Dr. Z. Jimmy Zhou, Jim Johnston
Papers # 2014 Chicago
Heat fusion joining method has been widely used for thermoplastic piping materials such as polyethylene to form the “one-material” pipeline system. No additional 3rd party materials such as an adhesive are needed. The fusion procedures to ensure the quality of the joints could be different for different materials and standards. ASTM polyethylene pipe heat fusion procedure in PPI TR-33 and ASTM F 2620 is not the same as the ISO procedure in ISO 21307. However, both fusion procedures have delivered high quality joints for about half century. While it is important to strictly following the fusion procedures, it does not mean that high quality joints cannot be obtained outside of the specified fusion temperature-interfacial pressure boxes in these two procedures. The questions therefore are raised that what is the fundamental basis behind these fusion procedures? Are these fusion procedures optimized? In addition to the procedure difference, different thermoplastic pipe have different material attributes to the quality of the joints. Semi-crystalline material is different from amorphous material. Due to the perception that the ability of heat fusion joining is a given feature to thermoplastic pipe, there are very limited researches for the essential variables of heat fusion joining of thermoplastic. The industry has focused on how to qualify the fusion procedures. In this paper, we will explore the material essential variables for heat fusion joining of thermoplastic pipe. Polyethylene pipe resin, as listed as PE4710, PE3608 and PE2708, is taken as a benchmark material. First, to form a joint that behaves like one uniform material, it is essential to have sufficient molecular chain entanglements and cocrystallization of the molecular chains from both sides of the pipe to be fused. Second, the melt flow behaviors as a function of temperature and shear rates are measured and modeled to understand the fusion procedures for various pipe diameters and wall thickness. Third, for a standard fusion procedure that is used for unimodal and bimodal polyethylene as well as high, medium and low density polyethylene, the essential material variables of these polyethylene materials must support the mutual heat fusion procedure. Both material characteristics and fusion procedure are critical to the quality of the joints.
