Polymer Modified Asphalt - What, How, Why?
The term “polymer” simply refers to a very large molecule made by chemically reacting many (poly) smaller molecules (monomers) to one another in long chains or clusters. The physical properties of a specific polymer are determined by the sequence and chemical structure of the monomers from which it is made. When polymers are added to asphalt, the properties of the modified AC depend on two things: 1) the polymer system used and 2) the compatibility of the polymer with the asphalt.
Elastomers and Plastomers - Engineer for performance
Specific binder and mix properties can be engineered by choosing the right polymer for the application, and making sure it is compatible with the asphalt. In general, elastomers are chosen to give a more resilient, flexible pavement, while plastomers result in mixes with higher stabilities and stiffness moduli. The results are highly dependent upon the concentration, the molecular weight, the chemical composition, and the molecular orientation of a particular polymer as well as the crude source, the refining process and the grade of the base asphalt used.
How does an asphalt change when modified by polymer?
Polymer modification causes significant changes in the stress-strain behavior, the creep response and the non-newtonian flow patterns. The ability of some polymers to elastically recover (creep response measured by monitoring the flow of a material under a load, and its elastic recovery when the load is removed) gives added durability to an asphalt. New tests have been developed to measure these properties. The Superpave binder test methods are more accurate than conventional methods (absolute and kinematic viscosities, ring and ball softening points, penetrations, etc.) at characterizing the physical properties of polymer modified asphalts, particularly at high and low temperatures. As an example, an AC-20 which grades as a PG 64-22, may become a PG 70-28 when modified.
Asphalt compatibility and polymer properties are key to performance
If a polymer is added to two different asphalts, the physical properties of the finished products may be very different. To be most effective, the polymer should form a continuous network within the asphalt. For this to happen, the chemistries of polymer and asphalt need to be compatible. The photomicrographs here are all SB or SBS polymers in different asphalts. The white is polymer, the black is asphalt. The first two have a continuous polymer network, have a stable structure that doesn’t separate, and take full advantage of the elastic properties of the polymer. The second two are not networked, separate on storage, and don’t have the same enhancement in properties. Some PMAC producers use special processes to compatibilize polymer and asphalt. When the technology is right, the binder properties can reduce rutting, raveling, thermal cracking and stripping in the mix, as well as increase the stability and fatigue life.
Manufacture, storage and construction practices vary with polymer system
Conventional manufacture, storage and construction practices may or may not need to be modified depending upon the polymer system used. Some systems require constant agitation to avoid segregation. Some polymer systems specify elevated temperatures for hot mix manufacture and construction, others can be run at conventional temperatures. Current methods of determining optimum mix and compaction temperatures do not take into account the changed flow properties of the modified binder. Recommended temperature and placement practices for a particular system should be followed.
Why modify?
Improvement in resistance to rutting, thermal cracking, fatigue damage, stripping, and temperature susceptibility have led polymer modified binders to be substituted for asphalt in virtually all paving and maintenance applications, including hot mix, warm mix - cold lay, cold mix, chip seals, hot and cold crack filling, patching, recycling, and slurry seal. They are used wherever extra performance and durability are desired. In many cases they are selected to reduce life cycle costs. Polymer modified binders have allowed the use of techniques previously not practicable, such as micro-surfacing and using emulsion chip seals on high traffic roads. And specifiers are finding that many of the Superpave binder grades require polymer modification to concurrently meet the requirements for high temperature resistance to rutting and low temperature resistance to thermal cracking.
To meet the needs of our customers, we have found that Stylink® polymer modified asphalt technology gives the best results in terms of microstructure, compatibility, physical properties, consistency of performance, ease of handling, and longer lasting, more cost-effective pavements. Our technical group has used the special properties of Stylink® technology in formulating products and systems that meet and exceed Superpave requirements, as well as virtually every type of tough paving application. We want to provide our customers with the solutions for building, maintaining and repairing longer lasting roads with lower life cycle costs. Contact your local Road Science™ representative for help in building structurally sound pavements with lower life cycle costs.