Advanced Thin Film Geomembrane Technology
For Biocell Liners and Covers
D. Martin, P. Eng
Research & Technology Manager, Layfield Geosynthetics and Industrial Fabrics Ltd,
11603 – 180 st, Edmonton, AB, Canada. T5S-2H6; PH (780) 451-7227; FAX (780)
455-5218; e-mail: dmartin@layfieldgroup.com.
Abstract
The purpose of this project was to complete a six year natural weathering study on a
series of geosynthetics an concurrently complete a 20,000 hour accelerated
weathering study. An energy equivalency method was used to establish a rough
relationship of approximately 1000 hours of accelerated weathering being equal to
one year of natural exposure. This relationship was seen to be consistent with the
results of the natural and accelerated studies, as well as to other examples of such
relationships in the literature. The 20,000 hour accelerated exposure allowed us to
compare the performance of highly UV resistant materials such as 1.5 mm HDPE
stabilized with carbon black, and thinner polyolefin geoemembranes stabilized with
pigment and additional UV stabilizers. The results of our study suggest that a
thinner (0.75 mm) geomembrane can perform equivalently to a 1.5 mm HDPE
geomembrane when sufficient UV stabilizing additives are utilized.
Introduction
The ability to maintain physical properties, and maintain containment of hazardous
materials, despite long-term exposure to ultra violet (UV) radiation, is an important
performance property for a geomembrane. Most modern geomembranes have a high
level of resistance to UV radiation, and as a result natural weathering studies must be
run for prohibitively long periods to give meaningful longevity estimates. The use of
accelerated weathering conditions to try and gauge the in-service weathering
performance of a material (but in a more timely fashion) is common in many
industries. However, relating an accelerated weathering exposure period to a natural
weathering service life is widely reported to be a difficult and elusive goal.
The purpose of this research project was to complete a six-year natural weathering
study on a series of geosynthetic materials, and concurrently complete a 20,000-hour
accelerated study. Sufficient overlap in materials existed between the two studies for
a comparison to be made between the two weathering processes. Certain moderately
UV stable materials, specifically lightly stabilized PVC based geomembranes,
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showed sufficient degradation during the six year natural exposure to act as
benchmarks for comparison to the accelerated exposure.
A calculation was also made, to compare the natural and accelerated exposures based
on the relative incident radiation between 300 and 320 nm. This calculation was
utilized to relate the accelerated exposure to an equivalent natural exposure period.
Benchmark materials were used to test the relationship.
Finally, the 20,000 hour accelerated exposure was useful in comparing the weathering
performance of highly stable materials. High Density Polyethylene (HDPE) 1.5 mm
thick, stabilized with 2 to 3% carbon black, was compared to thinner polyolefin
materials stabilized with a combination of either carbon black or titanium dioxide
with a proprietary UV stabilizer/Antioxidant additive package. The goal was to
evaluate if a thin film polyolefin geomembrane could perform equivalently to a
thicker HDPE material if an appropriate additive package was utilized.
Procedure
Natural Weathering Study
The geomembrane samples were mounted on plywood backing installed on the roof
of the Layfield building in Edmonton, Alberta , Canada. The first of the samples were
installed in 1996, with some additional samples added in 1997. The samples were
installed facing due south at an angle of 3 horizontal to 1 vertical. The samples were
left undisturbed until Sept. 2002 when they were removed for evaluation.
Four almost identical sets of samples were used for this exposure. For this study 1 set
of samples was destructively tested, leaving 3 sets of samples for further study. After
removal from the boards the samples were washed to allow for an accurate thickness
measurement to be taken. The retained control samples were unfortunately lost during
the six-year exposure, so physical properties had to be compared to original
specifications.
Accelerated Weathering Study
Geomembranes were exposed to cycles of UV radiation and condensation using a
QUV/SE model Accelerated Weathering Tester, operated in accordance with ASTM
G154. Three replicates of each geomembrane sample were added to the QUV
apparatus at the start of the study, with an equal number of samples retained as a
control.
Based on the experience of others in the accelerated weathering of highly UV
resistant materials such as HDPE 1.5 mm (Wagner and Ramsey, 2003) it was decided
to dramatically accelerate the weathering conditions when compared to natural
sunlight. Long Term natural weathering studies have also shown HDPE materials to
be highly resistant to the effects of UV light (Sangam, Rowe, Mlynarek and Sarazin,
2001). UVB bulbs were used for the exposure as they emit higher levels of UV
radiation with a wavelength between 300 and 320 nm than is found in natural
sunlight. The 300 nm wavelength radiation is considered the most damaging to
polyethylene, and 320 nm is considered the most damaging to PVC (Searle, 1999), so
clearly the UVB bulbs are more aggressive than natural sunlight in weathering these
materials. UVB bulbs also emit shorter wavelength (higher energy) radiation below
300 nm, which is not found in natural sunlight. The presence of this low wavelength
UV radiation also makes UVB bulbs more aggressive than natural sunlight in
weathering polymeric materials.
The accelerated weathering cycle for the first 10,000 hours of exposure was 8 hours
of UV light irradiance at 0.80 W/m2/nm (measured at the peak wavelength of 313
nm) and a temperature of 60 C; followed by a 4 hour condensation cycle at 50 C. For
the second 10,000 hours of the exposure the cycle was changed to 10 hours of UV
radiation (at the above conditions) followed by a 2-hour condensation cycle.
A second 20,000 hour exposure has now also been completed, using a cycle of 10
hours UV exposure (with the same conditions as above) followed by a 2 hour
condensation cycle.
Material Testing
The PVC based materials from the natural weathering study were tested according to
ASTM D882. The HDPE 1.5 mm samples from the accelerated weathering study
were tested using ASTM D638, with a Type IV die. All other materials from the
accelerated weathering study were tested with a modified version of ASTM D882,
where the sample strip thickness was reduced to ¼” in order to increase the number
of samples tested and raise the statistical validity of the results.
Results
Calculating a Relationship Between Accelerated and Natural Weathering
One of the goals of this study was to provide a method for extrapolating accelerated
weathering into expected service lives for geomembranes in the field. The difficulty
of quantifying this relationship has been noted by many sources, but examples of an
energy equivalency approach exist in the literature (Hsuan and Koerner, 1993). Our
attempt to develop a broad relationship between accelerated weathering and natural
service life was pursued in order to begin quantifying warranty periods for polyolefin
geomembranes.
In order to calculate a relationship between the two exposure methods we determined
the amount of incident radiation with wavelengths between 300 and 320 nm that the
samples had received. As noted, 300 nm is the wavelength considered to be the most
damaging to polyethylene, and 320 nm is considered to be the most damaging to PVC
(Searle, 1999). While the UVB bulbs emit radiation with wavelengths shorter than
300 nm, and this radiation is likely damaging to the geomembrane samples, natural
sunlight contains no UV radiation with wavelengths below 300 nm and therefore this
portion of the UVB bulb spectra could not be used in this comparison. For this reason
any comparison between fluorescent UVB exposure and natural sunlight is obviously
imperfect and should underestimate the natural service life of the material.
The average solar irradiance hitting a horizontal surface in Edmonton is roughly
4,640 MJ/m2/year (Mazria, 1979). The amount of radiation between 300 and 320 nm
is estimated at 0.6% (ASTM G154, 1998) giving a total energy in this region of 27.9
MJ/m2/year. Other sources estimate the UVB radiation (290 to 315 nm) in sunlight to
be 0.1% of the total energy (Grossman, 1977), which equates to 4.64 MJ/m2/year in
Edmonton.
For the purposes of our calculation we will assume that the total irradiance between
300 and 320 nm, which was received in a natural exposure by our geomembrane
samples, can be conservatively estimated at between 4.64 and 27.9 MJ/m2/year.
The samples for our natural weathering study were facing due south at an angle of 3
horizontal to 1 vertical. This will increase the intensity of the radiation exposure
compared to a horizontal surface. Therefore the above estimate may underestimate of
the actual values.
Based on the irradiance curve for fluorescent UVB bulbs (ASTM G154, 1998) and
our irradiance setting of 0.80 W/m2/nm measured at the peak emitted wavelength of
313 nm, we made an estimate of the area under the irradiance curve to determine the
total irradiance between 300 and 320 nm. Based on this rough estimate, the samples
in our accelerated weathering study received approximately 0.0429 MJ/m2/hour in
total energy between 300 and 320 nm.
These two calculated values give the following relation:
4.64 to 27.9 MJ/m2/year = 108 to 650 Hours of Accelerated UV exposure
0.0429 MJ/m2/hour
Year of Natural Exposure
Based on our cycle of 16 to 20 hours of UV radiation per day in the accelerated
weathering study, our calculation results in an estimate of roughly 200 to 1000 hours
of accelerated exposure equating to a year of natural exposure in Edmonton, Canada.
Wagner and Ramsey (2003) of GSE make reference of a loose correlation used in the
paint and coatings industry of 500 to 1500 hours of accelerated exposure equaling
approximately 1 year of real life exposure. The most conservative end of our
calculated relationship (1000 hours) falls in the middle of this range. We have
decided to use a relationship of 1000 hours of accelerated weathering equating to a
year of natural weathering as a conservative basis to begin looking at warranties.
Comparison of Weathering Study Results to Our Calculated Relationship
Results from the two weathering exposures conducted for this study were used to
compare accelerated and natural weathering of similar materials to the predictions
made by our calculated relationship. The most relevant results to test this relationship
were for regular flexible PVC and a commercially available PVC alloy.
The commercially available PVC alloy received a natural exposure of six years, and
an accelerated exposure of 3712 hours. Based on our calculations the natural
exposure should have had a slightly more severe effect on the material properties.
Figure 1 compares the elongation of the two weathered samples with a control
sample. The results show that the accelerated exposure was in fact more damaging,
suggesting our calculated relationship may be conservative.
% Elongation
PVC Alloy Weathering
700%
600%
500%
400%
300%
200%
100%
0%
593%
232%
38%
Control
Accelerated
3712 Hours
Natural 6
Years
Figure 1: Comparison of Accelerated and Natural
Weathering Exposures in a 0.75 mm PVC Alloy.
The regular flexible PVC received a natural exposure of 6 years, and an accelerated
exposure of 1712 hours. Again our calculated relationship predicts that the natural
weathering exposure should have had the more severe effect on material properties.
Figure 2 compares the elongation of the two weathered samples with a control
sample. The results again show that the accelerated exposure had a larger impact on
the elongation properties of 0.75 mm PVC. This second comparison also suggests
that our calculated relationship underestimates the severity of the accelerated
exposure.
% Elongation
PVC 30 Weathering
600%
500%
400%
300%
200%
100%
0%
515%
493%
304%
Control
Accelerated
1712 Hours
Natural 6
Years
Figure 2. Comparison of Accelerated and Natural
Weathering Exposures in a 0.75 mm PVC.
The UV Resistance of Highly Stabilized Materials.
The accelerated weathering portion of this study was run for a total of 20,000 hours in
order to compare the performance of some highly stabilized materials. Table 1 shows
the makeup of the samples used for this portion of the study.
Table 1: Highly Stabilized Samples for 20,000 hour Accelerated Study
Material
HDPE
Black
Polyolefin
White
Polyolefin
White
Polyolefin
Thickness
Titanium
Dioxide/
Carbon
Black
Baseline
Antioxidant
(mm)
1.5
(%)
2 to 3
(OIT)
100
Minutes
0.75
2 to 3
100
Minutes
Estimated
(HPOIT)
No
Additonal
A/0
2000
minutes
0.75
3.5
100
Minutes
2000
Minutes
2X*
0.75
3.5
100
Minutes
1000
Minutes
X*
Additional
Antioxidant
Additional
UV
Stabilizer
(ppm)
No
Additonal
UV Stab.
2X*
* The X and 2X values simply signify that the 2X samples contain twice the
additional UV stabilizer that the X sample contains.
The HDPE sample used for this study was manufactured by Columbia Geosystems.
The polyolefin samples are Enviro Liner© manufactured by Layfield Poly Films Ltd.
The baseline antioxidant is added to the base polymer at the time of manufacture,
with a minimum specification of 100 minutes Oxidative Induction Time (OIT) as per
ASTM D3895. The additional antioxidant and UV stabilizer are a proprietary blend
and were added at the extruder during the blown film process. The exact quantities of
UV Stabilizer and additional antioxidant are considered proprietary.
The results of the 20,000-hour exposure are shown in Figure 3. In this case we used
the percent of tensile strength retained as these results showed the same trend as the
percent elongation retained but showed greater deterioration in the samples.
The samples did not show clear signs of deterioration until the testing was done at
20,000 hours. The results show that both the white and black 0.75 mm polyolefin
samples that contained an additional 2X quantity of UV stabilizer (and sufficient
additional antioxidant for a 2000 minute HPOIT) performed as well as HDPE 1.5 mm
in this lengthy UV exposure. Based on our calculated relationship between this
accelerated exposure and a real life exposed service life, we would expect the HDPE
1.5 mil and the two polyolefins stabilized with 2X of our UV additive to exceed 20
years in an exposed service.
20,000 Hour Accelerated Weathering
120
HDPE (1.5 mm)
100
80
Black Polyolefin
(0.75 mm, 2X UV
Additive)
60
40
White Polyolefin
(0.75 mm, 2X UV
Additive)
20
0
2000
6000
10000
Hours of Exposure
20000
White Polyolefin
(0.75 mm, X UV
Additive)
Figure 3: Results of 20,000 hour Accelerated Weathering Exposure on
Highly Stabilized Geomembranes.
The white polyolefin sample stabilized with X amount of our UV additive (and
enough additional antioxidant for a 1000 minute HPOIT) lost nearly 50% of its
tensile strength after the 20,000-hour exposure. This material would have to be
considered seriously damaged and past it’s useful life after this sort of exposure.
Clearly there is a minimum required level of the UV stabilization and antioxidant
additive package to bring a thin film geomembrane up to an equivalent performance
with HDPE 1.5 mm.
HDPE (1.5 mm) and Black Polyolefin (0.75 mm) samples were included in our
natural weathering study, and were exposed to natural weathering for a period of six
years. At the end of this period they did not display any deterioration, which is
consistent with the results of our accelerated study based on our relationship of 1000
accelerated hours being equal to one year of natural exposure.
The results of HPOIT (High Pressure Oxidative Induction Time) testing on three of
the four samples (control and exposed samples) are shown in Table 2.
Table 2: HPOIT Testing on Exposed and Control Samples
Material
HDPE
Black
Polyol
-efin
White
PolyolEfin
Baseline
Antioxidant
(OIT)
100
Minutes
100
Minutes
100
Minutes
Additional
Antioxidant
Estimated
(HPOIT)
No
Additional
A/0
2000
2000
HPOIT
Before
Exposure
HPOIT
Following
Exposure
(minutes)
301
(minutes)
248
3930
3757
4205
2695
The HDPE and Black Polyolefin samples showed relatively minor decreases in
HPOIT as a result of the 20,000 hour exposure. The White Polyolefin had a more
substantial decrease in antioxidant level (36% loss of HPOIT).
The results for the second 20,000 accelerated weathering exposure are shown in Table 3.
In this case a 0.75 mm black polyolefin stabilized with only X amount of additional UV
stabilizers was tested side by side with a 1.5 mm HDPE stabilized with carbon black
alone.
Table 3: Tensile Testing Results Following a Second 20,000 Hour Accelerated Exposure
Material
Elongation Retained
1.5 mm HDPE with 2 to 3% 79%
carbon black
0.75 mil Polyolefin with 2
103%
to 3 % carbon black and X
amount of UV stabilizer.
Tensile Strength Retained
76%
97%
Once again the highly stabilized 0.75 mm Polyolefin retained more of it’s original
properties than a thicker HDPE sample stabilized with carbon black alone. In the first
exposure a UV additive content of 2X was used in the 0.75 mm black polyolefin sample,
but reducing this amount by half did not effect the durability of the polyolefin sample.
Conclusions
The approximate relationship that we calculated between our accelerated exposure
and real life exposed service life (1000 hours of accelerated exposure equating to 1
year natural weathering) appears to agree with other industry standard relationships,
and appeared to be conservative when compared to our weathering results.
With sufficient additional UV stabilizing and antioxidant additives both white and
black thin film polyolefin geomembranes performed as well as a thicker section of
HDPE stabilized with carbon black alone. It is feasible that a highly stabilized 0.75
mm geomembrane could provide an exposed service life in excess of 20 years, or at
least equivalent service lives to a 1.5 mm traditionally stabilized geomembrane.
References
American Society for Testing and Materials (ASTM). (1998). Standard G154-98,
West Conshohocken, PA.
Grossman, G.W. (1977) “Correlation of laboratory to natural weathering” Journal of
Coating Technology, Vol. 49, No. 633, pp45-54.
Hsuan, Y. G., and Koerner, R. M. (1993). “Can outdoor degradation be predicted by
laboratory acceleration weathering?” Geotechnical Fabrics Report, 11(8), 12 – 16.
Mazria, E. (1979) The Passive Solar Energy Handbook, Rodale Press, Emmaus, PA.
Sangam, H. P., Rowe, K. R., Mlynarek, J. and Sarazin, P. (2001) “Natural weathering
of a 14 year pre-aged geomembrane” Proceedings of the Geosynthetics Conference
2001, IFAI, Roseville, MN.
Wagner, N., and Ramsey, B. (2003) “QUV accelerated weathering study: Analysis of
polyethylene film and sheet samples”. Technical Document by GSE Lining
Technology, Inc., Houston, TX