Necessity may be the mother of invention, but in the industrial world, advancements are usually made in small, continual improvements rather than giant leaps. Nevertheless, advancements are made. Nowhere is this more evident than in the world of modified roofing membranes.
Unfortunately, the equipment and methodologies developed for testing commodity-type membranes cannot accurately measure the higher levels of tensile or tear strength found in some of today's high-performance membranes. Although newer testing methods exist that can accurately differentiate higher levels of tensile and tear strength performance, there is no recognized industry standard. As a result, there is no easy way for an architect, designer, or building owner to compare published performance specifications with any confidence that he is indeed comparing apples to apples.
Until such standards are established, the information following is intended to help end-users ask the right questions and demand appropriate independent testing methods to ensure reliable, accurate comparisons of competitive materials.
Radical Improvements in Mod-Bit Technology
During the last three decades, incremental advances in modified bitumen technology have radically improved the performance of roofing materials in four critical areas of performance:
• Waterproofing integrity.
• Flexibility in adapting to building movement.
• Ability to sustain thermal shock.
• Ability to remain stable when exposed to UV and weathering.
From APP (attactic polypropylene) to SBS (styrene butadiene styrene) to SEBS (styrene ethylene butadiene styrene) to SIS (styrene isoprene styrene), polymer modifiers - the alphabet soup of the membrane world - have allowed developers to continually adapt their formulations for significant long-term improvements in modified bitumen products.
Simultaneously, manufacturers have been making continual improvements in the reinforcement materials used as the backbones of these membranes. Material innovations - including non-woven glass reinforcement, orientated glass, polyester, and composite sheets - have significantly enhanced the stress-strain performance of the latest generations of modified bitumen membranes.
Traditional Testing Methods
Despite the fact that all these incremental improvements have added up to radically improved performance capabilities, performance testing for roofing membranes has remained relatively static. There have been some modest improvements in testing equipment, due to the introduction of electronics and computerization. But most of these improvements affect the read-and-storage capabilities of the testing instruments, rather than the testing methodologies.
The principal roof-membrane tests still involve the tearing, straining, flexing, or melting of materials. For the lower-tensile-strength products, traditional tensile-testing equipment continues to yield excellent and reproducible results. But when dealing with the new, high-performance membranes (products with greater than 500 pounds/inch tensile strength or 900 pounds tear strength), laboratory-to-laboratory variations are becoming more and more apparent.
Understanding Testing Variables That May Affect Results
Most of the deviations in test results with high-performance membranes are related to variations in the grip configurations and types. American Society for Testing and Materials (ASTM) procedures do not specify the use of a particular grip; they merely describe a typical configuration.
Specifically, ASTM D-5147 for tensile strength simply states that the sample must be held in place with "uniform pressure." ASTM D-4073 for tear strength merely dictates that the clamp must be "3 inches by 2 inches."
The primary variables that affect both tensile and tear test results are:
• The type of clamp, i.e., whether the clamping action is controlled manually or is pneumatic and self-adjusting.
• The size and shape of the gripping mechanism.
• The face type (flat, smooth, diamond-hatched, serrated, rubber-coated, etc.) and gripping pressure.
In regards to the clamping action, manual compression can be more subject to operator variances in technique, and therefore can lead to less consistent results. Pneumatic clamping ensures constant, repeatable pressure, grip after grip, and has the added benefit of being self-adjusting, as the gripped material becomes more and more compressed.
Although manual clamping has proved adequate for testing lower-tear-strength materials, when used to test higher-tear-strength materials, care must be taken to produce accurate test results. In other words, a high-performance membrane can appear to be stronger or weaker when tested using varying clamping methods. For the higher-tear-strength products, results may vary by as much as 25 percent. Similar results have been observed in tensile-strength analysis.
Size and Shape of the Gripping Mechanism
Variations in the size and shape of the gripping mechanisms can affect test results in two principal ways:
• Some grip styles are less able to hold on continuously to the sample as it becomes compressed. If the sample slips out of the gripping configuration, test results will be inconclusive.
• It is always preferable for a sample to break as near as possible to its center, at the farthest point away from the grip-faced edges. Some grip styles make it more likely for the break to occur close to the point of restraint. In such cases, the test results are somewhat suspect, since the grip-faced edges themselves may be adversely contributing to the observed effect.
Face Type and Gripping Pressure
The face type and gripping pressure applied are also critically important. A sample held against a smooth, polished face may slip more readily than one held against a serrated jaw face. Conversely, one held against a serrated jaw face may be more prone to crushing. The grip pressure appropriate for one type of face will be inappropriate for a face of a different type.
As can be seen, variations in testing equipment can make a significant difference in test results. The only way to ensure accurate comparisons of roofing materials is to request that products be independently tested using identical equipment and methods.
The 'Minimum' Nature of Performance Standards
Another important consideration is the fact that current ASTM protocols for modified bitumen membranes establish minimum, rather than recommended, performance values. ASTM D-6162, establishes baseline values for load-strain performance of SBS products as follows:
MD/XMD: lbF/in7580250Elongation at Max. Load
at 73 degrees F.: %243Tear Strength at 73 degrees F.: lbF6590280
Although this constitutes a reasonable standard for ensuring minimum performance, several products on the market today offer two to three times the performance of a Type III membrane. Such high-performance products offer significantly improved longevity in the dynamic conditions created by the thermal-related movement of roof systems.
Examining only the fracture mechanics of the membrane, it should be evident that a material offering as much as 900 pounds of tensile tear strength will outperform a competitive product that merely meets the Type III standard. Although it has been argued that 900 pounds of tensile tear strength is excessive, the performance characteristics of any roofing material will degrade over time. A higher level of load-strain performance may well make the difference between roof system success and failure, as the membrane reaches its 20th or 30th year.
Do Your Research
New roofing membrane technologies are making it more important than ever that customers take a more critical look at testing equipment and methods. Never accept published specifications at their face value. Always ascertain what testing equipment and methods were used, so that you can be certain you are comparing apples to apples when evaluating critical measures of performance such as tensile and tear strength.
Joe Mellott is vice president of technology at Momentum Technologies Inc., Uniontown, OH, and serves as chairman for the Technical Committee of the Roof Coating Manufacturers Association (RCMA) (www.roofcoatings.org).