Technical difficulties

Look in the bin of your average committed recycler and, odds are, the bulk of material will be mixed plastics. Getting these out of the bin would do wonders for recycling rates, but there are a few technical difficulties that must be addressed first. Will Simpson learns more

It remains one of the missing pieces in the UK recycling jigsaw, a fiendishly difficult nut that must be cracked if we are ever to become truly sustainable. Mixed plastics – the pots, tubs, trays (PTTs) and assorted films that carry a large proportion of our food – have always been problematic. With 35 per cent of local councils currently collecting them, and infrastructure lagging behind even this figure, clearly an effective collection and reprocessing system is still some way off. The major barrier to developing that infrastructure has traditionally been the sheer range of different polymer types and the associated technical difficulties in sorting them all.

Although WRAP has recently changed its tune in advising councils to think carefully before launching mixed plastics collections, it is still making optimistic noises on the topic. At the Resource Recovery Forum’s final ever conference this summer (‘Mixed Plastics: What’s Left To Sort?’), WRAP Programme Manager Gareth Hollinshead suggested in an upbeat presentation that some of these technical barriers might be on the verge of finally being overcome.

One of these is the sorting of food-grade polypropylene (PP), which accounts for some 40 per cent of non-bottle rigid plastic waste. There is a requirement for recycled food contact plastics that says 99 per cent of the pack has to have been previously used with food. Plastic bottles are fairly easy to sort for this manually, but for mixed plastics, with their huge array of sizes, shapes and colours, sorting has always been much more difficult.

WRAP has conducted research into a new technology called ‘laser diffraction’ that differentiates food-grade from
non-food-grade PP. This involves putting a number of tiny grated lines on a pack that is used with food. When these are illuminated by a laser, they create interference patterns that can then be picked up by a camera. In June, WRAP revealed in a report into the new technology that it worked in the lab scale tests.

Laser diffraction wasn’t the only technique that was tested. Researchers also looked at a system where fluorescent markers are put in a label lacquer with an optical sorter picking up a signal from the lacquers when they fluoresce. “We did some trials but it wasn’t as successful as we were hoping”, says Hollinshead. “We didn’t get a strong enough signal back from the markers. We still think it has potential, maybe it needs a bit more investigation. So we’re taking the laser diffraction work forward at the moment.” 

Yet much needs to be done before laser diffraction can be used on a large scale. As Hollinshead himself admits, cost remains a major unknown. “We haven’t investigated that aspect in any great detail yet. We’re also going to need plastics recyclers to want to invest in sorting equipment and we’ll need sorting equipment manufacturers to want to invest in developing commercial sorters.”

“The next phase of work is looking at the economic viability of it in terms of the ballpark costs for sorting systems and how fast the sorting speeds are. Speed is a key factor. It needs to be roughly the same as current optical sorting – if it’s too slow you won’t be able to sort the tonnage.” Hollinshead estimates that even if the next stage of WRAP’s work is successful, widespread diffraction grating still remains “two to three years away”.

"Some of the technical barriers to sorting mixed plastics might be close to being overcome"

Just as problematic as food-grade PP has been the sorting of black plastics. Manufactures prefer using them because they denote ‘quality’, hide food residues and make the colours of the food stand out. Perhaps most crucially, the most commonly used pigment – carbon black – is also cheaper than any of its main alternatives. The problem is that the near-infrared sensors used by optical sorters are unable to pick out carbon black, the result being that up to 60,000 tonnes worth of black plastic end up in landfill every year in the UK.

Other methods, such as mid-infrared sorters, have been tried but are not able to sort on the scale that would make it commercially viable at present. But in a WRAP report issued last September (‘Development of NIR Detectable Black Plastic Packaging’), it was revealed that there are a number of alternative black pigments that can be picked up by optical sorters under test conditions.

In terms of performance, the report gave a qualified thumbs up to these alternative pigments. “Tinting-wise, they’re not as strong as carbon black, which, because it consists of very, very fine particles, has a very high covering power”, explains Edward Kosior, Managing Director of Nextek, the plastics consultation firm that prepared the report. “So you need to use slightly more because of that difference in tint strength. But generally they work very well, give very pleasing colouration and everyone seems to be happy and accept that these things are suitable for making black trays.

“In the report, we outlined that we thought the impact on trade would be small. It may increase the price of trade slightly, but because pigment is always used in small quantities overall it won’t have a significant impact on net trade.”

However some industry stakeholders have raised doubts whether manufacturers will be keen to take up any innovation that pushes up costs even slightly. Jonathan Short, Managing Director of ECO Plastics is scathing about the idea: “Are you telling me, in these financially straightened times, that the retailers are going to pay more for a plastic tray because it’s got a tracer in which is going to help recycling? I don’t believe that for one moment.

"There are better ways of spending WRAP’s money in helping plastics recycling than trying to put some kind of tracer into black plastic trays"

“When things are buoyant, everyone’s making loads of money, the retailers want costs down. So, today, they’re even more cost-down orientated than they were before, so don’t be sold on that one by the retailers because it’s not going to happen. There are better ways of spending WRAP’s money in helping plastics recycling than trying to put some kind of tracer into black plastic trays.” Short also cautions that diverting black PET trays into the mixed-colour PET stream could damage the current end-market for this fraction; at the moment, ECO Plastics, for instance, sends its mixed-colour PET to the strapping industry, which Short notes has very stringent requirements.

Cost also remains an issue in that prices in general have been falling substantially lately, and mixed plastics are worth substantially less than bottle-grade plastics. Short, who reluctantly accepts that mixed plastics will find their way into the bales of bottle plastics he’s after, explains: “If you look at non-bottle plastics today, and you look at the weighted values of those – so the polypropylene, the butter tubs, the polystyrene yogurt tubs, the PVC trays, the black plastic trays – you put all those together as a weighted basket and the value would be virtually zero.”

And so, it remains a matter of some debate whether these two advances in sorting represent a breakthrough in plastics recycling. Hollinshead is guarded about their future viability. “It’s still a work in progress”, he admits. “But certainly with food-grade polypropylene there is massive potential. There are about 180,000 tonnes a year of it in the waste stream and a lot of that is used in food packaging, so there is a big demand for polypropylene in new food packaging, if only it were possible. I think brands and retailers will consider it a huge breakthrough if we can develop a process that enables them to put recycled PP back into food packaging.”

But he knows that equally as important is developing the mixed plastics recycling infrastructure. “Encouraging collection by local authorities and increasing the recycling capacity is crucial. And developing the end markets too – trying to get as much recycled plastic back into new packaging of products that would otherwise have used virgin plastics. Really, we need to look across the whole supply chain. How important are these technical innovations? You probably need to come back to us in two to three years and ask that question again.”