Mechanical Vs Chemical Recycling

Mechanical Vs Chemical Recycling

Recycling can often become cause for debate. That's why we thought it'd be useful to compile some key information, arguments, advantages and disadvantages etc. of mechanical and chemical recycling.

Mechanical vs Chemical Recycling: An Introduction

The subject of recycling can be a confusing one. So, for those of you who don't already know, mechanical recycling involves the processing of plastic waste into secondary materials or products, without making significant changes to the chemical structure of the material. With this method, there is no chemical disruption to the polymer chains of the materials. Key processes involved in this method of recycling are: sorting, washing, drying, grinding, re-granulating and compounding. On the other hand, the chemical recycling method involves the use of heat or chemicals to break down and recycle plastic waste into new chemicals and plastics that subsequently perform like virgin materials. In contrast to mechanical recyling, the structure of the polymer is altered and therefore converted into a chemical structure that can be used in high-quality applications; food packaging for example.

 

The British Plastics Federation (1) provide a very clear overview of the different plastic recycling methods and where they fit:

 

Chemical recycling diagram

 

Let's take a look at the pros and cons of each method in turn.

 

Chemical Recycling

What is it?

There are a few different types of Chemical recycling. Pyrolysis is one of the most commonly used chemical recycling processes and is often used interchangeably with the phrase. (2) This is the process of heating plastic waste to high temperatures (often around 500°) in the absence of oxygen, which breaks down the waste into its constituent chemicals. Gasification is a similar method, but involves the use of oxygen and steam to generate a gas that can be used as fuel, or to generate electricity. A third method is Depolymerisation, which involves using catalytic or chemical treatments to break down plastics into single monomers or short polymer chains - essentially, the basic building blocks of the plastic. As these building blocks are the same as those used in virgin plastics, they can then be used to create new plastics without any loss in quality.

 

Pros

In recent times, chemical methods of recycling have grown in popularity for a few reasons - primarily because of the profitability associated with them and the fact that it can remove packaging waste from landfill sites. Unlike mechanical recycling which downgrades the quality of plastics, chemical recycling methods such as depolymerisation can recycle plastics over and over again without loss of quality (3). And not only this, but in addition, chemical methods are more tolerant of contaminated materials (4).

The major factor driving support for chemical recycling is clearly the prospect of being able to truly 'recycle' plastic - depolymerisation opens up the prospect of being able to create new plastics of the same quality of the original - something that isn't possible with mechanical recycling, as mechanical methods shorten the polymer chains and therefore produce a lesser quality.

Alongside this, as The Astrum Group writes on LinkedIn, "According to industry experts, the chemical recycling market is poised for remarkable growth, estimated to reach $2.5 billion by 2025. This isn't just a green dream; it's a concrete business opportunity with real financial benefits." (5) This suggests that as well as having opportunity to provide environmental benefit, the industry is maturing to the point where it may be easier to produce economic returns.

 

Cons

On the other hand however, one criticism of this method is the high energy consumption and lack of maturity of the technologies. The various different chemical recycling processes are currently a lot less mature in development, when compared to mechanical recycling. One example cited by Benoit Perreau (Veolia) is that Pyrolysis applied on ‘plastic to plastic’ today shows a greenhouse gas footprint higher than virgin, and a very poor material recycling yield (max. 30 to 40 per cent after vapocracking), which makes the promise of ‘infinite recycling’ an illusion. Depolymerisation and dissolution, however, show slightly better GHG footprint than virgin and very good yields. Perreau states: “Mechanical recycling is and will remain the preferred recycling solution as it is by far the most efficient when applicable and has demonstrated itself to be the most beneficial to climate change mitigation.” (3)

Another point made by critics of the chemical methods is whether it can be called true 'recycling' at all. This question is well-described on the resource.co. blog, pointing out that "in order to be considered ‘true’ chemical recycling, the output must be a ‘recyclate’ and able to be used in its previous form; not used as fuel. Pyrolysis and gasification are, therefore, controversial forms of chemical recycling and are seen by some as energy-from-waste under a different name." (6)

 

Mechanical Recycling

What is it?

Similarly to chemical recycling, there are different methods and processes involved in mechanical recycling. Over the years, the overall process has been made simpler, now resulting in six basic steps: collection, sorting, washing, shredding, identification/separation and extrusion/compounding. Firstly, collection refers to the physical collecting of PCR (post-consumer recycled) content and materials from homes and businesses etc. Next in the recycling process, sorting, involves the separating of different types of plastics. Furthermore, plastics may then be sorted according to other attributes, such as: size, colour and use. After that, the plastic is then washed. Washing is a vital step in the process, as it ensures that all impurities are removed from the plastic. The fourth step in the mechanical recycling process is shredding. This is when the plastic is fed into shredders, breaking it down into smaller pieces, so that it can be processed in the next stages for reuse. Then, the identification/separation step is where the processed materials get tested for the quality and class. Finally, the particles of shredded plastic are compounded to form pellets, so they can be used in the manufacturing of new products.

 

Pros

Mechanical recycling has been the main form of plastic recycling, so the infrastructure is already widely in place for collection and sorting. By contrast, chemical recycling processes tend to be at a less-mature development stage, so will require more investment to make them commercially viable.

"PET is a prime example of successful mechanical recycling." (3) Bottles in 100% PCR PET have become more common - processes are constantly advancing and improving. When this can be achieved, this is true 'closed-loop' recycling, where the same or similar products are created again from the waste product, which lowers the carbon footprint dramatically and helps to preserve the resources of Earth.

 

Cons

"However, the picture is not so rosy when looking at non-PET materials." (3) It seems that the European Food Safety Authority's guidelines currently limit the uses which mechanical recycling streams can be put to. This has been one reason for the growth in popularity of the Chemical recycling methods.

The mechanical recycling method intrinsically reduces the quality of the end plastic product through the grinding and processes which take place. This often leads to 'downcycling' where only products of a lesser quality than the original can be produced - "In reality, only a fraction of ‘recyclable’ used plastic is recycled into the products for which they were originally produced". (6)

Suitability for all plastic types is another challenge - as Carlos Monreal, CEO of Plastic Energy puts it: "Unfortunately, not all plastics can easily be mechanically recycled. Certain types of plastics, such as films and flexibles, are typically either destined for landfill, incineration or end up in our oceans" (3)

Contamination and variable quality of the input stream is an issue in all types - particularly in mechanical recycling, "the quality of plastics collected (the feedstock for recycling processors) is, usually, inconsistent and contaminated leading to downcycling into lower value items." (7). This is especially important where the mechanical process itself reduces the quality of the end product - it is important that high quality of feedstock can be secured.

What is PCR packaging?

PCR (Post-Consumer Recycled) plastic is ever improving - find out more about what it is.

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