ACCELERATED AGING OF POLYETHYLENE PIPE GRADES IN CHLORINE DIOXIDE AND HYPOCHLORITE SOLUTION APPLYING A NEWLY DEVELOPED EXPOSURE DEVICE

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1 ACCELERATED AGING OF POLYETHYLENE PIPE GRADES IN CHLORINE DIOXIDE AND HYPOCHLORITE SOLUTION APPLYING A NEWLY DEVELOPED EXPOSURE DEVICE Márton Bredács and Andreas Frank, Polymer Competence Center Leoben GmbH; Leoben, Austria Amaia Bastero, Dow Chemical Ibérica, S.L., Spain Gerald Pinter, Polymer Engineering and Science, Montanuniversitaet Leoben, Leoben, Austria Abstract Chlorine based disinfectants are frequently dosed into the drinking water network to maintain the high quality of potable water. Considering the strongly oxidative nature of these chemicals their long-term impact on the aging of polyolefin pipes is a matter of interest. In this study three polyethylene pipe grades were objected to an accelerated aging in 10 ppm of chlorine dioxide (ClO 2 ) and 100 ppm of sodium hypochlorite (NaOCl) solution at 60 C. After a few weeks of conditioning the surface analysis with scanning electron microscopy (SEM) revealed several micro cracks, confirming a highly degraded superficial layer. The cross section SEM images displayed the degraded surface layer with a thickness of about a 100 µm. Furthermore thermal analysis such as the Oxidation Onset Temperature (OOT) indicated a significant stabilizer consumption in various depths for each sample. Introduction In the past decades polyolefin (PO) pipes have been successfully applied in the water distribution network due to their numerous advantages. To ensure the nowadays expected 100 years of service lifetime the possible effect of operation conditions on the long-term performance of the PO pipe must be thoroughly analyzed. Several drinking water disinfection techniques such as ozonation, ultraviolet (UV) radiation, chlorination (HOCl), the addition of chloramines, and chlorine dioxide (ClO 2 ) are used to eliminate disease-causing microorganisms in potable water [1 3]. The presence of disinfectants creates a strongly oxidative environment, which may lead to an accelerated consumption of stabilizers and even to the degradation of the materials. Chlorine based disinfectant are the most widely applied species as a result of their high effectivity against various pathogens and remarkably lower implementation and operation cost compared to ozonation and UV radiation. Considering the strong oxidizing potential of ClO 2, it is the most effective chemical substance against microorganisms among the chlorine based disinfectants [4,5]. When ClO 2 enters water it does not hydrolyze but remains as a dissolved gas. This gas is a stable radical which reacts fast due to single-electron transfer with organic compounds, forming mostly chlorite ions, but no chlorinated by-products. Another advantageous property of ClO 2 compared to HOCl is that it does not produce carcinogenic trihalomethanes (THM) [6,7]. The applied amount of ClO 2 for drinking water disinfection is generally in the range of 1-1,4 mg/l in the USA [8] and in the EU it is usually kept between 0,05-1 mg/l [7]. At higher concentrations of chlorite and chlorate ions are proved to have an adverse health effect [9], hence the maximum allowed amount of them is 0,8 mg/l [10]. Chlorination as a disinfection technique is more widely used then the addition of ClO 2. A significant drawback of this disinfection technique is the chlorinated by-products such as THM and further chlorinated organic molecules, which are proved to be carcinogen [2,11,12]. As a consequence in the USA the maximum residual disinfectant level is regulated to 4 mg/l measured as FAC [10]. The only regulation in the EU is the allowed maximal THM concentration in drinking water, which is set to 0,1 mg/l [13]. Chlorination has a lower disinfection capability so HOCl is frequently dosed into the drinking water network in higher concentration than ClO 2. In case of the replacement a part of the polymer pipe network sometimes shock chlorination or so called superchlorination up to 500 ppm is also applied for a short time period [14]. Various field case studies [15 18] revealed premature pipe leakage, which were in most cases attributed to the presence of disinfectant in the water distribution system. To investigate the effect of these two disinfectant on pipes the experiments frequently performed based on ASTM F2023 [19] and F2263 [20]. The internal pipe pressure tests are highly expensive and time consuming. Therefore in the relevant literature accelerated laboratory aging of pipe materials are intensively discussed [21 29] to gain useful data in shorter testing time. To perform reliable and reproducible experiments the exposure parameters must be constant well controlled during the aging period. The concentration of both disinfectants can decrease rapidly due to rapid reaction with organic compounds presented in the water and thermal decomposition, which can lead to significant loss even at room temperature. Several authors [30 33] pointed out that the concentration of ClO 2 in water can decline considerably in a matter of few hours. In case of HOCl similarly findings were reported in numerous publications [34 38,38]. Unfortunately in many SPE ANTEC Anaheim 2017 / 1960

2 relevant publications the only information provided in regard to the monitoring of disinfectant concentration is the replacement of the exposure media in every few days. To avoid any strong deviation in the concentration of aging media a continuous monitoring and adjusting is a crucial part of an accelerated aging experiment. Therefore a new exposure device was developed at PCCL in cooperation with Mechatronik Moharitsch GmbH (Austria). In spite of the obvious impact of disinfectants on PO lifetime the accurate degradation mechanism has not been understood yet. The aim of the current study is to gain a deeper insight into the relevant degradation process by accelerated laboratory aging of PO pipe grades. In order to investigate the effect of ClO 2 and HOCl on polyethylene (PE), an accelerated immersion test was performed. In the present work the main focus was put on the analysis of material property changes in various depths. Experimental The technical specifications of the newly developed exposure device are listed in Table 1. With this equipment precise immersion tests in ClO2 and chlorine solution could be performed, including the monitoring and the accurate controlling of the relevant aging parameters [39]. Table 1. Technical specifications of the PCCL exposure device. Chlorine dioxide Test medium Sodium hypochlorite Temperature max. 60 C ± 1 C Vol. sample chamber 50 l Flow rate max. 500 l/h Concentration ClO2 max. 10 ppm ± Concentration max. 100 ppm ± NaClO ph value ± 0.1 measurement free chlorine ± 0.1 ORP measurement ± 0.1 ppm For the formulation of ClO 2 solution the hydrochloric acid-chlorite-technique was applied. The ClO 2 stock solution is prepared by a device of the type Legio Zon CDL (ProMinent Dosiertechnik GmbH, Austria) and injected into a closed loop including the sample chamber via an integrated dosage pump. The chlorine solution is prepared from NaOCl stock solution. In the closed loop circulation a flow rate of 50 l/h was applied. Continuous monitoring of the disinfectant concentration, ph, ORP and temperature is verified by metering stations for acids/leaches and by a measurement and control plant DULCOMARIN II (ProMinent Dosiertechnik GmbH, Austria). All parts of the instrument are made of materials, which are resistant against chemical, mechanical and thermal loadings and meet the demands of ÖNORM M [40]. All exposure parameters are recorded in every five minutes, and stored for further analysis. In figure 1 and 2 the aging parameters over the exposure time for ClO 2 and HOCl are shown. The vertical dashed lines represent the stops of the machine due to sample removal. After starting each operation cycle all parameters have reached the selected values quickly and stayed on a constant level. These results clearly demonstrate the accuracy and the stability of the device. c ClO2 [ppm] sampling 1w (167 h) sampling 2w (328 h) sampling 3w (495 h) 10 ppm ClO 2 Exposure, T exp =60 C ClO 2 ph ORP/ t [h] sampling 3w3d (703 h) c ClO2 =10 ppm ph= 6.8 ORP=620 mv Figure 1. Aging parameters during the exposure to chlorine dioxide. NaOCl [ppm] sampling 4w (636 h) sampling 8w (1299 h) 100 ppm NaOCl Exposure, T exp =60 C HOCl ph ORP/ t [h] additional sampling 10w (1628 h) sampling 12w (2055 h) c HOCl =100 ppm ORP=940 mv 8 ph= 6.8 Figure 2. Aging parameters during the exposure to hypochlorite solution. Three different PE were subjected to accelerated aging in 10 ppm of ClO 2 and in 100 ppm of NaOCl solution, with a ph level of 6.8 at 60 C. In case of exposure to ClO mv of Oxidation-Reduction Potential (ORP) was measured. For the NaOCl solution the ORP value was found to be 930 mv. The immersion test in ClO 2 was conducted up to 570 h, and samples were 6 4 ORP/100 [-]; ORP/100 [mv] ph [-]; ORP/100 [mv] SPE ANTEC Anaheim 2017 / 1961

3 removed weekly. In NaOCl solution a three months long exposure was initiated including several sample removals, although in this study only a part of the results will be discussed. From 15 mm thick compression molded plates 10 mm wide and 30 mm long block specimens were prepared. To investigate the changes of material properties in various depths 10 thin film specimens were cut from the surface into the bulk after each sample removal. The 50 µm thin films were prepared via sectioning using a rotarymicrotome of the type RM 2255 (Leica Microsystems, Austria). In Fig. 2 the block samples and the sectioning of thin films are illustrated schematically. 50 µm A A-A In addition the OOT depth profiles of the PE samples aged in NaOCl solution is presented. Material properties after aging in ClO 2 The SEM images obtained from the surface of the block specimens are presented in Fig. 4. A high amount of surface micro cracks appeared on the surface of each material after the 576 h aging experiment. After 336 h of exposure more cracks were recognized on the surface of in comparison to and. Surface cracks have been identified in case of sample already after 168 h of aging. These findings indicate a slightly better resistance of and against aging in ClO 2 solution. (A) A Figure 3. Illustration of a block specimen, and the preparation of thin films. Characterization of the surface and the cross section morphology was carried out with scanning electron microscopy (SEM) of the type TESCAN VEGA II (Elektronen-Optik-Service GmbH, Germany). The film samples were subjected to oxidation test with a differential scanning calorimeter (DSC 4000 Perkin Elmer GmbH, Germany) to measure their oxidation onset temperature (OOT). Therefore the samples were continuously heated up from 25 C to C with a heating rate of 20 K/min in synthetic air atmosphere until they reached the onset of the oxidation. The calculation of the OOT values was carried out by shifting the baseline with 0.2 W/g [41]. With each thin film at least three OOT measurements were performed, and then the average values were used for further evaluation. To characterize potential changes in the chemical structure measurements were completed with a Spektrum GX Fourier Transform Infrared spectrometer (FTIR) in Attenuated Transform Reflectance (ATR) mode. Altogether 16 scans from 4000 to 650 cm-1 were recorded in ATR mode with a resolution 4 cm-1. The carbonyl index was calculated based on DIN [42], by the determination of the peak area ratio between and cm-1. Results and Discussion In this study the main focus is put on the results obtained from block specimens after aging in ClO 2. Beside the morphological analyses the changes in the amount of active antioxidant and in degradation product at different distance from sample surface is also discussed. 5 µm 5 µm 5 µm (B) 20 µm 20 µm 20 µm (C) 20 µm 20 µm 20 µm Figure 4. Surface SEM images of samples exposed to ClO 2 after 168 h (A), 336 h (B) and 576 h (C) of aging. The results of the cross section analysis at the end of the immersion test are illustrated in Fig 5. A degraded superficial layer with a thickness of about 100 µm can be recognized in case of the three PE materials. Images captured after 168 h and 336 h of conditioning displayed a similar thick degraded layer. No significant differences have been detected between the different materials 5 µm 5 µm 5 µm Figure 5. Cross section SEM images after 576 h of aging in ClO 2. The fact of the almost same thick degraded layer after each sample removal could lead to following assumption: The chemical species, which are responsible for the formation of the degraded layer, could rapidly diffuse into the material, and probably due to their high reactivity they are also consumed quickly. The ratio of the diffusion and SPE ANTEC Anaheim 2017 / 1962

4 reaction rate of the relevant molecules could give an explanation for the fast formation and for the unchanged thickness of the degraded material layer over the exposure time. To confirm this assumption on one hand the responsible must be identified, and on the other hand their physical properties need to be measured or at least estimated. Although in some publication a similar hypothesis has been already discussed [14,26,43], unfortunately the responsible species have not been identified yet. In regard to the diffusion and reaction rate of disinfectants some material constants have been already published in the scientific literature [44], although the available values mostly represent molecules produced by the chlorination of potable water. The activity and the amount of AO present in polymer sample could be well estimated by oxidation test. It is well known that the measured oxidation induction time (OIT) values are in linear relationship with the amount of the available AO. To examine the AO depletion process oxidation onset temperatures (OOT) were measured, which can be well correlated to the OIT test [45]. A significant advantage of OOT tests are less time consuming measurements, compared to the OIT analysis. The average OOT values plotted against the distance from the surface of the block specimens are illustrated in Figure 6 and 7. The lines in Figure 6 and 7 serve only as a guide for the eye. Lower OOT-s than C represent a very low or no active AO amount, indicating a strong AO loss. The OOT of about 270 C corresponds to the reference value, demonstrating the initial AO amount. Generally all samples show a strongly reduced OOT value at the surface and approximately the reference values in the bulk. Considering the rather short aging period this significant decrease of OOT demonstrates the fast chemical consumption of the AO. Similar findings were observed by other authors, revealing a rapid reaction between the AO and the ClO2 [28,46]. After 168 h of exposure (Figure 6) remarkable differences can be recognized in the AO depletion profile of the three PE samples. In case of a very low amount of AO can be observed in the first three thin films, and no AO consumption at a depth of µm. For the other two PE materials a highly similar trend can be recognized, although significant AO depletion was detected only in the first two layers. With increased exposure time (Figure 7) all three AO depletion profiles are shifted deeper into the bulk region, representing a highly reduced stabilization in a µm thick surface layer. These results indicate that has a slightly better stabilization than and against AO consumption in ClO 2 solution at an elevated temperature. For further precise conclusions an extended aging experiment should be performed, while the OOT profiles may tend to an asymptotic depth as proposed in the work of Colin et al. [47]. The approximately µm thick superficial layer presenting highly reduced OOT values is about two times thicker than it was observed from SEM cross section images. From this difference one could assume that different reactive species are responsible on the one hand for the AO consumption and on the other hand for the formation of a degraded material layer. = 168 h [ClO 2 ]=10 ppm, T Exp Figure 6. Oxidation Onset Temperature plotted against the distance from surface after 168 h. = 576 h [ClO 2 ]=10 ppm, T Exp Figure 7. Oxidation Onset Temperature plotted against the distance from surface after 168 h. Material properties after aging in NaOCl solution Samples of the three selected PE pipe grades after three different exposure times were subjected to dynamic oxidation. In Figure 10 the OOT depth profiles are shown after 636 h of aging in hypochlorite solution. Despite the longer exposure time than in case of ClO 2 showed constant OOT values indicating no remarkable AO consumption. In approximately 100 µm thick surface layer decreased active amount of AO was detected for and. On one hand after almost 8 weeks of aging no AO consumption was found for, as it is illustrated in Figure 11. On the other hand showed decreased OOT values through analyzed 350 µm of surface layer. For only a slightly decreased OOT values were recorded in the first three layers. SPE ANTEC Anaheim 2017 / 1963

5 = 2055 h = 636 h [HOCl]=100 ppm, T Exp Figure 8. OOT depth profile of three PE pipe grade after 636 h = 1299 h [HOCl]=100 ppm, T Exp Figure 9. OOT depth profile of three PE pipe grade after 1299 In Figure 12 for a significant change in the OOT depth profile can be recognized. The amount of active AO remarkably decreased in the investigated thickness. This behavior means that the reaction of hypochlorite and hypochlorous acid with the AO in the first 8 weeks of aging was suppressed by other dominating mechanism. Interestingly only in case of has been this behavior recorded. For and highly similar OOT values were measured than in case of, which correlates well with the findings of Colin [47]. Moreover the different effect of the two disinfectants on the AO consumption in the surface layer can be easily recognized. The decrease of the OOT values shows a more linear profile after aging in NaOCl solution, than in case of aging in ClO 2. The significantly longer aging times until considerably AO consumption in NaOCl solution, clearly indicates the higher oxidative potential of ClO 2. The observed different OOT profiles may be could be attributed the different nature of the disinfectants. In case of HOCl both chlorination and oxidation reactions can happen, compared to ClO2, which prefers oxidation over chlorination. [HOCl]=100 ppm, T Exp Figure 10. OOT depth profile of three PE pipe grade after 2055 Conclusion The accurate control of the exposure parameters like disinfectant concentration, ph level, ORP, and the temperature is highly important to precisely study the impact of ClO 2 and chlorine on the degradation of PO pipe materials. The presented results clearly indicate the capability of the new exposure device for stable and well monitored immersion tests in chlorine and ClO 2 solution. The surface and cross section analysis with SEM showed extensive degradation of the four materials. Micro cracks on the surface of have been found already after 168 h of aging. Furthermore the presented OOT and carbonyl profiles of displayed the thickest surface layer with significant AO consumption and increased CI values. These changes in case of were only detected in a thinner superficial layer. The represented results demonstrate very well that the analyses of OOT and CI profiles at different depths of block specimens is a useful tool for material ranking based on the resistance against aging in ClO 2. The aging in NaOCl solution led to considerably different OOT depth profiles, pointing out the diverse effect of the two disinfectants on the AO consumption. The obtained information is of high interest for material producers to develop more resistive stabilizer packages and PE pipe grades, when disinfectant are added to potable water distribution network. In order to obtain a more thorough understanding of the relevant aging processes, and to determine the responsible chlorine species for material degradation and AO consumption further accelerated aging studies will be performed. Acknowledgement The research work of this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and Federal Ministry for Economy, Family SPE ANTEC Anaheim 2017 / 1964

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