Ah, foam.
Its frothy goodness is responsible for everything from shaving cream to lattes.
But the word “foam” can describe /any/ liquid or solid with tiny pockets of gas inside.
Think back to when you were a kid – maybe you blew bubbles in your milk.
Those air-filled suds are a kind of foam, but so is a squishy memory foam mattress.
Thanks to the gases inside, foams can be really lightweight, useful materials for lots of
different purposes.
They could even save your life someday.
Take Silly String.
It’s awesome for pranks or party shenanigans, but there’s a lot of interesting chemistry
going on inside these cans.
The string part of this foamy toy is mostly made of an acrylic resin, a bunch of plastic
molecules linked up in long chains.
The original formula for Silly String had an acrylic resin called polyisobutyl methacrylate,
which is also found in Plexiglass.
This resin is mixed with a few other chemicals to give the strand some color, stickiness,
and more strength.
Now, the propellant is what forces all this stuff out when you squeeze the ­nozzle of
the can.
This lets the pressure drop inside the can, so the compressed liquid propellant turns
back into a gas, expanding to force the string goop out.
In the 1970s, Silly String used the ozone-depleting Freon-12 as a propellant, before chlorofluorocarbon
gases were banned from production.
Modern Silly String uses propellants called hydrofluorocarbons instead, which are cousins
that don’t damage the ozone layer, but are still potent greenhouse gases.
Squeezing out this mixture of ingredients creates thin foamy strands, with lots of help
from surfactants.
Surfactants are molecules that can be found in detergents around your home.
They have water-loving hydrophilic end, which interact with water molecules, and awater-repelling
hydrophobic end, which interact with other molecules like the ones in air or oil.
So, when the Silly String liquid gets exposed to air, surfactants like sorbitan trioleate,
basically stabilize the bubbles of air inside to make a fluffy, sticky foam.
It’s a fun and messy toy for kids, but this lightweight foam also has been used for serious
safety by the U.S. military – to detect trip wires near explosives.
If they spray Silly String into an empty room, and it falls to the ground, the room is safe.
But if there’s a trip wire or another hazard, the foam will hang off of it without setting
off the booby trap.
Another cool sometimes-canned foam is spray foam insulation, also called spray polyurethane
foam, or SPF for short.
It’s that whitish-yellowish stuff that you might see in houses under construction, or
even just in your garage.
And it’s designed to expand and harden to control heat and airflow.
To make this foam, two /separate/ mixtures are combined and sprayed at the exact same
time.
One container, usually called side A, contains some of the chemical building blocks of polyurethane,
the polymer SPF is named after.
[EYE-so-SIGH-un-ates] They’re called isocyanates, which are a
kind of organic chemical that all share a functional group of reactive atoms.
And they’re used in lots of coatings, glues, and foams.
[POLY-alls] Side B stores the other building blocks called
polyols, which have a bunch of hydroxyl functional groups.
Side B has some other stuff in it too, like fireproofing chemicals, catalysts to speed
up the reaction, surfactants, and a blowing agent, which helps expand the foam and affects
its density and stiffness.
When sprayed, the A and B sides mix together to create the super-tough polyurethane polymers
and some gases, to make a foam that hardens where it lands.
The mixture of gases in the foam bubbles makes it resist heat transfer really well.
So when you spray this foam onto walls and roofs, it acts as an insulator and reduces
unwanted airflow.
It’s good for weatherproofing /and/ it brings down the energy bill, which is never a bad
thing!
Finally, there’s the futuristic-sounding metal foam, which is just a solid metal — usually
something like aluminum — with lots of gas-filled pores.
There are a couple ways to make it.
You can inject some air, nitrogen, or argon gas straight into molten metal, creating gas
bubbles as the metal expands, cools, and hardens.
Or you can mix some blowing agents, like titanium hydride or calcium carbonate, into molten
metal.
These chemicals decompose when they’re heated, which also makes bubbles.
Metal foam is really useful because it acts just like its parent metal – with things
like corrosion resistance, strength, electrical conductivity, and thermal conductivity – but
at a fraction of the weight.
So it can be used for lots of things.
For example, car parts made of metal foam cut down on noise, reduce weight, and can
increase energy absorption during crashes so that we don’t get smushed.
It’s been used to create lighter animal prosthetics.
And, researchers are developing metal foam that mimics bone for biomedical implants that
hopefully won’t be rejected by the human body.
There’s even research into composite metal foam, which combines several metals for even
more strength, and could be developed into bulletproof armor.
So foam’s not only for parties.
It can save lives!
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