Page 256 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 225

sents a range of abrasive sizes and methods that have been found
favorable for abrasive cleaning of several common metal substrates.
Recommended abrasive sizes for metals are in the 80–325 grit range;
sizes recommended for polymeric substrates are in the 120–220 grit
range.
Certain low energy surfaces, such as polyolefins and fluorocarbons,
should generally not be abraded at all prior to application of an ad-
hesive or sealant. Abrasion and the resulting roughness on a low en-
ergy surface will only increase the probability of air pockets being
trapped in the crevices and valleys at the interface. These air pockets
contribute to stress concentration points and a generally weaker joint.
A general rule of thumb is that unless the adhesive makes a contact
angle of 90 degrees or less with the substrate, mechanical abrasion
and surface roughening should be avoided.
A wet-abrasive blasting process can be employed which may be more
adaptable than conventional dry blasting. With this process a 20–325
grit aluminum oxide or glass bead abrasive slurry is used as the blast-
ing medium. Generally, three parts of water by volume to one part by
volume of the abrasive is used. Wet-blasting units can handle a wider
range of abrasive materials, and a spray rinse automatically removes
the blasting residue.
Several specialty mechanical blasting systems have been developed
over the years for applications where standard grit blasting is not
appropriate. These methods include cryogenic, hydrodynamic, and pol-
ymeric blasting.
Cryogenic and hydrodynamic blasting have been used as an abra-
sive-free surface treatment prior to adhesive bonding. Both processes
are ideal when conventional abrasive media provides a contaminant
or when the collection and reprocessing of the abrasive are prohibitive.
These processes are often used in the field for surface preparation of
structures in the need of repair.
The cryogenic process uses peletized carbon dioxide at 100 Fasa
fluidized abrasive cleaning agent for surface preparation and removal
of corrosion and old coatings. 26 Not only does this provide an abrasion
mechanism, but certain inorganic salts and organic contaminants can
be dissolved with supercritical carbon dioxide.
High pressure water blast has been used for prebond surface treat-
ments to eliminate hazardous materials. The combination of high
pressure water abrasion with subsequent application of an adhesive
promoter/primer has been found to provide high strength and durable
aluminum bonds. 27,28
Polymeric blasting media has also been used for removal of paint,
coatings, and other contaminants for a variety of different applica-
29
tions. The abrasive medium consists of hard plastic material. Various