Note: Descriptions are shown in the official language in which they were submitted.
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TF~-TRANSI~ATI~~ HI/vos/W6/V08.04.1998
A mufti-layer, oriented, thermally sealable. vacuum-metallised ,~olpprop~rlene
film
This invention relates to a mufti-layer, biaxially oriented, thermally
sealable
~ polypropylene film which is vacuum-metallised with aluminium, wherein the
aluminium layer exhibits a good adhesive bonding capacity and a reduced number
of
pinholes and consequently exhibits a good barrier effect in relation to water
vapour,
oxygen and aromas. The invention also relates to a process for producing said
film.
The film to which the present invention relates has, as its characterising
features, a
core layer of isotactic polypropylene, an outer layer which is subjected to
corona
discharge, flame or plasma pretreatment or pretreatment with fluorine and
which
- comprises an olefinic homo-, co- or terpolymer, or a mixture of said
polymers, and
0.01 to 0.5 % by weight of anti-seizing particles which exhibit good, strong
bonding
to aluminium, a second outer layer comprising an olefinic homo-, co- or
terpolymer,
or a mixture of said polym~°rs, and 0.01 to 0.5 % by weight of anti-
seizing particles
which exhibit poor bonding; to aluminium, and an aluminium layer which is
vapour-
deposited under vacuum on the pretreated side.
On account of thein~ good b;~rrier properties in relation to water vapour,
oxygen, light
?0 and aromas, polypropylene; films which are vacuum-coated with metals or
metal
oxides are mostly used for tlhe packaging of sensitive foodstuffs.
The use of biaxially oriented polypropylene films which are vacuum-metallised
with
aluminium is widely knowm. Metallising improves the barrier effect in relation
to
?5 water vapour by a factor of 10 and improves the barrier effect of the film
in relation to
oxygen by a factor of 100. Further improvements are currently difficult, since
the
vacuum-deposited metal layer comprises pinholes. These pinholes constitute a
large
number of very small holes (of diameter about 1 pm) which are distributed on
the
metal layer in the form of clusters. Considerable amounts of oxygen can
diffuse
30 through the pinholes, which has a negative effect on the barrier properties
in relation
to oxygen.
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Thus EP-A 329336 describes a metallisable film which exhibits good barrier
properties. However, due to the high content of homopolymer in the layer to be
metallised, the bonding of the vapour-deposited layer to the substrate is not
~ satisfactory for current requirements. Moreover, in this vacuum-metallised
film there
is a large number of pinholes in the vapour-deposited metal layer.
EP-A 395204 describes a rr~etallisable film which exhibits improved adhesion
of the
metal. No success was achieved in solving the pinhole problem, however. This
film
therefore has disadvantages with regard to its properties as a barrier layer.
- EP-A 481266 describes a multi-layer, vacuum-metallised film in which, after
vacuum-
metallising, a protective layer of wax is vapour-deposited on the metal under
vacuum.
This film exhibits a. considerably reduced number of pinholes and its barrier
properties
1 ~ are improved. However, the; waxes described there result in wetting
problems during
- the processing step (covering/laminating) which follows vacuum coating,
particularly
when aqueous adhesive systems are used. Moreover, when composites are produced
there is no improvement in barrier properties which corresponds to that
achieved in
the prior art (see comparative example 3).
It has therefore proved to be; necessary to produce a metallised BOPP film
which has a
reduced number of pinholes and which thus exhibits improved barrier properties
in
relation to water vapour ;and oxygen, and which also exhibits good processing
properties (bondin~; to water-based adhesives).
This object is achieved by a film according to claim 1. The preferred feature
thereof is
that the film, which ~is prod.uced by means of a biaxial stretching process,
contains a
vacuum-metallisable outer layer which is subjected to corona discharge, flame
or
plasma pretreatment or which is pretreated with fluorine and which comprises
an
olefinic co- or terpolymer, or a blend of one or more olefinic co- or
terpolymers, and
anti-seizing particles which exhibit good bonding to vapour-deposited
aluminium, a
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core layer comprising an isotactic polypropylene having an MFI between 0.5 and
4.~
g/10 min, and a sealable oui:er layer, which is situated on the non-metallised
side and
which comprises an olefinic; co- or terpolymer, or a mixture of one or more
olefinic
co-or terpolymers, and anti-seizing particles which exhibit poor bonding to
alu-
~ minium. Surprisingly, the barrier properties, which hitherto could only be
definitively
influenced by the vacuum-rnetallising step, are considerably improved by the
use of
anti-seizing particles which exhibit poor bonding to aluminium and which are
situated
on the opposite side; to that which is metallised.
In order to ensure ;food running properties of biaxially oriented
polypropylene films
when they are processed on machines, it is necessary to incorporate additives
in the
- sealing layers or outer layers at least. However, when internal lubricants
such as
erucic acid amide or polydimethylsiloxane are incorporated in vacuum-
metallised
films, the latter exhibit poor bonding of the vapour-deposited layer to the
base film. In
order to impart good machine-running properties to the film despite a lack of
internal
- lubricants, it is neccasary to incorporate a sufficient amount of anti-
seizing particles in
the outer layers. Customary anti-seizing particles (average particle diameter:
1 to ~
pm, most preferably: 2 to 4 Vim) which exhibit good bonding to aluminium
consist of
silica, or of alkali alumin.osilicates, or of alkaline earth aluminosilicates,
or of
crosslinked polymethyl methacrylates, or of polyamide. Alkaline earth
aluminosilicates with an average particle diameter of 2 to 4 pm are
particularly
preferred. Anti-seizing particles (average particle diameter 1 to 5 pm) which
naturally
exhibit poor bonding properties comprise crosslinked olefines or crosslinked
silicones; particles with an amerage diameter of 2 to 4 pm are particularly
preferred.
Of the numerous materials which are used for sealable layers which can be
vacuum-
metallised, the following arc: preferably used:
- random propylene/ethylene copolymers
;0 - random propylene/olefine (1) copolymers
- random propylene/ethylene/olefine terpolymers
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- random propylene/olefine/ethylene terpolymers
- mixtures of two or three of the above polymers.
A material which is particularly preferred is a blend comprising a
propylene/ethylene
copolymer (90 to 9'9 % by weight) which is characterised in that it contains
1.0, to 10
by weight ethylene (preferably 2 to 6 % by weight) and melts between
110°C and
150°C, preferably ~~etween 120°C and 140°C, and an
ethylene/octene copolymer ( 1 to
% by weight) wlhich is characterised in that it has a density <0.915 g/cm3,
and 0.05
to 0.35 % by weight of a calcium aluminosilicate which is characterised in
that its
10 average particle diameter is between 1 and 3 pm.
A mixture which is particularly preferred contains 99.95 to 99.65 % by weight
of a
random propylene/butene/et:hylene polymer which is characterised in that it
has an
MFI of 5 to 10 g/10 min and a melting point of 134°C, and contains 0.05
to 0.35 % by
I S weight of crosslinked p~olydimethylsiloxane anti-seizing particles which
are
characterised in th;~t their average particle diameter is between 1 and 5 pm,
most
preferably 2 to 4 urn.
The films according to th<: invention are most preferably produced by
customary
processes, such as laminating, coating or melt coextrusion. Extrusion and
solidification of the; thick film is followed by biaxial orientation below the
crystallite
melting point of thf; PP layer. Biaxial orientation can be effected either
simultaneously
or sequentially. Thc: following process is particularly preferred:
After solidification. of the thick film on the casting roll, the film is
stretched in its
direction of travel. (longitildinally) at a stretching ratio of 4/1 to 7/1 and
at a
temperature of 90°C~to 150"C (preferably 110°C to 140°C).
The stretching ratio in the
transverse direction is preferably between 8/1 and 12/1 and transverse
stretching of
the film is effected at a temperature between 130°C and 170°C.
The subsequent
thermofixing step is preferably carried out at 100°C to 180°C
(preferably between
150°C and 170°C).. In order to ensure that the substantially
nonpolar film surface has
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an affinity for aluminium, it: is necessary to subject the film to corona
discharge (spray
discharge) pretreatment. In the course of this procedure, atmospheric oxygen
is
incorporated in thc: film surface in the form of carbonyl, epoxide, ether or
alcohol
groups. Other methods of pretreating polypropylene films include flame or
plasma
treatment and treatment with fluorine.
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Example 1
A biaxially oriented film was produced by means of the production process
described
above (area stretching ratio: 4~/1; longitudinal stretching temperature:
142°C;
transverse stretching temperature: 160°C), and had the following
structure:
Total thickness: 20 Vim; layer sequence: ABCD
See above for the definition of layer A.
Outer layer for vacuum-met<illisingJBl:
Thickness: 1 gm
Material:
99.75 % by weight propylene/ethylene copolymer 3.5 % by weight
ethylene
MFI: 5.0 g/10 min
density: 0.912 g/cm3
0.25 % by weight calcium aluminosilicate average particle size:
2.5 p m
Core la e'~r (Cl:
Thickness: 17 pm
Material:
100 % by weight isotactic polypropylene MFI: 3.0 g/10 min
Sealing lair (D):
Thickness: 2 pm
Material:
99.75 % by weight , propylene/butene/ MFI: 7.0 g/l0 min
ethylc°ne copolymer
0.25 % by weight cross Linked polydimethyl-siloxane average particle size:
;0 particles 2.0 ~m
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Comparative example 1
A biaxially oriented film w:as produced by means of the production process
described
above (area stretching ratio: 45/1; longitudinal stretching temperature:
142°C;
~ transverse stretching temperature: 160°C), and had the following
structure:
Total thickness: 20 pm; layer sequence: ABCD
See above for the definition of layer A..
Outer layer for vacuum-metallising.jB):
Thickness: 1 pm
Material:
- 99.75 % by weight propylene/ethylene copolymer 3.~ % by weight ethylene
MFI: 5.0 g/10 min
0.25 % by weight calcium aluminosilicate average particle size:
2.5 p m
Core layer lCl:
Thickness: 17 pm
Material:
100 % by weight isotac;tic polypropylene MFI: 3.0 g/10 min
Sealing layer (D~,
Thickness: 2 ~m
Material:
99.75 % by weight propylene/ethylene copolymer ethylene content:
3.5 % by weight
MFI: 7.0 g/10 min
0.25 % by weight calcium aluminosilicate average particle size:
2.5 pm
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Comparative exannple 2
A biaxially oriented film w;~s produced by means of the production process
described
above (area stretching ratio: 45/1; longitudinal stretching temperature:
I42°C;
transverse stretching temperature: 160°C), and had the following
structure:
Total thickness: 20 pm; layer sequence: ABCD
Outer layer for vacuum-metallising (BL
Thickness: 1 pm
Material:
99.75 % by weight propylene/ethylene copolymer 3.5 % by weight ethylene
MFI: 5.0 g/10 min
0.25 % by weight crosslinked polydimethylsiloxane average particle size:
particles 2.0 ~ m
Core la, ey r (Cl-
Thickness: 17 ~m
Material:
100 % by weight isotactic polypropylene MFI: 3.0 g/10 min
Sealing layer (Dl:
Thickness: 2 ~m
Material:
99.75 % by weight: prop;ylene/ethylene copolymer ethylene content:
3.5 % by weight
MFI: 5.0 g/10 min
0.25 % by weight crosslinked polydimethylsiloxane average particle size:
particles 2.0 p m
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Comparative example 3
A biaxially oriented film was produced by means of the production process
described
above (area stretching ratio: 45/1; longitudinal stretching temperature:
142°C;
transverse stretching; temperature: 160°C). During the in-line vacuum-
metallising step,
this sample was vacuum-coated with a wax layer W (see EP-A 481 266) to protect
the
aluminium layer (layer A).
Structure:
Total thickness: 20 p.m; layer sequence: WABCD
- Outer lair for vacuum-metallisin~ (Bl:
Thickness: 1 pm
Material:
99.75 % by weight propylene/ethylene copolymer 3.5 % by weight ethylene
- MFI: 5.0 g/10 min
0.25 % by weight calcium aluminosilicate average particle size:
2.5 p m
Core lager (Cl:
Thickness: 17 pm
Material:
100 % by weight isota<;tic polypropylene MFI: 3.0 g/10 min
?5 Sealing la er D):
Thickness: 1 pm
Material:
99.75 % by weight propylene/ethylene copolymer 3.5 % by weight ethylene
MFI: 7.0 g/10 min
0.25 % by weight calcium aluminosilicate average particle size:
2.5 pm
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The films described in the examples and comparative examples were vacuum-
metallised with aluminium to an optical density (OD) of 2.0 (layer A) and were
subsequently adhesiuely bonded to a transparent BOPP film by means of an
aqueous,
polyurethane-based dispersion adhesive.
The OD is understood to be defined by OD = log (Ia/I)
where Io is the intensity of the irradiated light is and I is the intensity of
the light
which is reduced b5~ the film..
The following Table shows that the film according to the invention is
considerably
superior to films comprising; symmetrical anti-seizing additives and films
comprising
a protective wax layer.
Nunnber O, perm. O, perm. of compositeAdhesive
o (cm3/m'-d (sample/adhesive/20~mbonding
pinholesbar) BOPP) capacity
xample ew 2 8 ood
1
omparison any 102 6 ood
1
omparison ediuun 83 32 ood
2 mount
Comparisonew 0 88 oor
3
Methods of measurement
?0 Determination of th erm abilitX to oxygen (Oz erm.
The permeability to oxygen was determined according to DIN 53380, Part 3, at
23°C
and 0 % relative atmospheric humidity.
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Number of pinholes
The light from a torch was shone through the film and the number of pinholes
was
assessed corresponding to the following classification:
none, few, medium amount, many
Determination of the bonding of anti-seizing particles to aluminium
Films were produced which contained anti-seizing particles in a concentration
of 2000
ppm/sealing layer. These films were placed on a BOPP film without anti-seizing
- particles, which ha<i been vacuum-metallised with aluminium but which had
not been
subjected to corona discharge pretreatment, and were loaded at 23°C for
a period of
90 minutes with a metal plate and weight such that the total mass of the plate
and
weight was 1 kg. After removing the metal plate and stripping the film
comprising
anti-seizing particles from tile BOPP film (which had not been pretreated)
which was
vacuum-metallised with aluminium, the number of pinholes in the aluminium
layer
was assessed by means of a torch. The number of pinholes could thus be
directly
correlated with the bonding of the anti-seizing particles to aluminium. The
following
?0 bonding properties were obtained:
ype of anti-seizin;~ umber of pinholesBonding to Al
particles
alcium aluminosilicate any ood
SiO, any ~ood
rosslinked polyethylenefew oor
rosslinked polydimethylsiloxaneew oor
rosslinked PIMA ~ any good
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Determination of the mel flow index IMFI)
The melt flow indea (abbreviated to MFI in the text) was measured according to
DIN
53 735 using a loading force of 21.6 N at a temperature of 23°C.
J
Adhesive bonding c:a aci
The laminated composite eras held for at least 3 days in a climate of
23°C and a
relative humidity of 50 %. Thereafter, the composite was separated by hand.
The
composite was classified as follows, based on the area of the aluminium layer
which
remained on the original side of the film or on the film which was laminated
by means
- of an adhesive.
Less than 50 % of the aluminium surface bonded to the film surface which was
l 5 laminated by mean, of an adhesive: poor.
More than 50 % of the aluminium surface bonded to the film surface which was
laminated by means of an adhesive: good.
25
