Basically there are two types of polymer modifiers: elastomers and plastomers. Elastomers increase modulus (stiffness) and also give the asphalt elasticity and stretchiness. Under load it can provide recovery under deflection. Plastomers are more limited in that they just stiffen the asphalt, so they don’t provide the kind of recovery you get with an elastic material, but still may be best suited for a particular application.

A monomer is an atom or a small molecule that binds chemically to other monomers to form a polymer, which is a large molecule made up of repeating structural units connected by covalent chemical bonds.

“A monomer is ‘one’ piece, an individual molecule,” Kluttz says. “A polymer is a bunch of monomers connected together. For example, a molecule of butadiene has only four carbons, but we can hook together a thousand of those end to end to make a polybutadiene chain that’s then 4,000 carbons long. We then hook polystyrene chains to each end of that. The result is a block polymer, which has blocks of polystyrene, and blocks of polybutadiene.”

Adding flexibility and strength to liquid asphalt by dissolving long-chain polymer molecules in it will improve the asphalt, Daranga says. “We will add different kinds of polymers, most commonly styrene-butadiene-styrene (SBS), but there is a huge choice of polymers for modification.”

“There are well over a hundred crude oil fields that are being accessed in North America . . . They will each respond somewhat differently to modification. Consistency is the issue.”

-Bob Kluttz, senior scientist, research and development, Kraton Polymers

Imagine a chain in which the links are monomers, Daranga says. “Polymer describes multiple monomers linked in a long chain,” he says. “All the chain links can be the same, or you can have two different kinds of chain links. The way you put them together opens up a whole world of possibilities. You can put them together one after the other, as in A-B-A-B-A-B. You can put them together as all As, and when you run out of those, all Bs. They can be assembled in a completely random or statistical fashion, or fancy patterns, AAA-BB-AAA-BB.

“SBS is just that,” Daranga told Better Roads. “It implies you have three blocks, SSS-BBB-SSS. But the chemist also can vary the length of the blocks and the ratio between them makes a huge difference in properties. SBS is a tri-block copolymer.”

The beauty and popularity of the SBS modifier comes from its union of the hard monomers of styrene – which provides stiffness and strength (think strong polystyrene plastic children’s toys) – with the flexibility of polybutadiene (think synthetic rubber).

“The best way to visualize an SBS molecule is as a nunchuk,” the Okinawan martial arts weapon, Daranga says. “You have two sticks with a flexible link in between them. The styrene is the stick and the butadiene is the flexible link, but not like a chain, instead like a rubber band. They also have the ability to stack one on top of the other, and create crystal-like domains. If you have a bunch of these molecules and put them together, they will try to stack up in a fashion almost like cross-linking, but it’s a physical, temperature-driven process that you can reverse by melting those crystal-like domains.”

The process is similar to vulcanization of rubber, where the rubber molecules or components are tied together with very strong sulfur bonds. “The same thing happens with SBS when it cools off,” Daranga says. “If it cools enough, and those styrene chains are close enough, they will pack together in a similar but weaker fashion to create a network, only not as strong as in a tire.”

“SBS polymers are probably the most common modifier used in paving globally,” Kluttz says. “What’s special about SBS – why it works so well – is that it’s a combination of two different polymers. It’s a block polymer, so it has blocks of polystyrene, and blocks of polybutadiene. When you mix SBS polymer into hot asphalt, it completely dissolves into a homogeneous liquid. But when it cools back down, the polystyrene domains come back out and come together to give the liquid asphalt increased stiffness or modulus.”

Modern PG binder specs sometimes also have an elastic recovery spec. “In the lab you stretch it out and it has to recover 60 or 70 percent of its original length,” Kluttz says. “Asphalt has near-zero elastic recovery, and for most of the binders today, we’re only putting in 2.5 to 3 percent polymer. So how does that little bit of polymer give that much ‘stretchiness’ to the asphalt?

“What happens is that the polybutadiene rubber polymer looks a whole like the light ends of asphalt,” Kluttz answers. “The polymer literally soaks up the light ends of asphalt – the nonpolar saturates and aromatics – so the 2 to 3 percent effectively becomes 20 or 30 percent, with the polymer swelling like a sponge soaking up water.”