Corrosion of Paint
227
recommended. Detergent cleaning is often used as a preliminary step to
mechanical treatments.
7.6.3.3 Mechanical Treatments
A solvent or detergent cleaning process must precede a mechanical treatment to prevent scrubbing surface contaminants into the roughened
surface. Physical scrubbing of plastic surfaces removes oxides and contaminated layers.
A commonly used procedure is either wet or dry sanding, using a grit of
40 to 400. The grit size depends on the amount of surface to be removed and
the surface finish desired. Softer plastics are more susceptible to damage.
Grit blasting, either wet or dry, and wire brushing are appropriate techniques to use on parts that have complex configurations. Grit size and type
can be varied to obtain the proper finish.
Regardless of the method employed, the roughened surface should be vacuumed or air-blasted to remove residual dust or grit. It is recommended that this
be followed by a solvent wipe or water rinse, followed by elevated drying.
7.6.3.4 Chemical Treatment
A chemical etch of the plastic to be coated is, in general, the most effective
surface preparation technique. Both physical and chemical characteristics
of the plastic can be modified to improve wet-out and film adhesion. To
reduce surface contamination and to obtain optimum interaction between
the chemical and the substrate, one or more of the cleaning operations must
be employed prior to the chemical treatment.
Chemical treatment involves the surface being washed or immersed in a
bath containing an acid, base, oxidizing agent, chlorinating agent, or other
highly active chemical. Regardless of the agent being used, it is necessary to
control the parts by weight of the active ingredient, the temperature of the
solution, and the elapsed time of immersion. Some procedures have a wide
range of ingredient ratios, while others are quite specific. The temperature of
the solution is inversely proportional to the time of immersion — the higher
the temperature, the shorter the immersion time. It is important that solution
strength be monitored and renewed as required.
All chemical etch techniques require a water rinse and elevated temperature drying.
Sulfuric acid etch.
•
The sulfuric acid-dichromate etch is recommended
for use on acrylonitrile-butyldiene-styrene (ABS), acetal, melamine, or
urea, polyolefins, polyphenylene oxide, polystyrene, polysulfone, and
styrene-acrylonitrile (SAS). A different ingredient ratio, immersion
temperature, and immersion time is recommended for each plastic.
Table 7.10 shows the parameters of the sulfuric acid-dichromate etch.
227
recommended. Detergent cleaning is often used as a preliminary step to
mechanical treatments.
7.6.3.3 Mechanical Treatments
A solvent or detergent cleaning process must precede a mechanical treatment to prevent scrubbing surface contaminants into the roughened
surface. Physical scrubbing of plastic surfaces removes oxides and contaminated layers.
A commonly used procedure is either wet or dry sanding, using a grit of
40 to 400. The grit size depends on the amount of surface to be removed and
the surface finish desired. Softer plastics are more susceptible to damage.
Grit blasting, either wet or dry, and wire brushing are appropriate techniques to use on parts that have complex configurations. Grit size and type
can be varied to obtain the proper finish.
Regardless of the method employed, the roughened surface should be vacuumed or air-blasted to remove residual dust or grit. It is recommended that this
be followed by a solvent wipe or water rinse, followed by elevated drying.
7.6.3.4 Chemical Treatment
A chemical etch of the plastic to be coated is, in general, the most effective
surface preparation technique. Both physical and chemical characteristics
of the plastic can be modified to improve wet-out and film adhesion. To
reduce surface contamination and to obtain optimum interaction between
the chemical and the substrate, one or more of the cleaning operations must
be employed prior to the chemical treatment.
Chemical treatment involves the surface being washed or immersed in a
bath containing an acid, base, oxidizing agent, chlorinating agent, or other
highly active chemical. Regardless of the agent being used, it is necessary to
control the parts by weight of the active ingredient, the temperature of the
solution, and the elapsed time of immersion. Some procedures have a wide
range of ingredient ratios, while others are quite specific. The temperature of
the solution is inversely proportional to the time of immersion — the higher
the temperature, the shorter the immersion time. It is important that solution
strength be monitored and renewed as required.
All chemical etch techniques require a water rinse and elevated temperature drying.
Sulfuric acid etch.
•
The sulfuric acid-dichromate etch is recommended
for use on acrylonitrile-butyldiene-styrene (ABS), acetal, melamine, or
urea, polyolefins, polyphenylene oxide, polystyrene, polysulfone, and
styrene-acrylonitrile (SAS). A different ingredient ratio, immersion
temperature, and immersion time is recommended for each plastic.
Table 7.10 shows the parameters of the sulfuric acid-dichromate etch.