Tag: liner

Geomembrane Composition and Antioxidants

The HDPE geomembrane contains antioxidant packages to protect the liner from oxidation during the production of the membrane and during the field life of the membrane. HDPE formulation consists of weight percentages 96-97.5% polyethylene (PE) Resin, 2-3% carbon black and up to 0.5% antioxidants. Carbon black is added to the geomembrane to limit potential damage by UV radiation. The amount of carbon black added is generally less than 3%. Amounts greater than that required to reduce opacity are wasted. The specific antioxidant package used in the HDPE formulation is dependent on the manufacturer of the HDPE. Antioxidants that are most widely used are indicated in Figure 8 Antioxidant Types (Koerner, 1998). Antioxidants are divided into two types; Primary, which limits oxidation during the geomembrane manufacturing process, and Secondary, which provide long-term protection during the service life of the material.

The chemical ageing of a geomembrane follows three stages:

  1. A period where the antioxidant is depleted.
  2. An induction period where the polymer degrades following the depletion of the antioxidant.
  3. Time to reach 50% degradation of a particular property (Half-Life)

The depletion of antioxidants is the result of the intrusion of oxygen into the structure of the geomembrane.

The rate of depletion of antioxidants is a function of among other things; the service temperature of the geomembrane and the presence of oxidizing liquid on one or both surfaces of the geomembrane. The rate of extraction of the antioxidant is controlled by the dissolution of the antioxidants at the surface of the geomembrane and the rate of diffusion from the interior to the surface of the geomembrane. It has been found that the consumption of antioxidants is three times faster in water than air (Smith, 1992).

Testing has been developed to measure the level of remaining antioxidants in a sample of geomembrane and thus provide an indication of the remaining service life of the HDPE. Two tests are:

OIT (Oxidative Induction Time). Samples are heated up under a nitrogen atmosphere, typically to 200C. Oxygen is then introduced to the sample cell, and the length of time before the onset of degradation, as seen by the initiation of an exothermic process in the DSC trace is measured

HPOIT (High Pressure Oxidative Induction Time). Similar to OIT testing but is carried out at lower temperature, 150C and higher pressure. The procedure was developed because heating to 200C can cause the decay of some types of antioxidants (HALS or Hindered Amine Light Stabilisers)and provide a false (low) indication of the level of antioxidant remaining in the formulation.

What Kind of Dam Liners We Should Use for a Pond

We are often asked what kind of liner we should use for a pond. The answer is not simple. There are many variables:

  • What is being stored? What is the PH of the liquid being stored? This will determine the type of polymer that is to be used if indeed a polymer is suitable at all. Liner manufacturers produce chemical resistance charts – these should be consulted.
  • Once the chemical compatibility is determined then we can decide what thickness of the liner is to be used. The thickness will be determined by the condition of the subgrade (How many rocks and what size and shape) and what life is required from the liner. The thicker the liner the more resistance there is to subgrade conditions and the longer the life in the sun. Resistance to rocks in the subgrade can be increased by the use of geotextile under the liner. It has been shown that a geotextile will act compositely with an HDPE liner to give better performance. HDPE is provided with antioxidants and UV stabilisers in its composition. The antioxidants are provided to give protection against oxidation during the manufacturing process and long term. The UV stabiliser is usually carbon black.
  • What colour liner. White liners expand less in the sun than black liners and produce smaller ripples when heated up. If the pond is an evapouration pond and is to be empty some or most of the time then the use of a white liner should be considered.
  • The batter slope and length will also have an effect on the type of liner. Where the batter length is long then the use of a rough-textured liner should be considered to give added frictional restraint to the sub-grade.

Liner type and thickness and configuration is a science on its own and has been the subject of extensive research. When making the call on a liner type consult someone with a background in the subject.