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low temperatures. The degradation mechanism may seemdifficult to explain given the relatively mild conditions ofcommercial use. The concepts below may shed some lighton the potential degradation pathway.One study5has shown that hypochlorous acid can reactwith iron(II) complex (Fe+2) in aqueous solution with therate constant 220 ± 15 dm3mol-1s-1. In this reaction,free hydroxyl radicals are formed in 27% yield. Thehydroxyl radical and chlorine radical can then initiate thedegradation of polyethylene pipe.Fe+2HOCl?HO.+ Cl.Reaction 8Another study6 showed that saturated alkanes can beoxidized by hypochlorous acid in darkness, in a two-phasesystem, and at relatively low temperatures (0°C-50°C).This study implicated Cl2O, generated from hypochlor-ous acid according to Reaction 9, as the radical generatingspecies.2HOCl ?Cl2O + H2OReaction 9The authors of this study proposed two possible mecha-nisms for the initiation of free-radical chains. The firstinvolves cleavage of the Cl-O bond in the Cl2O, whichthey claim is possible because of the high electronegativityof the Cl and O.Cl2O ?Cl.+ .OClReaction 10Another potential pathway for the free radical initiationis an electron transfer process between the polyethylenepipe and the chlorinating compound (Cl2O). The scien-tists drew an analogy between this reaction and the spon-taneous free radical fluorination of hydrocarbons by ele-mental fluorine.7R-H + Cl2O ?R.+ ClO- + Cl.+ H+Reaction 11The generation of R.and Reaction 10 andReaction 11 can lead to the accelerated degradation of thepolyethylene pipe. The polyethylene degradation pathwayis described below (Reaction 12 to Reaction 17).Cl.+ R-H ?HCl + R.Reaction 12R.+ O2?R-O-O.Reaction 13R-O-O.+ R-H ?R-O-O-H + R.Reaction 14R-O-O-H?R-O.+ H-O.Reaction 15R-O.+ R-H ?R-O-H + R.Reaction 16H-O.+ R-H ?H2O + R.Reaction 17ExperimentalIn the first phase of this study, commercial polyethylenepipes were obtained and immersed in deionized (DI)water, while another set was immersed in chlorine water.The concentration of chlorine in the chlorinated DI waterwas fixed at 5 ppm of free chlorine using calciumhypochlorite, at an initial pH of approximately 6.8. Thisstudy was carried out at 60°C for both the DI and Clwater. General Signal Blue M forced-air convection ovenswere used as the heating apparatuses. The DI water andCl water solutions were refreshed once a week. At one-week intervals, the OIT and carbonyl growth (via FTIRw/ATR) were measured and recorded. Oxidative induc-tion times (OIT) were measured following ASTMDesignation D3895-98.8The surfaces of the pipe sampleswere sliced using a diamond-tipped microknife blade. Theinfrared spectra of the sliced pipe samples were acquiredusing a single reflection diamond ATR accessory attachedto a Digilab UMA 600 infrared microscope. The micro-scope was coupled to the Digilab 7000e FTIR spec-trophotometer. The carbonyl band was located at 1715cm-1.SEM analysis was performed using a Zeiss DSM 982FEG-SEM equipped with a PGT EDX detector. Spectrawere collected at 20 KeV providing magnification as highas 5000x.In the second phase of the study, several developmentalcompounds were evaluated to measure their effect onincreasing the resistance of the polyethylene pipe to degra-dation caused by exposure to the strongly oxidizing freechlorine.In this part of the study, pipes were modeled with com-mercial-grade polyethylene resin. Additive packages werecompound-extruded on a Davis-Standard extruder with a1-inch single mixing screw. The extrusion temperaturewas set between 175°C and 195°C. Upon exiting thewater bath, the extrudate was pelletized and collected.The pellets were injection-molded into 120-mil plaques.An Arburg Allrounder injection molding machine(190°C-210°C) was used to produce the tensile bars andplaques.The plaques were also immersed in glass containerscontaining DI water or 5 ppm chlorinated water (week-20| PLASTICS ENGINEERING | OCTOBER 2011|

ly refreshed). Also on a weekly basis, the OIT, yellow-ness index, and total color change were measured andrecorded.ResultsPhase 1: Commercial Pipe EvaluationThe effects of hypochlorous acid on commercial-gradepolyethylene pipe were measured using standard ASTMOIT methods after exposure in a 60°C aqueous solutionof 5 ppm of free chlorine using calcium hypochlorite (7.3ppm of calcium hypochlorite). In parallel, samples weresoaked in 60°C deionized water to create a comparativebaseline to measure the extent of polymer degradation. Atseven-day intervals, the calcium hypochlorite solution andwater were renewed and the samples were re-soaked.Initial OIT of the commercial pipe was determined tobe 145 minutes. After immersion for five weeks in 60°Cwater, OITs of 124 minutes were measured and recorded.However, in the sample immersed in 5 ppm free chlorine,after five weeks at 60°C, the OIT significantly decreasedto 49 minutes. Table 1and Figure 2show the effects offree chlorine and water on commercial pipe samples.Table 1. OIT of Pipe Samples Immersed in Water and5 ppm Free Chlorine.Minutes (OIT at 200°C)Unexposed Commercial Pipe1452 weeks at 60°C in water1335 weeks at 60°C in water1241 week at 60°C in Cl1282 weeks at 60°C in Cl825 weeks at 60°C in Cl49Figure 2. OIT of pipe samples immersed in water and 5 ppmfree chlorine.The results indicate that in 60°C water, the polyethyl-ene pipe OIT decreases slightly over time. However, withjust 5 ppm of free chlorine, OIT decreases significantlymore quickly. After 5 weeks in water, the OIT was meas-ured and recorded at 86% of its original value, while inwater with 5 ppm free chlorine, the OIT was measured atonly 34% of its original value.The degradative oxidation of polyethylene can lead tothe formation of the carbonyl chemical functionality. Atechnique to determine the extent of degradation/oxida-tion of polyethylene is to evaluate and measure the car-bonyl functionality on the pipe's surface.FTIR/ATR is a method developed to measure the car-bonyl chemical functionality (1715 cm-1) at the pipe'ssurface in order to understand the extent of surface oxida-tion. Figures 3and 4show the results of carbonyl forma-tion of the pipe in water and free chlorine immersion.Figure 3. Infrared reflection spectra of HDPE after threeweeks in 60°C water with and without 5 ppm chlorine.Figure 4. Infrared reflection spectra of HDPE after five weeksin 60°C water with and without 5 ppm | OCTOBER 2011| PLASTICS ENGINEERING | 21