Latex Binders Polymer

Chemists who make latexes generally start by understanding how the product will be used. That’s because the end-use application of an emulsion polymer has significant implications for how it’s designed. Knowing the end use helps the chemist develop the right recipe, with all of the right ingredients, that results in an emulsion polymer with the right properties. 

In Stephen Covey’s well-known book, “The 7 Habits of Highly Effective People,” he observes that productive individuals always begin with the end in mind. Chemists who make latex binders would certainly adhere to this rule. That’s because the end-use application of an emulsion polymer has significant implications for how it’s designed. Knowing the end helps the chemist develop the right recipe, with all of the right ingredients, that results in an emulsion polymer with the right properties.

In this article, we’ll examine this recipe-building process to understand the decisions made by chemists when they design a latex binder from scratch. To do this, it helps to review the fundamentals of emulsion polymerization, the process used to make any synthetic latex. An emulsion polymer requires a number of ingredients, all of which get used during the polymerization process to create a polymer in water:

• Monomers
• Surfactants
• Initiator
• Carboxylic acids and specialty monomers

Monomers are the building blocks of the polymer. Most monomers, however, don’t like being in water, so surfactants are added to create cell-like environments — known as micelles — that form when hydrophobic (water-hating) tails of the surfactant gather toward the center while hydrophilic (water-loving) heads orient themselves outward, in contact with the water. Monomers then migrate from larger monomer droplets into the micelles, where they are shielded from water. Next, initiator chemicals enter micelles and trigger the chain-reaction polymerization that joins one monomer to another, until a long chain is formed. Finally, carboxylic acids and other specialty monomers, which are polymerized into the polymer backbone but exposed at the particle/water interface, help to stabilize the latex.

While the basic polymerization process is the same for any latex, each of the ingredients described above becomes a component that can be chosen by chemists to achieve a specific result.

The first decision confronting chemists is the choice of monomer, which comes down to understanding the environmental conditions of the final product. If the end-use application demands that the latex binder be exposed to sunlight, then the material runs the risk of degradation. This occurs as ultraviolet radiation, absorbed by the polymer, creates destructive free radicals, leading to issues ranging from loss of strength and flexibility to fading color and cracking.

Acrylic latexes made, for example, from methyl methacrylate and butyl acrylate monomers show better UV resistance than those made from styrene and butadiene. As a result, acrylic emulsions are good candidates for binders that must demonstrate excellent exterior durability and UV resistance. Styrene-acrylic emulsion polymers are another good option for exterior applications. In addition to UV performance, styrene-acrylic latexes offer enhanced water resistance, abrasion resistance and hardness, making them a suitable material for applications such as industrial coatings, wood coatings, concrete coatings, primers, filter media binders and traffic paint. Styrene-butadiene emulsion polymers are often the chemistry of choice when direct long-term UV exposure is not an issue. Styrene-butadiene latex binders are ideal for applications requiring excellent water resistance, high filler acceptance, good balance of tensile and elongation, and good adhesion to challenging substrates.

Another important consideration when selecting monomers is glass transition temperature, or Tg — the range of temperatures over which an amorphous polymer becomes less glassy and more rubber-like or vice versa. Chemists can choose monomers in specific combinations of hard and soft to achieve a certain Tg. For example, in a styrene-butadiene copolymer, as the styrene content increases, the Tg also increases. Conversely, if the butadiene content increases, the Tg decreases. By changing the ratio of styrene to butadiene, it’s possible to influence the Tg and produce polymers with different characteristics. The following mechanical properties are directly related to the glass transition temperature:

• Tensile modulus, which measures how much a polymer resists deformation
• Percent elongation, which is a measure of how much the length of a polymer changes when stretched
• Tensile strength, the amount of force a polymer can withstand before breaking