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DCBS: an accelerator for adhesion compounds and other tire applications.

In recent years concerns about the environment and toxicology have grown. Legislators all over the world are restricting and regulating more and more areas in the industry. The rubber industry is greatly affected by the legislation on N-nitrosamines in Germany. Of the sulfenamide accelerators, MBS and DIBS are suspected of forming carcinogenic N-nitrosamines. For MBS a number of alternatives within the group of sulfenamides were proposed, eg., CBS or TBBS alone or, if necessary, with the addition of a retarder. Also, mixtures of CBS and DCBS could be a viable solution. This article tries to summarize some of the properties of DCBS and gives an update on German N-nitrosamine legislation.



First stage mixing was done in an internal mixer. In a second stage, sulfur and accelerators were added on a mill.

Recipes were as follows:

NR masterbatch

TSR 5, Defo 700 100.0

N339 black 55.0

Ingralen 450 (arom. oil) 3.0

Antilux 111 (protective wax) 1.0

6PPD 2.0

TMQ 1.0

Stearic acid 2.5

ZnO 5.0

Total MB 169.5

S-SBR masterbatch

Buna SL 704 100.0

N234 black 80.0

Ingralen 450 (arom. oil) 30.0

6PPD 2.0

TMQ 1.0

Stearic acid 2.0

ZnO 5.0

Total MB 220.0

SBR masterbatch

Krylene 1500 100.0

N234 black 80.0

Ingralen 450 (arom. oil) 30.0

6PPD 2.0

TMQ 1.0

Stearic acid 2.0

ZnO 5.0

Total MB 220.0

NR adhesion compound

SMR 5, Defo 700 100.0

N326 black 50.0

Silica 10.0

6PPD 2.0

ZnO 10.0

Koresin (resin) 4.0

Cohedure RS 3.0

Total MB 179.0

For the adhesion compound,

Cohedur A 150 4.00

Rhenocure IS 60/G 5.62

Yielding total MB 188.62 were added to the compound one day after masterbatch mixing. Tests were executed according to international standards if not otherwise stated.

* Mooney scorch MV 2000 according to DIN 53 523

* Rheometer MDR 2000 ([+ or -]0.5 arc) according DIN 53 529

* Stress strain data according DIN 53 504

* Rheovulkameter PV 400 (Goettfert) (ref. 1)

* Cord pull out test according to a Bayer method (triple cords) cord: 67.5% Cu, construction 3 9 0.22

* Viscoelastic properties (Roelig) DIN 53513 (similar to ASTM D 2231).


Germany was the first country to restrict the occurrence of certain carcinogenic N-nitrosamines. In the German rubber industry, the "Chemicals Act" and the "Decree on Hazardous Chemicals" (Chemikalien Gesetz; Gefahrstoff-Verordnung) are well known and led to quite a change. The newest edition (ref. 2), with respect to N-nitrosamine compounds, was updated in the fall of 1994 and reads as follows:

Gefahrstoff-Verordnung (in der Fassung v. 19.9.1994, BGBI. IS. 2557) (ref. 2).

[sections] 15a General occupational prohibition and restriction

(1) Workers are not allowed to be exposed to the following hazardous chemicals, which are particularly carcinogenic:

* asbestos

* 2-naphthylamine and its salts


Clause (1) is not valid

* for the particularly dangerous N-nitrosamines according to (1), which are inevitable according to the technical state of art.

* for- N-nitroso-methyl-tert.-butylamine

* N-nitroso-dibenzylamine

* N-nitroso-dicyclohexylamine

* N-nitroso-ethyl-tert.-butylamine

* N-nitroso-n-butyl-tert.butylamine

* N-nitroso-diallylamine

* N-nitroso-proline

* N-nitroso-N-methyl-3-amino-pyridine

* N-nitroso-N-methyl-4-amino-pyridine

* Dinitroso pentamethylenetetramine and for those N-nitrosamines of (1) where examinations yield no indication of carcinogenic effect.

In other words this means:

* In principle, there should be no exposure to N-nitrosamines.

* There is an exemption for certain N-nitrosamines of which the dibenzyl and dicyclohexyl derivatives are of special interest. These could be called safe amines. For these N-nitrosamines, there is good evidence that they are not potentially carcinogenic. Other secondary amines, such as diisobutylamine and diisononyl, which are not yet listed, cannot be regarded as safe because detailed proof of their respective nitrosamines' non-carcinogenicity is not available today.

Other N-nitrosamines could expand the list of "safe amines" when further studies release them from suspicion of carcinogenicity.

German law tolerates N-nitrosamines as long as they are inevitable according to the technical state of the art.

The current German list of TRK values (Technical Reference Concentrations) calls for a concentration of nitrosamines in the workplace of not more than 1.0 [mu]g/[m.sup.3]. air in general, and, in critical areas, of not more than 2.5 [mu]g/[m.sup.3]. These values are seen as inevitable today. However, it is expected that these values will be lowered in the future as progress is made both in the technical state of the art and in field-validated analysis. Technical guidelines on how to comply efficiently with these limit values is published in the now revised TRGS 552: N-nitrosamines (March 1996 in BArbB1 3/1996, 4.3 (1), 1. and 2.). This excursion into N-nitrosamine legislation makes clear that DCBS is based on the "safe amine" dicyclohexylamine. This amine, as explained, is explicitly unrestricted in Germany and there is no limitation either for dicyclohexylamine nor DCBS.

Comparison of DCBS with other sulfenamides

An overview of the cure behavior and physical data of sulfenamides in three different polymers is given in tables 1-6. The long scorch safety of DCBS in all polymers is evident (figures 1 and 2). Although the molecular weight of DCBS is higher than the other sulfenamides and it is added into the compound at the same level, the modulus is only slightly lower in some cases. Additionally, it can be seen that DCBS is a suitable substitute for DIBS.


Mixtures of DCBS with CBS

An interesting approach is to mix the slowest and the fastest sulfenamides CBS and DCBS. Blends showed expected intermediate properties. If one compares the properties of a 1:1 mixture to MBS it is obvious that there is quite a good match. If not all of the long scorch is required, a smaller ratio of DCBS can be applied. In this way the general purpose accelerator CBS can be slowed down without the use of a retarder and the accompanying adverse effects.

Reversion resistance of DCBS

Reversion is often encountered when rubber chemicals are cured at high temperatures and/or for extended periods of time, resulting in reduced physical and dynamic properties. In practice, reversion is often noticed when curing large cross-section rubber articles. In this case, the outer parts are exposed to higher temperatures for longer times than the inner parts. Another instance of reversion can be encountered when parts are exposed to high service temperatures for longer periods of time. For both ways, the results are similar: The net crosslink density decreases, the number of S-atoms in the crosslink decreases and rearrangements along the polymer chain lead to a loss of properties. An indication for good reversion properties is a long t90R time in the rheometer test. This is the time when the torque declines (after reaching the maximum) to the same level as at t90. Another indicator is a good modulus retention. The modulus obtained at normal cure condition is compared to a modulus at more severe cure conditions, higher temperatures and/or longer cure time. Table 7 shows the cure properties and physical data of the sulfenamides CBS, TBBS, MBS and DCBS. In this straight forward NR compound with conventional cure, DCBS has the longest t90R-time at 150[degrees]C and 170[degrees]C and maintains the modulus at both cure conditions (like MBS). In addition, at 170[degrees]C DCBS offers the broadest plateau (t90r-t90).


Adhesion and flow properties

DCBS is always seen as the preferred accelerator for adhesion compounds. For many years DCBS has outperformed the other sulfenamides (refs. 3 and 4). Its main application is in steel cord adhesion compounds (refs. 5 and 6), and also in compounds with textile reinforcement (refs. 7 and 8). The pronounced scorch safety combined with the slow cure rate are prerequisites for good steel cord adhesion. These properties are obtained with the accelerators MBS, DIBS and DCBS, with the latter offering the longest scorch and the slowest cure rate. Since MBS and DIBS are to be phased out due to the possible formation of harmful N-nitrosamines (see above), only DCBS remains together with the recently introduced sulfenimide TBSI, which is compared with DCBS and TBBS in tables 8 and 9. From the data, the unmatched scorch safety of DCBS can be seen, which is significantly longer than TBBS and TBSI compounds. This low level of TBSI is at the borderline as the modulus is already slightly affected. All adhesion data of DCBS are significantly better than those of TBBS or TBSI. Table 9 shows flow properties of the adhesion compounds. The data were obtained from a rheovulkameter test. This test shows the "flowability" of a compound, which is important with respect to penetration to the cord construction. Since the compounds differ only by accelerator, the difference of "flowability" is only to be related to the relationship of scorch and cure rate.



An overview of the properties of DCBS in various compounds has been given. In the range of sulfenamides and sulfinimides it is the slowest accelerator. The good flow properties provided by DCBS have been demonstrated. The adhesion properties were still unmatched. Good reversion properties make DCBS also useful in other parts of a tire. DCBS is based on a safe amine, which does not release any suspected carcinogenic N-nitrosamines. German legislation has explicitly excluded dicyclohexylamines from any restrictions.


[1.] Goettfert, O. Kautt., Gummi, Kunstst. (1982). 35 (10), 849. [2.] Gefahrstoff-Verordnung, Bundesgesetzblatt I, 19.9.94, p. 2557. This information and its translation into English are supplied without any liability. Actual legislation should always be consulted. [3.] Albrecht, K.D., Kaut. Gummi, Kunstst. (1972), 25 (11), 531. [4.] Albrecht, K.D., Rubber Chem. Technol. (1973), 46 (4), 981. [5.] Buswell, A.G.; Eaton, D.C.; Tidswell, C.R., Rubber World (1975), 173 (2), 35. 6. Van Ooij, W.J.; Weening, W.E.; Murray, P.F., Rubber Chem. Technol. (1981), 54 (2), 227. [7.] Bukowski, R.A., J. Elastomers Plast. (1974), 6 (Oct.), 223. [8.] Bukowski, R.A., paper No. 47 presented at the 104th meeting of the Rubber Division, American Chemical Society, Denver, Colorado, 1973.


"DCBS: A preferred accelerator for adhesion compounds and other tire applications" is based on a paper given at the October, 1995 Rubber Division meeting. "Rubber to metal bonding" is based on a paper given at IRC `95. "Mooney stress relaxation testing for SBR processability" is based on a paper given at the May, 1996 Rubber Division meeting. "Aramid fibers and adhesion to elastomers: Application and performance" is based on a paper given at the October, 1995 Rubber Division meeting.
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Article Details
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Author:Ruetz, Lothar
Publication:Rubber World
Date:Nov 1, 1996
Previous Article:Aramid fibers and adhesion to elastomers: application and performance.
Next Article:Mooney stress testing for SBR processability.

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