Introduction

Single-ply membranes are currently the biggest segment of the commercial roofing market. Within that segment, thermoplastic polyolefin (TPO) is both the largest and the fastest growing sub-category. The first TPO roof in North America was installed as a demonstration project in 1986, and the membrane was commercialized around 1990.^1,2 Since then, the installed area of TPO is estimated to be in excess of 20 billion square feet.

TPO membranes have been extensively analyzed in laboratories and under accelerated weathering conditions. This work has demonstrated the ability of TPO to provide good heat aging performance and UV stability. The longevity of any material routinely exposed to the rigors of weather, sunlight and even pollution is difficult to predict from laboratory studies. This is true for materials such as paints, siding, paving, and, of course, roofing. Accelerated aging techniques can provide useful data indicative of long-term performance, but they don’t take into account the combinations of challenges seen by materials in real-world scenarios, which also depends on the quality of the installation, maintenance and ability to repair the membrane.

TPO membrane has now been installed on roofs for several decades, making a meaningful and representative field survey of its performance possible. The goal of this ongoing study is to evaluate TPO samples taken from older installations in terms of their properties as compared to their original specification. The installations were intended to cover a wide geographic range, in order to evaluate TPO’s performance in as variable a range of climactic conditions as possible. Specifically addressed are known failure modes of some manufactured TPO membranes, which include erosion of the cap (thickness over scrim) down to the scrim and surface cracking. In addition, having taken samples from aged roofs, it was possible to evaluate their repairability, should such a need arise.

Background

TPO Formulation

TPO membranes have been used in Europe since the 1960s. It wasn’t until the late 1990s and early 2000s that TPO membranes began to gain market share in the United States. Early formulations contained brominated fire retardants, which caused unanticipated weathering and premature degradation issues.³ By 1994, manufacturers were using magnesium hydroxide as the fire retardant and this remains the case today. Also in the early 1990s there were some significant variations in the polyolefin being used. But, by the mid-1990s these had narrowed to the same basic type that is in use today—i.e., propylene-rich ethylene-propylene elastomer.

Since the mid-1990s, TPO membranes have evolved with respect to the ultraviolet (UV) light and heat stabilizers being used to protect the polymer. This has come about both through improvements in stabilizer technology and a desire to extend service lifetimes. In addition, at least one manufacturer used more advanced stabilizers to increase membrane life when exposed to significantly higher-than-normal in-service temperatures.⁴ The focus on heat exposure came about because it appeared from in-service performance that heat and not UV was responsible for a large number of premature failures being experienced by some manufacturers.

During a 2009 ASTM TPO task group meeting, premature failure of TPO roofs was discussed. One manufacturer described membrane failures that were found to be related to unanticipated high heat loadings.⁵ In these conditions, the membrane was exposed to higher than normal temperatures due to situations such as reflections from nearby wall surfaces, HVAC units and neighboring taller buildings.

In early 2010, the Midwest Roofing Contractors Association’s (MRCA) Technical and Research Committee published an advisory on TPO.⁶ They noted that “information is being circulated in the industry indicating that high solar loading and elevated temperature lead to the premature exhaustion of anti-aging components such as antioxidants, UV absorbers and heat and light stabilizing compounds within TPO. This could lead to the breakdown of the sheet in affected areas.”

In this advisory, T.J. Taylor has noted that there can be several causes of excessive heat buildup on TPO roofs.^7,8 These include nearby highly reflective surfaces, dirt and directly adhered flexible solar panels. Subsequent testing of a large sampling of new membrane showed that there were large disparities in the accelerated aging performance of different manufacturers’ TPO membranes.^9,10 However, that testing showed TPO accelerated aging performance to have significantly improved versus the initial formulations of TPO.

TPO Field Testing

A limited number of field studies have previously been conducted to evaluate the long-term performance of TPO roof membranes. A European study examined three TPO roofs that had up to 12 years in service.¹¹ All roofs were found to be performing well with no issues or change in membrane thickness. The peel and shear strength values of the sampled seams were similar to or higher than nominal values required by Standards UNI EN 12316-2 and EN 12310-1&2. The researchers noted that “sampling actions on the roofs showed the perfect weldability and therefore the full possibility to repair membranes, even after years of operating exposure, by working on the inner side of the existing membrane.” This indicates the repairs were successfully conducted by welding new membrane to the “inner side,” also known as the core, or the bottom side of the aged membrane.

The Western States Roofing Contractors Association (WSRCA) conducted a 10-year study, beginning in 2000, with a final report being published in 2011.¹² It evaluated 60-mil white TPO membrane from four manufacturers, mechanically-attached, in four different climatic regions in Western North America.

The WSRCA researchers noted that “all of the TPO membranes examined in the field to date have proven to maintain their seam quality. All hot-air welded seams… are proving to have generally good weld integrity.” One membrane had some cracking that was associated with a sharp crease that had been created during the original installation. That same membrane also exhibited some micro-cracking and crazing in a limited section of the Las Vegas test roof. It was concluded that this resulted from UV and heat exposure, in combination with a potentially less-robust TPO formulation. The survey noted that “some formulations obviously withstand heat-loading better than others.”

WSRCA noted that additional preparation was needed for the repair of test cuts in some locations during the tenth year of exposure as compared to previous years. Specifically, a “solvent-scrub” step was added utilizing solvent and a scouring pad “to more aggressively remove a layer of oxidation on the surface.”

In 2011, Beers et al. published a long-term field study on FPO (European terminology for TPO) membranes in service for up to 20 years in Europe.¹³ The study predicted the in-service membranes would “fulfil their waterproofing function for further decades…provided they are used in compliance with the application and maintenance requirements,” stating that the conclusion is “restricted to conditions within the moderate Central European climate and does not hold for dramatic climatic changes.”

TPO Specification

ASTM standard specification development began in the early 1990s and it took over 10 years and 36 drafts before a consensus was reached. The first TPO standard specification was published in 2003—ASTM D6878, Specification for Thermoplastic Polyolefin Based Sheet Roofing. Published approximately 13 years after the membrane was introduced to the market, the standard prescribes various dimensional and physical properties, as well as compositional and accelerated aging requirements.

This Standard has been improved since its inception to incorporate more demanding tested-product performance, including stronger requirements for accelerated weathering and aging.

• In 2006, the UV exposure requirement was doubled from 5,040 kJ/m² to 10,080 kJ/m².
• In 2011, the heat aging requirement was increased from 4 to 32 weeks at 240 degrees Fahrenheit. The thickness over scrim specification was also changed from a minimum of 16-mils regardless of total thickness to a minimum of 30 percent of the total membrane thickness.
• In 2017, the heat aging requirement was changed to 32 weeks at 240F or 8 weeks at 275F. In addition, the retention of physical properties requirement was deleted and a specification that weight change be less than 1.5 percent after heat aging was added.
• In 2019, the standard was yet again strengthened to specifically identify the sampling procedures for heat aging. The exposures and pass/fail criteria were not modified.