Quality recycling

Growing environmental concern has fuelled consumer enthusiasm for products containing recycled plastics. But where to find enough recycled plastic, and of sufficient quality? XiaoZhi Lim reports

Reports of wide-scale plastic waste pollution have spurred companies into action. Some 80 global consumer packaging and retail companies have pledged to include between 15 and 50% recycled content in their packaging by 2025. Minimum recycled content is also being mandated in some countries. In the UK, consumer packaging companies now must pay a tax of £200 per ton to import or produce packaging not containing a minimum of 30% recycled material.

Now the challenge is finding enough recycled content, and with good enough quality, to meet increased demand for recycled plastic, says Jeremy Wallach from McKinsey & Company. Mechanical recycling of plastics, where clean, sorted material is melted and remoulded, has many limitations. Polymer macromolecules degrade during mechanical recycling as the heat reduces their molecular weight. An emerging worry is that all the chemical additives within a waste plastic get retained in the recycled product (see Box). Currently, most mechanically recycled plastics are downcycled into products with lower technical requirements.

Advanced recycling techniques could increase the types of recyclable waste feedstock. These techniques include pyrolysis or gasification, in which waste plastics are usually reduced to short-chain hydrocarbons. Most of these techniques can handle contaminated, mixed materials. ‘Part of the supply unlock is going to be broadening the amount of feedstock that gets recycled,’ says Wallach.

Advanced recycling techniques can also open markets for recycled plastic materials, particularly food-grade applications. ‘That’s probably where the greatest interest is now for chemically-recycled hydrocarbons,’ says Geoff Brighty of Mura Technologies. It is very difficult for mechanically recycled plastics to meet technical and safety standards for food-grade packaging, a big user of single-use plastic. Operations that collect only a limited, or even a single, type of plastic object – for instance, those that process only milk jugs – could produce mechanically recycled food-grade plastic. But ‘for most mixed waste plastics, that’s almost impossible,’ Brighty says.

If you want to be 100% circular, then you cannot just produce plastic of lower quality that you cannot use for food-grade applications.
Ina Vollmer Utrecht University, The Netherlands

Making recycled plastic that can be used in food and medical applications will be key towards building a circular economy. ‘If you want to be 100% circular, then you cannot just produce plastic of lower quality that you cannot use for food-grade applications,’ says Ina Vollmer from Utrecht University.

Many environmental groups have criticised traditional advanced recycling techniques such as pyrolysis for being energy intensive. Often, the output – resembling crude oil – may not be used to make new plastic and is instead sold as fuel. Emerging, less energy-intensive, recycling techniques include using supercritical water to crack polymer chains or low-temperature dissolution and purification techniques that extract whole polymer molecules while removing additives.

There is huge potential for growth. A recent price index shows that recycled polyethylene currently commands a price double that of virgin polyethylene, Wallach says. By Wallach and his colleagues’ analysis, with some $40bn investment over the next decade, advanced recycling could supply 20-40m t/year of plastics by 2030 – representing over 20% year-over-year growth. The biggest driving force comes from consumer goods and packaging giants who now need to use recycled material but are not satisfied with the quality from mechanical recycling, says Martin Schlummer from the Fraunhofer Institute. ‘This is a large playing field.’

Supercritical water

Polyolefins, especially flexible plastics, represent a feedstock that’s under-captured by mechanical recycling, says Wallach. Together with polyethylene terephthalate (PET), polyolefins like polyethylene and polypropylene are the top polymers used in packaging. But while PET enjoys substantial recycling, ‘there’s a dramatic shortage in the market of high-quality recycled polyethylene and polypropylene,’ Wallach says.

Mura Technologies is targeting just that feedstock, says Brighty. ‘What we want is polyolefinic-rich material because that’s the one that then yields us the best hydrocarbon mix.’ In Australia, Mura Technologies’ pilot plant processes mixed post-consumer plastic waste. The bales are first pre-sorted to remove contaminants that would damage the recycling process, such as metals or glass. The sorted plastic wastes are melted at high temperature and pressure in an extruder, then the melt is mixed with supercritical steam at 4500°C and 200bar. Under these conditions, the plastic effectively dissolves in the steam and the polymer molecules start breaking into shorter hydrocarbons, Brighty explains. Finally, the mixture goes through flash distillation to separate the hydrocarbon products.

This process yields a distillate gas and naphtha mixture that comprises about half of the product mix. This is the ideal feedstock for producing new plastic, Brighty says. Mura has partnered with Dow Chemical, to purchase this feedstock for making new resin.

Recycling plastics using supercritical water [Mura Technology]

Another product is a wax residue, which can be used to replace the bitumen in road surfaces. The lightest hydrocarbon gas products can be used to generate heat for the supercritical water process to improve its carbon footprint, Brighty says. Mura Technologies is also looking into other ways to lower its carbon footprint, for instance by switching to renewable energy for generating supercritical water. ‘We can’t recycle plastic at any cost,’ Brighty says. ‘We have to be mindful of our carbon intensity.’

Mura Technologies is currently building its first commercial plant in Teesside that would recycle 20,000t/year of plastic, managed by Mura’s subsidiary RenewELP. The commercial plant would be supplied with plastic waste from Geminor, an energy-from-waste company. Other than that, Mura Technologies is working to construct recycling plants in Germany and in Washington state in the US. One advantage of supercritical water is that the process is more predictable when scaled, as the plastics are not heated directly. In pyrolysis, which heats plastic directly, some of it burns to a char, says Brighty.

Dissolution purification

In theory, the hydrocarbon products from most chemical recycling techniques can be used to create virgin-quality plastic. But this does not always happen – often because of extreme price competition from virgin plastics. Dissolution techniques that keep polymer molecules intact, and only strip waste plastics of their additives, are more likely to provide new plastic, says Schlummer.

In Montreal, the company Polystyvert has operated a demonstration plant for recycling expanded polystyrene via dissolution for four years. First, the solvent, cymene, is used to dissolve polystyrene. Cymene is selective to polystyrene, says Polystyvert founder Solenne Brouard Gaillot, so other polymers would not dissolve and can be filtered off. The mixture then goes through a proprietary filtration and purification process to remove undissolved impurities until ‘eventually, you face soluble contaminants,’ says Gaillot. These include small molecule additives like inks or fire retardants. At this point, an anti-solvent – heptane – is introduced to precipitate polystyrene macromolecules. All these processes are performed at room or low temperatures. ‘We do not warm the polymer because [that] would impact the mechanical properties,’ says Gaillot.

The Montreal plant can operate continuously at 125kg per hour and is currently used to demonstrate the process for treating expanded polystyrene. It can accept material with up to 15-20% contamination, and for good reason. ‘If the material is clean, it should be mechanically recycled,’ says Gaillot. Dirty feedstock is also more affordable and along with the room or low temperatures conditions, that helps to keep operating costs down. While the recycled product is currently being sold to insulation firm Owens Corning, Gaillot does not view Polystyvert as a recycling company. Instead, the team performs demonstrations for polystyrene recyclers in hopes of licensing the dissolution process to them. The team is working to expand their process to other styrenic plastics. ‘Polystyrene is like the entry to all the styrenic products,’ Gaillot says. Polystyvert is working on a pilot plant for acrylonitrile butadiene styrene (ABS), which Gaillot hopes will be up and running in 2023.

At the Fraunhofer Institute, researchers have developed a solvent for dissolving almost every thermoplastic polymer, called CreaSolv liquids, says Schlummer. Fraunhofer’s processes can handle mixed waste containing as low as 50% of the target polymer. The material can also go through multiple consecutive processes to extract multiple polymers, allowing the recycling of multi-layer materials. In the EU funded MultiCycle project, Fraunhofer researchers demonstrated stepwise extraction of polyethylene, polypropylene, polyamide and PET from post-consumer flexible packaging. The recycled resins were used to create consumer packaging such as cosmetic sachets and flexible resealable pouches by packaging giant Amcor.

Mechanochemistry

Mechanochemistry is an emerging synthetic technique that grinds substances together in a ball-mill, or a twin-screw extruder. The technique has been successfully applied to prepare pharmaceutical co-crystals and metal-organic frameworks. Vollmer is hoping that mechanochemical recycling of plastics could result in a smaller distribution of products – a major problem with traditional pyrolysis or gasification.

‘With pyrolysis, you will have random scission everywhere,’ says Vollmer. Pyrolysis of polypropylene, for instance, produces over 500 different molecules. Ball-milling could offer higher selectivity, by cleaving polymer chains in the middle. ‘If we can achieve a narrower product distribution, you would not need to do so much post-processing or separation,’ Vollmer says.

A niche that mechanochemistry could fill is recycling thermoset plastics. Thermosets represent about 20% of plastics. But unlike thermoplastics, thermosets contain crosslinks that prevent them being melted and remoulded in a mechanical recycling process. The crosslinks also make thermosets energy-intensive to recycle with pyrolysis.

Researchers led by Qi Wang at the Polymer Research Institute of Sichuan University in Chengdu, China, showed that epoxy resins, one type of thermoset, could be recycled mechanochemically (ACS Sustainable Chem. Eng., doi: 10.1021/acssuschemeng.1c03221). Currently, waste epoxy resins – a major component of wind turbine blades – can only be pulverised, then used as a filler for composites. Wang’s team put waste epoxy resin through a specialised equipment in their laboratory called a solid-state shear mill. The mill has two inlaid grinding plates with multiple sectors that together impart three-dimensional shear forces on materials as they pass through. They found that after going through the mill, the crosslinks within waste epoxy resin were broken up. Hydroxyl and amino functional groups also appeared on the surface of the mechanochemically-processed epoxy powder. Using the recycled epoxy resin, the researchers then prepared new epoxy composites, which maintained about 80% of their mechanical properties compared with virgin epoxy.