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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.

    - - --'---529

    ( (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

    ."

    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 comparative surfacetension propert ies of the pigment and the vehicle, as well as the viscosity of the resultantmix.3.2 Particle Deaggregation and Deagglomer ationAfter the initial wetting stage, it isnecessary to deaggregate and deagglomerate the pigmentparticles. This isusually accomplished by mechanical action with devices such as ball milis,b ead mil is, and two-rol l mil is. As the p igment powder is b roken down to the ind iv idualparticles , h igher surface areas becorne exposed to the vehicle and larger amounts of i t arerequired to wet out newly formed surfaces. During this s tage of deaggregation, the amountoffree vehicle in the bulk diminishes; therefore, the viscosity ofthe dispersion increases. Athigher viscosities, shear forces are greater and the breaking down and separation ofparticlesbecome more efficien!. It is this process of mechanical breakdown of the aggregates andagglomerates that demands a high energy input and can become quite costly. Some easilydispersible pigments have been developed lately to aid in the reduction of energy require-mel,1 ts .Such pigments are produced by surface treatment of the pigment during manufac-ture, with the purpose of reducing or inhibit ing agglomeration-aggregation formation. Inmany cases , such treatments are highly specific to a single ink, paint , coating, or plast icmedium. .3.3 Dispersion StabilizationThe third stage of great importance inthe dispersion process is the stabilization of the pig-ment dispersion. This ensures that complete wetting and separation of the partic1es has beenreached, and also that the pigment particles are homogeneously distributed in the medium.I f the di spersion has not been s tabi li zed, f loccu la tion may occu r as a resu lt o f c lumpingtogether of the pigment particles. Flocculation is generally a reversible process. Floccula-t es typica lly b reak down when shear i s appl ied and wil l form aga in when the shear i s re-moved. Where a pigment dispersion is not s tabilized by the action ofresin molecules inthevehicle, the use of surfactants or polymeric dispersants can be considered. Such addit ives

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    PIGMENT DISPERSION 535 536 VERNARDAKISmay be uSt :d dir t:ct ly dur ing pigment manufacture, or they may be incorporated in the vehi-cJe.

    4.0 THE ROLE OF SURFACE ENERGYJ t i swel l known that mplecu la r fo rces at t he surf ace of a liquid are i n as ta te o f imbalance.The same is true of the surface of a sol i d, where the molecules or ions on the surface aresub ject to unbalanced forces of at lr act ion normal to t he sur face p lane. Such atoms do nothave al l t hei r f or ces sati sf ied by uni on with o ther a toms. As a result , t here is a net f or ce,wh ich tends topul l t he surf ace molecu les i nto the bul k. The opposing force , wh ich resis tsthisinwardlypullingforce,isknownasthesurfaceenergy or su rface tension. Tosat isfy these sur face forct :s, l iquids and sol ids tend to att ract and retain on their sur facesdissolved substances in the solut ion or gasses f rom the surrounding atmosphere. Theseforces are short- ranged at lr act ive forces , known asvan der Waals or London forces, andthey playa very important role in particJe aggregation, wetling, and dispersion stabi lization.

    Figure 5 Partial wetting of a salid surface by a liquid inaeeordance with the Young-Duprequation.

    4.1 Surface Energy and Surface Area(high surface tension), such aswater, wil l not entirely wet out a high energy surface. In this'case,thewetling energy is equal toor lessthan thesumoftheadsorption and interparticleatlract ion energies, and wetling may be either partial or nonexistent . The liquid will notspread entirely over the surface,asshown in Figure 5.Therelationshipthatdescribessuchasys tem is given by the Young-Dupr equation as follows:Pigments having a very small particJe size exhibit high surface arca and consequently highsurfact: energies. A~ large pigment particles are broken down into several smaller particles,

    new surfaces are constantly ereated, contributing to a higher surface arca and thus a highersurface energy.Let us assurne that a pigment powder has a surface area S o f 60 m2/g and a density r>of

    1 .0 g /cm3 . I ts b as ic particle diameter D fromYsv= YSL+Yl.v. cosO

    D=~pS

    where ysv, YSL,andYLVare theinterfacial energiesat thesolid-vapor, solid-liquid, andliq-uid-vaporinterfaces,respectively,ande is thecontactangle.Forcompletewetting, thecontact angle iszero (cose becomes unity), and the l iquid spreads entirely over the solidsur face . For e > O,wet ling e ither i s incomple te or does not occur a t a l! .

    4.2 Surface Energy and Pigment WettingSurface energies p'layan important role in thewetling ami stabilization of pigment disper-sions.Por wetting lo be efTeclive, thewetting energiesnf the pigment-vehicJe interfacernust begrealerthanthe surnnI'the adsorption energy (this is because01'substanceson thepigment surface)andtheattractive energy that holdsthe pigment partides together. Gener-ally, lower energy (Iow surfact: tension) liquids, such as aliphatic and aromatic hydrocar-bons. willspreadover.or wet. higherenergysurfaces.Quite nften, it happens that a liquiddoes not spreadover a pigrnentsurfaeecompletely. This occurs when a highenergyliquid

    4.3 Surface Energy and Destabilization of the DispersionSurface energies play an important role in the destabilization of the dispersion. ParticJesdispersed in liquid media are in constant motion (thermal or Brownian movement). As theymove through the medium, theycollidewithother pigmentparticJes.The frequencyof thesecollisions depends on the size of the particJes and on the viscosity of the medium. Duringsuch collisions, the particJeswill attract and mayjoin with other particles becauseof thepowerful short-range London-van der Waals attract ive forces, which in effect are surfaceenergies . These forces are e lect ri ca l and are due to the interac tion of the dipoles tha t a represent in the partic1es, as permanent dipoles (polar particles) or induced dipoles (nonpolarbut polarized particJes).Once the part ic les have come together, t hey may reaggregate or f orm fl occula tes if

    their sur face is not pro tected , and they wi ll sett le to the bott om of t he conta iner. This i s anundesirable effect for the ink, paint , or coatings manufacturero Therefore, to prevent reag-gregation or f locculat ion, such dispersions must be stabi li zed.

    wil l be 0.1 .im. I f these par ticles are cubic in structure, and i f, for the sake of simpl ic ity, weassume that a 1cm3 of p igment is broken down in to parti cles 0 .1 J .!min s ize, then 1 x 1015par ti cl es wi ll be pr oduced. We assume also that the par ticl es ar e i n perf ect cubic packing. Toget an idea of the area created by the new surface, we need only compare the surface area of6 cm2 for the 1 c m cubic particle to the surface area of 600,000 cm2 (60 m2) for the 1 x 1015cubic partic1es lhat are 0.1 11min size. The increase in surface area is 100,000-fold. The newsurfaces produced are tremendously large. The surface energies associated with these newsur faces are also qui te large. These van der Waals sur face energies create the att ract ionbetweenthesubmicrometer particlesthat come together to form the aggregatesand agglom-erateso

    4.4 Surface Energy and the Acid-Base ConceptThe idea of surface energy in pigments hasbeenclosely related to the acid-base concept,advanced by Sorensen,13 who has used it to descri be the in te rac tion between p igments,hinders, and s ol vents, to oblain optil1lall y stablc pigmcnt dispersi ons having the best appli-

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    (PIGMENT DISPERSION 537~ation propert ies in flu id ink systems. Such interrelation between the surface energies 01'these three components ina dispersion can be characterized by their acid-base properties.P igments can be c lass if ied as acidic (elec tron acceptors) , b asic (elec tron dono rs ), am-photeric (electron acceptors ami donors), or neutral. Binders and solvents can be similarlycharacterized.Acidic pigments should be used with basic resins (polyamide, melamine, alkyd), while

    basic pigments should be used with acidic resins (vinyl, acrylic, maleic). Amphoterics canbe used with both resins. Neutral pigments should besurface treated to improve their disper-s ion characteris tics . The solvent must have the same acid-base character as the pigment,whereby the interactinn between the solvent and the pigment surface isminimized and at thesame time the interaction between the resin binder and the pigment surface is maximized. Innther wnrds, there shnuld he no cnmpeti tion between solvent and binder for the pigmentsurface; to obtain maximum dispersion amI stlhi li ty , nnly the binder should adsnrb.

    5.0 MECHANISMS FOR THE STABILlZATION OF DISPERSION5.1 Charge Stabi lizationDispersions may become slable through two generally accepted mcchanisms: charge stabi-lization and steric or entropic stabilizalion. Charge stabilization is due to electrical repul-s ion forces, which are the result ofa charged electrical double layer surrounding the parti-cles asshown inFigure 6.The charged electrical double layer developed around the parti-cles extends well into the liquid medium, and since all the particles are surrounded by thesame charge (positive or negative), they repel each other when they come into close proxim-ity.5.2 St eric or Ent ropic StabilzationSteric stabilization is due to steric hindrance resuiting from the adsorbed dispersing agent,the chains 01'which become solvated in the liquid medium, thus creating an effective stericbarrier that prevents the other particles from approaching too close. This phenomenon isalso called entropic stabilization because, as the coated particles approach each other, thesolvated chains 01'the adsorbed dispersant lose some 01'their degrees 01'freedom, resuitingina decrease in entropy. Such lowering inentropy gives rise to repulsive forces, which keepthe particles away from each other. This type 01'steric or entropic stabilization isalso repre-sented in Figure 6.6.0 SURFACE TREATMENT6.1 SurfactantsSurface active agents or, simply,surfactantsare substances that are used to lower the inter-facial tension between a liquid and a solid. Such isthe case for pigments in fluid media, withlhe expressed purpose 01'improving pigment dispersihility by improving pigment wettingcharacleristics, (ueventing reaggregation, and increasing lhe stahilily 01'the dispcrsion. Asllrfactant molecule typically contains two grollps 01'opposite polarity and solubility. Thehydrophilic group isthe polar, water-Ioving part, while the lipophilic group is the nonpolar,oil-Inving part nI' the molecule.Surfactanls are characterized by lheir IILB value (hydrophile-lipophile balance), which

    isa rat io 01' the hydrophil ic to l ipophilic groups on the molecule and gives an indication 01'

    ( (538 VERNARDAKIS

    , ------// + "1 "

    I + + \/1 + + \/'+ +'I \I I\ + + + I\ I\ + I\ + 1" + /" + ,,"'-- - - - + -.,/./---

    E LECTRICALDOUBLELAYER

    CHARGE STABILIZATION

    STERIC OR ENTROPIC STABILIZATIONFigure 6 Charge and steric or entropic stabilizations.

    their solubil ity inwater oroil-solvent systems. High HLB values mean that the surfactant issoluble inwater (an abundance of hydrophil ic groups).Low HLS values, on the other hand,mean that the surfactant is soluble in oi! or solvents (an abundance of l ipophilic groups).Surfactants attach themselves to the pigment partic1es via preferential adsorption, as

    shown inFigure 7 for aqueous and nonaqueous systems. Inaqueous systems, the lipophilic(or hydrophobic) g roups are adsorbed on the par ti cl e su rface, and the hydrophi li c (orlipophobic) groups extend into the bulk 01' the vehicle. In the case of non-aqueous solventsystems, the hydrophilic grollpsol' the surfactant are attached to the particle surface and thelipophilic groups (lails) extend into and are solubil ized by the solven!.Surface treatments are effective for pigments because their surfaces contain polar or po-larized functional groups, which can serve as adsorption sites for the hydrophil ic or

    lipophilic groups ofthe surfactants. Por instance, organic pigments typically contain groupsI Isuch as nilro (-N02), hydroxyl (--01-1), carbonyl (-C =0), amide(-NH - C =0),methoxy (--O-CH3), chlorine (-Cl), hromine (-Sr ), sulf onate (-SOj), carboxylate(-COO-), ami metal ions such as Ba+2,Ca+2,Mn+2,and Cu+2,which can function as theanchoring sites for the hydrophilic or lipophilic groups 01'the surfactants.

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    PIGMENT DISPERSION 539 540 VERNARDAKISpoly(12-hydroxystearic acid)] with the carboxy group functioning as the anchor and thepolyester group as the solvated chain. Other s with multiple anchor groups, are fattypolyureas and polyurethanes, which may even contain polymeric solvatable groups insteadof the long fatty chains. .

    6.3 Surface Modifying Agents

    "-LIPOPHILlC GROUPS

    Surface modifying agents are another group of addit ives that can be used loaid the disper-sion of organic pigments inorganic media. These agents are often pigment derivatives (e.g.,large tlat dye molecules), which provide improved resis tance to tlocculat ion and greaters tabili ty to the dispersion. The pigment derivative isadsorbed onto the pigment surface vialhe van der Waals attracl ive forces, which acl over a large area, because such large planardye molecules l if t lal on lhe pigmenl surface. They may be used either alone or inconjunc-lion wilh a polymeric dispersan!. When used alone, lhey inlroduce or increase on the surfaceof nonpolar or low polarily pigmenls, lhe number of polar s ites , which are necessary to in-leract with lhe resin inthe vehic1es, toslabil ize lhe dispersion. When used logether with thepolymeric dispersanl, they provide anchoring siles on which the anchor groups of the dis-persanl will become attached. In this contexl, they can be used synergistically with dispers-ing agents, al which lime lhey are called colored synergists.

    AOUEOUS SYSTEM

    7.0 SURFACE TREATMENT DURING PIGMENTMANUFACTURENON-AOUEOUS SYSTEM Generally , surface-treated pigmenls are more easily dispersible, produce more stable dis-pers ion s in t lu id med ia wilh improved t low, and impar! h igher s lreng lh and g lo ss lo the

    printed films, when compared wilh untreated pigments. Surface treatments can be carriedoul at different s tages of pigmenl manufacture. Some pigments are prepared direclly as fin-ished producls, while others are inthe form ofa crude pigment that must be conditioned intothe pigmentary sta le.

    Use ofsurfactants is typically made at the initial s tage of pigment manufacture. Duringthe precipitation of the intermediate (e.g., diazo in the preparation of azo pigments), surfac-tants are used towet out and control the fineness of the precipita te, and they may also act aspromoters to accelerate the azo coupling teaction. At the second stage, during the precipita-tion ofthe pigment (e.g., in the azo coupling reaction), surfactants may be used in the disper-s ion ofthe pigment particles as they are being formed-for example, inazo yellows, whichare precipitated inthe pigmentary state, or inthe dispersion of the precursor (dyestuff), as inthe case of metallized azo reds (which are first formed as sodium salts), tocontrollhe saltformation (barium, calcium, etc.), and thus produce the final pigmento At the third stage,during the conditioning of the pigment, surface treatments are usedterpigment particle dis-persion, for coa ting the pigmenl sur face to prevent agg rega tion, and for cont rol ling thegrowth of crystal parlic1es. If partic1es are too difficult to fil ler, use of a specific addit ive(tlocculant) somelimes induces controlled flocculation and facilitates filtration. Complexformation with additives may also be carried out during this conditioning stage to stabilizethe partic1es and increase dispersibility, as is the case with diarylide yellows, which maybesurface treated with fat ty amines to produce Schiff base stabilization and result in easilydispersible pigmenls.

    Figure 7 Surfac!:lnl a ltachmenl 011pigmenl particles in aqueous and nonaqueous d ispers ions .

    It is well known, however, that classical surfactants do not always improve the disper-s ion characteris tics of pigmenls,especial ly when pigment surfaces are Iow in polarity ornonpolar and are dispersed in nonpolar vehic1es. With such pigments and vehicles, disper-sants and surface modifying agents ofother types must be used to improve wetting and dis-persibility and to prevent flocculation of pigment particles.6.2 Polymeric DispersantsPolymeric dispersants or "hyperdispersants" 14are claimed to be more effective dispersionstabilizers for nonaqueous systems. These substances have a two-part s tructure, one con-sis ting of an anchoring functioning group (or groups), and the other consist ing of a poly-meric solvatahle chain lo which the functional grnup isat lached. They are in effect poly-meric surfactants or dispersanls but were developed for use inspecific nonaqueous systems,where classical surfaclanls have limitations. When they are used as dispersants for organicpigments, il is prcferable thal they have mulliple anchoring groups on one polymeric chain,because organic particles are not as strongly polar as inorganic particles. Such dispersantsmay be 01' a fally polyester type, containing a carboxy group at lhe end [e.g.,

    ( (

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    (PIGMENT DlSPERSION 5418;0 SURFACE TREATMENT OF PIGMENTS: APPLlCATION8.1 Organic PigmentsThe published literature dealing with surface treatments of organic pigments patented orotherwise, isso extensive that noattempt ismade toreview it, although itmay be referred to,occasionally. Readers are urged however, to consult the review by Hayes,15 which coversthe role of classical surfactanls, polymeric dispersants, and pigment derivatives in surfacetreatments. Othcr reviews of interest are those by Topham,16 Merkle and Schafer,17 andHampton and McMillan,14 the latter dealing specifically with polymeric dispersants. Fur-ther examples will be presented here.

    It iswell known lothe pigment manufacturer that rosination is perhaps the oldest surfacetreatment known, especially for azo pigments, where rosin (abietic acid) isprecipitated ontothe pigment surface as the barium or calcium salt.1t can also be used to treat other pigments,such as copper phthalocyanine blue, 18and for a host of s imilar applications, ina polymer-ized formo Along lhe same lines , long chain carboxylic acids (fatty acids) have also beenused lo treat pigment surfacesl9. A likely arrangement of Ihese molecules adsorbed on thesurface isshown in Figure 8.The hydrophilic anchor groups are attached tothe surface, withthe lipophilic groups projecting outward. The use ofrosin has been mentioned because ofitshis torical s ignificance and because it iss t il l widely used today, s ince it is one of the leastexpensive surface treating agents.

    In the course of a study by the author for the development of a diarylide yellow AAOT(Colour Index Pigment Yellow 14) for flexographic ink applications, i t was found that adesirable product was one prepared inthe presence of an amine-type ethoxylated guanidineweakly cationic surfactant, incombination with a polar tetramethyl decynediol solvent.20 Itappears that these two surface active agents worked synergist ical ly to produce a strong,transparent and nonflocculating pigment, as opposed to products in which the surfactant orthe solvent or both were absent from the preparation. Transmission electron micrographs

    BARIUM SALT OF ABIETlC ACID FATTY ACI D

    PIGMENT

    Figure 8 Trcatmcnt of pigmcnl surfaccs with rosins and ral ly acids.

    ( (542 VERNARDAKIS

    (/)W..J~ 161-el:

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    PIGMENT DISPERSION 543

    /

    p.2 !-1m,

    ~DIAMAVER = 0.073 pm

    0.15 .200.10PART IC LE DIAMETER (p m)

    Figure 10 Transmission electron photomicrograph and particle size distribution of a sur-face-t reat ed di aryli de yell ow AAOT, Pi gment Yellow 14.

    12, on the other hand, represents the liquid ink prepared with the surface-treated pigmentand shows tha t thi s i s a non-flocculating pigment.

    Polymeric dispersants2l such as poly(12-hydroxystearic acid) are reportedly used bothas free acid and asa salt with a variety oforganic toners; these agents show more effective-ness when reacted with a pr imary amine (3- -

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    (PIGMENT DlSPERSION 545 546 VERNARDAKIS

    POLY (12 - HYDROXYSTEARIC ACID) SULFONATED PHTHALlMIDOMETHYLCOPPER PHTHALOCYANINE

    CuPe-SO] H : COPPER PHTHALOCYANINESULFONIC ACID

    3 - OCTADECYLAM INOPROPYLAMINE

    Figure 14 Sur fac e l rea lmen l o fc oppe r phlhalocy anine b lu e, P igmen t B lu e 15. w ilh a s ul fo nal edcopper phtha locyanine addit ive, s ll rface modifying agent .

    COPPE R PHTHALOCYANINE BLUEpreced ing cases, the p lanar sulfonated quinacr idone molecules appear to l ie f la t on thequinacridone pigment surface and thus improve considerably the dispersion propert ies ofthe pigment, especially when used incoating applications. Figure 15represents the arrange-ment of sulfonated quinacridone derivative on the pigment surface.Pigment derivatives of azo reds, 28oranges, and yellows 29have also been used for sur-

    face treating the corresponding pigments. With azo yellows, treatments can be carried out insitu with fat ty amines to produce easily dispersible products through a Schiffbase reactionIbetween the - e = o (carbonyl)groupsofthe pigmentand the-NH2 groupsofprimaryIamine s, t o form - e = N- Schiffbases.29-32Derivativesof monoarylideand diarylideyellow pigments can also beprepared by reacting the pigment with a primary diamine and aglycidyl ether33 toproduce a Schiffbase. The structure ofoneof these derivatives isshowninFigure 16for Pigment Yellow 12, AAA yellow. Again , the planar pigment molecule ap-pears to l ie on the pigment surface, and the long chains project outward into the vehicle 10produce stabilization of the dispersion.

    Figure 13 Sur fac e t re atment of c oppe r phlhaloc yan ine b lue , Pigmen t B lu e 15, s howing Ihe syn-ergislic effcct belween slllfonalcd copper phlhalocyanine and a polymeric dispersanl on the pigmentsurface.

    produce a synergistic effect on the pigment surface for improved dispersion. An example iscopper phthalocyanine sulfonic acid22.The mechanism of synergism is illustrated in Figure13 for the surf ace tre atment of copper phlhalocyanine blue. A great number of otherphthalocyanine derivatives have also been prepared and used as pigment stabilizers forphthalocyanine blue23.Phthalocyanine pigments may be conditioned from the crude state to the pigmentary

    form, for example, by milling the "crude" with a phthalocyanine derivative24 such as a sul-fonated phthalim idomethyl phthalocyanine25 in the absence of any milling of grindingaid26.These large planar molecules appear to lie flat on lhe copper phthaloeyanine surface,as shown in Figure 14, and they impart s tabili ty to the dispersions when used in printinginks, painL~,ano coalings, wilhout any aoditional conditioning of the milied product.Pigmenl deriva tives are by nomeans limited to phthalocyanines. Quinacridone pigmentshave been surt:lce lrealcd with sulfonateo quinacrioone oerivatives27 either as the slllfonicacio fonn or as lhe metal s ll lfona lc sa lt . wilh a wioe range o f metal s poss ible. As in the

    8.2 Inorganic PigmentsTitanium dioxide, in the two naturallyoccurring crystal forms,anatase and rutile, is themost important white pigment, which provides maximum opacifying power. Normally,Ti02 pigment s are not useo i n thei r pur e form beca use of their poor oispersibility ina varietyof resins and solvents. Generally , they are surface coated with small amounts of alumina,silica, or bolh (up to 3%total, onTi02) to increase the functionali ty of the surface (activeaosorption sites for the resin molecules) ano lo improve dispersibility ano impart stability lothe oispersion, especially in alkyo resin paint systems. For alumina-i:oated titanium diox-

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    PIGMENT DISPERSION 547 548 VERNARDAKIS

    SULFONATEO QUINACRIOONE RED

    ide, 34the highly basic sites onthe alumina surface, which are much more basic than the siteson the Ti02 surface, cause specific adsorption of the acidic functional groups of the alkydresin molecules . The remaining parts ofthe resin molecules (long chains) extend away fromthe surface, creating a considerable amount of steric hindrance around each pigment parti-cle, thus resulting in steric stabilization of the dispersion.Alumina-coated titanium dioxide, iron oxide red, and other inorganic pigments and fil l-

    ers can be surface treated with alkanolamines (aminoalkanols), having the general formu-las:M = H,AI,Mg,Zn,Cu,Ni,Cd,Cr,Co,Mn R-CH-CI-I2-NH2.I01-1 R2-CH-CH-R3, etc.I - I01-1 NI-12

    II Q U I N A C R I O O N E RED

    where R, R2, and R3are alkyl groups containing from 1to 22carbon atoms in the chain . 35The dispersibil ity of these pigments is increased considerably when used in paint formula-t ions containing air drying resin vehicles. The stabili ty of the dispersion is s imilarly im-proved because of the steric stabilization imparted to the pigment particles by the R, R2,and R3 long chain alkyl groups.Organic isocyanate adducts 36are uscd as effective dispersing agents for several classes

    of inorganic pigments, including zinc oxide, iron oxides, Prussian Blue, cadmium sulfide,ultramarine, vermilion, and chrome pigments (zinc, barium, and calcium chromates). Theseagents improve the dispersion characleristics and the flocculation resistance of the above-listed pigments when incorporated into conventional alkyd paint vehicles with organic sol-vents , where these systems also contain a substantial amount of t itanium dioxide.Figure 15 Surfacc trcatmenl of quinacridone red, Pigment Violct 19, with a sulfonatcd

    quinacridone additive, surface modifying agent.9.0 THE CHARACTERlZATION AND ASSESSMENT OF DISPERSION

    Q-'I ,'-'

    The extent to which a pigment isdispersed inthe medium or the degree of dispersion isnormally assessed interms of color strength, gloss, brightness, and transparency, and it alsohasan effect on therheological propertiesof thesystem37-39.Since all these properties aregovemed by Ihe size and distribution of the pigment particles in the dispersion, one can,tOday, measure these propert ies using any of the latest particle size analyzers based on thelight scattering principIeof the dispersed particles12.With theseinstruments,a verydilutesuspension isrequired,and it isnecessary toknow therefractiveindexand viscosity of thesuspending medium. The average particle diameters and the particle size dis tributions ob-tained are those of individual particles, aggregates, agglomerates, and flocculates in the dis-persion. Theadvantages of these inslruments are that they arequiteeasy to operate, theygive results rapidly, and they allow the dispersion process tobe followed at different l imesand at different s tages.One such instrument is the Coulter model N4 Submicron Particle Analyzer. Figure 17represents the particle size results for a green-shade phthalocyanine blue, CJ. Pigment Blue15:3, in an aqueous dispersion. The distribution is quite narrow,and the mean particledi-ameter is0 .117 .1m.These results are very similar to those obtained from inspection of thetransmission clectron micrographs of Figure 1forthe same phthalocyanine blue pigment inthe dry powder fonn, showing that very li!tle aggregalion exists in the dispersion.Such particle size analyzers , based on light scattering, can be used very effectively 10

    study particle size changes that occur during the dispersion of pigments in flu id systems.Furthermore, t ime studies may be carried out on the nocculation of pigments by determin-

    H2N-R-NH2 : PRIMARY OIAMINE, /0\ROCHrCH-CH2: GLYCIOYL ETHER

    H-N N-HI CI CI [email protected]

    C=OI --, _o, IH-C-N=N ,': : 1 N=N-C-HI '..-' ' I(R'oCH2CHCH~)2N-R-N=C C=N-R-N(CH2CHCH20R')2I c: I I IOH CH3 CH:3 OHOIARYLlDE YELLOW AA A (PIGMENT YELLOW 12) OERIVATIVE

    Figure 16 Diarylidc ycllow AAA, Pigmenl Yellow 12, derivative; Schiffbase.

    ( ( (

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    (PIGMENT DISPERSION 549

    CUMULANT RESULTS SDP INTENSITY RESULTSSAMPLE ID: CuPe AOUEOUS DISP. SAMPLE ID: CuPe AOUEOUS DISP.MEANDIAMETER= 118 NM _95% LlMITS = 118 TO 118 NMSTANDARD = 37 NMDEVIATION

    MEANDIAMETER=117 NM -S.D. = 30 NM C.V.= 26%

    SIZE S. D. AMOUNT1: 117 NM 30 NM 100 %

    20

    F igu re 17 Par li cl e s iz c res ul ts of a copper phtha lo cy an in e b lu e, P igment B lu e 15:3, a qu eou sd ispers ion by the Coulter Model N4Submicron Parlicle Size Ana lyzer, used 10assess the degree ofdispersion (SDP =Size Dislribution Program).

    ing p ar ti c1 e s iz e immediat ely af te r di sp er sion and the n lat er , a ft er the di sp er sion s h av e be enal lowed t o s tand for cer tain per iods. This gi ves a measure of the st abi li ty of t he dispersion.

    10.0 CONCLUSIONThere is no question as to the desirabil ity and effectiveness of a fully dispersed and stabi-l ized pigmented system. Such a dispersion brings out the optimum color propert ies of thepigment in terms of color strength, gloss , transparency, and rheology. When a pigment iscompletely dispersed, it contains a larger number of primary partic1es; therefore, a smalleramount is required to produce the necessary coverage and color strength than would benecessary for a p igment tha t was no t as wel l di spersed and contained a larger number o faggregates, agglomerates, and flocculates.The trend today is toward production of more and more easily dispersihle pigments, as

    counterparts to the easily dispersible azo yellows, which are already used widely in certainprinting ink systems. Pigment manufactures are always improving pigment dispersibility,through the use of surfacc treatments, in terms of surfacL1nts , polymeric dispersants , andpigment derivatives . The end result is the achievement of complete dispersion easily and

    ( (550 VERNARDAKISquickly . And since this isan energy intensive process , in terms of the dispersion equipmentuti lized, lessenergy isrequired, which results ingreatcr economic benefits for the pigmentuser.

    REFERENCES1. G. D. Parfitt, Ed., Dispers ion o/ Powders inLUuids 2nd ed. New York: Wiley, 1 973.2. T. C. Patton, Painl F/ow and Pigmenl Dispersion, 2nd ed ., New York: Wiley , 1 979.3. V. T. Crowl, J. Gil C%ur Chem. Assoc. , 55, 388 (1972).4. O. Hafner, J. Gi/ C%ur Chell l. Assoc ., 57, 268 (1974).5. W. Carr, J. Gi / C%ur Chem. Assoc. , 61, 397 (1978).6. D. M. Varley and H. H. Bower, J. Gi/ C%ur Chell l, Assoc ., 62, 401 (1979).7. H. M. Smith, PolYIII. Pailll C%rl., 175,660 (1985).8. P. A. Lewis, Ed., Pigmenl Handbook, Vol . 1, 2nd ed. New Yor k: Wi ley, 1988.9. G. D. Parfitt and K. S. W. Sing, Eds., Characlerizalion o/ Powder Sur/aces. l..ondon: Aca-

    demic Press , 1976.10. H . P. Pr euss, Pigmenls in Painl, Park R idge, NJ: Noye s, 1 974 .11. W. M .Mor gans, Gutlineso/Painl Techn%gy, Vols. 1and 2 ,2nd ed. l. .ondon: Charles Griff in

    & ea. , 1982.12. T . G . Verna rda ki s, Am. lnk Maker , 62(2), 24 1984.13. P. Sorensen, J. Painl Techno/., 47, 31 (1975).14. J. S. Hamptom and J. F. MacM il lan, Am. lnk Maker , 63(1), 16 (1985).15. B. G. Hays, Am. lnk Maker , 62(6), 28 (1984).16. A Topham, Prog. Grg. Coalings, 5,237 (1977).17. K. Mer kle and H. Schafer , i nPigmenl Handbook, Vol. III ,T .e. Patton, Ed. , New York: Wiley

    1973 pp. 157-167.18. A . E. Ambler and R. W. Tomli nson, U.s. Pat ent 3,296,001 ( Jan. 3, 1967) ; ICl.1 9. T . C . Rees a nd R . J. F lores , U .S . P at ent 4 ,0 32 ,357 ( June 28 ,1977) ; Sherwin-Wi ll iams.20. T . G . Verna rda ki s, Dyes Pigments, 2, 175 (1981).21. J. F. St3nsf ield and A. Topham, U.S. Patent 3,996, 059 (Dec. 7, 1976); ICl .22. P. K. Davi es, L. R Roger s, J. F. St3nsCi el d, and A. Topham, U. S. Patent 4,057, 436 (Nov. 8,

    1977; ICl.2 3. Anon. , B ri ti sh Paten t 1 ,5 44 ,839 (Apr . 2 5, 1 979) ; BASF.24. W. H. McKelli n, H. T. Lacey, and V. A. Giambalvo, U. S. Patent 2,855,403 (G ct . 7,1958);

    American Cyanamid.25. V . A . G iambalvo and W. Ber ry , U .S . Pa te nt 3 ,589,924; Ame ri ca n Cyanamid( June 29.1971>-26. S. L . J ohns on , G .McLa re n and G .H . Rober ts on , U .S . P at ent4,448 ,607 (May 15 , 198 4 ); SunChemical.27. E. E. Jaf fe and W . J. Marshal l, U.S. Pat ent 3,386,843 (June 4,1968) ; DuPont.28. J. Mi tchell and A. Topham, U. S. Patent 3,446, 641 (May 27, 1969); ICl.2 9. J. Mit ch el l and A . Topham, B ri ti sh Paten t 1,139 ,294 ( Ja n. 8, 1 969) ; ICI .3 0. Anon. , B ri ti sh Patent 1 ,080,115 (Aug. 23, 1967 ): KVK.31. F. Dawson, J. Mi tchel l, L. R. Roger s, W . Todd, and A. Topham , Br it ish Pat ent 1, 096,362

    (Dec. 29 , 1967 ); ICI .3 2. G . H . Rober ts on, U .S . P at ent 4 ,2 20 , 473 (Sep . 2 , 1980 ); Sun Chemica l.3 3. R . J . S chwar tz and T . Sulzb erg, U .S . P at en t 4,468,255 (Aug. 28, 1 984) ; Sun Chemical .34. M . J . B. Franklin, K. Gol dsbrough, G. D. ParCi lt , a nd J. Peacock, J. Painl Technol., 42,740

    (1970).3 5. H . L ind en , H . Rutze n, a nd B . Wegemund , U .S . P at en t 4 ,1 67 ,421 (Sep t. 11 , 1979 ); Henke l.3 6. F . Hauxwel l, J . F . SL1nsf ie ld , a nd A . Topham, U .S . P at en t 4,042 ,413 (Aug. 16 , 1977 ); ICl .37. W. Carr, J. Oil C%ur Chel/ !. Assoc ., 65, 373 (1982).3R. K. TsuL~ui and S. Ikcda , Prog. Org Coalillgs, 10,235 (19R2).39. R. Polke, Am. Ink Maker , 61(6), 15 (1983).

    A SDP DIFFERENTIAL INTENSITYM 15 AMOUNT(%>O SIZE (NM)UN 10 31.6 OT 46.4 OI 5 68.1 4N 100 53

    (%> O 147 4110 100 1000 215 2PARTICLE DIAMETER(NM> 316 O

    SDP DIFFERENTIAL INTENSITY464 O