Note: Descriptions are shown in the official language in which they were submitted.
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This application is a division of application Serial No. 255,563,
which issued on 12 February, 1980, as Canadian Patent No. 1,071,519,
entitled "Absorbent Article With Adhesive Strip".
FIELD OF THE INVENTION
This invention relates to articles Eor absorbing
fluids. An aspect of this invention relates to articles for
absorbing body fluids such as blood, urine, menses, and other
exudates, secretions, and excreta (including fecal matter).
Still another aspect of this invention relates to a sanitary
napkin provided with a pressure-sensitive adhesive means
for attaching the napkin to a supporting garment.
DESCRIPTION OF THE PRIOR ART
Absorbent structures have been developed which are
particularly well-suited for use with a supporting garment
or body-encircling member, wherein the absorbent structure
is temporarily attached to the member or garment with a
pressure-sensitive adhesive means. Typical examples of such
absorbent structures include sanitary napkins and diaper-like
inserts for attachment to the inside of rubber panties. The
inserts for rubber panties which serve, in combination with
the panties, as diapers, are similar in structure to the
sanitary napkins, except that they are wider and are designed
to receive and absorb urine and fecal matter. The sanitary
napkins are generally of two types--a highly absorbent type
for periods of heavy menstrual flow and a far-less absorbtive
type for the so-called light flow (which typically occurs
near the end of a menstrual period). An example of the
highly absorbtive type of sanitary napkin is described in U.S.
Patent 3,672,371 ~Roeder) issued June 27, 1972~ `
All of the aforementioned absorbent structures
generally comprise: (1) an elongated absorbent pad having
essentially two major surfaces, (2) an outer covering layer -
adhered to one or both of the two major surfaces, (3) an
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adhesive layer on the outer layer which is intended for contact
with the garment or body-encircling member, and (4) a release
liner means for covering and protecting the adhesive layer
until the absorbent structure is put to use.
Several types of pressure-sensitive adhesive systems
have been suggested for use as the aforementioned pressure-
sensitive adhesive layer on the outwardly-facing surface of
the outer layer of the absorbent structure. For example,
some work has been done with solvent-based pressure-sensitive
adhesive (PSA) systems. However, the generally low-viscosity
of such PSA systems can result in a rapid penetration or
impregnation of the absorbent pad, leading to poorly controlled
results with respect to coating of the PSA system. Furthermore,
any solvent-based system can have the usual problems of health
hazards from solvent vapors, solvent recovery to reduce costs,
etc., associated with it.
Water-based adhesive systems have also been suggested
for providing the PSA layer on the outer surface of the absor-
bent pad structure. It has been generally found, however,
that the water-based adhesives tend to "set" or dry down rather
slowly, particularly as compared to the solvent-based type.
Still another approach which has been attempted is
the use of transfer tapes to transfer a coherent layer of PSA
onto the desired portion of the pad structure. One difficulty
with this approach is that the resulting coherent layer of
adhesive can be used only as the PSA means for adhering the
pad to the garment. Since there is no impregnation of the
absorbent pad structure with the transferred adhesive, the
adhesive does not ordinarily add to or reinforce the integrity
of the absorbent pad structure. With a more fluid adhesive,
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it is at least possible to use the PSA system for a double
purpose; as the PSA layer on the outer surface and as a means
for cementing together seams (or the like) in the structure.
So-called hot-melt PSA's are known. However, these
PSA systems typically lack one or more performance requirements
for a sanitary napkin or diaper pad PSA. ~-
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SUMMARY OF THE INVENTION
A hot-melt PSA system has now been discovered which
can be used as the PSA for adhesive-coated absorbent structures.
Like many hot-melt systems, the hot-melt PSA used in this
invention comprises a tackifier, a rubbery-base material, and
a plasticizer or diluent, e.g. a hydrocarbon process oil, for
adjusting the tack, the viscosity, and the like. It has been
found that the rubbery base used in the hot-melt PSA should
comprise a rubbery block copolymer which imparts a variety of
desired characteristics. Two different types of block copolymers `
have been found to meet the requirements of a PSA for sanitary
napkins and, further, to provide some surprising advantages.
These two types of block copolymers both contain rubbery mid-
blocks and relatively stiffer polystyrene terminal blocks. In
one preferred embodiment of the invention, the block copolymer
is of the A-B-A type wherein the A blocks comprise polystyrene
and the B block is a rubbery poly (ethylene-butylene) center
block.
In a second preferred embodiment, the polymer is
a teleblock copolymer comprising molecules having at least
three branches radially branching out from a central hub, -
each of the branches having polystyrene blocks and a butadiene
segment in the center.
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A tacky, hot-melt pressure sensitive adhesive,
pourable at 150-200C, according to the present invention,
comprises:
(a) 10-30~ by weight of a rubbery copolymer
selected from the group consisting of a radial
block-copolymer, an A-B-A block copolymer, and
mixtures thereof, the radial block copolymer being
a teleblock copolymer comprising molecules having
at least three branches radially branching out from
a central hub, each branch having polystyrene
terminal blocks and a butadiene segment in the
center, the A-B-A block copolymer being a three-block
A-B-A block copolymer, the A blocks being polystyrene
and the B block being a rubbery ethylene-butylene
polymer;
(b) more than 15 but less than 40% by weight of
an essentially hydrocarbon oil selected from the
group consisting of naphthenic oils, paraffinic
oils, and mixtures thereof, the essentially hydrocarbon
oil having an initial boiling point above 200C,
and
(c) 40-65~ by weight of a terpene tackifier resin.
DETAILED DESCRIPTION
The following detailed description is directed not
only to the hot-melt pressure sensitive adhesive, which is
the subject of the present divisional application, but also
describes absorbent structures in accordance with Application
Serial No. 255,563, now issued as Canadian Patent No. 1,071,519,
of which the presen-t application is a division.
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As pointed out previously, a typical absorbent
structure of this invention is a sanitary napkin of the t~pe
disclosed in U.S. Patent 3,672,371 (Roeder), issued June 27,
1972. A structure of this general type can include an absor-
bent pad enclosed in a fluid-pervious wrapper which is over-
lapped to provide a seam, the seam typically running lengthwise ;
through the middle of a major surface of the pad. The layer
(or layers) of PSA (pressure-sensitive adhesive) can be applied
to this seam (overlapped) area. If the PSA has sufficient
fluidity when applied, it can permeate the seam, binding together
the overlapped area and sealing the absorbent pad inside the
fluid-pervious wrapper. A release liner over the PSA layer
protects it until the napkin is to be used, at which time the
liner is peeled off and the PSA layer is pressed against the
inside of an underwear garment or the like. The major surface
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on which the PSA is coated does not actually have to be fluid-
pervious, since the body fluids enter the absorbent structure
from the opposite surface. For this reason, the absorbent
structure can contain other elements such as a fluid-impervious
film integral with the PSA-coated surface and the absorbent
pad. Fur-thermore, the liner or envelope enclosing the
absorbent pad can comprise a plurality of layers of material
such as waterlaid sheets, nonwoven webs, and the like. The
absorbent pad itself can be a batt or frictionally entangled
mass of fibers with properties similar to an absorbent cotton
mass. Absorbent cotton itself can also be used as the absor-
bent pad material.
When the purpose of the absorbent structure is to
absorb a very light flow of blood and menses, the total structure
can be simpler. In this case the absorbent pad can be much
thinner and need not be surrounded or sealed in completely
by a fluid-pervious layer, due to the inherently greater
integrity of the thinner pad. For example, the entire structure
can consist essentially of the thin pad (totally exposed on
one major surface), a fluid-impervious film in adherent contact
with the other major surface of the pad (this film serves as
the outer covering layer), the PSA layer coated on the exposed
surface of the impervious film, and the release liner pro-
tecting the PSA layer.
In any of these structures, the PSA layer need not
be coextensive in area with the major surface on which it is
coated. On the contrary, the typical PSA layer is very much
narrower in width and can be somewhat shorter in length.
When the absorbent structure is an insert for rubber
panties and is intended to serve as a diaper, it is typically
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wider than a sanitary napkin and may be somewhat longer.
Otherwise, the structure is similar. On the other hand, if
the absorbent structure is intended for receiving wound
exudates, a different arrangement of elements is generally
desirable.
The properties of the PSA layer are peculiar to
this art. For example, the PSA, when applied, should have
a low enough viscosity to flow into place readily and pre-
ferably also to permeate the aforementioned seam area or
overlap area. However, the viscosity must not be so low that
the PSA composition will permeate through the outer covering
layer and into the absorbent pad itself. Furthermore, the ;
viscosity should be high enough to permit some degree of a
property similar to "hold-out"--that is, a significant amount
of the PSA composition should remain on the exposed surface
of the covering layer and form a coherent PSA layer. This
"hold-out" effect can be most effectively obtained if the PSA,
during application to the outer covering layer, has a signi-
ficant amount of thixotropy. For a hot-melt PSA, it is
ordinarily preferred that the composition have a viscosity
ranging from about 500 to 10,000 centipose (cps) in the pour
temperature range, which would typically be 100 - 250C.,
more typically 150 - 200C. (e.g. 300 - ~00F.).
Another important property is the ability to solidify
or congeal or "set" very rapidly--preferably almost instantan-
eously. Hot-melt PSA's are particularly desirable in this
regard, since they can cool down from the application temperature
to a solidification temperature almost instantly under normal
ambient conditions.
Among the most peculiar properties needed for the
PSA are the adhesive bond characteristics. These characteristics
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must be obtained wi-thout sacrificing room temperature tack
("probe tack" or "wet grab") and without creating a situation
where the adhesive will offset or transfer to the garment.
The requirements for the adhesive bond are that the PSA should
have adequate peel strength and yet have the ability to
release itself from the undergarment without pulling of fibers
or tearing the garment. There are presently no adequate
standardized tests for measuring these adhesive bond properties;
however, the PSTC-l test is helpful to give some indication
of the required amount of peel strength. See U.S. Patent
3,672,371, column 3, line 51 et seq. Standard 180 peel and
shear tests are also helpful. For example, a PSA of the
invention has been found to have a 180 peel strength ("Mylar"*
substrate, 12 in./min) ranging from about 4 to about 5 or 6
pounds per inch width (piw) and a tensile shear strength
(0.1 in./min) of about 15 to about 35 pounds per square
inch (psi).
The hot-melt PSA compositions which have been found
to have the best combination of properties for use in sanitary
napkins and diaper inserts comprise a rubbery or elastomeric
block copolymer, an essentially hydrocarbon oil, and a tacki-
fier resin, all of which are described in more detail subsequently.
The Rubbery Copolymer
-
The two basic types of rubbery or elastomeric block
copolymers particularly well-suited for use in this invention
are:
(a) an A-B-A block copolymer having polystyrene
end blocks and a rubbery polyolefin center or mid
block, the polyolefin being a rubbery poly(ethylene-
butylene) block, and
*Trade Mark
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(b) a teleblock copolymer comprising molecules
having at least three branches radially branching
out from a central hub, each said branch having
polystyrene terminal blocks and a butadiene segment
in the center.
The A-B-A block copolymer is considered a two-phase
polymer comprising polystyrene domains in a rubbery poly
(ethylene-butylene) matrix. In order to obtain desired levels
of tack, it (and this is also true of the teleblock copolymer)
is tackified with a resin and an oil. The A-B-A polymer is
a true elastomer (according to the ASTM definition of
"elastomer") and has an elongation at break well in excess of
200%, e.g. 500%. The elongation at break for the teleblock
(radial) copolymer is in approximately the same range or
slightly higher, e.g. 590%.
To ensure rubberiness or elastomeric behavior in
the radial copoLymer, the number of butadiene units should
be greater than the number of styrene units. Shore A hardness
for the teleblock (radial) copolymers can be in the range
typical of true rubbers, e.g. above 60 or 70.
The ultimate tensile strength of the rubbery copolymer
(whether of the A-B-A type or the radial type) is moderately
high, e.g. above 2000 p.s.i., more typically above 3000 psi.
.. .. .
Tensile strengths above 5000 psi are possible with the A-B-A
structure.
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The Essentially Hydrocarbon Oil
.
The oily liquids used in compounding PSA's of this
invention are essentially hydrocarbon process oils which are
preferably low in aromatic content. For example, an analysis
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of the types of carbon atoms in oils used in this invention
indicate that aromatic carbons comprise less than 5% of the
oil, while naphthenic carbons (i.e. carbons of cycloaliphatic
compounds and the like) can range from about 25 to 60% and
paraffinic carbons can range from about 35 to 75%. Accord-
ingly, these oils are typically referred to as "naphthenic"
or "paraffinic" process oils.
These oils are preferably very low in volatility.
Initial boiling points, under normal ambient pressure con-
ditions, can range from well over 400F. (i.e. above about
200C.) to almost 800F. (almost 430C.). The least volatile
fraction in the process oils can have a boiling point in
excess of about 1000F. (about 535C.), so that the entire
boiling range can cover several hundred degrees, e.g. 600-
1100F. (315 - 600C.).
The aromatic content of the oils as determined by
clay gel analysis (in weight percent) can range from less
than 1% up to 15~ or more; however, aromatic content should
be low and should not exceed about 20% by weight. The
molecular weight of the oil is typically above 200 and can
be above 600. Most naphthenic and paraffinic process oils,
however, tend to have a molecular weight within the range
of 250 - 600.
The Tackifier Resin
Although various types of tackifier resins are
known, such as hydrogenated rosin esters, esters of polyhydric
alcohols, phenol-aldehyde resins, and the like, the preferred
tackifiers are of the type known as "hydrocarbon resins".
In industrial practice "hydrocarbon resin" is a term of art
relating to resins in the molecular weight range of a few
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hundred up to about 6,000 or 8,000 which are obtained or
synthesized from rather basic hydrocarbonaceous materials
such as petroleum, coal tar, turpentine, and the like. A
good description of "hydrocarbon resins" can be found in
Kirk-Othmer, Encyclopedia of Chemical Technology, Second
Edition, Volume 11, Interscience, New York, 1966, pp242. Many
of the so-called "hydrocarbon resins" commercially available
today are "terpene" resins, i.e. polymers with repeating
terpene units. These polymers can be homopolymers, or copoly- ~ -
mers, (including terpolymers, etc.), since terpene is an olefin
which can be copolymerized with other olefins. Terpene-phenols
are also produced.
All hydrocarbon resins do not work with equal
effectiveness, and a commercially available resin known as
"Wingtack" 95 (trade mark of Goodyear Chemical Company) is
especially suited for use in this invention. This hydrocarbon `
resin is compatible with other components of the hot-melt PSA
and has a softening point (ball and ring method) of 100C. +
5. "Wingtack" 95 is believed to be a by-product of isoprene
or polyisoprene production and is also believed to be a
synthetic terpene resin derived from a mixed olefin feed
stock.
The hot-melt PSA compositions of this invention can
be formulated with techniques known in the art using heated
mixers and the like. The rubbery copolymer and the oil can
be blended together readily at moderately elevated temperatures
(e.g. 150 - 300F.). The tackifier resin can be added to the
copolymer oil mixture. If a pigment is included in the PSA
composition, it should be added to the copolymer/oil blend
before the tackifier resin is introduced into the composition.
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The resulting hot-melt PSA, once it is heated to
the temperature where it will flow readily, can be applied to
the outer covering layer of the absorbent structure or article
by any of the techniques known in the art, including flow
coating, roller coating, knife coating, or the like. The PSA
can also be extruded into place by using a hot extruder or
die face.
The Resulting Hot-Melt PSA Compositions
As is known in the art, various other components can
be added to modify the tack, rheology characteristics (including
melt viscosity, thixotropy), adhesive bond strength character-
istics, rate of "set", low temperature flexibility, etc.,
color, odor, etc., of a hot-melt PSA. For example, liquid
resins are sometimes used as a partial or total replacement or
substitution for process oils, although such a substitution
is not preferred in the context of this invention.
The proportions of components in a hot-melt PSA of
this invention are preferably selected to provide the adhesive
bond characteristics described previously. It is also important
that these adhesive characteristics remain reasonably constant
during storage or non-use in a hot-melt applicator and during
normal storage of the article (e.g. a sanitary napkin) prior
to use. Still further, it is desirable that adhesive properties
remain generally within the prescribed ranges from one
production batch to another. "Aging" tests of the A-B-A block
copolymer embodiment of the PSA indicate generally stable
adhesive properties at hot-melt applicator temperatures (e.g.
with the range of 100-250C., more typically 150 - 200C.)
and, after coating onto the article, with the range of
temperatures typically associated with storage and shipping
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(e.g. from about -20C. to about ~50C.).
With respect to this embodiment, the most noticeable
downward trend in adhesive properties detectable upon storage
of coated samples in 1 to 24-week aging tests appears to be in
the peel strength o~ samples stored at 120F. (48.9C.). Even
this trend does not indicate a significant deterioration in
peel strength.
In short, these data generally indicate a reasonable
degree of stability in the internal structure of the hot-melt
PSA. For example, the process oil is reasonably well retained
within a matrix or the like provided by the tackifier resin
and/or the rubbery block copolymer.
The tackifier resin (e.g. "Wingtack" 95, trade mark
of Goodyear Chemical Company), provides a number of desired
properties (e.g. increased shear strength of the adhesive bond)
particularly at levels in excess of 30 weight-% (e.g. 40 weight
-% or more). However, if the resin is increased at the expense
of the process oil content, there can be a significant loss of
tack and undesirable increases in viscosity. Accordingly, the
amounts of tackifier resin is preferably less than 70 weight-%
of the composition (e.g. 65 weight-% or less, more typically
50-60 weight-%).
The process oil content is preferably above 10% by
weight (e.g. at least 15 weight-~) to provide sufficient
tack and a low enough viscosity at the pour temperature or
temperature of application (e.g. 150 - 200C.). However,
excessive amounts of oil (for a given content of rubbery
base) can reduce shear strength and cohesive strength and
possibly lead to transference of PSA from the article to the
undergarment. Accordingly, it is ordinarily preferred that
the amount of process oil in the PSA be less than 40 welght-%.
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The rubbery copolymer base assists in providing many,
if not most, of the key properties contemplated for hot-melt
PSA's of this invention. However, minor amounts of these
rubbery materials are effective, and excessive amounts can
increase the viscosity drastically, interfering with flow
properties in the 100 - 250C. range discussed previously.
At amounts below 5% by weight, the contribution of the rubbery
base to the PSA is ordinarily insufficient, and amounts of
10 weight-% or more are preferred. At rubbery copolymer amounts
above 30% (e.g. 35%), the increase in viscosity (even at
elevated pour temperatures such as 350F.) rapidly approaches
inconvenient levels. Accordingly, the optimum amount of rubbery
copolymer appears to be within the range of 10 - 20 weight-%.
The principle and practice of this invention is
illustrated in the following nonlimiting examples.
Example 1
In this Example, a block copolymer with the A-B-A
structure was used. The commercial embodiment of the copolymer
is sold by Shell chemical Company under the trade mark "KRATON"
GX 6500 (more recently referred to as "KRATON" 1650). This
commercially available material is a three block copolymer
with polystyrene endblocks and a rubbery poly(ethylene-
butylene) midblock. It is considered a two-phase polymer
consisting of polystyrene domains in a rubbery poly(ethylene-
butylene)matrix. Typical properties of the copolymer are as
follows:
Tensile strength psi: 6000
Elongation at break : 500%
Modulus at 300% extension, psi: 950
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Solution viscosity (measured with a Brookfield
Model RVT Viscometer): 20% by weight in toluene,
centipoise: 2000
25% by weight in toluene,
centipoise: 12000
The product is available in particulate form, e.g.
in crumbs about 1/8 inch in size.
The formula for the hot-melt PSA of this Example,
was as follows:
10Parts by Weight Ingredient
30.0 Naphthenic process oil*
15.0 "KRATON" 1650 (trade mark;
formerly sold under the trade
mark "KRATON" GX 6500)
1.0 Pigment dispersion (43 parts
by weight of pigment dispersed
in 57 parts by weight of the
naphthenic process oil)
54.0 "Wingtack" 95 (trade mark of
Goodyear Chemical Co. for
hydrocarbon tackifier resin)
*See detailed description which follows.
After the "KRATON" was completely blended into the
process oil, the pigment dispersion was added under high shear
mixing. The mixing temperature was maintained below 300F.
After the pigment was dispersed in the "KRATON"/oil composition,
the "Wingtack" 95 was added, and mixing was continued until a
uniform composition was obtained.
The naphthenic process oil used in the composition
was "Tufflo 6204" (trade mark). This process oil has a
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viscosity (SUS/100F.) of 1965, a specific gravity (60/60)
of 9.9206, a flash point (COC) of 440F., a pour point of
0F., and a sulfur content which is less than 0.001%. The
molecular weight of the oil is 440, and clay gel analysis
indicates 0% asphaltenes, 9% polar compounds, or resins,
14.8% aromatics, and 85.2% saturates. Carbon type analysis
indicates 2% aromatic carbons, 52% naphthenic carbons, and ..
46% paraffinic carbons. The boiling range is 630 - 1018F.,
the initial boiling point being 630F., and 95% of the oil
being distilled at 1018F. Only 5% of the oil distills at
630F. and 50% distills at 710F.
Ten pounds of the resulting hot-melt PSA was tested
for peel strength and shear strength. The 180 peel strength
was conducted at 12 inches per minute and the tensile shear
test was conducted at 0.1 inches per minute. Results were :
as follows:
Aging Time* Peel (piw) Shear (psi)
Zero 4.9 29.1
8 hours 5.2 30.7
24 hours4.6 29.8
48 hours4.8 32.3
*Length of time adhesive was held in the hot-melt applicator
pot at 193C. (380F.) prior to application to the test
specimens.
Three samples were tested and averaged to arrive
a-t the values given in the above table.
Properties of the resulting hot-melt PSA were as
follows:
Ring and ball softening point: 202 - 240F.
Specific gravity: 0.922
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Brookfield Viscosity (Brookfield Thermosel [trademark] speed
20 rpm spindle SC-4-27, 8 gram sample):
1,300-1,650 cps at 350F.
3,800-5,000 cps at 325F.
700-900 cps at 375F.
This hot-melt PSA was found to be particularly well-
suited for sanitary napkin structures of the type described in
U.S. Patent 3,672,371.
Example 2 ~ -
This example illustrates the use of a radial
(teleblock) copolymer, the commercial embodiment in this case ;
being "SOLPRENE" 502, a trade mark of Philips Petroleum Company;
typical properties of "SOLPRENE" (trade mark) 502-CX are as
followsO
Specific gravity: 0.91
Melt flow, 190C./21.6Kg: 0.2
Molecular weight: 91000
Shore A hardness: 79 -
~utadiene/styrene ratio: 70/30
Tensile strength, psi: 3900
Modulus at 300% extension, psi: 540
Elongation at break: 590
Compression set (Method B) 22 hours, 78F.: 35~
The formula used in the case of the "SOLPRENE" polymer
was as follows:
Parts by Weight Ingredient -
29.0 Naphthenic process oil ("Tufflo"
6204, see Example 1)
15.0 "SOLPRENE" 502 (see preceding
description)
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2.0 Pigment dispersion (see Example 1)
54.0 "Wingtack" 95 (trade mark; see
Example 1)
The procedure ~or preparing the hot-melt PSA was
the same as that of Example 1. The specific gravity of the
resulting product was 0.922. The ring and ball soften.ing
point was 204 - 240E., the viscosity at 350F. was 750 - 940
centipoise (cps), determined on a Brookfield Thermosel at a
speed of 20 rpm, spindle SC-4-27, with an 8 gram sample.
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