New prospects for genuine recycling

The key to their success is partly the wide range of different types of polymer that are available. Special components and additives are, however, also vital, as they enable material manufacturers and compound scientists to vary the character of the plastics. Additives help to give plastics the properties that matter in the chosen application. Additives make plastics flexible and elastic, rigid and fireproof, extremely tough and resilient, colourful or antibacterial. Any and every plastics manufacturer keeps his formulations strictly confidential. Although the differences between polymer A and polymer B may well seem marginal at first glance, they are crucial for the application and, not least of all, for the company’s success.

One man’s meat is another man’s poison: the variety of different plastics available on the market presents a tremendous challenge to recyclers, particularly when the assignment is to manufacture recyclates from recovered plastics that can be used for high-quality applications. This is possible to a certain extent, when materials consisting of just one plastic are being processed, like bottles made of thermoplastic PET (polyethylene terephthalate). When a blend of similar plastics with merely minor differences is involved, “upcycling” – i.e. the manufacturing of products of the same or higher quality from recyclate – already proves to be more difficult. The only alternative in such cases is “downcycling” – i.e. the production of lower-quality plastics. Recovered plastics can be recycled thermally instead, e.g. to generate energy or heat. That is a proven solution but it is not idealdue to greenhouse gas emissions. In addition, some plastics, such as polyvinyl chloride (PVC), cannot be incinerated due to the generation of toxic chemicals.

Material scientists are working intensively not only on increasing recycling levels but also on optimising the potential of upcycling. In this context, they are dependent not least of all on the practical experience and know-how of experts. A substantial increase in the level of upcycling can only be achieved via fully effective co-operation between plastics manufacturers, compound scientists, processors and recyclers. Plenty of questions need to be answered in this context: would it, for example, make sense to provide plastics with tracers, with the help of which plastics could be identified and sorted more precisely by analytical means? Or should agreement be reached on a reduction in the number of different types of plastics that are used? The answers are not obvious, but experts will make them easier to find. Incidentally: what could be a better forum for interdisciplinary networking than K 2019 in Düsseldorf this October?

Scientists like Brett Helms and Peter Christensen from the Lawrence Berkeley National Laboratory (Berkeley Lab) at the University of California in the USA are adopting an interesting approach to the production of durable and in every respect sustainable plastics. Helms, Christensen and their colleagues are working on a polymer material that – put in simplified terms – acts like a set of Lego, in that it can be taken part into its tiniest components and then be put back together again and again in new and different forms, textures and colours – without any loss of performance or quality.

“It is rarely the case that any thinking is done about recyclability when most plastics are being manufactured”, says Peter Christensen. In addition, too much energy and money are as a rule required to return plastics to the material cycle as equivalents to new raw materials. The Berkeley Lab team now claim to have found a way that makes it possible to recycle plastics as it were fundamentally, i.e. at the molecular level, by dividing them up completely into their individual components and then putting them back together again intelligently to create something entirely new.

All plastics are polymers, i.e. they consist of large macromolecules based on compounds containing carbon (monomers) that have joined together to form chains (polymerisation). The plastic polyethylene (PE), for example, consists of macromolecules that in turn consist of numerous individual ethene molecules (ethylene), while the polypropylene (PP) monomer is propene (propylene). What are involved in both cases – ethene and propene – are gaseous, combustible hydrocarbon compounds that are obtained from oil or natural gas by what is known as steam cracking. They are polymerised in the course of catalysed chemical reactions.

Unfortunately polymers like PE or PP, i.e. plastics cannot be recycled bach to their original monomers. Recycling is further complicated when a plastic contains more than one monomer or additives. Even after separation, the additives remain closely bonded to the monomers in some cases – they have, as it were, been ‘baked’ together – and they stay that way when recycling is carried out too.

In most cases, recycling plants have to deal with plastic products that consist of different polymers with different property profiles. For example, if rigid plastics, stretch plastics, colourless plastics and colourful plastics are mixed together and ground up, the outcome of the recycling process is a raw material about which it is difficult to say what properties it will take over from the original plastics. The legacy of unknown and thus unforeseeable properties of a blended recyclate makes the manufacturing of high- or higher-quality products with a defined / specified property profile something of a gamble. And when a reusable shopping bag made of recycled plastic is finally worn out, it is not in most cases possible to produce new material by recycling it again. At the end of its useful life, it is either incinerated and is at best used to obtain heat, electricity or fuel – or, as is still standard practice in some parts of the world, it ends up on a landfill site, where it disintegrates in the course of time under external chemical and physical influences. In the worst-case scenario, the plastic bag finds its way into the sea and pollutes the environment there.

Incentives are required to stop the disposal of plastics on landfill sites and the environmental pollution this causes – and these incentives need to be meaningful and have a positive impact on added value into the bargain. After all, if a recyclate is of the same quality – comparable to a new raw material – so that product of the same quality can be manufactured as with a new raw material, what arguments can there be against using the recyclate?

A specific example: money doesn’t grow on trees, as the saying goes. True, but: every PET bottle that is carelessly thrown away represents the loss of a valuable raw material. So it is worthwhile collecting and recycling PET bottles. Because such high-quality products as bottles, films or fibres can be produced again from pure polyethylene terephthalate (PET).

Let us continue this train of thought: although increasing use is also being made of renewable resources, plastic is mainly produced from oil. This means: thermal recycling of used PET bottles to generate energy at the end of their useful life leads to emissions that impact the climate of the kind that are produced in conventional incineration processes as well. If we may be allowed to repeat ourselves: in contrast to other plastics, PET is not a plastic blend – it is a pure material that can be recycled and processed into high-quality products more effectively than practically any other polymer material.

In Germany, people’s interest in collecting PET bottles was triggered by the introduction of a deposit system, which has led to a recycling level of up to 98 per cent (for details see “Forum PET”). About one third of the recycled bottles are used to produce new PET bottles, while another third are processed into industrial films and one fifth are turned into textile fibres. It appears to be the case that PET recyclate gives companies new ideas, e.g. using former bottles for beverages to produce backpacks for schoolchildren, pullovers, sneakers, film surfaces for furniture or automotive parts.

Business with recycled PET bottles is facilitated by the installation of central collection facilities, where deposit bottles are squashed and compressed after being fed in manually. The recycling company sorts and cleans the material and then chops it into fingernail-size pieces which are melted and processed into granulate – the basis for manufacturing new, high-quality plastic products. Further collection systems that are being promoted by industry include the recycling of plastic window profiles or recovered agricultural films. Scientists are in the meantime working on optimisation of the plastic recycling process by biotechnological means. They are testing bacteria and enzymes that digest polymers and divide them up into their basic components. It can be expected that this interdisciplinary co-operation will lead to the creation of new jobs in future as well. After all, plastics are resources that have added value and can be recycled successfully if they are sorted appropriately. Efficient recycling will become increasingly significant in future – something that is, incidentally, a topic at K 2019 this October – and the foundations for this are already being laid. Which brings us back to the development carried out by Brett Helms and Peter Christensen from the Lawrence Berkeley National Laboratory (Berkeley Lab) in California / USA.

The plastic that Brett Helms, Peter Christensen and their colleagues have discovered and that can be recycled over and over again with no loss of performance is what is known as a polydiketoenamine (PDK). PDK consists of numerous triketones as well as aromatic or aliphatic amines, with water as the only by-product. It is largely coincidence that encouraged the research scientists to take a closer look at PDK as a sustainable plastic. This happened when the Berkeley scientists put various acids in vessels that were used to produce PDK-based adhesives.

PDK changed its structure when it came into contact with acid. The scientists were curious and wanted to know exactly what had taken place in the glass and how this kind of transformation was to be interpreted. So they analysed the molecular structure of the sample via NMR spectroscopy. They found that the polymer structure of the polydiketoenamine had broken down completely into the original monomers. Further tests revealed that the PDK had not only broken down into its monomers under the influence of acid; all the additives had been separated from the monomers too. The research scientists finally used their recycled PDK monomers to produce a new polymer. This operation was successful, without retaining the colour or other features of the original PDK.

The Berkeley scientists are convinced that their new, recyclable plastic might be a good alternative to many of the non-recyclable plastics that are in use today. “We are at a critical point, when we need to think about the infrastructure that is required to modernise recycling plants for the sorting and processing of waste in future”, says Brett Helms. He points out that if these plants are designed in such a way that they can recycle and process PDK and similar plastics just as completely as the laboratory experiment has demonstrated, the time when plastics still end up on landfill sites or litter the environment may be over in the foreseeable future. He thinks their development shows that plastics which are developed and produced in a process of thinking backwards can be recycled thoroughly and completely. As the next stage, the research scientists intend to develop PDK plastics with a wide range of thermal and mechanical properties for such different applications as textiles, 3D printing and foam plastics. They also aim to supplement the formulations by materials from plant and other sustainable sources.

Peter R. Christensen et al., Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds, Nature Chemistry 11 (2019) 442–448, http://dx.doi.org/10.1038/s41557-019-0249-2