Rubber compositions

A rubber composition for steel-belted automobile tires containing selected amounts of partial condensates of hexamethylolmelamine pentamethylether, cresol resin, sulfur and cobalt salts of organic acid, to improve its warm water to steel cords in the tire and increased tensile strength.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rubber compositions and has particular reference 
to rubber compositions suitable for use in the manufacture of steel-belted 
automobile tires. 
2. Prior Art 
Steel-belted tires are in widespread use for their inherent advantages of 
good driving stability, high wear resistance and other superior physical 
properties compared to conventional tires. Research, efforts have been 
focused, inter alia, on improving the of adhesion between rubber layers 
and steel cords which has an important bearing upon the quality of the 
tires. This adhesion parameter determines the service life of the tire 
exposed to various environmental conditions during and subsequent to its 
processing stage and shipment to the end user. In the manufacture of a 
steel-belted tire, the starting rubber possesses sufficient "initial 
adhesion" with steel cords at the time of vulcanization. Unvulcanized 
rubber, however, is susceptible to absorption of moisture and hence its 
adhesive property, namely "unvulcanized moisture resistant adhesion", 
tends to decline upon vulcanization. Vulcanized rubber is also liable to 
suffer a reduction in its adhesive property, namely "vulcanized moisture 
resistant adhesion", due to penetration of moisture in the air through the 
rubber layer into the steel cords during the use of tire. It is further 
possible that if the tire receives cuts, nail punctures and other damage 
to its tread portion during running of the automobile, which will allow 
permeation of water therethrough, this water is warmed up by the heat 
generated in the running tire and flows through the interstices of the 
steel cords, causing the rubber to separate due to a loss of its adhesive 
property; namely "vulcanized warm water resistant adhesion". 
The terms quoted above represent all aspects of the adhesive property 
required of rubber compositions for steel-belted tires. 
Various tire rubber composition are known which are capable of meeting 
some, but not all of the above adhesion requirements. 
The vulcanization process in the manufacture of steel-belted tires holds 
the key to productivity as a whole. Vulcanization time can be shortened by 
increasing its temperature. However, conventional rubber compositions 
containing cobalt salts of organic acids, though believed conducive to 
adhesion with steel cord, are liable to suffer a decline in their 
"unvulcanized moisture resistant adhesion", "vulcanized moisture resistant 
adhesion" and "vulcanized warm water resistant adhesion" because of the 
higher vulcanization temperature. 
It is known to choose high hardness and high modulus rubbers to cover hard 
steel cords for steel-belted tires. Such hard rubbers often incorporate 
large quantities of sulfur and carbon black and exhibit increased hardness 
and modulus upon vulcanization. At the same time, however, they tend to 
deteriorate in tensile strength and breaking extension. 
SUMMARY OF THE INVENTION 
With the foregoing difficulties of the prior art in mind, the present, 
invention seeks to provide rubber compositions that will have enhanced 
adhesive strength with respect to steel cords in terms of "initial 
adhesion", "unvulcanized moisture resistant adhesion", "vulcanized 
moisture resistant adhesion" and "vulcanized warm water resistant 
adhesion", with all these adhesive properties being retained even using 
high temperature vulcanization. Further that they will possess high 
hardness and excellent breaking extension property. 
The inventive rubber compositions are therefore particularly useful and 
effective in coating the steel cords used in steel-belted tires. 
The rubber composition according to the invention comprises 100 parts by 
weight of a starting rubber, and, based on the rubber, 1.0-5 parts by 
weight of partial condensates of hexamethylolmelamine pentamethylether 
0.5-5 parts by weight of a cresol resin 4-7 parts by weight of sulfur and 
0.1-0.8 parts by weight of a cobalt salt of organic acid based on the 
amount of cobalt element.

DETAILED DESCRIPTION OF THE INVENTION 
The rubber composition of the invention essentially comprises the following 
components. 
i) Starting Rubber 
Natural rubber (NR) alone or in combination with synthetic isoprene rubber 
(IR). 
ii) Partial Condensates of Hexamethylolmelamine Pentamethylether of the 
Formula 
##STR1## 
These partial condensates are commercially available such as for example 
Sumicanol 507 (tradename of Sumitomo Chemicals Co., Ltd.) which contains 
50% of hexamethylolmelamine pentamethylether partial condensates. Other 
polymethoxymethylolmelamines for example hexamethoxymethylmelamine are not 
eligible for the purpose of the invention. The hexamethylolmelamine 
pentamethylether according to the invention should be used in an amount of 
1.0-5 parts by weight per 100 parts by weight of the starting rubber. 
Smaller amounts than 1.0 weight parts would result in insufficient rubber 
hardness, and larger than 5 weight parts would lead to reduced breaking 
extension. 
iii) Cresol Resin 
This may be one readily available in commerce and is preferably meta-cresol 
resin which should be used in an amount of 0.5-5 parts by weight per 100 
parts by weight of the starting rubber. Less than 0.5 weight part would 
result in insufficient rubber hardness, and more than 5 weight parts would 
lead to reduced breaking extension and to objectionably increased heat in 
vulcanized rubber. 
iv) Sulfur 
The amount of sulfur to be added should be 4-7 parts by weight per 100 
parts by weight of the starting rubber. Less than 4 weight parts would be 
ineffective, while more than 7 weight parts would result in bloomed rubber 
surface. 
v) Cobalt Salts of Organic Acid 
Typical examples include straight chain or branched cobalt salts of 
monocarboxylic acid having a carbon number of 50-20 such as cobalt 
naphthenate, cobalt stearate, cobalt octylate and cobalt oleate. These 
cobalt salts should be added in an amount of 0.1-0.8 weight parts in terms 
of cobalt element per 100 weight parts of the starting rubber. Departures 
from this range would serve no useful purposes. 
vi) Other Additives 
There may be used other suitable additives such as carbon black, a 
vulcanization accelerator and the like depending upon the particular 
application for the rubber composition. 
Comparative and Inventive Examples 
For purposes of brevity, the formulations of the various rubber 
compositions for the Comparative Examples and Inventive Examples are shown 
in Tables 1 and 2 respectively along with their respective test results. 
Metal Adhesion Test 
1) Initial Adhesion 
Brass plated steel cords (1.times.5 strand) spaced 12.5 mm apart in 
parallel were coated from both sides with each of the rubber compositions 
into a 25 mm wide web which was subsequently vulcanized at 170.degree. C. 
for 20 minutes. The resulting test sample was subjected to drawing of the 
steel cords in accordance with the procedures of ASTM:D2229. Draw strength 
(index) and rubber coat percentage (%) were measured to determine the 
"initial adhesion" of the rubber composition. 
2) Vulcanized Warm Water Resistant Adhesion 
The test sample was immersed in 70.degree. water with lower end cords cut 
and thus disposed for four consecutive weeks, followed by drawing of the 
cords. 
Separation Test 
A tire fabricated with a belt layer consisting of steel cords coated with 
each of the rubber compositions was inflated to an air pressure of 1.2 
kg/cm.sup.2 and run on a test drum set with slip angle of .+-.3.degree., 
camber angle of 2.degree. and load of 127% (JATMA standards) at a speed of 
60 km/hr for a distance of 6,000 km. The tire was dismantled, and the 
extent or amount of separation of an end portion of the belt layer was 
measured, in which instance the value in Comparative Example 2 was taken 
as a reference index. Test results are better the smaller the index value. 
TABLE 1 
__________________________________________________________________________ 
Comparative Examples 
1 2 3 4 5 6 7 
__________________________________________________________________________ 
natural rubber (RSS #1) 
100 100 
100 100 100 100 100 
carbon black (HAF) 
60 60 60 60 60 60 60 
zinc oxide 10 10 10 10 10 10 10 
aging inhibitor 
1 1 1 1 1 1 1 
(phenylenediamine) 
cobalt-naphthenate *1 
3 3 3 3 3 3 
(0.3) 
(0.3) 
(0.3) (0.3) 
(0.3) 
(0.3) 
cobalt stearate *1 3 
(0.3) 
sulfur 6 6 6 6 6 6 6 
accelerator *2 
0.7 0.7 
0.7 0.7 0.7 0.7 0.7 
meta-cresol resin *3 
1 1 1 1 2 4 4 
hexamethoxy- 
2 4 6 4 4 4 6 
methylmelamine *4 
hexamethylolmelamine 
pentamethylether 
partial condensate *5 
vulcanization 
properties 
(160.degree. C. .times. 20 min) 
tensile strength 
250 238 
221 231 240 245 240 
breaking extension 
390 345 
305 340 355 365 340 
hardness (JIS A) 
74 76 77.5 
76.5 
76.5 
78 79 
initial adhesion 
(170.degree. C. .times. 20 min) 
draw strength 
97 100 
97 99 100 99 98 
rubber coat (%) 
95 95 95 95 95 95 95 
vulcanized warm water 
resistant adhesion 
(170.degree. C. .times. 20 min) 
draw strength 
60 64 65 64 62 63 67 
rubber coat (%) 
50 64 68 60 65 67 70 
separation test 
110 100 
90 90 90 85 85 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Inventive Examples 
1 2 3 4 
______________________________________ 
natural rubber (RSS #1) 
100 100 100 100 
carbon black (HAF) 
60 60 60 60 
zinc oxide 10 10 10 10 
aging inhibitor 1 1 1 1 
(phenylenediamine) 
cobalt-naphthenate *1 
3 3 3 3 
(0.3) (0.3) (0.3) (0.3) 
cobalt stearate *1 
sulfur 6 6 6 6 
accelerator *2 0.7 0.7 0.7 0.7 
meta-cresol resin *3 
1 1 2 4 
hexamethoxy- 
methylmelamine *4 
hexamethylolmelamine 
2 4 4 6 
pentamethylether 
partial condensate *5 
vulcanization 
properties 
(160.degree. C. .times. 20 min) 
tensile strength 255 246 250 255 
breaking extension 
400 380 375 360 
hardness (JIS A) 77.5 79.5 81 83 
initial adhesion 
(170.degree. C. .times. 20 min) 
draw strength 98 100 102 101 
rubber coat (%) 95 95 95 95 
vulcanized warm water 
resistant adhesion 
(170.degree. C. .times. 20 min) 
draw strength 73 78 80 79 
rubber coat (%) 78 82 86 85 
separation test 70 60 50 50 
______________________________________ 
Note: 
*1 each contains 10 weight % of cobalt element 
*2 N,Ndicyclohexylbenzothiazole sulfenamide 
*3 Sumicanol 610 (by Sumitomo Chemical Co., Ltd.) 
*4 Cyrez 964 (by American Cyanamid Co.) 
*5 Sumicanol 507 (containing 50% partial condensates of 
hexamethylolmelamine pentamethylether)