Curing fumes - a threat to the environment?

The rubber industry has in many countries and rather often, correctly or incorrectly, been pointed out as a threat to the health of the employees as well as having a bad influence on the environments outside the factories. These statements are probably related to the earlier history of industry, when very little or nothing at all was known about the hazards involved for the chemicals used, all kinds of material handlings were manual. ventilation and automation system technology was unknown and so on.

The industry has now a long tradition in improving working environment, thereby eliminating possible health hazards. During the latest decades, the industry has been more and more engaged in external environmental issues too, and the environmental conditions are increasing strictly regulated by the authorities.

The factors in rubber production that may have an influence on the environment are:

* Noise;

* Raw material dust;

* Smell;

* Emissions to water;

* Emissions of solvents; and

* Emissions of curing fumes.

The techniques for reducing most of these risk factors are well-known, even if the cost involved very often is hard to justify.

For the last factor, curing fumes, and its influence on the environment, discussions have started in recent years, especially in Germany and Scandinavia.

Curing fumes investigations

Several investigations in different parts of the world indicate that the risk for cancer may be slightly increased for workers exposed to curing fumes, even if the separate components in curing fumes generally show substantially lower values than the hygienic limit value, if such a regulation exists in the country in question (refs.1-5). Generally, the risks are higher for smokers than non-smokers.

The results originate from investigations covering long periods of time, normally up to 30 years, and thereby reflect an earlier situation when the working conditions and the knowledge of chemicals used by the industry was not as well developed as today.

To increase the understanding of curing fumes a lot of international projects have been reported. The most comprehensive work published has been a more than 10-year-project at Rapra Technology, supported by a number of international rubber manufacturers and some, but surprisingly few, raw material suppliers (refs.6 and 7).

Methods for laboratory curing and analyzing by GC-MS technique have been developed and about 25 typical industrial compounds have been analyzed. The result is a computer floppy disc, today available on the market, where i.e., the "parents" to quite a number of the fume-components could be recognized among the raw materials.

The Swedish Environmental Research Institute has worked on the problem to determine one or several substances (indicator parameters) which should be selected to represent the toxic effect of the total fume concentration in field measurements. The results (ref. 8) indicate that before such parameters are stipulated, the laboratory experiments must be completed with factory measurements and even then it may be impossible to find some general indicator parameters. It could be possible to choose a few parameters by studying a specific working place, and thereby improve the possibilities for a risk evaluation.

The German regulations with limit values for nitrosamines have generated a lot of knowledge about how to substitute accelerators, how to arrange air circulation at working places and about the analysis-technique (ref.9).

Curing fumes and the environment

The concern about the possible environmental influence from curing fumes shown by the Swedish Board for Environmental Protection, generated a "Nordic curing fume project" (ref. 10), covering:

* Analysis of factory emissions;

* Comparative analysis from laboratory curing;

* Estimation of human-toxicological influence;

* Estimation of eco-toxicological influence;

* Feasibility study of filtering technique.

Both sampling, analysis and the toxicological estimations were carried out by rubber industry independent experts.

Testing technique

For the investigation, six carbon-black filled compounds based on different polymers were used. The principal recipes are given in table 1.

Table 1 - carbon black filled rubber compounds
Compound    1      2        3        4     5      6
Polymer    NBR    EPDM     SBR     CR/BR   NR    EPDM
                         Reclaim
HA-oil     7.3    10.7     9.0      8.7    5.7
NA-oil            11.7                            5.0
S          0.7     0.3     0.7      0.4    0.9
MBTS       0.25    0.5     0.3
TMTD       0.2     0.2
CBS                                        0.5
TMTM                       0.1      0.2
DPG                        0.1      0.2
ZDBZ               0.2
ZDEC               0.2
Peroxide                                          2.0
TAC                                               0.8
CTP                                 0.5
TMQ        0.1             0.1      0.6
6PPD                                0.4

The curing method used for the sulfur cured compounds was rotorcuring at 170[degrees]C. The rotorcure technique was chosen for giving a constant stream of curing fumes. easy to measure.

The peroxide-cured EPDM compound was cured in a steam-tube operation and the volume of gases from the two methods cannot be compared, as the steam-tube did not allow a constant measure.

For comparison, a laboratory study was carried out in a gas transfer mold at 180[degrees]C by Rapra Technology Ltd.

The sampling of curing fumes in the factory investigation was made by use of:

* Absorbent tubes for sulfur compounds;

* Tenax tubes for organic compounds;

* Charcoal tubes for less volatile organic compounds:

* Quartz wool filters for aerosols and particles;

* Impinger flash for amines;

* Continuous registering instrument for THC.

The amount of fumes was determined both in a laboratory, by careful measuring of the loss in weight during curing, and in the factory trials .

Results received

The loss of weight during curing was in all cases between .04-.08%.

In the factory trial, a value of .044% was received and in thc laboratory investigation the values given as an average from 11 industrial compounds were:

  Curing temp. [degrees]C   Loss of weight, %
         140                     .040
         160                     .060
         180                     .082

The total emissions received in the rotorcuring of compounds 1-5 are given in table 2. The total emission varies very much with the recipes and is eight times higher from compound 2 (EPD M. high oil) than for compound 5 (NR-tire types).

Table 2- curing fume total emission
Compound                 1      2       3      4        5
Gas (mg/n[m.sup.3]      11     12       7      6        5
Aerosol (mg/n[m.su.3]   35     56      16      7.3      3.2
Total                   46     68      23     13.3      8.2
Gas flow (n[m.sup.3]/h  5370  5440    5180    5380     5380
Emission
- gas  (g/h)             59    65      36      32       27
- aerosol (g/h)         188    305     83      39       17
Total  (g/h)            247    370    119      71       44

In the analysis of the fumes, 221 substances were identified and thereof 100 were quantified and another hundred were quantified within wide limits as less than 50 ng/l. All substances were classified in respective chemical groups as: Aliphatic hydrocarbons; aromatic hydrocarbons; isothio cyanates; ketones; nitrosamines; thiazoles; aldehydes; amines; and sulfur compounds.

A typical example of classified substances is given in table 3.

[TABULAR DATA 3 OMITTED]

Human-toxicological evaluation

A risk identification of each substance was made in respect of available facts from: Animal testing; cell cultivation tests; epidemiological investigations; and clinic testing

A risk estimation for the area direct outside the factory (the factory yard) and for an area about 200 m outside the factory yard, where in the actual case a day care center is situated, was carried out.

The estimation was based on a yearly production of 25,000 tons of cured rubber, giving an estimated emission of 15 tons of curing fumes. For calculation of the concentrations in the respective areas, a meteorological gas diffusion model was used, and low risk values for the substrates were calculated according to a model recommended by the WHO (ref. 11).

The results, presented in tables 4 and 5, show that all calculated concentrations are far below the WHO low risk level with one exception. This exception is cyclohexyl-isothiocyanate where the calculated concentration and the low risk level are of the same order. It has to be mentioned that no information is available about effects of exposure to cyclohexyl-isothiocyanate, while TDI has been used to set up the low risk level.

[TABULAR DATA 4 and 5 OMITTED]

Furthermore, a mutagenicity test on the aerosols and the particles was carried out. A weak mutagenic effect was received in all samples, with the sample from the peroxide-cured compound being on the lowest detectable level.

Exo-toxicological evaluation

This study was carried out by making a comparison with data given in the USA Clean Air Amendment 1990 and the German TA Luft 1986.

The persistence of the different substrates to atmospheric influence was studied as in table 6, and estimations of the possible eco-toxicological influence was made in accordance with an OECD chemical program.

[TABULAR DATA 6 OMITTED]

Filtering of curing fume exhaust

A feasibility study of different filtering techniques was done, and the methods studied were: Oxidation, both thermal and catalytic; adsorption with active carbon; absorption by scrubbing; biofiltration; and condensation. Of all the methods studied, the adsorbation by active carbon was found to be the most economical one, and still judged efficient enough.

Due to the extremely high airflow in most rubber factories, the investment and the maintenance cost may be of an order that cannot be justified in light of the low concentration and the conclusion drawn from the toxicological estimations done. A calculation of cost is presented in table 7.

Table 7 - curing fume cleaning cost calculation
Product                 Sheeting   Tire    Mill
department                                 lining
Production
tons/year                6,000     5,000   4,500
Air flow
[10.sup.3][m.sup.3]/h       30       180     120
Investment
[10.sup.3] US$             355     1,4001,   100
Maintenance
[10.sup.3] US$/year        115       170     115

Conclusion

The most important conclusions reported by the independent specialists used in the project are:

* The quantity of vulcanization fumes can roughly be expected to be about 0.05% of the quantity of rubber vulcanized;

* Almost a hundred chemical compounds have been quantified in the vulcanization fumes and more than 200 compounds have been identified;

* The mutagenicity of the aerosols in the vulcanization fumes is weak;

* From the point of view of health hazards there is no reason for concern from the neighbors to a rubber factory;

* From an ecotoxicological point of view, the vulcanization fumes contribute to a very small extent to the general pollution level compared to the contribution from other selected sources, such as traffic and heating systems;

* The vulcanization fumes can, at times, give rise to a certain smell in the vicinity;

* It will be very expensive to apply a cleaning technique, both from an investment point of view and seen in relation to the value of finished rubber products. The small risk involved does not call for a filtering of the fumes, unless the claims concerning the smell call for it.

References

(1.) IARC. International Agency for Research on Cancer. Monograph on the evaluation of carcinogenic risk of chemicals to humans. Vol. 28. The rubber industry. Lyon, 1982. (2.) Fraser, D.A. and Rappaport, S., "Health aspects of the curing of synthetic rubbers," Env. Health Persp. 17, 45.53 (1976). (3.) Gustavsson, P., Hogstedt, C. and Holmberg, B., "Mortality and incidence of cancer among Swedish rubber workers, 1952-1981," Scand J. Work Env Health 12, 538-544 (1986). (4.) Sorahan, T., Parkes, H.G., Veys, C.A. and Waterhouse, J.A.H., "Cancer mortality in the British rubber industry: 1946-80," Brit. J. Ind. med. 43, 363-373 (1986). (5.) Buus, H., "Gummivulkanisering og sundhedsskader. Toksikologisk vurdering af kemiske stoffer udviklet ved vulkanisering av gummi Arbejdsmiljofondets forsknings- rapporter," Arbetsmiljofondet, Copenhagen 1986. (6.) Willoughby, B.G., "Investigations of additive performance and perspectives for process control. SRC-85," Copenhagen(1985). (7.) Willoughby, B.G. "Rubber fume identification," Europ. Rub. J. 166, 49-52 (1984). (8.) L. Rondahl and J. Mowrer, "Development of measuring method for curing fumes," IVL Publ. B 1003, Stockholm 1990. (9.) H-W, Engels, "Nitrosamine in der Gummiindustrie," Kautschuk und Gummi-Kunstoffe 43, 11/90. (10.) Nordic Curing Fume Project, internal report from the Nordic Rubber Ind. 1991. (11.) WHO: Air Quality Guidelines for Europe. WHO Regional Publication, European series No 23, 1987.

Acknowledgements

"Reclaim rubber usage and trends" is based on a presentation given at the October,1993 Rubber Division meeting.

"Surface area measurement of ground rubber using the B.E.T. surface area analyzer" is based on a presentation given at the April,1994 Rubber Division meeting.

"Curing fumes - a threat to the environment?" is based on a presentation given at the October. 1993 Rubber Division meeting.

"Improving fatigue resistance with neodymium polybutadiene" is based on a presentation given at the October, 1993 Rubber Division meeting.