Page 255 - Handbook of Adhesives and Sealants

224 Chapter Six

Dry abrasive blast consists of a uniform blasting of the adherend
surface with a clean, fine, non-metallic grit such as flintstones, silica,
silicon carbide, aluminum oxide, or glass beads. The particle size will
vary with the surface and the material. The degree of blasting for
metal substrates should be a ‘‘white metal blast’’—complete removal
of all visible rust, mill scale, paint, and foreign matter. Written defi-
nitions and visible standards as to the degree of surface abrasion are
available. A white metal blast is defined in SSPC-SP 5 (Steel Struc-
tures Painting Council) or NACE No. 1 (National Association of Cor-
rosion Engineers). 24 The degree of blasting for polymeric substrates
should be sufficient to remove the surface glaze from the part.
Each substrate reacts favorably with a specific range of abrasive
sizes. In many applications joint strength generally increases with the
degree of surface roughness. 3,25 Often the joint performance is more
dependent on the type of mechanical abrasion rather than the depth
of the abrasion. Table 6.9 shows that for stainless steel and aluminum,
sandblasted surfaces provide greater adhesion than when the surface
is only machined. However, excessively rough surfaces also increase
the probability that voids will be left at the interface, causing stress
risers that may be detrimental to the joint in service. Table 6.10 pre-

TABLE 6.9 Effect of Surface Roughness on Butt Tensile Strength of Joints Bonded
an Epoxy Adhesive 25
Butt tensile
Adherend Adherend surface† strength, lb/in. 2
6061 Al ................... Polished 4,720 1,000
6061 Al ................... 0.005-in. grooves 6,420 500
6061 Al ................... 0.005-in. grooves, sandblasted 7,020 1,120
6061 Al ................... Sandblasted (40–50 grit) 7,920 530
6061 Al ................... Sandblasted (10–20 grit) 7,680 360
304 SS .................. Polished 4,030 840
304 SS .................. 0.010-in. grooves 5,110 1,020
304 SS .................. 0.010-in. grooves, sandblasted 5,510 770
304 SS .................. Sandblasted (40–50 grit) 7,750 840
304 SS .................. Sandblasted (10–20 grit) 9,120 470
*74 C/16 h cure.
†Adherend surfaces were chromate-etched.

TABLE 6.10 Recommended Abrasive Methods and Size for Various Metal Substrates 14
Material Method Size
Steel..................................... Dry blast 80–100 grit abrasive
Aluminum ............................... Wet blast 140–325 grit abrasive
Brass .................................... Dry blast 80–100 grit abrasive
Brass .................................... Wet blast 140–325 grit abrasive
Stainless steel............................ Wet blast 140–325 grit abrasive