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5 time (OIT) (ASTM D-3895) or the oxidative-induction tem- industries' practices as a model for the development of tests perature (OITemp) (ASTM D3350), the presence of stabilizers for use with corrugated pipe resins. can be detected, offering some assurance that stabilizers are present (note that the OIT or OITemp tests do not predict Service Lifetime of PE long-term performance). The short-term properties used for this project included: The long-term service lifetime of HDPE is often presented in a graph like the one in Figure 1. An understanding of this 1. Density--basic property of PE; graph is necessary to develop test methods to characterize the 2. Melt Index (MI)--basic PE property relating to molecular long-term behavior of PE. weight; It shows three distinct stages of aging. The first stage is the 3. High Load Melt Index (HLMI)--ratio of 2 MI tests relates likely service lifetime for materials placed under significant loads to molecular weight distribution; (>30% of yield). Over time, because of the time-dependent 4. Percentage Color--organic fillers like colorants and process of creep, the material will fail by yielding or stretching carbon black; in a ductile manner. The second stage is at intermediate loads 5. Percentage Ash--inorganic fillers and contaminants; and involves failures by slow crack growth (stress cracking). 6. Differential Scanning Calorimetry (DSC)--detects pres- And, finally, under low stresses the material will fail only after ence of other semicrystalline polymers like PE; the additive package has been consumed and the HDPE under- 7. Flexural Modulus--indicator of stiffness; goes oxidation. 8. Tensile Yield Strength; The first stage is somewhat dependent on the material's yield 9. Strain at Break--sensitive to contaminants; strength, the service temperature, and the stress. Long-term 10. Notched, Stress-Crack Test--determines relative crack tests for yield strength usually involve placing the material initiation and growth; under a load less than its strength and waiting for failure to 11. Un-Notched, Stress-Crack Test--sensitive to contami- occur. This is called stress rupture if the sample is placed under nants that may initiate cracks; and a constant strain, or creep rupture if the sample is placed under 12. OIT and OITemp--indicators of stabilization. a constant stress. Temperature is sometimes used to accelerate the process and the results are analyzed through a method called time-temperature superposition (TTS), which assumes that Long-Term Properties higher temperatures and shorter times can be related to lower There is absolutely no doubt that the most important part temperatures and longer times. A specialized form of TTS is of this study is to generate reliable data concerning the long- called the stepped isothermal method (SIM) and has been used term performance of pipe made with recycled PE content. successfully on polyester, polypropylene, and PE reinforce- The approach for this project was to use the solid wall pipe ment products for civil engineering applications. The end of Equilibrium Yield Stress Stage II - Brittle Stage I - Ductile Log Stress (psi) Stage III - Oxidation Log Failure Time (Hrs) Figure 1. Hypothetical service lifetime for HDPE.

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6 this stage is characterized by the equilibrium tensile strength. mode. In the early days of pressure pipe, most of the failures Any service stresses lower than the equilibrium yield stress will occurred by slow crack growth. But, as the stress-crack resis- not cause a ductile failure during the service lifetime. tance of the resins got better and better, the primary failure The second stage involves brittle cracking through slow crack mode during service became ductile. growth. During this stage, a defect in the material can initiate The final stage occurs at service stresses below the stress that a craze, which can, in turn, become a running crack, eventually would cause a stress crack during the service lifetime. In this causing a break in the material. For a brittle crack to grow there case, the entire part becomes brittle through chemical oxidation has to be a significantly sized and shaped defect, and sufficient and fails by many cracks starting at the same time. This region load. HDPE materials have an inherent stress-crack resistance is controlled by the additive package that contains the long- that can be measured, but manufacturing defects and flaws can term antioxidants and/or light stabilizers. Oxidation is largely a accelerate cracking. For PE pressure pipe, the best way to pre- nonissue for properly stabilized resins. However, there should dict service lifetime is the long-term hydrostatic strength test. be some requirement for an OIT value or a specified minimum This entire technology has been developed for pressure pipe additive package placed in the final specification for resins con- and the results are relied upon for the design of pressure piping taining recycled PE. An OIT of 50 minutes should suffice and systems. ASTM D2837 describes a method called hydrostatic there are commercial additive packages that meet the criteria. design basis for evaluating the service lifetime of pressure pipe. The PE pressure pipe industry has used this failure envelope This involves high loads (ductile failure) at room temperature for many years to ensure the quality of resins used for gas to determine the long-term hydrostatic strength (LTHS) and and water distribution pipelines. The main protocol for this intermediate loads (slow crack growth) at elevated tempera- is ASTM D2837, "Standard Test Method for Obtaining Hydro- tures to validate that slow crack growth will not occur within static Design Basis for Thermoplastic Pipe Materials or Pressure 100,000 h. Similar tests for corrugated pipe have been presented, Design Basis for Thermoplastic Pipe Products." including the Federal Institute for Materials Research and For medium- and high-density PE pipe, the method requires Testing (BAM) Test (1, 2), the Florida Department of Trans- that at least 18 data points are generated at room tempera- portation (FL-DOT) junction test (3), the ring stress-crack ture with one point over 10,000 h (1.14 years). These points test (4), and the BAMFL-DOTFathead (BFF) Test (5). are then plotted as Log Stress vs. Log Time and the resulting It should be clearly stated that both Stage I and Stage II line extrapolated to 100,000 h (11.4 years). The 100,000 h stress service-lifetime plots are necessary to determine which process is called the LTHS. The LTHS value is then fitted within a range will limit the service lifetime of the pipe. If the point where the of values given in the standard to define the hydrostatic design two lines cross on the time scale is greater than the expected basis (HDB). An example is shown in Figure 2. service lifetime, the most important failure mode is ductile. If This shows a good example of a situation where only Stage I the crossover point on the time scale is less than the expected failures are observed during the test. The ASTM standard also service lifetime, then stress cracking is the life-limiting failure accounts for Stage II or Brittle Failure through slow crack 3.5 100,000 Hrs STAGE I 3.25 Log Stress (psi) LTHS = 1336 psi 3 HDB = 1250 y = -0.0241x + 3.2464 R2 = 0.9381 2.75 2.5 0 1 2 3 4 5 6 Log Time (Hrs) Figure 2. Determination of the hydrostatic design basis (HDB).

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7 3.5 STAGE I 100,000 Hrs 3.25 Log Stress (psi) STAGE II 3 LTHS = 977 psi HDB = 1000 psi 2.75 2.5 0 1 2 3 4 5 6 7 Log Time (Hrs) Figure 3. Determination of the HDB when Stage II is involved. growth. This is shown in Figure 3, and one can see that the yield stress or flexural modulus is not enough. One needs to failure mechanism has transitioned from Stage I to Stage II at determine the slope of the line in Figure 2 to get a good under- the stresses evaluated. It's important to understand that the first standing of long-term behavior under stress. example will undergo Stage II, it's just that it occurs at a time Once the HDB category is found, the hydrostatic design over 100,000 h. stress (HDS) is found by applying a design factor (DF) to the Figure 4 shows Stage I and Stage II for examples of both a HDB. The DF is similar to a factor of safety, where reductions HDB 1250 and HDB 1000 resin. are estimated for installation damage, lot-to-lot variability, and These examples clearly show that the differences seen so forth. In the pressure pipe industry, a DF of 0.50 (or 0.62) between resins is that the slope of the ductile line is shallower is applied to water pipe and a DF of 0.32 is applied to gas pipe. and, therefore, the transition from ductile to brittle (Stage I So, a 1250 HDB resin would have a HDS of 625 psi and 400 psi to Stage II) occurs later in time. This is why just specifying the in water and gas applications. 3.5 100,000 Hrs 3.3 LTHS = 1336 psi HDB = 1250 psi Log Stress (psi) 3.1 LTHS = 977 psi 2.9 HDB = 1000 psi 2.7 2.5 0 1 2 3 4 5 6 7 8 Log Time (Hrs) Figure 4. A comparison between the examples shown in Figures 2 and 3.