Powders, test methods powder flow

Other test methods are the powder bulk, aerated, and layer tests [137,138]. Several systems in-house built are available. All of these tests operate on the principle that a layer of the substance under investigation is heated in a circulating air oven as the temperature is increased. Air is transported through the sample (in the aerated test, the air flow is downward through the sample), and the temperature of the powder at several places is recorded. 

Other Methods of Flow Characterization A variety of other test methods to characterize flowability of powders have been proposed, which include density ratios, flow from funnels and orifices, angles of repose and sliding, simplified indicizer flow testing, and... 

Amidon GE. Physical test methods for powder flow characterization of pharmaceutical materials. A review of methods. Pharm Forum 1999 25 8298-8305. 

There is, of course, a direct way of testing flowability by measuring the time for a standard amount of powder to be discharged from a specified funnel or hopper. This is sometimes referred to as free flow time , as for example specified by an ASTM standard method40 for testing metal powders the funnel used is the same as in the International Standard ISO 3923, Part 1. The ASTM method was used in measuring the flow time of sands in a study of particle morphology (see Ref. 10, Chapter 17). 

There are several different methods available for determining the flow properties of powders, and there are many literature examples demonstrating correlations between a test method and the manufacturing properties of a formulation. Listed below are some of the more commonly used tests, together with references, detailing their use in pharmaceutical applications. 

Methods for evaluating the performance of powder coatings are the same as those used for conventional coatings. Test methods for coating powders include particle size distribution, powder free flow, sintering, fluidization characteristics, and others. They have been reviewed in detail and reported in the literature (120,121). In addition, the American Society for Testing and Materials has issued a comprehensive standard that covers the most important test methods for the evaluation and characterization of powder coatings (122). 

Compressibilities in excess of 20% tend to create particle bridges in hoppers while powders with a compressibility percentage greater than 40% are difficult to discharge (Table 1.6). At present the Hausner ratio is used more widely than compressibility, although Carr (1965c) used his parameter extensively to classify various types of powder into categories of excellent or very poor flow. The Carr approach to flow has now been subsumed into a standard test method for bulk solid flowability (ASTM D 6393-99 1999). 

Johanson claims that this new approach produces a similar powder flow function in one tenth of the time required by the Jenike test. However, Enstad and Maltby (1992) and Schwedes and Schulze (1990, 1992) criticised the Johanson method and doubted whether the measured failure stress was representative of a material s unconflned yield strength, even after the application of a correction factor. This observation was confirmed by Marjanovic et al. (1995, 1998) with five powders (Table 1.13). 

The science of soil mechanics was integrated with the related field of powder mechanics and reduced to industrial practice by Jenike [1] in 1964. Since then, it has been possible for industry to reliably measure the flowability of powders and relate the measurements, in engineering units, to the design requirements for silo flow. However, Jenike s publication was neither the first effort to quantify flowability nor the last. New testing methods continue to be introduced, with varying degrees of success. In many cases these alternative measurement methods are the result of an industrial necessity and reflect some shortcoming of the Jenike method. In other cases, they exist because the Jenike method is not known to the people involved or is not relevant to their problem. Business value can be derived from many different types of measurements. 

Gravitational Methods GONELL AIR ELUTRIATOR. This is the prototype of all analytical separators with laminar air flow. It consists of a cylindrical brass tube (or a series of tubes) with a conical base. An air inlet is provided in this base on the axis of the tube. The sample of powder is placed in the inlet cone, and air is blown thru the largest tube until separation is deemed complete, or for specified periods of time. The residue is removed, weighed, and transferred to a smaller diameter tube, and the test is repeated. The tube should have polished internal surfaces and should be periodically tapped or vibrated to disturb settled powder... 

---