Hausner ratio

2 Kostelnik and Beddow Ro-Tap method A Ro-Tap method was used by Kostelnik and Beddow (1970) for tap bulk densities and they proposed that when the aerated bulk density was determined for subsequent use in the Hausner ratio (Section 1.4.1) the same standard test cup should be used. Visual determination of the volume end point is difficult and thus the weight end point methodology is preferable. 

Forparticles in the rangeO.8. 8 mm, a 250 mL cylinder is recommended with mechanical tapping. The tapping unit is marketed as a Tap-Pak Volumeter (J. Engelsmann Company). Powder poured into this measuring cylinder is tapped 1000 times and the volume measured. From the initial weight of powder a tapped bulk density can be determined. 

The density of a powder compressed or compacted to a degree greater than that achieved by tap densities is, at times, required as a ceramic, metallurgical or pharmaceutical end product. 

A civil engineering uniaxial oedometer or consolidometer (BS 1377-7 1990 ISO/TS 17892-5 2004) consists of a cylindrical ring container which compresses, between two porous plates, a powdered sample at known normal forces in the range 10-3200 MPa. The porous plates allow fluids (gases/liquids) to flow through the compressed media. 

A coefficient of volume compressibility or coefficient of consolidation can be calculated from the thickness, volume or void ratio versus logarithmic applied stress relationship. 

There is another way of expressing the difference between the tap density and the aerated density, as compressibility defined as follows  

Compressibilities in excess of 20% are said to correspond to a tendency for creating bridges in hoppers and powders having values over 40% are very hard to discharge at all. 

There is no overwhelming case for using compressibility in favour of Hausner ratio, although it too is a dimensionless group of similar significance. The Hausner ratio is much more widely used at the moment. 

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As a consequence of the irregular and rough structure of the xylan particles, entanglements between particles are promoted and this fact may explain the poor flow properties of this polymer (Kumar et al., 2002 Nunthanid et al., 2004). Additionally, rheological parameters of xylan powder have also been studied, such as bulk and tapped densities, Hausner ratio, Carr s index, and angle of repose values, and they are summarized in Table 1. 

Controlled optimal particle size and size distribution ensures superior flow properties of coprocessed excipients and reduced reliance on addition of glidants. The volumetric flow properties of SMCC were studied in comparison with those of the physical mixture of its parent excipients (42). The particle size range of the two test samples was found to be similar, but the flow of coprocessed excipient was better than that of the physical mixture. A comparison of the flow properties of Cellactose with its parent excipients was also performed (5) by measuring the angle of repose and Hausner ratio, and Cellactose was found to have better flow characteristics than lactose or a physical mixture of cellulose and lactose. The spray-dried coprocessed product had a spherical shape and even surfaces, which resulted in improved flow properties. On similar terms, mechanically coating the 2% CSD over microfine cellulose powder resulted in improving its flow properties (43). 

A Carr index less than 21 for a powder mixture indicates good flow, while one between 21 and 25 suggests a marginal flow property. A more commonly used term is the Hausner ratio, which is simply p,/pb, or the tapped density divided by bulk density. This ratio was introduced by Hausner in 1967 to characterize metal powders,58 but is commonly used today for pharmaceutical powders. The higher the Hausner ratio, the poorer the flow. The Hausner ratio varies from about 1.2 for a free-flowing powder to 1.6 for cohesive powder. 

Flow Property Angle of Repose (deg) Carr Index (%) Hausner Ratio... 

Hausner Ratio and Powder Compressibility. The ratio between the loose and packed bulk density, Pbdl and Pbdp respectively is known as the Hausner Ratio (HR) and is used as an indication of the cohesiveness of the materials, see Ref. 20. In addition to the HR, the powder compressibility is also used to define cohesiveness. This is expressed as 100(/9bdp — Pbdl)/Pbdp-... 

Common indices of flowability are the Hausner ratio and the Can index (compressibility). The increase in bulk density of a powder is related to the cohesiveness of a powder. So measurement of the bulk density of a powder is essential to define the flow characteristics. Ratios of poured-to-tapped bulk densities are expressed in two ways to give indices of flowability ... 

The Hausner ratio varies from about 1.2 for a free-flowing powder to 1.6 for cohesive powders. Carr index classifications for flowability [2] are listed in Table 15. 

Given that the bulk volume associated with the particle mass is a mixture of air and solid material, the bulk density value is highly dependent on sample history prior to measurement. Calculation of the tapped density can then be achieved by tapping the bulk powder a specified number of times (to overcome cohesive forces and remove entrapped air) to determine the tapped volume of the powder. The tapped and bulk density values can be used to define the flowability and compressibility of a powder using Carr s index and the Hausner ratio. 

The angle of repose of granules prepared by five different methods was found to be primarily a function of surface roughness. The Hausner ratio (discussed later) has been related to the morphological properties of sands. ... 

As mentioned in previous sections of this article, the Carr index as well as other empirical tests such as the Hausner ratio continue be used, often in combination with other tests to characterize and predict the flow of pharmaceutical excipients and formulations. In addition to papers already discussed in this article... 

Additional test methods A brief review of additional test methods such as the angle of repose, Hausner ratio and Carr Index, and flow rate through an orifice. 

The typical density ratios are the Carr and Hausner ratios, given by... 

In addition to the direct measurements of tensile strength and cohesion, there are a number of other, indirect ways of assessing cohesiveness. These are discussed in sections 3, 5.2.4 and 4.8 which deal with categories of powders, Hausner ratio and flow-ability respectively. 

The reader is also referred to the use of the Hausner ratio to describe flowability, described in Section 5.2. As most other powder properties, flowability is greatly affected by humidity. 

Kostelnik and Beddow47 suggested and tested a Ro-Tap method for determining tap densities. They argued that, when measuring the tap density to be used in the Hausner ratio (see next section for its definition), its measurement should be consistent with the measurement of aerated bulk density in that it should use the same test cup. They considered the measuring cylinder as inappropriate because the visual determination of volume not only makes it difficult to determine the epd point but makes it impossible to determine the volume accurately enough. 

In understanding, the greatest question to be yet resolved is what is the best, yet simple, test to be used to monitor the cohesivity or flowability of powders There are, of course, the direct (or almost direct) ways of measuring it (the cohesion tester or the tensile testers) but neither the tests nor the equipment are really simple. Angles of repose or spatula are simple but, unfortunately, become quite meaningless as the powder gets more cohesive. I personally think that the way powders compact can be directly related to cohesivity the recent evidence of the usefulness of the Hausner ratio in many different applications points in this direction. The compaction test should also be better defined and more widely used. 

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