This invention is in the field of thermoplastic polymeric
films which may be used for the packaging of products, especially food products
such as meat and cheese. More specifically, the invention is in the field of polymeric
thermoplastic films that are thermoformable i.e., may be softened through
the application of heat, distorted to a desired shape, and cooled.
Polymeric films are widely used in the packaging field
for the packaging of products, especially food products. Films used for the packaging
of food generally contain multiple layers, in which each layer adds certain desired
physical or chemical properties to the completed film. For example, an "oxygen barrier
layer" serves to protect the packaged food from spoiling. Oxygen barrier properties
are necessary to protect many packaged foods over an extended shelf life. Additionally,
an "abuse layer" may serve to protect the packaged product from physical abuse or
stresses caused by the normal handling of the product during packaging, shipping,
or during commercialization.
Although each of the layers of a polymeric film serve a
particular purpose, the film itself must be stable, and the layers must remain together
and not delaminate.
One polymer typically used as a barrier layer is ethylene
vinyl alcohol copolymers ("EVOH") which are also known as saponified or hydrolyzed
ethylene vinyl acetate copolymers. In addition to its desirable properties as a
barrier to oxygen and other gases, EVOH is also an effective barrier to odors, fragrances,
solvents and oils. EVOH also has good properties of processability, i.e.
in comparison with other polymers it is relatively easy to process into a layer
of a multilayer film. EVOH and EVOH copolymers are generally classified according
to ethylene content, for example by mole percent. Typically, as the percentage of
ethylene increases in relatively low humidity applications, the gas barrier properties
decrease, moisture barrier properties improve, and the resin is more processable.
At higher levels of humidity, for example at levels that are common for the packaging
of meat, of from 90% to 92% humidity, higher ethylene content results in an increase
in the moisture barrier properties.
However, there are in general some difficulties with the
use of EVOH copolymer in polymeric films. For example, in comparison with other
resins EVOH copolymers have poor impact resistance, poor flex crack resistance,
and poor drawability.
In order to protect the EVOH layer, which is as previously
noted poor in impact and flex crack resistance, additional layers must be added
to cover the EVOH layer. One such layer which may be used to protect the EVOH layer
is a layer comprising a polyamide. A polyamide is a high molecular weight polymer
having amide linkages along the molecular chain structure. Nylon polyamides, which
are synthetic polyamides, have favorable physical properties of high strength, stiffness
and abrasion resistance.
It is known in the art of making polymeric films to construct
a multilayer film with a barrier layer of EVOH copolymer sandwiched by layers comprising
a nylon polyamide. The following patents are of interest in illustrating the prior
art polymeric films having a core layer of EVOH between two layers of a nylon.
U.S. Patent No. 4,284,674, to Sheptak
, teaches a polymeric thermal insulation product with EVOH core layer adhered
on each side to a nylon layer. The nylon layer is adhered to another layer of a
polyolefin, such as an oriented polypropylene.
U.S. Patent No. 4,355,721, to Knott
, teaches at column 5, lines 44 to 52, a multilayer polymeric film for
food packaging having a core barrier layer of EVOH sandwiched between two nylon
layers. An adhesive layer and HDPE sealant layer are disposed on one side of one
of the nylon layers.
U.S. Patent No. 4,640,852, to Ossian
, discloses a multilayer polymeric film with a core layer of EVOH sandwiched
between two nylon layers. The film may have additional fourth and fifth layers of
an adhesive and a heat sealable polymer, disposed on one side of the nylon layers,
and sixth and seventh layers of an adhesive and a LLDPE or propyleneethylene copolymer
disposed on the other nylon layer.
U.S. Patent No. 4,695,491, to Kondo
, teaches a heat shrinkable composite packaging film. The film has a core
layer of EVOH which is surrounded on one side by a first layer of a polymer having
a low permeability to hot water, and on the opposing side an outermost layer of
antiblocking polymer. The outermost layer may be a nylon.
U.S. Patent No. 4,729,926, to Koteles
, discloses at column 4 a polymeric film having the following structure:
- tie/nylon/EVOH/nylon/tie/LDPE/primer/outer layer The outer layer may be any
of various polymeric materials that are suitable barrier materials.
U.S. Patent No. 4,735,855, to Wofford
, teaches a seven-layer thermoformable polymeric laminate having the following
The sealant layer may be any of various sealants, including an ionomer. The abuse
layer also serves as a moisture barrier layer.
- abuse layer/tie/nylon/EVOH/nylon/tie/sealant
U.S. Patent No. 4,746,562, to Fant
, discloses a seven-layer polymeric film having the following layer structure:
Each of the outer LLDPE layers also comprises an antiblocking agent.
U.S. Patent No. 4,755,419, to Shah
, discloses an oxygen barrier oriented seven layer heat-shrinkable film
of the following layer structure:
The blend used for the outer layers may be a blend of LLDPE, LMDPE and EVA. Alternatively,
the outer layers may comprise a blend of ethylene propylene copolymer or polypropylene.
U.S. Patent No. 4,788,105, to Mueller
, teaches an oxygen barrier film which is adhered through use of an adhesive
layer to a second film comprising a nylon. The film may also comprise an LLDPE outer
U.S. Patent No. 4,816,304, to Nohara
, teaches a multilayer gas barrier vessel with a core layer of EVOH sandwiched
between two nylon layers, an outer layer of polyester, and an inner layer of polyester.
U.S. Patent No. 4,818,592, to Ossian
, teaches a core layer of EVOH sandwiched between two nylon layers. The
film may also include a fourth layer of an adhesive and a fifth layer of a heat
sealable polymer. In an alternative embodiment the film may also include a sixth
layer of an adhesive and a seventh layer of a heat sealable polymer disposed on
the fifth layer.
U.S. Patent No. 4,833,696, to Iwanami
, discloses a laminate with improved flex crack resistance, drawability
and modality, and excellent gas impermeability. The laminate comprises a first layer
of an EVOH copolymer and a thermoplastic polyester, and a second layer of a composition
consisting of a group including nylon.
U.S. Patent No. 4,855,178, to Langley
, discloses a chemical barrier fabric in which a fabric material is laminated
to a multilayer film of a layer of EVOH sandwiched between two layers of nylon.
A heat-sealable polyethylene layer is disposed on the outside of the sheet material.
U.S. Patent No. 4,909,726, to Bekele
, teaches a multilayer polymeric film for chub packaging of the following
The heat seal layer may be selected from the group consisting of ethylene alpha-olefin
copolymer, LDPE, and ethylene ester copolymer, and may include an antiblocking agent.
The abuse layer may be a very low density polyethylene or alternatively may be an
- heat seal/abuse layer/tie/nylon/EVOH/nylon/tie
U.S. Patent No. 4,983,431, to Gibbons
, teaches in figure 3 a five-layer polymeric film which is laminated to
a substrate such as paperboard. The film has the following layer structure:
One ionomer layer is laminated to the substrate while the second ionomer layer is
coated with a layer of LDPE.
U.S. Patent No. 4,937,112, to Schirmer
, teaches a multilayer blown polymeric film for use in chub packaging.
The film has a first outer layer of a heat sealable polymeric resin, such as LLDPE,
a first interim layer comprising a polymeric material of high molecular weight such
as LDPE, HDPE or EVA, a second interim layer comprising a nylon, and a second outer
layer comprising a self-weldable polymeric material.
U.S. Patent No. 4,999,229, to Moritani
, teaches a multilayered polymeric gas barrier film with an intermediate
layer of a composition having 50 to 97 weight percent EVOH and 45 to 3 weight percent
nylon. As disclosed in Example 1, a layer of nylon may be disposed in contact with
the intermediate layer.
U.S. Patent No. 5,068,077, to Negi
, teaches a multilayer polymeric film with a barrier layer of from 70 to
95 weight percent EVOH and from 5 to 30 weight percent nylon. The barrier layer
may be sandwiched between two layers of nylon.
U.S. Patent No. 5,194,306, to Blatz
, teaches a polymeric blend of a major portion of an amorphous nylon and
a minor portion of EVOH for use as a gas barrier in a multilayer polymeric film.
In an alternative embodiment at column 6, lines 8 to 11, the invention may comprise
a two layer structure with one layer of substantially EVOH and one layer of substantially
amorphous nylon- At column B, example 20 discloses a three layer structure comprising
a core layer of EVOH sandwiched between two nylon layers.
DE-A-41 30 486
discloses a 5-layered coextruded biaxially stretched casing having at
least 3 polyamide layers.
SUMMARY OF THE INVENTION
It has now been found that a clear multilayer polymeric
film of a unique structure, including a core layer of EVOH sandwiched between two
nylon layers, or two nylon inner layers and a nylon outer layer, represents an improved
multilayer thermoformable polymeric film. The films of the invention are coextruded
and then blown into a tubular shape. The films are then cooled by the method of
The films of the invention have improved physical properties
of "snap back" or "memory" over prior art films. The improved physical properties
mean that after the film is used in the packaging of a product the film shrinks
or tightly wraps around the product. The films of the invention create a much tighter
package than prior art films, and retain tightness over a longer period of time
than prior art films.
The films also have improved properties of gloss, and higher
clarity and lesser haze than the prior art. The resulting film of the invention
produces a packaging material having an improved structure and appearance. The advantages
of the films of the invention are especially apparent in thermoforming applications.
The improved physical properties and appearance of the
films of the invention are believed to result from the water quenching method in
which the films are made. The films are cooled by the application of air as the
coextruded film leaves the die. After air cooling the film is cooled by the application
of water in direct contact with the film.
The film comprises first and second core layers of nylon.
The nylon layers preferably comprise from 5 to 35 percent by weight of an amorphous
nylon copolymer that is blended with one or more of various other nylons. Each of
the first and second nylon layers may also comprise a nucleating agent.
Disposed between the first and second layers of nylon is
a third layer of an adhesive or tie resin. The adhesive resin may be a anhydride
modified polyolefin, such as an EVA-based or LLDPE-based adhesive, or any of the
various other polymeric adhesives commonly used in the art of making multilayer
films. Fourth and fifth layers of the adhesive are disposed in contact with the
first and second layers of nylon, respectively.
A sixth outer layer of a nylon is disposed in contact with
the fourth layer of an adhesive. Similar to the first and second layers of nylon,
the sixth layer preferably comprises from 5 to 35 percent by weight of an amorphous
nylon copolymer that is blended with one or more of various other nylons, and may
include a nucleating agent and an antiblocking agent.
The seventh layer of the film, the sealant layer, preferably
comprises a blend of LLDPE and LDPE. The sealant layer may also comprise EVA, the
linear polyethylene ULDPE, EMA, EAA, EMAA, an ionomer, or blends of any of these
Said thermoformable, multilayer polymeric film does not
contains an EVOH core layer.
The films of the invention may be of any thickness. A preferred
range of thickness is from 50,8 to 254 µm (2 to 10 mils), and a most preferred
range is from 63,5 to 190 µm (2.5 to 7.5 mils).
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE INVENTION
- Fig. 1
- depicts an embodiment of the films of the invention; and
- Fig. 2
- depicts a method of making the films of the invention.
Figure 1 depicts an embodiment of the films of the invention.
In this embodiment, the film does not contain a core layer of EVOH.
In its embodiment, the film may have any thickness, and
is preferably of a thickness of from 50.8 to 254 µm (2 to 10 mils), most preferably
from 63.5 to 190µm (2.5 to 7.5 mils).
The film has layers 21 and 22 of a nylon, preferably an
amorphous nylon copolymer that is blended with a nucleated nylon homopolymer, and
more preferably from 5 to 35 percent of an amorphous nylon copolymer. The amorphous
nylon copolymers of layers 21 and 22 are blended with various other nylons.
An amorphous nylon copolymer is a particular type of nylon
polymer that differs from crystalline or semicrystalline nylons. Amorphous nylon
copolymers are characterized by a lack of crystallinity, which can be shown by the
lack of an endotherm crystalline melting point in a Differential Scanning Calorimeter
("DSC") test ASTM D-3417.
A lesser degree of crystallinity indicates the ease with
which a film can be further fabricated, such as by thermoforming, orienting in the
solid state, laminating, or the like. The degree of crystallinity also correlates
to the brittleness of the film, and therefore the film's tendency to break or crack
when subjected to physical or thermal shock. Physical or thermal shocks generally
occur during further converting operations, such as in thermoforming and in the
handling involved in fabricating packages, in filling and sealing them, and in shipping
the filled and sealed packages in the product distribution system.
An amorphous nylon copolymer is generally a semicrystalline
polymer that is made amorphous by rapid quenching of the melt, thereby preventing
the development of a crystalline structure and producing a transparent solid. Alternatively,
chemical modifications can be made to the chemical backbone of the polymer in order
to significantly reduce or eliminate the ability of the polymer chains to organize
in an orderly, crystalline arrangement without need for rapid quenching of the melt.
It is known in the art of making polymeric films that amorphous
nylon copolymers, like other nylons, are ineffective as moisture barrier layers.
An example of an amorphous nylon copolymer that is suitable
for use in the films of the invention is Grivory® G21, manufactured
by EMS - American Grilon, Inc. of Sumter, South Carolina. Grivory®
G21 has a glass transition temperature by the DSC method of approximately 125°
C; a specific gravity of approximately 1.18, as measured by ASTM D792; a moisture
absorption after 24 hour immersion of 1.29%, as measured by ASTM D570; and a melt
flow index of 90 ml./10 min., as measured by DIN 53735.
Grivory® G21's physical properties include
a tensile strength of 10,400 psi (72 MPa), as measured by ASTM D638; 15% elongation
at break, as measured by ASTM D638; a flexural strength of 17,300 psi (119 MPa)
at ASTM D790; a flexural modulus of 416,000 psi (2868 MPa) at ASTM D790; and a Shore
hardness of 80 D-Scale.
In a preferred embodiment, the amorphous nylon copolymer
of layers 21 and 22 is blended with a nucleated nylon homopolymer.
In a preferred embodiment, a nucleating agent is added
to the amorphous nylon copolymer, or the amorphous nylon copolymer is blended with
a nucleated nylon homopolymer. In a more preferred embodiment, the nucleated homopolymer
is a high viscosity nucleated homopolymer, with a viscosity of approximately 120
or above. One such nucleating agent is a high viscosity homopolymer such as 3909
FN, manufactured by Allied Signal Chemical Company.
Disposed between layers 21 and 22 is layer 20 of a polymeric
adhesive. Additionally, disposed in contact with layers 21 and 22 are layers 23
and 24 of a polymeric adhesive. Layers 20, 23 and 24 may be any of various polymeric
adhesives commonly used in the art, for example an anhydride grafted polyolefin
Layer 25 comprises a nylon outer layer, which preferably
may comprise an amorphous nylon copolymer. Like layers 21 and 22, in a preferred
embodiment layer 25 comprises from about 5 to about 35 percent of the amorphous
nylon copolymer. The amorphous nylon copolymer of layer 25 is blended with various
other nylons. In a preferred embodiment, the amorphous nylon copolymer is blended
with a nucleated nylon homopolymer. The layer may also include an antiblocking agent,
including antiblocking agents which are common in the art of making polymeric films,
such as inorganic spheres (especially those derived from a combination of silica
and aluminum), talc, diatomaceous earth, silica, calcium carbonate, or other particulate,
or combinations of any of these agents. The antiblocking agent serves to roughen
the surface of the film, thereby lowering the coefficient of friction between films.
The antiblocking agent of layer 25 may be contained in a nylon carrier composition.
One such type of carrier composition is disclosed in commonly
U.S. Patent No. 5,109,049
, the disclosure of which is incorporated herein by reference. As disclosed
in the referenced patent at column 2, line 61 to column 3, line 6, nylon carrier
compositions are especially effective when used in forming an outer layer of a multilayer
packaging film. In a preferred arrangement, the nylon carrier composition is used
in an outer layer and a second outer layer is capable of forming a heat seal.
In one type of film made with the nylon carrier composition,
a polymeric material comprising 20 to 85 percent nylon is combined with 80 to 15
percent of a nylon carrier composition including antiblocking agent. The nylon carrier
composition, which may comprise any of various nylons including nylon 6, nylon 6,6
or nylon 6,66, is from 35 to 80 percent antiblocking agent and 65 to 30 percent
Alternatively, the film of the nylon carrier composition
may be a blend of 95 to 99.5 percent of the nylon polymer and 5 to 0.5 percent of
an additive concentrate, in which the additive concentrate includes the antiblocking
agent incorporated into a second nylon polymer composition.
A particular antiblocking agent that is preferred for including
in the nylon carrier composition is an antiblock agent manufactured by Zeelan Industries
of St. Paul, Minnesota, under the name "Zeeospheres". Zeeospheres are inorganic
ceramic spheres comprising silica and alumina. For example, zeeospheres are incorporated
as the antiblocking agent in the nylon carrier composition Reed Spectrum antiblock
Layer 25 is not a moisture barrier.
In a preferred version of the embodiment, each of the nylon
layers 21, 22 and 25 comprise a blend of amorphous nylon copolymer and a nucleated
homopolymer of nylon 6. In a more preferred embodiment, only two of the layers,
most preferably layers 22 and 25, comprise the blend of an amorphous nylon copolymer
and a nucleated nylon homopolymer.
Layer 26 is a sealant layer which is capable of forming
a heat seal with various other polymeric materials. Layer 26 may comprise any of
the various polymers used in a sealant layers, such as LLDPE, LDPE, EVA, EMA, EMAA,
an ionomer, or a blend of any of these polymers. A preferred sealant for this embodiment
is a blend of LLDPE and LDPE.
The films of the invention may be manufactured by any of
various methods common in the art of making polymeric films. Preferably, the films
of the invention are melt coextruded in a multilayer coextrusion die according to
any one of the layer structures of the various embodiments and then formed in the
general manner taught by expired
U.S. Patent No. 3,337,663, to Taga
, and expired
U.S. Patent No. 3,090,998, to Heisterkamp
, both of which are hereby incorporated by reference. The films are coextruded
in a tubular form, wherein the first outer layer of the finished film is the inner
layer of the tubular form or tube. The tube is then inflated by the admission of
air, cooled, collapsed, and wound up to form a finished roll or rolls.
Figure 2 depicts a preferred method of manufacturing the
films of the invention. A multilayer film according to any of the various embodiments
of the invention is coextruded through orifice 72 of die 71 as molten thermoplastic
material 75. The molten thermoplastic material 75 is pulled down through collapsing
shield 80, in the direction of arrows A to B, by nip rollers 73 and 74. Roller 73
turns upon its axis in a clockwise direction, while roller 74 turns upon its axis
in a counterclockwise direction. Molten thermoplastic material 75 is expanded into
a tubular form or "bubble" by inflation caused by a volume of gas injected through
gas outlet 76. Gas outlet 76 is located in die 72 such that the exhausting air inflates
Air rings 77 are located externally and/or internally of
the bubble. As the molten thermoplastic material 75 is inflated into a bubble, the
application of air from air rings 77 acts to control the cooling and stabilize the
molten thermoplastic material. The air rings 77 can apply air at various temperatures
and at a range of velocities. In addition to cooling by air rings 77, thermoplastic
molten material 75 is also cooled by water rings or mandrels 78, also located both
externally and/or internally of the bubble. Like the air rings, the water rings
78 can apply water at various temperatures and at a range of velocities.
A covering 79 may append from the die to surround the bubble
as it exits the die. The length and size of the covering will effect the relative
air pressures internally and externally of the bubble.
The method of making the films of the invention as disclosed
above results in thermoplastic multilayer films with improved physical properties
over prior art films. These improvements are believed to result from the degree
of control over the crystallinity of the polymeric film achieved by this method.
The crystallinity is first controlled by the application of air though air rings
77. The temperature and velocity of the air applied through air rings 77 controls
the degree of crystallinity. Subsequently, the rapid cooling caused by the method
of water quenching effectively freezes the amount of crystallization achieved during
air cooling in the finished polymeric film.
The water-quenched films of the invention have several
improved physical properties over prior art films that are cooled by more conventional
methods. For example, the films of the invention have improved clarity and processability.