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  • Research ArticleMicrostructure and Mechanical Properties of the Butt Joint inHigh Density Polyethylene Pipe

    Pashupati Pokharel, Yoonsang Kim, and Sunwoong Choi

    Department of Polymer Science and Engineering, Hannam University, Daejeon 305-811, Republic of Korea

    Correspondence should be addressed to Pashupati Pokharel; ppokharel@gm.hannam.ac.kr and Sunwoong Choi; swchoi@hnu.kr

    Received 19 May 2016; Revised 5 August 2016; Accepted 1 September 2016

    Academic Editor: Volker Schoeppner

    Copyright 2016 Pashupati Pokharel et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

    The microstructure and mechanical properties of the butt joint in high density polyethylene (HDPE) pipes were evaluated bypreparing the joints with increasing the cooling time from 10 s to 70 s before pressure created for fusion of the pipes. Here, coldfusion flaws in HDPE butt joint were created with increasing the cooling time around 70 s caused by the close molecular contactfollowed by insufficient interdiffusion of chain segments back and forth across the wetted interface. The tensile failure mechanismof the welded pipes at different fusion time was projected based on the tensile test of dog-bone shaped, fully notched bar typeas well as round U-notched specimens. The mechanical properties of the joints at different fusion time were correlated with thecorresponding fracture surface morphology. The weld seam as well as tensile fracture surfaces were etched using strong oxidizingagents. The crystallinity of surface etched weld zone by potassium permanganate based etchant was found higher than unetchedsample due to the higher susceptibility of amorphous phase of polyethylene with oxidizing agent.The U-notched tensile test of buttwelded HDPE pipe and surface etching of the weldments provided clear delineation about the joint quality.

    1. Introduction

    Polyethylene (PE) pipes have been used for carrying waterand gas throughout the world from many decades. PEpipe can solve a broad range of piping problems such ascorrosion and abrasion in industrial, municipal, mining,landfill, marine, and agricultural applications cost effectively.High welding capability of PE has been very useful to join PEmaterials through various fusion joining techniques [1]. Buttfusion welding, electrofusion welding, and extrusion weldingare the most widely used joining methods for high densitypolyethylene (HDPE) pipes [29]. Because of a low cost andrelatively simple procedure, hot plate butt fusion welding hasprolonged enormous popularity forwelding of PEpipes in theplastic pipe industry. In such welding process, melting pointis an important parameter and should be considered duringjoining the different grades of PE satisfactorily [8]. Duringwelding of PE pipes, themobilemacromolecules change theirlocation and the joint parts cool down under pressure fora sufficient period of time. Thus, the macromolecules reach

    a state of lower molecular mobility due to cooling and are lesscapable of changing their positions [9, 10].

    The developed interfaces after welding of plastic pipesare usually weaker than the parent pipes. The strength andthe quality of a joint depend on not only the geometry ofthe weld bead through a slight or a stronger notch effectbut also the microstructure of welded area [6, 11]. In thatline, various factors such as welding parameters and thermalhistory of polymer have been recognized on the strengthand the quality of a weld seam [12, 13]. Different mechanicaltesting methods are used to determine the weld seam forcetransmission. Conventional short-term tests of weld seamlike short-term tensile test and bend test have not shownstrongly differentiating statements about the quality of a weldseam [11]. The standard and nonstandard conditions of weldcan be distinguished in terms of morphological behavior[6, 11, 14]. Furthermore, the microstructures of weld zoneand heat affected zone determine the ultimate quality andmechanical properties of the joint. According to Barber andAtkinson [14], the heating and cooling process at the weld

    Hindawi Publishing CorporationInternational Journal of Polymer ScienceVolume 2016, Article ID 6483295, 13 pageshttp://dx.doi.org/10.1155/2016/6483295

  • 2 International Journal of Polymer Science

    Initial bead-up Heat soak Cooling time in themachine under pressure

    Initial bead-up pressure

    Heat soak pressure

    Fusion jointing

    Heat plate removal Time to achieve fusion

    Pres

    sure

    (bar

    )

    Time (s)

    joining pressure (t4)time (t3)

    Heater plate temperature = 231C

    pressure (p3) = 4.1bar

    (p2) 0.5 bar

    (p1) = 4.1bar

    (t5) = 720 stime (t2) = 110 stime (t1) 30 s

    Y

    X

    W-PE10; t3 + t4 = 10 sW-PE30; t3 + t4 = 30 sW-PE60; t3 + t4 = 60 sW-PE70; t3 + t4 = 70 s

    Figure 1: Schematic showing the butt fusion welding profile of HDPE pipes with the variation of fusion time.

    creates microstructure that differs from the base material.According to their study, the weld microstructure has 5distinct regions, namely, skin remnant, spherulitic slightlyelongated, columnar, boundary nucleation, and spherulitic.In addition, Schmachtenberg and Tu [6] reported well dis-tinguishable six regions as moving from the weld centerlineto the base material of hot plate welded polypropylene. Inthat study, they noticed the linear, coarse, fine, oriented,and stretched spherulitic morphology at the weld and heataffected zone. To understand the developed interfaces afterbutt fusion of HDPE pipes, it is essential to distinguish thepolymer chains segregation at the weld zone. Evaluation ofthe morphologies and mechanical properties of welded pipewith the variation of cooling time before pressure created forfusionmay provide the information about the quality of joint.

    To distinguish a defect caused by cold fusion, it is veryimportant to correlate the mechanical properties of joint atdifferent welding conditions with the respective weldmentsurfaces after etching. Etching the weldment using a mixtureof strong acid and oxidizing agent often reveals residualstresses by preferential attack of material in amorphousregions and/or of lower molecular weight (MW) crystallinematerial [11, 15]. Even though there are few earlier reportsabout the morphological feature of weld zone and heataffected zone [6, 11, 14, 16], enough studies are not foundin an open literature about the cold fusion flaws based onmicrostructure and mechanical properties of joints. Externalconditions, such as temperature, wind, and moisture, andcontaminants like pollen grains during welding at outdoorsituation also negatively impact the quality of the pipe fusionjoint. An incomplete fusion of the two pipe halves produces

    a cold fusion joint, possibly only a molecular level. Coldfusion is a specific type of defect that has been the mostelusive to detect in the HDPE butt or thermal fusion joints.The failure at a cold fusion joint after some service time maybe due to thermal ratcheting or fatigue or creep failure. It isessential to reliably identify cold fusion flaws in the joints toensure sound fusion properties of the joint. In this study, coldfusion flaws were developed at the butt joint with increasingthe cooling time before pressure created for fusion of thepipes and that was compared with butt joint at standardcondition. The microstructures of the butt joint at differentwelding conditionswere evaluated after etching theweldmentwith strong oxidizing agents.

    2. Materials and Methods

    2.1. Butt Fusion Welding of HDPE Pipes and Sampling ofWeldments. HDPE pipes (SDR 9, = 110mm) were weldedat 231C under 4.1-bar pressure using automatic butt fusionmachine (NUC-II WMS, world map system, Korea) [15].The details of the welding profile, that is, a plot of pressureversus time during the butt fusionwelding ofHDPEpipes, areshown in Figure 1. During butt fusion welding, the sum of theheater plate removal time (

    3) and the time to achieve fusion

    joining pressure (4) was varied from 10 to 70 s keeping other

    conditions constant. The material codes were given as W-PE10, W-PE30, W-PE60, and W-PE70 for butt joints havingfusion times 10 s, 20 s, 30 s, 60 s, and 70 s, respectively. Thepipe was cut longitudinally placing the welded zone at themiddle of sample for the preparation of dog-bone shapedas well as bar type specimens. Tensile tests of the butt weld

  • International Journal of Polymer Science 3

    samples at different fusion timewere performed at 232Cona universal testing machine (Shimadzu Corporation, Japan).Tensile test of dog-bone shaped specimens was performedwithout notching at a cross head speed of 50mm/min accord-ing to the ASTM D638-10, type I specification. Specimensfrom parent materials without welding were also preparedsimilarly and tested at the same condition for the comparison.The average values of the three samples were used for plottingthe yield strength and elongation at break. For bar typespecimens, sample size (thickness width) was fixed 10 10mm2 and full notch was made 35% of the thicknessat the weld zone of W-PE10, W-PE30, and W-PE60 usingnotching machine (AG-5000G Shimadzu). But, during thedevelopment of full notch at the joint of the rectangular barofW-PE70, the sample was broken into two pieces from joint.So, bar type W-PE70 samples without notch were used fortensile test. All the samples were tested with a cross headspeed 1mm/min at room temperature. We also performedthe tensile test of round notched specimens of butt weldedHDPE pipes. For this purpose, the outer bead of the pipewas removed and a round notch with a depth of 1.6mm wasmade on the joint of the pipe using cutter having U-shapedblades