Particles friability

The process conditions will influence the particle degradation by generating the stress on the individual particles on the one hand and by affecting the material properties and consequently the particle friability on the other. 

Breakdown of particles on impact can be tested either on single particles or on a quantity of the bulk solid, and the result is a measure of particle friability. The available tests have been well reviewed in the recent attrition report by the Board54 and only a mention of the main methods is given here. 

The main factors that affect the particle passage through sieve aperture are the method of sieve shaking, the ratio of open area of sieve to total area, particle size distribution, the number of particles on the sieve (sieve loading), and the dimension and shape of the particle. Friability and cohesiveness of solid particles can also affect the sieving operation. Difficulty can also arise with high aspect ratio particles (i.e., needle-shaped or flaky particles). 

In friability tests the material s susceptibility to attrition is evaluated. But it is not as simple as it may seem at first to select the suitable test procedure. In this context Pell (1990) gave a simple thought experiment to illustrate the difficulties If we took a batch of rubber stoppers and a batch of diamonds, and rubbed them on abrasive paper, we would conclude that the diamonds were more attrition resistant. If we instead struck the particles with a hammer we would conclude that the rubber were more attrition resistant. So, different test methods can rank materials differently with respect to their attritability. This effect was for example observed by Knight and Bridgwater (1985). They subjected spray-dried powders to a compression test, a shear test and a test in a spiral classifier. They found that each test gave a different ranking of the materials. Obviously, there is no... 

These difficulties are avoided in Gwyn s (1969) design (Fig. 3). Here, the attrition products are not kept inside the system but it is rather assumed that they are elutriated. In the enlarged diameter top section, gravity separation defines the limiting diameter of the elutriable particles. The attrition rate is assumed to be given by the elutriation rate. The steady-state elutriation rate can, therefore, be used as a friability index. 

Pneumatic Conveying Tests. In contrast to fluidized bed tests, no standard equipment exists that simulates the stress on particles in pneumatic conveying lines. There is no friability test quoted in the pertinent literature that is based on a specific pneumatic conveying system. 

When evaluating the effect of binder concentration on a number of tablet properties, surface area measurements were used to investigate the bond strength of the binder with the other particles [18]. A steady reduction in the surface area of the granules with increasing binder concentration indicated that the binder had covered or penetrated the particles, with the formation of particle-binder bonds. This was related to friability, and the increased bond strength was related to the decreased surface areas. 

Friability. The friability of the material is its tendency to fracture during normal handling. In general, a crystalline material will break along well-defined planes and the power required for crushing will increase as the particle size is reduced. 

Binder Different binders have different binding properties and the concentration of an individual binder may have to be changed to obtain similar binding of primary particles. Thus, the type of binder, and binder content in the formulation and concentration of the binder have major influence on granule properties. These properties include friability, flow, bulk density, porosity, and size distribution. 

Diluted binders are preferred because they facilitate finer atomization of the binder solution, provide control of the particle size, reduce friability and increase the bulk density, even though the tackiness or binding strength may suffer (8,68,78,82,82,97). 

In the United States, a number of physical tests are performed on silicon carbide using standard AGA-approved methods, including particle size (sieve) analysis, bulk density, capillarity (wettability), friability, and sedimentation. Specifications for particle size depend on the use for example, coated abrasive requirements (134) are different from the requirements for general industrial abrasives. In Europe and Japan, requirements are again set by ISO and JSA, respectively. Standards for industrial grain are approximately the same as in the United States, but sizing standards are different for both coated... 

The method employs a cylindrical porcelain jar mill fitted with three lifters that assist in tumbling the coal. A sample (usually 1000 g) of sized coal is tumbled in the mill for a specified time at a specified number of revolutions per minute. The coal is then removed and screened and the friability is reported as the percentage reduction in the average particle size during the test. For example, if the average particle size of the tumbled coal was 75% that of the original sample, the friability would be 25%. As with several other tests, the test parameters can be adjusted to suit the purpose of the investigation, but the precise parameters must be reported with the data. 

Friability tendency of coal particles to break down in size during storage, transportation, or handling quantitatively expressed as the ratio of average particle size after test to average particle size before test x 100. 

The vibrating fluidized bed is useful for processing particles that are difficult to fluidize because of stickiness, wide polydispersity, agglomerating tendency, large size, and extreme friability. High-frequency (10-100 Hz) and small-amplitude (1-10 mm) vibrations imparted to relatively shallow beds of particulate solids have been found to facilitate fluidization of difficult-to-fluidize solids. The use of such a fluidized bed for the chemical reactor has been evaluated by Mazumdar (1984). The detailed transport characteristics of a vibrating fluidized bed were recently outlined by Endesz et al. (1989). 

In the manufacture of tablets it is important to define and appreciate the physical properties of the active substance, in particular particle size and flowability. The technology involved in direct compression assumes great importance in tablet formulations because it is often the least expensive, particularly in the production of generics that the active substance permits. The limiting factors are the physical properties of the active substance and its concentration in the tablets. Even substances such as ascorbic acid, which are not generally suitable for direct compression owing to the friability of the crystals, can normally be directly pressed into tablets at concentrations of 30-40%. Ffowever, this technique is not as suitable if the content of ascorbic acid is higher. This limit may be shifted upward by special direct-compression auxiliaries, for example, Ludipress (BASF). 

The shape of particles is normally that of more or less regular spheres, dense or hollow, with smooth surfaces and sometimes cracks. This is related to the composition and the rate of solvent evaporation, with possible existence of internal pressure inside the drops when a rigid surface layer is being formed (Walton and Mumford 1999). All these characteristics will have some effect on handling properties of powders such as bulk and tapped densities, particle density, (mixing with other powders, storage) wettability and solubility, porosity, specific area (rehydration, instantisation) flowability (size, surface asperities), friability and creation/existence of dust, stability in specific atmosphere and medium (oxidation, humidification, active component release) (Huntington 2004). 

Two major classifications for diamond crystals are mono- or single crystalline and polycrystalline. The monocrystalline diamond particles tend to have more uniform surfaces and sharp edges. The abrasiveness of the monocrystalline diamond is mainly governed by its particle size. In the case of polycrystalline diamond, it is sometimes determined by the packing arrangement and the interaction of these single crystals that are related to friability [84]. Friability is defined as the readiness of a substance to crumble and form fine particles or fibers under the application of external pressures. 

Disintegration time, crushing strength, and friability of tablets, produced from Cop-MC using magnesium stearate as a lubricant, were found to be independent of the particle size of the prepared granules (Fig. 2.46). 

FIGURE 2.46 Effect of the particle size of the coprocessed excipient of chitin-mannitol on tablet crushing strength, disintegration time, and friability prepared from this excipient. The tablets were 9 mm in diameter and 180 mg in weight. All samples were lubricated with 0.5% (w/w) magnesium stearate. 

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