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Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content (2011)

Chapter: Chapter 4 - Conclusions and Suggested Research

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Page 49
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
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Page 49
Page 50
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
Page 50
Page 51
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
Page 51
Page 52
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
Page 52
Page 53
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content. Washington, DC: The National Academies Press. doi: 10.17226/14570.
×
Page 53

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49 Phase 1—Recycled PE Resins The purpose of this phase was to determine which types of recycled PE were available, where these materials could be obtained, and what their properties were. Both PIR and PCR PE were initially examined, but the effort focused on PCR because it is readily available, more consistent than PIR, and there is a trade association called the Association of Post- consumer Plastic Recyclers (APR). PIR should not be ruled out as a source of material, but PCR will be more consistent and more widely available. A total of 25 samples of recycled PE were obtained from nine different suppliers. There were three PIR samples and 22 PCR samples. Of the PCR samples, six were natural and 16 were colored. The natural resins came from milk bottles and the colored mostly from detergent bottles. The results showed that mixed-color PCR was an excellent candidate for use in corrugated HDPE pipe. The density aver- ages around 0.949 g/cm3, which is close to virgin pipe resins, so the strength and flexural modulus properties are in the AASHTO specified cell class for M294 pipe. The MI averaged around 0.5 g/10 min while the M294 maximum is 0.4 g/10 min. It also had poor stress-crack resistance and contaminants present that could be stress-crack initiation sites. A test method was developed to determine the percentage PP in the recycled HDPE. PP is a contaminant that comes from the tops and pour spouts of detergent bottles. It can be found in amounts of 10% or higher. Results showed that the PP content could be as high as 5% without hurting the stress- crack properties of the resin. Results also showed that melt filtration at a mesh size of 120 or greater can reduce the amount of contamination to about 0.5% and raise the elongation-at- break to over 100%. These values will be specified to ensure proper filtration occurs. The research needs from this phase involve the test method for percentage PE. The method needs to be written into a standard test method and evaluated through a consensus group such as ASTM, or developed through AASHTO. Also, FTIR spectroscopy is an alternative method to measure percent- age PP. It should be added to the method as an alternative to the DSC method presented in this report. Phase 2—Recycled PE Blends The purpose of Phase 2 was to determine how the addition of recycled HDPE would affect the properties of CPPI-certified resins and to select specific blends for trial corrugated pipe manufacturing. A total of 66 blends were prepared and their properties evaluated. One significant finding was that stress- crack resistance changed in an exponential manner when two different PE materials were blended together. Additionally, the yield strength changed in a linear manner. This means that when one blends two materials with different strengths and stress-crack resistances, the strength will change more quickly than the stress-crack resistance. This also means that one needs to find resins to blend with the recycled resins that have similar or greater strengths and significantly better stress-crack resistances. This project was limited by the original concept of blending recycled resins with PPI-certified corrugated pipe resins. Just since this project began, data has become available about the stress-crack resistance of bimodal HDPE resins. There are at least six companies in the United States that produce resins with densities in cell class 4 (required by AASHTO M294) and that have NCLS values in the thousands of hours. In fact, they are tested with a more aggressive stress-crack test called the PENT Test (ASTM F1473) and some resin suppliers report values greater than 10,000 h. Besides these “super” resins, a recycled supplier submitted a sample for this study recently that was made from 100% recycled material. Its NCLS stress-crack time was 220 h, and it lasted for over 600 h in the BFF test at 80°C/650 psi when 200 h suggests 1000 years of estimated lifetime. Research is needed to determine how the addition of these new, bimodal resins will affect the stress-crack resistance C H A P T E R 4 Conclusions and Suggested Research

of mixed-color PCR. It’s possible that the addition of just around 10% of these resins would raise the NCLS times of the recycled above the required value of 24 h. Of course, an unnotched stress-crack test like the BFF test would also be required to assess the long-term performance of the blends. Phase 3—Pipe Made from Recycled-Resin Blends The purpose of this phase of the project was to manufacture dual wall corrugated drainage pipe out of resin formulations containing recycled HDPE. Fifteen pipe samples were prepared at three different manufacturing plants. Each plant made two common formulations, so there was a total of 11 different blends evaluated. Once the pipe was made, the short-term properties were measured and compared with the require- ments of AASHTO M294. It was determined that it is not too difficult to create blends within the required cell classification required by AASHTO M294. It was more difficult to produce blends with adequate stress-crack resistance. If the 2010 requirement of 24 h on a plaque made from a finished pipe were in place, only eight of the 15 samples would meet the requirement. Again, this is a result of making blends with pipe resin with about 50 h of NCLS time. Resins tested in 2010 have had times greater than 150 h, which would make the resulting blends higher in stress-crack resistance. And, of course, the new bimodal HDPE resins could be used to raise the value even more. The long-term performance of the pipe formulations was also evaluated. Accelerated creep and creep rupture tests were performed by the SIM on tensile dumbbells taken from compression molded plaques made from pipe. These results showed that the materials all met the requirements of Section 12 of the LRFD Bridge Design Specification for 50- and 75-year service lifetimes. The long-term stress-crack resistance was determined with a new test called the BFF test. This test was found to be sensitive to contaminant particles, while other tests are not. Results were generated that suggested that recycled content-containing pipe formulations can be developed that have estimated ser- vice lifetimes greater than 1000 years. This allows one to apply a very conservative design factor of 0.10 for a 100-year service lifetime. The long-term oxidation resistance of pipe containing recycled HDPE is often thought to be a significant issue. However, studies over the past 20 years have consistently showed that in the absence of UV radiation, the oxidation rate for modestly stabilized HDPE materials is low. One example is the Pennsylvania Department of Transportation Deep Burial study in which the OIT times were measured after 20 years of buried service. The results showed that very little, if any, change had occurred to the OIT, except on the pipe ends exposed to the sun (13). Another is a recent study that showed that an additive package containing 0.05% Irganox 1010 and 0.10% Irgaphos 168 had an estimated antioxidant depletion time of 130 years (18). Oxidation failures are believed to be an un- likely event if the formulations are made with a minimum of 0.1% Irganox 1010 and 0.1% Irgaphos 168. The ultimate goal of this project concerning the long-term properties is to create a master curve, similar to those used for solid wall pressure pipe. An example for Sample B1 is shown in Figure 47. This curve is an estimate of the relationship between applied stress and service lifetime for a sample made from a 50 y = -0.0469x + 3.3278 R2 = 0.9608 y = -0.243x + 4.3822 R2 = 0.9827 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 0 1 2 3 4 5 6 7 8 Log Failure Time (hrs) Lo g St re ss (p si) 1188 psiStage I from SIM Stage II from BFF Shifted to 23°C Figure 47. Master curve at 23ºC for Sample B1.

PPI-certified, 100% Virgin HDPE. The information in this plot includes the following: 1. At applied loads less than 1,188 psi, the life-limiting failure mechanism is slow crack growth (Stage II). 2. At 900 psi, the lifetime estimate is 86 years. 3. At 500 psi, the lifetime estimate is 965 years. 4. The life-limiting load at 50 years is 1,026 psi 5. The life-limiting load at 75 years is 930 psi. 6. The life-limiting load at 100 years is 867 psi. This plot was generated from the SIM accelerated creep rupture test and from the BFF accelerated stress-cracking test. Both of these test methods require further development, but the example above should demonstrate the usefulness of the two. The test methods need standard methods written and round- robin tests conducted. There is interest in a SIM method in ASTM because it is already being specified for some drainage products and the only SIM standard available is for reinforcing geotextiles and geogrids. The estimates of long-term stress and strain and of long-term stress-crack resistance should be verified through accelerated laboratory testing on the pipes or field studies on buried pipe. There are some specific ways to do this. SIM tests should be performed on pipe samples to determine the relationship between a dumbbell test on a compression molded plaque and an actual section of pipe. This is easily done on 12-in. diameter pipe. All that is required is an appro- priate environmental chamber that can hold a 12-in. × 12-in. piece of pipe and a linear variable differential transformer (LVDT) to measure inside diameter displacement. The pipe sample would be placed between parallel plates and loaded with the appropriate mass. The results of this study will be an estimated service-lifetime curve. This curve will help deter- mine appropriate loads for actual field tests. For example, if the SIM results on pipe are similar to the results on a dumbbell from a plaque, one could use the esti- mated lifetime curve to select loads for a field study. The SIM creep rupture curve for sample B1 on a plaque is shown in Figure 48. Notice that one could apply loads of 40%, 38%, and 36% of the ultimate stress and create failure times in less than one year. The measured buckling load of Pipe B1 was 1,053 lb. So, the corresponding loads would be 442 lb, 421 lb, and 400 lb. These loads should create buckling failures in less than a year. This is the type of experiment that could validate the results of the SIM test on formulations containing recycled HDPE. Similar experiments on solid wall pressure pipe are ongoing in these laboratories. The SIM results on a plaque are being compared to sustained burst test results at room temperature. The slopes of the log stress vs. log time curve are nearly iden- tical, but the curves are shifted vertically. The pipe failures all occur at higher stresses than the dumbbells. It’s a linear offset and believed to be caused by sample geometry. There may also be an offset between a pipe plaque and a pipe in compression, but the slopes of the lines may be the same. If the results between a SIM test on a plaque and long-term compression tests on pipe are comparable, then a powerful new tool would exist for predicting long-term behavior. In summary, the suggested follow-on research pertaining to the SIM test and long-term service estimates are to do the following: 1. Develop a standardized method specific to performing SIM accelerated creep and creep rupture tests on HDPE. 51 B1 y = -0.0469x + 3.3278 R2 = 0.9608 2.9 3 3.1 3.2 3.3 3.4 0 1 2 3 4 5 6 7 8 Log Failure Time (hrs) Lo g St re ss (p si) Yield Stress = 3865 psi 40% = 902 hrs 38% = 2692 hrs 36% = 8524 hrs SIM Results Shifted to 23°C Figure 48. SIM creep rupture master curve for Sample B1.

2. Perform SIM on 12-in. diameter pipe samples to deter- mine the relationship between a dumbbell specimen and an actual pipe. 3. Validate the SIM results by laboratory tests on pipe under compressive loading at room temperature. The BFF test also needs to be validated in a similar manner to the SIM test. Multiple tests should be performed on the same samples across time to determine how reproducible the test is in a single laboratory. Then, tests should be performed by different laboratories to determine the lab-to-lab variability. Only then would the true value of this test be known. The stress-cracking master curve for sample B1, generated by the BFF test, is shown in Figure 49. Notice that the failure times for the brittle (Stage II) por- tion of the predicted curve are very long. Therefore, it is not practical to try to validate these results at room temperature. The same BFF test results shifted to 50°C (122°F) are shown in Figure 50. In this case, applying loads of 295 lb, 242 lb, and 190 lb to a pipe sample at 50°C (122°F) should create stress-crack failures in about a year. This experiment could be done in a room controlled to 50°C. The results from this study will show if the master curves generated by the BFF test actually relate to 52 B1 y = -0.243x + 4.3822 R2 = 0.9827 2.94 2.96 2.98 3 3.02 3.04 3.06 3.08 3.1 3.12 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 Log Failure Time (hrs) Lo g St re ss (p si) Yield Stress = 3865 psi 28% = 40 yrs 23% = 90 yrs 18% = 248 yrs BFF Results Shifted to 23°C Figure 49. BFF stress-cracking master curve at 23ºC for Sample B1. y = -0.2346x + 3.9008 R2 = 0.9787 2.6 2.7 2.8 2.9 3 3.1 3.2 2 2.5 3 3.5 4 4.5 5 Log Failure Time (hrs) Lo g St re ss (p si) Yield Stress = 3865 psi 28% = 1,299 hrs 23% = 3,005 hrs 18% = 8,543 hrs BFF data shifted to 50°C Figure 50. BFF stress-cracking master curve at 50ºC for Sample B1.

a corrugated pipe under a direct compressive load. This test is especially important for pipe formulations containing post- consumer mixed-color recycled HDPE because they are the ones that are more likely to stress crack over their service lifetimes. Validation of the BFF test results for predicting long-term stress-crack failures is very important because it is crystal clear that this is the main concern for corrugated pipe con- taining recycled HDPE. Of equal importance is validating the proposed BFF test under a single set of temperatures and stresses as a quality control test. Such a test is necessary to ensure that pipe formulations containing recycled HDPE have adequate stress-crack resistance over their intended service lifetime. Finally, there are 80 feet of pipe left for each of the 15 samples. The pipes have a variety of properties. Some do not meet the cell classification for AASHTO M294 pipe. Some display low density or low elongation at break or have poor stress-crack resistance. They vary in their stress-crack resistance in an unnotched test. The poorest had a failure time about 25% of the best’s time. There are some that have peak loads less than 20% deflection and others that are over 40% in pipe deflection tests. And, there are some that won’t meet the required 50-year creep modulus of 22,000 psi. These pipe samples are well suited to evaluate some of the assumptions and long-term projections made for corrugated drainage pipes. Unfortunately, the times to grow stress cracks are quite long in a field study. The research suggests that the maximum stress that will cause a stress crack instead of local buckling is 1,000 to 1,200 psi, depending upon the yield stress. So, in a field environment at 23°C, it would take over 25 years to grow a crack in a sample with 30% recycled content. Acceleration through temperature seems to be the only way to verify the results of the lifetime estimates based on the BFF test. 53

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 696: Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content provides potential specifications for corrugated drainage pipe manufactured with recycled high-density polyethylene (HDPE). The report includes proposed draft specifications for recycled HDPE, formulations of virgin and recycled HDPE, and drainage pipe containing recycled HDPE.

The following three appendixes of NCHRP 696 are available in electronic format only.

Appendix B: Recycled Polyethylene Resins

Appendix C: Recycled-Resin Blends

Appendix D: Pipe Containing Recycled HDPE

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