Transparent, biaxially oriented polyester film

Abstract

The invention relates to a transparent, biaxially oriented polyester film which comprises at least 80% by weight of thermoplastic polyester and also comprises an amount in the range from 0.05 to 0.5% by weight of a specific pigment system. The invention further relates to a process for producing the film. The film can be used as a packaging film, or for metalizing or ceramic coating, and in applications in the industrial sector, as a substrate for stamping foils.

1 . A transparent, biaxially oriented polyester film which comprises at least 80% by weight of thermoplastic polyester, wherein the film also comprises an amount in the range from 0.05 to 0.5% by weight, based on the total weight of the film, of a pigment system which has the following features: 
 a) the median grain diameter (d 50 ) is in the range from 1.5 to 5 μm and    b) the spread of the distribution, expressed via the SPAN 98, is less than or equal to 1.9.    
     
     
         2 . The transparent polyester film as claimed in  claim 1 , which comprises a pigment system which has a median grain diameter (d 50 ) in the range from 1.6 to 4.9 μm, preferably in the range from 1.7 to 4.8 μm.  
     
     
         3 . The transparent polyester film as claimed in  claim 1 , which comprises a pigment system which has a SPAN 98 of less than or equal to 1.8, preferably less than 1.7.  
     
     
         4 . The transparent polyester film as claimed in  claim 1 , whose thickness is in the range from 1 to 50 μm.  
     
     
         5 . The transparent polyester film as claimed in  claim 1 , whose gloss is greater than or equal to 150 and whose haze is less than or equal to 4.0%.  
     
     
         6 . The transparent polyester film as claimed in  claim 1 , whose roughness, expressed as its R a  value, is in the range from 30 to 150 nm, and whose value measured for surface gas flow is in the range from 4 to 200 s.  
     
     
         7 . The transparent polyester film as claimed in  claim 1 , whose planar orientation Δp is greater than or equal to 0.165.  
     
     
         8 . A process for producing a transparent polyester film as claimed  claim 1  by extrusion, in which the polyester is first compressed, plasticized, and thus homogenized in an extruder, at which juncture any additives provided may already be present in the polymer, and in which the melt is then pressed through a flat-film die, and the extruded melt is drawn off on one or more take-off rolls and solidified to give a prefilm, and the solidified prefilm is then biaxially stretched, and the biaxially stretched film is heat-set and, where appropriate, also corona- or flame-treated on the surface intended for treatment, which comprises using a longitudinal stretching temperature in the range from 80 to 130° C. and a transverse stretching temperature in the range from 90 to 150° C. and using a longitudinal stretching ratio in the range from 2.5:1 to 6:1 and using a transverse stretching ratio in the range from 3.0:1 to 5.0:1.  
     
     
         9 . The process as claimed in  claim 8 , wherein, after stretching, the film is heat-set at a temperature of from 150 to 250° C. for a period in the range from 0.1 to 10 s.  
     
     
         10 . The process as claimed in  claim 8 , wherein one or both surfaces of the film are corona- or flame-treated, the intensity of the treatment being adjusted so as to give a surface tension of the film in the range greater than or equal to 45 mN/m.  
     
     
         11 . The process as claimed in  claim 8 , wherein cut material arising during film production is reintroduced to the extrusion process as regrind in amounts in the range from 20 to 60% by weight, based on the total weight of the film.