Putting Membranes to the Test
A Comparison of Methods for Predicting Long-Term Performance of Commercial Roofing Membranes
The artificial weathering performance of commercial roofing membranes has been a vital characteristic to measure and evaluate in regard to predicting long-term field performance. Although single-ply membranes are the newer products in the market, they typically have more sophisticated methods for determining long-term performance than their asphalt-based counterparts. In particular, the single-ply membranes are subjected to longer term UV and thermal aging in order to predict performance, while the more established modified membranes generally use only black-oven aging to accelerate performance. This difference makes it very difficult for building owners and others to differentiate between the products. This paper attempts to compare the approaches for predicting long-term performance used in both the single-ply and modified asphalt membranes, as well as to offer some potential discussion for improving these methods in order to simplify comparisons across technologies.
In the 1990s the TPO membrane became part of the single-ply group of commercial roofing membranes, and in 2003 the American Society for Testing and Materials (ASTM, now known as ASTM International) established the first official standard specification, ASTM D6878. In this specification, weathering performance minimums were set for UV and thermal performance. This standard has been updated several times since 2003 as warranted by the advancements in polymer-aging technologies and, more importantly, as driven by field performance requirements. The ASTM was updated in 2006 to double the UV weathering standard; updated in 2008 to amend the post-UV weathering evaluation method; and finally updated twice in 2011, including an increase to the thickness-over-scrim requirements and referenced test method, as well as an increase to the thermal-performance (heat-aging) requirements. The most recent discussion on the TPO ASTM standard has focused on three specific aspects of heat (thermal) aging:
- Temperature for aging
- Duration of the test
- Post-aging evaluation methods
Currently, the ASTM D08.18 Subcommittee that governs TPO is conducting a round robin test program to understand all three of the variables noted above; the final results are still pending. This heat-aging study includes the evaluation of the failure method for the heat-aged samples. A new evaluation method is under discussion to match the UV-aged test method of determining the failure point by bending the exposed material around a defined geometry and visually observing if cracks are present. For both the UV-aged test method and the discussed heat-aged test method, the sample is pulled from the aging instrument and, if there are no cracks present, the sample is placed back into the instrument for a period of time. The evaluation is then repeated until the failure point is reached. Although this method has been used for some time in the roofing industry for UV-aged TPO samples, it has not been employed officially for heat aging TPO. Also, it is important to note that this method is not the preferred method in the plastics or polymer industries. This is due to the fact that the test process is a qualitative method and is subjective to the observer, the mandrel and even the specimen. In many plastics industries, percentage of weight loss is used as the primary means for determining the degradation of the material, including the stabilizers.,, The delayed increase in weight loss is related to the gradual depletion of the stabilizers so that the leftover polymer is completely exposed to oxidation. The principle is similar to the Oxidative Induction Time (OIT), which has been used in service life prediction of polyolefin geoliners. Yet OIT uses temperatures higher than the melting point of TPO to characterize the stabilizer capability so the TPO is not in the standard solid phase. This may render the method to be unsuitable. Standard weight loss analysis can be conducted at temperatures lower than the melting point of TPO, such that the material in degradation remains in the solid phase. There are articles pointing out that polyolefin has a breakpoint transition in degradation, so extrapolating the mechanism based on higher temperature degradation will predict a much longer service life.
Initial testing conducted on TPO roofing material has shown a strong correlation between cracking and the percentage of weight loss (see Figure 1). This test was conducted using commercially available TPO membranes. In this test, material was placed in an oven with temperatures set at 275°F (135°C). Samples were visually inspected and weighed every four weeks. The results indicate that there is a strong correlation between cracking and weight loss. This test was conducted at various temperatures ranging from 240º to 275ºF (116º to 135°C), yet they all showed similar results — i.e., as the weight loss increases sharply, the observation of cracking becomes more evident (see Figure 2).
More evidence to support this correlation comes from the sample preparation itself, primarily related to the size of the samples cut. The current ASTM specifies that material to be oven aged be of a large enough size to allow for it to be aged and the test pieces to be cut out of the aged sheet. A test was conducted using similar TPO membranes to understand the effects of cutting material into the appropriate sizes prior to heat aging versus cutting them from the larger pieces afterward (see Figure 3). The smaller samples precut to 1 inch by 4 inches show drastic weight loss and observed cracking when compared to a larger piece (which was 12 inches by 12 inches in this case). This phenomenon is often referred to as an “edge effect” and can be noted in various other polymeric samples, including modified roofing samples. Again, when considering this sample size study, the weight loss method is much more sensitive to this thermal degradation and provides a less subjective evaluation when compared to the crack observation method.
Evaluating Thermal Performance
Many discussions are taking place within the roofing industry regarding TPO membrane and the evaluation standards of thermal performance. The following tests were conducted in an effort to further understand the thermal degradation of TPO membrane and to support the movement for technical advances. The goal is that this information will contribute to these discussions and begin some new exchanges regarding the evaluation methods for aged TPO.
Although asphalt membranes have been used in commercial roofing much longer than single-ply membranes, the methods used for determining their future performance are fairly basic and outdated. The current methods, such as those contained with ASTM D5147, are generally based on flexibility retention after black-oven aging. The paving industry is much more advanced in developing testing methods and in employing new technology for predicting long-term performance. Many of the theories and test methods presented below were pioneered by the paving industry. As the demand from the market continues to drive toward longer term and sustainable solutions, the roofing industry must develop better methods for accelerating the aging impacts on asphalt membranes.
When developing methods for accelerating the aging of a material, it is important to understand how the material ages and ultimately fails. For example, as a membrane modified with SBS rubber ages, the SBS rubber itself begins to break down. For various performance-related reasons, most SBS membranes today are modified with a combination of radial and linear SBS polymers. Radial block SBS contains regions of coupled Butadiene Styrene diblocks (think balls for the Styrene portions and ropes connecting the balls as the Butadiene portions) shaped much like a cross. As the radial polymers age, the bond keeping the cross together breaks and the remaining polymer chains (called diblocks of Styrene Butadiene) separate. We can look for this aging using Gel-Permeation Chromatography (GPC) analysis. As the material ages, the radial peak becomes smaller and the diblock peak grows as the radial SBS breaks down.
Taking it to the Lab
Now that we have at least one view of how the material ages, the question becomes: How do we accelerate the aging in a laboratory setting? In other words, how do we force the breakdown of the radial SBS in such a way that we simulate actual weathering? To get this answer, we must first understand the basics of polymer science.
As the molecules in compounds such as SBS chemically combine to form polymers, the double carbon bonds in the backbones of the compounds donate electrons to other neighboring chains in a cascade of molecular bonding called polymerization. In order to reverse this process, we need to force the breakage of those bonds with energy sufficient to reclaim the electrons. One way of exceeding this activation energy is to subject the compound to high levels of oxygen at elevated temperatures in a pressurized environment. The oxygen will act as an electron scavenger and will begin to strip the radial polymers of their electron bonds, forcing the creation of diblock polymers. One tool that can be deployed to accomplish the oxidation of the polymer is called a Pressurized Aging Vessel (PAV). What we want to accomplish with such PAV aging is to simulate a similar percentage changeover of SBS from radial diblock to linear diblock polymers as that in real time, or at the very least black-oven aging.
One way to verify this is to compare the flexibility of the material after oven aging and PAV aging. We can do this quite easily with a Dynamic Shear Rheometer (DSR). A DSR exposes the material to shear stress and reports a value called a modulus, which is simply the stress applied to the material over the strain that it experienced. The higher the value generated, the stiffer the material. As the SBS material ages and the polymer starts to break down, the stiffer it becomes. The stiffness change of a modified blend subjected to various time intervals of PAV aging and compares this to oven aging. As can be seen from the chart, the flexibility change generated by aging the material in the PAV a given time simulates aging the material in a black oven.
Although there is still much more work to be done to develop an accurate method for accelerating the aging of bitumen-based roofing membranes, the data shown here demonstrates that such method development is very plausible.
Environmentally, as asphalt resources continue to reduce in quantity and quality, it will become much more critical for the roofing industry to improve its ability to understand the aging properties of asphalt and develop longer-term performance of these membranes in the field. And in the single-ply market, accurately predicting the long-term field performance of membranes is a critical component in improving the quality and longevity of roofing systems. A critical component of the development of these test methods is tying them to real-world aging. This effort would require industry involvement and the sampling of roofs across the country, but would be extremely beneficial to adding validity to the methods. In addition, as these test methods continue to evolve and improve, the industry should attempt to develop an ability to compare membranes across technologies to simplify options for the end users. The need for collaboration and cooperation to better determine long-term failures of these membranes is a challenge the roofing industry must embrace.
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