A new safe thiuram, TBzTD.

A new safe thiuram, TBzTD

The standards set by the European Inventory of Existing Commercial Chemical Substances (EINECS), combined with national chemical control acts, with all their merits also represent an enormous barrier against innovation which might be necessary for some reason.

For example, in West Germany the complementary regulations comprised in the "Gefahrstoff-Verordnung" (GStV), dated 1986, very precisely organized the conditions of handling of industrial chemicals, also quantifying the maximum permissible concentrations of given chemicals in the workplace atmosphere (TRK values). A very comprehensive review on the impact of these regulations on product research and development has been issued (ref. 1).

So industry felt to be restricted on what they were producing for years and only few new developments came up.

When in Western Germany the technical rules for hazardous material (TRGS 552, N-nitrosamines) came into force late in 1988 (ref. 2), it was obvious that this regulation of trace contaminants with proven carcinogenic potential in animal tests meant the beginning of the end of present vulcanization technology - or the beginning of highest efforts for new developments of better chemicals for the near future.

Assessment of nitrosamine formation

The starting point was the intensive search for the presence of nitrosamines and their precursors in many rubber chemicals including elastomers, black and white fillers, additives and of course vulcanization accelerators (ref. 3). Then we considered the possible mechanism of nitrosamine formation (ref. 4) which most likely is not as simple as presented in figure 1.

It is worthwhile to state that not only the formation but also the decomposition of nitrosamines may play an important role when one would try to calculate nitrosamine concentrations from known amine- and/or known [N.sub.x][O.sub.y] concentrations (ref. 3).

Preformed nitrosamines

We could find preformed nitrosamines in may rubber chemicals (ref. 3), figure 2.

Nitrosamines from vulcanization

Amines and nitrosating agents, [N.sub.x][O.sub.y], may be present in/ on elastomers, fillers and other compounding ingredients. During vulcanization these contribute to the formation of additional nitrosamines, possibly to levels some magnitude above those already existing in the uncured compound (figure 2).

Nitrosamines during storage of vulcanizates

Finally, we could define a third quantity of nitrosamines which were formed close to the surfaces of vulcanizates during storage, assembling or service (refs. 3, 5) (figure 3).

Method for isolation of volatile nitrosamines

A method for isolation of volatile nitrosamines from known vulcanizate surfaces could be developed and published (ref. 5). Thus, the contribution to the nitrosamine problem due to specific compound formulations can be assessed (figure 4).

Carcinogenicity from nitrosamines

It is known from literature (refs. 6-8) that it is most likely not the nitrosamines themselves but their metabolites from enzymatic hydrolyzation in organisms which might finally cause the diseases. Akzo has established, from quantum chemical calculations, a rule for judging the charge distribution on carbenium ions - products of the metabolism of nitrosamines (figure 5). This rule allows carbonium ions of different structure to be distinguished between those that could create tumors in organisms and those that cannot (ref. 9).

Theoretical assessments of new molecules

Some of these carbenium ions (or their diazoprecursors) obviously do not alkylate DNA (initiation of a tumor), if the electrical charge distribution would not favor SN 1- or SN 2- reactions in organisms.

We would like to leave this field to more specialized publications (ref. 10), but the pragmatically found rule supports research for new products in a very effective way: preliminary calculations on new chemicals and their metabolites indicate in a very early stage whether or not a replacing new product would be hazardous in this regard.

A first nitrosamine for which we would like to get an approval to be "innocent" is nitroso dibenzylamine, NDBzA (figure 6) and the relevant product in the class of thiurams on the basis of dibenzylamine is our development product, TBzTD (figure 7).

Without doubt, the greatest need for replacing chemicals is in the group of thiurams which are used s potent secondary accelerators in many applications if not as primaries in EV systems with good heat aging characteristics. The concern for sulfenamides should be much inferior having an extended product range within the existing chemicals without nitrosamine concern; nevertheless the greatest move so far is in the field of those sulphenamines (refs. 1, 11, 12).

Product development

Based on these findings, Akzo has developed the first safe thiuram that may be able to replace one or more of the currently used thiurams in the rubber industry: tetrabenzyl thiuram disulphide, TBzTD (figure 7). Some recent findings on product properties include:

* TBzTD only contains small quantities of the related nitrosamine NDBzA, and further, this nitrosamine is relatively non-volatile.

* TBzTD does not readily form additional NDBzA under normal vulcanization conditions nor during storage of its vulcanizates, in absence of nitrosating agents.

* NDBzA responds favorably to our quantum chemical calculations: it is not carcinogenic (refs. 6, 9).

* In more conventional toxicity tests, TBzTD has been judged as non-hazardous (ref. 13).

Application of TBzTD

Since the molecular weight of TBzTD is high (544) relative to the most common thiuram TMTD (240), higher parts would be expected to be needed to achieve the same level of active functional groups and of available sulfur for vulcanization. This is true speaking in terms of achievable compound moduli and rheological data (ref. 14).

On a molecular basis TBzTD should be applied 2.3 times more than TMTD, which is not acceptable.

Therefore, we evaluated the performance of the new thiuram in a 1:1 ratio per weight in comparison to existing thiurams. To our surprise it worked well when adding an additional small amount of free sulfur in EV - systems. In compounds with fairly high sulfur loadings, corrections in this regard are not necessary.

Black filled NR compounds

Tables 2 and 3 show the properties of TBzTD versus TMTD in black-filled NR-compounds and EV-cured. Table 1 shows the formulation

Table : Table 1 - TBzTD in black-filled NR compounds - formulation

Ingredients (phr)

Natural rubber (SMR 5)       100.0
Zinc oxide RS                  5.0
Stearic acid                   2.0
Carbon black N330             50.0
IPPD                           1.0
Process oil                    5.0
CBS                            0.6
Sulfur                   See below
Thiuram accelerators     See below
Compound no.                5144/1   5144/4   5144/5
DP-TBzTD                       2.5      2.5        -
TMTD                             -        -      2.5
Sulfur                         0.1      0.4      0.1

Table : Table 2 - TBzTD in black-filled NR compounds - test

                                results
                            TBzTD   TBzTD+S     TMTD
Compound no.               5144/1    5144/4   5144/5
Mooney scorch, min             31        23       11
[t.sub.90,] min              25.0      17.2     14.3
Tensile strength, MPa        24.2      28.8     28.3
Elongation at break, %        640       570      560
Modulus 100%, MPa             1.3       2.2      2.1
Hardness, [degrees] Sh A       57        63       62

Compression set

24 h/100 [degrees] C, % 52 43 40

Table : Table 3 - TBzTD in black-filled NR compounds - test

                          results after aging
                                         TBzTD   TBzTD+S     TMTD
Compound no.                            5144/1    5144/4   5144/5

After hot air aging, 3 days/125 [degrees] C

Tensile strength, MPa                      6.3      8.9      10.6
Elongation at break, %                     400      310       340
Modulus 100%, MPa                          1.2      2.0       2.1
Hardness, [degrees] Sh A                    50       60        61

Vulcanization was done in steam heated presses at 143 [degrees] C, individual [t.sub.90] times rounded up to the next full minute. Results are shown in table 2.

A little drawback for TBzTD is a slightly longer [t.sub.90] time, which by application of another 0.1 phr sulfur would be met, however. After hot air aging the same observation is valid, that a little bit more sulfur would have led to fully equivalent results (table 3).

This is surprising at first look, but if we follow the proposed mechanism of thiuram acceleration with initial sulfur solubilization and formation of polysulphides of the initial thiurams (ref. 15), then we easily understand the excellent scorch behavior of TBzTD and also its sulfur dependence in reaching equivalent physical levels in rubber compounds vice versa TMTD (and other thiurams).

Some recent applicational patents (ref. 16) describe the non-blooming properties of TBzTD, as well as its potential to exchange other thiurams in given formulations without major adaption. This and the fact that TBzTD does only give incentive to formation of very limited amounts of NDBzA, which for itself is not carcinogenic and which does not easily volatilize and hence will not come under the suspicion of the German TRG 552, was the major incentive for its development.

Black filled NBR compounds

At last some results from evaluation of TBzTD in blackloaded NBR compounds are demonstrated in tables 4-6. These are compounds where the thiurams take over the action of powerful secondary accelerators. It will be seen that the enhancing disadvantage of the big TBzTD molecule shows up again, for we did not compensate with additional small amounts of sulfur - the key issue with TBzTD, at least in some applications.

Table : Table 4 - TBzTD in black-filled NBR compounds - formulation

Ingredients (phr)

Nitrile rubber (28% ACN)   100.0
Zinc oxide RS              5.0
Stearic acid               1.0
Carbon black N550          60.0
Process oil                10.0
Wax                        1.0
ZMB                        2.0
ODPA                       1.5
CBS                        2.0
Sulfur                     0.5
Thiuram accelerators       See below
Compound no.               5324/1      5324/2
DP-TBzTD                   2.5         -
TMTD                       -           2.5

Table : Table 5 - TBzTD in black-filled NR compounds - test

                             results
                            TBzTD     TMTD
Compound no.               5324/1   5324/2
Mooney scorch, min            5.6      5.1
[t.sub.90], min               3.6      3.4
Tensile strength, MPa        16.9     16.4
Elongation at break, %        450      310
Modulus 100%, MPa             2.8      4.2
Modulus 300%, MPa            11.3     15.9

Compression set

24 h/100 [degrees] C, % 32 21

Table : Table 6 - TBzTD in black-filled NBR compounds - test

results after aging

After hot air aging, 3 days/125 [degrees] C

                                            TBzTD     TMTD
Compound no.                               5324/1   5324/2
Tensile strength, MPa                        17.6     18.0
Elongation at break, %                        320      210
Modulus 100 %, MPa                            4.9      7.5
Hardness, [degrees] Sh A                       70       75

Vulcanization was done at 165 [degrees] C at five minutes for both compounds. With this new development Akzo Chemicals believes to be on the right track having accepted the challenge from recent regulations in central Europe, which for sure will not concentrate there but be spread further.

References

[1.] H. Lohwasser, Einflub neuerer Rahmenbedingungen auf Produktentwicklungen in der chemischen Industrie, Kautschuk + Gummi, Kunststoffe 42 (1/1989) 22. [2.] Technische Regel fur Gefahrstoffe N-Nitrosamine, TRGS 552, BGes. Bl Sept. 1988. [3.] Akzo Chemicals, unpublished analytical results. [4.] WHO report nitrates, nitrites and N-nitroso compounds, Environmental Health Criteria 5, Geneva (1978) 23. [5.] D. Seeberger, G. Raabe: Entstehung von nitrosamine in vulkanisaten, Kautschuk + Gummi, Kunstsoffe 42 (1/1989) 27 - Generation of nitrosamines in vulcanizates. [6.] H. Druckrey, R. Preussmann, S. Ivankovic, D. Schmahl, J. Afkham, G. Blum, H.D. Mennel, M. Muller, P. Petropoulos and H. Schneider, Z. Krebsforsch 69 (1967) 103. [7.] P.N. Magee and J.M. Barnes, Adv. Cancer Res. 10 (1967) 163. [8.] H. Druckrey, Xenobiotica 3 (5/1973) 271. [9.] G. Raabe, D. Seeberger, P. Bolte: Ladungsverteilung in carbeniumionen und carcinogenitat von nitrosaminen, Kautschuk + Gummi, Kunststoffe 41 (11/1988) 1097- Charge distribution in carbenium ions and carcinogenicity of nitrosamines. [10.] Akzo Chemicals, in preparation. [11.] D.W. Chasar, A new thiocarbamyl sulfenamide accelerator, Kautschuk + Gummi, Kunststoffe 42 (1/1989) 31. [12.] K.M. Davies, D.G. Lloyd and A. Orband, The impact of N-nitrosamine regulations on sulfenamide selection, Kautschuk + Gummmi, Kunstsoffe 42 (2/1989) 120. [13.] Akzo Chemicals, unpublished tox. results. [14.] Akzo Chemicals, unpublished data. [15.] M. Coleman, Rubber Chemistry and Technology RCT 46 (1973) 938 and 957. [16.] DE 3709311-DE 3709313.

PHOTO : Figure 1 - nitrosamine formation, a possible reaction mechanism

PHOTO : Figure 2 - formation of nitrosamines during vulcanization

PHOTO : Figure 3 - formation of nitrosamines during storage of vulcanizates

PHOTO : Figure 4 - a method for isolation of volatile nitrosamines

PHOTO : Figure 5 - metabolism of nitrosamines leading to carcinogenicity (ref. 6)

PHOTO : Figure 6 - nitroso dibenzylamine and its related carbenium-ion

PHOTO : Figure 7 - structure of thiuram TBzTD