Rod Jones

P R O C E S S A N A L Y T I C S

Successful Particle Size Analysis of Dry PowdersA correctly set up dry pow-der measurement using alaser diffraction particlesize analyser is much eas-ier to perform than mea-surement of wet suspen-sions. However, incorrectmeasurement set-up canlead to errors and inconsis-tencies. This article exam-ines the key steps achiev-ing successful dry topowder measurement. Is-sues examined includepowder flowability andsampling, air pressure andsample dispersion, feedrate and the importance ofStandard Operating Proce-dures.

Successful particle sizeanalysis of dry powders

A correctly set up dry powdermeasurement using a laserdiffraction particle sizeanalyser is much easier toperform than measurementof wet suspensions. However,incorrect measurement set-up can lead to errors and in-consistencies. This article ex-amines the key steps inachieving a successful drypowder measurement.

A significant difference be-tween wet and dry measure-ments concerns sample con-ditioning. With a wetsuspension, there is ampleopportunity to condition thesample. Particle agglomera-tion is clearly signalled by thesoftware, allowing the addi-tion of appropriate additivesor stabilisers, and the use ofultrasonics. Repeat measure-ments can be performed untila satisfactory result is ob-tained, with further measure-ments confirming the stabilityof the suspension.

In contrast, there is no op-portunity to recycle dry pow-ders to continue treatment,as the sample passes throughthe laser beam just once. It istherefore important to ensurethat the dispersion process is

correct, so that the sizeanalysis accurately reflectsthe particle size distribution.

Sample flowability andsampling

A samples flowability has amajor impact on how itshould be dispersed. Free-flowing materials are proneto segregation when sub-jected to vibration, either inbulk transit or during trans-port by mechanisms such asvibrating hoppers.

The implications aretwofold. Bulk material shouldbe thoroughly mixed tocounter any segregation be-fore samples are withdrawn,and samples should be splitby a reliable technique, suchas spin riffling. The impor-tance of this cannot be over-stated: the standard deviation(SD) between riffle split poly-disperse samples can be aslow as 0.146 %, comparedwith 5.14% for scoop sam-pling [1]. If no riffler is avail-able, bulk samples should bemixed by half filling a screw-top container and tumbling itend-over-end at least 20times before withdrawing asample. The design of thespoon or spatula used to takesample from the pot is alsoimportant. Coarse particles

fall off the edges of flat spatu-las and this can give rise to asignificant bias. A suitablesmall spoon is preferable.

The second important con-sideration is to ensure thatthe measurement consumesthe entire sample: if signifi-cant segregation takes placewithin the sample duringtransport in a feeder, then abias may result. If there is anydoubt about the occurrence ofsegregation, this can be veri-fied by a simple series of re-peat, short duration measure-ments from a single tray fullof material. Where there is nodifference between the firstand final measurements in aseries, segregation is clearlynot a problem. If there is asignificant difference, thewhole series can be inte-grated and averaged to obtainan unbiased result for thesample. Subsequent measure-ments of such material should

be of sufficient duration to en-sure that the whole samplehas been seen. This providesautomatic integration, andeliminates any bias. Observa-tion of significant differencesbetween short duration mea-surements gives valuable in-formation about the behav-iour of a material and canmake a significant contribu-tion to understanding its bulkflow properties

Air pressure and dispersion

Cohesive materials are lessprone to segregation andsampling errors, and usuallyrequire more dispersive en-ergy to produce a cloud ofprimary particles. This bringsits own challenges. Thesmaller the particles, thegreater the surface attraction(Van der Waals forces). It hasbeen shown [2] that inter-particle attraction due to Van

Fig. 1: Pressure titration: as the dispersive pressure increases, the d10, d50and d90 plateau, indicating that 3 bar is the optimum pressure for correctdispersion

G.I.T. Laboratory Journal 2/2002, S. 46-48, GIT VERLAG, 64220 Darmstadt, Germany, www.gitverlag.com

P R O C E S S A N A L Y T I C S

der Waals forces is around100g (g being force of grav-ity). This increases to 1000gfor 1 micron particles and tobetween 104 and 105 g forparticles of 0.1 m.

A dry powder feeder issimilar in design to an air jet

mill. Tapering the particletransport tube within thefeeder accelerates airbornematerial close to the speed ofsound, creating shear forcesthat separate loose agglomer-ates. Particle collisions withthe walls of the feeder further

disperse the more cohesiveagglomerates.

For delicate materials, dis-persing at too high a pressurewill result in overly severecollisions, causing fracture ofthe material and milling ofthe product. Conversely, in-complete dispersion may oc-cur at too low a pressure,producing a larger than ex-pected size. To find the opti-mum between these two ef-fects, the performance of apressure titration (recom-mended in Section 6.2.3.2 ofISO13320) is advised. This isa series of repeat measure-ments at different dispersivepressures to determine themost suitable. Creation of a

trend graph of D(v,0.1),D(v,0.5) and D(v,0.9) versuspressure helps to visualisethe results. Fig. 1 shows atypical result, where a sizeplateau is reached with apressure of 3 bar and ismaintained at higher pres-sures, indicating that 3 bar issufficient for successful dis-persion. Sometimes with veryfriable material, a plateau isnever reached increasingthe pressure simply mills thematerial to finer sizes. Forlow density, freeze driedmaterials, good results willusually be obtained at disper-sive pressures as low as 0.25bar.

Fig. 2: Setting the feed rate so that the obscuration lies within the limits forthe duration of the measurement (using obscuration triggering) will ensureconsistency of results. However, too much reliance on obscuration trigger-ing in preference to regulating the feed rate will give less reliable resultsoverall.

Fig. 3: Overplot of wet dispersion and dry measurement on a dry powdercoating - perfect agreement at a dispersive pressure of 2 bar

G.I.T. Laboratory Journal 2/2002, S. 46-48, GIT VERLAG, 64220 Darmstadt, Germany, www.gitverlag.com

P R O C E S S A N A L Y T I C S

Feed rate

Feed rate is important in de-termining measurement suc-cess. If particle transport isthe only consideration, thenfree-flowing powders requirea lower feed rate than cohe-sive powders, which needmore energy to transportthem into the venturi of thefeeder. The ideal obscurationrange (Malvern Mastersizer2000) is between 0.5 to 5 %for a dry measurement, dif-fering markedly from theideal for wet measurements(between 5 % and 20 %). Thefiner the material, the lowerthe ideal obscuration can besince the determining factorin measurement success isthe number of light scatteringnuclei passing through thelaser beam during measure-ment. If particles are large,there will be fewer nuclei pergram, so sample loading canbe increased. If micronisedmaterials are being mea-sured, flow rates should becarefully regulated to achieveobscurations closer to 0.5 %.For very cohesive materials,such as magnesium stearate,placing the sample nearer tothe front of the sample feedtray will remove the need toincrease feed rates to trans-port the sample to the ven-turi. Aiming for lower obscu-rations when cohesive drypowders are being measuredhas the added benefit of max-imising the dispersive effectof the dry powder feeder ven-turi, since the impact of the

particles on the walls is bettercontrolled at lower feed rates.

On the Mastersizer 2000,Standard Operating Proce-dures (SOPs) in the softwarecan be set up to govern theideal feed rate and air pres-sure automatically. Sample ismeasured only within anideal obscuration range, sta-bilising measurements con-siderably. Further improve-ments in measurementquality are achieved by ad-justing the feed rate to ensurethat the obscuration remainswithin the limits for the dura-tion of the measurement (asshown in Fig. 2A and 2B).

Comparison with wet mea-surements the ultimatemeasure of success

When an SOP is set up for theanticipated lifetime of a prod-

uct run, and is expected to bereplicated globally, determin-ing the ideal settings by run-ning a pressure titration is agood investment of time. Con-firmation of the correctness ofthese settings is possible inmany cases by comparing thedry results with a wet mea-surement, as recommendedin Section 6.2.3 of ISO 13320.When in close agreement,these provide a useful defenceof the method if it must be au-dited for regulatory purposes.A perfect example of the con-firmation of the correctness ofa dry powder measurement isshown in Fig. 3.

Conclusion

Achieving optimum resultsfor dry powder measure-ments requires some simpleprecautions. Once the mea-suring system is set up for theoptimum flow of the powder,and a representative samplecan be provided to the instru-ment, the feed rate should beadjusted to give an accept-able obscuration. After this, apressure titration can be usedto determine the most appro-priate pressure. Whereverpossible, this should be con-firmed by comparison with awet measurement. The dura-tion of the dry measurementshould be adjusted to ensurethat the whole of the sampleis consumed.

If these steps are taken, theresult will be an SOP that al-

lows people with little or noexperience of particle sizing toperform good measurementsevery time. Fig. 4 shows atrend plot of 29 measurementsof cornflour performed by 3experts and 26 novices. Thenovices were shown where toplace the samples and weretold to follow the instructionson the computer screen. Thereis no significant variation inthe results obtained, so re-gardless of the fact that 3users had extensive experi-ence of performing particlesize measurements, the resultsthey and the novices producedare all in perfect agreement.

Literature[1] T. Allen Particle Size measure-

ment 5th Edition, Volume 1.

Chapman and Hall 1997. ISBN

0412 729504. Table 1.5, Page 38

[2] Aerosol Science, Ed. C N Davies,

Academic Press, London and

New York, 1966

Rod Jones is the Product Manager responsible forMalverns laser diffraction-based rangeof particle size analysers. With morethan 20 years experience of particlesizing within all branches of industry,Rod has accumulated a wealth of ap-plications knowledge through provid-ing practical solutions to customerssizing problems.

Malvern Instruments LtdEnigma Business ParkGrovewood RoadMalvern, Worcestershire, UKWR14 1XZTel.: +44 1684 89 24 56Fax: +44 1684 89 27 89www.malvern.co.uk

Fig. 4: Three of the measurements were performed by particle sizing ex-perts the rest were performed by people with no prior experience of par-ticle size analysis

G.I.T. Laboratory Journal 2/2002, S. 46-48, GIT VERLAG, 64220 Darmstadt, Germany, www.gitverlag.com