Updating pigment and polymer processes will boost your sustainability credentials, says Genevieve Pugh, Associate, Patent Attorney, Mewburn Ellis LLP.
Single-use plastic, particularly for packaging of household goods, is almost inescapable in our everyday lives. It is estimated that in the UK we throw away a staggering 1.7 billion pieces of plastic packaging each week.
Since the 2000s, the UK population have been urged to recycle it plastic waste, yet The Big Plastic Count 2024 shows that still only 17% of our plastic packaging gets recycled. One reason for this is that our recycling infrastructure struggles to cope with the vast amount of plastic thrown away including an abundance of coloured plastic. Despite being useful for providing visual appeal and helping categorising products, coloured plastic can contaminate recycling streams and causes problems with sorting.
Plastic recycling systems typically include mechanisms for sorting plastic waste by polymer-type and colour. Plastic waste is initially sorted using near-infrared (NIR) sensors which identify polymers by measuring their unique NIR absorption spectrum. Once sorted into types, optical sensors are then used to separate the plastic by colour. Clear and white plastic can be recycled for clear and white plastic production. However, coloured plastics typically can only be recycled into less versatile grey-ish or black plastic or may simply be sent to a landfill.
To avoid problems with coloured plastic recycling, some companies are switching to clear or white plastics (for example, Co-op have cleared their shelves of coloured milk bottle tops), or are switching to paper-based packaging. However, ultimately, if we are to maximise the amount of waste plastic participating in the plastic circular economy, maximising accuracy and efficiency of the processing of a broad range of plastic types is essential.
Coloured plastics, clear solutions
Pigments in plastic recycling streams make it hard to obtain recycled plastics with vibrant colours. For example, white plastics are often incorrectly sorted into clear plastic streams because optical detectors struggle to differentiate them from clear plastic. The presence of white plastic makes the recyclate murkier, significantly devaluing an otherwise clear plastic end-product.
Unfortunately, white pigments are essential components in the plastic packaging of certain products, such as UHT milk where an opaque light barrier is required to prevent photo-degradation, and so their elimination is not straightforward. Methods of removing inks from the surfaces of plastics have been developed. However, these processes do not remove the colour from the plastic itself. Hence, many companies are looking for solutions to the impact of pigmented plastic on the circular economy.
Titanium dioxide is the dominant white pigment used in plastics. In the Remove2Reclaim initiative, Chemours, Catalisti, INEOS and a handful of other collaborators have devised a sorting mechanism which identifies plastic containing TiO2, and a solvent-based extraction systems which recovers the pigment. This innovation allows both pigment and polymer to be separately recycled.
Black to the Future
Black plastic has long been a notable adversary to recycling efforts. Carbon black is by far the most popular black pigment due to its high colour strength and low cost. Unfortunately, carbon black also absorbs across the NIR spectrum, meaning characteristic peaks of a polymer type cannot be identified during sorting because no NIR wavelengths are reflected back to a detector. Hence, plastics containing carbon black are usually discarded and not recycled.
One way to tackle this problem is to switch from carbon black pigment to a NIR-detectable black pigment. Unilever, a founding member of The UK Plastics Pact, has done just this in their iconic TRESemmé® and Lynx® packaging. Many companies are now developing such pigments – for example, Heubach has 3 different detectable black pigments on the market, each one suitable for a different plastic type, and compatible with existing recycling technology. Lanxess has had its manganese ferrite black pigment Bayferrox® 303T on the market for decades. However, its potential as an NIR-detectable pigment is only now becoming apparent. BASF’s 2022 patent publication also describes multi-ring organic molecule NIR-transparent black pigments which can be used in recyclable plastic products.
Circular economy innovators SMX have also filed a range of patent applications directed to x-ray fluorescence (XRF) markers. These can be used as invisible signatures to label objects with information about their make-up, date of origin and place of origin etc. This technology can be applied to black plastic, allowing its polymer type to be detected so that it can be correctly sorted during recycling.
Another approach by Canon is to adopt sensors in recycling plants which use Raman spectroscopy instead of NIR. Raman spectroscopy is a slower technique, which previously hasn’t been quick enough to analyse plastic on a passing conveyor. Canon now have a system which uses recognition software to track plastic fragments as they move along a conveyor, providing enough time to obtain a Raman spectrum and sort by polymer type, whether carbon black is present or not. Although this requires adaptation of the current infrastructure, no modifications to the packaging itself is needed.
Looking forward
Innovation is broadening the types of plastic products we can recycle. As well as developing plastic alternatives, it is crucial that plastic already on the market can be sorted and recycled if we are to have an effective plastic circular economy.
