Quantum Study on the Performance and Durability of Polyurea Containment Liners versus Polyethylene Based Geomembranes

 

This study compared the durability and large scale puncture performance of our state-of-the-art polyurea geomembranes as tested and compared to various polyethylene materials.The truncated cone test (Large Scale Hydrostatic Puncture, ASTM D5514) was used to determine a critical cone height for a 60 mil polyurea liner and 60 mil HDPE liner. 

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A comparison was also made to CCH values previously published. The results of the study confirm previous findings that more flexible and elastic materials require greater cone heights to induce a puncture than less flexible materials. Polyurea is more flexible and behaves more elastically than any of the previously studied polyethylene materials and possessed a greater CCH. Considering the physical properties of seamless polyurea geomembranes, it is often a superior choice for secondary containment as well as primary containment applications such as lined pits for fracturing operations.Puncture resistance of geomembrane lining systems is a key property as even small punctures compromise the performance of the containment and cause environmental contamination. Challenging subgrade conditions often require geomembrane selection based on durability and the ability to resist puncture due to the presence of protruding stones, organic matter or other sub optimal materials. Surveys have shown that approximately 25% of the damage to HDPE liners occurs during installation with 79% of the damage attributed to seams (Nosko et. al. 1996, Nosko et. al. 2000, Forget et. al. 2005). Nosko further determined that 81% of geomembrane punctures are due to stones (Nosko, Andrezal, Greggor, Gainer 2005). Polyurea liners with their seamless installation and excellent puncture resistance over foreign objects eliminate the largest contributors to liner damage.

 

One strategy for resisting puncture is the ability to resist downward pressure of the effluent over a protruding object through high strength. A second and ultimately more realistic strategy is to stretch over the protruding object, conforming to the uneven subgrade and resisting puncture no matter how much downward pressure is applied. The design challenge of flexibility versus strength is common to many containment projects and data on newer materials such as polyureas needs to be generated.

 

Polyurea spray applied geomembranes have been used for roughly 15 years; during that time the technology has evolved greatly. Improved performance of polyureas as containment geomembranes has come from new formulations, improved application equipment and application procedures. QA/QC procedures for field projects have improved due to the above factors and polyureas provide superior environmental protection when compared to commonly used polyethylene liners. Polyureas behave in an elastic manner when impacted or stretched, returning to their original thickness when the stress is removed. This is an advantage over polyethylene systems, especially highly crystalline materials such as HDPE. The variety of polyethylene materials available tend to deform plastically and thin out irreversibly when impacted or stretched beyond their yield point. As a result they may fall below regulatory requirements without recovering.

 

Polyurea liners are often sprayed onto geotextiles when the subgrade is soil. The geotextile backing will have an influence on puncture behavior. In this study we did not consider geotextile cushioning due to the large number of suitable geotextiles and the required time to complete testing. It is our intention to expand our study and evaluate a number of geotextile/polyurea systems for future publication.

 

Specific test results of this study are available from Quantum. Briefly, the elastic material polyurea gave the greatest CCH and showed the best conformance to imperfections in the subgrade.This confirms the expected result that more flexible materials are more durable in this simulation of actual field conditions, and provide a higher level of environmental protection when compared to stronger and stiffer materials.The results show clearly that the high modulus semi-crystalline HDPE material punctures at much lower cone heights and is suitable only for very smooth subgrades or will require a high level of cushioning geotextile protection. As a comparison, a polyethylene copolymer reinforced with a fabric scrim (CSPE-R) was included as it is another very high strength material. It was unable to resist the downward forces exerted on the geomembrane and also failed at a relatively small cone height according to Hullings and Koerner, 1991. Less crystalline and more flexible polyethylenes such as Linear Low Density Polyethylene (LLDPE) gave much better performance than HDPE. It appears that critical cone height in polyethylenes improves as density decreases. The density of the HDPE tested in this study was measured at 0.951 g/ml, while the Geosynthetic Research Institutes GM specifications give a maximum density of 0.939 g/ml for LLDPE. 

 

In conclusion, seamless spray applied polyurea geomembranes possess outstanding durability and puncture resistance when compared to polyethylene geomembranes of varying density.Polyurea provides superior environmental protection for secondary containment applications such as tank farms, and particularly for primary containment applications such as synthetically lined ponds for fracturing operations.Very high strength fabric reinforced materials also do not compare to polyurea in terms of actual field performance. As the technology of formulations, application equipment, and application techniques has evolved, and installation advantages of a spray applied system over traditional sheet goods is better understood and accepted by industry and regulators, polyurea is quickly becoming a serious alternative to polyethylene in containment design. Major steps in field installation QA/QC of polyurea systems continue to evolve giving regulators and facility owners confidence in the level of containment provided by these products.

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