The effects of beating on the individual fibres are divided into four main groups swelling, fibrillation, cutting and the removal of the primary wall.
Swelling takes place in the amorphous hydrophilic hemicellulosic interfibrillar material. It involves a loosening of the fibre structure. The fibre wall is plasticised by the imbibed water and the fibre becomes more flexible . In an advanced state of swelling, the hemicellulose molecules are supposed to be partially dissolved in the surrounding water.
Fibrillation is caused not only by the direct action of the bars, but also by other treatments such as simple agitation or ultrasonic radiation. It is pointed out that a certain amount of swelling is needed to allow fibrillation and that fibrillation may be regarded as a natural consequence of progressive fibre swelling. Fibrillation first takes place after rather a long beating time.
A method is described by which it is possible to determine quantitatively the amount of primary wall on the surface of the fibres. The primary wall is torn off rapidly at the very beginning of the beating process and it is shown that the fibre surface free from primary wall can be correlated with the tensile strength.
When beating wood fibres, the removal of the primary wall and the swelling, in as much as it makes the fibre more flexible, seem to be the main effects in improving paper strength (fibre-to-fibre bonds), whereas fibrillation is of less or of no importance.