Fiber Length Distribution: The Number That Decides Softness

Two pulps both report 2.1 mm average fibre length. One produces a soft, uniform core. The other produces a core that feels gritty and drains slowly. The difference is not the average — it is how the fibres are spread around it.

Why the average hides the problem

Fibre length is measured on a population of millions of fibres. A single number can be produced by a narrow distribution centred at 2.1 mm, or by a mixture of 3 mm fibres and 1 mm fibres that happens to average 2.1 mm.

Those two pulps behave completely differently:

• Long fibres give strength and bulk, but slow down drainage and can wrap around forming screens.
• Short fibres give softness and fast drainage, but generate dust and reduce wet integrity.

How to read a length distribution

Ask for a fibre length histogram, not a mean. Look at three points:

1. The long-fibre tail (above 3 mm). More than 15% here usually means drainage problems and screen wrapping.
2. The fines fraction (below 0.2 mm). Above 8% and you will see dust in the converting line and a drop in machine yield.
3. The shape of the peak. A narrow, tall peak means a consistent raw material and a stable cooking process. A flat, wide peak usually means mixed furnish.

What causes a bad distribution

Distribution problems are almost always upstream of the pulp machine:

• Mixed species furnish. Softwood and hardwood chips cooked together give a bimodal curve.
• Uneven cooking. Temperature gradients in the digester leave some chips under-cooked and some over-cooked.
• Aggressive screening. Removing too much of the long fraction shifts the peak short and concentrates fines.

Why it matters commercially

Softness is the property consumers judge first, and it is largely a function of how evenly the fibre network resists compression. A narrow distribution compresses evenly. A wide distribution produces localised stiff points — the gritty feeling consumers describe as “cheap”.

You cannot fix a wide distribution downstream. You can only specify it, test it on arrival, and reject the lots that fall outside the band.