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How do bamboo and cotton absorb water?


Bamboo (also known as viscose or rayon) and cotton are two popular fabric types for cloth diapering because they absorb a lot of liquid. In this post, we'll discuss how they do this.

Warning! This post gets a little technical, and discusses the molecular structure of cotton and bamboo (which seems to be important for absorbency). I've tried to make it easy to understand with some pictures, but if you don't want to read all that, scroll to the summary at the bottom.

What are cotton and bamboo fabric made of, at a molecular level?

Cotton and bamboo fabrics are actually very similar at a molecular level. They are both long chains of cellulose with dangling 'OH' molecules, as shown below:

Some sources I checked indicate that cotton is 90% cellulose while others claim it is pure cellulose. Bamboo, the plant, is also comprised largely of cellulose, at around 70%, but keep in mind, bamboo the fibre (not the plant) is pure 100% cellulose derived from the plant pulp. 

The reason cellulose is significant is that it affects the durability and absorbency of the fibre.

Fun fact: Cellulose is actually a type of carbohydratea type that humans can’t digest.

How do cotton and bamboo absorb liquid?

According to several sources, it is the cellulose in the cotton and bamboo that is hydrophilic, meaning it absorbs water. Those loose 'OH' molecules that are on the outer edges of the chain (see image above) bond with water molecules (H2O) very easily, thus making both cotton and bamboo very absorbent.

But wait a second, that theory might be wrong

As with anything I Google these days, I am led down a rabbit hole, with lots of contradictory information surfacing.

It turns out that the "loose-OH-molecules-absorbing-water" theory might actually be too simplistic. According to Wikipedia,  the loose OH molecules do not bond with water but rather, with other OH molecules within the cellulose fibre itself. Take a look at this schematic to see what I mean:

Those internal bonds are what give the cellulose a crystaline, strong, shape.

So if it's not the loose OH molecules that are water-absorbent, what is?

Well after some more Googling, it looks like one possible answer might be a thing called 'hemicellulose' which is also found within cotton and bamboo (and all plants), albeit in a smaller percentage than cellulose. (I've seen it pegged at around 20% for most plants.) This hemicellulose isn't a nice organized chain. It's got lots of loose molecules, and looks more like a disorganized web of atoms. This is what it looks like:

The disorganization means the hemicellulose is absorbent, because it has more loose molecules for water to bond to it. It also means it's weaker than the nice, crystaline-shaped cellulose.

So we have our answer, right? Hemicellulose is responsible for water absorption? Umm...actually, no

The thing is: Upon further research, it turns out the hemicellulose is largely removed when the cotton and bamboo are cleansed using caustic soda (we’ll learn more about processing in an upcoming post). So if there's no more hemicellulose, something else must be responsible for cotton and bamboo's aborbency. But what?  

What actually causes cotton and bamboo to absorb water: the final answer

Drum roll please...

The real reason cotton and bamboo absorb water has to do with the structure of the cotton and bamboo strands. They are actually tubes with cells on the insides. At some point during the growth of the fibre, the cells inside the tube die and shrivel up leaving an empty tube like outer shell. The empty tube is ready to take on water through capillary action—like a straw.

Summary

It turns out there are many theories for how cotton and bamboo absorb liquid:

  • Theory 1: They are largely comprised of cellulose, which has OH molecules that bond to water (H20)
  • Theory 2: They contain some hemicellulose (~20%), which has loose OH molecules that bond to water (H20), more so that regular cellulose.
  • Theory 3: They both have tubular strands with a hollow center that absorbs water through capillary action

Of these three theories, the last seems the most plausible to me, given the fact that the first two have been refuted in various online sources.

However, I'm no expert! Please leave me a comment if you have more information and want to set me straight.

Happy cloth diapering!


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