How do baby diapers work?
It’s an interesting question. Before 1982, diapers relied on the absorbancy of cotton, paper and sponges to hold the, um, liquid in place. Unfortunately, those materials can only hold about 20 times their weight in water. The average diaper doesn’t really weight that much, so 20 times not very much equals leaks.
Today we have a much better situation. The use of superabsorbant polymer materials such as sodium polyacrylate has dramatically increased the “storage capacity” of your typical diaper. Sodium polyacrylate was originally developed as an agricultural product. Spread over crop fields it helps even out the drench-drought cycle by releasing moisture as needed. It is a super-absorbing polymer that can hold up to 800 times it’s weight in water. Again, compare that with paper, cotton or sponges which hold about 20 times their weight in water.
This superabsorber is commonly used in diapers to provide super absorbency. This also means it is easily available if you have a diaper (use a clean one – please!) on hand. To harvest the polymer powder, rip open a diaper and place it inside a one gallon zip lock bag. Seal the bag and shake to release the powder from the other materials inside the diaper. The powder that will collect in a corner of the bag is the superabsorber.
Imagine the possibilities for a science fair project where after harvesting the superabsorbant material from a variety of diapers you investigate the question of “Which diaper brand has the most absorbing capacity?”
Other uses for superabsorbers include potting soil where it is used as a moisture control agent. When water is added to the soil, the polymer will absorb and hold some water. As the soil dries, the polymer releases it’s water keeping the soil moist. Newer varieties of grass seed also incorporate a layer of the polymer to encase the seed in a moisture rich environment to promote germination.
A note on absorbancy of Sodium Polyacrylate
This polymer can absorb up to 800 times its own weight in distilled water, 300 times in tap water and 60 times in 0.9% NaCl (a standard solution to mimic urine absorbency). The difference in these three solutions is the electrolyte (often sodium) concentration. In deionized water, water is absorbed by the polymer to lower the sodium ion concentration inside the polymer.
Under low electrolyte concentrations (distilled water) osmotic pressure allows the polymer to swell and hold a great amount of water in a gel. For instance, adding distilled water to the polymer powder will create a gel. The addition of an electrolyte to the gel decreases osmotic pressure causing release of water from the polymer. For instance, sprinkling table salt (sodium chloride) on top of the gel will increase the electrolyte concentration outside the polymer and the gel will appear to “melt” as the water is released. The water exits the polymer to equalize the electrolyte (sodium ion) concentration.
A brief history of superabsorbers
In the early 1960s, the United States Department of Agriculture (USDA) was conducting work on materials to improve water conservation in soils. They developed a resin based on the grafting of acrylonitrile polymer onto the backbone of starch molecules (i.e. starch-grafting). The hydrolyzed product of the hydrolysis of this starch-acrylonitrile co-polymer gave water absorption greater than 400 times its weight. Also, the gel did not release liquid water the way that fiber-based absorbents do.
The polymer came to be known as “Super Slurper”. The USDA gave the technical know how several USA companies for further development of the basic technology. A wide range of combinations were attempted including work with acrylic acid, acrylamide and polyvinyl alcohol (PVA).
In the early 1970s, super absorbent polymer was used commercially for the first time – not for soil applications as originally intended – but for disposable hygienic products. The first product markets were feminine sanitary napkins and adult incontinence products.
In 1978, Park Davis used super absorbent polymers in sanitary napkins.
Super absorbent polymer was first used in Europe in a baby diaper in 1982 when Schickendanz and Beghin-Say added the material to the absorbent core. Shortly thereafter, UniCharm introduced super absorbent baby diapers in Japan while Proctor & Gamble and Kimberly-Clark in the USA began to use the material.
The development of super absorbent technology and performance has been largely led by demands in the disposable hygiene segment. Strides in absorption performance have allowed the development of the ultra-thin baby diaper which uses a fraction of the materials – particularly fluff pulp – which earlier disposable diapers consumed.
Super absorbents used in soil amendments applications tend to be cross-linked acrylic-acrylamide co-polymers (usually Potassium neutralized).
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