Malvern - Aero S Dry Dispersion

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Enhanced dry powder particle sizemeasurements using the Aero S disperser

PARTICLE SIZE

Understanding the behavior of dry powders is important in many applications, whetherit relates to the fundamental properties of the particles, their flowability for processing,or the ability to target the deposition of a material.

In order to characterize these materials accurately, it is essential for a dry powderdisperser to be able to control the aerodynamic dispersion of a wide range of materials;from friable to robust materials and from free-flowing to extremely cohesive materials.

The inter-particle forces that bind particles together include van der Waals forces,electrostatics and liquid bridges. Furthermore, as particles become finer the relativestrength of these forces increase and therefore smaller particles are much more of achallenge to disperse.

There are three main mechanisms available to overcome these inter-particle forcesand disperse dry powders. These mechanisms, in increasing order of aggression,are illustrated in Figure 3 (a) velocity gradients caused by shear stress, (b) particleto particle collisions and (c) particle to wall collisions. The importance of each ofthese mechanisms to a particular disperser will depend on the geometry, flow rate(or pressure drop) and material type. For any sample dry powder dispersion is a finebalance between effective dispersion and the risk of breaking the primary particles,associated with the more aggressive dispersion mechanisms.

The Aero SThe Aero S dry powder disperser unit has a modular design which allows theconfiguration of the dispersion unit to be optimized for different materials. Thisoptimization relates to both the flow of materials with different tray designs and anadjustable hopper in order to control the sample feed (Figure 2), as well as a rangeof venturis suitable for dispersing fragile, robust and cohesive materials. All of thesemodular components are automatically recognized by the software so the set upfor each sample is always recorded and can be locked into a standard operatingprocedure (SOP).

The modular design of the Aero S allows venturis with different geometries to be usedin order to make use of the different mechanisms available for dry powder dispersion.

The disperser designs were selected by evaluating the performance of severaldifferent geometries on a range of materials with different bulk powder properties. Thebulk powder properties were assessed by shear testing and the material were then

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2 Enhanced dry powder particle size measurements using the Aero S disperser

classified as being between not flowing, through very cohesive and cohesive to freeflowing.

Figure 1: Mastersizer 3000 and Aero S

Figure 2: Modular hopper, tray and venturi assembly

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Figure 3: Mechanisms for dry powder dispersion

The dispersion efficiency achieved for a particular material is evaluated by comparingmeasurements of the dry dispersion to a well dispersed wet reference result. Wetresults are considered to be the reference method as generally a higher energy isavailable for dispersion (due to the addition of surfactants, additives and ultrasound)without the aggression of the dry mechanisms. The degree of overlap between the twoparticle size distributions is defined as the dispersion efficiency, 100% being completeagreement and perfect dispersion. The two disperser designs which showed thehighest dispersion efficiency over the range of materials were then developed for usewith the Aero S. The fundamental study of dry powder dispersion is described in moredetail in a separate article [1].

Disperser geometriesThe two disperser geometries that can be used with the Aero S are a standard venturiand a high energy venturi.

The standard venturi provides effective dispersion without using the more aggressivedispersion mechanisms. It has no impaction surfaces and therefore uses velocitygradients and particle to particle collisions to disperse the particles. Figure 4 (a) showsa schematic of the standard venturi and the path taken by the particles. The sampledrops down from the sample tray into the funnel and the compressed air used todisperse the particles enters at right angles. The particles are then accelerated throughthe venturi and the dispersed sample passes directly through the measurement zone.The lack of an impaction surface makes this venturi particularly suited to dispersingfragile samples.

In the high energy venturi the particle are entrained into the airflow, and acceleratedin the same way as the standard venturi. The particles then flow through a 90degreebend creating an impaction zone. This impaction zone provides high energy dispersionby particle to wall collisions and is suitable for very cohesive and robust particles.

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Figure 4: Standard (a) and High Energy (b) venturi for the Aero S

The ability to choose between a standard venturi using mainly shear forces and ahigh energy venturi using impaction increases the range of materials for which drydispersion is suitable.

Assessing the state of dispersionThe degree of dispersion achieved by any venturi is dependent on the air pressureused. Hence, to assess the dispersion of a material the size distribution is measuredover a range of pressures (this is referred to as a pressure titration). Figure 5 showsthe results of a pressure titration carried out on a milk powder sample using thestandard venturi. In general a decrease in particle size is observed with increasingpressure. However, this decrease can be the result of two processes; firstly thedispersion of agglomerates within the sample and secondly breakage of the primaryparticles. It is thereforenecessary to determine the appropriate pressure at whichto measure the sample to achieve dispersion but not to break the primary particles.Comparisons of the dry results over a range of pressures (using the standard venturi)compared to the dispersed wet result (after ultrasound) are shown in Figure 6. Theseresults show that at low pressures (0.1bar and 0.5 bar are shown in this example) thereare agglomerates still present in the results which indicates that full dispersion has notbeen achieved. At higher pressures, 3bar, the agreement between the wet and dryresults is excellent, suggesting full dispersion has been achieved.

Figure 5: Pressure titration using the standard venturi

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Figure 6: Wet (blue curve)-dry comparison using the standard venturi

Figure 7: Milk powder pressure titration using the high energy venturi

Figure 8: Milk powder wet-(blue curve) - dry comparison, high energy venturi

A pressure titration for the same milk powder sample has also been measured usingthe high energy venturi, Figure 7.

For a given pressure a smaller size is measured using the high energy venturi(compared to the standard) due to the additional dispersion by impaction. Again acomparison of the wet and dry results has been used to assess the state of dispersionand to determine at which pressure dispersion but not particle breakage has occurred.Figure 8 shows the comparison of dry results using the high energy venturi at 1bar,3bar, and 4bar to the dispersed wet result. This shows excellent agreement betweenthe wet and dry results at low pressure, 1bar. However, at higher pressures (3barand 4 bar) the dry result is smaller than the wet result which indicates that the primaryparticles have been broken down at higher pressures. This example shows that bothventuris, using different dispersion mechanisms, can disperse the milk powder sample(although at different pressures) the next step is to choose which venturi is the mostappropriate for this sample. Figure 10 shows the pressure titrations (Dv50) on bothventuris and the Dv50 of the dispersed wet result for comparison. This shows that

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comparable results are achieved at 1bar using the high energy venturi and at between3bar and 4bar using the standard venturi. The range of pressures over which the wetand dry results agree can be used to determine which venturi is most appropriate. Thedecrease in size around 1bar is quite steep on the high energy venturi where as therelationship between size and pressure is more stable between 3bar and 4 bar on thestandard venturi indicating a greater robustness in the results to pressure. Thereforefor this relatively fragile material the standard venturi provides a much more robustresult over a range of pressures.

Figure 10: Pressure titration (Dv50) on both venturis compared to the wet result

Increased dynamic rangeDry measurements can be made on the Mastersizer 3000 from 0.1μm up to 3500μm.This extended dynamic range is an essential requirement for the analysis of coffeesamples, where the particle size affects both the flavor and the speed of brewing.Figure 9 shows the particle size distribution of a three grades of coffee (filter, smoothand espresso) where the size distribution extends from 10μm up to 3500μm.

Dry dispersion also offers advantages for measuring large or polydisperse materialsin terms of the mass of sample which can easily be measured. As the particle sizeincreases so does the mass of sample required in order to get a representativesample of the bulk. In dry diffraction measurements a greater mass of samplecan be measured as the sample is gradually fed through the measurement zoneat the appropriate concentration. Hence to measure a larger mass of sample themeasurement duration is simply increased.

At the other end of the dynamic range materials become much more difficult todisperse, as the inter-particle forces increase as the particle size gets smaller.

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Figure 9: Example of a measurement of filter, espresso and smooth coffee (standard venturi)

Figure 11: Example of a pigment measured dry (high energy venturi)

Figure 11 shows the particle size distribution of a pigment sample measured on theAero S. As this sample is very cohesive it has been dispersed using the high energyventuri at 4bar. The reproducibility of the dispersion has been tested by measuringseveral sub samples of the same batch the results of which are shown in Figure 12.The results show the coefficient of variance over the 10 repeat measurements is withinthe ISO limits for repeatability [2], indicating reproducible dispersion for this very finecohesive material.

ConclusionsThe wide dynamic range of the Mastersizer 3000 and the modular design of theAero S have opened up dry laser diffraction measurements to a much wider range ofmaterials.

The range of sample trays and an adjustable height hopper allow the sample flow to becontrolled for both free-flowing and cohesive materials.

Sample dispersion can then be optimized by the use of either the standard or highenergy venturi, providing solutions for materials that are fragile or robust, free-flowingor cohesive.

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Figure 12: Reproducibility over 10 measurements of a pigment sample

References[1] Investigating the Dispersion of Dry Powders. MRK1654-01[2] ISO 13320:2009 Particle size analysis -- Laser diffraction methods

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Malvern - Aero S Dry Dispersion