A primer on polymers:
A look at the basics
by Susan Brimo-Cox
products in the decorative concrete industry is common practice. Unfortunately, these products are also commonly misunderstood. What they are, how they work, what they do — “There are a lot of misconceptions about polymers,” observes Stan Stratton, director of technology development at L.M. Scofield’s West Coast production location in Los Angeles.
And as Ronald Borum, executive vice president of Crossfield Products in Rancho Dominguez, Calif., points out, “You have to buy the right tool for the right job.”
You don’t have to be a chemist to understand the basics. “Polymer” is a very generic term that refers to a chain of simple monomer molecules. If the monomers in the chain are the same it is a homopolymer. If the monomers are different you have a copolymer. When polymer particles are dispersed in water you have what is generally called a latex.
 Natural rubber latex is an organic polymer. D. Gerry Walters, a 46-year veteran in the industry and now a consultant based in Pine Knoll Shores, N.C., explains that synthetic polymers didn’t start to appear until the early 20th century. While there are all kinds of latexes commercially available today, most of them are not appropriate for use with hydraulic cements.
The polymers most frequently used with hydraulic cements include: styrene-butadiene rubber (SBR); styrene-acrylic; ethylene vinyl acetate (EVA; also called VAE); vinyl acetate homopolymers (PVA); and vinyl ester of versatic acid (VEOVA).
When polymers are added to concrete or cement mortar, the object is to have the polymer particles evenly dispersed throughout the mix in an appropriate ratio, so that, as the mixture cures, the polymers coalesce and form a co-matrix within the structure of the polymer-modified concrete or mortar.
Polymers are used for two primary reasons: good adhesion and water-resistance, explains Walters.
They also help cement-based products adhere to non-cementitious substrates, such as wood, observes Dan Cook, president of A.W. Cook Cement Products in Atlanta. But don’t equate adhesion with bond strength — they are not the same.
Other characteristics include “added flexural and tensile strength, [and] improved abrasion-resistance,” reports Matt Casto, vice president of technical services of Bomanite Corp. in Madera, Calif. What polymers don’t add, he says, is compression strength.
Stratton echoes that observation. “Polymers soften the cementitious matrix. Instead of resisting load, it gives a little bit, which [helps] eliminate cracking and reduces brittleness. On a bridge deck that’s an important property.”
And, while pour thickness is generally dependent on the size of the aggregate, polymers help enable very thin applications — as thin as one-sixteenth of an inch.
Each kind of polymer has individual properties, however, so it is important to select the right polymer for the project. Contractors should know the basic parameters of the products they use. What is adhesion? What is tensile strength? What do these properties mean? This information is usually listed on the product’s data sheets
The differences between polymers can mean success or utter failure.
“SBR is probably the best for adhesion and water resistance, but, if color is important, [you need to know] it will discolor in ultraviolet light,” observes Walters. “For adhesion, water resistance and color stability, go to an acrylic.” For good adhesion and good color when water resistance is not as critical — such as in an indoor or dry application — a VAE can be used, he says.
You’ll find polymers in a wide variety of decorative and repair products — self-leveling formulations; stampable, thick-section overlays and repair mortars; and micro-toppings. “The best polymer cements are the ones that have hybrid polymers,” says Casto. “Blends are better because you have the best of all worlds.” |
