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Tetraisobutylthiuram monosulfide (TiBTM) - a unique retarder/kicker in one molecule.

Recently, we reported (ref. (1)) the use of N,N,N',N'-tetraisobutylthiuram disulfide (TiBTD) as a low nitrosamine generating kicker for sulfenamides. When used as a kicker for benzothiazole sulfenamides, this thiuram had better scorch safety than, but cured at the same rate as, TMTD in similar recipes. In that publication we also briefly described a new material, N,N,N',N'-tetraisobutylthiuram monosulfide (TiBTM), which acted as a retarder as well as a kicker for 2-benzothiazole sulfenamides in SBR/BR.

Generally, a kicker or secondary accelerator increases the cure rate of a primary accelerator but at the same time shortens the scorch time (ref. (2)), sometimes an undesired side effect. Conversely, a cure retarder will increase the scorch delay but can slow the cure rate of the primary accelerator (ref. (3)). A single molecule possessing the characteristics of increasing scorch safety while increasing the cure rate when used with a specific primary accelerator is certainly unique and deserves further attention for potential exploitation.

In our first report (ref. (1)), we provided data demonstrating the above effect of TiBTM with TBBS, a primary accelerator, in SBR/BR, at varying levels of the two accelerators. It was not established, however, that the effect could be extended to other primary accelerators and rubbers, at what loadings the effect is most pronounced, and how effective these retarding/kicking characteristics are relative to commercial retarders and kickers. Thus, in this article, we explore these various ramifications with TiBTM.

[ILLUSTRATION OMITTED]

[Part 1 of 3]

Table 1 - comparison of TiBTM vs. CTP used with TBBS in an SBR/BR
stock

 1 2 3 4

SBR/BR masterbatch 214.00 214.00 214.00 214.00
TBBS 1.35 1.35 1.35 1.35
CTP .10 .20 .30
TiBTM
Sulfur 2.0 2.0 2.0 2.0

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L]
 N[??]m 2.7 2.7 2.7 2.7
Scorch time
 [(t.sub.2]), min 7.0 7.6 8.1 8.7
Cure time
 [(t.sub.9.sub.0]),
 min 13.4 14.6 15.6 16.6
Cure time
 [(t.sub.9.sub.0]),
 min 15.2 16.4 17.3 18.4
Cure rate index 13.2 12.0 11.0 10.2

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 17.8 19.9 22.5 24.5
Cure time
 [(t.sub.3.sub.5]),
 min 21.2 23.9 26.6 29.0
Cure index 3.4 4.0 4.1 4.5

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 17.5 18.1 16.1 17.9
Elongation, % 494 526 462 506
300% modulus, MPa 8.9 8.2 8.8 8.8

[Part 2 of 3]

Table 1 - comparison of TiBTM vs. CTP used with TBBS in an SBR/BR
stock

 5 6 7 8 9

SBR/BR masterbatch 214.00 214.00 214.00 214.00 214.00
TBBS 1.35 1.35 1.35 1.22 1.08
CTP
TiBTM .10 .20 .50 .10 .20
Sulfur 2.0 2.0 2.0 2.01 2.02

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L]
 N[??]m 2.8 2.8 3.0 2.7 2.7
Scorch time
 [(t.sub.2]), min 7.4 7.0 6.9 7.5 7.4
Cure time
 [(t.sub.9.sub.0]),
 min 12.9 12.3 10.7 13.4 12.7
Cure time
 [(t.sub.9.sub.0]),
 min 14.4 13.8 11.9 15.1 14.3
Cure rate index 15.1 16.4 21.9 14.4 16.0

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 19.8 20.1 19.5 19.9 21.1
Cure time
 [(t.sub.3.sub.5]),
 min 22.7 23.0 22.1 23.0 23.9
Cure index 2.9 2.9 2.6 3.1 2.8

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 12.9 12.3 14.4 16.4 16.8
Elongation, % 377 358 361 482 489
300% modulus, MPa 9.3 8.4 11.4 8.6 8.5

[Part 3 of 3]

Table 1 - comparison of TiBTM vs. CTP used with TBBS in an SBR/BR
stock

 10 11 12

SBR/BR masterbatch 214.00 214.00 214.00
TBBS .95 .81 .54
CTP
TiBTM .30 .40 .60
Sulfur 2.03 2.04 2.06

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L]
 N[??]m 2.7 2.7 2.7
Scorch time
 [(t.sub.2]), min 7.5 7.3 7.0
Cure time
 [(t.sub.9.sub.0]),
 min 12.8 12.4 12.1
Cure time
 [(t.sub.9.sub.0]),
 min 14.4 14.0 13.8
Cure rate index 15.6 16.6 17.1

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 20.0 19.9 21.1
Cure time
 [(t.sub.3.sub.5]),
 min 22.6 22.5 24.1
Cure index 2.6 2.6 3.0

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 16.2 15.9 15.0
Elongation, % 472 468 446
300% modulus, MPa 8.7 8.4 8.6





Experimental

The SBR/BR masterbatch, weight 214.0, used in this study contains OE-SBR, Ameripol 1712, 55; SBR, Ameripol 1500, 25; Taktene 1203, 35; zinc oxide, 3; stearic acid, 2; N234 carbon black, 70; Sundex 8125, 20; antioxidant, Agerite Resin D, 2; antiozonant, Antozite 67F, 2. These ingredients were combined in a BR internal mixer, and a masterbatch was obtained by combining a number of batches on a mill. The curatives were added on the mill. The NR masterbatch, weight 159.0, contains NR, 100.0; N234 carbon black, 45.0; Sundex Oil 790, 6.0; zinc oxide, 5.0; stearic acid, 2; and antioxidant, Agerite Resin D, 1.0.

The Monsanto oscillating-disc rheometer, Model R100, was run using a 1[degrees] arc. The commercial accelerators and retarders were used without further purification.

[Part 1 of 2]

Table 2 - comparison of TiBTM vs. CTP used with CBS in an SBR/BR
stock

 1 2 3 4 5

SBR/BR masterbatch 214.00 214.00 214.00 214.00 214.00
CBS 1.50 1.50 1.50 1.50 1.50
CTP .10 .20
TiBTM .10 .20
Sulfur 2.0 2.0 2.0 2.0 2.0

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 2.7 2.8 2.8 2.8 3.0
Scorch time
 [(t.sub.2]),
 min 6.2 6.3 7.1 6.5 6.9
Cure time
 [(t.sub.9.sub.0]),
 min 12.5 13.4 13.9 11.4 11.4
Cure time
 [(t.sub.9.sub.5]),
 min 14.2 15.3 15.5 12.9 12.9
Cure rate index 13.8 14.1 12.3 17.6 18.7

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 18.5 19.4 21.7 17.9 18.2
Cure time
 [(t.sub.3.sub.5]),
 min 21.1 22.3 24.7 20.0 20.4
Cure index 2.6 2.9 3.0 2.1 2.2

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 13.0 17.0 17.4 17.0 15.9
Elongation, % 386 470 497 468 424
300% modulus, MPa 1,308 1,317 1,277 1,331 1,434

[Part 2 of 2]

Table 2 - comparison of TiBTM vs. CTP used with CBS in an SBR/BR
stock

 6 7 8 9

SBR/BR masterbatch 214.00 214.00 214.00 214.00
CBS 1.35 1.20 1.05 0.90
CTP
TiBTM .10 .20 .30 .40
Sulfur 2.01 2.02 2.03 2.04

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 2.8 2.8 2.8 2.8
Scorch time
 [(t.sub.2]),
 min 6.9 7.0 7.45 7.0
Cure time
 [(t.sub.9.sub.0]),
 min 12.4 12.3 13.0 12.5
Cure time
 [(t.sub.9.sub.5]),
 min 14.2 13.9 14.9 14.5
Cure rate index 15.6 16.7 15.7 15.6

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 18.6 19.5 21.2 19.9
Cure time
 [(t.sub.3.sub.5]),
 min 20.8 21.8 23.8 22.4
Cure index 2.2 2.3 2.6 2.5

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 14.6 15.2 16.5 14.2
Elongation, % 421 435 458 407
300% modulus, MPa 1,317 1,316 1,324 1,336


[Part 1 of 2]

Table 3 - comparison of TiBTM vs. CTP/TMTM used with TBBS in an
SBR/BR stock

 1 2 3 4 5 6

SBR/BR masterbatch 214.0 214.0 214.0 214.0 214.0 214.0
TBBS 1.30 1.30 1.00 1.00 1.00 1.00
CTP 0.10 0.10 0.10
TMTD 0.15 0.15
TMTM 0.13 0.13
TiBTM
Sulfur 1.80 1.80 1.80 1.80 1.82 1.82

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 2.7 2.7 2.6 2.6 2.6 2.5
Scorch time
 [(t.sub.2]),
 min 7.5 7.8 6.2 6.7 7.4 7.8
Cure time
 [(t.sub.9.sub.0]),
 min 14.9 15.4 11.8 13.1 13.0 14.5
Cure time
 [(t.sub.9.sub.5]),
 min 17.0 17.2 13.8 15.0 14.9 16.5
Cure rate index 11.3 10.9 15.8 13.9 15.4 12.6

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 20.0 21.4 17.3 19.8 21.1 23.2
Cure time
 [(t.sub.3.sub.5]),
 min 23.8 25.9 19.8 22.5 23.8 26.4
Cure index 3.8 4.5 2.5 2.7 2.7 3.2

Stress-strain, opt
cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength,
MPa 16.8 13.9 12.3 14.5 15.4 15.9
Elongation, % 503 423 413 477 483 545
300% modulus, MPa 8.2 8.5 7.6 7.3 7.8 6.6

[Part 2 of 2]

Table 3 - comparison of TiBTM vs. CTP/TMTM used with TBBS in an
SBR/BR stock

 7 8

SBR/BR masterbatch 214.0 214.0
TBBS 1.00 1.17
CTP
TMTD
TMTM
TiBTM 0.23 0.11
Sulfur 1.82 1.81

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 2.7 2.7
Scorch time
 [(t.sub.2]),
 min 8.0 7.8
Cure time
 [(t.sub.9.sub.0]),
 min 14.4 14.5
Cure time
 [(t.sub.9.sub.5]),
 min 16.4 16.4
Cure rate index 13.1 12.4

Mooney scorch,
138[degrees]C
Scorch time
 [(t.sub.5]),
 min 23.3 22.5
Cure time
 [(t.sub.3.sub.5]),
 min 26.7 25.8
Cure index 3.3 3.3

Stress-strain, opt
cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength,
MPa 11.8 16.3
Elongation, % 371 500
300% modulus, MPa 8.7 8.0





Results and discussion

TiBTM was evaluated as a retarder/kicker for both N-t-butyl-2-benzothiazole sulfenamide (TBBS) and N-cyclo-hexyl-2-benzothiazole sulfenamide (CBS) in SBR/BR and natural rubbers. Additionally, TiBTM was compared to the retarder, N-cyclohexythio-phthalimide (CTP), and the kicker, N,N,N',N',-tetramethylthiuram monosulfide (TMTM) in these systems for any advantages/disadvantages.

Performance in SBR/BR rubber compounds

It was demonstrated earlier (ref. (1)) that increasing the loading of TiBTM above 0.2 phr did not increase the scorch delay or cure rate beyond that which was observed at 0.2 phr. The data in table 1 show that increasing the loading of TiBTM beyond 0.1 phr while keeping the loading of TBBS constant (recipes 5-7) did not improve the scorch time any further beyond the 2 min. increase compared to the control of TBBS alone (recipe 1). The cure rate, however, continued to increase because of the increasing total amount of accelerators present. Unfortunately, the stress-strain properties dropped off relative to those for the TBBS control. By replacing equivalent amounts of TBBS by TiBTM, e.g., removal of 10% of TBBS and replacing it by .10 parts of TiBTM, an improvement in the physical properties of the vulcanizates resulted (recipes 8-12). While all recipes showed an increase in the scorch time of about two minutes, the cure indexes increased by three units compared to the TBBS control. This approach results in some cost savings in that less primary accelerator is used.
Table 4 - comparison of TiBTM vs. CTP/TMTM used with CBS in an
SBR/BR stock

 1 2 3 4 5

SBR/BR masterbatch 214.0 214.0 214.0 214.0 214.0
CBS 1.44 1.44 1.15 1.15 1.30
CTP 0.10 0.10
TMTM 0.13 0.13
TiBTM 0.11
Sulfur 1.80 1.80 1.82 1.82 1.81

Monsanto rheometer,
160[degrees]C
[[M.sub.H]-M.sub.L], N[??]m 2.7 2.6 2.6 2.6 2.6
Scorch time [(t.sub.2]),
 min 6.2 7.1 6.7 7.0 7.0
Cure time [(t.sub.9.sub.0]),
 min 11.7 13.6 11.6 11.7 12.7
Cure time [(t.sub.9.sub.5]),
 min 13.4 15.2 13.2 13.2 14.3
Cure rate index 15.6 13.0 18.0 18.9 15.5

Mooney scorch, 138[degrees]C
Scorch time [(t.sub.5]), min 16.2 20.8 16.7 19.5 21.3
Cure time [(t.sub.3.sub.5]),
min 18.9 23.4 20.2 21.4 23.7
Cure index 2.7 2.6 3.5 1.9 2.4

Stress-strain, opt cure
[(t.sub.9.sub.5]) at
160[degrees]C
Tensile strength, MPa 15.1 16.4 13.0 13.7 15.4
Elongation, % 488 504 435 443 483
300% modulus, MPa 7.6 7.9 7.5 7.8 7.8


[Part 1 of 2]

Table 5 - comparison of TiBTM vs. CTP used with TBBS in a natural
rubber stock

 1 2 3 4 5 6

NR masterbatch 159.0 159.0 159.0 159.0 159.0 159.0
TBBS 1.00 1.00 1.00 1.00 1.00 0.90
CTP 0.1 0.20
TiBTM 0.10 0.20 0.10
Sulfur 2.00 2.00 2.00 2.00 2.00 2.01

Monsanto rheometer,
150[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 3.6 3.5 3.5 3.6 3.6 3.5
Scorch time
 [(t.sub.2]),
 min 7.5 8.9 9.4 8.1 8.3 7.9
Cure time
 [(t.sub.9.sub.0]),
 min 14.3 16.0 16.6 12.6 12.2 12.3
Cure time
 [(t.sub.9.sub.5]),
 min 15.8 17.4 18.0 13.6 12.9 13.3
Cure rate index 13.3 11.9 11.8 19.3 21.7 19.6

Mooney scorch,
130[degrees]C
Scorch time
 [(t.sub.5]),
 min 19.5 23.7 27.8 20.6 22.8 21.7
Cure time
 [(t.sub.3.sub.5]),
 min 22.2 27.1 32.3 22.4 24.7 23.3
Cure index 2.7 3.4 4.5 1.8 1.8 1.6

Stress-strain, opt
cure
[(t.sub.9.sub.5]) at
150[degrees]C
Tensile strength,
MPa 29.7 29.1 29.9 30.1 27.9 29.8
Elongation, % 560 551 569 559 511 561
300% modulus, MPa 13.1 13.0 13.0 13.5 13.7 13.0

[Part 2 of 2]

Table 5 - comparison of TiBTM vs. CTP used with TBBS in a natural
rubber stock

 7 8 9

NR masterbatch 159.0 159.0 159.0
TBBS 0.80 0.70 0.60
CTP
TiBTM 0.20 0.30 0.40
Sulfur 2.02 2.03 2.04

Monsanto rheometer,
150[degrees]C
[[M.sub.H]-M.sub.L],
 N[??]m 3.5 3.5 3.5
Scorch time
 [(t.sub.2]),
 min 7.8 7.8 8.1
Cure time
 [(t.sub.9.sub.0]),
 min 11.5 10.9 11.7
Cure time
 [(t.sub.9.sub.5]),
 min 12.2 11.5 12.3
Cure rate index 22.7 26.3 23.3

Mooney scorch,
130[degrees]C
Scorch time
 [(t.sub.5]),
 min 21.1 20.3 21.6
Cure time
 [(t.sub.3.sub.5]),
 min 22.7 21.7 23.1
Cure index 1.6 1.3 1.5

Stress-strain, opt
cure
[(t.sub.9.sub.5]) at
150[degrees]C
Tensile strength,
MPa 29.0 28.4 27.7
Elongation, % 535 530 514
300% modulus, MPa 13.6 13.4 13.5





The effective retarder CTP was also evaluated against TiBTM for comparison. One can see (recipes 2-4) that increasing the loadings of CTP continue concomitantly to increase the scorch time while decreasing the cure rate index. However, when CTP and TiBTM are used at low levels (recipes 2, 8), both provide similar scorch improvements while TiBTM increases the cure rate and CTP decreases it compared to that of TBBS alone. Thus, one can replace CTP when used at low levels by TiBTM to achieve the same scorch safety with a resulting improvement in cure rate, and lower the amount of primary accelerator at the same time.

It is generally recognized that CBS itself exhibits a shorter scorch time and a faster cure rate relative to TBBS (ref. (4)). Thus, we evaluated TiBTM as a retarder/kicker for CBS in experiments similar to those discussed above. These data are shown in table 2. One can see that a higher loading of TiBTM is needed to improve the scorch time relative to the control (recipes 6-9 vs. 1) while the cure rate index increases immediately. However, there is still a leveling out effect at 0.2 phr. CTP, on the other hand, shows an immediate effect on scorch time but to a lesser extent than with TBBS (recipes 2 and 3 vs. 1). Again, the cure rate tends to slow down when CTP is added. Thus, similarities exist in the interactions of TBBS and CBS with TiBTM and CTP but the magnitude of the changes are less pronounced with CBS.

Since TiBTM behaves as both a retarder and a kicker, it would be interesting to compare its processing properties to those of a CTP/TMTM recipe, since CTP is a retarder and TMTM is a kicker. The data from this study with TBBS as the primary accelerator are shown in table 3. For a true comparison, equivalent mole levels of TMTM and TiBTM should be used. Since the molecular weight is higher for TiBTM, 0.23 parts of TiBTM are equivalent to 0.13 parts of TMTM. One can see that TiBTM at .23 parts and even at half that level (recipes 7 and 8, respectively), provide a scorch delay and cure increase equivalent to that of a CTP/TMTM combination (recipe 6) over that of the control (recipe 1). As a comparison, a CTP/TMTD combination provides a cure rate increase but no scorch delay. The kicking effect of the TMTD overwhelms the scorch delay of the CTP.

[Part 1 of 3]

Table 6 - comparison of TiBTM vs. CTP used with CBS in a natural
rubber stock

 1 2 3 4

NR masterbatch 159.0 159.0 159.0 159.0
CBS 1.11 1.11 1.11 1.11
CTP .10 .20
TiBTM .10
Sulfur 2.0 2.0 2.0 2.0

Monsanto rheometer, 150[degrees]C
[[M.sub.H]-M.sub.L], N[??]m 3.5 3.4 3.4 3.5
Scorch time [(t.sub.2]), min 7.7 9.0 9.9 7.9
Cure time [(t.sub.9.sub.0]), min 13.2 14.9 16.2 11.5
Cure time [(t.sub.9.sub.5]), min 14.7 16.4 17.5 12.4
Cure rate index 15.9 14.3 13.0 23.3

Mooney scorch, 130[degrees]C
Scorch time [(t.sub.5]), min 22.2 26.1 31.2 22.4
Cure time [(t.sub.3.sub.5]), min 24.4 28.8 34.2 23.8
Cure index 2.2 2.7 3.0 1.4

Stress-strain, opt cure
[(t.sub.9.sub.5]) at 150[degrees]C
Tensile strength, MPa 29.0 30.6 30.1 29.5
Elongation, % 528 576 569 555
300% modulus, MPa 13.7 13.2 13.0 13.3

[Part 2 of 3]

Table 6 - comparison of TiBTM vs. CTP used with CBS in a natural
rubber stock

 5 6 7 8

NR masterbatch 159.0 159.0 159.0 159.0
CBS 1.11 1.00 0.89 0.78
CTP
TiBTM .20 .10 .20 .30
Sulfur 2.0 2.01 2.02 2.03

Monsanto rheometer, 150[degrees]C
[[M.sub.H]-M.sub.L], N[??]m 3.6 3.5 3.5 3.5
Scorch time [(t.sub.2]), min 8.0 7.9 8.0 8.1
Cure time [(t.sub.9.sub.0]), min 11.2 11.7 11.4 11.2
Cure time [(t.sub.9.sub.5]), min 12.0 12.6 12.0 11.9
Cure rate index 26.3 22.7 25.0 26.3

Mooney scorch, 130[degrees]C
Scorch time [(t.sub.5]), min 23.4 23.2 23.2 24.8
Cure time [(t.sub.3.sub.5]), min 24.8 24.6 24.8 27.8
Cure index 1.4 1.4 1.6 3.0

Stress-strain, opt cure
[(t.sub.9.sub.5]) at 150[degrees]C
Tensile strength, MPa 28.0 29.0 28.2 27.5
Elongation, % 535 570 550 543
300% modulus, MPa 12.8 13.0 12.3 12.4

[Part 3 of 3]

Table 6 - comparison of TiBTM vs. CTP used with CBS in a natural
rubber stock

 9

NR masterbatch 159.0
CBS 0.67
CTP
TiBTM .40
Sulfur 2.04

Monsanto rheometer, 150[degrees]C
[[M.sub.H]-M.sub.L], N[??]m 3.4
Scorch time [(t.sub.2]), min 8.0
Cure time [(t.sub.9.sub.0]), min 11.0
Cure time [(t.sub.9.sub.5]), min 11.5
Cure rate index 27.8

Mooney scorch, 130[degrees]C
Scorch time [(t.sub.5]), min 24.7
Cure time [(t.sub.3.sub.5]), min 25.9
Cure index 1.2

Stress-strain, opt cure
[(t.sub.9.sub.5]) at 150[degrees]C
Tensile strength, MPa 26.7
Elongation, % 527
300% modulus, MPa 12.1





A similar study was done with CBS as the primary accelerator and the results are shown in table 4. In this case, the TiBTM was used at half the molar loading compared to TMTM. The observed increase in scorch delay is larger for the TiBTM but with a smaller increase in cure rate relative to the CTP/TMTM system (recipe 5 vs. 4).
Table 7 - summary of changes in cure rates and scorch times for
TiBTM vs. CTP with TBBS and CBS in SBR/BR and NR stock(1)

 Mooney scorch Cure rate index
 time ([delta] min.) (% change)
Sulfenamide TiBTM CTP TiBTM CTP

 SBR/BR SBR/BR SBR/BR SBR/BR
TBBS 2.1 2.1 +9 -9
CBS 1.0 0.9 +13 +2

 NR NR NR NR
TBBS 2.2 4.2 +47 -10
CBS 1.0 3.9 +30 -10

(1) Loadings of TiBTM = CTP = 0.1 parts





Performance in natural rubber compounds

A similar series of experiments to those run in SBR/BR were run with natural rubber. With TBBS (table 5) or CBS (table 6) as the primary accelerator, TiBTM exhibits a smaller increase in the scorch delay but a substantial increase in cure rate compared to CTP recipes. In other words, TiBTM is much better as an accelerator than as a retarder in natural rubber although both effects exist. Again, increasing the loadings of TiBTM does not significantly improve the scorch delay relative to the lower loadings.

When a TiBTM recipe is compared to a CTP/TMTM recipe with TBBS or CBS as the primary accelerator, similar increases in scorch times and cure rates are observed, even when less than molar equivalent levels of TiBTM are used.

Conclusions

Table 7 shows a summary of the scorch time differences and percent changes in the cure rate indexes for TiBTM and CTP with the two primary accelerators in the two rubbers. In SBR/BR rubber, one can see that the changes in scorch time are of the same magnitude for TiBTM and CTP when used with the same primary accelerator. A similar comparison of cure rates shows, contrarily, a significant increase with TiBTM and typically no improvement or generally a loss with CTP. In natural rubber, CTP shows at least double the improvement in scorch time over that of TiBTM. The cure rate index change shows a 10% loss for CTP and a very large increase for TiBTM, in all examples, even when TiBTM is used at less than molar equivalent levels.

An explanation for the above behavior is not yet clear. Model reactions with monitoring of formed species may provide some insight into the observations. This work is in progress.

References

(1) . R.W. Layer and D.W. Chasar, Rubber Chem Technol., 67, 299 (1994).

(2) . M.A. Fath, Rubber World, 209 (1), 22 (1993).

(3) . M.A. Fath, Rubber World, 209 (3), 17 (1993).

(4) . M.A. Fath, Rubber World, 208 (5), 15 (1993).
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Author:Chasar, Dwight W.
Publication:Rubber World
Date:Aug 1, 1996
Words:4401
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