The strip washing question: The results!

To strip wash, or not to strip wash? Or, more precisely, when exactly is a nappy in need of strip-washing? This is the question that we, the Nappy Science Gang citizen scientists, set out to answer, having found that despite the wealth of (often conflicting) advice on how and why to strip wash, nobody had tried to figure out scientifically why nappies needed strip washing.

It’s commonly said that detergent builds up in a nappy over time and stops it absorbing properly, and the solution to this is to strip wash. However, what exactly a nappy looks like when it needs strip washing is far from clear, and if detergent build up is behind the multitude of symptoms has never been shown for sure. The citizen scientists decided first they had to establish a proper definition of a nappy in need of strip washing (INOSW from now on!), then work out what was causing this with some help from Shirley Technologies, and what we found surprised us all!

What we did.

First off, to come up with a solid definition of what a nappy INOSW was, we sent out a survey far and wide asking how they would define a nappy INOSW. We chose the most common response as our definition.

Then, the nappies to be sent off to the lab had to be selected. Many members sent in their nappies which were then scored on which was the smelliest when it was dry, after having water poured on it and after having urine poured on it (you can see the full protocol for this part here). The nappies which scored the worst were sent off to Shirley Technologies to be tested, along with a nappy which had lost much of its absorbency so we could see if the same effect was causing them both to have such poor performance.

nappies inosw.png


We asked Shirley Technologies to see if there was any build up in those nappies and to compare them to an ordinary nappy. They did three tests for us:

  1. Successive solvent test to find anything soluble embedded in the nappy
  2. Bioburden test to see how many bacteria were living in the nappy
  3. Ash test to find anything inorganic embedded in the nappy by putting the sample in a furnace for long enough to burn up any organic matter

We also went to visit the Nanotechnology and Nanoscience Centre at Nottingham University to take a look at the nappy samples through a scanning electron microscope so we could see what was going on on a microscopic level.

A full test of which strip washing method was best was not carried out due to lack of time, but Shirley Technologies did wash one of the samples INOSW at 90 degrees to see what happened.

The results: What we found

So, what does a nappy in need of strip-washing look like? Well, the main reasons why people strip-wash (based on 291 survey responses) are: “nappy smells strongly after one wee” and “nappy smells stale when freshly laundered” followed by “received pre-loved”. The number one smell of a nappy INOSW is “ammonia”, with runners up of ”barnyard smell” and ”stale”.


The nappies which were sent in as candidates for this definition were bamboo fitted nappies (little lambs or bamboozles). These nappies are often used as night nappies, so it would certainly be interesting to look further at whether nappies being filled with wee for 12 hours contributes to nappies becoming INOSW, as well as at the different fabrics likelihood of being INOSW as we make some progress towards below.

The nappies selected as being most INOSW were:

A: LL bamboo INOSW, size 2, washed with a half dose of liquid detergent at 40 degrees, I year old

D:LL bamboo INOSW, size 2, washed with half a dose of bio powder and vanish at 60 degrees with extra rinse, 9 months old

E:Totsbots bamboozle INOSW, washed with non bio powder at 40 degrees, 2.5 years old

G: TJ pocket nappy Control Washed with Eggcorn at 60 degrees


The nappy showing very poor absorbency which was also sent in was:

B: Totsbots V3 Low absorbency, pre-loved, washing instructions say 60 degree wash

Shirley Technologies chose to test nappies D and B in this first round of testing, so we weren’t able to compare all the nappies INOSW. We are now having these tests done on the other selected nappies too, but for now we still learned plenty of interesting things from the two nappies tested, as we’ll see below.

First up, the successive solvent test.

Both the nappies D and B had very little (and statistically similar) solvent-soluable build up. You can see what this build up was made up of in the tables below. Shirley Technologies thought that because none of the build up was made of oily materials (any of the oily build up should have dissolved in the solvent) it was unlikely that oily build up was casing either of the nappy’s poor performance, including the B: Totsbots V3’s lack of absorbency which we thought might have been the case. They also thought that the alkyl benzyl indicated the presence of some detergent residue, though whether is the main culprit in causing nappy’s poor performance was not clear.

D: LL bamboo INOSW

Solvent Extract (%w/w) Composition
Petroleum ether 0.09 Hydrocarbon mineral oil; traces of fatty acids and ester-type materials
Methanol 0.11 Alkyl benzene sulphonate; amide-type material; soaps
Water 0.35 Inorganic carbonate salt; celulosic material; inorganic suplhate

B Totsbots V3 Low absorbency

Solvent Extract (%w/w) Composition
Petroleum ether 0.09 Hydrocarbon mineral oil; polyester oligomer,traces of fatty acidsand ester-type materials
Methanol 0.11 Dyestuffs, polyester oligomre, Alkyl benzene sulphonate
Water 0.35 Inorganic carbonate salts; carboxylates

Tables showing the chemical composition of the solvent-soluable build up of two nappies.

Now, onto the bioburden test.

By looking at the table below, we can see both nappies B and D had extremely high (and statistically similar) bacteria levels which suggested there was probably allot of leftover soiling in the nappies.

We can also see that although bamboo-based nappies are often marketed as being anti-bacterial, in this test the bamboo nappy had a similar bacteria level to the non-bamboo nappy tested. Shirley technologies also noted that LL bamboo nappy lists it’s composition as 90% Oeko-tex bamboo, but this does match with any of the legal fibre names for products sold in the UK (PD ISO/TR 11827:2012 and Annex 1 of EU Regulation 1007/2011). It’s legally correct name is actually viscose. Bamboo closer to it’s natural state may be naturally anti-bacterial, but viscose isn’t. Viscose is really entirely artificial.

Sample Average CFU/ml of 5 trials
D:LL bamboo INOSW 3400
B: Totsbots V3 Low absorbency 2300
G: TJ Pocket nappy Control Zero

Table showing the bacteria levels of the solvent-soluable build up of two nappies. CFU = colony forming units

And finally, the ash test.

The two nappies tested showed very different amounts of inorganic build up. As you can see from the table below, the D:LL bamboo nappy had a really high amount of ash left over, while the B: Totsbots V3 had a low amount similar to the control nappy.

Sample Ash content (%)
D:LL bamboo INOSW 20.7
B: Totsbots V3 Low absorbency 0.6
G: TJ Pocket nappy Control 0.3

Table showing the ash content of the samples of nappies tested.

The technician who performed the test said it was the highest ash content he’d ever seen. Our advisor, Mark Smith, commented that it was the highest ash content he’d ever even heard of.

Shirley Technologies looked at the composition of the deposit ash from D:LL bamboo and found it was hydroxl apatite (a type of calcium phosphate) which was intimately associated with the fibres of the nappy, and formed a skeletal residue on the nappy even after the ash had been removed.

Shirley Technologies thought that these deposits were probably caused by phosphates from detergents combining with calcium ions and redepositing itself onto the material. Calcium ions are much higher in hard water, which would suggest this type of build up would be more prevalent in hard water areas. More investigation is needed to see if nappies INOSW are really more prevalent on hard water areas, or if there are additional causes to nappies becoming INOSW.

However, to add to the excitement, when the Nappy Science Gang went on a trip to the Nanotechnology and Nanoscience Centre at Nottingham University, we were able to take a look at the hydroxl apatite build up through a scanning electron microscope. The images were pretty amazing, as you can see below!


Imaged produced with a scanning electron microscope showing build up in the LL bamboo nappy.

A full examination of stripwashing methods was not carried out, but Shirley Technologies did wash a small sample of the D:LL bamboo INOSW nappy at 90 degrees (in a laboratory procedure supposed to represent a domestic washing machine) to see what happened. They found that this removed all of the bacteria build up, and significantly reduced the hydroxl apatite build up (ash content 6.6%, compared to 20.7% before). However, the laboratory wash was at 90 degrees when most domestic washing machines don’t actually get that hot, and since they were washing a small sample of a nappy and not the whole thing with crevices and all it may not really represent a domestic 90 degree wash. Though we should certainly include a simple 90 degree wash in any further investigations we do into stripwashing procedures!

So, in summary:

  • The Little lamb bamboo nappy INOSW had a high bioburden probably caused by residual soiling, as well as hydroxl apatite deposit on the material fibres which could be causing it’s poor performance
  • The reason for Totsbots V3’s low absorbency is less clear. It had a high bioburden but did not have anywhere near the same levels of ash remains as the little lamb bamboo did. The build up it did have was not made up of oily components so it was not repelling water the this way as we expected.
  • Not all of the INOSW nappies were tested in this experiment, but further testing of these nappies will be going ahead.
  • The LL bamboo nappy lists it’s composition as 90% Oeko-tex bamboo, when it’s legal fibre name is actually viscose.

2 thoughts on “The strip washing question: The results!

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