Hard-to-adhere metals

Key Points Note

While metals have high-energy surfaces, some types are difficult for paints to adhere to. Surfaces that are easily adhered to are covered with water molecules, and the presence or absence of -OH groups plays a crucial role in determining adhesion properties. 

1. Reasons for Difficulty in Adhesion to Certain Metals 

Characteristics of Metals like Aluminum, Stainless Steel, and Copper: These metals form oxide or passive films on their surfaces, making it difficult for the functional groups of paints to adhere. On the other hand, iron readily adsorbs moisture from the air, and paints with hydrogen-bonding functional groups adhere well to it. 

The Role of -OH Groups in Metal Surfaces: The lack of -OH groups on metal surfaces is thought to cause poor adhesion. Copper, covered with an unstable basic film, has poor adhesion. Zinc, while similar, has a basic film that is dense and hard like aluminum. 

2. Methods to Improve Adhesion 

Surface Treatment of Difficult-to-Adhere Metals: For aluminum, etching primers are effective, and for stainless steel and copper, blasting processes remove the inactive films. Stainless steel is protected by a chromium oxide film, but on galvanized zinc with chromate films, which primarily consist of chromium hydroxides, adhesion is good. 

Reaction on Galvanized Zinc Surfaces: On galvanized zinc surfaces without chromate films, water-soluble substances like zinc chloride and ammonium chloride exist and react with infiltrated water to form zinc hydroxide Zn(OH)2. Over time, this can lead to blistering and peeling of the paint from the galvanized layer. 

The presence or absence of -OH groups on metal surfaces appears to be a crucial factor in determining the difficulty in adhesion. 

3. Adhesion Theory 

Polymer-Metal Adhesion Mechanism: Previous experiments using Langmuir balances to measure the tension of monolayer films of epoxy resin revealed that the content of hydroxyl groups significantly affects adhesion strength. 

Influence of Hydrogen Bonding: This adhesion mechanism is based on the formation of hydrogen bonds, suggesting that the adhesion strength of polymers to metals and wood depends on hydrogen bonding. Although this theory has been supported for a long time, various theories exist, such as the proposal that epoxy resin does not form hydrogen bonds with water or that acid-base interaction forces are the main factor in adhesion strength. 

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