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The effects of chlorinated water on polyethylene pipes.

Chlorine chemistry is complex. Under certain common conditions, chlorinated compounds can react with polyethylene pipes, causing premature failures (bursts/leaks). It is important to understand this degradation mechanism in order to prevent premature pipe failure. Recently, a specific additive system, when incorporated into polyethylene pipe resin, was demonstrated to significantly improve the pipe's resistance to degradation caused by chlorine exposure.

Background

Before 1915, unsafe drinking water caused a significant number of deaths due to cholera, dysentery, hepatitis A, and typhoid fever. Abel Wolman, the chief engineer of the Maryland State Department of Health from 1922 to 1939, (1) made the important contribution of chlorinating the drinking water supply for the city of Baltimore. The cities of New York, Detroit, and Columbus (Ohio) quickly followed in chlorinating their drinking water. By the late 1920s, this practice was widely accepted throughout the United States, and an 85% drop in deaths from typhoid fever was reported.

Chlorination of drinking water also offers the additional benefits (2) of reducing many disagreeable tastes and odors; eliminating slime bacteria, molds, and algae; reducing hydrogen sulfide, ammonia, and other nitrogen compounds; and removing iron and manganese from water.

Since 1974, the U.S. Environmental Protection Agency (EPA) has had the authority to set water-quality standards. Even though the EPA requires a minimum level of disinfectants in the water, maximums are set as follows: 4 mg/l for elemental chlorine and 4 mg/l for chloramine.

Thermoplastic pipe was first used to transport drinking water in the 1940s. Since then, technical advances have greatly expanded plastic-pipe applications. Today, plastic pipe offers improved long-term performance, corrosion resistance, scaling resistance, abrasion resistance, physical property flexibility, cost efficiency (lower labor cost, ease of installation), coilability, low coefficient of friction, and lightweighting compared with metal pipes. These advantages have allowed thermoplastic pipes to replace metal-and-brick water-distribution technology.

Today, plastic pipes are used in communication-cable protection, hot-water heating, wastewater transport, potable-water distribution, and irrigation.

Chemistry

While thermoplastic pipe has good corrosion resistance, it unfortunately is not impervious to attack by chlorine-based disinfectants. The most common disinfectants for drinking water are chlorine gas, chloramines, and sodium hypochlorite/calcium hypochlorite. They all work by generating "free chlorine" (HOC1 and O[Cl.sup.-]). With the expanding application of plastic pipes and the use of chlorinated water, the physical properties of polyethylene pipe are under severe degradation stress. The following reactions illustrate the formation of the disinfecting free chlorine (HOC1 and O[Cl.sup.-]) from chlorine gas, chloramines, and metal hypochlorite.

Disinfecting Agent Formation by Chlorine

[Cl.sub.2] + [H.sub.2]O [right arrow] HOCl + [H.up.+] + [Cl.sup.-1] Reaction 1

HOCl [right arrow] O[Cl.sup.-] + [H.sup.+] Reaction 2

Disinfecting Agent Formation by Chloramines

N[H.sub.2]Cl + [H.su.2]O [right arrow] N[H.sub.3] + HOCl Reaction 3

HOC1 [right arrow] O[Cl-.sup. ]+ [H.sup.+] Reaction 4

Disinfecting Agent Formation by Metal Hypochlorite (NaOCI/CaOCl)

NaOC1 + [H.sub.2]O [right arrow] O[Cl.sup.-] + [Na.sup.+] + [H.sup.+] + O[H.sup.-] Reaction 5

O[Cl.sup.-] + [H.sup.+] + O[H.sup.-] [right arrow] HOCl + O[H.sup.-] Reaction 6

Hypochlorous acid (HOCI) is considered an oxidizer (the active sanitizing agent) that can neutralize harmful germs, bacteria, and pathogens, as well as react with polyethylene pipe. The concentration of HOCl is highly dependent on pH. At a pH of 5.5, HOC1 is estimated to be undissociated, while at pH of 11, HOC1 is completely dissociated. Also, at a pH of less than 1, [Cl.sub.2] gas formation can be expected.

The following illustrates the reaction pathway for the dissociation and undissociated HOC1, which is dependent on p[H.sup.3] (see Figure 1).

O[Cl.sup.-] + [H.sup.+] + O[H.sup.-] [right arrow] HOCl + O[H.sup.-] Reaction 6

HOCL + [H.sup.+] + [Cl.sup.-] [right arrow] [Cl.sub.2] + [H.sub.2]O Reaction 7

[FIGURE 1 OMITTED]

The recommended pH for safe and effective sanitizing is in the range of 6.5 to 7.5.

It has been documented (4) that polyethylene pipes undergo degradation, but little has been written to explain how this polymer can be degraded by hypochlorous acid in an environment that is heterogeneous (solid phase and aqueous phase), free of harmful UV energy and at relatively low temperatures. The degradation mechanism may seem difficult to explain given the relatively mild conditions of commercial use. The concepts below may shed some light on the potential degradation pathway.

One study (5) has shown that hypochlorous acid can react with iron(II) complex ([Fe.sup.+2]) in aqueous solution with the rate constant 220 [+ or -] 15 [dm.sup.3] [mol.sup.-1] [s.sup.-1]. In this reaction, free hydroxyl radicals are formed in 27% yield. The hydroxyl radical and chlorine radical can then initiate the degradation of polyethylene pipe.

[Fe.sup.+2]

HOCl [right arrow] HO * + Cl * Reaction 8

Another study (6) showed that saturated alkanes can be oxidized by hypochlorous acid in darkness, in a two-phase system, and at relatively low temperatures (0[degrees]C-50[degrees]C). This study implicated [Cl.sub.2]O, generated from hypochlorous acid according to Reaction 9, as the radical generating species.

2HOC1 [right arrow] [Cl.sub.2]O + [H.sub.2]O Reaction 9

The authors of this study proposed two possible mechanisms for the initiation of free-radical chains. The first involves cleavage of the C1-O bond in the [Cl.sub.2]O, which they claim is possible because of the high electronegativity of the C1 and O.

[Cl.sub.2]O [right arrow] Cl * + * OCl Reaction 10

Another potential pathway for the free radical initiation is an electron transfer process between the polyethylene pipe and the chlorinating compound ([Cl.sub.2]O). The scientists drew an analogy between this reaction and the spontaneous free radical fluorination of hydrocarbons by elemental fluorine. (7)

R-H + [Cl.sub.2]O [right arrow] R * + ClO- + Cl * + H+ Reaction 11

The generation of Re and Cle in Reaction 10 and Reaction 11 can lead to the accelerated degradation of the polyethylene pipe. The polyethylene degradation pathway is described below (Reaction 12 to Reaction 17).

Cl * + R-H [right arrow] HCl + R * Reaction 12

R * + [O.sub.2] [right arrow] R-O-O * Reaction 13

R-O-O* + R-H [right arrow] R-O-O-H + R * Reaction 14

R-O-O-H [right arrow] R-O * + H-O * Reaction 15

R-O * + R-H [right arrow] R-O-H + R * Reaction 16

H-O * + R-H [right arrow] [H.sub.2]O + R * Reaction 17

Experimental

In the first phase of this study, commercial polyethylene pipes were obtained and immersed in deionized (DI) water, while another set was immersed in chlorine water. The concentration of chlorine in the chlorinated DI water was fixed at 5 ppm of free chlorine using calcium hypochlorite, at an initial pH of approximately 6.8. This study was carried out at 60[degrees]C for both the DI and C1 water. General Signal Blue M forced-air convection ovens were used as the heating apparatuses. The DI water and CI water solutions were refreshed once a week. At one-week intervals, the OIT and carbonyl growth (via FTIR w/ATR) were measured and recorded. Oxidative induction times (OIT) were measured following ASTM Designation D3895-98. (8) The surfaces of the pipe samples were sliced using a diamond-tipped microknife blade. The infrared spectra of the sliced pipe samples were acquired using a single reflection diamond ATR accessory attached to a Digilab UMA 600 infrared microscope. The microscope was coupled to the Digilab 7000e FTIR spectrophotometer. The carbonyl band was located at 1715 [cm.sup.-1].

SEM analysis was performed using a Zeiss DSM 982 FEG-SEM equipped with a PGT EDX detector. Spectra were collected at 20 KeV providing magnification as high as 5000x.

In the second phase of the study, several developmental compounds were evaluated to measure their effect on increasing the resistance of the polyethylene pipe to degradation caused by exposure to the strongly oxidizing free chlorine.

In this part of the study, pipes were modeled with commercial-grade polyethylene resin. Additive packages were compound-extruded on a Davis-Standard extruder with a 1-inch single mixing screw. The extrusion temperature was set between 175[degrees]C and 195[degrees]C. Upon exiting the water bath, the extrudate was pelletized and collected. The pellets were injection-molded into 120-mil plaques. An Arburg Allrounder injection molding machine (190[degrees]C-210[degrees]C) was used to produce the tensile bars and plaques.

The plaques were also immersed in glass containers containing DI water or 5 ppm chlorinated water (weekly refreshed). Also on a weekly basis, the OIT, yellowness index, and total color change were measured and recorded.

Results

Phase 1: Commercial Pipe Evaluation

The effects of hypochlorous acid on commercial-grade polyethylene pipe were measured using standard ASTM OIT methods after exposure in a 60[degrees]C aqueous solution of 5 ppm of free chlorine using calcium hypochlorite (7.3 ppm of calcium hypochlorite). In parallel, samples were soaked in 60[degrees]C &ionized water to create a comparative baseline to measure the extent of polymer degradation. At seven-day intervals, the calcium hypochlorite solution and water were renewed and the samples were re-soaked.

Initial OIT of the commercial pipe was determined to be 145 minutes. After immersion for five weeks in 60[degrees]C water, OITs of 124 minutes were measured and recorded. However, in the sample immersed in 5 ppm free chlorine, after five weeks at 60[degrees]C, the OIT significantly decreased to 49 minutes. Table 1 and Figure 2 show the effects of free chlorine and water on commercial pipe samples.

[FIGURE 2 OMITTED]

The results indicate that in 60[degrees]C water, the polyethylene pipe OIT decreases slightly over time. However, with just 5 ppm of free chlorine, OIT decreases significantly more quickly. After 5 weeks in water, the OIT was measured and recorded at 86% of its original value, while in water with 5 ppm free chlorine, the OIT was measured at only 34% of its original value.

The degradative oxidation of polyethylene can lead to the formation of the carbonyl chemical functionality. A technique to determine the extent of degradation/oxidation of polyethylene is to evaluate and measure the carbonyl functionality on the pipe's surface.

FTIR/ATR is a method developed to measure the carbonyl chemical functionality (1715 [cm.sup.-1]) at the pipe's surface in order to understand the extent of surface oxidation. Figures 3 and 4 show the results of carbonyl formation of the pipe in water and free chlorine immersion.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

At the third and fifth week of immersion at 60[degrees]C water, the commercial pipe sample showed no appreciable surface formation of the carbonyl functionality. At 1715 [cm.sup.-1] wavelength, the formation of the absorption peak associated with carbonyl groups is insignificant. On the other hand, after three and five weeks of immersion at 60[degrees]C water containing 5 ppm of free chlorine, a well-defined carbonyl peak at 1715 [cm.sup.-1] was observed at the surface of the commercial pipe. This peak can be attributed to the accelerated oxidative degradation at the surface of the commercial polyethylene pipe.

Another analytical method employed to evaluate the extent of degradation in pipe is scanning electron microscopy (SEM), which offers a hyper-magnification visual inspection of the surface, and can allow observation of the surface imperfections, such as microcracks. Microcracks less than 1 um can be visually observed. Figures 5 and 6 illustrate the visual surface inspection via SEM techniques.

[FIGURE 5 OMITTED]

Visual microscopic inspection via SEM indicated the commercial pipe immersed in water for two, five, and nine weeks showed no microcracks at 500x, 1000x, and 3000x magnification. See Figure 6. However, at magnifications of 1000x and 3000x, microcracks between 0.5 and 1.3 um were observed in pipe samples immersed in 5 ppm free-chlorine water for five weeks, and larger cracks appeared after nine weeks of immersion. See Figure 5.

[FIGURE 6 OMITTED]

Phase 2: Chlorine Resistance of Polyethylene With New Specialty Additives

After defining the deleterious effects of free-chlorine water on polyethylene pipes, three proprietary specialty additives were evaluated to counter the degradation effects of free chlorine on polyethylene.

The first test included the OIT measurements on control and stabilized samples. The proprietary specialty additives are identified as Compounds A, B, and C in Table 3. The OIT of the unexposed sample was 104 minutes. After three weeks in 5 ppm free-chlorine water at 85[degrees]C, the sample OIT dropped to 42 minutes (60% decrease from original value). However, with the incorporation of a specialty additive, Compound A, the OIT was measured and recorded at 100 minutes (less than 4% decrease from original value). New Compounds B and C did not show a positive effect.

Color stability can be important in certain applications. See the pictures of the plaques below. It was generally noted that with prolonged exposure to free chlorine, samples yellowed and then darkened.

[ILLUSTRATION OMITTED]

After being immersed for six weeks in 5 ppm free-chlorine water at 60[degrees]C, the control plaque increased 20.4 units in yellowness index (YI), while the Compound A plaque increased only 12.9 YI units.

[ILLUSTRATION OMITTED]

Conclusion

In this study, the degradation of commercial-grade polyethylene pipe accelerated under exposure to free chlorine. Accelerated degradation was observed with a significant decrease in OIT versus samples that did not contain free chlorine. Compared with a sample exposed to "water only," early carbonyl formation was observed at the surface of the pipe sample exposed to free chlorine. Premature formation of microcracks was observed at the surface of the pipe exposed to free chlorine.

Several additive compounds were evaluated in order to increase the pipe's resistance to free-chlorine oxidative degradation. Compound A was found to maintain the pipe's original OIT even under exposure to 5 ppm of free chlorine after three weeks at 85[degrees]C. Further studies are planned to optimize and enhance the performance of polyethylene pipe under exposure to free chlorine.

Acknowledgments The authors extend their gratitude to Jeff Jenkins, Kimberly Fallo, Jerome O'Keefe, and Chermeine Rivera, who performed the work that generated most of the test results. Special thanks also go to Brent Sanders and Sari Samuels for their valuable insight. The authors' appreciation also goes to the management of Cytec Industries Inc. for the support and permission to publish this paper.

References

(1.) http://en.wikipedia.org/wiki/Abel_Wolman.

(2.) http://www.enviroliteracy.org/article.php/1017.php.

(3.) Drinking Water Chlorination, Chlorine Chemistry Council, Arlington, Virginia (February 2003).

(4.) S. Chung, K. Oliphant, et al., An Examination of the Relative Impact of Common Potable Water Disinfectants on Plastic Piping System Components, Jana Laboratory.

(5.) W. McGlynn, Guidelines far the Use of Chlorine Bleach as a Sanitizer in Food Processing Operations, Oklahoma State University, FAPC-116-2.

(6.) L.P. Candeias, M.R. Stratford, and P. Wardman, Free Radic Res, 1994 Apr. 20(4):241-9. "Formation of hydroxyl radicals on reaction of hypochlorous acid with ferrocyanide, a model iron(II) complex."

(7.) Fontana, Minisci, et al., Free Radicals in Synthesis and Biology, Chlorination of Hypochlorous Acid. Free Radical Versus Electrophilic Reactions, pp. 269-82.

(8.) ASTM Designation D3895-98, Standard Test Method for Oxidative Induction Time of Polyolefins by Differential Scanning Calorimetry.

The authors first presented a version of this paper at SPE's Polyolefins 2009 Conference.

Jerry Eng, * Thomas Sassi, * Thomas Steele, * and Giacomo Vitarelli * * * Cytec Industries, Stamford, Connecticut, USA * Cytec Industries, Milano, Italy
Table 1. OIT of Pipe Samples Immersed in Water and
5 ppm Free Chlorine.

                                      Minutes (OIT at
                                      200[degrees]C)

Unexposed Commercial Pipe             145
2 weeks at 60[degrees]C in water      133
5 weeks at 60[degrees]C in water      124
1 week at 60[degrees]C in C1          128
2 weeks at 60[degrees]C in C1         82
5 weeks at 60[degrees]C in C1         49

Table 2. Initial OIT of Polyethylene Sample.

Minutes (OIT at 200[degrees]C)
Unexposed Control Sample              104

Table 3. OIT of Polyethylene Samples Immersed
in Water With 5 ppm Free Chlorine.

Three Weeks at         Minutes (OIT at
85[degrees]C, 5 ppm    200[degrees]C)
Free Chlorine

Control Sample         42
Additive Compound A    100
Additive Compound B    37
Additive Compound C    15
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Author:Eng, Jerry; Sassi, Thomas; Steele, Thomas; Vitarelli, Giacomo
Publication:Plastics Engineering
Geographic Code:1USA
Date:Oct 1, 2011
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