When all is SED and done

Manufacturers are under regulatory pressure to reduce this risks associated with solvent use; here Mark Nursall looks at the options available.

Halogenated solvents such as trichloroethylene and n-propyl-bromide (nPB) are tremendously effective cleaning and degreasing products; their speed and effectiveness makes them ideal for high-volume production applications.

Unfortunately, many solvents do have their side effects.

The risks associated with the inhalation of solvent vapours or prolonged exposure to skin contact are well known and can include cancer, effects on fertility and risks during pregnancy.

The European Union has recognised these risks and implemented the Solvent Emissions Directive (EC Directive 1999/13/EC) which places strict limits on the emissions of volatile organic compounds (VOCs) from industrial processes.

Companies that rely on such solvents for cleaning parts and subassemblies during manufacture have until 2007 to comply with the directive.

There is a broad range of reduction options available for users who currently rely on halogenated solvents to give acceptable part cleaning.

These options range from the adoption of abatement equipment to minimise employee exposure to emissions to the complete replacement of the harmful chemicals with alternative cleaning technologies.

A comprehensive solution to the solvent emissions issue is the replacement of solvents with an aqueous cleaning process.

Water based systems eliminate emissions concerns but substituting such a process into an existing production configuration can be less than straightforward.

Aqueous cleaning processes are typically several times slower than equivalent solvent processes - largely because the drying of water-wetted parts takes considerable longer.

In a production environment, this often means a much larger, more expensive machine and bigger batch sizes to maintain acceptable throughput rates.

The introduction of a drying stage to the cleaning process also consumes floor space and substantial amounts of energy, while the difficulty in achieving a thorough dry after aqueous cleaning can also create corrosion problems in some applications.

Finally, water-based systems result in the generation of substantial quantities of contaminated effluent, which will need to be treated on-site before disposal or recycling.

An alternative approach is the replacement or enhancement of existing solvent-cleaning equipment.

Modern solvent-based cleaning machines are extremely well designed to minimise emissions during use.

Advanced solvent recovery systems ensure that the operator is exposed to the minimum of harmful vapour and that the maximum possible amount of solvent is recovered for reuse.

This has the added advantage of keeping overall solvent consumption to a minimum, which reduces costs and could keep consumption levels down below the action levels set out in the SED.

Unfortunately, for solvents such as trichloroethylene and nPB, which have particularly low exposure and emission limits, even high quality, modern equipment can still produce vapour concentrations above the new limits.

To reduce operator exposure and emissions, therefore, these systems often require the installation of an abatement system - for example, passing contaminated air through an activated carbon filter.

Abatement systems can be effective, but come with their own capital costs, energy consumption and the requirement for regular maintenance and absorbent media replacement.

The use of toxic solvents also means that careful consideration has to be made of storage, handling and disposal activities, since poor practice here can result in substantial emissions and exposure for operators, as well as environmental damage.

Of course, all solvents are not the same.

It is possible to substitute high-risk solvents for recently developed alternatives with much lower risks and higher exposure limits.

Citrus-derived degreasers alleviate many of the emissions-related issues associated with chlorinated solvents, but they may leave a residue on the part surface, which can be problematic in some applications.

Citrus-based products can also be flammable, which creates its own storage and handling issues.

For many users who face difficulty finding a solution to their solvent emissions issues, a new generation of solvents based on hydrofluoroether (HFE) may provide the answer.

HFE solvents such as 3M's Novec range of engineered fluids are not associated with cancer or cell mutation and have operator exposure limits at least twice as high as trichloroethylene or nPB.

In many cases this means that, if used in good modern equipment, there is no need for a costly abatement system.

In addition, as HFE solvents do not have certain "risk phrases" associated with them, emission limits are less stringent.

HFE fluids work effectively in standard machinery to standard cycle times and their high stability means that they are long lasting.

Their low surface tension and low boiling point allows them to thoroughly wet the most complex parts, remove soils and contamination and then quickly evaporate off without leaving a residue.

Minimal capital investment and low fluid consumption means that Novec fluids can be cheaper to operate over the long term than lower cost, but more toxic equivalents.

Beyond their effect on the health of operators, different cleaning products also vary considerably in their wider environmental effects, including global warming potential.

It is highly likely that the regulatory environment governing these effects will also begin to tighten in the next few years, a factor that should be taken into consideration when selecting an alternate cleaning solvent.

While the task of compliance with ever-tightening legislation may seem daunting, a phased approach can provide the answer in many cleaning applications.

With appropriate solvent substitutions and the gradual upgrading or replacement of equipment, manufacturers can minimise capital and operating expenditure while maintaining the high standards of cleaning required by their processes and customers.

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