Mixed plastics under the lens - recycling PET
The effects of recycling PET in the presence of polylactic acid (PLA) has come under scrutiny in the last few years. A UK collaborative project by the Materials Knowledge Transfer Network (KTN) and Smithers Rapra, UK, aims to uncover the facts.
The focus of the work is to investigate claims that a small amount of PLA entering a recycling stream can have a negative impact on the physical properties of extruded rPET, such as on molecular weight, making the material unfit for use.
To test the process, PET from Closed Loop Recycling, Dagenham, UK, was dried at 130ºC and blended with extrusion grade PLA, and dried at 80ºC, in a twin-screw extruder. This process was used to observe the viscosity changes of PLA contamination, and to assess whether changes are physical or chemical. The processed blends were then collected by stranding through a water bath and pelletising.
Jan Czerski of the Materials KTN, UK, explains, ‘The reason why the subject of contamination of the PET recycling stream by PLA has invariably been taken further was because there was little evidence of tests being done to indicate the degree of change when PLA was present. It seemed to be that comments had been made on the basis of assumptions, rather than fact. This report, which contains PLA samples from Smithers Rapra, did in fact show that there were effects [in PET].’
In the trials, twin-screw compounding results revealed that when one per cent of PLA was introduced to 99% PET, colour changes are visible. The viscosity changes in PET have been measured at different ratios of PET to PLA.
The reduction in viscosity was visible at 97% PET and three per cent PLA, where viscosity was reduced to 82%. At 95% PET and five per cent PLA, viscosity fell to 55%.
However, Czerski warns, ‘At the moment there is still very little PLA in bottles and the amount of PLA entering the recycling stream. Consequently, the percentages included in these [tests] are extreme to some extent. But, in the future, their effect is likely to increase.’
Czerski is also keen to highlight that the work is still at early stages and while it does provide a basis for future research, it is still only ‘an initial study’.
The next phase will look at the study of melt strength and blow ability for making bottles, physical strength and clarity.
‘We are waiting for reaction and talking to various biocentres in New York and PET recyclers. What we are trying to do is illustrate the support the KTN can give to various industries. We wanted to set the grounds for research,’ he says.
The project is now open for organisations and KTN members to take part in the next stages. Potential collaborators can contact the KTN and view the full report, Assessment of the Rheology of PET Containing Various Amounts of PLA, at https://ktn.innovateuk.org/web/polymers
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