Thus, in an SBS-modified asphalt pavement, the polystyrene adds strength while the polybutadiene adds flexibility and elasticity. “Styrene helps control the organization of the butadiene,” Daranga says. “If I just put the elastic parts in there, they would float all over the place, looking like a ball of spaghetti. But if I can tie the spaghetti here and there with the crystal-like structure provided by the styrene, that’s much better.”

A related, popular polymer modifier is SBR. “If SBS is styrene-butadiene-styrene, then SBR is styrene-butadiene-rubber,” Kluttz says. “They are similar, but not exactly the same. With SBR, the styrenes and butadienes are all mixed together, randomly, compared to SBS, which as a block copolymer is organized in blocks.”

While SBS is supplied to the liquid asphalt supplier in small, solid pellets, and has to be dissolved in the liquid asphalt using mixing equipment, SBR is a liquid latex, which can be injected into the liquid asphalt by the binder supplier or at an asphalt plant.

It’s supplied in drums, tankers or even rail cars. These products will be 30 percent water, compared to SBS which are pellets. If blended into an emulsion for surface treatments, SBR may be cationic or anionic to enhance performance with existing aggregates.

Ethylene-Based Polymers

Another popular family of polymer modifiers are the cyclic ethylene-based polymers, and the pure ethylene-based polymers like ethylene-vinyl-acetate (EVA).

EVA is the copolymer of ethylene and vinyl acetate. The weight percent of vinyl acetate usually varies from 10 to 40 percent, with the remainder being ethylene. As a plastomer it’s a polymer modifier but it doesn’t have the “stretchy” properties of an elastomer. EVA is said to be an older technology, but it still can be used to bring liquid asphalt up to a PG rating.

“If I just put the elastic parts in there, they would float all over the place, looking like a ball of spaghetti. ”

- Codrin Daranga, Ph.D., technical manager, Blacklidge Emulsions

That’s because EVA is a polymer that approaches elastomeric materials in softness and flexibility, yet can be processed like other thermoplastics. EVA typically is added at concentrations between 2 to 5 percent by weight of asphalt binder, and is typically dispersed to the hot asphalt binder at temperatures between 300 to 340 degrees F.

“Benefits of this polymer modifier include the fact that only moderate agitation is necessary,” says blending equipment manufacturer Ecopath Holdings. “The blends can be stored for weeks without succumbing to separation. Consistent with many other polymer modified asphalts, the compatibility of the EVA and asphalt binder is vital for achieving the desired properties.”

Studies have shown that at lower polymer contents (3 percent by weight), EVA modified binders exhibit dispersed polymer particles in a continuous bitumen matrix, Ecopath reported. “EVA modified binder properties, such as morphology and storage stability, are influenced by the characteristics of the base bitumen and binders,” Ecopath says. “Generally, increases in EVA concentration yield greater improvements in the binder; however, these increases also lead to reductions in storage stability. Other studies have shown that, when EVA and SBS modified binders are compared to neat binders, SBS binder exhibits a significantly higher elastic recovery than neat binders. Also, EVA binders tend to exhibit fewer improvements in elastic recovery while losing ductility and elastic recovery at a greater rate.”

And that’s not all. “There are chlorinated polyethylenes, oxidized polyethylenes and polypropylenes,” Blacklidge Emulsions’ Daranga says. “Depending on how long or short the molecules are, you get into the waxes or paraffins. There are ring-based polymers such as DuPont’s Elvaloy. Cellulose-based polymers can be used as reinforcers. The possibilities are limited only by our imagination.”

Response to Superpave

The advent of the PG binder rating system after the first Strategic Highway Research Program (SHRP) and the Superpave system of asphalt mix design was the prime driver of the current interest in asphalt modifiers.

“It definitely highlighted the need for us to modify asphalt to get the better-performing binder properties that we needed out of the same old asphalts we were using,” Daranga says.