Page 266 - Handbook of Adhesives and Sealants

234 Chapter Six

tablishing how much coating weight is required. This is best done by
the actual testing of finished parts.
For aluminum, anodizing provides the most water durable adhesive
joints. It is used by many automotive and aerospace suppliers. The
corrosion protection is provided by anodizing the clean deoxidized alu-
minum surface in either chromic or phosphoric acid electrolytic baths.
In the U.S., phosphoric acid anodizing is often used because of its
lower toxicity and easier disposal. Anodizing creates an oxide under
controlled voltage and temperature conditions, thereby creating a
more protective surface. 31 As with phosphating, care must be taken to
ensure the optimum coating thickness.
Chromate conversion coatings are used with some metals, such as
aluminum, zinc, magnesium, and copper alloys, to enhance adhesion
and to protect the surface. They are tough, hydrated gel structures
that are usually applied by immersion in heated solutions of propri-
etary, chromium containing compounds. Chromium compounds have
also been elecrolytically applied to steel.

6.5.1.2 Chemical treatment of polymeric surfaces. The chemical modi-
fication of low energy polymer surfaces may be carried out by treat-
ment with chromic acid, metallic sodium complex dispersions,
bleach/detergents, potassium iodate/sulfuric acid and other mixtures.
The chemical used and the treating conditions are dependent on the
type of polymer being treated and the degree of adhesion required.
Chemical treatment of polymeric surfaces is generally more difficult
than metallic surfaces and requires special considerations. Polymeric
products often contain pigments, antioxidants, slip agents, mold re-
lease agents, etc. that can migrate to the surface and interfere or alter
a surface treatment process. Slight changes in the polymer formula-
tion or its fabrication process may result in changes in the surface
condition and the effectiveness of treating operations. Depending on
the exact formulation and method of manufacture, the surfaces can be
considerably different on parts manufactured from the same generic
plastic and having the same bulk physical properties. Polymers that
are molded against a hot metallic surface may have very different
surface characteristics, for example, than the same polymer that is
formed without contact to a metal surface. Melt temperature and cool-
ing rate can also affect the surface properties of the polymer. The con-
trol of surface crystallization during plastic processing is important.
Molding conditions can alter the surface crystalline content when com-
pared to the bulk polymer. Normally crystalline regions are much less
susceptible to etching than amorphous regions.
Most important, however, is the fact that surface treatments on
polymeric materials are subject to the degree of handling after treat-