Sealable, biaxially oriented polyester film

Details Sealable, biaxially oriented polyester film Sealable, biaxially oriented polyester film

2001-03-27

2002-07-23

Chen, Vivian (Department: 1773)

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Including components having same physical characteristic in...

C428S215000, C428S323000, C428S327000, C428S346000, C428S347000, C428S349000, C428S354000, C428S480000, C428S910000, C428S141000, C264S173100, C264S173120, C264S173150, C264S288400, C264S289300, C264S290200

Reexamination Certificate

active

06423401

ABSTRACT:

The invention relates to a transparent, sealable, coextruded, biaxially oriented polyester film composed of at least a base layer (B) and, applied to the two sides of this base layer, outer layers (A) and (C). The invention also relates to the use of the film, and to a process for its production.
BACKGROUND OF THE INVENTION
Sealable, biaxially oriented polyester films are known in the prior art. These known prior art films either have good sealing performance or good optical properties or acceptable processing performance.
GB-A 1 465 973 describes a coextruded, two-layer polyester film, one layer of which is composed of copolyesters containing isophthalic and terephthalic acids and the other layer of which is composed of polyethylene terephthalate. No useful information is given in the specification concerning the sealing performance of the film. Due to lack of pigmentation, the film cannot be produced reliably (cannot be wound), and its capability for further processing is limited.
EP-A 0 035 835 describes a coextruded sealable polyester film where, to improve the winding and processing characteristics, the sealing layer contains particles whose average particle size exceeds the thickness of the sealing layer. The particulate additives create surface protrusions preventing the undesirable blocking and sticking of the film to rolls or guides. No further information is provided on the incorporation of antiblocking agents with regard to the other, nonsealable layer of the film. It remains moot whether this layer contains antiblocking agents. Choosing particles having a larger diameter than the sealing layer and the concentrations reported in the examples has an adverse effect on the sealing characteristics of the film. The reference provides no information on the sealing temperature range of the film. The seal seam strength is measured at 140° C. and found to be in the range from 63 to 120 N/m (which corresponds to 0.97 to 1.8 N/15 mm of film width).
EP-A 0 432 886 describes a coextruded, multilayer polyester film which has a first surface on which has been arranged a sealable layer, and has a second surface on which has been arranged an acrylate layer. The sealable outer layer here may also be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters. The coating on the reverse side gives the film improved processing performance. The patent gives no indication of the sealing range of the film. The seal seam strength is measured at 140° C. For a sealable layer thickness of 11 &mgr;m the seal seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coating is that this side is now not sealable with respect to the sealable outer layer, and the film therefore has only very restricted use.
EP-A 0 515 096 describes a coextruded, multilayer, sealable polyester film which comprises a further additive on the sealable layer. The additive may comprise inorganic particles, for example, and is preferably applied in an aqueous layer to the film during its production. Using this method, the film is claimed to retain its good sealing properties and to be easy to process. The reverse side comprises only very few particles, most of which pass into this layer via the recycled material. This patent again gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is above 200 N/m (3 N/15 mm). For a sealable layer of 3 &mgr;m thickness the seal seam strength given is 275 N/m (4.125 N/15 mm).
WO 98/06575 describes a coextruded, multilayer polyester film which comprises a sealable outer layer and a nonsealable base layer. The base layer here may have been built up from one or more layers, and one of these layers is in contact with the sealable layer. The other (outward-facing) layer then forms the second nonsealable outer layer. Here, too, the sealable outer layer may be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters, but these comprise no antiblocking particles. The film also comprises at least one UV absorber, which is added to the base layer in a weight ratio of from 0.1 to 10%. The base layer of this film has conventional antiblocking agents. The film has good sealability, but does not have the desired processing performance and has shortcomings in optical properties (gloss and haze).
It was therefore an object of the present invention to provide a sealable, biaxially oriented polyester film which does not have the disadvantages of the prior art films mentioned and which in particular has improved sealability and improved processability, while otherwise its optical properties remain the same or even improve. It was a particular object of the present invention to extend the sealing range of the film toward low temperatures, and to improve the seal seam strength of the film. In addition, the film was also to be processable on high-speed processing machinery. It should also be ensured that any cut material arising during film production can be reintroduced as recycled material to the production process in amounts of up to 60% by weight, based on the total weight of the film, without adversely affecting the physical or optical properties of the film.
SUMMARY OF THE INVENTION
According to the invention, the object is achieved by providing a coextruded, biaxially oriented, sealable polyester film with at least a base layer (B), with a sealable outer layer (A) and with another outer layer (C), where the sealable outer layer A has a minimum sealing temperature of not more than 110° C. and a seal seam strength of at least 1.3 N/15 mm of film width, and the topograhies of the two outer layers (A) and (C) have the following features:
Sealable outer layer (A):
R
a
≦40 nm
Value measured for gas flow within the range from 300 to 4000 s.
Nonsealable outer layer (C):
COF≦0.5
40 nm≦R
a
≦150 nm
Value measured for gas flow≦140 s.
Number of elevations N
c
per mm
2
of film surface correlated with their respective heights h via the following equations:
A
C1
-B
C1
·log
h
/&mgr;m<log
N
C/
mm
2
<A
C2
-B
C2
·log
h
/&mgr;m
0.01 &mgr;m≦h≦10 &mgr;m
A
C1
=0.29 B
C1
=3.30
A
C2
=1.84 B
C2
=2.70.
The subclaims give preferred embodiments of the invention, and these are described in more detail below.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, the film has at least three layers and then embraces the base layer (B), the sealable outer layer (A) and the nonsealable outer layer (C).
Polymers Used for the Base and for the Outer Layer
Base Material
At least 90% by weight of the base layer (B) is preferably composed of a thermoplastic polyester. Polyesters suitable for this purpose are those made from ethylene glycol and terephthalic acid (polyethylene terephthalate, PET), from ethylene glycol and naphthalene-2,6-dicarboxylic acid (polyethylene 2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclohexane and terephthalic acid (poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else made from ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalate dibenzoate, PENBB). Particular preference is given to polyesters at least 90 mol %, preferably at least 95 mol %, of which is composed of ethylene glycol units and terephthalic acid units, or of ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units derive from other aliphatic, cycloaliphatic or aromatic diols and, respectively, dicarboxylic acids, as may also occur in the layer (A) or in the layer (C).
Other examples of suitable aliphatic diols are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HO—(CH
2
)
n
—OH, where n is an integer from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol, 1,5
pentanediol and 1,6-hexanediol) and branched aliphatic glycols having up to 6 carbon atoms. Among the cycloaliphatic diols, mention should be made of cyclohexanediols (in particular 1,4-cyclohexanediol). Examples of other suitab

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