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BackgroundBefore 1915, unsafe drinking water caused a significantnumber of deaths due to cholera, dysentery, hepatitis A,and typhoid fever. Abel Wolman, the chief engineer of theMaryland State Department of Health from 1922 to1939,1made the important contribution of chlorinatingthe drinking water supply for the city of Baltimore. Thecities of New York, Detroit, and Columbus (Ohio) quick-ly followed in chlorinating their drinking water. By thelate 1920s, this practice was widely accepted throughoutthe United States, and an 85% drop in deaths fromtyphoid fever was reported.Chlorination of drinking water also offers the additionalbenefits2of reducing many disagreeable tastes and odors;eliminating slime bacteria, molds, and algae; reducinghydrogen sulfide, ammonia, and other nitrogen com-pounds; and removing iron and manganese from water.Since 1974, the U.S. Environmental Protection Agency(EPA) has had the authority to set water-quality stan-dards. Even though the EPA requires a minimum level ofdisinfectants in the water, maximums are set as follows: 4mg/l for elemental chlorine and 4 mg/l for chloramine.Thermoplastic pipe was first used to transport drinkingwater in the 1940s. Since then, technical advances havegreatly expanded plastic-pipe applications. Today, plasticpipe offers improved long-term performance, corrosionresistance, scaling resistance, abrasion resistance, physicalproperty flexibility, cost efficiency (lower labor cost, easeof installation), coilability, low coefficient of friction, andlightweighting compared with metal pipes. These advan-tages have allowed thermoplastic pipes to replace metal-and-brick water-distribution technology.Today, plastic pipes are used in communication-cableprotection, hot-water heating, wastewater transport,potable-water distribution, and irrigation.ChemistryWhile thermoplastic pipe has good corrosion resistance, itunfortunately is not impervious to attack by chlorine-based disinfectants. The most common disinfectants fordrinking water are chlorine gas, chloramines, and sodiumhypochlorite/calcium hypochlorite. They all work by gen-erating "free chlorine" (HOCl and OCl-). With theexpanding application of plastic pipes and the use of chlo-rinated water, the physical properties of polyethylene pipeare under severe degradation stress. The following reac-tions illustrate the formation of the disinfecting free chlo-rine (HOCl and OCl-) from chlorine gas, chloramines,and metal hypochlorite.Disinfecting Agent Formation by ChlorineCl2+ H2O ?HOCl + H++ Cl-Reaction 1HOCl ?OCl-+ H+Reaction 2Disinfecting Agent Formation by ChloraminesNH2Cl + H2O ?NH3+ HOClReaction 3HOCl ?OCl-+ H+Reaction 4Disinfecting Agent Formation by Metal Hypochlorite(NaOCl/CaOCl)NaOCl + H2O?OCl-+ Na++ H+ + OH-Reaction 5OCl-+ H+ + OH-?HOCl + OH-Reaction 6Hypochlorous acid (HOCl) is considered an oxidizer (theactive sanitizing agent) that can neutralize harmful germs,bacteria, and pathogens, as well as react with polyethylenepipe. The concentration of HOCl is highly dependent onpH. At a pH of 5.5, HOCl is estimated to be undissociat-ed, while at pH of 11, HOCl is completely dissociated.Also, at a pH of less than 1, Cl2gas formation can beexpected.The following illustrates the reaction pathway for thedissociation and undissociated HOCl, which is dependenton pH3(see Figure 1).OCl-+ H+ + OH- ?HOCl + OH- Reaction 6HOCl + H++Cl-?Cl2+ H2OReaction 7Figure 1. Effect of pH on hypochlorous acid content.The recommended pH for safe and effective sanitizingis in the range of 6.5 to 7.5.It has been documented4that polyethylene pipes under-go degradation, but little has been written to explain howthis polymer can be degraded by hypochlorous acid in anenvironment that is heterogeneous (solid phase and aque-ous phase), free of harmful UV energy and at relativelywww.4spe.org | OCTOBER 2011| PLASTICS ENGINEERING | 19

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 Cl.in 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| www.4spe.org