Modified bitumen roof systems were introduced on the United States commercial roofing market in the late 1970s. The technology for modified bitumen systems was developed in Europe in the 1960s. There are two primary types of modified bitumen systems:
1. Atactic polypropylene (APP).
2. Styrene-butadiene-styrene (SBS).
Atactic Polypropylene (APP) technology was developed in Italy out of necessity. The Italians did not have the proper blowing equipment to apply blown asphalt on the roof area. This prohibited them from converting conventional asphalt flux used in built-up roof systems (BURs) into a product that was capable of withstanding the high temperatures reached on a roof system. An additive or modifier in the asphalt was required to provide the properties required for a proper roll roofing product.
They discovered that a waste by product of polypropylene called amorphous polypropylene when added to asphalt over the proper reinforcement sheet provided a sufficient roll roofing material. The reinforcement is a heavy spun bonded polyester mat. The polyester mat provided the proper durability and elongation stability necessary for the APP modified asphalt.
APP sheets are applied to the substrate by torch application. The APP membrane sheets can be torch applied because the APP modifiers extend the asphalt without changing its basic characteristics. This allows for a thicker asphalt content to be applied to the base of the membrane sheet. The additional asphalt content of the sheet - when torched - provides the adhesive capacity of the membrane to the substrate. The polymer chemistry of APP modified asphalt consists of 25 percent to 30 percent of polypropylene added to asphalt mixed and blended for four to six hours.
Styrene-butadiene-styrene (SBS) membrane sheets were developed by the French and Germans in the 1960s. They required an asphalt with basic properties to be applied in their colder climate regions. SBS asphalt uses 10 percent to 15 percent of a rubber polymer called styrene-butadiene-styrene as an additive to the asphalt modifier. When the rubber polymer is added to the asphalt it goes through an inversion phase from a compound into an isometric form. This form of modified asphalt has many of the characteristics of rubber, including its excellent elongation capacities.
When applied over fiberglass reinforcement, SBS membrane sheets have the inert capacity to stretch and recover to their original shape. Tests conducted have indicated that SBS modified asphalt with 10 percent rubber content can be stretched to six times their original length without breaking and have the capacity to fully recover to their original shape.
SBS membrane sheets can be applied with hot asphalt or solvent-based and waterborne liquid adhesives.
The blending technology of SBS modified bitumen is critical to the success of the sheet. When the bitumen and synthetic rubber polymer are blended, there is a tendency for high polymer liquids not to mix with each other. This causes a phase separation, which can be clearly observed under a microscope. The SBS blend typically consists of the SBS polymer, asphalt and other low-level fillers, such as limestone (15 percent content). Some manufacturers add substances that inhibit the oxidation of the asphalt. The items are blended together with the reinforcement at 350 to 390 degrees Fahrenheit for three to five hours.
There are two types of reinforcements that are primarily used with modified bitumen systems:
The use of polyester reinforcements is widespread in SBS modified bitumen systems. The primary benefit of using polyester in SBS systems is for its high elongation capabilities. This is well suited to the performance of the SBS modified bitumen coating. Polyester also exhibits high puncture resistance, great tear strength and the durability to withstand high roof traffic.
Polyester’s ability to withstand high temperatures has made for its predominant use with APP modified bitumen. Polyester can remain dimensionally stable during the torching application or when applied in hot asphalt.
Fiberglass exhibits superior dimensional stability and tensile strength to polyester. Fiberglass is not affected by heat or tension. When a fire retardant formulation is applied, fiberglass can provide excellent fire resistance on SBS systems. Fiberglass has excellent tensile strength and when used in SBS sheets they will resist roof movement until the stress building up in the mat forces it to break.
Modified bitumen sheets are manufactured to widths of 36 to 39 inches. The thickness of the modified sheets range from 120 to 170 mils.
In the past decade modified bitumen’s market share has surpassed BUR. Often referred to as a hybrid system, modified bitumen has become known as “the new BUR.” The increase in market share is due in part to decline of BUR use and modified’s application versatility in more user-friendly procedures. Modified also posses some attributes that are superior to conventional BURs: most notably low-temperature flexibility and strain energy.
Low-temperature flexibility is defined as the lowest temperature that a membrane sample can be bent to 180 degrees. The test method used by ASTM to determine membrane flexibility is conducted by placing a 1-inch wide sample over a mandrel. The material is bent to 180 degrees after it is placed in cold and heat conditioning chambers. The material passes the test if the material does not exhibit flexural cracking during the test method. In most cases, the material is subjected to a series of tests at various (hot and cold) temperatures until failure is reached. Modified exhibits excellent flexibility in low-temperatures.
Strain energy is the measurement of the membrane’s ability to absorb energy or movement from the substrate. In roofing applications, relative movement can occur from differential thermal contraction of the membrane and the substrate or substrate movement. Modified bitumen possesses a high breaking strain energy, which allows for maximum elongation when it is under tension. This allows the material to perform in cases of differential movement, over stress cracks and at insulation joints.
Another advantage of modified bitumen systems is versatility of configurations. The systems can be applied in one-ply to multi-ply configurations. These system configuration methods make the materials suitable to meet most project and budget constraints. Self-adhered products do away with the hazards associated with open flames and recent developments include adhesives with low VOCs and cap sheets with pre-manufactured reflective coatings that allow systems to meet Energy Star and LEED requirements.