Adhesive composition and method of preparation

Adhesive composition comprised of an unvulcanized block polymer product and a compatible tackifier resin. Said adhesive composition is required to be prepared by (A) mixing a solution of a block styrene/isoprene polymer product with a solution of a tackifying resin and (B) optionally removing the solvent.

FIELD OF INVENTION 
This invention relates to adhesive compositions and method of preparation. 
The invention has particular reference to pressure sensitive adhesives of 
both the solution cast and the hot melt application types. 
BACKGROUND OF INVENTION 
Various adhesives can individually be advantageously used as pressure 
sensitive, laminate or hot melt adhesives. Most are generally known for 
their ability to form articles by bonding laminates together. Some are 
more particularly known as pressure sensitive adhesives and some as hot 
melt adhesives. 
Pressure sensitive adhesives and adhesive-coated masking tapes and the like 
are well known. These products commonly contain a mixture of natural 
rubber or styrene/isoprene/styrene block polymer and a thermoplastic 
tackifier resin compatible therewith. The adhesive, coated on a flexible 
backing to form a tape, is tacky at normal room temperatures and adheres 
instantly and tightly to surfaces against which it is pressed without 
requiring moistening or heating. It typically is required to have a 
balance of good shear strength, a satisfactory 180.degree. peel strength, 
and good tack property in order to be commercially useful. 
Adhesives are many times based on mixtures of rubbery materials and 
tackifiers. Pressure-sensitive adhesives typically require more 
sophisticated properties than simple tackified rubber compositions. They 
must have especially balanced properties of adhesion, cohesion and 
elasticity in addition to peel and shear strengths to be useful for 
adhering materials together on a pressure-sensitive basis. 
Many times, pressure-sensitive adhesives can be based on mixtures of 
tackifiers with natural rubber or with block copolymers of isoprene and 
styrene. However, such adhesive composites are often deficient in 
desirable properties. 
It is, therefore, an object of this invention to provide an adhesive 
composition of a tackifier and a rubbery block polymer product of styrene 
and isoprene. 
DISCLOSURE AND PRACTICE OF INVENTION 
In accordance with this invention, an adhesive composition has been 
discovered which is comprised of an admixture of an unvulcanized block 
polymer product and a compatible tackifier resin therefor prepared by the 
method which comprises (A) mixing (1) 100 parts by weight of a solution 
comprised of 100 parts by weight of an unvulcanized block styrene/isoprene 
polymer product with an overall styrene polymer content in the range of 
about 10 to 35 weight percent and a solvent therefor in the range of about 
300 to about 1100, preferably about 400 to about 1000, parts by weight 
with (2) about 50 to about 200, preferably about 100 to about 150, parts 
by weight of a solution comprised of 100 parts by weight of a tackifying 
resin for said block polymer product and a solvent therefor in the range 
of about 25 to about 1100, preferably about 100 to about 1000, parts by 
weight and (B) optionally removing said solvent from the prepared mixture 
prior to its application as an adhesive; wherein the total of said 
solvent(s) is a solvent for the combination of said block polymer and said 
resin; wherein said block polymer is (i) produced by the process of 
contacing isoprene and a suitable amount of divinylbenzene under solution 
polymerization conditions with a polystyryllithium catalyst, said amount 
of divinylbenzene being from about 0.5/1 to about 10/1, preferably about 
0.5/1 to about 3/1, moles of divinylbenzene per mole of active lithium 
catalyst or is (ii) of the linear or radial type styrene/isoprene/styrene 
block copolymer prepared by sequentially polymerizing styrene and isoprene 
with a lithium based initiator and wherein said resin has a softening 
point in the range of about 50.degree. C. to about 170.degree. C., 
preferably about 70.degree. C. to about 115.degree. C. 
The (B) option of removing the solvent before application would be used for 
hot melt adhesive applications. For solution adhesive applications, 
solvent could be retained or the mixture dried and redissolved prior to 
application. 
It is an important feature of this invention that the adhesive composition 
can be prepared by directly mixing the polymerizates of either or both the 
block polymer product composed of (A-1) and/or the resin polymerizate 
composed of (A-2). 
Thus, the adhesive composition can be formed by a method selected from (A) 
mixing the block polymer polymerizate with a solution of the solid resin 
dissolved in its solvent, (B) mixing a solution of the solid block polymer 
product dissolved in its solvent with a solution of the solid resin 
dissolved in its solvent, (C) mixing a solution of the solid block polymer 
dissolved in its solvent with the resin polymerizate or (D) mixing the 
block polymer polymerizate with the resin polymerizate. 
The term polymerizate is intended to refer to a solution of the block 
polymer product or resin as a direct result of its preparation or 
polymerization, of its precursor monomers in the presence of its solvent 
and catalyst, deactivated and optionally removed. 
The terms solid block polymer product and solid resin refer to such 
materials in their recovered form from the polymerizates. When they are 
dissolved in their solvent they become a solution. It is readily 
recognized that this solution-forming procedure from the solid product 
requires an additional step and effort as compared to the utility and/or 
convenience of directly using the polymerizate. 
In the practice of this invention, the blending or mixing process can be 
generally accomplished, for example, by using a turbine blade agitation 
system for a period of about three hours at about 20.degree. C. to 
30.degree. C. 
The adhesive composition can conveniently be recovered, for example, by 
application of conventional steam stripping and drying procedures, such as 
oven drying, to remove solvent. 
It was unexpectedly found that by using the solution preparation process of 
this invention, a resulting adhesive composition was observed having 
increased tack, faster molten solution time and excellent tack retention 
of the hot melt blend properties as compared to simply dry blending the 
block polymer product and tackifying resin. 
A disclosure of such dry blending can be found in U.S. Pat. No. 4,172,860 
filed Oct. 2, 1978, which is incorporated herein by reference. 
In order to effectively provide a solution of the resultant adhesive 
composition by the method of this invention, it is readily understood that 
solvents or solvent mixtures are selected in which the block polymer 
product and the resin are mutually soluble. For example, the block polymer 
product is generally soluble in aromatic hydrocarbons such as toluene and 
benzene as well as some cycloaliphatic hydrocarbons such as cyclohexane, 
cyclopentane and methylcyclopentane. Tackifying resins are also usually 
soluble in such solvents with some being soluble in aliphatic hydrocarbons 
and chlorinated aliphatic hydrocarbons. Mixtures of solvents can be used 
with solubility advantages tailored to both the block polymer and resin. 
It is generally preferred that the solvent or solvent mixture of the 
resulting mixed solutions contain about 80 to about 100 weight aliphatic 
hydrocarbon solvent based on the solvent(s) alone. Thus, the choice of 
solvent or solvent mixture must depend upon the type of resin used as well 
as the block polymer product which can readily be determined by one having 
skill in the art. The term hydrocarbon solvent is intended to refer to 
liquid hydrocarbons and liquid halo-substituted hydrocarbons. 
In the practice of this invention, said tackifier is a resin preferably 
having a softening point in the range of about 50.degree. C. to about 
150.degree. C., preferably about 70.degree. C. to about 115.degree. C., 
and is preferably selected from at least one of terpene resins and 
synthetic hydrocarbon derived resins of diolefin/olefin copolymers. 
Coumeroneindene type resins may be satisfactory. The diolefin/olefin 
copolymers are generally of the type prepared by copolymerizing in the 
presence of a catalyst selected from aluminum chloride or boron 
trifluoride or boron trifluoride etherate, a mixture of piperylene and at 
least one olefin containing 4 to 6, preferably 5 to 6, carbon atoms in a 
diolefin/olefin mole ratio in the range of about 0.6/1 to about 2.5/1, 
preferably about 0.8/1 to about 1.6/1. Optionally, said copolymer can be 
modified by copolymerizing therewith about 5 to about 20 weight percent, 
based on the total monomers, of at least one additional monomer selected 
from .alpha.-methylstyrene and dicyclopentadiene. Some of such resins can 
have melting points according to ASTM E28-58T less than the required 
range. However, lower softening resins can be conveniently mixed with 
higher softening point resins to achieve the required range. 
A particularly desirable type of tackifying resin is prepared from 
piperylene and 2-methyl-2-butene in about 0.8/1 to about a 1.6/1 mole 
ratio, with the optional inclusion of .alpha.-methylstyrene and/or 
dicyclopentadiene. Aluminum chloride would be a preferred catalyst. 
In the further practice of this invention, the unvulcanized elastomeric 
block copolymer (i) preferably has its first polystyrene block with a 
molecular weight in the range of about 8,000 to about 45,000 with the 
overall styrene content in the copolymer being in the range of about 10 to 
about 35 weight percent, preferably 10 to 20 percent. 
Additional description of a required block copolymer (i) for this invention 
and methods for its preparation are found in U.S. Pat. No. 3,949,020 and 
it is herein incorporated by reference. 
In the block copolymer (i), the molecular weight of the polystyrene block 
and the molecular weight of the polydiolefin block can range quite 
broadly. Also, the weight ratio between the polystyrene and the 
polydiolefin can range quite broadly. The polystyrene block molecular 
weight can range from about 8,000 to about 45,000. The polydiolefin block 
molecular weight, ignoring the effect of the divinylbenzene (DVB), can 
range from about 10,000 to about 100,000. It should be understood that one 
skilled in the art could adjust these molecular weight limitations and 
ratios to produce a host of materials ranging from soft, relatively weak 
(tensile at break 500 psi) adhesive substrates, through tough 
thermoplastic elastomeric, to plastics. Because these materials are 
branched, they are more easily processed than their linear counterparts. 
The polystyryllithium catalysts useful in the preparation of the block 
copolymer are those that correspond to the formula RLi, in which R is 
polystyrene prepared by reacting alkyllithium with a styrene monomer 
solution forming polystyrene with a lithium atom at one end of the styrene 
chain. The alkyllithiums which can be utilized in the instant invention 
are n-butyllithium, secondary butyllithium, isopropyllithium, amyllithium, 
etc. Similarly, substituted styrenes such as p-methylstyrene, 
p-t-butylstyrene, or other substituted styrenes may be used in place of 
styrene, although for this invention, generally only styrene is preferred. 
For instance, if a styrene-isoprene block polymer branched along the 
polymer chain of the polyisoprene block to other styrene-isoprene units is 
to be prepared, the n-butyllithium is reacted with a styrene solution to 
make a polystyryl lithium catalyst which can then be added to an isoprene 
solution containing the branching agent and polymerizing the isoprene 
solution until the desired block polymer is formed. 
The polymerization reaction is usually carried out in an inert solvent such 
as a hydrocarbon. It has been found however that all hydrocarbons will not 
be particularly conducive. Some hydrocarbons that can be utilized for 
preparing the copolymer are benzene, toluene, cyclohexane, cyclopentane 
and methyl cyclopentane. 
The unvulcanized elastomeric block copolymer (ii) of the linear or radial 
type which can be used in this invention comprises nonelastomeric terminal 
styrene polymer blocks having a molecular weight in the range of about 
5000 to about 30,000 and internal elastomeric isoprene polymer blocks 
having a molecular weight in the range of about 50,000 to about 150,000, 
where the styrene polymer block(s) is about 10 to about 30 weight percent 
of the copolymer. The characteristics of the internal or middle block, 
which is elastomeric, might be expressed in ASTM Special Technical 
Bulletin No. 184 as follows: "A substance that can be stretched at room 
temperature to at least twice its original length and, after having been 
stretched and the stress removed, returns with force to approximately its 
original length in a short time." 
The terminal nonelastomeric blocks constitute polymeric blocks which do not 
meet this definition. Furthermore, the difference between glass transition 
temperatures of the end blocks and middle blocks should be at least 
40.degree. C. and preferably at least 100.degree. C. 
Typically, the unvulcanized elastomeric copolymers (i) can be prepared by 
first polymerizing styrene with a lithium based initiator. A suitable 
lithium based initiator is, for example, lithium metal, alkyl lithium 
compounds, lithium hydrocarbyls and organo lithium amides. The alkyl 
lithium compounds are preferred. Particularly preferred alkyl lithium 
compounds are branched chain, preferably secondary alkyl lithiums. Such 
alkyl lithium compounds especially include secondary butyl lithium, 
isobutyl lithium, isoamyl lithium and secondary amyl lithium. 
After polymerizing the styrene to an average molecular weight of from about 
2000 to about 100,000, and preferably from about 5000 to about 30,000, 
isoprene and additional amounts of styrene are added to the polymerization 
mixture. 
The polymerization then continues to provide an elastomeric middle block 
isoprene polymer having an average molecular weight of from about 25,000 
to about 1,000,000, and more preferably between about 50,000 and about 
500,000, followed by a non-elastomeric styrene block polymer to provide 
the block copolymer effect. 
Typically, the preparation of the elastomeric block copolymer is made by 
polymerizing the monomers at temperatures in the order of about 
-20.degree. C. to about 100.degree. C., preferably between about 
20.degree. C. and about 65.degree. C. in an inert hydrocarbon solvent such 
as an alpha olefin or aromatic hydrocarbon. Typically inert hydrocarbon 
solvents are aliphatic hydrocarbons (including cycloaliphatic) such as 
cyclohexane and a typical aromatic hydrocarbon is benzene. Usually the 
amount of initiator should be maintained as low as possible, but may be 
varied over a relatively wide range such as from about 1 to about 200 
parts per million, based on the weight of the monomers present. 
In the practice of this invention, the adhesive composition is useful for 
bonding substrates together. Although utility as a solution cast adhesive 
can be shown, the adhesive composition prepared by this invention has been 
found to particularly provide an enhanced usefulness in hot melt adhesive 
applications. Apparently the solution mixing as a method of preparation is 
an important factor. 
The adhesive composition of this invention can suitably bond substrates 
together such as by simply applying it as a hot melt to substrate surfaces 
and adhering the surfaces together with a thin film of the mixture 
therebetween. Alternately, it can be applied in its solution form. The 
mixture solution is applied to at least one of the substrate surfaces, 
followed by at least partially drying it to enhance its tack, bringing the 
substrate surfaces together with the application of pressure and drying 
the adhesive to enhance the bond. 
Pressure-sensitive tapes can be prepared by applying a solution of the 
adhesive composition to a flexible substrate and drying it. 
The composition, as a hot melt pressure sensitive adhesive, can be prepared 
by mixing the block copolymer, tackifying resin, and thermoplastic polymer 
such as polyethylene, polyisobutylene, polystyrene, 
poly-.alpha.-methylstyrene, ethylene-vinyl acetate copolymers and a minor 
amount of an oil such as petroleum-derived or coal tar-derived oil such as 
mineral oil or rubber processing oil. The hot melt adhesive is applied by 
heating the mixture, applying it to a substrate and cooling. 
In the preparation of the adhesives of this invention it is understood that 
minor amounts of various antioxidant and fillers can be mixed therewith.

The following examples further illustrate the invention and are not 
intended to be limiting. The parts and percentages are by weight unless 
otherwise indicated. 
EXAMPLE I 
A resin is prepared as follows: 
To a reactor is charged 200 parts of heptane and 6 parts of anhydrous 
aluminum chloride at a temperature of about 25.degree. C. While stirring 
the mixture, 400 parts of a hydrocarbon mixture containing unsaturated 
hydrocarbons is slowly added to the reactor over a period of about 90 
minutes. The temperature of the reaction is maintained at about 25.degree. 
C. to 30.degree. C. for about 60 minutes. The hydrocarbon mixture has the 
following composition: 
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Component Parts 
______________________________________ 
2-pentene 5.3 
2-methyl-1-butene 2.3 
2-methyl-2-butene 37.5 
Isoprene 0.2 
1,3-pentadiene (piperylene) 
45.0 
Cyclopentene 7.5 
Unsaturated hydrocarbons containing 
5 to 6 carbon atoms 2.3 
100.1 
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After the polymerization, 25 parts of water, 8 parts of particulate 
hydrated calcium hydroxide and 8 parts of Attapulgus clay are added to the 
polymerizate. Agitation is provided to suspend the hydrated calcium 
hydroxide at about 90.degree. C. and continued for about 120 minutes. The 
mixture is then filtered to remove the insoluble catalyst residue, excess 
calcium compound and clay. The filtrate is then steam distilled to a pot 
temperature of about 235.degree. C. The resulting molten resin is poured 
from the reactor onto an aluminum tray and cooled to about 25.degree. C. 
to form 173 parts of a hard brittle resin having a pale yellow color, and 
having a softening point, according to ASTM Method E28-58T of about 
98.degree. C., although it has been observed that softening points in the 
range of about 95.degree. C. to about 101.degree. C. are generally 
obtained. 
EXAMPLE II 
A styrene/isoprene block copolymer is generally prepared according to the 
method of Example II of U.S. Pat. No. 3,949,020. 
EXAMPLE III 
Adhesives were prepared by solution masterbatch according to this invention 
and by individual blending and compared. The tackifying resin was prepared 
generally according to Example I and the styrene/isoprene block polymer 
product was prepared according to the general method referred to as 
Example II. 
In the practice of this invention, it is preferable that the polystyrene 
block has a molecular weight in the range of about 10,000 to about 25,000, 
based on number average, for hot melt applications. 
The solution masterbatch was prepared by blending the unvulcanized block 
polymer dissolved in solvent with the tackifier dissolved in solvent using 
a turbine blade agitator at about 25.degree. C. The masterbatch was mixed 
for 3 hours and the adhesive composition recovered in a conventional 
stripping and drying manner. The adhesive composition was oven dried at a 
temperature of about 50.degree. C. for about 6 hours. 
The adhesive composition was applied as a pressure sensitive adhesive by 
mixing with additional toluene while agitating to form a 33.3 weight 
percent solution. The solution was cast on one mil thickness film of Mylar 
(a trademark of the E. I. DuPont de Nemours & Co. for a polyester film) 
which was pulled between two steel rolls at a steady rate which were 
positioned at approximately a 30.degree. vertical angle to each other for 
a doctoring effect for the coating. The coated film was oven dried for 
about 5 minutes at about 70.degree. C. to form a one mil thickness dry 
coating on the film. 
The same adhesive composition could be applied as a hot melt by first 
drying to remove solvent and then heating the mass to about 190.degree. 
C.-205.degree. C., coating the Mylar film therewith and doctoring by 
pulling the coated film through the rolls which would be heated to about 
190.degree. C.-210.degree. C. Thus, the one mil adhesive film coating can 
be deposited and formed in this manner without the use of solvent. 
The individual blending of an adhesive composition was accomplished by 
sequentially mixing toluene with the block polymer product, then the 
tackifying resin and then a small amount of antioxidant to form a 
solution. 
The solution of the individually blended mixture was applied as a pressure 
sensitive adhesive by coating it on a one mil Mylar film and doctored by 
pulling through two rollers which were at approximately a 30.degree. 
vertical angle to each other at a steady rate. The sheet of coated film 
was oven dried for about 5 minutes at about 70.degree. C. to form a one 
mil thickness dry film coating. 
Comparative data of the two types of pressure sensitive adhesive 
compositions is shown in the following Tables 1 and 2. Table 1 relates to 
the compositions and Table 2 relates to data pertaining to the 
compositions. The compositions are identified herein as experiments, or 
compositions, A-D. Experiment D represents the invention utilizing the 
mixing of individual solutions of block copolymer and tackifying resin 
followed by drying the solution mixture. The dried composition was 
redissolved in toluene and dried to yield the adhesive. The block 
copolymer was used directly in its polymerizate form where cyclohexane had 
been used as the polymerization solvent. The tacifying resin had been 
previously recovered and dried from its polymerizate followed by 
redissolving it in cyclohexane. 
Composition A-C were prepared by simply mixing toluene with the dried block 
polymer, adding the dried resin thereto to form a solution and then drying 
the solution mixture. 
TABLE 1 
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Adhesive Compositions 
A B C D 
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styrene/isoprene/styrene 
56 -- -- -- 
block copolymer.sup.1 
styrene/isoprene/styrene 
block copolymer.sup.2 
-- 56 -- -- 
styrene/isoprene block 
copolymer with DVB.sup.3 
-- -- 56 -- 
styrene/isoprene block 
copolymer mixed with 
tackifying resin.sup.4 
-- -- -- 100 
Tackifying resin.sup.5 
44 44 44 -- 
Phenolic antioxidant 
1 1 1 1 
Toluene 200 200 200 200 
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.sup.1 Obtained as Kraton 1107, a product of the Shell Chemical Co., 
reportedly containing 14% styrene end blocks 
.sup.2 Obtained as Solprene 423P, a product of the Phillips Petroleum Co. 
reportedly containing 15% styrene end blocks 
.sup.3 Prepared generally according to Example II 
.sup.4 Tackifying resin was prepared according to Example I. Block 
copolymer was prepared by method of Example II. The resin was recovered 
and dried from its polymerizate, then solution mixed with the block 
polymer polymerizate in its cyclohexane solvent in a copolymer/resin rati 
of 56/44 and then dried. 
.sup.5 Prepared according to method of Example I. 
TABLE 2 
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Adhesive Composition Data 
A B C D 
______________________________________ 
Brookfield Viscosity (cps) 
(Spindle No. 2 @20 rpm) 
840 1105 510 500 
Rolling Ball Tack (In.) 
Initial 2.4 2.9 2.2 1.9 
Aged.sup.1 2.6 3.1 2.4 1.6 
180.degree. Peel Adhesion (Oz.) 
Initial 53 55 51 48 
Aged.sup.1 54 54 53 48 
90.degree. Peel Adhesion (Oz.) 
Initial 38 38 38 35 
Aged.sup.1 36 35 38 33 
Dynamic Shear (psi) 
Initial 43 46 44 41 
Aged.sup.1 38 39 39 37 
Temperature to Failure (.degree.F.) 
1000 G/l sq. in. 
249 252 202 199 
Solution Appearance 
Light Cloudy.sup.2 
Light Light 
Straw Amber Amber 
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Film Appearance 
Initial 
##STR1## 
Aged.sup.1 
##STR2## 
UV Exposure - 96 hrs 
##STR3## 
Light Source - 14-1/2 inches 
Surface Temperature 92.degree. F. 
Exposed Film Tack.sup.3 
(DAYS) 3 1 1 1 1 
12 2 3 2 1 
23 1 2 2 1 
30 1 VL VL 2 
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.sup.1 Oven aged for 7 days at 70.degree. C. 
.sup.2 Probably caused by a partitioning agent 
.sup.3 Number "1" is most observed tack, and "VL" is very little observed 
tack. The observation is a relative value on the day observed. 
EXAMPLE IV 
Hot melt pressure sensitive adhesives were prepared by compounding the 
block copolymers used for the adhesive compositions of A-C of Example III. 
The dry, mixed copolymer/resin of Compound D of Example III shows the 
invention. The recipe using the block polymers and Composition D of 
Example III are shown in Table 3 and identified as AA and DD, 
respectively. 
To prepare the hot melt from block polymers, additional resins, rubber 
processing oil and zinc dibutyl dithiocarbamate were mixed and the mixture 
brought to a molten solution. The block polymer was then added. 
The hot melt from Compound DD was similarly prepared except that an 
additional 5 parts of the block polymer product according to Example II 
and 90 parts of Compound D described in Example III were mixed together. 
The recipes are shown in the following Table 3. 
TABLE 3 
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HOT MELT PRESSURE SENSITIVE ADHESIVES 
AA DD 
______________________________________ 
Tackifying resin.sup.1 80 40 
Coumarone-Indene resin; 155.degree. C..sup.2 
25 25 
Naphthenic petroleum oil 40 40 
Zinc dibutyl dithiocarbamate 
3 3 
Block copolymer of A, B or C from Example III 
55 -- 
Block polymer product of Example II 
-- 5 
Dried Compound D from Example III 
-- 90 
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.sup.1 According to Example I 
.sup.2 Ring and Ball softening point 
The hot melt solution times are shown in the following Table 4. These times 
are particularly important because it shows that by prior solution 
masterbatching the block polymer and tackifying resin, the molten solution 
time is reduced. Long molten solution compounds are generally undesirable 
and typically increase preparation of hot melt costs. The molten solution 
time is the span of time between the addition of the first in ingredient 
in the recipe of this example until the mixture was a smooth, workable 
mass. 
TABLE 4 
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Hot Melt Solution Time 
Time to Molten 
Compound.sup.1 
Experiment Solution (minutes).sup.2 
______________________________________ 
A AA 70 
B AA 67 
C AA 40 
D DD 36 
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.sup.1 Compounds from Example III. 
.sup.2 Pot temperature about 175-190.degree. C. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the invention, it will be apparent to those 
skilled in this art that various changes and modifications may be made 
therein without departing from the spirit or scope of the invention.