Corrosion of Paint
225
7.6.2.7 Zinc and Cadmium
Zinc and cadmium substrates are treated the same as galvanized steel
(see above).
7.6.3 Plastic Substrate Preparation
As with metallic substrates, surface preparation has the greatest impact on
film adhesion. Film adhesion to a plastic is primarily a surface phenomenon
and requires intimate contact between the substrate surface and the coating.
Without appropriate conditioning and cleaning, intimate contact with the
plastic surface is not possible.
Plastic surfaces present a number of unique problems. Many plastics, such
as polyethylene and fluorinated polymers, have a low surface energy, which
means that few materials will readily adhere to the surface. Plastic materials are often formulations of one or more polymer types, or have various
amounts of inorganic fillers. In addition, the coefficient of thermal expansion
is usually quite high for plastic compounds and can vary widely, depending on polymer blend, filler content, and filler type. The flexibility of plastic
materials puts more stress on the coating, and serious problems can develop
if film adhesion is low as a result of poor surface preparation.
Surface preparation, depending on the polymer, is required to:
1. Remove process oils, dirt, grime, waxes, mold release agents, and
poorly retained plasticizers.
2. Match the finish on the plastic to the coating viscosity for improved
film adhesion.
3. Convert the surface of the plastic to provide an interface that is more
like, or compatible with, the chemical structure of the coating.
4. Promote oxide formation to produce a higher level of surface
activity.
5. Control absorbed water, which can interfere with adhesion.
There are as many procedures for cleaning and preparing plastic surfaces as there are polymer types. Most plastic types have recommended
procedures for achieving the best finish for coating. Many polymers can be
blended together to achieve specific properties. Fillers and plasticizers are
also included in the resin matrix to yield certain characteristics. The same
preparation procedure may not be suitable in each of these cases.
Advice as to the appropriate surface preparation procedure to use should
be available from the manufacturer of both the plastic and the coating.
However, because of the possible unique needs of the user, even the manufacturer may not have a definite answer. Experimentation may be required to
identify the most suitable technique for a specific application.
225
7.6.2.7 Zinc and Cadmium
Zinc and cadmium substrates are treated the same as galvanized steel
(see above).
7.6.3 Plastic Substrate Preparation
As with metallic substrates, surface preparation has the greatest impact on
film adhesion. Film adhesion to a plastic is primarily a surface phenomenon
and requires intimate contact between the substrate surface and the coating.
Without appropriate conditioning and cleaning, intimate contact with the
plastic surface is not possible.
Plastic surfaces present a number of unique problems. Many plastics, such
as polyethylene and fluorinated polymers, have a low surface energy, which
means that few materials will readily adhere to the surface. Plastic materials are often formulations of one or more polymer types, or have various
amounts of inorganic fillers. In addition, the coefficient of thermal expansion
is usually quite high for plastic compounds and can vary widely, depending on polymer blend, filler content, and filler type. The flexibility of plastic
materials puts more stress on the coating, and serious problems can develop
if film adhesion is low as a result of poor surface preparation.
Surface preparation, depending on the polymer, is required to:
1. Remove process oils, dirt, grime, waxes, mold release agents, and
poorly retained plasticizers.
2. Match the finish on the plastic to the coating viscosity for improved
film adhesion.
3. Convert the surface of the plastic to provide an interface that is more
like, or compatible with, the chemical structure of the coating.
4. Promote oxide formation to produce a higher level of surface
activity.
5. Control absorbed water, which can interfere with adhesion.
There are as many procedures for cleaning and preparing plastic surfaces as there are polymer types. Most plastic types have recommended
procedures for achieving the best finish for coating. Many polymers can be
blended together to achieve specific properties. Fillers and plasticizers are
also included in the resin matrix to yield certain characteristics. The same
preparation procedure may not be suitable in each of these cases.
Advice as to the appropriate surface preparation procedure to use should
be available from the manufacturer of both the plastic and the coating.
However, because of the possible unique needs of the user, even the manufacturer may not have a definite answer. Experimentation may be required to
identify the most suitable technique for a specific application.