PET and PEF

Recycling of barrier materials in the PET stream

The trend of moving towards packaging solutions that fit the circular model has become so apparent during the last years, it hardly needs any introduction. At the same time, there has been a strong increase in the demand for barrier packaging solutions. Barrier layers and -additives can be very effective in improving CO2 or O2 barrier for rigid or flexible PET packaging. As such, these developments can result in dramatic weight reduction; the foremost important driver in a sustainable circular approach. However, the barrier materials that are currently commercially available can encumber or even obstruct recycling of such packages and us such true circularity.

Poly(ethylene 2,5-furandicarboxylate) (PEF) is a bio-based polyester with a known good barrier performance /1/ /2/. As such it can compete with commercially available (passive) barrier materials such as MXD6 and EVOH. One of the drawbacks of using these existing barrier materials is that they typically have poor compatibility with PET, resulting in poorer performance of resulting resin blends. Therefore, there is a need to have proper separation of the barrier layer from the recycled PET stream, for example by flaking (with the aim to delaminate the barrier layer) and subsequent sorting. When applying a passive thin barrier layer the latter case, converters need to balance between easy delamination of the barrier layer from the PET and the performance of the multilayer article during its use. Data published on bottles made of blends with PET and PEF /3/, suggests that PEF would not necessarily need to be separated from the PET stream as it hardly influences the processing behavior of recycled PET and its performance in bottles. Additional studies have led to an interim approval by EPBP of PEF to enter the European PET bottle recycling stream at up to 2% of the total volume of that recycling stream /4/.

A recent article on the miscibility of PET and PEF /5/ indicates however that PET and PEF blends are not intrinsically miscible. It is important to mention that the blends for this study were prepared by first dissolving both polymers in a solvent and subsequently mixing these solutions (solvent mixing). This method prevents any transesterification to occur. Only during DSC measurements, the resulting blends were shortly brought in melt, then quenched and subsequently a heating ramp was applied to determine the glass transition(s) in the enthalpic data. The resulting DSC traces show that for these PET/PEF blends the time in melt (without the presence of shear) was not sufficient to allow for transesterification, leading to a blend with phase separation between PET and PEF. Apparently, the transesterification of PET and PEF requires more time, and possibly shear, to fully take place, leading to a PET/PEF co-polyester with only one glass transition as observed in the previously mentioned study on bottles made from PET/PEF blends /3/. In the latter study, the PET and PEF are dry blended and subsequently injection molded into preforms. Obviously, this latter procedure allows for more transesterification, as the residence time in melt is longer. An actual bottle-to-bottle mechanical recycling route will allow for even more transesterification as this procedure does not only include an extrusion step, but also a solid state polymerization in addition to the injection molding step. Therefore, only an injection molding step is a worst case estimate of the amount of transesterification that can take place. Here, some additional PET/PEF blend data as produced by dry blending and injection molding will be presented, supporting the statement that PEF is a bio-based barrier solution that can enhance the recyclability of (multilayer) barrier containers.

Injection molding of blends

The mixture of PET and PEF was fed predried and dry blended into a Boy 50M injection molding machine at 170°C. The resin was molded into 76x76 mm2 plaques with a nominal thickness of 1 mm and a shot weight of about 13 g. The molding parameters were kept constant, with a melt temperature of 280°C, injection time of around 0.6 s and total cycle time of 24 s. The resulting total residence time of the polymer melt in the barrel is estimated to be 5-9 minutes, while the mold was kept at 20°C. Other than the injection pressures, no significant differences were observed during molding the plaques from the different blends. In addition to the PET and PEF blends, a PET and MXD6 blend (at a ratio of 95/5 wt%) was molded. Visually, there is a clear difference between the different blends as seen in Figure 1 (top), showing in transparency the logo and caption through 4 plaques of 1 mm thickness stacked.