TWI Industrial Member Report Summary 897/2008
By G J Carter (TWI) and I Pengelly (Health and Safety Laboratory)
It is common practice in the automotive and white goods industries to resistance weld steel that has a thin organic coating (TOC) and/or a coating of oil. During welding, the coatings are heated over a range of high temperatures, giving rise to the emission of fumes and gases that may be harmful to health.
Even with knowledge of the composition of the coatings, it is difficult, if not impossible, to predict the range of compounds that will be emitted and such information is not usually available on Material Safety Datasheets (MSDSs). Typically, the MSDS data is restricted to carbon dioxide and water, although clearly this is only the case when temperatures are sufficient for complete degradation (<1000°C). Consequently, it is impossible to perform a proper assessment of the risks to health arising from exposure to the emissions, neither is it possible for occupational hygienists to single out the fume components that should be monitored when measuring workplace exposures, identifying a need to generate emissions data for these purposes.
Recognising this need, members of the standardisation committee, CEN/TC121/SC9/WG5, produced a Technical Specification (ISO/TS 15011-5. Health and safety in welding and allied processes - Laboratory method for sampling fume and gases - Part 5: Identification of degradation products generated when welding or cutting through products composed wholly or partly of organic materials) for evaluation over a trial period, with the aim of generating sufficient data to develop a full standard.
The Technical Specification requires that screening tests, using a pyrolysis technique with the heating source connected to a gas chromatography - mass spectrometry (GC-MS) analysis system, be performed, to provide a preliminary indication of the thermal degradation products. Results from the screening tests then form the basis of a measurement strategy to be used in welding tests, to generate the data for inclusion on datasheets. However, welding tests are expensive to perform and, providing the data generated in pyrolysis tests could be shown to provide reasonable correlation with welding generated data, a better solution would be to use the pyrolysis technique only. Further, the development of a standard method would ensure a uniform approach and give consistency to the data generated.
- Perform resistance welding and pyrolysis tests on weld-through organic coatings to identify the thermal degradation products emitted.
- Compare the amount and compositions of the thermal degradation products generated during the pyrolysis tests with those obtained by resistance welding.
- Use the data obtained to recommend a test procedure for standardisation for generating safety data for organic weld-through products.