The shop in the Students Union here have a new brand of pre-packed wrap in their meal
deal, and these boast that all of the packaging is environmentally friendly and recyclable.
But… you can see that they’re still wrapped in this transparent plastic material, which
doesn’t seem very sustainable at first glance.
So, in order for this to be more environmentally friendly, it’s got to be a very different
material to our usual plastic.
The first sign that it’s definitely a different material is how it behaves.
I have, here, the plastic sleeve from something else, and when you crumple it up, it springs
right back.
It’s almost impossible to crease.
But this newer one, crumples almost like paper.
Clearly, these don’t behave the same way, they’re not going to be the same material.
A good rule of thumb is that this one, which doesn’t crumple but springs back, is going
to be a petroleum-based plastic, probably polyethylene or polypropylene.
While the other, which will crumple and stay that way, is some kind of bio-polymer.
That’s a plastic-like material, but using biological materials as its starting point.
That’s not always going to be true, but it’s a reasonably reliable way of telling
the two apart.
And then, I thought, hang on, I’m in a chemistry building.
We can literally find out what these things are made of.
So, let’s head to the lab.
Anyway, we’ll do this next bit with voiceover because the lab technicians and fume hoods
were making a lot of noise.
What we have here is an infrared spectrometer.
For the uninitiated, infrared is light that has lower energy than the visible light we
can see with our eyes.
t’s off to the red side of the electromagnetic spectrum.
One reason we can’t see it is that infrared doesn’t normally have enough energy to start
chemical reactions, but it can make molecules vibrate in different ways.
The chemical bonds in molecules will stretch or twist in response to specific wavelengths
of infrared light, which we measure with a spectrometer.
And these exact wavelengths are usually characteristic of particular types of bonds and groups in
molecules.
If we see a broad response around three and a half thousand wavenumbers, it’s probably
an alcohol, if there’s a big, sharp peak around seventeen hundred wavenumbers, it’s
likely a carbonyl.
And so on…
It’s the same technique that was recently used by the James Webb Space Telescope to
characterise the atmosphere of an exoplanet.
Let’s go ahead and acquire a background scan, first, so the instrument is calibrated
right…
So, now we have the background, let’s add the plastics and acquire a spectrum.
This specific version of infrared spectroscopy is known as Attenuated Total Reflectance,
or ATR.
In ATR, the infrared light bounces inside a diamond crystal.
The light never actually goes through the sample, we just place it on top of the crystal
push it down to contact the surface as tightly as possible.
This makes the technique incredibly quick and convenient for characterising materials
like this.
No mess… let’s swap to the second one, and record it.
Of course, some spectra are incredibly complicated, and difficult to analyse by hand.
But, each spectrum will be characteristic of a certain material.
That means we can use a computer to look up existing spectra, and try to match it to known
samples.
Previously, we had to do this by hand in huge books of spectra.
But now the computers can flip through them in under a second.
I’ll add the polymer database, and run these samples through that.
And, as expected, we have one that is petroleum-based polyproplyene.
That’s one of the most common petroleum polymer sin the world, making up about 16
percent of all plastics.
It’s a slight variant on the most common one, which is about a third of all plastics.
So far, so expected.
And if we let the computer identify the other one, it tells us that it’s… cellophane.
Cellophane is a type of polymeric material based on cellulose.
Now, depending on the country you’re from, or when you grew up in, “cellophane” may
mean different things.
It’s originally the brand name for this specific material, with a trademark held by
DuPont for nearly a century.
But, because cellophane was one of the first transparent plastics ever made and mass produced
for packaging, the name has caught on as a generic term for any transparent plastic.
Even if it’s made of petroleum products.
But, here, we’re talking very specifically the biopolymer.
To make cellophane.
we need to talk about cellulose, which is a polymer formed by connecting glucose molecules
together – so, basically a long chain of sugar molecules.
For anyone remembering their school-level biology, you’ll know cellulose and starch
are the same thing, both common carbohydrates, but differ in how the glucose molecules connect
together and arrange.
The result being that starch is edible and digestible by us, but cellulose isn’t.
Animals that eat cellulose-rich plants digest their food in a different way.
So, we certainly couldn’t eat this cellophane wrap, even if it is a plant-based material.
You’ve probably come across cellulose in the form of paper.
In fact, running paper through our ATR spectrometer shows it up as cellulose or cellophane.
The database is a little confused by that one for some reason.
To get cellulose, we start with wood pulp, and dissolve it in such a way to separate
out the cellulose components from the tougher materials like lignin.
To turn this pulp into cellophane, though, you need to react it with carbon disulfide.
This is a sulfur analogue of carbon dioxide, except it’s a liquid instead of a gas and
very… very toxic.
About as toxic as carbon monoxide.
The vapour can be lethal on the parts-per-million scale.
And it has a long history of harming and killing workers throughout the 20th century through
the production of synthetic fibres like Rayon.
Which might be a bit of a problem if you want to claim this stuff is environmentally friendly.
But, that said, there are alternative processes to make it.
I can’t guarantee that this was made using toxic carbon disulfide or using any of the
other processes.
So, it might have been produced quite cleanly.
And that’s fine.
It’s just something to consider if you’re evaluating how environmentally friendly a
product is.
However…
The other key caveat is that this is not a recyclable material.
This is based on cellulose.
It’s more closely related to paper than it is to polyethylene, but can’t be recycled
with paper because it’s been reformed with sulfur.
One of the ways we try to recycle plastic materials is to shred it up, and melt it back
into re-usable pellets.
But cellophane doesn’t actually melt.
It burns.
And definitely smells a bit, too.
It’s like burning marshmallow, which makes sense if this is basically polymerised sugar.
If this gets into your plastic recycling stream, you have a problem.
It’s going to ruin or destroy anything you’re trying to recycle, and all that plastic is
going to have to go to landfill, or the resulting recycled material will be far lower quality.
So, it is important that this doesn’t go into a recycling bin along with other forms
of possibly recyclable plastic,
As a biopolymer, what it is, is degradable… or biodegradable.
If you leave this out in damp, oxygenated conditions, it will break down far faster
than plastic.
No all biopolymers decompose in home composting bins very quickly, but it will certainly break
down to nothing, and completely harmless byproducts far faster than polyethylene will.
So, while this plastic will take decades to decompose, and polyethylene will take centuries,
if you bury it in the ground, this one could be completely gone within a matter of months.