Spatula, angle

The classification system introduced by Carr [29,30] was used to evaluate the flow properties of the sorbitol powders. In Carr s system, a flowable powder is defined as free flowing and will tend to flow steadily and consistently. This is to be contrasted with a floodable powder, which will exhibit an unstable, discontinuous, and gushing type of flow. The parameters in Carr s system include the angle of repose, angle of spatula, compressibility, cohesion, and dispersibility. Based on these parameters, flowability and floodability indices are calculated to determine the handling properties of bulk solids. 

Angle of Spatula and Angle of Internal Friction. The angle of spatula is a quick measurement of the angle of internal friction. This is the angle, measured from the horizontal, that a material assumes on a... 

The third factor in Carr s characterization scheme is the angle of spatula. As mentioned previously, Carr designed a special method for measuring this angle. According to him, the angle of spatula is an indirect measure of cohesion, surface area, size, shape, uniformity, fluidity, porosity, and deformability. 

Angle of repose Compressibility Angle of spatula Uniformity coef. Cohesion ... 

Closely related to the angle of repose is the angle of spatula which is measured by spooning out powder with a 7/8 in (22 mm) wide flat-blade spatula. According to Carr36, the spatula is inserted into the bulk powder parallel to the bottom of the container and then lifted straight up and out of the powder, whilst keeping it parallel with a horizontal plane. 

The spatula may then be vibrated or tapped in a specified manner and the angle of the tangent to the powder surface at the edge of the spatula is measured, to the horizontal plane. 

The angle of spatula is a simple measure of the flowability of the powder and, according to Carr36, for a material to be considered free-flowing, its angle of spatula should be under 40 degrees. Materials that have a wide range of particle size and shape may require the use of a broader blade of spatula than the width specified above. 

The angle of spatula is clearly useful as a very simple and rough measure of flowability but the errors in measurement are bound to be very large. 

Another, this time indirect, method of assessing flowability was proposed by Carr36. This was based on measuring four properties of the powder (angles of repose and spatula, compressibility and uniformity coefficient, all as defined by Carr36) and, by awarding points (out of 25) for each, the sum total was then a measure of flowability and called Carr s Index . A commercial piece of equipment is available (Hosokawa Powder Characteristics Tester) for measuring the properties necessary for Carr s Index. 

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. 

Stainless-steel spatula, 5-8mm wide, bent to obtain 135° angle. 

Test A - Measurement of Carr angle of repose Test B - Measurement of Carr angle of fall Test C - Calculation of Carr angle of difference Test D - Measurement of Carr loose bulk density Test E - Measurement of Carr packed bulk density Test F - Calculation of Carr compressibility Test G - Measurement of Carr cohesion Test H - Measurement of Carr uniformity Test I - Measurement of Carr angle of spatula Test J - Measurement of Carr dispersibility... 

---