Geomembranes

Geomembranes / Evaluating specifications

HDPE Geomembrane specifications

Geomembranes can be manufactured from four main polyolefins: three from the polyethylene family, namely high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and very low-density polyethylene (VLDPE), and polypropylene (PP). Lower densities mean more flexible materials and increased weldability of the product, offset by reduced chemical resistance and mechanical strength, increased gas permeability and, in some cases, such as LLDPE, a 30% reduction in UV resistance.

In the specific case of PP, this material can provide the same flexibility as VLDPE but with significant chemical resistance, meaning it can be classified as a sort of hybrid material for special applications.
HDPE Geomembrane specifications are determined by a series of parameters that describe both the type of raw material used and the quality of the geomembrane in terms of durability (UV resistance, chemical resistance to stress cracking and temperature). They also give an indication of the quality of the geomembrane transformation process and hence the manufacturer itself.

HDPE Geomembrane sheet

Atarfil’s technical HDPE Geomembrane sheet group the properties of each of these concepts to make them easier to understand.

The following description gives a non-exhaustive summary of the meaning of certain properties to aid understanding and help users determine evaluation criteria. Please do not hesitate to contact Atarfil Technical Services for further information about the concepts that are displayed in the HDPE Geomembrane sheet below.

 


    • Regarding one of the most important geomembrane liner specifications, the density of the geomembrane is given by adding the density of the resin and the weighted density of the percentage of the incorporated carbon black. International regulations set a limit of >0.94 gr/cm3 for classification as HDPE, although this figure can be misleading, since it is technically possible to obtain an HDPE geomembrane from an LLDPE resin with sufficient carbon black. Only values >0.92 g/cm3 guarantee the geomembrane has been manufactured from HDPE resins.
      The Melt Flow Index (MFI) is another important parameter. This value describes the flow of the resin in its transformation, making it possible to distinguish between HDPE and LLDPE. Generally speaking, an MFI 190/5 kg < 1.5 will always correspond to HDPE, whereas above this figure, we know we are dealing with LLDPE. This distinction is important because LLDPE has significantly lower chemical resistance, gas impermeability and UV resistance (30% lower) than HDPE, since the latter is crystalline (>45%).

      • Continuing with the most relevant geomembrane liner specifications, the antioxidant does not affect the final mix, largely because it protects the product inside the machine, where it is subject to high pressures and temperatures in an anaerobic environment without oxygen, before gradually absorbing the atmospheric oxygen as it emerges from the head.

    There will be a trace of antioxidant in the final product, which can be evaluated using a series of tests: Oxidative Induction Time (OIT) at high and low pressure, oven ageing, lamp ageing and oxidation.
    The resin quality is partially covered above although the NCTL stress cracking test is much more useful.

    This test evaluates the strength or toughness of the HDPE molecule to crack propagation (the crystallinity of HDPE is >45%). A crack in a secure waste containment product means a leak and this cannot be allowed to occur during the mineralisation or deactivation period for the waste. However, at the outset we do not know the exact length of this period. The environmental and geographic conditions at different waste sites and secure storage, together with the physical and chemical processes of the waste inside create a wide range of conditions, such as chemical and physical attacks, temperature oscillations and the possible presence of gases and chemicals in the land, including the intrinsic weaknesses of the biannual installation process when the product is subject to the thermal changes caused by welding. Geomembranes are security products and given the uncertainty surrounding the processes to which they may be subject, at the Atarfil Group we believe this should provide the highest level of security possible based on the available technology. The most modern HDPE resins can reach NCTL strengths > 3,000 hours and Atarfil is committed to this standard, even though the minimum value in many international specifications is just 500 hours. We believe this value should be taken as the absolute minimum.
    When it comes to developing more competitive products, taking this commitment into account should be the real objective for all parties involved in order to create a more sustainable world when it comes to waste management. We take responsibility for this task.


      • Of all these HDPE Geomembrane liner specifications, the most important is elongation, both elastic and at break. Calculating elastic elongation requires specialist test operators and yields a narrow range for both textured and smooth products.

    In contrast, elongation at break has a wider range of values and permits a more straightforward explanation, since it can be used to evaluate the quality of the process for transforming the geomembrane.

    Let us first concentrate on HDPE. If we could obtain a sample of a transformed geomembrane in a laboratory machine under optimal ambient conditions of humidity and temperature, the sample would allow us to determine the maximum elongation of the material, which is intrinsic to the material itself and cannot be exceeded. This optimum elongation value is approximately 1100% for HDPE.
    In the large-scale manufacturer of geomembranes, the ambient conditions at the manufacturer’s plant, in addition to their choice of heat parameters and the configuration of the machine used for the transformation process reduce value of this parameter. This means that the further we are from the optimum value, the lower the quality of the transformation process.

    International regulations set an acceptable limit of >700% for geomembrane products. If the figure is below this value, the transformation is low quality, while the nearer we get to the optimum value of 1100%, the higher the quality of the manufacturer’s transformation process.
    All this is based on a given resin quality, which can be very high.