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 chlorine.www.4spe.org | OCTOBER 2011| PLASTICS ENGINEERING | 21
At the third and fifth week of immersion at 60°C water,the commercial pipe sample showed no appreciable sur-face formation of the carbonyl functionality. At 1715cm-1wavelength, the formation of the absorption peakassociated with carbonyl groups is insignificant. On theother hand, after three and five weeks of immersion at60°C water containing 5 ppm of free chlorine, a well-defined carbonyl peak at 1715 cm-1was observed at thesurface of the commercial pipe. This peak can be attrib-uted to the accelerated oxidative degradation at the sur-face of the commercial polyethylene pipe.Another analytical method employed to evaluate theextent of degradation in pipe is scanning electronmicroscopy (SEM), which offers a hyper-magnificationvisual inspection of the surface, and can allow observationof the surface imperfections, such as microcracks.Microcracks less than 1 um can be visually observed.Figures 5and 6illustrate the visual surface inspection viaSEM techniques.Figure 5. Commercial pipe immersed in free chlorine(HOCL and OCl-).Visual microscopic inspection via SEM indicated thecommercial pipe immersed in water for two, five, andnine weeks showed no microcracks at 500x, 1000x, and3000x magnification. See Figure 6.However, at magnifica-tions of 1000x and 3000x, microcracks between 0.5 and1.3 um were observed in pipe samples immersed in 5ppm free-chlorine water for five weeks, and larger cracksappeared after nine weeks of immersion. See Figure 5.Phase 2: Chlorine Resistance of Polyethylene With NewSpecialty AdditivesAfter defining the deleterious effects of free-chlorine wateron polyethylene pipes, three proprietary specialty addi-tives were evaluated to counter the degradation effects offree chlorine on polyethylene.The first test included the OIT measurements on con-trol and stabilized samples. The proprietary specialty addi-tives are identified as Compounds A, B, and C in Table 3.22| PLASTICS ENGINEERING | OCTOBER 2011| www.4spe.org Low Magnification: Macro crack not observed via SEM Analysis (200x and 500x) High Magnification: Onset of micro cracks observed via SEM Analysis (1000x and 3000x) Low Magnification: 2, 5 and 9 weeks (500x) High Magnification: 5 and 9 weeks (1000x and 3000x) Figure 6. Commercial pipe im mersed in DI water. Continued on page 24 ??