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Durability of High Density Polyethylene Geomembranes

Durability of High Density Polyethylene Geomembranes. Dr. Grace Hsuan Civil & Architectural Engineering. Landfill Liner System. GT. GN. GCL. Gravel w/ perforated pipe. GM. GG. CCL. Landfill Cover System. GG. Geosynthetic ECM. Cover Soil. GC or GN. GT. GM. GCL. GP or GC.

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Durability of High Density Polyethylene Geomembranes

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  1. Durability of High Density Polyethylene Geomembranes Dr. Grace Hsuan Civil & Architectural Engineering

  2. Landfill Liner System GT GN GCL Gravel w/ perforated pipe GM GG CCL

  3. Landfill Cover System GG Geosynthetic ECM Cover Soil GC or GN GT GM GCL GP or GC

  4. Type of Geomembranes

  5. Compositions(approximate percentage)

  6. Function of Carbon Black • The primary function is as an ultraviolet light stabilizer to protect polymer being degraded. • Carbon black absorption coefficient increases with loading up to ~ 3%.

  7. Function of Antioxidants • The function of antioxidants is to protect polymers from being oxidized during the extrusion process and service lifetime. • For polyolefines, antioxidants is vital to the longevity of the product. • Antioxidant depletion will be the focus of this course.

  8. Oxidation Degradation • Oxidation takes place via a series of free radical reactions. • Oxidation leads to chain scission that results in decrease of Mw and subsequently on mechanical properties.

  9. Different Degradation Stages

  10. Types of Antioxidants • Primary antioxidants react with free radical species • Secondary antioxidants decompose ROOH to prevent formation of free radicals

  11. Types of Antioxidants

  12. Depletion of Antioxidants Two mechanisms: • Chemical reactions – by reacting with free radicals and peroxides • Physical loss – by extraction or volatilization

  13. Arrhenius Plot A E act R ln R r 1 high temperature low temperature (lab tests) (site temperature) Inverse Temperature (1/T)

  14. Experimental Design • Incubation environment should simulate the field (i.e., landfill environment) • Limited Oxygen • Some degree of liquid extraction • Utilize elevated temperatures to accelerate the reactions. • 55, 65, 75, and 85oC

  15. 1 10 Piezometer Load Insulation Perforated steel loading plate Sand Heat tape Sand Geomembrane Incubation Device

  16. Tests Performed • Oxidative inductive time (OIT) for antioxidant content. • Melt index for qualitative molecular weight measurement. • Tensile test for mechanical property

  17. Oxidative Induction Time (OIT) • OIT is the time required for the polymer to be oxidized under a specific test condition. • OIT value indicates the total amount (not the type) of the antioxidant remaining in the polymer.

  18. OIT Test for Evaluation of Antioxidant (AO) • OIT Tests: • ASTM D3895-Standard OIT (Std-OIT), or • ASTM D5885-High Pressure OIT (HP-OIT) • HP-OIT test is used for AOs which are sensitive to high temperature testing

  19. Thermal Curve of OIT Test

  20. 150 100 Std-OIT HP-OIT Density Melt Index 50 Yield Stress Yield Strain Break Stress Break Strain 0 0 5 10 15 20 25 30 Test Results Percent Retained Incubation Time (month) Changes in Eight Properties with Incubation Time at 85°C

  21. Analysis of OIT Data • Determine OIT depletion rate at each temperature. • Utilize Arrhenius Equation to extrapolate the depletion rate to a lower temperature. • Predict the time to consume all antioxidant in the polymer.

  22. 4.5 4 3.5 3 ln OIT (min.) 2.5 55°C 2 65°C 75°C 1.5 85°C 1 0 5 10 15 20 25 Incubation Time (month) a) - OIT Depletion Rate

  23. -1 Standard OIT HP-OIT -2 -3 -4 -5 0.0027 0.0028 0.0029 0.0030 0.0031 b) –Arrhenius Plot y = 17.045 - 6798.2x R^2 = 0.953 y = 16.856 - 6991.3x R^2 = 0.943 ln (OIT Depletion Rate) 1/T (°K)

  24. c) Lifetime of Antioxidant ln(OIT) = ln(P) – (S) * (t) where: “OIT” is the value of unstabilized HDPE geomembrane “P” is the OIT value of unaged HDPE geomembrane “S” is the OIT depletion rate at 20oC “t” is the lifetime of antioxidant in the geomembrane

  25. c) Lifetime of Antioxidant • The OIT value for unstabilized HDPE geomembrane was found to be 0.5 min. • For this particular antioxidant package, the lifetime is • t = 200 years at 20oC

  26. Summary • Antioxidants are essential in protecting the properties of the geomembrane. • OIT test has found to be an straight forward method to assess the antioxidant remaining in the geomembrane. • The lifetime of antioxidant package in the HDPE geomembrane can be predicted using Arrhenius equation.

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