PROPERTIES OF POLYETHYLENES IN GEOMEMBRANES

Polyethylene is by far the most widely used polymer to manufacture geomembranes. Polyethylene resins are manufactured in very-low-density, low-density, linear low density, medium-density and high-density varieties. The density range for all polyethylene geomembrane polymers falls within the general limits of 0.85 to 0.960 g/cm3.Molecular weight, molecular weight distribution and crystallinity (i.e. density) are the three most important characteristics of polyethylene resins and play a major role in determining the durability and end-use performance properties of HDPE and LLDPE geomembranes. Polyethylene is classified into several categories based on its density and branching. HDPE has little branching, giving it stronger intermolecular forces and higher tensile strength than lower density polyethylene, thereby making it ideal for geomembrane applications. HDPE is defined as having a density of equal to or greater than 0.941 g/cc.The density of polyethylene is primarily controlled by the frequency and length of the side branches (which in turn are determined by the type and level of comonomer). The side branches prevent the PE chains from packing closely together, so the longer the side branches, the more open the structure and hence the lower the density. Homopolymer HDPE has a density greater than 0.960 g/cc while copolymers have densities less than 0.960 g/cc.

Typical comonomers are butene, hexene and octene which are carbon chains with 4,6 and 8 carbons respectively. These comonomers are denoted as C4, C6 and C8 for simplicity. They all have a reactive double bond at the end of the chain and are referred to as alpha olefins. The ‘olefin’ indicates they contain a C=C bond in their structure while alpha indicates the double bond is between the first and second carbon atoms.The type of comonomer used determines the end-use performance characteristics ofthe resin. Hexene and octene copolymers are tougher and more flexible; however butene copolymers are typically less expensive. It is important to emphasize that HDPE geomembranes are actually manufactured using a polyethylene resin with a density 0.932–0.940 g/cm3 which falls into the MDPE category as defined in ASTM D-883. It is the addition of carbon black that pushes the final density of the geomembrane up into the density range between 0.941 and 0.950 g/cm3which corresponds to a HDPE as defined in ASTM D-883. For this reason the ‘HDPE’ nomenclature is used to describe most black polyethylene geomembranes.

HDPE is the most common field-fabricated geomembrane material primarily due to its low material cost, broad chemical resistance and excellent mechanical properties. MDPE is a substantially linear polymer, with high levels of short-chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g.1-butene, 1-hexene and 1-octene). LLDPE is a substantially linear polymer, with significant numbers of short branches,commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g.1-butene, 1-hexene and 1-octene). As its name implies, Linear Low Density Polyethylene is a lower density polymer (<0.939 g/cm3), with increased material flexibility. LLDPE is mainly used for liners where large settlements are anticipated for long term consolidation, such as for landfill covers. Capping contaminants with LLDPE geomembranes, not only makes it possible to control the release of carbon dioxide and methane (by-products oft he decomposition of organic matter), but allows their capture and reuse. The flexibility of the LLDPE is also useful for geomembrane liners that are installed on subgrades prone to differential settlement. LLDPE has a higher tensile strength and higher impact and puncture resistance than LDPE. It is very flexible and elongates under stress. It can be used to make thinner sheets, with better environmental stress cracking resistance. It has good resistance to chemicals and to ultraviolet radiation (if properly stabilized). However it is not as easy to process as LDPE, has lower gloss and a narrower operating range for heat sealing. Hence it finds application in plastic sheets (where it permits use of lower thickness profile than comparable LDPE), coverings of cables, geomembranes and flexible tubing.

LLDPE geomembranes are available in a smooth, textured or single textured finish.The comonomers used to produce the resin can include hexene or octene. Low-density polyethylene (LDPE) has very poor environmental stress crack resistance and rather poor mechanical properties and so it does not find application as a geomembrane. LDPE has a high degree of short- and long-chain branching, which means that the chains do not pack to form a dense crystal structure as well. It has therefore less strong intermolecular forces, as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility.VLDPE is most commonly produced using metallocene catalysts and is a highly flexibleand ductile material.