Better sorting in the blink of an eye

Progress in mechanical recycling

The sector of mechanical plastic recycling has been around for a long time and is currently the most widespread. It uses various techniques including sorting, crushing, washing and extrusion. The polymers’ molecular structure does not change much through the cycle. This simple and relatively inexpensive process is ideal for recycling homogeneous end-of-life plastic products or those made up of easily-separable elements. However, it does not allow the extraction of many components that are intricately mixed with plastics (additives, such as dyestuffs or undesirable substances, other materials, etc.) and the polymer can only withstand a limited number of successive mechanical recycling cycles if the aim is to preserve properties close to those of the virgin material. However, its low cost makes it a rapidly profitable process provided that the polymers are carefully sorted beforehand.

After washing and crushing, some plastics can be molded again and thus begin a second life

Black plastics, the former bugbears of recyclers

Sorting plastics is no easy task. It would take quite an eye to see the difference between polypropylene and polycarbonate. However, this is an essential operation because it is much more cost-effective to recycle identical polymers together.

This is why sorting centres are often equipped with optical sensors capable of identifying the different types of plastics according to their light refraction properties. These electronic eyes are able to distinguish flexible and transparent plastics from opaque and/or coloured rigid plastics as well as the main polymers. Until recently, this type of system still had some limitations. The first of these relates to multilayer objects that are not recognised by optical scanning systems, and the second relates to black plastic objects because the carbon black used to make this colour absorbs the infrared rays of the optical sensors.

Sensors open their eyes

Aware of the challenges, optical sensor manufacturers have addressed the problem and found the solution. Thus, in recent months, sensors equipped with high-intensity lasers have appeared on the market. Directing the beam at any plastic object causes electrons to be ripped off, creating a plasma. The latter is analysed by optical spectrometry, which makes it possible to determine the atomic composition of the material and therefore the nature of the polymer.

The latest generation of optical sorters can now tell the difference between subtly-different types of polymers.

The machine is particularly effective, regardless of the plastics’ colour and composition, with an error rate of less than 5%. In case of doubt, the object is ejected and can be analysed once more. If the analysis fails again, the object will then be permanently ejected from the conveyor. In addition, this technology allows the detection of brominated flame retardants (BFRs) commonly found in plastics used in electrical and electronic equipment. Currently, polymers containing them are only recyclable if they have a very low bromine content (less than 2000ppm).

However, without a precise reading, doubt sometimes remains and some perfectly recyclable objects are not always recycled. By succeeding in measuring the exact bromine rate, the sorting process will therefore be much finer, the risk of errors reduced to almost zero, and many end-of-life plastics will be given a new life.

Polymers with their hearts on their sleeves

One of the main bottlenecks of mechanical recycling, as well as chemical recycling sometimes, is the limitations of sorting processes. While laser scanning is effective, it remains an expensive and relatively slow technology, taking on average one second to analyse an object. Other avenues are being explored, such as chemical tracers which are already being used to combat counterfeiting. This involves integrating a tracer into packaging, at the time of the manufacturing process, which contains all the information relating to the polymers and possibly the additives used. They can be readable by the naked eye or by a specific device such as an ultraviolet lamp or a smartphone. Digital tattooing is also being studied. This process enables all types of information to be encoded directly onto an object in the form of computer bits. Like a simple barcode, they can easily be read by an optical device.

The barcode and its cousin, the RIFD chip, have long been perceived as the solution to the sorting problem. However, they were quickly abandoned because they in turn posed recycling problems since they had to be "printed" on a medium stuck onto the packaging or any other object.

Finally, studies conducted a few years ago showed that by adding minute quantities of rare earth oxides (around 1,000 ppm) to black polypropylene (PP) (a polymer widely used in the automotive industry), so-called X-ray fluorescence spectrometric (XRF) optical sensors were able to immediately recognise the material. XRF has the advantage of not in any way affecting the physical-chemical properties of PP and not degrading it either.

Integrating chemical tracers containing all types of information into polymers enables optical readers to recognise the type of polymer and above all to combat counterfeiting, particularly in the pharmaceutical industry.

However innovative they may be, these techniques, although promising, are still widely tested today, to determine their sustainability and to accurately measure their cost-effectiveness.

Technical achievements, major innovations, sorting and recycling processes are evolving in the right direction and although some have already been fully developed others should be able to demonstrate their effectiveness in the coming months. Manufacturers are also working in the field of product design to make their products more easily recyclable, in particular in the field of packaging, where after having revolutionized the food sector by considerably extending expiry dates and thus combating waste, manufacturers are now seeking to offer packaging that is as efficient but less complex and therefore more easily recyclable (see our March 2019 issue).