Colloquia

Summary

Mono-material multi-layer flexible films are promising plastic packaging materials well-suited for recycling. However, processing mono-material films is challenging due to the narrow sealing window when using the same polymer for the different functional layers in flexible packaging. Therefore, new polymer grades need to be developed with the required thermal and physical properties to expand the sealing window. To accelerate the development of these materials, Borealis AG, a manufacturer of polyolefin packaging films, is researching new methods to predict the sealing properties of materials at an early stage of the development process.

This thesis proposes a rapid test method to predict the sealing window of polymer films using fast-scanning differential scanning calorimetry (DSC). Seal strength development relies on the polymer chains’ ability to become mobile and move across the interface over a range of temperatures. In this work, fast-scanning DSC was applied to study the crystallisation behaviour of polymer films during a thermal profile comparable to the heating and cooling response encountered during the production and sealing of the packaging. The ability of the polymer to re-crystallize also depends on the polymer chain mobility and was indirectly used to estimate the sealing window. The method is suggested to be used as a first selection tool to pick promising sealants out of a set of newly developed materials. This will require only a few nanograms of each material, whereas conventional seal testing methods require significantly larger sample batches.

The proposed test method was verified using two different grades of bimodal LLDPE that strongly differed in sealing properties. For both materials, melting and rapid recrystallisation of the low melt phase were observed during the heating and isothermal step, which resembled the thermal profile of sealing the polymer film. The range during which the recrystallisation occurred matched well with the sealing window of both materials. Therefore, the low melt phase was shown to be essential for the development of seal strength. Once the high melting phase became mobile, which was close to the melting point of the polymer, the sealing window ended due to compromised structural integrity. The proposed test method showed to be a promising rapid characterization method to determine the sealing window of LLDPE films. Further research should focus on the applicability of the method on different polymers.