TBzTD: a secondary accelerator for stable crosslink systems in tire applications.

Tetrabenzylthiuram disulfide (TBzTD)/sulfenamide accelerator system offer a unique combination of characteristics which include excellent reversion resistance, efficient vulcanization with sufficient scorch delay and increased nitrosamine safety.

Blends of TBzTD (0.25 to 0.5 phr) and N-t-butyl-2-benzothiazole-2-sulfenamide (TBBS 1.0 to 1.25 phr) evaluated in NR tire compounds are discussed here to illustrate the advantages of TBzTD/sulfenamide systems over sulfur donor/sulfenamide and lower molecular weight thiuram/sulfenamide accelerator systems. Abbreviations used throughout are compiled in table 1.

Table 1 - abbreviations

CBS      N-cyclohexyl-2-benzothiazole sulfenamid
DTDM     4,4-dithiodimorpholine
MBDS     2-(morpholinothio)benzothiazole disulfide
MBS      2-(morpholinothio)benzothiazole
MBT      2-mercaptobenzothiazole
NR       Natural rubber
OTOS     N-oxydiethylenethiocarbamul-N'-oxydiethylene
         sulfenamide
SRB      Styrene butadiene rubber
TBBS     N-t-butyl-2-benzothiazole-2-sulfenamide
TBTD     Tetrabutylthiuram disulfide
TBZTD    Tetrabenzylthiuram disulfide
TMTD     Tetramethylthiuram disulfide
TMTM     Tetramethylthiuram monosulfide

The TBzTD/sulfenamide blends take advantage of lower sulfur rank crosslinks provided by the thiuram and the scorch safety imparted by the sulfenamide. Low levels (0.25 to 0.50 phr) are effective due to the improved efficiency provided by the combination of a thiuram/sulfenamide system. The benefits of a thiuram/sulfenamide system have been previously exemplified in the literature (ref. 1). However, a majority of the studies focused on accelerators that produce nitrosamines considered hazardous by many occupational, environmental and government groups. The high molecular weight of TBzTD is responsible for its success in these systems by improving the scorch safety and the nitrosamine safety of the system.

Background

Tire compounders continuously search for stable vulcanization systems in order to increase the life of their product without negatively affecting the processing characteristics of the uncured rubber. Due to the dynamic service life of a tire, the ideal vulcanization system provides thermal stability, low hysterisis and high flex fatigue resistance without negatively affecting the processing requirements of the tire compound. To achieve these properties from a conventional cure system, the system must impart low sulfur ranked crosslinks, minimize the formation of sulfur bridges and reduce the amount of polysulfide crosslinks. This system, commonly known as a semi-efficient cure system, is a compromise between the thermally stable crosslinks of an efficient cure system (EV) and the dynamic fatigue resistance of conventional systems (ref 2).

Semi-efficient vulcanization systems

Semi-efficient vulcanization systems generally consist of higher levels of a primary accelerator, a sulfur donor and lower levels of sulfur. Commonly employed primary accelerators in the tire industry are 2-Mercaptobenzothiazole (MBT) and delayed action sulfenamides (MBS, TBBS or CBS) in the part level range from 1.0 to 2.0 phr. These accelerators, when used with conventional levels of sulfur (1.5 to 2.5 phr), provide excellent scorch safety, good flex fatigue resistance, but poor thermal stability. The flex fatigue resistance is imparted by the formation of lower bond strength, flexible, less heat resistant sulfur crosslinks. The poor thermal stability of the systems can be compensated by the use of sulfur donors (0.25 to 1.6 phr) and lower levels of sulfur (0.5 to 1.0). The use of sulfur donors and the low free sulfur (0.5 to 1.0 phr) impart better thermal stability to the vulcanizate. The crosslinks of the vulcanizate usually consist of lower sulfur rank crosslinks that offer a compromise of the thermal stability of monosulfide crosslinks and the flexibility of polysulfide crosslinks (ref. 3).

Sulfur donors, such as 4,4-Dithiodimorphohne (DTDM), thiurams and dithiocarbamyl sulfenamides liberate sulfur at elevated temperatures. The latter two types of sulfur donors, thiurams and dithiocarbamyl sulfenamides, also are very effective accelerators. Both have been proven to cure natural rubber without the presence of sulfur (ref. 4). The liberated sulfur results in the formation of thermally stable lower sulfur rank crosslinks (ref. 5).

Thiurams

For example, highly efficient thiuram sulfurless cure systems produce thermally stable monosulfidic crosslinks (ref 6). The monosulfidic crosslinks are formed by the reaction of zinc oxide and the dithiocarbamate groups generated by the decomposition of the thiuram accelerator. The result is a vulcanized compound with approximately 85:15 blend of monosulfide: disulfide crosslinks (ref. 7). Thiurams fall into a class of chemicals called ultra accelerator because of their short scorch safety and rapid rates of vulcanization. Therefore, they are often used as secondary accelerators or "kickers" with benzothiazole sulfenamides or MBT. The sulfenamides and MBT retard the onset of vulcanization without significantly affecting the rate of cure (ref. 8).

Thiurams and sulfenamides/MBT

The improved scorch safety, rapid cure rates, and higher crosslink densities provided by combinations of thiurams and sulfenamides/MBT are a valuable tool for the rubber compounder. Compared to any of the accelerators used independently, the blend of TMTD and sulfenamides/MBT offer the following benefits (ref 9):

* TMTD/sulfenamide (or MBT) vs. thiuram - better scorch safety; improved flex fatigue; improved cut growth resistance.

* TMTD/sulfenamide (or MBT) vs. sulfenamide (or MBT) - improved sulfur efficiency and higher state of cure; lower sulfur rank crosslinks; improved thermal stability.

However, this combination, depending on the thiuram employed, can be too scorchy in NR and SBR compounds that experience a high degree of heat history during processing, such as extruded tire components.

Thiocarbamyl sulfenamides

Thiocarbamyl sulfenamides offer a solution to the thiuram scorch issue. For example, N-Oxydiethylenethiocarbamyl-N'-oxydiethylene sulfenamide (OTOS) exhibits the scorch of a sulfenamide and the efficient cure of a thiuram or dithiocarbamate. Thiocarbamyl sulfenamides offer improved thermal stability, lower heat build-up, and lower permanent set when compared to benzothiazole systems. The improved properties are imparted by the dithiocarbamate group's efficient utilization of sulfur, which like the thiurams improves the properties by promoting lower sulfur rank crosslinks. The thiocarbamyl moiety is approximately 30% more efficient than the benzothiazole counterpart. However, unaged tear and flex fatigue are inferior to benzothiazole sulfenamides, which use sulfur less efficiently and promote more flexible polysulfide crosslinks (ref. 10).

Why TBzTD/sulfenamides?

The above mentioned systems offer unique properties. However, they all require a compromise. Most sulfur donors produce hazardous nitrosamines and are not as efficient as thiuram or thiocarbamyl sulfenamides. Most thiurams also produce hazardous nitrosamines and are often too scorchy even when used with delayed action accelerators to be effectively utilized in the tire industry. Thiocarbamyl sulfenamides present the best compromise in properties, efficiency and processing safety, but the commercial alternatives also produce hazardous nitrosamines. We will propose and discuss systems that offer the least compromise for certain compounding applications. TBzTD is a high molecular weight thiuram accelerator/sulfur donor with improved scorch safety and increased nitrosamine safety (compared to other nitrosamine generators). Combinations of TBzTD and TBBS not only exhibit very similar properties to the systems previously mentioned, but also improve the nitrosamine safety. The combination provides an excellent alternative to sulfenamide/sulfur donor systems.

Nitrosamines and accelerators

Accelerators and sulfur donors, such as TMTD, DTDM, OTOS and MBS, are based on secondary amines that can form hazardous nitrosamines. During vulcanization, these accelerators liberate secondary amines that react with a nitrosating agent to form nitrosamines. Primary amines are also a common component of accelerators, but form an unstable reaction product that readily decomposes. Table 2 lists common accelerators and the corresponding amine potentially generated in the vulcanization process. TBzTD, based on high molecular weight, low volatility, secondary amine, does not generate nitrosamines as readily as lower molecular weight secondary amines.

Table 2

                 Amine evolved                      Boiling
Ingredients      Primay            Secondary          point
                                                    [degrees] C
TMTD, TMTM                         Dimethylamine        7.4
TBBS             tert-butylamine                         44
TETD                               Dimethylamine         56
MBDS, MBS                          Morpholine           128
DTDM, OTOS       Cyclohexylamine
CBS                                Dibutylamine          135
[TB.sub.Z]TD                       Dibenzylamine         300(d)

d = decomposes before it boils

TBzTD forms dibenzyl amine (DBA) during vulcanization. The DBA has an extremely low volatility, a boiling point of 300[degrees]C (decomposes before it boils), and low reactivity with nitrosating agents (ref. 11).

Experimental

Compounds used were mixed in a laboratory internal mixer (approximately 1.3 liter) with a two pass mixing procedure. The first pass consisted of a 3 to 5 minute cycle time with a dump temperature of 135 -140[degrees]C. The second pass, or curative pass, was performed in an internal mixer or a laboratory mill reaching temperatures of 80-100[degrees]C. Rheometers were run per ASTM D2084. Mooney scorches were run per ASTM D1646. Physical properties were tested per ASTM D412, and flex to fatigue tests were run per ASTM D4482. A dynamic spectrophotometer was used to measure viscoelastic properties. All heat agings were completed in a hot air oven.

Results and discussion

TBzTD and TMTD sulfurless cure system in NR compound Sulfurless cure system consisting of TBzTD, TMTD or thiuram/sulfenamides, illustrated the sulfur donating capabilities of the thiurams and also the improved efficiencies of the blend of thiurams and sulfenamides (table 3). Equivalent molar levels of accelerator were used in the experiment, but in cases 2 and 4 half the amount of sulfur donating accelerator, TBzTD or TMTD, was replaced with the sulfenamide, TBBS. Although the sulfur available for crosslinking was reduced by 50%, the delta torques, modulus, and tensile for stocks 2 and 4 were approximately 70% of stocks containing equivalent molar amounts of solely TBzTD or TMTD. The sulfurless thiuram cure systems provided thermally stable, lower sulfur ranked crosslinks, as illustrated by little or no reversion in the rheometer curves. The TBzTD data confirmed the known efficiency of thiuram/sulfenamide systems.

[TABULAR DATA 3 OMITTED]

TBzTD vs. TMTD as secondary accelerators in NR TBzTD (high molecular weight) and TMTD (low molecular weight) were compared as secondary accelerators with TBBS in a natural rubber compound (tables 4-6). The evaluation focused on scorch safety, cure times, and efficiency (equal or higher modulus with the same molar level of accelerators/sulfur). The control vulcanization system was TBBS (1.25 phr/5.3 [x10.sup.-3] mol) and sulfur (1.75 phr/5.5 x [10.sup.-2] mol). The experimental system consisted of similar total molar accelerator levels (TBBS at 1.0 phr/4.2 x [10.sup.-3] mol thiuram at 0.25 phr or 0.50 phr) and equivalent or lower levels of sulfur.

[TABULAR DATAS 4-6 OMITTED]

The scorch safety of the TBzTD/TBBS system (compounds 7 and 8) was longer or only slightly shorter (<0.2 minutes) than the control, while the TMTD "kicker" (compounds 9-11) significantly reduced the safety by 1.2 - 5.6 minutes. Advantages in reversion resistance were also noted with the TBzTD system as illustrated in figures 1 and 2. The cure rates followed a similar trend: TMTD systems being faster than the TBzTD. The efficiency of thiuram/sulfenamide systems was illustrated in the improved reversion resistance and the higher modulus of the compounds. The thiuram system at lower and equal accelerator levels offered similar or higher modulus, respectively, than the control system (tables 5 and 6). As the efficiency of the accelerator system and the amount of lower rank sulfur crosslinks increases, one would expect a decrease in the unaged tear strength. In the case of the above systems, the tear strength and the flex properties were not affected until the thiuram level was greater than 1.0 x [10.sup.-3] mol (compound 11). Both TBzTD and TMTD systems provided better retention of 300% modulus than the control after aging 14 days at 70[degrees]C in a hot air oven. Agings conducted at 100[degrees]C were too severe to illustrate differences in the systems.

The results suggested that at lower thiuram levels (<1.0 x [10.sup.-3] mol), a combination of lower and higher sulfur rank crosslinks develop, which as a result provided a valuable blend of thermal stability and flex fatigue resistance. Since, the sulfur levels were held relatively constant and improvements in reversion and modulus were noted, it was concluded that the thiuram/sulfenamide systems offered efficient utilization of the sulfur resulting in lower rank sulfur crosslinks. The use of TBzTD improved the scorch safety and the hazardous nitrosamine issues related to TMTD systems, while retaining the physical properties of a lower molecular weight thiuram.

TBzTD vs. DTDM as sulfur donors in NR truck tread A comparison of TBzTD (compound 14) versus DTDM (compound 13) as sulfur donors in combination with TBBS was completed in a NR truck tire tread (tables 7 and 8). These semi-efficient accelerator/sulfur donor systems imparted thermal stability and moderate unaged flex fatigue resistance required for this application. A sulfenamide system [TBBS (1.0 phr)/sulfur (2.0 phr)] was also included as a benchmark. The benefits of TBzTD over DTDM were TBzTD acted as both an accelerator and a sulfur donor DTDM acts only as a sulfur donor), TBzTD promoted higher modulus compounds at equal phr levels, and TBzTD improves nitrosamine safety. Since TBzTD possesses accelerator activity, the scorch retarder, N-cyclohexylthiophthalimide (CTP - 0.2 phr), was added to provide additional scorch safety.

Table 7 - TBzTD vs. DTDM in NR truck tread
compound(*) - processing data

                                 12      13      14
TBBS                           1.00    1.00    1.00
CTP                            0.20      --    0.20
DTDM                             --    0.50      --
TBzTD                            --      --    0.50
Tire sulfur                    2.00    1.50    1.50

Mooney scorch (small rotor @ 132 [degrees] C)
3 point rise (minutes          17.1    15.2    15.2

Rheometer @ 160 [degrees] C
TS2 (minutes)                   3.8     3.5     3.0
T50 (minutes)                   5.5     5.1     3.8
T90 (minutes)                   7.6     6.7     5.0
MH-ML (dNm)                    30.4    31.4    31.8

Rheometer @ 177 [degrees] C
TS2 (minutes)                   1.8     1.7     1.5
T50 (minutes)                   2.6     2.5     2.0
T90 (minutes)                   3.4     3.0     2.3
MH-ML (dNm)                    27.7    28.7    28.5

(*) #37248 - SIR 10 (100.0 phr), N-220 (42.0), silica (12.0),
aromatic oil (6.5), ZnO (5.0), stearic acid (2.5) 6PPD
(2.5), tackifier (3.0), was (2.0)
Table 8 - TBzTD vs. DTDM in NR truck tread
compound(*) - physical properties

                                 12      13      14
TBBS                           1.00    1.00    1.00
CTP                            0.20      --    0.20
DTDM                             --    0.50      --
TBzTD                            --    1.50    1.50

Physical properties - cured 20' @ 160 [degrees] C
Unaged
Tensile (MPa)                  23.9    25.1    25.8
300% modulus (MPa)              9.3     9.9    12.2
% elongation at break           580     570     550
Shore A hardness                 62      63      62
Die C tear (KN/m)              89.3    78.8    94.5

Aged - 3 days @ 100 [degrees] C - % change
Tensile (%)                     -26     -41     -26
300% modulus (%)                 49      42      20
% elongation at break (%)       -36     -37     -29
Shore A (point change)            7       4       4
Die C tear (%)                  -59     -53     -67

Physical properties - cured 10' @ 177 [degrees] C
Unaged
Tensile (MPa)                  20.9    23.3    24.3
300% modulus (MPa)              7.8     8.4    10.7
% elongation at break           590     590     560
Shore A hardness                 60      60      59
Die C tear (KN/m)              73.5    68.3    47.3

Aged - 3 days @ 100 [degrees] C - % change
Tensile (%)                      -7     -14     -18
300% modulus (%)                 97      82      41
% elongation at break (%)       -37     -36     -30
Shore A (point change)           10      10      11
Die C tear (%)                  -43     -44     -19

Viscoelastic properties - 10 Hz, 1%
strain (cured 20' @ 160 [degrees] C)
G' x [10.sup.7]                2.78    2.91    3.01
G" x [10.sup.7]                0.41    0.42    0.38
Tan delta @ 75 [degrees] C    0.148   0.145   0.127

(*) #37248 - SIR 10 (100.0 phr), N-220 (42.0),silica (12),
aromatic oil (6.5), ZnO (5.0). stearic acid (2.5) 6PPD (2.5),
tackifier (3.0), was (2.0)

Processing data, unaged and aged physical properties, and viscoelastic properties illustrated the advantages of the TBzTD system (tables 7 and 8). The scorch safety of both the TBzTD (ref. 14) and the DTDM system were similar, but slightly shorter than the TBBS alone. As expected with the thiuram system (TBzTD), the cure rate was significantly faster. The TBzTD system provided a much tighter crosslink network as illustrated by the higher modulus and lower tangent delta a 75[degrees]C attained with similar levels of sulfur (compared to the DTDM system and the TBBS alone). Also, the unaged tear was not affected by the use of the TBzTD at the 0.5 phr level. As a result of the tighter crosslinks, the retention of physical properties after heat aging of the TBzTD system was superior to the other accelerator packages. The TBzTD/TBBS system improved the thermal stability of the compound, provided similar scorch safety, higher modulus and lower hysterisis than the DTDM/TBBS system.

Why TBzTD promotes advantageous properties at low levels The mechanism that results in the formation of lower sulfur rank crosslinks has been extensively studied. Layer et al. previously illustrated the benefits of mixing a benzothiazole sulfenamide with TMTD or OTBS. These very efficient combinations formed lower sulfur rank crosslinks that resulted in thermally stable and higher modulus compounds. The reaction of zinc oxide and the thiuram generates sulfur for crosslinking, resulting in lower sulfur rank vulcanization. Thiocarbamyl sulfenamides produce similar physical properties and are believed to react similarly to TMTD because of the dithiocarbamate groups liberated by the decomposition of the sulfenamide (ref. 12).

For example, OTBS (w/o sulfur) will cure natural rubber and achieve similar modulus and rheometer torque to TMTD sulfurless systems when the equivalent amount of dithiocarbamate groups are used (1 mole TMTD = 2 moles OTBS). The addition of benzothiazole sulfenamide improves the effectiveness of the dithiocarbamate sulfur donating group. Without the benzothiazole sulfenamide, a portion of the dithiocarbamate groups must act as accelerators and are unable to donate sulfur. Therefore, benzothiazole sulfenamide allows for more dithiocarbamate groups to act as sulfur donors. Layer claims that four dithiocarbamyl groups are required to form one monosulfidic crosslink. One molecule of thiuram is required to form the monosulfidic crosslink, while the other molecule is used to generate sites for crosslinks in the rubber by removing hydrogens (ref. 13).

The proposed mechanism for TBzTD/TBBS accelerator systems was developed from the literature and performance data (figure 3). The performance data and work completed by Layer illustrated the improved efficiency caused by the substitution of a portion of TBBS with similar or lower molar equivalents of thiuram accelerator. The overall improvement in properties offered by these systems is believed to be completed by a combination of vulcanization pathways. The mechanism in figure 3 depicts the speculated combinations. TBzTD can react with zinc oxide to form a polysulfide that will form lower sulfur rank crosslinks or react with zinc oxide and the benzothiazole moiety of the TBBS. The latter would theoretically double the efficiency of the TBzTD molecule by allowing more dithiocarbamate groups to act as sulfur donors. The similar tear strength and flex fatigue of sulfenamide systems versus lower thiuram/sulfenamide blends (<1.0 x [10.sup.-3] mol of thiuram) implies the formation of higher sulfur ranked crosslinks. This formation of higher sulfur rank crosslinks is explained by the third pathway, conventional sulfenamide vulcanization.

Conclusions

TBzTD (0.25 to 0.50 phr)/TBBS (1.0 to 1.25 phr) systems provide a powerful tool in applications requiring scorch safety, efficient cures and nitrosamine safety. Although advantages of lower molecular weight thiuram/sulfenamide systems are common knowledge, these systems require a compromise in scorch safety and the formation of hazardous nitrosamines. The high molecular weight of TBzTD and the amine used to fabricate the molecule result in accelerator properties that lower molecular weight thiurams cannot duplicate.

The addition of low levels (<0.5 phr) of TBzTD to sulfenamide cure systems improves the efficiency of the cure system, while not significantly reducing flex properties. TBzTD, like most thiurams, acts as both a sulfur donor and an accelerator. TBzTD reacts with zinc oxide to liberate sulfur that forms lower ranked, thermally stable crosslinks. In the combination with sulfenamides, the efficiency of the thiuram is greatly improved. This allows for lower levels of thiuram and in turn better scorch safety. TBzTD performs like lower molecular weight thiuram and other sulfur donors, but without the decreased scorch safety and the more hazardous nitrosamines. TBzTD/sulfenamide systems can be extremely effective in tire components that require thermal stability, lower hysterisis and moderate flex fatigue resistance. Truck tire tread, base tread and bead filler are applications for this system.

[Figures 1 to 3 ILLUSTRATION OMITTED]

References

[1.] Layer, R. W., "A Study of thiocarbamyl sulfenamide/zinc oxide and TMTD/zinc oxide sulfurless cure system," Rubber Chemistry and Technology 36, 1993, p. 513. [2.] Fath, M.A "Cure system design," Rubber World, Feb. 1994, p. 18. [3.] Ibid. p. 18. [4.] Layer, Rubber Chemistry and Technology 36, 1993, p. 513. [5.] Hofmann, Werner, Rubber Technology Handbook, Hanser Publishers, New York, 1989, p. 223. [6.] Ibid. p. 227. [7.] Layer, Rubber Chemistry and Technology 36, 1993, p 513. [8.] Hofmann, p. 247. [9.] Moore, K. C, "OTOS/MBT derivative vulcanization system," Elastomerics, June 1978, p. 36. [10.] Layer, R.W., "Curing SBR with thiocarbamyl sulfenamide accelerators," Rubber Chemistry and Technology 59, 1986, p. 278. [11.] Thomson M.A and TL Jablonowski, "Nitrosamines: international regulations," presented to The Akron Rubber Group, October 24, 1991, p. 3. [12.] Layer, Rubber Chemistry and Technology 36, 1993, p. 513. [13.] Layer, Rubber Chemistry and Technology 36, 1993, p. 523-525.