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18| PLASTICS ENGINEERING | OCTOBER 2011| www.4spe.orgChlorine chemistry iscomplex. Under cer-tain common condi-tions, chlorinatedcompounds can react with poly-ethylene pipes, causing prematurefailures (bursts/leaks). It is impor-tant to understand this degrada-tion mechanism in order to pre-vent premature pipe failure.Recently, a specific additive sys-tem, when incorporated into poly-ethylene pipe resin, was demon-strated to significantly improve thepipe's resistance to degradationcaused by chlorine exposure.Jerry Eng,* Thomas Sassi,* Thomas Steele,* and Giacomo Vitarelli***Cytec Industries, Stamford, Connecticut, USA**Cytec Industries, Milano, ItalyPhoto by Nicole-Koehler

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 | OCTOBER 2011| PLASTICS ENGINEERING | 19