Powders true density

The true density of a solid is the average mass per unit volume, exclusive of all voids that are not a fundamental part of the molecular packing arrangement [55]. This density parameter is normally measured by helium pycnometiy, where the volume occupied by a known mass of powder is determined by measuring the volume of gas displaced by the powder. The true density of a solid is an intrinsic property characteristic of the analyte, and it is determined by the composition of the unit cell. 

When measuring the true density of a powder, it is important to use a gas that is not adsorbed by the material being measured. Both helium and nitrogen obey the ideal gas law at ambient pressures and temperatures however, helium is preferred because of its smaller size [6]. It is also important to outgas the sample before the measurement to obtain consistent and reliable results. 

Bulk powder density must be distinguished clearly from the true density of parti-eles. Bulk powder density is simply the mass of a powder bed divided by its volume. The volume of the powder bed ineludes the spaces between agglomerates, between primary particles, and the volume of micropores within the partieles. These voids... 

If the powder has no porosity the true density can be measured by displacement of any fluid in which the solid remains inert. The accuracy of the method is limited by the accuracy with which the fluid volume can be determined. Usually, however, the solid particles contain pores, cracks or crevices which will not be completely penetrated by a displaced liquid. In these instances the true density can be measured by using a gas as the... 

When the fluid displaced by powder does not penetrate all the pores, the measured density will be less than the true density. When densities are... 

Mercury porosimetry provides a convenient method for measuring the density of powders. This technique gives the true density of those powders which do not possess pores or voids smaller than those into which intrusion occurs at the highest pressure attainable in the porosimeter and provides apparent densities for those powders that have pores smaller than those corresponding to the highest pressure. 

A porous particle contains many interior voids known as open or closed pores. A pore is characterized as open when it is connected to the exterior surface of the particle, whereas a pore is closed (or blind) when it is inaccessible from the surface. So, a fluid flowing around a particle can see an open pore, but not a closed one. There are several densities used in the literature and therefore one has to know which density is being referred to (Table 3.15). True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. True density is also referred to as absolute density or crystalline density in the case of pure compounds. However, this density is very difficult to be determined and can be calculated only through X-ray or neutron diffraction analysis of single-crystal samples. Particle density is defined as the mass of a particle divided by its hydrodynamic volume. The hydrodynamic volume includes the volume of all the open and closed pores. Practically, the hydrodynamic volume is identified with the volume included by the outer surface of the particle. The particle density is also called apparent or envelope density. The term skeletal density is also used. The skeletal density of a porous particle is higher than the particle one, since it is the mass of the particle divided by the volume of solid material making up the particle. In this volume, the closed pores volume is included. The interrelationship between these two types of density is as follows (ASTM, 1994 BSI, 1991) ... 

Powders are porous materials and their bulk and relative densities can change with consolidation (6). However, a powder s true density is the density of its solid phase only and thus is independent of the state of consolidation. The true density of organic excipients typically ranges from 1.0 to 1.6g/cm3 while inorganic excipients (e.g., calcium phosphate) show values greater than 2g/cm3. True density is used to determine powder or compact solid fraction (SF) (see below) and it may be a consideration when selecting excipients if segregation is a concern. True density is often determined by gas pycnometry. 

Powder weight =0.3 (punch diameter) x (punch contact area) x (true density)... 

The true density of lead azide is 4.8, but the loose powder has an apparent density of about 1.2. 

Several catalyst densities are used in the literature. True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. In a strict physical sense, this density can be calculated only through X-ray or neutron diffraction analysis of single crystal samples. The term apparent density has been used to refer to the mass divided by the volume including some portion of the pores and voids, and so values are always smaller than the true density. This term should not be used unless a clear description is given of what portion of the pores is included in the volume. So-called helium densities determined by helium expansion are apparent densities and not true densities since the measurement may exclude closed pores. 

Based on the definition of density, two new terms are defined. Porosity is defined as the proportion of a powder bed or compact that is occupied by pores and is a measure of the packing efficiency of a powder and relative density is the ratio of the measured bulk density and the true density ... 

The equation of Heckel has been discussed again and again. One main issue of critique is that pharmaceutical powders are not purely plastically deforming materials and thus particle size and deformation mechanisms influence the derived parameters [129, 130]. Already very small errors in displacement determination or the measurement of true density can induce huge errors in the derived parameters [75-77, 129, 131, 132], Spnnergaard [126] referred the equation of Walker and Bal shin for his characterization of materials. He criticized further that the yield strength derived from the Heckel equation is directly dependent on the true density of the powders [127]. 

Mercury Porosimetry Method Mercury is a nonwetting liquid that must be forced to enter a pore by application of external pressure. Consequently it is an extremely useful and convenient liquid for measuring the density of powders and/or particles. This method can measure the apparent and true density of one sample by... 

The particle density or true density of a particulate solid or powder is the density of the particles that make up the powder, in contrast to bulk density, which measures the average density of a large amount of the powder in a specific medium (usually air). The normal particle density of geological formations is assumed to be approximately 2.65 g/cm3, although a better estimation can be obtained by examining the lithology of the particles. 

Fumed silica appears as a fluffy white powder characterized by an extremly low bulk density down to the range of about 20-50 g f. In contrast, the submicron fumed silica particle consists of amorphous silicon dioxide and, hence, its true density is about 2200 g 1 ... 

The bulk density of a powder is obtained by dividing its mass by the bulk volume it occupies. The volume includes the spaces between particles as well as the envelope volumes of the particles themselves. The true density of a material (i.e., the density of the actual solid material) can be obtained with a gas pycnometer. The bulk density of a powder is not a definite number like true density or specific gravity but an indirect measurement of a number of factors, including particle size and size distribution, particle shape, true density, and especially the method of measurement. Although there is no direct linear relationship between the flowability of a powder and its bulk density, the latter is extremely important in determining the capacity of mixers and hoppers and providing an easily obtained valuable characterization of powders. 

Anticipating problems in the physical mixing of powders and the homogeneity of intermediate and final products because significant differences in true densities can result in segregation,... 

The relative density of a powder bed at a point when the applied pressure equals zero, ptH is used to describe the initial rearrangement phase of compaction as a result of die filling. is determined experimentally and is equal to the ratio of the bulk density at zero pressure to the true density of the powder. The relative density, p, indicates the... 

Therefore. Equation (112) can be used to predict the true density of binary mixtures. After calculating predicted true densities (Equation (112)] and substituting Equations (110) and (111) into Equations (108) and (109). we can obtain dm and for the binary mixture based upon the corresponding values of n, d. and k of the constituent powders and the true densities. Upon obtaining the values of dm and Am itie tensile strength of binary tablets (aim) can be derived for a given relative density of the mixture (Om) using Equation (106), i.e.. 

TABLE 4 Measured True Densities for Powder Systems Considered... 

Density can be defined as ratio of the mass of an object to its volume therefore, the density of a solid is a reflection of the arrangement of molecules in a solid. In pharmaceutical development terms, knowledge of the true density of powders has been used for the determination of... 

Information about the true density of a powder can be used to predict whether a compound will cream or sediment in a metered dose inhaler (MDI) formulation. The densities of the hydrofluoroalkane (HFA) propellants, 227 and 134a, which are replacing chlorofluoro-carbons (CFCs) in MDI formulations, are 1.415 and 1.217 g/cm , respectively. Therefore, suspensions of compounds that have a true density less than these figures will cream (rise to the surface), and those that are denser will sediment. Those that match the density of the propellant will stay in suspension for a longer period (Williams III et al. 1998). It should be noted, however, that the physical stability of a suspension is not merely a function of the true density of the material. 

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