Thermal Welding Liners - How It Began
In 1990 EPI began wedge welding PVC Geomembrane field seams. The original goal was to extend the liner construction season in Michigan by developing a welding technique for PVC that could be used in cooler weather. Geomembrane welding would allow earlier Spring start ups and delay shut down until late Fall.
Wedge welders are hand-held devices that allow us to heat the two sides of the liner that we want to weld together to then press them and seal them. The material is sealed at the molecular level by the heat and compression. This new geomembrane welding technique is an excellent choice for many liner applications.
Changing To Hot Air Welders
After several years of experimentation and numerous consultations with equipment manufacturers, EPI made a wholesale change to hot air welders for welding PVC. We recieved considerable help from Bruno Zurmuhle of Leister and J.B. Budny from Heely-Brown Company. Developing the procedures for using hot air welders on thinner, more flexible PVC materials was a challenging task. But the problems were conquered and EPI welding technicians began to develop the skills to professionally weld PVC geomembrane in any thickness in almost any weather condition.
Thermal Liner Welding - The Air Channel Test
What evolved from these problem solving sessions and the reams of test data developed, was the absolute belief that the air channel test could be used to verify the physical strength of PVC liner welding. In 2001, TRI Environmental agreed to do some testing and research on PVC thermally welded seams. Rick Thomas also became intrigued that PVC seams could be tested for peel strength using an air channel test. In 2002 burst testing research was initiated on hot air and hot wedge welded PVC seams in 30 and 40 mil PVC. The result of this testing and other research was a graph of pressure vs. sheet temperature for air channel testing PVC geomembranes that verifies a minimum of 15 lb/in peel strength for the full length of the test section. This temperature and pressure correlation is necessary to correct the test to the same conditions required in the laboratory when peel testing PVC seams.
In 2002, Mark Wolschon, Quality Control Manager of EPI, introduced to ASTM the idea of a standard air channel test for PVC. ASTM Committee D35 established an ASTM Task Group to develop a new standard. After over two years of extensive discussions, ASTM D7177 Standard Specification for Air Channel Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed Geomembranes was adopted by ASTM in 2005. ASTM D7177 is now the recognized standard for air channel testing of PVC field seams.
Simply stated, poorly made thermal welds in PVC geomembrane peel open when subjected to air channel testing according to ASTM D7177. This test stresses the entire length of the seam, so any weak areas no matter how small will be immediately located. Any failing seam should be replaced. EPI has also experienced field welds with passing destructive samples removed, failing an air channel test in a small area of the same seam. A seam of this type must be rewelded to insure the customer receives the best possible product.
EPI has compiled qualitative test results and statistically analyzed the data. We have found through this process peel strength of the seams is generally stronger and the seams can be verified for conformance in minutes, rather than days, after production. Air channel testing is used for seam continuity as well as verifying peel strength for the entire length of the seam!
This histogram of field seam peel tests on 516 specimens of 30 and 40 Mil PVC shows that 80% of the results are between 20 and 45 pounds per inch width, with peek values as high as 55 lbs/in/width.
This hot air dual track welding and testing method assures you that the PVC field seam strength will exceed all minimum quality standards.
ALL field seams must be tested and T-seams can be difficult to air channel test if not welded properly. T-seams are defined as a point in the seam where three layers of material overlap each other. This occurs at the point that a dual track field weld crosses a factory seam, usually at a 90 degree angle. We have had hundreds of questions from engineers and customers regarding the air channel testing of "T" seams, the seam created along the end of a PVC geomembrane panel. EPI air channel tests ALL field seams, including the seams along the ends of factory panels.
While panels can be fabricated square, PVC panels are typically fabricated in rectangles, longer than they are wide. The panels are created using a large number of individual strips from rolls of PVC, welded together edge to edge. These edge welds terminate at the end of each panel and will overlap an adjacent panel when deployed in the field. This end panel overlap must be welded properly in order to air channel test the resulting weld.
Specimen of EPI "T" Seam in 30 mil PVC
There is a potential at each "T" to have a very tiny hole at the junction of the three layers of material. This is another key reason why air channel testing of every seam is critical to the integrity of the liner system, finding and eliminating these holes. Special care is taken by the welding technicians when setting up the liner welder to make sure this type of overlap is completely sealed, so the air channel test can be used to verify strength and continuity of these seams also.
EPI factory seams have no loose edge, so the process for welding T-seams is relatively easy. Slowing the welding machine's rate of travel will allow the melted PVC material to flow together at the junction of the three layers of material, providing the necessary seal and weld strength. For fabricators who leave a loose edge on the factory seams, then each loose edge will need to be trimmed, similar to the process used on field welds which intersect other seams.
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