Bulk powder properties

Bulk density and packed bulk (or tapped) density are important properties. The bulk density determines the weight of resin that can be stored in a vessel and the amount a mixer can hold. It also has a major influence on extruder output rates. The bulk density of a resin depends upon its porosity, particle shape and particle size distribution. For suspension resin, bulk density is typically in the range of 450-650 kg nr3. 

The packed density is based on the ability of the resin to occupy the smallest space without physical distortion and is usually 10-30% greater than the bulk density. 

Another important consideration is powder flow time as it governs the speed and ease of polymer discharge from a silo or from a mixer. This is influenced by the PVC particle shape, size distribution and compound formulation ingredients. 

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Evaluation of the morphology of a pharmaceutical solid is of extreme importance, since this property exerts a significant influence over the bulk powder properties of the material. In addition to providing insights into the micromeritic properties of the solid, microscopy can also be used to develop preliminary estimations of the particle-size distribution. A determination can be easily made regarding the relative crystallinity of the material, and it is often possible to deduce crystallographic information as well. Unknown particulates can often be identified solely on the basis of their microscopic characteristics, although it is useful to obtain confirmatory support for these conclusions with the aid of microscopically assisted techniques. 

Bulk powder properties are important in understanding the handling properties of an excipient or a granulated product. Common parameters measured are bulk and tap density. From these values the compressibility can be calculated using the following ... 

The morphology of a pharmaceutical solid is of importance since this property can influence the bulk powder properties. The six crystal systems are cubic, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic crystals. The observed overall... 

Bulk powder characteristics are important in understanding the handling properties of an excipient or a granulated product. A classification system to evaluate the flow properties of powders has been introduced by Carr.P A flowable powder is defined as free-flowing and tends to flow steadily and consistently, whereas a floodable powder exhibits an unstable, discontinuous, and gushing type of flow. A number of studies have investigated the bulk powder properties of starch " and granulations made with starch. The starch materials were found to exhibit poor to borderline flow properties. 

Bulk powder properties are extremely important in pharmaceutical processing. Knowledge of the true and bulk densities of the drug substance as well as of the excipients is extremely useful in... 

The next step is to predict the aggregate strength, a, on the basis of known bulk powder properties and compare this parameter with the aerodynamic stress. Based on the model proposed by Kendall and Stainton (72), the balance of forces can be approximated (24) by... 

Based on an evaluation of the physicochemical properties of the active compound, several initial formulations (generally, four to six) are selected and screened in this step (Dobry et al. 2009). A small-scale spray dryer designed for maximizing yields from SDD batches of less than 100 mg is used. This dryer is not designed to replicate optimized bulk powder properties (e.g., particle size, density) of larger scale spray dryers, but rather is used to guide formulation decisions based on physicochemical properties and fast, efficient formulation-screening studies. 

The effect of particle shape on bulk powder properties has been illustrated by Rupp (5). The effect of particle size and shape on the bulk density and flow rate is illustrated in Figures 2 and 3. As illustrated in these examples, packing of powder in the bulk becomes more efficient as the shape factor or loss in sphericity increases. The flow rate becomes worse with loss in sphericity. 

Powders can be regarded as being either two- (solid-liquid) or three-phasic (solid-liquid-gas) systems which, at times, can be treated as a single continuum. Because there is a great variety in the bulk powder properties and characteristics of such assemblies there is a need to understand the mechanico-physical properties of discrete solid particles as well as the physico-chemical interaction of adsorbed solid-fluid boundaries on particles. 

There are many of empirical instruments which may measure bulk powder characteristics, which affects the behaviour of powders in processing plants. And some of these empirical laboratory instruments have now been commercialised. However, the sophistication of instrumentation available for the measurement of bulk powder properties in real time is not as advanced as the instruments currently used to measure particle size on a micro- or nano-scale. The phenomenon of flow of a powder out of any orifice or from a specified piece of process plant equipment should be unambiguously termed rate of powder flow and not confused with the bulk powder property of flowability which can be used discriminate the handabil-ity of bulk powders and may eventually contribute to enhance the rate of powder flow. 

The common definition of flowabihty is the ability of a powder to flow from a specific item of plant equipment at a desired degree of flowability. Flowability is generally quantified by a range of mobility, from free flow to non-flow which can, at times, be measured in a laboratory with specific flow property instrumentation. A bulk powder may be described as being free flowing, cohesive or non-flowing, and thus powder flowability tends to become an integral part of the description of bulk powder property measurement. Correlations between powder bulk properties in terms of both micro-scaled particulate factors as well as macro-... 

Since in the physical world there are only three dimensions mass, length and time bulk powder properties can only be described in terms of the inter-relationship between these three dimensions. 

The largest category of bulk powder behaviour, expressed in the terms of the three-dimensional parameters, is either a mass-time or a volume-time relationship. This category measures the rate of discharge from orifices, as opposed to the bulk powder properties of flowability and/or floodability of powders (Table 1.1). 

This is probably the most widely used bulk powder property although different industries have different protocols in its measurement (ISO 3923-1 1979 ASTM B329-98 2003). 

There is no specialised national or international standard procedure to test all the mechanical and physical tests which may be applied to highly dense and compacted materials. A selection of national or international standards allied to bulk powder properties is, however, shown in Table 1.7. 

Table 1.7 A selection of National and International Standards allied to bulk powder properties.

Since a powder assembly can generally be regarded as a biphasic system the permeability of a powder, or the ability of a gas to pass through a collection of particles or bed of powder, has now been recognised as a bulk powder property. The magnitude of permeability as a function of bulk density may be used in the determination of the powder flow rate and discharge of powders from hoppers. 

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