assessment of pigment dispersion (progress in organic coatings)

7/28/2019 Assessment of Pigment Dispersion (Progress in Organic Coatings)

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161rogress in Organic Coatings, 4 (1976) 161 - 1880 ELsevier Sequoia S.A., Lausanne - Printed in the Netherlands

ASSESSMENT OF PIGMENT DISPERSION

W. CARRPigments Divi sion, Ciba-Geigy Plastics & Additi ves Company, Roundthorn I ndustrial Estate,Wythenshawe, Manchester M23 9ND (Gt. Britain)

ContentsIntroduction, 161Criteria, 162Classification, 163Wet systems, 1644.1

4.2

Dry5.1

5.2

Indirect methods, 1644.1.1 Flow properties4.1.2 Dielectric properties4.1.3 Optical densities4.1.4 Hindered settlingDirect methods, 1694.2.1 E lectronic methods4.2.2 Sedimentation methods4.2.3 Photosedimentometer attachmentsystems, 180Indirect methods, 1805.1.1 Colour strength5.1.2 Gloss5.1.3 Optical densitiesDirect methods, 1845-2.1 E lectron microscope

Conclusion, 188References, 188

1. IntroductionFrom measurements of nitrogen surface areas and densities, the orderof magnitude of the basic particle sizes of pigments can be readily calculatedfrom the simple formula:SC 6

Pdwhere S is the nitrogen surface area, p is the density, and d is the mean dia-meter of the basic particles.. _


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The calculations show that the basic particle size of most pigments isbelow 1 pm. With many organic pigments and most carbon blacks, the basicparticle size is below 0.1 pm. These figures are the diameters of equivalentspheres. On the other hand, in the full strength powder pigments sold com-mercially, the smallest speck of powder must be at least 20 pm in size, other-wise the Pigments would not be a powder but would be in the form of adust or smoke.

In the commercial powder forms of pigments, therefore, the basic par-ticles must be present in the form of closely packed aggregates, each speckof powder containing at least lo6 basic particles.In use, the pigment is incorporated into a suitable vehicle and the sys-tem is ground, or milled or dispersed in order to break down these aggregatesas far as possible. This process takes equipment, energy and time, and requiressupervision; it therefore costs money and there is a strong financial induce-ment to control the dispersion process to avoid unnecessary cost.As the dispersion proceeds, the technological properties of the systemalter. The colour strength increases, as does the brightness and gloss. Theseimprovements are usually welcome. Less welcome may be increases in vis-cosity, greater deviation from Newtonian flow, and possible decreases inopacity. To get the correct combination of properties, and to ensure stan-dardisation from batch to batch, again requires control of the dispersingprocess.The control of the dispersion process is therefore important, both fromthe cost and performance angles. Control requires measurement of the dis-persion state so that the process can be stopped at any desired stage. It isthe purpose of this paper to survey actual and potential methods of assessingpigment dispersion and to comment critically and impartially on their suita-bility.The assessment of pigment dispersion in any system means, in practice,the determination of the actual size of the pigment particles or the aggregztesof particles present in the system. If the particles or aggregates are not uni-form in size, then the particle size distribution has to be measured.

2. CriteriaA considerable number of methods have been described or suggested inthe literature for determining the particle sizes of solid particles in suspen-sion, whether they are pigmentary or non-pigmentary. A useful survey ofthis field was published in 1963 by the Particle Size Group of the Societyfor Analytical Chemistry [ 11, and it is understood that this survey is currently

being brought up to date.To help decide whether any particular technique is or would be suitablefor pigments, we need some criteria by which the technique can be judged.These criteria readily select themselves to anyone familiar with pigmenttechnology. They are: (i) sensitivity, (ii) versatility, and (iii) applicability.


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163The first criterion, sensitivity, is the most important one as it is basic.It is now well established, from a variety of techniques, that in practical pig-mented systems, the pigment particles are in the sub-micron range. According-

ly, any method of size determination will not be suitable for pigments if itcannot deal with the sub-micron range.The second criterion, versatility, stems from the wide variety of pig-ments and vehicles that are met with in practice. The pigments compriseorganics, inorganics and carbon blacks. Organic pigments include azos, tonersand specialities such as phthalocyanines, quinacridones, dioxazines, carbazoleviolets, isoindolinones and many others. Inorganics include titanium dioxide,oxides of iron, chromes and cadmium pigments among others. Together withcarbon blacks, these constitute a very varied range, and if the proposedmethod for particle size analysis involves any chemical analysis of pigments

to determine the amount in specific size fractions, many of the pigments onthis list would present difficult problems.The vehicles can be aqueous,emulsion, oil based or solvent based sys-tems. They will usually include resins in appreciable quantities and othermaterials such as surfactants, driers, etc. in minor quantities.Suitable methods for pigmented systems must therefore be capable ofuse with all types of vehicles, aqueous and non-aqueous, and be unaffected

by the presence of solutes.The third criterion, applicability, stems from the colloidal nature ofpigment dispersions. As already mentioned, in most systems the pigmentparticles are less than 1 pm and the system is wholly or partly in the colloidalrange. This means that a pigmented system is an equilibrium system, repre-senting a balance between the forces of attraction and repulsion. Ideally,any particle size determination technique for pigments should not involveany alteration to the system, to ensure that the equilibrium is not affected.If the system has to be diluted. for the purpose of the experiment, then thereis always a possibility of dilution shock occurring which will give coarsersize distributions than the original true one. If the measuring techniqueinvolves the application of shear to the system, then it is possible that thesize distributions obtained will be finer than the original true one. This sus-ceptibility of the pigmented system to the effect of external forces andalterations must always be kept in mind.

3. ClassificationUsing these three criteria, we can now survey various size-measuringtechniques to determine their suitability for application to pigmented sys-tems. To ensure clarity and order, the foilowing classification wiIl be used:

(i) wet systems, (ii) dry systems.As the name implies, the wet systems will include materials such aspaints, printing inks, paper-coating mixtures, etc. before they are appliedand while they are still in a fluid state. The dry systems will include the films


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derived from such products, as well & mass pigmented products-such asplastics, rubber and viscose.In each of these ttio major classifications, two sub-divisions will beused, namely: (i) indirect, (ii) direct.Indirect methods deal with the measurement of some property of thesystem, other than particle size, which is connected in some known way withthe particle size of the pigments.Direct methods deal with the actual physical measurements of the sizeor size distributions of the pigment particles in the system.

4. Wet systemsThis field would include paints, printing inks, paper-coating mixtures,stainers and pigment pastes.

4-l I ndi rect methods4.1 .I F low propert iesIt is known from practical experielice that as the level of pigment dis-persion increases, the flow properties of the resultant dispersion can alter.The alteration appears to be greater with higher pigment concentration. Mea-surement of flow properties would therefore appear to offer a means of

following pigment dispersion and measuring it.Such an approach has many obvious attractions. In the first place, nodilution of the systems would be involved and therefore no danger of dilutionshock. Secondly, any shear applied to the system would only be small andtherefore unlikely to affect the dispersion level. Thirdly, flow measurementsusing modern instruments are quick and easy. Sophisticated cone and plateinstruments such as the Shirley Ferranti viscometer and the WeissenbergRheogoniometer are available, as well as concentric cylinder instrumentssuch as the Rheomat and Brookfield viscometers. These only require smallamounts of the material, are rapid in use, can be made recording instrumentsand will cover a wide range of applied shears. As a technique, flow measure-ment can be applied equally well to aqueous and non-aqueous systems andto all types of pigments, organic, inorganic and-carbon blacks, because thereare no analytical steps involved. The advantages of this indirect method aretherefore many and obvious and much work has and is being done in thisfield 12).The only major drawback to this method is in the interpretation of theresults, that is, in relating the flow properties of the system ,to the dispersionlevels of the pigment in the system. Many workers have attempted to derivea theoretical relationship between-flow properties and pigment dispersion,but with only limited succ&s~ and an acceptable .welI-def&d relirtionshiphas not yet emerged,-This is not altogether surprising considering.the variablesinvolved.. These include: 1 -~ ~. ;- ._.~ .-


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~1~~~~0.1 02 0.3 0.4C5Cr% hneter in microns

,oo L_ 1 , / ;tTi_r--;t-r; ) jy-: I I 1 i i--i.-;-~+-&----;--IT--? , . i ;--T~j-0.1 0.2 0.3 0.4Xl% diameter in microns

Fig. 1. Flow properties of Blue GLSM lithographic inks. 25 C, 7000 dyne cme2. Pigmentconcentration: --- 25%, - - - - 20%, - .--- 15%, - 10%.Fig. 2. Flow properties of Rubine 4BP lithographic inks, 25 C. 7000 dyne crnm2. Pigmentconcentration; - - - - 22.570, --- 20%, -.-.- 17.5%, - 15%,------ 10%.

(a) the rate of shear applied(b) the pigmentation level(c) the dispersion level(d) the interaction between the surface of the pigment particles and theactual vehicle.With regard to this interaction, it is commonly assumed that, in non-aqueous systems, dispersion stability is obtained by adsorption of long-chainmolecules from the vehicle on to the surface of the pigment. The effect ofthis type of absorption on flow properties is difficult to predict in the lightof present knowledge.Experimentally, the type of results, found in the authors laboratories133 and shown in Figs. 1 and 2, illustrate the difficulties experienced inrelating fiow to dispersion levels. These graphs show the relationship betweenapparent viscosity and dispersion at different levels of pigmentation for a&phthalocyanine blue and a calcium 4B toner in a lithographic varnish. Theviscosity was measured at a shear stress of 7000 dynes cm- on a ShirleyFerranti viscometer. The dispersion levels were determined using an indirectmethod of Colour measurement described later.From the graphs for both pigments, it can be seen that at low concen-trations the viscosity is indepe_ndentof particle size. As the pigment concen-tration rises, the viscosity is independent of particle size until a certain size-.>


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is reached, and further reduction in the size markedly increases the viscosity.At high levels of pigmentation, the viscosity is very dependent on size through-out the whole size range.Curves of the type illustrated in Figs. 1 and 2 demonstrate another fact.Even if the relationship between dispersion level and flow properties is wellknown, flow properties are only a sensitive guide to dispersion levels at high

pigmentation levels. In the examples shown, the flow properties would givelittle information on dispersion levels at pigmentation levels of 15% andbelow.To sum up, the measurement of flow properties as a guide to assessingdispersion level has many attractions but some weaknesses. If the relationshipbetween the two could be more clearly defined, then the use of flow mea-surements would be a valuable guide to dispersion, particularly in systemswhich normally use a high level of pigmentation. These would include litho-graphic inks, ball mill bases, and speciality pigment pastes.4.1.2 Dieiec tric propertiesAt various times it has been suggested that the dispersion process isprobably accompanied by changes in dielectric properties of the system, andtherefore measurement of these changes in dielectric properties would pro-vide a convenient method for following the dispersion process.if this idea worked, it would have some important advantages. Therewould be no necessity for diluting the system, therefore there would be nodanger of dilution shock. There would be no necessity to apply any form ofshear to the system. On the other hand, shear could be applied to the systemdeliberately, in order to break down thixotropy, while measurements wereactually in progress, and this might give an important and valuable insightinto thixotropy. It was envisaged that a condenser would be set up with thepigmented system as the dielectric medium between the plates. The capaci-tance of the cell could then be readily measured by known techniques. If astirrer was present in the dielectric medium, it could be switched on and offat controlled intervals, to follow the development of any structure. It wasfelt that this technique would enable the dispersion process to be monitored,as well as the build-up of structure with time.These theoretical advantages seem attractive enough to warrant someinvestigation, but there is little published work on the subject.Some preliminary work at the Paint Research Station was reported ata Seminar held there in 1974. This preliminary work seemed to indicate thatalthough dielectric changes did occur in some pigmented systems as disper-sion progressed, the extent of the changes varied markedly from pigment topigment. With most pigments, the extent of the changes was so small as tomake the method unsuitable. With the other pigments, although the overallchanges were considerable, the pattern of the changes was not well suited tofollowing dispersion. In the regions of coarse dispersion, say from 5 pm downto 1 pm, as the dispersion improved, there was appreciable change in dieiectricproperties, but the rate of change decreased as the dispersion improved. Con-


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167sequently, in systems where the particles were less than 1 pm, the dielectricproperties varied little as dispersion was improved Further. Consequently thesensitivity of dielectric changes as a measure of dispersion level is very poorin the sub-micron range. This is a major drawback, as all practical experienceconfirms that this is the important region of dispersion. It is also believedthat dielectric measurements are best suited to non-aqueous iystems andleast to aqueous systems.This specificity with regard to pigments and vehicles, coupled with poorsensitivity in the sub-micron area, if confirmed, would be major drawbacks.The potential attractions of such a technique, however, would appear towarrant a thorough and authoritative investigation.4. I .3 Op t i c a l dens i t i e sThe optical density of a suspension can be measured on a transmissionspectrophotometer in a suitable optical cell. The optical density of differentconcentration suspensions can be measured and plotted against the concen-tration of the dispersed phase. if the graph is linear, the extinction coefficientcan be calculated from its slope. For a pigment, the extinction coefficient isa function of the optical constants of the pigment and its particle size in thesuspension. The extinction coefficient will tnerefore be related to the par-ticle size of the pigment particles in suspension.Experimentally, optical densities can be measured readily on transmissionspectrophotometers using cells of known path lengths. With pigmented sys-tems, the measurement should be carried out at the wavelength of maximumabsorption of the pigment. The main problem is reducing the optical densityof the system to a level at which it can be conveniently measured. Mostcommercial pigmented systems are far too concentrated for the purpose, evenwhen a short path-length cell is used. Consequently, they have to be diluted,and the degree of dilution is considerable. Using a cell of 100 nrn path length,pigmented systems have to be diluted to a concentration of 0.5% pigment orless. The optical densities of dilutions with 0.5% pigmentation, and lowerlevels, are measured and the optical densities are plotted against pigment con-centration. The resultant graphs should be linear and the extinction coeffi-cients can be calculated from their slope,. The extinction coefficient for anygiven pigment is related to its particle size.

The exact nature of this relationship can be determined empirically ortheoretically. Empirically, the particle sizes of the pigment dispersions haveto be determined directly by another method such as centrifugal sedimenta-tion ].4]. Extinction coefficients can then be plotted against particle size togive a master curve for the particular pigment. With a subsequent dispersionof the same pigment, the extinction coefficient is obtained from opticaldensity measurements and the corresponding particle size is read off fromthe master curve of extinction coefficient ue rsus particle size. Theoretically,if the optical constants n , t he refractive index, and 12, he absorption coeffi-cient, are known, it is possible to calculate the relationship between extinctioncoefficient and particle size using the hlie theory [5], and hence a theoretical


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168curve of extinction coefficient uersus particle size can be drawn. Once thishas been done for a particular pigment, then, as before, for any given disper-sion of that pigment, optical density measurements will give the extinctioncoefficient, and, with this value, the corresponding mean particle size can beread off from the theoretical master curve.There is no real experimental difficulty in measuring the optical densi-ties of diluted paint stainers, liquid inks or paper-coating systems and in cal-culating their extinction coefficients. Master curves of extinction coefficientsuersus particle size can be determined both empirically and theoretically sothat the relationship between the two can be well defined.Optical density measurements can therefore be used as an indirect wayof determining the level of pigment dispersion. As a technique it has manyattractions. It is fairly rapid and relatively simple, and the apparatus requiredis standard equipment in many laboratories. The curves of extinction coef-ficient uersus particle size, whether determined empirically [4] or theoretic-ally [ 53, have quite steep slopes in the sub-micron region, so that the methodis sensitive enough for most purposes. It is applicable to all types of systems,aqueous or non-aqueous, and is not affected by the presence of solutes inthe liquid phase providing they are colourless.There are, however, three drawbacks to the use of this technique. Thesystem has to be diluted considerably, a master curve is necessary, and themethod only gives a mean diameter and not a size distribution.

The dilution step is certainly fraught with danger as there is always apossibility of dilution shock. This can be guarded against by diluting in steps,slowly, with stirring, and by incorporating resin or surfactant into the dilut-ing medium. It is interesting to note that in general, aqueous pigmented sys-tems are more resistant to dilution shock than non-aqueous systems [ 61. Thedangers associated with diluting must never be overlooked, but in the majo-rity of cases they can be overcome.Master curves are necessary if extinction coefficients are to be trans-lated into particle sizes. Empirical curves require a disc centrifuge or similarinstrument for their determination, but once available they can be used inde-finitely with that particular pigment. Theoretical master curves can be ob-tained with the help of a suitable computer programme, from a knowledgeof n and k and an understanding of the Mie theory. It is of course possiblethat, in ftiture, pigment manufacturers may supply master curves as an in-tegral part of their technical data sheets, in the same way that they nowsupply reflectance curves. Without master curves, extinction coefficients canonly be used as comparative measures of dispersion.The extinction coefficient is a function of all the pigment particlespresent in the dispersion, whether big or small, and can therefore only givea value for the mean particle diameter, It can give no information on theparticle size distribution.4.1.4 H i nde r e d se t t l i n gThis is another technique that can be applied to pigmented systems andwhich is claimed to give information on the degree of dispersion. To obtain


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hindered settling, the concentration of pigment particles must be 1% or more_This ensures that the pigment phase tends to settle out en masse rather thanas individual particles. There is a boundary line between the pigment phaseand the clear medium. The settling takes place under gravity and the rate ofsettling is measured as well as the final sediment volume. From these mea-surements it can be deduced whether the pigment particles have flocculatedand, if so, whzt the mean size of the flocculates are.If the measurements are carried out on a pigment dispersed in a solventalone and then dispersed in the solvent containing resin, information can bededuced on the dispersing power of the resin. It has been suggested [7] thatthis technique could be used to assess dispersion levels of pigments, but nopapers have as yet appeared describing such applications. It is believed thatthe technique provides a simple, inexpensive way of classifying dispersionsas good or bad, but that it cannot be used for following or identifying smallchanges in dispersion.4.2 Direct methodsIn this section we are surveying techniques which can, or may, measuredirectly the particle size distribution of pigments in pigmented systems whilethey are still in the fluid state. Again our criteria are sensitivity, versatilityand applicability.

Possible methods can be discussed under two headings, electronic andsedimentation.4.2.1 Electronic methods

The CouIter Counter is the best known example of electronic size-mea-suring methods. It is based on the Coulter principle of letting the dispersionflow through a narrow aperture_ Each particle wilI displace its own volumeof media, and if there is a difference in the conductivities of the solid andliquid phases, the displacement will give rise to a change in coilductivity thatwill be proportional to the particle volume.There is a tremendous amount of literature on this principle, and onthis instrument and its application to a wide variety of systems. There is nodoubt that this instrument represents a major step forward in particle sizemeasurement. It is simple in function, quick and accurate. However, it isfairly generally accepted that it is not sensitive enough to deal with particlesin the sub-micron range. On this ground alone, it is not suitable for pigmentedsystems. It is also doubtful whether it could be used successfully in mostcommercial non-aqueous systems, because the differences in conductivitybetween the pigments and the media are much smaller.4.2.2 Sedimentation methodsThese methods of size analysis have many attractive features. The theoryand practice of free sedimentation, based as it is on Stokes law, is well under-stood and accepted. Moreover, this theory is applicable to all dispersed sys-tems, whether they are aqueous or non-aqueous, and is not affected by thepresence of solutes such as resins, surfactants, etc. in the continuous phase.The one thing that is crucial to all sedimentation work using Stokesequations is that the sedimentation must be free and unhindered. Each par-


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170title must fall freely, independently of its neighbours. In practice this meansthat the pigment content of the system must be about 0.1% or less, and thismeans, in turn, that the original system must be diluted considerably. Thisbrings in the danger and possibility of dilution shock, i .e. of coarsening ofthe dispersion. If this takes place, then the results obtained will not be re-presentative of the original system.The possibility of dilution shock is a real danger and must always bekept in mind. It is a possibility with all techniques and instruments that arebased on free sedimentation, whether it is gravity or centrifugal sedimenta-tion. The danger is, however, not sufficient to write off such methods, buttests must always be made to see if dilution shock is present. If it is, steps canusua.lly be taken to guard against it. These steps include the incorporation ofresin or surfactant into the diluting media, and making the reduction in steps.Where the system is very viscous, as in, say, some litho inks, then the physicalincorporation of the diluting medium into the ink can present problems.Sometimes considerable shear is required to bring about the actual mixingof the viscous ink and the diluent, and this shear could conceivably increasethe level of dispersion. One way round this problem is to warm the ink toreduce its viscosity, and to make the dilution in stages.Another requisite of sedimentation techniques in general, which caneasily be overlooked, is that the pigment particles should not dissolve in thesedimenting liquid. This does not arise with inorganic pigments and carbonblacks, but it is known that many common azo pigments have a distinct bleedor slight solubility in the systems in which they are used. Arylamide yellowsand toluidine reds are important examples of this. Would the bleed ofthese pigments, while sedimenting at low pigmentation levels, be sufficientlygreat to vitiate the particle size results? The literature is silent on this subject.The procedure wit.h all sedimentation techniques is fairly standard:dilution to about 0.1% solid phase or less, sampling from a fixed knownpoint at fixed known times, and analysis of the samples to determine thecontent of the sohd phase. From the depth of the sampling point from thesurface and the time of sampling, the maximum size of the pigment particlesin the sampIe can be calculated. As the time increases, the maximum size ofparticles in the samples decreases. From the data a size distribution curvecan be built up, although the mathematics can be tedious.

Gravity sedimentation is too slow to classify particles which are all inthe sub-micron range. Centrifugal sedimentation can separate such particlesin a reasonable time if the speed of revolution is high enough. Commercialapparatuses for determining particle size distribution, based on centrifugalsedimentation, are now on the market and have been and are being used withpigmented systems. The best known of these instruments is probably theJoyce Loebl disc centrifuge, and the authors experience with this instrumentwill be described in order to illustrate the advantages and disadvantages ofthe technique [8, 91.The instrument consists of a hollow, accurately machined disc, mountedvertically, which can rotate at selected speeds up to 8000 rpm. The originaldisc was made of Perspex, but solvent-resistant discs are now available.


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With this instrument a line start technique is used and a sample of thesuspension is removed automatically down to a fixed depth by suctionthrough a probe. The time at which the sampling is carried out can be presetand the sampling is done automatically.The maximum size of particles in any sample is determined by the timeof spinning and the speed of spinning, and for any given system these are themain operating variables. For a typical experiment, eight to ten spinningsare made giving eight to ten samples representing different size fractions.These samples are analysed for t.heir pigment content, and in this way a com-plete size distribution curve is obtained_ The line start technique simplifiesthe mathematics of the calculations. It will be seen from this brief descrip-tion that the method has three stages: dilution, fractionation, and analysis.

The dilution stage is necessary to ensure free unhindered sedimentation,and the dangers inherent in this step have already been pointed out. Again itmust be pointed out that the dilution stage is necessary for all sedimentationexperiments based on Stokes equations, and does not apply solely to theJoyce Loebl instrument and technique.

The fractionation stage is largely automatic and trouble-free and givesa number of samples of pigment suspensions of differing maximum particlesizes.

In the analysis stage, the pigment content of each of these fractions hasto be determined, and the analysis can present problems. These problemsstem from the small amounts of pigments present in each sample and thefact that pigments are by definition substantially insoluble in most solvents.

The amount of pigment present in each size fraction is of the order oftwo milligrams, and the liquid phase may have small amounts of resin, driersand surfactants present. With these small amounts of pigment, dry weightdeterminations are not sufficiently accurate. Most azo organic pigments canbe handled readily enough by getting them into solution and measuring theoptical density of the solution under standard conditions. If the liquid phasein the fraction is water, the water is removed by evaporation and the pigmenttaken up in dichlorobenzene or dimethyl formamide. If the liquid phase isnon-aqueous, the same technique can be used. With some pigments, the dis-solving solvent can be added directly to the size fraction, but with most, theevaporation stage is required_

The optical densities are converted to pigment weights by the use of apreviously determined calibration graph of optical density uersus concentra-tion.

For non-azo pigments, such as phthalocyanines, dioxazine and carbazoleviolets, isoindolinones, etc., a suitable technique is to evaporate off the liquidphase from the fraction and dissolve the pigment in concentrated sulphuricacid. An aliquot of this solution is rapidly diluted with water containing asmall amount of surfactant and then the optical density of the resultant clis-persion is quickly measured, i.e. within 20 minutes of the dilution. Duringthis time interval, the suspension is fine enough to obey Beers Law. Again,the optical density can be converted into a pigment weight by reference to apreviously calibrated graph.


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172The reader will appreciate that the analytical problems associated withinorganic pigments are likely to be more difficult because of their greaterinsolubility. Where the analysis is possible, it is likely to be so tedious and

time-consuming as to rule it out of court for practical purposes.With titanium oxide size fractions, for example, the fraction has to beevaporated to dryness; then the residue is submitted to a caustic fusion, andthen the mass is extracted with water. This will give the titanium in a solubleform and then it can be treated with a suitable reagent to give a characteristiccolour which can be used as a basis for analysis. For day to day purposes,this analytical step is far too lengthy.With carbon black size fractions, the analytical problems seem to beinsuperable.The technique of centrifugal sedimentation using a line start techniqueand sample removal would therefore appear to be restricted to use withorganic pigments because of the analytical problems associated with otherpigments. We are left with the frustrating position of being able to fractionateall types of pigmented systems into their various size fractions, but unableto determine the amounts of pigment in these size fractions.With organic pigments, optical density measurements represent a con-venient and accurate way of measuring the pigment contents of the fractions.The disc centrifuge has worked well with organic pigment dispersionsof all kinds, both aqueous and non-aqueous. The method gives size distribu-tion curves which are reproducible down to sizes as low as 0.025 pm, and thetheory of the method is sound. The method, therefore, as far as organic pig-ments are concerned, appears to satisfy our three criteria of sensitivity, ver-satility and applicability, provided care is taken at the dilution stage.

Figures 3 and 4 illustrate size distribution curves that have been obtainedin this manner for a number of practical pigmented systems. These bear outthe above claims for the technique.

On the other hand, the method has a number of weaknesses and limita-tions. As we have already indicated, the system has to be diluted and there-fore dilution shock has to be guarded against. The method is essentially res-tricted, in practice, to non-bleeding organic pigments because of the analyticalproblems associated with inorganic pigments.Even with organic pigments, the method is slow. Normally eight to tenseparate spinnings have to be carried out in order to get eight to ten differentsize fractions, then these fractions have to be evaporated to dryness, takenup in a solvent and their pigment content determined by analysis. All thistakes time and, from start to finish, a particle size distribution curve willtake about one and a half days to determine.This means that the method can only be considered as a research tool.4.23 Photosedimentometer attachmentMany of these drawbacks can be overcome by the use of a Photosedimen-tometer Attachment (PSA), which was a development designed for use withthe disc centrifuge. I n this development, a narrow beam of light is shonethrough the disc and falls on to a photocell. The resulting voltage is amplified


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800.sc?d 60Ez.v

40

20

0 0.01 0.i 1.0particle diameter in microns

Fig. 3.. Particle size distribution curves obtained with a disc centrifuge for Irgazin Violet6RLT ball-milled paint stainers, after milling for 2 hours (A), 12 hours (33) and 72 hours(CL

01part& diameter m microns

Fig. 4. Particle size distribution curves obtained with a disc centrifuge for a Calcium 4Bgravure ink using Ca/Zn resinate in 50/50 SBP5: toluol. Initially x - x, after 3 months o - o.

and fed to a pen recorder which can, therefore, plot a curve of turbidityveisus time. The light passes through the suspension in the disc at the samedepth from the surface that the probe reaches.The method of operating the instrument is the same as before and againuses a line start technique. One ml of a diluted suspension is injected into thespinning disc containing a suitable spin fluid and a curve is obtained of turbi-dity v&rsus time. From the speed of the disc, the instrument constants, the


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174densit.y of the pigment and the spin fluid, the time axis can be converted toparticle size and the graph is redrawn to give a curve of turbidity uersus par-ticle size, with the latter axis increasing arithmetically.

Ideally, this curve should start at zero turbidity and return to zeroturbidity, and if it does not, then it is extrapolated to do so. The curve isthen normalised. The area under the curve is measured and this is representedas 100% weight of the pigment injected into the disc.To obtain the weight of particles under a given size, the ordinate cor-responding to that size is drawn and the area under the curve to the left ofthe ordinate is measured, and this is expressed as a percentage ratio of thewhole area to give the weight percentage of pigment particles below thatparticular size. By drawing more ordinates and repeating this step, a completepicture of the size distribution can be obtained and an integral curve can bedrawn of weight per cent undersize versus particle size. This is the same typeof curve that. is obtained by using the earlier technique without the PSAattachment.Let us examine, in detail , the effects of using the PSA technique. In thefirst place, the actual experimental time is cut down dramatically becauseonly one spinning is required. This one spinning gives the data for a completesize distribution curve, whereas; in the original method, one spinning wouldonly give the data for one point on the size distribution curve. Furthermore,no analytical step is required; this again cuts down the time factor. Also, ifthe spinning time selected is, say, 45 minutes, the actual experiment, togetherwith the normalising of the resultant curves, can be completed in about twohours. This compares with one and a half days by the original method.The absence of any analytical stage has an even more significant effect.I t immediately widens the scope of the method to include all pigments,whether organic, inorganic or carbon black. Extenders and emulsions andresin dispersions can also be handled.The reduction in time and the extension of technique to include alltypes of dispersions are very significant advances. The reduction in timemeans that the technique can be considered for factory control as well asfor research purposes. Furthermore, the disc centrifuge, when fitted with asolvent-resistant disc and a photosedimentometer attachment, can handle allthe systems normally used in the surface-coating industries, irrespective ofthe nature of the disperse phase or the continuous phase.It would therefore appear to satisfy completely two of our assessmentcriteria, namely sensitivity and versatil ity. It will also satisfy the third cri-terion, namely applicability, if dilution shock is guarded against when theoriginal system is diluted.As might be expected, the photosedimentometer technique does havesome disadvantages. Because it is a sedimentometer technique, the originalpigmented systems have to be diluted.pd this always brings on the dangersof dilution shock.Because only one spinning is used, the speed at which this is carried outhas to be carefully chosen, so that both coarse and fine particles in the.sys-


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175

=z 160=.i 1205 80

Tme of spinning converted toparticle diameter (Microns)Fig. 5. Disc centrifuge with PSA. Recorder curve for phthalocyanine blue decorativepaint stainer. Faster disc speed.z5 160g 120g 80zF: 40-k

1.0 0.4 0.3 0.2 0.1Tme of spinning converted toparticle diameter (Microns)

Fig. 6. Disc centrifuge with PSA. Recorder curve for phthalocyanine blue decorativepaint stainer. Slower disc speed.

tern can be accurately sized. Experience has shown that most pigmented sys-tems have a fairly wide size distribution varying from, say, 0.05 to 2.0 I_tm,i.e. by a factor of 40. With such systems, a high spinning speed will give arecorder curve of turbidity (T) against time similar to that illustrated in Fig. 5.This means that although the curve drops to zero and all the fine particles areassessed, the coarse end of the.curve is cramped and the measurement of thecoarse particles will be inaccurate. On the other hand, a slow spinning speedwill give a curve similar to that illustrated in Fig. 6. The coarse end will beopened up, but the time of spinning will be excessively extended to accountfor the fine particles. An excessive spinning time may bring in other errorssuch as spin fluid evaporation and temperature rises. To avoid these, thespinning may have to be stopped before the curve reaches zero and consi-derable extrapolations made. A medium speed will fall in between these twoextremes_

In our experience, the photosedimentometer is not suitable for pigmen-ted systems with a very wide range of particle sizes because only one spin-ning is involved. We restrict its use to systems where the particle sizes rangefrom approximately 0.1 to 1.0 pm. Further evidence for this is given a littlelater. The method only gives comparative results. As the grinding of anygiven system proceeds, the weight percentage undersize ue rsus particle sizecurves obtained by the photosedimentometer technique become progressive-ly finer, but the curves are only comparative. They do not give absolute size


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176_ .

TABLE 1Comparison of sizes from absolute and PSA measurements. Dioxazine violetin alkyd based decorative paint stainerBall milling time, hours 6 12 24 9650% diameter PSA method), p 0.56 0.42 0.26 0.2150% diameter (absolute method),w 0.35 0.26 0.21 0.18

distributions. At least the size distributions obtained by this technique differ,and differ considerably, from those obtained for the same dispersions bythe original method. An indication of the sort of differences Dbtained isgiven in Table 1, where the results for a paint stainer based on dioxazineviolet in an alkydiesin system are given:We believe that the original method gives absolute results while the PSmethod gives values which are in error because of the uncertainty of therelationship between concentration and turbidity at low particle sizes.The marked deviation of the PS results from absolute values is a distinctdrawback to the wider use of the method. However, it can be largely over-come by the use of calibration curves. For any one specific pigment, forexample a /3-phthalocyanine blue, a suitable pigmented system is made upand its particle size distribution is determined both by the original absolutemethod and by the PS method. To obtain the calibration graph for thispigment, log (c/lzT) is plotted against log d on log-log paper. The value ofc, the weight of pigment in the spinning disc whose particles are below d pmin diameter, is derived from the weight percentage undersize results obtainedusing the absolute method. The term k T i s t he arbitrary turbidity or opticaldensity reading obtained on the vertical axis of the chart record, and d iscalculated from the instrument constants and the time and speed of spinning.The calibration graph for this pigment is shown in Fig. 7, and it will be seenthat the graph is linear between sizes 0.1 and 1.0 pm.

Other organic pigments give straight-line graphs by this method, althoughthe lines differ markedly in slope.There is a certain amount of scatter about the lines and a very pro-nounced deviation from linearity at diameters greater than 1.0 pm . This maybe due to the collection efficiency of the probe being poor at coarse sizesand/or the turbidity readings for these sizes being inaccurate because thesingle spinning used in the PS method is too fast for them. The straight linescan be better defined by plotting c/lzT against d on linear graph paper,drawing the best curve through the points and using this curve to obtain thelog-log plot.In Table 2, the slopes of the straight lines have been listed for a numberof organic pigments representing all parts of the spectrum. The figures inthis t.able demonstrate conclusively that there is no single master graphsuitable for the calibration of all pigments.


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177

0 O-lDiameter (microns)Fig. 7. Calibration curve for Copper Phthalocyanine Blue GLSM.

TABLE 2Slopes of calibration graphsPigment C.I. index

referenceSlope ofcalibrationgraph

Green DBN Green 8 0.42Violet 6RLT Violet 34 0.45Green GLN Green 7 0.50Blue GLSM Blue 15 0.60Grange F2G Orange 34 0.72Magenta TCB Violet 2 0.80Red M2B Red 48 0.83Red CBN Red 53 0.90Yellow GTN Yellow 1 0.95Red RLY Red 49 1.02Rubine 4BP Red 57 1.05Yellow 2GP Yellow 17 1.28Yellow BO Yellow 12 1.37Yellow LBAW Yellow 13 1.68

Armed with a calibration graph for a pigment, the determination ofthe size distribution of that pigment in surface coatings becomes relativelystraightforward.After diluting to 0.5% pigment content, the dispersion is injected intothe spinning disc fitted with the PSA and allowed to settle through a suitablespin fluid. The recorder gives a plot of h T against time. The graph is redrawn,with the time axis converted to particle diameter d . For each selected valueof d , t h e corresponding k T reading is multiplied by the value of c/VT readoff from the calibration graph for that size. This gives a term c which is afunction of pigment concentrationThe values of c are plotted against the: -1


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178values of d on linear graph paper, and the graph is normalised as before togivea graph of weight per cent undersize uersus particle diameter.The size distribution curve is now an absolute curve. This :an be checkedby repeating the experiment using the absolute method without the PSA.Only one spinning has been required and no analytical step was necessary.Armed with the requisite calibration curve, a disc centrifuge fitted with asolvent-resistant disc and a photosedimentometer attachment can give anaccurate, absolute size distribution curve of any dispersed system with onlyone spinning. The time of this spinning, together with the necessary re-drawing and normalising of the curve, takes about two hours.Once determined, a calibration curve can be used with all subsequentdispersions and products based on that pigment. To obtain a calibrationcurve requires particle size determination by both methods. This can be donewith organic pigments, as these are well suited to the absolute method.Inorganic pigments are less well s*uited o the absolute method, due toanalytical difficulties. In many cases, however, these can be overcome withtime and care, and hence calibration graphs can be drawn for them. The cali-bration graph for a rutile titanium oxide is shown in Fig. 8. The minimum inthis graph is probably associated with the fact that the scattering power ofthis pigment goes through a maximum as its particle size is reduced.Garbon blacks cannot be measured by the absolute method, so thatcalibration graphs cannot be directly obtained for them. A way round thisdifficulty has been suggested recently [lo J , namely to use for carbon blacksthe calibration curve obtained for Pigment Green ELIf the beam of light used as the sensing mechanism in the photosedimen-tometer technique is replaced by a stream of X-rays, there is little, if any,difference between the results of the X-ray sedimentation technique andthe absolute method. This is probably associated with the fact that thewavelength of the X-rays is much smaller than the particle diameters. How-ever, the absorbing capacity of pigments for X-rays is a function of theiratomic numbers, and consequently X-ray instruments only appear to besuitable for inorganic pigments, and not for organic pigments and carbonblacks [ 111.X-ray centrifugal sedimentometers are now available commercially,and are being used, with satisfaction, by the manufacturers of titaniumoxides and inorganic pigments.As mentioned earlier, sedimentation experiments for determining par-ticle sizes are based on Stokes law and are therefore believed to be theore-tically sound. One criticism of such experiments, when applied to the verysmall particles met with in pigment technology, has been voiced by one ofthe writers colleagues [7 3. In dry pigment powders we know that the basicparticles are clustered together very tightly. On milling, grinding or dispersingthe powders into a liquid phase, the clusters are rapidly broken down intosmaller clusters and possibly basic particles. These smaller c!usters, or basicparticles, can reaggregate or flocculate through the existence of very shortrange attractive forces. When they do so, they may well trap some of the


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17910

c x10-k-T

lo-

.6

IO -

O-

.l 10

Fig.

I II II I

01 l-o 10Particle diameter (microns)

8. Calibratiorr curve for rutile titanium dioxide.

liquid phase inside, and the aggregate plus trapped liquid will settle undergravity or centrifugal force as a single entity. The density of this compositewill differ from the true density of the pigment and, as it is the true densitythat is used in the calculations, the particle size distributions obtained willbe in error. This criticism cannot be lightly dismissed, but it is difficult toknow how to determine the degree of error, if any, that would be introducedin this way.From our experience, based on hundreds of size distribution experi-ments, the repeatability of the size distributions falls off when the meandiameters are greater than about 0.6 pm, but is very good with dispersionsfiner than this. The loss in accuracy above 0.6 pm may well be due to thedensity problem outlined above. Fortunately, the size regions of greatestinterest, both practically and theoretically in pigment technology, are from0.5 pm downwards, and we have always found the reproducibility to be goodin this region. We would also expect the incidence of trapped liquid in thissize region to be insignificant.We can now sum up both indirect and direct methods of assessing dis-persion in the wet state.I n d i r e c t me t hods . Measurement of flow properties is very attractive frommany points of view, but the interpretation of the results is uncertain.Optical density measurements are straightforward but require massivedilution. To give absolute results, a master curve is required of extinctioncoefficient ue rsus particle size. This can be obtained either theoretically orempirically.

Di rec t me thods . Centrifugal sedimentation is the only technique .sensi-tive enough for the sub-micron range. Using a photosedimentometer attach-ment, the size distributions of organic pigments can be readily determined.Carbon blacks can be measured to a close approximation. An X-ray centri-fugal sedimentometer is much better suited to inorganic pigments.


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180With all sedimentation experiments, dilution shock must be guardedagainst. Photo or X-ray sedimentometers are best suited to reasonablynarrow size distributions, say from 0.1 to 1.0 pm.

5. Dry systems5 .1 In d i r ec t m eth ods

51.1 Co lour st r eng t hIt has long been known that as grinding proceeds, colour strength willoften increase. For many pigments the increase in colour strength can be con-siderable. Colour strengths can be compared very accurately by the humaneye and they can also be measured instrumentally by a variety of spectro-photometers. Colour strength would, therefore, appear to provide a suitablemeans for following changes in pigment dispersion.For accurate comparisons, the pigmented specimens must be preparedunder identical conditions. For surface coatings, it is preferable that thepaint or ink is adequately reduced with a suitable standard opaque white sothat the resultant films are opaque and the colour strengths are independentof the film thickness. The film with the higher colour strength contains themore highly dispersed pigment.I f colour strength is to be used to measure the absolute degree of dis-persion, rather than for purely comparative purposes, then a master curvemust be prepared of colour strength ue rsus particle size, using standardised

conditions for measuring the colour strength.Particle size measurements can be carried out using centrifugal sedimen-tation as already described, and hence the master curves can be determinedexperimentally_ Their determination is quite lengthy, but their usefulnessis considerable.Master curves can also be calculated theoretically using the Mie theory,provided the optical constants of the pigment are known. Figures 9 and 10give some master curves of colour strength versus particle size for some well-known organic pigments [12]. The curves for phthalocyanine blue andPigment Green B are believed to represent the extremes; that is, the mastercurves for all organic pigments lie between these two curves and follow thesame general pattern. This pattern gives a slow, regular increase with reducingparticle size in the coarser regions, but as the particle sizes are reduced belowabout 0.4 Drn, the subsequent increases in colour strength are much greater.These increases can be of the order of 200 - 300% as the particle size reducesfrom 0.4 to 0.15 pm. This means that the colour strength is a very sensitiveindication of dispersion level in this region.The actual instrumental measurement of the colour strength of surfacecoatings is only a matter of minutes. With paints, however, the films maywell require overnight drying. On the other hand, many films can be preparedand allowed to dry simultaneously and can be quickly examined for colourstrength the next day.


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181

5x

45

4.0

3.5

3.0

iris 25

2x

1.5

1.c

0.1 -

Fig. 9. Particle size of paint stainers us. colour strength of titanium dioxide reductions(1:12.5). A, Copper Phthalocyanine Blue: beta form (Irgalite Blue GLS): 3, sand ground;x , ball milled; stabilised alpha form (Irgalite Blue BCS): *, sand ground; A, ball milled.B, Pigment Green B (Cl Pigment Green 8 Irgalite Green DBN): 0, sand ground; 0, ballmilled.

Colour strength measurement, as a means of measuring dispersion, isstraightforward and very sensitive, particularly in the regions of good dis-persion. Essentially a comparative method, it can be converted into anequally sensitive absolute method if a master curve of colour strength uersusparticle size is available.On the other hand, it must be appreciated that colour strengths are onlyindicative of the pi,ment dispersion in the final dry film. It is usually assumedthat this dispersion level is the same as that in the wet paint or ink, but recentwork has suggested that this is not always the case [ 13]_ Moreover, pheno-mena can take place in the drying stage, such as flooding of the white or

coloured pigment, which will give erroneous values of the colour strength.Colour strength measurements can never give information on the par-ticle size distribution. On the other hand, it is difficult to represent a par-ticle size distribution by a single figure. Mean diameters, however determined,give little information on the size distributions, and it is possible for variouspigmented systems to give similar mean diameters while differing markedlyin size distributions. Colour strength measurements can give a single figure,usually the value of k /s under standard conditions, which is more represen-tative of the true state of dispersion; k is the absorption coefficient and s thescattering coefficient. Every individual particle or aggregate of particles con-tributes to the colour strength of the end system, but the smaller particlescontribute very much more on a weight basis. The colour strength sums up


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0.6

0.5 -

0.0 -

OJ

0.2 -

0.1 -

C

IIIf II-\i i\\_. _I;\!,

3.8SO/. diamclcr In microns

Fig. 10. Colour strength us. particle size. l/75 reductions in Beckosol P470. - - - Car-bazole Violet (Pigment Violet 34). - Monarch 71 (Pigment Black 7), -. - - - Phthalo-cyanine Green (Pigment Green 7), -I--- Diarylide Yellow (Pigment Yellow 17).the effect of all the particles, and it is unlikely that systems with equal colourstrengths wil l have any but slight differences in particle size distribution.In our experience, colour strength measurements represent.an easy,straightforward, sensitive and accurate way of assessing pigment dispersionin the sub-micron range, especially for air-drying decorative paints and oilinks, For such systems, only one master curve is needed for absolute values,and this can readily be determined for the paints. The oil inks can be usedas stainers or tinters for the white paint [ 3 3 as they are usually fully compa-tible with it. We suspect that the method is not so suitable for other systemssuch as stoving paints, emulsion paints, and liquid inks. To apply the tech-nique on an absolute basis to these systems, separate master curves wouldbe required because dispersion levels of the titanium oxide in the reductionswould be different. The colour strengths are measured on the final dry films,and there is a possibil ity that the dispersion levels in these films differ fromthose in the corresponding wet systems. With the airdrying paints and oil


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183

inks, the evidence to would seem indicate that is noalteration of pigment dispersion on film drying.Colour strength determinations also have the decided psychologicalmerit that they represent the main property that the user is interested in.5.1.2 GlossIt is also known in a general way that, as dispersion increases, the glossof the subsequent films increases. Specular gloss can be readfly measured,and gloss measurements could therefore be used as a technique for followingdispersion changes. Published evidence [14] to date, however, shows thatwith paint films, specular gloss does increase with dispersion in the coarserregion but tends to level off once the mean diameter is reduced to about0.3 - 4 /.lnl.Gloss is therefore not suitable for assessing dispersion levels in the sizeregions below 0.4 pm. Gloss is also known to vary with time even whenpanels are stored in the dark [ 13 3It is highly doubtful whether gloss would be of any use at aU in assessingdispersion in surface coatings of low gloss.5.1.3 Optical densitiesProvided a film is not opaque in itself, and has been prepared on a trans-parent substrate, its optical density can be measured if the film thickness isknown. The measurement takes only a matter of minutes on a transmissionspectrophotometer. If the level of pigmentation is varied, and the experimentrepeated, the optical density can be plotted against pigment concentration.If the graph is linear, the extinction coefficient of the pigment in the filmcan be determined from the slope of the graph. It is dependent on the opticalconstants n and k of the pigment and the particle size of the pigment in thefilm. There is, therefore, a direct relationship between the extinction coef-ficient of the film and the particle size of the pigment in the film.Optical density measurements can therefore be used to compare dis-persion levels of a given pigment. They can be used to determine the absolutelevel of dispersion if the exact relationship between extinction coefficientand particle size is known.As before, this relationship can be determined experimentally or it canbe derived from the application of the Mie theory.Experimental determination means measuring the pigment dispersiondirectly, and this can only be done in the wet product. One has then to makethe assumption that, on application and drying, the dispersion level is unaltered.One can then plot a graph of extinction coefficient versus particle size andthis is a master curve for that pigment. Subsequent determinations of extinc-tion coefficients can be converted into pigment particle sizes by reading offthe corresponding sizes from the master curve.The technique is similar in principle to that used for the wet paints. Asuitable substrate for the film is a glass microscope slide. The paint or inkhas to be diluted considerably to enable the optical density to be measured.As a satisfactory film has to be prepared from the diluted system, the dilutionwill usually have to be carried out with the vehicle itself, i.e. with a solution


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184

of resin in a solvent. Dilution with a solvent itself will give a system whoseviscosity may be too low for satisfactory film formation, A pigmentationlevel of 0.5% or less is usually adequate with film thicknesses ranging from25 to 100 pm. With liquid inks, the incorporation of a plasticiser may benecessary to obtain a satisfactory film.The films can usually be applied by a bar coater or block applicator togive films of known wet film thickness. Films can also be applied to theglass by spraying, in which case the film thickness can be determined fromweighings of the slide before and after.As in the wet state, the curves of extinction coefficient versus particlesize have quite steep slopes in the sub-micron region, so that the method issensitive. Again, it is applicable to all types of systems, aqueous or non-aqueous.

Again, the system has to be diluted drastically and the films have to beprepared on a transparent substrate.One of the most interesting things about using optical density measure-ments as a measure of dispersion levels is that it is probably the only techni-que that can be applied to both the wet and dry states. I t does, therefore,offer a means whereby dispersion levels in the dry films can be directly com-pared with the corresponding dispersion levels in the wet products fromwhich were derived. It be used study the ofmechanisms pigmentDirect methods5.2.1 E lectron nz croscopeI f a photograph of the pigment dispersion in the dry state can be ob-tained, whether from a surface coating or a mass pigmented product, thesize of the particles can be measured at leisure and a size distribution curvecan be obtained. For particles known to be in the sub-micron region, opticalmicroscopy is not adequate and electron microscopy is required. Withsuitable microtomes, very thin cross-sections of surface-coating films andsegments of mass pigmented products can be obtained and electron micro-graphs prepared. Some outstanding photographs of pigmented systems havebeen obtained in this way and published in the literature. The photographs

give permanent records and can be studied at leisure. Many classes of pigmentshave been examined in this way in various types of systems. Electron micro-scopy as a direct method of assessing pigment dispersion scores on the groundsof sensitivity, versality and applicabil ity.However, it is not without drawbacks. The equipment is still expensive.It requires a specialist operator to get the best results, and in this field theoperator should also have a good knowledge of pigment technology. It isnot a rapid method, because the preparation of specimens for the electronmicroscope is a delicate task which cannot be hurried.Even when satisfactory photographs have been obtained, there are con-siderable difficulties to be faced. These arise from the fact that the pigmentparticles are never uniform in size; there is always a size distribution. Even in


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185

Fig. 11. Electron micrograph of decorative paint film (X 10,000). Dioxazine Violet inalkyd resin. 50% diameter = 0.15 p.


26/28

1 8 6r -~ " - ~ . . . . . ~ . " . : ' :. " - " . ' . :- " . ~ ~ . . , :: . ~ - . ~ - ~ " '7 ~ ', . .~ "~ ; ~- ~ -. . . - - ., . . ~ , . ~ ~ . . ~ ' ~ ; ~ - , . . . ~ - ~ - ' ~ : . ~ . : _ ~ , . : ~ . ~ .~ ~ ~ ~ . . . . . ~ - : ~ ~ , ~ . . . ~ , . ~ . ~ . ~ . - .

~ ' ~ ~ ~ - ~ _ . ~.~ ~ : , ~ i - ~ : - : = ~ = ~ . . ~~_ -~ :~ ~ ~ . :. .; .-~ -~ .~ - , - [ . - . _ - - . ~ . . _ ]~ . , . ~ - ; . . . ]~ . .~ .~ , ,~~ ~ ~ , ~ ] 5 ~ g ~ . . - ~ . . . . . . . . - . " ~ ' : . ' : : . : . ~ ; ] . ' : ~ o ~ g . . ~ ' : " - - ~ - ' ~ . . ~~ l ~ . _ ~ ~ -. , , ~ - ~ ' - ~ . . . . - . ; - - . : ~ . - ~ - :- . . ~ : ~ : .

. . . . ~ H~" ~ .~ " . ] ~ ~ - . ._ ~ ,~ - . ~ . . '.. ' ~ " ~ ~ . . , , ~ ~ . ~ ~,-m , - - ,~ , .

~ - - . . . : . ~ . ~ . ~ , ~ . ~ , : . . : , , - . .~ . . ~ , ~ ' . ~ - - ~ ' . ~ , ~ : ~ . . : - .~ ' , ~ ~ - " , r - ;~ ,~ ~ -~ . . '- ~ F . ;~ ~ 1 : ' ~ . .~ . ~ ; b " ~ ' "~ : . ,~~ . ~ - . t - ~ , ~ " . ~ . ~ . . e ' :~ . . . . ~ . 'A . . ~ . ~ " , ~ . . ' - ~ = ig . - " . % : " - ~ [ - , - ~ " .~ . . , ~ , ~ - w , - ~ . . - . ~ ~ , ~ - - . ~ ~ . " " ~ . . ~ . ' - ' ~ ' .} ~ e ' - = ' ~ ' . ~ " ~ " " ~ L " " . ' - ~ ~ ' ; - : " , ' ~ - ' ~ - ,. . " " ~ - ~ ~ ' - ' , : ~r m . , . - - * ~ C i r . . ~ : " q ~ . . ' 4 1 ~ ' . ~ . , , ~ e , ; _ ~ . . : . ~ - ~ ~ - . x . . ' , . - :- . . . . . . - . . ~ . ~

' ~ , ~ " ' " ~ ~ ,~ , ,.,.:~ . _, - ; " "O " , -v .~ - , - . . , ~ .,~" ~?-

F i g . 1 2 . E l e c t r o n m i c r o g r a p h o f d e c o r a t i v e p a i n t f i l m ( x 1 0 , 0 0 0 ) . D i o x a z i n e V i o l e t ina l k y d r e s in . 5 0 % d i a m e t e r = 0 . 2 5 / ~ n .


27/28

187

very well-dispersed systems, the range of sizes is usually 25-fold, going from,say, 0.04 to 1.0 pm. This sort of size distribution introduces two problems.Firstly, in systems where the mean particle sizes are known to be fairlyclose together (from other methods of assessment), the eye can see littledifference in the photographs and finds it difficult to decide which is thebetter dispersion. This is illustrated in the two electron micrographs shownin Figs. 11 and 12. These micrographs are of cross-sections of films of deco-rative paint stainers based on a dioxazine violet in an alkyd resin system.There is little difference between the dispersion levels visible to the eye.The mean particle sizes (50% wjw diameter) of the pigments in the twopaints are 0.15 and 0.25 pm respectively, and their colour strengths (h/svalues) when measured as l/75 reductions with titanium oxide are 0.42 and0.28, respectively. The two stainers are markedly different in terms of theso-called flocculation test. Despite these differences in mean diameters,colour strengths and application behaviour, the eye can detect little, if any,difference between them in terms of dispersion or particle size. This is be-cause the difference in mean diameters is small compared with the range ofsizes present in both.Secondly, to determine a size distribution from a micrograph, the sizesof a considerable number of particles, taken at random, have to be measured[lo]. The number of measurements to be made will depend on the degree ofaccuracy and the actual range of sizes in the picture. In most cases at least1000 individual particles will have to be measured, and usually the figurewill be much more.To carry this out by hand is out of the question as a regular procedure,and it is commonly agreed among microscopists that size distributions canonly be obtained from electron micrographs when the counting and sizingcan be done automatically [ 151.

Automatic scanners, analysers and counters are available commercially,but they are very expensive. The area of the cross-section of a film viewedin a single photograph is small, and consequently quite a number should beprepared from different areas of the specimen in order to get a truly repre-sentative size distribution. This increases the time taken and emphasises theneed for automatic counting.Given the necessary equipment, and this includes, besides the micro-scope, the preparation equipment and an automatic scanner and counter,particle size distributions can be determined directly for pigmented filmsand mass pigmented systems A specialist operator is reqquired, and even thenthe whole procedure is fairly slow and it is difficult to see it being used ona routine basis. It might be argued that the best role for electron microscopy

in the field of particle size measurement would be to act as a check on otherquicker and less sophisticated methods. Certainly, comparisons of thisnature would help to demonstrate the soundness or limitations of othermethods. The literature to date, however, does not give any comparisons ofthis nature in the sub-micron region.


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1886. Conclusion

The authors views on the position at the present time regarding theassessment of pigment dispersions in the sub-micron region are as follows.For wet systems, centrifugal sedimentation has proved itself, in practice,to have the necessary sensitivity and versatility for organic pigments. Whenfitted with a light beam as the sensing mechanism, the experimental time isdramatically reduced. The technique is very suitable for organic pigments andcarbon blacks in all types of systems. When an X-ray beam is used as a sensingmechanism, the technique is very suitable for inorganic pigments.For dry systems, colour strength measurement is a very useful indirecttechnique well suited for day to day work. For direct accurate measurement,electron microscopy and sophisticated counting and scanning equipmentare required.For comparisons of dispersion levels, in the wet and dry forms of thesame pigmented system, optical density measurement would appear to betheonly method available, as this is the only technique that can be appliedto both wet and dry systems.

References1 Classification of methods for determining particle size, Analyst, 88 (1963) 156.2 C. C. Mill (Ed.), Rheology of Disperse Systems, Pergamon Press, New York, 1959.3 W. Carr, J. Oil Colour Chem. Assoc., 55 (1972) 794.4 W. Car-r, Proc. XIIIth FATIPEC Congr., 1976, p. 164.5 G. Mie, Ann. Phys., 25 (1908) 377.6 W. Car-r, J. Oil Coiour Chem. Assoc., 54 (1971) 155.7 R. B. McKay, J. Appl. Chem. Biotechnol., 26 (1976) 55.8 E. Atherton, A. C. Cooper and M. R. Fox, J. Sot. Dyers Coiour., 80 (1964) 521.9 M. Mermann and B. Honigmann, Farbe Lack, 75 (1969) 337.

10 G. A. Lombard and W. Car-r, J. Oil Colour Chem. Assoc., 58 (1974) 246.11 T. Alien and L. Svarovsky, Powder Technoi., 10 (1974) 23.12 W. Can-, J. Paint Technol., 42 (1970) 694.13 W. Car-r, Paper presented at an O.C.C.A. Symposium in Manchester, April 1976 (in

the press).14 W. Car-r, J. Oil Colour Chem. Assoc., 54 (1971) 1093.15 L. J. Venuto and W. Hess, Am. Ink Maker, 45 (1967) 42.

assessment of pigment dispersion (progress in organic coatings)

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