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I am looking to use the absorbent material found in nappies to improve the water retention of the soil in my garden.

To this end, I have two questions that I have not found definitive answers too.

  1. Are disposable nappies made with Sodium or Potassium Polyacrylate?
  2. Is Sodium/Potassium Polyacrylate biodegradable?
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This class of polymer is also referred to as superabsorbent polymer(SAP).

  1. Disposable diapers/nappies are likely to be the sodium form. Sodium forms are typically cheaper. Further, the potassium form of the salt is not as soft and would be less suitable for cloth-like wearables.

  2. Polyacrylate SAP does not biodegrade in soil very fast.
    For the main chain polymer:

...if at all, 0.12-0.24 % per 6 months.

Refs:
1: A manufacturer of agricultural/hygiene polyacrylate SAP

2: Wilske, B.; Bai, M.; Lindenstruth, B.; Bach, M.; Rezaie, Z.; Frede, H.-G.; Breuer, L. Biodegradability of a polyacrylate superabsorbent in agricultural soil. Environmental Science and Pollution Research 2013, 21 (16), 9453–9460.Springer

Side note: All diapers with an absorbent layer are SAP, either polyacrylate or polysaccharide (rarely). Polysaccharide is the more expensive / less effective 'green' alternative, and if your product contains this, it will tout it on the label. (link limit reached)

babygearlab.com/a/11113/What-Is-Inside-Those-Disposable-Diapers
https://blog.bolandbol.com/product-reviews/diapers-review/
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.608.22&rep=rep1&type=pdf
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    $\begingroup$ This is little more than a link-only answer. Since links to the www could theoretically decay quickly, this site’s policy is to include (i.e. quote, using > at the beginning of lines) the essential information here but leave the link as a citation. Please edit to do so or this posting may be eligible for removal. $\endgroup$ – Jan Mar 29 '17 at 14:59
  • $\begingroup$ Sorry about that. New to the process. $\endgroup$ – Pete Mar 29 '17 at 16:05
  • $\begingroup$ Should be sorted now. $\endgroup$ – Pete Mar 29 '17 at 19:04
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To keep it simple: there are as many different polymers as suppliers of the superabsorbent polymer used in diapers.https://en.wikipedia.org/wiki/Superabsorbent_polymer#Current_synthesis.

So, answer to #1. Some are, some aren't. As far as #2, define what you mean by "biodegradable". Take a diaper, put it into a composting bin, and turn it over every week for a million years, and I'd expect little to remain. It is likely based on your question that you are unaware that some diapers are marketed to be "biodegradable" and some aren't. The reality is that the term has no standard meaning - it's marketing hype. However, it's also true that some polymers will be inherently much more prone to decomposition than others and that some of these are incorporated into some diapers.

Your best bet is to call (or visit) the customer service of the specific diaper manufacturer you are interested about. Note that the nih link provided in a different answer doesn't mention any use of the sodium salt for any major brand. Pampers (P&G), I think, dominates the market and sells 9 different disposable diaper brands. I didn't see any mention on their site about biodegradability, which was an important controversy some generations back. Since even "highly biodegradable" materials can take centuries to decompose in landfills, the question is a bit meaningless. (If you want biodegradable, use toilet paper (not seriously, since TP is designed to fall apart rapidly, too rapidly for practical use in a wearable product).)

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