Page 242 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 211

ical abrasion, or active surface modification. Passive surface treatment
processes do not alter the chemistry of the surface but only clean and
remove weakly attached surface layers (i.e., solvent washing, mechan-
ical abrasion). Active surface treatment processes cause a chemical
change to the surface (i.e., anodizing, etching, plasma treatment). Ta-
bles 6.2 and 6.3 characterize surface treatments for metallic and poly-
meric substrates respectively.
More than one surface treatment may be required for optimum joint
properties. A four-step process that is often recommended for achiev-
ing high strength joints with many substrates consists of degreasing,
mechanical abrasion, repeated degreasing, and chemical treatment or
etching. Table 6.4 shows the relative bond strengths that can be re-
alized when bonding aluminum after various surface treatment pro-
cesses. Note that when low or medium strength is sufficient for the
application, only minimal surface preparation is required.

TABLE 6.2 Characterization of Common Surface Treatments for Metals 10
Pretreatment type Possible effects of pretreatment
Solvent Removal of most of organic contamination
Mechanical Removal of most of organic contamination. Removal
of weak or loosely adhering inorganic layers, e.g.
mill scale. Change to topography (increase in surface
roughness). Change to surface chemistry
Conversion coating Change to topography (increase in surface
roughness). Change to surface chemistry, e.g. the
incorporation of a phosphate into the surface layers
Chemical (etching, anodizing) Removal of organic contamination. Change to
topography (increase in surface roughness). Change
to surface chemistry. Change in the thickness and
morphology of metal oxide

TABLE 6.3 Characterization of Common Surface Treatments for Polymers 11
Pretreatment type Possible effects of pretreatment
Solvent Removal of contaminants and additives. Roughening (e.g.
trichloroethylene vapor/polypropylene). Weakening of surface
regions if excessive attack by the solvent
Mechanical Removal of contaminants and additives. Roughening
Oxidative Removal of contaminants and additives. Introduction of
functional groups. Change in topography (e.g. roughening with
chromic acid treatment of polyolefins)
Plasma Removal of contaminants and cross-linking (if inert gas used).
Introduction of functional groups if active gases such as oxygen
are used. Grafting of monomers to polymer surface after
activation, e.g. by argon plasma