Note: Descriptions are shown in the official language in which they were submitted.
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HOT MELT ADHESIVE HAVING
IMPROVED WET STRENGTH PROPERTIES
Background of the Invention
The present invention relates to hot melt adhesives which
exhibit improved open-times, and more specifically to hot melt adhesives
having improved wet strength properties and which find usefulness in the
manufacture of disposable nonwoven garments.
Description of the Prior Art
The prior art is replete with numerous examples of hot melt
adhesives which are employed for the construction of disposable soft goods.
Specific applications for these prior art adhesives have included disposable
ri
diapers, sanitary napkins, surgical drapes, hospital pads and adult
incontinence
products to name but a few. Moreover the prior art methods of application
of these prior art adhesives have included but are not limited to extrusion
(mufti-bead or slot) and spray or wheel application systems.
Those skilled in the art will readily recognize that many
different polymers have been used in adhesives employed in the construction
of disposable soft goods. In this regard typical hot melt adhesives have em-
ployed polymers which have included S-I-S (styrene-isoprene-styrene); SBS
(styrene-butadiene-styrene); SEBS (styrene-ethylene-butylene-styrene); EVA
(ethylene vinyl acetate); and APAO (amorphous poly alpha olefin). While
these polymers, when properly blended provide acceptable adhesion to most
substrates employed in typical diaper construction, and further provide
acceptable adhesion under dry conditions, they have had several shortcomings
which have detracted from their usefulness.
One of the most noteworthy shortcomings of the prior art
adhesives concerns the manner in which the adhesive reacts when exposed to
liquids, such as water, urine, or the like. For example, when the prior art
adhesives are applied to polyolefin substrates (such as polyethylene,
polypropylene or nonwoven substrates, as the case may be), almost all adhe-
sives will give good dry bonds and will normally maintain an acceptable bond
strength when wet. However, when substrates such as tissue or core fluff are
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used, certain problems present themselves. For example, while many of the
adhesives formulated with the polymers, noted above, will give excellent dry
bond strengths, these same adhesive bonds appear to be easily broken when
the substrates are exposed to water, urine, or other water-based discharges or
solutions. This bond delamination of the various substrates can cause the
fluff '
core to sag, and further results in a poor fit, absorption problems and
leaking
of the garment especially when the garment is of the type which includes
disposable diapers, or incontinence products.
More recently, manufacturers of disposable nonwoven
garments such as diapers have endeavored to produce products which are
much thinner in their overall thickness and profile and which incorporate
super absorbent materials in place of the fluff, which is normally in the
core.
In this regard, it should be understood that the nonwoven fibers comprise long
wood pulp fibers which are interwoven together to form a given mass. In
contrast, super absorbent material includes small sphere shaped particles
which are not connected together to form a coherent mass like the nonwoven
material. Therefore, core integrity becomes an even more important issue
with these thinner garments. Therefore, it has long been known that it would
be desirable to have a hot melt adhesive which is useful for bonding to
substrates which are typically employed in the construction of nonwoven
garments, such as polyethylene; polypropylene; nonwoven; tissue; or fluff and
which further maintains acceptable bond strength with the above identified
substrates following exposure of same for prolonged periods of time to water,
urine or similar materials, and which further is substantially nonblocking
under normal storage, and usage conditions.
Summary of the Invention
It is therefore an object of the present invention to provide
an improved hot melt adhesive which is useful for the manufacture of dispos-
able soft goods.
A further object of the present invention is to provide a hot
melt adhesive which can be employed as a construction adhesive, and which
further can be applied by employing extrusion, or spray techniques to at least
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one polyolefin, or nonwoven substrate and at least one polyethylene,
polypropylene, nonwoven, tissue or fluff.
Another object of the present invention is to provide a hot
melt adhesive which has the following preferred composition:
about 25%, by weight, of a butene-1-ethylene copolymer;
about SO%, by weight, of a tacldfying resin;
about 25%, by weight, of a plasticizer; and
about 0.1 to about 3%, by weight, of a hindered phenolic
antioxidant, and wherein the hot melt adhesive composition
displays an improved open-time and room temperature flow properties, and
further has a viscosity of about 2,000 cP at 325°F, and a melting point
of
about 170°F to about 200°F.
Another object of the present invention is to provide an
adhesive composition which, when applied to a tissue laminate in an amount
equal to about 3 milligrams per square inch and at a temperature of about
300°F, and further with an open time of about 1 second, produces a peel
strength, when measured by utilizing a dry Instron peel strength evaluation,
of
about 100 grams.
Another object of the present invention is to provide a hot
melt adhesive composition and wherein the adhesive composition is substan-
tially nonblocldng under normal storage conditions, and further when applied
to a nonwoven, and tissue substrate in an amount which is equal to about 2
milligrams per square inch, and at a temperature of about 300°F, forms
a
bond which has an average peel strength of about 50 grams following expo-
sure to water for at least one hour duration.
Another object of the present invention is to provide a hot
melt adhesive composition which utilizes a butene-1-ethylene copolymer, and
wherein the adhesive composition possesses an excellent balance of high
specific adhesion, elevated temperature resistance and acceptable viscosity
during normal application temperatures.
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In an embodiment of the invention the hot melt adhesive will bind to
polyethylene, polypropylene, absorbent core, and nonwoven top sheet or tissue
of a
disposable article and further has a propensity to flow into the fibers of the
absorbent core
or tissue thereby establishing a mechanical bond which resists degradation
when exposed
to a solution which includes or comprises water.
Further objects and advantages of the present invention are to provide a hot
melt adhesive composition for the purposes described, and
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which is durable, easy to apply utilizing conventional manufacturing tech- -
niques; and which further does not have the numerous shortcomings attribut-
able to the prior art adhesives used for substantially identical.purposes
heretofore.
De~,~~'~n off' t~ eferred Embo l anent
The preferred adhesive composition of the -present invention w
includes:
about 10% to about 60%, by weight, of a butene-1-ethylene
copolymer;
about 20% to about 80%, by weight, of a tackifying resin;
about 5% to about 30%, by weight, of a plasticizer; and
about 0.1% to about 3%, by weight, of an antioxidant, and
wherein the hot melt adhesive composition has a viscosity of about 1,000 to
about 50,000 cP at 325°F, and a melting point of about 170°F -
200°F.
IS Butene-1-homopolymers and copolymers which are useful in
the present invention are primarily linear chain molecules with regular and
spatially ordered arrangements of ethyl side groups. These side groups are
the result when butene-1 is polymerized across the 1, 2, carbon double bond,
and along an ethylene chain backbone. This is described in further detail in
U.S. Patent Number 3,362,940. When cooled from a melt, the ethyl side
groups initially align in a tetragonal spatial arrangement. With time the
tetragonal crystalline phase form transfers into a stable hexagonal spatial
arrangement with a subsequent development of improved physical properties.
A more thorough discussion of the polymer utilized herein may be found in
the reference to Mostert, U.S. Patent Number 4,937,138. As will be seen from
the disclosure above, the present polymer is useful in amounts of about 10% to
about
60%, by weight.
A suitable commercially available butene-1-ethylene copoly-
mer.can be secured from Shell Chemical Company of Houston, Texas under
the trade-mark Duraflex 8910 PC.
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As noted above, tackifying resins which are useful within the
scope of the present invention have a weight of about 20% to about 80%, by
weight. The tackifyi.ng resins can be selected from any of the nonpolar types,
which are commercially available. An example of a commercially available
tac~fying resin which is useful for the present invention ,includes the resin
which is identified commercially by the- trade-mark-Escorez 1310 LC
and which is manufactured by Exxon Chemical Company. Normally, nonpolar
tackifying resins which are useful with the present invention include resins
which have partially, or completely hydrogenated C9 or Cs based hydrocarbon
resins with softening points that are in a range of approximately 70°C
to
approximately 125°C. Tackifying resins which are useful for the present
invention can perhaps include polar tac)afying resins, however, the choice of
available polar tacldfying resins is limited in view of the fact that many of
the
polar resins appear only partially compatible with the butene-1-homopolymer,
and copolymers. As nosed above, the tackifying resin selected which is useful
with the present invention will be about 20% to about 80%, by weight, of the
entire adhesive composition and more preferably about 50% thereof.
The plasticizer that finds usefulness in the present invention
can be any number of different plasticizers but the inventors have discovered
that a plasticizer which includes a mono-olefin po~,lymer such as what is
commercially available under the trade-mark lndopol H~ 100, and which
is manufactured by Amoco, is particularly useful in the present invention. As
will be appreciated, plasticizers have typically been employed to lower the
viscosity of the overall adhesive composition without substantially decreasing
the adhesive suength and/or the service temperature of the adhesive. As
noted above, the present adhesive composition has a viscosity of about 2,000
cP to about 50,000 cP at 325°F, and a melting point of about
170°F to about
200°F. In view of the unexpectedly favorable viscosities, the adhesive
compo-
sition of the present invention has improved spraying characteristics when
employed with conventional manufacturing equipment.
As earlier discussed, the hot melt adhesive composition of the
present invention includes about 0.1% to about 3%, by weight, of an antioxi-
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dant. As a general matter it is understood that antioxidants and stabilizers
are important to protect the otherwise vulnerable copolymer and thereby the
adhesive composition from the deleterious effects of thermal and oxidative
degradation which is frequently experienced during the manufacture and
application of adhesive compounds as well as in the ordinary use of the final
product. It should be understood that this degradation usually manifests
itself
by the deterioration of the adhesive composition in appearance, physical
properties and performance. Among the most useful stabilizers are high
molecular weight hindered phenols, and multifunctional phenols, such as
sulfur, and phosphorous-contain phenols. In this regard, hindered phenols are
well known to those skilled in the art, and may be characterized as phenolic
compounds which also contain sterically bulky radicals in close proximity to
the phenolic hydroxyl group thereof. In particular, tertiary butyl groups
generally are substituted onto the benzene ring in at least one of the ortho
positions relative to the phenolic hydroxyl groups. The presence of the
sterically bulky substituted radicals in the vicinity of the hydroxyl group is
believed to retard its stretching frequency and correspondingly its
reactivity.
This steric hindrance is believed to provide the phenolic compounds with its
stabilizing properties.
A suitable antioxidant for use with the present invention is
commercially available from CIBA-GEIGY Company under the trade-mark
Igranox 1010.
The performance of these stabilizers are well known in the
art and may be further enhanced by utilizing in conjunction therewith, syner-
gists such as for example, thiodipropionate esters and phosphites; and
chelating agents and metal deactivators, such as, for example, ethylenedi-
aminetetraacetic acid, salts thereof and disalicylalpropylenediimine.
The hot melt adhesive composition of the present invention
may be formulated using any of the techniques known in the art. A represen-
tative example of the prior art procedure involves placing all of the
plasticizes
and stabilizer in a jacketed mixing kettle, and preferably in a jacketed heavy
duty mixer of the Baker-Perkins* or Day* type and which is equipped with
* trade-mark
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rotors and thereafter raising the temperature of this mixture to a range of
about 250° to about 350°F. It should be understood that the
precise tempera-
ture to be used in this step will depend upon the melting point of the particu-
lar ingredients. When the initial mixture, noted above, has been heated, the
mixture is blanketed in carbon dioxide at a slaw flow rate, and the resin
described above is slowly added. When the resin i~ melted; and the desired
temperature is reached, the copolymer is added to the mixture. The resultant
adhesive composition mixture is then agitated until the copolymer is com-
pletely dissolved. A vacuum is then applied to the mixture to remove any
entrapped air.
Optional additives may be incorporated into the hot melt
construction adhesive compasition in order to modify particular physical
properties. These additives may include colorants, such as titanium dioxide
and fillers such as talc and clay. However, these additional additives do not
include nucleating agents such as those that are described in U.S. Patent
Number 4,937,138 to Mostert, in view of the undesirable effects imparted to
the adhesive composition when these nucleating agents are added to same.
The invention is further illustrated by way of the examples
noted below. In this regard, an adhesive composition in accordance with the
teachings of the present invention was made by the general procedure de-
scribed above. This adhesive composition had the following constituent
elements:
about 25%, by weight, of a butene-1-ethylene copolymer;
about SO%, by weight, of a tackifying resin;
about 25%, by weight, of a plasticizer; and
about 0.1% to about 3%, by weight, of a hindered phenolic
antioxidant and wherein the hot melted adhesive composition following
application displayed an improved open time, and room temperature flow
properties, and further had a viscosity of about 2,000 cP at 325°F, and
a
melting point of about 17Q°F.
More specifically, butene-1-ethylene copolymer was commer-
dally secured from Shell Chemical Company under the trade-mark Duraflex
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8910-PC; the tackifying resin was commercially secured from Exxon Chemical
Company under the trade-mark Escorez 1310 LC; the plasticizes was commercially
secured from Amoco Chemical Company under the trade-mark Indopol H-100*; and
the antioxidant was commercially secured under the trade-mark Irganox 1010
from
$ the CEBA-GEIGY.
The resulting hot melt adhesive composition, upon testing,
was found useful for a multibead or multiline construction of nonwoven
garments. Further it was sprayed and extruded on various substrates and
found to be generally acceptable for all intended purposes. The specific
performance characteristics of this preferred formulation and the examples
which follow are discussed in greater detail hereinafter.
A second example of the adhesive composition was formulat-
ed and had the following cha.racteristia:
about 40%, by weight, of a butene-1-ethylene copolymer;
about 40%, by weight, of a tackifying resin;
about 20%, by weight, of a plasticizes, and
about 1%, by weight, of a hindered phenolic
antioxidant, and wherein the adhesive composition has a viscosity of about
4000 cP at a temperature of about 325°F and a melting point of about
180°F.
A third example of the adhesive composition was formulated
and had the following characteristics:
about $0%, by weight, of a butene-1-ethylene copolymer;
about 30%, by weight, of a tackifying resin;
about 20%, by weight, of a plasticizes; and
about I %, by weight, of a hindered phenolic
antioxidant, and wherein the adhesive composition has a viscosity of about
5$00 cP at a temperature of about 325°F and a melting point of about
188°F.
A fourth example of the adhesive composition was formulat-
ed and had the following characteristics:
about 25%, by weight, of a butene-1-ethylene copolymer;
about 65%, by weight, of a tackyif~ring resin;
about 10%, by weight, of a plasticizes; and
* Indopol H-100 is a polybutene plasticizes.
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about 1%, by weight, of a hindered phenolic
antioxidant, and wherein the adhesive composition has a viscosity of about
1500 cP at 325°F and a melting point of about 173°F.
In each of the examples, noted above, it was found that
' S adhesives formulated in accordance with these examples were useful for
constructing nonwoven, disposable soft goods.
As noted earlier, traditional hot melt adhesives which have
been formulated using SIS, SBS, SEBS and APAO have a propensity when
exposed to water for prolonged periods of time, to lose their bond strengths,
with the result that laminations prepared with such adhesives will fail when
exposed for prolonged periods of time to water.
As should be understood, such delaminations, of a nonwoven
garment, such as a diaper or the like may cause the garment's core integrity
to
fail with resulting undesirable effects such as leaking or poor fit.
In order to demonstrate the novel and unexpected results
achieved by the present invention, the inventors compared and contrasted the
adhesive compositions of the present invention with the prior art adhesive
compositions which have been commercially available for some time and
which further utilize the polymers, such as SIS, SBS, SEBS, and APAO, as
noted above. For example, the prior art adhesive compositions were applied
in an amount equal to about approximately three milligrams per square inch
and at a temperature of 300°F, and further with an open time of about
0.5
seconds to form a tissue laminate. It was discovered that the present adhe-
sive, when measured by a dry Instron peel strength evaluation, under similar
coating conditions, produced a bond strength of approximately 100 grams, this
bond strength being comparable to more traditional adhesive compositions.
However, when the same adhesive composition was compared to the more
traditional adhesives following a one hour soak in tap water, it was
discovered
that the traditional adhesives, as noted above, produced bond strengths in the
range of approximately 5 to about 25 grams. In contrast, adhesives manufac-
tured in accordance with the teaching of the present invention produced bond
strengths in excess of SO grams thereby making them a very attractive adhe-
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sive for use in constructing a nonwoven garment,-and more specifically a
diaper which may be exposed to water for prolonged periods of time.
The present composition was further tested by means of
applying it to a nonwoven, and tissue substrate to form a laminate. The
adhesive was applied in an amount equal to about 2 milligrams per square
inch and at a temperature of about 300°F and further with .an open time
of
about 1.0 second. Following formation of the laminate, it was submerged in
water for approximately one hour. Following submersion, the peel strength of
the adhesive was tested in a manner similar to that which was discussed
above. The test results demonstrated that the lamination had an average peel
strength of about 50 grams following exposure to water for one hour. Tradi-
tional adhesive compositions displayed greatly reduced bond strengths along
the ranges noted above. What was surprising, however, was the percentage' of
bond strength retained. For example, a standard adhesive such as H2120-O1
and which may be secured from Findley Adhesives, had a bond strength
following a soak of one hour of about 5.7 grams which represents about 2% of
its dry bond strength. In contrast, the adhesive of the present invention,
following a one hour soak retained a bond strength of about 56.6 grams,
which represents approximately 36% of its dry bond strength.
When tested relative to multibead construction, and wherein
polyethylene, and polypropylene are bound together to form a lamination, the
present adhesive composition when applied to the selected substrates, as
discussed above, in an amount equal to about 1.5 milligrams per square inch
of adhesive at 300°F and at a 0.5 second open time produced a bond
equal to
about $55 grams. In contrast, when compared to prior art adhesive composi-
tions, the average peel strength under the same coating conditions resulted in
bond strengths of about 661 grams. When further compared against tradition-
al adhesive compositions which are used to form laminations of polyethylene,
and polypropylene and wherein the adhesive composition of the present
invention is applied to the selected substrates in an amount equal to about 3
milligrams per square inch, and at a temperature of 300°F, the present
formulation produced bond strengths, as reflected by an average peel strength
* trade-mark
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as measured by an Instron tensile tester, which are comparable to that
produced by the conventional hot melt adhesives. Still further, when
laminations were formed uzider the same conditions and which included a
polyethylene substrate which is bound to a nonwoven, the average peel
strength of the present adhesive was significantly improved and quite unex-
pected relative to that produced by a conventional hot melt- adhesive composi-
lion. For example, under the conditions noted above, the average peel
strength for a lamination which is composed of a polyethylene and nonwoven
substrate, and wherein the adhesive composition of the present invention was
applied in an amount equal to about 3 milligrams per square inch, and at a
temperature of about 300°F, was about 339 grams, as compared to
traditional
hot melt adhesive compositions which may be commercially secured from
Findley Adhesive Inc. under the trade-mark H2275-02 and which
produced an average peel strength of about 208 grams.
As noted earlier, the adhesive composition of the present
invention is substantially nonblocking under normal storage conditions. For
example, the inventors prepared a lamination of poly~;thylene which was
combined with a nonwoven to form a laminate. The adhesive of the present
invention was applied to form the laminate in an amount equal to about 1.5
milligrams per square inch, and at a temperature of about 300°F. The
adhesive was applied with about a 0.5 second open time. Further, a laminate
was prepared under identical coating conditions by using a traditional hot
melt adhesive composition such as what is commercially available from
Findley Adhesives Inc, of Wauwatosa, Wisconsin and which can be purchased
under the trade-mark H2275-02. When identical blocking samples were
prepared and were aged in an oven for 48 hours and at a temperature of
140°F, the traditional hot melt adhesive, such as the formulation I-
I2275-02,
displayed an average peel strength of approximately 35 grams. In comparison,
the adhesive composition of the present invention displayed an average peel
strength of about 8.5 grams. These results were surprising in view of the fact
that the present adhesive blocking values were significantly less than the
state-
* trade-mark
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of the-art adhesive which is considered to be an excellent non-blocking
adhesive for nonwoven garment assembly.
These results are significant in view of the conditions under
which most nonwoven garments are employed. It is important, to understand
that it is very beneficial that an adhesive composition flow for a time
thereby
allowing it to penetrate the nonwoven fibers thus establishing a mechanical
bond with same. Once the flowing has occurred, however, it is important that
'the hot melt adhesive set up and be relatively hard and nontacky (non-
blocking). This, of course, decreases the potential for blocking (sticking) to
an adjoining substrate or surface such as may be the case when diapers are
packed in a box and shipped at elevated temperatures; or worse yet, the
possibility that the adhesive will, under some conditions, such as elevated
temperature, adhere to the skin of a young infant, or during storage, flow
completely out of the bond interface. As would be expected, an adhesive
which would remain tacky under these circumstances could make an associat-
ed nonwoven article extremely uncomfortable to wear, as might be the case
when such adhesives are utilized in garments such as incontinence garments,
and feminine care articles or the like, which may be placed in an orientation
where it is exposed to body hair.
In all the tests noted above, the peel strengths were measured
at a rate of 12 inches per minute. Further, five samples were tested in each
instance. Additionally, the blocking test was employed to simulate compact
packaging. More specifically, the laminations employed in the blocking test
were placed on top of nonwoven garments such as a diaper or the like with
the nonwoven portion of the laminate placed in contact with the nonwoven
substrate of the diaper. Samples were then compressed with a weight of 200
grams per square centimeter for a period of 48 hours, and at a temperature of
140°F. The samples were then allowed to cool to room temperature. The
samples were then tested for their peel strength in an Instron tensile tester
at
a rate of 12 inches per minute.
A review of the information above will reveal that the
adhesives of the present invention give similar peel strengths as compared
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with more traditional hot melt adhesives such as Findley Adhesives H2275-0Z,
and further show improved bond strengths when exposed to water. Further,
the present adhesive composition appears to exhibit no noteworthy blocking
characteristics, thereby suggesting that the adhesive will be very useful for
S assembling nonwoven goods which will be packaged in a compact fashion.
In addition to the foregoing, the adhesive composition of the
present invention has a low viscosity, and melting point, which makes this a
very desirable adhesive for use with the more traditional application methods.
Therefore, it will be seen that the hot melt adhesive of the
present invention provides a fully dependable and practical means for adhe-
sively assembling a disposable soft good such as a disposable diaper, feminine
napkin or the like, and which further avoids the detriments associated with
the prior art practices which includes, among others, employing several
different adhesive compositions for assembling a disposable soft good. In
addition to the foregoing, the improved hot melt adhesive of the present
invention shows surprising and unusually desirable manufacturing viscosities
when compared with the prior art, and further has peel strengths, following
exposure to water which are greatly improved in relative comparison to the
prior art.
It will be apparent to those skilled in the art that the forego-
ing example has been made for the purpose of illustration and that variations
may be made in the proportions, procedures, and material without departing
from the scope of the present invention. Therefore, it is to be understood
that this invention not be limited except by the claims which follow:
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