The flowability of the powder is related to the shape, size, surface state, density, porosity, etc. of the particles, and the complex relationship between internal friction and adhesion between the particles, the fluidity of the powder cannot be used. A single physical value is expressed. However, the fluidity of the powder has a great influence on the weight difference of granules, capsules, tablets and the like, and is an important link to ensure product quality. There are many forms of powder flow, such as gravity flow, vibration flow, compression flow, fluidized flow, etc., and the corresponding fluidity evaluation methods are also different. When quantitatively measuring the fluidity of the powder, it is best to adopt and treat it. The method corresponding to the process, Table 1 lists the evaluation methods of the flow form and the corresponding liquidity.
Table 1 Flow form and its corresponding liquidity evaluation method
kind
Phenomenon or operation
Liquidity evaluation method
Gravity flow
Flow out of the bottle or hopper. Rotating container type mixer, filling
Outflow velocity, wall friction angle, angle of repose, outflow limit aperture
Vibration flow
Vibrating feeding, vibrating screen filling, outflow
Angle of repose, outflow speed,
Compressibility
Compressed flow
Compression forming (tableting)
Compression degree, wall friction angle, internal friction angle
Fluidized flow
Fluidized layer drying, fluidized layer granulation Air transport of granules or tablets
Angle of repose, minimum fluidization speed
(1) Evaluation and measurement methods of liquidity
Angle of repose
The angle of repose is the maximum angle formed by the free slope of the powder accumulation layer and the horizontal plane. Commonly used measurement methods include injection method, discharge method, tilt angle method, etc., as shown in FIG. The angle of repose can be determined not only directly, but also by measuring the height of the powder layer and the radius of the disk. That is, tan θ = height / radius.
Figure 1 Schematic diagram of the angle of repose
The angle of repose is measured by the gravity and the friction between the particles when the particles slide on the free slope of the volume layer of the powder pile and is in a static state. It is the easiest way to check the fluidity of the powder. The smaller the angle of repose, the smaller the friction and the better the fluidity. It is generally considered that the demand for production fluidity can be satisfied when θ ≤ 40°. The powdery powder or the powder having a particle diameter of less than 100 to 200 μm or less has a large interparticle interaction force and a poor fluidity, and accordingly, the angle of repose is large. It is worth noting that the data obtained by the measurement method is different and the reproducibility is poor, so it cannot be regarded as a physical constant of the powder.
2. Outflow speed
The flow velocity is described by adding the material to the funnel to determine the time required for all materials to flow out. The assay device is shown in Figure 2. If the flowability of the powder is poor and it is impossible to flow out, a glass ball assist flow of 100 μm is added, and the amount (w%) of the glass ball required for free flow is measured to express the fluidity. The more the amount added, the worse the fluidity.
3. Compression
Compressibility: A certain amount of powder is gently loaded into the measuring cylinder to measure the initial loose volume; the tapping method is used to make the powder in the tightest state, and the final volume is measured; the loosest density Ï0 is calculated and the closest Degree Ïf; The degree of compression c is calculated according to the formula 12-31.
Figure 3 tapping the analyzer
The degree of compression is an important indicator of the fluidity of the powder, and its size reflects the cohesiveness and softness of the powder. When the degree of compression is 20% or less, the fluidity is good, and when the degree of compression is increased, the fluidity is lowered. When the C value is 40% to 50%, the powder is difficult to automatically flow out from the container.
4. Internal friction coefficient μ
(II) Influencing factors and improvement methods of liquidity
1. Increase the particle size appropriately
2. Control the humidity of the powder
3. Add lubricant
4. Particle morphology and surface roughness
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