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 Geomembranes Limited 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)

Type Resin Carbon

Black

Anti-oxidant

HDPE 95-97 2-3 1-0.5

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 StagesInduction

PeriodDegradation

Period

Aging Time (log)

Pro

pe

rty

Re

tain

ed

(%

)

Unstabilized Polyethylene

InductionPeriod

DegradationPeriod

Aging Time (log)

Pro

pe

rty

Re

tain

ed

(%

)

stabilized Polyethylene

AntioxidantDepletion Period

Types of Antioxidants

• Primary antioxidants react with free radical species

• Secondary antioxidants decompose ROOH to prevent formation of free radicals

Types of Antioxidants

Type Chemical Type Example

Primary Hindered phenols Irganox 1076 or 1010

Santowhite crystals

Hindered amines Various of Tinuvin,

Chemassorb 944

Secondary Phosphites Irgafos 168

Sulfur compounds Dilauryl 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 Rr 1

Eact

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 85oC

Piezometer

Insulation

Perforated steel loading plateSand

Sand

Heat tape

Geomembrane

Load

1 10

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

Time (min)

Ex

oth

erm

En

do

the

rm

N235 kPa

O2

Intercept

OxidativeReaction

200°C

Isothermal

OIT

at 35 kPa

Test Results

3025201510500

50

100

150

Std-OITHP-OITDensityMelt IndexYield StressYield StrainBreak StressBreak Strain

Incubation Time (month)

Pe

rce

nt

Re

tain

ed

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

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25

55°C65°C75°C85°C

ln O

IT (

min

.)

Incubation Time (month)

b) –Arrhenius Plot

0.00310.00300.00290.00280.0027-5

-4

-3

-2

-1Standard OIT

HP-OIT

1/T (°K)

ln (

OIT

Dep

leti

on R

ate)

y = 17.045 - 6798.2x R^2 = 0.953

y = 16.856 - 6991.3x 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 20oC

“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 20oC

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.