There are a few things people use as pad cores that are not ideal.
“Cotton batting” – This is what you buy to put into quilts. It is not generally recommended to use this, as it is primarily designed to trap air, in quilts that are infrequently laundered – so it is not designed for absorbing liquid and being washed a lot. Some people find it shreds in the wash and become unusable. Some people have reported that it is nowhere near as absorbent as a cotton terry. However some people do use it, and some people have no trouble with it. Using it can be a risk, and you are better off using other options.
While you can buy bamboo batting, which will be more absorbent than cotton batting – it’s still not designed for the usage that cloth pads will be subjected to. As it’s not much cheaper than bamboo fleece, it really is not good value for money.
Polarfleece – This is a synthetic fabric, and does not actually absorb. While it may appear to hold liquid for a pad core (since there is nowhere else for the blood to go), it is really not ideal for a pad core, being bulky without being properly absorbent.
Acrylic Felt – as with polarfleece, while it may appear to absorb liquid, it’s really not absorbent, so not ideal. I’m not sure why someone would actually think this is a good core fabric, but I have seen it being used!
Wool Felt – Again, while this can absorb, it is not absorbent like other fabrics, and adds more bulk than absorbency. It is actually better as a natural leak-resistant backing.