CA 02771447 2012-02-17
POLYPROPYLENE FILM WITH OPENING AID
The present invention relates to a biaxially oriented polypropylene film with
perforations and packaging made from biaxially oriented polypropylene film
with perforations as an opening aid.
The success of biaxially oriented plastic films, particularly films made from
thermostatic polymers and especially biaxially oriented polypropylene films,
is
essentially based on their outstanding mechanical strength properties in
conjunction with a comparatively low weight, good barrier properties and good
weldability. The polyolefin film protects the package contents from quickly
drying out or from loss of flavour with very little material input.
The consumer's need for hygienic, visually appealing, tightly sealed,
resistant
packaging is incompatible with the desire for easy, controllable opening. The
latter is a subject of complaint among consumers of polyolefin film packaging
and regarded as a disadvantage as compared with paper packaging.
Uniaxially oriented films exhibit low tear resistance in the orientation
direction
and can easily be torn into and the tear propagated. However, uniaxially
oriented films cannot be used in many areas due, among other things, to their
lack of mechanical properties in the transverse direction. The biaxial
orientation produces, on the one hand, the desirable high strengths (moduli)
in both dimensions; on the other hand, however, the preferred directions are
also thereby equalised. The consequence of this is that in order to open film
packaging (e.g. biscuit packaging), a high force must first be overcome, in
order to tear the film. However, if the film is damaged or torn, a tear is
uncontrollably propagated even when using very low pulling force. These
defective usage properties involving excessively high initial tear resistance
combined with very low tear propagation resistance reduce the acceptance of
film packaging on the end-user market, despite the advantages mentioned
above.
It was proposed in the state of the art that the film should be provided with
a
predetermined breaking point. During opening, the film is torn into at this
predetermined breaking point. However, the tear often propagates
CA 02771447 2012-02-17
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uncontrollably, since although these solutions make the film easier to tear
into, they do not actually help with controlled tear propagation.
A further known solution is the mechanical inclusion of a predetermined
breaking point in the form of perforations or notches along an imaginary
opening line. However, even these kinds of perforations frequently do not
guarantee controlled tearing. During opening, the tear only follows the
perforated line to begin with and is then propagated in a randomly deviating
manner through the material.
This uncontrolled tear propagation behaviour is problematic particularly in
the
case of biaxially oriented polypropylene films, since the tear propagation
resistance of this material is particularly low. This problem particularly
affects
packaging containing individual items, which are not packed loosely, but in an
ordered fashion, such as packs of cigarettes, Weetabix, crispbreads, biscuit
rolls and the like. These types of packaging focus particularly on the fact
that
the consumer only removes single items to begin with and would like to keep
the remainder in the packaging, so that further units can removed at a later
time. Uncontrolled tear propagation in the film packaging is particularly
annoying for the consumer in this application.
WO 98/2312 describes packaging which is precut by a laser beam. This
packaging exhibits a multi-layer structure. In particular, a metallic
intermediate
layer is provided, which is intended to prevent the laser beam from cutting
through the film. This packaging is time-consuming and costly, due to the
lamination with a metal layer. Nothing is disclosed in the description about
the
precise design of the packaging.
In addition, the perforated film must exhibit adequate mechanical stability,
so
that it can be used for the packaging of individual items. Methods are
described in the state of the art whereby perforation is integrated as a
processing stage in the packing process. This solution prevents any
mechanical loads on the perforated film, e.g. during rolling and unrolling.
However, it is not possible to integrate perforation in all packing processes.
On the one hand, the spatial conditions where there is an existing plant are
often such that there is simply no space to attach a device of this kind.
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Furthermore, packing processes in the cigarette industry run at very high
speeds, which are possibly too quick for perforation. Therefore, it is not
always possible to incorporate a film-perforating device into a plant used to
wrap groups of cigarette packs. However, adequate mechanical stability of the
film is in conflict with the simultaneous desire for easy-to-open packaging.
The problem addressed by the present invention involved providing improved
packaging for organised individual items.
The problem underlying the invention is solved by a biaxially oriented
polypropylene film with perforated lines, in which the film exhibits at least
two
perforated lines, which run parallel to one another and are located at a
maximum interval of 10 mm and wherein the perforations of the two lines are
offset relative to one another and there are intervals of undamaged film
between the perforations and the intervals between the perforations are
longer than the perforations themselves.
The problem is further solved by a method of wrapping individual items, in
which a perforated film is wound from a roll and used to package the
individual items.
Fig. 1 shows an embodiment of the film (2) with circular perforations (1) with
a
length (diameter) B. The length of the intervals between the perforations
corresponds to length A. The two perforated lines running in parallel are
disposed at an interval C relative to one another. In the embodiment
according to Fig. 1, the intervals A are longer than the length of the
perforations B. The perforated lines are offset relative to one another, so
that
the centre of the perforation in each case is aligned with the centre of the
opposite interval.
Figure 2 shows a film with circular perforations of length B, which is
significantly greater than the length of the intervals A.
Figure 3 shows perforations with a rectangular geometry, in which here, too,
B<A.
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Fig. 4 shows a film with rhomboid perforations, such as those produced by
rotating knives, for example.
Fig. 5 shows a film with elliptical perforations. Here, too, B<A.
Fig. 6 shows a possible needle perforation geometry in which propeller-like
perforations are produced.
Fig. 7 shows a film web exhibiting several double perforated lines in a
longitudinal direction.
Fig. 8 shows individual item packaging (3), which is opened by breaking off
along the double perforated line (4).
The common feature of all these embodiments according to Figures 1 to 8 is
that the intervals A between the perforations are longer than the length B of
the perforation itself.
The two perforated lines running parallel and offset to one another create a
predetermined breaking point in the film, in which a tear is propagated in a
very controlled manner following the initial tear. After the opening tear, the
tear runs along one of the perforated lines or between the two parallel
perforated lines in a controlled manner. The film packaging is only opened to
the desired extent.
The packaging according to the invention is particularly suitable for arranged
or stacked individual items, e.g. cigarette packs, and also for soft goods,
such
as butter, sausage and the like. It will be possible for the consumer to tear
the
film lengthwise along a predetermined breaking point in such a way that
individual items can be removed without additional units falling out unchecked
in the case of individual items. The individual segments between the parallel
perforated lines are adapted according to the size of the unit to be removed.
This makes it possible to control whether one or more units can easily be
removed with one opening.
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The perforated lines may be applied to the film web in a suitable pattern. The
lines will preferably run straight. However, it is also possible for the line
to be
applied using any geometric form, so that the predetermined breaking point
later corresponds to the shape and dimensions of the packaged individual
5 items.
The perforated lines may be produced by mechanical perforation using
needles or knives, for example, or by a laser beam or in some other way, so
that a continuous hole is produced in the film. However, perforations within
the meaning of the present invention also refer to thin points at which there
is
no continuous hole produced in the film, but the film is simply made thinner
at
the corresponding points by removing material, using a laser beam that only
penetrates to a predetermined depth, for example. In these embodiments, 10
to 80 % of the original film thickness, preferably 20 to 60 % of the original
film
thickness, is retained close to the thin point. The packaging thereby remains
sealed, so that it continues to meet all hygiene requirements and its barrier
properties are not detrimentally affected.
There also follow along the perforated line holes, incisions or thin points
(perforation B) alternating with undamaged film sections, of which the length
A
is indicated by the perforation interval. Intervals within the meaning of the
present invention are therefore the areas of undamaged film extending from
the end of one perforation to the start of the next perforation in the same
perforated line.
The geometric shape of the perforation may vary and depends on the method
of perforation. Needles or rotating knives may be used for perforation. For
perforated lines made up of non-continuous thin points, lasers or ultrasound
are preferred. Needle perforations produce circular or elliptical or propeller-
shaped forms, which are produced by the needle and film being in conduct
during perforation and moving relatively to one another. Knife perforations
produce more rectangular or rhomboid incisions or thin points. Laser and
ultrasound methods produce different geometric forms. Except for the case of
circular perforations, all perforations have a greater dimension in one of the
two basic directions (longitudinal direction), which is aligned along the
perforated line.
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The perforations usually have a length B of 0.1 - 6 mm, preferably 0.3 to 4
mm, particularly 0.5 to 2 mm, in the aforementioned longitudinal direction.
The
size of the perforation perpendicular to this longitudinal direction is
correspondingly smaller and usually amounts to 5 to 80 %, preferably 10 to 50
%, of its dimension in the longitudinal direction. Circular and roughly
circular
perforations have a diameter of 0.1 - 5 mm, preferably 0.3 to 3 mm.
For a particularly low tear force, perforations with a length > 1 to 6 mm are
particularly preferred, particularly also lengths of 1.5 to 6 mm or 2 to 4 mm,
are suitable. Surprisingly, even these comparatively large or long
perforations
do not adversely affect the film's mechanical strengths. These embodiments
can also be rolled up into a film roll superbly well and unrolled again on the
packing machine without any difficulty.
The length A of the sections, i.e. of the undamaged film between the
perforations, is > 0.1 to 10 mm, preferably 1 to 6 mm, and depends on the
length of the perforations. For embodiments with long perforations of 1.5 to 6
mm, for example, intervals > 4 to 10 mm are preferred. The dimensions of the
intervals A are chosen in relation to the perforation, such that the length of
the
intervals is greater than the length of the perforations. The length A of the
intervals is generally at least 10 % greater than the length B or the diameter
B
of the perforations. The length B of the perforations is generally 5 to 60 %
of
the length A of the intervals, preferably 10 to 50 %.
The dimensions indicated for the perforations and intervals also apply in the
same way to perforations in the form of non-continuous thin points, which for
their part may likewise be circular, elliptical or elongated in form.
The dimensions of intervals between perforations along a line are generally
constant or vary only slightly in the context of the usual production
accuracy.
In the same way, the length of the individual perforations along a line does
not
vary or varies only slightly (up to 10 %), i.e. the diameter of the holes or
the
length of a perforation is constant, for example. Likewise, the perforations
and
intervals between the two parallel perforated lines have a predetermined
breaking point with roughly the same dimensions.
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Essential to the invention is that a predetermined breaking point provided is
formed from two perforated lines running in parallel, which are disposed at a
distance C of maximum 10 mm. The distance C of these two lines is
preferably 0.5 to 8 mm, particularly 0.8 to 5 mm.
The second feature of the predetermined breaking point essential to the
invention is the configuration of the perforations in the two perforated lines
running parallel to one another. It was found that an offset arrangement of
perforations in the two lines led to a surprising improvement in the control
over tear propagation. In this offset arrangement, a perforation in the second
line does not start at the same level as the opposite perforation in the first
line.
In this way, the perforation in the second line partially covers the opposite
interval in the first perforated line. In a preferred embodiment, the centre
of
the sections of the first perforated line is aligned with the centre of the
opposite perforations in the second perforated line in each case, so that a
symmetrical pattern of perforations is created.
Surprisingly, the arrangement of two parallel perforated lines offset relative
to
one another improves control of the tearing process significantly. The
invention also enables the packaging to be broken into at the predetermined
breaking point above the edges of the individual items, if the individual item
exhibits corresponding mechanical stability and the individual items are
arranged in the packaging in such a manner that their edges run level with the
double line.
The perforated lines are generally applied in the machine direction of the
film.
The perforation may be simultaneously combined with the cutting of the film
into narrower film widths and thereby be carried out in one working cycle.
Alternatively, perforation may also take place before or after the film is cut
to
the finished width. In all these process variants, the film to be perforated
may
for example be fed over rollers with suitable perforation tools, for example
these rollers may be equipped with needles or knife blades. In the case of
laser or ultrasound perforations, corresponding tools are positioned above or
below the film web. With these methods, the film being perforated is wound off
a roll, the perforations according to the invention are applied and the
perforated film is then rolled up again and the roll used on the plant for
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packaging individual items, i.e. unrolled and used to wrap up goods.
Surprisingly, the perforations do not affect the film's rolling behaviour or
only
negligibly, so that the perforated film can be rolled up and unrolled using
the
customary devices. It also emerged, surprisingly, that the perforation
according to the invention does not affect the film's mechanical stability, to
the
extent that tears, cuts, thick points or stretching occurs when the film is
rolled
up and unrolled.
The roll of perforated film is used to produce the packaging according to the
invention. The packaging according to the invention is particularly suitable
for
arranged or stacked individual items and also for soft goods, such as butter,
sausage and the like. It will be possible for the consumer to tear the film
lengthways along a predetermined breaking point, so that individual items can
be removed, without others falling out uncontrollably in the case of
individual
items. The individual segments are adapted according to the size of the unit
being removed. This makes it possible to control whether one or several units
can be conveniently removed after opening.
A biaxially oriented polypropylene film is generally used as the film.
Depending on the type of packaging, the film may be a translucent to
transparent or opaque film. "Opaque film" within the meaning of the present
invention means a non-transparent film with a light permeability (ASTM-D
1003-77) of maximum 70 %, preferably maximum 50 %. The film may be
single- or multi-layered in principle. Laminates are also suitable for the
packaging according to the invention; these are preferably formed from the
films described herein.
For transparent embodiments, the formulation of the film and the type of laser
may be coordinated with one another, so that the laser beam leaves behind a
white or coloured line close to the thinning. This means that the packaging
later exhibits a discernible marking, showing the consumer where the tear
used to open the packaging should run, to make for ease of handling.
Possible thermoplastic synthetics for the film include polyolefins made from
olefin monomers with 2 to 8 C-atoms. Particularly suitable are propylene
polymers, ethylene polymers, butylene polymers, cyclo-olefin polymers or
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mixed polymers of propylene, ethylene, butylene units or cyclo-olefins are
preferred. The layers of the film or the layer in the case of single-layer
embodiments generally contain at least 50 % by weight, preferably 70 to 99 %
by weight, particularly 90 to 98 % by weight, of the thermoplastic polymer,
relative to the weight of the layer in each case.
Propylene polymers are preferred as polyolefins. These propylene polymers
contain 90 to 100 % by weight, preferably 95 to 100 % by weight, particularly
98 to 100 % by weight, propylene and have a melting point of 120 C or
higher, preferably 130 to 170 C, and generally a melt flow index of 0.5 g/10
min to 15 g/10 min, preferably 2 g/10 min to 10 g/ 10 min, at 230 C and a
force of 21.6 N (DIN 53 735). Isotactic propylene homopolymers with an
atactic proportion of 15 % by weight and under, copolymers of ethylene and
propylene with an ethylene content of 10 % or less, copolymers of propylene
with C4-Ca olefins with an olefin content of 10 % by weight or less,
terpolymers
of propylene, ethylene and butylene with an ethylene content of 10 % by
weight or less and with a butylene content of 15 % by weight or less represent
preferred propylene polymers for the core layer, wherein the isotactic
propylene homopolymer is particularly preferred. The weight percentages
indicated relate to the respective polymer.
Furthermore, a mixture of the aforementioned propylene homopolymers
and/or copolymers and/or terpolymers and other polyolefins, particularly of
monomers with 2 to 6 C-atoms, is suitable, in which the mixture contains at
least 50 % by weight, particularly at least 75 % by weight, propylene polymer.
Other suitable polyolefins in the polymer mixture are polyethylenes,
particularly HDPE, LDPE, VLDPE and LLDPE, in which the proportion of
polyolefins does not exceed 15 % by weight in each case relative to the
polymer mixture.
If necessary a layer, preferably the base layer or an intermediate layer, of
the
film may also contain pigments and/or vacuole-initiating particles in the
customary amounts in each case for opaque embodiments.
The film according to the invention may be single-layered, the film is
preferably multi-layered. To achieve this, a one-sided or two-sided
CA 02771447 2012-02-17
intermediate layer and/or top layers may be applied to the base layer.
Accordingly, multi-layered embodiments of the film exhibit intermediate layers
and top layers where necessary, in addition to the base layer.
5 These additional top layers and/or intermediate layers are generally made
from polyolefins. They contain at least 70 % by weight, preferably 75 to 100 %
by weight, particularly 90 to 98 % by weight, of a polyolefin. The same
polymers are suitable in principle for these additional layers as are
described
above for the base layer.
Suitable for the top layers are
copolymers of
ethylene and propylene or
ethylene and butylene or
propylene and butylene or
ethylene and another olefin with 5 to 10 carbon atoms or
propylene and another olefin with 5 to 10 carbon atoms or
a terpolymer of
ethylene and propylene and butylene or
ethylene and propylene and another olefin with 5 to 10 carbon atoms or
a mixture or blend of two or more of the aforementioned homo-, co- and
terpolymers.
Particularly preferred among these are
statistical ethylene propylene copolymers with
an ethylene content of 2 to 10 % by weight, preferably 5 to 8 % by weight, or
statistical propylene-butylene-1 -copolymers with
a butylene content of 4 to 25 % by weight, preferably 10 to 20 % by weight,
relative to the total weight of the copolymer in each case, or
statistical ethylene-propylene-butylene-1 terpolymers with
an ethylene content of 1 to 10 % by weight, preferably 2 to 6 % by weight, and
a butylene-1 content of 3 to 20 % by weight, preferably 8 to 10 % by weight,
relative to the total weight of the terpolymer in each case, or
a blend of an ethylene-propylene-butylene-1terpolymer and a
propylene-butylene-1 copolymer
with an ethylene content of 0.1 to 7 % by weight
CA 02771447 2012-02-17
and a propylene content of 50 to 90 % by weight
and a butylene-1 content of 10 to 40 % by weight,
each relative to the total weight of the polymer blend.
The co- or terpolymers described above generally have a melt flow index of
1.5 to 30 g/10 min, preferably of 3 to 15 g/10 min. The melting point lies
within
the range 120 to 140 C. The blend of co- and terpolymers described above
has a melt flow index of 5 to 9 g/10 min and a melting point of 120 to 150 C.
All the melt flow indices indicated above are measured at 230 C and a force
of 21.6 N (DIN 53 735). Layers of co- and/or terpolymers preferably form the
top layers of sealable embodiments of the film.
The total film thickness may vary within broad limits and depends on the
intended application. Preferred embodiments of the film have total
thicknesses of 5 to 250 pm, wherein 10 to 100 pm, preferably 20 to 80 pm,
are preferred.
The base layer within the meaning of the present invention is the layer that
accounts for more than 50 % of the total film thickness. Its thickness results
from the difference between the total thickness and the thickness of the top
and intermediate layer(s) applied and may therefore vary within broad limits
in
a similar way to the total thickness. Top layers form the outermost layer of
the
film and measure between 0.5 and 5 pm, preferably 1 to 3 pm. The
intermediate layer lies between 1 and 20 pm, preferably 1 to 10 pm.
In order to improve certain properties of the polypropylene film according to
the invention still further, both the base layer and also the intermediate
layer(s) and the top layer(s) may contain additives in an effective amount in
each case, preferably hydrocarbon resin and/or antistatic agents and/or an
anti-blocking agents and/or anti-friction agents and/or stabilising agents
and/or neutralising agents, which are compatible with the polymers of the core
layer and the top layer(s), with the exception of the usually incompatible
anti-
blocking agents.
The films are produced according to the extrusion method known in the art. In
the context of this method, the melts corresponding to the individual layers
of
film are extruded by a flat film extrusion die. The film obtained in this way
is
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drawn off for solidification on one or more roller(s) and cooled. The
temperature of the drawing-off roller or rollers is 10 to 90 C, preferably 20
to
60 C.
The film is then stretched biaxially. The biaxial stretching may be carried
out
simultaneously or consecutively, wherein consecutive biaxial stretching, in
which the film is stretched longitudinally (in the machine direction) to begin
with and then transversely (perpendicular to the machine direction), is
particularly favourable. In a longitudinal direction, the stretching of the
film is
preferably 3:1 to 7:1 and takes place at a temperature of under 140 C,
preferably within the 125 to 135 C range. In a transverse direction,
stretching
is preferably 5:1 to 12:1 and is carried out at a temperature of over 140 C,
preferably 145 to 160 C. Longitudinal stretching is advantageously carried
out with the help of two high-speed rollers running at different speeds,
depending on the stretch ratio being sought, and transverse stretching is
carried out using a corresponding biaxial tenter. It is possible in principle
for
film to be stretched simultaneously in a longitudinal-transverse direction.
These simultaneous stretching processes are known in the art.
For thermofixing (heat treatment), the film is then kept at a temperature
between 110 and 150 C for roughly 0.5 to 10 sec. If necessary, one or both
surface(s) of the film can be corona- or flame-treated following biaxial
stretching using one of the known methods.
If necessary, the film may be laminated, coated, melt-coated, varnished or
clad following production but prior to perforation using further processing
stages, in order to lend the film further advantageous properties. Composites
of polypropylene films and polyethylene films are particularly preferred as
laminates. Composites of this nature may be produced by laminating
individual films. A further technically advantageous variant for the
production
of PP/PE laminates is the extrusion-coating of a suitable polyethylene on a
biaxially-oriented polypropylene film. Extrusion coatings of this kind are
known
in the state of the art. It has been found that laminates made from PP/PE
films
are advantageous when treated by laser beam and are less easy to perforate
accidentally.
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The packaging according to the invention is characterised by controllable tear
properties. If necessary, the force required to initiate a tear at the edge of
the
film may be reduced by means of a notch, preferably V-shaped. The
packaging tear can be propagated significantly more easily and with greater
control. Moreover, the packaging still exhibits all the advantages also
displayed by customary film packaging, such as high mechanical strength,
water and oxygen barriers, good optical properties.
The packaging according to the invention is particularly advantageous for use
with individual items, particularly stacked units, such as biscuits, cigarette
packs or compressed food products.
The film with the perforations according to the invention facilitates a new
kind
of packaging solution for cigarettes packs. The cigarette packs are stacked
and wrapped in the film with the perforations according to the invention.
These cigarettes packs can then be opened at the predetermined breaking
point by breaking along the perforated line.
The invention is described in greater detail by the following examples:
Example 1
A transparent, three-layer ABA film with a symmetrical structure and a total
thickness of 20 pm was produced by coextrusion followed by gradual
orientation in a longitudinal and transverse direction. The top layers had a
thickness of 0.6 pm in each case.
B-base layer:
approx. 90 % by weight propylene homopolymer with a melting point
of 162 C and a melt flow index of 3.4 g/10
min
0.15 % by weight N,N-bis-ethoxyalkylamine (antistatic agent)
0.30 % by weight erucic acid amide
A-top layers:
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approx. 75 % by weight statistical ethylene propylene copolymer with
a C2 content of 4.5 % by weight
approx. 25 % by weight statistical ethylene propylene butylene
terpolymer with an ethylene content of 3 %
by weight and a butylene content of 7 % by
weight (residual propylene)
0.33 % by weight Si02 as anti-blocking agent with a mean
particle size of 2 pm
0.90 % by weight polydimethyl siloxane with a viscosity of
30,000 mm2/sec
The production conditions in the individual process stages were:
Extrusion: Temperatures base layer: 260 C
top layers: 240 C
Temperature of the
drawing-off roller: 20 C
Longitudinal
stretching: Temperature: 110 C
Longitudinal stretch ratio: 5.5
Transverse
stretching: Temperature: 160 C
Transverse stretch ratio: 9
Fixing: Temperature: 140 C
Convergence: 20 %
The film was then coated on one surface with a sealable acrylate coating.
The film was then cut to a width of 350 mm into narrow sections and wound
up. These narrow sections (rolls) were fed over a roller bearing needles in a
second working stage, the film thereby being provided with perforated lines in
a longitudinal direction, which were disposed in parallel at a 6 mm interval.
The perforations had a propeller-like form with a length of 3 mm. The length
of
the intervals between perforations was 4 mm. The film perforated in this way
was wound into a roll of perforated film.
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The film perforated in this way was then used to wrap a stack of 12 packs of
cigarettes. The packs were arranged so that the edges of the packs coincided
with the perforated line. The packs could be removed by breaking off along
the perforated line, without any uncontrolled propagation of the tear in the
film.
5
Comparative example
A single row of perforations was made in the film described in Example 1,
which exhibited perforations of the same size and at equal intervals to those
10 in Example 1. The film was used in the same way for the wrapping the stack
of 12 cigarette packs. The packaging could not be opened by breaking off.
When attempts were made to tear the film along the perforated line, the tear
propagated uncontrollably alongside the perforated line in 3 out of 10
attempts.
