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Materials Chemistry

The chemistry of disposable nappies: absorbency, wetness indicators and waste

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Five thousand: that’s the number of nappy changes the average child will need. There are several nappy choices available to parents, but disposable nappies make up a large portion of the market – and there’s a fair amount of chemistry behind how they keep a baby dry.

Several different polymers make up disposable nappies. Polymers are long-chained molecules formed from many small, repeating motifs – essentially the links in the chain. There are at least five different polymers in your average disposable nappy, each serving a slightly different purpose.

The nappies themselves are formed from several different layers. The top-sheet, closest to the baby’s skin, is commonly made of polypropene. Some brands also have a thin layer of lotion on the top-sheet to protect the baby’s skin. This layer of the nappy allows the baby’s urine to pass through and into the further nappy layers below.

The layer below the top-sheet is the acquisition layer. This layer is usually composed of cotton and polyester and absorbs the urine, moving it away from the baby’s skin. Cotton absorbs up to 27 times its weight in water. Salts and other compounds in urine reduce the amount that it absorbs relative to this figure, but it still does a pretty good job of soaking it up. On its own, however, it wouldn’t be long before your baby’s nappy would start to leak.

That’s where our next layer comes in. The absorbent layer underneath the acquisition layer contains some cotton too but also contains another polymer: sodium polyacrylate. This is a superabsorbent polymer which is able to absorb a whopping 800 times its own weight in distilled water. As with cotton, the amount of urine it can absorb is reduced by other compounds found within urine. Even so, it still manages a pretty impressive 30 times its own weight. As it absorbs the urine, it forms a gel, preventing wetness inside the nappy.

The average six-month-old produces approximately 15 grams of urine per hour. Most nappies only contain around 2–4 grams of sodium polyacrylate, but this, along with the absorbent cotton, is sufficient to absorb several hours’ worth of urine, keeping babies dry through the night.

The final disposable nappy layer is the water-resistant back-sheet. Commonly composed of polypropene and polyethene, this layer prevents the wetness contained in the nappy from transferring to the baby’s sheets or clothes.

In more recent years, the outside surface of some nappies for younger babies have included a ‘wetness indicator’. This is commonly in the form of a coloured line, which changes colour when the interior of the nappy is wet, letting parents know when the nappy needs to be changed.

What’s the chemical story behind these indicating lines? I contacted several nappy companies to try and find out, with mixed success. One company simply informed me that they didn’t sell their nappies in the UK any more (not what I asked!). Another was slightly more forthcoming and pointed me towards some general information on their site. This confirmed my suspicion that the lines used a pH indicator to provide the indication and a search through disposable nappy patents filled in the rest of the detail.

Many nappies that contain a wetness indicator seem to use a chemical called bromophenol blue. Bromophenol blue is a pH indicator – that is, it changes colour depending on the surrounding acidity or alkalinity. In nappies, bromophenol blue appears yellow when the nappy is dry, but the slightly alkaline pH of urine causes its colour to change to blue when the nappy is wet. Other patents suggest that some other nappies use chemicals that are sensitive to moisture as indicators, though it’s unclear how these compounds cause a colour change to appear.

Having discussed the chemistry inside disposable nappies, it’s time to discuss what happens to them after use. Those 5000 nappies per child add up – in the UK alone, they account for 2–3% of all household waste. And they stick around: it’s estimated that a disposable nappy takes around 450 years to fully decompose in landfill. In 2008 the UK’s Environment Agency estimated that, over the time a typical child wears nappies, disposables would create 550kg of carbon emissions through their manufacture, distribution, and disposal.

Given this potential environmental impact, turning to reusable nappies might seem like a no-brainer. However, the same study cautioned that taking into account the impacts of laundering reusable nappies, their carbon emissions impact was comparable to that of disposable nappies. This was based on a few assumptions, and the study identified that air drying instead of tumble-drying, reusing nappies with a second child, and several other factors could all bring down the impacts of reusables. It’s also the case that reusables don’t generate the same volume of waste that disposable nappies do.

Regardless of the debate of the benefits of reusables versus disposables, disposable nappies remain the more popular of the two. To try and combat the quantities of waste sent to landfill, some companies have started to investigate how the materials in used nappies can be recycled. One such plant near Venice in Italy aims to turn plastics from nappies into bottle caps and clothing. If enthusiasm for disposable nappies continues, perhaps this is the answer to the waste problem they pose – though it will need to be carried out at a much larger, worldwide scale.

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