Paved roadways have been around since ancient Babylonian civilizations began blending asphalt with gravel to accommodate travelers and merchants. In more modern times, roads constructed from asphalt, or bitumen, appeared in the late 1700s as chemists started to better understand the binding qualities of natural asphalt in surface performance. 

 

Fast forward to today, and the Earth is traversed by over 40 million miles of roadways with higher traffic and larger vehicles than ever before. Increasingly, departments of transportation, municipalities, and agencies in charge of implementing and maintaining these roads are looking for higher performing asphalt mixes that will last longer while withstanding everything from high travel volume to extremes in weather conditions. 

 

While an asphalt mix in its purest form consists of binder and aggregate, the use of polymer modified bitumen (PMB), or polymer modified asphalt, has been on the rise since the 1970s. These blends are designed to maintain the integrity of the roadway while enhancing the functional performance characteristics of the mix. 

 

Here, we will take a closer look at polymer modified bitumen including what it is and how it helps improve roadway performance in asphalt mixes around the world. 

Polymer modified bitumen is a special blend of bitumen and polymers designed to enhance the properties of the bitumen and the asphalt mixes based on such PMB, with the aim of creating more high-performance roadways.  

 

Virgin bitumen is produced as a byproduct of crude oil refining. It is a highly viscous material that acts as a binding agent when mixed with an aggregate. Bitumen is naturally made up of hydrocarbons that oxidize over time, causing the binder to degrade and the roadway to age. However, most virgin bitumen lacks the complete properties needed in quality to build high performing modern roadways. 

 

Adding a polymer, or multiple polymers, to the bitumen to create a polymer modified bitumen can help strengthen and toughen the resulting roadway. 

Virgin bitumen is a black, sticky substance that can even appear nearly solid depending on the quality and composition produced by the refinery. The hydrocarbon molecules in bitumen naturally group together while smaller molecules move about freely between them. This makes asphalt viscoelastic, meaning that: 

 

  • At high temperatures, the bitumen flows like a liquid and behaves viscously.  
  • At low temperatures, the bitumen appears like a solid and has elastic behavior. 
  • At mid-range temperatures, the bitumen shifts between states depending on its chemical composition. 

 

. Adding polymers to create a polymer modified bitumen can add stability to the binder. This is because the larger, chained polymer molecules help create a more physically sound binder structure.  

Polymer modified bitumen is formulated to provide more stiffness and strength to an asphalt mix. This can include improving the following properties: 

 

  • Resisting rutting deformation 
  • Improving damage tolerance 
  • Preventing cracking and softening  
  • Enhancing water resistance 
  • Diminishing stripping 
  • Increasing rigidity and stability   

Polymer is a general term for very large molecules that chain together in repeating units and are known for their elasticity, strength, and flexibility. Combining asphalt with certain polymers can enhance the performance characteristics through the adjustment of physical properties.  

 

There are two main categories of polymers when it comes to polymer modified bitumen: elastomeric and plastomeric. Occasionally, crumb rubber modified bitumen (CRMB) is also included in discussions surrounding PMBs. 

 

Elastomeric polymers can enhance the binder’s flexibility and ability to maintain performance across a wide range of temperatures. The most common elastomeric polymer in PMBs is styrene-butadiene-styrene (SBS). 

 

Plastomeric polymers can help improve aging properties in asphalt mixes as well as enhance stiffness in the resulting roadway. An example of a plastomeric polymer used in PMBs is ethylene vinyl acetate (EVA).  

  

Another method for modifying asphalt mixes is to incorporate crumb rubber into the blend. Sourced from recycled rubber products such as tires and manufacturing scraps, crumb rubber exhibits physical characteristics similar to elastomeric polymers when added to an asphalt mix.  

A typical PMB is manufactured by bringing together polymer and bitumen at high temperatures with stirring. The exact conditions of temperature, intensity of mixing, and time of mixing will vary with the type of polymer used and the amount of polymer used to make the PMB. 

Both neat bitumen (unmodified) and PMB modified bitumens have a good history of performance in mixes used on roadways. The choice of whether to use one or the other depends on the climactic conditions (heat or cold), expected traffic conditions (high traffic conditions favor selection of PMB), expected durability or lifetime of the roadway (PMB roadways generally have a better expected lifetime), and budget considerations (PMBs are considerably more expensive than unmodified bitumens). 

 

To cope with thermal stresses and strains experienced on a roof, asphalt binder must possess elasticity, ductility, and capability to resist cracking. Polymer modified bitumens (PMBs) are an excellent choice for roofing membranes because they have a good history of providing these characteristics. Additionally, the superior characteristics of PMB membranes allow for fewer overlapping layers on a roof, thus reducing the cost of the roof. Finally, PMB membranes also show better resistance to penetration holes caused by nails or walking in order to provide for a leak-free roof. 

Polymer modified bitumen and PGXpand® 

A unique bitumen-friendly polymer additive, PGXpand is designed to enhance the high-temperature properties of bitumen without negatively affecting low temperature properties. Mixes that incorporate PGXpand benefit from lower viscosity and improved workability. In addition, functional properties can be enhanced at low doses of PGXpand. The result is a roadway that has excellent rutting resistance, roadway durability, and fatigue properties – all with a lower carbon footprint than most PMBs.