Multifunctional acrylates as anti-reversion agents in sulfur cured systems.

Physical properties of vulcanized rubber systems are influenced by the curing process and chemical nature of the formed crosslinks. The bond strength of linkages can help predict the performance of a given vulcanizate in specific applications. Peroxide cured systems are often employed in static or compressive conditions where low set and heat resistance are required. A network derived from polysulfidic linkages is preferred for components subject to dynamic strain where improved flexural fatigue and tear properties are desired. Bond dissociation energies are often used to quantify the strength of the crosslinks and differentiate the systems (refs. 1 and 2). Carbon-carbon linkages possess high dissociation energies and resist failure to a limiting point, then fail catastrophically. Having lower dissociation energies, polysulfidic bonds break more readily under strain, but due to their chemical nature also possess the ability to reform and alleviate stresses (refs. 3 and 4).

The dynamic nature of polysulfidic bonds characterizes the utility of the system, but also facilitates reversion. Reversion is defmed by a loss in physical properties associated with degradation of network integrity. The reversion process is thermally initiated and primarily associated with overcure or high temperature service conditions. Reversion involves reactions that lead to the desulfuration of polysulfidic linkages and mainchain modification (cis-to-trans isomerization) which results in weaker network structures (refs. 5 and 6). As desulfuration progresses, the distribution shifts from polysulfidic to monoand di-sulfidic crosslinks, and eventually crosslink density is also lost. The result is a degradation of physical properties and a decrease in the performance of the rubber article (ref. 7).

Many strategies have been developed to prevent the reversion process. Reversion can be minimized in cure systems which promote mono- and disulfidic crosslinks (efficient vulcanization). Sulfur-donor systems or polysulfide polymers can be utilized (ref. 8). It has recently been proposed that certain zinc methacrylate salts, when used as activators in accelerated sulfur vulcanization, promote crosslinks of lower sulfur rank and minimize reversion (ref. 9). These attempts to mediate reversion also alter the original structure of the network itself, typically decreasing dynamic and tear properties that are the direct benefits of polysulfidic linkages.

Alternately, the inclusion of more stable crosslinks in addition to the sulfur network has been proposed (ref. 10). Similarly, the mixing of peroxide and conventional accelerated sulfur cure systems has been attempted (ref. 11). A disadvantage of this compounding method is the need to reformulate in order to compensate for the new, structurally different, crosslinks in order to maintain the desired cured properties.

A more popular approach is to compensate for the destruction of crosslinks by adding chemical additives that react when reversion processes are initiated. In such systems, the desulfuration mechanism is offset by the formation of new, stable crosslinks. Ideally, the processes have similar rates such that crosslink density is maintained. It has been shown that acrylate esters are effective anti-reversion agents that display this behavior (ref. 12). Earlier work has also demonstrated the utility of acrylates to improve the heat-aged properties of rubber compounds, suggesting a similar mechanism (ref. 13).

The present study evaluates the performance of a multifunctional acrylate ester product (SR534) specifically optimized to prevent reversion in rubber compounds. SR534 is compared to the imide-based anti-reversion agents 1,3-bis(citraconimidomethyl)benzene (CIMB) and N,N'-m-phenylenedimaleimide (PDM). CIMB has been promoted as capable of crosslink compensation reactions (ref. 14), while PDM is commonly used as a secondary accelerator. Cure characteristics and anti-reversion properties will be compared. Tensile, viscoelastic and flexural fatigue properties are evaluated. Special attention is placed on comparing the efficiency with which the various additives maintain physical properties as a function of reversion.

Experimental

Chemicals

The chemical additives evaluated in this study are given in table 1. The commercially available products were used without further treatment or purification.

Rubber compounding

Using an internal mixer, a non-productive masterbatch was prepared containing all ingredients except anti-reversion agents and curatives. Table 2 outlines the formulation used for the masterbatch and the specific cure packages. Conventional (CV), semi-efficient (SE-V) and efficient (EV) vulcanization systems were evaluated. These are model formulations that have not been optimized for a specific application. The productive stage was mixed using a two-roll mill. Milling times were approximately five minutes. Productive stock was aged overnight prior to testing.

Physical testing

The determination of vulcanization behavior was performed on a Tech Pro MDPT moving die rheometer (MDR) according to ASTM D 5289. Cure temperature for sample preparation is 160[degrees]C unless otherwise noted. Reversion was calculated as the difference in torque between the highest initial recorded value ([M.sub.H]) and that at 60 minutes ([M.sub.60]). The individual calculated [t.sub.90] times (optimal cure) were used for subsequent test sample preparation. In addition, reverted samples were prepared by curing to 60 minutes. Stress-strain and tear data were acquired on a Thwing-Albert materials tester following ASTM D 412 and D 624 (Die C). The MDR was also employed to evaluate both unvulcanized and cured dynamic properties under sinusoidal shear conditions at frequencies and amplitudes defined later in the text. DeMattia flex fatigue was determined according to ASTM D 813 under adiabatic conditions starting at room temperature. Complete testing results are provided in appendices 1 and 2.

Swelling experiment

Samples were mixed in a prep mixer using the same formulation in table 2 with the carbon black omitted from the masterbatch. The unfilled compounds were mixed for four minutes at 60 rpm and 60[degrees]C. The MDR was used to determine optimal cure times, and vulcanizates were prepared. Swelling experiments were conducted on cylindrical samples of known weight and dimensions. Crossrink density was determined using the experimental methodology outlined by Flory and Rhener (ref. 15) and modified with equivalent terms derived from theories of rubber elasticity. The Z parameter used for the toluene/cis-poly(isoprene) system was 0.37.

Results and discussion

Model formulation development

In order to compare the given anti-reversion agents, it is best to understand what formulation characteristics contribute to the reversion process. Elastomers with low degrees of unsaturation provide the least amount of reversion, while reversion is promoted in unsaturated rubber containing high allylic hydrogen content. Figure 1 correlates the extent of reversion as a function of allyric hydrogen for a series of elastomers. The formulation used was identical to table 2, substituting various polymers (ethylene-propylene-diene monomer, EPDM; emulsion styrene-butadiene rubber, SBR; cis-butadiene rubber, BR; cis-isoprene rubber, IR; and natural rubber, NR).

[FIGURE 1 OMITTED]

It has been suggested that, while the amount of sulfur controls the overall reaction kinetics, the amount and type of accelerator determines the overall length of the sulfidic linkages (refs. 16 and 17). The sulfur-to-accelerator ratio is used as a guideline for determining the type of cure package in use and the distribution of sulfur crosslink rank (ref. 18). Table 3 provides an outline of cure types, sulfur-to-accelerator ratios and typical crosslink distributions.

The rheometer cure profiles for the listed cure packages are provided in figure 2. It is clear that as the sulfur-to-accelerator ratio decreases (CV to S-EV), reversion was reduced. In an efficient vulcanization (EV), sulfur donors are utilized and no reversion was evident. Kinetic and cure state data from these vulcanizations are provided in table 4.

[FIGURE 2 OMITTED]

Having the highest allylic hydrogen concentration, cis-1,4 poly(isoprene) (synthetic and natural rubber) disolaved the

greatest loss in crosslink density. In addition, the data indicate that reversion was most pronounced using a cure package which promotes polysulfidic crosslinks. Therefore, model formulations based on natural rubber using both CV and S-EV cure systems have been selected to best evaluate the behavior and efficiency of anti-reversion agents.

Reversion resistance

SR534 is a multifunctional acrylate ester compound that has been optimized for use in rubber compounding. In a more traditional role as coagents for peroxide cure systems, acrylate and methacrylate esters are available in a wide range of structural variation. SR534 was developed specifically to address reversion resistance in sulfur vulcanized, unsaturated rubber compounds. Figure 3 compares the reversion resistance properties of SR534 to a diacrylate (SR213, 1,4-butanediol diacrylate) and trimethacrylate (SR350, trimethylolpropane trimethacrylate) at 5 phr in the model formulation. SR534 is a blend of selected multifunctional acrylates and outperforms the diacrylate. The data demonstrate that the methacrylate was also not effective, a finding reported in earlier work (ref. 12). SR213 and SR350 appeared to limit cure ([M.sub.H]), while SR534 matched the crosslinking efficiency of the control. Reversion data are outlined in table 5. A higher torque value at [M.sub.60] compared to [M.sub.H] indicates marching modulus and, therefore, no reversion.

[FIGURE 3 OMITTED]

The imide-based commercially available anti-reversion agents PDM and CIMB were also evaluated in the same formulation. The reported mode of activity for CIMB is to compensate for the loss of crosslink density as a function of reversion by generating additional, non-sulfur, crosslinks that are thermally stable (refs. 10 and 14). A Diels-Alder reaction has been proposed. PDM is a secondary accelerator also utilized for reversion resistance, but is more active and leads to the addition of non-sulfur crosslinks during the vulcanization step through a similar reaction scheme.

The use of acrylate ester products as coagents for peroxide-cured systems is a major application, and also demonstrates their ability to participate in free-radical initiated addition and grafting reactions (ref. 19). Given the chemical environment of the model sulfur-cured formulation, a crosslinking mechanism relying on radical intermediates may be unlikely. Containing acrylate ester functionality, the structure of SR534 is fundamentally different than the imide-based products. However, the ability to participate in similar crosslinking reactions has also been proposed (refs. 20-22). Figures 4 and 5 provide a comparison of these agents in both the CV and SE-V formulations, at 5 phr of additive and a cure temperature of 160[degrees]C. Table 6 outlines the reversion data in these formulations at both 2 and 5 phr of additive. It is shown that both SR534 and CIMB provide similar rheological profiles that are distinguished by a lack of reversion. Both additives not only matched the [M.sub.H] of the control, but maintained torque with time. PDM increased [M.sub.H] and lost considerable torque during overcure.

[FIGURES 4-5 OMITTED]

As the most desirable aspects of sulfur-cured systems are attributed to the polysulfidic nature of the crosslinks, the addition of non-sulfur crosslinks during the initial vulcanization step may be undesirable. Therefore, SR534 was developed considering that an ideal anti-reversion agent would provide minimal impact to optimally cured ([t.sub.90]) properties, and primarily react to maintain crosslink density during conditions that initiate reversion. CIMB has also been promoted to exhibit this type of behavior. Its form differs from PDM by altering the attachment and sterics of the imide structure, favorably affecting the activity of the compound. Similarly, SR534 was developed by designing a structure that exhibits activity closely matching CIMB, without incorporating amine structures.

The same compounds were evaluated at elevated cure temperature as well. While extending cure time at 160[degrees]C provides a comparison of additive performance under overcure conditions, curing at 180[degrees]C can simulate reversion under high temperature service conditions. Table 7 outlines the reversion resistance of the additives when cured to 60 minutes at 180[degrees]C. Again, both SR534 and CIMB virtually eliminated reversion under these more demanding conditions, while PDM allowed significant reductions in measured torque over time. The cure behavior of the CIMB and SR534 at elevated temperatures and 5 phr loading was characterized by a marching modulus (figure 6). Such behavior demonstrates the ability to provide adequate reversion protection under high temperature conditions.

[FIGURE 6 OMITTED]

Both SR534 and CIMB were included in an expanded study to determine the optimal loading of anti-reversion agent in the model CV formulation at 160[degrees]C. Reversion was determined for loadings between 0.5 and 5.0 phr of SR534 or CIMB. The data are presented in figure 7. The reduction of reversion as a function of loading was similar for each compound. In the given formulation, reversion is effectively eliminated at three phr. The levels of additive required in actual formulations may be significantly less, given the model formulation was developed to display a maximum amount of reversion.

[FIGURE 7 OMITTED]

Tensile properties

The physical properties of rubber compounds employing SR534, CIMB and PDM were compared with each sample cured to the individually calculated [t.sub.90] cure times. Cure temperature was 160[degrees]C. Data were normalized to the control (no anti-reversion agent) and are provided in the CV formulation at both 2 and 5 phr (figures 8 and 9, respectively). The data highlight the departure from the physical properties of the control formulation. It is clear that PDM reacts preferentially during the initial vulcanization step, as both 100% and 300% modulus were significantly higher than the control. The modulus values of SR534 and CIMB were also elevated in this model formulation. However, both SR534 and CIMB displayed a significant improvement over PDM in terms of modulus contribution during vulcanization. The loadings of additive used in this study were selected to provide data points both above and below that which eliminated reversion (three phr, figure 7). It is suggested that an optimized loading would typically be chosen based on a balance between reversion resistance and modulus increase. The optimal loading may be different for each additive studied.

[FIGURES 8-9 OMITTED]

Test samples of the, same compounds were intentionally subjected to conditions that would lead to reversion. Overcure conditions were simulated by curing past [t.sub.90] times to 60 minutes. Following the methodology outlined above, the data collected in the reverted state were also normalized to the original control values (no additive, cured to [t.sub.90]). In order to provide perspective on how physical properties change as a function of induced reversion, the deviation in normalized properties is presented. Deviation is calculated according to this equation:

Deviation = [property @ 60 minutes cure/control property @ [t.sub.90] cure] [property @ [t.sub.90] cure/control property @ [t.sub.90] cure]

By using this equation, the efficiency with which the anti-reversion compounds maintain properties as a function of heat history can be compared. The control itself was also subjected to the same treatment, offering a baseline for the relative amount of loss in properties. Figures 10 and 11 present the results in the CV and S-EV formulations at 5 phr additive. A negative deviation represents a loss in properties with cure time.

[FIGURES 10-11 OMITTED]

The data show that compounds containing PDM lost the most amount of modulus as a function of exposure to reversion conditions, followed by the control. SR534 and CIMB increased modulus. In both formulations, SR534 had the least deviation in modulus, while CIMB gained considerably. Perhaps most importantly, the deviation in 300% modulus was the least for SR534. Following the relative magnitude of deviation in modulus is the loss of elongation and tear properties.

By considering the overall deviation, the efficiency with which each additive maintains physical properties can be compared. Taking the sum of the deviations for each compound (absolute values are used, as any departure from optimally cured properties would be undesirable), the relative performance can be compared. The data are presented in this manner for the CV and SE-V formulations in figures 12 and 13, respectively. SR534 and CIMB displayed similar trends in their behavior as a function of reversion. However, the net deviation from the optimally cured properties is lowest for the multi functional acrylate ester product. SR534 was superior to CIMB in maintaining physical properties when subjected to overcure conditions.

[FIGURES 12-13 OMITTED]

Interestingly, net deviations increase at higher additive loading (2-5 phr). These results are primarily driven by the tendency of these compounds to increase modulus when overcured. Again, optimization of additive level balancing both modulus rise, and reversion resistance was not considered in this fundamental study and is best addressed in a specific formulation.

Viscoelastic properties

The viscoelastic properties of the CV compounds were also compared. After curing to [t.sub.90] in the MDR, the samples remained in the test cavity and were subjected to various strain amplitudes at a frequency of 1 Hz and a temperature of 100[degrees]C. Actual values are provided in appendix 2. The normalized results are given in figure 14 for compounds containing 5 phr additives. The addition of anti-reversion agents resulted in decreased tangent delta (1 Hz, 10% amplitude) versus the control. A lower tangent delta value can be correlated to improved compound hysteresis. High strain shear modulus (G', 1 Hz, 50% amplitude) data are also provided. PDM increases G' considerably compared to the control, while SR534 and CIMB produce values slightly below that of the control. Overall, higher loadings of anti-reversion agent produced increased G' values. The addition of SR534 produces the lowest shear modulus. SR534 is the only liquid product and may contribute to lower compound shear viscosity (cured).

[FIGURE 14 OMITTED]

Viscoelastic properties were subsequently evaluated as a function of reversion. The samples were cured for 60 minutes in the MDR and subjected to the strain sweep described previously. These results are provided in figure 15 as a deviation from optimal properties (normalized) according to the methodology outlined previously. SR534 maintains the lowest tangent delta values in the reverted state. The data suggest that not only does SR534 maintain network integrity when subjected to overcure conditions, but may also reduce the heat build-up associated with applications experiencing severe dynamic flex, reducing the auto-acceleration quality of reversion associated with these service conditions. Both the control and PDM compounds lost considerable shear modulus upon reversion (typically >40%), while SR534 and CIMB limited the reduction in G'. The loss in viscoelastic properties was generally reduced at higher additive loading.

[FIGURE 15 OMITTED]

Flex fatigue

SR534, CIMB and PDM compounds were also evaluated for flex fatigue resistance. The addition of non-sulfur crosslinks or crosslinks of lower sulfur rank will produce negative effects in dynamic cut growth testing. To illustrate the trend, figure 16 presents the results using the control compounds. The control series correlates cut growth with sulfur rank distribution. For this series, 100% modulus values were similar. The flex fatigue performance became progressively worse as the concentration of mono- and disulfidic crosslinks increased at the expense of polysulfidic. Crack growth rate increases EV > S-EV > CV.

[FIGURE 16 OMITTED]

Figure 17 outlines the results of fatigue testing when incorporating the anti-reversion agents in the CV compound (5 phr) and curing to [t.sub.90]. SR534 performed equivalent to the CV control formulation, while CIMB had a slightly elevated growth rate. The compound using PDM showed the highest cut growth. Interestingly, the performance of SR534 was superior despite having an initial modulus higher than the control, establishing evidence that the inclusion of an anti-reversion agent can provide benefits in dynamic flex fatigue resistance.

[FIGURE 17 OMITTED]

Test specimens of the same compounds were cured for 60 minutes and tested. The results are provided in figure 18. In the reverted state, flex fatigue performance now correlates with modulus. SR534 outperformed CIMB, and these compounds increased modulus compared to the results in figure 17. The modulus of the control and PDM was lower, the outcome of reversion. Anti-reversion agents SR534 and CIMB are not fully reacted when the compounds are optimally cured, and may help de-couple the dependence of modulus on flex fatigue performance by improving the cured network during testing. However, the vulcanizates that were intentionally overcured prior to flexing would not have this mechanism available. Therefore, modulus would again strongly influence the results. Flex testing from optimally cured samples may most accurately reflect actual conditions. By interpreting the results displayed in figures 17 and 18, the benefits of including anti-reversion agents can be realized.

[FIGURE 18 OMITTED]

Swelling experiments

Equilibrium swelling experiments are utilized to determine the crosslink density ([V.sub.x]) of vulcanizates. An unfilled (no carbon black) version of the CV model formulation was used to create compounds using both SR534 and CIMB additives (5 phr). A control compound was also included. Optimal cure times were calculated using a rheometer, and samples cured to both [t.sub.90] and 60 minutes were prepared. Crosslink densities were calculated for each vulcanizate. The results are provided in figure 19. The control loses considerable network integrity with reversion. SR534 provides crosslink density values most similar to the control at optimal cure, and maintains crosslink density when subjected to overcure conditions. CIMB appeared to partially react during vulcanization and increased crosslink density prematurely. CIMB also lost density as a function of cure residence time. Swelling experiments may best characterize the advantages possible when incorporating SR534 into a reversion-prone formulation.

[FIGURE 19 OMITTED]

Summary and conclusions

The ideal anti-reversion agent would provide reversion resistance in a manner that does not significantly alter the physical properties of the compound. Of the several strategies that result in a network capable of maintaining crosslink density under conditions that would normally lead to reversion, only those additives which compensate for the loss of network integrity at a similar rate through the addition of new, heat-stable linkages meet the above criteria. Alternately, products which establish non-polysulfidic crosslinks during vulcanization negatively affect desirable properties such as dynamic flex fatigue and tear, and lead to higher cured modulus. The activity of PDM characterizes the behavior of this type of anti-reversion additive. Compensation for the loss in performance necessitates reformulation.

SR534 is a multifunctional acrylate ester product that contains no amine groups and has been developed to minimize the effects on optimally cured properties and provide maximum protection under conditions which induce reversion. In the model formulation, SR534 demonstrated reversion resistance performance similar to CIMB in overcure and high temperature conditions. Both products reduced the impact on physical properties and flex fatigue compared to PDM when cured to [t.sub.90]. However, SR534 performed best for maintaining physical properties when subjected to overcure conditions. Dynamic properties were also improved using SR534. Swelling experiments showed that crosslink density was maintained at elevated temperatures using SR534, while both the control and CIMB lost network integrity.

The data provided suggest that SR534 is capable of forming additional, heat-stable crosslinks under conditions that would normally result in reversion. By eliminating reversion and best maintaining desirable physical properties, SR534 is an effective amine-free alternative to available anti-reversion products.

Appendix 1--physical property data

                                           CV formulation

Control   Cure time, minutes                 4.51    60.00
          Tensile strength, MPa             21.08    14.26
          Elongation, %                       580      515
          100% modulus, MPa                  1.62     1.24
          300% modulus, MPa                  8.16     6.82
          Tear strength (Die C),
            kN/m                            84.00    33.25
          Tan delta (1 Hz, 10%)             0.160    0.199
          G' (1 Hz, 50%)                   865.17   397.50
          phr                          2        2        5       5
SR534     Cure time, minutes        7.14    60.00     7.00   60.00
          Tensile strength, MPa    21.50    18.26    22.36   17.05
          Elongation, %              515      465      505     375
          100% modulus, MPa         2.31     2.51     2.69    3.34
          300% modulus, MPa        10.30    10.30    11.47   12.78
          Tear strength (Die C),
            kN/m                   75.24    33.25   72.564   38.50
CIMB      Cure time, minutes        6.07    60.00     6.50   60.00
          Tensile strength, MPa    24.39    21.46    24.84   20.74
          Elongation, %              635      460      645     430
          100% modulus, MPa         2.07     3.07     2.31    3.45
          300% modulus, MPa         8.89    12.61     8.96   13.44
          Tear strength (Die C),
            kN/m                   85.75    38.50    92.75   49.88
PDM       Cure time, minutes        6.11    60.00     6.35   60.00
          Tensile strength, MPa    23.12    17.29    22.87   19.19
          Elongation, %              490      490      450     435
          100% modulus, MPa         2.89     2.10     3.82    3.24
          300% modulus, MPa        12.33     8.89    14.47   12.30
          Tear strength (Die C),
            kN/m                   64.75    48.13    49.00   41.13

                                          SE-V formulation

Control   Cure time, minutes                   5.33   60.00
          Tensile strength, MPa               21.46   15.67
          Elongation, %                         600     590
          100% modulus, MPa                    1.52    1.52
          300% modulus, MPa                    8.03    6.06
          Tear strength (Die C),
            kN/m                             101.50   30.63
          Tan delta (1 Hz, 10%)
          G' (1 Hz, 50%)
          phr                            2        2       5        5
SR534     Cure time, minutes          5.45    60.00    6.01    60.00
          Tensile strength, MPa      25.42    23.08   25.56    22.05
          Elongation, %                545      535     580      470
          100% modulus, MPa           2.51     2.45    2.34     2.82
          300% modulus, MPa          12.06    11.09   10.96    12.23
          Tear strength (Die C),
            kN/m                   100.625   41.125   92.75   42.875
CIMB      Cure time, minutes          5.57    60.00    5.86    60.00
          Tensile strength, MPa      24.39    23.80   24.94    21.98
          Elongation, %                630      480     640      435
          100% modulus, MPa           2.03     2.93    2.24     3.45
          300% modulus, MPa           9.44    13.06    9.37    14.02
          Tear strength (Die C),
            kN/m                    118.13    49.88   97.13    44.63
PDM       Cure time, minutes          6.26    60.00    7.15    60.00
          Tensile strength, MPa      25.05    22.01   25.05    21.36
          Elongation, %                480      500     435      450
          100% modulus, MPa           3.45     2.51    4.62     3.41
          300% modulus, MPa          14.71    11.44   17.16    13.26
          Tear strength (Die C),
            kN/m                     70.88    42.88   63.00    46.38

Appendix 2--viscoelastic property data

                                              CV formulation

Control   Cure time, minutes             4.51               60.00
          Tan delta (1 Hz, 10%)         0.160               0.199
          G' (1 Hz, 50%)               865.17              397.50
          phr                           2        2          5        5
SR534     Cure time, minutes         7.14    60.00       7.00    60.00
          Tan delta (1 Hz, 10%)     0.093    0.181      0.096    0.152
          G' (1 Hz, 50%)           789.74   546.60     820.91   694.76
CIMB      Cure time, minutes         6.07    60.00       6.50    60.00
          Tan delta (1 Hz, 10%)     0.098    0.190      0.102    0.165
          G' (1 Hz, 50%)           833.06   749.75     850.11   806.36
PDM       Cure time, minutes         6.11    60.00       6.35    60.00
          Tan delta (1 Hz, 10%)     0.091    0.193      0.096    0.176
          G' (1 Hz, 50%)           958.04   513.47   1,204.48   733.84

References

(1.) L.H. Palys and P.A. Callais, Rubber World 229 (3), 35 (December 2003).

(2.) P.R. Dluzneski, Rubber Chem. Technol. 74, 451 (2001).

(3.) W. Cooper, J. Polym. Sci. 28, 195 (1958).

(4.) L. Bateman, ed., "The chemistry and physics of rubber-like substances," MacLaren, London, 1963.

(5.) M. Mori, Rubber Chem. Technol. 76, 1,259 (2003).

(6.) C.H. Chen, J.L. Koenig, J.R. Shelton and E.A Collins, Rubber Chem. Technol. 54, 734 (1981).

(7.) U. Shankar, Rubber Chem. Technol. 25, 241 (1952).

(8.) J. Hahn, P. Palloch, N. Thelen and H.J. Weidenhaupt, Rubber Chem. Technol. 74, 28 (2001).

(9.) S. Henning, Paper #10, Spring Technical Meeting, Rubber Division, ACS, San Antonio, TX, May 16-18, 2005.

(10.) T. Kleiner, J. Neilsen, H.J. Weidenhaupt, W. Jeske and H. Buding, Rubber & Plastics News, January 14, 2002, p. 14.

(11.) J.B. Class, U.S. #6,245,861 (2001).

(12.) E.J. Blok, M.L. Kralevich and J.E. Varner, Rubber Chem. Technol. 73, 114 (2000).

(13.) Y. Hirata, et al. JP #63-150339 (1988).

(14.) A.H.M. Schotman, P.J.C. van Haeren, A.J.M. Weber, F.G.H van Wijk, J.W. Hofstraat, A.G. Talma, A. Steenbergen and R.N. Datta, Rubber Chem. Technol. 69, 727 (1996).

(15.) P.J. Flory and J.J. Rhener, J. Chem. Phys. 11, 521 (1943).

(16.) M.R. Krejsa and J.L. Koenig, Rubber Chem. Technol. 66, 376 (1993).

(17.) A.Y. Coran in "Science and technology of rubber," F.R. Eirich, ed, Academic Press, New York, 1978.

(18.) C.H. Chen, E.A. Collins, J.R. Shelton, and J.L. Koenig, Rubber Chem. Technol. 55, 1221 (1982).

(19.) S. Henning, and R. Costin, Paper E, Spring Technical Meeting, Rubber Division, ACS, San Antonio, TX,, May 16-18, 2005.

(20.) C.J. Albisett, N. G. Fisher, M.J. Hogse, and R.M. Joyce, J. Am. Chem. Soc. 78, 2637 (1956).

(21.) W.K. Johnson, J. Org. Chem. 24, 864 (1959).

(22.) T. Unukai and T. Kojima, J. Ong. Chem. 31, 1121 (1966).

Table 1--anti-reversion agents

Chemical additive                       Code

Multifunctional acrylate ester         SR534
1,3-bis(citraconimidomethly)benzene     CIMB
N,N'-m-phenylenedimaleimide              PDM

Table 2--formulations

              Ingredient, phr                CV       S-EV         EV

Non-          Natural rubber (CV-60)      100.0      100.0      100.0
productive    Carbon black (N 330)         50.0       50.0       50.0
              Zinc oxide                    5.0        5.0        5.0
              Stearic acid                  1.5        1.5        1.5
Productive    Antioxidant (a)               1.0        1.0        1.0
              Anti-reversion agent      0, 2.0,    0, 2.0,    0, 2.0,
                                            5.0        5.0        5.0
              Sulfur                        2.5        1.2        0.2
              TBBS (b)                      0.6        1.6        2.0
              TMTD (c)                                            2.0

(a)- 2,2,4trimethyl-1,2-hydroquinoline

(b)- N-t-butylbenzothiazole-2-sutfenamide

(c)- Tetramethylthiuram disulfide

Table 3--cure type and bond
distributions

                                  CV       S-EV          EV

Sulfur-to-accelerator ratio    1-1.7    1.7-0.4    0.4-0.08
Polysulfidic %                 60-80      30-50        5-25
Mono-, disulfidic %            20-40      50-70       75-95

Table 4--vulcanization data by cure type

                           CV     S-EV       EV

1/2 (minutes)            1.42     3.22     1.83
[t.syb.90] (minutes)     4.51     5.33     4.81
Delta torque (dNm)      29.60    28.35    26.46
Reversion (dNm)         14.85     8.41        0

Table 5--reversion data for various
additives, 160[degrees]C

          Phr   [M.sub.H] (dNm)   [M.sub.60] (dNm)   Reversion (dNm)

Control    --             31.05              16.20             14.85
SR534       5             30.60              33.25              0
SR213       5             24.54              22.30              2.24
SR350       5             25.29              16.60              8.69

Table 6--reversion data for various
additives, 160[degrees]C
CV formulation, 160[degrees]C

                  [M.sub.H]    [M.sub.60]    Reversion
           Phr        (dNm)         (dNm)        (dNm)

Control     --        31.05         16.20        14.85
SR534        2        27.57         23.70         3.87
CIMB         2        29.60         26.20         3.40
PDM          2        36.03         24.40        11.63

S-EV formulation, 160[degrees]C

Control     --        29.61         21.20         8.41
SR534        2        30.04         26.10         3.94
CIMB         2        29.44         29.00         0.44
PDM          2        40.87         31.70         9.17

                  [M.sub.H]    [M.sub.60]    Reversion
           Phr        (dNm)         (dNm)        (dNm)

Control
SR534        5        30.60         33.25         0
CIMB         5        32.99         32.89         0.10
PDM          5        48.58         36.80        11.78

S-EV formulation, 160[degrees]C

Control
SR534        5        29.74         30.00         0
CIMB         5        28.54         30.68         0
PDM          5        51.96         42.40         9.56

Table 7--reversion data for various additives, 180[degrees]C

CV formulation, 180[degrees]C

                  Reversion           Reversion
           Phr        (dNm)    Phr        (dNm)

Control     --        14.30
SR534        2         2.33      5         0
CIMB         2         0         5         0
PDM          2         9.79      5         8.61

S-EV formulation, 180[degrees]C

                  Reversion           Reversion
           Phr        (dNm)    Phr        (dNm)

Control     --        12.24
SR534        2         2.89      5         0
CIMB         2         0         5         0
PDM          2         9.71      5         7.60