Melt extrusion processes and films produced thereby

Abstract

The invention relates to a process for melt extruding a film layer of a thermoplastic olefin which comprises extruding the film having physical characteristics under process conditions selected by reference to the Draw ratio Dr and the Aspect ratio A such that draw resonance is avoided in which in addition the process conditions are defined by reference to the Deborah number such that the molten polymer is extruded at a Draw ratio of at least 30 in a high elasticity region at a λ c value of characteristic time as defined herein of from 0.001 to 0.6 s, a Deborah number as defined herein in excess of 0.005 and an A value as defined herein lower than 0.8. Narrower ranges of these parameters may be defined for application of the invention to different polymer types.

We claim:  
     
         1 . A film layer prepared by a melt extrusion process, the film layer having a thickness of from 3 to 75 microns, a molecular weight distribution of from 2 to 4, a machine direction tensile strength of from 50 MPa to 120 MPa and a transverse direction tensile strength higher than 45 MPa.  
     
     
         2 . The film layer of    claim 1   , wherein the film layer has a thickness of from 3 to 75 microns, a molecular weight distribution of from 2 to 4, a machine direction tensile strength of from 55 MPa to 100 MPa and a transverse direction tensile strength higher than 45 MPa.  
     
     
         3 . The film layer of    claim 1   , wherein the melt extrusion process is a melt extrusion of a thermoplastic olefin having at least 80% by weight of ethylene derived units, a melt index of from 0.2 to 8, and a melt index ratio of less than 30.  
     
     
         4 . The film layer of    claim 1   , wherein the film layer has a density of from 0.910 to 0.935 g/cm 3  and a transverse Elemendorf tear strength greater than 500 g/mil.  
     
     
         5 . A film layer prepared by a process comprising: 
 (a) selecting a thermoplastic polyolefin having a characteristic time (λ c ) determined by fitting a curve of dynamic viscosity (η*) vs. frequency of oscillation (ω) such that    η*(ω)=η 0 /1+(λ c ω) m ,     where η 0  is the zero shear viscosity and m is a power law index, and said λ c  is a value of from 0.001 to 0.6 s;    (b) setting the aspect ratio (A) of the air gap length (X) to one-half an extrusion die orifice width (L 0 ) to lower than 0.8, where X is defined as the distance from said die orifice of a melt extruder to the location on a chill roll where extrudate of said polyolefin is solidified;    (c) extruding the film through said extrusion die orifice onto said chill roll while maintaining a ratio (Dr) of film velocity at said chill roll (u f ) to film velocity at said die orifice (u o ) of at least 30; and    (d) maintaining the product (De) of said λ c  times the ratio of said film velocity at the die orifice (u 0 ) to said air gap length (X) at a value greater than 0.005.    
     
     
         6 . The film layer of    claim 5   , wherein the film layer has a thickness of from 3 to 75 microns, a molecular weight distribution of from 2 to 4, a machine direction tensile strength of from 50 MPa to 120 MPa and a transverse direction tensile strength higher than 45 MPa.  
     
     
         7 . The film layer of    claim 5   , wherein the film layer has a thickness of from 3 to 75 microns, a molecular weight distribution of from 2 to 4, a machine direction tensile strength of from 55 MPa to 100 MPa and a transverse direction tensile strength higher than 45 MPa.