Figure 1
The use of polyethylene film and foil in the beverage industry has become a popular alternative to traditional packaging products, such as cardboard. This is mainly due to the strength, shrinking and expansion behavior, tear and moisture resistance, transparency, and inexpensiveness of PE. Thermal analysis is an excellent technique that can determine the shrinking behavior of PE films.
The example below shows how the Mettler Toledo’s TMA/SDTA840 is used to investigate the shrinking behavior of polyethylene (PE) film in two different directions. This yields valuable information about the direction of film movement.
Samples of porous polyethylene film were heated from 25 °C to breaking point at a heating rate of 5 K/min in a nitrogen atmosphere with a load of 2 mN. The load used was slightly higher than the minimum force to ensure the films remained stretched during the application process. Samples were taken in two different directions of the film (crosswise, TD and lengthwise, MD). To verify the results two samples were measured in each direction.
The results in Figure 1 illustrate that the sample starts shrinking as soon as it is heated in the lengthwise direction. At around 130 °C, the sample is so viscous that it can no longer withstand the force applied and therefore starts flowing, which leads to a fast expansion. In the crosswise direction, the sample slightly expands up to about 90 °C and then starts shrinking. Again, at about 130 °C, it starts flowing due to the viscosity of the sample. In both directions, the shrinkage occurs in several different steps. This can be explained by the subsequent melting of different crystals that are present in the sample. The difference in the shrinkage behavior of the two directions indicates that the film has been drawn in a lengthwise direction.
The Mettler Toledo’s TMA/SDTA840 is an excellent tool that allows the simple, rapid and reliable characterization of PE film shrinking behavior. The different shrinking/expansion velocities identify melting processes of differently sized and shaped crystals that may have formed during the production process. This yields valuable information on both the quality and suitability of the PE used for packaging bottles or cans into six packs. The TMA is well suited to this task and is therefore also an extremely valuable instrument for quality control within the beverage industry.
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