From the GMA Techline
Subject: Welding LLDPE/HDPE How do you achieve a satisfactory extrusion weld between a 1mm LLDPE geomembrane and a 2mm HDPE geomembrane? What constitutes a satisfactory extrusion weld?
Reply: You have two things going against you in your situation: thickness differences and material differences. Regarding the thickness situation, 1mm is difficult not to overgrind in order to have the extrudate placed upon it. You will need to put your best personnel on this task. It is difficult but possible.
Regarding the material differences, LLDPE has a broader, and somewhat lower, melting window than HDPE, but they do overlap considerably. In this regard, wedge welding of the two different resins is more difficult than extrusion welding. I don’t think this is as much a problem as is dealing with the thinner sheet. Clearly, many trial (or test) strips will be necessary on your project.
You also ask what would be an acceptable extrusion weld. If you go to our website at www.geosynthetic-institute.org under “specifications” and look at GRI-GM19 you will find that 1.5mm HDPE requires 525N in shear and 340N in peel. Conversely, the thinner and weaker 1mm LLDPE requires 263N in shear and 250N in peel.
Niall, I am afraid that all you can ask for in this case is the lower values associated with the 1.0mm LLDPE.
Subject: Welding follow-up Our landfill project was brought to a standstill today by the CQA engineer. We were attempting to weld a textured 1mm LLDPE cap to a textured 2mm HDPE basal liner at the crest of a cell slope using LLDPE welding granulate. Production weld samples are passing normal peel and shear destructive tests (according to me) because the 1mm liner is ribboning out and ultimately breaking and the weld remains intact (an SE 2 type break).
The CQA engineer, however, is insisting on destructively testing the bond between the LLDPE extradite and the HDPE liner. The resulting extradite-to-liner destructive peel test is resulting in an AD1 adhesion failure, so he is then determining that the weld has failed and must be repaired. We have argued all day but he is sticking to his opinion.
What do you think? Are there any industry guidelines relating to the welding and testing of HDPE to LLDPE connections? Would it be better to use HDPE welding granulate?
Reply: There are no criteria to my knowledge for welding dissimilar geomembranes of any type including LLDPE to HDPE. This is what I wrote to you [previously] and the best you can probably get is the average of the two values for strength.
Extrudite-wise, I would certainly try a HDPE rod and I think it would have been my first choice. The temperature control will be critical and get it just high enough to melt the rod but not the LLDPE.
Subject: RE: Welding follow-up Thanks for your help! We switched from using LLDPE welding granulate to HDPE welding rod and the test samples are now passing—thank God—and everybody is happy!