What makes glue harden with exposure to air?
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Depends on the glue. But most glues use a solvent (water, alcohol, or something else) to suspend an adhesive. When exposed to air, the solvent evapourates leaving the hardened adhesive behind.
Cyanoacrylates like Super/Krazy Glue react with water. The humidity in the air causes cyanoacrylate to chemically change and seal the mating surfaces together.
The mechanism depends on the adhesive. Can you be more specific?
Ok, others covered drying off a solvent by evaporation, and anionic polymerization initiated by water.
The last major group you’d encounter are moisture-cured isocyanates (for example “Gorilla Glue”). These are triggered by water vapor in the air and/or physisorbed on the surfaces being joined. The reaction mechanism is a combination of hydrolysis, followed by condensation polymerization.
Glues like Wood glue and White glue work by evaporation. The are water based and as the water evaporates the glue hardens and creates a strong bond. They stay liquid in the bottle because the water can’t easily escape.
Glues like Cyanoacrylates (super glue) and polyurethane (Gorilla glue, PL Premium) work by chemically reaction. When they are exposed to water like the moisture in the air then undergo a chemical reaction that cures them. They stay liquid in the container because water can’t get to them.
This is why blowing on superglue helps it cure, it’s not the air it’s the moisture in your breath that does it
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