Note: Descriptions are shown in the official language in which they were submitted.
~%~
This application is related to prior cop ending Canadian
application Serial No. ~19,695 filed on January 18, 19~3.
FIELD OF TIRE INVENTION
This invention relates to heat shrinkable, thermoplastic pack-
aging films. In particular, the present invention is directed to shrink
films utilizing linear low density polyethylene or linear medium density
polyethylene resins as a constituent of a core layer in a multi-layer film.
BACKGROUND OF THE INVENTION
The present invention is directed to new and useful heat shrink
able film formulations. One distinguishing feature of a shrink film is
the film's ability) upon exposure to a certain temperature, to shrink or
if restrained from shrinking, to generate shrink tension within the film.
Y22LB/kd - 1 -
I'
I
The manufacture of shrink films, as is well known in the hut,
may be generally accomplished by extrusion of the resinous neutrals
which have been heated to their flow or melting point frown an extrusion
die in tubular or planar (sheet) form. After a posy extrusion quenching
to cool, the extradite is then reheated to its orientation temperature
range. The orientation temperature range for a given film will voyeur with
the different resinous polymers and blends thereof which comprise the
film. However, the orientation temperature range may generally be stated
to be above room temperature and below the melting point of the film.
the orientation temperature range for a given film is readily determine
able by those of ordinary skill in the art without undue experimentation.
The terms "oriented" or "orientation" are used herein to desk
crime the process and resultant product characteristics obtained by
stretching and immediately cooling a resinous polymeric material which
has been heated to its orientation temperature range so as to revise the
molecular configuration of the material by physical alignment of the
molecules to improve mechanical properties of the film such as , for
example, shrink tension and orientation release stress. Both of these
properties may be measured in accordance with ASTM D 283~-69 (reproved
1975). When the stretching force is applied in one direction uniaxial
orientation results. ennui the stretching force is applied in two direct
lions biaxial orientation results. orientation is also herein used
interchangeably with "heat shrinkability" with these terms designating a
material which has been stretched and set by cooling at its stretched
dimensions. An oriented (i.e., heat shrinkable) material will tend to
return to its original unstretched dimensions when heated to an appear-
private temperature below its melting temperature range.
Returning to the basic process for manufacturing the film as
discussed above, it can be seen that the film once extruded and initially
quenched to cool is then reheated to its orientation temperature range
and oriented. The stretching to orient may be accomplished in many ways
such as, for example, by "blown bubble" techniques or "tinter framing".
These terms are well known to those in the art and refer to orientation
steps whereby the material is stretched in the cross or transverse direct
lion (TO) and/or in the longitudinal or machine direction (MD). After
being stretched, the film is rapidly cooled to quench and thus set or
lock-in the oriented molecular configuration.
~22LB2/kd
4536-540
After locking-in the oriented molecular configuration the
film may then be stored in rolls and utilized to tightly package
a variety of items. In this regard, the product to be packaged is
first enclosed in the heat shrinkable material by heat sealing the
shrink film to itself where necessary. Thereafter, the enclosed
product is subjected to elevated temperatures by, for example,
passing the product through a hot air or hot water tunnel. This
causes the film to shrink around the product to produce a tight
wrapping that closely conforms to the contour of the product.
The above general outline for manufacturing films is
not meant to be all inclusive since -this process is well known to
those in the art. For example, see U. S. Patent Nos. 4,274,900;
4,229,241; 4,194,039; 4,188r443; 4,048,42~; 3,821,182 and
3,022,543.
Many variations on the above discussed general processing
theme are available to those in the art depending upon the end use
for which the film is to be put and the characteristics desired
to be instilled in the film. For example, the molecular of the
film may be cross-linked during processing to improve the films
abuse resistance and other characteristics. Cross-linking and
methods for cross-linking are well known in the art. Cross-linking
may be accomplished by irradiating the film or, alternatively, may
be accomplished chemically through the utilization of peroxides.
Radiation dosages are referred to herein in terms of
the radiation units "fad", with one million fads or a mergarad being
designated as "MY". The degree of molecular cross linking is
expressed in terms of the radiation dosage that induces the cross-
linking. "Radiation" as used herein generally means ionizing
--3--
~2~2~ ~53~-540
radiation such as X-rays, gamma rays, and electrons which directly
induce molecular cross-linking. (However, when used in conjunction
with cross-linking agents dispersed within a material, both heat
and light can be considered forms of radiant energy which induce
cross-linking.) Electrons are the preferred form of radiant energy
and are preferably produced by commercially available accelerators
in the range of 0.5 to 2.0 me.
Another possible processing variation is the application
of a fine mist of silicone spray -to the interior of the freshly
extruded material to improve the further process ability of the
material. A method for accomplishing such internal application is
disclosed in cop ending Canadian Application Serial No. 403,969
filed in the Canadian Patent Office on Ma 28, 1982. Alternatively
an anti-fog agent can be internally applied. Anti-fog agents have
also been found to be of beneficial use in improving process ability.
In an addition to those materials disclosed in my cop ending
application serial no. 403,969, anti-fog materials have demonstrated
the ability to eliminate internal tape welding of an extended tube.
The polyolefin family and, in particular, the polyethyl-
one family of shrink films provides a wide range of physical and performance characteristics such as shrink force (the amount of
force that a film exerts per unit area of its cross-section during
shrinkage), the degree of free shrink (the reduction in linear
dimension in a specified direction that a material undergoes when
subjected -to elevated temperatures while unrestrained), tensile
strength (the highest force that can be applied to a unit area of
film before it begins to tear apart), sealability, shrink tempera
lure curve (the relationship of shrink to temperature), tear
I
4536-5~0
initiation and resistance (the force at which a film will begin
to tear and continue to tear), optics (gloss, haze and transpire-
envy of material), and dimensional stability (the ability of the
film to retain its original dimensions under different types of
storage conditions). Film characteristics play an important role
in the selection of a particular film and they differ for each
type of packaging application and for each package. Consideration
must be given to the product size, weight, shape, rigidity,
number of product components, other packaging materials which may
be used along with the film, and the type of packaging equipment
available.
In view of the many above discussed physical character-
is tics which are associated with polyethylene films and in
further view of the numerous applications with which these films
have already been associated and those to which they may be
applied in the future, it is readily discernible that the need
for ever improving any or all of the above described physical
characteristics of -these films is great and, naturally, ongoing.
OBJECTS OF THE INVENTION
Accordingly, it is a general object of the invention to
provide a heat shrinkable polyolefin film that will be an
improvement over those films already utilized in the prior art.
It is another object of the present invention to provide
a polyolefin film having improved ball burst properties.
Yet a further object of the present invention is to
provide a polyolefin film having high elongation.
In another object of the present invention is to provide
an improved polyolefin shrink film having improved optical quell-
ties.
--5--
eye
4536-5~0
A still further object of the invention is to provide
a polyolefin shrink film having a wide shrink temperature range.
Another object of the present invention is to provide
an improved polyolefin shrink film having improved sealability.
Furthermore, yet another object of the present invention
is to provide a polyolefin shrink film having improved resistance
to tear propagation.
An even further object of the invention is to provide
a polyolefin shrink film having an improved machinability.
Yet another object of the present invention is to
provide an improved polyethylene shrink film which utilizes either
a linear low density or a linear medium density polyethylene as a
constituent of a core layer.
Still another object of -the present invention is to
provide an improved process for manufacturing the film.
These and other objects are achieved by the polyolefin
shrink film which is disclosed herein.
DEFINITIONS
Unless specifically set forth and defined or limited,
the terms "polymer" or "polymer resin" as used herein generally
include homopolymers, copolymers, terpolymers, block, graft
polymers, random, and alternating polymers.
Polyolefin or olefin polymer or polymers as used herein
includes not only polymers of unsaturated aliphatic hydrocarbons
of the general formula CnH2n but also copolymers of olefins with
other monomers such as ethylene with vinyl acetate.
--6--
~536-540
The term "melt flow" as used herein or "melt flow index"
is the amount, in grams, of a thermoplastic resin which can be
forced through a given orifice under a specified pressure and them-
portray within ten minutes as described in ASTM D 1238.
The term "core" or "core layer" as used herein means a
layer in a multi-layer film which is enclosed on both sides by
additional layers.
The term "skin" or "skin layer" as used herein means
an outer (i.e., surface) layer of a multi-layer film.
The term "low density polyethylene" (LOPE) as used
herein refers to homopolymers of ethylene having a density of
from 0.910 to 0.925.
The term "linear low density polyethylene" (LLDPE) as
used herein refers to a copolymer of ethylene and 8% or less of
butane, octane or Helene having a density of from 0.910 to 0.925
and in which the molecules comprise long chains with few or no
branches or cross-linked structures.
The term "linear medium density polyethylene" (LMDPE~
as used herein refers to a copolymer of ethylene and less than 8%
butane, octane or Helene having a density of from 0.926 to 0.940
and in which the molecules comprise long chains with few or no
branches or cross-linked structures.
The term "ethylene vinyl acetate copolymer" (EVA) as
used herein refers to a copolymer formed from ethylene and vinyl
acetate monomers wherein the ethylene derived units are present
in major amounts and the vinyl acetate derived units are present
in minor amounts. Preferred ethyl vinyl acetate copolymers are
those having from 2 - 12% vinyl acetate derived units.
-pa-
I
~536-5~0
The term "gauge" as used herein refers to -the dimension-
at thickness of a film. 100 gauge is equal to 1 mix which is equal
to 0.001 inch.
The term "racking ratio" as used herein refers to the
ratio of expansion, (i.e. stretching) during orientation, of a
film. For example, a racking ratio of four (4) or four (4) to one
(1) would mean that the film had been stretched -to four times its
original, unstretched, dimension during orientation.
-6b-
2~3~
Swallower OF THE lo lo
It has been discovered that a Flexible, heat shrinkable thermos
plastic packaging film having a desirable combination of physical char-
acteristics such as syrinx tension, optical characteristics, curability,
sealability, shrink temperature range, and tear resistance has breed
achieved by the multi-layer flexible, thermoplastic packaging film of the
present invention. This rnulti-layer film has a "core" layer that come
proses a linear low density or linear medium density polyethylene resin.
A preferred three layer embodiment comprises, in addition to the above
identified "core" layer, two skin Ayers each comprising an ethylene
vinyl acetate copolymer. Preferably, the multi-layer film is irradiated.
It is also preferable to orient the film so that it is heat shrinkable in
at least one direction.
The multi-layer film may be combined with other polymeric
materials for specific applications. For instance, relatively thin
layers may be added on either or both sides of the basic preferred three
Mayer structure to improve seal strength or to lower gas and moisture
permeability.
The invention also includes an improved process for manufacturing
the film. One important aspect of the improved process is the utilize-
lion of racking ratios id the ridge of 2.5 - I to 1 in the transverse
direction and 2.5 to 4.2 in the longitudinal direction.
BRIEF DESCRIPTION OF THE DRAYING
Figure I is a cross sectional view of a preferred three layered
embodiment of the present invention.
Figure If is a schematic representation of a preferred process
for manufacturing the film of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, which is a cross sectional view of a
three layered preferred embodiment of the present invention, it is seen
that this embodiment comprises core layer 2 and skin layers I and 3. A
thickness ratio of the three layers of 1/2/1 is demonstrated in Figure I.
However the thickness of each skin layer may range between JO - 30% of
the total film thickness thus, the thickness of the core layer may range
ibid
-1-
4536-540
between 40 - 80% of the total film thickness. A preferred core
layer 2 constituent comprises a linear low density polyethylene
polymer. However, a linear medium density polyethylene polymer
may be substituted as a core layer constituent without substantial
alteration of the characteristics of the final film product.
LLDPE is herein utilized as an abbreviation for linear
low density polyethylene as defined above. LMDPE is herein
utilized as an abbreviation for linear medium density polyethylene
as defined above.
My experimentation has revealed that an especially
preferred core layer formulation consists essentially of a linear
low density polyethylene. This material may be obtained from the
Dow Chemical Company under the trademark Dow X2045.
Returning to Figure 1 and, in particular, to skin
layers 1 and 3, my experimentation has also determined that a
particularly preferred skin layer formulation consists essentially
of an ethylene vinyl acetate copolymer. This material may be
obtained from Dupont under the trademark Elvax 3128. Elvax 3128 is
a copolymer of ethylene and vinyl acetate having from 8.4 - 9.4% of
vinyl acetate having a Melt Flow of 2.0 -I 0.2. Alternatively,
the material may be obtained from El Paso (formerly Reaction) under
the -trademark PUCKS. PUCKS is a copolymer of ethylene
and vinyl acetate having from 3.3 to 4.1% of vinyl acetate having
a Melt Index of 2.0 + 0.5 and a density of 0.9232 - 0.9250.
Throughout this specification and claims all percentages
are "by weight" percentages.
Throughout the specification and claims all references
to density are in gm/cc at 23C.
~536 540
In summary, my experimentation has determined that a
particularly preferred embodiment of the present invention comprises
a core layer consisting essentially linear low density polyethylene
and skin layers consisting essentially of an ethylene vinyl
acetate copolymer of an ethylene vinyl acetate copolymer.
Although the above-discussed three layer formulations
are generally preferred over structures having more than three
layers as a result of the economics of manufacture, I have also
produced various five layer formulations which are also sails-
factory from a physical characteristics point of view. However the cost of manufacturing a five layer film is greater than
that of a three layer film.
Those skilled in the art will readily recognize that
all of the above disclosed, by weight, percentages are subject to
slight variation. Additionally, these percentages may vary
slightly as a result of the inclusion or application of additives
such as the silicon mist and anti-fog agents discussed above or
agents such as slip and anti-block events. A preferred anti-block
agent is silica which is available from Johns Manville under the
trademark White Mist. Preferred slip agents are Erucamide
(available from Hummock Chemical under the trademark Kemamide E),
and Strummed (available from the Hummock Chemical Company under
the trademark Kemamide S) and, N, No Dioleoylethylenediamine
(available from Glyco Chemical under the trademark Acrawax C).
A preferred Silicon spray is a liquid polyorganosiloxane manufac-
lured by General Electric under -the trademark General Electric
SF18 polydimethylsiloxane.
The general ranges for application of these additives
~2~2~;
4536-5~0
are as follows:
(l) Silica - 250 - 3000 PAM
12) Acrawax C: 200 - 4000 PAM
(3) Erucamide 200 - 5000 PAM
(4) Strummed: 200 - 5000 PAM
(5) Silicon Spray: .5 mgft2 _ and up.
When utilized within the specification and claims of
the instant application the term "consisting essentially of" is
not meant to exclude slight percentage variations or additives
lo and agents of this sort.
Additional polyolefin layers and/or minor amounts of
additives of the types described above may be added to the 3-layer
structure of the present invention as described but care must be
taken not to adversely alter -the desirable shrink tensions,
shrink properties, optics and other characteristics or the multi-
layer film of the present invention.
In the preferred process for making the multi-layer
linear low or linear medium density polyethylene shrink film of
the present invention the basic steps are blending the polymers
for the various layers, coextruding the layers to form a multi-
layer film, and then stretching the film to biaxially orient.
These steps and additional desirable steps will be explained in
detail in the paragraphs which follow.
-pa-
329~;
he process begins by blending the raw neutrals (i.e. polyp
metric resins) in the proportions and ranges desired as discussed above.
the resins are usually purchased from a supplier in pellet form and can
be blended in any one of a number of commercially available blenders as
is well known in the art. During the blending process any additives
and/or agents which are desired to be utilized are also incorporated.
The blended resins and applicable additives and/or agents are
toed fed into the hoppers of extrudes which feed the coextrusion die.
For the three-layer film at least Tory extrudes need to be employed if
each layer is to have a different composition. two extrudes are fed the
materials desirable for the inner and outer skin layers and the other
extrude is fed the linear low or linear medium density polyethylene
material which is desired for utilization in the core layer. Additional
extrudes may be employed, if desired. Preferably the materials are
coextruded as a tube having a diameter. The final diameter of the coed-
trued film will be dependent on the original diameter of the extruded
tube and the subsequently applied racking ratio. the coextruded tube is
relatively thick and is referred to as the "tape". Circular or tubular
coextrusion dies are well known in the art and can be purchased from a
number of manufactures. In addition to tubular coextrusion, slot dies
could be used to coextrude the material in planar form. Well known
single or multi-layer extrusion coating processes could also be employed
if desired.
An additional process step which may be utilized is to irradi-
ate the tape or unexpanded tubing or sheet by bombarding it with high-
energy electrons from an accelator to cross-link the materials of the
tape. Cross-linking greatly increases the structural strength of the
film or the force at which the material can be stretched before tearing
apart when the film materials are predominately ethylene such as polyp
ethylene or etheylene-vinyl acetate. Irradiation also improves the
optical properties of the film and changes the properties of the film at
higher temperatures. If an irradiation step is employed a preferred
irradiation dosage level is in the range of 0.5~ to ]2.0MR. MY is an
abbreviation for megarads. A megarad is 1 x 10 fads with a fad being
the quantity of ionizing irradiation that results in the absorption of
loo ergs of energy per gram of irradiated material regardless of the
~L~]01kd
I I
4536-5~0
source of the radiation. In some instances, it may be desirable
to stretch the multi-layer film first and then irradiate it, or,
if sequential coating is employed one layer or a group of layers
could be irradiated and then another layer or layers could be
added before the final step of stretching and orienting.
As stated above, an additional optional process step
is the application of a fine silicon spray to the interior of the
newly extruded tape. The details of this process step are
disclosed in Canadian Application Serial Number 403,969 filed on
May 28, 1982.
Following coextrusion, quenching to cool, and if desired
irradiation, the extruded tape is reheated and is continuously
inflated by internal elf pressure into a bubble thereby trays-
forming the narrow tape with thick walls into a wide film with thin
walls of the desired film thickness. This process is sometimes
referred to as the "trapped bubble technique" of orientation or
as "racking". After stretching, the bubble is then deflated and
the film is wound onto semi-finished rolls called "mill rolls".
The racking process orients the film, by stretching it transversely
and, to some extent, longitudinally to rearrange the molecules
and thus impart shrink capabilities to the film and modify the
film's physical characteristics. Additional longitudinal or
machine direction stretching may be accomplished by revolving
the deflate rollers which aid in the collapsing of the "blown
bubble" at a speed greater than that of the rolls which serve to
transport the reheated "tape" to the racking or blown bubble area.
All of these methods of orientation are well known in the art.
It has now been found that utilization of racking ratios
~22~
4536-540
of from 2.5 to 4.2 in the transverse direction and 2,5 - 4.2
in the longitudinal direction result in films having lower degrees
of orientation along with improved physical characteristics such
as, for example, improved elongation as measured in accordance
with ASTM 882.
To further disclose and clarify the scope of my invent
lion to those skilled in the art the following examples are
presented.
-ha-
I
E~.~IPLE 1
A three layered structure having an approximate layer thickness
ratio of l/2/l was extruded by supplying four extrudes. Extrudes
number 2 and 3 which supplied the die orifice for the core layer were
provided with 100% linear low density polyethylene (Dow X2045 (0.920
density, Melt index lo Extrudes No. l and 4 each supplied a die
orifice for a skin layer and both were supplied with 100% ethylene vinyl
acetate copolymer having 3.3 - 4.1h vinyl acetate. IPE204-CS95 (0~9232 -
0.9250 density, Melt Index 2.0 0.5)].
The temperature of extrude Dumber 1 was set in the temperature range of from DOW. Extrude No. 2 was maintained at a temperature
range of from 425-485F. Extrude No. 3 was maintained at a temperature
range of from 425-495F and Extrude No. 4 was maintained at a tempera-
lure from 350-375F. The circular die was maintained at a temperature
range of 400~F.
The tape was extruded at 53 feet per minute.
After extrusion of the layers through the 6 inch circular die
orifice the tubular extradite which had a tape thickness of approximately
9 mill and a tubular width of approximately 5 5/8 inches was quenched to
cool by passing through a cold water bath. Upon extrusion of the tape a
wine silicon mist was applied to the interior of the extruded tube at the
rate of 6 my. ft. . The tape was wound up. Thereafter, the tube was
passed through a irradiation unit at approximately 68 feet per minute to
accomplish an irradiation level of 6 MR. The irradiated tubular extra-
date was then reheated to orient by passing though a heating zone or
oven. Thereafter the film was cooled by water quenching to lock-in the
oriented molecular structure. final film thickness was approximately 75
gauge.
EXPEL II
The compositions utilized were the same as in example I.
All of the process parameters were the same as example one with
the exception that the tape was extruded at 68 feet per minute and was 6
mix in thickness. The tape was transversely racked at 3.6 to 1 and
longitudinally racked at 3.14 to ]. final film thickness was I gauge.
~22LB12/kd
~2~9~6
E~PLE. III
A three layered structure having an approximate layer thickness
of 114/1 was extruded by supplying 4 extrudes. ~xtruders number 2 and 3
which supplied the die orifice for the core layer were provided with 100%
linear low density polyethylene DOW X2045 (0.920 density, welt Index
I ExLruders number 1 and 4 each supplied a die orifice for a skin
layer and both were supplied with 100% ethylene vinyl acetate copolymer
having from 8.4 - 9.4% vinyl acetate and a melt flow of 2 Q.2.
The temperature of extrude number 1 was set in the temperature
range of from 350-375F. Extrude number 2 was set in the temperature
range of from 425-475F. Extrude number 3 was set in the temperature
range of 425-495F. Extrude number 4 was set in the temperature range
of 350-375. The circular die was set at a temperature of 400F.
After extrusion of the layers through the 6 inch circular die
orifice the tubular extradite which had a tape thickness of approximately
9 mill and a tubular width of 5 I inches was quenched to cool by pass-
in through a cold bath at approximately 48 feet per minute. Upon extra-
soon of the tape a fine silicon mist was applied to the interior of the
extruded tube at the rate of 9 my. ft. .
The tube was next passed through an irradiation unit at approx-
irately 48 feet per minute and irradiated to a level of 6
The irradiated tube was then reheated to orient by passing
though a heating zone or oven. After being heated the tubular extradite
was transversely stretched "racked" approximately 3.7 to 1 and longitu-
finally stretched approximately 4.2 to 1. Thereafter the film was cooled
by water quenching to look-in the orientation. Final film gauge was 75
gauge.
The data obtained upon testing of these materials is located in
Table I below.
Y22LBl3/kd
lo
ill ;2%9Z~$
TABLE I
Example I II III
Bayer Ratio 1 lt2/1 1/2j1 1/4/1
Tensile Strength x ]00 (PSI)
ED 138.6 ]22.5 129.4
TO ]56.6 ]60.1 ~53.0 jig
Elongation (%)
HO 202 ]94 ]89
TO 3 129 179 197
Hades x ]000 (PSI)
HO 25.7 23.5 22.6
TO 4 25.8 24.0 21.1
Tear Propagation (gyms.) If,
HO 9.25 4.31 7.00
TO 5 4.31 2.81 8.38
Tear Resistance (lobs.)
MD 0.64 0.51 0.76
TO 0.69 0.44 0.86
Ball Burst a 73F
0.50 In. Dia. Sphere
Ho. (cm - kg) 15.1 ]0.3 20.6
Shrink Properties
at ~85F 6
Free Shrink (%)
MD , 20 19 22
ED 7 22 22 24
Shrink tension (PSI)
ED 310 235 336
TO 405 449 373
At 245F 6
Free Shrink I
MD 65 63 63
TO 7 So 68 63
Shrink 'tension (PSI)
MD 294 ~85 328
TO 8 384 362 330
Optics
Haze (%) 0.9 1.2 0.7
Gloss (45) 95 94 94
Total Transmission 92.6 92.6 92.8
Y22~B14/kd
Footnote to Table I 1 2 Z9 2 9 6
3ASTM D 882
ASTM D 882 ;
3ASTM D 882
4ASTM D 1938
5ASTM D 1004
ASTM D 2732
7ASTM D 2838
8ASTM D ]003
All of the above tabulated Table I data are averages obtained
by procedures in accordance with the designated ASTM standard.
it should be understood that the detailed description and specific
examples which indicate the presently preferred embodiments of the invention
are given by way of illustration only since various changes and modifications
within the spirit and scope of the invention will become apparent to those
of ordinary skill in the art upon review of the above detailed description and
examples.
In view of the above:
Y14LC15/kq
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