The global economic order is built upon the exploitation of numerous non-renewable resources. The unsustainability of this model in the long term is increasingly hard to ignore and is evident not only in the depletion of raw materials but also in numerous negative impacts on the environment. In 1972, an essential book on this topic was published. It is called ‘The Limits to Growth’, written by various scientists from the Massachusetts Institute of Technology at the request of the Club of Rome. The authors used computer models to predict an ecological crisis that would occur during the 21st century due to resource depletion and pollution. The 2012 version, ‘Les limites à la croissance (dans un monde fini)’, updated and revised through the concept of ecological footprint, essentially agrees with the earlier version.
In this context, there arises the need to replace traditional, non-linear flows with circular flows that minimize resource extraction and waste generation. This is known as a circular economy, which takes inspiration from biological systems and is scientifically grounded in the concept of ‘Industrial Ecology‘, formalized by Thomas E. Graedel in 1996. Nature produces no waste. Accordingly, and from the point of view of Industrial Ecology, waste is the consequence of imperfect production processes that need to be overcome in favour of a circular flow of materials. It is very challenging -probably impossible- for an industrial society to operate in a fully circular manner, but the reality is that the current production flow is highly non-circular, leaving ample room for improvement.
According to Plastics Europe, global plastic production reached 390.7 million metric tons (Mt) in 2021 (excluding polymers not used in the conversion of plastic parts and products such as textile fibers, adhesives, or coatings). In 2020, the amount of plastic waste recovered in the EU 27+3 (EU plus UK, Norway, and Switzerland) amounted to 29.5 Mt, which is slightly over half of the plastic production in these countries. Approximately one-third of the collected plastic waste is recycled (although only half, 5.5 Mt, is incorporated into new products). A quarter of those 29.5 Mt goes to landfills (23.4%, 36% in Spain), and the rest is incinerated. Across the countries, the plastic cycle is even further from being circular. According to the OECD, plastic waste increased from 156 Mt in 2000 to 353 Mt in 2019. Out of this, 55 Mt could be collected for recycling, and only 33 Mt could be reintroduced into the production cycle (9.3%).
In this context, the Directive 850/2018 amending Waste Directive 1999/31/EC established that Member States must take necessary measures to ensure that by 2035 the amount (weight) of municipal waste deposited in landfills is reduced to 10%, at least, with respect to the total amount of municipal waste generated. This text and the local regulations transposing it establish recycling targets and the need to set up separate collection of domestic-origin textiles and biowaste. The key to the expansion of reuse and recycling lies in Extended Producer Responsibility schemes that embody the idea that producers and distributors are responsible for the waste generated by their products throughout their entire lifecycle. Depending on the type of waste, different schemes of that type exist, including packaging, tyres, and electronic devices among others.
Plastic has not been mentioned so far, as current recycling systems do not always have specific provisions for it. In Spain Ecoembes established a recycling system for packaging made from any material (including wood) and elements such as caps or closures, but not non-packaging plastics. According to Ecoembes, in 2021, they processed 1.6 million tons of packaging waste that representing 84% of domestic packaging put on the market, although the figure is controversial and has been lowered by independent estimations to around 30%. Small containers, dark-coloured items, multi-layered (Tetra Brik) containers are not recycled, or their recycling is very limited. All these materials end up in landfills or, in the best case, are incinerated. Plastic kitchen utensils, toys, pens, coffee capsules, and, in general, any plastic material that is not packaging or a part of it, are currently outside recycling schemes. It’s important to highlight that many objects are made from multiple materials and, therefore, are very difficult to recycle. A typical example of composite material is diapers, with outer and inner layers of polyethylene and polypropylene, respectively, and polyacrylate as the absorbent material. Limiting to packaging is indeed a weakness, although it’s also important to acknowledge that packaging constitute a quantitatively significant portion of plastic waste: 39.1% of the plastic demand in EU 27+3 is devoted to packaging production.
Another important aspect to be highlighted is that plastic recycling is not unlimited. After a few cycles, the polymer loses quality due to degradation during use or reprocessing, or due to the presence of additives from previous uses. It is possible to use degraded plastic to manufacture products with lower requirements, but sooner or later, it will have to be discarded. It’s a well-known fact that polyethylene terephthalate can be recycled into textiles (polyester): five large bottles can produce a t-shirt. Recycling materials for textiles is a positive step, although it’s not an absolute solution. Garments incorporating recycled polyester still release microplastic fibers, and furthermore, their incorporation into textiles disrupts circularity in the packaging sector, given the nearly non-existent recyclability of synthetic textiles.
The recycling of plastics is typically mechanical. It involves shredding the material to produce recycled pellets used by compounders. When this is not feasible for technical reasons, chemical recycling can still be used, which involves techniques such as thermal cracking or hydrolysis to convert the macromolecules that constitute the plastic into products suitable for new uses. The new uses could involve the production of new polymers or not. For example, Plastic Energy operates two plants in Almería and Seville with the capacity to produce 10,000 tons/year of pyrolysis oil from agricultural plastics, which can be used as fuel or in catalytic cracking units to produce plastic monomers. Gasification to obtain synthesis gas (a mixture of carbon monoxide and hydrogen) is another form of chemical recycling. In Spain, Repsol, Enerkem, and Agbar have announced the construction of a gasification plant for non-recyclable municipal solid waste (including plastics) to produce methanol. Finally, energy recovery allows for harnessing the energy contained in the chemical bonds of waste. This practice is less common in Spain than in other countries (20% compared to 42% of plastics recovered in EU 27+3), but it is always preferable to landfill plastics that cannot be treated otherwise.
Certainly, there are materials easier to recycle than others. Polyethylene terephthalate (beverage containers) or high-density polyethylene (common in numerous plastic objects) are relatively straightforward to recycle. Others like polyvinyl chloride (water pipes, synthetic leather) or polystyrene (insulation, packaging material) present greater complexity. Bags and films (low-density polyethylene) are difficult to recycle because they tend to produce jams. Composite materials are generally not recyclable. Hence the need to manufacture single-material products, especially for high-demand items like packaging. Reducing the need for virgin material, extending the lifespan of objects (including textiles), investing in selective collection systems, limiting single-use plastic items, and using rational fiscal policies with fees for suboptimal waste management (Pay-as-you-throw) are some of the main tools available to enhance circularity in the plastic market.
Science, Plastic and Environment 