Prevention of Nitrosamine Exposure in the Rubber ... - ACS Publications

Chapter 4

Prevention of Nitrosamine Exposure in the Rubber Industry B. Spiegelhalder and C.-D. Wacker

Downloaded by UNIV OF LEEDS on November 3, 2014 | http://pubs.acs.org Publication Date: March 28, 1994 | doi: 10.1021/bk-1994-0553.ch004

Department of Environmental Carcinogens, FSP03, German Cancer Research Center, Im Neuenheimer Feld 280, D-6900 Heidelberg, Germany

The occurrence of carcinogenic nitrosamines at workplaces in the rubber and tire industry is still an unsolved problem. Recent air measurements in Germany (1988-1991) showed nitrosamine concentrations up to 41µg/m .New regulations require levels not greater than 2.5 µg/m . The major source of nitrosamines is the use of certain vulcanization accelerators such as thiurams, dithiocarbamates and sulfanamides. These accelerators are nitrosated during the vulcanization process. The origin of the nitrosating agent are oxides of nitrogen adsorbed on the large surface of inorganic rubber additives, e.g., zinc oxide and carbon black. We investigated two different approaches to prevent the formation of carcinogenic nitrosamines: (1) The nitrosating potential within a rubber mixture can be eliminated or at least drastically reduced by the addition of scavengers, like primary amines, urea or sulfamic acid. (2) Following the concept of "safe amines", the amine moiety of the accelerators does not form carcinogenic nitrosamines, a number of new accelerators with good technical properties was synthesized. Both preventive measures can be used effectively to improve workplace hygiene. 3

3

The occurrence of carcinogenic nitrosamines at workplaces in the rubber and tire industry was first described by Fajen et al. in 1979 (J). Subsequent studies in other countries (2-4) proved that nitrosamines can be found in nearly all factories producing rubber at levels up to >380 ng/m . Recent air measurements in Germany (1988-1991) showed that nitrosamines at workplaces in the rubber industry is still an unsolved problem and concentrations up to 41 ng/m could be observed. Table I summarizes maximum exposure levels at different work places measured in the early eighties. New regulations on occupational exposure limits in Germany require levels not greater than 2.5 ng/m . 3

3

3

NOTE: Dedicated to Rolf Preussman for his 65th anniversary.

0097-6156/94/0553-0042$08.00A) © 1994 American Chemical Society

In Nitrosamines and Related N-Nitroso Compounds; Loeppky, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Nitrosamine Exposure in Rubber Industry 43

4. SPEEGELHALDER & WACKER

Table I: Maximum exposure levels at different work places in the rubber industry (jig/m ) 3

NDMA

Location or process Raw material handling

0.9

2

Milling, extruding, calendering

2

9

Assembly & building

1

3

130

380

Inspection & finishing

10

20

Storage & dispatch

19

17

Curing Downloaded by UNIV OF LEEDS on November 3, 2014 | http://pubs.acs.org Publication Date: March 28, 1994 | doi: 10.1021/bk-1994-0553.ch004

NMOR

The major source of nitrosamines is the use of certain amine based vulcanization accelerators such as thiurams, dithiocarbamates and sulfenamides. The secondary amine moiety of the accelerators is nitrosated during the vulcanization process. The origin of the nitrosating agent are oxides of nitrogen adsorbed on the large surface of inorganic rubber additives, e.g., zinc oxide and carbon black or nitrosating rubber chemicals. A nitrosation of the released amines can also occur in the air by oxides of nitrogen. Figure 1 is a proposed reaction scheme (5). Figure 2 shows the nitrosation reactions of typical accelerators.

Use of amine precursors (vulcanisation accelerators) Use of NOx-releasing chemicals

Processing (Heat) Release of amines

Nitrosamine formation in the product

NOx Nitrosamine formation in the air

Degassing of nitrosamines

EXPOSURE

Figure 1: Reaction scheme of nitrosamine formation in the rubber industry


44

NITROSAMINES AND RELATED JV-NITROSO COMPOUNDS

3 \

1

II

1

/

3

NO

CH V

ON - N

N - C - S - S - C - N

/

3 V

CH

7

X

3

CH 3

S

I

Zn | S - C - N

CH

CH

/ 49 \

Zn S - C -

NDBA

ON—N

CH

C

49


NDMA

CH

NO

H

49

V

o

^

NPIP

ON-N'

NO. ON—N

O

NMOR

Figure 2: Formation of nitrosamines from their corresponding accelerators The different approaches to prevent the formation of carcinogenic nitrosamines are listed as follows: •

Avoidance of nitrosating chemicals (e.g.: "Retarder A", blowing agents)

•

Reduction of oxides of nitrogen in workroom air

•

Use of amine-free accelerators (e.g.: peroxide accelerators)

•

Inhibition of nitrosamine formation during vulcanization and/or destruction of nitrosation potential on surface of inorganic rubber additives (carbon black)

•

Use of accelerators derived from "safe amines"

In our research program we focused on possibilities to inhibit the nitrosamine formation during the vulcanization process, as well as to synthesize accelerators based on safe amines. To identify effective inhibitor systems model experiments were carried out in a reaction mixture, which simulates vulcanization conditions in rubber at elevated temperatures (>120°C). Paraffin oil containing nitrosatable rubber chemicals was used (6). Using morpholino-2-benzthiazolsulfenamide (MOZ) as a nitrosatable compound inorganicfillers,like zinc oxide, silica or carbon black, could be identified to have a nitrosation potential. Oxides of nitrogen which are adsorbed on the large


4. SPIEGELHALDER & WACKER

Nitrosamine Exposure in Rubber Industry

surface of the inorganic material are probably the nitrosating species. In Figure 3 the nitrosating potential of different inorganic rubber additives is shown. The nitrosamine formed is N-nitrosomorpholine (NMOR). The amount of nitrosamine formed is directly proportional to the amount of N O determined as nitrite in ZnO (Figure 4). x


NMOR formed [ug]

ZnOHS ZnO A ZnO S Sil S Sil Ζ CB N2 CB N3 CB N5 Type of inorganic Filler Figure 3: NMOR formation in model nitrosation experiments using different types of fillers (ZnO = zinc oxide, Sil = silica, CB - carbon black)

NMOR formed

r = 0.99 ..1

*****

0

1

2 3 4 Nitrite in ZnO [ppm]

5

Figure 4: NMOR formation by different ZnO types with different nitrite content.


45

46

NITROSAMINES AND RELATED iV-NITOOSO COMPOUNDS


The nitrosating potential within a rubber mixture can be eliminated, or at least drastically reduced, by the addition o f scavengers or inhibitors, like primary amines or diamines, urea or amidosulfonic acid (tf). The efficacy o f these nitrosation inhibitors can be demonstrated in model experiments simulating the rubber vulcanization. The results for M O Z as nitrosatable accelerator are shown in Figure 5. Mixtures o f amidosulfonic acid and urea were most effective in rubber mixtures preventing nitrosamine formation. A s an example N M O R concentrations in carbon black reinforced rubber vulcanizates are shown in Figure 6. The inhibitor system amidosulfonic acid / urea was studied in more detail over a range o f different relations o f the two inhibitors in a rubber containing thiuram and M O Z .

NMOR formed [ug]

Réf.

DAO

ASa

Urea pABa AcSa Type of Inhibitor

Hexa

AsP

F i g u r e 5: I n h i b i t i o n of N M O R formation d u r i n g simulated vulcanization using M O Z as accelerator; Ref - preparation without i n h i b i t o r , D A O diaminooctane, A S a - amidosulfonic acid, p A B a = para-amino benzoic acid, A c S a - acetyl salicylic acid, H e x a = hexamethylenetetramine, A s P = ascorbyl palmitate. The resulting inhibition o f the formation o f N D M A and N M O R is graphically represented over a range from 0 to 5 phr (parts per hundred rubber) o f each inhibitor. It can be seen, that the inhibition o f N D M A formation shows a different characteristic than the inhibition o f N M O R formation (Figure 7). The approach to inhibit nitrosamine formation was also investigated by other authors. Chasar (7) described the use o f alkaline earth oxides and hydroxides (CaO, C a ( O H ) and B a ( O H ) ) and could achieve a 70-95% reduction o f nitrosation. The use o f Na-hydroxymethane sulfinate by Schmieder et al. (8) resulted in nitrosamine levels < 10 ppb in synthetic rubber. Schuster et al. (9) showed that α-tocopherol may successfully prevent nitrosamine formation in rubber vulcanizates, especially in black rubber types. Maleic and fumaric acid can also serve as nitrosation inhibitors in rubber (JO). 2

2


4.

SPIEGELHALDER & WACKER


NMOR [ppm]


[E1N550 BBN33Q • N 2 2 p ]

Figure 6: Different inhibitor systems in natural rubber vulcanizates reinforced with different carbon black types and MOZ as accelerator; Ref = without inhibitor, AP1600 and RH1987 = amidosulfonic acid/urea preparations.

Figure 7: Inhibition of NDMA and NMOR formation in black NBR rubber using thiuram and MOZ as accelerator by different amidosulfonic acid / urea combinations (phr = parts per hundred rubber). American Chemical Society Library 1155 16th St. N. W. In Nitrosamines and Related N-Nitroso Compounds; Loeppky, R., et al.; Washington, C. 20036 ACS Symposium Series; American Chemical Society:D. Washington, DC, 1994.

47

48

NITROSAMINES AND RELATED AT-NITROSO COMPOUNDS

Aromatic hydroxydithiocarbonate


Aromatic hydroxydithiocarbonic acids can serve as nitrosation inhibitors and as accelerators as well (77). The nitrosamine reduction by polyoxymethylene described by Lheureux et al. (72) results from scavenging the amine moiety in a vulcanization system containing methylphenylamine derived accelerators. The chemistry of the miscellaneous inhibition methods of nitrosamine formation can be summarized by the following mechanisms: • a reduction of oxides of nitrogen to either NO by antioxidants, • a reduction to nitrogen by scavengers and • a reaction with the amine moiety of the accelerator or free amine to prevent nitrosation. The best method for avoiding the formation of nitrosamines may be the use of nitrogen-free accelerators. Compounds like peroxides or thiophosphates, which were suggested as substitutes (5) are only of limited to specific rubber mixtures, however In most cases thiurams, dithiocarbamates and sulfonamides are still necessary for rubber to achieve good technological properties. The widely used sulfenamides can in most cases easily be substituted against accelerators containing primary amines (t-butyl-, t-amyl-, cyclohexylamine) instead of secondary amines like morpholine. Some of these substitutes are already commercially available and are used as safe accelerators in the tire industry (75-76). Much more difficult to exchange are thiurams and dithiocarbamates. Only dibenzylamine derived accelerators can be obtained commercially for use as substitutes (17,18). The wide range of different rubber products require more thiuram and carbamate accelerators to cover the wide range of applications with different needs. Our aim Was to synthesize new accelerators based on "safe amines", which either do not form nitrosamines or which form non carcinogenic nitrosamines. The following structures represent a selection of "safe amines" (Figure 8).

Figure 8: Chemical structures of selected "safe amines". In Nitrosamines and Related N-Nitroso Compounds; Loeppky, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

4.


SPIEGELHALDER & WACKER

Following the concept of "safe amines'* a number of new accelerators with good technical properties was synthesized (19 20). Thiuram and carbamate accelerators based on methylpiperazine are among the most suitable substitutes for recompounding rubber mixtures. As an example for suitability the vulcanization properties of safe accelerators the methylpiperazine derived thiuram and dithiocarbamate are compared against commercial accelerators in Figure 9 and 10. Vulcasafe ZMP and Vulcasafe MPT are good acting vulcanization accelerator with similar properties as the technical standards. Vulcasafe ZMP and Vulcasafe MPT appear to be good substitutes for tetramethylthiuramdisulfide (TMTD), a widely used thiuram accelerator as well as for some of the traditional dithiocarbamates. These new accelerators offer the possibility of a "safe" rubber vulcanization without formation of carcinogenic nitrosamines. In the case of the methylpiperazine derivatives the toxicological evaluation is still in progress. The corresponding nitrosamine is probably non carcinogenic or at least a very weak carcinogen. Some of our "safe amine" accelerators are currently being tested by a number of German rubber producers for their applicability in specific rubber formulations.


t

Monsanto - Rheometer (140° C) Torque

1

0

0

[lb inch]

80 4 // 4 /y

60 ;t

40

/,

//

V-1

••••••

St

ZDMC

i

ZDEC

/

2

20

-

1DBC

Vulcasafe ZMP

J 0

2

4

6

-A 1 \—\— 8 10 12 14 16 18 20 22 24 time [min]

Figure 9: Vulcanization curves of dithiocarbamates; ZDMC = Zn-dimethyl-, ZDEC = Zn-diethyl-, ZDBC = Zn-dibutyl-, Vulcasafe ZMP = Zn-methylpiperazine-dithiocarbamate.


49

50

NITROSAMINES AND RELATEDtf-NITROSOCOMPOUNDS

Monsanto - Rheometer (150* C ) Torque

1

0

&

[lb inch]

/* 60


4a

i

$ t1

Prevention of Nitrosamine Exposure in the Rubber ... - ACS Publications

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