HDPE in Liner Specifications: Material First, Application Second
HDPE is the material named most often in liner enquiries. It is a capable membrane and often the right choice, but it is usually requested before the application has been defined. This is the most common reason a quote slows down. The details that settle the choice, such as thickness, grade and standard, then have to be requested before the quote can be prepared.
Specify the Outcome, Not the Material
A liner specification is most useful when it describes what the liner has to achieve rather than naming the material upfront. With that information, the supplier can propose the appropriate membrane and explain the reasoning behind the selection. A performance-based specification leads to a better-matched liner and a faster quote.
The information that allows a specification to work this way includes:
- What is being contained, and how aggressive it is to the membrane
- Whether the subgrade is expected to settle, evenly or unevenly
- How steep the batters are
- Whether the liner is exposed to UV or will be covered
- Whether it has to meet a potable water standard such as AS/NZS 4020
Thickness Does Not Predict Performance
Material type and thickness on their own say little about how a liner will perform in service. In an EPA puncture test programme, a 0.5 mm PVC membrane matched the puncture resistance of a 1.5 mm HDPE membrane, three times its thickness. The membrane type, its formulation and its suitability for the application carry more weight than the thickness figure alone.
HDPE is inflexible and highly crystalline. It yields at around 12% elongation, can be affected by environmental stress cracking, and requires a well-compacted, smooth and flat subgrade because of its rigidity and the thermal movement it undergoes in service. Flexible membranes stretch considerably further before yielding, are far less susceptible to stress cracking, and conform to uneven ground and subgrade settlement. Specifying HDPE over ground that is expected to move may cause serviceability problems a few years into the installation’s life.
Reading the Spec Sheet
For those working from a manufacturer’s data sheet, the following properties are worth understanding before making a material comparison:
- Thickness: the figure to check is the minimum average, not the nominal. The two are not the same, and the minimum average is the value that governs performance.
- Grade and standard: for HDPE the usual reference is GRI GM13, the accepted minimum world standard for HDPE geomembranes. Many manufacturers exceed it, and the degree to which they do varies.
- Environmental stress crack resistance (ASTM D5397): values range from 500 hours, which is the standard minimum requirement, up to 3,000 hours. This is an important consideration for exposed geomembranes subject to prolonged and aggressive swings in ambient temperature.
- Oxidative induction time: an indicator of the quantity of stabilising chemicals added to the polymer, representing its antioxidant protection. This governs durability under sustained heat and UV exposure.
- Carbon black content: typically, 2 to 3% for HDPE, and the primary source of its UV protection.
- AS/NZS 4020: a specific certification required where the liner is in contact with drinking water. This standard applies to the finished product, not the raw material alone.
What the Specification Needs to Include
A specification that describes the application, the subgrade conditions, the UV exposure and the standard the liner is required to meet gives the supplier the information needed to recommend a material and explain the choice. Where only the material is provided, the remaining details have to be sought before the assessment can begin, and the quotation waits on the answer.