This article reprinted from Western Roofing magazine May/June 2007.

How “Green” is TPO?

TPO Fits Into the Eco-Friendly Environment

by Randy Ober, thermoplastics product manager, Carlisle Syntec Inc.

Lately, “green” has become a very popular buzzword in the roofing and construction industry. While my company has manufactured green TPO membrane for special projects, the actual color of the membrane is not what is being referenced by this term. Instead, the industry is speaking about the environmental friendliness of particular building products. There are many facets related to the environmental friendliness of a roofing membrane. I will pick just a few to discuss in this column.

One significant concern regarding construction materials is the sheer volume that must be disposed of on an annual basis. Whether a building is being remodeled or an existing building is being torn down and a new one erected in its place, all of the old materials must be removed and either recycled, incinerated, or placed in a landfill. The actual roofing membrane (especially if it is a single-ply) comprises a small volume compared to other types of construction materials, but nonetheless, it still must be removed from the project site.

TPO membranes are inherently environmentally friendly. When incinerated, TPO membranes are reduced to carbon dioxide, water vapor and ash, none of which are particularly harmful to the environment. Likewise, due to the chemical make-up of TPO membrane, disposal in a landfill is not an issue. TPO manufacturing plants are typically very efficient and very little scrap material is hauled off to the landfill. The scrap TPO that is generated during the manufacturing process can be recycled and put back into the bottom ply of the membrane.

Even the process of installing TPO membrane is environmentally friendly. No smoke or irritating odors are produced during the installation process and the amount of trash that is generated is minimal due to the relatively small number of components necessary to install the system.

Today, mechanically fastened systems comprise the majority of TPO membrane installations. In this type of system, the membrane is secured to the roof deck by fasteners and plates. Therefore, the only components necessary for installation are membrane, fasteners, plates, and hot air. No solvents are used during the installation of a mechanically fastened TPO system since all welds are completed using hot air (no primers or adhesives are required). Cleaning the membrane prior to welding is not typically necessary unless it is exposed to the elements for an extended period of time prior to welding or is exposed to high traffic or construction dirt (i.e. a roof tear-off adjacent to the application of the new membrane).

Another method for securing TPO roofing membrane to a substrate is through the use of an adhesive. These are typically solvent-based contact adhesives that are applied to the substrate and the membrane. The adhesive is then allowed to flash off before closing, rolling, or brooming the membrane. The solvents used in these adhesives typically contain a relatively high amount of VOCs (or volatile organic compounds). VOCs contribute to the production of ozone (which is a common air pollutant) by reacting with nitrogen oxides and sunlight. In certain regions of the country (specifically California) the maximum VOC content of adhesives is limited by law and many of the standard solvent-based TPO bonding adhesives are excluded from use.

Certain solvents are exempt from the VOC restrictions since they do not react as quickly as other solvents in forming ozone. Several manufacturers have introduced TPO contact adhesives based on these types of solvents to allow contractors in areas that have VOC restrictions to use a solvent-based product. The adhesives based on these VOC-exempt solvents produce an odor similar to the standard bonding adhesives and also have similar application characteristics (they roll out similarly and have similar dry times).

Another available alternative to comply with the restrictive VOC requirements is water-based bonding adhesives. While the water-based contact adhesives have some advantages such as little to no odor and compliance with the VOC restrictions, they are not as contractor-friendly as solvent-based adhesives due to the extended time required for the water to dry. Since these are typically contact adhesives, additional labor is expended while waiting until the membrane can be closed, rolled, or broomed into the adhesive. Typically, the solvent-based adhesive will provide better uplift resistance as well. Water-based adhesives can only be used in temperatures above freezing and most manufacturers actually recommend minimum temperatures in the 40 to 50°F range. The water-based adhesives must also be protected from freezing since they can be permanently damaged if they are frozen and thawed prior to usage.

The last alternative that addresses the environmental aspects associated with fully adhered systems is TPO membrane coated with a pressure sensitive adhesive. The technology associated with pressure sensitive adhesives has continued to progress over the years and these membranes now provide a viable alternative to standard membrane with field applied liquid adhesives. The pressure sensitive, self-adhesive TPO membrane will adhere to a variety of substrates, such as polyisocyanurate, DensDeck® Prime, OSB, and plywood without the use of primers. Since the adhesive is pre-applied to the bottom ply of the membrane, the labor associated with application of the adhesive as well as waiting for the adhesive to flash-off is eliminated. No solvents are necessary for the application of this type of sheet since no primers are required. Seams are typically made using hot air welders but some manufacturers offer contractors the ability to make the seams with tape (some of these tape systems require primers which may contain VOC levels in excess of that allowed in California). Parapet walls and other vertical applications typically require primers as well to enhance the adhesive bond with the self-adhering TPO membrane.

Overall, TPO roofing systems provide applicators, building owners, and their tenants with an outstanding “green” method for waterproofing their buildings while protecting the environment and causing minimal disturbance to the occupants and neighbors. •••