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FIELD OF THE INVENTION
The present invention relates to the use of both a pro-rad type of chemical
cross-linking agent and irradiation in the manufacturing process for
making an oriented, i.e. heat-shrinkable, polymeric film. Generally in the
manufacture of heat shrinkable films, polymer layer(s) are melt extruded
as a "blown bubble" through an annular die to form a tube which is cooled
and collapsed, and then stretched (oriented) typically by inflating the
extruded tube of film with a gas to form a "bubble" in the tube.
Typically, irradiation is employed prior to orientation. By the practice
of this invention, the orientation speed is improved.
Heat shrinkable polymeric films are widely used for packaging of fresh and
processed meats, cheeses, poultry and a large number of non-food items.
Some of the films are formed into heat shrinkable bags or pouches which
are supplied to a meat packer. Typical polymers employed are linear low
density polyethylene (LLDPE), low density polyethylene (LDPE), ethylene
vinyl acetate (EVA) copolymer, and the like.
BACKGROUND OF THE INVENTION
It is generally well known in the art that irradiation, such as by electron
beam irradiation, of certain polymeric film materials results in the
irradiative cross-linking of the polymeric molecular chains contained
therein and that such action generally results in a material having
improved heat shrink properties, abuse resistance, structural integrity,
tensile strength, puncture resistance, and/or delamination resistance.
Such physical improvements from irradiation, in particular the improved
heat shrink properties, are discussed in U.S. Pat. No. 3,022,543 (1962) to
Baird et al. Many of the other physical improvements also are discussed at
columns 2 and 8 of U.S. Pat. No. 4,178,401 (1979) to Weinberg and at
column 4 of U.S. Pat. No. 3,741,253 to Brax. Furthermore, it is also known
from U.S. Pat. No. 4,525,257 (1985) to Kurtz et al that low level
irradiation under 2 MR of narrow molecular weight distribution, linear low
density ethylene/alphaolefin (LLDPE) particulate copolymer sufficient to
introduce cross-links into the particulate copolymer but insufficient to
provide for significant measurable gelation produces improved copolymer
rheology providing increased extensional viscosity during film
fabrication, i.e. the bubble is more stable as the LLDPE is more easily
stretchable.
That cross-linking of polymers may be accomplished chemically through
utilization of chemical cross-linking agents is also well known to those
of skill in the art. For instance, cross-linking agents, such as allyl
compounds or organic peroxides, have been used to cross-link polyethylene
polymers and copolymers. A general discussion of chemical crosslinking can
be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer
Science and Technology, Plastics, Resins, Rubbers, Fibers published by
John Wiley & Sons, Inc. and copyrighted in 1966. This document has a U.S.
Library of Congress Catalog Card Number of 64-22188 and the referenced
pages are hereby incorporated by reference. Typically, the chemical
cross-linking agents react with the polymer to form a solid or highly
viscous cross-linked copolymer. Furthermore, it is also known from U.S.
Pat. No. 4,515,745 (1985) to Churma et al assignors to Union Carbide and
U.S. Pat. No. 4,675,364 Churma et al assignors to Viskase that when an
organic peroxide chemical cross-linking agent is used in manufacturing an
extruded film having at least one layer of EVA, in a controlled amount
small enough (0.05 to 0.015% agent by weight of the EVA layer) not to form
gel measurable by ASTM test method number D 2765 (ASTM D2765 is further
discussed below), i.e. no measurable gel means the EVA containing a
crosslinking agent is soluble in a solvent such as xylene, then the
rheology during extrusion is modified, i.e. the bubble is more stable as
the EVA is more easily stretchable. A drawback of organic peroxides is the
difficulty in controlling the amount used. Organic peroxides are free
radical generators and will react as soon as they come in contact with the
heat of the extruder. Thus, they can gum up the workings of the extruder
before the polymeric film has a chance to exit the extruder. U.S. Pat. No.
4,614,764 (1986) to Colombo et al shows greater bubble stability in the
blown tubular film extrusion process by controlling free radical generator
type of cross-linking agents such as organic peroxides, by first modifying
LLDPE in the molten state with a free radical generator and then blending
the resultant with LLDPE, so the reaction has already occurred prior to
blending this modified polymer with LLDPE in the extruder during blowing a
film.
Also of interest is that electron beam irradiative cross-linking of the
polyethylene insulation of high voltage power cables has a foaming problem
which is alleviated by use of a chemical cross-linking agent, such as
ethylene glycol dimethacrylate, diallyl maleate, dipropargyl maleate,
dipropargyl monoallyl cyanurate, or triallyl cyanurate, which agent
accelerates the electron beam so low megarad dosage of irradiation can be
employed. This is discussed in "Development of Radiation Cross-linking
Process for High Voltage Power Cable", Sasaki et al, Japan Atomic Research
Institute, Takasaki, Japan, (1979) Vol. 14 Radiation Physical Chemistry,
pages 821-830, and "Suppression of Discharge Breakdown of Polyethylene
Insulation During Electron Beam Irradiation to Power Cable", Sasaki et al,
Takasaki Radiation Chemistry Research Establishment, JAERI, Takasaki,
Japan, (1981) Vol. 18, No. 5-6, Radiation Physical Chemistry, pages
847-852. Of related interest is U.S. Pat. No. 4,576,993 (1986) Tamplin et
al assignors to Raychem which discloses LDPE cross-linked by 0.2 to 5
weight percent of pro-rad such as triallyl cyanurate and 2 to 80 megarads
of irradiation for use in dimensionally recoverable articles such as high
voltage insulation.
Also of interest is U.S. Pat. No. 4,374,223 issued Feb. 15, 1983 to
Raamsdonk et al assignors to Olympia Werke which shows an elastic covering
for closing off a nozzle outlet area containing at least one nozzle
orifice of an ink printing head filled with an aqueous liquid, said
covering comprising as elastomeric material having viscoelastic properties
for filling exceedingly small cavities and interstices in the area to be
closed off, comprising by weight: 100 parts ethylene-vinyl acetate
copolymer having an ethylene: vinyl acetate ratio of 40:60; 20-40 parts
talc, as mineral filler; 0.5-15 parts polycarbodimide hydrolysis
protection agent; 0.2-1 parts age retardant (antioxidant); 0.5-2 parts
triallyl cyanurate; 3-5 parts 1-1-Bis(tert-butyl peroxy)-3,3,5-trimethyl
cyclohexane; and 2-10 parts polyethylene glycol of molecular weight about
200.
However, it has been unexpectedly discovered that the combination of a
pro-rad type of chemical cross-linking agent with irradiation has not only
obviated the problem of carefully controlling the amount of chemical
cross-linker so as to avoid gumming up the extruder but also has resulted
in the orientation process for manufacturing polymeric films having a
decreased time, i.e. increased speed or rate.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
heat-shrinkable film manufactured by a process including the combination
of pro-rad type of chemical cross-linking and irradiative cross-linking.
In another aspect, the present invention provides for a method for the
manufacture of a heat shrinkable polymeric film wherein the time for the
orientation step in the manufacture of such films can be decreased, i.e.
the orientation can be achieved at a greater speed than was heretofore
possible. It is an unexpected advantage of the invention that the films
can be made with lower extruder amps and pressure.
Therefore, the present invention provides a multilayer heat-shrinkable
(oriented) film comprising at least one layer of cross-linkable polymer,
said layer (1) originally containing a pro-rad agent and (2) being
irradiated, both being to an extent sufficient to provide an amount of
cross-linking effective to accomplish an increased orientation rate.
By "extent sufficient to provide an amount of cross-linking effective to
accomplish an increased orientation rate" as that phrase is used herein,
it is intended to mean that the amount of cross-linking from the presence
of both the amount of pro-rad agent originally contained in the feed for
the at least one layer of cross-linkable polymer in combination with the
dosage of irradiation, regardless of whether or not the amount of
cross-linking from either or both can be measured by the gel test, will
increase the orientation rate during processing of the film as compared
with the orientation rate of the corresponding polymeric film that (I) is
only irradiated, (II) only contains a pro-rad agent, or (III) is both (a)
free of a pro-rad agent and (b) not irradiated. In the Examples below the
increased orientation rate of the various films is indicated by a greater
processing speed in feet/minute for the inflated "bubble" during the
orientation step of processing.
The present invention also provides a process for increasing the
orientation rate during manufacturing a heat-shrinkable (oriented)
polymeric film comprising: (a) introducing a pro-rad agent to a
cross-linkable polymer by blending the cross-linking agent with the
cross-linkable polymer, (b) extruding the blended cross-linkable polymer,
(c) subjecting the extruded polymer to irradiation, (d) orienting the
extruded polymer in at least one direction, and (e) recovering an oriented
polymeric film, wherein (f) both the original amount of pro-rad agent and
the dosage of irradiation are to an extent sufficient to provide an amount
of cross-linking effective to accomplish an increased orientation rate
during the orientation step.
The present invention also provides a multilayer heat-shrinkable (oriented)
film comprising at least one layer of cross-linkable polymer, said layer
(1) originally containing from about 0.001 to about 5% by weight of
pro-rad type of cross-linking agent and (2) being subjected to irradiative
cross-linking at a dosage from about 0.5 to about 20 MR.
DETAILED DESCRIPTION
Pro-rad compounds are compounds which have poly-functional, i.e. at least
two, moieties such as C.dbd.C, C.dbd.N or C.dbd.O. Suitable pro-rad type
of chemical cross-linking agents include, but are not limited to,
polyfunctional vinyl or allyl compounds, for example triallyl cyanurate
(TAC), triallyl isocyanuarte (TAIC) or pentaerythritol tetramethacrylate
(PETMA), glutaraldehyde (GA), ethylene glycol dimethacrylate (EGDMA),
diallyl maleate (DAM), dipropargyl maleate (DPM), or dipropargyl monoallyl
cyanurate (DPMAC).
Typically, in the manufacture of films, a suitable polymer usually in the
form of pellets or the like, is brought into a heated area where the
polymer feed is melted and heated to its extrusion temperature and
extruded as a tubular "blown bubble" through an annular die. Other
methods, such as "slot die" extrusion wherein the resultant extrudate is
in planar form, as opposed to tubular form, are also well known. After
extrusion, the film is then typically cooled and then stretched, i.e.
oriented by "tenter frasing" or by inflating with a "trapped bubble", to
impart the heatshrinkable property to the film, as is further described
below. Irradiation, typically via an electron beam, preferably takes place
prior to the stretching for orienting the film. Below, first is described
in detail introducing the chemical cross-linking agent to the
cross-linkable polymer followed by the general process for making and
orienting film. Then irradiation is described in detail.
Any cross-linkable polymer may be employed in the present invention.
Suitable cross-linkable polymers are the polyolefins which are further
described in the "Definitions" section below.
The terms "cross-linking," "cross-linkable," "cross-linked", and the like,
as used herein relate to, but are not limited to, a gelation test, namely
ASTM D-2765-84 "Standard Test Methods for Determination of Gel Content and
Swell Ratio of Cross-linked Ethylene Plastics," pages 557-565 (January,
1985). In the test, the polymer is dissolved in decahydronapthalene or
xylene, and the insoluble fraction produced by cross-linking is determined
by the amount that dissolves versus the amount that gels. In the present
invention, it is irrelevant whether the amount present of irradiation
alone would be so minimal as not to cause measurable gelation, or whether
the amount present of pro-rad type of chemical cross-linking agent alone
would be so minimal as not to cause measurable gelation. There are other
methods, such as infrared spectrophotometry, size exclusion
chromotography, mass spectrophotometry, nuclear magnetic resonance, or
shift in the polymer melt index, for measuring the effects of small
amounts of irradiation or chemicals, said methods being well known to
those skilled in the art, which methods could easily be employed for
measuring the effect of a minimal amount of cross-linking or molecular
weight increase whether by irradiation or by chemical agent. Accordingly,
what is required is only that the combination of both a pro-rad type of
chemical cross-linking agent and irradiation be sufficient to cause an
improved faster orientation, i.e increased orientation rate.
In the practice of the invention, the pro-rad type of chemical
cross-linking agent is preferably introduced by blending it into the solid
pellets of polymer feed for the at least one layer of cross-linkable
polymer, such as by melt blending with heat and/or by the use of a solvent
such as isopropyl alcohol, thereby forming a master batch (MB) of pro-rad
agent and polymer which may be stored. This master batch could be used as
is for the polymer feed. Alternatively, it may be further mixed with
pellets of the same or different polymer for the feed at an amount such
that the resultant blend originally contains the desired concentration of
chemical cross-linking agent.
While it is not intended to be bound to any theory, it is believed that the
pro-rad does not react from the heat of melt blending or of the extruder
but rather reacts when initiated by the electron beam irradiation. In
other words, it is believed that if melt blending is used, the temperature
from the heat will not be high enough to carry out the reaction of the
pro-rad agent, and typical extrusion temperatures are not hot enough to
carry out the cross-linking reaction of the pro-rad.
Thus, statements that the at least one layer of cross-linkable polymer
"originally" contains or contained a pro-rad type of chemical
cross-linking agent or statements about the "original" amount of
cross-linking agent present are intended to be references to the amount
measurable of pro-rad agent added into the polymer feed for the polymer
layer. The resulting structure of the polymer irradiated in the presence
of a pro-rad will not be the same as that irradiated without the pro-rad.
The pro-rad type of chemical cross-linking agent must be originally present
to an extent sufficient in the polymer so that the combination of both
chemical cross-linking agent and irradiation will provide an effective
amount of cross-linking to accomplish an increased orientation rate. To
achieve this sufficient extent, the pro-rad agent should be originally
present in the composition of polymer feed for at least one film layer in
an amount from about 0.001 to about 5.0% by weight, more preferably about
0.005 to about 2.5% by weight, even more preferably about 0.015 to about
1.0% by weight.
More particularly, the manufacture of shrink films may be generally
accomplished by extrusion (single layer films) or coextrusion (multi-layer
films) of thermoplastic resinous materials which have been heated to or
above their flow or melting point from an extrusion or coextrusion die in,
for example, either tubular or planar (sheet) form, followed by a post
extrusion cooling. The stretching for orientation may be conducted at some
point during the cool down while the film is still hot and at a
temperature within its orientation temperature range, followed by
completing the cooling. Alternatively, after the post extrusion cooling,
the relatively thick "tape" extrudate is then reheated to a temperature
within its orientation temperature range and stretched, to orient or align
the crystallites and/or molecules of the material and then cooled down.
The orientation temperature range for a given material or materials will
vary with the different resinous polymers and/or blends thereof which
comprise the material. However, the orientation temperature range for a
given thermoplastic material may generally be stated to be below the
crystalline melting point of the material but above the second order
transition temperature (sometimes referred to as the glass transition
point) thereof. Within this temperature range, the material may be
effectively oriented.
The terms "orientation" or "oriented" are used herein to describe generally
the process steps and resultant product characteristics obtained by
stretching transversely, longitudinally, or both (whether during the post
extrusion cool down or during reheating after the post extrusion cool down
as described in the paragraph above) and substantially immediately cooling
a resinous thermoplastic polymeric material which has been heated to a
temperature within its orientation temperature range so as to revise the
intermolecular configuration of the material by physical alignment of the
crystallites and/or molecules of the material to improve certain
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 2838-81. When the stretching force is applied in
one direction, monoaxial orientation results. When the stretching force is
simultaneously applied in two directions, biaxial orientation results. The
term oriented is also herein used interchangeably with the term
"heat-shrinkable" with these terms designating a material which has been
stretched and set by cooling while substantially retaining its stretched
dimensions. An oriented (i.e. heat-shrinkable) material will tend to
return to its original unstretched (unextended) dimensions when heated to
an appropriate elevated temperature.
Returning to the basic process for manufacturing the film as discussed
above, it can be seen that the film, once extruded (or coextruded if it is
a multi-layer film), is then oriented by stretching within its orientation
temperature range. The stretching to orient may be accomplished in many
ways such as, for example, by "trapped bubble" techniques or "tenter
framing". These processes are well known to those in the art and refer to
orientation procedures whereby the material is stretched in the cross or
transverse direction (TD) and/or in the longitudinal or machine direction
(LD or MD). After being stretched, the film is quickly cooled while
substantially retaining its stretched dimensions which thus sets or
locks-in the oriented molecular configuration.
The film which has been made may then be stored in rolls and utilized to
package a wide variety of items. If the material was manufactured by
"trapped bubble" techniques the material may still be in tubular form or
it may have been slit and opened up to form a sheet of film material. In
this regard, a product to be packaged may first be enclosed in the
material by heat sealing the film to itself where necessary and
appropriate to form a pouch or bag and then inserting the product therein.
Alternatively, a sheet of the material may be utilized to overwrap the
product. These packaging methods are all well known to those of skill in
the art.
When a material is of the heat-shrinkable type (i.e. oriented), then after
wrapping, the enclosed product may be subjected to elevated temperatures,
for example, by passing the enclosed product through a hot air tunnel.
This causes the enclosing heat shrinkable film to shrink around the
product to produce a tight wrapping that closely conforms to the contour
of the product. Such heat-shrinkable films are especially desirable for
packaging food products such as cheese and meat. As stated above, the film
sheet or tube may be formed into bags or pouches and thereafter utilized
to package a product. In this case, if the film has been formed as a tube
it may be preferable first to slit the tubular film to form a film sheet
and thereafter form the sheet into bags or pouches. Such bags or pouches
forming methods, likewise, are well known to those of skill in the art.
Alternative methods of producing films of this type are known to those in
the art. One well-known alternative is the method of forming a multi-layer
film by an extrusion coating in combination with an extrusion or
coextrusion process as was discussed above. In extrusion coating a first
tubular layer or layers is extruded and thereafter an additional layer or
layers is simultaneously or sequentially coated onto the outer surface of
the first tubular layer or a successive layer.
The above general outline for manufacturing of films is not meant to be all
inclusive since such processes are well known to those in the art. For
example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443;
3,741,253; 4,048,428; 3,821,182; and 3,022,543. The disclosures of these
patents are generally representative of such processes and are hereby
incorporated by reference.
Many other process variations for forming films are well known to those in
the art. For example, conventional thermoforming or laminating techniques
may be employed. For instance, multiple substrate layers may be first
coextruded via a blown bubble tube with additional layers thereafter being
extrusion coated or laminated thereon, or two multi-layer tubes may be
coextruded with one of the tubes thereafter being extrusion coated or
laminated onto the other.
In the preferred embodiment as illustrated in the examples below, the
multi-layer film of the invention contains a barrier layer. The barrier
layer may be composed of a layer comprising vinylidene chloride copolymer
(PVDC, which is commonly known as saran), or composed of a layer
comprising hydrolyzed ethylene-vinyl acetate copolymer (EVOH), preferably
hydrolyzed to at least about 50%, most preferably to greater than about
99%, or composed of both a layer comprising vinylidene chloride copolymer
and a layer comprising EVOH. When the barrier layer is composed of a layer
comprising EVOH, the mole percent of vinyl acetate prior to hydrolysis
should be at least about 29%, since for lesser amounts the effectiveness
of the hydrolyzed copolymer as a barrier to fluids such as gas is
substantially diminished. It is further preferred that the barrier
copolymer have a melt flow being generally compatible with that of the
other components of the multi-layer film, preferably in the range of about
3-10 (melt flow being determined generally in accordance with ASTM D1238).
The gas of main concern is oxygen and transmission is considered to be
sufficiently low, i.e. the barrier material is relatively gas impermeable,
when the transmission rate for the barrier material is below 70 cc/m.sup.2
/mil thickness/24 hours/atms, as measured according to the procedures of
ASTM Method D-1434. The barrier layer of this multi-layer barrier shrink
film embodiment of the present invention has a transmission rate below
this value. EVOH can be advantageously utilized in the film of the
invention since irradiative high energy electron treatment of the fully
coextruded film does not degrade an EVOH barrier layer, as could be the
case for a vinylidene chloride copolymer barrier layer.
When as further discussed below, a vinylidene chloride copolymer (PVDC) is
employed instead of or together with EVOH as the barrier layer, then the
irradiation preferably should take place prior to application of the saran
layer to avoid degradation thereof. This application may be achieved by
well known extrusion coating methods, as discussed above. More
particularly, the extrusion coating method of film formation is preferable
to coextruding the entire film when it is desired to subject one or more
layers of the film to a treatment which may be harmful to one or more of
the other layers. Exemplary of such a situation is a case where it is
desired to irradiate with high energy electrons one or more layers of a
film containing a barrier layer comprised of one or more copolymers of
vinylidene chloride (i.e. saran), such as of vinylidene chloride and vinyl
chloride or such as of vinylidene chloride and methyl acrylate. In other
words, the barrier layer includes a saran layer in addition to or instead
of an EVOH layer. Those of skill in the art generally recognize that
irradiation with high energy electrons is generally harmful to such saran
barrier layer compositions, as irradiation may degrade and discolor saran,
making it turn brownish. Thus, if full coextrusion followed by high energy
electron irradiation of the multi-layer structure is carried out on a film
having a barrier layer containing a saran layer, the irradiation should be
done at low levels with care. Alternatively, this situation may be avoided
by using extrusion coating. Accordingly, by means of extrusion coating,
one may first extrude or coextrude a first layer or layers, subject that
layer or layers to high energy electron irradiation and thereafter
extrusion coat the saran barrier layer and, for that matter,
simultaneously or sequentially extrusion coat other later layers (which
may or may not have been irradiated) onto the outer surface of the
extruded previously irradiated tube. This sequence allows for the high
energy electron irradiative treatment of the first and later layer or
layers without subjecting the saran barrier layer to the harmful
discoloration effects thereof.
Irradiative cross-linking may be accomplished by the use of high energy
electrons, ultra violet rays, X-rays, gamma rays, beta particles, etc.
Preferably, electrons are employed up to about 20 megarads (MR) dosage.
The irradiation source can be any electron beam generator operating in a
range of about 150 kilovolts to about 6 megavolts with a power output
capable of supplying the desired dosage. The voltage can be adjusted to
appropriate levels which may be for example 1,000,000 or 2,000,000 or
3,000,000 or 6,000,000 or higher or lower. Many other apparatus for
irradiating films are known to those of skill in the art. The irradiation
is usually carried out at a dosage between about 0.5 MR and about 20 MR,
with a preferred dosage range of about 1 MR to about 12 MR. Irradiation
can be carried out conveniently at room temperature, although higher and
lower temperatures, for example, 0.degree. C. to 60.degree. C. may be
employed.
In the Examples below the multi-layer films were made by combining tubular
coextrusion (colloquially called the hot blown bubble technique) with
extrusion coating to achieve an oriented (heat-shrinkable) film. A tubular
process was utilized wherein a coextruded tube of a two layer substrate
core (a first sealing layer and a second layer) was extrusion coated with
third layer of saran and another fourth layer simultaneously, then the
four layer structure was cooled and collapsed, and then reheated and
biaxially stretched in the transverse direction and in the longitudinal
machine direction via inflating the tube with a bubble. Then the stretched
bubble was cooled and collapsed, and the deflated film wound up as
flattened, seamless, tubular film to be used later to make bags, overwrap,
et cetera (with layer 4 as the package outside and sealing layer 1 as the
package inside). As illustrated in the Examples below, the deflate speed
(also referred to in the art as the "racking speed") i.e. how fast the
oriented bubble of film can be cooled and collapsed, was significantly
improved, i.e. the orientation processing was increased, when the film
both (1) contained a pro-rad agent, and (2) had been irradiated.
In the Examples below that involve irradiation, prior to the coating of the
saran layer and the additional layer, the two-layer substrate was guided
through an ionizing radiation field; for example, through the beam of an
electron accelerator with layer 2 being beam-side to receive a radiation
dosage in the range of about 4.5 megarads (MR). Thus, when sealing layer 1
of a multi-layer film contains no pro-rad but layer 2 of the film contains
pro-rad, then the sealing layer of the film will be cross-linked less by
the same beam dosage than the second layer of the film will be. The second
layer will receive an "effective" higher amount of cross-linking because
of the presence of both the pro-rad and the electron beam irradiation. If
there were no pro-rad present, then for layer 2 to receive that higher
amount of cross-linking, a greater dosage of electron beam irradiation
would have to be employed. That would mean sealing layer 1 would also be
receiving that greater dosage which could interfere with its sealing
properties.
DEFINITIONS
As used herein the term "extrusion" or the term "extruding" is intended to
include extrusion, coextrusion, extrusion coating, or combinations
thereof, whether by tubular methods, planar methods, or combinations
thereof.
An "oriented" or "heat shrinkable" material is defined herein as a material
which, when heated to an appropriate temperature above room temperature
(for example 96.degree. C), ill have a free shrink of about 5% or greater
in at least one linear direction.
Unless specifically set forth and defined or otherwise limited, the terms
"polymer" or "polymer resin" as used herein generally include, but are not
limited to, homopolymers, copolymers, such as, for example block, graft,
random and alternating copolymers, terpolymers, etc. and blends and
modifications thereof. Furthermore, unless otherwise specifically limited
the term "polymer" or "polymer resin" shall include all possible molecular
configurations of the material. These structures include, but are not
limited to, isotactic, syndiotactic and random molecular configurations.
The term "polyethylene" as used herein, which "polyethylene" is a type of
polyolefin that may be employed in the film of the invention, refers to
families of resins obtained by substantially polymerizing the gas
ethylene, C.sub.2 H.sub.4. By varying the comonomers, catalysts and
methods of polymerization, properties such as density, melt index,
crystallinity, degree of branching, molecular weight and molecular weight
distribution can be regulated over wide ranges. Further modifications are
obtained by other processes, such as halogenation, and compounding
additives. Low molecular weight polymers of ethylene are fluids used as
lubricants; medium weight polymers are generally miscible with paraffin;
and the high molecular weight polymers are resins generally used in the
plastics industry. Polyethylenes having densities ranging from about 0.900
g/cc to about 0.935 g/cc are called low density polyethylenes (LDPE) while
those having densities from about 0.935 g/cc to about 0.940 g/cc are
called medium density polyethylenes (MDPE), and those having densities
from about 0.941 g/cc to about 0.965 g/cc and over are called high density
polyethylenes (HDPE). The older, classic low density types of
polyethylenes are usually polymerized at high pressures and temperatures
whereas the older, classic high density types are usually polymerized at
relatively low temperatures and pressures.
The term "linear low density polyethylene" (LLDPE) as used herein, for a
type of polyethylene which may be employed in the film of the invention,
refers to the newer copolymers of a major amount of ethylene with a minor
amount of one or more comonomers selected from C.sub.3 to about C.sub.10
or higher alpha olefins such as butene-1, 4-methyl pentene-1, hexene-1,
octene-1, etc. in which the molecules thereof comprise long chains with
few side chains or branched structures achieved by low pressure
polymerization. The side branching which is present will be short as
compared to non-linear polyethylenes. The molecular chains of a linear
polymer may be intertwined, but the forces tending to hold the molecules
together are physical rather than chemical and thus may be weakened by
energy applied in the form of he | | |
