Plastic shopping bags
We recycle about 500,000 tons of plastic bags and other films every year, and the amount is growing. Unfortunately, that’s only about 5% of what we use, but it’s a start. Have you ever wondered what happens to all those recycled plastic bags?
Most plastic shopping bags come from low-density polyethylene (LDPE, no. 4 in the recycling triangle). Like other plastics, it is a petrochemical, neither renewable nor biodegradable. These bags can do many kinds of environmental damage, especially as litter.
Shopping bags aren’t the only problem, of course. LDPE also makes bread bags, the produce bags groceries provide, and all manner of films used for packaging.
Recycled plastic bags and other films can become new plastic shopping bags, composite lumber, or various nanotechnology products.
Plastic bags from your house to a processor
Processing plastics for recycling
You can’t just put them in your regular recycling. They get tangled in the sorting equipment your recycling center uses and therefore interfere with its operation.
In most places, however, you can return plastic bags and other films to a grocery store and put them in the designated recycling container. Eventually, they get from the store to a processing company.
About 20% of plastic bags that get back to the store for recycling still have receipts in them. The receipts are a contaminant that the processing facility must remove. It does so by shredding the bags and then plunging them into a vat of water. The receipts and any other paper get waterlogged and sink. The plastic floats.
With the receipts out of the picture, the bags get dried and shredded into small flakes. It is these flakes that manufacturers use to make new products. In principle, plastic can go through the cycle of manufacturing and recycling indefinitely. That is, its quality does not degrade no matter how many times it gets recycled.
In practical terms, it means that bags from virgin plastic and bags from recycled plastic don’t require separate handing and can be used to make the same products.
Composite lumber
Composite lumber might be the single most important product made from recycled plastic bags. In turn, that lumber can make decks, siding, fences, benches, door and window frames, playground equipment, and more. A 500-square foot deck made of composite lumber requires about 140,000 plastic bags.
Trex and Advanced Environmental Recycling Co. are the largest manufacturers of composite lumber in the US.
Composite lumber has two main ingredients: polyethylene and cellulose fiber.
The polyethylene comes from waste plastic, either post-consumer waste such as recycled plastic bags and bottles or post-industrial waste.
The cellulose often comes from scrap wood from cabinet-makers and flooring factories. It can also come from wheat straw, rice hulls, and other agricultural wastes. In other words, composite lumber does not require the use of virgin materials
The plastic is cleaned and made into granules about the size of short rice. The cellulose is ground into a fine flour-like powder and dried. These materials are mixed together, with some additives, at high heat and extruded through a die. After it cools, it is imprinted with a wood-like grain.
Carbon nanotubes
A carbon nanotube. Carbon nanotubes are cylinders of ten carbon atoms across. IBM Fellow Dr. Phaedon Avouris created a fully functional transistor based on a single carbon nanotube molecule that is about 50,000 times thinner than a human hair.
I have seen explanations of a couple of ways of converting plastic bags and films to carbon nanotubes. I’ll describe only one of them.
First of all, though, what’s a carbon nanotube?
Nanotechnology deals with making materials as little as one atom thick. Imagine a nanosheet of pure carbon rolled into a tube. It can be either a single-walled or multi-walled nanotube. In either case, it will be fifty thousand times or more thinner than a human hair and stronger than nano steel.
Among other things, the manufacture of tennis racquets, golf balls, and other sports equipment uses carbon nanotubes.
The polyethylene in plastic bags is a chain of carbon and hydrogen molecules. Heat it to a high enough in the presence of oxygen, it burns. But instead of burning it, making nanotubes uses a process called pyrolysis. Argon gas expels the oxygen from the chamber, so nothing can burn.
The pyrolysis chamber contains membranes with cylindrical pores arranged in hexagons. At a temperature of about 850ºC, the carbon and hydrogen split apart. The carbon fits in the pores. Once a chemical process removes the membrane, carbon nanotubes remain.
I don’t suppose carbon nanotubes can ever use up as much waste plastic film as composite lumber. On the other hand, it is a new and revolutionary technology that’s constantly finding new uses. There may be other new technologies on the horizon that can provide additional uses for recycled plastic bags.
Sources:
7 things you probably didn’t know can be made from recycled plastic / SC Johnson, Mother Nature Network
How to make plastic wood / Craig Webb, ProSales. June 10, 2011
Plastic bags: where they come from and where they end up / David Brand, Global Citizen. February 23, 2018
Turning plastic bags into carbon nanotubes / mdogangun, Sustainable Nano. March 4, 2014
What happens to plastic grocery bags when they get recycled? / SeattlePi
Photo credits:
Plastic bags. Public domain from Wikimedia Commons
Plastic recycling process. Some rights reserved by Argonne National Laboratory
Wood-plastic composite. Public domain from Wikimedia Commons
Carbon nanotube. Some rights reserved by IBM Research
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