Sealable polyester film with high oxygen barrier, its use...

Details Sealable polyester film with high oxygen barrier, its use... Sealable polyester film with high oxygen barrier, its use...

1999-03-24

2002-05-07

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Hollow or container type article

Polymer or resin containing

C428S336000, C428S480000, C428S910000, C428S458000, C264S290200

Reexamination Certificate

active

06383585

ABSTRACT:

The invention relates to a biaxially oriented polyester film having a base layer B at least 80% by weight of which is composed of a thermoplastic polyester, and having two 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
In many cases, there is demand for food and drink packaging to have a high barrier effect with respect to gases, water vapor and flavors. For this reason, use is usually made of polypropylene films which have been metalized or have been coated with polyvinylidene chloride (PVDC). However, metalized polypropylene films are not transparent and are therefore not used in cases where the view of the contents is likely to have added promotional effect. Although films coated with PVDC are transparent, the coating, like the metalizing, takes place in a second operation which makes the packaging markedly more expensive. Ethylene-vinyl alcohol copolymers (EVOH) likewise exhibit a strong barrier effect. However, films modified with EVOH are particularly highly sensitive to moisture, and this limits their range of application. In addition, because of their poor mechanical properties they have relatively high thickness or have to be laminated with other materials at high cost, and they are also difficult to dispose of after use. Many packaging systems, furthermore, demand sealability of the films as well as an effective barrier.
It is therefore an object of the present invention to provide a transparent, sealable, biaxially oriented polyester film which has a high oxygen barrier, is simple and cost-effective to produce, has the good physical properties of the known films, and does not give rise to disposal problems.
DESCRIPTION OF THE INVENTION
The object is achieved by means of a biaxially oriented polyester film having at least three layers and having a base layer B at least 80% by weight of which is composed of (at least) one thermoplastic polyester, and having two outer layers A and C, wherein the outer layer A is composed of a polymer or a mixture of polymers which comprises at least 40% by weight of ethylene 2,6-naphthalate units and up to 40% by weight of ethylene terephthalate units and/or up to 60% by weight of units from aliphatic, including cycloaliphatic, or aromatic diols and/or dicarboxylic acids, and the opposite outer layer C has a sealing initialization temperature of ≦200° C., preferably ≦130° C., or between from about 100° C. to 200° C. The novel film generally has an oxygen permeability of less than 80 cm
3
/(m
2
bar d), preferably less than 75 cm
3
/(m
2
bar d), particularly preferably less than 70 cm
3
/(m
2
bar d). The novel film is preferably transparent.
Preference is given to a polyester film in which the polymers of the outer layer A comprise at least 65% by weight of ethylene 2,6-naphthalate units and up to 35% by weight of ethylene terephthalate units. Among these, particular preference is in turn given to a polyester film of the type in which the polymers of the outer layer A comprise at least 70% by weight of ethylene 2,6-naphthalate units and up to 30% by weight of ethylene terephthalate units. The outer layer A may, however, also be composed completely of ethylene 2,6-naphthalate polymers.
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), or branched aliphatic glycols having up to 6 carbon atoms, and cycloaliphatic diols having one or more rings and if desired containing heteroatoms. Among the cycloaliphatic diols, mention may be made of cyclohexanediols (in particular 1,4-cyclohexanediol). Examples of suitable aromatic diols are those of the formula HO—C
6
H
4
—X—C
6
H
4
—OH where X is —CH
2
—, —C(CH
3
)
2
—, —C(CF
3
)
2
—, —O—, —S— or —SO
2
—. Besides these, bisphenols of the formula HO—C
6
H
4
—C
6
H
4
—OH are also very suitable.
Preferred aromatic dicarboxylic acids are benzenedicarboxylic acids, naphthalenedicarboxylic acids (for example naphthalene-1,4- or -1,6-dicar-boxylic acid), biphenyl-x,x′-dicarboxylic acids (in particular biphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylic acids (in particular diphenylacetylene4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylic acids, mention may be made of cyclohexanedicarboxylic acids (in particular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C
3
-C
19
-alkanedioic acids are particularly suitable, where the alkane moiety may be straight-chain or branched.
The present invention also provides a process for producing this film. It encompasses
a) producing a film from base and outer layers A and C by coextrusion,
b) biaxial orientation of the film and
c) heat-setting of the oriented film.
To produce the outer layer A, it is expedient to feed granules of polyethylene terephthalate and polyethylene 2,6-naphthalate directly to the extruder in the desired mixing ratio. At about 300° C. and with a residence time of about 5 min, the two materials can be melted and can be extruded. Under these conditions, transesterification reactions can occur in the extruder and during these copolymers are formed from the homopolymers.
The polymers for the base layer B are expediently fed in via another extruder. Any foreign bodies or contamination which may be present can be filtered off from the polymer melt before extrusion. The melts are then extruded through a coextrusion die to give flat melt films and are layered one upon the other. The coextruded film is then drawn off and solidified with the aid of a chill roll and other rolls if desired.
The biaxial orientation is generally carried out sequentially. For the sequential stretching, it is preferable to orient firstly in a longitudinal direction (i.e. in the machine direction) and then in a transverse direction (i.e. perpendicularly to the machine direction). This causes an orientation of the molecular chains. The orientation in a longitudinal direction may be carried out with the aid of two rolls running at different speeds corresponding to the stretching ratio to be achieved. For the transverse orientation, use is generally made of an appropriate tenter frame.
The temperature at which the orientation is carried out can vary over a relatively wide range and depends on the properties desired in the film. In general, the longitudinal stretching is carried out at from 80 to 130° C., and the transverse stretching at from 90 to 150° C. The longitudinal stretching ratio is generally in the range from 2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio is generally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1.
During the subsequent heat-setting, the film is held for from 0.1 to 10 s at a temperature of from 150 to 250° C. The film is then wound up in a conventional manner.
A great advantage of this process is that it is possible to feed the extruder with granules, which do not block the machine.
The base layer B of the film is preferably composed to an extent of at least 90% by weight of the thermoplastic polyester. Polyesters suitable for this 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-bishydroxymethylcyclo-hexane and terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also from ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalate bibenzoate, PENBB). Particular preference is given to polyesters which are composed to an extent of at least 90 mol %, preferably at least 95 mol %, of ethylene glycol units and terephthalic acid units or of ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units

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