Saturday, August 23, 2008

Solvent Cleaners

Solvent cleaners should be limited to precleaning and should be used to remove large excesses of oil and grease. Vapor degreasing tends to remove oil from solid dirt particles without always removing the solids. The dirt particles left on the surface are very hard to remove because they are now dried out, subsequent alkaline cleaning will have no effect on them. Solvents remove fats and oils, pitch, resins, waxes, parafin, and the like. An ideal grease solvent should have the following properties.
  1. Degrease when cold or hot, or in the vapor state.
  2. Non flammable
  3. Chemically inert to metals
  4. Stable
  5. Nontoxic
  6. Boiling point under 125 oC
  7. Low specific heat and latent heat of vaporization
  8. Vapors heavier than air
  9. Liquid phase of high specific gravity and low surface tension
  10. Meet air pollution requirements
  11. Be acceptable under the Occupational Safety and Health Act Standard
Such solvents as tri- and tetrachloroethylene, when properly inhibited, most nearly exhibit the properties desired. Both are toxic, however and care must be taken in designing equipment for their use, including provision for adequate ventilation.

By addition of oil-soluble emulsifiers to solvents to solvent-emulsion cleaners are produced. These are reported to afford very good removal of solid soils when used as diphase cleaners; they involve simultaneous contact of surface with water and a solvent that preferentially wets the metal. This system is particularly useful in removing dust and other solid particles mixed with oils and greases or pigmented drawing and buffing compounds. The metal still retains an oil film, however, so that it is necessary to alkaline clean before electroplating. A preliminary solvent emulsion treatment greatly decreases the load on the final cleaning bath. Care must be taken to remove all the solvent cleaner from recesses in metal parts before they are placed in the plating bath, or trouble will be encountered with plating solutions that contain surface active agents.