Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED PRESSURE SENSITIVE LABELING ADHESIVE
Technical Field
This invention relates to novel pressure
sensitive adhesive compositions, and in particular to such
pressure sensitive adhesives which are best suited to the
manufacture of pressure sensitive labels.
Background of the Invention
Pressure sensitive adhesives are materials which
have tack properties at room temperature. The pressure
sensitive adhesive firmly adheres to a variety of
dissimilar surfaces without the need of more than finger or
hand pressure.
During label manufacture a laminate, formed of a
face stock, a pressure sensitive adhesive layer, and a
release liner, is passed through apparatus which converts
the laminate to yield commercially useful labels and label
stock. The processes involved in the converting operation
include, for example, printing, die cutting and matrix
stripping to leave labels on a release liner, butt cutting
of labels to the release liner, marginal hole punching,
perforating, fan folding, guillotining and the like. Die
and butt cutting involve cutting of the laminate to the
face of the release liner. other procedures involve cut-
ting entirely through the label laminate and include, for
instance, hole punching and perforating, and guillotining.
The cost of converting a laminate into a finished
product is a function of the speed at which the various
processing operations can be done. While the nature of all
layers of the laminate can impact cost of convertibility,
historically the adhesive layer has been the greatest
limiting factor in the ease and cost of the conversion
operation. This is true because of the viscoelastic nature
of the adhesive. The fact that the adhesive is
vlscoelastic hampers precise and clean penetration of a die
in die cutting operations, and promotes adherence to
cutting blades and the like in any type of cutting
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operation. The fact that adhesives can be stringy also has
an impact on matrix stripping operations, which are often
done after die cutting operations.
To date, achieving good convertibility has not
automatically coincided with achieving excellent adhesive
performance. Adhesives must be ~ormulated to ~it
predetermined needs. Important properties include peel
adhesion, tack, shear properties, viscosity at various
temperatures, and the like. Good general purpose adhesives
may exhibit poor convertibility simply because the adhesive
iB difficult to cleanly cut. Such an adhesive may stick to
a die or blade. In label manufacture, die cutting and
matrix stripping operations by necessity occur at a variety
of speeds ranging up to 1000 feet per minute or more.
Within that range, an adhesive may provide regions where a
matrix will break despite the fact that successful matrix
stripping can occur at speeds on either side of the region.
It is a goal to provide adhesive systems wherein the
adhesive can be cleanly cut and the matrix stripped at
substantially any practical operating speed.
Sasaki et al, U.S. Patent No. 5,290,842 shows a
combination wherein a styrene-butadiene block copolymer is
combined with a styrene-isoprene-styrene block copolymer to
show two separate and distinct glass transition temperature
peaks, allegedly to improve the convertibility. Other
patents of interest are Downey, U.S. Patent No. 3,880,953;
Tindall, U.S. Patent No. 3,509,239; Feeney, U.S. Patent No.
4,060,503; and Korpman, U.S. Patent No. 3,932,328.
This invention relates to improvements to the
products described above and to solutions to some of the
problems raised thereby.
Summary of the Invention
This invention provides a pressure sensitive
adhesive which exhibits improved convertibility, that is,
the ability to achieve cutting o~ the adhesive in
processing operations involving cutting through a face
stock andladhesive to at least the release liner o~ the
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laminate, while at the same time providing superior
adhesive performance.
These results can be obtained by combining a
typical styrenic block copolymer, blended with tackifying
resins, whether natural or hydrocarbon based, and
including, in place of some or all of the plasticizer, a
styrene isoprene di-block copolymer. Both~the styrenic
block copolymer and the styrene isoprene di-block copolymer
are miscible together with the tackifier to produce a
pressure sensitive adhesive that exhibits a single glass
transition temperature (Tg) value. The expected result of
this partial or complete replacement of the plasticizer is
mainly the reduction of the tendency of the plasticizer to
migrate from the adhesive and cause staining of the face
stockr the release liner, or both. Also, however, this
partial or complete replacement of the plasticizer produces
the unexpected results of substantially higher temperature
performance and a very substantial improvement in the
ability of the adhesive to be die cut.
Previous attempts to improve the die cutting of
hot melt pres~ure sensitive adhesives have had detrimental
effects on other characteristics. For example, increasing
the resin content increases the tangent delta value of the
pressure sensitive adhesive compared to the tangent delta
value measured in the absence of increased resin content,
thereby increasing the ease of die cutting, but also
significantly increases the glass transition temperature,
Tg. The Tg can be increased to a point where the adhesive
no longer maintains its pressure sensitive characteristics.
In addition, the application and service temperature ranges
are condensed. Incorporation of or increasing the level of
typical processing oils will improve the processability,
but will decrease an adhesive's ability to be die cut, and
will diminish high temperature performance. The addition
of a low molecular weight styrene-isoprene di-block
copolymer improves processability and improves the tangent
delta values at 20~C, while maintaining an acceptably low
Tg. This enhancement in ability to be die cut and the
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enhancement in high temperature performance are unexpected
benefits. A pre~erred plasticizer is composed of a styrene
isoprene di-block copolymer and a processing oil in a
weight ratio of about 1:2 to 1:0.5.
Other ob~ects and advantages of the invention
will become apparent hereinafter.
!
Detailed Descri~tion of the Invention
The present invention is directed in general to a
pressure sensitive adhesive formed from a mixture of base
polymers, in particular styrenic block copolymers, and a
tacki~ier. As is generally conventional, the styrenic
block copolymers may be selected from any of several types,
including but not limited to styrene-isoprene-styrene block
copolymers such as Vector 4111 available from Dexco,
styrene-isoprene-styrene/styrene-isoprene block copolymers
such as Kraton D1107 or Kraton D1112, manufactured and sold
by Shell, styrene-butadiene-styrene block copolymers such
as ~ector 6241D available from Dexco, styrene-butadiene-
styrene/styrene-butadiene block copolymers such as Kraton
D1102 also from Shell, and styrene-butadiene block
copolymers such as Solprene 1205, manufactured and sold by
~ousmex, Inc. The above examples are unsaturated midblock
copolymers. Saturated midblock copolymers are also usable,
including but not limited to styrene-ethylene-butylene-
styrene block copolymers such as Kraton G1652, availablefrom Shell, styrene-ethylene-butylene-styrene/styrene-
ethylene-butylene block copolymers such as Kraton G1657,
also ~rom Shell and styrene-ethylene-propylene--
styrene/styrene-ethylene-propylene block copolymers such as
Septon 2063, manufactured by Kuraray, a Japanese company,
and distributed in the United States by Arakawa Chemical
Ind., Ltd.
Generally conventional tackifier systems can
include, as one example of many choices available, a
normally solid tackifier such as Wingtack 95, an aliphatic
petroleum resin, manu~actured by Goodyear, or Escorez 1310
LC, an aliphatic petroleum resin, manu~actured by Exxon,
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together with a plasticizer system, usually including a
plasticizer oil such as Shellflex 371, from Shell, or a
normally liquid tacki~ier such as Wingtack 10, an aliphatic
t petroleum resin, available from ~oodyear, or some
5 combination of both.
The invention calls for the addition of a styrene
isoprene di-block copolymer having a low viscosity,
preferably a viscosity such that the styrene isoprene di-
block copolymer is a liquid at 25~~. This styrene isoprene
10 di-block copolymer used in the adhesive composition of the
present invention has the configuration of A-B, wherein "A"
is a styrene polymer block and 'IB'' is an isoprene polymer
block. These "di-block" copolymers used in the present
invention differ significantly from "tri-block" polymers
15 having an A-B-A con~iguration, with "A" and "B" each
representing separate, distinct polymers. Di-block
copolymers are also distinguished from "radial polymers"
(ABX~ which consist of a central "A" block of a polymer
with numerous arms of a ''B'' block of a different polymer
20 extending from the central A polymer block. The preferred
styrene isoprene di-block copolymers of the present
invention have an absolute molecular weight of less than
70,00Q, with the most preferred styrene isoprene di-block
copolymers having an absolute molecular weight of less than
50,000, or a polystyrene equivalent molecular weight of
less than 70,000. Preferred A-B styrene isoprene di-block
copolymers of the present invention will have the styrene A
block amounting to about 5~ to about 50~ of the mass o~ the
copolymer molecule, most preferably about 5~ to about 25~.
In particular, LVSI-101 is one such styrene isoprene di-
block copolymer available from Shell, and LIR-310 is
another, available from Kuraray through Arakawa, for this
purpose. The plasticizers referred to above, and
particularly the oils, have tended to be subject to
migration, causing adver~e e~ects on substrate and
adhesive performance. Since styrene isoprene di-block
copolymer is substantially less migratory than these
plasticizers, the addition o~ the styrene isoprene di-block
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copolymer in place of some or all of the normally liquid
tackifier and/or plasticizer oil was expected to have the
effect of reducing staining. What was not expected,
however, was that the inclusion of the styrene isoprene di-
S block copolymer provided broad end-use temperature ranges
while at the same time substantially improving die-cutting
performance.
In order to demonstrate and verify these e~fects,
several formulations were prepared, using generally
standard preparation processes and methods, according to
the Examples 1 through 4 shown in the following tables, and
certain testing procedures were employed.
Dynamic Mechanical Analysis, or DMA, is used to
measure how a material responds to an imposed strain or
deformation. This response will either be a viscous,
elastic or vis-coelastic response. DMA is used to model and
predict how a material will respond to real-world phenomena
such as coating, die-cutting, aging and other conditions.
DMA is also used to predict adhesive performance. The DMA
testing used to profile these pressure-sensitive adhesives
was run on 25 mm parallel plates using a fre~uency of 10
radians per second and a 1 or 5 percent strain, as
indicated. Temperature sweeps were performed from 140~ to
-40~C with an initial gap of 1.6 mm.
The "tangent delta" is the ratio of the viscous
re~ponse (G") to the elastic response (G') of a particular
DMA cu ~e. The glass transition temperature (Tg) is
indicated by a peak on the tangent delta curve of a
temperature sweep. Die-cutting performance for the
formulations referred to herein can be approximated by
comparing the tangent delta values at 20~C ~or the
di~ferent formulations. A higher tangent delta value at a
given temperature indicates a material will respond in a
more viscous fashion. The greater the viscous response,
the more likely a material will flow when subjected to an
outside force. Consequently, materials with higher tangent
delta values at a temperature of interest will tend to die-
cut better. They flow and separate when subjected to
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cutting forces, as compared to materials with low tangent
delta values which deform and then recover. Hence, the
higher the tangent delta value at 20~C, the better the die-
cutting performance. The tangent delta value and the Tg of
a formulation, along with various performance tests, will
determine the suitability of a product for a pressure-
sensitive application. The addition of the styrene
isoprene di-block copolymer in the present invention
results in a pressure sensitive adhesive having an
increased tangent delta value of 5~ or greater when
compared to a similar pressure sensitive adhesive which is
identical except for lacking the addition of the styrene
isoprene di-block copolymer.
The pressure sensitive adhesive compositions of
the present invention may be formulated using any of the
techniques well known in the art. A representative example
of a prior art procedure involves placing all of the oil
substances and any optional stabilizer substances in a
jacketed mixing kettle, and preferably in a jacketed heavy
duty mixer of the Baker-Perkins or Day type, equipped with
rotors. The materials are mixed and the temperature of the
mixture is raised to a range from about 250~F to about
350~F. As should be understood, the precise temperature to
be used in this step will depend on the melting point of
the particular ingredients. When the initial mixture has
been heated,- the mixture is blanketed with CO2 at a slow
flow rate and the required tackifiers, or resins, described
above, are slowly added. When all of the ingredients are
melted at the desired temperature, the copolymers are added
to the mixture. The resultant pressure sensitive adhesive
composition mixture is agitated until the copolymers are
completely dissolved. A vacuum is then applied to remove
any entrapped air.
The preparation of coated stock for pressure
sensitive adhesive testing is described as follows.
Adhesive is coated onto a release liner ~SBL 42 SC FlU from
Akrosil or equivalent) such that a 2.25 inch wide
continuous pattern is applied at 1.0~0.05 mils (adhesive
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dry weight). A two mil polyester film is laminated to the
adhesive within 5 to 5.5 seconds ~web speed of ap-
proximately 10 ft./min.) ~or use in 180~ peel and SAFT
testing.
One type of testing, 180~ peel testing, i8
designed to measure the strength of an adhesive bond to a
st~n~d 2 inch by 5 inch stainless steel surface. The
strength of the bond is determined by measuring the force
required to peel a strip of adhesive-coated polyester stock
from a test panel at an angle of 180~ after a specified
dwell time. The stock is conditioned in a controlled
environment (72i2~F/50i3~ relative humidity) for a minimum
of 12 hours before l inch by 10 inch strips are cut from
it. Each strip is applied to the required stainless steel
panel, adhesive side down, with a mechanical roll-down
machine that uses two passes of a 4.5 pound rubber roller
mo~ing at 12 in./min. Each sample is allowed to dwell for
15 minutes before separating at a rate of 12 in./min. on a
tensile tester. The peel angle must be maintained at 180~.
The resulting peel force is measured and reported in pounds
per linear inch. A m; n ~ mllm o~ three trials should be run
and then the average taken.
Another type of testing is the Shear Adhesion
Failure Temperature (SAFT) test. This is a measure o~ the
internal strength o~ an adhesive at elevated temperatures.
An adhesive-coated polyester stock sample is applied to a
standard 2.5 inch by 2.5 inch stainless steel panel. A
constant load is applied and the temperature raised 3~F per
minute until the sample fails from the panel. The stock is
conditioned in a controlled environment (72i2oF/5oi3%
relative humidity) for a mln;mnm of 12 hours before 1 inch
by 3 inch strips are cut from it. Each strip is applied to
the required stainless steel panel using sufficient
pressure such that a cohesive failure results when the
sample fails. Each sample is allowed to dwell for one hour
before placing in an oven capable o~ raising the
temperature at a constant rate of 3~F per minute. The test
panels are hung vertically in the oven. A 1000 gram weight
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~unless otherwise specified) is attached to each sample and
allowed to hang until the sample fails. The oven must be
conditioned to a constant temperature of 90~F and held for
7 one minute before raising the temperature. The test is run
5 until all samples fail from their respective panels. The
temperature at which failure occurs is then recorded. All
failures should be cohesive in nature. A cohesive failure
results when there is residual adhesive on both the test
panel surface and the coated substrate. An adhesive
10 failure, indicating delamination from either the coated
substrate or the test panel, with no residue, will not
result in a representative value. It indicates the
temperature at which the adhesive failed from the test
panel or substrate, and is no indication of the internal
15 strength of an adhesive. A minimum of three trials should
be run and then the average taken. The pressure sensitive
adhesive of the present invention preferably demonstrates a
ri~e or a minimal decrease in SAFT. This preferable
minimal decrease corresponds to a decrease of 5~F or less
2~ for a pressure sensitive adhesive of the instant invention,
when compared to a similar pressure sensitive adhesive
which is identical except for lacking the addition of the
styrene isoprene di-block copolymer.
As to each example formulation, 180~ peel tests
25 and shear adhesion failure temperature tests were
conducted. The results of those tests on the improved
adhesives provided by this invention were compared with the
results of those tests conducted on prior art adhesives as
controls, and the results tabulated. The glass transition
30 temperature Tg and tangent delta were also calculated and
are presented in the same tables.
The pressure sensitive adhesives of the present
invention preferably demonstrate a minimal increase in Tg.
This minimal increase in Tg corresponds to an increase of
35 3~C or less for a preferred pressure sensitive adhesive of
the instant invention, when compared to a similar pressure
sensitive adhesive which is identical except for lacking
the addition of the styrene isoprene di-block copolymer.
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Table 1 shows controls where, besides the use of
Kraton 1107 as the base polymer, and Piccotac 95, from
Hercules, as a normally solid tackifier, the mixture
includes 15~ by weight of conventional plasticizers.
Shellflex 371, a hydrocarbon processing oil, is used as the
conventional plasticizer in Control 1, while Wingtack 10, a
normally liquid tackifier, is used as the conventional
plasticizer in Control 2. In Example 1, LVSI-101 is the
only ingredient used as a plasticizer. Example 2 includes
use of a combination of two-thirds LVSI-101 and one-third
Shellflex 371 as a plasticizer, while Example 3 includes
use of a combination of one-third LVSI-101 and two-thirds
Shellflex 371 as a plasticizer. In each of these
formulations Irganox 1010, a phenolic antioxidant, is added
to reduce oxidative degradation during processing and to
improve shelf life performance. The DMA testing for each
of the formulations shown in Table 1 was performed at 5
strain. As can be seen below, each of the examples has
peel adhesion, equal or superior to Control 2, which has
the liquid tackifier as plasticizer, and clearly superior
to Control 1, which has the processing oil as the
plasticizer. Further, as indicated by the Shear Adhesion
Failure Temperature figures, the examples show
signi~icantly increased high temperature strength over
conventional adhesives.
Moreover, while Control 2 does have the highest
tangent delta value at 20~C, its Tg is also much higher
than any of the examples. This fact indicates that the low
temperature performance of this formulation has been
compromised to achieve that high a tangent delta value. In
fact, because the Tg is above 15~C, whereas the Tg of each
of the examples is below 10~C, the adhesive or pressure
sensitive properties of the formulation of Control 2 will
be generally unacceptable for the great majority of
conventional applications. The examples all have Tg values
comparable to Control 1, and all the examples show
sig~i~icant increases in tangent delta values at 20~C over
Control 1. The pressure sensitive adhesives of the present
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invention exhibit a single Tg value. This single Tg value
is the result of the styrene isoprene di-block copolymer
and the remaining elastomers being miscible with the
tackifier. Accordingly, the examples possess wider
temperature ranges than the controls and the improved
tangent delta values indicate a substantial improvement in
ease of conversion.
Table 1
Control 1 Control 2 Example 1 F.Y~mrl~ 2 E~ample 3
.~h~.llfl~Y 371 15 - - 5 10
Wingtack 10 - 15 - - -
LVSI-101 - - 15 10 5
Piccotac 95 55 55 55 55 55
Kraton 1107 30 30 30 30 30
Irganox 1010
180~ Peel to 4.9 5.5 5.5 5.6 5.9
Stainless Steel
Shear Adhesion164 162 179 168 168
Failure T~ d~u-~;
(~F)
2 0 T, (~C) 7.17 16.34 9.37 8.99 7.12
Tangent Delta at 1.069 2.404 1.179 1.213 1.173
20~C
Table 2 compares Example 17 from Sasaki et al,
U,S, Patent No. 5,290,842, here labeled Control 3, to a
25 formulation provided by the invention, indicated as ~xample
4. The difference between the two formulations is the
replacement of half of the processing oil, Shellflex 371,
with the styrene isoprene di-block copolymer LVSI-101. The
DMA testing for each of the formulations shown in Table 2
was performed at l~ strain. Again, as can be seen, the
180~ peel test and shear adhesion failure temperature tests
yield much improved figures for Example 4 as compared to
Control 3, indicating a substantial improvement in adhesive
properties over a formulation described in the Sasaki
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patent as having "excellent adhesive properties". Further,
the tangent delta ~igures indicate much easier converting,
again over a formulation described by Sasaki as exhibiting
"excellent convertibility".
Table 2
Control 3 Exa~ple 4
LVSI-101 - 12
Solprene 1205 21.2 21.2
K~aton D-1112 16.8 16.8
Escorez 1310 LC 38 38
10.~h~llflrY 371 24 12
Ethyl 330a 0.7 0 7
Cyanox LTDPb 0.7 0 7
180~ Peel to.St~in~ Steel 2.8 3.9
Shear Adhesion Failure T ~ c~ 130 154
(O~
Tg ~~C) -7.12 -4.9
Tangent Delta at 20~C~ 0.556 0.696
~phenolic antioxidant; ~propionic aci~ antioxidant.
Finally, the effect of other, non-styrene-
containing low molecular weight polymers was compared, by
comparing the ~ormulation from Example 1 above to
formulations using these polymers. In particular, low
molecular weight isoprene rubber polymers LIR-30 and LIR-
50, from Kuraray, through Arakawa, were selected ~or
Controls 4 and 5. Here again, the DMA testing for each o~
the formulations shown in Table 3 was performed at 5~
strain. Here the control formulations provide neither the
peel adhesion nor the high temperature performance of a low
molecular weight styrene-isoprene copolymer.
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Table 3
Control 4 Control 5Example 1
~VSI-101 - - 15
Piceotac 95 55 55 55
Kraton 1107 30 30 30
LIR-30 15 - -
LIR-50 - 15
Irganox 1010
180~ Peel to Stainless Steel 4.1 4.8 5.5
Shear Adhesion Failure T~ JC-~IUI~ 154 167 179
1 0 (~F)
T~ ~~C) 4.12 3.87 9.37
Tangent Delta at 20~C 1.124 0.928 1.179
While the adhesives hereinbefore described are
effectively adapted to ~ul~ill the aforesaid objects, it is
15 to be understood that the invention is not intended to be
limited to the speci~ic pre~erred embodiments of pressure
sensitive labeling adhesives set forth above. In
particular without limitation, it should be understood that
the formulations discussed herein may include additional
oils, fillers, extenders, pigments, dies, indicators,
stabilizers, and other such ingredients as may be desired
to achieve certain desirable or desired properties or avoid
certain undesirable or undesired properties. Additionally,
the formulations discussed herein may be free o~ organic
solvents. The claimed invention, however, is to be taken
as including all reasonable equivalents to the subject
matter of the appended claims.
. . ~
t
