Page 247 - Handbook of Adhesives and Sealants

216 Chapter Six

TABLE 6.6 Maximum Allowable Time between Surface Preparation and Bonding or
Priming of Metal Substrates 14
Metal Surface Time
Aluminum ............................ Wet-abrasive-blasted 72 h
Aluminum ............................ Sulfuric–chromic acid etched 6 days
Aluminum ............................ Anodized 30 days
Stainless steel ......................... Sulfuric acid etched 30 days
Steel.................................. Sandblasted 4 h
Brass ................................. Wet-abrasive-blasted 8 h

Passive mechanical processes use physical or mechanical means to
remove some of the surface material and its contaminants, thereby
exposing a fresh, clean, and chemically active surface. Mechanical sur-
face treatments are usually not sufficient by themselves. Some form
of chemical or solvent cleaning is also necessary to remove organic
contaminants from the surface. Chemical or solvent cleaning is per-
formed before mechanical abrasion and again immediately afterward
to remove dust and other remnants from the abrasion process. Sand-
ing, abrasive scrubbing, wire brushing, grit blasting, grinding, and
machining are common examples of mechanical processes. Surface ab-
rasion is important because it increases the substrate surface area
that is in contact with the adhesive in addition to removing weak
boundary layers. Although mechanical abrasion processes are fast and
quickly expose the bulk material, they often have a high material cost
and labor content. Care must be observed regarding the contamination
of the abrasive media and possible recontamination of cleaned sub-
strates. Therefore, the abrasive media must be checked and changed
often.
Passive chemical cleaning processes merely remove obvious surface
contamination, including soil, grease, oil, fingerprints, etc. by chemical
means without altering the parent material. Vapor degreasing, solvent
washing, alkaline and detergent cleaning, and ultrasonic cleaning are
typical examples. As with any process utilizing cleaning solutions,
sudden or gradual contamination is always a possibility and must be
considered in a quality control plan. There are significant new devel-
opments occurring with these processes due to safety and health is-
sues and new environmental regulations. 15,16 New equipment and pro-
cesses have been designed to eliminate or reduce harmful emissions,
and new ‘‘safety solvents’’ have been developed to replace harsh clean-
ing solvents and chemicals.

6.4.1 Passive chemical surface treatment
Passive chemical surface treatments remove soil and organic contam-
inants from the surface. They include such common processes as sol-