         24 . The process of said claim  1 , wherein said multilayer film is formed by co-extruding the first and second polymers through a slotted die system to form a multilayer film comprising a substrate layer of the first polymer and a surface layer of the second polymer and thereafter orienting the film in the machine direction followed by orienting the film in the transverse direction to form a biaxially-oriented multilayer film.  
     
     
         25 . A process for the production of a multilayer film having a substrate layer and a surface layer comprising: 
 (a) providing a first polymer to form the substrate layer of a multilayer film;    (b) providing a second polymer comprising a syndiotactic propylene polymer comprising a melt flow index of less than 2 grams/10 minutes produced by the polymerization of propylene in the presence of a syndiospecific metallocene catalyst effective to form a heat-sealable surface layer of said multilayer film; and    (c) co-extruding said first and second polymers through a slotted die system at a temperature within the range of 150′-260° C. to form a film comprising a substrate layer of said first polymer and a surface layer of said second polymer of a thickness which is less than the thickness of said substrate layer.    
     
     
         26 . The process of  claim 25 , wherein the surface layer of said second polymer is effective in producing a heat seal with itself at a seal temperature of no more than 110° C.