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Why Polymer Modified Asphalt?

Asphalt pavements are flexible by design.

Asphalt is a plastic material which melts and flows at high temperatures and becomes hard and glassy at low temperatures. Therefore, it's ability to perform successfully as a binder is limited by temperatures and stresses applied to the pavement. The Federal government's Strategic Highway Research Program has identified the limitations of asphalt as a binder by developing new test equipment and methods to better classify asphalt. As the result, a new grading system for asphalt has become accepted across the nation. This system, known as Performance Grading, classifies an asphalt's ability to adequately perform at both high and low temperatures. Under this system, asphalts are given a high and low temperature identification in their name, such as PG 76-22. Many of these new grades of asphalt can only be made with the addition of polymers.

Polymers impart significant improvements in an asphalt's ability to withstand temperatures extremes. Polymer modified asphalt is more viscous at high temperatures which improves a pavement's resistance to rutting and shoving. At low temperatures, polymer modified asphalt is more ductile, less glassy. As a result, pavements constructed with polymer modified asphalt are less likely to ravel or become brittle at low temperatures. The elastic component that polymers add to an asphalt makes the pavement tougher and more resistant to fatigue cracking as well as provide a more adhesive binder that adheres to aggregates better even under damp conditions found on tree covered streets.

But what are polymers? Are they all the same?

Polymers are simply very large molecules that are made by connecting smaller molecules called monomers together to form long chains. The physical properties of these polymer chains is dependent upon properties and structure of the monomers and how the monomers are connected. Polymers used for asphalt generally fall into two types being either elastomers or plastomers. Plastomers merely stiffen the asphalt while elastomers make asphalt more flexible at low temperatures as well as stiffer at high temperatures. Polymers of choice for asphalt modification are generally elastic in nature.

For a polymer to be effective it must disperse to form a swollen polymer network in the asphalt. The most economically effective polymers used to modify asphalt are Styrene Butadine polymers in either solid pellet form or liquid latex form. Within the family of styrene butadiene polymers are many types of which only a few are useful and swell appropriately in asphalt to form an effective network. Solid styrene butadiene polymers such as SB and SBS materials must be melted and sheered into hot asphalt at a refinery or terminal. SBR latex, the liquid form of styrene butadiene polymer, is a microscopic dispersion of polymer in water that can be added directly in the production of hot mix at the contractor's plant. Due to the microscopic particle size, the polymer disperses rapidly into asphalt without the need to be melted or sheered in with costly equipment. SBR latex offers many other advantages to both the contractor and engineer to produce higher quality pavements at the lowest cost.


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