Surface preparation methods, such as degreasing alone or degreasing followed by abrasion and removal of loose particles, are generally adequate for most adhesive bonding applications. However, to achieve optimal strength, consistency, and long-term durability of a bond, a chemical or physical pretreatment may be necessary. This article will provide examples of such specialized pretreatments.
Acid Etching
A metal substrate’s visible surface is not often made of pure metal and is much more commonly a combination of oxides, chlorides, sulfides, and other atmospheric contaminants, which make for a mechanically weak surface.
Acid etching is a widely recognized technique for eliminating surface oxidation layers, typically weakly adhered to the metal surface, and replacing them with a strongly bonded layer that is both mechanically and chemically compatible with the adhesive.
Various acid treatments are employed depending on the type of metal substrate being treated. For instance, chromic acid is used for aluminum, sulfuric acid for stainless steel, and nitric acid for copper.
Anodizing
The aerospace industry extensively uses anodizing as a surface pretreatment for titanium and aluminum alloys. Anodizing aims to deposit a porous oxide layer atop the oxide layer formed post-etching. This porous oxide layer facilitates easy penetration of adhesive (or primer) into the pores, resulting in a robust bond.
For effective bonding, the oxide layer produced by anodization should remain unsealed. In the case of hard-anodized aluminum alloy, stripping is necessary, achieved through abrasive blasting or etching, as the hard anodized finish inherently provides poor bonding properties.
Application of Primer
Another form of surface pretreatment is the application of primer, which is generally used for materials like metals, ceramics, and glass. Primer is typically the last stage of a multi-step pretreatment process.
Some substrates have surfaces that are difficult to bond, such as copper, meaning that they are not easily wetted. The primer, usually a similar composition to the adhesive but with added solvent, easily wets the substrate. When the adhesive is applied to a chemically compatible surface, it forms a strong bond once cured.
Care must be taken when preparing chemical pretreatment solutions, not only due to handling hazards but also due to incorrect preparation that could result in lower bond strengths when compared to no chemical treatment.
Time of application is also significant: too short an application may not allow for sufficient surface activation, whereas excessively long exposure to the chemical(s) can lead to chemical reaction products that will interfere with adhesion.
Following chemical pretreatment, surfaces should be thoroughly washed with enough clean water. It is strongly recommended to use de-ionized (demineralized) water for the final rinse.
Learning Lab: How to Prepare Metals for Adhesive Bonding
Want to achieve a good preparation for metals ?. Video Credit: Huntsman Advanced Materials
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This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.
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