- Green Technology
How is Plastic Recycled?
Identify the Type of Plastic
The international Plastic Identification Code can help us all identify the different types of plastic. Different types of plastic are processed and recycled in a different manner depending on the characteristics of the scrap plastic.
Essentially, the plastic types can be established from the number in the triangular recycling symbol, and can be summarized as follows:
1. PET eg. cool drink and bottled water, salad domes.
2. HDPE eg. milk bottles, buckets, irrigation piping, shopping packets, detergent bottles.
3. PVC eg. plumbing pipes, garden hose pipe, tubing.
4. LDPE eg. soft packets and bags, refuse bags, sandwich bags and some irrigation piping.
5. PP eg. food and clothing film, buckets, garden furniture.
6. PS eg. expanded Polystyrene fast food packaging, foamed meat trays, rigid cutlery.
7. Other eg. incorporates a vast number of specialized, industrialized plastics, including ABS for computer monitor and television housings, Nylon and others.
Sorting and Separating Plastic
Many of us want to recycle and separate plastics before sending them to the local landfill or MRF (Materials Recovery Facility). At that site, further sorting and separation is carried out.
The various plastics are sorted according to their specific type, as set out above. There are inherent differences between those plastics and generally, they are not compatible. They must be properly separated because an inseparable mixture of all types is almost useless in any commercially viable sense.
Experienced pickers can quickly differentiate most plastic types and various recyclers are then able to process those materials according to their properties. Possibly the most obvious separation is whether the plastic floats or sinks. Broadly speaking, chopped pieces of plastic will float if they fall under numbers 2, 4 or 5 above and the rest, will sink! This is a great clue and processing tool to separate those plastic types from the others and is used extensively in recycling technology. More on this below, under the washing and drying section.
Granulating and Grinding
Your plastic has now been separated into the various types. The next step is to cut if down to a manageable size for the processing steps that follow.
The plastic is fed into machines that having various spinning blades and chop the plastic into smaller pieces.Ordinarily, those pieces are anywhere from the size of a fingernail to the size of your hand's palm. The machines that perform this task are called granulators, shredders and grinders, but the essence is simply to reduce the size of the plastic pieces.
Think of the large sheets of plastic that cover a new piece of furniture or mattress. Those pieces when folded over upon each other cannot be effectively washed because of all the various nooks and crannies in which dirt can hide. However, once those sheets are chopped into smaller pieces, the task of washing becomes easier.
Similarly, it it not easy for automated washing processes to wash bottles and buckets of different shapes and sizes. If, however, those various items of plastic are all chopped into generally uniform shapes and sizes, for example, flakes the size of your fingernails, they become much easier to process.
A further reason for chopping the differing plastic items down to a similar size, is for transport reasons. If you think of transporting a truck (or car) full of plastic drums, you could fit relatively few drums into a large space. The obvious reason for this is because your would be transporting air - the drums are all hollow. Conversely, if those drums were all chopped into flakes, you could fit far more weight of plastic into a far smaller volume. The truck-load of plastic drums might now fit into a few garbage bags.
Washing and Drying
Right! We now have manageable sized flakes of plastic - relatively uniform. For this reason, it can now be processed through a washing and drying plant.
In the instance of the plastics mentioned above that float, they are processed through a plant in which they float on a bath of water and are moved along and agitated through a series of paddles. This agitation loosens dirts and separates it from the plastic, conceptually the same as what your washing machine does with your clothing.
In the case of plastics that sink (try it and cut a piece of the various plastics from household garbage, before putting them in your kitchen sink filled with water) the plastics are moved along under water with augers or other methods. Think of using your hand to move and wash plastic that has sunk to the bottom of your sink or bath...
Once the plastics have been washed (in more sophisticated plants, hot and cold water is used, and sometimes detergents too) it is important to dry them. The reason for this is because the next step in the process involves melting the plastic using heat. If the plastic is wet, the heat causes any moisture to vaporize and effectively become gas (steam). This gas causes bubbles in the molten plastic and reduces it quality as a recycled product. Imagine if your plastic outdoor furniture was filled with millions of tiny bubbles, instead of being solid plastic - it is easy to understand how this would weaken the plastic product.
There are numerous drying techniques that use squeezing, centrifugal force (really, squeezing using the forces of physics), heating and blowing (often combined by blowing plastic with hot air), amongst others. Many recyclers have proprietary methods, learned from experience, but I am sure that you get the picture. Again, you could use the image of drying clothes to understand the general concept.
Extruding and Pelletising
The final step is extruding and pelletising the chopped, cleaned and dried plastic. In many instances, the plastic is by now, further reduced in size to even smaller flakes, no bigger than a smallish fingernail.
The flakes must now be melted down to make one smooth molten flow that is fed through an extruder. This simply means that the flakes are pushed through a heated barrel or tube, until they have melted.
The melted plastic is forced out of small holes at the end of the barrel and emerges as strings of molten plastic, sometimes referred to as spaghetti. Continuous strings of plastic are then cooled by running them through a long bath of cold water, to harden the plastic spaghetti strings.
The last step in the process is a simple chopping machines that constantly chops the long strings into short, little pieces, known in the industry as pellets. It is useful to think of a chef using his knife to chop a carrot or piece of celery into short pieces, but on a smaller scale. The plastic strings that are chopped up are generally thinner than pencil and the plastic pellets are only a few millimeters in diameter.
In the interesting case of PET (number 1 in the recycling triangular symbol), the plastic is turned into thin fiber that makes the stuffing in your wind-breaker jackets and duvet! That stuffing inside is often recycled cold drink bottles!
Your garbage and scrap plastic has now been turned from a broad variety of dirty bottles, bags, pipes, drums and other containers or furniture, into a single colour and uniform size of plastic piece that is ready for the next step in the process - conversion into a plastic product of some sort.
The process described above has produced the raw material (admittedly recycled, but inherently the same thing) that is used in manufacturing most plastic products. The recycled plastic pellets are slightly bigger than grains of rice and easy to handle, transport, store and work with. Plastic is extruded and turned into recycled pellets.
The conversion of plastic can be understood to mean that plastic raw material is processed into a final consumer product. There is a massive list of plastic products, as a quick glance around any room in your home will tell you.
Plastic pellets (the raw material of the plastic manufacturing industry) are converted by melting them down again and forcing the resultant, malleable (almost fluid) ooze into molds and using other processes to make a final product. The reader could picture melting candle wax and molding it into a shape while still hot, before cooling it down to take its final shape.
Your recycled plastic is used to make sheeting, packets, piping, containers, bottles and all sorts of other products. Your recycling is meaningful - and I hope that this article will encourage you to recycle all your waste/garbage, not just plastic.