Molecular weight in geomembrane

Molecular weight (Mw) is basically the length of the polymer chains. Polymer chains are very long (made up of thousands of carbon atoms linked in series) and hence are

also referred to as macromolecules. In general terms, as the polymer molecular weight increases, the geomembrane strength increases. The molecular weight of the polymer can affect physical properties such as the tensile strength and modulus, impact strength, puncture resistance, flexibility and heat resistance as well as its long-term durability properties.

It is difficult to measure the molecular weight directly so generally a simpler way of expressing molecular weight is by the melt index (MI) (also referred to as melt flow index (MFI) or melt flow rate (MFR)). The melt index is inversely proportional to the polymer’s molecular weight. For example, a low melt index value indicates higher molecular weight and stiffer melt flow behavior (i.e. higher melt viscosity) while a high melt index value indicates a lower molecular weight and easier melt flow (i.e. low melt viscosity) (Scheirs, 2000). Note: MFI is not applicable to PVC polymers.

Table shows the effect of molecular weight and melt index on polymer properties. HDPE geomembrane resins are generally high MW resins and therefore have low melt flow index values (see Figure ). For this reason they are referred to as ‘fractional melt’ and ‘HLMI’ (high load melt index) resins. The term ‘HLMI’ HDPE refers to those polyethylene resins that should really be called High Molecular Weight resins with an HLMI of less than 15 g/10 min using ASTM D1238, Condition F (21.6 kg load).

The relationship between polymer molecular weight and melt index is summarized in Table. In addition to the length of the polymer chains (i.e. the molecular weight) the mechanical and physical properties of the plastics are also influenced by the bonds within and between chains, chain branching and the degree of crystallinity.