durability of high density polyethylene geomembranes dr. grace hsuan civil & architectural...
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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.