Inhibiting ozone deterioration of rubbers

Rubbers are prevented from ozone deterioration without discoloring or staining by the addition of low toxic thiodipropionitrile of the formula, EQU N.tbd.C--(CH.sub.2).sub.2 --S--(CH.sub.2).sub.2 --C.tbd.N.

The present invention relates to a new method for inhibiting oxone 
deterioration of rubbers without problems due to discoloring, staining or 
toxicity, which method is very effective for protecting natural or 
synthetic rubbers from the ozone deterioration, i.e. generation and growth 
of cracks. 
Natural or synthetic rubber is generally deteriorated by the action of 
oxygen or ozone resulting in the remarkable deterioration of its physical 
properties. Particularly in recent years, the deterioration caused by 
trace amounts of ozone in atmosphere has become a serious problem. 
There have been developed various kinds of method, for the purpose of the 
prevention of deterioration by ozone, and it has been found that some 
amine compounds belonging to p-phenylenediamine derivatives act 
effectively as an antiozonant. These amine compounds, however, have such a 
property that they are discolored as early as on a rubber compounding step 
and particularly on a vulcanization step, or they are increasingly 
discolored to larger extent by the subsequent action of heat or sunlight 
thereby causing the discoloration of rubber itself. Alternatively, the 
compounds have such a property that they stain the surface of materials 
which have been brought into contact with the rubber. For this reason, the 
amine compounds can substantially be used only for the rubber products 
incorporated with carbon black, irrespective of their excellent inhibiting 
property of ozone deterioration. 
Furthermore, it is well known that the amine compounds generally have a 
high toxicity which causes a serious problem. This promoted the 
development of a non-discoloring, non-staining and low-toxic antiozonant 
which is also available for the white rubber products, as a result of 
which it was found that phenol type or thiourea type compounds have an 
excellent inhibiting effect of ozone deterioration. In recent years, 
however, an antiozonant having as the major component the formula, 
##SPC1## 
Was found to be a non-discoloring and non-staining antiozonant that is much 
superior to the phenol type and thiourea type compounds above mentioned, 
and the product has become to be used in a large amount. This product 
surely has excellent non-discoloring and non-staining properties, however 
its inhibiting effect of ozone deterioration is not necessarily 
satisfactory compared with the amine compounds. The inventors have 
synthesized and tested a wide range of compounds for the purpose of 
developing a non-discoloring, non-staining and in addition low-toxic 
antiozonant, and found that thiodipropionitrile of the formula, 
EQU N .tbd. C -- (CH.sub.2).sub.2 -- S -- (CH.sub.2).sub.2 -- C .tbd. N 
has a non-discoloring and a non-staining properties and effective 
protective ability for ozone deterioration of vulcanized rubber. Owing to 
the non-discoloring and non-staining properties, the nitrile can also be 
used for producing the white, pale or colored rubber products to which the 
conventional amine type antiozonants would be unpreferably applied. 
Furthermore, the nitrile can not only be prepared in an industrial scale 
economically and advantageously, but also the toxicity to human body is 
very low (LD.sub.50 = 5,300 mg/kg), and therefore they have an important 
significance in a practical application. 
Rubbers which can be used in the present invention are natural rubber, 
styrenebutadiene rubber with cis-1,4-structure content of 5-20 percent, 
chloroprene rubber, butadiene rubber with cis-1,4-structure content of 
5-20 percent, isobutylene-isoprene rubber, isoprene rubber, 
nitrile-butadiene rubber or ethylene-propylene terpolymer. 
Furthermore, thiodipropionitrile has such an outstanding characteristic 
that it can be applied in combination with the conventionally available 
vulcanizing agents, vulcanization accelerators, inhibiting agents for 
heat-aging or flex-cracking, antioxidants, filler pigments and other 
rubber-compounding agents, without giving any adverse effect on the 
vulcanization property and other physical properties of rubber. 
The amount of thiodipropionitrile to be used is generally 0.01 to 10 
percent by weight based on rubber, and preferably 0.1 to 5 percent, and in 
addition the performance thereof can outstandingly be increased by the 
combination with petroleum waxes.

The present invention will be illustrated with reference to the following 
examples, which are only given for the purpose of illustration and not to 
be interpreted as limiting. Parts are by weight. 
EXAMPLE 1 
Two parts of each of the compounds in Table 1 (sample numbers A and B) and, 
as a counterpart, Antiozonant AFD (a registered trademark of Bayer Co. 
Ltd.) represented by sample number C were individually mixed thoroughly 
with the following rubber compound, 
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pale crape No. 1 100 parts 
stearic acid 2 " 
zinc oxide 5 " 
white carbon (Caplex No. 80, 
a registered trademark of 
Shionogi Seiyaku Co. Ltd.) 
30 " 
light calcium carbonate 
40 " 
titanium dioxide 10 " 
softener (Circosol 42 XH, 
a registered trademark of 
Sun Oil Co. Ltd.) 10 " 
activator (Acting SL, 
a registered trademark of 
Kawaguchi Kagaku Co. Ltd.) 
1 " 
sulfur 0.5 part 
dibenzothiazyl disulfide 
0.8 " 
tetramethylthiuram monosulfide 
0.1 " 
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and each resulting mixture was well milled on a 2-roll mill of 10 inches in 
diameter and then vulcanized thereon at 140.degree.C. for 20 minutes. A 
blank test was also carried out at the same time for comparison, using the 
rubber compound alone above mentioned. 
Then, Dumbbel No. 2 test pieces were prepared from each vulcanized batch 
thus obtained according to ASTM D-412-51T. The static and dynamic ozone 
deterioration tests were made on the test pieces on the Ozone 
Weather-O-Meter produced by Toyo Rika Co. Ltd. In the static test, the 
Dumbbel No. 2 test pieces were kept at a 20 percent elongation in the 
stream of ozone during the test, and during the latter test, the test 
pieces were given repeated elongation of from 0 up to 20 percent once a 
second in the stream of ozone. 
The tests were carried out under the condition that the ozone concentration 
be 35 .+-. 5 pphm and the test temperature be 50.degree. .+-. 
10.degree.C., and the time required for the generation of visually 
observable cracks on the surface of the vulcanized rubber was taken as a 
crack-generating time which was used as the measure of ozone-resistance. 
The results of the static and dynamic ozone deterioration tests are as 
shown in Table 2 and Table 3, respectively. 
Table 1 
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Sample No. Sample 
______________________________________ 
A N .tbd. C -- (CH.sub.2).sub.2 -- S -- (CH.sub.2).sub.2 -- C 
.tbd. N 
a mixture of No. A and petroleum 
B (Suntight S.sup.(1)) in a weight ratio 
of 1:1 
C Antiozonant AFD 
______________________________________ 
Note: .sup.(1) A registered trademark of Seiko Kagaku Co. Ltd. 
Table 2 
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Static ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
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A 2 20 
B 2 25 
C 2 10 
no addition 2 
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Table 3 
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Dynamic ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
______________________________________ 
A 2 24 
B 2 24 
C 2 12 
no addition 2 
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EXAMPLE 2 
Two parts of each of the compounds in Table 1 (sample numbers A and B) and, 
as a counterpart, Antiozonant AFD represented by sample number C were 
individually mixed thoroughly with the following rubber compound, 
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SBR No. 1502 100 parts 
stearic acid 1 " 
zinc oxide 5 " 
white carbon (Carplex No. 80) 
30 " 
light calcium carbonate 
20 " 
titanium dioxide 10 " 
softener (Circosol 42 XH) 
10 " 
activator (Acting SL) 1 " 
sulfur 2 " 
dibenzothiazyl disulfide 
1.5 " 
tetramethylthiuram 
monosulfide 0.2 part 
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and each resulting mixture was well milled on a 2-roll mill of 10 inches in 
diameter and then vulcanized thereon at 150.degree.C. for 30 minutes. A 
blank test was also carried out at the same time for comparison, using the 
rubber compound alone above mentioned. The test pieces prepared from each 
vulcanized batch were tested under the same condition of ozone 
deterioration test as described in Example 1. 
The results of the static and dynamic ozone deterioration tests are as 
shown in Table 4 and Table 5, respectively. 
Table 4 
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Static ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
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A 2 16 
B 2 22 
C 2 6 
no addition 1 
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Table 5 
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Dynamic ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
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A 2 21 
B 2 20 
C 2 9 
no addition 2 
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EXAMPLE 3 
Two parts of each of the present compounds in Table 1 (sample numbers A and 
B) and, as a counterpart, Antiozonant AFD represented by sample number C 
were individually mixed thoroughly with the following rubber compound, 
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chloroprene rubber WR T 
100 parts 
stearic acid 0.5 part 
zinc oxide 5 parts 
magnesia 4 " 
light calcium carbonate 
45 " 
titanium dioxide 5 " 
2-mercapto imidazoline 
0.5 part 
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and each resulting mixture was well milled on a 2-roll mill of 10 inches in 
diameter, and then vulcanized thereon at 150.degree.C. for 30 minutes. A 
blank test was also carried out at the same time for comparison, using the 
rubber compound alone above mentioned. 
The test pieces prepared from each vulcanized batch were tested under the 
same condition of ozone-deterioration test as described in Example 1 
except that the ozone concentration was 90 .+-. 5 pphm. 
The results of the static and dynamic ozone deterioration tests are as 
shown in Table 6 and Table 7, respectively. 
Table 6 
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Static ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
______________________________________ 
A 1.5 200 
B 1.5 220 
C 1.5 140 
no addition 20 
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Table 2 
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Dynamic ozone deterioration test 
Sample Amount added Crack-generating 
No. (part) time (hour) 
______________________________________ 
A 1.5 220 
B 1.5 230 
C 1.5 160 
no addition 40 
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EXAMPLE 4 
Vulcanized rubber was tested on its discoloring and staining properties by 
the following test method. A strip test piece was prepared from each white 
vulcanized batch obtained in Example 1. Each piece was placed on the paper 
coated with a nitrocellulose based white lacquer, and the whole was 
attached, with the test piece faced outward, to an exposure frame which 
faced toward south at 45.degree. of inclination angle, and exposed to sun 
light for 15 days. The results of the exposure tests are as shown in Table 
8. 
Table 8 
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Amount Shades of test 
Shades of lacquer 
Sample added piece after coated surface 
No. (part) test after test 
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A 2 pale yellow very pale yellow 
B 2 very pale white 
yellow 
C 2 pale yellow very pale yellow 
no addi- pale yellow white 
tion 
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It can clearly be understood from the test results that thiodipropionitrile 
is equivalent to superior to the well known antiozonant AFD which is said 
to have the most excellent non-discoloring and non-staining properties, 
and that it is also remarkably superior to Antiozonant AFD in the 
inhibiting effect of ozone deterioration.