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 1960’s. It wasn’t until the late 1990’s and early 2000’s 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 1990’s 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.

Roof Sampling Program

The intent of this study was to evaluate field-aged TPO roof membrane performance and the ability to repair membranes as they age. Membrane samples were collected from roofs around the United States that were at least 12 years in service; the oldest sample was installed 18 years before this study began. Most of the roofs evaluated were installed between 2005 and 2006. All samples were from the same manufacturer and were predominantly 45- and 60-mil thick smooth-back membranes. Samples were taken from mechanically attached, induction-welded and adhered roofs. Self-adhered membranes were excluded from this study.

Samples were taken from 20 different roofs across the U.S., as indicated in Figure 1. The buildings included offices, manufacturing facilities, retail outlets, libraries, automotive repair shops, warehouses and a grocery store.

Figure 1: Approximate locations of the 20 roofs sampled.

The roof projects, membrane attachment and thickness, and age are summarized in Table 1. Attachment type is described as adhered (A), induction-welded (IW), or mechanically attached (MA).

Table 1: Locations #1-3 were part of the initial process to coordinate efforts with teams across the country for consistency. These roofs are a few years shy of the requirements set forth within this study.

Sample Selection

Two samples were taken whenever possible. Each sample was approximately 2 feet by 3 feet and captured a field-welded seam. One sample was taken from the field of the roof (see Figure 2) and another was taken in a location that resulted in increased heat exposure by being near a south-facing parapet wall (see Figure 3). To date, the increased heat exposed samples were located adjacent to the north parapet wall. In the future, this ongoing study will include areas where sunlight reflects off adjacent glazing or metal.

The large samples were cut into smaller pieces to evaluate membrane thickness, thickness over scrim, brittleness, heat aging and weather resistance, ply adhesion of existing welds, and ply adhesion of repair welds. Each test was conducted on five unique specimens from each location on the roof.

Figure 2: Typical field sample (2 feet by 3 feet),  Figure 3: Typical sample cut near a parapet

The large samples were cut into smaller pieces to evaluate membrane thickness, thickness over scrim, brittleness point, heat aging and weather resistance, ply adhesion of existing welds, and ply adhesion of repair welds. Each test was conducted on five unique specimens from each sample, and the results averaged.

Test Program

The testing program evaluated those membrane properties that would be indicative of the roof’s ability to maintain water-tightness. All tests were conducted in a commercial test laboratory. Whenever appropriate, comparisons were made to the ASTM D6878 TPO material specification. It is important to note that the 2019 version of the specification is more stringent than the version in place when the membrane materials were manufactured. Instances where this could impact the conclusions have been noted in the results section. In every case, the ASTM D6878 specification being referred to here is the latest version that was promulgated in 2019.

In addition to water-tightness, cool roof membranes such as TPO are used due to their ability to reflect the sun’s energy and thereby lower air conditioning loads and mitigate urban heat island effects. Therefore, the solar reflectance (SR) of the samples was also evaluated.

Membrane Thickness and Thickness of Coating Over Scrim

TPO membranes consist of two polymer layers of TPO—the cap (topside) and the core (bottom side)—which are laminated together with a polyester reinforcing scrim in between. Following ASTM D751 and D7635/D7635M, Standard Test Methods for Coated Fabrics and Standard Test Method for Measurement of Thickness of Coatings Over Fabric Reinforcement respectively, the overall membrane thickness and the thickness of the coating over the scrim (TOS) were measured. This measurement was compared to the current ASTM TPO standard requirements for new membranes to evaluate how they are weathering.

Heat Aging and Weather Resistance

ASTM D6878 evaluates heat aging and weather resistance using ASTM D573, Standard Test Method for Rubber—Deterioration in an Air Oven, and ASTM G151/G155, Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials, respective test methods, which include a visual inspection of the membrane surface at 7X magnification when bent over a 3-inch mandrel for surface cracking. For the purposes of this survey, because the samples were field-aged, the ASTM D6878 visual inspection pass/fail criterion was used without applying the heat aging requirements.

Brittleness Point

Brittleness point, sometimes referred to as low temperature flexibility, was evaluated per ASTM D2137, Standard Test Methods for Rubber Property—Brittleness Point of Flexible Polymers and Coated Fabrics, method B. Specimens taken from mechanically-attached membranes were examined at 5X magnification for any visible fracture or crack in the cap layer after having bent the specimens to an angle of 180 degrees in the same direction caused by the test impact.

Low temperature flexibility testing was not conducted on adhered membranes, as remnants of the coverboard or insulation facer were adhered to the membrane core. This rendered the specimens too stiff to adequately test.

Aged Ply Adhesion

ASTM D1876, Standard Test Method for Peel Resistance of Adhesives, also referred to as the T-Peel Test, was used to evaluate weld integrity and membrane ply adhesion, as shown in Figure 4. The initial peak load caused by breakage at the edge of the weld area, and the series of lower peak loads during delamination of the membrane were recorded. The ply adhesion values reported are the average of the maximum load values at the initial break.

Figure 4: Illustration of the ply adhesion test and a full film-tearing bond, indicating a complete weld.

Ply adhesion testing of a proper seam weld will fail cohesively within one of the plies, exposing the underlying scrim. This is referred to as a “film-tearing bond” (FTB) and indicates the integrity of the weld. For the purposes of this evaluation, anything over 70 percent film-tearing bond was considered a proper weld. Figure 4 shows a 100 percent film-tearing bond at the end of the test, indicating a complete weld.

It is important to note that aged weld strength can be impacted by both the long-term weather exposure and the initial weld quality.

Aged Membrane Repairability

New TPO membrane that was commensurate in type and thickness of the existing membrane, was welded to the aged membrane roof samples to evaluate the ability to repair older roofs. Repairs with new membrane welded down onto the cap of the aged membrane—also called a top down repair—were evaluated, with Figure 5 indicating the general process. While this repair process is the most common, in some instances it is necessary to weld repair membrane to the core (the underside) of the aged roof, also called a bottom up repair, as indicated in Figure 6. Both approaches were evaluated in this study. Note that weld strength to the core was not evaluated for cases of adhered membrane roof samples due to remnants of adhesive and/or facer from the insulation or cover board. Given the remnants attached to the underside of adhered membranes, repairs to the core would not be reliable.

Figure 5. Repair of aged TPO membrane with a new patch welded to the aged cap. Photo courtesy of WSRCA.

Figure 6. Repair of aged TPO membrane with a new patch welded to the aged core. Photo courtesy of WSRCA.

In both cases, industry-standard cleaning protocols were followed in preparation of the test specimens for used to measure weld strength and film-tearing bond. For consistency and to eliminate variables, a robotic welder was used at 12.1 feet per minute at 1148F.

Aged Membrane Solar Reflectance

The solar reflectance of the aged membrane samples was measured according to ASTM C1549. While the test method allows for sample rinsing, that was not carried out in this study. Thus, this study is indicative of actual solar reflectance that is experienced by the roofs. Therefore, long-term adherence to energy efficiency and/or HVAC equipment sizing assumptions by the building designer can be checked.

Results

As located in Figure 1, samples were taken from 20 different roofs across the U.S. Collectively, these roofs have seen over 1,000 days of hail, almost 400 feet of rain, nearly 4,000 days of over 90F and wind gusts of up to 120 mph. On average, a typical roof membrane in this study was installed between 2005 and 2006 has been exposed to 157 days of hail, over 50 feet of rain, over 500 days of over 90F and wind gusts of up to 92 mph.

To put this into perspective, the roof membrane from Orlando, Fla. has been exposed to the elements for over 17 years and has weathered 32 hurricanes, 56 feet of rain, 1,660 days of over 90F and wind gusts of up to 105 mph. Given this real world weathering by mother nature, all artificial aging requirements for testing were waived.

Membrane Thickness and Thickness of Coating over Scrim

TOS is a critical characteristic of single-ply membranes, because it is a measurement of the quantity of the weathering layer, which provides the UV and heat stabilization properties of the membrane. Erosion of the membrane down to the reinforcing scrim layer may compromise the weather tightness of the membrane, indicate failure, and require repair or replacement as shown in Figure 7.

Figure 7:  Premature failure as cap erodes and exposes scrim. Photo courtesy of Rene Dupuis.

The membrane thickness over scrim (TOS) data for the field samples are shown in Figure 8. With one exception, all of the membranes tested show TOS values above the ASTM D6878-19 specification. For 60-mil membrane, this is 18-mil and for 45-mil membrane, it is 13.5-mil. The results suggest that from the perspective of erosion, all of the membranes have significant remaining life.

Figure 8. Membrane TOS 60-mil samples in blue, 45-mil in gray, with their respective ASTM D6878 minimum specification. Note that for Project 5, 60-mil membrane was installed at the perimeter and 45-mil membrane was installed in the field.

As indicated in Figure 8, sample 3 is 1 mil below the current ASTM minimum for TOS. However, this sample would have complied with the published ASTM requirements at the time of manufacture.

Total sample thicknesses for the field samples are shown in Figure 9.

Figure 9. Membrane thickness, nominal 45-mil in gray, nominal 60-mil in blue. Samples taken within the field of the roof. Note projects 1 and 2 used a 50-mil membrane and comply with the ASTM thickness requirements.

ASTM standard D6878-19 requires the as-produced membrane to be within +15 percent, -10 percent of stated thickness, and not less than 39-mils. Even after 12 or more years of aging, most of the 45 and 60-mil membrane samples complied with the current ASTM requirements for newly manufactured TPO membranes. Total membrane thickness in Projects 17 (an 18-year field aged membrane) is 2 mils shy of the requirement for new ASTM membranes while Projects 18-19 are 1 mil shy of the requirement.

It is somewhat in conflict with the TOS values, which are generally at or above the minimum. The apparent discrepancy between the two measurements suggests that the products included in this study were manufactured with a significant weathering layer, and after many years in service have maintained the critical thickness over scrim.

Samples taken from the perimeter might be expected to exhibit increased erosion of the weathering layer and therefore, have reduced TOS versus the field samples. However, as indicated in Figure 10, this was not observed.

Figure 10. TOS for the field versus perimeter samples. The diagonal line would be the result if no differences in weathering were observed. Samples in the field and perimeter were permitted on 9 of the roofs in this study.

As seen in Figure 10, at least for the roofs where permission to obtain field and perimeter samples was granted, no consistent trend in terms of weathering were identified. This is an indication that the data are within the error tolerances of the measurement technique.

Heat Aging and Weather Resistance

Surface cracking was evaluated by visual inspection of the field-aged roof membranes at the time the samples were collected, and via inspection at 7X magnification when bent over a 3-inch diameter mandrel. This evaluation is an important indicator of long-term performance.¹⁰ Surface cracking on a roof is not indicative of an immediate problem until such cracks propagated down to the reinforcing scrim, as seen in Figure 11. Surface cracks are suggestive of the stabilizer content being substantially depleted, which could lead to more rapid deterioration of the membrane. None of the samples—neither the 45- nor 60-mil membranes—exhibited any signs of surface cracking when bent over the mandrel and viewed at 7X magnification.

Figure 11: Testing performed by Rene Dupuis illustrates the reason why we look at cracking of the membrane. The test sample is from laboratory exposure and the real world sample is from an aged roof.

Brittleness Point

The aged roof samples were evaluated for brittleness point, an indicator as to whether the membranes become more susceptible to cracking during extreme cold conditions. Only those samples that had been mechanically attached were tested, with the results being shown in Figure 12.

Figure 12. Brittleness point of tested samples—nominal 45 mil in gray, nominal 60 mil in blue. Not all samples were tested for brittleness.

ASTM standard D6878 requires new membranes to have a brittleness point of -40F or lower. All except one of the 60-mil samples tested still meet cold temperature flexibility requirements after field aging. The 45 mil samples showed initial signs of cracking at -35F. The slight rise seen for these samples is indicative of low temperature stiffening due to oxidative crosslinking. Roofing membrane issues caused by rising brittleness point were previously observed with early versions of PVC, which exhibited cracking and shattering during winters.¹³ However, such issues were essentially eliminated by improving the formulations and by the use of reinforcing scrim. The scrim used in TPO is the same basic design and type as used in PVC and the small changes in brittleness point observed in some of the sampled TPO roofs are not a cause for concern. However, the data supports the use of thicker membranes for longer-term performance.

Aged Ply Adhesion

While failures associated with welded seam delamination are not widespread, ply adhesion or weld strength is used as a quality control tool in the field. However, there is not a clear industry consensus on the minimum strength requirement to evaluate weld strength. ASTM D6878 does not provide a minimum value for weld strength. Therefore, weld strength is compared to the values published in a study of new TPO membranes conducted by Structural Research, Inc. (SRI). This study included a broad TPO sampling of all industry manufacturers.¹⁴ This study which evaluated the weld strength of all thicknesses of TPO from all manufacturers averaged a ply adhesion (T-Peel) value of 40 lbf/inch, with a minimum value of 29.3 lbf/inch. Previously, however, Simmons et al. found that the ply adhesion tests typically failed adhesively, meaning there was not a strong bond between the welded TPO layers and the film-tearing bond was 0 percent, when the ply adhesion of the seam weld was 26 lbf/inch or less.¹⁵

The ply adhesion values of the aged TPO membrane seam welds were on average, 50 lbf/inch, which is above the average ply adhesion value from the SRI study on new TPO membranes, as shown in Figure 13. As expected, the aged welds appear to be performing well and are of adequate strength. Note that for Location 11, the value of 20.7 lbf is indicative of a suspect quality weld and would require further investigation.

Figure 13. Weld strengths of the aged roof samples.

Film tearing bond, as shown in Figures 14 and 15, was also analyzed as it is relied upon in the field to assess the qualitative integrity of a weld.  If the sample fails and the scrim is not exposed, or the film tearing bond is not greater than 70 percent, the heat and/or speed of the welding equipment must be adjusted.

Figure 14. Film tearing bond of nearly 100 percent, from Location 6.

The film-tearing bond at Location 11 was of suspect quality, summarized in Figure 15. This is also suggested by the weld strength data at this location.

Figure 15. Percent film-tearing bond of the aged roof samples. Zero film-tearing bond at locations 4 and 11 suggest poor quality workmanship.

All of the samples except Locations 4 and 11 met the threshold requirements for percent film-tearing bond. Comparing the weld strengths and film-tearing bond percent, it is clear that the roofs at locations 4 and 11 might warrant further investigation due to the combination of low weld strength and film-tearing bond. In the case of location 2, the aged membrane has a weld strength of 53.9 lbf with a film-tearing bond less than 70 percent. This suggests that the issue on this roof could be localized, possibly to the installation time of day and ambient conditions.

The ply adhesion values of the aged TPO membrane seam welds were on average, 15 percent above the average ply adhesion value from the SRI study on new TPO membranes. Therefore, as expected, the aged welds appear to be performing well and are of adequate strength.

Aged Membrane Repair-Ability

To address questions around the ability to repair aged TPO membranes, this study examined the adhesion of new membrane to aged membrane. Two approaches were examined; welding the new membrane to the cap of the aged membrane, as well as new membrane welded to the core of the aged membrane.

The ply adhesion of the new membrane to the aged cap averaged 44.7 lbf/inch (standard deviation 8.9 lbf), is shown in Figure 16. For the new membrane welded to the aged core, the ply adhesion averaged 58.8 lbf (standard deviation 7.5 lbf), as shown in Figure 17. Both values represent the combined 45- and 60-mil thicknesses.

Figure 16. Ply adhesion of the new membrane to the aged cap. The average ply adhesion was 44.7 lbf/inch. Not all samples were tested for ply adhesion.

Figure 17. Ply adhesion of the new membrane to the aged core. The average ply adhesion was 58.8 lbf/inch. Adhered membranes were excluded from this evaluation.

The ply adhesion values of new repair membrane to the core of the aged TPO membrane are above the average ply adhesion value of 40 lbf/inch from the large independent study of new TPO membranes conducted by SRI. This provides validity to the integrity of repairs to aged TPO membranes and the ongoing maintainability of these roofs. It should be noted that ultimate ply adhesion is mainly a concern for wind uplift of mechanically fastened systems as shown in Figure 18.

Figure 18. Schematic showing the three main forces applied to a mechanically attached membrane during a high wind event.

Typically, repairs of single-ply membranes are made to punctures within the field of the membrane. As such, they usually represent a very small fraction of the membrane area and essentially the overall mechanical strength is not significantly impacted. The reinforcing scrim can redistribute loads around a small repair. For systems where the membrane is adhered to the substrate, ultimate membrane and ply adhesion are also not as critical.

Aged Solar Reflectance

The aged solar reflectance values are shown in Figure 19. With the exception of locations 17 and 18, the roofs showed an average value of 0.665. This is only slightly below the 3-year aged solar reflectance of 0.68 reported by the Cool Roof Rating Council (CRRC) for this membrane.¹⁶  

Figure 19. Solar reflectance values of the aged cool roof membranes.

The 0.44 and 0.38 values exhibited by Locations 17 and 18, respectively, suggests that the area sampled was particularly contaminated—a condition that could possibly be resolved by cleaning. Many roofs have localized contaminated areas, such as those that occur near drains or low points. Taken overall, the results suggest that the published three-year solar reflectance values generally can be used to model long-term energy efficiency of these roof systems.

Interim Conclusions

While more evaluations are underway, the data collected to date on the 20 aged TPO roofs evaluated suggests that the membranes are performing well. There were no geographical differences noted and the results indicated that the roofs are capable of achieving their expected service lives. The authors will continue to analyze data and look for trends between climate zones, exposure, membrane thickness, application method, and more as data becomes available.

The ply adhesion values of new repair membrane to the aged TPO membrane are primarily above the average ply adhesion values anticipated for new TPO membranes. This provides validity to the integrity of repairs to aged TPO membranes and the ongoing maintainability of these roofs.

In a few instances, the film-tearing bond was below expectations. The data suggest that there were issues with the workmanship in these few cases, which points to the importance of weld quality checks throughout a roof installation. These would involve test welds to determine the percent film-tearing bond.

The TOS values for all roofs tested suggest that there is little to no erosion of these membranes. Also, no instances of surface cracking when bent over a mandrel were observed. Taken together, these two observations suggest that the membranes are aging very well and in line with the requirements outlined in ASTM D6878.

Solar reflectance data were largely in line with published three-year aged values from the CRRC. This suggests that the generally held assumption that the three-year data are indicative of long-term solar reflectance is correct. Therefore, where that value is used to model building energy efficiency and/or specify HVAC equipment sizes as a result of membrane choice, the membrane is performing to long-term expectations.

The findings to date illustrate the robust performance of TPO membranes as they age. Given the inherent flexibility and fungal resistance of TPO, and the UV and heat stabilizers, this comes as no surprise. However, the ability to repair aged TPO membranes has been undefined and anecdotal to date. The interim findings of this study clearly demonstrate the weld integrity of properly executed repairs to aged TPO membranes.

In summary, the aged TPO membrane roofs in this study are performing well and in most instances, meeting the current ASTM D6878-19 requirements for newly manufactured membranes.

• Even after 12 or more years of aging, both the 45- and 60-mil membrane samples exceeded current thickness requirements for newly manufactured TPO membranes.
• Both the 45- and 60-mil membranes analyzed in this study are still in compliance with these newly manufactured membrane requirements, with the thickness over scrim averaging over 40 percent of the actual aged membrane thickness.

• All of the samples, both the 45- and 60-mil membranes, exhibited no signs of cracking when bent over the mandrel and viewed at 7X magnification.

• All of the 60-mil samples tested to date still meet cold temperature flexibility requirements after 12 or more years of aging. The 45-mil samples showed signs of cracking at -35F. While this is still good performance and aged membranes cannot be expected to perform at the same level as new membranes, the data supports the use of thicker membranes for longer performance.
• Ply adhesion values of the aged TPO membrane were 15-percent above the average ply adhesion value from the SRI study on new TPO membranes. As expected, the aged welds appear to be performing well and are of adequate strength.
• Ply adhesion values of new repair membrane to the aged TPO membrane are above the average ply adhesion value for new TPO membranes. This provides some validity to the integrity of properly executed repairs to aged TPO membranes.

Resources

gaf.com/tpostudy

¹ Demonstration/test roof, material supplied by Montell Corporation (later incorporated into Basell Corporation, which subsequently became Lyondell-Basell).

² H. R. Beer, “Longevity and Ecology of Polyolefin Roofing Membranes,” Proceedings of the Fourth International Symposium on Roofing Technology, Gaithersburg, MD, Sept. 17-19, 1997.

³ H. Hardy Pierce, C. McGroarty, and T. J. Taylor, “An Unprecedented Study Shows Surprising Differences Among TPO Membranes,” Professional Roofing, Aug. 2015, pp. 38-42.

⁴ ASTM D08.18.05.08 Meeting, Atlanta, December 7 – 9, 2009.

⁵ T&R Committee Advisory on TPO, Midwest Roofing Contractors Association, Inc., February 10, 2010.

⁶ T. J. Taylor and L. Xing, “TPO Membranes, UV and Heat Aging. What are the latest findings?,” MRCA 61st Annual National Conference, Indianapolis, October 28 - 29, 2010.

⁷ Midwest Roofer, Midwest Roofing Contractor Association, January/February 2011, pp. 8-9.

⁸ C McGroarty and T. J. Taylor, “A Study of Longevity – Long-Term Performance of TPO Roof Membranes Can Vary,” Professional Roofing, February 2014, 44 – 50.

⁹ T. J. Taylor and L. Xing, “Accelerated Aging of TPO Membranes—Prediction of Actual Performance,” Roofing Research and Standards Development, Vol. 8, ASTM Int., 2015, pp. 139-152.

¹⁰ S. Croce and M. Fiori, Polyolefin Roof Membranes On Site Durability Evaluation, 11th International Conference on Durability of Building Materials and Components, Istanbul, Turkey, May 11-14th, 2008.

¹¹ C. Chapman, K. C. Barnhardt, C. Madsen, J. Carlson, A. H. Delgado, R. M. Paroli, R. Ober, D. Wacenske, M. Ludwig, D. Hunt, W. Collins, and S. Elliot, “TPO Roof Research and Testing Project,” 10th-Year Update Report, Western States Roofing Contractors Association, Western Roofing, July, 2011, pp. 1-18.

¹² Hans-Rudolf Beer, et al. Long-term Field Studies and Residual Service Life Prediction of FPO Roofing Membranes, DBMC XII, April 2011.

¹³ F. J. Foley, J. D. Koontz, and J. K. Valaitis, “Aging and Hail Research of PVC Membranes,” 12th International Roofing and Waterproofing Conference, Orlando, Fla., Sept. 25-27, 2002, pp. 1-25.

¹⁴ Dupuis, Rene M., “Final Report on 2013-2014 Broad Sampling TPO Test Program,” 2015 pages 1-7.

¹⁵ Simmons, T. R., Runyan, D., Liu, K. K. Y., Paroli, R. M., Delgado, A. H., and Irwin, J. D., “Effects of Welding Parameters on Seam Strength of Thermoplastic Polyolefin Roofing Membranes,” Proceedings of the North American Conference on Roofing Technology, 1999, pages. 56–65.

¹⁶ Cool Roof Rating Council, coolroofs.org/products/results