An electron zap turns
flimsy plastic into
sturdy shrink wrap
Published with permission from Symmetry Magazine, A joint Fermilab/SLAC publication
If you bought a Butterball turkey this Thanksgiving,
you have particle accelerators to thank for its
freshness. For decades now the food industry has
used particle accelerators to produce the sturdy,
heat-shrinkable film that Butterballs come
“Particle accelerators tie the molecules of plastic
together and make the film tougher mechanically.
It doesn’t crack or tear,” says Marshall
Cleland, a technical advisor at IBA Industrial, an
international company that has been manufacturing
particle accelerators for commercial use
Understanding how accelerators give cross-linked
shrink film its unique properties requires
a refresher course in chemistry.
Heat-shrinkable film—commonly known as
shrink wrap—is made of polyethylene plastic. The
plastic molecules, called polymers, are long
chains of carbon atoms strung together like pearls.
Each carbon atom also connects with two
hydrogen atoms, leaving it no room to bond with
“The fully saturated carbon had its full meal,
including dessert, and becomes chemically
inert,” Cleland says. “If you heat it to the boiling
point of water, it will turn into a syrupy mess.”
However, when hit with a beam of electrons
from a particle accelerator, the plastic’s polymer
strings become chemically active.
The electron beam knocks hydrogen atoms off
the polymer chains, leaving the polymers hungry
to fill those vacancies. If conditions are right, the
carbon atoms in one chain bond with carbons
in neighboring chains—and those carbon-carbon
bonds are incredibly strong.
“The whole thing starts to knit together. Instead
of being loose threads, it is sort of like a fishnet
where everything is tied together,” Cleland says. “It
is what we call a cross-linking reaction.”
When fully cross-linked, the plastic “becomes
elastic if you heat it to boiling temperature, but
it won’t melt,” Cleland says. After electron-beam
treatment, the plastic is stronger and more heatresistant.
It can be heated and stretched into
a thin film without ripping. When cooled to room
temperature, the cross-linked plastic retains its
expanded shape. Place something inside it,
such as a Butterball turkey, and apply heat, and
the plastic shrinks back down to its original size,
resulting in an air-tight wrapping.
The food industry purchases these cross-linked
products from plastic manufacturers in large rolls
or bags, depending on how the film will be used.
You will find cross-linked shrink film wrapped
around many items in the grocery store, such as
turkeys, produce, and baked goods, as well as
around board games, video games, DVDs, and
CDs. “It’s a big business,” Cleland says.
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