Adhesive composition for skin adhesion and bandage applications

A pressure sensitive adhesive composition for skin adhesion and bandage applications which comprises (a) 100 parts by weight of a styrene-isoprene-styrene triblock or multiarm copolymer which has a coupling efficiency of 20 to 50%, an overall absolute arm molecular weight of 33,000 to 100,000, a polystyrene content of 18 to 30%, and a polystyrene block weight average molecular weight of 11,000 to 20,000, and (b) from 100 to 400 parts by weight of a styrene-isoprene diblock copolymer which has an overall absolute molecular weight of 20,000 to 40,000, a polystyrene content of 10 to 25%, and a polystyrene block weight average molecular weight of 4000 to 7000.

CROSSREFERENCE TO RELATED APPLICATION 
This application claims the benefit of U.S. provisional application Ser. 
No. 60/029,303 filed Oct. 24, 1996. 
FIELD OF THE INVENTION 
This invention relates to novel pressure sensitive adhesive compositions 
which are especially formulated for use in skin adhesion and bandage 
applications. More particularly, the present invention relates to the 
pressure sensitive adhesive compositions for such applications which 
contain low coupled styrene-isoprene-styrene triblock or multiarm 
copolymers and low molecular weight, low styrene content styrene-isoprene 
diblock copolymers. 
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. It is known that the adhesive should be formed of a 
composition with sufficient internal strength to prevent leaving a residue 
of the adhesive on the application surface after the adhesive has been 
removed. The problem of providing excellent skin adhesion with block 
copolymer adhesives has persisted. In addition soft polyolefin films or 
films produced from styrenic block copolymers cannot be used with adhesive 
systems which have low molecular weight components such as oil and 
tackifying resins unless a barrier coating is used to block migration of 
these components. 
It is known that block copolymers of vinyl aromatic hydrocarbons and 
conjugated dienes make good adhesives for a number of adhesive 
applications. For example, styreneisoprene-styrene (SIS) triblock 
copolymers are known to make good adhesives for a number of tape 
applications such as packaging tape. However, only certain SIS adhesives 
systems along with a tackifing resin work well for skin adhesion and 
bandage applications. Prior work, such as described in U.S. Pat. No. 
5,274,036, has shown that combinations of low coupling efficiency SIS 
polymers such as KRATON.RTM. 1112 and KRATON.RTM. 1117 block copolymers 
made by Shell Oil Company, with low molecular weight liquid rubbers, such 
as Kuraray LIR 310 (a liquid isoprene-styrene diblock copolymer), Kuraray 
LIR 50 (a liquid isoprene polymer), and Kuraray LIR 290 (a liquid 
hydrogenated isoprene polymer), have utility as medical skin adhesives 
because there are little or no low molecular weight species such as oil or 
tackifying resin which can cause skin irritation. 
However, these combinations exhibit only a marginal peel strength to skin. 
The peel strength needs to be increased to make commercially successful 
products. The present invention provides a specific narrowly defined 
combination of low coupled SIS block copolymers with higher styrene 
contents than the above triblock copolymers and low molecular weight SI 
diblock copolymers with a low styrene content. 
SUMMARY OF THE INVENTION 
This invention provides pressure sensitive adhesive compositions which have 
unusually high peel and shear values for skin adhesion and bandage 
adhesives and which do not require the presence of low molecular weight 
species such as tackifying resins and oils which can cause skin irritation 
and also migrate into flexible olefin or elastomeric bondage film 
substrates. The skin adhesion and bandage pressure sensitive adhesive 
compositions of the present invention are comprised of 100 parts by weight 
(pbw) of a high molecular weight coupled styrene-isoprene-styrene 
copolymer and from 100 to 400 parts by weight of a low molecular weight 
styrene-isoprene diblock copolymer. 
The SIS triblock copolymer must be a low coupled triblock or multiarm 
copolymer wherein the coupling efficiency may range from 20 to 50 percent 
by weight. The polystyrene content of the polymer may range from 18 to 30 
percent by weight. The overall weight average molecular weight of each arm 
(diblock before coupling) of the copolymer of the present invention ranges 
from 33,000 to 100,000 and the polystyrene block molecular weight of these 
copolymers may range from 11,000 to 20,000. The overall absolute molecular 
weight of the SI diblock may range from 20,000 to 40,000. The polystyrene 
content of the SI diblock may range from 10 to 25 percent by weight. The 
polystyrene block weight average molecular weight may range from 4000 to 
7000. 
DETAILED DESCRIPTION OF THE INVENTION 
As is well known, polymers containing both aromatic and ethylenic 
unsaturation can be prepared by copolymeri-ing one or more polyolefins, 
particularly a diolefin, in this case isoprene, with one or more alkenyl 
aromatic hydrocarbon monomers, in this case styrene. The copolymers may, 
of course, be random, tapered, block or a combination of these, in this 
case block. The blocks in the copolymers of this invention are linear or 
multiarm. 
Polymers of conjugated diolefins and copolymers of one or more conjugated 
diolefins and one or more alkenyl aromatic hydrocarbon monomers such as 
the present S-I-S or SI block copolymers comprised of linear or multiarm 
polymeric blocks are frequently prepared in solution using anionic 
polymerization techniques. In general, when solution anionic techniques 
are used, these S-I-S block copolymers are prepared by contacting the 
monomers to be polymerized simultaneously or sequentially with an 
organoalkall metal compound in a suitable solvent at a temperature within 
the range from about -150.degree. C. to about 300.degree. C., preferably 
at a temperature within the range from about 0.degree. C. to 100.degree. 
C. Particularly effective anionic polymerization initiators are 
organolithium compounds having the general formula: 
EQU RLi.sub.n 
Wherein: 
R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted aromatic 
hydrocarbon having from 1 to about 20 carbon atoms; and n is an integer of 
1 to 4. 
The concentration of the initiator can be regulated to control the 
molecular weight of the overall composition and of the polystyrene blocks. 
Generally, the initiator concentration is in the range of about 0.25 to 
about 50 millimoles per 100 grams of monomer. The required initiator level 
frequently depends upon the solubility of the initiator in the hydrocarbon 
diluent. The ratio of the initiator to the monomer determines the block 
size, i.e. the higher the ratio of initiator to monomer the smaller the 
molecular weight of the block. 
In general, any of the solvents known in the prior art to be useful in the 
preparation of such polymers may be used. Suitable solvents, then, include 
straight- and branched-chain hydrocarbons such as pentane, hexane, 
heptane, octane and the like, as well as, alkyl-substituted derivatives 
thereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, 
cycloheptane and the like, as well as, alkyl-substituted derivatives 
thereof; aromatic and alkyl-substituted aromatic hydrocarbons such as 
benzene, naphthalene, toluene, xylene and the like; hydrogenated aromatic 
hydrocarbons such as tetralin, decalin and the like; and linear and cyclic 
ethers such as methyl ether, methyl ethyl ether, tetrahydrofuran and the 
like. 
As described in U.S. Pat. No. 4,096,203 the disclosure of which is herein 
incorporated by reference, usually the styrene is contacted with the 
initiator. Next, the living polymer in solution is contacted with 
isoprene. The resulting living polymer has a simplified structure 
A--B--Li. It is at this point that the living polymer is coupled. 
There are a wide variety of coupling agents that can be employed. Any 
polyfunctional coupling agent which contains at least two reactive sites 
can be employed. Examples of the types of compounds which can be used 
include the polyepoxides, polyisocyanates, polyimines, polyaldehydes, 
polyketones, polyanhydrides, polyesters, polyhalides, and the like. These 
compounds can contain two or more types of functional groups such as the 
combination of epoxy and aldehyde groups, isocyanate and halide groups, 
and the like. Various other substituents which are inert in the treating 
reaction can be present such as hydrocarbon radicals as exemplified by the 
alkyl, cycloalkyl, aryl, aralkyl and alkryl groups and the alkoxy, 
aryloxy, alkyhio, arylthio, and tertiary amino groups. Many suitable types 
of these polyfunctional compounds have been described in U.S. Pat. Nos. 
3,595,941; 3,468,972, 3,135,716; 3,078,254, and 3,594,452, the disclosures 
of which are herein incorporated by reference. When the coupling agent has 
two reactive sites such as dibromoethane, the polymer will have a linear 
ABA structure. When the coupling agent has three or more reactive sites, 
such as silicon tetrachloride, the polymer will have a branched structure, 
such as (AB).sub.n BA. 
In the prior art, such as that exemplified by U.S. Pat. Nos. 3,595,941 and 
3,468,972, the disclosures of which are herein incorporated by reference, 
the effort was always made to select the particular coupling agent or 
reaction conditions that resulted in the highest coupling efficiency. 
Lower coupling efficiencies are desired herein. Coupling efficiency is 
defined as the weight of molecules of coupled polymer divided by the 
weight of molecules of coupled polymer plus the weight of molecules of 
uncoupled polymer. Thus, when producing an SIS linear polymer, the 
coupling efficiency is shown by the following relationship: 
##STR1## 
Coupling efficiency can be determined theoretically from the 
stoichiometric quantity of coupling agent required for complete coupling 
or coupling efficiency can be determined by an analytical method such as 
gel permeation chromatography. Typical prior art coupling efficiency is 
from about 80% to almost 100%. In U.S. Pat. No. 4,096,203, coupling 
efficiency is controlled from about 20% to about 80%, preferably about 30% 
to about 70%. It is also within the scope of the present invention to 
blend polymers from processes of differing coupling efficiency. For 
example, if a 60% efficiency is desired, then polymers from processes 
having an 80% efficiency and a 40% efficiency may be blended together or a 
100% triblock may be blended with a 100% diblock in a 60:40 ratio. 
This coupling efficiency is controlled by a number of methods. One method 
to reduce coupling efficiency is to add less than the stoichiometric 
amount of coupling agent required for complete coupling of the polymers. 
Another means of reducing coupling efficiency is by the premature addition 
of a terminator compound. These terminators, such as water or alcohol, 
respond very quickly and could easily be employed to cut short complete 
coupling of the polymers. In addition, by performing the coupling reaction 
at elevated temperatures, such as above about 190.degree. F., thermal 
termination of many of the living polymer groups (A--B--Li) occurs prior 
to coupling. The typical coupling conditions include a temperature of 
between about 150.degree. and about 170.degree. F. 
Following the coupling reaction or when the desired coupling efficiency has 
been obtained, the product is neutralize such as by the addition of 
terminators, e.g., hydrogen, water, alcohol or other reagents, for the 
purpose of terminating any residual unreacted lithium anions. The product 
is then recovered such as by coagulation utilizing hot water or steam or 
both. 
The adhesive compositions of this invention should contain 100 parts by 
weight (pbw) of a high molecular weight low coupled SIS copolymer and from 
100 to 400 pbw of low molecular weight diblock copolymer. If less than 100 
pbw of the diblock is used, then the adhesive formulation will have poor 
tack and peel properties. If more than 400 pbw is used, then the shear and 
holding power properties suffer. It is preferred that the amount of 
diblock range from 200 to 300 pbw because this gives a good balance of 
properties. 
The overall absolute molecular weight of each arm of the SIS copolymer 
should range from 33,000 to 100,000. The term "arm molecular weight" is 
used because the SIS block copolymer may be linear or multiarmed. The 
polystyrene content should range from 18 to 30% by weight, preferably 20 
to 24% by weight, to obtain good shear and peel properties. The coupling 
efficiency ranges from 20 to 50% by weight, preferably 25 to 45% by 
weight, to wet out the rough skin surface and to provide good peel 
properties. The polystyrene block weight average molecular weight 
should,be from 11,000 to 20,000 to provide sufficient strength. 
The overall weight average molecular weight of the low molecular weight 
diblock should range from 20,000 to 40,000 to provide low viscosities and 
good tack properties. The polystyrene content should range from 10 to 25% 
by weight, preferably 11 to 18% by weight, to provide good tack 
properties. The polystyrene block weight average molecular weight should 
be from 4000 to 7000, preferably 4000 to 5000, to provide the appropriate 
balance of low viscosity, tack properties, and peel properties. The low 
molecular weight diblock is important for achieving pressure sensitive 
properties and low viscosities for hot melt processing. 
The molecular weights of linear polymers or unassembled linear segments of 
polymers such as mono-, di-, triblock, etc., or the arms of star polymers 
before coupling are conveniently measured by Gel Permeation Chromatography 
(GPC), where the GPC system has been appropriately calibrated. For 
anionically polymerized linear polymers, the polymer is essentially 
monodisperse (weight average molecular weight/number average molecular 
weight ratio approaches unity), and it is both convenient and adequately 
descriptive to report the "peak" molecular weight of the narrow molecular 
weight distribution observed. Usually, the peak value is between the 
number and the weight average. The peak molecular weight is the molecular 
weight of the main species shown on the chromatograph. For polydisperse 
polymers the weight average molecular weight should be calculated from the 
chromatograph curve. The materials used in the columns of the GPC are 
styrene-divinyl benzene gels or silica gels. The solvent is 
tetrahydrofliran and the detector is a refractive index detector. The 
absolute molecular weight can be calculated by calibrating the GPC with 
known polystyrene standards, measuring the styrene block molecular weight 
during the polymerization, and then measuring the total percent styrene of 
the molecule by NMR. The absolute arm molecular weight of the total 
molecule is then calculated by dividing the styrene block molecular weight 
by the % styrene in the arm. 
Methods of controlling the molecular weights of the blocks and the overall 
polymer are quite well known. For instance, such are disclosed in U.S. 
Pat. No. 3,149,182, which states that the amount of monomer can be kept 
constant and different molecular weights can be achieved by changing the 
amount of catalyst or the amount of catalyst can be kept constant and 
different molecular weights can be achieved by varying the amount of the 
monomer, and in U.S. Pat. No. 3,231,635, the disclosures of which are 
herein incorporated by reference, and many others. 
A typical block copolymer composition within the scope of the present 
invention has 100 parts by weight of an SIS block copolymer which has an 
overall absolute molecular weight of 141,000 (and thus, an absolute arm 
molecular weight of 70,500), a coupling efficiency of 35%, a polystyrene 
block weight average molecular weight of 15,500, a polystyrene content of 
22%, and 250 parts by weight of a diblock copolymer. A typical low 
molecular weight diblock copolymer has an overall absolute peak molecular 
weight of 30,000, a polystyrene content of 13% by weight, and a 
polystyrene block molecular weight of 4000. 
It is not necessary to add a tackifying resin that is compatible with the 
elastomeric conjugated diene block as is required in most pressure 
sensitive adhesive compositions. Low levels can be added if necessary to 
adjust tack. The composition of the present invention has sufficient tack 
by virtue of the particular specific combination of polystyrene contents, 
and dangling rubber chain ends, etc. for it to be extremely useful in the 
intended applications. 
It is preferred that the adhesive composition of the instant invention do 
not contain plasticizers, such as rubber extending plasticizers, or 
compounding oils or liquid resins because they are unnecessary and may 
cause other problems such as migrating into certain flexible substrates. 
Optional components of the present invention are stabilizers which inhibit 
or retard heat degradation, oxidation, skin formation and color formation. 
Stabilizers are typically added to the commercially available compounds in 
order to protect the polymers against heat degradation and oxidation 
during the preparation, use and high temperature storage of the adhesive 
composition. 
Stabilizers known in the art may also be incorporated into the adhesive 
composition. These may be for protection during the life of the disposable 
article against, for example, oxygen, ozone and ultra-violet radiation. 
However, these additional stabilizers should be compatible with the 
essential stabilizers mentioned herein-above and their intended function 
as taught herein. 
The adhesive compositions of the present invention are typically prepared 
by blending the components at an elevated temperature, preferably between 
about 130.degree. C. and about 200.degree. C., until a homogeneous blend 
is obtained, usually less than three (3) hours. Various method of blending 
are known to the art and any method that produces a homogeneous blend is 
satisfactory.

EXAMPLES 
A number of compositions were prepared for use in the following 
experiments. These compositions and their characteristics are described 
below. The polystyrene content was determined by NMR spectroscopy. The 
polymers used are identified in Table 1 below. 
The SAFT (shear adhesion failure temperature) was measured by 1".times.1" 
Mylar to Mylar lap joint with a 1 kg weight. SAFT measures the temperature 
at which the lap shear assembly fails under load. Rolling Ball Tack (RBT) 
is the distance a steel ball rolls on the adhesive film with a standard 
initial velocity (Pressure Sensitive Tape Council Test No. 6). Small 
numbers indicate aggressive tack. Holding Power (HP) is the time required 
to pull a standard area (1/2 in..times.1/2 in.) of tape from a standard 
test surface (steel, Kraft paper) under a standard load (2 kg), in shear 
at 2.degree. antipeel (Pressure Sensitive Tape Council Method No. 7). Long 
times indicate high adhesive strength. 180.degree. peel was determined by 
Pressure Sensitive Tape Council Method No. 1. Large numbers indicate high 
strength when peeling a test tape from a steel substrate. Polyken probe 
tack (PPT) was determined by ASTM D-2979. Loop tack (LT) was determined 
using TLMI loop tack tester. High numbers for PPT and LT indicate 
aggressive tack. 
TABLE 1 
______________________________________ 
Polymer Identification 
Sample ID CE(%)* PSC (wt. %) 
Arm Mw PS Block Mw 
______________________________________ 
KRATON .RTM. 1117 
66 17.4 61,000 10,600 
KRATON .RTM. 1119 
35 22 70,500 15,500 
KRATON .RTM. 1112 
61 15 73,000 10,900 
LVSI 101 0 13 31,000 4,000 
KRATON .RTM. 1111 
84 22 70,500 15,000 
PP5586 0 13.8 30,800 4,300 
PP5590 0 13.8 34,800 4,800 
PP5468 0 24 25,800 6,200 
Polyisoprene 
0 0 30,000 0 
PP5469 0 13.9 23,700 3,300 
______________________________________ 
*Polymers with a coupling efficiency of 0 are diblocks. 
TABLE 2 
______________________________________ 
Properties of 1.5 Mils of Adhesive on 1 Mil Mylar 
Control 
Control Control Control 
1 2 3 4 
______________________________________ 
KRATON .RTM. 1117 
100 100 
KRATON .RTM. 1119 100 100 
LVSI 101 250 350 250 350 
Irganox 1010 2 2 2 2 
Test Data 
Adhesion to steel.sup.1 
10.2 12.6 24.4 20.6 
ozs/inch 
Adhesions to skin.sup.2 
2 3.5 4 7 
ozs/inch 
Mass Transfer to skin.sup.3 
0 0 0 0 
Tack (% paper fiber.sup.4 
1 15 40 80 
removal) 
______________________________________ 
.sup.1 180.degree. peel 
.sup.2 90.degree. peel pulled by hand using a Normark "weighin" digital 
scaler. Adhesive was applied to the inside of the arm where there was no 
hair using a 4.5 pound rolling (rolled forward and back one time) for 15 
minutes before pulling. The tape was pulled at 90.degree. at roughly 12 
inches per minute by hand. 
.sup.3 Visual observationadhesive transfer is a qualitative test. The 0 
indicates no transfer detectible visually. 
.sup.4 Adhesive strip was rolled down on "yellow legal pad" paper and 
jerked off quickly by hand. Percent area of adhesive containing paper 
fibers is reported. The fiber tear is a qualitative test. It is a rough 
determination of the percent area that fiber can be detected on the 
adhesive surface. 
From the limited study above, it is obvious that the higher styrene/low 
coupled polymer KRATON.RTM. 1119 polymer is much superior to KRATON.RTM. 
1117 polymer. 
__________________________________________________________________________ 
Formulation A B C D E F G H I J K 
__________________________________________________________________________ 
KRATON .RTM. 1112 
100 100 100 100 100 
KRATON .RTM. 1117 100 100 100 100 
KRATON .RTM. 1119 100 100 
LVSI 101 250 250 
Polyisoprene (30,000) 
250 
PP-5469 250 
PP-5586 250 250 250 
PP-5590 250 
PP-5468 250 250 250 
IRGANOX 1010 
1 1 1 1 1 1 1 1 1 1 1 
Rolling Ball (cm) 
1.6 1.3 1.6 1.9 1.6 1.2 1.3 1.5 2.1 1.3 2 
Polyken Probe (Kg) 
0.44 
0.64 
0.66 
0.89 
0.61 
0.65 
0.62 
0.61 
0.72 
0.63 
0.88 
Loop Tack (oz/in) 42 40 53 38 -- 45 37 
180 Peel (pli) 
0.2 0.6 2 1.8 1.6 1.6 2 2.3 1.7 2.1 2.2 
Holding Power Steel (min) 
1.9* 
26* 51* 1030* 
2150** 
2490** 
1630** 
5430** 
5780** 
5950** 
&gt;6,000** 
Holding Power Kraft (min) 
3* 3* 12* 24* 
160** 
180** 
350** 
3330** 
2120** 
-- &gt;6,000** 
SAFT/Mylar (C) 67 69 81 71 78 70 83 
SAFT/Kraft (C) 56 58 68 60 59 57 67 
__________________________________________________________________________ 
*Holding power 1/2" .times. 1/2" 1kg 
**Holding Power 1" .times. 1" 2 Kg 
180 peel (PSTC test) 
SAFT 1" .times. 1", 500 grams PSTC test 
Rolling Ball PSTC test 
The conclusion that can be drawn from the data above is 
(1) The styrene block of the low molecular weight S-I diblock must be 
greater than approximately 4,000 molecular weight or the peel and shear 
properties are poor. (Formulations A and B compared to C, D, E, F, G, H, 
I, J, K) 
(2) KRATON.RTM. 1119 formulations have superior peel properties SAFT and 
shear properties to rough substrates such as Kraft paper (Formulation G 
compared to E and F, and Formulation K compared to H, I, and J) 
(3) Adhesive formulations based on 1119 and liquid S-I diblocks do not 
contain any low molecular weight, mobile components such as oils and 
tackifying resins. This should allow the use of flexible substrates based 
on poly alpha olefins, low density polyethylene, or styrenic block 
copolymers (SBS, SIS, SEBS, or SEPS) as mentioned in U.S. Pat. No. 
4,024,312) or combinations thereof. Normally these flexible films would 
absorb oils and tackifying resins of normal adhesive formulations, thereby 
changing the adhesive properties over time. Since the low coupled SIS and 
SI liquid diblocks are not mobile or fugitive, the adhesive properties 
will be stable with time and should be ideal adhesives for these flexible 
substrates. 
(4) Formulations containing liquid S-I diblocks which have styrene contents 
greater than 13 percent styrene and styrene endblocks &gt;4000 show improved 
holding power and peel properties, particularly when combined with low 
coupled SIS polymers which have styrene contents greater than 15 percent.