Cohesion of materials

It would be desirable to have simple tests capable of characterising the fluidisation behaviour or flowability of particulate materials on the basis of their bulk properties. To this end, Carr19 developed a system to characterise bulk solids with respect to flowability. Table 6 summarises the properties which are determined. In Carr s method a numerical value is assigned to the results of each of these tests, and is summed to produce a relative flowability index for that particular bulk material. Given the extensive use of these empirical techniques in academia and industry, a brief review on the subject is reported here. Nevertheless, it should be emphasised that these techniques allow measurements of the flow-ability or cohesion of materials solely in their stationary or compressed status and at ambient conditions. A direct relationship between these... 

Considering only the adhesion of material B on substrate A and the cohesion of material B, the ideal adhesive strength amax(B/A) and cohesive strength Oma (B), are given by [70] ... 

Relationships between structure of polymers and cohesion of materials... 

The apolar component of the free energy of cohesion of material 1 is ... 

The success of the compaction operation depends pardy on the effective utilization and transmission of appHed forces and pardy on the physical properties and condition of the mixture being compressed. Friction at the die surface opposes the transmission of the appHed pressure in this region, results in unequal distribution of forces within the compact, and hence leads to density and strength maldistribution within the agglomerate (70). Lubricants, both external ones appHed to the mold surfaces and internal ones mixed with the powder, are often used to reduce undesirable friction effects (71). For strong compacts, external lubricants are preferable as they do not interfere with the optimum cohesion of clean particulate surfaces. Binder materials maybe used to improve strength and also to act as lubricants. 

Sintering. A ceramic densiftes duriag sintering as the porosity or void space between particles is reduced. Additionally, the cohesiveness of the body iacreases as iaterparticle contact or grain boundary area iacreases. Both processes depend on and are controlled by material transport. 

For a given slurry, the maximum filtration rate is determined by the minimum cake thickness which can be removed—the thinner the cake, the less the flow resistance and the higher the rate. The minimum thickness is about 6 mm (0.25 in) for relatively rigid or cohesive cakes of materials such as mineral concentrates or coarse precipitates like gypsum or calcium citrate. Sohds that form friable cakes composed of less cohesive materials such as salts or coal will usually require a cake thickness of 13 mm (0.5 in) or more. Filter cakes composed of fine precipitates such as pigments and magnesium hydroxide, which often produce cakes that crack or adhere to the medium, usually need a thickness of at least 10 mm (0.38 in). 

A number of general principles govern the selection of crushers [Riley, Chem. Proce.ss Eng. (January 1953)]. When the rock contains a predominant amount of material that has a tendency to be cohesive when moist, such as clay, any form of repeated pressure crusher will show a tendency for the fines to pack in the outlet of the crushing zone and prevent free discharge at fine settings. Impact breakers are then... 

Fig. 1. (a) Adhesive vs. cohesive failure, (b) Close-up view of adhesive failure in the pre.sence of an interphase. The locus of failure may be adjacent to or within the interphase (as shown), and particles of material may be ejected during the debonding process. 

It is for this reason that the discovery by Ulrich was of significant importance to the successful development of acrylic PSAs. He found that by copolymerizing polar monomers, such as acrylic acid, one could greatly increase the cohesive strength of the polymer allowing PSA articles coated with this type of material to sustain a load without premature shear failure. These polar monomers commonly... 

Polyvinyl chloride is also widely used. Rigid polyvinyl chloride is introduced to the mold in powder form. The material is chosen for durable constructions because of its chemical resistance and ease of processing. It incorporates functional additives and demolds easily. Plasticized polyvinyl chloride can be used to produce flexible parts such as balls and soft toy parts. The polyvinyl chloride is introduced to the mold as either a plastisol or powder. A plastisol is a suspension of granules in a plasticizing agent. When heated, the polymer granules absorb the plasticizer and fuse to form a cohesive, flexible material. 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

Group C powders are of small particle size, cohesive by nature and hence, difficult to fluidize. Either the fluidized bed lifts as a solid plug of material or forms stable channels of air flow, which allows the fluidizing gas to escape. The latter phenomenon is referred to simply as channelling . It may be possible to fluidize such powders by mechanical agitation (e.g., stirring, vibration). 

The bulk density (pt,uik) °f a material depends on a number of factors, such as the size, shape, and cohesion of the particles [60,62], Large particles, which may leave spaces, will result in a lower bulk density than smaller particles, which pack closer together and leave smaller spaces. The way in which the measurement is done can also affect the bulk density. 

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