Note: Descriptions are shown in the official language in which they were submitted.
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WO96/16802 PCT/US95/14838
COLD SEALABLE COATING
This invention relates to a cold sealable emulsion
polymerized cohesive composition which is low in surface
tack.
In the past, in the preparation of films useful for
packaging purposes, heat sealable coatings, such as
acrylic coatings, were coated on one side of the film
substrate and another heat sealable coating, such as
polyvinylidene chloride (PVDC) was coated on the other
side. The acrylic coated side was generally the outside
of the film, the side in direct contact with the hot
sealer surfaces, where good hot slip and jaw release
characteristics are required. The PVDC coating was
usually on the inside of the film and provided the high
seal strength, good hot tack characteristics and barrier
properties required for such packaging. These heat
sealable coatings have glass transition temperatures
("Tg"s) which are higher than room temperature. Such a
coated film is disclosed in U.S. Patent No. 4,403,464.
Similarly, U.S. Patent No. 4,456,741 discloses heat
sealable terpolymer compositions useful as
pressure-sensitive adhesives for use with, for example,
backing materials including paper, polyester film and
foamed polymers. The terpolymer heat sealable
pressure-sensitive adhesive composition comprises butyl
acrylate, N-vinyl-2-pyrrolidinone and styrene. Other
heat sealable coatings are disclosed in U.S. Patent No.
3,696,082; and East German Patent No. DD-146,604.
In packaging products which are sensitive to heat,
such as candies, chocolates, ice cream and the like, in
plastic film or paper packages, the use of heated
elements must be avoided in order to prevent melting of
the products. Therefore, the use of heat sealable
coatings to package heat sensitive products has presented
serious difficulties often requiring isolation of the
product from the heated elements. Cold sealable
pressure- sensitive adhesives were developed which did
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not require the use of a heated element to seal the
packages. However, these adhesives had high surface tack
characteristics which made them adhere to uncoated
surfaces of the packaging film, making these adhesives
difficult to use due to the resulting blocking (i.e.
sticking) of the film.
Findley 207-939, a polyisoprene adhesive
manufactured by Findley Adhesive, Inc., is a cold
sealable pressure-sensitive adhesive coating which
exhibits good crimp seal strength on oriented
polypropylene film and has a Tg of -1.6C. This adhesive
has a high surface tack which often results in blocking
of the packaging film.
Another such pressure-sensitive adhesive composition
is disclosed in U.S. Patent No. 4,500,683 (Hori et al.).
The pressure sensitive adhesives of this patent contain
as a polymer component an addition polymerization polymer
of an acrylate or methacrylate polymer and one or more
ethylenically unsaturated monomers, such as
acrylonitrile, capable of forming homo- or copolymers
having a glass transition temperature of at least 273-k.
The composition is made by solution polymerization or
bulk polymerization. This composition forms a viscous
adhesive composition which is tacky at room temperature,
thus presenting the blocking problems when used on
packaging films.
Various other pressure sensitive films have been
disclosed. U.S. Patent No. 2,795,564 (Conn et al.)
discloses quick tack adhesive films made by emulsion
polymerization of a soft-polymer- forming monomer of
alkylacrylate, an ~, B-unsaturated monovinylidene
carboxylic acid and a hard-polymer-forming monomer such
as acrylonitrile.
Similar emulsion polymers have been described for
different uses in U.S. Patent No. Re: 24,906; (Ulrich)
and in U.K. Patent No. 1,003,318 (Smith et al.).
U.S. Patent No. Re: 24,906 discloses a pressure
sensitive adhesive which adheres to paper and is
-
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WO96/16802 PCT~S95/14838
cohesive. This composition may have high blocking
properties since a low adhesion liner or coating is
suggested for purposes of protecting the adhesive surface
from forming a permanent bond. U.K. Patent No.
1,003,318 discloses an alkali-soluble emulsion
polymerized copolymer as an adhesive surface coating.
Acrylic-based formulations are used as protective
outside coatings for packaging films used in wrapping
products. These acrylic-based coatings improve
machineability, printability and flavor and aroma
protection. However, the known low blocking pressure
sealable coatings such as those described in U.S. Patent
No. 5,070,164 (Min et al.) tend to block (stick) to
acrylic-based formulations. This blocking causes serious
difficulties during packaging and material handling.
Thus, the related art has disclosed heat sealable
coatîngs and pressure-sensitive, cold sealable adhesives
useful in the packaging art. The related art has not,
however, disclosed a cold sealable, pressure-sensitive
cohesive formulation, which is cohesive only when placed
under pressure in contact with other cold sealable
cohesive coated surfaces and does not block to acrylic-
h~ outside formulations.
An object of the present invention is to provide a
cold sealable cohesive formulation having the unique
improved combination of properties, e.g., low surface
tack to acrylic-based coatings, good coating uniformity,
good seal strength and wetting ability, on substrates
such as plastic and paper film.
A further object of the invention is to provide a
cold sealable, emulsion polymer cohesive formulation
which, when coated on a first side of a film or paper
substrate coated on a second side with an acrylic-based
formulation, imparts high cold seal strength, i.e.,
bonding under only pressure, and improved surface
properties, e.g., low surface tack to the acrylic-based
coating.
Another object of the present invention is to
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WO96/16802 PCT~S95/14838
provide improved packaging film or paper coated with a`~
cold sealable cohesive formulation having a smooth,
non-tacky surface which will not block at room
temperature.
Another object of the invention is to provide
improved coated packaging film having good cold seal
strength.
These and other objects are achieved according to
the present invention by providing an emulsion
polymerized, cold sealable, cohesive composition
prepared from the emulsion polymerization of an
ethyleneically unsaturated nitrile and one or more
monomers selected from group (a) and group (b) and
particulates ranging in size from about 6 to about 9
microns. The group (a) monomer is a soft monomer. The
soft monomer is selected from the group consisting of
ethyl acrylate, hexyl acrylate, iso-octyl acrylate (such
as 2-ethylhexyl acrylate), butyl acrylate, isobutyl
acrylate, methyl acrylate, vinylidene chloride, 1,3-
butadiene, vinyl-~a ~ `a~e and mixtures thereof. The group
(b) monomer is a functional monomer. The functional
monomer is characterized by a reactive group, such as an
acidic group. The functional monomer is selected from
the group consisting of methacrylic acid, acrylic acid,
2S it~con;c acid, crotonic acid, sulfoethyl methacrylate,
maleic acid and mixtures thereof.
The resulting latex formulation adheres to packaging
film substrates, such as polyolefin films, specifically,
oriented polypropylene ("OPP") or paper and other films.
The coated surface is cohesive at room temperature and
under pressure to other similarly coated surfaces and
exhibits low blocking tendencies to acrylic-based
formulations. The coated surface presents a smooth,
non-tacky surface.
The invention is directed to a pressure sealable
emulsion polymerized cohesive polymer which has low
blocking t~nApncies to an acrylic-based formulation,
comprising: an acrylonitrile monomer and one or more of:
CA 0220~368 1997-0~-14
WO96116802 PCT~S95/14838
- (a) a soft monomer selected from the group ~`
consisting of methyl acrylate, ethyl acrylate, hexyl
acrylate, iso-octyl acrylate, butyl acrylate, isobutyl
acrylate, isopropyl acrylate, vinylidene chloride, 1,3
butadiene and vinylacetate; and
(b) a monomer selected from the group consisting of
acrylic acid, methacrylic acid, itaconic acid, crotonic
acid, sulfoethyl methacrylate and maleic acid; the
monomers being combined in weight percent amounts based
on the total weight of the monomer sufficient to achieve
a glass transition temperature ranging from at least
about -35-C, typically about -30 C to about 15-C, the
polmer having low tack characteristics and cohesiveness
to similarly coated substrates when cold sealed under
pressure.
The emulsion pol-ymerized cold sealable pol-ymer
formulations of the present invention are, typically,
prepared by an emulsion polymerization process. The
polymerization is carried out in the presence of water,
an emulsifying~agent and a free radical catalyst.
Typical free radical catalysts include hydrogen peroxide,
ammonium or potassium persulfate or a redox type, such as
mixtures of persulfates with alkali metal bisulfites,
thiosulfates or hydrosulfites. Generally, the total
amount of catalyst employed is in the range of from about
0.1% by weight to about 2~ by weight based on the total
weight of the monomer. The emulsion polymerization is
typically carried out in the presence of one or more
anionic, nonionic or cationic emulsifiers such as, for
example, the alkyl carboxylic acid salts, phosphoric acid
salts, the alkyl sulfate salts, the alkyl sulfonate
salts, the alkyl aryl ether alcohols and the alkyl aryl
polyether sulfate salts. Generally, the total amount of
emulsifier employed is from about O.Ol to about 2.0
percent by weight based on the total amount of water.
A chain transfer agent, such as, isooctyl
thioglycolate, thioglycerol or dodecylmercaptan can be
employed in the emulsion polymerization process. Usual
- . - . = - }
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WO96/16802 pcT~sssll4838
amounts range from about O.l to 5% by weight based on the
weight of total monomer.
The polymerization can be conducted in a redox
system or in a higher temperature thermal process using a
persulfate-type initiator or an azobis isobutrylnitrile
initiator.
In general the polymerization is carried out at a
temperature of from about 40C to about 100C, preferably
about 60C to about 80-C, at a pressure in the range of
from about 0 to about 30 psig (about lOl to 308 kPa). A
thermal polymerization is carried out at the higher range
of these temperatures typically above about 70-C. The
reaction can be conducted in glassware with a reflux
condenser. This stage is usually carried out in the
presence of an inert gas, such as nitrogen. The
polymerization is generally carried out for a time of
from about l to about 8 hours, preferably about 3 to 4
hours. After completion of the polymerization reaction,
the pH of the polymer can be adjusted to up to lO, more
specifically, from about 6 to about lO.
A two-stage polymerization process is also
contemplated, generally the second stage polymerization
is performed under the same temperature and pressure
conditions as in the first stage. A preformed seed latex
is made to which up to about 95% of the remaining amount
of monomer feed is gradually fed in a second stage over a
period of from about 2 to about 5 hours. The-total
reaction time of the second stage will usually range from
about 4 to about 6 hours.
In one embodiment of the invention there is a two-
stage polymerization in which the seed, or core, latex
comprises up to about 50 wt.% of a different polymer such
as a polymer described in U.S. Patent No. 5,070,164. To
this is added, as the r~m~;n;ng monomer feed, a monomer
feed of this invention.
In accordance with the present invention, the --
.
polymers prepared from the above described emulsion
polymerization process are prepared from a monomer feed
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WO96/16802 pcT~ss~ll4838
comprised of an ethyleneically unsaturated nitrile
specifically any acrylonitrile, even more specifically, a
first monomer selected from the group consisting of
acrylonitrile and methacrylonitrile, a second soft
monomer selected from the group consisting of ethyl
acrylate, hexyl acryLate, iso-octyl acrylate, butyl
acrylate, methyl acrylate, isobutyl acrylate, 1,3-
butadiene, vinyl acetate and vinylidene chloride; and, a
third functional monomer selected from the group
consisting of methacrylic acid, acrylic acid, itaconic
acid, crotonic acid, sulfoethyl methacrylate, and maleic
acid. Usually, one monomer is selected from each group
to produce a terpolymer.
The glass transition temperature of acrylonitrile is
about 97 n C. The glass transition temperature of
homopolymers from the second and third monomeric groups
are listed in Table 1.
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WO96/16802 PCT~S95/14838
TABLE 1
Homopolymer
- Tg's
Second Monomer GrouP; Tg foC)
Ethyl acrylate -22
Hexyl acrylate -57
Isopropyl acrylate -3
Iso-octyl acrylate -50.15
Butyl acrylate -54
Methyl acrylate 8
Vinylidene chloride -17
Isobutyl acrylate -43
1,3-Butadiene -78
Vinylacetate -32
Third Monomer Group;
Acrylic acid 106
Methacrylic acid 185
Itaconic acid ---
Sulfoethyl methacrylate ---
Maleic acid ---
Crotonic acid ---
In general, the amounts of each monomer of the
present invention will depend on the preferred glass
transition t~mr~rature of the final formulation.
Typically, the polymer will contain about lO to 65 wt.%
of acrylonitrile, about 30 to 85 wt.% of the second
monomer group, and about 0.5 to 5 wt.%, preferably about
1 to 3 wt.%, even more preferably 1 to 2 wt.%, of the
third monomer group.
A crosslinking agent may be useful. Low levels,
usually less than 5 wt.%, of a crosslinking agent, such
as divinylbenzene or 1,3-butylene glycol dimethacrylate
or any other crosslinking agent known in the art, may
also be employed, but are not required. It was found
that the effect of the chain transfer agent, typically
iso-octyl thioglycolate, to lower the molecular weight
could be balanced by the effect of the crosslinking agent
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WO96/16802 PCT~S95/14838
__9__
to increase the molecular weight: therefore, when higher
levels of crosslinking agent are used, higher levels of
the ch~;n transfer agent are usually used and vice versa.
This helps to balance the properties of cohesive strength
and sealability.
The emulsion polymers of the present invention have
a weight average molecular weight ranging to about
150,000, preferably from about 50,000 to about 90,00O as
determined by gel permeation chromatography (GPC). The
polymer has a calculated glass transition temperature
(Tg) of from at least about -35-C, usually about -30-C to
about 15-C. We found that lower glass transition
temperatures, typically less than 3-C, more typically
less than O-C and even more typically about -5-C, exhibit
better pressure sealable properties, but this can vary
dep~n~;ng upon the comonomers used. The diameter of the
polymer particles are generally in the range of from
about 0.05 to about 0.3 microns.
The glass transition temperature ("Tg") of the
polymer formul-ation is inversely related to the pressure
sealability of the coating. So that when the Tg of the
polymer decreases, the pressure sealability increases.
Tg of the polymer is related to the ratios of the weight
fractions of the monomeric components and the Tg's of
these components, so that when a terpolymer made from
three monomers is being analyzed,
1 Wfl Wf2 Wf3
___ = ___ + ___ + ___
Tg Tgl Tg2 Tg3
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WO96/16802 - PCT~S95/14838
----10----
where:
Tg = the Tg of the terpolymer;
Tgl = the Tg of the first monomer;
Tg2 = the Tg of the second monomer;
Tg3 = the Tg of the third monomer;
Wfl = the weight fraction of the first monomer;
Wf2 = the weight fraction of the second monomer;
and,
Wf3 = the weight fraction of the third monomer.
After drying, the resulting latex formulation
adheres to the packaging film substrates even after
exposure to moisture. That is, they retain a significant
amount of their original bond strength even after
immersion in water. Once solidified by drying, the
coated surface does not adhere to non-treated, uncoated
film surfaces such as untreated polypropylene or acrylic-
h~ surface coatings.
The low adhesion to acrylic-based coatings is an
important feature of the invention because these coatings
impart the combined properties of machineability and
printability to untreated and release-treated
polypropylene. The untreated and release-treated
poly~ylene, while being machineable, is not printable.
The latex coating presents a smooth, non-tacky surface
which will not block to acrylic-based surface coatings
under normal operating conditions. However, if similarly
coated surfaces are placed in contact with each other and
under pressure, and elevated temperatures, usually up to
about 150F, (65.6C) room temperature or even below room
temperatures, usually about 60-F, (15.6C), then the
coated surfaces become cohesive forming a strong bond
between each other. With certain polymers, the bond
created has been found to be stronger than the film
itself. The seal temperature required can be effected by
the thickness of the composition on the substrate: that
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----11----
is, thicker substrates can require higher temperatures
for pressure sealability.
Thus, the emulsion polymer formulations of the
present invention are very useful in imparting high cold
seal bond strength to packaging film substrates,
particularly polyolefin film, such as polypropylene film
substrates. Substrates which are particularly
contemplated for use with the cold sealable polymer
formulations of the present invention include cast or
oriented polypropylene, coextruded films, nylon, white
opaque film, such as film made from opaque oriented
polypropylene cont~;~;ng a strata of voids formed by
void-initiating particles such as polybutylene-
terephthalate as descirbed in U.S. Patent No. 4,377,616,
cellophane, paper, polyesters, high density polyethylene
and linear low density polyethylene. When a
polypropylene film is employed, the sheet is usually
about 20 to 40, specifically about 30 to 35 microns in
thickness.
In one specific aspect of this invention the film
substrate is primed with a material that helps to anchor
the topcoat. Typical primers include epoxy-type primers
such as a polysLy~ene latex which contains at least one
epoxy functionality, melamine formaldehyde or
polyethyl~n~;~;ne.
The polymeric portion of the contemplated acrylic
based coatings should contain at least about 80 wt.%
acrylic, preferably greater than 90 wt.% acrylic. Small
amounts of other materials such as acrylonitrile and
ethylene acrylic acid can also be present in the acrylic-
based polymer, typically less than about 10 wt.%. The
amount of non-acrylic materials should be limited as they
can cause blocking. The acrylic-based coating can be
formulated with particulates such as polyethylene,
silicon and silicone, silica, talc, or other particles
ranging in size from 0.5 to 10.0 microns, specifically
about 6 to 9 microns, depending upon whether the final
film product is opaque or transparent. For opaque films
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--12--
the particulates are about 8 microns; for transparent
films, particulates are less than 8 microns. The size of
the particulates can be important for purposes of
blocking and mach;~eAhility as well as storage stability
of the coating formulation because larger particles tend
to settle out of the formulation. The solid particulates
loading is usually less than 15% solids, h~ on the
total weight of the coating. Preferably, the solids
concentration is less than 10% solids.
Commonly known techn;ques can be employed to apply
the emulsion polymer formulation of the present invention
to the film or paper substrate. For example, when
impregnating or saturating the substrate, it may be
dipped or sprayed. If the substrate is coated, this may
be accomplished by dipping, spraying or by employing a
roller, spreading knife, brush or the like. Generally,
for the best crimp sealability, the emulsion polymer
formulation should be applied at a low level, typically,
applied in an amount of from about 0.5 to 5 g/lOOO sq.
in. (g/6,451 cm2), preferably about 1 to 1.5 g/lOOO sq.
in. (g/6,451 cm2) to the film substrate.
The emulsion polymerization formulation of the
present invention may be compounded with, or have mixed
therein, other known ingredients or stabilizers,
antifoaming agents, dying adjuvants, pigments, waxes,
corn starch, silica, talc and the like or other
compounding aids to control surface tack and other
surface properties. Thickeners or bodying agents may be
added to the polymers so as to control the viscosity of
the polymer and thereby achieve the proper flow
properties for the particular application desired.
The following examples are illustrative of the
invention.
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WO96/16802 PCT~S95/14838
--13--
EXAMPLE 1
This example illustrates the preparation of an
emulsion polymerized, cold sealable, terpolymer
formulation within the scope of the invention.
Using a semi-continuous batch process, a latex was
prepared by cont;ml~lcly adding and metering 404 g. of
the total monomer feed over a period of 2.5 hours to a
reactor con~;n;~g 475 g of water, 1 g of 30% sodium
lauryl sulfate, as an emulsifier, and 1.6g 70% tert-
butyl hydroperoxide as free-radical catalyst. The
premixed total monomer feed is comprised of 80.2 wt.%
ethyl acrylate, 15.8 wt.% acrylonitrile, 3 wt.%
methacrylic acid, and 1 wt.% isoctyl thioglycolate (as a
chain transfer agent), based on the total monomer feed.
A feed cont~;n;ng 7g of 30% sodium lauryl sulfate, 0.7g
of sodium formaldehyde sulfoxolate (as a catalyst
activator) and 45g water was added over a three hour
period. The emulsion polymerization reaction was
main~ at 60-C to 70-C and the reaction was performed
in an atmospheric pressure reactor equipped with a reflux
~o~nc~ and in the pr~c~nc~ of inert nitrogen gas.
Sufficient agitation was used to uniformly disperse the
monomers in the reactor. The reactor batch was held for
about 1 hour after the addition of all the feeds.
The latex was subsequently cooled and filtered
through a 200 m~sh scr~ên. The lat~x stability OL the
resultant EA/ACN/MAA terpolymer was excellent. The
terpolymer has the structure:
C C~ C ~ C
C ~ ~ a ~ _ ~ C -
Q C~X3 08
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WO96/16802 PCT~S95/14838
-14--
- Polymer (a:b:c = 81:16:3)
After polymerization, the terpolymer was blended
with a typical combination of wax and talc. This occurs
after removal from reactor.
EXAMPLE 2
This example illustrates the preparation of a
crosslinked polymer. The procedure of this example was
substantially the same as Example 1 except that it was a
thermal-initiated polymerization. Using a semi-
continuous batch process, a latex was prepared by
continuously adding and metering 406 g of the total
monomer feed over a period of 2.5 hours to a reactor
containing 475 g water, 1 g of 30% sodium lauryl sulfate,
as an emulsifier, and 12 g of 10% aqueous ammonium
persulfate (as a free radical initiator) that was
prepared immediately prior to the start of the
polymerization and added to the reactor approximately two
minutes prior to beginning the addition of the monomer
feed. The pre-mlxed total monomer feed is comprised of
79.8 wt.% ethyl acrylate, 14.8 wt.% acrylonitrile, 3 wt.%
methacrylic acid, 1 wt.% divinyl benzene (80% pure), and
1.5 wt.% iso-octyl thioglycolate (as a chain-transfer
agent), hA~ on the total monomer feed. A feed
cont~ining 9 g of 23% sodium dodecyl benzene sulfonate
(as a stabilizing emulsifier) and 41 g water was added
over a 3-hour period. The emulsion polymerization
reaction was maintained at 75 to 85C and the reaction
was performed in the same laboratory glassware that was
used in Example 1. After all feeds were in, the batch
was held at 80-C for about 1 hour at which time 17 g of
30% aqueous ammonia (diluted to 400 g with water) was
added. When the temperature again reached 80-C, the
batch was held an additional hour at 80-C and then cooled
and removed from the glassware.
The properties of the polymers prepared by Examples
1 and 2 are summarized in Table 2 below:
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WO96/16802 PCT~S95/14838
--15--
TABLE 2
Characteristics of Polymer Latex
~- ExamPle 1Example 2
Latex
7. 5 Total Solids (%) 42.8 31.5
Particle Size (nm) 130 87
pH 6.1 9.5
EXAMPLE 3
Polymer latex formulations prepared in accordance
with the emulsion polymerization processs of both
Examples 1 and 2 were applied by direct gravure to the
sealable portions of one side of a stAn~Ard acrylic
coated 92 gauge OPP film primed with polyethyleneimine
(which helps to anchor the topcoat to the substrate).
The other side of the film was primed with the
polyethyleneimine and coated with an all-acrylic polymer
formulation. The coatings were dried at a temperature of
about 220F.
The coating weights for both pressure sealable
coatings were about 1.2 (+/- 0.4) g/1000 sq. in. (g/6,451
cm2) while the coating weight for the acrylic-based
coating was about 0.6 g/1000 sq. in. (g/6,451 cm2).
The properties of the coated films were tested and
the results of the testing, in comparison with the
results of testing a cold sealable terpolymer described
in U.S. Patent No. 5,070,164, made by a 1-step
polymerization process, are summarized in Table 3.
= -
CA 02205368 1997-05-14
WO 96/16802 PCT/US95/14838
3O o 3O ~
~ --I C
--I O
U F'
O
O ~ ~ g
C ~
O ~ ~ g
CO U~ ~ ~ O ~ C,~
O
* *
~ 1~ O C) C
V V U ~ ~ ` C
~ ~ V V .Y ~ 5
,¢ o ~-- ,t c~ --I J H
C U
UO ~ ~ p, ~ _
0 ,~ o O ~ ~''
m ~ ~ .c
~ ~ u ~ ~
a
~ u ~ a) o
c ~ ~ ~ ~ u a _ c ~ c
o ~ ~ ~ u
~1 ~ ~ X ~ ~ ~ ul ~ c _ I ~ c
~1 ~ z z ~ ~ c ~
U 10 1'5: ~ U ~-1 U ~ :~. O R O .C tO
~1 0 ~ ~ '¢ C
E~ Q ~ ~ ~ O _I U ~ ~
,r O ~ ~3 ~ h ~ ~ ~ O ~ C~ ~ r1
u ~ ~ C
3 O
o
N a, N a) --I 11 11 11 11 11 11 11 --I ~ O
O ~ ~
z ~ ~: a m a) ~n
~x ~x ~ w~:Xm~
,~
1~3 ~ O Q al C,~ ~r1
~1) N O O U~
r1 C C U~ _
ta s~ o ~ o )~ r
~r1 ~ a) ~r1 Gl ~r1 ~ r~ S
F ~ E3 S ~ ~.
S ~ r~ -' r~
o e o e o
~:
~ o u~ o ~
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WO96/16802 PCT~$95/14838
--17--
EXAMPLE 4
This example demonstrates the advantage of employing
additives in which a significant particulate population
is in the size range of 6 to 9 microns.
Cold seal polymers described herein were formulated
with the particulatcE d~ccribed below in amounts of 1, 3
and 5 parts per hundred.
Particulate Approximate Size
(A) Corn Starch (food grade) 2-10
(B) Zeospheres x-120 1-7
(C) Zeospheres 200 1-9
(D) KMP 590 Skin Etsu 2
(E) Syloid 63 W.R. Grace, Davidson 6-8
Chemical
The coatings were evaluated for surface tack and for
cold seal strength 24 hours after forming the seal. The
plot of Figure 1 shows that Syloid 63 (Polymer E), which
cont~inD~ a significant amount of particles in the size
range of 6-8 microns, achieved the best balance between
low surface tack and cold seal strength tested 24 hours
after forming the seal.
The seal strengths were determined by placing the
sealing surfaces into the jaws of a crimp sealer to form
a seal at 80 psi (551 kPa), 0.5 seconds dwell time and a
temperature of about 23-C. The seal strength was
measured by trying to pull the seal apart in a 90 peel
test (e.g. using the INSTRON) at between 10 and 30
minutes after the seal was formed.
The low pressure self tack was determined by placing
the pressure sealable surface to itself under no applied
pressure for 24 hours at ambient temperature. The sample
was then placed into a blocking jig which exerted
pressure of 0.7 psig (106 kPa) for 4 hours. The force
required to separate a sample was measured using the 9oo
peel test (e.g. using the INSTRON).
