College of Engineering
Plastics: Durable, Diverse, and Indestructible
in MSE Spotlight Faculty, MSE Spotlight Research, Spotlight Faculty, Spotlight Research
PBS broadcast NOVA invites Professor Malika Jeffries-EL (Chemistry, MSE) to explain plastics.
Watch the full PBS segment here
Brief episode abstract: Through a series of lab experiments, “Beyond the Elements” host David Pogue learns that there are tens of thousands of grades of plastics, each tailored for a specific purpose. What do we do with them when their job is finished?
Transcript:
- Plastics: Durable, Diverse, and Indestructible
Published: February 12, 2021
David Pogue: Malika Jeffries-EL plays with the molecular building blocks of plastic for a living. She’s a polymer chemist at Boston University
So clearly, there’s all kinds of different plastics, but is there something that unites them all that makes a plastic a plastic?
Malika Jeffries-EL: Plastics are a subset of polymers, in that they are known not just for having their macromolecular structure but the processing and mechanical properties that come from as a result of that structure.
David Pogue: Like bendy-ness and strength.
Malika Jeffries-EL: Exactly, strength, exactly, flexibility. Rigidity would be another property.
David Pogue: Like rubber, all plastics are polymers, long molecules made up of subunits called monomers. What makes each of these polymer-based materials distinct, are the combinations of the different monomers used to make them.
Malika Jeffries-EL: For example, this is actually really hard and rigid, and one of the units in here is styrene, and this is polystyrene.
David Pogue: Not hard and rigid at all.
Malika Jeffries-EL: Not hard and rigid at all, but when you blend in the other molecules, you get different properties.
David Pogue: Wow.
But it’s not all chemistry. Processing can turn the same plastic into very different products.
Malika Jeffries-EL: These were actually molded and blown into this bottle shape, and in this case, really small fibers were spun from the polymer and then processed to make this.
David Pogue: And it comes out soft and comfortable.
Malika Jeffries-EL: Comes out soft and comfortable.
David Pogue: Our Age of Plastics isn’t very old. It was this guy, Leo Baekeland, who gets credit for the first fully synthetic plastic. He called it Bakelite and by the 1920s it had become a big hit—in all kinds of products, from radios to kitchenware to kids’ toys and coming in a variety of colors.
Malika has offered to whip up some of this landmark plastic.
It’s made from two monomers, phenol—a ring of six carbon atoms bonded to five hydrogens, and an oxygen bonded to a hydrogen—and formaldehyde, one carbon atom bonded to two hydrogens and double bonded to an oxygen.
After dissolving the solid phenol into the formaldehyde solution Malika adds two acids to start up the process. Then we wait.
Malika Jeffries-EL: There should kind of be this “Aha” moment and it should just go.
David Pogue: Are you saying it’s gonna harden?
Malika Jeffries-EL: Yeah, it should get cloudy and polymer should come crashing out. I feel like it’s getting pinker, which is an indication that the chemistry is changing.
David Pogue: Oh did you see that? Like instantaneously.
Right before our eyes, the phenol and formaldehyde molecules link up, giving off water molecules while creating long polymer chains.
You made plastic. Look at that. Genuine, crusty, hard, hard plastic.
Malika Jeffries-EL: So this is an example of a thermoset plastic. Once it’s set into place with heat, you can’t reform it or reshape it with additional heat.
David Pogue: Oh okay, so unlike a plastic drink bottle…
Malika Jeffries-EL: That’s right.
David Pogue: …you can’t melt this down and reform it into something else.
Malika Jeffries-EL: No.
David Pogue: This is Bakelite now and forever.
Malika Jeffries-EL: That’s stuck like that forever. Yep
David Pogue: In a thermoset plastic like Bakelite, the bonds between the polymer chains are extremely strong. By the time you’ve applied enough heat to break them, the chains themselves have decomposed. So you can’t re-melt thermoset plastics or reshape them for recycling. But not all plastics are thermoset.
There’s nylon, the first commercially successful plastic that wasn’t. It came to public attention at the 1939 World’s Fair as a substitute for silk in women’s stockings. And its importance grew during World War II. At the time, the main source of silk for parachutes was America’s enemy, Japan. So the military recruited nylon as a replacement.
Malika wants to show me one more example:
And this time what are we going to make?
Malika Jeffries-EL: Um, so for this demonstration I thought I would show you how we make polyurethane foams.
David Pogue: And what do we use polyurethane foam for in the world?
Malika Jeffries-EL: Polyurethane is used in like seat cushions, and also insulations. You think about blown foams and things like that.
David Pogue: Oh yeah, E.T. blown foam. Yeah I remember that.
There are two key reactants: First up is a type of molecule with an oxygen-hydrogen hook at either end. Aside from its role in polyurethanes, this one shows up in paintballs and laxatives, too. The other reactant carbon-backboned isocyanate molecule with the nitrogen-carbon-oxygen hooks at either end.
Malika Jeffries-EL: And we stir those together. And so you can already see it’s starting to react because it’s starting to get milky and it’s starting to grow and you can see it’s rising up a little bit.
David Pogue: The two molecules begin to link up to form a polyurethane polymer. At the same time, one ingredient also reacts with some water generating carbon dioxide gas. That’s what causes the bubbling and ultimately the foam when the polyurethane grows rigid.
I know I’m tacky but—Oh! And the cup’s entombed inside there.
Malika Jeffries-EL: Yeah the cup is gone.
David Pogue: At this point…
Malika Jeffries-EL: Polycarbonate
David Pogue: …you are probably getting the idea…
Malika Jeffries-EL: Polyethylene terepthalate, PETE
David Pogue: …that there are lots of different plastics…
Malika Jeffries-EL: Polyvinylchloride, PVC
David Pogue: …each made out of polymers…
Malika Jeffries-EL: These are examples of polyamides, commercially known as Nylon
David Pogue: …constructed sort of the same way,
Malika Jeffries-EL: Polystyrene
David Pogue: …but out of different subunits
Malika Jeffries-EL: Polypropylene, PP
David Pogue: …to obtain very different material properties.
Malika Jeffries-EL: Low-density polyethylene, LDPE
David Pogue: And then if you start throwing in additives and fillers…
Malika Jeffries-EL: Polyvinylalcohol, PVA
David Pogue: …like colorants…
Malika Jeffries-EL: High-density polyethylene, HDPE
David Pogue: …flame retardants, glass or carbon fibers…
Malika Jeffries-EL: Polymethylmethacrylate, PMMA
David Pogue: …you end up with tens of thousands of “grades” of plastic…
Malika Jeffries-EL: Polyoxymethylene, POM
David Pogue: …each tailored for a specific purpose, which has created the problem: What do we do with them when that job is finished?
Mostly, we throw them out. 91% of all the plastic we make ends up in landfills. Or burned. Or just escapes into the environment. The remaining 9% is recycled. But first, the plastic has to be carefully separated by type, those recycling number symbols. Any mix-up there can contaminate any otherwise reusable plastic, rendering it worthless.
