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    (Coatings Teehnology Handbook, D.Sotos, Ed., New York:Mareel Dekker, 1991.

    1.0 INTRODUCTION

    61Pigment Dispersion

    Theodore G. VernardakisSun Chemjcal Corporation

    Cjndnnati. Ohjo

    The dispersion of pigments in fIuid media isof great technological importance to the coat-ings manufacturers who deal with pigmented systems. The basic aim isto change the physi-cal state of pigments to achieve desired effects inspecific application systems. The disper-sion process involves the breaking down and separation ofthe aggregated and agglomeratedparticles that are present inall p igmets in their normal form after their manufacture. Dis-persion is not considered to be a process of pulverization, but rather a process of partic1eseparation, homogeneous distribution of the particles in a medium, and stabilization of theresultant system to prevent reaggregation, reagglomeration, fIocculation, and settling. Theprocess of dispersion must bedone efficiently and inthe shortest t ime possible to draw outof the pigment i ts maximum color propert ies at the least cosI.

    The topic of pigment dispersion infIuid media has been covered extensively in the litera-ture1~. The theoretical aspects of pigment dispersion apply equally well to inorganic andorganic pigments. In this chapter, the practical examples of surface treatments apply pri-marily to organic pigments, but similar treatments can be carried out on inorganic pigmentsas well. '

    2.0 A BRIEF INTRODUCTIONTO PIGMENTS2.1 Pigment Defin it ionMaterialsarecoloredby theuse of pigmentsor dyes.Pigmenls arecolored,black,while, ortluorescent particulate organic or inorganic solids, 'usually they are insoluble in, and essen-l ially physically and chemically unaffected by, lhe vehic1e or substrale in which they areincorporaled. They alter appearance by seleclive adsorption and/or by scattering of lighl?

    Pigmenls usually are dispcrsed in vehicles or substrales for applicalion (e.g. , in inks,painls, plaslics,or olhcr polymeric material). Pigments relain a cryslal or particulale struc-tllre throllghoul the coloration process.

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    ( (530 VERNARDAKISAs a result of the physical and chemical characteris tics of pigments, p igments and dyesdiffer in lheir application: when a dye isapplied, i tpenetrates the substrate insoluble form,

    afler which ilmay or may not become insoluble. When a pigmenl isused tocolor or opacifya substrate, the finely divided, insoluble solid remains throughoUl the col'oration process.

    Organic p igments a re highly colored, inert syn thel ic compounds tha t a re u sual lybrighler, purer, and richer in color than inorganic pigments. Generally , however, they areless resis tant tosunlighl (some fade badly on exposure to light), to chemicals (greater len-dency to bleed insolvents), and to high processing temperatures (Iower heat stability); quiteoften 100, lhey are more expensive than inorganic pigments. Pigmenls are c1assified by the"Colour Index" according lo specific pigmenl name and conslilution number. For example,phthalocyanine blue is known by the C.I. name Pigment Blue 15 and, its el. number is74160, while l itanium dioxide is C.I. Pigment White 6, C.I. 77891. The greal number andvariely oforganic amiinorganicpigments make it impossibleto t reat them al! in this chap-ter. References should be consulted for information on pigmenl types, chemical and physi-cal properties, melhods ofpreparation, grades, specifications, and applications. See, for ex-ampie, References 8-11.2.2 Pigment ParticlesPigments are normally produced ina wel presscake form, which upon drying and grindingo r spray d rying assl lmes the form of afine d ry powder. P resscakes , e ither a t the ir no rmalpigment content (20-40%) or as "high solids"(50-60%), are used by the manufacturers ofaqueous pigment dispersions for paint, textile, and ink applications, as well as by those whoproduce fIushed colors for oil ink orcoatings applications. Dry pigment powders are used ina host of other systems such as solvent inks,coatings, and plastics. Pigments in the presscakeo r dry powder form ar-ecomposed of f ine part ic les, no rmal ly in the submicrometer s izerange. Their color properties are generally influenced by particle size and partic1e size dis-tribution; therefore, an assessment on the degree of dispersion must, above all , be consid-ered interms of these critical measurements 12.In general, color properties, such asstrength,transparency, gloss , rheology, and lightfastness of all p igmented systems, are affected toagreater or lesser extent by the size and dislribution ofthe pigment partic1es in the dispersion, .For example., phthalocyanine blue is first prepared commercially ina "crude" pigment formhaving a large partic1esize, upto 25 11m.As such,it has little color value and must thereforebe reduced to smaller, finer particles to enhance its coloristic properties. After partic1e sizereduction (down to 0.03--0.15 11m),an excellent pigment is obtained, which exhibits a highdegree oftinctorial strength, transparency, and gloss. Typical electron micrographs of thesetwo materials , showing partic1e size, are reproduced in Figure 1. and associated crystals

    Pigment particles normally exist in the form of primary pa~ticles,laggregates, agglomer-ates, and fIocculates. Primary particles are individual crystals as they are formed during themanufacturing process . (Fig. 2). They may vary insize depending on the c09ditions ofpre-cipitat ion and growth, which are controlled by the pigment manufacturero The scanningelectron photomicrograph ofFigure 2 for micronized sodium chloride (although this is not apigment), is used only to illustrate the individual and associated crystals that make up theprimary particles of a compound.

    Aggregates are collect ions of primary particles thal are attached to each other at theirsurface~ or crystal faces and show a tightly packed structure. Agglomerates consist of pri-mary particles and aggregates joined at the comers and edges in a ooser type of arrange-menl. Aggregates are formed during the manufacturing process inthe course of the ripeningperiod of the precipitates. Agglomerates, most often , are formed during the drying of the

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    PIGMENT DISPERSION 531

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    0.5 ~m

    Figure 1 Scannil1geleetron photomierograph of copper phlhalocyanine blue crude (top) andIransmissiol1 electron pholomicrograph of copper phlhalocyanine blue pigment (bollom) showingparlicle sizc differences; Pigment Blue 15.

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    532 VERNARDAKIS

    Figure 2 Scanning electron photomicrograph showing primary parlicles: (a) individual cryslalsand(b) associatedcryslalsof micronizedNaCI.

    Figure 3 Transmi~sion electron photomicrograph showing (a) aggregatedand (b) agglomera\cdpigmentparlicles.D&C Red No. 30, Vat Red 1.

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    " (PIGMENT DISPERSION 533

    Figure 4 Transrnission electron photornicrograph showing flocculated pigrnent particles.Dirnethylquinacridone rnagent.1,Pigrnent Red 122.

    presscakes and thesubsequent drymillingof the pigment lumps.Figure 3 shows typicalarrangements of aggregated andagglomerated pigment particles.Flocculates consist of primary particles, aggregates, and agglomerates, generally ar-ranged in a fairly open structure, as shown in Figure4. Flocculates maybe broken downeasily under shear,but they will form againwhen such shear forcesare removed and thedispersionis allowed to stand undisturbed.3.0 THE DISPERSION PROCESSThe primary purpose of dispersion is lobreak down pigment aggregates and agglomeratesto their optimum pigrnentary particulate size (down to individual single partic1es, if possi-ble) and to distribute these pigment particles evenly throughout a medium (Le., the carrier).Usually the carrier isa l iquid or a solid polymeric material that is deformable at high tem-peratures during processing. To achieve the optimum benefits of a pigment, bolh visual andeconornic, itis necessary toobtain as full a reduction as possible to the primary particle size.After all, the color strength ofa pigrnent depends on its exposed surface area: the smaller theparticle size, the higher the surface area, and thus, the stronger the color. Furthermore, thepigment isgenerally the l1Iostexpensive constituenl of any pigmented system; therefore theuser normally wants toobtain optimum performance with the smallest possible amount ofpigmen!. [deally, a good pigment dispersion consists chiefly of primary particles, with only

    ( (534 VERNARDAKISa minimum ofloose aggregates and agglomerates. In practice, reduction to the primary par-t icle size is largely determined by the nature of the pigment (Le., i ts dispersib il ity), by thedispersion systern and processing equipment, and by the end-use requirements of the prod-uc!. .

    Dispersion should not be confused with pulverization. The lat ter iss imply a comminu-t ion process whereby large pigment lumps are broken down tosmaller units , which consti-tute the powder formoPulverization does not break down the aggregated, agglomerated, andflocculated particles into primary particles. Dispersion, however, accomplishes this effec-tive[y.3.1 Pigment Wetting[t isgenerally recognized that the dispersion process consists of three dis tinct s tages: wet-ting, deaggregation-deagglomeration, and stabilization. The wetting st.1ge involves the re-moval from the surface of the pigment particles of adsorbed molecules of gas, l iquid, andother mater ia ls and the ir replacement with molecules of the vehic le . [n o ther words , th epigment-air interface in dry pigment powders or the pigment-water interface inpresscakes is replaced by the pigment-vehicle interface. This isaccomplished through pref-erential adsorption. The efficiency ofwetting depends primarily on the

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