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HDPE geomembranes are excellent barriers against containment transport. However, we’re constantly working to develop the next generation of polyethylene (PE) solutions that combine improved performance, processability and sustainability. GRI-GM13 Standard Specification for Test Methods, Test Properties and Testing Frequency for High Density Polyethylene (HDPE) geosynthetic-institute.org/grispecs/gm13.pdf Smooth and Textured Geomembranes has achieved acceptance and adoption in many markets around the world. The specification has been modified more than 18 times in an effort to improve it since the original adoption in 1997. The testing protocols and frequencies have been updated in an effort to keep things current. However, over time, the barriers market has significantly expanded in scope and range. There currently are a large range of geomembrane applications that vary significantly in duration, expected performance levels and associated properties.
A “one size fits all” approach, while appropriate in the past, is no longer adequate to address the industry’s needs. While it is understood that the values within GRI-GM13 can be changed for differing applications, in practice, this rarely occurs. Further, there are some requirements and statements in the document that are no longer appropriate for the broad range of applications. Moreover, there are multiple pathways and formulations that exist to attempt to meet the specification requirements. Most of these pathways are appropriate for an outcome of differing markets and techniques. However, some manufacturers are optimizing “performance to the specification,” as contrasted with optimal performance for the applications. In the long run, this is a disservice and harmful to the geosynthetics industry.
Considering the above situation, a new high-performance HDPE specification has been developed at GSI. In 2018, at the prompting of the IGS barrier workshop in Munich, Germany, a technical task group was formed within GRI to address this issue and create a “high performance” HDPE specification. The group decided that the purpose of the specification was to be directed at manufacturing quality control (MQC) of a better performing HDPE geomembrane beyond GRI-GM13. This infers that if a specifier has unique or extenuating circumstances for a particular project, a premium HDPE geomembrane could now be specified. The result is GRI-GM42 Standard Specification for Test Methods, Test Properties and Testing Frequency for High Performance-High Density Polyethylene (HP-HDPE) Geomembranes Used in Extreme Conditions.
This specification covers HP-HDPE geomembranes with a formulated sheet density of 0.945 g/ml or higher in the thickness range of 1.5 mm (60 mil) to 3.0 mm (120 mil). Both smooth and textured geomembranes are covered under this specification. This specification sets forth a set of minima, physical, mechanical and chemical properties that must be met or exceeded by the geomembrane being manufactured. In a few cases, a range is specified. The entire specification in its latest revision is available for download from the institute’s website at geosynthetic-institute.org/grispecs/gm42.pdf. HP-HDPE geomembranes enable outstanding resistance to high temperatures, chemicals, antioxidant depletion, stress cracking and mechanical properties beyond conventional HDPE geomembranes. It should be noted that all endurance testing for GRI-GM42 should be performed on 0.24mm (10 mil) film made from the respective geomembranes. All film preparation is carried out on homogenized particles produced for the HP-HDPE geomembranes. After mixing, samples were pressed into plaques for thermal annealing and exposure. This protocol has been standardized into GRI-GM40 Standard of Practice for Preparation of Film for Accelerated Oxidation Resistance Testing of Polyolefin Geomembranes.
This practice is intended for the preparation of thin film samples used for determining an accelerated assessment of product durability in order to reduce the time necessary to assure compliance to specifications. It needs to be acknowledged that the thickness of polyolefin geomembrane contributes to its durability and thus service life. (i.e., the thicker the geomembrane, the longer it will last). The encouraged use of thin samples or “film” is only made with the objective of accelerating the durability evaluation. Real-world geomembranes (1.5 mm (60 mil)) or greater will have much greater durability than the “manufactured films” used in this practice. However, it has been shown that there is a correlation between geomembrane and film performance. The use of film instead of the geomembrane’s actual thickness was adopted for the development of accelerated assessments of product durability in order to 1) reduce the time necessary to assure compliance to durability specifications and 2) encourage the quality control testing of raw material blends by manufacturers via more routine and frequent testing, especially for high-performance geomembranes.
While thickness of polyolefin products contributes to durability and service life, the objective of accelerating the evaluation of durability performance of formulations encouraged the use of thin samples. Choosing the film is the first step in the process. In all accelerated testing work performed in this study, 0.25 mm (10 mil) samples were manufactured from provided products to accelerate the aging process. This thickness has the advantage of matching the thickness specified in ASTM 8117 and ASTM D5885 for standard and high-pressure OIT measurements respectively. This approach is practical in nature and serves to stress the product formulation and achieve both test acceleration and differentiation between stabilizing chemistries.
The next step in the process involves a water leaching procedure or “annealing” of the film via heat treatment that stabilizes properties of the film prior to endurance exposure. It is used to make materials more uniform for endurance testing by reducing internal stresses. The film is heated in an 85°C water bath for a period of 48 hours, then slowly cooled to room temperature. The practice used is GRI Standard Practice GM41 for Accelerated Oxidation Resistance Testing of Polyolefin Geomembranes.
This practice is largely informed by procedures found in ISO 13438, the screening test method for determining the resistance of geotextiles and geotextile-related products to oxidation. This practice involves the immersion of film in reagent grade water (RGW). It is typically run in a mason jar with distilled, deionized, deaired water. The film should be totally submerged and resting away from the jar walls. This can be accomplished by creating a conical sample and completely immersing it in RGW.
There should be only one specimen per container. The jars are typically inverted in the water bath at 85 + 1°C and are exposed for 48 hours in the water bath. After exposure, the samples are removed from the jar and allowed to dry at standard laboratory environment until constant mass is achieved. Accelerated aging tests then are conducted on the annealed film.
There are three accelerated aging procedure requirements under GRI-GM42 for endurance exposure:
- ASTM D5721 Standard Practice for Air-Oven Aging of Polyolefin Geomembranes at 85 + 2°C for 350 hours
- ASTM D7238 Standard Test Method for Effect of Exposure of Unreinforced Polyolefin Geomembrane Using Fluorescent UV Condensation Apparatus at an accelerated cycle of 20 hours at 0.78 W / (m2 • nm) and 340 nm, irradiance QUVA at 75°C followed by four hours of condensation at 60°C for 350 light hours, and
- ASTM D5322 Standard Practice for Immersion Procedures for Evaluating the Chemical Resistance of Geosynthetics to Liquids with 10 +/- 1 PPM chlorine solution at 50°C for 350 hours. Chlorine concentration shall be controlled, monitored and recorded over the 30-day incubation period. The chlorine solution needs to be checked weekly at a minimum to assure stability.
With HP-HDPE materials, the goal is to specify materials used when containment is critical due to environmental and/or economic concerns. There is also a desire to provide materials that will be optimized for lifespan and durability. This material specification for HP-HDPE is designed to be the best possible. The raw materials should be made from resins specifically designed and tailored for use as geomembranes. Regrind and recycled materials should not be used. Minimum thickness of these materials should be 60 mil/1.5 mm or greater.
Testing frequencies for long-term durability are increased compared to GRI-GM13. Lifespan and durability testing is conducted under harsher and/or longer conditions than other grades and may require specific testing unique to a specific project/application. These materials should be able to withstand high-temperature water (near 212°F/100°C) containment. Regularly, this containment application also includes chlorine contained in the water in various oxidative states. This is considered to be a critical application. It varies from other specifications and applications in that different resins are used to manufacture these geomembranes. The resins are specifically designed for geomembranes used at elevated temperatures.
Acknowledgements
This specification represents the combined efforts of the resin and geomembrane manufacturer organizations over the course of several years. The authors sincerely appreciate the participants’ efforts and congratulate them in the development of this specification. Sincere appreciation is also extended to all GSI members, in particular owners/consultants/government agencies and test laboratories for their review and comments.