Fibre consistency

The fibre consistency determines how many fibres will absorb the, in each impact, applied energy. Normally the fibres are refined at a consistency of 3.5 to 4.5% fibres in water.

Refining at a high fibre consistency means that there are many fibres in the refiner gap to absorb the applied energy.

If the consistency is decreased, the number of fibres being able to absorb the applied energy decreases. The stress on the individual fibres becomes greater. The risk for damaging the fibres increases.

When refining the fibre walls are treated. The primary wall, P, is more or less peeled off. The same happens to the outer secondary wall, S1. The secondary wall, S2, with its large number of OH-groups, is released. The outer part of the S2 layer starts to split up and the fibrils partially come loose. This is called outer fibrillation.

Even the inner part of the fibre wall, S2, is influenced by the refining. The fibre wall starts to delaminate; called inner fibrillation. When the outer fibre walls P and S1 are removed and the fibre wall S2 is delaminated, water can penetrate. The fibre wall begins to swell. The water absorption increases the fibre’s softness and pliancy.

Later, when the water between and inside the soft fibres evaporates in the drying section fibres will easily collapse. When the web is finally dried the fibre surfaces come in close contacts and bind strongly to each other.

Thus, the main reason to refine is to attain an outer and an inner fibrillation but some undesired things happen.

When the fibre wall is split up, parts of the wall are peeled off and a lot of fine material is created. The fibre is bent and wrinkled. The fibre may even be pressed together so hard that it will be cut off.

A too intensive refining may be detrimental, the average fibre length decreases, the fibre walls are weakened, the fine material content becomes too high.

The fibre changes may lead to, a stock difficult to dewater, a lower paper strength.

During the refining some substances are released from the fibres. Small amounts of lignin, hemicellulose and wood resin will dissolve or disperse in the water.

source: Lennart Stolpe