Growing consumer demand and legislative requirements drives innovation in recycled plastics.
Yet recycling rates are too low and the quality of recycled materials are generally lower than virgin. How can we improve this situation?
FuturePack was a large Norwegian R&D project which ended in 2021. It was supported by the Norwegian Research Council and led by Norner Research with the aim to evaluate and develop new technologies for renewable plastics and improved recycling of plastics packaging. Some key studies were carried out on how recycling can be tuned to give better quality and value which we will discuss here.
One of the key findings of our research was how antioxidant is consumed during recycling, resulting in poorer quality plastic materials. Adding new stabilizers can play an important part in increasing the quality of recycled plastics. Another interesting study of the research project was related to the odour identification and reduction strategies for recycled plastics.
Improved quality by antioxidants
Antioxidants are crucial to avoid degradation of the polymer in recycling. Figure 1 shows three scenarios for how the MFR (viscosity) increases and mechanical properties are altered with repeated melt-extrusion. This happens due to chain scission of the PP polymer chains. The brown line/bars represent a case where we did not add any antioxidant and the MFR increased dramatically while the mechanical properties were reduced. In the case represented by the blue curve we added antioxidant at the fifth extrusion which shows that the degradation was stopped or reduced. But, even better, if antioxidants are added from start (1000 ppm B215), and at every recycling step, as in the green curve/bars, the degradation can be avoided (figure 1a) and the mechanical properties is kept (Figure 1b). This shows clearly the impact and importance of continuous stabilisation to maintain the properties and performance.
Figure 1 a) MFR increase of PP without extra stabilization, and the effect of adding extra antioxidants added after 5 extrusions and added from start and at every recycling step. b) Effect of mechanical properties after 5 times extrusion without extra stabilization and mechanical properties after 4 “recycling steps” with continuous re-stabilization (1000 ppm B215) from start and at every recycling step, i.e., every second extrusion.
Improved quality by de-volatilisation
Recycled plastics are normally more or less contaminated and this can cause smell of different nature and intensity. This is a main challenge and it is crucial to be able to minimise. The FuturePack project studied various techniques and principles for odour reduction. Additionally, we developed analytical methodology and strategies for the assessment of the smell.
In a case study we used hot washed flakes of HDPE from the Norwegian recycler IVAR, produced from household waste. These were sent to EREMA for recycling in their pilot centre where we could test the efficiency of the EREMA Refresher technology for odour reduction.
The principle of the EREMA Refresher and its place in the value chain is shown in the figure 2. This process relies on hot air flushing of pellets to remove any high and low volatile substances including odour components. The pellets are already hot after the extrusion process and are maintained at a temperature of at least 60° C and flushed with hot air for an extended period. The intention is that this will reduce the odour due to de-volatilisation of the pellets.
Figure 2: The Refresher is the last process step in above description where pellets are treated with hot air for a fixed amount of time.
The flakes were extruded at EREMA and Refreshed for 0, 3, 5, 7 and 22 hours. The resulting samples were further studied by Norner. First, they were evaluated by an odour panel. The odour intensity results after the EREMA refresher treatment of the HDPE PCR are shown in Figure 3. This demonstrates a clear reduction of the odours versus increased time and treatment in the EREMA Refresher. Secondly the samples were analysed by GC-FID which gives a quantitative measurement of the total content of volatile components which is strongly correlated to the odour reduction.
Also, a third analysis strategy has been developed which use a GC-O-MS. This means that the components are identified as being with/without smell by an operator sniffing and smelling one part of the gas flow before the detector is identifying which substance it is. In this way we can evaluate the odour by number of substances and their possible risk by the identification. We could see that the number of odour components were drastically reduced 90% from 54 to 5 with 22h time in the refresher, but even after 3h the number was reduced with 50%.
We have also compared the results with a virgin HDPE reference. Please notice that the content of volatiles (VOC) in this virgin reference is significantly higher even if the odour contribution is clearly lower in the panel. This shows that analysis of total volatile content is not the best way to assess the odour quality.
Figure 3a+b): Norner forced ranking by odour panel plus GC-FID and GC-O results of HDPE PCR from household processed at EREMA Refresher for 0, 3, 5, 7 and 22 hours
The two cases clearly shows that the re-additivation and de-volatilisation are two important contributors to the quality and value of recycled plastics.
The FuturePack project had following Norwegian industry partners; Bama, BEWi, Elopak, Grønt Punkt, Mills, Norgesgruppen, Nortura, Orkla, ROAF and Tine, representing various parts of the value chain for food production, packaging and recycling. The participating R&D institutes were Norner Research, Nofima, RISE PFI, Norsus and NTNU IKP.