Note: Descriptions are shown in the official language in which they were submitted.
2UC)~'~~ i
F-4798-L 1
ORIF~1TED OPAQUE MULTIiAYER F~'AT SEALABLE FILM
This invention relates to a multilayer film arxi, more
particularly, to a flexible multilayer heat-sealable film especially
suited for packaging; and to a method of forming the same.
In the packaging of certain types of foods: for example,
snack foods like corn-based products, potato chips arid cookies it is
common practice to employ multilayer film having two or more
polymeric layers wherein one of the layers is known to be an
effective heat-seal layer. In the packaging process, a supply of
such a multilayer film can be shaped into a tube in a vertical
form-and-fill madiine. Marginal regions of the heat-seal layer are
brought in face-to-face relationship and heat sealed together to
form a tube. Next the pac3caging machine automatically forms both a
horizontal heat-seal to define a bag and a horizontal severance-too--.w.: ...-
detach the base of the bag from downstream. , Product is dispensed.-
into the open end of the bag; -and, finally, a second horizontal
heat-seal and severance is effected to provide a product packaged in
a tube which is heat-sealed at both ends and along a seam joining
the sealed ends. While the food or other product is being
dispensed into the package, air also is present in the package and
this air assists in protecting and cushioning the product during
subsequent shipment of the finished packages. During shipment of
the product, particularly with the larger sized bags, e.g., those
containing 0.5 kilogram (16 ounces) of product, the bags have a
terr3ency to split or burst at the end seals.
U.S. Patent No. 4,565,739 describes an oriented heat
sealable multi-layer structure including a polyolefin film
substrate. A second layer, positioned ca at least one surface of
the film substrate, includes a member selected from the group
consisting of a medium density polyethylene, a high density
pol~~ylene arxi mixtures thereof and polypropylene. A primer
- 20047p5
F-4798-L 2
coating is positioned on at least one surfare of the second layer
and, finally, a heat sealable layer is positioned on the primer
coating. The heat sealable layer includes a member of the group
consisting of an acrylic polymer layer and a vinylidene chloride
polymer layer.
U.S. Patent No. 4,377,616 discloses an opaque, biaxially
oriented polymeric film structure cauprising a thermoplastic core
matrix having a strata of voids; said voids being created by the
inclusion within the matrix material of spherical void-initiating
solid particles which are incompatible with the matrix material.
The void space occupied by the particle is substantially less than
the volume of the void. The polymer matrix material is extruded in
the form of a film and positioned on opposite surfaces of the film
are void free, transparent thermoplastic skin layers adhering to
said surface. The voids present in this structure cause light
diffraction resulting in an opalescent appearance.
The present invention seeks to provide a heat-sealable film
suited to packaging and having improv6d properties, especially
greater heat-seal capabilities and impraved~fluid transport
properties.
In accordance with the present invention, there is provided
a heat-sealable structure, preferably an oriented mufti-layer
heat-sealable structure, prising:
(a) a substrate comprising an opaque, preferably
cite, polyolefin film;
(b) a second layer ooextruded on at least ccie surface
of the substrate (a), the second layer (b) comprising a
heat-sealable tbmopolymer, coQolymer or blend thereof;
(c) a primer voating on at least one surface of the
second layer (b), the surface having been previously
treated to increase the energy of the surface; aryd
(d) a heat-sealable layer o~n the primer coating (c),
the heat-sealable layer (d) including a vinylidene
chloride copolymer o~ntaining at least 50$ by weight of
vinylidene chloride.
~~~-~'~ a
F-4798-L 3
Where layer (b) is coextruded on both surfaces of substrate (a), the
layer (bl) may either be identical to or different from layer (b).
This invention also provides a proress for producing a
heat-sealable structure of the present invention, which process
cccmprises:
(i) coextruding a substrate (a) including an opaque
polyolefin film with at least one second layer (b)
including a heat-sealable ha~polymer, copolymer
or blend thereof;
(ii) biaxially orienting the coextruded product of step
(i);
(iii) treating the coextruded product of step (i) to
increase the energy of the surface;
(iv) applying a primer coating (c) to a surface of the
second layer (b); and
(v) applying a heat-sealable layer (d) on primes
coating (e);-the heat=seaiable-iayer (d) including. w
a vinylidene dz.l.oride copolymer containing at
least 50~ by weight of vinylidene chloride.
This invention further provides the use, in a multi-layer
heat-sealable structure, of a substrate which has been opacified by
cavitation.
The polyolefin contemplated for use in the substrate (or
core) material (a) of the multi-layer, heat-sealable film structure
of this invention includes polyethylene, polypropylene, polybutene,
a copolymer or a blerx3 thereof, preferably tx~polymeric
polypropylene. Particularly preferred is an isotactic polypropylene
containing at least 80~ by weight of isotactic polypropylene. It
is also preferred that the polypropylene has a melt flow irxiex fran
4 to 8/g/10 minutes. The substrate or core layer (a) of the
present invention is opaque. Opacity can be imparted to the
substrate layer in any known manner; for example, the polyolefin
substrate can become opacified in the manner described in U.S.
Patent 4,377,616. In addition, the substrate or core layer (a) of
F-4798-L 4 .~ 2 0 0 4 7 p 5
the present invention is preferably white. The whiteness can be
i~arted to the substrate layer in any known manner. For example,
the white opaque substrate or core (a) can be made by mixing a major
proportion of polyolefin with a second material of higher melting
. point and which is in the form of mierospheres. The mixture is
heated to melt the polyolefin and the microspheres are dispersed
throughout the molten material. The mixture is then coextruded as
a core layer and the layer is biaxially oriented to create
cavitation in the layer. In addition. 1 to 3$ Q02 can be added
to impart a further degree of whiteness. In a most preferred
embodiment, the white opaque polyolefin film substrate is opacified
by cavitation.
The homopolymer, copolymer (including terpolymer) or blerx3
thereof contemplated for use in the layer (b) functions. in some way
not completely understood, to increase the overall borx3 strength
formed in FSM seals or crimp seals when employing vinylidene
chloride polymer heat seals.... Desirably,_it_may.~rise.high
- density polyethylene, medium,density polyethylene,:or.linear low
density polyethylene.
The ~lymer contemplated herein may comprise any copolymer
(which may be a random or a block copolymer) typically employed in
the manufacture of multi-layered films. For example, an ethylene
propylene copolymer, preferably an ethylene-propylene random
copolymer (which is formed by the simultaneous polymerization of the
respective monomers in admixture) can be used to form layer (b).
Effective formation of a random copolymer of ethylene and propylene
is accomplished when the ethylene is present simultaneously with the
propylene in an amount sufficient to result in from 0.5 to 10$ by
weight of ethylene in the resulting copolymer. Where the copolymer
is a block copolymer it may include from 10 to 30 weight percent of
ethylene.
The copolymer may comprise a terpolymer. The contemplated
terpolymers are comparatively low stereoregular polymexs. The
terpolymers can have a melt flow rate at 446~F ranging from 2 to 10
_ ~ 2p047p~
F-4798-L 5
grams per 10 minutes and preferably from 4 to 6 grams per 10
minutes. The crystalline melting point can range from less than
2S0~F to somewhat greater than 271~F. The terpolymers will
pred~ninate in propylene, and the ethylene and 1-butene morr.~mers can
~ be present in approximately frcm 0.3:1 to 1:1 mole percentage in
relation to eac3-i other .
The preferred base substrate layer can be homopolypropylene
having a melting point range from 321~F to 336~F. A commercially
available material of this description is FI~1 W828. * Forthe
to ooextruded skin layers, preferred terpolymers such as,
ethylene-propylene-butene terpolymers, would have melting points of
266~F and 255~F, respectively. The materials are coaanercially
available as "Chisso"* XF7500 and "Chisso"* XF 7700. These terpolymers
contain fr~n 1 to 7 wt. $ ethylene and from 1 to 7 wt. $ 1-butene.
In preparing the mufti-layer structure of the present
invention, the substrate layer (a) and layer (b) are coextruded so
that layer (b) is from 2 tow25$-of the total thiclmess of the t~
layers. For sane purposes, layer (b)-can be present on-bot~i
surfaces of a core layer of polypropylene (a) in which ~ase'both
layers (b) would amount to from 4 to 50$ of the total thickness of
the three layers. In preparing the ooextruded film it has been
fourr3 advantageous and convenient to recycle certain quantities of
scrap extrudate back into the base homopolymer polypropylene or
other polyolefin. Thus, the homr~polymer polypropylene or other
polyolefin can have from 0 to 30% of reclaimed material blended
therein.
It has been found that heat seal layers, such as vinylidene
chloride copolymer heat seal layers, do not adhere well to polyolefin
film surfares, especially polypropylene film surfaces, even when the
latter have been subjected tro well known pretreatment operatiocis
such as, for example, treatment by corona discharge, flame or
oxidizing ~emicals. The same has been fourx3 to be true in adhering
the heat seal layers contemplated hexein to the surface of layer (b).
However, it has been found that the use of a primer coating (c)
*Trademark
- 20047p5
F-4798-L 6
intermediate layer (b) and the heat seal layer provides an
unexpectedly high level of adherence.
The primer materials contemplated for enhancing the bond
between layer (b) and the vinylidene chloride copolymer heat seal
layer (d) include the reaction product of an epoxy resin and an
acidified aminoethylated vinyl polymer. The contemplated epoxy
resins are glycidyl ethers of polyhydroxy ca~unds. Typical
polyhydroxy compounds which may be used include bisphenol A; a ring
substituted bisphenol A; resorcinol; hydroquinone;
phenol-formaldehyde; a novolac resin; an aliphatic diol, such as
ethylene glycol, propylene glycol) 1,4-butaner3iol, 1,6-hexanediol;
glycerol; a lower alkyl hydantoin or a mixture thereof.
The preferred epoxy resins of the present invention are
those made by the glycidation reaction between epichlorohydrin and
bisphenol A. Epoxy resins of this type are commonly classified by
their epoxy equivalent weight (E~W) which is defined as the weight of
resin in grams which caztains one gram equivalent of epoxy groups.
Resins with an EEW ranging from 170 to 280 may be use~d.in-the--present
invention, but the- preferred- range is from 180- to 210: ~~
A variation in the c~aposition of the epoxy resin component
is one in which a hydantoin compound is substituted for the bisphenol
A. For example, 1,1-dimethyl hydantoin may be employed in a low
rmlecular weight epoxy resin since resins based on this material are
completely water soluble, thereby eliminating the necessity for
emulsification.
Although the specific structure of the epoxy resin is not
critical to the primer employed in the present invention. important
arisiderations in the selection of the epoxy resin depend upon its
physical state. For example, it must be liquid and capable of
~~g r~ily dispersed or dissolved with the secand component or
curing agent as described hereinbelow. If the epoxy resin is of
low viscosity, it may be stirred directly into the second component,
i.e., curing agent. However, it is preferred to employ the epoxy
resin in an aqueous emulsion.
.~ ,
=2047p5
F-4798-L 7
The second component in the epoxy primer compositions of the
present invention is an amino modified acrylic polymer which is
water soluble. This polymer is a curing agent for the epoxy
compound. The preferred material is described in U.S. Patent
No. 3,719,629 and may be generically described as an acidified
amino-ethylated interpolymer having pendant aminoalkylate groups.
This material is produced by polymerizing acrylate, methacrylate,
styrene or other suitable monomers with sufficient methacrylic or
acrylic acid to give a -COON content from 7.5 to 12.5$. Solvent
polymerization techniques are preferred. The polymer is then
reacted with ethyleneimine monomer and acidified with hydrochloric
acid to render the polymer water soluble.
The primer coating may be applied to the layer (b) as a
dispersion or as a solution, from an organic vehicle, for example,
an alcohol or an aromatic hydrocarbon, such as xylene or a mixture
thereof.
In one embodiment of the. present inventiai, a liquid epoxy...___
resin is emulsified in a solution.of the tying agent-by rapid =-- - ~.
stirring. Thereafter, the resultant dispersion is diluted with
water to the desired concentration for coating, which typically
includes from 2 to 25% solids.
When mixing the epoxy resin with the curing agent, it is
generally preferred to use a stoichianetric equivalent balanre of
epoxy and amine groups. However, it has been fourr3 that the
stoichiometric ratio may be varied over a wide range, for instance.
from one epoxy group to three amine groups too three epoxy grazps to
one amine group and, preferably, from one epoxy group to two amine
groups to two epoxy groups to one amine group, without seriously
affecting the product's usefulness as a primer coating.
It is to be understood that the solution or dispersion of
epoxy resin and curing agent can contain small amount of wetting
agents in order to facilitate the application of the primPx material
to the surface of layer (b). Conventional non-ionic wetting agents
which can be employed include the hexyl or benzyl ether of ethylene
A
- 20 447p5
F-4798-L 8
glycol, the hexyl ether of diethylene glycol, butyl aloahol, hexyl
alcohol. octyl alcd~ol and diaretone alcohol.
Although mixtures of the epoxy resin and the curing agents
will cross-link or cure without the necessary addition of a catalyst,
it has been found advantageous in certain instances to use an amine
catalyst. Such catalysts include propylene diamine and
hexamethylene diamine.
It is to be understood that the present multi-layer structure
is not limited to the specific class of primer compositions defined
hereinabove. Other operable primer compositions include those
defined in U.K. Patent No. 1,134,876 which discloses a primer
produced by condensing a monoaldehyde with an interpolymer of
acrylamide or methacrylamide and at least one other unsaturated
monomer. U.K. Patent No. 1,174,328 discloses a material which has
utility as a primer in the present invention. The material
described is the product resulting from condensing aminoaldehyde
with acrylamide or methacrylamide arr3, subsequently,
interpolymerizing the condensation product with at least one other
unsaturated monomer in the presence of a Oi-C6 alkanol.
A preferred primes voating resin of this type ccomprises a
cbpolymer containing up to 90% by weight of styrene, up to 80% by
weight of an alkyl acrylate, up to 15$ by weight of methacrylic acid
and from 5$ to 25% by weight of acxylamide, which has been condensed
with a solution of formaldehyde in n-butarx~l containing from 0.2 to 3
equivalents of formaldehyde for each amide group in the copolymer.
A particularly preferred primer resin is a 50% solids solution of a
copolymer resins containing 38.5 parts of styrene, 44 parts of ethyl
acrylate, 2.5 parts of methacrylic acid and 15 parts of acrylamide.
which has been condensed with 5.2 parts of formaldehyde in n-butanol.
A composition of the vinylidene chloride polymer latex which
is employed as the heat sealable top coating material layer (d) on
the primer coating (c), is not critical to the practire of the
invention. Commercially available vinylidene chloride latexes
having a vinylidene chloride content of at least 50$ and,
_v ~.Zp047p5~
F-4798-L 9
preferably, from 75% to 92$ may be employed. The other
ethylenically unsaturated oomonomers may include alpha, beta
ethylically unsaturated acids, such as acrylic and methacrylic
acids; alkyl esters containing 1 to I8 carbon atoms of said acids,
such as methylmethacrylate, ethyl acrylate and butyl acrylate. In
addition, alpha, beta ethylenically unsaturated nitriles, such as
acrylonitrile arr3 methacrylonitrile can be employed, as well as
monovinyl aromatic compounds, such as styrene and vinyl chloride.
Specific vinylidene chloride polymer latexes ccxltemplated
include 82$ by weight vinylidene chloride. 14$ by weight ethyl
acrylate and 4$ by weight acrylic acid. Alternatively, a polymer
latex cocmmrising 80$ by weight vinylidene chloride, 17$ by weight
methyl acrylate and 3$ by weight methacrylic acid can likewise be
eirq~loyed. In addition, the heat seal couposition described in
U.S. Patent No. 4,058,649 can be emplayed.
The multi-layer structure of the present invention can have
an overall thicbness within a wide range, but is preferably from 0.5
up to 3.0 mils in thic}aness ~_..
Before applying the primer caat~g.ition~-to the second
ZO layer, the exposed surface of layer (b) is treated to increase its
surface energy and therefore ensure that the coating layer (c) will
be strongly adherent thereto thereby eliminating the possibility of
the coating peeling or being stripped fran the film laminate. This
treatment can be accomplishe'3 employing )mown techniques such as,
for example, film chlorination, i.e., exposure of the film surfare
to gaseous chlorine, treatment with oxidizing agents such as chrc~ic
acid, hot air or steam treatment, or flame treatment. Although any
one of these techniques is effectively employed to pretreat the film
surfare, a particularly desirable method of treatment is the so
called electronic treatment method which comprises exposing the film
surface to a high voltage corona discharge while passing the film
between a pair of spaced electrodes. After electronic treatment of
the film surface, the coating composition is then applied thereto.
The primer dispersion to be applied to the surface of layer
(b) can have a solids concentration from 5$ to 25%.
F-4798-L 10 2 0 ~ 4 7 p 5
Zhe primer dispersion can be applied to the surface of layer
(b) utilizing standard coating techniques so that a dry coating
weight frcxn 0.0S to 0.25 gram/1,000 square inches of film is
obtained. The thus coated film can be subsequently gassed through
a hot air oven to remove caatpletely water and solvent. Thereafter,
this primer coated film can be coated with the selected vinylidene
chloride polymer latex also using standard coating techniques such
as gravure and roll coating. The vinylidene chloride polymer
coated system can then be dried by passing it through a conventional
hot air oven.
The amount of vinylidene chloride polymer applied to the
multi-layer film may be varied over a wide range depending upon the
specific properties desired in the final film product. Coating
weights frcen 1 to 6 grams per 1,000 square inches may be employed.
If a t~-sided coated film is desired, the beneficial corona treating
of the substrate coating with layer (b) the priming and the
vinylidene chloride polymer application can be repeated, either in
line or out-of-line.
the following E~cample illustrates the invention.
Known conventional coextrusion and orientation techniques
were employed in preparing a number of PVDC coated, oriented polymer
films within the scope of this invention and, for comparison
purposes, PVDC coated, oriented polymer films outside the scope of
the invention.
The PVDC coated white opaque films were tested for crimp seal
strength. The type of polymer used in the skin layer and the
percent of the core layer that the polymer represented were varied.
Crimp seals were tested skin to skin and the force needed to separate
the seals was determined.
The results are shown in the table below. It is clear from
the table that samples caztaining PVDC coated white opaque films with
ethylene propylene random apolymer, ethylene-propylene block
2~(~~-~ t~
F-4798-L 11
copolymers and ethylene-propylene butene-1 terpolymer had heat seal
strengths significantly greater than the control. In fact. the
strength of the seal in the film with ethylene propylene randan
copolymer present at 15$ of the core layer was outstanding. At the
same time, the PVDC coated white opaque films presented unusually
good appearance characteristics.
TABLE: FILM SFALA.BILITY
Skin Resin with PVDC Coating Skin $ Crimp Seal @ 260~F(g/in)
Ethylene propylene random
copolymer 5 855
Ethylene propylene random
copolymer 15 1185
Ethylene propylene block
copolymer 5 548
Ethylene-propylene-butene-1
terpolymer 5 875
Polypropylene h~mpolymer
(control) 5 360
Polypropylene homopolymer
(control) 15 380
Zhe multi-layer, heat-sealable structure of this invention
affords many highly desirable characteristics. The enhanced
sealability is especially effective for the packaging of
comparatively large quantities of product since there has been a
tendency for such packages to split or burst at the end seams. At
the same time, this packaging material is effective for the
packaging of ca~paratively small quantities of product.
In addition, the second layer provides excellent wet-out to
water based inks aryd coatings and higher bond strength between that
layer and a substrate. Furthermore, the opaque film presents an
attractive appearance while providing low light transmission. low
~~t~'~~~
F-4798-L 12
water vapor transmission rate arr3 low oxygen transmission rate since
the cavitation included therein provides no path through which
light, liquid or oxygen can traverse.
