Cell walls delamination

Figure 5.9 Cross section of (a) a Kraft pulp and (b) a sulfite pulp, demonstrating the greater delamination of the cell wall of the latter.

During the delamination of the cell wall which takes place during refining (Figure 5.9), there is an increase in the water accommodated within non-crystalline zones, and the cell wall water content typically rises from around 1 to 3 g per g of dry fibre (Figure 5.10). 

More is being achieved than just the physieal separation of the fibres, as a separate but otherwise undamaged fibre would be far too stiff and rigid to make good paper. The internal structure of the cell wall is being delaminated. The fibre experiences both external and internal fibrillation. Also large amounts of desirable... 

This chapter is based both on results reported in the literature and on recent studies of the effect of aging, on the structure of dry archaeological wood. Various insects cause major structural breakdown in dry wood, but enclosure of wood or the presence of toxic extractives in wood appear to have protected many artifacts from attack by insects. Under dry conditions, the effects of age on wood structure appear minimal up to an age of 4400 years. Structural changes are observed only at the ultrastructural level when using transmission electron microscopy. Delaminations in the middle lamella region or in the secondary cell walls are the most commonly reported phenomena. Fissures and loosening of fibers have also been observed. 

The wood of Finus pinea was reported (8) as being almost intact . Microtomy for the SEM studies showed that the cell walls often bent over, making it diflScult to obtain a smooth surface. Weakening of the middle lamella was evident by the separation of individual tracheids or rows of tracheids from each other. TEM studies revealed delamination in the middle lamella-Si region, where the fibrils in Si were loosened. 

Figure 4. As for Figure 3, but showing a more detailed photo of the secondary cell-wall layers of a fiber. The fibrillar structure generally appeared quite intacty with no obvious delaminations between secondary wall layers. 

Homogenization has also been applied to various types of primary wall cellulose (e.g. sugar beet, potato tuber, banana rachis) where the microfibrils are more loosely organized than in the secondary wall of wood cells [28-31]. Hence, the disintegration requires less energy and the fibers are more easily delaminated. However, the material still mainly consists of bundles of microfibrils. 

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