Wetting and Dispersing Additives - 上海海沛化工科技有 · PDF file ·...

Wetting and Dispersing Additives

TEGO® Dispers

76TECHNICAL BACKGROUND / WETTING AND DISPERSING ADDITIVES


While wetting and dispersing additiveslower the viscosity of a paint formulation,that is certainly not their only effect. Thisclass of additives has a major influenceon a wide range of essential character-istics of the formulation:

Color strength: Color strength is a meas-ure of the ability of a pigment to absorbincident light and color a medium. It haspractical importance in coloring whitebase paints: the stronger the tintingagent the more cost-effective it is. Thecolor strength of such a preparationdepends mainly on the intrinsic ab-sorbance and average particle size ofthe pigment. The smaller the pigmentparticles, the greater is their effectivesurface area and therefore the highertheir absorbing power. Higher absorbingpower goes hand-in-hand with highercolor strength. Smaller pigment parti-cles must be stabilized using dispersingadditives, so that they remain finely dis-tributed and do not clump together intoaggregates.

Hiding power: The hiding power of acoating is its ability to cover a color orthe color difference with the substrate.How well the substrate is covered de-pends on the thickness of the coating,the color of the substrate and also on

the scattering power of the pigment andthe refractive indices of the pigmentand binders. In a coating with stronghiding power, the pigment particlesscatter the incident light so stronglythat hardly any of it reaches the sub-strate. Any remaining light reflectedfrom the substrate is then so stronglyscattered that it no longer reaches theeye. For a coating to scatter light opti-mally, an even, fine distribution of pig-ments stabilized by dispersing additivesis essential.

Flocculation: Flocculation is the re-agglomeration of previously dispersedparticles. In the dispersion process, pig-ments are broken up by inputting energyand forming new surfaces. However thisis an unstable state. Flocculation canoccur at any time during paint manu-facture, storage or application. Pig-ments also tend to flocculate after thepaint has been applied to a surface.Carbon blacks, with large surface areas,show this tendency and, if this happens,a substrate cannot be evenly covered inblack. Dispersing additives inhibit floc-culation of the pigment particles.

Gloss: Light reflected from a surfacecontains specular and diffuse compo-nents. High gloss occurs when the sur-

The function of wetting and dispersing additives

Anyone who has ever stirred up sandand water in a bucket knows how diffi-cult it is to form a stable, homogeneousmixture from an insoluble solid and aliquid. A similar process must be mas-tered in the manufacture of pigmentedpaints and coatings. Only pigments with

very small particle sizes between 0.05and 0.5 µm can impart optimum colorand protection. It is therefore necessaryto reduce the size of the pigments ascommercially supplied. In the dispersionprocess the insoluble pigment is brokendown, finely dispersed and stabilized in

the liquid paint formulation. It is almostimpossible to do this without using wet-ting and dispersing additives. These ad-ditives are extremely important for thecoatings industry because of their verywide range of functions.

Figure 1: Flocculation of carbon black in a solvent-borne coatings system

face is very smooth and the diffuse com-ponent of the reflection is very small.Pigment particles or flocculates whichprotrude from the surface disturb thespecular reflection. Flocculated pigmentscan also affect leveling. Poor leveling re-duces the gloss even further. In as far aswetting and dispersing additives inhibitflocculation, they improve the gloss of acoating.

Flooding and floating: These phenom-ena occur with mixed pigments whenthe densities and/or particle sizes of thepigments differ strongly. With its highdensity, titanium dioxide tends to con-centrate at the bottom of the dryingpaint film. Organic colored pigmentshave a noticeably lower density thantitanium dioxide and therefore concen-


trate in the upper part of the film. Thisvertical flooding, also called floating,causes the paint film to appear morecolored than intended. Flooding can bedemonstrated by rub-out tests (see page86).

Horizontal flooding occurs by separationof the colored pigments in the dryingpaint and stems from solvent flows.Larger particles are carried along by thesolvent stream and islands with dif-ferent pigmentation occur. These, so-called, Bénard cells are clearly visible(see Technical Beckground ”Slip andFlow Additives“, p. 61).

This unwelcome separation of pigmentparticles can be prevented if suitabledispersing additives are used to inducecontrolled flocculation.

Pigments, dyes andfillersTo understand the complex functions,and therefore the importance, of wet-ting and dispersing additives in a formu-lation, one must first examine the com-plexity of pigments, dyes and fillers.

Pigments and dyes are termed colorants,materials which are used to color othermaterials. In contrast to dyes, which aresoluble in the application medium, pig-ments consist of particles which are in-soluble.

Pigments, dyes and fillers can be naturalor synthetic. Traditionally, they are sep-arated into organic and inorganic. Or-ganic pigments and dyes are mainlycharacterized chemically by their aro-

matic groups1. Inorganic pigments areusually oxides, oxyhydroxides, sulfides,silicates, sulfates or carbonates2. Oftenformulations contain several coloranttypes. Fillers, like pigments, are insolublein the application medium. They aremainly used to raise the solids contentof the formulation. However they canaffect other properties such as the me-chanical strength of the paint film.

Pigments are used primarily to impartcolor. They often contribute to the me-chanical properties of the film and im-

prove its weathering and corrosion re-sistance and its processing properties.Particle sizes of pigments cover a widerange (10 to 1000 nm) but the bestrange is between 50 and 500 nm. Pig-ment particles of this size allow opti-mum values of color strength, purity,gloss, hiding power, lightfastness andweathering resistance to be achieved. Asthe color properties are directly relatedto the particle sizes, these must keptwithin tight limits to guarantee reliableproduct properties.

a) b) c) d)

Figure 2: a) carbon black, b)+c) iron oxide yellow, d) magnetic pigment pictures: a) Degussa, b)-d) Lanxess

Figure 3: Primary particles, aggregates and agglomerates: grouped according to DIN 53206


The geometric structures and particleshapes of pigments are numerous. Ironoxide occurs as needles, carbon black isparacrystalline, titanium dioxide is al-most spherical, barium sulfate is quadri-lateral or rhomboid and aluminumbronzes are flakes.

An industrial pigment always exhibits adistribution of particle sizes which af-fects the wetting behavior. A high pro-portion of smaller particles needs ahigher amount of additive. Under someconditions, very dense spherical packingincreases the dispersion time.

Other physical properties, such as elec-trical charge and magnetism, also havea major effect on the grinding of a pig-ment. If the surface of the pigment is

modified, the characteristics of the par-ticle change. Crystal lattices of inor-ganic pigments are sometimes stabilizedwith foreign ions. The surfaces of or-ganic pigments are already partiallytreated with surface-active wetting ad-ditives.

Just these few general properties of thepigment give some idea of the potentialcomplexity of relationships in a formu-lation. It is easy to imagine how thiscomplexity can grow when the many in-fluences of the different binders andtheir polarities are included. To controlthe associated phenomena, the usermust often combine several additives.The additive manufacturer is faced withthe challenge of developing multifunc-tional additives.

The dispersion processThe dispersion process can be separatedinto three individual steps which occurboth sequentially and consecutively:wetting, dispersion, stabilization.

WettingIn order that pigment particles can befinely dispersed in a liquid, the particlesmust be wetted by the liquid. Air incor-porated in the pigment powder must becompletely replaced and the pigmentparticle completely surrounded by liq-uid. The process of wetting a solid witha liquid can be approximately describedby the Young equation:

With spontaneous wetting or spreadingthe contact angle is zero so that the co-sine term is 1. In this case:

�s = �sl + �l · cos �

or�s – �sl = cos ��l

where

�s = free surface energy of thesolid

�sl = solid/liquid boundary surfaceenergy

�l = surface tension of the liquid

� = contact angle between solidand liquid

�s – �sl = �l


For the liquid to wet the solid, the sur-face tension of the former must be lessthan that of the latter. A liquid withlower surface tension wets pigmentsbetter than a liquid with higher surfacetension. An additive that promotes wet-ting must therefore primarily reduce thesurface tension. However the interfacialsurface tension also has an effect. If anadditive reduces the surface tension be-tween liquid and solid phases it alsopromotes wetting. The additive is ad-sorbed during wetting onto the surfaceof the pigment and the pigment parti-cles are encapsulated.

The interactions between pigment parti-cles are thereby reduced and the viscos-ity of the mill-base is lowered.

DispersionThe pigment particles are now wetted byan enclosing binder solution and theirsurfaces are covered with surface-activeadditive. The additive reduces the inter-actions between the pigment particlesand thus lowers the viscosity of the mill-base. This permits higher pigment load-ings which is of special importance forthe mechanical dispersion process. Mill-base formulations which have been op-timized in this manner are suitable for

all modern milling equipment, such asdissolvers, pearl mills and triple-rollmills.

In the dispersion process itself the pig-ment aggregates are destroyed and re-solved into primary particles and smallerpigment aggregates. Aggregates cannotgenerally be broken down because theprimary particles are so strongly boundsurface-to-surface. Account is taken ofthis in pigment manufacture and theproportion of aggregates carefully con-trolled. Their amount can affect thecolor tone of a pigment.

It is clear from the previous discussionthat optimal wetting occurs with a wet-ting angle of � = 0. The time necessaryfor dispersion cannot be estimated fromthe Young equation although this wouldbe interesting for industrial pigmentgrinding because it affects costs. Thewetting of a pigment can be treated asa porous powder bed through which aliquid flows. Physically this powder bedcan be envisaged as a bundle of capillar-ies. A relationship between the distanceflowed l(t) of a liquid front in a porouspowder bed as a function of time isshown by the Washburn equation:

The pigment-specific constant C de-scribes the orientation of the micro-capillaries within the pigment bed. Theliquid medium is characterized by itsviscosity and surface tension. Both thepigment surface and the liquid mediumaffect the contact angle via the interfa-cial surface tension. The equation showsthat generally the flow path of the liq-uid medium in a pigment powder islarger the smaller the contact angle andthis can be achieved by, for example, useof dispersing additives which reduce thesurface tension of the liquid.

where

l(t) = flow distance of the liquidfront

C = pigment-specific constantr = average pore radius of the

agglomerate�l = surface tension of the liquid

phase� = contact angle between

pigment and liquid� = dynamic viscosity of the

liquid phaset = time


The lowering of the surface tension isimportant for another reason. Every dis-persion process (breaking up of aggre-gates and agglomerates) requires inputof energy. The work required is calcu-lated from:

The equation shows that for an increasein area dA of the surface during disper-sion – by breaking up agglomerates – anenergy dW, proportional to the surfacetension �, is necessary. The smaller thesurface tension, the greater the changein surface area for a given amount ofdispersion energy. It can equally well besaid that for a certain change in surfacearea in the presence of a dispersion ad-ditive – that is with lowered surface ten-sion – less dispersion energy is needed.This is why wetting and dispersion addi-

tives are so important in dispersing pro-cesses. They reduce the dispersion timeby reduction of the contact angle, asindicated in the Washburn equation,reduce the necessary energy input andprevent re-agglomeration during dis-persion.

StabilizationAgglomerates (fig. 4a) are broken downinto primary particles and smaller ag-gregates by the dispersion process. Theformation of primary particles is accom-panied by an increase of interfacial area with the liquid medium (fig. 4b).The higher the interfacial tension thegreater is the driving force for the solidto reduce its interfacial area. This causesthe particles to re-agglomerate to formso-called flocculates (fig. 4c). A floccu-late is an agglomerate in suspension.Dispersing additives suppress the for-mation of flocculates.

To stabilize the fine pigment dispersion,the additive molecules must be firmlyadsorbed on the pigment surface. Thisrequires the presence of groups or seg-

ments in the additive molecule whichcan interact strongly with the pigmentsurface via ionic bonds, dipole interac-tions or hydrogen bridges. Depending onwhether the formulation is water orsolventborne, various mechanisms arepossible.

In waterborne coatings, electrostatic re-pulsion has traditionally been regardedas the most important stabilization fac-tor. Interactions within the formulationcan be described by the DLVO theory(named after Derjagin, Landau, Verweyand Overbeek). The interplay betweenthe forces of attraction and repulsionmust be considered in interpreting thestability of waterborne dispersions. Thepigment particles are surrounded by anelectrochemical double layer. Each pig-ment particle is virtually situated in a so-lution of oppositely charged ions whichare relatively firmly attached to the pig-ment. The strengths of the attracting andrepelling forces are a function of the dis-tance between particles. In a stronglypronounced double layer, repulsion pre-dominates and the dispersion is stable.

dW = � ·dAwhere

W = interfacial surface energy� = surface tension

A = interface area

Figure 4: Schematic diagram of (a) agglomerated, (b) dispersed, and (c) flocculated pigments


If the electrochemical double layer isdisrupted, by, for example, the additionof electrolyte, the attractive forces gainthe upper hand and the dispersion breaksdown. The electrostatic interactions canbe described quantitatively by the Zetapotential �, which is a measure of thepotential at the shear layer of a movingparticle in a dispersion. As � approacheszero, the tendency of the particles toagglomerate increases. Dispersing addi-tives enlarge the electrochemical doublelayer thus stabilizing the dispersion.

Solventborne paint systems cannot bestabilized electrostatically. Instead, thepreferred way of preventing particleflocculation is by steric stabilization us-ing polymeric additives with groupswhich have an affinity for pigments.These groups attach themselves to thepigment surface and ensure adsorptionof the additive. The polymer segmentsare responsible for compatibility in theorganic system and also stabilize thedispersion by protruding into the sol-vent. If they come too close the polymer

segments interpenetrate, their mobilityis restricted thus lowering the entropy.The pigment particles surrounded bypolymer migrate apart again to com-pensate for this entropy loss.

To enable wetting and dispersing addi-tives to meet the complex demandsmade on them, it makes sense to com-bine electrostatic and steric effects. Thisprinciple, sometimes called electrosteric

stabilization, is the modus operandi ofmodern wetting and dispersing addi-tives. Only such additives can fulfill thehigh demands made on stabilization anddurability.

Another way of preventing pigment par-ticles approaching each other and thusflocculating is ”controlled flocculation“.In this process, the structure of the ad-ditives causes mutual interaction. The

Figure 5: Electrostatic, steric, and electrosteric stabilization of dispersed particles

Figure 6: Simplifiedrepresentation

(not to scale) of con-trolled flocculationby cross-linking ofadditive molecules


individual additive molecules adsorb onthe pigment surface and build a three-dimensional network by interacting. Thisnetwork results in a change in the rheo-logical properties: in a state of rest theviscosity is very high so that the pig-ments can no longer easily settle. Thediffering mobility of the pigments,which is also partly responsible forfloating, is prevented by bonding thepigments to flocculates of the samecolor.3

The three-dimensional network can,however, disrupt the flow of the paintand reduce gloss. Controlled flocculationis therefore used primarily in solvent-borne primers and fillers. In waterbornesystems, this method is not efficient.Similar characteristics can be achievedwith associative thickeners.

Wetting and dispersing additives areamphiphilic compounds, i.e., they areboth hydrophilic and lipophilic. Theirmolecular structure enables them to en-able or promote dispersion of pigmentsand filling agents in solvents. They shouldalso stabilize the disperse state.

Wetting and dispersing additives can beclassified in various ways. In the litera-ture they are classified according tochemical structure or by dividing intoionic and non-ionic products. They mayalso be grouped according to area of ap-plication, i.e. waterborne and non-wa-terborne, or by pigment type, i.e. organicor inorganic. Molecular size is also apossible criterion whereby wetting addi-tives are defined as low molecular anddispersing additives as high molecular.Because of the complexity of am-phiphilic substances, which moreoverexhibit unique functions, it is not easyto develop a simple model. The transi-tions are fluid because the desired

multi-functionality of combinations ne-cessitates various wetting agent com-ponents.

One of the factors determining the effi-cacy of a wetting and dispersing addi-tive is the number of its “anchor groups”.With only one single anchor group perdispersing additive molecule, displace-ment by a solvent molecule leads imme-diately to complete detachment of theadditive molecule from the surface. Thiswould result in flocculation of the pig-ment. Additives with several functionalgroups cannot be so easily detached andare particularly high performance (fig. 7).

In organic pigments, aromatic groupsare the main chemical structure givingrise to the additive properties. That iswhy, in the case of organic pigments,dispersing additives which contain, forexample, phenyl or naphthyl units areparticularly suitable for lasting stabi-lization of the dispersion.

Figure 7: Schematic of the structure of surfactant compounds with various types of head groups

Structure of wetting and dispersing additives


Inorganic pigments can also be dividedinto chemical classes. These are largelyoxides, oxyhydroxides, sulfides, silicates,sulfates or carbonates.3 These pigmentsare distinguished by polar molecularstructures so that their interactionswith acid groups, such as carboxy, phos-phate or sulfate groups, are particularlypronounced.

Carbon blacks are a special case. Thesurface area of such pigments is manytimes greater than that of organic oreven inorganic pigments. On the onehand, this necessitates significantlymore dispersing additive to effectivelycoat a surface; on the other carbonblack has neither a classic organic aro-matic nor an inorganic crystalline struc-ture. Experience has shown that nitro-gen-containing dispersing additives arethe most effective.

The structure of oligomeric and poly-meric dispersing additives is particularlysuitable for steric stabilization. Blockand graft polymers are more suitablethan homo- or copolymers with a ran-dom statistical distribution. Groups withan affinity for pigments are incorpo-rated in the polymer additive to suit thetype of pigment and application (fig. 8).

Modern high-performance polymericwetting and dispersing additives whichare suitable for all types of pigment,possess multiple examples of all theabove mentioned adhesive groups, e.g.TEGO® Dispers 755 W.

Figure 8: Copolymers for steric stabilization


Particle sizeThe prime criterion for the quality ofdispersion is particle size distribution.The end point of the dispersion processcan be recognized easily by determiningthe particle size or maximum particlesize. The simplest method of measuringthe maximum particle size of inorganicpigments is the Grindometer drawdown. A sample of the mill-base ispainted on a grindometer. Large parti-cles are displaced by a doctor blade andgenerate stripes in the draw down. Thesize of the largest particles can be readdirectly off a scale.

With a little practice, the maximumparticle size of the mill-base can bequickly and simply determined using aGrindometer. The particle size distribu-tion cannot, however, be measured inthis way. The Grindometer is of no usewith binder-free dispersions which dryvery rapidly and with particle sizes lessthan 5 µm.

Very small particles and particle size dis-tributions can be measured by moresophisticated methods such as laser dif-fraction or ultrasound which are, how-ever, generally unsuitable for routinelaboratory practice because of their highoperating expense.

It is possible to determine very reliablyby indirect means whether the desiredparticle size has been achieved. For ex-ample, the color intensity of organic

pigments is dependent on particle size.The endpoint can be determined bymeasuring the development of colorintensity at intervals during dispersing.

Color intensityTo determine the color intensity, a sam-ple of the mill-base is let down in asuitable formulation. The result is as-sessed optically or using a spectropho-tometer and compared with that of astandard grind. The amount of mill-baseis adjusted until the optical impressiongiven by both samples is the same. Therelative color intensity of the sample in% of the standard can be calculatedtaking into account the various amountsused. This method is very time-consum-ing but gives meaningful comparativedata and is used mainly by pigmentmanufacturers.

Absolute values can be obtained by amethod based on the Kubelka-Muncktheory which relates remission to trans-mission. Summing the remission valuesover the whole wavelength range givesa value of the color intensity.

In practice, this method suffers from asystematic error since it is based on theassumption of an infinite film thicknessand a constant degree of reflection. It isunsuitable for pigment development.

Evaluation of wetting and dispersing additives

Color intensity according to Kubelka-Munck:

Rub-outA rub-out test is suitable for checkingstabilization of the pigment particles. Itcan be used to assess the compatibilityof pigment concentrates, the tendencyof pigment particles to flocculate andflooding phenomena. An area of themoist but partly dry paint film is rubbedwith a finger or brush. If the pigmentshave demixed or are strongly floccu-lated, the mechanical process of rubbingre-establishes homogeneous distribu-tion. Viscosity in the dry film has alreadyincreased strongly. This stabilizes thepigment particle distribution which isnow homogeneous again. The extent ofpigment separation or flocculation isrecognizable from the color differencefrom that of the unrubbed film. Thecolor difference is usually quoted as theseparation of the chromaticity �E. �E isdimensionless. For �E less than 0.5, nocolor difference is visible, between 0.5and 1.0 the color difference is onlyslightly visible. �E values greater than 1 are not acceptable.


ViscosityThe viscosity of a mill-base must beadapted to suit the dispersing unit. If theviscosity of the mill-base is excessive,the unit may be damaged. If it is too low,input of shear forces will be inadequateto break down the pigment agglomer-ates. Viscosity is also an important indi-cator of the stability of a pigment con-centrate. If it changes during storage,the pigments are usually inadequatelystabilized.

The dynamic viscosity of a mill-base canbe quickly and easily determined usinga Brookfield viscometer. This methodcan only be used, however, for qualitycontrol. Mill-bases exhibit pseudoplas-tic flow characteristics. Their viscositychange is dependent of the shear en-ergy. A complete flow curve is requiredto obtain an exact picture of the flowcharacteristics of a mill-base. This is ob-tained by measuring the viscosity of themill-base at different shear rates using arotational viscometer. Such flow curves

provide information on the flow behav-ior of the material from manufacture,through transport to application. Inter-actions in the dispersion can also bequickly detected.

Detailed information on the theory ofviscosity and rheology is given in “Tech-nical Background: Rheological Addi-tives”(p. 98).

What to do if problems still occurDespite innovative, high performancewetting and dispersing additives, disper-sion of pigments is no easy task. Unex-pected problems occur frequently espe-cially with waterborne formulations.

We can provide advice and back-up toenable you to exploit the performanceof our additives to the full.

where

FS: Color strengthK: Coefficient of absorptionS: Coefficient of scatteringR: Reflection at infinite film

thickness (hence no change indegree of reflection)

Literature:[1] DIN 55987: 1981-02

[2] W. Herbst, K. Hunger, Industrial Organic Pigments, VCH Verlagsgesellschaft mbH, Weinhein, Germany, 1993, p. 4ff

[3] G. Buxbaum, Industrial Inorganic Pigments,VCH Verlagsgesellschaft mbH, Weinhein, Germany, 1993, p. 9


FAQ:Why isn’t electrostatic stabilization pos-sible in solventborne formulations?A prerequisite for electrostatic stabiliza-tion is dissociation of the additive mol-ecules into adsorbed anionic and freemobile cationic components. Even inpolar solvents, this dissociation is stillonly very limited. A double layer cannottherefore form around the pigmentparticles and electrostatic stabilizationis not possible.

What are the advantages of combiningwetting and dispersing additives withwetting agents?Frequently, the terms wetting agent anddispersing agent are erroneously usedsynonymously. As the name implies,wetting agents are excellently suited forwetting surfaces. In the case of pigmentdispersion, they are able to wet the pig-ment surface very well but are also ableto stabilize the finely-dispersed parti-cles. A combination of wetting agentand wetting and dispersing additive canprovide the advantages of both classesof additive. The wetting agent is rapidand mobile and promotes penetration ofthe pigment powder by the liquid thusshortening the wetting and dispersingtime. It smoothes the way for the poly-meric wetting and dispersing additivewhich ensures stabilization of the pig-ment particles.

Why do inorganic pigments achieve ahigher hiding power than organic pig-ments?Hiding power is an expression of theability of the coating to prevent lightreaching the substrate surface. Thesmall amount which still reaches thesurface is absorbed on the return path.Hiding power is largely determined bythe degree of light scattering. Whenpassing through the coating, light is de-flected so often that it does not reachthe substrate surface.

The ability of a substance to refract lightis described by the refractive index. Thegreater the difference between the re-fractive index of the substance and thatof the surrounding medium, the morestrongly is the incident light refracted orscattered. The refractive index of inor-ganic pigments is very high. Organicpigments are known for their ability toabsorb light, quantified by a very highspecific coefficient of absorption. Thewavelengths which are not absorbed arescarcely refracted at all. Coatings withorganic pigments thus do not hide aswell as those containing inorganic pig-ments.

Can wetting and dispersing additives beadded subsequently?Wetting and dispersing additives reducethe interfacial surface tension betweenpigment and water thus improving wet-ting of the pigment powder. During dis-persion, the additive is then anchored tothe pigment surface. Such anchoring isnecessary to enable the additive to sta-bilize the fine pigment particles against

flocculation. It is definitely recom-mended that the wetting and dispersingadditive be added before dispersion sothat its full effect can be developed.

If the viscosity is lowered sufficientlyduring dispersion, further additive canbe added. As long as the pigment sur-face is not fully coated, additive mole-cules can still anchor and carry out theirfunction. Once wetting and dispersingare complete, addition of additive isalmost useless.

Post addition of compatibilizers to thefinished grind can, however, increasethe compatibility of pigment concen-trates and the base paint. Even after dis-persion, such compatibilizers adsorb onthe pigment surface and improve pig-ment paste uptake and compatibility.

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