TCE is valued for its solvent capacity in adhesive and paint applications, as well as its ability to solubilize contaminants such as oil, grease and buffing compounds on many types and configurations of metal parts. In 2005 almost 90% of the TCE use reported in Massachusetts was as an adhesive or paint solvent. Only approximately 11% of the reported uses in 2005 were associated with degreasing applications, however occupational exposure is a particular concern in this use application.
Alternatives for Adhesive and Paint Applications
Solvent-based adhesives are used in a variety of applications, from upholstered furniture and bedding applications to flexible vinyl mounting applications to substrate lamination applications. Adhesives use a carrier to transport, often in aerosol form, the tackifying resins and polymer solids that bond substrates together.
There are hundreds of adhesive formulations available on the market, many of which use a solvent carrier such as TCE. Typical solvent-based formulations use methylene chloride, acetone, acetone/heptane blends and n-propyl bromide. Each of these solvent-based adhesive formulations has potential human health and/or environmental health impacts that make them questionable alternatives to TCE-based formulations with respect to being “safer”. Other solvent adhesive formulations available use mineral spirits, petroleum solvents, petroleum distillates and VM&P naphthas.
However non-solvent based alternatives are available for many applications, including:
- Aqueous-based carriers using latex or latex-synthetic blends, and
- Hot melt applications, where a solvent-free 100% solids method is used in adhesion.
TCE is a relatively common carrier used in solvent-based paints. Alternatives include lower solvent content paints containing other solvents, or switching to aqueous-based latex paints. In addition, depending on the application, 100% solids paint processes such as powder coating may be appropriate alternatives.
The TURI lab has created an extensive database of safer alternatives to TCE (and other toxic solvents and cleaners) used for degreasing purposes. This database is available on the TURI lab website at www.cleanersolutions.org.
Alternatives for Degreasing
Over the years, preferred degreasing solvents have included chlorofluorocarbons (CFCs), trichloroethane (TCA), TCE and perchloroethylene (perc). CFCs and TCA were classified as Class I ozone-depleting substances and were phased out under the Clean Air Act in 1990.
For vapor degreasing, key physical properties include: low vapor pressure, low latent heat, low boiling point, low flash point, low surface tension and high solvency powers. TCE is generally used in degreasing operations because of its high boiling point, which allows the solvent to remove soils and waxes that lower-boiling solvents may not. The stability of TCE makes it particularly useful in airless degreasing systems. TCE is also used in “cold” degreasing applications, where the liquid is applied to the contaminant directly.
There are proven alternatives for metal degreasing, including hydrocarbon solvents (e.g., terpenes, alcohols, acetone, ketones, and acetates) and aqueous and semi-aqueous processes, including ultrasonic processing. Performance evaluations conducted by the TURI lab indicate that aqueous degreasing alternatives perform comparably to or better than TCE for a wide variety of contaminants and parts configurations. Resistance to adopting aqueous cleaning continues, however, and is often attributed to the following challenges:
- Reduced efficiency at producing clean and dry parts.
- Inability of the aqueous cleaner to get into small, blind holes. Solvents in the vapor form are more successful at cleaning these hard to reach areas.
- Reluctance to write off sunken capital associated with existing vapor degreasing equipment.
Effective “drop in” solvent alternatives (i.e., solvents that can be used in existing equipment) include n-propyl bromide (nPB), hydrochlorofluorocarbons, hydrofluorocarbons, hydrofluoroethers and volatile methyl siloxanes. These drop-in solvents have purchase costs that range from 3 to 43 times that of TCE on a per gallon basis.
An important consideration in degreasing applications is the need for removal of contaminants. Companies can evaluate their manufacturing operation to identify potential process modifications that would eliminate the need for this cleaning step.
N Propyl bromide (nPB) is closest in cost and performance to TCE, and is frequently chosen as a drop-in substitute for TCE in vapor degreasing. However nPB has been shown to produce severe health effects for workers, affecting both the central nervous and reproductive systems. California’s Proposition 65 lists nPB as a reproductive toxicant.
Endnotes: International Programme on Chemical Safety and the Commission of the European Communities; WHO; Environ Health Criteria 50: Trichloroethylene, 1985; Morris, et al, 2000. Alternative Adhesive Technologies in the Foam Furniture and Bedding Industries: A Cleaner Technologies Substitutes Assessment. Volume 1: Cost and Performance Evaluation.