Stabilization of tire compounds with QDI.

Higher mileage tires demand longer service life from rubber vulcanizates. Physical properties of rubber compounds depend upon a variety of factors above and beyond just the type of rubber, filler and cord reinforcement used in the compounds. Natural rubber is the elastomer of choice for heavy service tires such as track, aircraft and off-the-road tires. Natural rubber provides the outstanding strength and fatigue characteristics required for these severe applications (ref. 1). Elastomers combining long fatigue life with good toughness or tear resistance require sulfur vulcanization (ref. 2). The type of accelerator and ratio of sulfur to accelerator determine the characteristics of the sulfur crosslinks (ref. 3). The sulfur rank or the average number of sulfur atoms per crosslink largely determines the characteristic properties of the resulting vulcanizate (ref. 4). Networks rich in long polysulfidic crosslinks (i.e., 4, 5, 6 or more sulfur atoms per crosslink) provide better fatigue and tear resistance than networks rich in mono- and disulfidic crosslinks. In general, networks rich in disulfide and monosulfide crosslinks are thermally and oxidatively more stable than those dominated by polysulfidic crosslinks. This result is primarily due to two factors. First, the polysulfidic networks are prone to reversion. Second, the polysulfidic crosslinks are susceptible to oxidative degradation. Conversely, monosulfidic and disulfidic networks provide a measure of oxidative protection while being resistant to reversion. Monosulfidic crosslinks are expected to decompose into sulfenic acid derivatives, which are postulated as powerful antioxidants (ref. 5). However, networks rich in monosulfidic and disulfidic crosslinks provide poor fatigue and tear resistance properties. Thus, compounding for severe applications (i.e., applications requiring high flex-fatigue resistance and tear strength) requires networks rich in polysulfidic crosslinks. When this is the goal, compounding with antidegradants that help to preserve the rich polysulfidic networks would also be desirable.

Reversion is the degradation of the sulfur network and results in a degeneration of vulcanizate physical properties. Reversion occurs upon overcure and during cure at elevated temperatures (ref. 6). It is catalyzed by accelerator fragments complexed with zinc ions and/or zinc ions and amines (ref. 7). Amines, in particular, have a deleterious effect on reversion resistance. In fact, even though sulfenamides provide delayed-action-fast cures compared to 2-mercaptobenzothiazole and 2-mercaptobenzothiazole disulfide, reversion is greatly accelerated. When added to model vulcanizate mixtures, amines, zinc-amine complexes or zinc-amine-accelerator complexes likewise accelerate reversion (ref. 8). Thus, traditional amine based antidegradants, while providing protection against oxidative degradation, may inadvertently promote reversion of the sulfur network. Maintenance of the polysulfidic network upon aging would help to provide improved dynamic mechanical properties, improved fatigue resistance and help to maintain tear strength upon aging.

Possible methods to prevent reversion promoted by an amine derived antidegradant would be to employ non-amine antidegradants or use a polymer-bound amine based anti-degradant. However, non-amine antidegradants do not sufficiently protect conventional sulfur vulcanizates from oxidation and ozone degradation. The use of only polymer-bound antidegradants is also unacceptable. Both ozone and oxygen cause degradation predominantly at the surface of the rubber, thus requiting diffusible antidegradants (ref. 9). Ozone is so reactive that ozone degradation is considered to be a surface phenomenon, while degradation due to oxygen occurs not only at the surface, but also deeper into the rubber. Thus, polymer bound antioxidants can provide a certain amount of protection, but must be used in conjunction with antiozonants which diffuse in rubber.

Quinonediimines are multifunctional rubber chemicals (ref. 10). Quinonediimines become polymer bound during mixing, processing and during vulcanization, thereby providing non-fugitive antioxidant characteristics. Raevsky et al. micro-milled polyisoprene with and without N,N'-diphenylquinonediimine, and measured relative oxygen induction times (ref. 11). The quinonediimine was found to inhibit oxidation of the rubber. After dissolving the milled rubber in benzene followed by precipitation in alcohol (repeated three times), a persistent ESR (electron spin resonance) signal of decreasing intensity and corresponding to the quinonediimine radical continued to be observed. The authors concluded that the material had chemically bonded to the polymer, forming relatively stable high molecular weight radicals by the reaction of the polymer radical with the quinonediimine.

Cain et al. studied black-filled, sulfenamide cured NR compounds containing various quinonediimines (ref. 12). Thin layer chromatography and UV spectroscopy of the azeotropic extracts of the NR compounds showed 60-75% of the original quinonediimine converted to the corresponding p-phenylene diamine (PPD). The remaining quinonediimine could not be detected. (In vulcanizates compounded with PPDs, 90-98% of the p-phenylene diamine was recovered.) As also found in Raevsky's work (ref. 11), the extracted (quinonediimine protected) vulcanizates showed two to three-fold increases in time to 1% oxygen uptake when compared to similarly extracted vulcanizates protected by the corresponding PPD. This additional evidence reinforced the claim that some of the quinonediimine had been bound to the rubber network and resulted in antioxidant activity.

In natural rubber compounds, 30-40% of added quinonediimine becomes polymer bound during vulcanization. This bound antidegradant provides persistent antioxidant characteristics. However, since it is bound to the polymer backbone, it may not contribute to the degradation of the polysulfidic crosslinks. The purpose of this study is to determine if a quinonediimine antidegradant provides better network stabilization than the corresponding para-phenylene diamine. The network stabilization will be assessed in terms of retention of the polysulfidic network and the retention of dynamic mechanical properties upon aging under aerobic conditions.

Experimental

All rubber chemicals, including N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine were used without further purification. Compound formulations are shown in table 1. Compound mixing was done in two stages. In the first stage, all ingredients except curatives were added in a laboratory internal mixer with a chamber volume of 1.0L. The dump temperature was 145-150 [degrees] C. The masterbatch was kept for 24 hours before finalizing in a second stage on a two-roll mill. Compounds were then characterized for processing and vulcanization properties in a Mooney viscometer and a moving die rheometer, respectively. The NR compounds were cured at 150 [degrees] C to a state equivalent to t90 times and samples were prepared from those vulcanizates for mechanical and dynamic mechanical properties. Stress-strain properties of the vulcanizates were measured using a universal testing machine in accordance with ISO 37. Other physical testing procedures used were as follows: Tear strength (Crescent 1 mm cut) - ISO 34:1994; abrasion to DIN - DIN 53516; fatigue to failure (Cam #14) - ASTM D4482-85

Table 1 - NR rubber formulations

Ingredients            1     2     3     4

NR SMR 10            100   100   100   100
Carbon black N-330    50    50    50    50
Zinc oxide             5     5     5     5
Stearic acid           2     2     2     2
Aromatic oil           3     3     3     3
6PPD                  --   2.0    --   1.0
QDI                   --    --   2.0   1.0
CBS                  0.6   0.6   0.6   0.6
Sulfur               2.3   2.3   2.3   2.3

Aging was done in accordance to ASTM D865-88. Azeotropic extractions were accomplished by placing the specimens in a soxhlet extraction and refluxing with an acetone-chloroform-methanol azeotrope for 16 hours (unless otherwise noted). The samples were then air dried for at least 24 hours before aging. Viscoelastic properties of the vulcanizates were determined using a dynamic analyzer. These tests were carded out at 15 Hz and -20 [degrees] C/+60 [degrees] C with a dynamic strain of 1%.

The structural elucidation of the vulcanizates was done by equilibrium swelling in toluene using the method reported by Ellis and Welding (ref. 13). The Vr value so obtained was converted to the Mooney-Rivlin elastic constant ([C.sub.1]) and finally to the concentration of chemical crosslinks by using equations described in the literature (ref. 14). The proportions of mono-, di- and polysulfidic crosslinks were determined using the method reported elsewhere (ref. 15).

Results

The QDI molecule is unique in that it has both antioxidant performance and scorch inhibition characteristics. The amine-type antidegradants normally reduce the scorch time, but QDI improves the scorch safety in NR formulations as shown by the data presented in table 2. The stress-strain properties of the vulcanizates (stocks 1-4, table 1) are shown in table 3. Extraction, followed by aging, serves to mimic service conditions of the rubber. It is evident from the data presented in table 3, that QDI provides an increase in modulus. This is not unexpected as it has been demonstrated that QDI does react with the polymer chains. With respect to the retention of tensile and elongation at break values, QDI provides much higher retention of the tensile and elongation at break after extraction when compared to the 6PPD control (table 3).

Table 2 - cure and processing data

Properties                          1      2      3      4

Cure data @ 150 [degrees] C
Delta S, Nm                      1.66   1.55   1.60   1.56
ML, Nm                           0.24   0.25   0.20   0.22
Ts2, min.                         4.3    4.4    5.0    4.6
T90, min.                        12.1   11.5   11.8   11.6
Cure rate, t90-ts2                7.7    7.1    6.8    7.0
Mooney scorch,                   29.8   28.0   32.1   32.7
  (t5 min.) at 121 [degrees] C
Table 3 - properties of the vulcanizates
(cure 150 [degrees] C/t90)

Properties                          1      2      3      4

Modulus, 100%, MPa
  Unaged                          3.4    3.3    3.7    3.5
  Aged (5 days/80 [degrees] C)    4.2    4.3    4.8    4.7
  Extracted/aged                  3.9    4.5    5.3    5.4

Modulus, 300%, MPa
  Unaged                         16.5   15.6   17.5   16.1
  Aged (5 days/80 [degrees] C)   17.9   18.4   20.1   19.9
  Extracted/aged                   --     --     --     --

Tensile, MPa
  Unaged                         26.6   28.4   28.6   28.4
  Aged (5 days/80 [degrees] C)   20.2   27.3   26.2   27.7
  Extracted/aged                  9.4   16.9   21.7   20.7

Elongation, %
  Unaged                          450    415    450    510
  Aged (5 days/80 [degrees] C)    330    450    390    420
  Extracted/aged                  200    260    300    300

Extraction: Azeotropic mixture -acetone/chloroform/methanol
(12 hr./50 [degrees] C)

Positive influences on the hysteresis properties can be expected if QDI improves the interaction with carbon black. The viscoelastic properties are shown in table 4. Stocks containing QDI show reduced energy losses (tangent delta) which are expected to positively influence the rolling resistance of pneumatic tires. This improvement in rolling resistance does not adversely affect the traction characteristics as implied in figure 1.

[GRAPH OMITTED]

Table 4 - vicoelastic properties at
15Hz/ 60 [degrees] C/1% strain

Properties                            1       2       3       4

Elastic modulus, E', MPa
  Unaged                            9.8     9.3     8.9     9.3
  Aged (3 days/100 [degrees] C)     9.9    11.0    11.2    10.8
  Extracted/aged                   10.0    11.2    10.3    10.5

Viscous modulus, E", Mpa
  Unaged                           1.10    1.07    0.85    1.07
  Aged (3 days/100 [degrees] C)    1.29    1.35    1.25    1.18
  Extracted/aged                   1.21    1.31    1.01    1.14

Tangent delta
  Unaged                          0.112   0.115   0.096   0.114
  Aged (3 days/100 [degrees] C)   0.130   0.123   0.111   0.110
  Extracted/aged                  0.121   0.117   0.097   0.109

Extraction: Water for 14 days/50 [degrees] C;
aging - hot air 3 days/100 [degrees] C

In order to correlate the better aging performance of the QDI vulcanizates, the network structure of the vulcanizates (before and after aging) was analyzed. Total crosslinks, poly-,di- and monosulfidic crosslinks for the NR formulations are shown in table 5.

Table 5 - crosslink density and distribution of
crosslink types (cure 150 [degrees]/t90)

XL-density(*)                Control   6PPD    QDI   1/1 phr
                               (1)      (2)    (3)     (4)

Total                         4.95     5.01   5.20    5.05
Poly-                         3.96     3.98   4.02    4.01
Di-                           0.84     0.85   0.88    0.85
Mono-                         0.15     0.18   0.30    0.19

Aged 5 days/80 [degrees] C

Total                         5.15     5.20   5.30    5.15
Poly-                         2.31     3.10   3.76    3.61
Di-                           0.64     0.70   0.77    0.72
Mono-                         2.20     1.40   0.77    0.72

(*) Crosslink density expressed in gm.mole/gm.
Rubber hydrocarbon x [10.sup.5].

Sidewall and tread formulations comprised of NR/BR blends were also studied in a similar fashion. The formulations for the tread and sidewall compounds are given in tables 6 and 8. The results of the network studies are given in tables 7 and 9. With the exception of the aged tread compound (table 7), all of the studies show that the total crosslink density of the network, both before and after aging, is slightly higher than that of the control compounds containing 6PPD. After aging, the most notable characteristics observed correlate to a reduction in the extent of measurable reversion by network analysis. A higher percentage of the polysulfidic network is retained and fewer monosulfide crosslinks are measured.

Table 6 - NR/BR tread formulation

Ingredients          1(a)   2(a)   3(b)

SMR CV                 60     60     60
BR1203                 40     40     40
Carbon black N-339     55     55     55
Sundex 8125             5      5      5
Zinc oxide              3      3      3
Stearic acid            2      2      2
6PPD                    2     --     --
QDI                    --      2      2
Wax (Sunolite 240)    1.5    1.5    1.5
TBBS                    1      1      1
Sulfur                  2      2      2

(a) Antidegradant added to the 1st stage mix.

(b) Antidegradant added to the 2nd stage mix.
Table 7 - NR/BR tread formulation

XL-density(*)                  6PPD in     QDI in     QDI in
                                first       first     second
                              stage mix   stage mix    stage
                                 (1)         (2)      mix (3)

Total                           5.25        5.40       5.38
Poly-                           3.37        3.40       3.42
Di-                             0.70        0.75       0.73
Mono-                           1.18        1.25       1.23

Aged 3 days/100 [degrees] C

Total                           6.12        6.02       6.00
Poly-                           1.90        2.22       2.22
Di-                             0.65        0.67       0.70
Mono-                           3.57        3.13       3.05

(*) Crosslink density expressed in gm.mole/gm.
Rubber hydrocarbon x [10.sup.5].
Table 8 - NR/BR sidewall formulation

Ingredients                1      2      3

SMR CV                    50     50     50
BR1203                    50     50     50
Peptizer (Renacit-11)   0.25     --     --
Q-Flex QDI                --   0.25     --
Carbon black N-330        20     20     20
Carbon black N-550        20     20     20
Stearic acid               1      1      1
Zinc oxide                 3      3      3
Sundex 8125                4      4      4
Resin (Picco 6100)         6      6      6
Wax (Sunolite 240)       2.5    2.5    2.5
6PPD                       3     --     --
QDI                       --   2.75   3.00
TBBS                     1.0    1.0    1.0
Insoluble sulfur        2.28   2.28   2.28
Table 9 - NR/BR sidewall formulation

XL-density(*)                 Control    QDI (2) as       QDI (3)
                                (1)     peptizer and       all in
                                        antidegradant   second stage
                                                            mix

Total                          4.65         4.78            4.72
Poly-                          2.91         2.97            2.86
Di-                            0.66         0.65            0.63
Mono-                          1.08         1.16            1.23

Aged 3 days/100 [degrees] C

Total                          5.43         5.51            5.60
Poly-                          1.11         1.54            1.52
Di-                            0.60         0.62            0.58
Mono-                          3.72         3.35            3.50

(*) Crosslink density expressed in gm.mole/gm.
Rubber hydrocarbon x [10.sup.5].

Discussion

Networks experiencing reversion exhibit a reduction in polysulfidic crosslink content and an increase in monosulfidic crosslink content. In addition, zinc sulfide, cyclic sulfides,and conjugated dienes and trienes form when reversion occurs. All of the features of reversion tend to degrade the physical properties of the compound.

The most notable feature from this study is evidence that upon aerobic aging, the extent of reversion is reduced when quinonediimine antidegradants are used. When compared to rubber containing 6PPD, in each case studied, a higher percentage of polysulfides remained in the QDI protected compounds after aging. In addition, fewer monosulfidic crosslinks are formed upon aging rubber containing QDI as an additive. These data suggest that compounds containing QDI have less free amine to attack the polysulfidic crosslinks, thereby leaving more crosslinks of higher sulfur rank intact. Furthermore, model compound studies show that, when QDI reacts with carbon free radicals, attachment can occur on the alkyl nitrogen atom and on the quinoidal ring as well. Once QDI becomes attached to the polymer backbone, its mobility is limited and its chemistry may well change to that of a PPD containing a tert.-amine group. These tertiary amine sites could effectively complex zinc ions and zinc-accelerator fragments, thereby restricting their mobility in the vulcanizate. These proposed polymer bound PPD-zinc complexes may account for less reversion, by preserving polysulfidic crosslinks, and provide persistent antioxidant activity. In any event, a change in the mechanism of conventional protection of a vulcanizate composition appears to occur when QDI reacts with the polymer backbone.

It is interesting to note that while reversion is considered to be significant only in NR compounds, evidence presented here suggests that reversion is measurable even in NR/BR blends which may not appear to revert significantly in rheometer experiments.

The quinonediimine antidegradant converts primarily to a PPD or (as evidenced by previous studies) becomes bound upon vulcanization. Protection by a quinonediimine anti-degradant should mimic that of a PPD, except that the bound portion offers persistent antidegradant properties when the rubber is exposed to aggressive environments (i.e., solvents, acid rain, aqueous detergents, etc.). Aging behavior in this study demonstrates that oxidative aging performance of QDI protected vulcanizates is similar to PPD protected compounds, but reversion under aerobic conditions is reduced.

Conclusions

Protecting vulcanizates, based on NR or NR blends, with quinonediimine antidegradants provides protection similar to that achieved with conventional PPD antidegradants. In an oxidative environment, reversion is reduced when compounds are protected with QDI antidegradants. Dynamic mechanical properties are maintained better with quinonediimine antidegradants compared to PPDs due to a reduction in reversion chemistry upon aging.

References

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