In a porous agglomerate, such as pellets, three separate phases can be distinguished:
water, air and the solid material from the particles. These particles can be held together
by liquid necking. The force of the bond is dependent on the surface tension (Ap) of the
binding liquid y (N m- ’ >, and the radius ( r) of the neighbouring particle (m> (Rumpf,
1958). The binding pressure of the bond (Nl is given by the equation of Laplace
Ap = 2 X (y/r), in the case where the radii of the neighbouring particles are equal. In
pelleted feeds this is normally not the case, since milling induces a particle size
distribution (with subsequent larger and smaller particles) of the diet ingredients. From
the above equation it can be easily derived that with decreasing radius of the particles,
binding strength becomes higher. This is in line with the generally accepted principle
that with finer grind, better pellets are produced (Payne, 1978). The binding agent
(water) between the different particles can be redistributed around the particles without
loss of the established bond. Elevated temperatures cause redistribution of water via
evaporation and condensation and this will increase the mobility of water between
particles (Friedrich and Robohm, 1968). In the case where pellets’ moisture level is
decreasing, then the moisture bridges will shrink and total binding forces will decrease
due to a smaller number of moisture bridges. However, force per bond will increase
since water in the larger capillaries is evaporated first. Subsequently, it follows that the
remaining water establishes bonds between smaller particles. From the equation given it
follows that binding force between neighbouring particles is stronger with decreasing
radius. In the case where large quantities of water fill all pores, there will be a two-phase
system of water and particles, with no capillary force present to maintain structure in the
pellet. It is shown by Knacke and Pohl(1959) that an optimum exists in binding strength
of clay agglomerates depending on the amount and surface tension of the water added.