Note: Descriptions are shown in the official language in which they were submitted.
WO90/0742? PCT/US90/00101
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Resealable Packaq~inq 7~aterial
Flexible packaging materials comprising layers of
thermoplastic polymers are used to package a variety of
products, particu~arly food. For example, cereals, potato
chips and other snack foods are packaged in such materials.
The packages are formed in packaging machines in which the
material is sealed to itself by heated sealing jaws.
However, when a package is opened by pulling apart one of the
heat seals, nolmally at the top of the package, ~he heat seal
is not reasealable without the application of heat as well as
pressure. This invention pro~ides a flexible packaging
material that is resealable by the appli~ation of pressure
alone at the a~ea of a ~eal formed by a packaging machine.
The material of this invention comprise~ a substrate
laye~, a layer of tacky adhesive adjacent the substrate
layer, and a skin layer covering the layer of tacky
adhesive. When a ~eal formed by sealing the packaging
mate~ial tc itself i5 pulled apart, the tacky adhesive is
exposed at the surface of the seal area, thereby permitting
the material to be re~ealed to itself at the area of the seal
by the application of manual pres~ure alone ~o reclose the
package.
The substrate layer may comprise any material ~hat is
conYentionally used fo~ flexible packaging aeplications, such
as pla~tic film, me~al foil, paper or a combination thereof.
Plastic film substrate~ may be ~ingle layer or multilayer and
may be oriented or unoriented. The film can be polyamide,
polye~ter, or olefin polymer or copolymer. Suitable
substrates include poly~ethylene terephthalate~ film, paper,
oriented polypropylene film, and nonwoven ma~erials.
As used herein, the term "tacky adhe~ive~ ~efers eo an
adhesive that, in the practice of this invention, forms a
re~ealable bond by the application of manual pressure alone
at room temperature. The re~eal strength is preferably
between about 0.5 and 4 pounds per inch (0.1 and 0.7 kg/cm),
morç preferably between about one and two pounds per inch
(0.18 and 0.36 kg/cm). Such adhe~ives may include cold seals
W0~0/07427 PCT/US90/00101
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and tacky lacquers. Cold seals are formed from aqueous latex
emulsions of an elastomer. The tacky adhesive is preferably
a pressure sensitive adhesive, which is a blend o~ an
elastomer and a tackifier. The pressure sensitive adhesive
preferably comprises ~rom about 40 to 80 percent of a
thermoplastic elastome~, 20 to 60 percent of a tackifier, and
up to about 15 percent of other components, ~uch as a
fragrance and conventional additives, such as an antioxidant
and an oil.
~ hen the adhesive layer in the packaging material
contains a fragrance, the fragrance is not appreciably
noticeable until the material is formed into a package and
the package is opened because the fragrance i~ e~sentially
encapsulated in the packaging material. When the package is
opened, the fragrance is liberated, thereby enhancing the
appeal of the product in the package. Best re~ults are
obtained u~ing an oil-based fragrance that is absorbed by the
elastomer component of the adhesive.
Particularly suitable elastomers include styrene
copolymers, such as styrene-butadiene copolymer~ sold under
tha trademark Stereon by Fire~tone and styrene-isoprene
copolymer~ sold under the trademark Kraton by Shell Chemical
Company. Other suitable ela~tomers include polyurethane and
ethylene-vinyl acetate copolymers having a vinyl acetate
content of about 18 percent or higher.
The tackifier may be any tackifier conventionally u~ed
with ela6tomers to for~ pres~ure ~en~itiYe adhesives.
Suitable tacki~ier~ include terpene resins, such as re~ins
sold under the trademark Zonatac by Arizona Chemical Company,
and petroleum hydrocarbon re~ing, ~uch as re~ins sold under
the trademark Es~orez by Exxon Chemical Company.
The pres~ure ~en~itive adhesive is pre~erably
~oextrudable. The coextrudabili~y of the pre~sure sensitive
adhe~ive may be enhanced by utilizing a thermoplastic
elastomer having a low melt index and by increa~ing the
proportion of the elasto~er in the adhesive. The melt index
of the adhesive iB preferably le~s than 30, more preferably
lé~s than 10, for coextrucion wi~h conventional polyolefins.
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The pressure sen~itive adhesive, o~ o~her tacky adhesive,
may be applied to the substrate by methods o~her than
extrusion, such as adhesive lamination, cotogravure,
flexographic or silk screen printing, Meyer rod, spray
coating, charged particle coating, or vacuum deposition. The
adhesive may be applied as 100 percent solids or by using
water or a solvent as a vehicle.
Since the tacky adhesive is required only in the area
where the packa~e is to be opened, the adhesive may be coated
onto the substrate in register only where it is required.
The skin layer may be similarly applied in register over the
adhesive alone, such as by printing the adhesive layer and
the skin layer onto the substrate sequentially u~ing a
central impression press. However, the ~kin layer may be
applied to other surface areas as well, including the entire
surface of the packaging material.
The tacky adhe~ive may also be applied to the sub~trate
by tran~ferring it from a carrying medium. This technique i~
commonly u~ed to transfer a thin layer of metal, ~uch as that
formed by vapor deposition, from one substate to another in
the manufac~ure of metallized packaging materials. The skin
layer may al~o be tran~ferred with the adhesive by this
technique. The carrying medium i~ ereferably paper coated
with a silicone relea~e agent. The paper substrate may be
recycled and may be an endless belt in a continuous proces~.
If the paper bearing the adhesive layer is rolled up, it is
preferably coated with the release agent on both side~ ~o
prevent blocking.
The thicknes6 of the layer of tacky adhe~ive is
preferably between about 0.1 and 2.0 mil (2.5 and 51
microns)~ more pre~erably between about 0.3 and 0.7 mil (7.6
and 18 microns).
The skin layer perform~ two critical functions. First,
it ~roduces a coefficient of friction on the surface of the
packaging material such that the material i8 machinable in a
conventional packaging machineO Second, the s~in layer
become~ broken ~o that when the ~eal i8 pulled apart, the
tacky adhe~ive i8 exposed at the surface of- the ~eal. There
W~90/07427 PCT/US90/00101
are two mechanisms by which this occurs. In one mechanism
(First Mechanism), the skin layer is disrupted by the
shearing action of the sealing jaws in the packaging machine
such that the tacky adhesive becomes present at the intec~ace
where the material is sealed to itself. The ~kin layer i5
preferably disrupted over ~o~t of the area of the seal. By
thi~ mechanism, it is po~sible to produce the initial seal
from ehe tacky adhesive alone, thereby obviating the need to
apply heat to the sealing jaws. In the other mechanism
(Second Mechanism), the skin layer is heat ~ealed to it~elf,
preferably using flat sealing jaws, and remains substantially
intact until it ruptures through to the adhesive layer when
force is initially applied to pull the heat seal apart. The
~kin layer then delaminates from the sub~trate layer when
additional force is applied until the end of ~he &eal i~
reached. The delamination may occur between the ~kin layer
and the adhesive layer, ba~ween ehe adhesive layer and the
substrate layer, or, preferably, in a plane lying in the
adhesive layer so the adhe~ive is present on both surfaces of
the seal area after the ~eal ha6 been pulled apart. The
latter type of delamination i5 promoted by heating both
fiealing jaws when the heat ~eal i~ formed.
When the Second ~echani~m i~ employed, the tacky adhesive
i~ preferably an extrudable pres~ure ~ensitve adhe~ive which
is coaxtruded with the skin layer, either with the ~ubstra~e
layer or onto the fiubstrate layer. When the ~ub~trate layer
i~ coextruded with ehe skin layer and the adhe6ive layer,
either blown film coextru~ion or ca~t film ~oextru~ion may be
employed. The extrudate may contain multiple layer~ of the
pres~ure sensitive adhe~i~e.
The skin layer may be a layer of a thermoel2stio
film-forming polymer which i~ conventionally u~ed a~ a heat
~eal layer in packaging materials. The polymer preferably
has a tensile strength such ~hat at the desired thicknesc of
the Rkin layer the heat 6eal i8 easy to open, preferably by a
force between about 0.4 and 0.9 kg/cm. The polymer also
preferably has a low elongation at ~reak, preferably less
than 400 percent. The polymes is preferably a copolymer of
PCT/U590/~0101
W~0/07~27
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s
ethylene and an ethylenically unsaturated comonomer selected
fro~ the group con~isting of carboxylic acids and esters,
salts and anhydrides thereof. Example~ of such comonomers
include acrylic acid, methacrylic acid, methyl acrylate,
ethyl acrylate, methyl methacrylate, vinyl acetate, and
maleic anhydride. Such copolymers are well known
commercially available polymer~ commonly used to form heat
~eal layers in flexible packaging materials. Other polymers,
~uch as polyolef in8, may be blended with the copolymer in a
minor amount. The copolymer is preferably an ionomer. The
term ionomer refers to a ~ilm-forming copolymer of an olefin
and an ethylenically un~aturated monocarboxylic acid wherein
at least 10% of the carboxylic acid yroup~ are neutralized by
a metal ion, preferably zinc. The olefin ha~ the formula
RCH=CH2 where R is hydrogen or an alkyl group ba~ing up to
8 carbon atoms. The olefin i~ preferably ethylene. The
carboxylic acid i~ preferably acrylic acid, methacrylic acid,
or methylmethacrylic acid. The proportion of olefin unit~ in
the ionomer i8 at lea~t 50, preferably at least 80, mole
percent. Suitable ionomer~ are described in U.S. Patent No.
3,355,319 and are available commercially under the trademark
Surlyn.
The skin layer i8 preferably relatively thin, preferably
~hinner than the heat seal layer of conventional packaging
material~ 80 the layer -fractures when the sealing jaw~ are
applied or when the seal i~ pulled apart. The thicknes~ of
the skin layer i8 preferably between about 0.05 and 0.5 mil
(1.3 and 13 micron~), more preferably between about 0.075 and
0.2 mil (1.9 and 5 micron~).
Nhen the First Mechanism i~ employed, the ~kin layer may
be any substance that produce~ the desired coefficient of
~riction and that is sufficiently disrupted by the sealing
~aw~ that the adhesive becomes present at the ~eal
interface. The substance may even be finely divided solid
particulate matter (dust), such as talc or starch particles.
The particles may be applied to the ad~e~ive layer in
accordance with ~onventional ~ethods, such as by using Oxydry
(trademar~) apparatus.
WO90/07~27 PCT~S90/00101
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The ~kin lay~r and the layer o~ pressure sen~itive
adhe~ive may be coextruded together generally in accordance
with conventional coextrusion coating methods to form the
packaging maeerial of the invention. The components of the
~ressure sensitive adhesive are preferably maintained under
an inert atmosphere, such as nitrogen, to minimize oxidation
at the extru~ion temperature, which produces degradation and
odor.
The packaging material may be sealed in accordance wi~h
methods typically used for sealing flexible packaging
ma~erials in packaging machines. When the First Mechani~m is
employed, the packaging material is preferably sealed by
means of sealing jaws that impart shear to the skin layer,
~uch a~ desc~ibed in U.S. Patent 4,5B~,555. The shear
promotes fracture of the skin layer and exposure of the tacky
adhesive when the ~eal i~ opened. When the Fir~t Mechanism
is employed, the temperature of the sealing jaws is
preferably less than the melting point of the skin layer. In
some embodiment~, the jaws may be at ambient temperature so
the material may effectively be ~cold sealed" rather than
heat sealed. When the Second Mechanism i~ employed, the skin
layer is heat sealed to itself at a conventional temperature
of about 230F (110C). In either mechani~m, the seal has a
bond strength that is sufficient to en~ure the package does
not open during normal handling. This bond ~trength is
preferably at least aboue two pounds per inch (0.4 kg~cm).
However, the seal i8 alBo preferably easy to open by being
pulled apart manually, so the bond ~trength is preferably
les6 than about five pounds per inch (0.9 kg/cm).
Un~il the skin layer i~ fractured, the coefficient of
friction of the ~kin layer is determined by the subs~ance
forming the ~kin layer. Accordingly, the material of thi6
invention may be u~ed in conventional packaging machine~,
such as form, fill and seal machines, in the same manner as
conventional flexible packaging ma~erials. A material having
a tacky adhe~ive- on its surface could not be used in such
machines, of course, becau~e of the high coefficient of
friction of the adhe6ive. The ability of a packaging
WO90/07.J27 PCT/U~90/00101
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matsrial to be used in packaqing ma~hine~ is referred to in
the art as machinability. ~he material of this in~ention has
good machinability. In pre~erred embodiments, each surface
of ~he mate~ial has a coe~ficient of ~iction against
~tainless steel which is within the preferred range for
machinability, which i6 from about 0.1 to 0.5.
Example 1
A laminate was formed by coextruding a skin layer and a
pressure sensitive adhesive onto a substrate. The sub6trate
was a polytethylene terephthalate) film haYing a thickne~s of
12.2 microns (Dupont Mylar). The skin layer, which was
extruded at 218C, consisted of polyEtyrene (Dow Styron
625). The pressure senEitive adhe~ive, which was extruded at
about 163C, consisted of 68 percent ~tyrene-isoprene-~tyrene
block copolymer (Shell Kraton D-1107~, 30 percent aliphatic
pe~soleum hydrocarbon re~in (Exxon Escorez 1310), and 2
percent phenolic antioxidant (Ciba-Geigy Irganox 1010). The
hydrocarbon re6in was injected into the ba~rel of the
extruder. The thick~esR of the polyztyrene laye~ wa~ about
0.1 mil (2.5 microns) and the thickne~s of the layer of
pressure ~encitive adhe~ive wa~ about 0.5 mil (12.7
microns). The laminate was heat sealed to itself u~i~g the
heat seal jaws described in U.S. Patent 4,582,555 by applying
a pressure of BO p8i ( 5.6 kg/cm) for two ~econd~ at a
temperature of 110F. A for~e of 3.3 pounds per in~h width
(0.59 kg/cm) wa~ required ~o open the heat ~eal. The opened
heat seal wa6 then ~esealed, opened and resealed again twenty
time6 by applying a pre~sure of 14 p~i (0.99 kg/sq.em) at
room temperature for les~ than 0.1 secoAd. The a~erage
regeal stre~gth was 1.03 pound~ per inch width ~184 gram~cm).
~ his example illustrate~ thae the paekaging material of
this in~ention is e~peGially suitable for packaging cereal~
and snack foods in packages that may be easily resealed after
each intermittent use.
.
PCT~US90/0010l
WO90/07427
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Example 2
A lamina~e was formed by applying a coa~ing of molten
pressure sensitive adhesive onto a substrate, cooling the
adhe6ive, and then applying a coating of polyvinylidene
chloride (PVDC) emulsion on top of the adhesive. The
coatings were applied using a Meyer rod. The ~ub~trate was
poly(ethylene terephthalate) film ha~ing a thickness of one
mil (25 microns). The pres~ure sensitive adhesive wa~ heated
to about 150C and applied at a coating weight of about 14
pounds per ream (23 grams per square meter). The pres~ure
sensitive adhe~ive, which is commercially available from
Findley Adhesives, has a vi~co~ity at 150C of about 42,000
centipoise. The PVDC was applied at a coating weight of
about 4 pounds per ream t7 gram~ per square me~er) and dried
at about 93C. The lamina~e was t~en sealed to itself using
sealing jaws that impart high shear. The jaw~ were heated to
71C and applied at a pres6ure of 80 p~i (5.6 kg/sq.~m). A
force of ~.2 pounds per ineh (0.57 kg/cm) was required to
open the seal. The opened 6eal was then re~ealed, opened and
re~ealed 20 time~ by applying a pressure of 14 psi tO.99
kg/6q . cm) at room temperature for le~ than one second . The
average re~eal strength wa~ 0.53 pound~ pPr inch (95 grams
per cm).
The Example illu~rate~ the preparation of packaging
material of thi~ invention employing the First Mechani~m.
The material may be u6ed to package cereals and snack foods
in packages that may be easily re~ealed after each
intermittent use.
ExamPle_3
A laminate was formed by coextruding a ~kin layer and a
layer of pre~sure sensitive adhegive onto a film of high
density polyethylene having a thieknezs of two mils (51
microns). The skin layer, which wa~ extruded at 230C,
consist2d of an ionomer available co~mercially from Dupont.
The pres~ure sensitive adhesive, which wa~ extruded at 196C,
i8 a~ailab-le commercially ~rom Findley Adhesive~. The
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adhesive ha~ a viscosity of over one million centipoises at
177C. The basis weight of the adhesive was S pounds per
ream (8 grams per square meter). The basis weight of the
skin layer was 4 pounds per ream (7 grams per ~quare meter).
The laminate was then heat sealed to itself using flat
6ealing jaw~ applied at a pressure of 40 psi (2.~ kg/cm) for
one second, with one jaw being at room temperature and the
other jaw at 116C. A force of 2.4 pounds per inch (0.43
kg/cm) was required to open the heat seal. The opened heat
~eal was then resealed, opened and resealed again 20 times by
applying a pres~ure of 14 p8i (O. 99 kg/sq.cm) at room
temperature for less than one second. The average reseal
strength was 1.03 pounds per inch (184 grams per cm).
Thi Example illustrate~ the preparation of packaging
material of this inven~ion employlng the Second Mechanism.
The material may be u~ed to package cereals and snack foods
in packages that may be easily resealed after each
intermi~ent use.
