Two pulps can both be described as “high absorbency” and still be wrong for the same product. One absorbs quickly and holds little; the other absorbs slowly and holds a great deal. The product decides which matters.
The two measurements
- Absorption rate — how fast liquid enters the fibre network, usually measured in seconds to reach a set percentage of capacity.
- Capacity — total liquid held per gram of fibre under a defined load, usually in g/g.
Rate is governed by surface chemistry and pore structure. Capacity is governed by fibre wall porosity and the degree of bonding. They respond to different process levers, which is why optimising one usually costs the other.
Which product needs which
- Baby diapers need fast rate at the acquisition layer, because urine arrives in bursts and must be taken up before it leaks. Capacity matters in the core but rate matters at the surface.
- Adult incontinence needs capacity, because the volume per event is larger and the product is worn longer.
- Feminine hygiene needs a balance, with a strong emphasis on rewet — how much liquid comes back under pressure.
- Pet pads and industrial pads need capacity at low cost; rate is secondary.
How to test both on your own pulp
Use a consistent method. The two common ones — demand wettability and a simple dip-and-drain — give different absolute numbers but the same ranking, so pick one and stay with it.
Test at least three lots before drawing a conclusion. Fluff pulp varies between lots, and a single measurement is a data point, not a specification.
The specification mistake
Most purchase specifications list capacity and omit rate. That works until a lot arrives with acceptable capacity and slow rate, and the line starts rejecting product for leakage rather than for absorbency. Both numbers belong in the specification, with the tolerance band set by what your product can tolerate — not by what the supplier finds convenient to guarantee.