Polyethylene is one of the most widely used plastics, with applications in packaging, consumer goods, and coatings, to name just a few. According to The Plastics Industry Trade Association, the use of polyethylene took off after the second world war when a variety of high and low density versions were developed. Large-scale production of these materials reduced their cost dramatically, enabling them to compete with the older plastics and with the more traditional materials such as wood, paper, metal, glass, and leather. The introduction of alloys and polymer blends made it possible to tailor properties to fit certain performance requirements that a single resin could not provide.
Polyethylene starts with naphtha, or petroleum, which is extracted from crude oil and heated to release ethylene, which forms branch-like structures to become polyethylene. Polyethylene exists in many different branch structures. As explained on Plastics Europe, the Association of Plastics Manufacturers web site, different characteristics such as stiffness or elasticity can be imparted to the polyethylene during production, depending on the density of the material and its liquidity in melted form. The density and liquidity also largely depend on the amount of pressure applied during production. Producing polyethylene at low pressure forms straight, robust and tightly packed branches. The result is dense polyethylene with a firm and stiff structure. Manufacturing polyethylene at high pressure causes the particles form a crisscross of branches and side branches, resulting in a lighter, more elastic material.
Whether polyethylene has a liquid character or not depends on its melting index, meaning how slowly or quickly the melted mass flows through a gap. Polyethylene melts are usually characterized rheologically in small amplitude oscillatory shear (SAOS) as this mode of deformation can be obtained easily on a rotational rheometer. However, most technical processes such as blow molding are dominated by extensional deformation that interfere uni- or multiaxially with the shear flow field. Thus extensional deformation is needed together with SAOS and steady shear to obtain a complete picture of a sample’s rheological behavior.
One of the main goals of extensional testing is to probe for the strain hardening behavior, or the increase of the extensional viscosity of a certain molecular architecture independent of the strain rate. This strain hardening is mainly dominated by long-chain branches; more branches lead to a more pronounced viscosity increase. However, strain hardening does not depend solely on branching but also on molecular weight and its distribution. The impact of high molecular weight fractions can be monitored at low deformation rates in contrast to the branching influence.
To characterize the extensional behavior of polymer melts and viscoelastic solids, extensional and rotational rheometers can be used together. The testing principle is based on stretching the sample on two counter-rotating drums. As both drums rotate with the same speed in inverse directions, the deformation field applied is a purely uniaxial one. Read the application note, Characterizing Long-chain Branching in Polyethylene with Extensional Rheology to see how the characteristics seen in extensional tests can be used to model certain processing steps such as blow-molding or film production, and for quality control applications.