Process for preparing rubber compositions

A process for preparing rubber compositions comprised of a (a) 100 parts rubber material including 65-100 parts of cis-polyisoprene and optionally at least one additional high unsaturation rubber, (b) 40-70 parts Carbon Black and (c) conventional vulcanization and processing additives. The composition is mixed in two consecutive stages, in the first stage all the carbon black is mixed with at least 50 parts of the cis-polyisoprene, and in the second stage the remaining polyisoprene and any optional additional rubber is added to the mix. This method of mixing material has been observed to produce a rubber composition having enhanced tear and adhesion properties as compared with conventional processes where the rubber is mixed with the carbon black in one mixing stage. Thus, the invention is also directed to the resulting rubber composition and to a composite of such composition containing a filament reinforcement.

FIELD 
This invention relates to the preparation of rubber compositions by the 
incorporation of carbon black therein. It particularly relates to a 
process of mixing carbon black and rubber by a two stage process and the 
resulting rubber compositions are particularly useful for the manufacture 
of tire reinforcement plies. 
It is known that when two or more elastomers are compounded with carbon 
black, that the carbon black is unequally distributed amongst the 
different polymer phases. In a paper by Sircar and Lamond, Rubber Chem 
Tech 46 (1973) 178, a number of different rubbers are compounded with 
polybutadiene in 50:50 mixtures and the carbon black added in various 
methods to the different polymers. It was demonstrated that the carbon 
black migrated from a low unsaturation polymer to a high unsaturation 
polymer irrespective of how the black was added. 
A second paper by Sircar, Lamond and Pinter, Rubber Chem Tech 47 (1974) 48 
studied the effect of the carbon black distribution on the properties of 
polymer blends. The blends were all formed from a 50:50 mixture of two 
polymers, for example natural rubber (NR) and polybutadiene (BR) and the 
black added to one polymer and then diluted with the second. The results 
showed that a carbon black-rich compound of the first elastomer was 
suspended in a carbon black-free second elastomer. It was noted that NR/BR 
blends which contained unloaded NR have cut growth properties not 
significantly different from conventionally mixed compounds, and that 
superior cut growth resistance was observed for NR/BR blends containing 
unload BR. 
Another paper by Hess & Cherico, Rubber Chem Tech 50 (1977) 301 also 
studied the effect of carbon black distribution on polymer blend 
properties. Again 50:50 blends of two polymers were taken, but this time 
each polymer was mixed with carbon black to form a master batch prior to 
blending with the other of the two polymers. 
In all the master batch blending studies it was shown that the 
black/polymer phase distribution reflected the initial carbon black 
loadings thus demonstrating that there is no significant phase transfer of 
the carbon black from one polymer to the other. For NR/BR blends it was 
shown that tear strength was highest with a 75% loading of black in the NR 
and lowest with high black loading in the BR. This is apparently related 
to a high black loading in the continuous polymer phase and in NR/BR the 
most continuous polymer mesh was formed by adding black preferentially to 
the NR. This type of mixing gave higher tear resistance compounds. 
None of the above papers demonstrates or discusses the possibility of 
obtaining a non-uniform carbon black distribution in a single polymer, or 
a polymer blend having a single polymer as a major component thereof. 
DISCLOSURE AND PRACTISE OF THE INVENTION 
Accordingly there is provided a process for preparing a rubber composition 
comprised of 
(A) 100 parts by weight of rubber including 65 to 100 parts by weight of 
cis 1-4polyisoprene and 0-35 parts by weight of at least one additional 
high unsaturation rubber; 
(B) 40-70 parts by weight of carbon black and 
(C) rubber vulcanisation aids including accelerators and sulphur, and other 
additives, such as zinc oxide, stearic acid or stearate and resins. 
The process comprising mechanically mixing processing the composition under 
shear conditions in at least two consecutive stages, in the first stage 
all the carbon black is mixed with at least 50 parts by weight of the cis 
1-4 polyisoprene characterized in that at the first stage, the number of 
parts of cis-polyisoprene does not exceed more than 80% of the total 
polyisoprene content of the rubber and in the second stage the remaining 
cis 1-4 polisoprene and other rubber, if present, is mixed into the 
composition. 
In one embodiment of the invention, the 100 parts by weight of rubber 
consists of natural rubber, (preferably 60-70 parts in first stage) and by 
mixing the natural rubber with carbon black, (preferably 55 to 60 parts by 
weight) in two stages it has been observed that the tear strength of the 
natural rubber composition is unexpectedly increased. This is somewhat 
contrary to the findings of the Sircar, Lamond and Pinter paper referenced 
above, in which it was indicated that blends containing unloaded NR did 
not have significantly different cut growth properties as compared with 
conventionally mixed compounds. 
In a second embodiment of the invention, the 100 parts by weight of rubber 
include 1-35 parts, preferably 10-20 parts by weight of polybutadiene. 
Preferably the cis 1-4 polyisoprene and polybutadiene rubbers are blended 
together prior to adding at the second stage. The addition of the unfilled 
polyisoprene and polybutadiene rubber introduces unfilled phases into the 
composition and it is considered that the unfilled polyisoprene will 
provide improved tear strength and the unfilled BR gives improved adhesion 
and fatigue resistance. By blending the polyisoprene and the BR prior to 
addition at the second stage we obtain an interaction between the two 
rubbers and when added as a blend at the second stage, this gives improved 
fatigue resistance.

The invention will be described by way of the following examples which are 
representative of the scope of the invention. All parts in the 
compositions of the various compounds are given in parts by weight. 
Rubber compounds which are particularly useful for tire reinforcement 
composites in particular carcass plies and breaker plies were formulated 
by mixing various amounts of Natural Rubber and polybutadiene (total 
rubber=100 parts) together with about 60 parts of Carbon Black (HAF type), 
and conventional rubber compounding ingredients such as zinc oxide, 
stearic acid, silica, antidegradants, peptisers, resin(s), sulphur and 
accelerators. 
It is to be noted that the compositions used in the following examples are 
identical except that they have only differing amounts of Natural Rubber 
(NR) and polybutadiene (BR). 
The conventional single stage mixed control compositions were mixed in a 
laboratory according to the following procedures: 
(a) Into a 3 liter Shaw Intermix were added all the ingredients except 
accelerators, sulphur, some peptizer and some resin. After about 4 minutes 
mixing time the mix temperature reached about 160.degree. C. and the batch 
was dropped from the mixer. 
(b) After an interval of about 24 hours the batch was further mixed for 
about 2 minutes until it reached 120.degree. C. and was dropped from the 
mixer. 
(c) After a further interval of about 24 hours the batch was further mixed 
for 1-2 minutes with the sulphur, accelerator, resin and peptizer until 
the mix temperature reached about 100.degree. C. 
This final batch was then processed by a two roll mill into sheet form to 
produce test samples. 
The two-stage mixed compositions were produced according to the invention 
as follows: 
The first stage mix comprises: 
(I) For step (a) above except that only a portion of the natural rubber is 
placed in the 3 liter Shaw Intermix. 
(II) As for (b) above. 
The second stage mix comprises: 
(III) The remaining natural rubber plus any polybutadiene is added to the 
material and the batch mixed for about 4 minutes until a temperature of 
160.degree. was reached and then dropped from the mixer. 
(IV) as for step (b) above. 
(V) as for step (c) above. 
The following tests were used to evaluate the rubber compositions: 
______________________________________ 
(a) Tensile Modulus at 300% Extension 
(ASTMS D412) 
(b) Rebound Hot (ASTMS D1054) 
(c) Demattia Flexlife (ASTMS D813) 
(d) Wire adhesion (S.B.A.T.) 
(ASTMS D2229) 
______________________________________ 
Also the following non-standard tests were used: 
(e) Compound Adhesion Test 
Samples of the elastomeric test material are formed in sheets (152 
mm.times.101 mm.times.2.4 mm) and two sheets are laminated to form a 
sandwich with polyester film layer between the two sheets. The film has 
rectangular apertures 60 mm.times.5 mm so that the two sheet contact each 
other through the apertures. Each outer side of the elastomeric material 
is then backed by a reinforcing fabric layer, having cords in the 
direction of tear. After curing the test sheets are cut into 2.54 mm wide 
strips with the apertures running down the middle of the strip. The two 
end tabs at one end of the strip are placed in the jaws of a stress-strain 
tester and pulled apart at 50 cm/min thereby pulling apart the joint 
formed at the aperture between two elastomeric layers. The test was 
performed at 100.degree. C. 
(f) Hot Instron Tear Test 
Samples of the elastomeric material are cured in 152 mm.times.76 
mm.times.12.7 mm slabs which are aged for 7 days at 90.degree. C. and are 
then cut into 152 mm.times.25.4 mm strips. The strips are then fitted into 
a jig and cut along both longitudinal edges to leave an uncut 
longitudinally extending portion in the center of the stip of a width of 
6.4 mm-7.6 mm. One end of each strip is then also cut to a depth of about 
57 mm to provide tab ends for placing in testing jaws. 
The tab ends are then fitted into the jaws of a stress-strain tester and 
pulled apart at 50 cm/min. 
After tearing, the tear width is measured and the tear value calculated. 
(g) Textile Cord Adhesion 
Samples of the cord are calendered with the elastomeric test material. 
Samples of 75 mm.times.140 mm are cut with the cord extending parallel 
with the 75 mm sides of the sample. The sample is then cut into two 75 
mm.times.75 mm portions and a 25 cm.times.75 cm strip of holland cloth 
placed along the top edge of one portion running across the cords. The 
second portion is placed on top forming a two layer 75 cm.times.75 cm pad 
which is then cured. Two 25 mm strips parallel with the cord are cut out 
of the pad on each side of its center line so that each strip has two 25 
mm long tabs at one end. The strips are heated to 120.degree. C. and the 
tabs placed in the jaws of a stress-strain tester. The jaws are separated 
at a speed of 5 cm/min. The adhesion is reported as load required to 
separate the two cord reinforced layers. 
Note: Test is to be preformed whilst samples are at 120.degree. C. 
EXAMPLE I 
Using 100 parts by weight of Natural Rubber which was mixed as detailed 
below in Table I and with reference to Page 4, and tested according to the 
methods detailed on Pages 5 and 6. 
TABLE I 
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Addition of Rubber (Parts) 
Mixing Sequence 
Control EX Y EX Z 
______________________________________ 
1st stage mix 100 65 50 
Remill * * * 
2nd stage mix -- 35 50 
Remill -- * * 
Addition of sulphur, 
* * * 
accelerators etc. 
______________________________________ 
The rubber compositions resulting from the above mixing procedures were 
tested with the results shown in Table II below. 
TABLE II 
______________________________________ 
Con- 
Physical tests trol EX Y EX Z 
______________________________________ 
Modulus (300%) (MN/m.sup.2) (ASTM D412) 
18 18 18 
Rebound hot (%) ASTM D1054 
65 68 69 
Hot Compound adhesion ASTM KN/M 
9 50 50 
Hot Instron Tear KN/M ASTM 
40 55 52 
**Adhesion to cord KN/M static 
61 94 -- 
Demattia Flex life ASTM D813 
5 18 -- 
Wire adhesion SBAT 298 -- 398 
Aged Wire adhesion 
3 Days Water 90.degree. C. 
266 -- 304 
10 Days Air 110.degree. C. 
294 -- 291 
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*These operations performed as part of the mixing sequence. 
** Adhesion to aromatic polyamide treated cord. 
Thus it can be seen from the test results, that mixing all the carbon black 
with a portion of the natural rubber, and then diluting this mix with the 
remaining natural rubber has effected some physical properties, when 
compared with the physical properties of the traditional one-stage control 
mix. 
For example, the fatigue life has been increased, the adhesion and tear 
properties have been greatly increased, and the hysteresis loss has been 
reduced. All these benefits have been achieved only by differing the 
mixing procedures. 
EXAMPLE II 
Using a mixture of natural rubber and polybutadiene rubber with a total 
rubber content of 100 parts, the rubber contents and mixing sequences are 
detailed below in table III. 
The NR and BR were blended together prior to addition at the second stage. 
The ratio of the NR/BR for the different samples was as shown in Table 
III, however, it was found that it is preferable where possible to add the 
second stage rubber material as a 3:1 blend by weight of NR:BR 
respectively. 
The rubber compositions resulting from the mixing sequences of Table III 
were then tested with the results as detailed in Table IV below. 
TABLE III 
__________________________________________________________________________ 
Addition of Rubber (Parts) 
Mixing Sequence 
Control 
A B C D E F G 
__________________________________________________________________________ 
1st stage 100 87.5 NR 
85 NR 
70 NR 
50 50 50 50 
addition of rubber 
12.5 PBD 
15 PBD 
30 PBD 
Remill * * * * * * * * 
2nd stage -- -- -- -- 37.5 NR 
32.5 NR 
25 NR 
17.5 NR 
addition of rubber 12.5 PBD 
17.5 PBD 
25 PBD 
32.5 PBD 
Remill -- -- -- -- * * * * 
addition of Sulphur, 
* * * * * * * * 
Accelerators, etc. 
__________________________________________________________________________ 
*These operations performed in the mixing sequence. 
TABLE IV 
__________________________________________________________________________ 
PHYSICAL TESTS 
Control 
A B C D E F G 
__________________________________________________________________________ 
300% Modulus KN/m.sup.2 
18 18 -- 16 17 17 16 16 
Hot Rebound % 66 68 66 66 69 72 70 70 
Compound 7 15 7 20 30 33 31 32 
Adhesion KN/m 
Instron Tear KN/m 
45 30 20 20 45 53 38 40 
Demattia Flex 5 -- 21 -- 28 -- -- -- 
Life hours 
Wire Adhesion KN/m 
298 337 
425 
435 
348 
418 
480 
416 
Aged Wire Adhesion KN/m 
3 days 266 246 
-- -- 294 
236 
278 
262 
Water 90.degree. 
10 days 294 309 
-- -- 318 
312 
312 
285 
Air 110.degree. 
Textile Cord 78 -- 99 -- 116 
-- -- -- 
Adhesion KN/m 
__________________________________________________________________________ 
It can be seen from the results that the tear strength, and adhesion 
properties of the material are improved for the two-stages mixed compounds 
D, E, F, and G, containing BR; as compared with the conventional one-stage 
mixed compounds A, B, or C, containing BR. Furthermore there is also a 
slight improvement in hysteresis and an improvement in fatigue properties 
over and above the single stage mix compounds. Thus it can be seen that 
the phase mixed NR/BR compounds have the improved fatigue life of such 
compounds when compared with the NR control, but with no sebsequent loss 
of tear properties normally associated with mixing Br into NR. 
Whilst the invention has been described with reference to the use of 
polybutadiene as an optional additional high unsaturation rubber, it is 
considered that other high unsaturation rubbers such as vinyl 
polybutadiene could be used. By vinyl polybutadiene is meant a 
polybutadiene rubber containing greater than 20% of vinyl 1,2 structure. 
Otherwise, the term "polybutadiene" as used herein refers to cis 
1,4-polybutadiene which typically contains less than about 5 percent of a 
vinyl 1,2-structure. 
In the practice of this invention and as a preferred embodiment thereof, 
for the second mixing stage, the remaining cis 1,4 polyisoprene, 
preferably as natural rubber, and the high unsaturation rubber, preferably 
as polybutadiene rubber, are added as a blend composed of a weight ratio 
in the range of 3/1 to 1/3 of polyisoprene to polybutadiene. 
In one aspect and as an additional embodiment of this invention, a rubber 
composition is provided which is prepared according to the process of this 
invention. 
In another aspect and as a further embodiment of this invention, a 
composite is provided which comprises the rubber composition of this 
invention containing a filament reinforcement. Such filament can typically 
be composed of one or more filaments. In the case of multiple filaments, 
they can be in the form of a cord of filaments twisted together. 
Contemplated filamentary material includes metal wire and organic polymer 
textile filaments formed from organic polymers such as, for example, nylon 
and aramid materials. 
In practice, it is comtemplated that the process of this invention is 
suitable for the preparation and making of a tire reinforcing ply coating 
composition. 
Whilst the invention has been illustrated with reference to the described 
example, it is believed that it is within the scope of the skilled man to 
make minor modifications thereto without departing from the scope and 
spirit of the invention.