Page 263 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 231

During the surface treatment, the part or area to be bonded is usually
immersed in a chemical solution at room or elevated temperature for
a matter of minutes. Tank temperature and agitation must also be
controlled. Chemical solutions must be changed regularly to prevent
contamination and ensure repeatable concentration. After chemical
treatment, the parts are immediately rinsed with water and dried. The
rinse operations are to remove the residue of the surface treatment
steps. Chemical treatments are usually specified by the following par-
ameters:

Solution used. Such as caustic etch, sulfuric–dichromate, sodium
etch, ferric chloride–nitric acid, etc.
Solution temperature. Usually room temperature to 100 C.
Immersion time. Usually seconds to minutes.
Type of rinse. Usually running tap water followed by distilled water.
Type of drying. Usually in air at room temperature. However, ele-
vated temperatures or other conditions are not uncommon.

Chemical treating solutions should be prepared in containers of
glass, ceramic, or chemical resistant plastic. Stirring rods should be
made of the same material. Metals others than those to be treated
should not touch the solution. For solutions containing chemicals that
attack glass and polyolefin containers (e.g., hydrofluoric acid, fluo-
rides), TFE fluorocarbon containers should be used. Solutions in plas-
tic trays can be heated by immersion in hot water baths. Hot plates
or infrared heaters can be used for glass and porcelain trays.
Many chemical treating solutions have been developed over the
years for both metallic and non-metallic substrates. These are de-
scribed in detail in the final sections of this chapter and in Appendices
C-1 through C-4. Many proprietary metal surface treatment solutions
have also been developed by companies that specialize in this field.
These usually contain mixtures of acids and specialty chemicals to
control over-etching. Since these solutions come already mixed, their
hazardous nature is less than if one had to formulate a solution from
raw materials. However, the commercial solutions are still hazardous
and provide environmental problems particularly in regard to the dis-
posal of spent liquid. Some paste-type etching products are also avail-
able that simultaneously clean and chemically treat surfaces. They
react at room temperature and need only be applied to the specific
area to be bonded. Thus, these treatments are very useful for complex
part geometries and for parts that cannot be immersed in a chemical
solution. However, these paste etchants generally require much longer
treatment time than acid-bath processes.