White, sealable, flame-retardant, biaxially oriented polyester film, its use and process for its production

DETAILED DESCRIPTION OF THE INVENTION The non-sealable outer layer C preferably has a coefficient of friction COF smaller than 0.
5, average roughness R.
sub.
a of from 40 to 100 nm, a value of less than 120 s measured for gas flow, gloss smaller than 100 (measurement angle 20.
degree.
) and a number N of elevations per mm.
sup.
2 of film surface correlated with the respective height h via the following equations:
A.
sub.
C1 - B.
sub.
C1 *log h/.
mu.
< N.
sub.
C /mm.
sup.
2 <
A.
sub.
C2 -
B.
sub.
C2 *log h/.
mu.
m
0.
01 .
mu.
m < h < 10 .
mu.
m
A.
sub.
C1 = 0.
29 B.
sub.
C1 = 3.
30
A.
sub.
C2 = 1.
84 B.
sub.
C2 = 2.
70
The whiteness of the polyester film is preferably greater than 70 and its planar orientation less than 0.
165, together with compliance with construction materials classes B2 and B1 to DIN 4101 Part 2/Part 1 and passing of the UL 94 test.
The film of the invention comprises at least one flame retardant fed by way of what is known as masterbatch technology directly during film production, the concentration of the flame retardant being in the range from 0.
5 to 30.
0% by weight, preferably from 1.
0 to 20.
0% by weight, based on the weight of the thermoplastic.
The ratio maintained between flame retardant and thermoplastic during masterbatch production is generally in the range from 60:40% by weight to 10:90% by weight.
Typical flame retardants include bromine compounds, chloroparaffins, and other chlorine compounds, antimony trioxide, and alumina trihydrates, but the halogen compounds are disadvantageous due to the halogen-containing by-products produced.
Another extreme disadvantage is the low lightfastness of films provided therewith, alongside the evolution of hydrogen halides in the event of a fire.
Examples of suitable flame retardants used according to the invention are organophosphorus compounds, such as carboxyphosphinic acids, anhydrides of these, and dimethyl methylphosphonate.
It is important for the invention that the organophosphorus compound is soluble in the thermoplastic, since otherwise the required optical properties are not obtained