Apatite for Destruction – the hypotheses so far

ash test crucible

The ash ‘skeleton’ left in the crucible after a sample of nappy had been heated to 600degC for 16 hours

Our project is investigating the mineral exoskeleton found on some nappies in our experiments last year. So far it’s mainly been looking stuff up on the internet, digging around, and talking through what we can do and what our priorities are.

To give you a bit of a flavour, here are the bare bones of the facts about our test nappies from last year’s experiment. And below that, the list of hypotheses the group have come up with so far. (A hypothesis is a kind of educated guess, based on what you know or observe. The point of the hypothesis is to use it to make predictions, which you can then test. It’s a starting point for further investigation. Even if it turns out to be wrong, a hypotheses has been useful, if it helps move the research forwards.)

The test nappies were all sent to us because they were experiencing problems – mainly persistent smells. They were tested by Shirley Technologies Limited, a forensic and industrial testing laboratory in Manchester who specialise in textiles and laundry testing. You can read more about how we chose the nappies for testing here.

Nappy A
Nappy type: Little Lamb size 2 fitted, viscose (from bamboo)
Hydroxyl apatite content: 3.8% w/w
Washing history: 1/2 dose Method liquid at 40degC, tumble dried
Water: Hard

Nappy B
Nappy type: Tots Bots V3 AIO, polyester
HA content: 0.6% w/w
Washing history: Unknown (preloved)
Water: unknown

Nappy D
Nappy type: Little Lamb size 2 fitted, viscose (from bamboo)
Hydroxyl apatite content: 20.7% w/w
Washing history: 1/4 dose (30ml) Daz bio powder + Vanish Oxi action at 60°C + extra rinse. Washed every 3-4 days (sits for 3-4 days in pail until full load achieved). Indesit innex xwe 91683 9kg 1600 spin. About ten months old. Line dried.
Water: Very hard

Nappy E
Nappy type: Tots Bots Bamboozle, 80% viscose (from bamboo), 20% polyester
Hydroxyl apatite content: 21.9% w/w
Washing history: non bio at 40°C. Washed once or twice a week. (150-180 washes over 2.5 yrs)
Water: Moderately Hard, “phosphate added for plumbosolvency”

Nappy G
Nappy type: TJ’s Pocket Nappy ‘charcoal fleece’, polyester
Hydroxyl apatite content: 0.5% w/w
Washing history: Unknown (preloved)
Water: Unknown


Our group have come up with the following long list of hypotheses about hydroxyl apatite and nappies, and are now discussing how we might test each one, and which ones are most important.

1. The calcium in the hydroxyl apatite is from hard water.
2. The more liquid a nappy can absorb, the higher the HA content.
3. HA will only form on viscose nappies.
4. The phosphate in the HA comes from urine (which is raised as a result of a dairy diet in infants)
5. The phosphate is in the water (added to stop lead leaching from old pipes)
6. The phosphate comes from detergents.
7. the phosphate is from some as of yet unidentified source
8. All these sources of phosphate can contribute.
9. A nappy with HA will contain more live bacteria after washing than one with no HA under the same conditions
10. HA will be found on nappies washed in water where there is added fluoride.
11. HA is deposited from solution i.e. in the wash
12. HA is deposited from the solution left on the nappy as it dries.
13. The HA is deposited on the dirty nappy before washing.
14. Insufficient detergent makes hydroxyl apatite growth more likely.
15. Bacteria will be seen to find little nooks and crannies of the perfect size for snuggling in on the surface of the hydroxylapatite.
16. Nappies with a high bioburden will have fibre damage which is conducive to nucleation of hydroxyl apatite crystals.
17. HA may be found on nappies where there are no ‘user problems’
18. HA is more likely to form on nappies that are in contact with urine for prolonged periods e.g night nappies.
19. If urine is involved in the formation of the HA, then there will be more HA in the ‘wee zone’ of the nappy, than in the back.
20. HA has been found in (USA) baby formula. Could it be building up on nappies of bottle fed babies?
21. Once hydroxyapatite starts to form, it forms a substrate for more crystals of HA to attach to and so the problem gets rapidly worse once it has started.

If you have anything to add about hydroxyl apatite and it’s chemistry, the way it builds up on laundry, or tests we should be thinking about doing, then please get in touch.

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