Current geosynthetic membranes are available in a range of polymer materials, including Butyl, CSPE, Polychloroprene, EPDM, HDPE,
LLDPE, fPP and EIAs. These are homogeneous constructions whereby the membrane is made of the same polymer throughout
A membrane’s Chemical Resistance is determined by the polymer’s molecular properties. In particular, Polarity is a key characteristic in
determining chemical resistance; it defines the material’s solubility and it follows that the higher the solubility, the higher the permeation
It is important to make the distinction between a membrane being resilient to a hydrocarbon and a membrane being effective as a barrier
to permeation. HDPE is widely regarded as chemical resistant but it is not a particularly effective barrier against hydrocarbons. HDPE is a
hydrocarbon and since “like is soluble in like”, hydrocarbons permeate relatively easily through HDPE.
Contaminated soil risk assessments commonly report on over 60 contaminant hydrocarbons. Some of the more significant are illustrated
in Fig. 2 which compares permeation rates between common homogeneous geomembrane polymer materials and a new patented barrier
membrane, Puraflex, specifically developed for resistance to hydrocarbons.
Limited chemical resistance data is generally available and material selection is often made on a ‘best fit’ basis, relying on industry
standard chemical resistance test results (discussed below) and basic chemical resistance charts which usually provide a traffic light
opinion i.e. Resistant (Green), Limited Resistance (Amber) and Unsuitable (Red). Thereafter, permeation rates can only be reduced by
increasing a membrane’s thickness; a membrane’s thickness has to be doubled to halve the permeation rate.
The term ‘Hydrocarbon Resistant’ may simply mean that a membrane has passed a chemical resistance test which has no obvious
relationship to the membrane’s ability to block a challenge hydrocarbon to a level that ensures adequate protection over time. Furthermore
the vagueness of the word “hydrocarbon” can lead to false claims that a barrier good for one hydrocarbon (e.g. methane) is automatically
good for a very different type of hydrocarbon (e.g. benzene).
Most materials offered as ‘Hydrocarbon Resistant’ are no more than HDPE membranes. Aluminium laminates are sometimes specified but
these materials, originally developed as methane gas barriers, are not recommended as a barrier to hydrocarbons or for use in moist or
acidic soil environments.
The Aluminium layer is susceptible to oxidation which negates any barrier properties. Since the aluminium layer itself is thin and
vulnerable, it has to be sandwiched in between two layers of polyethylene and reinforced to prevent any tearing by ground movements.