Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTILAYER POLYMERIC FILM HAVING UNSEALED PORTIONS WITH
CONTROLLEDSHAPE
The present invention relates to bi- or multilayer films of plastic (in
particular polyolefin)
materials, containing portions of controlled shape, area, number and
arrangement, wherein at
least two adjacent layers are unsealed (i.e. not sealed to each other), and to
a process for
producing them.
Such films constitute a new class of materials, particularly fit for use in
fields where the
visual perception of regular and defined designs (like decorative patterns,
words, logos) is
desirable, such as for instance in packaging, laminated items, banknotes. In
fact the unsealed
portions, normally including air, are visually distinct from the background
constituted by the
rest of the film, where all the layers are sealed to each other.
The closest materials in the prior art are the so called "cellular" or
"corrugated" sheets,
obtained by laminating to each other films or sheets of thermoplastic
polymers.
As the purpose of such materials is to achieve a cushioning effect in
packaging applications
and/or an improvement in mechanical properties, such as rigidity and impact
resistance, they
contain cells or patterns having a substantive thickness, largely exceeding
the total thickness
of the superimposed layers, so that they substantially differ in shape and
thickness from a
multilayer film.
The processes for preparing such cellular or corrugated sheets comprise the
separate
preparation of flat layers and a lamination step where the layers are sealed
to each other by
way of means capable of producing cells or corrugations having substantive
thickness, like
embossing rolls.
Examples of this kind of processes and of the materials obtainable from them
are disclosed
in published patent applications EP 166 312, EP 399 965 and WO 96/01185.
In addition to the said substantial differences in terms of properties and
size of the obtained
materials, such prior-art processes are incapable of providing oriented
materials and
therefore they cannot achieve, particularly in the case of polypropylene
materials, the
remarkable improvement of mechanical properties deriving from molecular
orientation.
Therefore the present invention provides bi- or multilayer polymeric films,
preferably
oriented, in particular mono- or bi-axially oriented, comprising portions of
controlled shape
wherein at least two adjacent layers are unsealed (unsealed portions), said
portions being
visually distinct from the background constituted by other portions of the
film, where
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preferably all the layers are sealed to each other.
In particular, the present invention provides bi- or muItilayer polymeric f
lms, preferably
oriented, in particular mono- or bi-axially oriented, comprising portions of
controlled shape
wherein at least two adjacent layers are unsealed (unsealed portions), said
portions being
visually distinct from the background constitued by the rest of the film,
where all the layers
are sealed to each other.
For "controlled shape" it is meant that the shape of the unsealed portions, as
delimited by
their contours, is regular, as opposed to the irregular and random shape of
defects of sealing
possibly occurring when the film-production process is incorrectly run.
For "visually distinct" it is meant that such unsealed portions are detectable
by the human
eye and appear distinguished from the said background when looking at the
surface of the
films or looking through them.
As previously mentioned, such effect is for instance achieved when a thin
layer of gas, in
particular air, is entrapped in the unsealed portions.
A further advantage of the films of the present invention is that they retain
the advantageous
optical properties typically achievable by the blown-bubble extrusion process
(mainly
through a proper control of the rate of cooling of the film), at least in the
background
portion. Also the mechanical properties typical for films obtained by the said
blown-bubble
extrusion process can be easily retained in the films of the present
invention.
Preferred features for the films of the present invention are (taken singly or
in whichever
combination):
- a total thickness in the background portion from 12 to 100 Nxn, more
preferably from 20
to 75 pxn;
- an additional thickness in the unsealed portions from 0.01 to 4 p,ln;
- a thickness of single layers from 0.1 to 50 wm, more preferably from 0.5 to
40 wm;
- a number of layers of from 2 to 10, more preferably from 2 to 6;
- a percentage of the overall area of the unsealed portions with respect to
the total area of
the film of from 0.1 to 75%, more preferably from 1 to 30%;
- a stretch ratio for bi-axially oriented polypropylene films from 3 to 12,
more preferably
from 4.5 to 8, in both directions (namely MD, i.e. Machine Direction, and TD,
i.e.
Transverse Direction), while for other oriented films (for example of PET,
PVC, LLDPE,
PA) stretch ratios could be different and are well known in the art;
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- a blow up ratio for non oriented films from about 3 to about 8;
- a Gardiner haze for bi-axially oriented polypropylene films (according to
standard
ASTM D 1003) from 2 to 6 units (unsealed portions may have a haze value up to
0.5
units higher than sealed portions);
- a 60 degree gloss for bi-axially oriented polypropylene films(according to
standard
ASTM D 2457) from 120 to 145 units;
- a tensile strength for bi-axially oriented polypropylene films (both MD and
TD ASTM D
882) from 180 to 200 N/mm2.
Whenever the term "area" is used, an area measured on whichever of the two
faces
(surfaces) of the film is meant.
Shape, spacing and arrangement of the unsealed portions are not critical and
can be largely
dictated by aesthetic criteria, depending upon the desired subject to be
represented by the
unsealed portions, singly or in combination (for instance words, scripts,
logos, patterns,
watermark-like drawings).
The layers constituting the films of the present invention can be made of or
comprise any
thermoplastic or elastomeric polymer capable of being sealed in a bi- or
multilayer assembly
under temperature and pressure conditions advantageously employable in the
industrial
practice. Preferably at least one layer is made of or comprises one or more
olefin polymers,
for instance polyethylene (HDPE, LDPE, LLDPE), low temperature sealing
polymers, such
as ethylene/vinyl acetate, ethylene/butyl acrylate or ethylene/methyl
methacrylate
copolymers, vinylidene polymers or copolymers, crystalline and isotactic
olefin polymers,
elastomeric or elastomeric-thermoplastic olefin polymers or blends of the such
polymers.
In particular, at least one layer can be made of or comprise one or more
homopolymers or
copolymers, or their mixtures, of R-CH=CH2 olefins where R is hydrogen or a C1-
C6 alkyl or
an aryl radical. Particularly preferred are the following polymers:
i) isotactic or mainly isotactic propylene homopolymers;
ii) random copolymers of propylene with ethylene and/or C4-Cg a-olefins, such
as for
example 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene, wherein the total
comonomer content ranges from 0.05% to 20% by weight, or mixtures of said
copolymers with homopolymers i);
iii) heterophasic copolymers comprising (a) a propylene homopolymer and/or a
random
copolymer ii), and an elastomeric fraction (b) comprising one or more
copolymers of
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ethylene with propylene and/or a C4-C8 a-olefin, optionally containing minor
amounts of a dime, such as butadiene, 1,4-hexadiene, 1,5-hexadiene, 5-
ethylidene-2-
norbornene.
Preferably the amount of diene in (b) is from 1 % to 10% by weight.
The heterophasic copolymers iii) are prepared according to known methods by
mixing the
components in the molten state, or by sequential copolymerization, and
generally contain the
copolymer fraction (b) in amounts ranging from 5% to 80% by weight.
Examples of polymeric materials different from polyolefm, employable for the
layers of the
films of the present invention, are polyvinylchlorides, polyamides, polyesters
and
polycarbonates.
In order to achieve an easy sealability between the layers, it is preferred to
include in the
multilayer structure a sufficient number of heat-sealable layers made of or
comprising heat-
sealable polymers with low sealing temperature and good seal strength (the
latter being
measured in tenors of load or force to be applied to open the seal).
Such polymers are well known in the art; for instance, in the case of
polyolefins, they can be
selected among the above mentioned low temperature sealing polymers and the
said random
propylene copolymers ii) with a comonomer content high enough to achieve the
said seal
features (indicatively, not less than 5% by weight of the said comonomers with
respect to the
total weight of the copolymer).
When two or more layers are not easily sealable, it is convenient to alternate
each of them
with at least one of the said heat-sealable layers.
The polymer layers may comprise additives commonly employed in the art, like
stabilisers,
pigments, fillers, nucleating agents, slip agents, lubricant and antistatic
agents, flame
retardants, plasticizers and biocidal agents.
As a further object, the present invention provides a process for producing
the said bi- or
multilayer polymeric films, said process comprising the steps of superimposing
two or more
layers, for instance by superimposing two mono- or multilayer webs, and
locally sealing the
superimposed layers by passing them through a pair of rolls (sealing nip
rolls), at least one of
which has engravings on its surface, such engravings having the same shape as
the unsealed
portions to be obtained in the films.
For example, in a flat film process, either cast film or stenter, the web can
be slit down in the
middle and the two webs thus formed can be superimposed (for instance by
guiding them
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over angled rollers or turner bars) and sealed together.
It is also possible for the process to be carried out as an off line
operation, by unwinding the
two webs from separate offwinds and sealing them together in a heated nip.
In particular, the present invention provides a blown-bubble extrusion process
for producing
the said bi- or multilayer polymeric films with unsealed portions having
controlled shape,
said process comprising (A) the formation of a blown bubble of polymer,
followed by a
collapsing step (B) where the bubble is flattened and superimposed layers are
obtained, and a
sealing step (C) where the flattened bubble is split by cutting it at the two
edges and the
superimposed layers are locally sealed by passing them through a pair of rolls
(sealing nip
rolls), at least one of which has engravings on its surface, such engravings
having the same
shape as the unsealed portions to be obtained in the films.
The term "engravings" is used here to designate the hollow areas (recesses)
resulting from
the act of engraving.
As deducible from the previous description, the local sealing defines the
background
portions of the films and is obtained in correspondence to the surfaces of the
sealing nip rolls
wherein no engravings are present, while the engravings correspond to and
define the
unsealed portions of the films.
The size of the unsealed portions substantially corresponds to the size of the
engravings.
Number and arrangement of the unsealed portions are determined by the
arrangement of the
engravings in the roll or rolls.
The blown bubble is obtained by conventional techniques, by blowing with a
gas, in
particular with air, a tubular polymer melt obtained by extrusion through an
annular die.
Such die is preferably a co-extrusion die, to enable production of multilayer
films.
In a preferred embodiment of the invention, the inner layer of the tubular
polymer melt and,
consequently, of the bubble, is made of or comprises one or more of the
previously said heat-
sealable polymers.
The gas used for blowing is trapped in the bubble by the die at one end and by
a pair of nip
rolls (main nip rolls) at the other.
The bubble is collapsed (flattened) by passing it through the said main nip
rolls.
Before the main nip rolls the bubble is preferably passed through converging
means, for
example trains of rolls, slats or air cushions.
After coming out of the die and while being blown, the polymer melt is
generally cooled by
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way of appropriate cooling means, such as air or water cooling rings.
According to an alternative and preferred embodiment, which is illustrated in
annexed
Figure 1, the said tubular polymer melt, after coming out of the extrusion die
S, is quenched
with an appropriate quench system (for instance a water quench system 6),
where it is cooled
to a temperature under the melting point of the polymer material in form of a
tube that can be
transported over rolls to subsequent parts of the process. The temperature at
which the tube
is kept is preferably from 10 to 65°C, more preferably from 20 to
30°C.
This quenched tube subsequently passes through one or more pairs of nip rolls
7 which
control the speed of the tube. Because the rate of extrusion and the diameter
of the tube have
previously been controlled, the speed of the last pair of nip rolls also
controls the tube
thickness.
During and/or subsequent to running through the last of the said nip rolls,
the tube is
preheated to a temperature at which it is ductile (using for instance infra-
red heaters 8 placed
before andlor after the said nip rolls) and blown to form a bubble I which is
then collapsed,
both blowing and collapsing steps being carried out as previously described.
Blowing is achieved by introducing a large volume of gas (in particular air)
into the tubular
plastic melt exiting the annular die or into the preheated tube.
As previously said, the bubble is collapsed by passing it through a couple of
main nip rolls 1
that restrict loss of gas from the bubble, preferably preceded by converging
means 4.
Moreover these rolls are preferably set to run at a faster speed with respect
to the polymer
feeding speed (determined by the nip rolls conveying the tube, when a quenched
tube is
produced), so that the polymer, as well as being stretched in the Transverse
Direction by the
air in the bubble, is simultaneously stretched in the Machine Direction.
As the polymer emerges from the main nip rolls, it is in the form of a
collapsed tube,
commonly called "layflat tube".
The speed of the main nip rolls and the width of the layflat tube determine
the film thickness
at this point.
When the layflat tube retains stresses (generally caused by chain to chain
distortions that
have not exceeded the limit of elastic distortion), they should be reduced in
order to flatten
the tube and, consequently, the film.
This can be done in an annealing section 3, installed after the main nip
rolls.
The annealing section can for instance consist of an air oven, where the
layflat tube is
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contacted with hot air at such a temperature as to induce a small degree of
film shrinkage.
The hot air temperatures are generally from 100 to 180°C, more
preferably from 120 to
160°C, while illustrative film shrinkages are in the order of from 2 to
4% MD and 1 to 3%
TD at 120°C.
It is also possible to carry out the annealing step by using a stenter
equipment or running the
film over a series of heated rolls.
Complete shrinkage is prevented by maintaining suitable temperatures and
applying suitable
tension in the layflat tube both in the Machine and in the Transverse
direction.
In the Machine Direction, the said tension can be controlled by adjusting the
speed
differential between the sealing nip rolls and the main nip rolls.
In the Transverse Direction, the tension can be applied by threading the
layflat tube over a
pair of rods running in the machine direction along each side of the annealing
section.
Preferably, the rods taper inwards to. allow the correct amount of transverse
shrinkage to
occur without film snagging and breaking. To assist in running the layflat
tube over the rods
at high temperature, a gas (air) supply is preferably fed to the outer edge of
the rods to
lubricate the contact with the fold (i.e. the two edges) of the layflat tube.
It is believed that
this gas supply might also have the effect of favouring the formation of a
thin gas layer in the
unsealed portions of the films.
Before being fed to the sealing nip rolls, the layflat tube is split by
cutting it at the two edges
i.e. at the two lines where the layflat tube folds.
This can be achieved by fixing the said rods to points outside the tube, so
immediately
before the mounting points, the tube is slit into webs.
After splitting the layflat tube, the so obtained superimposed layers are heat-
sealed to obtain
the bi- or multilayer film.
The sealing step is carried out simultaneously as the two webs are brought
together.
According to an alternative embodiment, when stenter equipment is used for the
annealing
step, the two webs are obtained by cutting the layflat tube before annealing.
Also, when
using stenter equipment, the sealing step can be carried out either before or
after annealing.
As previously said, heat-sealing is achieved by passing the superimposed
layers through a
couple of sealing nip rolls 2.
According to an alternative embodiment, both the said collapsing step (B) and
sealing step
(C) can be carned out by means of the main nip rolls, which in this case
coincide with the
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sealing nip rolls in terms of function and features.
There are two principal and equally preferred constructional alternatives for
the sealing nip
rolls making up the sealing nip.
According to one of these alternatives, one of these sealing nip rolls has a
smooth surface
(non-engraved roll} and is heated at the required sealing temperature while
the other is
engraved. If, for instance, a propylene copolymer or terpolymer is used as
heat-sealable
polymer, the sealing temperature is preferably in the range of 95 to
140°C, more preferably
105 to 125°C; if other heat-sealable polymers, such as LDPE or blends
of LDPE with EVA,
EBA or EMA are used, the sealing temperature is lower.
The non-engraved roll is preferably made of metal (in particular of steel) and
can be heated
for instance by way of internal electrical resistances, internally flowing
heating fluids or
inductive coupling. Alternatively, heat input could be from an external
source.
The engraved roll is preferably made of metal (in particular of steel) as
well, but covered
with a layer of polymeric material, in particular of rubber, containing the
engravings.
The hardness of the polymeric material should be such that it does not distort
significantly
under the nip pressures applied. Preferably, the hardness should range from 50
to 100 on the
International Rubber Hardness Durometer Scale (I R H D), more preferably from
65 to 90.
Specific examples of polymeric materials that can be used for the engraved
roll are Hypalon
chlorosulfonated polyethylene, polybutadiene, natural rubber, EPDM,
styrene/butadiene
copolymer, chloroprene, silicone rubber, nitrile rubber, polyurethane, styrene
rubber,
carboxylated nitrile rubber and Viton fluoroelastomers.
The rubber thickness should be preferably 1 to 50 mm, more preferably 4 to 15
mm.
The depth of the engravings is such that heat sealing does not occur in their
correspondence.
Preferably, the depth of the engravings should be in the range of from 0.1 to
5 mm, more
preferably from 0.5 to 2 mm.
The nip pressure should be preferably in the range of from 0.5 to 2.5 kg per
linear cm, more
preferably from 1 to 1.5 kg per linear cm.
According to the other preferred alternative, one of the said sealing rolls
has an engraved
surface and is heated to the required sealing temperature, while the other has
a smooth, non
engraved surface. The required sealing temperature is similar or identical to
that used with
the previous alternative.
In this case, the engraved roll is preferably made of metal (in particular of
steel). When it is
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of steel, it can be electro-plated and/or electrodeless plated or otherwise
coated with other
metal or metals (preferably copper, nickel or chromium, or combinations
thereof] or with
other heat conductive materials.
The engraved roll is, for instance; heated by way of internal electrical
resistances, internally
flowing fluids or inductive coupling. Alternatively, heat input could be from
an external
source.
The depth of the engravings is such that heat sealing does not occur in their
correspondance.
Preferably the depth of the engravings should be in the range of 0.1 to 5 mm,
more
preferably from 0.1 to 1 mm.
The smooth roll is preferably made of metal (in particular steel) as well but
is covered with a
layer of polymeric material, in particular rubber. The hardness of this
polymeric material
should range from 30 to 100 in the International Rubber Hardness Durometer
Scale (I R H
D), more preferably from 50 to 90.
Thickness and specific examples of polymeric materials that can be used for
the smooth roll
are the same as those given for the first alternative. Also the nip pressure
should preferably
be in the same ranges as in the first alternative.
After coming out of the sealing rolls, the film can be subjected to finishing
treatments, like
corona discharge treatments, for example.
The so obtained film can be coupled by lamination to other mono-, bi- or
multilayer plastic
films or to other materials, like paper or metal foils.
Coupling can be effected by laying adhesives onto the appropriate surfaces and
using
conventional lamination equipment.
The following example is given for illustrative purposes and does not limit
the invention
itself.
Example
Using a blown-bubble extrusion apparatus as described above with reference to
Figure 1,
films according to the invention are produced.
In detail, a tube composed of 3 layers is produced by extrusion through a co-
extrusion die
and cooling in a water quench system, then the tube is heated and blown and
the so obtained
bubble is collapsed by passing it through a couple of main nip rolls. The
layflat tube so
obtained is annealed by means of an air oven, split by cutting it at the two
edges and the
resulting superimposed layers are heat-sealed through a couple of sealing nip
rolls having the
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previously described features (namely, a heated non-engraved roll of steel and
a roll of steel
covered with an engraved layer of rubber).
The film layers are produced by co-extruding the following polymeric
materials:
- Inner layer: propylene terpolymer Adsyl 5 C30 F (Montell) having a Melt Flow
Rate,
according to standard ISO 1133, of 5.5 g/10 min. and a density, according to
standard
ISO 1183/A of 0.89 g/cm3.Core layer: propylene homopolymer KF 6100 (Montell),
having a Melt Flow Rate, according to standard ISO 1133, of 3 g/10 min.;
- Outer layer: propylene homopolymer RF 6100 (Montell), having a Melt Flow
Rate,
according to standard ISO 1133, of 8 g/10 min. or, in alternative, propylene
copolymer
Moplen EP 3 C 39 F (Montell), having a Melt Flow Rate of 5 g/10 min. and a
density,
according to standard ISO 1183, of 0.9 g/cm3.
The following working conditions are used
Inner Extruder output 18 Kg/hr
melt temperature 230 °C
Core Extruder output 124 Kg/hr
melt temperature 230 °C
Outer Extruder output 18 Kg/hr
melt temperature 230 °C
Quench water temperature 25 °C
speed 11.0 m/min.
diameter 165 mm
Bubble main nip speed 69.5 m/min.
layflat width 1600 mm
TD draw ratio 6.2
MD draw ratio 6.3
draw temperature 170 °C
Annealing width reduction 100 mm
speed reduction 4m/min.
air temperature 155 °C
Sealing roll temperature 125 °C.
The thickness of the single layers results to be:
Inner layer 1.7 p,rrl
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Core layer 11.6 pln
Outer layer 1.7 ~tm.
The,total thickness of each of the two webs before sealing is about 15 pm.
The total thickness of the film after sealing the two webs is about 30 prrl.
Depending upon the kind of engravings present in the engraved roll, films
comprising
unsealed portions of various shape, extension and arrangement are obtained,
with properties
falling within the previously reported ranges.
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