Bitumen forms the foundation of a good quality blend. Not all bitumen is receptive to modification and blending with the other components of the final material. Additionally, lower quality bitumen results in poor performance and premature aging of the entire product. Therefore, a manufacturer’s asphalt source is extremely important.

While fillers are the lowest cost material used, they have important roles to play in the final formulation. Filler is the material that stabilizes the blend and provides the finished product with many of its engineered properties. Fillers are typically mineral-based. They are used to stabilize the formula, bolster fire resistance, and increase ultra-violet light (UV) and weathering resistance. For example, the addition of carbon and graphite can increase fire performance. Limestone and basalt can decrease the cost of the formula and aid in absorbing excess oil. Waxes and tackifying oils are added to increase self-adhesive properties of the formula. Most modified bitumen formulas contain between 25 percent and 35 percent filler. If a formulation has too much filler, more than 35 percent in filler additives, the final blend degrades.

Polymers are the most expensive element of a modified bitumen blend. There are three common polymers used to modify asphalt today: SBS (styrene butadiene styrene), APP (atactic polypropylene), and IPP (isotactic polypropylene). We’re going to focus on the two most common polymer blends currently used in roofing: APP and SBS.

APP

APP is short for Atactic Polypropylene. It is a plastic-type polymer that imparts rigidity to the final material. A good APP-modified bitumen material will typically have 20-30 percent APP in the final formulation. APP modified bitumen materials have a higher softening temperature and are harder or have a higher durometer than SBS modified bitumens.

SBS

SBS, or Styrene-Butadiene-Styrene, is a rubber-type polymer that comprises roughly 10 to 13 percent of the total blend. An SBS-modified bitumen blend is a softer, more malleable final product than an APP blend. This additive allows the blended SBS to achieve greater elongation than other asphaltic systems.

Reinforcing scrim

Reinforcing scrim acts as a carrier for the modified bitumen. The scrim is embedded within the sheet and provides dimensional stability for the roll, improves fastener retention, and increases puncture resistance. There are three primary types of reinforcing scrim: fiberglass, polyester and composite or hybrid mats.

The type of scrim or reinforcement will influence different properties of the material. Fiberglass reinforcement is more fire resistant and provides dimensional stability while polyester reinforcement is more puncture resistant and has better elongation and tear resistance.

The modified bitumen sheets can be specified based on the type of reinforcement using different ASTM specification numbers. In the case of SBS rolls, ASTM D 6163 denotes a fiberglass reinforcement, ASTM D 6164 is for polyester reinforcement and ASTM E6162 is the specification for SBS sheets with a combination or hybrid reinforcement. In addition to specifying the ASTM specification number, it is valuable to also reference the Type and Grade of the material. The Type further indicates the mass and physical properties of the sheet: Type II materials have a larger net mass, peak load, and tear strength than Type I. The Grade indicates the surfacing of the sheet, either G for granule or S for smooth.

Summary

All of the components of a modified bitumen sheet - the bitumen, filler, and polymer - come together through a blending process. Adding a polymer changes the dispersal and consistency of all the products throughout the blend, making the blending protocol incredibly important. The blending time and temperature can greatly affect the dispersal and consistency of the filler and polymer within the bitumen matrix, changing the properties of the final material. The modified bitumen is then extruded into rolls with the reinforcement of choice to make the final roll used on a building.

Mosaic Ultra Urban Living Apartments

Dallas, Texas

Photo courtesy GAF/Siplast

The completed roof renovation at Mosaic Ultra Urban Living Apartments project in Dallas, Texas.

The Mosaic project involved the 2007 renovation of the former Fidelity Union Bank and Insurance complex, two vacant downtown Dallas office towers. Mosaic is one of Dallas’ most striking high-rise residential communities.

The challenge
These former office towers required full removal of up to four layers of existing roofing and insulation from 15 different roof levels. Inadequate drainage had created severe ponding issues on all roofing levels, as well.

The solution
Products including highly energy-efficient insulation, a strong, resilient, non-woven polyester mat coated with weather-resistant, APP polymer-modified asphalt, and self-adhered SBS helped create an updated and effective roofing system. The roofing renovation happened on time and within budget. Tapered polyiso foam insulation was used to provide high thermal efficiency and to virtually eliminate ponding water. A durable non-woven glass mat, coated with flexible, polymer-modified asphalt, was also used, which combined the strength of fiberglass reinforcement with the elongation characteristics of SBS modified asphalt.

The self-adhered modified bitumen roof systems earned praise for being clean and environmentally friendly. This project was the Winner of the ARMA Quality Asphalt Roofing Case-Study Award.

Installation and Application Options

Commercial asphalt roofing systems are multi-ply systems consisting of two or more waterproofing layers. This redundancy provides building owners with durable protection for their building. Beyond the material properties, asphaltic or bitumen roof systems are adaptable and can be installed using various methods with multiple detailing options and surfacing varieties.

Image courtesy GAF/Siplast

Figure 5. Installed image of a modified bitumen system.