Patent ID: 7588816

Claim:
A biaxially oriented polyester film, comprising at least 80% by weight of ethylene-terephthalate-derived units and of from 1 to 8% by weight of ethylene-isophthalate-derived units (based in each case on the total weight of polyester), said film further comprising from 0.01 to 0.5 weight percent, based on the weight of the layer, of first particles whose median diameter d 50 is from 0.1 to 2.0 μm, and from 0.01 to 0.5 weight percent, based on the weight of the layer, of second particles whose median diameter d 50 is from >2.5 μm to <8 μm, wherein said film exhibits (i) a roughness R a of greater than 35 nm and less than 100 nm, as measured via DIN 4762 with a cut-off of 0.25 mm, and (ii) a water vapor permeation of less than 20 gm/(m 2 ·d)H 2 O based on 15 micron film thickness, 23° C., 50% r.h. measured via DIN 53122.