AA

What it is

Acrylic acid is the simplest unsaturated carboxylic acid, CH₂=CH–COOH (molecular weight 72.06 g/mol). It is a colorless, corrosive liquid with a sharp, acrid odor that freezes at about 13 °C and boils at about 141 °C. It is fully miscible with water.

Role in coatings

In emulsion and solution acrylics, AA is a functional monomer, usually used at a few percent of total monomer. The carboxyl groups it adds:

• improve colloidal, mechanical and freeze-thaw stability of latexes;
• help wet and stabilize pigments and improve adhesion to many substrates;
• make alkali-swellable thickening possible when used at higher levels (often with methacrylic acid);
• provide reactive sites for crosslinkers such as zinc, aziridines, carbodiimides and epoxies.

AA is also the feedstock for acrylate esters such as butyl acrylate.

Chemistry & key properties

AA copolymerizes readily through its vinyl group. In a latex, much of it ends up near the particle surface, where neutralization with ammonia or amines ionizes the carboxyl groups.

• Acid number: the carboxyl content of the resulting polymer is often expressed as acid number (mg KOH/g).
• Too much acid raises water sensitivity and viscosity, so levels are kept low in exterior and protective coatings.
• AA vs MAA: methacrylic acid is more hydrophobic and gives a stiffer polymer; formulators choose based on stability and water resistance.

Handling & storage

Acrylic acid is corrosive to skin and eyes, flammable, harmful by several routes and very toxic to aquatic life. It is stabilized with an inhibitor (typically MEHQ) that needs dissolved oxygen, so it is stored under air, not nitrogen, within a narrow temperature band. If it freezes, the inhibitor can separate; thaw only at moderate temperature following the supplier’s procedure, because overheating can trigger violent polymerization.