Disposable Diapers Are A Tribute To Material Science

It’s a really tough problem that has been solved to an amazing level. How do you capture and contain urine from a floppy, curved, and moving human infant? Ah, but the problem is a bit harder than that. You also want to keep that liquid away from the soft skin of the newborn and keep the exterior of your overall system dry too. From an R&D point of view the nice thing is that the customer base is huge — everyone needs some type of diapers. And what we have achieved thus far is a huge accomplishment of material science. [Bill Hammack], The Engineer Guy, takes on the engineering of baby diapers in his latest video.

A diaper uses three inner layers to sweep urine away from baby’s skin. The first layer actually repels water — being injected between skin and this layer, liquid passes through the holes in the material. But the moisture repellent property prevents it from moving in the opposite direction because of the next two layers encountered. The second layer uses capillary action to pull the moisture toward the third (and to act as a one-way moisture valve). The third layer contains a super-absorbent polymer. That layer starts off very thin and swells with absorption.

Bill explores just a bit about how these materials are actually manufactured. The layers are non-woven to form the necessary structures. The absorption layer uses cotton fibers to ensure moisture doesn’t form a dam between polymers. Whether you have a little one in your own household or not, the science behind this solved problem is fascinating and well worth the six minutes you’ll spend on the video below.

53 thoughts on “Disposable Diapers Are A Tribute To Material Science

    1. YMMV… we did a 4 year cost analysis using cloth diapers… then we switched to disposable and noticed that our water/power bill decreased the same amount as our disposable diaper expenditure… perhaps it depends on water/sewer/power/brand ect. for things to be so much cheaper. for us it turned out to be equal cost either way.

        1. But thousands of tons of CO2 and pollution ends up in the atmosphere, and other stuff ending up in the dump.

          Cost in dollars means energy and resources spent elsewhere in the economy.

  1. my polymers prof said that it would increase the cost of a diaper by $0.03 to make it also biodegradable, but that’s too much money for these companies to spend…

    1. I’d love to see this, but I think it’s a more difficult problem than just the cost per diaper. Most likely what we need is diapers that are industrially compostable. This is because you want a system where the materials will be broken down and reused in some way with sanitation in mind. Few cities have this but some, like San Francisco, are quite successful. If this infrastructure was in place there would be a lot more demand for disposables that work with it, including diapers.

    2. So they won’t raise the price of most diaper package at all to offset cost increase? It’s easy money, the companies could label them as biodegradeable, better than cheap disposable, and make it $5 per 24-pack more.

          1. It is an atypical example, most dumps are not like that, which creates a more urgent problem because methane is a very powerful greenhouse gas. They can’t have it both ways can they. :-) Fortunately the methane can be captured and used for electrical energy production, in fact this is now commonly practised.

        1. Nothing, they are idiots, huge amounts of methane are generated in dumps due to decomposition. There are very few places where bacteria cannot exist and the longevity of most plastics in biologically active soils is often overstated. Much of the plastic coming out of China is so poorly blended that it breaks up prematurely anyway.

        2. Its packed so tightly that it all just sits there. There have been studies where they core drill into old dumps and newspapers decades old are not decomposed as well as a lot of other stuff down there.

          1. Will you please stop spreading that fallacy, just because one dump had a dill core go through a stack of papers that were in a plastic bag it doesn’t mean all dumps are the same. In fact I’d say the opposite, from what I have seen they are an incredibly diverse and complex environment that is highly dependant on many factors including the local geology, water table and climate.

      1. Not going to dig through the archives at userfriendly.org but there’s a strip where Miranda asks A.J. what kind of underwear he uses, while he’s absentmindedly working.

        What kind of underwear do you wear?
        Oh my…
        On the weather! It depends on the weather!

  2. I worked as an intern at a diaper plant when I was in college. The machines were about 100 yards long, and the superabsorbant came in 2000 lb bags. Everything except the superabsorbant and the adhesives came on spools and there were over a dozen continuous webs of material feeding into the process until it got cut into shape.

  3. Tell me as soon as engineers develop an disposable adult diaper that allows for at least 4 hours of safe use by an active man with bladder incontinence. Every disposable adult diaper becames a useless something, filled with jelly bulge squeezed off the crothch area in less than an hour. Then it leaks.
    As for me, for active and otherwise healthy adults the cloth diaper is still the only viable way.

    1. Maybe a piece of tubing and a bottle? While I have never tried it, I have read about some hardcore sports fans (in a stadium), who devised something like that so they wouldn’t have to miss a second of the action.

    2. Go and get evaluated for an electrode implant, if it works you can turn the flow on and off like a tap, or even rig it up to an Arduino and play tunes when you do go to pee. No I am not taking the piss out of you, this is a real option.

    3. Well, baby diapers seems to last for 4 hours but they are pretty cumbersome. The diaper miniaturization process nowadays is not good enough so you have to trade time for comfort (and look). The same goes for hygienic pads. Good luck.

    1. I once saw a guy spray his house with this superabsorbent gel when a forest fire was on its way there. The large amount of water in the gel will keep heat from reaching the underlying material and protects the house.

      1. I WILL try that out! I’ll have several identical pieces of wood.
        One plain for a control, one wet with water, and one coated with water saturated gel.
        And a flamethrower. (Those small ones they sell for burning weeds.)

  4. The gel, AFAIK, is a sodium salt of acrylic acid. It has been used in polymer gel muscles; the gel swells when alkaline, and shrink in acid. Or around a charged platinum electrode.

    Very strong, abysmally slow, unless made in very thin layers. Obviously, not practical. Several patents available.

    I like Hirai’s plasticized PVC muscle https://www.youtube.com/watch?v=dJl2gFUgkEc

    I’ve studied it and his papers quite a bit, should have commented on the polymer muscle thread a week or so back, but I missed it. Our muscles, IIRC, are ~70mJ/cc energy density. His PVC, by my reckoning, .7mJ/cc, are not too slow, but quite feeble. That is, if you apply the material as he did.

    I was shocked and astounded that plasticized PVC can have a dielectric constant approaching 10,000 ! Maxwell-Sillars-Wagner diffusion? It shows a Warburg impedance. The plasticizer and PVC on their own only have an Er of 2 – 4.

    Although he has a patent, I sure there’s prior art, as I saw an article in a pop tech mag over 20 years ago suggesting soaking black electrical tape (PVC) in acetone. Acetone evaporates quickly. The effect doesn’t show up until you get over 2:1 ratio plasticizer to polymer. Electrical tape is probably plasticized with pthalate, which isn’t electroactive as the adipate (aliphatic ester) is. 3M sells an adipate plasticizer at Menards I’ve yet to try. Perhaps boil some veg. oil with lye to esterify it?

    Much more I could say. But I wasn’t impressed with the muscle thread. Robots won’t really proliferate without and until cheap and simple muscles are available. How about 3D printed bio-polymers? Sort out some actin, myosin, tubulin, titin, et. you get from dumpster-diving for rotten hamburger. No big deal, right?

  5. Years ago I worked in the engineering dept. for a company that made the stainless steel honeycomb panels that were part of the blowform drums used by a particular diaper company to form their absorbent pads. It never ceased to amaze me how often new design iterations came in that mainly just tweaked the pad density in specific areas. Many tens of thousands of dollars and months of engineering/manufacturing to make these machine components with only slight variations in overall pad design (to my eyes anyway).

    At one point they sent us a crate of the super absorbent polymer so we could use it to test the durability of the SS mesh that overlaid the honeycomb panels using a sandblaster (it was amazing how quickly this stuff wore out under the barrage of this material on the production line). Naturally the area around the sand blaster got covered in a fine layer of this polymer. One evening I was leaving work after a rainy afternoon and the parking lot was filled with mushy footprints left by the people that picked up the dust on the bottoms of their soles. Effective stuff.

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