Small amounts (~1-3%) of vinyl acid monomer such as acrylic acid or methacrylic acid are commonly added to latexes to improve colloidal stability. They may also serve other functions, such as improving adhesion to substrates for coatings applications. However, unlike typical hydrophobic monomers, it is often not simple to know or measure where the functional monomer resides at the end of the polymerization. Ideally it is primarily located at the particle surface, where it can serve its intended role by providing additional electrostatic stabilization. However, it can also be buried within the particles or remain in the serum (water phase) as water-soluble copolymers or oligomers.

Buried acid groups are usually not detrimental to latex or film properties, although they represent “lost” acid groups. However the formation of water-soluble polymers can have a detrimental effect on properties. Careful analyses can reveal how your acid is partitioned between these three regions. Although often not appreciated, it is important to recognize is that how you conduct your process has a profound impact on where the acid groups reside and therefore on your final product properties. Important considerations in addition to the specific acid monomer used are pH, monomer feeding profiles, initiator type and temperature. Even with the same formulation, you may obtain very different acid group distribution if you change your process conditions.