Cellulose
and the water has a strong plasticizing effect. Since the wet fiber wall has a modulus ( E ) of around one to ten MPa (Nilsson et al. 2001) a capillary radius ( r ) of one micrometer (a reasonable dimension considering the dimensions of the fiber) would lead to a strain ( )of the wet fiber wall, Fig. 8 Schematic description of the making and breaking of fiber-fiber joints. The wet fibers are pulled together by the capillary pressure and the wet fiber wall will yield in the wet zone. In the dry joint there will be areas in molecular contact and areas not in molecular contact but still close enough to allow for
an interaction between the surfaces. Of the proposed mecha- nisms, only van der Waals and ionic interactions are significant when there is no direct molecular contact (during the making of the joints in the wet state)
cellulose-hemicellulose, and cellulose-lignin interac- tions as evaluated by contact adhesion testing (Gustafsson et al. 2012). Considering the function of these components in trees, this result is also very logical since there should be a good adhesive inter- action between these components to create a strong structure. As the dry joint is strained, all the separate interactions across the interface will act to resist the load and breakage can occur either through adhesive failure of the joint or cohesive failure of the fiber wall (Fig. 8). The cohesive interactions inside the fiber wall are similar to the interactions that exist between fibers, which means that in an ideal joint it is difficult to identify the actual interface. The presence of water, in the form of moist air, will have several effects. First, it will soften the fiber wall due to moisture adsorption. Further, as water adsorbs at the interface, specifically into those regions where the molecular contact between the fibers is poor, the water will weaken the dispersion interaction, enable disentanglement of molecules that might have moved across the interface and naturally also disrupt possible H-bonds. The making and breaking of fiber-fiber joints is an interplay between the deformation of the macroscopic fiber wall due to massive capillary forces which allows close contact between dry fibers, and the development of molecular interactions at those contact points,
2 c cos h Er
¼
between 1.5 and 15% assuming a fully wetted fiber surface (cos h ¼ 1) and using the surface tension of pure water c ¼ 72 mN m - 1 , and as the capillaries narrow, the deformation will be even higher. This means that just as for the fibrils (see above) the fibers are pulled closely together during drying. However, unlike the elementary fibril, the fiber surface is not molecularly smooth, which means that only a part of the interface between the fibers will be in molecular contact whereas other parts will be separated. As mentioned earlier, H-bonds are specific and short- ranged, and practically require direct contact to form, whereas other forces (ionic and van der Waals) can be significantly long-ranged at this scale to act over the separation distance. It has been shown that the migration of polymers across the interface has a significant influence on the dry adhesive properties of the fiber-fiber joints (Johansson et al. 2009) and that the thermodynamic work of adhesion is similar for cellulose-cellulose,
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