Composition containing vinylpyridine-styrene-butadiene copolymer, and use thereof as adhesive

A composition comprising a vinylpyridine-styrene-butadiene copolymer and at least one amino acid selected from the group consisting of proline, leucine, isoleucine, homoserine, valine, .alpha.-aminobutyric acid, .gamma.-aminobutyric acid, citrulline, lysine, ornithine, arginine, glutamic acid, aspartic acid, and salts of these amino acids. The composition may optionally contain sulfur and/or sulfur donor.

FIELD OF THE INVENTION 
This invention relates to a composition comprising a 
vinylpyridine-styrene-butadiene copolymer (to be referred to hereinbelow 
as a VSB copolymer) and a specified amino acid; a composition comprising 
the VSB copolymer, sulfur and/or a sulfur donor, and the specified amino 
acid; and use of these compositions as adhesives. 
BACKGROUND OF THE INVENTION 
An RFL adhesive is mainly used as an adhesive for bonding tire cords to 
rubber in tires. The RFL adhesive is a mixture of a 
resorcinol-formaldehyde resin and a rubber latex. In the early stage, a 
natural rubber latex was used as the rubber latex, but later superseded by 
synthetic rubber latices. Among them, a latex of a VSB copolymer (VSB 
latex) is known to produce an RFL adhesive having the best performance. In 
addition to the RFL adhesive, the VSB latex can also be used as an 
adhesive in the form of a mixture with an isocyanate, or with an 
isocyanate and an ethyleneurea compound. 
The problem still desired to be improved in the aforesaid adhesives based 
on a VSB copolymer, particularly an RFL adhesive based on a VSB copolymer 
is that the adhesion strengths of these adhesives decrease with time. For 
example, a reduction in adhesion strength between tire cords and rubber 
occurs during the running of an automobile, and may sometimes result in 
the breakage of the tire. Although the cause of the decrease of the 
adhesion strength in a tire has not yet been completely elucidated because 
of the great complexity of actions exerted on the tire, it is believed to 
be at least certain that the temperature rise of the tire owing to 
internal heat buildup is one cause. During the running of an automobile, 
the tires undergo repetitive stresses of various types such as tensile, 
shearing, compressive and flexural stresses, and the resulting hysteresis 
loss (i.e., energy loss) causes heat buildup within the tires. 
The present inventor performed an experiment simulating the temperature 
rise of a tire owing to internal heat buildup (the experiment will be 
described in detail hereinbelow), and found that the VSB copolymer has 
extremely poor resistance to high temperatures, and when heated to 
160.degree. C., it decomposes to evolve gases and consequently deforms 
(Experiment A). When sulfur is added to the VSB copolymer, the generation 
of gases and, consequently, the deformation of the VSB copolymer become 
remarkable as a result of heating (Experiment B). The present inventor 
found that the addition of a small amount of a certain amino acid to the 
VSB copolymer markedly reduces the generation of gases and the deformation 
of the copolymer owing to heating (Experiment C). The present inventor 
also found that when sulfur is added to the VSB copolymer containing the 
amino acid produced in Experiment C, the generation of gases and the 
deformation of the copolymer owing to heating are further reduced 
(Experiment D). In other words, when the VSB copolymer alone is used, the 
addition of sulfur accelerates the generation of gases and the deformation 
of the copolymer, but when the amino acid is present together with the VSB 
copolymer, the addition of sulfur unexpectedly inhibits the generation of 
gases and the deformation of the copolymer. 
As shown by these Experiments, the thermal stability of the VSB copolymer 
can be improved by adding a specified amino acid to the VSB copolymer. 
This is desirable for removing the decrease with time of the adhesion 
strength of adhesives based on the VSB copolymer, especially the RFL 
adhesive. 
SUMMARY OF THE INVENTION 
This invention provides a two-component composition comprising a 
vinylpyridine-styrene-butadiene copolymer (VSB copolymer) and at least one 
amino acid selected from the group consisting of proline, leucine, 
isoleucine, homoserine, valine, .alpha.-aminobutyric acid 
.gamma.-aminobutyric acid, citrulline, lysine, ornithine, arginine, 
glutamic acid, aspartic acid and salts of these amino acids. 
The invention also provides a three-component composition comprising the 
aforesaid-two-component composition and sulfur and/or a sulfor donor. 
The two-component and three-component compositions of this invention are 
suitable for use as adhesives. 
DETAILED DESCRIPTION OF THE INVENTION 
Tires for automobiles and air planes are usually produced by preparing an 
RFL adhesive from a mixture of an aqueous solution of a condensate of 
resorcinol and formaldehyde and a VSB latex, dipping tire cords in the RFL 
adhesive for a predetermined period of time, withdrawing the tire cords 
and drying them to obtain tire cords coated with the RFL adhesive, 
embedding the tire cords in a natural or synthetic rubber (for example, a 
styrene/butadiene rubber) containing a vulcanizing agent such as sulfur, 
and then heating the assembly to the vulcanization temperature (e.g., 
130.degree. to 180.degree. C.), whereby the vulcanization of rubber and 
the bonding of rubber to the tire cords are performed simultaneously. 
In view of the fact that the above manufacturing method is usually 
practiced to produce tires, the present inventor produced a plate-like 
specimen of the VSB copolymer, and heat-treated it at 160.degree. C. for 
30 minutes. In 10 to 20 minutes, malodorous gases were generated, and the 
surface of the plate-like specimen became rough. [Experiment A]. 
A plate-like specimen was produced from a uniform mixture of the VSB 
copolymer and 2% by weight of sulfur or a sulfur donor, and heat-treated 
at 160.degree. C. for 30 minutes. Evolution of gases was greater than in 
Experiment A using the VSB copolymer alone and the gases were more 
malodorous. As a result, the plate-like specimen had innumerable cracks on 
its surface and an uneven thickness. [Experiment B]. 
From the results of Experiment A and B, the present inventor assumed the 
following mechanism as at least one case of the reduced adhesion strength 
between rubber and tire cords after long-term running of an automobile 
tire. The temperature of the tire rises to a fairly high temperature 
(e.g., 100.degree. C. or higher) owing to internal heat buildup during 
running, and in the meantime, sulfur (vulcanizer) contained in rubber 
constituting the tire migrates partly to the RFL adhesive. Hence, the same 
experimental conditions (the heat-treatment of VSB copolymer in the 
presence of sulfur) are produced. This may cause the generation of gases 
and the decrease of the adhesion strength between the rubber and the tire 
cords. 
The present invention produced a plate-like sample from a uniform mixture 
of the VSB copolymer and 2% by weight of the amino acid specified in this 
invention (e.g., proline), and heat-treated it at 160.degree. C. for 30 
minutes. It was unexpectedly found that as compared with Experiment A in 
which the VSB copolymer was used alone, the generation of gases decreased 
markedly, and the heat-treated plate-like sample showed much higher 
smoothness. [Experiment C]. 
A plate-like specimen was prepared from a uniform mixture of the VSB 
copolymer and 2% by weight each of proline and sulfur (or a sulfur donor), 
and heat-treated it at 160.degree. C. for 30 minutes. It was found 
surprisingly that the generation of gases was reduced further as compared 
with Experiment C using a mixture of the VSB copolymer and proline and was 
scarcely noted. No substantial change occured in the surface condition of 
the plate-like specimen as a result of the heat-treatment. [Experiment D]. 
In Experiment D, the cure curve of the VSB copolymer led to the 
confirmation that it can be cured with sulfur. The resulting reaction 
product had good properties as vulcanized rubber. 
The results of Experiments A to D demonstrate that when sulfur or a sulfur 
donor is added to the VSB copolymer, heat-treatment of the mixture 
accelerates the generation of gases, whereas the addition of an amino acid 
specified in this invention to the VSB copolymer inhibits the generation 
of gases by heat-treatment. This fact has been completely unknown in the 
past. It was also discovered that when sulfur or a sulfur donor is added 
to a mixture of the VSB copolymer and the specified amino acid used in 
this invention, the generation of gases owing to heat-treatment is reduced 
to a greater degree. This fact is quite unexpected in view of the fact 
that sulfur or a sulfur donor rather accelerates the generation of gases 
when added to the VSB copolymer in the absence of the amino acid. 
As is apparent from the above statement, the compositions of this invention 
are suitable for use as adhesives for rubber, and can afford an RFL 
adhesive of improved performance by substituting them for the VSB 
copolymer conventionally used in the RFL adhesive. 
The VSB copolymer itself used in this invention is known. For use in this 
invention, a VSB copolymer composed of 65 to 95% by weight of butadiene, 
10 to 20% by weight of styrene and 2 to 20% by weight of vinylpyridine is 
preferred. A VSB copolymer composed of 75% by weight of butadiene, 15% by 
weight of styrene and 15% by weight of vinylpyridine is most preferred. 
The specified amino acid used in this invention is at least one member 
selected from the group consisting of proline, leucine, isoleucine, 
homoserine, valine, .alpha.-aminobutyric acid, .gamma.-aminobutyric acid, 
citruline, lysine, ornithine, arginine, glutamic acid, aspartic acid and 
salts of these amino acids. Of these, proline, leucine, isoleucine, and 
homoserine are preferred. As will be shown hereinbelow in Example 2, the 
use of amino acids other than those specified in this invention, for 
example serine, tryptophan, threonine and cystine, cannot inhibit the 
generation of gases caused by the heat-treatment of the VSB copolymer. 
In respect to the salts of the amino acids, alkali metal salts, inorganic 
mineral acid salts and lower aliphatic carboxylic salts are contemplated. 
By the term "lower alphatic" is meant those acids having 2-5 carbon atoms 
in their structure. Examples of the preferred salts are sodium or 
potassium salts of glutamic or aspartic acid, the monohydrochloride or 
lysine, ornithine or arginine and salts produced by adding one molecule of 
a lower alphatic carboxylic acid selected from acetic acid, propionic 
acid, butyric acid and valeric acid to lysine, ornithine or arginine. The 
structural formulas for certain of the inorganic and organic acid salts of 
these common acids will be set forth below: 
##STR1## 
The sulfur donor used in the composition of this invention denotes a 
substance which releases active sulfur at the crosslinking temperature. 
Such a sulfur donor is known, and includes, for example, sulfur compounds 
such as sulfur monochloride, sulfur dichloride, morpholine disulfide, 
alkylphenol disulfides, N,N'-dithiobis(hexahydro-2H-azepinone-2), and 
phosphorus containing polysulfides; thiazole compounds such as 
2-(4'-morpholinodithio)benzothiazole; and thiuram polysulfide compounds 
such as tetramethylthiuram disulfide, activated tetramethylthiuram 
disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, 
N,N'-dimethyl-N,N'-diphenylthiuram disulfide, dipentamethylene disulfide, 
dipentamethylenethiuram disulfide, dipentamethylenethiuram hexasulfide, 
cyclopentamethylenethiuram disulfide and mixed alkylthiuram disulfides. 
The amino acid content of the composition of this invention is 0.01 to 30 
parts by weight, preferably 0.1 to 5 parts by weight, per 100 parts by 
weight of the VSB copolymer. The content of sulfur and/or the sulfur donor 
in the composition of this invention is 0.01 to 30 parts by weight, 
preferably 0.1 to 5 parts by weight, per 100 parts by weight of the VSB 
copolymer. 
An RFL adhesive can be prepared from the composition of this invention by 
first forming a latex containing the VSB copolymer and the amino acid and 
optionally sulfur and/or the sulfur donor in the proportions described 
hereinabove, and then mixing the latex with an aqueous solution of a 
resorcinol-formaldehyde resin. Preferably, the VBS copolymer content in 
the latex is 35 to 45% by weight. Usually, the concentration of the entire 
so lids of the RFL adhesive is 10 to 20% by weight, and the RFL adhesive 
contains 10 to 30 parts by weight, preferably 15 to 20 parts by weight, of 
the resorcinol-formaldehyde resin per 100 parts by weight of the VSB 
copolymer. 
The composition of this invention may optionally contain other additives 
such as fillers and thickeners. 
The following Examples illustrate the present invention in more detail. The 
tensile strength, elongation and modulus were measured at a pulling speed 
of 200 mm/min. using a Schopper tensile tester in accordance with JIS 
K-6301. The hardness was measured by using a hardness tester of type A of 
JIS. The proportions of the ingredients of rubber compositions are 
expressed in parts by weight.

EXAMPLE 1 
Three types of compositions (two-component compositions in accordance with 
this invention) having the recipes shown in Runs Nos. 1 to 3 in Table 1 
were prepared in a customary manner using an open roll. Paraffin paper for 
prevention of tackiness was applied to a mold, and each of these 
compositions was heat-treated for 30 minutes with a steam press held at 
160.degree. C. 
For comparison, the VSB copolymer alone was heat-treated under the same 
conditions. 
After the heat-treatment, each of the samples taken out of the mold was 
observed. The sample in Comparative Run No. 1 showed a great deformation 
of its surface caused by the evolution of gases, whereas the samples 
obtained in Runs Nos. 1 to 3 showed a marked reduction in gas generation 
and surface deformation. 
TABLE 1 
______________________________________ 
Comparative 
Run No.1 
Run No.2 Run No.3 Run No.1 
______________________________________ 
VSB copolymer.sup.1 
100 100 100 100 
Proline 5 -- -- -- 
Leucine -- 5 -- -- 
Homoserine -- -- 5 -- 
Degree of defor- 
mation of the 
surface owing to 
Small Small Small Large 
gas generation 
______________________________________ 
Note:- 
The VSB copolymer.sup.1 was the one obtained by drying Nipol 2518 FS.RTM. 
a product of Nippon Zeon Co., Ltd. 
EXAMPLE 2 
Three-component compositions shown in Runs Nos. 1 to 7 in Table 2 were 
prepared in a customary manner using an open roll. Each of the 
compositions was heat-treated at 160.degree. C. for 20 minutes with a 
steam press. The amino acids used are shown in Table 3. With the 
three-component compositions in Runs Nos. 1 to 7 in Example 2, the 
generation of gases was more effectively inhibited than with the 
two-component compositions of Runs Nos. 1 to 3 in Example 1, and as a 
result, plates of the vulcanized VSB copolymer having a good surface 
condition could be produced. The properties of the vulcanized VSB 
copolymers obtained are shown in Table 3. 
For comparison, the composition of Comparative Run No. 1 composed only of 
the VSB copolymer and sulfur was heat-treated at 160.degree. C. for 20 
minutes. The VSB copolymer taken out of the mold developed numerous cracks 
on its surface while evolving gases. Furthermore, it had a greatly uneven 
surface, and the properties of the copolymer could not be measured. 
For further comparison, compositions containing serine, tryptophan, 
threonine and cystine, respectively, which are the amino acids outside the 
scope of the invention were prepared, and heat-treated at 160.degree. C. 
for 20 minutes in Comparative Runs Nos. 2 to 5. As in Comparative Run No. 
1 described hereinabove, the samples were heavily deformed owing to the 
generation of gases, and the effect of adding the amino acids was not 
noted at all. 
TABLE 2 
______________________________________ 
Comparative 
Runs Nos. 
Comparative Runs Nos. 
1 to 7 Run No. 1 2 to 5 
______________________________________ 
VSB copolymer.sup.1 
100 100 100 
SRF L carbon 
40 40 40 
black.sup.2 
Sulfur 2 2 2 
Amino acid.sup.3 
2 -- 2 
______________________________________ 
Note 
.sup.1 Same as in Example 1. 
.sup.2 Selfreinforcing low structure carbon black added as a reinforcing 
agent. 
.sup.3 The amino acids used in Runs Nos. 1 to 7 are shown in Table 3, and 
the amino acids used in Comparative Runs Nos. 2 to 5 were the four amino 
acids outside the scope of the invention. 
TABLE 3 
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300% Elonga- 
Tensile 
Run modulus tion strength 
No. Amino acid (kg/cm.sup.2) 
(%) (kg/cm.sup.2) 
Hardness 
______________________________________ 
1 Proline 68 450 103 50 
2 Leucine 73 450 112 50 
3 Homoserine 91 390 120 54 
4 Valine 76 390 105 49 
5 .gamma.-Aminobu- 
74 400 103 49 
tyric acid 
6 Citrulline 72 380 94 48 
7 Isoleucine 78 450 122 50 
______________________________________ 
EXAMPLE 3 
A three-component composition was prepared from the VSB copolymer shown in 
Run No. 1 of Table 4, morpholine disulfide (sulfur donor) and leucine (the 
amino acid specified in this invention), and heat-treated under the same 
conditions as in Example 2. Gas generation and deformation could be 
effectively inhibited, and a vulcanized VSB copolymer having a good 
surface condition was obtained. The properties of the vulcanized copolymer 
are shown in Table 4. 
For comparison, a composition consisting only of the VSB copolymer and 
morpholine disulfide was heat-treated under the same conditions as above. 
Gases were generated, and heavy deformation occured in the heat-treatment 
product. The properties of the product could not be measured. 
TABLE 4 
______________________________________ 
Comparative 
Run No. 1 Run No. 1 
______________________________________ 
VSB copolymer.sup.1 
100 100 
SRF-L carbon black.sup.2 
40 40 
Morpholine disulfide 
2 2 
Leucine 2 -- 
300% Modulus (kg/cm.sup.2) 
38 -- 
Elongation (%) 540 -- 
Tensile strength (kg/cm.sup.2) 
74 -- 
Hardness 40 -- 
______________________________________ 
Note 
.sup.1 The same as in Example 1. 
.sup.2 The same as in Example 2.