Plastic recycling 101

In the wild world of plastics, there are many more types than the layperson might realise. And while most are recyclable, not all are. Some ingredients are not worth recycling. And both chemical and mechanical recycling have their pros and cons. Let’s take a deep dive into how it all works. 

Being in the recycling industry, I often get asked questions like: 

• Are all plastics recyclable?
• Can you use 100 per cent recycled plastics to make new bottles?
• Can you recycle plastics indefinitely?
• How many times can you recycle plastic?

So far so good. But then most people expect clear and simple answers, and that’s where the problem lies. 

The right answer to most of the above questions is: “it depends”. 

Or, to say it with American journalist Henry Louis Mencken: “for every complex problem, there is a solution that is neat, plausible, and wrong.”

Not all plastics are created equal and recycling does not equal recycling. But, because I get asked these questions quite a lot, here are some relatively simple answers. 

Most plastics are recyclable, but not all ingredients within a plastic product may be recyclable and not all plastics are worth recycling. 

What does this mean, you ask? Well, think about the pigments of colour used to make coloured plastic. We can recycle the plastic, and we can get rid of the pigment, but we typically cannot or do not recycle the pigments. The main reason is that it’s just not viable to do so.

Generally, plastics or resin types are given a code to indicate what they are and that also gives us an idea about their recyclability:

• Code 1 pertains to polyethylene terephthalate (PET)
• Code 2 is high-density polyethylene (HDPE)
• Code 3 is polyvinyl chloride (PVC)
• Code 4 is low-density polyethylene (LDPE)
• Code 5 is polypropylene (PP)
• Code 6 is polystyrene (PS) and expanded polystyrene (EPS)
• Code 7 is “other” plastics

Recycling plastics codes 1 to 6 is pretty much standard today. Code 7 plastics are more tricky, as they often are mixtures of resins with all sorts of additives to give the desired performance criteria. Putting it simply: the more additives, the more difficult the recycling.

When answering the question whether we can make new bottles, say from 100 per cent recycled plastics, it gets a bit more complex.

At a very high level, the simple answer is “yes”, followed by a pretty big “but”.

There are two main reasons for the “but”:

1. We have to differentiate between mechanical recycling and chemical recycling
2. We have to understand the cost and yield of recycling

Let’s quickly explain mechanical and chemical recycling

Mechanical recycling is sort of self-explanatory. 

Let’s take a milk bottle you may have in your fridge. It is made from HDPE. The typically blue coloured lid is made from PP and is connected to a noose that “seals” the lid to the bottle. You should rinse the empty milk bottle before putting it into the yellow recycling bin. 

As we put all sorts of recyclables into the yellow lid bin, we commingle materials that don’t belong together. They are then taken to a materials recovery facility (MRF), which has the job of un-commingling the recyclables. If it is a good MRF, it sorts HDPE from PET and so on. But even if it does that, the PP lid and noose are still with the HDPE bottle. 

Now comes the mechanical recycling story

The HDPE bottle, baled up with thousands of others, and goes to an advanced plastic recycling facility where the bale is broken up, the bottles are shredded and the non-HDPE components separated from the HDPE, typically by optical sorting equipment. 

Then the shredded HDPE is washed up to three times. First a cold wash, then a friction wash, and then a hot or caustic wash to clean the HDPE properly, and remove all contaminants and labels. 

The washed pieces are dried, again mechanically, and sorted again to remove any remaining non-HDPE. 

It will then be finely shredded to make a flake product, which some customers want. 

Or, it will be extruded and go through a laser filter, which removes any remaining pieces of non-HDPE that were missed in the previous steps. The actual extrusion is done by a screw-type piece of metal that can be heated. The extrusion melts the plastic just enough to make it malleable to form a sausage that is then cut into little pieces called pellets. The pellets are small – so measuring equipment can accurately allocate by milligrams how much HDPE goes into a mould to make a new HDPE bottle.

The US Food and Drug Administration approved the HDPE recycling process of a particular company safe for food contact in 2019. So food-grade recycled HDPE (rHDPE), including new rHDPE milk bottles, can be made this way. 

HDPE can only go through this type of process a limited number of times. The mechanical process and the extrusion process degrade the rHDPE resin to a degree. That’s why virgin resin is mixed with rHDPE resin to ensure the HDPE bottle performs exactly the way we want or need it to perform. Food grade rPET has been around for longer than rHDPE.

The “yield” of recycling simply means: how much rHDPE do you get out (output) of the baled HDPE bottles coming in (input)? That obviously depends to a large degree on the level of contamination of the milk bottles, hence the advice to rinse before putting it into the bin. 

The yield varies between 10 and 25 per cent, so you can conservatively say that out of 1000 kilograms of HDPE bottles we can make 750 kilograms of new HDPE bottles.

But, as said before, we need to mix some virgin HDPE with the rHDPE, as we do not know which pieces of HDPE have been recycled how many times.

Chemical recycling

The story gets more complicated with chemical recycling. There are at least three different types with a number of sub-categories. I quote from the CSIRO Report Advanced recycling technologies to address Australia’s plastic waste from August 2021. 

There are multiple conversion technologies, such as gasification, pyrolysis, hydrocracking, and hydrothermal. What they all have in common is that heat is used in the process. 

Then there are depolymerisation technologies such as: 

• Enzymolysis, where enzymes (a biological process) are used to break down the plastics
• Chemolysis, where a chemical process is used to break down plastics
• Solvolysis, where solvents are used to break down the plastics

Finally, there is a process called purification, which also uses a solvent to dissolve the plastics.

Chemical recycling goes through a number of steps to break down plastics and reduce them to monomers, so they can then be put together again as polymers, which have the same performance parameters as virgin polymers.

Going back to the above question whether we can make a bottle from 100 per cent recycled plastic – the answer is absolutely! If we need to add virgin polymers to the mechanically recycled polymers, we can use chemically recycled resins. 

We can now also answer the last question asked above: can plastics be recycled indefinitely? The answer is: yes, with chemical recycling, they can. Again, there is a “but” to be added. 

The problem with chemical recycling 

I wouldn’t be telling the whole story if I didn’t mention a few little hiccups along the way.

The CSIRO Report is silent about yield from chemical recycling. The issue with chemical recycling is that it has very low yields. Of the 100 per cent plastic going in as input, a smaller amount than in mechanical recycling finds its end-use as plastics. 

There are a number of by-products, such as chars, tars and oils, which can be used as fuels or in other applications, but not in making plastics. Of course, a lot depends on the amount of contamination in the input. But generally speaking, yields in chemical recycling are a lot lower than in mechanical recycling. Asking experts, I have heard that most processes have yields below 60 per cent, with some even a lot less than that.

Another error the CSIRO Report makes is to say that “generally, the products of mechanical recycling do not meet the requirements for food contact compliant applications”. 

That’s not correct, as explained above with the HDPE milk bottle. It depends on the processes chosen and the washing and cleaning steps chosen within the mechanical recycling process.

Then there is another error, one that is commonly made. The report states that plastics are valued for their high calorific value (CV) in Energy from Waste (EfW) processes. 

That is also not correct. EfW plants make most of their money by charging a gate fee. Their boilers are designed for a certain thermal capacity. 

The more high-CV materials (i.e. plastics) they accept, the less they can charge, as the plastics use up the thermal capacity of the boiler. In actual fact, EfW boilers are designed for low CV fuels, such as residual MSW (household waste).

Despite these errors, the CSIRO has produced a really good and necessary report.

Getting the right mix 

What is important to note is that chemical recycling is a good process for the vast majority of plastics going to landfill right now, most of which are from commercial and industrial sources and from the building and construction industry. 

When it comes to plastic-to-plastic recycling, chemical recycling has an important role to play. But it uses more energy, is more expensive and has a lower yield than mechanical recycling. 

We need both.

Now you know all you need to know about plastic recycling. 

Simple. Right?

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