TBzTD and CBBS - alternative accelerators for reducing nitrosamine generation.

Nitrosamines are a hot topic of discussion in worldwide rubber circles, with the regulations currently in place in Germany (TRGS 552) and other regulations possibly coming to other parts of Europe and probably the U.S. in the not too distant future. Much has been written about the possible hazards of nitrosamines, mostly NDMA (nitrosodimethylamine) and NMOR (nitrosomorpholine), chemicals that produce these nitrosamines, and chemicals to eliminate or minimize the generation of these materials (refs.1-3).

This article introduces two chemicals, TBzTD (N,N,N'N'- tetrabenzylthiuramdisulfide) and CBBS (N-cyclohexylbis-(2-benzothiazolesulfen)amide that have the potential to replace several accelerators that generate these nitrosamines. The article shows TBzTD replacing thiuram accelerators such as TMTD and TETD, and CBBS, a delayed action sulfenamide, is compared to MBS, DCBS and other similar accelerators.

Regulations

At the present time, the German government requires that the nitrosamine levels in the calendering, extrusion, molding, salt bath and UHF curing areas, as well as the quality control and warehousing areas, be under 2.5 ug/[m.sup.3]. The other areas of rubber factories have the limit of 1 ug/[m.sup.3]. In cases where it is technically infeasible by available control technology to keep the nitrosamines under the 2.5 ug/[m.sup.3] limit, the companies will be able to apply for an exemption. In the worst case, the law states a company would have to cease production until the limits are met.

In 1987, the West German safety and health authorities showed that 73% of the work sites were under the 2.5 ug/[m.sup.3] limit. The principal nitrosamines found were:

* N-nitrosodimethylamine;

* N-nitrosodiethylamine;

* N-nitrosodibutylamine;

* N-nitrosomorpholine;

* N-nitrosopiperidine;

* N-nitrosomethylphenylamine;

* N-nitrosoethylphenylamine;

It is currently thought that the European Community will harmonize their air standards in 1992.

At the present time, there are no regulations in the United States regarding the amount of nitrosamines allowed in the air at rubber plants. It is anticipated that OSHA may follow the policies of the CEN-European Committee of Standardization which could go into effect in 1992.

Nitrosamine generation

Nitrosamines are generated when a secondary amine reacts with a nitrosating

agent, typically an oxide of nitrogen. Primary amines form an unstable diazonium hydroxide that decomposes to an alcohol and nitrogen, while tertiary amines do not readily form nitrosamines. Many rubber accelerators generate secondary amines during mixing and curing operations, and thus can generate nitrosamines. MBS (2-morpholinothiobenzothiazole), a chemical that forms the secondary amine morpholine, generates the nitrosamine NMOR (N-nitrosomorpholine), while CBBS generates the primary amine cyclohexylamine, which does not yield a nitrosamine.

TBzTD can form the secondary amine dibenzylamine (DBA), but because of the low volatility and reactivity of this amine compared to other secondary amines, the related nitrosamine NDBzA (nitrosodibenzylamine) has not been observed during processing and curing operations. Also, NDBzA has not been found to be carcinogenic (ref. 3) and is not a regulated nitrosamine per TRGS 552 (ref. 4), hence TBzTD and its related zincdithiocarbamate (ZBzDC) are suggested as replacements for accelerators that do generate hazardous nitrosamines.

Table 1 lists the amines that are generated from many of the common rubber accelerators and their boiling point. Boiling points are useful as an indication of the volatility of the amine formed, and hence a relative measure of the propensity of the amine to react with nitrogen oxides in the air to form a nitrosamine. [TABULAR DATA 1 OMITTED]

Nitrosamines are also found in some polymers (as a result of shortstops used to control polymerization) and many non-rubber items, including processed food (including bacon, hot dogs, etc.), cigarette smoke and beer.

Experimental

Compounds for the studies were mixed as a masterbatch with curatives and accelerators added in a second mixing pass. The masterbatches were mixed in an internal mixer, to dump temperatures of 135 [degrees] C to 140 [degrees] C and cycle times of 2-3 minutes. The curatives were added in a second internal mixer pass at 80 [degrees] C with a three minute mix cycle.

Curemeters were run per ASTM D2084. Mooney scorches were run per ASTM D1646. Physical properties were tested per ASTM D412. Tests for nitrosamines were done per procedures described elsewhere (ref. 5).

Typical properties of TBzTD and CBBS are shown in table 2.

Table 2 - typical physical properties
                            TBzTD      CBBS
Chemical name               (1)        (2)
Specific gravity            1.31       1.42
Molecular weight            544.8      429.6
Melting point, degrees C.   128 min.   131 min.
(1) N,N,[N.sup.1],[N.sup.1]-tetrabenzyl thiuramdisulfide
(2) N-cyclohexyl-bis-(2-benzothiazolesulfen) amide

Tire compounds

Tire compounds normally require a delayed action accelerator, typically a sulfenamide, for their unique processing and curing requirements. MBS has been one of the main accelerators used in this application because of its excellent scorch characteristics. In the tire model compound shown in table 3, two typical tire cure systems, one using MBS alone, the other using MBS/TMTD, are compared to CBBS and CBBS/TBzTD systems. A curing system using TBBS/CTP (CTP is the prevulcanization inhibitor N-cyclohexythiophthalimide) is also shown for comparison.

Table 3 - tire model compound
NR              30.0
BR              30.0
SBR             40.0
N299 black      70.0
Aromatic oil    35.0
Zinc oxide       3.5
Stearic acid     1.0
Antiozonant-1    2.0
Wax              2.5
Sulfur           2.0
                 A      B      C     D      E
MBS              1.6    1.15   -     -      -
TMTD             -      0.2    -     -      -
CBBS             -      -      1.6   1.15   -
TBzTD            -      -      -     0.2    -
TBBS             -      -      -     -      1.5
CTP              -      -      -     -      0.2
Nitrosamines, ppb (extraction, cured [15.sup.1]/160 [degrees] C)
NMOR             1200   335    -     -      -
NDMA             -      134    -     -      -

Nitrosamine testing showed NMOR and NDMA were formed in the compounds that contained MBS and TMTD. CBBS, TBzTD, TBBS and CTP did not show the generation of detectable nitrosamines. Data on scorch and cure rate characteristics (figure 1), showed CBBS to compare favorably with the MBS cure system, while the CBBS/TBzTD system had slightly better scorch. CBBS with a slightly higher level of TBzTD would be needed to' more closely match the cure rate and scorch of the MBS/TMTD system. The TBBS/CTP system had scorch and cure time intermediate between the MBS and MBS/TMTD systems. Cured properties showed all compounds having similar modulus, tensile and elongation.

Natural rubber

An evaluation of TBzTD in NR vs. TMTD as shown in table 4. Curemeter data of the direct replacement show TBzTD to have a slower rate of cure than TMTD, with a lower MH, but its reversion is much less than that of TMTD.

Table 4 - TBzTD vs. TMTD in NR
SMR 5 CV                       100
N660                           50
Naphthenic oil                 12
Zinc oxide                     3
Stearic acid                   1
Sulfur                         2.5
MBTS                           0.5
TMTD                           0.5      -       -
TBzTD                          -        0.5     0.5
Urea (surface treated)         -        -       1.5
Total                          169.5    169.5   171
Rheometer at 160 [degrees] C, 1 deg. arc, 1.7 Hz
MH, dNm                        32.1     24.5    32.5
tc50, min.                     3.2      3.8     3.1
tc90, min.                     3.6      4.7     3.5
Reversion in [10.sup.1], dNm   2.5      0.8     0.9
Press cure [5.sup.1]/160 [degrees] C, 1 deg. arc, 1.t Hz
300% modulus, MPa              9.3      6.3     9.8
Tensile strength, MPa          19.2     18.6    19.1
Elongation, %                  490      550     470

It was found that a very effective activator for the TBzTD is a surface-treated urea (STU). The addition of this activator at 1.5 phr gave similar cure characteristics to the control compound and still maintained its minimal reversion. An experimental design was evaluated to determine the best levels and ratios of TBzTD to STU, with the results plotted in figures 2-4. These figures indicate that, to match the cure characteristics and physical properties of the control compound containing 0.5 phr TMTD, 0.25-0.50 phr of TBzTD and 1.0 - 1.5 phr of STU would be required.

TBzTD was also found to have benefit as a less-reverting sulfur donor when compared to DTDM (dithiomorpholine) in NR. Our tests show curemeter curves at 175 [degrees] C which indicate the minimal reversion seen with TBzTD vs. DTDM. The scorch time and cure time, however, are much shorter with TBzTD than with DTDM. The addition of the retarder CTP at 0.6 phr along with the 0.6 phr of TBzTD gives a compound that matches up well vs. the compound containing 0.6 phr of DTDM as seen in table 5.

Table 5 - TBzTD vs. DTDM in NR
SMR 5 CV                                100
N330                                    58
Naphthenic oil                          6
Zinc oxide                              7
Stearic acid                            2
Antiozonant-1                           2
TBBS                                    0.6
Sulfur                                  1.5
                                        A      B      C
DTDM                                    0.6    -      -
TBzTD                                   -      0.6    0.6
CTP                                     -      -      0.6
Press cure [15.sup.1]/160 [degrees] C
300% modulus, MPa                       15.6   16.7   15.0
Tensile strength, MPa                   24.5   24.9   22.7
Elongation, %                           445    435    430
Press cure [15.sup.1]/160 [degrees] C
300% modulus, MPa                       15.9   17.0   14.9
Tensile strength, MPa                   24.0   23.7   24.6
Elongation, %                           430    435    430

CBBS was evaluated vs. MBS in a standard NR recipe. The results show very comparable scorch and cure characteristics as well as cured properties.

In a NR breaker compound, CBBS was compared to MBS and DCBS (N,N-cyclohexyl-2-benzothiazylsulfenamide). DCBS is typically the sulfenamide accelerator used in this application because its slow cure rate gives good adhesion to brass coated steel wire. The data, shown in table 6, indicated that CBBS and MBS had shorter scorch and cure times compared to DCBS. All three accelerators gave comparable unaged adhesion and cured physical properties, with CBBS and MBS having comparable, slightly better aged tensile and adhesion.

Table 6 - CBBS vs. MBS and DCBS in NR
           breaker compound
SMR 5 CV                                       100
N351 black                                     55
Aromatic oil                                   7
Zinc oxide                                     5
Stearic acid                                   1.5
RA (resorcinol-asetaldehyde)                   2
Antiozonant-1                                  1
Antiozonant-1                                  1.5
TCBQ                                           1
                                        A      B      C
MBS                                     0.7    -      -
CBBS                                    -      0.7    -
DCBS                                    -      -      0.7
80% Insoluble sulfur                    5.0    5.0    5.0
MS/135 [degrees] C
Scorch time, min.                       9.6    10.5   15.9
Curemeter at 177 [degrees] C
tc90 min.                               4.0    3.9    4.5
Press cure, [10.sup.1]/177 [degrees] C
300% modulus, MPa                       13.9   13.2   13.7
Tensile strength, MPa                   23.4   21.9   22.3
Elongation, %                           460    450    430
Oven aged, 3 days/100 [degrees] C
Tensile strength, MPa                   12.5   12.3   10.6
Elongation, %                           130    130    120
Wire adhesion at 121 [degrees] C
(brass coated steel), N
Unaged                                  370    380    400
Oven aged 3 days/100 [degrees] C        430    430    370
Steam aged 3 days/100 [degrees] C       410    400    320

SBR

In an SBR v-belt type compound (table 7), the nitrosamine-generating cure system of MBDS/TMTD was replaced by CBS/TBzTD with comparable cure rate and physical properties as a result. The CBS/TBzTD compound had better scorch characteristics and a slightly slower cure rate compared to the control system. !!1 BEGIN TABLE Table 7 - TBzTD/TBBS vs. MBDS/TMTD in SBR

SBR 1502                                85
SMR 5CV                                 15
N774 black                              125
Aromatic oil                            10
Zinc oxide                              5
Stearic acid                            2
Antioxidant-2                           1
Antioxidant-3                           1
                                        A      B
MBDS                                    0.7    -
TMTD                                    0.2    -
TBBS                                    -      1
TBzTD                                   -      0.3
Sulfur                                  2      2

MS/135 [degrees] C

Scorch time, min. 10.6 19.3

Curemeter at 150 [degrees] C

tc50, min.                              8.2    11.5
tc90, min.                              19.1   17.4

Press cure, [25.sup.1]/150 [degrees] C

100% modulus, MPa                       9.9    19.3
Tensile strength, MPa                   18.3   16.6
Elongation, %                           240    210

EPDM

A standard EPDM sheeting compound containing TMTD and TETD was evaluated for nitrosamines, with NDMA and NDEA found. To eliminate the TMTD and TETD, a cure system using TBzTD, MBTS and STU was developed. The resulting compound gave comparable scorch and cure characteristics, as well as comparable original and aged physical properties.

EPDM is widely used in automotive hose compounds, but these compounds generally have high levels of nitrosamine-generating accelerators to get good set characteristics and fast cures. A typical hose liner compound and a suggested replacement is shown in table 8. The replacement compound matches up favorably to the control in all respects, including compression set.

Table 8 - EPDM hose compound
EPDM-3                                  75
EPDM-2                                  25
N650 black                              175
Paraffinic oil                          75
Zinc oxide                              5
Stearic acid                            1
                                        A        B
ZBDC                                    1.50     -
DTDM                                    1.00     -
TMTD                                    0.80     -
MBTS                                    4.00     4.00
TBBS                                    -        0.75
ZBsDC                                   -        1.50
TBzTD                                   -        0.40
Urea (surface treated)                  -        1.20
Sulfur                                  1.25     1.00
Total                                   364.55   364.85
MS/135 [degrees] C., scorch, min.       7.7      8.0
Curemeter at 165 [degrees] C
tc90, min.                              7.6      7.7
Press cure, [15.sup.1]/165 [degrees] C
Tensile strength, MPa                   13.9     12.8
Elongation, %                           280      350
Oven aged, 70 hr/150 [degrees] C
Tensile strength, MPa                   15.0     15.0
Elongation, %                           179      180
Compression set, %
22/70 HR/100 [degrees] C                59/73    52/69

NBR

A model compound for evaluations of TBzTD in NBR is shown in table 9. The direct replacement of TMTD with TBzTD gave lower modulus and higher compression set values, probably because of the higher molecular weight of the TBzTD vs. the TMTD. The addition of ZBzDC improved the modulus and compression set dramatically. Recent data show that increasing the sulfur slightly will improve both properties to be a better match for the TMTD control.

Table 9 - NBR model compound
NBR-1                                          100
N990 black                                     100
Stearic acid                                   1
Zinc oxide                                     5
CBS                                            2
Spider sulfur                                  0.25
                                        A      B      C
TMTD                                    3      -      -
TBzTD                                   -      3      3
ZDBzC                                   -      -      1
Press cure [10.sup.1]/70 [degrees] C
Hardness, Shore A                       71     71     68
100% modulus, MPa                       3.0    2.0    2.2
300% modulus, MPa                       12.8   7.6    9.2
Tensile strength, MPa                   13.4   11.9   12.5
Elongation, %                           340    530    450
Compression set, %
70 hr/125 [degrees] C                   31     44     36
Curemeter at 170 [degrees] C
ts2, min                                1.6    3.0    2.0
tc50, min                               3.3    5.1    3.8
tc90., min                              4.5    7.0    6.7
MH, dNm                                 56.5   39.5   44.8

Conclusions

Concerns about exposure to nitrosamines in the workplace are increasing on a worldwide basis. Regulations that are now in place in Germany are expected to expand to other countries, including the United States, in the coming years. Although levels of nitrosamines can in some cases be lowered somewhat by nitrosamine scavengers and plant environmental controls, the best way to eliminate the volatile nitrosamines is to eliminate the chemicals that cause them from the rubber compounds. TBzTD and CBBS are two new chemicals that have minimized the generation of nitrosamines associated with many of the common rubber accelerators in use today.

Substitutions of volatile nitrosamine-generating accelerators in model compounds of various polymers and applications showed TBzTD to be a good replacement for accelerators such as TMTD and TETD, although some curative adjustments are usually necessary. CBBS matches up well with MBS, another delayed action accelerator.

Other accelerators that can be used are TBBS, which seems to compare favorably with MBDS, CBS, MBTS, MBT, ZBzDC, DPG and others.

The coming years will prove to be both exciting and challenging in the accelerator arena, by developing new, environmentally responsible cure systems and by taking full advantage of those accelerators that are available to us today.

References

[1.] Ehrend, H. and Leo, T., "Crosslinking systems that avoid nitrosamines," Rubber and Plastics News, 10/30/89, p. 16-18. [2.] Davies, K.M., Lloyd, D.G. and Orband, A., "The impact of N-nitrosamine regulations on sulfenamide selection, Kautschuk + Gummi, 2/89, p. 120-123. [3.] Gupta, H., International Polymer Science and Technology, V13, #11, 1986, p. 6-8. [4.] Technische regel fur gefahrstoffe N-nitrosamine, TRGS 552, Bundesarbeitsblatt Sept. 1988. [5.] Hajdasz, D.J., Analysis for N-nitrosamines in elastomeric compounds, Paper #42, Rubber Division, ACS meeting, Detroit, 10/91.