Unexpected obstacles in mechanical recycling of polypropylene labels: Are ambitious recycling targets achievable?

Abstract

Recycling initiatives like the ‘Green Deal’, aspire to achieve complete recyclability for packaging materials and previously overlooked recycling fractions, but they might face practical challenges. We investigated the mechanical, processing, and toxicological properties of polypropylene labels during mechanical recycling to gain insight into dealing with as yet unutilized recycling fractions. Possible challenges during recycling include label pigments shifting infrared spectra bands and promoting β-spherulite nucleation, which can cause homopolymer melting peaks to resemble copolymers or multilayered structures. Label blends also do not conform to the straightforward rules of linear mixing. This means that predicting mold and machine parameters for blends of virgin and recycled material becomes challenging. The elongation at break of these blends is also influenced by the presence of labels, with noticeable deterioration effects even at label concentrations as low as 20 wt.%. Moreover, labels contain substances classified as Cramer Class III, with DNA-reactive mutagenicity responses present in processed and unprocessed labels. Despite labels ultimately being recyclable and potentially repurposable for extrusion or injection molding applications, their complex processing requirements, safety concerns, and limited economic viability may make them more valuable indirectly, on a systemic level, providing useful information for consumers including on the disposal and recyclability of packaging.