Raising the Roof: The Next Generation PVC
About 25 years ago, PVC roofing made its way from Switzerland and Germany to the United States, bringing with it a host of desirable features. When it first arrived, seams were solvent-welded and later, as contractors became trained, PVC was heat-welded with a hot-air gun. The new roofing material also saved considerably on time and labor, was lightweight, offered many color choices, resisted animal fats and greases, and provided good reflectivity. However, in the late ‘80s, thin and un-reinforced PVC roofing membranes began shattering in cold weather, due to plasticizer migration and polymer degradation. In addition, government restrictions were placed on the use of heavy-metal lead-based heat stabilizers, a standard of the industry at the time.
These events led to important changes in PVC additives and product design. Like a sandwich, polyester or fiberglass reinforcement was inserted between two layers of thicker PVC film for added dimensional stability, strength, and durability. New additives such as the latest biocides and heat stabilizers were introduced to ensure a variety of properties. Consequently, PVC roofing systems were greatly improved and became one of the single-ply roofing industry's stars.
The Power of TwoOver time, virtually all roofing manufacturers adopted similar PVC resins and low-volatile plasticizers-both with high molecular weight and linear orientation, blending in "richer" compounds with less filler content. Inevitably, this led to long-term performance improvement for all membranes. This evolution continued throughout the ‘80s, leveling the playing field as each manufacturer began using the improved chemistry for resin and plasticizer.
PVC compounds normally include impact modifiers, process aids, lubricants, pigments and more. PVC formulators will add ultraviolet light stabilizers to their compositions when the intended end-use involves direct exposure to sunlight. Typically these stabilizers are UV absorbers that protect membranes by absorbing and dissipating the light energy that would otherwise age roofing systems prematurely. Until recently, another class of UV stabilizers, namely the hindered amine light stabilizers (HALS), were not used in PVC applications. When PVC degrades, it releases hydrochloric acid and, in such an acidic environment, HALS were thought to be ineffective. However, in polyolefins and in styrenic polymers, HALS are known to be powerful UV stabilizers.
In order to boost the long-term performance and flexibility of its PVC roofing membranes for customers, GenFlex Roofing Systems, Maumee, Ohio, looked to UV stabilization for further reducing plasticizer loss and polymer degradation. The company tested this theory by adding a double dose of UV absorber to its chemistry as well as those of some of its competitors. Surprisingly, the company found that this did nothing to enhance the performance and longevity of any of the membranes tested. GenFlex then turned to Ciba Specialty Chemicals, Basel, Switzerland, to see if together the two companies could find a way to increase the life expectancy of their roofing membrane.
Ciba Comes Up with a "Quantum Leap Forward"In response, Ciba Specialty Chemicals developed a new class of light stabilizers, based on HALS chemistry, and it has clearly been demonstrated that this brand new type of HALS (called NOR HALS) can perform extremely well in highly acidic polymers such as PVC. GenFlex began the long and arduous testing cycle by preparing PVC sample panels with UV stabilizers and sending them to Ciba for accelerated weathering. Using Xenon arc weatherometers, Ciba tested for appearance and physical properties such as tensile strength and elongation.
After more than two years of bombarding samples with extreme sunlight and weathering, one formulation stood out. The new Ciba® TINUVIN® XT 833 was tested with a variety of different formulations, including those of leading competitors.The first formulation had a high filler loading and low plasticizer. After 6,000 test hours, elongation retention was six times higher with the addition of Ciba TINUVIN XT 833 than with the standard UV stabilizers tested. At 8,000 hours, it had twice the retention compared to the standard.
The second formulation, with low filler and high plasticizer loading, was designed to be more robust for long-term weathering. It achieved 70 percent elongation retention after 6,000 hours vs. 18 percent using standard UV stabilizers, and after 8,000 hours, it had 10 times the elongation retention. Used with the KEE (Elvaloy® polymeric plasticizer) formulation, Ciba TINUVIN XT 833 effected a 45 percent jump at 6,000 hours and a 35 percent boost in retention of properties after 8,000 hours over the standard UV stabilizers tested. Testing demonstrated that it not only performs better than currently used UV absorbers, it also greatly increases the life expectancy of PVC membrane.
Invisible for Roofing ContractorsCiba TINUVIN XT 833 enhances the current UV absorber in GenFlex's PVC sheet formulation and makes it even more effective. A simple analogy would be making coffee taste sweeter. Adding sugar doesn't change the basic product (ground coffee beans and boiling hot water) but it does make for a sweeter coffee taste. A number of years ago, coffee got a "technological advancement" in the form of an artificial sweetener. Like sugar, adding the artificial sweetener doesn't change the coffee, but it does provide a low-calorie substitute for sugar. These enhancements don't alter the basic product chemistry but they do make the product perform better.
Understandably, changes can make people nervous. However, while adding Ciba TINUVIN XT 833 greatly enhances UV protection, roofing contractors won't have to adapt to anything new. There is no change in pricing or application of the roofing membrane. The product will still meet code requirements, and there are no changes in how it is installed.
Flexing Some "Muscle" for Better PerformanceLike most of its competitors, GenFlex previously used calender lamination to manufacture thermoplastic roofing membranes. By 2000, demand exceeded plant capacity for both TPO and PVC. Clearly a new plant was required and there were plenty of options-GenFlex could build a coating, calendering or extruder facility. Studies by GenFlex revealed that excessive or long-term heat exposure during membrane manufacture negatively affected the plasticizer with increased migration, degradation and loss of membrane flexibility. Therefore, a critical decision was made to design the company's Muscle Shoals plant to facilitate a new, advanced extrusion method.
This method introduces less heat, exposing the PVC sheet to heat for only 20 seconds or less, as opposed to the much longer time in traditional PVC manufacturing. At the same time, this new method allows the manufacture of 12-foot sheets of both TPO and PVC on the same line. Since each sheet has to be fused at the seam on the job site, fewer seams equal less labor time. So wider is better (also easier and faster), especially in fully adhered systems. Consequently, roofing contractors can provide higher quality roofing systems in less time.
With new extrusion technology replacing calendering, the PVC mixture is run through dies that squeeze out two films with reinforcement in the middle-all at once. GenFlex also implemented key changes to the biocides used in PVC membranes to provide additional resistance to algae and fungus.
The positive result of pairing Ciba TINUVIN XT 833 with an extrusion process using the existing formula (which includes high molecular weight resin and plasticizer, and low concentrations of filler) greatly improved performance over the standard UV stabilizers tested in terms of longevity, tensile strength, elongation, color hold, glass transition temperature and more. Together, the two companies have made a good product even better-in fact, one that outperforms all competitive formulations tested in the aged sample weather testing.
What's Next?GenFlex successfully introduced its enhanced PVC roofing product, GenFlex RM, to the marketplace in January 2004. Ciba Specialty Chemicals plans to officially launch Ciba TINUVIN XT 833 globally later this year. Already, the company has taken orders from outside the United States.
The use of Ciba TINUVIN XT 833 appears to have raised the bar for PVC membranes, and both Ciba and GenFlex believe that it will also raise customer expectations. There are numerous other outdoor applications that could conceivably benefit from Ciba TINUVIN XT 833. GenFlex is evaluating it for performance enhancement in TPO. Other divisions of GenFlex's parent company, OMNOVA Solutions Inc., are evaluating it in many other products. The greatest potential is seen in those applications where, like PVC, an acidic environment is a source of difficulty.
We tested Ciba TINUVIN XT 833 in a broad enough range of formulation types to feel confident in saying that any external use of flexible PVC products-awnings, pool liners, coated fabrics, etc.-could potentially benefit from this technology. Since one of the most demanding applications-roofing-has already benefited from this new product, Ciba TINUVIN XT 833 is likely to function quite well in many of these other PVC areas.
If It's So Good, Why Doesn't Everybody Use It?With the push for longer warranties, the pressure is on for roofing systems that will outlive these extended warranty periods. It's human nature, though, to say, "If it ain't broke, don't fix it." And, yes, the evaluation time, field testing and code approvals are daunting and time-consuming- these took over three years to complete. However, Ciba also tested the new Ciba TINUVIN XT 833 product in several leading competitive membrane chemistries compared to just conventional UV absorbers. In each case, the product significantly improved the performance of every one of the competitive formulations.
GenFlex believes Ciba TINUVIN XT 833 from Ciba Specialty Chemicals can elevate the entire PVC roofing industry to new heights by increasing the performance of PVC roofing products across the board.