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

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

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

Type of Geomembranes Widely Used GeomembranesLimited Used Geomembranes High density polyethylene (HDPE) Chlorosulfonated polyethylene (CSPE) Linear low density polyethylene (LLDPE) Ethylene interpolymer alloy (EIA) Polyvinyl chloride-plasticized (PVC-p) Ethylene propylene trimonomer (EPDM) Flexible polypropylene (f-PP)

Compositions (approximate percentage) TypeResinCarbon Black Anti- oxidant HDPE

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%.

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.

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.

Different Degradation Stages

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

Types of Antioxidants TypeChemical TypeExample PrimaryHindered phenols Irganox  1076 or 1010 Santowhite crystals Hindered amines Various of Tinuvin , Chemassorb  944 SecondaryPhosphites Irgafos  168 Sulfur compoundsDilauryl thiodipropionate Distearyl thiodipropionate Hindered amines Various of Tinuvin , Chemassorb  944

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

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

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 85 o C

Piezometer Insulation Perforated steel loading plate Sand Heat tape Geomembrane Load 110 Incubation Device

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

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.

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

Thermal Curve of OIT Test

Test Results Std-OIT HP-OIT Density Melt Index Yield Stress Yield Strain Break Stress Break Strain Incubation Time (month) Percent Retained Changes in Eight Properties with Incubation Time at 85°C

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

a) - OIT Depletion Rate °C 65°C 75°C 85°C ln OIT (min.) Incubation Time (month)

b) –Arrhenius Plot Standard OIT HP-OIT 1/T (°K) ln (OIT Depletion Rate) y = x R^2 = y = x R^2 = 0.943

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 20 o C “t” is the lifetime of antioxidant in the geomembrane

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 20 o C c) Lifetime of Antioxidant

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.