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Evaluation of TBSI.

Concerns over potential health hazards from stable n-nitrosamines have led the rubber industry to seek alternatives for several commonly used secondary amine-based rubber chemicals. In many cases, substituting a non-secondary amine based chemical while maintaining key scorch, cure and physical properties has been relatively successful. For example, Helt, et al (ref. 1) and Tisler, et al (ref. 2) have shown that thiuram cure systems can be (effectively replaced by using statistical optimization techniques.

On the other hand, replacing secondary amine based sulfenamide accelerators such as MBS, DIBS or DCBS has not been an easy task. These materials offer long scorch delay and slow cure rate - properties that are required for:

* optimum steel cord to rubber adhesion,

* efficient processing of high viscosity compounds,

* minimization of reversion in thick rubber articles and many other uses (ref. 3).

Initial evaluations to replace secondary amine based sulfenamides involved the addition of CTP prevulcanization inhibitor to primary amine based sulfenamide accelerators such as TBBS and CBS (ref. 4). As expected (ref. 5), the long scorch delay was easily achieved but the slow cure rate was not obtained. It became readily apparent that primary amine based sulfenamide accelerators could not effectively duplicate all of the characteristics of the secondary amine based materials.

This article discusses an evaluation of a non secondary amine based accelerator: N-t-butyl-2-benzothiazole sulfenimide (TBSI). Although derived from a primary amine, this accelerator is characterized by the long scorch delay and slow cure rate typically associated with the secondary amine based sulfenamides.

Using a statistically designed screening experiment, TBSI is compared to MBS, TBBS, DCBS and DIBS in a bead filler compound. Contour plots are used to compare trends in key cure characteristics, physical properties and reversion resistance over a wide range of accelerator and sulfur concentrations. The accelerators were also compared
 Table 1 - rubber formulations
 Bead filler Carcass stock
SMR 5CV 50.00 -
SIR 20 - 67.00
SBR 1500 50.00 11.00
E-BR 8405 - 22.00
N330 carbon black 80.00 -
N660 carbon black - 50.00
Naphthenic oil 12.00 21.00
Zinc oxide 5.00 3.50
Stearic acid 2.00 -
TMQ 2.00 1.00
Sulfur Variable 2.88
Accelerator Variable 1.50
CTP Variable -

in a typical tire cord carcass stock in order to confirm several statistically predicted trends.


Rubber formulation

The formulations used for this evaluation are in table 1. The high viscosity and thick sections of the bead filler strip make the compound an ideal candidate for a statistical comparison of accelerators having long scorch delay and slow cure rate. The carcass stock was used to confirm reversion resistance results.

Concentrations were statistically varied in the bead filler stock between 0.8 and 2.0 phr for accelerator, 1.0 and 4.0 phr for sulfur, and 0 and 0.3 phr for CTP. The prevulcanization inhibitor was included with TBBS since this combination is considered to be a viable replacement for secondary amine based sulfenamides.

Statistical design

Multiple linear regression analysis was used to calculate constants for the following equation:

[Mathematical Expression Omitted]

where Z is the independent variable (i.e., measured property), [B.sub.0] is the y-intercept, and [B.sub.1] and [B.sub.2] are coefficients that relate the independent variables (i.e., sulfur and accelerator) to the measured property in question.

The equation for Z was then used to generate contour plots that show trends in the measured property over a wide range of accelerator and sulfur concentrations.

The contour plots were liner for all measured properties since interactions between the independent variables (eg., [x.sub.1.sup.2], [x.sub.1] times [x.sub.2]) were not included in the design. The results are used to compare trends in various key properties with respect to accelerator and sulfur concentration rather than to predict properties or optimize concentrations.

Test Methods

Mooney scorch was measured on a MV2000E Mooney viscometer at 121[degrees]C according to ASTM test method D1646-89. Curing characteristics at 150[degrees]C and 170[degrees]C were determined on an MDR2000E moving die curemeter. Following the manufacturer's recommendation, the arc was set at 0.5[degrees]. Stress strain specimens were prepared and cured to t'90 and 5 x t'90 at 150[degrees]C according to ASTM test method D3182-89 and physical properties were measured using ASTM D412-87. Standard conditions of 500mm/min. crosshead speed and 25mm gauge length on the extensometer were used.

Results and discussion

The raw data from the bead filler compound used to calculate the coefficients is available from the author. The mid-points were repeated in each evaluation in order to take into account normal (or typical) variation in laboratory procedures and measurements. With a few exceptions, reproducibility was within acceptable limits ([+-] 10%).

Table 2 contains the values for [B.sub.0], [B.sub.1] and [B.sub.2] used to generate the contour plots. Values for R-squared indicate


good correlation for most of the measured properties except reversion resistance.

Scorch safety

Figure 1 shows a comparison of the scorch safety for TBSI to DIBS, MBS, TBBS and DCBS. It is readily apparent that changes in sulfur concentration have a significantly greater effect on scorch safety compared to changes in accelerator concentration.

The slope (or shape) of the lines suggests that scorch safety is affected in the same manner to changes in TBSI concentration as it is by other accelerators. Assuming that a steeper slope indicates less effect of concentration on t5, the accelerators rank as follows:

TBBS>MBS>TBSI>DCBS=DIBS (decreasing effect of concentration)

Based on absolute values of t5, the accelerators rank as follows:

DCBS>TBSI>DIBS>MBS>>TBBS (decreasing scorch safety)

The differences in scorch safety between accelerators are greatest in the semi-E.V. range (1.2-1.5 phr accelerator and 1.0 to 1.5 phr sulfur) and become smaller as the concentrations approach a conventional system (0.8 to 1.0 phr accelerator and 2.0 to 2.5 phr sulfur).

The absolute scorch safety and the effect of concentration on t5 for TBSI lies at the highr end of the secondary based sulfenamides even though it is a primary amine based accelerator.

Cure rate

Cure rate is given as the rate at the steepest part of the cure curve. The results are shown in figure 2. As with scorch safety, the sulfur concentration exhibits a much larger effect on rate compared to accelerator concentrations.

A visual comparison of slopes (i.e., change in cure rate with accelerator concentration) indicates that TBSI exhibits similar behavior to DCBS and DIBS. MBS and TBBS show a larger effect in this respect. Based on the absolute values of cure rate, the accelerators rank as follows in the concentration range of conventional cure system:

TBBS>MBS=TBSI>DCBS>DIBS (decreasing cure rate)

The same relationship holds true in the semi-E.V. range except that in this range MBS has a faster rate compared to TBSI.

Primary amine based sulfenamides can replace their secondary amine based counterparts by using a prevulcanization inhibitor such as CTP to adjust the scorch safety. By adding 0.25 phr CTP to the bead filler compound (figure 3), the scorch safety obtained with TBBS is almost equal to that of TBSI. The contour plot at the bottom of figure 3 shows, however, that while the cure rate of TBBS is decreased by the addition of CTP, it matches the cure rate of TBSI only at high sulfur and low accelerator levels. The cure rate for TBBS is still significantly faster in the conventional and semi-EV ranges.

Thus, CTP in combination with a primary amine based sulfenamide accelerator can match a secondary amine accelerator in scorch safety but cannot match the cure rate.

Maximum torque

A comparison of maximum torque values from the moving die curemeter is shown in figure 4. Again, sulfur concentration has a greater effect than accelerator concentration.

The direction of the slopes suggest that each accelerator exhibits a similar influence on maximum torque with respect to concentration.

Maximum torque is often used to compare the activity of accelerators. At equal concentration, a material giving a higher torque value is considered more active. With this premise in mind, the accelerators compare as follows in terms of activity:

MBS[is greater than or equal to]TBSI[is greater than or equal to]TBBS [is greater than]DIBS=DCBS (decreasing activity)

The magnitude of difference between TBSI, DIBS and DCBS increases with increasing accelerator concentration.

These results suggest that less TBSI would be required to achieve similar physical properties to DIBS and DCBS. As previously demonstrated, cure properties would not be sacrificed with the smaller amount of TBSI since the accelerator concentration has a small effect

Physical properties

The response of modulus, tensile strength and elongation to changes in curative concentration again show that sulfur concentration has a larger effect compared to the accelerator concentration and that TBSI shows a similar response to the sulfenamides.

With the exceptions of DIBS, TBSI shows a relatively

Table 3 - reversion resistance carcass stock
 1 2 3 4 5
TBBS 1.5 - - - -
MBS - 1.5 - - -
DCBS - 1 1.5 - -
DIBS - - - 1.5 -
TBSI - - - - 1.5
Rheometer @ 170[degrees] C. (MDR 2000, 0.5[degrees] arc)
ML, dn-m 0.76 0.74 0.76 0.79 0.76
MH, dn-m 10.62 9.63 7.33 8.06 9.31
tS1, minutes 2.09 2.12 2.73 2.50 1.70
tS2, minutes 2.47 2.65 3.80 3.48 2.47
T'50, minutes 2.81 3.16 4.73 4.48 3.32
T'90, minutes 3.73 4.25 7.00 6.48 4.83
Peak rate, dn-m/min. 9.5 5.6 1.9 3.3
Torque @ 60',dn-m 7.55 7.20 5.79 6.36 7.83
% Retained torque 71% 75% 79% 79% 84%
 @ 60'

Table 4-reversion resistance carcass stock
 1 2 3 4 5
TBBS 1.5 - - - -
MBS - 1.5 - - -
DCBS - - 1.5 - -
DIBS - - - 1.5 -
TBSI - - - - 1.5
Unaged stress/strain
t'90 Cure @ 170[degrees]C
Shore A hardness 54 52 47 49 52
100% modulus, MPa 2.12 1.93 1.33 1.43 1.84
200% modulus, MPa 5.79 5.20 3.41 3.78 4.92
300% modulus, MPa 10.42 9.46 6.38 7.07 9.02
Ult. tensile, MPa 22.28 21.81 19.28 19.82 22.10
Ult. elongation, % 537 550 606 600 574
10 X t'90 cure @ 170[degrees]C
Shore A hardness 48 47 42 45 48
100% modulus, MPa 1.48 1.34 1.10 1.17 1.48
200% modulus, MPa 4.10 3.56 2.85 3.06 3.99
300% modulus, MPa 7.72 6.77 5.49 5.85 7.44
Ult. tensile, MPa 18.02 18.88 14.81 15.94 18.52
Ult. elongation, % 547 589 586 586 570
% retained
Delta Shore A
Delta Shore A -6 -5 -5 -4 -4
100% modulus 70% 69% 83% 80%
Ult. tensile 81% 87% 77% 80% 84%
Ult. elongation 102% 107% 97% 98% 99%

higher level of modulus development in the range of conventional cure systems. The magnitude of difference decreases as the accelerator concentration increases towards semi-EV levels.

In terms of tensile strength, the acceleration (at equal concentration) rank as follows:


It should be noted, however, that the magnitude of difference in tensile strength is not very large. AT 2.0 phr sulfure and 1.0 phr accelerator, for example, the difference between the highest and lowest tensile strength values is 3.2 MPa.

Reversion resistance

Reversion resistance was measured in two ways:

* cure properties using the moving die curemeter at 170[degrees]C, and

* retention of physical properties on ovencure (5 x t'90).

Figure 5 shows the response of retained maximum curemeter torque at 60 minutes to curative concentrations. The slope of the lines suggests that the concentration of TBSI influence reversion resistance to a higher degree than the concentration of sulfenamides. In the conventional cure, TBSI shows significantly between reversion resistance. The difference decreases, however, with increasing accelerator concentration and the reversion resistance with TBSI is equal to the sulfenamides in the semi-EV range.

Test results which show retained modulus and tensile strength after ovencure tend to confirm the curemeter results. The correlation coefficients for these contour plots are low enough, however, to cast doubt on the trends.

In order to confirm the reversion resistance results, therefore, the sulfemanide accelerators and TBSI were evaluated in a tire cord carcass stock. The results are given in Table3. The reversion resistance as measured on the curemeter indicates significant advantage for TBSI compared to the sulfenamide accelerator.

Retention of physical properties confirm this advantage for TBSI when compared to BMS and TBBS, TBSI gives similar performance of DIBS and DCBS. Table 4 also confirms the activity comparisons that were discussed earlier.


* The effects of TBSI concentration on scorch, cure, and physical properties are similar to those of sulfenamide accelerators.

* The scorch safety and cure rate of TBSI is in the same range as MBS, DIBS, and DCBS.

* TBSI showed higher activity compared to DIBS and DCBS, equal activity to MBS, and less activity than TBBS.

* TBSI shows improved reversion resistance compared to suflfenamide accelerators.

* Sulfure concentration had a much larger effect on cure and physical properties compared to accelerator level.

This evaluation shows that TBSI can be an effective replacement for secondary amine based sulfenamide accelerators such as MBS, DIBS, or DCBS.


[1] Helt, W.F., To, B.H., Paris, W.W., Rubber World, Vol. 204, No. 5, 1991.

[2] Tisler, A.T., To. B.H., Paris, W.W., "Alternate non-secondary amine compounding for butyl innertubes and improvements for long term ozone resistance," Presented at ACS Rubber Division Meeting, Oct. 1991.

[3] Luecken, J.J., Monsanto Bulletin: "Santocure TBSI".

[4] Lederer, D.A., "Replacement of secondary amine based vulcanziation systems," presented at a meeting of the New England Rubber Group, April 1991.

[5] "Santogard PVI for increased productivity in processing and curing operations," Monsanto Bulletin IC/RC-28.
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Title Annotation:N-t-butyl-2-benzothiazole sulfenimide
Author:Zaper, Mia
Publication:Rubber World
Date:Apr 1, 1992
Previous Article:Curing rate and flowing properties of silicone rubber at injection molding.
Next Article:DSM.

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