Carboxylated vs. non-carboxylated?
When asking the question of carboxylated vs. non-carboxylated, the answer comes down to adhesion. Some applications demand a coating that bonds well to the substrate and others need a much lower level. Paints are a good example of the former, expected to remain bonded for years, while peel-able protective films and those “Hello, my name is ...” stickers should release easily.
Carboxylation is the process by which a specialist manufacturer of polymer emulsions increases the adhesion provided by a styrene-butadiene emulsion. How this is done happens deep in the heart of the polymerization process.
Surfactants and polymerization
Polymerization, whether of styrene and butadiene or other compounds, entails persuading the molecules to join up in chains. The extent of chain formation is governed by the addition of a surfactant. Part hydrophobic, part hydrophillic, the surfactant strongly influences properties like particle size and distribution and the viscosity of the emulsion.
Despite it's importance for polymerization, the presence of surfactant is a negative in many coating applications. Free surfactant (the proportion not bonded to the polymer chains) in particular tends to agglomerate, which degrades moisture sensitivity. This shows up in comparison of MVTR numbers for carboxylated and non-carboxylated coatings.
Including a strong acid monomer like acrylic acid, methyl methacrylate acid, or itaconic acid in the polymerization reaction adds carboxylic acid to the polymer. This stabilizes micelle formation and reduces the amount of surfactant needed.
A second effect of carboxylation is to add cross-linking capability. Cross-linking is the degree to which the polymer chains join to one other, and can be measured by the “acid number.” A high number indicates more carboxylic acid groups, quantifying the amount of acid present.
If a polymer is visualized as a single strand or chain, cross-linking weaves these together, increasing strength and toughness. In the end coating, surface hardness and gloss are higher than in non-cross-linked emulsion products. Resistance to grease, solvents, abrasion, heat and UV all improve, and the pigment binding capacity is enhanced. In addition, a carboxylated emulsion provides good adhesion to a substrate, and increases over time.
Applications for carboxylated polymer latex emulsions
With excellent water resistance (due to low levels of surfactant plus the cross-linking), good adhesion, and high strength and elasticity, carboxylated products are appropriate in coatings for metals, plastics and glass. They also find application in paper and non-woven coatings .
Applications for non-carboxylated latex emulsions
Whenever low initial adhesion is needed, and it is desirable that it not build over time, a non-carboxylated product is appropriate. Temporary protective coatings such as film-type products are a good application, as are removable adhesives for use in stickers or decals.
Consider the application requirements
When selecting a latex emulsion, the formulator must consider the properties, particularly the level of adhesion, needed in the end product. By managing the level of carboxylation, Mallard Creek Polymers can tailor adhesion and other properties like water resistance and hardness to meet the specific application requirements.