Note: Descriptions are shown in the official language in which they were submitted.
CA 02359323 2001-07-10
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HEAT-SHRINKABLE, IRRADIATED, POLYETHYLENE MONO-LAYER FILM
The present invention refers to a heat-shrinkable, irradiated, mono-layer
polyethylene
film characterized by a very low thickness variation, to a process for the
manufacture thereof
and to the use of said film as a packaging material.
Bi-axially oriented, heat-shrinkable, films are films that have been oriented
by
stretching in two perpendicular directions, typically the longitudinal or
machine direction
(MD) and the transverse or crosswise direction (TD), at a temperature
comprised between the
Tg and the melting point of the resin employed, i.e. at a temperature where
the resin is not in
the molten state.
1o Bi-axially oriented, heat-shrinkable, films are made by extruding the
polymer from a
melt into a thick sheet that is quickly quenched to prevent or delay polymer
crystallization,
and then oriented by stretching under temperature conditions, as indicated
above, where
molecular orientation of the film occurs and the film does not tear. Upon
subsequent re-
heating at a temperature close to the orientation temperature, the oriented,
heat-shrinkable.
film will tend to shrink in seeking to recover its original dimensional state.
Bi-axially oriented, heat-shrinkable polyethylene mono-layer films are known
in the
literature and widely used in the market.
They are typically obtained by extruding the polymer through a rov.md die to
get a
tubular thick film called "tape", that is immediately and quickly quenched by
means of a
2o water bath or cascade, re-heated, with or without prior irradiation, at the
suitably selected
orientation temperature and stretched bi-axially, while at this temperature,
by the so-called
"trapped bubble" technique that uses internal gas pressure to expand the
diameter of the tape
to form a large "bubble" and advancing the expanded tube at a faster rate than
the extrusion
rate so as to obtain transverse and machine directions of orientation
respectively. The film is
then cooled and rolled up in the cooled state so as to retain the property of
heat-shrinkability.
Generally in the above process the tape is cross-linked prior to stretching in
order to impart
greater mechanical strength thereto and thus allow a better control of the
process via an
increased stability of the blown bubble.
The films obtained via the trapped bubble technique always show a poor
thickness
CA 02359323 2001-07-10
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control and thickness variations of at least about 25-30 % are typically
obtained.
EP-A-319,401 describes an alternative process for the manufacture of bi-
axially
oriented, heat-shrinkable, mono-layer films of ethylene-a-olefin copolymers.
Said process
provides for the extrusion of the co-polymer through a flat die in the form of
a sheet, and
after a quenching step, for the heating of the sheet to a first orientation
temperature and the
stretching thereof in the longitudinal direction, followed by heating of the
longitudinally
stretched film to a second orientation temperature higher than the first one,
and by the
transversal stretching thereof. No irradiation step is foreseen in said method
as, unlike the
blown bubble process, there are no problems of process ("bubble") stability in
the flat
orientation process.
The films obtained by the method described in said prior art document show an
extremely high thickness variation. Considering the working examples contained
in EP-A-
319,401, the film made from an ethylene-octene-1 co-polymer with density of
0.919 g/cm'
and MI of 6 g/10' (resin A) shows a thickness variation > 25 %, while that
made from an
~5 ethylene-octene-1 co-polymer with density of 0.917 g/em' and MI of 2.3
g/10' (resin B)
shows a thickness variation > 33 %. These values, that might be acceptable
when the film is
obtained via the trapped bubble method, as the use of a rotating platform
would allow the
distribution of the thicknesses along the film width, are unacceptable in case
of a flat
orientation process where no thickness distribution can be achieved. In this
latter case in fact
a thickness variation as indicated would not allow the winding up of the end
film into
acceptable rolls.
It has now been found that it is possible to control the thickness variation
in a heat-
shrinkable, mono-layer polyethylene film obtained by the flat orientation
process, by
irradiating the sheet prior to stretching. It has been found that depending on
the irradiation
level it is possible to achieve a thickness variation less than 20 %,
preferably less than 18
and even more preferably less than 15 %.
A first object of the present invention is therefore a heat-shrinkable,
irradiated, mono-
layer polyethylene film characterized by a thickness variation of less than 20
%, preferably
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less than 18 % and even more preferably less than 15 %.
A second object of the present invention is a process for manufacturing a heat-
shrinkable polyethylene mono-layer film characterised by a thickness variation
of less than
20 %, preferably less than 18 % and even more preferably less than 15 %, which
process
s comprises extrusion of the film resin through a flat die, quenching of the
cast extruded sheet,
irradiation thereof, re-heating to the suitably selected orientation
temperature and stretching
of the irradiated sheet.
A third object of the present invention is the use of a heat-shrinkable,
polyethylene,
irradiated. mono-layer film characterized by a thickness variation of less
than 20 %,
to preferably less than 18 % and even more preferably less than 15 %, in the
packaging of food
or non-food products.
DEFINITIONS
As used herein, the term "film" is used in a generic sense to include plastic
web,
regardless of whether it is film or sheet. Typically, films of and used in the
present invention
15 have a thickness of 100 um or less, preferably they have a thickness of 80
p.m or less, more
preferably a thickness of 50 p.m or less, still more preferably a thickness of
35 ~m or less,
and yet, still more preferably, a thickness of 25 p.m or less.
As used herein, the phrase "thickness variation" refers to the percent value
obtained
by measuring the maximum and minimum thickness of the film, calculating the
average
2o thickness value and applying these numbers to the following formula:
film thickness~max~ - film thickness~m;",
Thickness variation (%) = x 100.
film thickness,,~e~
2S
The maximum and minimum thicknesses are determined by taking a total of 10
thickness measurements at regular distance intervals along the entirety of the
transverse
direction of a film sample, recording the highest and lowest thickness values
as the maximum
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and minimum thickness values, respectively, while the average value is
determined by
summing up the same lU thickness measurements and dividing the result by ten.
The
thickness variation is then computed (as a percent value) using the formula
above. A
thickness variation of 0 % represents a film with no measurable differences in
thickness. A
s thickness variation over 25 % is unacceptable industrially while a thickness
variation below
20 % is acceptable.
As used herein, the phrase "machine direction", herein abbreviated "MD",
refers to a
direction "along the length" of the film, i:e., in the direction of the film
as the film is formed
during extrusion and/or coating.
As used herein, the phrase "transverse direction", herein abbreviated "TD",
refers to a
direction across the film, perpendicular to the machine or longitudinal
direction.
As used herein, the phrases "orientation ratio" and "stretching ratio" refer
to the
multiplication product of the extent to which the plastic film material is
expanded in the two
directions perpendicular to one another, i.e. the machine direction and the
transverse direction.
15 As used herein, the phrases "heat-shrinkable," "heat-shrink," and the like,
refer to the
tendency of the film to shrink upon the application of heat, i.e., to contract
upon being heated,
such that the size of the film decreases while the film is in an unrestrained
state. As used herein
said term refer to films with a total free shrink (i.e., free shrink in the
machine direction plus
free shrink in the transverse direction), as measured by ASTM D 2732, of at
least 30 percent at
zo 120 °C, more preferably at least 40 percent, still more preferably,
at least 50 percent, and, yet
still more preferably, at least 60 percent.
As used herein, the term "monomer" refers to a relatively simple compound,
usually
containing carbon and of low molecular weight, which can react to form a
polymer by
combining with itself or with other similar molecules or compounds.
2s As used herein, the term "co-monomer" refers to a monomer that is co-
polymerized
with at least one different monomer in a co-polymerization reaction, the
result of which is a
copolymer.
As used herein, the term "polymer" refers to the product of a polymerization
reaction,
4
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WO 00/41872 PCT/EP00/00127
and is inclusive of homo-polymers, and eo-polymers.
_ As used herein, the term "homo-polymer" is used with relerence to a polymer
resulting
from the polymerization of a single monomer, i.e., a polymer consisting
essentially of a single
type of mer, i.e., repeating unit.
As used herein, the term "co-polymer" refers to polymers formed by the
polymerization
reaction of at least two different monomers. For example, the terns "co-
polymer" includes the
co-polymerization reaction product of ethylene and an a-olefin, such as 1-
hexene. However.
the lern~ "co-polymer" is also inclusive of, for example, the co-
polymerization of a mixture of
ethylene, propylene, 1-hexene, and 1-octene. The term "co-polymer" is also
inclusive of
I o random co-polymers, block co-polymers, and graft co-polymers.
As used herein, the term "polyethylene" refers to ethylene homo- and co-
polymers.
As used herein the term "ethylene homopolymer" identify polymers consisting
essentially of an ethylene repeating unit. Depending on the polymerization
process employed,
polymers with a different degree of branching and a different density can be
obtained. Those
~5 characterized by a low degree of branching and showing a density higher
than 0.940 glcm'
are called HDPF while those with a higher level of branching and a density up
to 0.940 g/cm'
are called LDPE.
As used herein the term "ethylene copolymer" refers to the copolymers of
ethylene
with one or more other olefins and/or with a non-olefinic comonomer
copolymerizable with
2o ethylene, such as vinyl monomers, modified polymers thereof, and the like.
Specific
examples include ethylene-a-olefin copolymers, ethylene-vinyl acetate
copolymers,
ethylene-ethyl acrylate copolymers, ethylene-butyl acrylate copolymers,
ethylene-methyl
acrylate copolymers, ethylene-acrylic acid copolymers, ethylene-methacryIic
acid
. copolymers, ionomer resins, ethylene-alkyl acrylate-malefic anhydride ter-
polymers, ctc..
z5 As used herein, terminology employing a "-" with respect to the chemical
identity of a
copolymer (e.g., "an ethylene-a-olefin copolymer"), identifies the co-monomers
which are co-
polymerized to produce the copolymer
As used herein, the phrase "heterogeneous polymer" refers to polymerization
reaction
5
CA 02359323 2001-07-10
wo ooiaisn rcr~Paoiooia~
products of relatively wide variation in molecular weight and relatively wide
variation in
composition distribution, i.e., typical polymers prepared, for example, using
conventional
Ziegler-Natta catalysts. Heterogeneous polymers are useful in various layers
of the film used in
the present invention. Although there are a few exceptions (such as TAFMERTM
linear
s homogeneous ethylene-a-olefin copolymers produced by Mitsui Petrochemical
Corporation,
using Ziegler-Natta catalysts), heterogeneous polymers typically contain a
relatively wide
variety of chain lengths and co-monomer percentages.
As used herein, the phrase "homogeneous polymer" refers to polymerization
reaction
products of relatively narrow molecular weight distribution and relatively
narrow composition
1 o distribution. Homogeneous polymers are structurally different from
heterogeneous polymers, in
that homogeneous polymers exhibit a relatively even sequencing of co-monomers
within a
chain. a mirroring of sequence distribution in all chains, and a similarity of
length of all chains,
i.e., a narrower molecular weight distribution. Furthermore, homogeneous
polymers are
typically prepared using metallocene, or other single-site type catalysts,
rather than using
15 Ziegler Natta catalysts.
More particularly, homogeneous ethylene-a-olefin copolymers may be
characterized
by one or more methods lmown to those of skill in the art, such as molecular
weight distribution
(M"/M"), composition distribution breadth index (CDBI), and narrow melting
point range and
single melt point behavior. The molecular weight distribution (M"/M"), also
known as
2o polydispersity, may be determined by gel permeation chromatography. 1'he
homogeneous
ethylene-a-olefin copolymers useful in this invention generally have (M"/M")
of less than 2.7;
preferably from about 1.9 to about 2.5; more preferably, from about 1.9 to
about 2.3. The
composition distribution breadth index (CDBI) of such homogeneous ethylene-a-
olefin
copolymers will generally be greater than about 70 percent. The CDBI is
defined as the weight
25 percent of the copolymer molecules having a co-monomer content within 50
percent (i.e., plus
or minus 50%) of the median total molar co-monomer content. The CDBI of linear
polyethylene, which does not contain a co-monomer, is defined to be 100%. The
Composition
Distribution Breadth Index (CDBI) is determined via the technique of
Temperature Rising
6
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WO 00141872 PCTIEP00100127
Elution Fractionation (TREF). CDBI determination clearly distinguishes the
homogeneous
copolymers used in the present invention (narrow composition distribution as
assessed by
CDBI values generally above 70%) from VLDPEs available commercially which
generally
have a broad composition distribution as assessed by CDBI values generally
less than 55%.
s The CDBI of a copolymer is readily calculated from data obtained from
techniques know,i in
the ari, such as, for example, temperature rising elution fractionation as
described, for example,
in Wild et. al., J. Poly. Sci. Poly. Phvs Ed , Vol. 20, p.441 (1982).
Preferably, the
homogeneous ethylene-a-olefin co-polymers have a CDBI greater than about 70%,
i.e., a
CDBI of from about 70% to about 99%. In general, the homogeneous ethylene-a--
olefin co-
polymers in the multi-layer films of the present invention also exhibit a
relatively narrow
melting point range, in comparison with "heterogeneous copolymers", i.e.,
polymers having a
CDBI of less than 55%. Preferably, the homogeneous ethylene-a-olefin
copolymers exhibit an
essentially singular melting point characteristic, with a peak melting point
(Tm), as determined
by Differential Scanning Calorimetry (DSC), of from about 60°C to about
110°C. Preferably
t5 the homogeneous copolymer has a DSC peak T", of from about 80°C to
about 105°C. As used
herein, the phrase "essentially single melting point" means that at least
about 80%, by weight,
of the material corresponds to a single T", peak at a temperature within the
range of from about
60°C to about 110°C, and essentially no substantial fraction of
the material has a peak melting
point in excess of about 11S°C, as determined by DSC analysis. Melting
information reported
2o are second melting data, i.e., the sample is heated at a programmed rate of
10°Clmin. to a
temperature below its critical range. The sample is then repeated (2nd
melting) at a
programmed rate of 10°C/min. The presence of higher melting peaks is
detrimental to film
properties such as haze, and compromises the chances for meaningful reduction
in the seal
initiation temperature of the final film.
25 A homogeneous ethylene-a-olefin copolymer can, in general, be prepared by
the co-
polymerization of ethylene and any one or more a-olefins. Preferably, the a-
olefin is a Ca-C,~,
a-mono-olefin, more preferably, a C4-C,, a-mono-olefin, still more preferably,
a C4-Cg a-
mono-olefin. Still more preferably, the a-olefin comprises at least one member
selected from
7
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WO OOI41872 PCTIEPOOI00127
the group consisting of butene-1, hexenc-1, and octene-1, i.e., 1-butene, 1-
hexene, and 1-octene,
respectively. Most preferably, the a-olefin comprises octene-1, and/or a blend
of hexene-1 and
butene-I.
Processes for preparing and using homogeneous polymers are disclosed in U.S.
Patent
s No. 5,206,075, U.S. Patent No. 5,241,031, and PCT International Application
WO 93/03093,
each of which is hereby incorporated by reference thereto, in its entirety.
Further details
regarding the production and use of homogeneous ethylene-a-olefin copolymers
are disclosed
in WO-A-90/03414, and WO-A-93/03093.
Still another genus of homogeneous ethylene-a-olefin copolymers is disclosed
in U.S.
Patent No. 5,272,236, to Lai, et. al., and U.S. Patent No. 5,278,272, to Lai,
et. al.
As used herein, the phrase " ethylene-a-olefin copolymer" refer to such
heterogeneous
materials as linear low density polyethylene (LLDPE), linear medium density
polyethylene
(LMDPE) and very low and ultra low density polyethylene (VLDPE and ULDPE); and
homogeneous polymers such as metallocene-catalyzed FXACTT"' linear homogeneous
15 ethylene-a-olefin copolymer resins obtainable from the Exxon Chemical
Company, single-site
AFFINITYTM linear homogeneous ethylene-a,-olefin copolymer resins obtainable
from the
Dow Chemical Company, and TAFMERTM linear homogeneous ethylene-a-olefn
copolymer
resins obtainable from the Mitsui Petrochemical Corporation. All these
materials generally
include co-polymers of ethylene with one or more co-monomers selected from C4
to C,o a-
20 olefin such as butene-l, hexene-l, octene-1, etc. in which the molecules of
the copolymers
comprise long chains with relatively few side chain branches or cross-linked
structures. The
heterogeneous ethylene-a-olefin co-polymer commonly known as LLDPE has a
density
usually in the range of from about 0.915 g/cm' to about 0.930 g/cm', that
commonly known as
LMDPE has a density usually in the range of from about 0.930 g/cm' to about
0.945 g/cm',
25 while those commonly identified as VLDPE or ULDPE have a density lower than
about 0.915
g/cm'.
As used herein, the phrase "free shrink" refers to the percent dimensional
change in a 10
cm x 1 p cm specimen of film, when subjected to selected heat (i.e., at a
certain temperature),
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WO 00/41872 PCT/EP00/00127
with the quantitative determination being carried out according to ASTM D
2732, as set forth in
the 1990 Annual Book of ASTM Standards, Vol. 08.02, pp.368-371. "Total free
shrink" is
determined by summing the percent free shrink in the machine direction with
the percentage of
free shrink in the transverse direction.
For the purpose of the present invention, the film "haze", i.e. the percentage
of
transmitted light which is scattered forward while passing through the sample,
is measured
by ASTM D 1003 (Method A).
For the purpose of the present invention the film "gloss", i.e. the surface
reflectance
of a film specimen, is measured according to ASTM D 2457 - 90 at a 60°
angle.
DETAI1;ED DESCRIPTION OF THE INVENTION
The film according to the present invention preferably comprises an ethylene-a-
olefin
co-polymer.
Ethylene-a-olefin copolymers that can suitably be employed for the film
according to
the present invention are heterogeneous and homogeneous ethylene-a-olefin
copolymers
having a density of from about 0.880 g/cm' to about 0.940 g/cm;, preferably
from about
0.890 g/cm' to about 0.935 g/cm', more preferably from about 0.900 g/cm' to
about 0.930
g/cm;, and still more preferably from about 0.905 g/cm' to about 0.925 g/cm'.
Preferably said film will contain at least about SO % by weight of one or more
ethylene-a-olefin copolymers, more preferably at least 60 % by weight of one
or more
2o ethylene-a-olefin copolymers, still more preferably at least 70 % by weight
of one or more
ethylene-a-olefin copolymers, and yet still more preferably at least 80 % by
weight of one or
more ethylene-a-olefin copolymers.
Preferably, the ethylene-a-olefin copolymer of the film according to the
present
invention has a melt index of from about 0.3 to about 8 g/10 min, more
preferably from about
0.5 to about 7 g/10 min, still more preferably from about 0.6 to about 6 g/10
min, even more
preferably from about 0.8 to about 5 g/10 min (as measured by ASTM D1238).
Other polymers that can be blended with the ethylene-a-olefin copolymers in
said
preferred film of the present invention arc polyolefins compatible therewith,
e.g. ethylene,
9
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propylene, and butene homo- or co-polymers.
_ In a most preferred embodiment said other polymers are selected from the
group
consisting of ethylene homo-polymers and ethylene co-polymers with a non-
olefinic
comonomer copolymerizable with ethylene, such as ethylene-vinyl acetate co-
polymers,
ethylene-(meth)acrylic acid copolymers, ethylene-alkyl (meth}acrylate
copolymers,
ionomers, and the like polymers.
Thus in a most preferred embodiment of the present invention the film consists
essentially of one or more ethylene-a-olefin co-polymers of different
densities or of a blend
thereof with one or more ethylene homo-polymers and/or ethylene co-polymers
with a non-
olefinic comonomer copolymerizable with ethylene.
The polymers) used for the film of the present invention may contain
appropriate
amounts of additives as known in the art. These include slip and anti-block
agents such as talc,
waxes, silica, and the like, antioxidants, fillers, pigments and dyes, cross-
linking enhancers, UV
absorbers, antistatic agents, anti-fog agents or compositions, and the like
additives known to
~ 5 those skilled in the art of packaging films.
The film according to the present invention is obtained by extrusion of the
resin or
blend of resins through a flat die, cooling quickly the mono-layer sheet
exiting from the
extrusion die by means of a chill roll, irradiating the cast sheet thus
obtained to a suitably
selected irradiation dosage, repeating this tape to the orientation
temperature, and stretching the
~0 heated tape in both directions, MD and TD, by any sequential or
simultaneous tenter apparatus.
The obtained bi-axially oriented heat-shrinkable film may then optionally be
stabilized by an
annealing step, and finally cooled and wound up.
In a preferred embodiment the film will comprise up to 30 % by weight,
preferably up
to about 20 % by weight and even more preferably up to about 1 U % by weight
of recycle
25 material from the manufacture of polyolefin films. When orientation is
carried out by means
of a tenter, the film edges that have been clipped during orientation need to
be trimmed off at
the end of the process before the end bi-axially oriented heat-shrinkable film
is wound up.
These trimmed edges are collected, pelletized and recycled, preferably in-
line. In a most
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preferred embodiment, therefore, said recycle material will come from the
manufacture of the
same polyethylene film and the scrap recycle will be carried out in-line.
The bi-axially oriented, heat-shrinkable, mono-layer film prepared in
accordance with
the present invention can have any total thickness desired, so long as the
film provides the
desired properties for the particular packaging operation in which the film is
used, e.g. optics,
modulus, seal strength, ete. . In a most preferred embodiment however the
thickness of the film
is lower than 25 p.m; typically it is comprised between about 6 and about 20
p,m; and even more
preferably between about 7 and about 19 Vim.
The process according to the present invention involves feeding the solid
polymer beads
to an extruder, where the polymer beads are melted and then forwarded into a
flat extrusion die.
The obtained cast sheet, that is preferably from about 0.2 mm to about 3 mm
thick, is then
chilled on a chill roll, typically with the aid of an air knife that keeps the
sheet in contact with
the chill roll. Preferably the chill roll is partially immersed in a water
bath at a low temperature
(e.g. from about 5 to about 60 °C). Alternatively the cooling step can
be carried out by using a
liquid-knife as described in WO-A-95/26867 where a continuous and
substantially unifornl
layer of water or of any other cooling liquid flows onto the surface of the
sheet that does not
contact the chill roll. Any other known means for cooling the cast web can
however be
employed.
The cooled sheet is then fed through an irradiation unit, typically comprising
an
~o irradiation vault surrounded by a shielding, the flat sheet is irradiated
with high energy electrons
(i.e., ionizing radiation) from an iron core transformer accelerator.
Irradiation is carried out to
induce cross-linking. The flat sheet is preferably guided through the
irradiation vault on rolls. It
is thus possible by suitably combining the number of rolls and the path of the
traveling web
within the irradiation unit to get more than one exposure of the sheet to the
ionizing radiation.
Preferably, the sheet is irradiated to a level of from about 15 to about I50
kGy, more
preferably of from about 20 to about 130 kGy, still more preferably of from
about 25 to about
I 10 kGy, and yet still more preferably of from about 30 to about 100 kGy,
wherein the most
preferred amount of radiation is dependent upon the polymers employed and the
film end use.
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As an example polymers with a high MFI requires a higher dosage of irradiation
to obtain the
desired thickness control while with polymers with a iow MFI, low irradiation
dosages are
sufficient. Furthermore highly branched polymers are more susceptible to
irradiation than the
more linear ones and thickness control with an highly branched polymer can be
achieved using
a low irradiation dosage. The person skilled in the art can set the minimum
irradiation dosage
required to get the desired thickness control by one or just few simple tests.
While irradiation needs to be earned out on the extruded cast sheet just
before
orientation, in order to get a suitable control of the end film thickness, an
additional irradiation
step could be carried out also after orientation in order to further broaden
the film heat-sealing
i o window.
fhe irradiated cast sheet is then fed to the pre-heating gone of a sequential
or
simultaneous flat stretching apparatus.
In a sequential flat stretching apparatus, the film is generally first
stretched in the MD
and then in the TD. The MD stretching is accomplished by drawing the heated
sheet between
~ s sets of heated rolls with the downstream set moving at a higher speed. The
TD stretching is
on the other hand obtained by means of a tenter frame, a machine that consists
of two
continuous chains on which are mounted clamps gripping the two edges of the
film and
carrying it along as the chain is driven forward. The two chains gradually
move part and as
they do they draw the film in the TD between them.
2o Conventional stretching ratios for the flat, sequential orientation process
are up to
about 8:1, preferably from about 5:1 to about 7:1 in MD and up to about 12:1,
preferably
from about 6:1 to about 10:1, in TD.
The temperature of the oven in the pre-heating cone, the length thereof and
the time
spent by the traveling web in said zone (i.e. the web speed) can suitably be
varied in order to
25 bring the sheet up to the desired temperature for MD orientation. In a
preferred embodiment
the MD orientation temperature is comprised between about SO °C and
about 100 °C
depending on the composition of the polyethylene film, and the temperature of
the pre-
heating zone is kept between about 60 °C and about 120 °C. The
longitudinally oriented film
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WO 00/41872 PCT/EP00/00127
is then heated to the suitably selected TD orientation temperature that is
generally higher than
the MD one. In particular, suitable TD orientation temperatures in the
sequential flat
stretching process according to the present invention are comprised between
about l00 °C
and about 140 °C depending on the composition of the polyethylene film,
and the
temperature of said second pre-heating zone is therefore kept between about
105 °C and
about 1 SO °C. In said second pre-heating zone the sheet is clipped but
it is not yet stretched.
Thereafter, the resulting hot, irradiated, and clipped sheet is transversally
stretched by means
of a tenter apparatus.
Alternatively the extruded mono-layer polyethylene sheet is bi-axially
stretched by
means of a simultaneous tenter apparatus providing for a simultaneous
stretching of the sheet
in both the machine and the transverse directions.
Simultaneous stretching of a continuous traveling flat sheet in the
longitudinal and
transversal directions is a technique known in the literature since many
years. tJS-A-
2,923,966, issued in 1960, described an apparatus for carrying out such a
simultaneous flat
~5 stretching. The apparatus there claimed comprised two endless conveyors,
positioned on the
two sides of the web and disposed along divergent paths, said conveyors being
formed of a
plurality of links pivotally interconnected at their ends to provide a lazy-
tongue structure and
carrying a series of spaced clips to grip the web edges.
The use of endless loop linear motor systems for the simultaneous stretching
of a
2o continuous traveling flat sheet has later been described, e.g. in US-A-
3,890,421, and
improvements thereto, with particular reference to the problem of controlling
synchronism,
have been described in e.g. US-A-4,825,111, US-A-4,853,602, and US-A-
5,051,225.
Actually there are various commercial simultaneous bi-axial film tenters.
A suitable line for simultaneous stretching with linear motor technology has
been
25 designed by Brueckner GmbH and advertised as LISIM~ line. The configuration
of the
tenter can be varied depending on the stretching ratios desired. Using a
synchronous linear
motor tenter, the stretching ratios that may be applied are generally
comprised between about
3:1 and about 10:1 for MD stretching and between about 3:1 and about 10:1 for
TD
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WO 00/41872 PCTIEPOO/OOI27
stretching. Preferably however stretching ratios of at least about 4:1 in both
directions are
applied, wherein stretching ratios of at least about 5:1 are more preferred,
and stretching
ratios of at least about 6: I are even more preferred.
In case of a simultaneous flat stretching process, there is only one pre-
heating zone
and the temperature in said pre-heating zone is kept between about 105
°C and about 145 °C,
i.e. close to the orientation temperature that is generally comprised between
about 100 °C and
about 140 °C.
Thereafter, the resulting hot, irradiated, and clipped sheet is directed to
the stretching
zone of the simultaneous tenter, and stretched simultaneously in both
directions.
In both flat stretching processes, the bi-axially stretched film is then
transferred in a
relaxation or annealing zone, heated to a temperature of about 70-90
°C.
Following the annealing step the film is transferred to a cooling zone where
generally
air, either cooled or kept at the ambient temperature, is employed to cool
down the film. The
temperature of said cooling zone is therefore typically comprised between
about 20 and about
~5 40 °C. At the end of the line, the edges of the film, that were
grasped by the clips and have
not been oriented, are trimmed off and the obtained bi-axially oriented, heat-
shrinkable
polyethylene Flm is then wound up, with or without prior slitting of the film
web to the
suitable width.
The heat-shrinkable, mono-layer, irradiated polyethylene film thus obtained
has a
2o thickness variation of less than 20 percent, preferably less than 18
percent, and more
preferably less than 15 percent.
The bi-axially oriented, heat-shrinkable, mono-layer polyethylene film,
obtained by the
above process, has a total free shrink, at 120 °C, of from about 60 to
about 170 percent,
generally, from about 70 to about 170 percent, typically, from about 80 to
about 160 percent.
25 When the film is obtained by a simultaneous flat stretching process, the
free shrink
properties thereof are more balanced in the two directions and differences of
less than 15,
preferably less than 10, and even more preferably less than 5, between the %
free shrink in
MD and the % free shrink in TD are obtained.
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The film thus obtained, when heated under restraint, exhibits a shrink tension
in either
directions of at least 40 psi, and preferably of at least 50 psi. Shrink
tension is measured in
accordance with ASTM D 2838.
The obtained film may then be subjected to a corona discharge treatment to
improve
the print receptivity characteristics of the film surface.
As used herein, the phrases "corona treatment" and "corona discharge
treatment" refer
to subjecting the outer surfaces of the film to a corona discharge treatment,
i.e., the ionization of
a gas such as air in close proximity to a film surface, the ionization
initiated by a high voltage
passed through a nearby electrode, and causing oxidation and other changes to
the film surface,
~ 0 such as surface roughness. Corona treatment of polymeric materials is
disclosed in e.g. US-A-
4,120,716.
The invention is further illustrated by the following examples, which are
provided for
the purpose of representation, and are not to be construed as limiting the
scope of the invention.
Unless stated otherwise, all percentages, parts, etc. arc by weight.
t 5 Examples 1 to 3
Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1
copolymer with a density of 0.920 g/cm' and a melt index of 1.0 g/10 min
(DowlexTM 2045
by The Dow Chemical Company) have been manufactured on a sequential tenter
frame line,
with an MD stretching ratio of 5:1 and a TD stretching ratio of 8:1.
20 The thickness of the extruded sheet and the stretching ratios were set to
obtain end
heat-shrinkable films 15 ~m-thick.
In Examples 1 to 3 all the parameters of the manufacturing process but the
level of
irradiation were exactly the same. In particular, the MD pre-heating zone was
kept at about
95-105 °C, the MD stretching was carried out at about 78-88 °C,
pre-heating for the TD
zs stretching was kept at about 125-135 °C, TD stretching was carried
out at about 120-125 °C
and the relaxation was kept at about 80-85 °C. The irradiation level
was 0 kGy (Example 1 ),
27 kGy (Example 2) and 54 kGy (Example 3).
The film thickness was continuously monitored with a beta-gauge instrument and
the
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obtained values have then been statistically evaluated.
Following Table I reports the average thickness, the maximum thickness, the
minimum thickness, the 2a value, and the thickness variation (%) of the films
of Examples I
to 3.
The 26 value is the double of the standard deviation a, is expressed in pm and
is the
parameter employed industrially to evaluate thickness control. It is
calculated by the
following equation
26 = 2. n.E" ~ x;' - (E"~ x;)'-/(n-1 )
wherein "n" is the number of thickness measurements, which is at least ten,
and "x;"
~ o are the actual readings.
TABLE I
Film of Ex. Av. thick.Max. thick. Min. 2a 'thick. variation
thick.
(!gym) (lrm) (gym)
~ s 1 15.5 30.9 11.9 7.0 123
2 14.3 16.8 I1.5 2.4 37
3 15.0 16.9 14.5 0.9 16
The optical properties of the film of Example 3 were excellent : haze was 2.5
% and
20 gloss was 134 gu.
The shrink properties also were excellent as the film of Example 3 showed a
total free
shrink of 126 % at 120 °C.
Examples 4 to 6
The procedure of the Examples I to 3 has been repeated using however a
25 heterogeneous ethylene-butene-1 co-polymer with density of 0.919 g/cm' and
a melt index of
1.0 g/10 min (FlexireneT"' FG 20 by Yolimeri Europa). While the non irradiated
film
(Example 4) could not be obtained due to a breakage of the film in the middle
during
transversal orientation, the results obtained with a level of irradiation of
27 kGy (Example 5)
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WO 00/41872 PCTlEP00/00127
and 54 kGy (Example 6) are reported in following Table II.
TABLE II
Film of Ex. Av. thick. Max. thick. Min. thick. 2a Thick. variation
(gym) (p,m) (p.m)
4 _ _ _ _ -
5 15.0 19.8 13.3 2.7 43
6 15.0 15.9 14.1 0.7 12
l0 Example 7
A mono-layer, heat-shrinkable film of a heterogeneous ethylene-octene-1
copolymer
with a density of 0.902 g/cm' and a melt index of 3.0 g/10 min (TeamexT"'
1000F by DSM)
has been manufactured on a sequential tenter frame line, with an MD stretching
ratio of 5:1
and a TD stretching ratio of 8:1. The thickness of the extruded sheet was set
to obtain an end
heat-shrinkable film 15 ~m-thick.
In the orientation process the MD pre-heating zone was kept at about 60-
65°C, the
MD stretching was carried out at about 50-55 °C, pre-heating for the TD
stretching was kept
at about 105-110 °C, TD stretching was carried out at about 100-105
°C and the relaxation
temperature was kept at about 70-75 °C. The irradiation level was 72
kGv.
2o The results thus obtained are reported in following Table I1I.
TABLE I1I
Fihn of Ex. Av. thick. Max. thick. Min. thick. 2a Thick. variation %
(gym) (pm) {p,m)
7 17.4 18.2 16.6 0.8 9
Examples 8 to 10
Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1
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WO 00/41872 PCTlEP00/00127
copolymer with a density of 0.911 g/cm' and a melt index of 6.6 g/10 min
(StamylcxT"' 08-
076 by DSM) have been manufactured on a sequential teeter frame line, with an
MD
stretching ratio of 5:1 and a TD stretching ratio of 8:1. The thickness of the
extruded sheet
was set to obtain an end heat-shrinkable films 15 p.m-thick.
s The orientation temperatures were the same in both cases : the MD pre-
heating zone
was kept at about 70-7s °C, the MD stretching was carried out at about
60-65 °C, pre-heating
for the TD stretching was kept at about 105-110 °C, TD stretching was
carried out at about
100-105 °C and the relaxation temperature was kept at about 70-75
°C. The irradiation level
was 0 kGy (Example 8), 54 kGy (Example 9) and 72 kGy (Example 10).
io The results thus obtained are reported in following Table IV.
TABLE 1V
Pilm of Ex. Av. thick.Max. thick. Min, 2a Thick. variation
thick.
(>anl) (Eim) (Irm)
~ s 8 18.2 30.1 12.8 6.2 95
9 15.3 19.8 12.7 3.1 46
IO 18.8 20.7 18.1 1.3 14
The optical properties of the film of Example 10 were excellent : haze was 1.0
% and
2o gloss was 145 gu.
The shrink properties also were excellent as the film of Example 10 showed a
total
free shrink of 160 % at 120 °C.
Exam,~les I 1 to 13
Three mono-layer, heat-shrinkable films of a homogeneous ethylene-octene- I
25 copolymer with a density of 0.920 glem~' and a melt index of 0.9 gll0 min
(EliteT"' S 100 by
The Dow Chemical Company) have been manufactured on a sequential teeter frame
line,
with an MD stretching ratio of S:1 and a TD stretching ratio of 8:1. The
thickness of the
extruded sheet was set to obtain an end heat-shrinkable films 15 p.m-thick.
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WO 00/41872 PCT/EP00/00127
The orientation temperatures were the same in all the three cases : the MD pre-
- heating zone was kept at about 105-1 15 °C, the MD stretching was
carried out at about 88-98
°C, pre-heating for the TD stretching was kept at about 135-145
°C, TD stretching was
carried out at about 130-135 °C and the relaxation temperature was kept
at about 90-95 °C.
The irradiation level was 0 kGy (Example 11 ), 27 kGy (Example 12), and 54 kGy
(Example
13).
The results thus obtained are reported in following Table V.
TABLE V
Film of Ex. Av. Max. thick.Min. thick.2a Thick. variation
thick.
(!gym) (wm) (N~m)
11 16.0 27.8 12.1 5.8 98
12 15.G 17.1 11.9 2.2 33
13 15.4 16.8 14.5 0.8 l5
The optical properties of the film of Example 13 were excellent : haze was 2.
~ % and
gloss was 135 gu.
The shrink properties also were excellent as the film of Example 13 showed a
total
free shrink of 132 % at 120 °C.
2o Examples 14 to 16
Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1
copolymer with a density of 0.910 g/em' and a melt index of 2.2 g/10 min
(StamylexT"' 08-
026F by DSM) have been manufactured on a sequential tenter frame line, with an
MD
stretching ratio of 5:1 and a TD stretching ratio of 8:1. The thickness of the
extruded sheet
was set to obtain an end heat-shrinkable films 15 pm-thick.
'fhe orientation temperatures were the same in both cases : the MD pre-heating
zone
was kept at about 70-75 °C, the MD stretching was carried out at about
60-65 °C, pre-heating
for the TD stretching was kept at about 105-110 °C, TD stretching was
carried out at about
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WO 00/41872 PCT/EP00/00127
100-105 °C and the relaxation temperature was kept at about 70-75
°C. The irradiation level
was 0 kGy (Example 14), 54 kGy (Example 15), and 72 kGy (Example 16).
The results thus obtained are reported in following Table VI.
TABLE VI
Film of Ex. Av. thick.Max. thick. Min. 2a Thick. variation
thick.
(lam) (N~m) (!gym)
14 17.7 30.4 13.0 6.5 98
18.U 24.1 15.3 2.7 49
16 16.8 18.9 15.9 1.3 18
The optical properties of the film of Example 16 were excellent : haze was 0.9
% and
doss was 145 gu.
The shrink properties also were excellent as the film of Example 16 showed a
total
15 free shrink of 158 % at 120 °C.
The films obtained according to the present invention can be used in the
packaging of
food and not food products as known in the art. To this purpose they can be
used in the flat
form to be wrapped up around the product to be packaged or they may be first
converted into
bags or pouches by conventional techniques well known to the person skilled in
the art. They
2o can also be coupled or laminated to other films or sheets to obtain a
packaging material of
improved performance.
Although the present invention has been described in connection with the
preferred
embodiments, it is to be understood that modifications and variations may be
utilized without
departing from the principles and scope of the invention, as those skilled in
the art will readily
understand. Accordingly, such modifications may be practiced within the scope
of the
following claims.
