4.3.1 Cleaning

Any barriers to surface contact will reduce an adhesive’s bond strength. Such barriers include accidental contaminants like dirt, rust and oil or intentionally applied materials like detergents or protective films. Metal may be contaminated with cutting oil or corrosion inhibitors. Injection-moulded plastics typically have mould release on their surfaces. Table 9 shows some common sources of contamination.

Figure 82 Surface contamination on substrate preventing adequate adhesion
Table 9 Common Sources of Contamination
Source of Contamination Contamination
Manufacturing processes Release agents, graphite content in cast iron, drawing lubricants
Handling Oils
Environment Moisture, dirt and dust
Machining processes Cutting fluids, swarf, corrosion inhibitors
Inactive layers Oxide layers (metals) protective oil, e.g. black oxide steel bolts
Residues from galvanic processes Metallic protective coatings like zinc, nickle, phosphate or chromate
Cleaning processes Residues from alkaline baths or industrial cleaners, corrosion inhibitors

One cleaning step to remove all contaminants is preferable. However, depending on the nature of the contamination on a substrate, multiple methods may be required. Multiple forms of contamination are typically present, especially on used parts. In addition to removing contaminants, the cleaner used must be compatible with the substrate and the adhesive. Cleaners must also follow health, safety and environmental standards, such as those for volatile organic compounds (VOC) and biodegradability. The drying time of a cleaner and the required equipment must also be considered in the choice of cleaners. Table 10 outlines considerations when choosing a cleaning process.

Table 10 Cleaning Application Details and Considerations
Application Detail to Consider Importance of Consideration
Type of Contamination The cleaner should be selected based on the chemistry of the contamination; polar/nonpolar, inorganic/organic.
Process Equipment: dip tank, spray, wipe, etc. Time: solvent evaporation, secondary residue removal
Parts being cleaned Compatibility – does not chemically attack the substrate
Adhesive system Compatibility – does not affect cure mechanism; no residue to act as a release agent
Health and Safety of Operators Cleaners should comply with health and safety standards to protect workers and the environment.

Multistep cleaning should follow a particular order. While new parts may only need solvent or aqueous cleaning to be ready for bonding, parts with loose particulate, rust, old paint or old adhesive will require multiple cleaning steps. Before abrasion with sandpaper, wire wheel, grit blasting or other aggressive, mechanical abrasion, the surface should be cleaned with a solvent or aqueous cleaner to prevent contamination from being driven further into the surface. After mechanical cleaning or abrasion, a solvent will remove loose debris and lingering contaminants. Aqueous cleaners should not be used on freshly mechanically cleaned or abraded metal parts due to the risk of flash rust. This sequence also applies to mechanical abrasion intended to increase bond strength by roughening.

Once cleaned, parts should be bonded immediately or stored where they are protected from recontamination. Metals prone to oxidation should be bonded or treated with a rust preventative shortly after being cleaned.

The following cleaning methods may be used alone or in combination, depending on the needs of each part.

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