SBS-modified roofing membranes are just one type of modified bituminous roofing. They are flexible membranes manufactured in rolls and are thermoplastic in nature but display variable elastomeric properties, primarily because of the modifiers blended with the bitumen. These membranes are generally manufactured using three materials: (1) modified bitumen, (2) reinforcement fibers, commonly woven as a sheet and embedded within the membrane (these fibers may be glass, polyester, or a blend of the two), and (3) a surface coating, such as stone or ceramic granules. The performance of these membranes (for example, their flexibility or ability to retain their shape under higher temperatures) may be compromised by subtle, seemingly inconsequential changes in composition.
The formulation of the bitumen is critical, particularly when its physical properties are modified by the addition of polymers. Bitumen is most commonly modified with styrene butadiene styrene (SBS) or atactic polypropylene (APP; see separate section in this Manual). The manufacturer must ensure that a suitable degree of compatibility exists between the bitumen and the polymer and that a thorough mix has been obtained. The careful selection and combination of bitumen and polymer (the type of modifier, the percentage of polymer used, etc.) determines the physical properties and long term performance of the modified bituminous “binder” (the meaning of this sentence is unclear; what exactly is the 'binder' and how is it related to the bitumen and polymer used to modify the bitumen? - isn't the "binder" the same as the polymer such as SBS or APP, or is it something else? The structure of this sentence suggests that the polymer and the binder are actually two different things). The binder provides the membrane with its waterproofing capability and may impart the following properties:
- low temperature flexibility
- elasticity
- high temperature resistance to flow
- resistance to aging
The bitumen utilized for oxidized bituminous membranes has been “blown” (clarify), similar to roofing asphalt (see Section 3.9.1). This produces a higher softening point and, therefore, a more useful temperature range. (This means what? That the membrane can be applied using a wider spectrum of temperatures? That it retains its shape under higher temperatures? If so, then let's say that, i.e. "Injecting oxygen into the bitumen results in a higher softening point, which makes installation easier and means the membrane will retain its shape under higher temperatures".)
The physical properties of the binder are further enhanced by applying it to or embedding it in a carrier or reinforcing mat, most commonly a textile such as:
- non-woven spun-bonded polyester
- woven polyester scrim
- fibreglass mats
- woven fibreglass cloth
- combinations of the above
The type of reinforcing selected depends on the end use and desired physical properties of the finished products. The weight, quality, tension during manufacturing, and method of saturation and coating of the reinforcing are some of the factors governing the final product's performance. Some polyester modified membrane manufacturers use oxidized bitumen to saturate the reinforcement, particularly when “heavyweight” material is used.
Membranes are usually surface treated with a parting agent so they do not stick in the roll. Surfaces that will be hot asphalt applied are usually sanded while torch-applied surfaces use polyethylene or other thermofusible films (materials that can be bonded with heat). Exposed surfaces may be embedded with mineral granules (usually ceramic chips or slate flakes) or laminated with metal foil for ultra-violet protection. As an alternative on roofs with minimal slopes, a pour coat and gravel surfacing may be installed over some membranes, although this makes the quality of application difficult to inspect and may lead to problems with membrane slippage due to the weight of the cap sheet and surfacing.
