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Plastics in the recycling process: from waste to waste.

The very first and important stage of the recycling process is sorting waste at home. But what happens later?
Where does the waste end up and how does a PET bottle transform into a fleece sweatshirt?
Let’s follow through the most important process in our trade: the recycling cycle of plastics.
First of all, it’s worth noting that not all plastic is the same! The term “plastics” covers hundreds or maybe even thousands of different synthetic polymers, composite materials and many mixes of plastic, which have been produced and continuously improved for about 100 years now.
Bin lorries collect our rubbish, which then, depending on the fraction, is sent to proper waste-sorting plants or municipal establishments. In order to reprocess and recover materials, they have to be thoroughly sorted, which means there is the second-level sorting process of plastics.

Sorting plant is a place where machines and human workers separate waste. Plastic is divided into PET, HDPE, PVC and many more fractions. At this stage we eliminate highly soiled elements that could disrupt the next processes.
We are going to sort PETs, let’s begin!
The conveyor belt (sorting line) is made up of different sorters, including the optical ones. Thanks to their advanced technology which uses X-ray or infrared radiation, the sorters can detect wave spectrum of waste, classify it and properly separate it. Another type of sorters are special sieves with eyelets of specific diameters – sometimes very small elements are not recyclable, therefore they have to be eliminated. Weight sorters react to high weight of certain elements, because they are programmed to know how much a PET packaging of specific dimensions weighs. Too light or too heavy elements do not go to the next stages.
However advanced the machines are, they might make error and need human oversight. People also work along the conveyor belt, between the sorters and at the end of the line. Their reliable eyes detect details even the most up-to-date machines couldn’t eliminate.

When the waste arrives at the recycling facility, it is firstly separated into two groups: municipal and industrial. If the recovered raw material is going to be used for consumer purposes (e.g. packaging, textiles), we process waste from the same, primal source. If it’s going to be used in the automotive industry and for example, become a car battery’s case, then it can be sourced from industrial waste.


We are moving on to the loudest stage, which is shredding and crushing. Now we need crushers, fast-spinning blades and other machines suitable for specific recyclable materials. PETs are relatively hard, but easily breakable, so they end up in the crushers. Rubber, on the other hand, is very ductile and flexible. If it landed in a crusher, it would behave like a chewing gum in your mouth. It wouldn’t crush and crumble, so it has to go to the cutting chamber. The whole scheme is to grind it in the most optimised and efficient way.
Now let’s go back to our PET – after crushing and pressing, we get small elements, irregular in diameter of around 1 cm.
Shredding is important for two reasons:
Firstly, not all sorting plants or recycling facilities are equipped with all essential conveyors, sorters and machines that enable full processing from waste to recyclate. Sometimes the recycling process is divided between more facilities. It’s cheaper and more efficient to transport shredded materials than full-size waste.
Secondly, even if the recycling operation takes place in one place, it’s easier to process fragments than full-sized packaging, which we will explain in the next part of the text.

Collected waste can sometimes carry substantial marks of wear and tear. The most soiled elements are eliminated at the first stage of sorting.
Now it’s time to get rid of the labels, dust and dirt. All unwanted elements are “blown out” in a special air-chamber. Lighter elements of dirt are separated from heavier PET flakes.
Now that the air cleaned our materials, let’s give them a proper bath.
The cleaning stage is not only about eliminating dirt. It has a second important  function: final waste sorting. All plastics have specific rates of density. Our PET should float on the water surface. If some elements fall on the bottom, it means there is pollution that is heavier than PET. We don’t want it in the next stages of the process, so we eliminate it.

Drying and autoignition

Clean materials are dried, which is not that easy as it may seem. All sorting plants and recycling facilities are in danger of spontaneous ignition (combustion) of plastic. It may be caused by overheating at the stage of drying, uncontrolled build-up of gases, electrifying of the plastic fragments or too much dust in the air.
The knowledge about possible dangers can help implement suitable precautions and preventive measures at all stages of the processing.
If our material is clean, dry and we are 100% sure it’s free of all contaminants, we divide it according to its colour. It’s a task dedicated exclusively to a special optical sorter. It measures waves of colours and sorts it – with the highest precision.

Final recyclate – extrusion

The last stage of the recycling process is melting the plastic in the extruder. The plastic pellets are placed in the machine. Inside, they are heated and liquified. Next, the plastic is forced into moulds, where it cools and hardens. The solidified material is then extruded in the form of long strands. It can be coiled or cut into smaller pieces. It depends on the extruder’s technology and the recyclate’s final destination.
Finished recyclates are sent to factories. The material is used in 100% in new products or is added to virgin plastic. And so we stand a chance of seeing our old plastic on a store shelf. We can take it home, use its hidden treasure and later put the empty packaging in the proper waste bin!
The above description is of course a simplified picture of the plastics recycling, but it’s enough to understand that it’s a technologically complex and multi-stage process.

We should be aware that everything starts at our homes and we are the ones who can make the recyclers’ job a lot easier by properly sorting our waste. Not only household, but also processing plant owners should know they are responsible for the first step of the recycling process.