The Synchrotron Radiation Source at Daresbury, UK, has previously been reliant upon the use of trichloroethylene (tradename "Triklone") for the cleaning of its vacuum components. Recent European legislation comes into force on 31st October 2007, placing stringent limitations upon environmental emissions of volatile organic compounds [1]. Henceforth the use of trichloroethylene at Daresbury will become prohibitively expensive.
Using measurements of (i) static outgassing rates [2], and (ii) electron stimulated desorption rates [3], the vacuum group at Daresbury have made a long term study of the efficacies of various solvents for the cleaning of UHV components [3,4,5]. As a result, water based solvents have been universally rejected, whilst solvents such as n-propyl bromide and isopropyl alcohol have been demonstrated to offer acceptable cleaning properties.
In contrast, hydrofluoroethers have been clearly identified as offering improved cleaning power over trichloroethylene. Importantly, hydrofluoroethers are low-toxicity and non-ozone depleting, with relatively short atmospheric lifetimes of the order of 10 to 50 months. However, their manufacture is costly with consequent limitations on their use in the large quantities required for cleaning plant operations.
This work presents the results of recent studies of the application of hydrofluoroethers as a "co-solvent" in two-stage cleaning processes: a second, cheaper solvent provides an additional cleaning cycle, thus minimising the long term contamination of the more expensive hydrofluoroether cycle.
Although more work remains to further clarify the most efficient co-solvent combination, it is clear that a viable system is achievable, and that the use of trichloroethylene can be entirely replaced at Daresbury, cost-effectively and without detriment to existing standards of machine cleanliness.
[1] EC directive 1999/13/EC.
[2] P.A. Redhead; J.Vac.Sci.Tech., A20(5), 1667 (2002).
[3] J.D. Herbert and R.J. Reid; Vacuum, 47(6-8), 693 (1996).
[4] J.D. Herbert, A.E. Groome, R.J. Reid; J.Vac.Sci.Tech., A12(4), 1767 (1993).
[5] K.J. Middleman, J.D. Herbert, R.J. Reid; Vacuum, 81(6), 793 (2007).
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