Hot melt adhesive having both high peel and tensile shear strength

Blends of certain block copolymers and epoxy resins, which have superior thermoplastic adhesive properties are disclosed. The blends are prepared by mixing hydroxy-terminated poly(ester/ether) block copolymers with epoxy resins. These unreacted blends are useful as hot melt thermoplastic adhesives having very high peel strength, high tensile shear strength, creep resistance at elevated temperatures, and good viscosity stability.

This invention relates to hot melt thermoplastic adhesive blends of 
copolymers and epoxy resins and to their method of preparation. 
Hot melt adhesives are applied in a heated, molten state to substrates such 
as metals, glass, wood, and the like. Upon cooling, the hot melt adhesive 
forms a bond between the substrates. Thermoplastic adhesives are hot melt 
adhesives which form a bond that is substantially heat reversible such 
that the adhesive will again soften and flow at elevated temperatures with 
a resulting loss in bond strength. 
A superior hot melt thermoplastic adhesive can be one that has a high peel 
strength and a high tensile shear strength. This results in an adhesive 
bond that is both tough and elastomeric. Adhesives that have good tensile 
shear strength but poor peel strength are tough, but brittle. Adhesives 
that have good peel strength but poor tensile shear strength are 
elastomeric, but relatively weak. Another property that is highly 
advantageous for a hot melt adhesive is an acceptable creep resistance at 
elevated temperatures. One drawback of many hot melt adhesives is that 
they have a relatively short pot life; that is, they often cannot be held 
at their application temperatures for extended time periods without losing 
their ability to be applied to substrates or without losing their adhesive 
strength. Short pot life is usually evidenced by significant increases or 
decreases in viscosity while the adhesive is held in the pot at its 
application temperature. 
It is known from publications such as Cella, Journal of Polymer Science: 
Symposium No. 42, pages 727-740 (1973); and U.S. Pat. No. 3,723,568 and 
No. 3,784,520 that certain hydroxy-terminated poly(ester/ether) block 
copolymers have thermoplastic properties. These polymers, which are also 
starting materials in copending U.S. Ser. No. 590,622, exhibit relatively 
high tensile shear strengths but very low peel strengths when they are 
used alone as hot melt thermoplastic adhesives. Epoxy resins are often 
used as adhesive curing agents and have wide application in cross-linking 
or otherwise reacting with various substances in forming thermoset 
adhesives. When epoxy resins are used alone, they are very brittle solids 
at room temperature and exhibit virtually no tensile shear or peel 
strength. 
Accordingly, it is an object of this invention to provide improved hot melt 
thermoplastic adhesive blends that have high tensile shear strengths, 
exceptionally high peel strengths, creep resistance at elevated 
temperatures, and long pot life. 
Another object of the present invention is an improved product and a method 
of forming blends that are improved hot melt thermoplastic adhesives which 
are both tough and elastomeric. 
One other object of this invention is to provide an improved hot melt 
thermoplastic adhesive which exhibits superior adhesive properties, 
particularly high peel strength, even when used on untreated substrates. 
It is another object of this invention to form improved hot melt 
thermoplastic adhesives that have a reasonable viscosity at application 
temperatures and that have the ability to be held at these elevated 
application temperatures for long periods while exhibiting viscosity 
stability and adhesive strength retention. 
This invention covers hot melt blends of epoxy resins with certain 
hydroxy-terminated poly(ester/ether) block copolymers of the formula (I): 
##STR1## 
wherein R' and R" are alkyl, alicyclic, acyclic, aryl, or arylakyl of from 
2 to 12 carbon atoms, p is a number of from 2.4 to 136.0. a is a number 
such that the "hard" segment within the first set of brackets makes up 
about 70 to 20% by weight of the copolymer, and b is a number such that 
the "soft" segment within the second set of brackets makes up about 30 to 
80% by weight of the copolymer. The blends are formed by heating and 
mixing an epoxy resin and a copolymer (I) until a compatible, 
thermoplastic mixture is formed. In use, the hot mixture may be applied to 
substrates and allowed to cool, thereby forming a thermoplastic bond of 
the substrates. These substrates may be pre-or post-heated, if necessary, 
to improve the bond strength. 
Other objects and advantages of the present invention will be apparent to 
those skilled in the art from the detailed description of the invention as 
follows: 
The blends of this invention are based on the discovery that mixtures of 
copolymer (I) with epoxy resin improves the adhesive features of the 
copolymer. These mixtures do not undergo any appreciable chemical 
reaction, but the epoxy resin interacts with the copolymer to improve its 
hot melt thermoplastic properties. The mechanism by which this phenomenon 
is brought about is not precisely known. It is presently believed, 
however, that the long pot life and viscosity stability of the compatible 
mixtures at their application temperatures can be explained at least in 
part by the accepted observation that generally hydroxy groups are very 
sluggish reactors with epoxy resins in the absence of an acidic catalyst. 
The copolymers (I) of the present blends are substantially linear, 
low-to-moderate molecular weight (about 4,000 to 25,000) 
hydroxy-terminated poly(ester/ether) block copolymers. Molecular weights 
of these polymers, when used throughout herein, are average molecular 
weights determined by conventional and group analyses for hydroxy groups, 
utilizing titration with succinic anhydride. 
The hydroxy-terminated, substantially linear poly (ester/ether) block 
copolymer (I) is a polymeric reaction product of: (1) one or more of an 
aromatic, aliphatic, or cycloaliphatic dicarboxylic acid or ester-forming 
derivative thereof; (2) one or more of a low molecular weight aliphatic, 
alicyclic, acyclic, or aromatic diol; and (3) one or more of a 
difunctional polyether, including the poly(alkylene ether) glycols. 
Suitable aromatic dicarboxylic acids include but are not limited to 
terephthalic acid, phthalic acid, isophthalic acid, bibenzoic acid, 
bis-(p-carboxyphenyl) methane acid, p-oxy(p-carboxyphenyl) benzoic acid, 
ethylene bis-(p-oxybenzoic) acid; 1,5-napthalene dicarboxylic acid, 
2,6-napthalene dicarboxylic acid, 2,7-napthalene dicarboxylic acid, 
phenanthrene dicarboxylic acid, and 4,4'-sulfonyl dibenzoic acid. 
Ester-forming derivatives include, for example methyl, ethyl, phenyl, and 
monomeric ethylene glycol esters, and acid halides, such as acid 
chlorides, of such aromatic dicarboxylic acid. 
Representative aliphatic and cycloaliphatic dicarboxylic acids include 
sebacic acid, 1,3-cyclohexane dicarboxylic acid, 1,4-cyclohexane 
dicarboxylic acid, adipic acid, succinic acid, malonic acid, oxalic acid, 
azelaic acid, suberic acid, pimelic acid, maleic acid, fumaric acid, 
glutaric acid, 4-cyclohexane-1, 2-dicarboxylic acid, 2-ethylsuberic acid, 
2,2',3,3'-tetramethyl succinic acid, cyclopentane dicarboxylic acid, 
4,4'-bicyclohexyl dicarboxylic acid, 3,4-furan dicarboxylic acid, and 
1,2-cyclobutane dicarboxylic acid. Also included are ester derivatives 
such as those mentioned relative to the aromatic dicarboxylic acids. 
Suitable low molecular weight diols include dihydroxy compounds such as 
ethylene glycol, propylene glycol, tetramethylene glycol, pentamethylene 
glycol, 2,2-dimethyltrimethylene glycol, hexamethylene glycol, 
decamethylene glycol, 1,2-propanediol, 3-methyl-1,5-pentanediol, 
1,3-cyclobutanediol, 1,4-cyclohexane-B,B-diethanol, 
1,4-cyclohexane-dimethanol, 1,3-cyclopentane dimethanol, 
1,4-cyclohexanediol, resorcinol, hydroquinone, 1,5-dihydroxy naphthalene, 
bis-(p-hydroxy) diphenyl, bis-(p-hydroxyphenyl) methane, and 
bis-(p-hydroxyphenyl) propane. 
The difunctional polyethers are represented by the general formula: 
##STR2## 
wherein R includes H and CH.sub.3, x is an integer from 1 to 11, and n is 
a number from 2.4 to 136.0. Representative of such compounds are the 
following poly(alkylene ether) glycols: poly(ethylene ether) glycol, 
poly(propylene ether) glycol, poly(tetramethylene ether) glycol, 
poly(pentamethylene ether) glycol, poly(hexamethylene ether) glycol, 
poly(heptamethylene ether) glycol, poly(octamethylene ether) glycol, 
poly(nonamethylene ether) glycol, and poly(decamethylene ether) glycol, 
such polymers having an average molecular weight within the range of about 
400 to 6,000. 
The hydroxy-terminated poly(ester/ether) block copolymer (I) includes two 
types of blocks, one being a "soft" segment that provides the polymer with 
a relatively low glass transition temperature and has an elastomeric 
character, the other being a "hard" segment that provides the polymer with 
a crystalline domain having a relatively high melting point to lessen 
chain slippage in the absence of elevated temperatures. For example, the 
preferred hydroxy-terminated copolymer (I) is prepared from dimethyl 
terephthalate, dimethyl isophthalate, butane-diol-1,4 (tetramethylene 
glycol) and poly(tetramethylene ether) glycol. Both R' and R" are 
(CH.sub.2).sub.4. The "hard" segment has an average molecular weight of 
about 220 and the following structure: 
##STR3## 
The "soft" segment has an average molecular weight of about 1,130 and a 
structure: 
##STR4## 
where p is an integer of from about 8 to about 23. 
The "soft" segment makes up about 30 to 80% by weight of the total polymer, 
preferably about 40 to 70% by weight. The "hard" segment makes up about 70 
to 20%, by weight of the total polymer, preferably about 60 to 30% by 
weight. 
Epoxy resins that are suitable for forming the mixed blends of the present 
invention include those based on bisphenol A and epichlorohydrin that 
exhibit epoxide equivalents within the approximate range of from about 175 
to 4000 and average molecular weights of from about 350 to 3800 and are 
represented by this general formula: 
##STR5## 
Also included are phenol nolovak epoxy resins having the following 
formula: 
##STR6## 
The epoxy resin may also be the tetraglycidyl ether of 
1,1,2,2-tetra-bis-(hydroxyphenyl) ethane or a cycloaliphatic epoxide such 
as 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexane 
carboxylate. 
These epoxy resins must be molten at the application temperature of the 
particular copolymer (I), which is also molten at such temperature. This 
temperature will vary with the extent of elevated temperature creep 
resistance desired. Typical application temperatures for the blends of 
this invention will range between about 130.degree. C. and about 
205.degree. C. The epoxy resins must also be capable of forming compatible 
mixtures with the copolymer (I) to form a homogeneous blend. The epoxy 
resin is preferably present in the blend at levels of about 5 to about 50 
percent by weight, while the copolymer (I) makes up about 50 to 95 weight 
percent of the total blended mixture. 
The blends are thermoplastic adhesives which melt at a preselected 
application temperature and which are suitable for bonding such substrates 
as metals, glass, wood and the like. They exhibit a very high tensile 
shear strength between about 900 to about 1800 pounds per square inch 
(psi). Tensile shear strength measurements throughout this disclosure are 
made by testing a 1 .times. 1 inch lap bond on unprimed cold rolled steel 
at 25.degree. C. The blends exhibit an exceptional peel strength between 
about 50 to about 150 pounds per linear inch (pli). Peel strength 
measurements throughout this disclosure are made by testing on unprimed 
aluminum (approximately 25 mils thick) at 25.degree. C. The blends can 
pass creep resistance tests at 300.degree. F. when the copolymer (I) has a 
melting point of not less than about 160.degree. C. (about 345.degree. 
F.). Creep resistance measurements throughout this disclosure are 
determined by forming a 1 .times. 1 inch lap bond on cold rolled steel, 
suspending a 2,000 gram load therefrom, and placing the bonded steel in an 
oven at 300.degree. F. (about 149.degree. C.). The blend "passes" the 
creep resistance test if the bond holds for greater than 100 hours. 
These blends also possess a suitable initial viscosity upon being raised to 
their respective application termperatures. Within a range of typical 
application temperatures between about 130.degree. C. and 205.degree. C., 
the initial viscosity of the just-formed blend will range between about 
100 and 800 poises. An additional feature of these blends is that these 
initial viscosities remain relatively stable while the blends are held at 
application temperature for several hours. It is a feature of the products 
of this invention that their pot life is especially long; that is, the 
initial application temperature viscosity neither decreases nor increases 
excessively when held at that temperature for as long as 8 hours or more. 
An appreciable increase in viscosity is undesirable since such increases 
will severely reduce the workability of a hot melt adhesive at its 
application temperature. The amount of the viscosity increase observed in 
the blends of this invention while they are being held at hot melt 
application temperatures in the hot melt pot is believed to be controlled 
primarily by the very sluggish reaction between the epoxy resins and the 
hydroxy-terminated copolymers (I). In addition, an appreciable decrease in 
viscosity is undesirable for a long-pot-life hot melt adhesive, since such 
a decrease is usually accomplished by a reduction in adhesive strength. 
The adhesive strength and toughness of the thermoplastic bonds formed by 
the present blends are retained whether the blend was applied shortly 
after the application temperature was reached or whether the blend was not 
used until after it had been in the hot melt pot for up to 8 hours or 
more. 
The method of the present invention calls for heating the copolymer (I) and 
the epoxy resin until each becomes molten. It is then possible to mix or 
blend the two molten components until a compatible and substantially 
homogeneous blend is formed at about the application temperature. The 
heating step preferably raises the components to a temperature between 
about 130.degree. C. and about 205.degree. C. The blended components may 
then be held at the approximate application temperature for up to 8 hours 
or more. The blend is then applied, at its application temperature, to the 
substrates to be bonded. Preheating or postheating the substrates, 
especially in the case of metals, may be desirable to obtain more complete 
"wetting" of the bonded surfaces and resulting higher bond strengths. The 
substrates are mated and the assembly (substrates bonded with the blend) 
is allowed to cool to ambient temperature, at which time the thermoplastic 
bond is formed. 
The hydroxy-terminated copolymer (I) is formed to a moderate molecular 
weight of about 4,000 to 25,000 by a polymerization reaction among the 
dicarboxylic acids or esters, the diols, and the difunctional polyethers 
described herein. Preferably, the initial reaction is carried out under 
nitrogen gas at a pressure within the approximate range of 1 to 15 mm Hg, 
preferably 5 to 10 mm Hg, at a temperature of approximately 
150.degree.-250.degree. C., preferably about 190.degree.-210.degree. C., 
usually in the presence of an ester interchange catalyst and an 
antioxidant or stabilizer. During this process, methanol distills over, it 
being a reaction by-product. Once the methanol distillation has ceased, 
the temperature is increased to about 220.degree.-280.degree. C., 
preferably about 240.degree.-260.degree. C. and the pressure is maintained 
within the range of about 1 to about 15 mm Hg for about 1 to 6 hours so as 
to form a low-to-moderate molecular weight (about 4,000 to 25,000) 
polymer. The molecular weight increases with the length of reaction time. 
Suitable ester interchange catalysts include: organic titanates, such as 
tetrabutyl titanate and tetraisopropyl titanate, either alone or in 
combination with magnesium or calcium acetate; complex titanates, such as 
MgHTi(OR).sub.6 or NaHTi(OR).sub.6 from alkali or alkaline earth metal 
alkoxides and titanate esters; inorganic titanates, such as lanthanum 
titanate; calcium acetate/antimony trioxide mixtures; and magnesium 
alkoxides. 
The stabilizers may be a phenol derivative, such as 
4,4'-bis(2,6-ditertiary-butyl phenol); 
1,3,5-trimethyl-2,4,6-tris-(3,5-ditertiary-butyl-4-hydroxy 
benzyl)-benzene; and 4,4-butylidene-bis(6-tertiary-butyl-m-cresol). Other 
appropriate stabilizers include inorganic metal salts or hydroxides as 
well as organic complexes such as nickel butyl dithiocarbonate, manganous 
salicylate, and copper 3-phenyl salicylate, and copper 3-phenyl 
salicylate. Also capable of utilization as the stabilizer are mixtures of 
hindered phenols with esters of thiopropionic acid, mercaptides and 
phosphite esters. Preferred for use in this invention are amine 
stabilizers, including: p,p-dioctyldiphenyl amine; 
N,N-bis(betanaphthyl)-p-phenylene diamine; N,N-bis 
(1-methylheptyl)-p-phenylene diamine; 
N-phenyl-N'-(p-toluenesulfonyl)-p-phenylene diamine; 
N-(3-hydroxybutylidene)- -naphthyl amine; diphenyl amine-acetone 
condensate; and N-phenyl- -naphthyl amine-acetone condensate. 
Representative of the polymerization reaction forming an hydroxy-terminated 
poly(ester/ether) block copolymer (Formula I) is the following reaction, 
wherein the dicarboxylic acid or ester is a mixture of dimethyl 
phthalates, the diol is a glycol and the difunctional polyether is a 
polyalkylene ether glycol: 
##STR7## 
wherein R' and R" are alkyl, alicyclic, acyclic, aryl, or arylakyl of from 
2 to 12 carbon atoms, p is a number of from 2.4 to 136.0, a is a number 
such that the "hard" segment within the first set of brackets makes up 
about 70 to 20% by weight of the copolymer, and b is a number such that 
the "soft" segment within the second set of brackets makes up about 30 to 
80% by weight of the copolymer. 
The actual values of "a" and of "b" are functions of the reactants utilized 
and of the molecular weights thereof. For example, in the preferred 
embodiment, the dicarboxylic acid or ester is a combination of about 0.50 
to 0.90 moles of dimethyl terephthalate to about 0.10 to 0.50 moles of 
dimethyl isophthalate, the diol is 1,4-butanediol, and the difunctional 
polyether is poly(tetramethylene ether) glycol thAt ranges between a 
molecular weight of from about 600 to 2000. In this preferred embodiment, 
the value of "a" ranges between about 0.45 (whereby the hard segment is 
about 20% by weight) to about 0.96 (whereby the hard segment is about 70% 
by weight) and the value of "b" ranges between about 0.55 (whereby the 
soft segment is about 80% by weight) to about 0.04 (whereby the soft 
segment is about 30% by weight).

The following examples are set forth as illustrative embodiments of the 
invention and are not to be taken in any manner as limiting the scope of 
the invention which is defined by the appended claims. 
EXAMPLE I 
A hydroxy-terminated poly(ester/ether) block copolymer having a molecular 
weight of about 20,920 was made by reacting the following ingredients 
under nitrogen atmosphere in a 2 gallon Ross mixer: 
1,224 grams of polytetramethylene ether glycol (molecular weight of about 
1,000) 
1,274 grams of dimethyl terephthalate 
546 grams of dimethyl isophthalate 
1,102 grams of 1,4-butanediol. 
In this formulation, the mole ratio of the dimethyl terephthalate to the 
dimethyl isophthalate is 70 to 30. This reaction was carried out in the 
presence of tetrabutyltitanate/magnesium acetate, an ester interchange 
catalyst, and octylated diphenylamine, an antioxidant. 
Initially, the reaction temperature was held at 200.degree. C. until all 
methanol ceased distilling over, which was about 1 hour after the 
200.degree. C. temperature had been reached. The pressure was then reduced 
to 6 mm/Hg, and the temperature was increased to 250.degree. C. These 
conditions were maintained for 2 hours. The reaction mixture was cooled to 
200.degree. C., and the resulting hydroxy-terminated poly(ester/ether) 
block copolymer (I) was recovered. This polymer had a molecular weight of 
about 19,670, a melting point of 140.degree. C., and is identified as 
"P-140" in Table 1. It was then heated to its melting point along with 
various ratios of epoxy resins. Two different resins were blended in 
amounts such that the final blend contained 10, 15 or 20 weight percent of 
the epoxy resin and 90, 85 or 80 weight percent, respectively, of the 
copolymer (I). One of the resins, identified as A, is the reaction product 
of Bisphenol A with epichlororohydrin having an epoxide equivalent weight 
of about 900, while the other resin, identified as B, is a similar product 
with an epoxide equivalent weight of about 5000. The test results for the 
various thermoplastic products thus prepared are summarized in Table 1. 
EXAMPLE II 
The procedure of Example I was repeated with the sole exception that the 
mole ratio of dimethyl terephthalate to dimethyl isophthalate was changed 
so that it was 80 to 20. This resulted in a copolymer (I) having a 
molecular weight of about 21,500 and a melting point of 160.degree. C. It 
is identified as "P-160" in Table 1. 
EXAMPLE III 
The procedure of Example I was repeated with the sole exception that the 
mole ratio of dimethyl terephthalate to dimethyl isophthalate was changed 
so that it was 90 to 10. This resulted in a copolymer (I) having a 
molecular weight of about 18,700 and a melting point of 180.degree. C. It 
is identified as "P-180" in Table 1. 
EXAMPLE IV 
The procedure of Example I was repeated with the sole exception that the 
mole ratio of dimethyl terephthalate to dimethyl isophthalate was changed 
so that it was 100 to 0. This resulted in a copolymer (I) having a 
molecular weight of about 21,600 and a melting point of 195.degree. C. It 
is identified as "P-195" in Table 1. 
Table 1 
__________________________________________________________________________ 
TENSILE 
LAB NUMBER % EPOXY SHEAR PEEL CREEP 
OF BLEND COPOLYMER 
EPOXY USED STRENGTH 
STRENGTH 
at 300.degree. F 
__________________________________________________________________________ 
P140 P140 0 -- 990psi 16pli Fail 
P140-10-A P140 10 A 940 110 Fail 
P140-15-A P140 15 A 1010 140 Fail 
P140-20-A P140 20 A 1040 138 Fail 
P140-10-B P140 10 B 1060 150 Fail 
P140-15-B P140 15 B 1170 145 Fail 
P140-20-B P140 20 B 1280 85 Fail 
P160 P160 0 -- 1070 16 Pass 
P160-10-B P160 10 B 1240 61 Pass 
P160-15-B P160 15 B 1340 80 Pass 
P160-20-B P160 20 B 1560 65 Fail 
P180 P180 0 -- 1070 10 Pass 
P180-10-A P180 10 A 1060 27 Pass 
P180-15-A P180 15 A 1240 34 Pass 
P180-20-A P180 20 A 1370 38 Pass 
P180-10-B P180 10 B 1320 51 Pass 
P180-15-B P180 15 B 1350 74 Pass 
P180-20-B P180 20 B 1590 62 Pass 
P195 P195 0 -- 980 2 Pass 
P195-10-A P195 10 A 1310 6 Pass 
P195-20-A P195 20 A 1420 8 Pass 
P195-10-B P195 10 B 1520 16 Pass 
P195-20-B P195 20 B 1710 11 Pass 
__________________________________________________________________________ 
EXAMPLE V 
The P140 copolymer alone and the present blends of P140 with the epoxy 
resin A, all as prepared in Example I, were tested for viscosity stability 
over time periods that would correspond to pot lives advantageous for 
commercial adhesives. Each sample was held at 400.degree. F., and the 
viscosity of each was measured at various time intervals. The results are 
tabulated in Table 2. As can be seen, the unblended copolymer P140 went 
through a marked viscosity decrease, resulting in loss of toughness and 
adhesive strength. The P140-10-A and the P140-20-A blends actually showed 
an increase in viscosity until the gel state was reached after about 5 
hours. The P140-15-A blend showed remarkable viscosity stability, with the 
viscosity remaining relatively constant over the 8-hour test period. No 
loss in toughness of the later-formed thermoplastic bonds was observed. 
Table 2 
______________________________________ 
Viscosity (poises) at 400.degree. F. 
P-140 P-140-10-A P-140-20-A P-140-15-A 
______________________________________ 
0 hr. 765 540 500 465 
1 hr. 654 320 500 560 
2 hr. 540 170 540 490 
3 hr. 460 140 590 410 
4 hr. 410 160 690 360 
5 hr. 370 460 1030 350 
6 hr. 330 GEL GEL 360 
7 hr. 290 -- -- 430 
8 hr. 270 -- -- 570 
______________________________________ 
EXAMPLE VI 
This example illustrates that blends of epoxy resins with copolymers other 
than the copolymers (I) included in the blends of the present invention do 
not bring about improvements in the thermoplastic properties of such other 
thermoplastic copolymers, even though these copolymers blend in a 
compatible manner with the epoxy resins. 
A blend was made using 20 parts of epoxy B with 80 parts of a commercially 
available block co-poly(ester-amide) known as Montac 1050. Montac is a 
brand designation of Monsanto Company. The following bond strengths were 
obtained on substrates post heated to 454.degree. F.: 
______________________________________ 
Tensile Shear, 
T-peel, 
Steel Aluminum 
32 mil 25 mil 
______________________________________ 
Montac 1050 1330 psi 60 pli 
30 parts Montac 1050 
1830 psi 36 pli 
20 parts epoxy B 
______________________________________ 
Some increase in tensile is probably due to the partial crosslinking of the 
copolymer by the epoxy resin. 
Obviously, many modifications and variations of the invention as 
hereinbefore set forth may be made without departing from the spirit and 
scope thereof, and only such limitations should be imposed as are 
indicated in the appended claims.