Binder:powder ratios

It is intuitively obvious that a certain amount of binder is required in the powder mass before enough will be present on the surface to ensure stickiness. This critical, minimum amount of binder is an important characteristic of the system and must be determined beforehand. Equally important is the time span over which the binder either spreads on the granular surface and/or penetrates into the pores of the powder. Both, the critical binder/powder ratio and the characteristic spreading/penetration time will be discussed first before the theory of growth kinetics is presented. 

Table 3. Critical Binder/Powder Ratios for Some Selected Powder-Liquid Systems...

Material Particle Size [pm] Bulk Density [lb/ft3] Critical Binder Powder Ratio [%] ... 

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. 

Critical Binder Liquid/Powder Ratio. When a liquid is mixed into a bulk powder made of fine particles, the liquid distributes itself first into small spaces between particles forming liquid bridges as can be seen in Fig. 12a. For very small amounts of liquid, these bridges are randomly spaced and do not influence the bulk properties of the powder. Upon introduction of larger... 

Figure 5 Experimental torque vs. binder/solid ratio for a porous sodium carbonate powder.

For rayon fiber based eomposites (Seetions 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropie piteh earbon fiber eomposites (Seetion 5) were manufaetured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous sereen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrieation proeess allows the manufaeture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold eontoured plates, and tubes, to near net shape via this synthesis route. 

The electrodes were prepared by mixing graphite powder with the NMP-predissolved PVDF binder (grade KF 9306, available from KUREHA Advanced Materials Div., Tokyo, Japan) at the ratio 9/1 of C/PVDF, respectively. The viscosity of the slurry was adjusted at 5,000 cps by adding more solvent (NMP). 

The instrument and the procedure to determine the critical binderpowder ratio were developed (Tardos, 1994) based on the above observations. The instrument consists essentially of a low shear mixer, schematically represented in Fig. 14, in which the powder is slowly agitated and the binder is... 

Some of the operating variables are characteristic of the equipment (i.c., pressure, rhythm, and frequency of vibration to accelerate the flow of powder into the die), while others depend on the charge and, for a given powder, on the granulometry, the kind and concentration of binders, and the lubricants. The ratio of the diameter of the powder particles to that of the final granules should fall between 1/20 and 1/50. 

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