 Kurdistan Journal of Applied Research (KJAR) Print-ISSN: 2411-7684 | Electronic-ISSN: 2411-7706 Website: Kjar.spu.edu.iq | Email: kjar@spu.edu Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 166 Investigation of the Properties of High-Density Polyethylene Pipes used in Kurdistan for Piping System of Potable Water Sarkawt Rostam Department of Mechanical Engineering/ Production College of Engineering Sulaimani Polytechnic University Sulaimani, Iraq sarkawt.rostam@spu.edu.iq Article Info ABSTRACT Volume 6 – Issue 2– December 2021 DOI: 10.24017/science.2021.2.16 Article history: Received 8/11/2021 Accepted 26/1/2022 High-density polyethylene (HDPE) pipes are recently used in the water distribution network in Kurdistan to replace the old pipes. In this investigation, two types of HDPE pipes (namely A and B) available in the local market have been studied and their properties were compared. Mechanical properties through tensile tests have been investigated and valuable data were collected that could provide a guideline reference for the designers and end-users utilizing these pipes for water supply networks. Furthermore, the HDPE samples were analyzed by differential scanning calorimetry (DSC). Results showed that the ultimate tensile strength recorded for pipe B was greater than pipe A by 8%. Besides, both the elongation at break and strain at break for pipe A was outperformed by almost 6%. On the other hand, the tests showed that the transition from elasticity behavior to ductility behavior for pipe B occurs earlier in comparison to pipe A. It was noted from the gathered information that the two tested pipes were within the standards with variations in their characteristics. Keywords: Polyethylene pipes, mechanical properties, differential scanning calorimetry, oxidation induction time, glass transition temperature. Copyright © 2021Kurdistan Journal of Applied Research. All rights reserved. 1. INTRODUCTION High-density polyethylene pipes are extremely used for water, gas, and sewage systems and are of yearly increased demand. Recently, PE pipes are widely used in piping water [1] and gas systems due to their superior properties of low cost, excellent mechanical and chemical properties [2-4], long-term resistance to environmental effects, and ease of installation as well as lower weight, high resistance to corrosion and ease of joining. Conversely, the repair and technical efforts are of high costs. So, it is necessary to study the reliability of these pipe systems [5-8]. Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 167 The lifetime of plastic pipes used in water and gas supply is estimated to be of 50 years. For this long time of usage, various tests to prove the usability and reliability of these pipes are crucial to avoid high costs of failure during installation and repair during the service [9]. Tensile behavior of recycled HDPE was studied by Ries et al. [10]. The tests were conducted at room temperature for different strain rates. Three stress-strain curves were used to identify the material parameters that can be used in engineering problems. The results show a significant effect of strain rate on ductility, modulus of elasticity, and tensile strength of HDPE. Tensile and oxidation induction time tests were conducted by Castagnetti et al. [11] to show the effect of chlorinated and non-chlorinated water on HDPE pipes used in urban networks for water conveyance. The tests show that the mechanical properties were not affected by the chlorine oxide for the duration of exposure up to 2000 hrs. Draghicescu et al. [12] presented results of tensile tests for HDPE pipelines used for water supply networks to characterize their mechanical properties. It is shown that an increase of Young’s modulus and stiffness depends on the pipe’s diameter. Wu et al. [13] studied the mechanical behavior through the uniaxial tensile test of buried PE pipe under load subsidence. They proposed that the results could be used as a reference for safety and maintenance strategy for the used pipes during service. A predictive model to predict the lifetime of PE pipes was proposed by Wang et al. [14] using the oxidative induction time (OIT) test. The results showed that the internal pressure has a direct impact on the PE pipes. Wu et al. [15] developed a finite element model to analyze the factors affecting the strength of PE pipes. Their results showed that both the strain rate and transition section length have a significant effect on the pipe’s strength. Kratochvilla el al. [16] use a cracked round bar to characterize two types of PE pipes namely PE 100 and PE 100-RC (resistance to crack). The results were evaluated by comparing them to other tests such as notched pipe, creep, and Charpy impact tests. Mechanical properties of polyethylene-layered silicate nanocomposites studied by Alexandre et al. [17]. The study showed that the tensile and shear properties (including Young’s modulus, shear modulus, and strain behavior) were improved by the addition of hydrogen. In addition to mechanical properties, Frank et al. [5] investigate the morphology, stabilization, and structure of materials used in PE pipes. The focus was on the remaining lifetime of the pipes used in gas and water distribution systems. To assess the reliability levels of HDPE pipes, Khelif et al. [6] present a probabilistic characterization of this pipe and its lifetime. Their analysis showed that the influence of the operating temperature of the used pipes on the pipe safety was larger than the gas pressure fluctuations. Expanding the findings from previous works on the mechanical properties of PE pipes leads to practical recommendations and guides for dealing with such pipes in field conditions. In this paper, mechanical properties through tensile test and response to heat through thermal oxidation resistance using DSC for HDPE pipes were investigated experimentally. The aim of this investigation is to gather data about the HDPE pipes available in local markets and used in the area for potable water distribution networks. This is to ensure reliable and safe use of the pipes within the granted period given by the manufacturers as their repair and maintenance are of high costs. The outcomes from this investigation supply the designer, as well as the users with valuable data about HDPE pipes used in water distribution networks, were the research covered the outputs of tensile tests for different samples taken from the two investigated types of HDPE pipes such as tensile strength and its corresponding strain, elongation at break, and elongation at yield and they were compared within the ISO standard of PE pipes used for the piping system of potable water. In addition, the research covered the values of glass transition temperature and time, and oxidation induction time from the DSC test. 2. METHODS AND MATERIALS 2.1. Materials The high-density polyethylene black with blue strip pipes has been used in the tests. They were provided by the local market for two manufacturers providing the market with PE pipes Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 168 and used in the piping system of potable water. Through this paper, the two manufacturers are named “A” and “B”. The pipes specifications are listed in Table 1. As is clear from the table that the type of pipes used is PE 100 from manufacturer “A” while manufacturer “B” uses the type which has resistance to crack and is called PE 100-RC. Other specifications are the same as explained in the table. Table 1: Specifications of PE pipes Specifications Manufacturer A Manufacturer B Type PE 100 PE 100-RC Outside diameter x wall thickness 110 x 10 (mm) 110 x 10 (mm) Nominal pressure (PN) 16 bars 16 bars Standard dimension ratio (SDR) 11 11 Standard ISO 4427 ISO 4427 Color Black with blue strips Black with blue strips Usage Potable water supply Potable water supply 2.2. Sample preparation for tensile test Pieces of 220 mm length were prepared from pipes of 110 mm diameter and 10 mm wall thickness. From the prepared pieces, PE testing samples with a cross-sectional area of 10 x 10 mm were cut according to ISO 6259-3 using a computerized numerical (CNC) milling machine (Figure 1). Figure 1: CNC milling machine for sample preparation Figure 2 and Table 2 show the dimensions of the testing sample. Figure 2: PE test pieces according to ISO 6259-3 Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 169 Table 2: Dimensions of test specimens according to ISO 6259-3 Symbol Description Dimension, mm A Overall length (minimum) 150 B Width of ends 20 ± 0.2 C Length of narrow, parallel-sided portion 60 ± 0.5 D Width of narrow, parallel-sided portion 10 ± 0.2 E Radius 60 F Gauge length 50 ± 0.5 G Initial distance between grips 115 ± 0.5 2.3. Mechanical test The tensile samples of high-density PE pipes were tested using universal machine test (Testometric- 50 kN, Figure 3) according to ISO 6259. Three samples from each manufacturer (A and B) were taken from the pipes and cut by a CNC milling machine into the shape as shown previously in Figure 2. The samples were designated as PE-A1, PE-A2, PE-A3, PE-B1, PE-B2, and PE-B3. Force, stress, strain, and elongation at different stages were being measured. Figure 3: Universal machine test 2.4. Differential Scanning Calorimetry test The response of PE pipes to heat was investigated through differential scanning calorimetry (DSC). In this work, the device of DSC 200-F3 type has been used (Figure 4). It contains two pans in its measurement chamber. The first pan is filled with punched samples of PE pipes for both A and B manufacturers in separate tests and the other pan, which is called the reference pan remains empty. The change of the temperature rate within the time of experimentation is monitored through a connected computer to the DSC device. The oxidation induction time (OIT), glass transition, and other related indicators for each pipe were measured. The input parameters for experiments are shown in Table 3. Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 170 Figure 4: Differential scanning calorimetry Table 3: Input parameters to DSC 200-F3 Parameter Value Material PE PE sample mass 15 mg Reference mass 0 mg Crucible mass 23 mg Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 171 Purge 1 Oxygen Purge 2 Nitrogen Protective purge Nitrogen Start temperature 30˚C End temperature 200˚C Emergency reset temperature 210˚C Heating rate 20 K/min Nitrogen flow rate 50 ml/min Oxygen flow rate 50 ml/min 3. RESULT AND DISCUSSION 3.1. Tensile test results Samples of high-density PE-100 pipes from two different manufacturers have been tested using a universal machine test. The conditions of the conducted experiments are pretension of 10 N, gauge length of 50 mm, sample length of 220 mm, and a test speed of 50 mm/min. Figures 5 and 6 show the stress-strain curve for the two types of PE pipes under investigation. As it appears from the curves that the PE specimen reaches the upper yield point which gives ultimate tensile stress and then an elastic extension can be shown until it reaches its breakpoint giving maximum strain at break. The tensile test is usually used to show the mechanical behavior of thermoplastics. The results from such a test are used to find out the ultimate strength, applied load at peak, yield strength, strain, elongation at yield, elongation at LOP, and elongation at break. The test conditions are highly affecting the behavior of the thermoplastics so the results from the tensile test can provide a behavior index during the usage of the pipes. Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 172 Figure 5: Stress-strain diagram for sample PE-A1 Figure 6: Stress-strain diagram for sample PE-B2 Table 4 summarizes the results of tensile strength and the corresponding strain. It is clear from the table that the pipe from manufacturer “B” gives higher strength in comparison to the one from manufacturer “A” by almost 8%. Table 4: Ultimate tensile strength and corresponding strain from the tensile test Sample Ultimate tensile strength (MPa) Strain % PE-A1 24.59 20 PE-B2 26.75 18 The relationship between force and elongation for the PE tested samples of manufacturer A is shown in Figure 7 and for a manufacturer B is shown in Figure 8. Both figures show that the maximum applied load is between 2379 - 2459 N for PE-A samples and 2524 - 2675 N for PE-B. These values are corresponding to elongation at break as follows: from 200.855 – 314.07 mm and from 201.810 – 306.806 mm for PE-A and PE-B samples respectively. 0 5 10 15 20 25 0 50 100 150 200 250 300 350 400 450 S tr e ss , M P a Strain, % 0 5 10 15 20 25 30 0 50 100 150 200 250 300 350 400 450 S tr e ss , M P a Strain, % Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 173 (a)PE-A1 (b)PE-A2 (C)PE-A3 Figure 7: Force-elongation relationship for samples PE-A 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 0 25 50 75 100 125 150 175 200 225 F o rc e , N Elongation, mm 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 0 25 50 75 100 125 150 175 200 225 250 F o rc e , N Elongation, mm 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 0 25 50 75 100 125 150 175 200 225 250 275 300 325 F o rc e , N Elongation, mm Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 174 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 0 25 50 75 100 125 150 175 200 225 F o rc e , N Elongation, mm (a)PE-B1 (b)PE-B2 (C)PE-B3 Figure 8: Force-elongation relationship for samples PE-B 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 0 25 50 75 100 125 150 175 200 225 F o rc e , N Elongation, mm 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 0 50 100 150 200 250 300 350 F o rc e , N Elongation, mm Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 175 To calculate the elongation at break as a percentage value, the following formula is used: 𝜀 = 𝑙− 𝑙𝑜 𝑙𝑜 x 100% (1) Where: ε = elongation at break as a percentage value, l = gauge length at break, mm lo = initial gauge length, mm. By applying the above formula and using the values of elongation at break in mm from Figures 7 and 8 above, Table 5 can be drawn. The last two columns of the table showed that the elongation at break for all tested specimens is within the standard value of ISO 4427-2 (≥ 350%). Table 5: Results of elongation at break for the tested samples Sample Elongation at break mm Elongation at break % Elongation at break, % According to ISO 4427-2 PE-A1 200.855 401.710 ≥ 350 % PE-A2 246.916 493.832 PE-A3 314.070 628.140 PE-B1 201.810 403.620 PE-B2 210.320 420.640 PE-B3 306.806 613.806 As a summary of the results recorded from the tensile test, Table 6 was prepared as a guide table for designers and end-users of the investigated pipes. The table shows that the values of elongation at break, elongation at yield, and strain at break for the PE-A group outperform while the values of force at peak and elongation at LOP for the PE-B group outperform. Therefore, the designer has the right to choose the preferred PE pipe (A or B manufacturer) depending on the requirements during the pipe installation in water networks. Table 6: Results from tensile tests S a m p le D ia m e te r m m T h ic k n e ss m m W id th m m A r e a m m 2 F o r c e a t P e a k N E lo n g a ti o n a t B r e a k m m E lo n g a ti o n a t Y ie ld m m E lo n g a ti o n a t L .O .P . m m S tr a in a t B r e a k % PE-A1 110 10 10 100 2459 200.855 8.735 0.865 401.586 PE-A2 110 10 10 100 2429 246.916 8.276 0.686 493.792 PE-A3 110 10 10 100 2379 314.070 8.350 0.590 628.014 Average 2422.33 253.94 8.453 0.713 507.79 PE-B1 110 10 10 100 2524 201.810 7.860 0.730 403.539 PE-B2 110 10 10 100 2675 210.320 8.500 0.800 420.640 PE-B3 110 10 10 100 2535 306.806 7.686 0.656 613.563 Average 2578 239.64 8.015 0.728 479.24 Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 176 3.2. DSC test results Figure 9- 12 shows the glass transition for polymer under investigation where the mechanical properties of PE-100 change from elastic material to brittle material. Figures 9 and 10 show the glass transition temperature (Tg) for PE-A and PE-B samples respectively. While Figure 11 and 12 shows the time required for this transition. To get a clear view of the Tg values for the investigated samples from both manufacturers A and B, Figure 13 has been drawn. The figure illustrates that from the onset to the end, the transition of PE-A (126.8˚C – 137.1˚C) is above that for PE-B (119.1˚C- 133.6˚C). Regarding the transition time, this time for the pipe of manufacturer A starts from 146.8 min and ends at 163.7 min while for manufacturer B starts at 119.3 min and ends at 128.2 min. This explains that the transition from elasticity behavior to ductility behavior for pipe B occurs earlier in comparison to pipe A. Figure 9: Glass transition temperatures for PE-A samples Figure 10: Glass transition temperatures for PE-B samples Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 177 Figure 11: Glass transition time for PE-A samples Figure 12: Glass transition time for PE-B samples Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 178 Figure 13: Glass transition temperatures for PE-A and PE-B at different stages During the service, water leakage and consumption reaction may occur which causes depletion of antioxidants. Therefore, to protect the pipes from thermal oxidation, the OIT test was conducted to monitor this case. The level of the remaining antioxidants in the pipe is measured through OIT testing. The exothermic reaction or as called the onset to oxidation will be recorded as a OIT (in minutes) during the test. From the OIT figures this value was determined by drawing a tangent to the exotherm at its maximum point [9]. Figures 14 and 15 show the OIT for PE-A (136.3 min) and PE-B (102.7 min) respectively. According to ISO 4427-2, this time for test temperature of 200˚C is ≥ 20 min. So, both pipes are within the standards. Figure 14: Oxidation induction time for PE-A samples 118 120 122 124 126 128 130 132 134 136 138 140 Onset Mid Inflection End G la ss t ra n si ti o n t e m p e ra tu re , T g ( ˚C ) Glass transition stages PE-A PE-B Kurdistan Journal of Applied Research | Volume 6 – Issue 2 – December 2021 | 179 Figure 15: Oxidation induction time for PE-B samples 4. CONCLUSION Mechanical properties and response to the heat of PE pipes utilized in potable water distribution networks in Kurdistan have been investigated experimentally. Both tensile test and DSC were used. Two sets of samples from two PE pipe manufacturers were tested. The concluded points are summarized as follows: 1. The ultimate tensile strength value was 26.75 MPa for the pipe from manufacturer B corresponding to 18% strain. While the value of strength decreased to 24.59 MPa for manufacturer A with 20% corresponding strain. 2. The maximum elongation at break was 628.14% for the pipe of manufacturer A. 3. The maximum percentage for strain at break was 507.79% for the pipe of manufacturer A. 4. The glass transition of pipe A was above the value of pipe B, where the recorded maximum temperature was 126.8˚C at onset and 137.1˚C at the end. 5. The range of oxidation induction time was from 136.3 min to 102.7 min for pipes A and B respectively. ACKNOWLEDGMENTS The experimental test of this work was conducted at Sulaimani Civil Construction Laboratory. The author extends special thanks to Engr. Aryan Wahid for his help with the testing. REFERENCE [1] D. Brocca, E. Arvin, and H. Mosbæk, “Identification of organic compounds migrating from polyethylene pipelines into drinking water,” Water Res., vol. 36, no. 15, pp. 3675–3680, Sep. 2002. [2] S. Kanagaraj, F. R. Varanda, T. V. Zhil’tsova, M. S. A. Oliveira, and J. 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