Both membranes have properties such as:
TPO, or Thermoplastic Polyolefin, combines the durability and weatherability of ethylene-propylene (EP) rubber with the heat weldability of polopropylene. TPO membranes are being marketed as a product that combines the properties of EPDM and PVC, without the associated drawbacks that the latter two materials have. In other words, TPO is supposed to be as UV-resistant and as heat-resistant as EPDM, and as heat-weldable as PVC. TPO is specifically formulated for long term weather resistance without the use of plasticizers which have a poor track record with migration. “Thermoplastic” is a generic term in polymer science; it encompasses a class of polymers that soften when heated in a reversible process. The term “olefin” is even more generic, being an old chemical name for any molecule containing carbon-carbon double bonds (the modern name for this family of molecules is alkenes). Any polymer formed by chemically linking up many olefin molecules is termed “polyolefin.” Most TPO products have undergone product reformulation over the last 20 years.
TPO roofing membranes were first introduced into the United States around 1987. TPO has increased in popularity in the roofing industry. TPO is experiencing a surge in the market but continues to be in the experimental phase.
One major product performance advantage that a few TPO membrane companies have over PVC appears to be the issue of reflectivity. TPO washes slightly more clean with rainfall whereas PVC does not seem to do this as well. Some TPO companies that issue reflectivity warranties for their white roofs that need to meet California Title 24 energy requirements. However, both TPO and PVC need to be washed in order to maintain the original installed brilliant white reflectivity property.
PVC, or Polyvinyl Chloride, has the longest track record with the first membranes being installed in Europe around 1960. PVC is a molecule comprised of carbon, hydrogen and chlorine (taken from salt). PVC has a plasticizer that tends to migrate over time. Conventional PVC membranes rely on very high levels of liquid plasticizer to manufacture a flexible membrane. Over time, heat, UV, chemicals and other environmental contaminants can extract the plasticizer to the surface of the membrane where it is washed away by wind and rain. However, some PVC membranes such as Durolast have an advantage in that they contain no filler material on the back side. Durolast is PVC all the way through the product. Most, if not all TPO manufacturers have implemented filler material into their product that is now beginning to have some problematic feedback with product shelf life and seems to contribute to material weld “pops” if not welded precisely at the correct temperature and condition. The TPO shelf life appears to be a concern in that the product is preferred to be installed within months of being made or else welds may not adhere as well. Does the roofing installer use old material laying around in his stock supply? Hopefully not but the reality seems to be that few roofing companies are unwilling to “eat” left over TPO materials (dispose of them) and that means an increase in the chance that the material will be used past the acceptable shelf life. The PVC plasticizer migration is speculated to reduce flexibility over time and leave the membrane susceptible to damage from thermal shock, hail impact and foot traffic. However, PVC roofs are standing the test of time, 30 years in many cases. PVC single ply enjoys a lengthy track record and manufacturers such as Durolast can show roofs without incidence that are around 30 years old while TPO has a much shorter time on the market of 10 to 15 years at most.
A TPO roof is a hybrid between PVC and EPDM. It has weldable seams and is available in white and light colors (for EnergyStar ratings) like PVC, but doesn’t appear to raise as much environmental concerns found in PVC membranes. Keep in mind that PVC is used in many household items such as irrigation pipes, though not exposed to UV damages. The bottom line is clear. PVC does appear to go the distance whereas TPO is still yet to prove itself in the same roofing arena over time.
There is evidence showing long-term health and environmental damage caused by persistent organo-chlorines in the environment. These substances include dioxins, furans, and polychlorinated biphenyls (PCBs). Dioxins can be created when PVC is burned under less-than-ideal conditions, as in building fires or waste incineration. In many countries emissions from waste incineration is well controlled. To the credit of the chlorine and PVC industries and government regulatory bodies, vast improvements have been made in manufacturing processes over the past twenty years, and many of the worst environmental offenders such as DDT, dieldrin, and CFCs are already gone or on their way out. The residual vinyl chloride gas in PVC products is claimed to be reduced to insignificant levels, compared with two decades ago. The environmental and health risks associated with PVC are greatest at the two ends of its lifetime: during manufacturing and then disposal (if by incineration). A task force of the USGBC concluded recently that PVC was not enough of an environmental issue to require that it be banned from certain LEED projects. PVC has been banned, however, from states such as Washington because minute traces of environmental hazards were found in the roof water runoff.
“Thermoplastic” means that when heated sufficiently, the single ply material temporarily changes from a solid to a semisolid state enabling the sheets or panels that are overlapped to be fused together and return to a solid upon cooling, yielding one continuous membrane. It is this feature that enables the seam overlaps of vinyl roof membranes to be fused or heatwelded together. To accomplish the welding, specialized, electrically-powered welding equipment that is either self-propelled or handheld is used. These units operate on electricity and inject heated air into the seam area, softening the membrane surfaces. A roller that is either hand-held or part of the self propelled unit, presses the seam overlap together. As the welder moves away from a given seam location, the membrane quickly cools down to ambient temperature and the heat weld is made, providing a watertight bond.
Built-up roofing (BUR) and modified bitumen roofing systems require the construction of multiple waterproofing membrane layers. Consequently, they can be subject to a number of installation errors. Combine these factors with poor resistance to UV, poor chemical resistance, poor performance in ponding water scenarios, and poor energy efficiency, and what you have by many objective standards is basically an inferior roofing product. Because none of these factors are an issue with single-ply roofing membranes, they continue to take market share from asphalt roofs.
A definite downside to Single Ply roofing is aesthetics. Single Ply really is not a good looking roof for say, visible residential areas.
Summary: Both PVC and TPO are viable roofing options that will provide you with an excellent weather proof roof solution. The debate between the two is complex. GAF, JM, and Firestone are currently building new single ply roof membrane manufacturing plants. All of these have chosen to build TPO rather than PVC plants. However, some say that is a function of market share yielding lower productivity costs to produce TPO and therefore granting the manufacturer significantly higher profit margins had they chosen to produce PVC instead.
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