The engineering in this — a modern disposable diapers — stuns me. It’s a triumph of
polymer science. A superabsorbent polymer soaks up urine, but that’s only part of
the story: its brilliant engineering lies in the creation of interlinked mechanisms
that operate on different time scales. They whisk urine from a baby's skin and distribute
it to the diaper’s superabsorbent, which traps the urine. Let’s work our way from
the outside in. The outermost layer is a thin soft covering, almost impossible to remove
in one piece. It’s there for the parents’ comfort: It feels like cloth, but it’s made
from plastic. The next layer is a barrier sheet—typically it’s polyethylene, the
same stuff used in plastic grocery bags. This plastic barrier is there to contain feces.
Along the diaper’s edges are elasticized leg cuffs, which as noted in one of the many
patents, help contain “explosive liquified bowel movements.” So, containing solids
is simple, but where the diaper excels, where its engineering astonishes me, is its handling
of urine. To sweep it away from a baby’s skin and to keep it away is the task of the
diaper’s three inner layers. The topsheet rests on the child’s skin, bonded to it
is the surge layer — called in the industry the acquisition/distribution layer — and
then the absorbent core. A cross-section of the diaper show the relative thickness of
these three inner layers: the top sheet is very thin, the surge layer a little thicker,
and the absorbent core the thickest. Each has a different function. The topsheet’s
behavior surprises me: most liquid goes right through the mesh, but when I gently place
a drop of liquid on the topsheet it beads up. I’ve been able to get it to stay there
six hours. I’d expect the topsheet to be highly absorbent — to soak up water — but,
the fibers in the topsheet, the part touching the child’s skin, repel water! While at
first counterintuitive, this action is central to keeping a child dry. Urine passes through
the holes of the topsheet, which is made from polypropylene, because it strikes the topsheet
at over six miles per hour! Once through the topsheet, it’s water repellence helps prevent
the stagnant pool of evacuated urine from flowing back onto the child’s skin. As I
“insult” the diaper, the liquid passes through the topsheet and enters the surge
layer, which swells rapidly. Watch as that layer distributes this localized “insult”
throughout the length of the diaper. As the surge layer empties the absorbent core swells.
To whisk the liquid from the topsheet to the absorbent core the surge layer uses capillary
action. You can see that with this device. As I fill
it the blue liquid climbs up each of the capillary tubes. Most absorbent material uses this action:
for example, the threads of a washcloth form channels throughout the fabric: by capillary
action liquid “climbs” into these channels. Note: it isn’t the fibers that absorb the
liquid, but the spaces or channels between the fibers. In an absorptive material the
size of the channels determine how much liquid is moved and how fast. Notice here that the
smallest diameter tube moves liquid farther than the largest diameter tube, but, of course,
the smaller tube moves a smaller volume of liquid. The channels in the surge layer are
arranged like these four tubes. Near the baby’s skin the surge layer has large channels: this
moves a lot of liquid quickly — fast uptake — and then further from the skin a more
dense network of fibers, which creates smaller diameter channels. Not only does this arrangement
of channels whisk the urine away, it also works like a one-way valve to prevent urine
from flowing back toward the baby’s skin. You would think that something this absorbant
would be woven like cloth, but to weave a fabric with varying channel size would be
extraordinary difficult and expensive. Instead to construct the surge layer diaper companies
use “nonwovens.” In a woven fabric the strains are carefully interlaced, but in a
nonwoven polymer fibers are suspended in air and then deposited on a moving belt where
the warmed fibers adhere to each other. It’s a lot like making cotton candy: there the
fibers matte together to make something that resembles cloth, but clearly isn’t woven.
As the urine leaves the surge layer it enters the absorbent core, which stores the urine
until the diaper is disposed. It must hold urine for up to nine hours or so as the child
sleeps: a number-four-sized diaper can hold about 400 ml of urine. The urine is trapped
in a superabsorbent polymer. Here I have five or six grams of that polymer. I’ll mix it
with 160 or so grams of water.
As the polymer absorbs the water, it swells and turns into a gel.
Now, these superabsorbents are truly amazing things. This bag contains hundreds
of small spheres — each 4 millimeters in diameter — made of a superabsorbent polymer
similar to what’s in diapers. Watch what happens as I set a single sphere in a watch
glass and cover it with colored water. Over four hours it soaks up the water and grows!
The superabsorbent contains long polymer chains — long chains of carbon molecules that create
channels, like the fibers in a wash cloth. These channels between the polymer chains
absorb the water by capillary action. The water also “sticks” to the superabsorbent:
the chains are designed to chemically attract water. It forms this sphere because the
polymer chains are attached to each other — it’s called cross-linking. As the polymer
attracts water here, the water, so to speak, becomes caught in a “net” of polymer.
The superabsorbent turns into a gel, and traps, the water, or urine in the case of a diaper.
In fact, I can roll this sphere around in my hand and it still holds water, even though
it’s some 99% water and 1% polymer. Superabsorbent is so effective that when it forms a gel with the urine it
can block the flow of urine to unused parts of the absorbent core. So, in many diaper
brands it’s mixed throughout with 15% or so cotton, which provides channels for water
to reach the unused superabsorbent.The details I’ve shared with you about how a diaper
works astonish me, but equally stunning is the speed at which this precision object is
made: diaper lines produce around 1,000 diapers a minute. You’ll likely never see one in
person: the way each manufacturer designs their apparatus to move a flappy object like
a diaper at many miles per hour is a tightly-held trade secret. I’m Bill Hammack, the engineer guy.