Summary
- In our previous experiments, we got five nappies tested in a lab, and found that two of them were 20% by weight hydroxl apatite (HA), so we decided to get a few more nappies tested.
- This time around the nappies contained 1-5% minerals after the ash test. Some of this was Hydroxyl apatite, and the rest was calcium carbonate (CC). Calcium Carbonate is a salt that is formed when detergent reacts with calcium (probably from hard water). In some cases the amount of calcium carbonate exceeded the amount of HA on the nappies/wipes.
- 5 viscose nappies and 2 viscose wipes were tested (viscose from bamboo).
We hypothesised that the wipes would contain less HA because they had been exposed to less wee. This hypothesis was disproven (we were wrong!). One wipe contained 15% minerals and the other 5% minerals. The minerals were a combination of CC and HA, with the proportion of CC being higher than that of HA.
- For 2 of the nappies, samples were taken from wee zone (crotch of nappy) and non-wee zone (near Velcro) for comparison. We hypothesised that the wee zone would contain more HA than the non-wee zone due to increased exposure to urine. We saw this pattern on one nappy, but not the other. More data would be needed to draw conclusions.
- Some of the nappies had been washed in a full dose of detergent, and others in half-dose of detergent. We hypothesised that nappies washed in a half dose of detergent would contain more HA. This did not appear to be the case – the half-dose nappies we tested actually contained less mineral content than the full dose nappies. However, more data would be needed to say this for sure.
- We also asked the people who donated the nappies to complete a detailed survey on washing, drying, dry pailing and storage routines, as well as information about their child and their diet. I have not spotted any obvious correlations (links) between the lab results and the survey data, but why not look for yourself, and let us know if you spot anything interesting.
If you are in a hurry, key points / discussion questions are highlighted in red.
Background to experiments
In the course of our Nappy Science Gang investigations into reusable nappies, we found some nappies with an ‘exoskeleton’ of hydroxyl apatite (a mineral found in bones and teeth). Two out of the five had about 20% w/w of hydroxyl apatite in the fibres i.e. 20% of the weight of the dry nappy was made up of this mineral.
The Royal Society for Chemistry then awarded us a grant for a new user-led public science project about nappies: Apatite for Destruction.
Read more about the background to the project here.
During the course of our discussions, the group came up with two hypothesis:
- The calcium in the hydroxyl apatite is from hard water.
- Insufficient detergent makes hydroxyl apatite growth more likely.
Nappies sent to Shirley Technologies Limited
5 viscose nappies and 2 viscose wipes were selected and sent to Shirley Technologies Limited.
We asked for samples to be taken from the wee zone (crotch area) and from the non-wee zone (near Velcro) – see image below.
- We hypothesised that the wipes would contain less HA because they had been exposed to less wee.
- We hypothesised that the wee zone would contain more HA than the non-wee zone due to increased exposure to urine.
- Some of the nappies had been washed in a full dose of detergent, and others in half-dose of detergent. We hypothesised that nappies washed in a half dose of detergent would contain more HA than those washed in a full dose of detergent.
This is what Shirley Technologies did with our nappies:
“A sample of material from the test area of all submitted samples was ashed in a silica crucible in a muffle furnace at 6000C for 16 hours. The amount of residual material was calculated as a percentage of the amount of original material. Subsequent to quantification for the ash content, the nature of the residual material was analysed using Fourier Transform infra red spectroscopy (FTIR)” (Quote from their report).
If you want to learn more about these tests, there is a simple explanation with pictures at the bottom of this blog post.
Results of Ash Test
This time around the nappies contained 1-5% minerals after the ash test. Some of this was Hydroxyl apatite, and the rest was calcium carbonate (CC). This was a surprise finding.
In some cases the amount of calcium carbonate (CC) exceeded the amount of hydroxyl apatite (HA).
The order in which the terms are written is an indication of relative proportions.
HA = hydroxyl apatite, CC = calcium carbonate
| Owner | Ash Content (% w/w)
Wee zone |
Ash Content (% w/w) Non-wee zone |
Full / half dose |
| ASR | Not tested | 2.46
HA |
Full dose |
| ASR | 3.33
HA |
1.58
HA |
Half dose |
| EM | Not tested | 3.36
CC & HA |
Full dose |
| EM | 1.67
HA & CC |
1.94
CC & HA |
Half dose |
| ER | 5.28
HA & CC |
4.29
CC & HA |
Full Dose |
| ASR wipe | 15.60
CC & HA |
| EM wipe | 5.57
CC & HA |
This is the commentary Shirley Technologies Limited provided in their report:
“STL would comment that this hydroxyl apatite is a common by-product of insoluble components of detergents redepositing onto washed materials of cellulosic composition. This effect is known to be exacerbated by the presence of calcium or magnesium salts in the water, as such the effect is more prevalent in so called ‘hard water’ areas of the country
STL would also comment that some of the tested residual matter also contained calcium carbonate, another inorganic salt formed by reaction between detergent and calcium salts originating from hard water. In some cases the proportion of calcium carbonate actually exceeded that of hydroxy apatite.
Further analysis showed that in all cases the residual material was intimately associated with the fibres of the tested fabric, and formed a skeletal residue when the ashing removed the cellulosic fibre.”
Unfortunately, it is not possible for Shirley technologies to tell us exactly how much of the ash content is HA and how much is CC. They can only tell us which there was more of (one relative to the other).
It is interesting to note that in the non-wee zone, there is more CC than HA, but in the wee zone, there is more HA than CC. Could wee be contributing to that?
The wipes also contained more CC than HA.
Comparing Results to hypothesis
- We hypothesised that the wipes would contain less HA because they had been exposed to less wee.
This hypothesis was disproven (we were wrong!).
One wipe contained 15% minerals and the other 5% minerals. The minerals were a combination of CC and HA, with the proportion of CC being higher than that of HA.
- We hypothesised that the wee zone would contain more HA than the non-wee zone due to increased exposure to urine.
We saw this pattern on one nappy, but not the other.
More data would be needed to draw conclusions.
- We hypothesised that nappies washed in a half dose of detergent would contain more HA than those washed in a full dose of detergent.
This did not appear to be the case – the half-dose nappies we tested contained less mineral content than the full dose nappies.
However, more data would be needed to say this for sure.
We also asked those who donated nappies to complete a detailed survey, the results of which are in the tables below:
Washing History
All nappies donated were Little Lambs made from Viscose (usually from Bamboo).
| Owner | Washing Powder | Water type | Wash routine | Nappy ‘Issues’ and attempted resolutions |
| ASR (full Dose)
2.46 HA only |
Sainsburys biological powder – 170ml | Very hard water | 60 degrees, pre wash, full wash, extra rinse, no water save, 1600 rpm spin
Full drum |
Smelly, crunchy
Always been an issue (obtained new) Line dry where can, double wash very occasionally |
| ASR
(half dose) 3.33 and 1.58 HA only |
Sainsburys biological powder -60ml | Very hard water | 30 or 40 degrees, pre- wash, main wash, rinse, long spin at 1600 rpm. Half-full drum |
Smells, hard / rough
Always been an issue (obtained preloved) Occasional 60 degree wash |
| EM
(full Dose 3.36 CC & HA |
Fairy non-biological powder – 200ml | Very hard water | Cotton cycle 40; pre-wash, long wash & rinse, 1000 rpm spin
¾ full drum |
Crunchy. / ammonia smell
Gradually getting worse Wash at 60; hang on line in rain |
| EM
(half dose) 1.67 and 1.94 CC & HA |
Fairy non biological powder – 100ml | Very hard water | Cotton 40- plus pre-wash, long wash & extra rinse (2hr 45), 1000 rpm spin
¾ full drum |
None stated |
| ER
5.28 and 4.29 HA and CC |
Daz biological powder – 200ml | Hard water | 5 minute “quick wash” cycle (20C) with aprox 4tbsp powder, followed by full length cottons cycle @60C with full dose powder aprox 2.5hr cycle no additional rinses. 1400 rpm spin.
¾ full drum |
Slightly crunchy and stale smell. Smells very strongly of ammonia when used.
Noticed after approx. 12 months of use Hung out in rain for a few days
|
Useage History
All nappies were used with a fleece liner and PUL wrap.
One difference from last year is that all but one of the nappies have been out of use and in storage for a period of time. What might happen to HA under different storage conditions?
| Owner | Drying | Storage | Creams | Useage |
| ASR (full Dose)
2.46 HA only |
Dehumidifier in bathroom with nappies on an airer
24-36 hours to dry |
Dry pail 3 days
|
Bepanthen | Nappy obtained new. Now nearly 3 years old Washed for 18 months low dose Out of use in storage for last 9 months in plastic, lidded container or vacuum bag |
| ASR
(half dose) 3.33 and 1.58 HA only |
Dehumidifier in bathroom with nappies on an airer
24-36 hours to dry |
Dry pail 24 hours (small stash) | Bepanthen | Every 2-3 days for 18 months
Nappy was pre-loved when obtained and is now 3 years old Used as both night and day nappy Out of use in storage for last 12 months in a plastic container. |
| EM
(full Dose 3.36 CC & HA |
Line dry / airer only
Up to 48 hrs |
Wash Sat/Sun and Wednesday | Bepanthen | Once per week
Nappy obtained new in Jan 2016 (~1 year old) Used as night nappy
|
| EM
(half dose) 1.67 and 1.94 CC & HA |
Line dry / airer only
Up to 48 hrs |
Every 2 days | Used to use coconut oil | Once per week
Nappy obtained new in May 2015 (~1 yr 8 months old). Used as night nappy Not currently being used. Stored for just under 1 year in a chest of drawers (no containment) |
| ER
5.28 and 4.29 HA and CC |
Line dry / airer only
2 days on airer by radiator in winter, 6-8 hours on warm 5.75 oz dry, 9oz wet out machine |
Approx 3-4 days | Occassional Bepanthen – less than once a month. | Weekly
Nappy obtained new and is now 20 months old. Used as both night and day nappy Not currently being used. Stored for 9 months in airing cupboard |
Child / Children
I was interested to note the high incidence of a dairy free diet in our test nappies.
Is this just coincidence or could it be relevant?
| Owner | Child/ren | Diet |
| ASR (full Dose)
2.46 HA only |
1 boy, 1 girl | Breast and Solids
Dairy free
|
| ASR
(half dose) 3.33 and 1.58 HA only |
1 boy | Breast and Solids
|
| EM
(full Dose 3.36 CC & HA |
1 boy | Breast and Solids
Dairy free
|
| EM
(half dose) 1.67 and 1.94 |
1 boy | Exclusively breastfed
I was dairy free |
| ER
5.28 and 4.29 HA and CC |
1 girl | Breast and Solids
Strict dairy free diet by mum and baby. |
Results from Bamboo Wipes
The wipes had a higher ash content than the nappies. This is not what we had expected. Our hypothesis was that the calcium in hydroxyl apatite was coming from urine. We had predicted that the wipes would contain less HA, as they had been exposed to less urine.
However, I did notice a few things that might be worth further discussion / investigation:
- The wipes contained more calcium carbonate (CC) than HA.
Perhaps there is something about the way we use and wash wipes (compared to nappies) that leads to more CC? - Wipes are soaked in water, used and then put in a bucket. In a hard water area, they are therefore saturated in hard water + a small amount of urine, then stored (probably still damp) until washed. Could CC and HA be forming in the bucket?
- The wipe that was washed separately contained nearly 3 times as much ash content as the wipe that was washed with the nappies. Why?
- We predicted that materials which take longer to dry after washing would contain more HA. However, cloth wipes dry faster than nappies, yet had a higher mineral content? Is this again something to do with the difference in the way CC and HA form?
| Owner | Ash Content (% w/w) | Wash / use history |
| ASR wipe | 15.60
CC & HA |
Washed separately at 60 with full dose bio for last 10 months. Non bio, full dose 60 degree prior that. Cheeky wipes, in last 6 months I’ve used their own oil on dirty bottom wipes only |
| EM wipe | 5.57
CC & HA |
Washed with nappies – half dose until around 7/8 months then full dose.
No oils and wet cloths on demand |
Conclusion
It is difficult to develop conclusions based on this set of experiments. It seems instead to have generated more questions – especially around the formation of calcium carbonate vs hydroxyl apatite. But that is the nature of science sometimes!
What did Shirley Technologies actually do?
You can read a bit more about the laboratory procedures Shirley Technologies did below. If you have any questions, please ask them in the Facebook group or contact us.
Summary of techniques used:
- Ash contents were determined by ignition loss at 600°C for 16 hours.
- Ash compositions were identified by FTIR using the potassium bromide disc technique.
Ash contents
Shirley technology told us: “A sample of material from the test area of all submitted samples was ashed in a silica crucible in a muffle furnace at 6000C for 16 hours. The amount of residual material was calculated as a percentage of the amount of original material.”
In a nutshell, they weighed the nappy sample, burnt it, then weighed what was left! This is what was reported to us as a % (final weight (w)/ original weight (w)) x 100 = % w/w
In a little more detail:
A crucible and its lid are pre-weighed after thorough drying. The sample is added to the completely dry crucible and lid and together they are weighed to determine the mass of the sample. The cruible containing the sample is then placed in the hot furnace long enough so that complete combustion of the sample occurs. The crucible, lid and ash then are re-weighed.
The residues after a sample is completely burnt consist mostly of metal oxides.
During complete combustion, lots of air (oxygen) is present, so all the carbon can fully burn off. This is what we want for our purposes here.
Incomplete combustion occurs when air (oxygen) is limited. This would produce contaminates if it occurs during this ashing process (these would be detected during the FTIR stage).
After being weighed, the ash is put through the FTIR machine, to find out what it contains.
FTIR (Fourier transform infrared spectroscopy)
Spectroscopy measures how well a sample absorbs light at each wavelength. FTIR makes use of infra-red radiation (light). Some radiation is absorbed by the sample and some passes through (is transmitted).
The machine used will have looked something like this:
Image By RobertsBiology (Self-photographed) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/4.0-3.0-2.5-2.0-1.0)%5D, via Wikimedia Commons
To put the sample inside the FTIR machine, it must sit on something. Glass and plastic both absorb infrared light, so would affect the results. Our ash samples were therefore smeared between two discs of potassium bromide. Potassium bromide does not absorb infrared light at the wavenumbers we are interested in. (Potassium has the chemical symbol ‘K’, so KBr is Potassium Bromide)
FTIR produces a molecular ‘fingerprint’ of the sample. Different chemical structures (molecules) produce different ‘fingerprints.’
Example HA spectrum (not from our results)
Spectra from: https://www.researchgate.net/figure/276253103_fig1_Fig-1-FTIR-spectra-of-the-hydroxyapatite-powders
Example calcium carbonate spectrum (not from our results)
The machine measures the vibrations of atoms. Atoms can vibrate in different ways – these show up as separate peaks on the graph. Heavier atoms vibrate slower than lighter ones; Stronger bonds vibrate faster than weaker ones.
Follow this link to watch a fun animation of vibrating molecules: https://en.wikipedia.org/wiki/Infrared_spectroscopy
Based on these graphs, the computer attached to the FTIR machine (with a bit of help from the operator), can tell us what substances were in our sample – in our case hydroxl apatite and calcium carbonate.
