When one pulls on nylon fibers, they do something special. They stretch for awhile, but then,
after the fibers have stretched so far, the fibers get very strong and resist being stretched
any further. Once the fibers have been stretched, or drawn, they are strong enough to
use to make things like textiles and rope. Nylon isn't the only material that can be cold
drawn like this to make strong fibers. Polyesters
can also be cold drawn. In fact, cold drawing was discovered by
Julian Hill in 1930 when he was trying to make
fibers from a polyester he had just made, years before the invention of nylon in 1935.
Now let's take a closer look at what happens to a nylon filament when it is cold-drawn. When an undrawn nylon filament is stretched, it becomes thinner in some sections. The more the filament is stretched, the longer the thin drawn portions become, and the thicker undrawn middle section gets shorter and shorter until the whole filament is thin and drawn. To get a better idea of what is going on, use the simulation below to see for yourself what nylon filament looks like during cold-drawing:
When one cold-draws a real nylon filament, the filament is easy to stretch. But once it has been stretched to the point where that thick middle part of the filament has all been exhausted, the whole filament has been made thinner (drawn), it will suddenly resist stretching.
Obviously, it is the thick undrawn part of the filament that stretches. Once it is gone, little
stretching occurs. This is good, because now the filament will make a strong fiber. Why does
the thin, drawn filament resist stretching?
Before drawing, the polymer chains are
tangled up in a disorganized manner. We call them
amorphous
when they're tangled up
like this. But drawing forces the polymer chains to stretch out long and straight, as you can
see in the picture below:
As the filament is cold drawn, the nylon polymer molecules first stretch out straight at the ends of the filament. This is why the ends of the filament becomes thin before the thick middle portion of the filament. Take a closer look at a filament during the cold-drawing process:
When the polymer chains are stretched out long and straight, they can then form crystals. This means the polymer chains are arranged in an orderly manner. They stay in this orderly manner because they are held together by strong hydrogen bonds between the amide groups in the polymer chains. Because nylon polymer molecules are held tightly together, they can no longer stretch or deform easily.
When nylon is made commercially, cold-drawing is carried out after the nylon polymer has been spun into filaments. Cold-drawing is a separate process from spinning, and is carried out using a machine called draw twister.
In real-life, cold drawing doesn't always happen as neatly as in the pictures on this page. Usually, the narrowing of the fiber, called necking, occurs at more than one place. A real fiber being cold drawn might look more like this:
In a real draw twisting machine the draw pin keeps the necking from happening in more than one place.
References
1. Carothers, Wallace. Journal of the American Chemical Society, 1932, 54, 1980.
2. Hermes, Matthew. Enough for One Lifetime: Wallace Carothers, Inventor of Nylon. Washington, D.C.: American Chemical Society; Philadelphia: Chemical Heritage Foundation, 1996, pp. 115-6.
Image credit
Julian Hill re-enacting the discovery of cold-drawing: Gift of Joe Labovsky.