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.

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.

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.