Advanced Thin Film Geomembrane Technology For Biocell Liners and Covers. D. Martin, P. Eng

Transcription

1 Advanced Thin Film Geomembrane Technology For Biocell Liners and Covers D. Martin, P. Eng Research & Technology Manager, Layfield Geosynthetics and Industrial Fabrics Ltd, st, Edmonton, AB, Canada. T5S-2H6; PH (780) ; FAX (780) ; 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,

2 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 The samples were installed facing due south at an angle of 3 horizontal to 1 vertical. The samples were left undisturbed until Sept 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,

3 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

4 300 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/m 2 /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/m 2 /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/m 2 /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/m 2 /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/m 2 /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 MJ/m 2 /hour in total energy between 300 and 320 nm. These two calculated values give the following relation: 4.64 to 27.9 MJ/m 2 /year = 108 to 650 Hours of Accelerated UV exposure MJ/m 2 /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.

5 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. PVC Alloy Weathering % Elongation 700% 600% 500% 400% 300% 200% 100% 0% 593% 38% Control Accelerated 3712 Hours 232% 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.

6 PVC 30 Weathering % Elongation 600% 500% 400% 300% 200% 100% 0% 515% 304% Control Accelerated 1712 Hours 493% 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 Thickness Titanium Dioxide/ Carbon Black Baseline Antioxidant (mm) (%) (OIT) HDPE to Black Polyolefin White Polyolefin White Polyolefin to Additional Antioxidant Estimated (HPOIT) No Additonal A/ minutes Additional UV Stabilizer (ppm) No Additonal UV Stab. 2X* 2X* X* * 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.

7 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 Hours of Exposure HDPE (1.5 mm) Black Polyolefin (0.75 mm, 2X UV Additive) White Polyolefin (0.75 mm, 2X UV Additive) 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

8 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 Baseline Antioxidant (OIT) HDPE 100 Black Polyol -efin White Polyol- Efin Additional Antioxidant Estimated (HPOIT) HPOIT Before Exposure (minutes) HPOIT Following Exposure (minutes) No Additional A/ 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.

9 Table 3: Tensile Testing Results Following a Second 20,000 Hour Accelerated Exposure Material Elongation Retained Tensile Strength Retained 1.5 mm HDPE with 2 to 3% 79% 76% carbon black 0.75 mil Polyolefin with 2 to 3 % carbon black and X amount of UV stabilizer. 103% 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, pp Hsuan, Y. G., and Koerner, R. M. (1993). Can outdoor degradation be predicted by laboratory acceleration weathering? Geotechnical Fabrics Report, 11(8), 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.

10 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