Binder surface tension

In a real process, both binder surface tension as well as viscosity will act to dissipate energy and ensure sticking and coalescence, but no simple analytical solution exists for this case. It was also demonstrated by Ennis (Ennis et al., 1991) that conditions based on viscous dissipation are more restrictive then those based on capillary forces and hence the discussion of granulation regimes is limited, in this paper, to the former. 

Cases where the binder consists of two or more components are of great importance. The introduction of filler leads to the redistribution of fractions of both components between the surface layer and brdk. It was shown that filler incorporation into a two-component binder essentially changes the dependence of binder surface tension (determined from their respective contact angles) on the composition. This effect is connected with the above-mentioned redistribution of components between the surface layer and brdk. 

It will be assumed for the present considerations that sufficient binder is present in the granulator as determined by the binder/powder ratio and that the binder is appropriately spread on enough granular surfaces so as to ensure that most random collisions between particles will occur on binder-covered areas. It will also be assumed that the particles are more or less spherical having a characteristic dimension, a. The simplified system of two colliding particles is schematically shown in Fig. 21. The thickness of the liquid layer is taken to be h, while the liquid is characterized by its surface tension yand its viscosity /x. The relative velocity U0 is taken to be only the normal component between particles while the tangential component is neglected. 

During this phase, the solvent evaporates, and the diameter of the drop increases. The energy required to form a drop is the product of the surface tension and the new surface area. About 0.1 cal/g is needed to subdivide 1 g of water into l-(xm droplets. The air pressure required to atomize the binder liquid is set by means of a pressure regulator. The spray pattern and spray angle are adjusted by adjusting the air cap. 

The addition of fines to a powder system allows a larger top size of particle to be agglomerated due to the attendant increase in cohesive forces caused by a decrease in surface mean particle diameter and increase in agglomerate density. Thus, although eqn. (1) indicates a top size of about 150 pm for monosized particles with aqueous binders, the top size of feed for industrial disc pelletizers is usually higher at 30 to 50 mesh (300 to 600 /zm) with the provision that at least 25% should be finer than 200 mesh (75 /zm) [7]. Other liquids with surface tensions lower than that of water, or liquid/ solid systems in which the particle surface is imperfectly wetted, require finer particle sizes to make successful balling possible. 

Kollicoat IR is a unique polymer for pharmaceutical applications prepared by a graft polymerization process of polyethylene glycol (25%) with polyvinyl alcohol (75%). Kollicoat IR dissolves quickly in water and aqueous solutions of acid and alkali and reduces the surface tension of aqueous solutions to allow the solutions to have high spray rates. The polymer film is very flexible, not tacky, and easily colored. The polymer can be used as instant release coating, pore former, binder, protective colloid, etc. 

The adsorption process can be affected by the surface tension and the viscosity of a binder, and more energy may be consumed during granulation. As shown in Fig. 2, the surface tension and the viscosity of a binder play important roles in granulation because these properties influence the liquid bridges between the particles, as well as the distribution of the binder... 

A low surface tension value correlates with a small contact angle. The binder with the smaller contact angle has improved spreadabiiity and can wet powders more effectively (65,84). A surfactant can also be added to the binder solution to improve wettability, especially for hydrophobic powders, and functions to lower both the surface tension as well as the contact angle of the liquid. If the contact angle, 6, is less than 90, then the powder wetting is spontaneous. However, if the contact angle is closer to 180 then the powder would be considered unwettable by the liquid. The pore space within a particle assembly can be simplistically considered as a model capillary. The capillary pressure, Pc, of a liquid is related to the surface tension by the following equation ... 

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