|TPO Roofing Membranes
ASTM Specification Adds Market Credibility to New Roofing Material
by Randy Ober and Paul Oliveira
Thermoplastic polyolefin membrane is based on polymers defined by ASTM D 5538, Practice for Thermoplastic Elastomers Terminology and Abbreviations, as a thermoplastic elastomer - olefinic. A thermoplastic elastomer is a diverse family of rubber-like materials that, unlike conventional thermoset rubbers, can be reprocessed and recycled like thermoplastic materials. Thermoplastic polyolefins are in the thermoplastic elastomer family and are commonly referred to as TPO in the single-ply roofing industry.
TPO-based products have been used in various applications, including the automobile industry since the 1980s, and have seen their usage continue to expand. In 1989, TPO-based membrane moved into the roofing industry as a non-reinforced sheet. In 1993, the original non-reinforced TPO membrane was replaced with membranes containing reinforcing fabric. Since that time, the TPO single-ply roofing market has grown to hundreds of millions of square feet annually with TPO membrane comprising the fastest growing segment of the U.S. single-ply roofing industry.
Currently, there are at least six manufacturers of TPO roofing membrane in the United States as well as several marketers. Until recently, there has been no ASTM standard established for TPO reinforced roofing membrane. The approval and release of an ASTM TPO standard has defined minimum physical properties of the membrane, providing needed standardization to the industry.
The task group challenged with the development of this ASTM specification, D 6878, Specification for Thermoplastic Polyolefin Based Sheet Roofing, is under the jurisdiction of Subcommittee D08.18 on Polymeric Materials, part of Committee D08 on Roofing, Waterproofing and Bituminous Materials. The task group is made up of individuals representing roofing material manufacturers, consultants, architects, specifiers, roofing contractors, governmental agencies and other organizations. The task group meetings for the development of the TPO standard have always been popular and well attended and many members travel to committee week specifically for this event. With all of the input from various interests that has been incorporated into this specification, D 6878 will serve the roofing industry well.
TPO roofing membrane is typically based on polypropylene and EP (ethylene-propylene) rubber polymerized together using state-of-the-art polymer manufacturing technology. This technology enables the production of TPO membranes that are flexible at low temperatures without the use of polymeric or liquid plasticizers. Unlike some other popular thermoplastic roofing membranes, the TPO polymer does not contain chlorine and no chlorine-containing ingredients are added during sheet production. This lack of chlorine has allowed TPO marketers to tout their membrane as an environmentally safe, green product.
The TPO resin is compounded with other components including a weathering package, fire retardants and pigments for color to create a product that can withstand the elements associated with rooftop exposure. The membrane is comprised of TPO based top and bottom plies encapsulating a reinforcing fabric that enhances the physical properties of the sheet (see Figure 1). The combination of the fabric and TPO plies provide reinforced membranes with high breaking and tearing strength and puncture resistance. Sheet widths are available up to 3.66 m (12 ft) and membrane thickness is typically 1.1 or 1.5 mm (0.045 and 0.060 in.), however, membrane up to 2.0 mm (0.080 in.) is available.
TPO roofing membrane is typically installed using mechanical fasteners and plates placed along the edge of the sheet and fastened through the membrane and into the roof decking (see Figure 2). Adjoining sheets of TPO membrane are overlapped, covering the fasteners and plates, and joined together with a minimum 40 mm (1 1/2 in.) wide hot air weld.
In the field of the roof, proper membrane width and fastener spacing is determined after calculating potential uplift loads based on building height, surrounding terrain and the wind zone in which the building is located. The fastener/deck combination must exhibit adequate pull-out resistance to withstand the fastener loading generated during uplift. The membrane and the welded seam must also resist the forces generated by wind uplift. The ability of a mechanically fastened system to withstand these forces is typically evaluated using a 3.7 x 7.3 m (12 ft x 24 ft) uplift table where the system may be pressurized until failure (see Figure 3). Along the perimeter of the roof, where the highest wind loads are experienced, reduced width membrane (40 to 60 percent of the field sheet width) is installed to reduce the fastener and seam loading.
The membrane may also be fully adhered to an insulation or deck material using an adhesive. Insulation is typically secured to the deck with mechanical fasteners and the TPO membrane is adhered to the insulation (see Figure 4). This type of system is highly resistant to wind and its associated uplift forces. Since the sheet is 100 percent affixed to the substrate, the membrane does not flutter due to associated wind forces. With no sheet movement due to the 100 percent attachment of the membrane, the fully adhered system is ideal for very visible roofs such as domes or other high slope applications.
One of the primary benefits of TPO membrane is the ability to fuse the sheets together with a hot air weld (see Figure 5). The welding process results in a bond that is actually stronger than the sheet itself. Flashing details, such as exhaust vents, pipes and parapet corners are also completed using hot air welds and flashing material (typically non-reinforced).
When properly compounded, TPO membrane has a wide window of weldability, allowing the roofing applicator to complete hot air welds consistently in a broad temperature range. Since weld integrity is critical to the performance of the roofing system, it is imperative that the membrane welds properly in a wide range of ambient temperatures. TPO membrane can be welded at speeds up to 80 mm/s (16 ft/min) using a self-propelled hot-air welder, however, optimum welding speeds are 60 to 70 mm/s (12 to 14 ft/min) with the air temperature at the welding nozzle at approximately 540º C (1000º F). TPO is a thermoplastic material and does not cure on the rooftop once installed. This allows the sheet to be repaired with proper surface preparation after rooftop exposure.
Wide Sheet Technology
Another benefit that TPO membranes brought to the roofing community were wide width sheets. Currently, TPO membrane is available in the market in widths up to 3.66 m (12 ft). These wider sheets provide installed cost savings by reducing the total number of seams to be completed in the field and the labor associated with the seaming/welding process. Wider sheets also require fewer fasteners and plates for membrane securement, fewer rolls for the roofing applicator to handle on the roof, and less membrane being utilized in the seam overlaps.
The ASTM specification for TPO roofing membranes outlines a variety of physical properties and their minimum performance requirements. These properties include top ply thickness over scrim, elongation, breaking strength, tearing strength, linear dimensional change, weatherability and a myriad of others. Establishing these minimum requirements will provide a guideline for sheet manufacturers and will help ensure TPO membrane produced under this specification provides long-term rooftop performance.
Thickness of top-ply coating over scrim is a very important element of the roofing membrane and has a direct effect on a sheets ability to provide long-term weathering, abrasion resistance and weldability. The top ply thickness is measured at the intersection of reinforcing fibers. Breaking and tearing strength are also critical properties since each has a direct influence on the performance of the membrane when subjected to wind uplift forces.
To evaluate long-term weather resistance, the xenon-arc weatherometer is specified. The xenon-arc weather resistance test exposes membrane samples to the combined effect of ultraviolet and infrared radiation, ozone, heat and water spray, to greatly accelerate outdoor weathering.
Some thermoplastic membrane weather test results are published in total exposure hours, but the ASTM TPO standard specifies total radiant exposure in kilojoules per square meter (kJ/(m2.nm)). Total radiant exposure is the total energy per unit area analogous to the total sunlight required to sunburn skin. The irradiance is the intensity of the sunlight measured in units of power (watt) per unit area. Since the irradiance can be set independently on each xenon arc weatherometer, specifying the total radiant exposure rather than total number of hours reduces confusion.
The ASTM TPO standard outlines accelerated aging tests that contain some of the most severe conditions of all single-ply roofing materials. Xenon arc testing for TPO membranes (per ASTM G 151, Practice for Exposing Nonmetallic Materials in Accelerated Test Devices that Use Laboratory Light Sources, and G 155, Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials) must be conducted at a black panel temperature of 80º C (176º F) with samples exposed to 5,040 kJ/(m2.nm) at 340 nm prior to removal and inspection. (After xenon-arc exposure, the specimens are inspected under a microscope at 7X magnification to look for cracks or crazing. Physical property testing is also specified for TPO after exposure within the xenon arc chamber. Breaking strength and elongation may not drop below 90 percent of the value generated with unaged material). No other ASTM specification for single-ply membranes requires a higher specified xenon arc weatherometer black panel temperature. As expected, the higher the black panel temperature, the more severe the test. A common rule of thumb used with xenon arc testing is that the severity of the heat-aging component is doubled for each 10º C increase in the black panel temperature.
Heat-aging (ASTM D 573, Test Method for Rubber-Deterioration in an Air Oven) is another test used to accelerate outdoor weathering, and no other ASTM specification for single-ply roofing membranes uses a higher temperature (116º C/240º F) than the TPO specification.
TPO membrane, reinforced and non-reinforced, is 100 percent recyclable during the production process. During production, if the need arises, the membrane can be ground into rework and this regrind can be incorporated into the bottom ply during the extrusion process to produce new TPO product. This process results in 100 percent reuse of recycled product.
Another important characteristic of TPO membrane is its high level of reflectivity. Because of this, white TPO membranes can meet and even substantially exceed the U.S. Environmental Protection Agencys ENERGY STAR performance levels. Those ENERGY STAR marks that have been posted on refrigerators, water heaters and other appliances for years now can be seen on rolls of white TPO membrane. White TPO membranes typically display reflectivity ratings in the high 80 percent range when new (ENERGY STAR specifications require 65 percent minimum) and in the low 80 percent range after three-year rooftop exposure with cleaning (ENERGY STAR specifications require 50 percent minimum). TPO membranes are highly resistant to mold and algae growth, which can degrade the overall reflectivity of the roof and reduce anticipated energy savings.
Over the past decade, TPO membranes have succeeded in gaining strong roofing industry credibility based on their physical property characteristics, wide sheet technology and long-term rooftop performance throughout the world. The newly approved ASTM TPO specification adds credibility by providing definitive guidelines for minimum membrane properties, thus helping to ensure product quality, durability and longevity. //
Copyright 2003, ASTM