Page 248 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 217

vent wiping, vapor degreasing, and chemical cleaning. Contaminants
removed by passive cleaning include dirt, oil, mold release, moisture,
grease, fingerprints and other foreign substances on the surface. All
cleaning methods are improved by additional agitation that can take
the form of scrubbing, aggressive stirring, or ultrasonic agitation. Ta-
ble 6.7 provides an indication of the wide variation in surface clean-
liness as a function of cleaning medium and level of agitation.
Most cleaning methods require the use of solvents or chemicals;
thus, safety and consideration of the environment are of prime impor-
tance. Toxicity, flammability, materials incompatibility, and hazardous
equipment are all important safety factors that must be considered in
choosing the proper cleaning or surface treating system. Environmen-
tal factors that must be considered are volatile emissions and waste
handling, storage, and disposal.

6.4.1.1 Solvent cleaning. Solvent cleaning is the process of removing
soil and organic contaminants from a substrate surface with an or-
ganic solvent. Where loosely held dirt, grease, and oil are the only
contaminants, simple solvent wiping alone will provide surfaces for
weak- to medium-strength bonds. Solvent cleaning is widely used and
should precede any chemical or abrasive surface preparation. How-
ever, it is the least effective substrate treatment in that it only cleans
the surface of organic contaminants and loosely held particles.
Volatile solvents such as toluene, acetone, methyl ethyl ketone,
methyl alcohol, and trichloroethylene are acceptable. However, the lo-
cal and most recent safety and environmental restrictions must be
consulted before selecting any solvent. For many years 1,1,1-
trichloroethane was the work-horse for most cleaning processes be-
cause of its excellent solvency, low toxicity, nonflammability, and high
permissible exposure levels. However, because it is a depleter of strat-
ospheric ozone, it has been phased out. 18 In its place are substitutes
like aqueous cleaners and chlorinated solvents such as methylene
chloride, perchloroethylene, and trichloroethylene. Today, trichloroeth-
ylene and mineral spirits are perhaps the most commonly used sol-

TABLE 6.7 Efficiency of Degreasing as a Function of
Process 17
Cleaning
efficiency
Degreasing method (%)
1. Pressure washing with detergent solution 14
2. Mechanical agitation in petroleum solvent 30
3. Vapour degreasing in trichloroethylene 35
4. Wire brushing in detergent solution 92
5. Ultrasonic agitation in detergent solution 100