Effect of insufficient homogenization during the extrusion of polyethylene pipes on butt fusion joint integrity

Abstract

High-density polyethylene (HDPE) is increasingly used for pipe applications owing to its lower cost and higher resistance to chemical corrosion and biological attack compared with metallic materials. Hence, HDPE with improved mechanical properties for pipe application were developed. In addition, additives are used to improve the longterm performance of PE pipes. Carbon black (CB) is one of the most widely used additives in PE pipes due to its low cost and absorption of UV which contributes to the resistance to photo degradation. However, it is difficult to achieve good homogenisation of HDPE and additives by the single-screw extrusion method. Insufficient homogenisation is observed as black (where HDPE and CB are well mixed) with white striations (only HDPE without CB); the white striations are called "windows". It has been shown that windows could lead to reduction of HDPE pipe performance and several methods have been developed to detect them. However, there is lack of study on the effect of windows on the integrity of butt fusion joints and there is only one method reported in open literatures which can quantify the window. However, this method causes material wastage.

Visual, Non-Destructive, and Destructive Investigations of Polyethylene Pipes with Inhomogeneous Carbon Black Distribution for Assessing Degradation of Structural Integrity

“…It was also reported that tear initiation was observed at the window-black compound boundary at cold drawing strains; such premature interfacial separation was attributed to the observed property reduction. Similarly, for butt fusion joints, elongation [22,31] or energy to break [31,35] decreased with windows. Such a reduction was first shown with a dog-bone tensile specimen (ISO 527 Type IA) [36] removed from a butt fusion area [22] and later with waisted tensile specimens (ISO 13953) [31,35,37].…”

“…Similarly, for butt fusion joints, elongation [22,31] or energy to break [31,35] decreased with windows. Such a reduction was first shown with a dog-bone tensile specimen (ISO 527 Type IA) [36] removed from a butt fusion area [22] and later with waisted tensile specimens (ISO 13953) [31,35,37]. In both cases, the elongation was significantly reduced with the mediumand high-level windows specimens.…”

“…There exists very limited literature concerning the effect of windows' presence on the structural integrity of pipes and their butt fusion joints [21][22][23][29][30][31][32][33][34][35]. Rapid crack propagation field failures were noted on in-line compound 315 SDR 11 PE water pipes and butt fusions in non-safety class nuclear power installation [21].…”

“…The mechanical and fracture behaviors of windowed pipes and butt fusion joints involving controlled windows' levels (low, medium, and high levels) were recently reported [22,23,30,31,35]. They indicated the post-neck strains decreasing drastically, and a transition from normally ductile to brittle fracture surface was observed when windows above a certain level were present [22,23,30,31].…”

“…In terms of observing window patterns and quantifying % windows, most of the work done used microtomed thin films obtained from small sections (e.g., 10 mm × 10 mm) taken from the whole pipe cross section [23,24,29,31,35,38]. The grayscale threshold and pixel count method were used to establish % windows in both in-flow and cross-flow directions for pipes and butt fusion zones [23,31].…”