Salt bath vulcanization is a liquid curing method (LCM), a common continuous vulcanization (CV) method. CV methods are desirable for producing products such as tubing, hoses and weather stripping. Salt is a good choice for a CV method because it requires relatively short-length curing units—it has good heat exchange properties and can be used at the necessary high temperatures (177 to 260°C). Also, the salt does not cause surface oxidation, and it is easy to clean off with water. The entire operation involves at least four main processes: the rubber is fed through a cold-feed vented (or vacuum) extruder, conveyed through the salt bath, rinsed and cooled and then cut and processed according to specification. The extrudate is either immersed in or showered by the molten salt, which is a eutectic (easily fusible) blend of nitrate and nitrite salts, such as 53% potassium nitrate, 40% sodium nitrite and 7% sodium nitrate. The salt bath is generally enclosed with access doors on one side and electric heating coils on the other.
A disadvantage of the salt bath LCM is that it has been associated with the formation of nitrosamines, which are suspected human carcinogens. These chemicals are formed when a nitrogen (N) and an oxygen (O) from a “nitrosating” compound bind to the amino group nitrogen (N) of the amine compound. The nitrate and nitrite salts used in the salt bath serve as nitrosating agents and combine with amines in the rubber compound to form nitrosamines. Rubber compounds that are nitrosamine precursors include: sulphenamides, secondary sulphenamides, dithiocarbamates, thiurams and diethylhydroxylamines. Some rubber compounds actually contain a nitrosamine, such as nitrosodiphenylamine (NDPhA), a retarder, or dinitrosopentamethylenetetramine (DNPT), a blowing agent. These nitrosamines are weakly carcinogenic, but they can “trans-nitrosate”, or transfer their nitroso- groups to other amines to form more carcinogenic nitrosamines. Nitrosamines that have been detected at salt bath operations include: nitrosodimethylamine (NDMA), nitrosopiperidine (NPIP), nitrosomorpholine (NMOR), nitrosodiethylamine (NDEA) and nitrosopyrrolidine (NPYR).
In the United States, both the Occupational Safety and Health Administration (OSHA) and the NIOSH consider NDMA to be an occupational carcinogen, but neither has established an exposure limit. In Germany, there are strict regulations for occupational exposures to nitrosamines: in general industry, the total nitrosamine exposure may not exceed 1 μg/m3. For certain processes, such as rubber vulcanization, total nitrosamine exposures may not exceed 2.5 μg/m3.
Eliminating the nitrosamine formation from CV operations can be done by either reformulating the rubber compounds or using a CV method other than a salt bath, such as hot air with glass beads or microwave curing. Both changes require research and development to ensure that the final product has all the same desirable properties as the former rubber product. Another option to reduce exposures is local exhaust ventilation. Not only does the salt bath need to be enclosed and properly ventilated, but also other areas along the line, such as places where the product is cut or drilled, need sufficient engineering controls to ensure that worker exposures be kept low.