In geothermal energy projects, geosynthetic materials are exposed to a unique combination of extreme conditions. A thorough understanding of their material properties and degradation mechanisms is essential for selecting reliable and long-lasting geomembranes.
This chapter describes specific material properties of High-Density Polyethylene (HDPE) and XR elastomeric alloy polymers, including chemical resistance, thermal stability, mechanical behavior, and UV resistance, followed by the various degradation mechanisms that these materials may encounter in geothermal environments, including chemical, thermal, mechanical, and biological degradation.
Material Properties and Performance
Chemical Resistance
Geothermal fluids naturally contain a complex variety of dissolved minerals, salts, acids, and alkalis, which can be highly aggressive. In these applications, a geomembrane must withstand prolonged exposure to these chemicals without significant degradation.
HDPE
HDPE exhibits excellent chemical resistance to a wide range of chemicals, including most acids, alkalis, and many organic solvents. However, it’s vulnerable to attack from strong oxidizing agents and some hydrocarbons.
While strong oxidizing agents and hydrocarbons might not be universally present in all geothermal fluids, they can occur in certain situations. For example, near-surface geothermal systems or those influenced by atmospheric oxygen can produce oxidizing agents. Strong oxidizing agents can trigger oxidative degradation in HDPE, leading to chain scission and embrittlement.
The presence of hydrocarbons in geothermal fluids also depends on the geology of the geothermal formation. Specifically, areas with sedimentary formations or those near hydrocarbon reservoirs can introduce hydrocarbons like methane or aromatic hydrocarbons. If hydrocarbons are present, they can cause swelling and plasticization (a softening of the polymer), which reduces HDPE’s mechanical strength.
XR Elastomeric Alloy Polymers
XR polymers are engineered for exceptional chemical resistance, often surpassing HDPE. Even at elevated temperatures, they can withstand exposure to highly aggressive chemicals, including strong acids, bases, and solvents. Even so, prolonged exposure to extremely strong oxidizing agents like concentrated nitric acids and strong peroxides could lead to surface oxidation and changes in mechanical properties.
Prolonged exposure to aromatic hydrocarbons like benzene, toluene, and xylene could cause swelling or plasticization, similar to HDPE. These effects reduce the material’s mechanical strength and could trigger dimensional changes. However, XR polymers are generally more resistant to hydrocarbons than standard polyethylenes.
Regardless of the material, the rate of these chemical reactions is significantly increased by the elevated temperatures in geothermal systems. It’s essential to recognize that HDPE and premium XR geomembranes are vulnerable to other chemicals that aren’t naturally present in geofluids. However, those chemicals could be introduced through contamination, human-related activity, or cleaning and maintenance activities. It should be a strict policy to analyze any chemicals that might come in contact with geomembranes.
Thermal Stability
With their characteristic elevated temperatures, geothermal environments are considerably challenging to geosynthetics’ performance over the long term. Understanding a material’s tolerance to heat helps evaluate its suitability and expected lifespan.
Multiple degradation mechanisms come into play when polymers are exposed to high temperatures. Chain scission, the breaking of polymer chains, reduces molecular weight and mechanical strength. Thermal oxidation, a reaction with oxygen, disrupts the polymer’s structure and weakens bonds, making the material brittle and prone to cracking.
For semicrystalline polymers like HDPE, elevated temperatures can melt crystalline regions, softening the material and reducing its density.
The rate of thermal degradation depends on the polymer type, temperature, how long it’s exposed, and whether oxygen or other chemicals are present. To minimize the degradation effects, choose materials with high thermal stability and compare their tolerance to the long-term operating temperatures. Certain additives, such as antioxidants and heat stabilizers, can enhance thermal resistance by limiting damage even as temperature and duration of exposure increase. Check with your supplier for specs on the materials you’re considering.
Factors Affecting Thermal Stability
A geosynthetic’s thermal stability is influenced by several factors. While the unique molecular structure of a polymer inherently produces a certain degree of thermal stability, it can be enhanced by incorporating additives such as antioxidants and heat stabilizers. These additives can help limit damage even as temperature and duration of exposure increase. Still, other environmental conditions, such as oxygen or chemicals, can accelerate thermal degradation processes.
In geothermal applications, geosynthetic materials must withstand prolonged exposure to elevated temperatures without significant degradation. The required thermal stability specs will depend on the operating temperature of the geothermal system. For specific information on the thermal resistance of our high-performance XR geomembranes, don’t hesitate to contact our technical team.
While some temperature-induced changes can be reversed upon cooling, always prioritize materials designed for sustained high-temperature exposure.
Mechanical Behavior
Geomembranes used in geothermal applications must withstand the stresses and strains encountered during installation and extended operations. This ability is influenced by the material’s tensile strength, elongation, and puncture resistance.
Tensile strength refers to a material’s ability to resist pulling forces, while elongation describes its ability to stretch before breaking. Puncture resistance is the material’s ability to withstand concentrated forces that could cause tearing or perforation.
XR geomembranes are uniquely engineered for exceptional mechanical strength. The proprietary elastomeric alloy formulation provides inherent flexibility and toughness, while a nylon woven reinforcement enhances tensile strength, puncture resistance, and tear resistance. The EIA coating provides excellent abrasion resistance and substantially improves the material’s overall durability, making our XR geomembranes an outstanding solution for withstanding the demanding conditions of geothermal environments.
UV Resistance
While geothermal applications often involve buried or covered geosynthetics, UV resistance remains crucial since even brief periods of exposure (i.e., during installation or maintenance) can initiate continuing degradation processes that affect long-term performance.
When UV photons are absorbed by a polymer, the photon’s energy can break chemical bonds and create free radicals along the polymer chains. As free radicals are drawn to attack and weaken molecular bonds, they propagate a cascade of reactions that degrade the polymer, including chain scission, crosslinking, and oxidation. The cumulative effect of these chain reactions is a degraded, brittle polymer with reduced mechanical strength and a tendency to crack.
Geosynthetic materials used for critical containment must be formulated to withstand the challenges of UV exposure so they maintain integrity over time. Additives such as UV stabilizers are often incorporated into the polymer matrix to absorb or block UV radiation and protect the polymer chains from damage. The effectiveness of UV stabilizers depends on factors such as the type and concentration of the additive, the intensity of UV radiation, and the duration of exposure. Even in applications where geosynthetics are eventually covered, good UV resistance is a vital property that ensures the material will perform reliably over the long term.
We encourage you to contact our technical team for the latest information on our high-performance XR geomembranes and how they can meet the demands of your geothermal projects.
