Thiourea accelerators in the vulcanization of butyl elastomers.

Butyl rubber, produced via the cationic polymerization of isobutylene and isoprene, is a random copolymer that displays excellent oxidation stability and barrier properties. Reactivity ratios of isobutylene and isoprene are equivalent. Therefore, when compounded, a homogeneous vulcanizate is obtained that has good fundamental mechanical properties and has low permeability to air. Of the many parameters influencing final butyl compound properties, the vulcanization or cure system is of considerable importance. There are five main factors that influence the current development of butyl and halobutyl rubber cure system technology (refs. 1 and 2):

* Elimination of accelerators that may generate nitrosamines;

* accelerators for improved scorch resistance and improved ease of compound processing during final product assembly, e.g., a tire;

* vulcanization systems suitable for high temperature curing, with associated benefits in productivity;

* improved reversion resistance; and

* improvement in compound attributes such as mechanical or dynamic properties.

As end product cure temperature increases, greater reversion resistance is required in addition to maintaining fundamental mechanical properties. This infers no marching modulus and maintenance of a stable crosslink network. Cure systems with primary accelerators such as mercaptobenzothiazole disulfide (MBTS) or sulfenamides such as tert-Butyl-2-benzothiazole sulfenamide (TBBS) have been used to address such requirements. The fundamental structures of accelerators used in the vulcanization of rubber fall into one of five groups, including: the thiazole, thiocarbamyl, alkoxythio carbonyl, dialkylthio phosphoryl or diamino-2,4,6-triazinyl (figure 1) (refs. 2-5). Depending on the cure system, thiurams, dithiocarbamates and xanthates may show poor reversion resistance.

[FIGURE 1 OMITTED]

Butyl rubber is used in products such as tire innertubes, curing bladders, engine mounts, sheeting and pharmaceuticals where resistance to oxidation, reversion and heat stability are important performance criteria. Therefore, compound cure systems must be stable and not readily susceptible to reversion. It has been reported that thioureas, on their own, do not vulcanize elastomers. However, in combination with TMTD and MBTS, a more rapid vulcanization is possible. In polychloroprene compounds, thiourea based efficient vulcanization systems (EV) have been reported to demonstrate little reversion resistance when compared to sulfur-sulfenamide based vulcanization systems. This work wishes to explore the effect thiourea accelerators have on basic butyl rubber vulcanizate properties.

Experimental methods

Table 1 contains a butyl rubber model formula in which five thiourea accelerators were screened. The accelerators studied were dimethylthiourea (DMTU), diethylthiourea (DETU), dibutylthiourea (DBTU), diphenylthiourea (DPTU) and diorthotolylthiourea (DOTTU). The structure of the accelerators can be found in figure 2.

[FIGURE 2 OMITTED]

Vulcanization kinetics were measured using a MDR rheometer as described in ASTM D5289, and cure rates were measured at 140[degrees]C to 180[degrees]C with 0.5[degrees] arc. Reported data include cure state (delta torque Mh-M1 or [DELTA]T), induction time (time to a 10% torque rise, [t.sub.10]), [t.sub.90] or time to 90% of maximum torque, and cure rate at 50% of cure ([t.sub.50]).

A cure rate index (C.R.I.) was calculated from equation 1 below. This calculation readily permits a relative ranking of compound cure rates.

(1) C.R.I. = 100/([t.sub.90] - [t.sub.l0])

The Arrhenius equation (2) is typically written as

(2) k = [Ae.sup.-Ea/RT]

where k is the rate coefficient, A is a constant, Ea is the energy of activation, R is the gas constant and T is the absolute temperature in [degrees]K. In order to obtain an insight on the mechanism of vulcanization, an 'apparent' activation energy was calculated by rearrangement of the Arrhenius equation 2 solving for [E.sub.a] (equation 3). When rheometer data at only two temperatures were available, the apparent activation energy could be readily estimated from the simple relationship in equation 3 where [T.sub.1] and [T.sub.2] are the specific test temperatures and k' and k" are the corresponding reaction rates.

(3) Ea = 4.576 [T.sub.2][T.sub.1] (log k" - log k')/ ([T.sub.2] - [T.sub.1])

This approach was used becasue the activity of thiourea derivatives studied in the vulcanization of butyl rubber, may be different at 140[degrees]C to 150[degrees]C and 170[degrees]C to 180[degrees]C temperature ranges.

The thiourea screening compounds are illustrated in table 2. Each of the five thiourea accelerators was evaluated in place of TMTD at 2.0 phr and 4.0 phr.

Results and discussion

Preliminary screening of thiourea accelerators

As noted, the five accelerators were evaluated at 2.0 phr and 4.0 phr for screening purposes. The MDR rheometer results of the screening can be found in table 3. At 140[degrees]C, the [t.sub.90] for all compounds was in the 24 to 26 minute range. The induction time ([t.sub.10]) for the TMTD compound was longer than that for the thioureas. Also, the cure rate at [t.sub.50] and the cure rate index of the thioureas was lower than the TMTD compound. The cure state as measured by rheometer torque increases through the cure cycle. Additionally, the delta torque ([DELTA]T) of the compounds is decreasing.

For all compounds at 160[degrees]C, the [DELTA]T is greater than the [DELTA]T at 140[degrees]C, indicating the activity of the thiourea derivatives may be temperature dependent. However, when compared to TMTD, the cure time, [t.sub.90], was longer and induction time, [t.sub.10], shorter, together again showing a slower rate of cure. Of the thiourea derivatives, DBTU and DOTTU tended to show the faster cure rates. Compared to TMTD, [E.sub.a] was higher for the thioureas, due to the lower reaction rate.

At 170[degrees]C, the [DELTA]T for TMTD, DMTU and DETU are equivalent. Rheometer [t.sub.10] induction runes are satisfactory, though the [t.sub.90] times are too long. At 180[degrees]C, the [DELTA]T for TMTD, DMTU, DETU and DBTU are similar, but the thioureas have slower cure rates and consequently longer [t.sub.90]. [E.sub.a] was similar to TMTD at 170[degrees]C and 180[degrees]C, suggesting that the optimum temperature of activity of these accelerators is above 160[degrees]C. These results are similar to comparative studies of dicyclohexyl benzothiazole sulfenamide (DCBS) and diisopropyl benzothiazole sulfenamide (DIBS), where DCBS displays little activity at temperatures in the range 100[degrees]C to 120[degrees]C.

Cure rate at [t.sub.50] or at the point where the rheometer torque gradient is maximum is lower for the thioureas than that for TMTD (table 4).

Viewing the test data from this preliminary screening study, a number of additional observations is evident:

* The [t.sub.90] or time to 90% of maximum torque achieved with the thiourea accelerators is higher than that for TMTD, with concentration having little effect on reducing the cure time.

* In the temperature range from 140[degrees]C to 160[degrees]C, 'apparent' activation energies for all of the thiourea accelerators are higher than those for TMTD. This suggests the formation of a sulfurating agent or other complex that might readily crosslink butyl rubber is not easily generated.

* Cure states obtained with use of DMTU, DETU and DBTU are equivalent to that for TMTD. However, the thioureas containing phenolic groups DPTU and DOTTU have displayed lower states of cure. Figure 3 illustrates the cure profile at 180[degrees]C for each of the thioureas at 4 phr and TMTD at 1.0 phr.

[FIGURE 3 OMITTED]

* Reversion resistance shown by DBTU cure systems is superior to both TMTD and the other thiourea accelerators.

By use of DBTU in the cure system for butyl rubber, improvements in reversion resistance and high state of cure similar to that obtained through use of TMTD may be achieved. Also, further optimization of DBTU based cure systems may enable development of specific cure profiles to meet a given set of defined final properties. Thus, an optimization of DBTU concentration through use of a designed experiment was explored.

Optimization of DBTU level in a model butyl compound

The model compound described in table 1 was used for the study. Zinc oxide was fixed at 5.0 phr. DBTU, MBTS and sulfur levels were evaluated using a three-variable central composite design, as follows:</p> <pre> Phr Variable Minimum Maximum MBTS

1.0 3.0 DBTU 1.0 3.0 Sulfur

1.0 2.5 </pre> <p>A three variable standard central composite design was utilized as shown in figure 4. Compound test data can be found in tables 5 and 6, with designed experiment statistics located in table 7.

[FIGURE 4 OMITTED]

The tabulated compound data were imported to SAS JMP, and regression equations calculated using the relationship:

(4) Dependent variable = aX + bY + cZ + d[X.sup.2] + e[Y.sup.2] + f[Z.sup.2] + gXY + hXZ + jYZ + constant

This quadratic model allows identification of any potential interactions. By use of squared terms in such models, it is possible to describe response patterns of the dependent variables that may not be linear. Furthermore, the three-factor central composite design to which this relationship is suited uses 15 runs or compounds and is more economic than a full factorial design which may use up to 27 runs or compounds.

ASTM methods used include ASTM D412 for tensile strength, ASTM D2240 for durometer A hardness and ASTM D624 for tear strength. Three dimensional contour plots and cube plots were constructed and analyzed, enabling identification of trends in compound properties, including:

* No linear correlations were found between the independent variables, DBTU, MBTS and sulfur, and the dependent variables such as rheometer data and selected physical properties.

* Upon constructing the second order regression equations, satisfactory correlation coefficients were noted for vulcanization parameters such as cure rate, torque and [DELTA]T. However, fundamental mechanical properties such as tensile strength showed weaker correlation coefficients. Tensile strength tended to be within a narrow range of 9.7 MPa to 11.3 MPa (table 7).

* Contour plots were constructed plotting DBTU versus MBTS, and the following observations were noted (figures 5 and 6):

[FIGURES 5&6 OMITTED]

* The retarding effect of MBTS was observed as scorch time increased with increasing amounts of MBTS (ref. 2).

* Increasing DBTU content led to an increase in cure state ([DELTA]T).

* Increasing DBTU and MBTS led to a longer time to reach [t.sub.90] at 160[degrees]C. However, at 180[degrees]C, DBTU had no noticeable impact on [t.sub.90].

* Cure rate decreased with increasing DBTU levels and all concentrations of MBTS.

* DBTU had little effect on activation energy, particularly with lower amounts of MBTS.

The construction of additional contour plots of MBTS versus DBTU can be found in figure 7.

[FIGURE 7 OMITTED]

* Though modulus varied within a relatively narrow range of 2.3 MPa to 2.7 MPa, increase in DBTU level led to an increased 300% modulus.

* Tear strength (ASTM D624, Die B) varied within a range of 28 KN/m to 37 KN/m and passed through an optimum when DBTU was in the order of 2.0 phr and sulfur or MBTS were at lower concentrations.

Viewing graphical representations of sulfur versus DBTU, a number of similar observations was noted which may be pertinent to offering an interpretation of the mechanism of DBTU in butyl rubber vulcanization (figure 8).

[FIGURE 8 OMITTED]

As DBTU level increased:

* Scorch time decreased at all sulfur levels;

* activation energy showed little change with increase in DBTU, but decreased as sulfur level increased from 1 phr to 2.5 phr; and

* compound modulus increased with both an increase in sulfur and DBTU.

It is noted that though trends are evident, the magnitude is small, as illustrated in the cube plots of activation energy and rheometer [t.sub.90] at 160[degrees]C (figure 9).

[FIGURE 9 OMITTED]

The [t.sub.90] for the TMTD control is 19 minutes with a cure rate index of 5.92 (table 3). The prediction profiler in SAS JMP was run using delta torque ([DELTA]T) at 160[degrees]C, [ts.sub.2], [t.sub.50], [t.sub.90], cure rate index and activation energy (figure 10). The profiler also illustrates how these parameters may be adjusted through changes in the concentration of the independent variables MBTS, DBTU and sulfur. For example, MBTS and sulfur have a much greater effect on reducing activation energy than DBTU. However, the cure state ([DELTA]T) is affected by the presence of thiourea.

[FIGURE 10 OMITTED]

Analysis of results

The synergistic activity and behavior found with binary accelerator systems in rubber vulcanization is well accepted, though not fully understood. Thiourea accelerators such as ETU have been used with metal oxides in the vulcanization of polychloroprene. It has been reported that ETU and DETU are the most effective thioureas for accelerating the high-temperature vulcanization of polychloroprene. ZnO and MgO are also added in order that the vulcanization time and degree of vulcanization can be controlled. The processing safety of ETU accelerated vulcanization can be further adjusted with TMTD, which, in combination with processing aids, can then more readily meet product target properties (rcf. 6).

Various alkyl substituted thioureas can also be used as partial or complete replacement of ZnO (ref. 7). Such molecules are effective secondary accelerators for zinc mercaptobenzothiazole, and the compounds can show resistance to copper staining and improved ozone resistance of natural rubber latex film. The use of thioureas in specialty rubbers, such as chloro-sulfonated polymethylene polyacrylic rubber, chloro-butyl rubber, epichlorohydrin rubber and ethylene-propylene terpolymer has also been presented (ref. 7).

Unsymmetrical thioureas where the substitute group, R, = H or Me, have been evaluated as secondary accelerators in EV systems based on a high sulfenamide and low sulfur content in tire tread compounds (ref. 8). At 140[degrees]C, thiourea alone in an EV system gave a short scorch time with maximum cure rate and crosslinking. With sulfenamides, it was possible to obtain a more balanced level of optimum cure, maximum crosslinking and processing safety in comparison with a conventional cure system. By increasing the vulcanization temperature from 140[degrees]C to 160[degrees]C, the same compositions of thioureas in an EV system gave faster cure rates, showing that this efficient vulcanization system was more sensitive to higher temperatures, as inferred in this study. In natural rubber compounds, a significant amount of reversion was noted, even at very low levels of sulfenamide accelerators. When the thiourea based system was evaluated at 180[degrees]C, none of the thiourea compounds displayed reversion, while the standard control compound with a normal level of sulfur and accelerator had excessive reversion. The thiourea-based EV systems produced vulcanizates that were more stable towards the influence of heat, even in natural rubber (ref. 8).

Given the reported use of thioureas for curing of polymers other than polychloroprene, and the improvements in reversion resistance, its evaluation in butyl rubber cure systems becomes of interest. Butyl rubbers are used in curing bladders where reversion resistance at up to 200[degrees]C is required, and also in innertubes where heat and aging resistance are needed. Industrial replacements for ETU have been pursued. Resorcinol in combination with a sulfur donor or triazine derivatives has been investigated, but successful replacement has not been achieved (refs. 9-11). Probably the most effective solution is use of higher molecular weight molecules, such as DMTU, DETU, DBTU and the aromatic derivatives DPDU and DOTTU, which is the basis of this work in butyl rubber.

Thioureas have not been subjected to the same degree of investigation as thiazoles or sulfenamide accelerators. It has been suggested that in latex vulcanization systems, where a thiourea is used as a secondary accelerator, it acts as a nucleophilic reagent enabling cleavage of sulfur bonds in the primary accelerator such as a thiuram (TMTD) or sulfenamide (CBS) at low temperatures of 100[degrees]C to 120[degrees]C. This had been proposed from work on 1-phenyl-2,4-dithiobiuret and 1,5 diphenyl-2,4-dithiobiuret (figure 11) (ref. 12). Comparatively better tensile properties and good retention of these properties after aging were also shown by these vulcanizates. The optimum concentration of the thiourea derivative secondary accelerator required for these vulcanization reactions was in the order of 0.5 phr to 1.5 phr.

[FIGURE 11 OMITTED]

Based on the subsequent work, it was concluded that thioureas will engage in a thioanion-disulfide interchange to form a perthioanion XSS-. In the presence of zinc stearate, sulfurating complexes can then be formed which, in turn, can react with the elastomer (RH) to form a rubber-bound intermediate compound, R[S.sub.n]SX, where X would be the accelerator pendent group (refs. 2 and 13). The scheme proposed is illustrated in figure 12.

[FIGURE 12 OMITTED]

The perthioanion readily participates in the reaction leading to formation of sulfurating complexes, which in turn react with the rubber polymer chain to create a rubber bound pendent intermediate. For maximum acceleration effects of thiourea in low or sulfur-free cure systems, low concentrations of the primary accelerator such as TMTD or MBTS, would be preferred (ref. 13). Since the formation of XSS- might be low due to the low amount of TMTD, the thiourea is therefore more effective. Thioureas will also react more rapidly than zinc oxide with disulfides such as TMTD or MBTS. As a point of general interest, it was also noted that in the case of TMTD-accelerated sulfur vulcanization, digression from a first order reaction rate is prevented by the addition of stearic acid. Stearic acid reacts with ZnO to form zinc stearate. Zinc stearate being more soluble in rubber compounds, aids the reaction with a thiuram such as TMTD to form the dithiocarbamate, or thiazole such as MBTS to form ZMBT. Stearic acid was also reported to lower the activation energy of thiuram accelerated vulcanization.

1-phenyl-3-(N,N'-diphenylamidino) thiourea has been used as a secondary accelerator with TMTD and MBTS in the sulfur vulcanization of natural rubber (figure 13) (refs. 14 and 15). It was found that the more nucleophilic the secondary accelerator, the less the optimum cure. Such observations would be in agreement with the better reversion resistance found with DBTU compared with DPTU and DOTTU.

[FIGURE 13 OMITTED]

Applying their interpretation to the dibutyl substituted thiourea (DBTU) in the systems investigated in this work, DBTU may also first react with MBTS in the formation of the sulfurating complex (figure 14).

[FIGURE 14 OMITTED]

Table 8 summarizes the vulcanization kinetics for the thiourea derivatives investigated in this study. The decrease in induction time with use of thioureas is in agreement with earlier reports that a thioanion--disulfide interchange occurs to form a perthioanion XSS-. In this case, the complex would be formed with MBTS. Time to optimum cure, t90, is extended and this is also reflected in the relative drop in cure rate index. However, these data should be viewed with the rheometer cure profile (figure 3), which illustrates that DMTU, DETU or DBTU may be optimized to match that for TMTD. The apparent activation energy, [E.sub.a], was obtained from the cure profiles at 140[degrees]C and 160[degrees]C. Compared to DMTU and DETU, DBTU has shown the lowest activation energy and highest cure rate index (C.R.I.).

Reversion resistance is an important compound parameter in applications such as curing bladders that use butyl rubber. Figure 15 illustrates cure state and cure system reversion resistance for the five thiourea derivatives, and suggests DBTU would be the preferred molecule for further optimization. This resistance to reversion displayed by DBTU and also DPTU and DOTTU might be attributed to the preferential formation of monosulfidic crosslinks. Additional work would be of value to assess if carbon-carbon crosslinks are formed.

Reaction with MBTS will be slower than that with TMTD, possibly due to the steric bulk of the thiazole groups. DBTU is more effective at building modulus than MBTS (figure 7), which may also act as a retarder (ref. 2). Higher modulus compounds can also be achieved by increasing both sulfur and DBTU content, as curvature suggests that the relationship may be synergistic.

Summary

Butyl rubber is used in a variety of applications such as tires, curing bladders and in products where aging and long-term retention of compound properties are required. The vulcanization system must therefore be stable under a variety of conditions.

Thiourea derivatives can meet many of the requirements for vulcanization systems, such as crosslink stability and use in high temperature curing. DBTU has shown to be particularly effective in this regard. For DBTU--MBTS--sulfur cure systems for butyl robber, the optimum level of dibutylthiourea is in the order of 2.0 phr to 2.5 phr. DBTU potentially reacts with MBTS to form a perthioanion, which ultimately leads to formation of a sulfurating complex. Depending on the thiourea derivative selected, the butyl compound may display excellent reversion resistance, leading to improved product performance.

References

(1.) E.N. Kresge, R.H. Schatz and H.C. Wang, "Isobutylene polymers, "Encyclopedia of Polymer Science & Engineering. vol. 8, 2nd edition, pp. 423-448, John Wiley & Sons, Inc., 1987.

(2.) S. Solis, B. Rodgers, N. Tambi, B.B. Sharma and W.H. Waddell, "A review of the vulcanization of isobutylene-based elastomers, "Rubber Division, ACS, San Antonio, 2005.

(3.) L. Batman, C. G. Moore, M. Porter and B. Saville, "Chemistry of vulcanization, "Chemistry and Physics of Rubber-Like Substances, p. 465, editors, L. Batman and M. Porter. Maclaren Press, London, 1963.

(4.) F. Ignatz-Hoover, "Review of vulcanization chemistry," Rubber World vol. 220, pp. 24-29, 101-102. August 1999.

(5.) G. Alliger and I.J. Sjothun. Vulcanization of Elastomers. Reinhold Publishing, New York, 1963.

(6.) U. Eholzer and T. Kempermann, Kautschuk Gummi Kunstsoffe, vol. 33, pp. 696-699, 1980.

(7.) D. W. Yochum and T.L. Puthenpurackal, Rubber World, vol. 166, pp. 51-57, 1972.

(8.) N.D. Ghatge and R. G. Gokhale, Rubber World vol. 160, pp. 76-81, 1969.

(9.) A.Y. Coran, "Vulcanization, " Science and Technology of Rubber, editors J.E. Mark, B. Erman and F.R. Eirich, 2nd edition, Academic Press, 1994.

(10.) K. Mori and Y. Nakamura, Rubber Chem. & Technol., vol. 57, pp. 34-47, 1984.

(11.) H. Kato and H. Fugita, Rubber Chem. & Technol., vol. 55, p. 949-960, 1982.

(12.) G. Mathew and A.P. Kuriakose, J. Appl. Polymer Sci., vol. 49, pp. 2,009-2,017, 1993.

(13.) V. Duchacek. J. Appl. Polymer Sci., vol. 22, pp. 227-237. 1978.

(14.) C. Mathew, V.T.E. Mini, A.P. Kuriakose and D.J. Francis', J. Appl. Polymer Sci., vol. 59, pp. 365-391, 1996.

(15.) A.P. Susamma, V.T.E. Mini and A.P. Kuriakose, J. Appl. Polymer Sci., vol. 79, pp. 1-8, 2000.

Table 1--model screening formula

Material                                        (Phr)

Butyl rubber 268                                100.0
N660 carbon black                                70.0
Paraffinic oil                                   25.0
Phenolic tackifying resin                         4.0
Stearic acid                                      1.0
Zinc oxide                                        5.0
Tetramethylthiuram disulfide (TMTD)               1.0
Mercaptobenzothiazole disulfide (MBTS)            0.5
Sulfur                                            2.0

Table 2--butyl rubber screening formulas

Compound:                         1            2            3

Butyl rubber 268              100.0        100.0        100.0
Carbon black,                  70.0         70.0         70.0
  N660
Paraffinic oil                 25.0         25.0         25.0
Phenolic                        4.0          4.0          4.0
  tackifying resin
Stearic acid                    1.0          1.0          1.0
ZnO                             5.0          5.0          5.0
TMTD                            1.0
MBTS                            0.5          0.5          0.5
Sulfur                          2.0          2.0          2.0
DMTU                                         2.0          4.0
DETU
DBTU
DPTU
DOTTU

Compound:                         4            5            6

Butyl rubber 268              100.0        100.0        100.0
Carbon black,                  70.0         70.0         70.0
  N660
Paraffinic oil                 25.0         25.0         25.0
Phenolic                        4.0          4.0          4.0
  tackifying resin
Stearic acid                    1.0          1.0          1.0
ZnO                             5.0          5.0          5.0
TMTD
MBTS                            0.5          0.5          0.5
Sulfur                          2.0          2.0          2.0
DMTU
DETU                            2.0          4.0
DBTU                                                      2.0
DPTU
DOTTU

Compound:                         7            8            9

Butyl rubber 268              100.0        100.0        100.0
Carbon black,                  70.0         70.0         70.0
  N660
Paraffinic oil                 25.0         25.0         25.0
Phenolic                        4.0          4.0          4.0
  tackifying resin
Stearic acid                    1.0          1.0          1.0
ZnO                             5.0          5.0          5.0
TMTD
MBTS                            0.5          0.5          0.5
Sulfur                          2.0          2.0          2.0
DMTU
DETU
DBTU                            4.0
DPTU                                         2.0          4.0
DOTTU

Compound:                                     10           11

Butyl rubber 268                           100.0        100.0
Carbon black,                               70.0         70.0
  N660
Paraffinic oil                              25.0         25.0
Phenolic                                     4.0          4.0
  tackifying resin
Stearic acid                                 1.0          1.0
ZnO                                          5.0          5.0
TMTD
MBTS                                         0.5          0.5
Sulfur                                       2.0          2.0
DMTU
DETU
DBTU
DPTU
DOTTU                                        2.0          4.0

Table 3--rheometer results at 140[degrees]C and 160[degrees]C

Compound:                        1        2        3        4

TMTD                           1.0
MBTS                           0.5      0.5      0.5      0.5
Sulfur                         2.0      2.0      2.0      2.0
DMTU                                    2.0      4.0
DETU                                                      2.0
DBTU
DPTU
DOTTU
MDR rheometer
  at 140[degrees]C
Minimum torque                 1.7      1.9      1.9      1.8
  ml (dNm)
Maximum torque                 7.3      4.3      4.0      4.3
  mh (dNm)
  mh-ml (dNm)                  5.7      2.5      2.1      2.5
Induction time [t.sub.10]      6.9      2.5      1.7      2.5
  [t.sub.50]                  12.3     11.9     11.5     11.3
  [t.sub.90]                  24.6     25.8     25.7     25.6
Cure rate, at [t.sub.10]       0.5      0.2      0.2      0.2
Cure rate index                5.7      4.3      4.2      4.3
MDR rheometer
  at 160[degrees]C
Minimum torque                 1.4      1.6      1.7      1.6
  ml (dNm)
Maximum torque                 9.9      7.9      7.9      7.9
  mh (dNm)
  mh-ml (dNm)                  8.5      6.4      6.3      6.4
Induction time [t.sub.10]      2.3      1.3      1.3      1.3
  [t.sub.50]                   5.4      8.8      6.8      7.6
  [t.sub.90]                  19.2     25.0     25.3     24.0
Cure rate, at [t.sub.50]       1.7      0.8      0.8      0.8
Cure rate index                5.9      4.2      4.2      4.4
Activation                    89.7    120.6     95.0    123.3
  energy, [E.sub.a]

Compound:                        5        6        7        8

TMTD
MBTS                           0.5      0.5      0.5      0.5
Sulfur                         2.0      2.0      2.0      2.0
DMTU
DETU                           4.0
DBTU                                    2.0      4.0
DPTU                                                      2.0
DOTTU
MDR rheometer
  at 140[degrees]C
Minimum torque                 1.8      1.7      1.7      1.7
  ml (dNm)
Maximum torque                 4.3      4.3      4.1      3.6
  mh (dNm)
  mh-ml (dNm)                  2.5      2.6      2.5      1.9
Induction time [t.sub.10]      1.5      2.9      2.5      2.0
  [t.sub.50]                   9.8     11.9     10.4     10.1
  [t.sub.90]                  25.1     25.7     25.2     25.0
Cure rate, at [t.sub.10]       0.3      0.2      0.2      0.2
Cure rate index                4.2      4.4      4.4      4.4
MDR rheometer
  at 160[degrees]C
Minimum torque                 1.6      1.4      1.4      1.5
  ml (dNm)
Maximum torque                 8.0      7.3      7.1      5.3
  mh (dNm)
  mh-ml (dNm)                  6.5      5.9      5.7      3.9
Induction time [t.sub.10]      1.2      1.3      1.1      1.2
  [t.sub.50]                   7.6      6.9      6.7      7.3
  [t.sub.90]                  23.4     21.6     23.0     23.0
Cure rate, at [t.sub.50]       0.9      0.8      0.9      0.6
Cure rate index                4.5      4.9      4.6      4.6
Activation                    97.7    110.5    117.1     91.8
  energy, [E.sub.a]

Compound:                                 9       10       11

TMTD
MBTS                                    0.5      0.5      0.5
Sulfur                                  2.0      2.0      2.0
DMTU
DETU
DBTU
DPTU                                    4.0
DOTTU                                            2.0      4.0
MDR rheometer
  at 140[degrees]C
Minimum torque                          1.7      1.7      1.6
  ml (dNm)
Maximum torque                          4.2      3.6      3.7
  mh (dNm)
  mh-ml (dNm)                           2.5      2.0      2.1
Induction time [t.sub.10]               3.1      2.4      2.8
  [t.sub.50]                           13.6     10.4     11.4
  [t.sub.90]                           25.1     24.8     25.0
Cure rate, at [t.sub.10]                0.2      0.1      0.1
Cure rate index                         4.6      4.5      4.5
MDR rheometer
  at 160[degrees]C
Minimum torque                          1.4      1.4      1.4
  ml (dNm)
Maximum torque                          5.5      5.0      5.0
  mh (dNm)
  mh-ml (dNm)                           4.1      3.6      3.6
Induction time [t.sub.10]               1.3      1.1      1.2
  [t.sub.50]                            6.4      5.2      4.8
  [t.sub.90]                           21.7     21.2     20.7
Cure rate, at [t.sub.50]                0.6      0.6      0.6
Cure rate index                         4.9      5.0      5.1
Activation                            100.5    104.4    111.2
  energy, [E.sub.a]

Table 4--rheometer results at 170[degrees]C and 180[degrees]C

Compound:                            1       2       3       4

TMTD                               1.0
MBTS                               0.5     0.5     0.5     0.5
Sulfur                             2.0     2.0     2.0     2.0
DMTU                                       2.0     4.0
DETU                                                       2.0
DBTU
DPTU
DOTTU
MDR rheometer
  at 170[degrees]C
Minimum torque                     1.3     1.5     1.6     1.4
  ml (dNm)
Maximum torque                    10.1    10.5    10.4    10.0
  mh (dNm)
mh-ml (dNm)                        8.9     9.1     8.8     8.5
Induction time [t.sub.10]          1.4     1.0     1.1     0.9
[t.sub.50]                         3.4     7.3     7.3     5.9
[t.sub.90]                        12.9    22.6    21.3    21.8
Cure rate, [t.sub.50]              2.9     1.6     1.4     1.7
CRI(cure rate                      8.6     4.6     4.9     4.8
  index)
MDR rheometer
  at 180[degrees]C
Minimum torque                     1.2     1.4     1.5     1.4
  ml (dNm)
Maximum torque                     9.7    10.7    10.0    10.6
  mh (dNm)
  mh-ml (dNm)                      8.6     9.2     8.5     9.1
Induction time [t.sub.10]          0.9     0.7     0.7     0.7
[t.sub.50]                         1.9     4.0     3.8     3.5
[t.sub.90]                         6.7    14.1    11.4    15.5
Cure rate, at [t.sub.50]           5.2     2.7     2.5     2.8
CRI (cure rate                    17.2     7.5     9.3     6.7
  index)
Activation                        99.9    88.0    97.0    84.1
  energy, [E.sub.a]

Compound:                            5       6       7       8

TMTD
MBTS                               0.5     0.5     0.5     0.5
Sulfur                             2.0     2.0     2.0     2.0
DMTU
DETU                               4.0
DBTU                                       2.0     4.0
DPTU                                                       2.0
DOTTU
MDR rheometer
  at 170[degrees]C
Minimum torque                     1.4     1.3     1.3     1.3
  ml (dNm)
Maximum torque                     9.8     8.3     8.3     5.9
  mh (dNm)
mh-ml (dNm)                        8.4     6.9     7.0     4.5
Induction time [t.sub.10]          0.9     0.9     0.8     0.9
[t.sub.50]                         5.9     4.3     4.8     5.3
[t.sub.90]                        22.9    19.2    21.8    19.9
Cure rate, [t.sub.10]              1.6     1.7     1.6     1.0
CRI(cure rate                      4.5     5.4     4.8     5.3
  index)
MDR rheometer
  at 180[degrees]C
Minimum torque                     1.5     1.3     1.3     1.3
  ml (dNm)
Maximum torque                    10.2     8.3     9.8     5.7
  mh (dNm)
  mh-ml (dNm)                      8.7     7.0     8.5     4.4
Induction time [t.sub.10]          0.6     0.6     0.6     0.7
[t.sub.50]                         3.2     2.3     3.0     2.9
[t.sub.90]                        14.2    12.5    15.5    11.2
Cure rate, at [t.sub.50]           2.6     2.8     2.8     1.5
CRI (cure rate                     7.4     8.4     6.7     9.5
  index)
Activation                        80.6    86.6    89.9    68.8
  energy, [E.sub.a]

Compound:                                    9      10      11

TMTD
MBTS                                       0.5     0.5     0.5
Sulfur                                     2.0     2.0     2.0
DMTU
DETU
DBTU
DPTU                                       4.0
DOTTU                                              2.0     4.0
MDR rheometer
  at 170[degrees]C
Minimum torque                             1.3     1.3     1.3
  ml (dNm)
Maximum torque                             5.8     5.5     5.2
  mh (dNm)
mh-ml (dNm)                                4.5     4.2     3.9
Induction time [t.sub.10]                  0.9     0.8     0.8
[t.sub.50]                                 4.1     3.5     2.9
[t.sub.90]                                18.2    18.7    16.3
Cure rate, [t.sub.10]                      1.1     1.1     1.2
CRI(cure rate                              5.8     5.6     6.4
  index)
MDR rheometer
  at 180[degrees]C
Minimum torque                             1.3     1.3     1.2
  ml (dNm)
Maximum torque                             5.4     5.3     4.9
  mh (dNm)
  mh-ml (dNm)                              4.1     4.0     3.7
Induction time [t.sub.10]                  0.6     0.6     0.6
[t.sub.50]                                 2.3     2.0     1.6
[t.sub.90]                                10.4    10.7     8.8
Cure rate, at [t.sub.50]                   1.6     1.9     2.0
CRI (cure rate                            10.2     9.9    12.2
  index)
Activation                                71.8    86.7    89.7
  energy, [E.sub.a]

Table 5--designed experiment results from Mooney viscosity, Mooney
scorch and rheometer testing

Compound                       1         2         3         4

Butyl rubber 268          100.00    100.00    100.00    100.00
Carbon black,              70.00     70.00     70.00     70.00
  N660
Paraffinic oil             25.00     25.00     25.00     25.00
Phenolic                    4.00      4.00      4.00      4.00
  tackifying resin
Stearic acid                1.00      1.00      1.00      1.00
Zinc oxide                  5.00      5.00      5.00      5.00
MBTS                        1.40      2.60      1.40      2.60
DBTU                        1.40      1.40      2.60      2.60
Sulfur                      1.30      1.30      1.30      1.30
Mooney ML1+4@               48.0      48.1      47.1      46.6
  100[degrees]C
Mooney scorch @
  125[degrees]C
Minimum viscosity           12.0       6.4      10.4       7.1
Time to 1 point rise        20.3      39.2      16.7      23.2
  (minutes)
Time to 5 point rise        25.8      44.5      22.0      27.3
  (minutes)
Time to 10 point rise       33.8      51.9      29.5      33.3
  (minutes)
Time to 35 point rise       36.4      36.5      35.3      35.9
  (minutes)
MDR rheometer @
  160[degrees]C
ML (dNm)                     1.5       1.5       1.5       1.5
MH (dNm)                     8.6       8.1       9.3       9.2
Delta torque [DELTA]T        7.1       6.6       7.8       7.7
  (dNm)
ts2 (minutes)                5.5       5.4       5.2       4.8
[t.sub.50] (minutes)        12.2      11.3      13.3      13.0
[t.sub.90] (minutes)        42.0      41.9      43.1      44.2
Cure rate index @            2.7       2.7       2.6       2.5
  160[degrees]C
MDR rheometer @
  180[degrees]C
ML (dNm)                     1.3       1.3       1.3       1.3
MH (dNm)                     8.0       8.1       8.8       8.8
Delta torque [DELTA]T        6.7       6.8       7.5       7.5
  (dNm)
ts2 (minutes)                1.6       1.6       1.5       1.4
[t.sub.50] (minutes)         3.0       3.1       3.1       3.3
[t.sub.90] (minutes)        10.7      11.8      10.6      11.1
Cure rate index @           11.0       9.8      10.9      10.3
  180[degrees]C
Apparent activation        113.0     103.4     115.9     114.3
  energy

Compound                       5         6         7         8

Butyl rubber 268          100.00    100.00    100.00    100.00
Carbon black,              70.00     70.00     70.00     70.00
  N660
Paraffinic oil             25.00     25.00     25.00     25.00
Phenolic                    4.00      4.00      4.00      4.00
  tackifying resin
Stearic acid                1.00      1.00      1.00      1.00
Zinc oxide                  5.00      5.00      5.00      5.00
MBTS                        1.40      2.60      1.40      2.60
DBTU                        1.40      1.40      2.60      2.60
Sulfur                      2.20      2.20      2.20      2.20
Mooney ML1+4@               47.5      47.3      45.9      46.5
  100[degrees]C
Mooney scorch @
  125[degrees]C
Minimum viscosity           12.2       6.6      10.2       7.0
Time to 1 point rise        19.4      37.2      15.5      21.3
  (minutes)
Time to 5 point rise        24.0      41.6      19.8      25.3
  (minutes)
Time to 10 point rise       30.5      47.7      26.0      30.3
  (minutes)
Time to 35 point rise       35.8      35.8      34.8      35.6
  (minutes)
MDR rheometer @
  160[degrees]C
ML (dNm)                     1.4       1.5       1.5       1.5
MH (dNm)                     9.4       8.8      10.3      10.2
Delta torque [DELTA]T        8.0       7.4       8.8       8.7
  (dNm)
ts2 (minutes)                4.8       4.9       4.6       4.2
[t.sub.50] (minutes)        12.6      11.7      13.9      13.4
[t.sub.90] (minutes)        43.1      43.3      44.7      45.3
Cure rate index @            2.6       2.6       2.5       2.4
  160[degrees]C
MDR rheometer @
  180[degrees]C
ML (dNm)                     1.3       1.3       1.3       1.3
MH (dNm)                     9.1       9.1      10.3      10.3
Delta torque [DELTA]T        7.9       7.9       9.0       9.0
  (dNm)
ts2 (minutes)                1.4       1.4       1.3       1.3
[t.sub.50] (minutes)         3.2       3.3       3.5       3.7
[t.sub.90] (minutes)        12.2      13.2      12.8      13.5
Cure rate index @            9.3       8.5       8.7       8.2
  180[degrees]C
Apparent activation        103.4      96.2     102.4      98.8
  energy

Compound                       9        10        11        12

Butyl rubber 268          100.00    100.00    100.00    100.00
Carbon black,              70.00     70.00     70.00     70.00
  N660
Paraffinic oil             25.00     25.00     25.00     25.00
Phenolic                    4.00      4.00      4.00      4.00
  tackifying resin
Stearic acid                1.00      1.00      1.00      1.00
Zinc oxide                  5.00      5.00      5.00      5.00
MBTS                        1.00      3.00      2.00      2.00
DBTU                        2.00      2.00      1.00      3.00
Sulfur                      1.75      1.75      1.75      1.75
Mooney ML1+4@               47.3      47.3      48.0      45.4
  100[degrees]C
Mooney scorch @
  125[degrees]C
Minimum viscosity            9.8       5.5       9.1      10.1
Time to 1 point rise        15.2      29.5      40.3      17.0
  (minutes)
Time to 5 point rise        19.6      34.0      46.4      21.9
  (minutes)
Time to 10 point rise       26.5      39.7      54.3      28.0
  (minutes)
Time to 35 point rise       35.4      36.4      36.7      35.1
  (minutes)
MDR rheometer @
  160[degrees]C
ML (dNm)                     1.5       1.5       1.5       1.5
MH (dNm)                     9.5       9.4       8.0       9.8
Delta torque [DELTA]T        8.0       7.9       6.5       8.4
  (dNm)
ts2 (minutes)                4.8       4.7       5.5       4.7
[t.sub.50] (minutes)        12.6      12.4      11.2      13.7
[t.sub.90] (minutes)        41.9      43.5      42.1      45.0
Cure rate index @            2.7       2.6       2.7       2.5
  160[degrees]C
MDR rheometer @
  180[degrees]C
ML (dNm)                     1.3       1.3       1.3       1.3
MH (dNm)                     8.8       9.4       7.9      10.0
Delta torque [DELTA]T        7.5       8.0       6.7       8.7
  (dNm)
ts2 (minutes)                1.4       1.4       1.6       1.3
[t.sub.50] (minutes)         3.0       3.4       3.1       3.5
[t.sub.90] (minutes)        10.5      12.7      12.7      12.6
Cure rate index @           11.1       8.9       9.0       8.9
  180[degrees]C
Apparent activation        115.3     101.1      97.5     104.3
  energy

Compound                                13        14        15

Butyl rubber 268                    100.00    100.00    100.00
Carbon black,                        70.00     70.00     70.00
  N660
Paraffinic oil                       25.00     25.00     25.00
Phenolic                              4.00      4.00      4.00
  tackifying resin
Stearic acid                          1.00      1.00      1.00
Zinc oxide                            5.00      5.00      5.00
MBTS                                  2.00      2.00      2.00
DBTU                                  2.00      2.00      2.00
Sulfur                                1.00      2.50      1.75
Mooney ML1+4@                         47.8      46.4      46.9
  100[degrees]C
Mooney scorch @
  125[degrees]C
Minimum viscosity                      9.1       8.1       9.0
Time to 1 point rise                  23.5      18.9      20.5
  (minutes)
Time to 5 point rise                  28.4      23.4      25.1
  (minutes)
Time to 10 point rise                 35.6      28.8      31.1
  (minutes)
Time to 35 point rise                 36.6      36.3      37.0
  (minutes)
MDR rheometer @
  160[degrees]C
ML (dNm)                               1.5       1.5       1.5
MH (dNm)                               8.4      10.2       9.4
Delta torque [DELTA]T                  6.9       8.7       7.9
  (dNm)
ts2 (minutes)                          5.4       4.2       4.8
[t.sub.50] (minutes)                  11.8      12.6      12.6
[t.sub.90] (minutes)                  41.1      44.0      43.4
Cure rate index @                      2.8       2.5       2.6
  160[degrees]C
MDR rheometer @
  180[degrees]C
ML (dNm)                               1.3       1.3       1.3
MH (dNm)                               7.9      10.4       9.5
Delta torque [DELTA]T                  6.6       9.1       8.2
  (dNm)
ts2 (minutes)                          1.6       1.3       1.4
[t.sub.50] (minutes)                   2.9       3.5       3.4
[t.sub.90] (minutes)                   9.6      13.5      12.5
Cure rate index @                     12.4       8.2       9.0
  180[degrees]C
Apparent activation                  121.5      96.4     101.3
  energy

Table 6--designed experiment results for tensile strength, hardness
and tear strength

Compound                         1            2            3

Exxon Butyl 268             100.00       100.00       100.00
Carbon black,                70.00        70.00        70.00
  N660
Paraffinic al                25.00        25.00        25.00
Phenolic                      4.00         4.00         4.00
  tackifying resin
Stearic acid                  1.00         1.00         1.00
Zinc oxide                    5.00         5.00         5.00
MBTS                          1.40         2.60         1.40
DBTU                          1.40         1.40         2.60
Sulfur                        1.30         1.30         1.30
Tensile strength             10.0          9.70        10.80
  (MPa)
Elongation (%)                 745          715          730
100% modulus                  0.7           0.8          0.8
  (MPa)
200% modulus                  1.5           1.5          1.6
  (MPa)
300% modulus                  2.4           2.3          2.5
  (MPa)
Hardness                        43           44           43
  (durometer A)
Tear die B (KN/m)             32.3         34.0         35.6
Tear de C (KN/m)              32.4         31.0         30.6

Compound                         4            5            6

Exxon Butyl 268             100.00       100.00       100.00
Carbon black,                70.00        70.00        70.00
  N660
Paraffinic al                25.00        25.00        25.00
Phenolic                      4.00         4.00         4.00
  tackifying resin
Stearic acid                  1.00         1.00         1.00
Zinc oxide                    5.00         5.00         5.00
MBTS                          2.60         1.40         2.60
DBTU                          2.60         1.40         1.40
Sulfur                        1.30         2.20         2.20
Tensile strength             10.00        10.50        10.40
  (MPa)
Elongation (%)                 750          765          755
100% modulus                   0.9          0.8          0.8
  (MPa)
200% modulus                   1.6          1.6          1.6
  (MPa)
300% modulus                   2.4          2.4          2.5
  (MPa)
Hardness                        45           42           46
  (durometer A)
Tear die B (KN/m)             30.4         30.1         38.2
Tear de C (KN/m)              31.0         33.0         30.9

Compound                         7            8            9

Exxon Butyl 268             100.00       100.00       100.00
Carbon black,                70.00        70.00        70.00
  N660
Paraffinic al                25.00        25.00        25.00
Phenolic                      4.00         4.00         4.00
  tackifying resin
Stearic acid                  1.00         1.00         1.00
Zinc oxide                    5.00         5.00         5.00
MBTS                          1.40         2.60         1.00
DBTU                          2.60         2.60         2.00
Sulfur                        2.20         2.20         1.75
Tensile strength             10.10        10.00        10.30
  (MPa)
Elongation (%)                 750          740          740
100% modulus                   1.0          1.0          0.9
  (MPa)
200% modulus                   1.8          1.8          1.8
  (MPa)
300% modulus                   2.7          2.6          2.7
  (MPa)
Hardness                        45           48           45
  (durometer A)
Tear die B (KN/m)             33.0         33.6         36.7
Tear de C (KN/m)              30.2         34.8         35.0

Compound                        10           11           12

Exxon Butyl 268             100.00       100.00       100.00
Carbon black,                70.00        70.00        70.00
  N660
Paraffinic al                25.00        25.00        25.00
Phenolic                      4.00         4.00         4.00
  tackifying resin
Stearic acid                  1.00         1.00         1.00
Zinc oxide                    5.00         5.00         5.00
MBTS                          3.00         2.00         2.00
DBTU                          2.00         1.00         3.00
Sulfur                        1.75         1.75         1.75
Tensile strength              9.70        10.30        10.60
  (MPa)
Elongation (%)                 750          755          740
100% modulus                   0.9          0.8          0.9
  (MPa)
200% modulus                   1.6          1.5          1.6
  (MPa)
300% modulus                   2.4          2.3          2.4
  (MPa)
Hardness                        45           42           44
  (durometer A)
Tear die B (KN/m)             31.4         34.6         28.2
Tear de C (KN/m)              30.0         36.7         31.1

Compound                        13           14           15

Exxon Butyl 268             100.00       100.00       100.00
Carbon black,                70.00        70.00        70.00
  N660
Paraffinic al                25.00        25.00        25.00
Phenolic                      4.00         4.00         4.00
  tackifying resin
Stearic acid                  1.00         1.00         1.00
Zinc oxide                    5.00         5.00         5.00
MBTS                          2.00         2.00         2.00
DBTU                          2.00         2.00         2.00
Sulfur                        1.00         2.50         1.75
Tensile strength              9.50        11.30        10.60
  (MPa)
Elongation (%)                 725          740          735
100% modulus                   0.8          0.9          0.9
  (MPa)
200% modulus                   1.4          1.7          1.7
  (MPa)
300% modulus                   2.1          2.6          2.6
  (MPa)
Hardness                        42           44           45
  (durometer A)
Tear die B (KN/m)             36.4         33.2         37.0
Tear de C (KN/m)              32.5         31.6         33.9

Table 7--designed experiment statistics

Property                                      [R.sup.2]

ML 1+4 @ 100[degrees]C viscosity                 0.9547
Mooney relaxation time                           0.6859
Minimum viscosity @ 125[degrees]C                0.9156
Mooney scorch [ts.sub.10] @ 125[degrees]C        0.9783
MDR rheometer @ 160[degrees]C
Delta torque                                     0.9861
[ts.sub.2]                                       0.9850
[t.sub.50]                                       0.9438
[t.sub.90]                                       0.9369
MDR rheometer @ 180[degrees]C
Delta torque                                     0.9851
C.R.I. at 160[degrees]C                          0.8907
C.R.I. at 180[degrees]C                          0.9633
Activation energy, [E.sub.a]                     0.9775
Tensile strength                                 0.6481
300% modulus                                     0.8396
Hardness                                         0.7791
Die B tear strength                              0.7256

Property                                        F-ratio

ML 1+4 @ 100[degrees]C viscosity                 11.705
Mooney relaxation time                            1.213
Minimum viscosity @ 125[degrees]C                 6.028
Mooney scorch [ts.sub.10] @ 125[degrees]C       25.1331
MDR rheometer @ 160[degrees]C
Delta torque                                    39.4330
[ts.sub.2]                                      36.6138
[t.sub.50]                                       9.3373
[t.sub.90]                                       8.2584
MDR rheometer @ 180[degrees]C
Delta torque                                    36.7498
C.R.I. at 160[degrees]C                          4.5277
C.R.I. at 180[degrees]C                         14.6057
Activation energy, [E.sub.a]                    24.1803
Tensile strength                                 1.0235
300% modulus                                     2.9091
Hardness                                         1.9594
Die B tear strength                              1.4693

Property                                    Probability

ML 1+4 @ 100[degrees]C viscosity                 0.0073
Mooney relaxation time                           0.4375
Minimum viscosity @ 125[degrees]C                0.0310
Mooney scorch [ts.sub.10] @ 125[degrees]C        0.0012
MDR rheometer @ 160[degrees]C
Delta torque                                     0.0004
[ts.sub.2]                                       0.0005
[t.sub.50]                                       0.0121
[t.sub.90]                                       0.0158
MDR rheometer @ 180[degrees]C
Delta torque                                     0.0005
C.R.I. at 160[degrees]C                          0.0555
C.R.I. at 180[degrees]C                          0.0044
Activation energy, [E.sub.a]                     0.0013
Tensile strength                                 0.5192
300% modulus                                     0.5192
Hardness                                         0.2375
Die B tear strength                              0.3504

Table 8--summary of vulcanization
kinetics

Accelerator          TMTD         DMTU         DETU

Level (phr)           1.0          2.0          2.0
Temp.                 160          160          160

([degrees]C)          2.3          1.3          1.3
[t.sub.10]           19.2         25.0         24.0
[t.sub.90]            8.5          6.4          6.4
[DELTA]T              5.9          4.2          4.4
CRI                  89.6        120.6        123.3

Accelerator          DBTU         DPTU         DOTTU

Level (phr)           2.0          2.0          2.0
Temp.                 160          160          160

([degrees]C)          1.3          1.2          1.1
[t.sub.10]           21.6         23.0         21.2
[t.sub.90]            5.9          3.9          3.6
[DELTA]T              4.9          4.6          5.0
CRI                  110.5        91.8        140.4

Figure 15--selection of DBTU for further
optimization

             Time (minutes to     Rheometer delta torque
                reversion                 (dNm)

TMTD               1.25                   8.56

DMTU               2.00                   8.45

DETU               0.40                   8.69

DBTU               0.00                   8.48

DPTU               0.00                   4.11

DOTTU              0.00                   3.68

Note: Table made from bar graph.