Page 252 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 221

which they condense on the substrate surface is generally too low to
affect the part.

6.4.1.2 Chemical cleaning. Strong detergent solutions are often used
to emulsify surface contaminants on both metallic and nonmetallic
substrates. These methods are popular on polymeric surfaces where
solvent cleaning may degrade the part or on parts where the contam-
ination is more easily removed by an aqueous cleaner (e.g., salt films,
dirt). Chemical cleaning is generally used in combination with other
surface treatments. Chemical cleaning by itself will not remove heavy
or strongly attached contaminants such as rust or scale. All of the
chemical cleaning processes should be preceded and followed by sol-
vent cleaning (when possible) or water rinsing.
Detergents, soaps, and other cleaning chemicals are the least ex-
pensive and easiest cleaning agents to handle. Two basic environmen-
tally friendly cleaning products are available: aqueous cleaners and
non-chlorinated solvents.
Aqueous cleaners are generally manufactured in concentrated forms
and are diluted with water before use. They come in three major clas-
ses: acidic, neutral, and alkaline. The alkaline cleaners are generally
used for cleaning metal surfaces prior to bonding. The following types
of aqueous cleaners are available. 21

Caustics (sodium or potassium hydroxide): Caustics work well on
steel, removing scale, smut, light rust and heavy oils, but cannot be
used on aluminum. They are also dangerous if not handled carefully.
Silicates (sodium meta silicate): Silicated cleaners work well on alu-
minum, but often are too aggressive on brass or copper.
Amines (triethanolamine, monoethanolamine): Amines are good on
all metals, but pose odor concerns.
Phosphates (trisodium phosphate, tetra potassium pyro phosphate):
Phosphates are very safe for the user (9.5 to 10.5 pH), but often are
not aggressive enough for heavy cleaning unless they are combined
with large amounts of wetting agents and/or solvents.
Acids (phosphoric, hydrofluoric, citric, etc.): Acids are very effective
on metals that require oxide removal, but they often require strict
safety precautions.
Chelates (EDTA): In small doses, chelates can help extend the life
of cleaning solutions. However, they often prompt wastewater con-
cerns because of the dissolved metals in the spent waste.

There are many detergents that are capable of cleaning substrates
prior to bonding. Generally, one to five ounces of liquid detergent per