An unprecedented study shows surprising variations among TPO membranes
In "A study of longevity," February 2014 issue, page 44, TPO membranes' long-term performance was discussed. The article also reviewed the general history of TPO and concluded the material generally is performing well in the almost 30 years it has been in service. However, some manufacturers have had issues when membranes were installed in high-heat situations. Problems occurred typically on roofs located in extremely hot, sunny climates or on roofs that had an unusual amount of dirt buildup, leading to loss of reflectivity. As the industry learned about these issues, ASTM International made the accelerated testing requirements in ASTM D6878, "Standard Specification for Thermoplastic Polyolefin Based Sheet Roofing," more stringent.
It's important to note accelerated testing subjects a material to conditions (ultraviolet [UV] light, heat, etc.) in excess of normal service parameters to uncover faults and failure modes. These often harsh conditions are used to get what would take years in the real world in a matter of months. The results of an accelerated aging test are an important proxy for the expected service life of a TPO membrane. For example, a poorly formulated and/or manufactured membrane will not even pass a low bar when subjected to accelerated aging while a well-formulated and manufactured sheet can last three to four times as long in accelerated aging tests.
Conditions and membrane failure
For the purposes of this article, membrane failure refers to either cracking or erosion down to the scrim in the field of the material. Once either of these things happens, membrane breakdown occurs rapidly. There are two in-situ temperatures the industry needs to be concerned about:
- 160 F—Many manufacturers' design guides reference this temperature as an upper limit for sustained roof temperatures. It's worth noting most roofs don't exceed this; white membranes are designed to stay cool, and most don't exceed 140 F to 150 F even after several years in the field.
- 195 F—There are several circumstances that can lead to membranes reaching temperatures this high for sustained time periods. These include membranes with directly attached solar panels, significant dark dirt buildup that blocks reflectivity, high-altitude locations and nearby highly reflective surfaces such as parapet walls.
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