Improving flex fatigue and dynamic ozone crack resistance through antidegradants.

Tire manufacturers continue to try to achieve a better performance tire which would differentiate their products from their competitors, to achieve a longer lasting tire or a lower weight tire which would not only reduce the cost of the tire, but also provide lower rolling resistance. In order to achieve these goals, it is necessary to have better performance antidegradants.

Several different components are required in the construction of radial ply passenger tires. All tire components possess key properties which relate to tire performance. Internal components, such as carcass, bead filler, belt skim, cushion, base tread and bead wire compounds are protected by antioxidants. External tire components such as sidewall, rim flange/ chafer, toe strip and cap tread are exposed to oxygen, sunlight and ozone. These external components are protected by antiozonants/antioxidants and wax. Especially chemical antiozonants are required to protect all of the external tire compounds from ozone, which are based on traditional rubbers containing unsaturated backbones, such as NR, SBR and BR. Rubber chemical producers try to continue to develop better performance antidegradants which would not only provide better protection, but also a cost advantage over existing products. However, it is very difficult to develop lower cost/ performance antidegradants. Also, tire compounders continue their efforts to improve tire performance by developing better fatigue resistant and dynamic ozone crack resistant rubber compounds using various antidegradants and elastomers.

In this article, various antidegradants and their blends have been evaluated in tire sidewall (NR/BR), carcass (NR/ SBR/BR) to improve dynamic ozone crack resistance and flex fatigue property for longer lasting tires. Also, a proposed mechanism has been explained with the performance data.

Proposed mechanism

Antiozonants

It is reported that the protection effect of the PPD (p-phenylenediamine) type antiozonant on the rubber compounds results from a "scavenger-protective film" mechanism (refs. 2 and 3). It is known that the antiozonant reacts with the ozone at a rate much faster than the backbone carbon-carbon double bonds of the rubber on the rubber surface (ref. 4). The rubber is protected from the ozone attack until the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the rubber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against the ozone. A thin film developed from the antiozonant/ozone reaction products on the rubber surface also offers protection.

Under the static condition, what protection mechanism is effected by the PPD antiozonant in the protection against the ozone attack? The diffusion coefficient, measured under static condition, for 6PPD [N-(1,3 dimethylbutyl)-N'phenyl-phenylenediamine] and TAPDT [2,4,6-tris(N-1,4-dimethylpentyl-p-phenylenediamino)-1,3,5-triazine] in a NR compound was reported in the order of [10.sup.-5] [cm.sup.2]/hr magnitude (ref. 5). At this rate, diffusion of these PPD antiozonants from the inner parts to the surface could perhaps be too slow to be an important contributing mechanism against the ozone attack. Also, under the static condition, there may be only limited formation of the thin film since the PPD concentration is limited to those available on the surface. Consequently, there may not be sufficient thin film formation on the rubber surface to offer a meaningful protection. If these were tree, then the static ozone protection would depend mostly on the scavenging role effected by the antiozonant, that is the reactivity of the antiozonant and its initial concentration available on the surface of the rubber. In this report, our data suggests that under the static condition, the scavenging mechanism is effected by the antiozonant playing the key role in the protection against the ozone attack.

In a PPD molecule, the aryl alkyl-substituted NH group is more reactive towards ozone than the bisaryl-substituted NH group owing to the higher charge density on the N-atom of the aryl alkyl-substituted NH group (refs. 1 and 4). This correlates very well with the literature report that aryl alkyl-PPD (e.g., 6PPD) produced nitrone, while the bisalkyl-PPD such as 77PD [N-(1,4-dimethylpentyl)-N'-phenyl-p-phenylnylene-diamine] produced dinitrone instead (refs. 2 and 3). Apparently, the stabilizing effect of the N-aryl group on the nitrone (figure 1) retards further reaction of the nitrone with ozone. A simplified reaction mechanism for the aryl-alky PPDs such as 6PPD is depicted in figure 1.

[ILLUSTRATION OMITTED]

TAPDT resembles an aryl alkyl-PPD. The chemical reaction mechanism of TAPDT (mw 694) with ozone would be similar to that of the 6PPD (mw 268). However, each molecule of TAPDT contains three active aryl alkyl-PPD moieties and TAPDT would scavenge more ozone molecules than the 6PPD on an equal basis. A simplified reaction mechanism for TAPDT is depicted in figure 3. Each of the three PPD moieties in the TAPDT molecule reacts with ozone ([O.sub.3]) to form the tris-(quinonediimino) intermediate (QDI) via the two intermediates shown in figure 3. The QDI intermediate can react with ozone to form a nitrone. Further reaction of the nitrone with ozone would be slow as in the case with 6PPD shown in figure 1.

[ILLUSTRATION OMITTED]

In summary, under the conditions where the antiozonant diffusion is limited, the key protection mechanism would be the scavenging role effected by the antiozonant. This reasoning is supported by the data we observed with TAPDT, which provides better static ozone protection than 6PPD.

N,N'-disubstituted PPDs are also known to provide excellent protection for rubber compounds against oxidation. Like secondary aromatic amines, the N,N'-disubstituted PPDs are considered primary antioxidant. The primary role of this class antioxidant is acting as chain,breaker through the abstraction of the hydrogen atom in the reaction with alkylperoxy radicals [RO.sub.2] (ref. 5). A mechanism of the PPD acting as auto-oxidation chain-breaker is shown in figure 4 (ref. 6).

[ILLUSTRATION OMITTED]

Antioxidants

The secondary aromatic amines such as polymerized 1,2-dihydro-2,2,4-trimethylquinoline (TMQ), a high temperature reaction product of diphenylamine and acetone (BLE) and 4,4'-bis([Alpha],[Alpha]-dimethylbenzyl)diphenylamine (AO 445) are commonly used antioxidants. These three antioxidants nonetheless provide different anti-fatigue efficiency for rubber compounds. In this report, we would like to explore the difference in the reaction mechanisms for the anti-fatigue and the anti-oxidation. A rationale is also proposed to explain why these three amines perform differently under various conditions. Chemical structures of TMQ, BLE and AO 445 are shown in figures 5 and 6.

[ILLUSTRATIONS OMITTED]

Fatiguing of rubbers at room temperature is a degradation process caused by the shear of repeated mechanical stress under limited access of oxygen. The mechanical shear generates macroalkyl radicals (R*). A small fraction of the macroalkyl radicals reacts with oxygen to form alkylperoxyl radicals, leaving behind still a high concentration of the macroalkyl radicals. Consequently, removal of the macroalkyl radicals in a catalytic process constitutes a prevailing anti-fatigue process (ref. 7).

On the other hand, the macroalkyl radicals are rapidly converted to the alkylperoxyl radicals under air-oven heat aging. The auto-oxidation propagated by the alkyperoxyl radicals thus dominates the degradation process. Therefore, removal of the alkylperoxyl radicals becomes the primary function of an antioxidant

It has been shown that the diarylamines (II) are good anti-fatigue agents and that diarylamine nitroxyl radicals (I) are even more effective than the parent amines (ref. 8). The anti-fatigue mechanism of the amine anti-degradants, shown in scheme 2, has been proposed with the formation of the intermediate nitroxyl radicals playing an active role (ref. 8). Generation of the nitroxyl radicals (1) from the free amines (2) is depicted in scheme 1. In the fatiguing process (scheme 2), the macroalkyl radicals are generated (equation 1). Removal of the macroalkyl radicals by the nitroxyl radicals is shown in equations 2 and 3. The resulting hydroxylamine (3) can be re-oxidized by alkylperoxyi to re-generate the nitroxyl radicals in an auto-oxidation chain-breaking process (equation 4). The nitroxyl radicals (1) can be partially converted back to the free diarylamine (2) during vulcanization through the reductive action of thiyl radicals of thiols (equation 5). The free diarylamine (2) thus regenerated, would repeat the scheme 1 to form more nitroxyl radicals (1).

[ILLUSTRATIONS OMITTED]

Reactivity of the nitroxyl radicals is affected by delocalization, stearic hindrance and substitutions (ref. 9). Delocalization of the unpaired electron into the aromatic ring increases the number of reactive sites and decreases its stability. Ring substitution at the para-position reduces the side reaction of the nitroxyl radicals. Stearic hindrance would obviously reduce the reactivity of the nitroxyl radicals as an anti-fatigue agent. Thus, the stearic hindrance in the TMQ-nitroxyl radical (4) reduces its reactivity in intercepting the macroalkyl radicals (example eq. 3); consequently, the TMQ-nitroxyl radical (4) is less efficient as an anti-fatigue agent than a less hindered diarylamine nitroxyl radical (1). Thereby, the free amine precursors of the nitroxyl radicals (1) would be more efficient anti-fatigue agents than the free amine precursors of the nitroxyl radicals (4). Therefore, the diarylamines in general, such as the BLE and AO 445, are more efficient anti-fatigue agents than the TMQ. This argument is in agreement with the data reported both here and in a previous report (ref. 8).

Generation of the alkylperoxyl radicals is rapid during air-oven heat aging of rubbers, in contrast to the fatiguing process. The alkylperoxyl radicals propagate an auto-oxidation degradation process. The auto-oxidation mechanism is depicted in scheme 3. Removal of the alkylperoxyl radicals became the primary function of an antioxidant (scheme 4).

Scheme 3 - auto-oxidation mechanism

(a simplified form)

                                   Initiation

RH                 [right arrow]   R*

Propagation

R* + [O.sub.2]     [right arrow]   [RO.sub.2*]
[RO.sub.2*] + RH   [right arrow]   ROOH + R*

Termination

[2RO.sub.2*]       [right arrow]   Inert product
[RO.sub.2*] + R*   [right arrow]   ROOR
2R*                [right arrow]   R-R
Scheme 4 - mechanism of antioxidant action

(a simplified form)

[RO.sub.2*] + AH   [right arrow]   ROOH + A*

A*                 [right arrow]   Dimers, ... other stable product

The heat aging is conducted at elevated temperatures; volatility of the anti-fatigue agents often plays important roles in the heat aging process, in addition to the reaction mechanism discussed above. The GPC analysis indicates the molecular weight of BLE (i.e., Mw 390 and Mn 230) is much lower than,that of TMQ (i.e., Mw 820 and Mn 560). Apparently, BLE would be more volatile than the TMQ. This is confirmed by a TGA study, which gives percent weight loss for the BLE at 45.3% compared to the TMQ at 7.5% in 60 minutes at 177 [degrees] C. A separate TGA study concluded that TMQ is more volatile than AO 445 (ref. 10). Thus, the volatility of the antioxidants in descending order is BLE [is greater than] TMQ [is greater than] AO 445. The loss due to volatility would explain why, after heat aging, the anti-fatigue efficiency of the BLE became only slightly better than the TMQ, even though BLE was significantly better than TMQ when unaged. In addition, BLE may not be as effective an antioxidant as TMQ for heat aging, due at least partly to the volatility.

Based on the anti-fatigue mechanism and the volatility, we came to the following conclusion: For fatigue protection of unaged rubber compounds, AO 445 would be better than TMQ. After heat aging, the efficiency of the AO 445 would become much better than TMQ. On the other hand, the anti-fatigue efficiency of the BLE would be better than the TMQ for unaged rubber compounds, but the difference in the efficiency between BLE and TMQ would be reduced after heat aging.

Evaluation of NR/BR/SBR carcass compound with TMQ, BLE and AO 445 antioxidants and TMQ/BLE blends for improved flex fatigue property (ref. 11)

Carcass

Tire body or carcass rubber compounds must form strong bonds to the adhesive coated (resorcinol formaldehyde latex dipped) fabric. Usually, polyester, rayon or nylon fibers are used to make cords for tire carcass, except for steel monoply truck tires. Carcass strength and durability should be adequate enough to insulate the tire cords and hold them in their paths. The carcass compound must, however, be soft enough to permit a slight change in cord angles when the tire is flexed. The body rubber serves as insulation between the fabric plies. Outstanding fatigue resistance is required of the carcass compounds in order to withstand cyclic deformation. The carcass is not exposed to the outside, so that it does not require ozone protection. However, due to cyclic flexing, it is necessary to have good flex fatigue resistance and heat aged properties.

Experimental

Currently, most tire companies use TMQ for protecting carcass compounds. In this experiment, BLE, AO 445 and BLE/TMQ blend systems have been evaluated in a NR/BR/SBR carcass compound, along with TMQ alone, which is shown in table 1. Processing data measurement, unaged and aged physical properties and unaged/aged flex fatigue tests have been carried out to demonstrate the improved flex fatigue properties and heat aged properties.

Table 1 - across compound

                            MB-1

NR                          60
BR 1205                     20
SBR 1707                    27
N-660                       50
Zinc oxide                   4
Naphthenic oil               7.5
Stearic acid                 1.5
Octylphenol formaldehyde     2.0
Resorcinol resin             2.0

Total                      174.0

                                    A-0    A-1    A-2    A-3    A-4

MB-1                                174    174    174    174    174
TMQ                                        1.0                  0.5
BLE                                               1.0           0.5
AO 445                                                   1.0
Methylene donor                     1.0    1.0    1.0    1.0    1.0
MBTS                                1.2    1.2    1.2    1.2    1.2
DPG                                0.25   0.25   0.25   0.25   0.25
80% insoluble sulfur               3.00   3.00   3.00   3.00   3.00

Results and discussion

Mooney viscosity at 100 [degrees] C, Mooney scorch and curometer at 177 [degrees] C of all five compounds were measured (table 2). No significant differences were obtained. In order to minimize experimental variations, one masterbatch without antioxidants and curatives was prepared in an internal mixer. Using the same masterbatch, antioxidants and curatives were added in a second mixer. The identical mixing procedure was employed for all five batches. Clearly, the results shown in table 4 indicated that BLE, BLE/TMQ blend and AO 445 would provide better flex fatigue properties, while TMQ and AO 445 have the best heat protection, which is shown in table 3. However, after aging 70 hours at 100 [degrees] C, all the batches were inferior to flexing (table 4). This would illustrate the mechanism of flex fatigue improvement. Antioxidants should migrate to the surface for improved flex fatigue; however, lower volatilities such as BLE, heat aged property is inferior to TMQ.

Table 2 - processing data

                                       A-0    A-1    A-2    A-3    A-4

TMQ                                           1.0                  0.5
BLE                                                  1.0           0.5
AO 445                                                      1.0

Mooney viscosity at 100 [degrees] C     49     49     48     49     48

Mooney scorch at 132 [degrees] C
  3 points rise time (minutes)        11.9   11.6   11.6   11.8   11.4

Curometer at 177 [degrees] C
  ML, N-M                             0.40   0.40   0.41   0.41   0.40
  MH, N-M                             3.45   3.43   3.49   3.48   3.45

  t1, minutes                         0.93   0.94   0.95     97     93
  tc50, minutes                       1.71   1.72   1.74    174   1.73
  tc90, minutes                       2.96   3.00   3.05   3:04   3.03
Table 3 - unaged and aged physical properties

                                 A-0    A-1    A-2    A-3    A-4

TMQ                                     1.0                  0.5
BLE                                            1.0           0.5
AO 445                                                1.0

Cured 10 minutes at 177
    [degrees] C
  Tensile at R.T., MPa          19.5   18.8   19.1   16.7   19.0
  % elongation                   450    460    470    430    460
  300% modulus, MPa             10.5   10.2   10.1   10.5   10.3
  Shore A hardness                57     56     55     54     55
  Tear strength (die c), kN/m     35     33     34     35   37.0

Aged 2 weeks at 70
    [degrees] C
  Tensile at R.T.,MPa           12.3   14.8   13.8   13.4   14.7
    % retention                   63     79   74.4     81   77.7
  Elongation                     238    300    295    260    300
    % retention                   53     65   62.7     60     66
  Change in Shore A hardness      +5     +4     +6     +6     +5
  Tear strength (die c)         16.0     23     24     25     25
    % retention                   61     68     71     70     69
Table 4 - dynamic flex fatigue property

                              A-0       A-1     A-2    A-3    A-4

TMQ                                     1.0                   0.5
BLE                                             1.0           0.5
AO 445                                                 1.0

DeMattia flex
  Kilocycles to full
    cracking unaged              133    361     603    460    538
  Aged 70 hours at 100
    [degrees] C                   25   80.5     102   90.5    105
Monsanto flex fatigue
  Kilocycles to failure
    unaged                Not tested   68.2   110.0   79.0   98.0
Aged 70 hours at 100
    [degrees] C           Not tested    118    12.0   16.0   15.5

Conclusion

* TMQ, AO 445 and TMQ/BLE blend provide better heat aged properties than BLE alone.

* BLE and TMQ/BLE blends have better flex fatigue properties than TMQ and AO 445.

* All the antioxidants provide better protection than the blank.

* BLE/TMQ and AO 445 not only have better flex fatigue than TMQ alone, but also provide protection from heat aging similar to TMQ and better than BLE.

Evaluation of NR/BR tire black sidewall compound with various types of antiozonants and their blends for improved dynamic flexing and dynamic ozone crack resistance (ref. 12)

Sidewall

Typically, tire sidewall contains either NR/BR or NR/BR/ SBR compounds for improved cut growth resistance, abrasion resistance and dynamic flexing. The sidewall is exposed to the outside environment. Therefore, it must be protected from ozone or oxidation cracking. Typically, 2.5 to 4.0 phr of alkyl aryl paraphenylene diamine such as 6PPD or IPPD are used in combination with blended wax for static and dynamic ozone cracking resistance and improved flex fatigue resistant properties. Also, NR and cis BR would lower hysteresis for lower running temperatures. This lower temperature improves the flex fatigue property.

Experimental

Four compounds were prepared using the same NR/BR masterbatch for evaluating flex fatigue, dynamic ozone flexing and static ozone crack resistance (table 5). Mooney scorch, unaged and aged physical properties, dynamic flexing, and dynamic and static ozone cracking resistance were measured.

Table 5 - tire sidewall
test compound

SMR 5CV            50.0
CIS 4 BR           50.0
N-326 black        50.0
Aromatic oil        5.0
Zinc oxide          3.0
Antiozonant    As shown
Stearic acid        1.0
Blended wax         1.5

TBBS                1.0
Sulfur              2.0

Results and discussions

Four parts of each 6PPD and TAPDT and 2/2 blends of 6PPD/TAPDT were prepared along with the blank compound. Unaged and aged physical properties indicated that there were no significant differences among them except for the Mooney scorch value (table 6). TAPDT would provide a longer scorch than 6PPD. Flex fatigue values were significantly better with 4.0 phr of 6PPD or 2/2 TAPDT/6PPD blends than TAPDT alone. Due to the slow migration of TAPDT (higher molecular weight antiozonant), the accelerated flexing test was inferior to both blends and 6PPD (table 7). However, longer static protection with TAPDT and blends of TAPDT/6PPD were achieved (table 8).

Table 6 - processing and physical properties

                                    B-0    B-1    B-2    B-3

6PPD                                       2.0    4.0
TAPDT                                      2.0           4.0

Processing
  Mooney scorch at 135
    [degrees] C                    14.5   13.3   10.5   16.0
Unaged physical properties -
    cured 15' at 160 [degrees] C
  Tensile, MPa                     23.2   22.7   22.3   21.3
  300% modulus, MPa                 8.0    6.1    6.3    6.2
  Elongation, %                     590    640    620    620
  Hardness, Shore A                  58     58     57     57

Aged 96 hours in oxygen at 70
    [degrees] C
  Tensile, % retention               54     80     86     85
  Elongation, % retention            63     65     74     74
  Hardness, points change             6      9      7      6
Table 7 - dynamic properties

                                      B-0      B-1      B-2      B-3

6PPD                                           2.0      4.0
TAPDT                                          2.0               4.0

Outdoor dynamic flexing -
    Naugatuck, CT
Continuous flexing - aged six
    months prior to testing
  KC to crack                       2,609   39,560   39,561   16,951
Intermittent flexing, 8 hours of
    flex + 16 static exposure -
    aged 6 months
  KC to crack                       2,969   33,302   31,091   23,724
Ozone flexing, 25% extension,
    50 PPHM ozone, 38 [degrees]
    C Unaged samples
Hours to crack                          4       48      144       24
Table 8 - static properties

                            B-0    B-1        B-2      B-3

6PPD                               2.0        4.0
TAPDT                              2.0                 4.0

Outdoor static exposure
Bent loop; Naugatuck, CT
Hours to crack               78   1,845(*)   1,607   1,845(*)
                              C      OK          C      OK
Bent loop; aged 6 months      8   1,080         24   1,080
  prior to testing            C      OK          C      OK
20% extension, unaged         2   1,128        816   1,128
                              C      OK         VS      OK
20% extension; aged 6         4     840         24   1,080
  months prior to testing     C       C          C      OK

(*) exposure still in progress at 1,845 days

C = cracked; OK = no visible cracking;

VS = yew slight cracking

Conclusion

* TAPDT will provide scorch safety in tire black sidewall compounds, while 6PPD has reduced the scorch safety.

* 6PPD and the blends of TAPDT and 6PPD have better flex fatigue and dynamic ozone crack resistance than TAPDT itself.

* TAPDT and the blends provide better static protection than 6PPD alone.

* The combination of 6PPD (lower molecular weight AOs) and TAPDT (higher molecular weight AOs) will provide longer term protection for both static and dynamic ozone cracking.

References

(1.) S.W. Hong, P.K. Greene and C-Y. Lin, ACS Rubber Division 155th Conference, Chicago, IL, Paper No. 65 (April 13-16, 1999).

(2.) R.P. Lattimer, E.R. Hooser, R.W. Layer and C.K. Rhee, Rubber Chemical and Technology, 56, 431 (1983).

(3.) R.P. Lattimer, E.R. Hooser, R.W. Layer and C.K. Rhee, Rubber Chemical and Technology, 53, 1170 (1980).

(4.) D. Bruck, Dormagen and H.W. Engles, Kautschuk & Gummi Kunstoffe 44, 1,014-18, Nr. 11 (1991).

(5.) (a) J.R. Shelton and D.N. Vincent, J. American Chemical Society, 85, 2,433 (1963); (b) J.R. Shelton, Rubber Chemical and Technology, 45, 359 (1972).

(6.) (a) K. Adamic, D.F. Bowman and K.U. Ingold, J. American Oil Chemical Society, 47, 109 (1970); (b) C.E. Boozer, G.S. Hammon, C.E. Hammilton and J.N. Sen, J., American Chemical Society, 77, 3,223 (1955).

(7.) G. Scott, Rubber Chemical and Technology, 58, 269, 1985.

(8.) H.S. Dweik and G. Scott, Rubber Chemical and Technology, 57, 735, 1984.

(9.) F.A. Forrester, J.M. Hay and R.H. Thompson, "Organic chemistry of stable free radicals," (1968), Academic Press, London, p. 180.

(10.) Uniroyal Chemical Technical Bulletin for Naugard 445.

(11.) S. W. Hong and R.A. Mazzeo, "Improved tire performance through the use of antidegradants," presented at the ITEC, Akron, OH, September 1996.

(12.) D.A. Birdsall, S.W. Hong and D.J. Hajdasz, The Tyre Tech '91 Conference, Berlin, Germany, October 24-25, 1991.