Less than a decade ago, Britons disposed of all plastic bottles by simply popping them in the bin with the household waste; as recently as 2001, we landfilled a whopping 97 per cent of the things. In 2007, by remarkable contrast, the UK collected 35 per cent of the waste stream, saving roughly 182,000 tonnes of plastics – 4,525 million bottles – from an unseemly end, rotting slowly in a tip.
So, we’ve come a long way and can expect to keep improving, but collecting

the plastic is only part of the battle – it must also be reprocessed, and to do that, it must be sorted. And if the blip in the recyclables export market showed us anything, surely it’s that there’s room for expansion in the domestic sorting and reprocessing market.Optical sorting technology to efficiently separate plastics has existed since the 1990s, though, and its use is on the increase in the UK. Specialist plastic facilities such as the Closed Loop Recycling plant in Dagenham and Jayplas’

bottle sorting site in South Normanton, Derbyshire, use optical sorters to sort plastics by type and colour; Closed Loop recycles HDPE and PET bottles back into food-grade plastic, and the Jayplas plant reprocesses HDPE, PET, polypropylene (PP), polystyrene (PS) and PVC waste.Many sensor technologies are able to identify plastics to some extent. Visual spectrometry, for example, is useful for classifying colours, and just a couple of short decades ago, X-ray transmission was often used to separate newcomer PET from the ubiquitous high-value plastic of the day, PVC. X-ray technology differentiates objects based on atomic density, and could sort PET from PVC because of the chlorine atoms in the latter. As most plastics are now made from fossil fuels and thus are mainly hydrocarbons with similar densities, costly X-ray machines have gone out of favour.These days, most companies sort plastics using near infrared spectroscopy. Near infrared radiation has a wavelength just longer than the light the human eye can detect. Even plastics that look the same to us mere mortals will have unique spectra in the infrared range through which machines can identify them.Before they are analysed, plastics are spread out in a single layer along a conveyor belt. Then, according to Closed Loop Project Manager Nick Cliffe: “The bottles pass under a light source on which a sensor is mounted. The sensor detects the amount of near infrared light that the bottle itself absorbs by looking at the change in the reflected light.” A computer then decides if a specimen’s near infrared spectrum matches a reference spectrum. Plastics that meet the criteria are isolated from the rest of the material through the use of pneumatic ejectors, which hit either the selected or rejected items with a jet of compressed air as they fall off the conveyor belt, dividing the material intotwo streams.Jonathan Clarke, Country Manager of TiTech, which manufactures optical sorters, says: “An optical sorter can only do one operation, can only separate out one material at time.” To really refine the streams, therefore, the plastic must pass through a series of optical sorters at these specialised plants; Jayplas’ Derbyshire site receives plastics that have been picked out of the dry recyclables waste stream at MRFs, removes contaminants and then uses 15 automated sorters to identify plastic types and colours, for instance.TiTech says its optical sorters achieve purity rates typically between 90 and 93 per cent, though in some applications a rate of 98 per cent is obtained. For input into closed loop recycling processes, the International Life Sciences Institute recommends a minimum of 99 per cent food-grade polymer, so optical sorters cannot be used on their own in such systems. Closed Loop in London combines four optical sorters with trommel sorting, magnetic and eddy current separation of metals, air classification, manual sorting and other processes because, as Cliffe puts it: “No single stage is foolproof.”Purity levels are usually raised to above 99 per cent after the bottles are granulated. Significantly, the bottle bits must be separated from the cap bits, which are usually coloured HDPE. At Closed Loop, this is achieved in the PET stream through sink-float separation, since PET sinks but HDPE floats. For flakes from HDPE bottles, a colour sorter that uses a digital camera to detect coloured flakes purifies the material.Whereas in Closed Loop’s process the HDPE is then melted to molten form at over 200?C, effectively eliminating any remaining contamination, the PET is cleaned with caustic soda and then must go through a colour sorter and a final laser sorter. The PET flakes drop down an inclined plane and pass under green and fluorescent lasers that classify them using Raman spectroscopy.Raman spectroscopy relies on the inelastic scattering of monochromatic light to identify materials. It could take years of studying physics to truly understand the concept, but Cliffe sums up the processthus: “The energy from the laser is absorbed by the flake and then that energy is reemitted with a particular spectrum – each and every material has a different spectrum. So, by using a laser light, you can tell the difference between all the different kinds and colours of plastic and all sorts of other foreign materials.”Laser technology, though at the moment very expensive (Closed Loop’s Unisensor laser sorter cost £500,000, whereas its optical sorters were a meagre £150,000 each, for instance), is incredibly accurate. Indeed, Clarke predicts lasers will be the next big thing for the sorting industry. Researchers are currently developing another analytic sorting technique using lasers called laser-induced plasma spectroscopy (LIPS). Clarke explains: “A high-powered laser hits material for a fraction of a second and burns a tiny bit to create plasma with a clear register identifiable through elemental spectroscopy.” All elements emit light in characteristic frequencies when excited to plasma form, so LIPS can, in theory, identify the exact elemental composition of all materials, making plastic identification quick and accurate.According to Cliffe, laser technology is even capable of detecting and sorting out bioplastics, whereas optical sorters are not. He says: “Bioplastics are inconvenient for most plastic recyclers. In our case if they get into the kiln, they melt or burn causing clumps to form and generally gum up the works. Our main optical sorters have trouble distinguishing between PET and bioplastic bottles. We brief our manual sorting teams to look out for the bottle brands which we know are bioplastics.”Clarke’s opinion of bioplasticsdiffers somewhat; he says: “It’s not a problem to separate them out.” However, he still thinks they’re ‘terrible’: “They encourage a finite use of material and there’s nothing you can do with them – no one’s recycling them.” Clarke insists it is only worth recycling a material if there’s a sizeable amount of it in the waste stream and there simply isn’t with bioplastics at the moment.And indeed, this opinion applies to the proliferation of petroleum-based polymers as well. Clarke says: “It’s best to make things out of as few polymers as possible because then people will definitely pull them out and recycle them.” Cliffe agrees, insisting he cannot understand why anyone would use PVC in food packaging, for instance. There’s no doubt that current and future automated sorters can distinguish between many, if not all, plastic types, but the technology is only useful insofar as it picks out plastic to be reprocessed. There are, after all, collection, sorting and reprocessing sides to the equation and we must ensure that all of them fit together to solve the plastic problem.