1

Controlling the absorption dynamic of water-based ink into porous pigmentedcoating structures to enhance print performance

Cathy J. Ridgway and Patrick A.C. Gane

Keywords: absorption, ink setting, digital printing, water-based inks, ink holdout, flexography, porous structures,coating structure for digital printing, digital paper coating,silica, calcium carbonate

SUMMARY: Previous work (Gane, P.A.C. and Ridgway, C.J.,Advances in Printing Science and Technology Volume 27,2001) has shown that the absorption of the fluid phase from a water-based flexographic ink is affected by the polymer content in the fluidphase, filtercake formation within the contacting ink layer and thedistribution of dissolved polymer between the filtercake and thecoating structure. The study showed that enhanced setting rate couldbe obtained by controlling the surface area to porosity ratio of theprinting surface. This paper now reports the effect on absorptiondynamic and ink component holdout of introducing two differentmaterials, each with a different form of porosity-creating structure, asconstituents of consolidated tablets. These are (i) a high surface areahighly porous precipitated silica, which under compression generatesa structure with fine pores, and (ii) a novel relatively high surfacearea deformable calcium carbonate structure consisting of porousspherical aggregates covered with small plate-like crystals whichconsequently create a high permeation tortuosity. Both the materialshad a median particle size of approximately 1 µm and were studiedboth in combination with standard dispersed ground calciumcarbonate as well as on their own. A porous network model, Pore-Cor, was used to exemplify the important structural features of theporous structure controlling the inertial and viscous fluid uptakedynamics which in combination act to make broad pore sizedistribution structures containing fine pores absorb faster than moremonosize distributions consisting predominantly of either fine orlarge pores. The centrifuged fluid phase of a flexographic ink wasused as representative of a complex polymer-containing water-baseddye ink which could be considered for standard flexography or asmight be encountered in digital ink jet printing. Residual pigment inthe centrifuged fluid phase was shown to be excluded from all thestructures tested. The dissolved dyes readily enter the silicainterstitial structures but not the novel carbonate structures,illustrating the effect of using high adsorptive surface area actingtogether with extended wetting front residence time (lowpermeability) to aid retention of dyestuff at the surface of a coating.This indicates that the novel carbonate can be used in paper coatingsto give an enhanced print colour density along with a reduction in inkdemand.

ADDRESS OF AUTHORS: Omya AG, CH-4665Oftringen, Switzerland.

Water-based digital and flexographic printing, with their useof environmentally friendly inks are increasing in importanceespecially for ‘point of sale packaging’ due to the requirementto create marketing appeal for cartons and boxes, andpersonalised products. The development of flexography overthe years has been described by Moir (1994). High standardsof print quality are nowadays expected and, because of this,coated grades of bleached board and unbleached Kraft linerare in increasing demand. For such printing using water-basedflexographic inks, rapid drying characteristics are essential,and can be achieved through evaporative loss aided byadditives which reduce surface energy and increase fluidphase partial pressure, and by providing strongly absorptivecoating layers, which are of particular importance whenconsidering multi-colour formats. The development of print-

on-demand ink jet technologies is more recent and placessimilar demands on print surface structure design for bothprinting and writing papers as well as board. For increasedresolution and print quality, coated grades often containspeciality pigments and formulations such as precipitatedsilica and polymer additives. Such absorptive coating layers can be produced using theblocky particle morphology of ground calcium carbonate,generally of relatively fine particle size, as a basis andadditional high surface area pigments are of benefit to enhancethe capture of dyes and to promote controlled absorption ofthe fluid phase of the ink. To achieve coverage, multi-layercoating is often employed in which the precoat layers can havelarger pore size and greater porosity than the subsequent finetopcoat. However, the rate of absorption is not controlledwholly by the absolute level of porosity: the distribution ofpore sizes in the coating structure network is equally importantespecially when considering the rate of absorption. Theimportance of preferred pathway absorption has beendescribed by Schoelkopf et al. (2000a). There is a need for thehigh capillarity developed in the finer pores in order to ensurerapid absorption during the short times in which the ink is incontact with the coating layer during high-speed printingprocesses to avoid blocking, i.e. marking of adjacent surfaces,feathering and bleeding. This high capillarity is required inparticular to overcome the additional drag which is created bythe partially viscosifying ink layer. Xiang and Bousfield(1998) proposed a model for the formation of an interfacefiltercake between the ink and the coating surface and itsretardational drag effect. For optimal absorption properties itis necessary to consider the force balance between thecapillary absorption into the network and the permeabilitycharacteristics of any retained pigments and polymers formingthe filtercake rather than simply strive for ever greaterporosity. Previous work (Gane, Ridgway 2001) has shown that theabsorption of the fluid phase from a flexographic ink isaffected by the following: (a) the polymer content in the fluidphase of the ink, (b) filtercake formation within the contactingink layer, which acts as a filter for the capture of dissolvedpolymer, (c) distribution of dissolved polymer between thefiltercake and the coating structure which controls the rate offluid absorption. There can also be a retardational effect due tosolved polymers coating the pore surfaces of the networkcoating structure. An enhanced setting rate for a flexographicink could be obtained by controlling the surface area toporosity ratio of the printing surface. It was thereforepredicted that an enhanced setting rate of a flexographic inkcould be obtained whilst maintaining some residual porositywithin the immobilised ink structure. This polymer adsorptioneffect is equally applicable to ink jet inks which contain bothsurfactants and dyes. The present paper studies the quantification of the effectsexperimentally and theoretically of introducing two differentforms of increased coating porosity that can be generated byusing (a) a high surface area porous precipitated silica1 with amedian pore diameter of 4 µm and an oil absorption volume of

1 GASIL Silica 35M, Crosfild Limited, Warrington, England, WA5 1AB,UK.

2

200 g/100 g, which generates a structure with a multitude offine pores, and (b) a novel high surface area compressiblecalcium carbonate structure used to introduce reducedpermeability whilst retaining high adsorption power. Experimental absorption rates are studied using a techniqueof fluid absorption into porous compressed pigment tablets(Schoelkopf et al. 2000b). This is effectively a wickingexperiment. A computer model, Pore-Cor2, is used to simulate thepermeation of the centrifuged fluid phase of a flexographic inkinto the 100 % silica porous structures by applying a wettingalgorithm for fluids undergoing both inertial and viscousdynamical absorption. Pore-Cor cannot model the potentialclogging effects of dissolved polymer. Its use in the analysisis therefore limited to highlighting the principal importantqualitative structural features of the structures. Understandingthe features responsible for the absorption dynamic assistsgreatly in the design of suitable coatings for fluid inkapplications.

The Pore-Cor model of porous structuresPore-Cor is a computer model that simulates the void-spacestructure of porous materials. It has been used to simulate thestructures of a wide range of porous materials includingsandstones (Matthews et al. 1996), medicinal tablets (Ridgwayet al. 1997) and soil (Peat et al. 2000). Pore-Cor uses a unitcell with 1 000 cubic pores in a three dimensional 10x10x10array, connected by up to 3 000 throats, (i.e. one connected toeach cube face), which in this work are assumed doubleconical (11), representing the realistic divergence andconvergence of geometries associated with spherical andblocky pigment morphologies (Toivakka, Nyfors 2001). Eachpore is equally spaced from its neighbouring pores by the‘pore row spacing’ Q, and each unit cell is a cube of sidelength 10 Q. There are periodic boundary conditions, whichare applied serially during wetting, with each cell connected toanother identical unit cell in each direction. When used tosimulate actual samples the pore- and throat-size distributionsof the unit cell are optimised so that the simulated non-wettingpercolation curve fits as closely as possible to theexperimental mercury intrusion curve (Matthews et al. 1995),corrected for mercury compression, penetrometer expansionand sample compression (Gane et al. 1996). The distributionsof pore and throat sizes are characterised by two parameters,'throat skew' and 'truncated pore skew'. The throat skew is thepercentage number of throats of the smallest size. Thedistribution of throat sizes is log-linear and pivots about itsmid-point at 1 %. The truncated pore skew increases the sizesof the pores by a constant multiple. However, the pores whosesizes then become larger than the experimentally determinedmaximum are truncated back to the size of the largest throat,thus giving a peak in the distribution at the maximum size.The positions of the pores and throats within the definedmatrix are random, determined by a pseudo-random numbergenerator. A matrix of values of throat skew and connectivity (theaverage number of throats per pore over the whole unit cell)are refined on a trial basis, and the void structure which haspercolation characteristics most closely matching theexperimental intrusion curve is selected (Matthews et al.1995). The percolation characteristics of the network areinsensitive to Q. Therefore, the Q value of the chosen structureis adjusted so that its porosity matches the experimental value 2 Pore-Cor is a software program of the Environmental and Fluids ModellingGroup, University of Plymouth, PL4 8AA U.K.

while ensuring that no pores overlap. If the experimentalporosity value cannot be simulated then the truncated poreskew parameter needs to be used, thus introducing larger poresand allowing greater porosity values to be modelled. It is notnormally possible to represent the full true complexity of thevoid network of a natural sample using the relatively simplegeometry of the Pore-Cor unit cell. One reason is that the sizeof the unit cell is often smaller than the representativeelementary volume (REV) of the sample. Therefore, differentunit cells must be generated using different seeds for thepseudo-random number generator. The algorithm is designedso that different structural parameters in conjunction with thesame seed of the pseudo-random number generator produce afamily of unit cells which are similar to each other; forexample, all may have a selected group of large pores in thesame region (Peat et al. 2000). Different stochasticgenerations use a different pseudo-random number generatorseed, and can either use the original Pore-Cor optimisationparameters or can be re-optimised to the experimental data. The wetting equations derived in a previous publication(Ridgway et al. 2001) were used to calculate the wetting fluxin each pore and throat in the void network. It was assumedthat inertial flow occurred when fluid began to enter eachthroat, initially wetting the throat in the form of a monolithicblock of fluid (Schoelkopf et al. 2000a). The inertia of thefluid caused a retardation of wetting of the larger features, atshort times after initial entry into a throat feature, relative tothat predicted by the Washburn equation. Once a double-conical throat was completely full, the volumetric flow rate atthe throat exit was calculated, and used to calculate the rate offill of the adjacent adjoining pore. A pore could be filled bythe fluid from more than one throat, which could start to flowinto it at different times. Once a pore was full, it started to fillthe throats connected to it that were not already full and whichwere not already filling from other pores. If at any stage theoutflow of a pore exceeded the inflow, then a restriction due toconservation of mass was applied, which removed anyimbalance and restricted the flow of fluid further into thenetwork.

Materials and methodsPigments - ground calcium carbonate, silica, novelcalcium carbonateThree pigments were used in these studies and a summary oftheir properties is given below in Table 1. Silica has the highest surface area and smallest particle sizedistribution followed by the novel calcium carbonate and thenthe HC603 which has the broadest particle size distribution andthe lowest surface area. The novel calcium carbonate is derived from standard groundcalcium carbonate and consists of a rose-like morphology offine spherical porous aggregates, the surface of which isformed from smaller platelets, as shown in Fig. 1. The use ofsuch a pigment as a filler for improved calenderability ofsupercalendered paper grades is discussed elsewhere (Blixt etal. 2001).

3

HC60 Novel calciumcarbonate

Silica

Surface area (BET) /m2g-1 6.5 62 262.4% by weight less than< 10 µm 99< 5 µm 92 99 100< 2 µm 60 82 91< 1 µm 40 35 57< 0.5 µm 23 18 24< 0.2 µm 12 14 6Median particle size (D50%) /µm 1.5 1.3 0.9Top cut (D98%) /µm 10 5 3

Table 1. Surface area and particle size distribution of pigments.

Fig. 1. Novel calcium carbonate used to make compressibletortuous tablets.

Making the tablets and blocksMany valuable investigations can be performed directly onpaper surfaces or on model coating layers. However, a papercoating has a thickness, depending on grade, of between 5-15µm, and a practical single ink layer is typically 0.25 µm (or intotal on a four colour press ~1 µm). It is to be expected thatusing even volumes as small as a micro litre directly onto sucha microscopic surface volume is distinctly limited bysaturation phenomena when it comes to understanding thecomplexities of coating imbibition. Because of theselimitations, macroscopic blocks of consolidated pigment wereformed over a range of compressions which allowed for theinvestigation in detail of capillary absorption and therelationship of initial pore filling to factors such as surfacechemistry and geometry of the pigmented pore structure as afunction of sample compression. For the preparation of these pigment tablets a 4.5 cm internaldiameter cylindrical hardened steel die attached to a baseplatewith a single acting upper piston is used which is suitable for awide range of pigment particle sizes, chemistries andmorphologies. The die is divisible into two parts to aidremoval of the compacted pigment sample and the walls of thedie are protected with a strip of plastic film to prevent stickingof the powder to the wall and to reduce edge friction, Fig. 2.

cylinder

base part

piston

cellulose membrane

strip of plastic film

pigment powder

cellulose membrane

Fig. 2. Schematic cross-section of the die showing the plasticlining and cellulose membrane (Gane et al. 2000).

A number of tablets were made using mixtures of a spraydried predispersed natural ground calcium carbonate derivedfrom limestone (HC60)3 with progressive additional amountsof silica and novel calcium carbonate. The weight ratios ofadditional high surface area pigment to HC60 were chosen tobe 0:100, 5:95, 10:90 and 20:80. Samples were also madeusing 100 % of the high surface area pigments. The pigmentswere mixed using a mechanical stirrer, and then used to maketablets over a range of pressures. The pigment was compactedin the hydraulic press for 5 minutes at a predeterminedpressure. The range of applied press forces Fp, the press areaAp, and the resulting pressures are listed in Table 2 . The tablets were cut and ground to form several 12 mm cubicblocks, using a rotary disc grinder and a specially constructed,precisely adjustable jig. To reduce artefacts caused by thewetting of their outer surfaces during the absorptionexperiments, samples were coated with a thin barrier line ofsilicone around the base of the vertical edges arising from thebasal plane. The remainder of the outer planes were not coatedto allow for the free movement of displaced air during liquidabsorption and to minimise any interaction between thesilicone and the absorbed liquid.

3 HC60 is used to denote Hydrocarb 60, a product name of Omya AG,Baslerstrasse, CH 4665 Oftringen, Switzerland

4

Cross-sectional area, Ap = 17.35 cm2 (±0.16)Applied force Fp Effective pressure P=Fp/Ap

/ kN (±5) / MPa (±2.7)

300 172.9400 230.6450 259.4

Table 2. Conversion table of applied forces to resulting pressures experienced in the tablet press.

Wetting apparatusThe rate of fluid uptake was measured using an automatedmicrobalance, namely a PC-linked Mettler Toledo AT460balance with a precision of 0.1 mg, capable of 2.7measurements per second. To provide a sufficiently slow andprecise approach of the sample down to the liquid surface, aspecial sample holder was constructed according to Gane et al.(2000), Fig. 3. The chamber around the balance base plateenabled a controlled atmosphere to be established, shieldedfrom external air and humidity. There was a gas inlet, whichcould be used to keep the cell under a steady stream of an inertgas such as nitrogen.

Liquid

Sample clamp movable in z-direction

Closed chamber

Possibility forNitrogen flow

Cuboidsample

Edge protection

Microbalance

Fig. 3. Gravimetric wetting apparatus (Schoelkopf et al. 2001)

All apparatus, gases and samples in this study weremaintained at 23.0 ±1.5°C. Prior to the absorptionexperiments, each porous block sample was placed in a 5 litrechamber. The chamber was then flushed through with drynitrogen, and the sample left to equilibrate for 48 hours. Theabsorption experiments themselves were performed in thebalance chamber described above, which was flushed with asteady stream of dry nitrogen flowing at 1 litre min -1. Theevaporation rates of the fluid phases from the supersourcereservoir were measured with a dummy non-wetting, non-absorptive sample block in place of the experimental sampleso the absorption data could be corrected for this apparentweight difference. Using this technique the micro-scale absorptioncharacteristics of structures could be scaled to observablyrealistic volumes under supersource conditions.

Mercury porosimetryA Micromeritics Autopore III mercury porosimeter was usedto measure the percolation characteristics of the samples. Themaximum applied pressure of mercury was 414 MPa (60 000psia), equivalent to a Laplace throat diameter of 0.004 µm.The equilibration time at each of the increasing appliedpressures of mercury was set to 60 seconds. The mercuryintrusion measurements were corrected using the software

Pore-Comp 4 for the compression of mercury, expansion of theglass sample chamber or 'penetrometer' and compressibility ofthe solid phase of the sample which adopts the followingequation from Gane et al. (1996):

−−Φ−−

++−=

ss

111

bulk

101

bulkblankobsint

)(exp1)1(

18201log)(175.0

MPP

V

PVVVV δ

[1]Vint is the volume of intrusion into the sample, Vobs theintruded mercury volume reading, δVblank the change in theblank run volume reading, V1

bulk the sample bulk volume atatmospheric pressure, P the applied pressure, Φ1 the porosityat atmospheric pressure, P1 the atmospheric pressure and Mss

the bulk modulus of the solid sample. The volume of mercuryintruded at the maximum pressure, once corrected for samplecompression effects, can be used to calculate the porosity ofthe sample.

Characterisation of the centrifuged fluid phase of aflexographic inkThe absorption studies made in this work compare thecomplexity of a flexographic ink with that of water. Otherwater-based ink types will be expected to show similarqualitative effects. To make the study universally applicable,the fluid phase of the ink was studied separately. The multi-phase system constituting a flexographic ink isseparable into a liquid phase and a solids-containing structuralphase. A sample of a typical flexographic ink5 was centrifugedin a Sigma laboratory centrifuge 2-156 at 35 000 min -1 for 45minutes. The top (supernatant) phase was syringed into aseparate vessel and centrifuged again. This process wasrepeated four times. The resultant separated continuous phasewas examined by a Mastersizer S7. It was shown to contain aresidual small quantity of pigment particles with a sizedistribution shown in Fig. 4. We see that the particles ofpigment are large relative to the fine pores expected in a paperor board coating and, as we see later, this results in exclusionof the pigment from the structures under study. The fluid alsocontained soluble red dye which was used as an adsorptiontracer during the subsequent absorption experiments.

4 Pore-Comp is a software program developed by the Environmental andFluid Interactions Group, University of Plymouth, PL4 8AA, U.K.5 Rot VSW 230, a product of Printcolor Gravoflex AG, CH-4800 Zofingen,Switzerland.6 Merck eurolab, Merck KGaA Darmstadt, Germany.7 Malvern Instruments Ltd., Malvern, UK.

5

Density/ kgm-3

Viscosity/ kgm-1s -1

Surfacetension / Nm-1

Contact angle forfluid on dispersedcalcite / o

Polymerconcentration/ %

1 035 0.0113 0.0338 16.65 26.27

Table 3. Properties of centrifuged fluid phase from flexographic ink.

particle diameter / µm

0.1 1.0 10.0 100.0 1000.00

10

50

40

30

20

10

0

100

90

80

70

60

volu

me

/ %

Fig. 4. Particle size distribution in centrifuged ink fluid phase.

The fluid phase obtained by centrifugation has also beenshown to contain film forming polymers (Gane, Ridgway2001). FTIR spectra which were run over a wave-numberrange from 400 to 4 000 cm-1 in transmission mode using asilver chloride carrier showed characteristic absorptions forsuch substances as styreneacrylate and resin. There were alsoabsorptions indicating alkyl ketone dimers together with somebands in the centrifuged fluid phase due to incompleteremoval of what was assumed to be organic pigment. Otherabsorptions were assignable to a range of compoundsincluding glycols, esters, alcohols, fatty acids and salts. The viscosity of the centrifuged fluid phase was shown to bea constant at constant shear stress independently of time. Anyrheological effect arising from dissolved polymer wastherefore assumed to be small at least at the initiation ofconcentration, i.e. at the starting solution concentration, and acharacteristic viscosity was therefore assumed. Further measurements were made of the density, surfacetension, contact angle and polymer content, the results ofwhich are shown in Table 3 and the methods used to obtainthe values are described in a previous publication (Gane,Ridgway 2001).

Results and discussionMercury intrusion100 % silica and 100 % novel calcium carbonatetabletsThe mercury intrusion curves, after the Pore-Comp correctionshad been performed, are shown in Fig. 5. The intrusion curves for the silica tablets showed a decreasein intrudable volume as the pressure used for making thetablets was increased. The porosity for the samplescompressed at 300 kN, 400 kN and 450 kN are 58 %, 53 %and 50 %, respectively, showing the tablets are becomingmore dense at higher pressures. The 50 % intrusion diameters,d50, are 0.008 µm, 0.007 µm and 0.007 µm for the 300 kN,400 kN and 450 kN compacted tablets, respectively, showingthere is little change in the actual median void size despite thereduction in porosity. The pore size distributions are fairly

monosize and the effect of increased pressure is to introducesome finer pores which play an important role in theabsorption dynamic together with effective closure of someparts of the structure. This is in contrast to previous studieslooking at a broad size distribution pharmaceutical gradecrystalline lactose and an anti-inflammatory compoundpowder compression where increased pressure on the tabletresulted in a reduced d50 value (Ridgway et al. 1997). Fig. 6shows the structure for a 100 % silica tablet compressed to300 kN. The narrow size distribution is clearly seen.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.001 0.01 0.1 1 10

diameter / µm

intr

uded

vol

ume

/ cm

3 g-1

300 kN compaction force

400 kN compaction force

450 kN compaction force

HC60 tablet - same porosity as novelc a l c i u mcarbonates t r u c t u r e s

silica

novelcalciumcarbonate

Fig. 5. Intrusion curves for 100 % silica and 100 % novelcalcium carbonate tablets

Fig. 6. Electron micrograph of silica taken from a tabletcompressed to 300 kN.

On the other hand, the intrusion curves for the novel calciumcarbonate tablets show the same volume of intrusion for allthree compaction forces, (the novel calcium carbonateintrusion curves overlie one another in Fig. 5). It is known thatthe structure is deformed during the calendering process ofpaper making where a pressure of up to 50 MPa is applied.Pressures far in excess of this value are applied when thetablets are made and therefore the outer layer of the sphericalaggregates are deformed or squashed. Fig. 7a and Fig. 7bshow the novel calcium carbonate structures after compactionforces of 300 kN and 450 kN, respectively. The surface

6

aggregates are no longer apparent. The extent to which thestructure is compressed cannot, therefore, be clearlydetermined using mercury porosimetry, and as specific surfacearea of the tablets themselves were not determined it is notpossible to specify the effect on potential adsorption. Furthercompression of the tablets does not result in any furtherporosity reduction. The intrusion data show a d50 of 0.018 µm,and a porosity of 33 % for all the tablets. These porosityvalues were verified by known oil mass absorption (atsaturation) and also by actual volume and densitymeasurements.

a)

b)

Fig. 7a and Fig. 7b. Electron micrographs of novel calciumcarbonate taken from a tablet compressed to a) 300 kN and b)450 kN.

Also shown in Fig. 5 is the mercury intrusion curve for acompressed tablet of HC60 where the resulting porosity was33 %, the same as that for the novel calcium carbonate tablets.The d50 value is clearly larger with a value of 0.18 µm. Anelectron micrograph of an HC60 tablet compressed to 100 kNis shown in Fig. 8. The broad void size distribution is clearlydistinguishable when this picture is compared with that of thesilica in Fig. 6.

Fig. 8. Electron micrograph of HC60 taken from a tabletcompressed to 100 kN.

Silica or novel calcium carbonate mixed with HC60A range of porosities was achieved for each ratio of additionalhigh surface area pigment to HC60 by changing thecompression pressure, Fig. 9. The addition of the high surfacearea pigments caused an increase in the porosity which wasmore pronounced for silica than for the novel calciumcarbonate. It is clear that with different additions of the novelcalcium carbonate the same porosity could be achieved over asignificant range of applied pressures indicating theconformable nature of the material. This gives the opportunityfor strongly adjusting the experimental surface area to porousvolume ratio by increasing the amount of the novel calciumcarbonate additive. Conversely, in the case of the silica, thesurface area to porosity ratio follows more linearly withamount of silica used.

20

25

30

35

40

45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

sample

po

rosi

ty /

%

silica/HC60 novel calcium carbonate/HC60

5:95

10:90

20:80

Fig. 9. Porosities of HC60/high surface area pigment mixes.

The mercury intrusion curves for these samples were correctedas previously described for mercury compression andpenetrometer expansion using the program Pore-Comp. Foursamples over the mix range for each additional pigment wereselected with the aim of isolating the porosity effects and alsoisolating the effects introduced by the addition of the highsurface area pigment. This was achieved by selecting twosamples for the 5 % addition level which had differentporosities, one sample from the 10 % addition level which hadthe same porosity as the more porous of the 5 % samples, andfinally one at 20 % addition which had yet higher porosity. The mercury intrusion curves give an indication of theinternal void structure of the tablets. These are shown below,

7

in Fig. 10 and Fig. 11 respectively, for the novel calciumcarbonate and the silica mix tablets.

0

0.04

0.08

0.12

0.16

0.2

0.001 0.01 0.1 1 10 100 1000diameter / µm

5 % novel calcium carbonate, porosity 25.1 %

5 % novel calcium carbonate, porosity 29.2 %10 % novel calcium carbonate, porosity 28.8 %20 % novel calcium carbonate, porosity 32.7 %

HC60, porosity 25.6 %HC60, porosity 32.2 %

Fig. 10. Mercury intrusion curves for HC60/novel calciumcarbonate samples.

0.00

0.05

0.10

0.15

0.20

0.25

0.001 0.01 0.1 1 10 100 1000diameter / µm

5% silica, porosity 27.06 %5% silica, porosity 31.35 %10 % silica, porosity 30.4 %20 % silica, porosity 35.85 %

HC60, porosity 27.6 %HC60, porosity 37.3 %

Fig. 11. Mercury intrusion curves for HC60/silica samples.

Fig. 10 and Fig. 11 additionally show the intrusion curves fortwo 100 % HC60 tablets of porosity equal to the highest andlowest values of the respective mixed tablets. The mercuryintrusion curves for the HC60/novel calcium carbonate tabletsshow that the pore sizes are reduced once the new pigment isintroduced as the mercury intrusion occurs at a higherpressure. The permeability of the structures will be reduceddue to the smaller pore sizes. The surface area will thereforehave been increased for the HC60/novel calcium carbonatetablet, which has a more closed structure, when compared witha tablet of the same porosity made purely of HC60. The novelcalcium carbonate has filled the larger pores and added poressimilar to the smaller sizes resulting in a smaller mono modalsize distribution. The HC60/novel calcium carbonate structures have a d50 ofapproximately 0.1µm. In contrast, the HC60 mercury intrusioncurves have a steeper gradient and show a larger d50 value thanthe equivalent HC60/novel calcium carbonate structures. Adding silica to HC60 increases the porosity much more thanthe addition of novel calcium carbonate, Fig. 11. The poresizes are reduced as before and there is a much greaterincrease in the number of finer pores as indicated by a step inthe intrusion curves at a diameter of 0.01 µm. The size of thestep correlates directly with the amount of silica added. Thesilica is again filling in the original larger pores of the HC60structural packing but also adding smaller pores. There islikely to be an increase in the permeability due to the largeincrease in porosity, and also an increase in surface area whichgets greater the more silica is added. These structures are moreopen than the equivalent HC60 structures and have a bi-modalvoid size distribution.

Absorption of fluids - the role of polymer as anabsorption retardantThe absorption dynamics of water versus the centrifuged fluidphase of the flexographic ink were studied to establish theaction of dissolved polymer in the fluid phase. Depending onthe absorption rate, pore size distribution and absolute level ofconcentration, the deposition, adsorption or precipitation ofpolymer can occur either within a forming interface filtercakeor within the porous absorbing structure or both. For example,the effect of surfactant on the absorption rate of water into aporous structure is well documented and is retarded comparedwith simple wetting fluids due to the dependence of thewetting front progress on the diffusion of the polymer and itsadsorption-desorption rate onto the side walls of the porousmedium. Furthermore, size exclusion of polymer from theporous structure would lead to blanking off of entry poreseffecting a further contribution to drag. This extra dragcontribution is clearly shown in Fig. 12 where theexperimental absorption dynamic for the centrifuged fluidphase is compared with that of pure water as they are absorbedinto a 100 % silica porous structure. The absorption isexpressed as the mass of fluid absorbed per unit area of theexternal sample surface contacting the fluid reservoir.

0

1

2

3

4

5

0 500 1000 1500 2000 2500time / s

water in silica

fluid phase in silica

Fig. 12 Absorption into a 100 % silica porous block comparingthe ink fluid phase with pure water.

The absorption curves have been corrected for both the effectof evaporation and for the initial wetting jump that occurswhen the tablet sample is first lowered into the fluid using themethod of Schoelkopf et al. (2000b). The equations for thewetting jump correction are shown in the Appendix. The absorption curves for the ink fluid phase into thepreviously chosen 100 % silica and 100 % novel calciumcarbonate structures are shown in Fig. 13. The absorptioncurves are truncated at the point where the evaporation rate ofthe fluid phase is equal to that of the absorption of the fluidphase. The absorption of centrifuged fluid phase into a 100 %HC60 tablet is also shown for comparison.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 500 1000 1500 2000 2500 3000time / s

mas

s ab

sorb

ed p

er u

nit

area

/ kg

m-2

100% silica, compressed to 300 kN, porosity 58%100% silica, compressed to 450 kN, porosity 50%100% novel calcium carbonate, compressed to 300 kN, porosity 33%100% novel calcium carbonate, compressed to 450 kN, porosity 33%100 % HC60, porosity 27%

Fig. 13. Absorption curves for 100 % silica and 100 % novelcalcium carbonate consolidated tablets compared at differentformation compressions and with 100 % HC60.

mas

s ab

sorb

ed p

er u

nit a

rea

/ kgm

-2

Intr

uded

vol

ume

/ cm

3 g-1In

trud

ed v

olum

e / c

m3 g-1

8

The absorption curve for the HC60 tablet has a much steeperinitial rise; this is due to the broader size distribution withinthe tablet (see Table 1). The effect of broadening a sizedistribution on the absorption behaviour was showntheoretically with the network model Pore-Cor by Ridgwayand Gane (2001). The large voids act as reservoirs feeding thesmaller voids which fill faster at the short time scales. Thecurve also has a point where the rate reduces which is wherethe polymer starts to seal the void structure due to sizeexclusion and evaporation at the wetting front outer surface. The initial gradient of the absorption curves for the 100 %high surface area pigment structures are much lower. Thenovel calcium carbonate has an extremely narrow sizedistribution, as can be seen from Fig. 7 (SEM) where theaggregates have almost identical sizes and, as shown by thesteepness of the mercury intrusion curves, so have the pores. Itis already known from other work (Ridgway, Gane 2001) thata narrow size distribution leads to an initial low absorptionrate. The difference between the two novel calcium carbonatecurves is thought to be due to the deformation of the structureof the pigment during the tablet compression, however, asdiscussed previously, the exact changes in the pore structureare not known precisely as the high pressure intrudingmercury acts further to compress the deformable structurerendering differentiation difficult. The absorption curves for the silica tablets lie above those forthe novel calcium carbonate. The lower porosity of the twosilica tablets has the faster absorption. The absorption rates forthe silica follow the pattern expected from earlier modellingwork (Ridgway, Gane 2001) with the block made at higherpressure, and consequently having the lower porosity, givingthe higher absorption rate due to the promotion of preferredpathway wetting in combination with a broadening of the poresize distribution with a small increase in the number of thefinest pores. A model of polymer uptake has previously beenproposed by the authors, Gane and Ridgway (2001), whichwould suggest that the silica tablet may become more clogged.

Colouration of tablets after absorptionmeasurementsAs well as studying absorption rate, the distribution ofcolouration was also observed. The only red colouration in the case of the 100 % novelcalcium carbonate, seen after the absorption, was at thesurface interface of the tablet. In contrast, the red dye was seento have penetrated into the silica blocks. The broad sizedistribution HC60 also showed colouration within the samplereducing in intensity toward the wetting front. These effectsare shown in the schematic in Fig. 14.

CaCO3 SiO2 Novel CaCO 3

Fig. 14. Colour in blocks after the absorption experiment.

The exclusion of the red dye dissolved in the fluid phase fromthe novel calcium carbonate samples can be ascribed to the

very high surface area of the novel calcium carbonatecombined with its low permeability providing a greatercapability for adsorption whereby the dye is held at the surfaceof the block while the remaining fluid continues to beabsorbed. This effect is similar to that seen by Rousu et al.(2001) in the case of the chromatographic separation of offsetink fluid components which was seen to be increased as afunction of pigment surface area and residence time at thewetting front (high surface area, low permeability givesmaximum chromatographic separation). Subsidiaryexperiments have confirmed this where aqueous dye solution(Nickel (II) phthalocyanine-tetrasulphonic acid8, 0.24 % solidcontent) placed on the surface of a silica block penetratedrapidly to a depth of 2 mm as observed by sectioning theblock, whereas on the surface of a novel calcium carbonateblock aqueous dye solution was adsorbed and the colourremained on the surface as a more intense hue. The redness of the surface of the base of the tablets wasmeasured using a Datacolor International Elrepho 3000 series.The instrument reports the colour in the form of the CIEcolour standard. Measurements of a (extending from green tored), b (extending from blue to yellow) and L* (the brightness)are given. The results are shown in Table 4 below and Fig. 15shows the relevant quartile of a cross section of the CIELABmodel. These results show that the novel calcium carbonatestructures give values that lie closer to the red axis (lower bvalues) than the silica tablets which appear to have a higheryellow colour content. The colour hue between the pairs oftablets is consistent and the silica tablets have a higher huethan the novel calcium carbonate tablets.

8 A product from Fluka Chemika AG, CH-9470 Buchs, Switzerland.

9

Novel calciumcarbonate

Novel calciumcarbonate

silica silica

Compactionforce / kN

300 450 300 450

L* 71.08 74.28 69.25 71.25a 43.06 39.16 41.65 34.95b 25.64 20.14 37.03 32.23

Table 4 CIE analysis results for 100% silica and 100% novel calcium carbonate tablets after the absorption of the centrifuged fluidphase.

1 2 3 4 5 a

b

5

4

3

2

1

Red

Yellow

30

90

60

0

Hue - Grade

novel calcium carbonate

silica

Fig. 15. CIELAB model showing colour measurement of silicaand novel calcium carbonate tablets.

The adsorption of the dyestuff in the surface structure of thenovel calcium carbonate structure acts to enhance the printcolour density and is considered as a way to reduce the inkdemand of coatings. Inspection of the mixed sample blocks including silicashowed that the colouration from the fluid phase was speckledas a result of the suspected heterogeneity of the mixturebetween HC60 and the silica, as shown experimentally in Fig.16 (5:95 silica to calcium carbonate). This illustrates veryclearly the presence of an absorption competition. Thespeckled areas that were lighter in colouration indicated thatthe solved dyestuff enters less into, or is less adsorbed onto,the pigment which is rich in that area.

Fig. 16. Inhomogeneous dye adsorption in a tablet made froma blend of HC60 and silica.

Analysis of the elemental inhomogeneity was made usingenergy dispersive X rays (EDX). This is a micro analyticaltechnique that is based on the characteristic X-ray peaks thatare generated when the high energy beam of an electronmicroscope interacts with the specimen. Each element yields acharacteristic spectral fingerprint that may be used to identifythe presence of that element within the sample. The relativeintensities of the spectral peaks may be used to determine therelative concentrations of each element in the specimen. TheX-ray signal is detected by an energy dispersive solid-statesilicon-lithium detector and the construction and efficiency ofthis detector sets a lower limit on the atomic number that maybe detected. Using EDX-Analysis the element content of thesample in Fig. 16 is shown in Table 5 .

Elements Mass % Atomic %Bright spot: Si 2.4 3.4 Ca 97.6 96.6

Dark spot: Si 15.2 20.3 Ca 84.8 79.7

CompoundsBright spot: SiO2 2.1

CaCO3 97.9Dark spot: SiO2 13.3

CaCO3 86.7

Table 5. Element content of HC60/silica mix tablet shown inFig. 16

The dye absorption/adsorption is less in the case of thestandard carbonate-rich areas compared with the areas of highsurface porous silica. The void sizes in the silica-rich areas aresmaller than in the standard calcium carbonate areas and sothe absorbed and/or adsorbed colour will also appear moreintense in the silica-rich regions.

Absorption characteristics of complete inkThe 100 % high surface area pigment blocks of both silica andnovel calcium carbonate showed no measurable absorptionwhen the whole ink was used. This is assumed, afterinvestigation, to be due to the colour pigment particles andpolymers blocking the surface pores of the tablet block andresulting in an impermeable rapidly solidifying layer as theinitial rapid surface wetting occurs causing virtuallyinstantaneous drainage of fluid from the ink at the point ofcontact. In contrast, however, the ink fluid penetrated readily into themixed blocks and once again the mottled effect was clearlyvisible in the case of the HC60/silica mix indicating again thecompetitive adsorption of the dye and/or competitiveabsorption of the fluid.

10

Minimumdiameter

/ µm

Maximumdiameter

/ µm

Throatskew

Truncatedpore skew

Connectivity Porosity

/ %

Unit cell size

/ µm

300 kN 0.0036 0.0309 0.96 1.3 2.9 58.4 0.319

450 kN 0.0036 0.0309 1.27 1.3 2.9 50.3 0.324

Table 6. Pore-Cor parameters for 100 % silica tablets.

Modelling the absorption of 100 % silica tabletsThe fully corrected experimental mercury intrusion curvesprovide the percolation characteristics necessary for thegeneration of the model void structures using Pore-Cor. ThePore-Cor structures were determined for the 450 kN and300 kN compaction force 100 % silica tablets and theabsorption of the centrifuged fluid phase into these structureswas simulated to illustrate the structural features associatedwith absorption dynamic. As mentioned previously, the Pore-Cor simulation cannot model the longer-time polymerabsorption effects in terms of clogging, however the fluidproperties of the actual fluid phase are used and Pore-Corhighlights the important qualitative structural features.Furthermore, the longer time scale absorption is not relevantfor Pore-Cor due to the limited volumes provided by the singleunit cell, and it is assumed that the polymer clogging effectoccurs after longer time scale absorption. The correspondingsize distribution of pores and throats for the Pore-Cor structurefor the most compact silica tablet are shown in Fig. 17. It wasnecessary to use a truncated pore skew of 1.3 to match theexperimental porosity. This truncated pore skew generated apeak in the distribution at the maximum experimentallyobserved pore size which is used as the truncation point of thepore size distribution.

0.1

1

10

100

0.001 0.01 0.1diameter / µm

perc

enta

ge n

umbe

r

throats

pores

Fig. 17. Pore and throat size distribution for the 450 kNcompaction force 100 % silica tablet.

Table 6 summarises the optimum Pore-Cor parameters for thetwo comparative samples and Fig. 18 shows the simulatedstructure for the same sample as determined by the pore- andthroat-size distribution illustrated in Fig. 17.

Fig. 18. Pore-Cor unit cell for silica compressed to 450 kN.

Fig. 19 shows the simulated wetting of the samples expressedas mass per unit area as a function of time for the centrifugedfluid phase using the properties shown in Table 2.

0.00000

0.00001

0.00002

0.00003

0.00004

0.00005

1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02time / s

mas

s ab

sorb

ed p

er u

nit a

rea

/ kgm

-2

100 % silica compressed to 450 kN

100 % silica compressed to 300 kN

Fig. 19. Simulated absorption into 100 % silica tablets

The absorption rate into the more highly compacted tablet(450 kN) is greater than that into the less compacted tablet(300 kN), reproducing the observed order of the experimentalresults already seen in Fig. 13. Despite the short time scaleused in the modelling, and hence the lower absorbed volumescompared with experiment, it is seen that the relativequantitative differences between the silica samples under thetwo compressions follows those seen in the practicalexperiment, Fig. 13. The correlation between modelling and experiment permitsus to demonstrate the importance of including inertialretardation and inertial wetting within our understanding ofabsorption into a porous network. Omission of thesedynamical effects results simply in a reverse of the modelled

11

absorption dynamic between the more highly compressed andless compressed samples which would be contrary toexperimental observation. The use in the model of a pure fluid without polymer content,with the same viscous and surface tension properties as theexperimental ink fluid, illustrates that the role of polymeradsorption, if leading to clogging, is minor when consideringthe relative differentiation in absorption rate between thedifferent experimental structures, but, as shown by Ridgwayand Gane (2001), can determine overall absorption rate andcan be assumed to occur for structures having pore sizedistributions similar to these experimental samples despitevery different surface chemistry, e.g. silica versus dispersedcalcium carbonate. This statement of course ignores the effectof polymer clogging in a real ink filtercake and concentratesonly on the effect in the porous structure. Therefore, clogging,if occurring, can be assumed to be a sample surface or nearsurface phenomenon controlling the delivery of fluid to thebody of the structure. The necessary distribution of pore sizes and pore shape,considered here in the model by using the double-conicalthroat construction for structures made from low aspect ratioparticles, to develop a certain absorption dynamic forcontrolled ink fluid imbibition, can therefore be initially pre-determined. The model predicts that in the case of theexperimental fluids applied in this work, the inclusion of finepores into a structure with an inherent broad size distributionof pores increases absorption rate (throat skew 1.27 versus0.96 simultaneous with reduced porosity - Table 6).Complementing this with reduced permeability,experimentally shown by the addition of the novel calciumcarbonate, then increases the potential for ink holdout and, ifsupplemented by high dye adsorptive power, a high printdensity.

ConclusionsThe network model Pore-Cor has been used to illustrate thestructural features responsible for the differences in absorptionrates between the unhindered wetting absorption of silicatablets formed at two different compaction forces based oninertial and viscous considerations. These results help toexplain the absorption dynamic seen for a range ofexperimental pore size distribution based on addition of highsurface area additive. It is shown that the need for optimisedcontrol of the setting characteristics of water-based inkscannot be met realistically by increasing the absorption rate tothe point at which the ink immobilises instantly as thisprevents ink levelling and leads to uneven pickup. Thereforethe structure of the coating should be of high porosity but witha well-controlled absorption dynamic and permeability. Homogeneous blends of high surface area porous pigmentsand standard ground calcium carbonate pigment are shownexperimentally to provide the necessary control parameters foroptimisation. The coating structure should contain aproportion of fine pores to provide sufficient capillarity.Advantageously, these fine pores should also have the abilityto adsorb or capture dyestuff so as to enhance the print colourdensity. An example of a novel speciality calcium carbonatestructure is used to create this effect by controlling theabsorption rate into a structure of high surface area but lowpermeability. The novel calcium carbonate provides sufficientresidence time during absorption to promote adsorption of thedye colour and therefore gives potential for higher printdensity. Low permeability in combination with highadsorptive power gives the possibility of retaining ink solids

and dyes at the coating surface, thus increasing ink holdoutand print density with the consequent benefit of reducing inkdemand and increasing resolution.

Literature

Blixt, T., Balzereit, B., Laufmann, M. (2001): "A New CalciumCarbonate Concept for SC Papers", Proceedings of the CPPA,Montreal, Canada.Gane, P.A.C., Kettle, J.P., Matthews, G.P., Ridgway, C.J.(1996): "Void Space Structure of Compressible PolymerSpheres and Consolidated Calcium Carbonate Paper-CoatingFormulations", Industrial and Engineering Chemistry Research,35, 1753.Gane, P.A.C., Ridgway, C.J. (2001): "Absorption Rate Studiesof Flexographic Ink into Porous Structures: Relation toDynamic Polymer Entrapment During Preferred PathwayImbibition", Advances in Printing Science and TechnologyVolume 27, 56.Gane, P.A.C., Schoelkopf, J., Spielmann, D.C., Matthews,G.P., Ridgway, C.J. (2000): "Observing Fluid Transport intoPorous Coating Structures: Some Novel Findings", TappiJournal, 85 , 1.Matthews, G.P., Ridgway, C.J., Small, J.S. (1996):"Modelling of Simulated Clay Precipitation Within ReservoirSandstones", Marine and Petroleum Geology, 13, 581.Matthews, G.P., Ridgway, C.J., Spearing, M.C. (1995): "VoidSpace Modeling of Mercury Intrusion Hysteresis in Sandstone,Paper Coating, and Other Porous Media", Journal of Colloidand Interface Science, 171, 8.Moir, W.W. (1994): "Flexography - a Review", JOCCA-SurfaceCoatings International, 77, 221.Peat, D.M.W., Matthews, G.P., Worsfold, P.J., Jarvis, S.C.(2000): "Simulation of Water Retention and HydraulicConductivity in Soil Using a Three-Dimensional Network",European Journal of Soil Science, 51, 65.Ridgway, C.J., Gane, P.A.C. (2001): "Dynamic Absorption intoSimulated Porous Structures", TRI/Princeton InternationalWorkshop, Princeton,Ridgway, C.J., Ridgway, K., Matthews, G.P. (1997):"Modelling of the Void Space of Tablets Compacted Over aRange of Pressures", Journal of Pharmacy and Pharmacology,49, 377.Ridgway, C.J., Schoelkopf, J., Matthews, G.P., Gane,P.A.C., James, P.W. (2001): "The Effects of Void Geometryand Contact Angle on the Absorption of Liquids into PorousCalcium Carbonate Structures", Journal of Colloid andInterface Science, submitted,Rousu, S.M., Gane, P.A.C., Eklund, D.E. (2001): "Influence ofCoating Pigment Chemistry and Morphology on theChromatographic Separation of Offset Ink Constituents",Twelfth Fundamental Research Symposium, Oxford 2001.Schoelkopf, J., Gane, P.A.C., Ridgway, C.J., Matthews,G.P. (2000a): "Influence of Inertia on Liquid Absorption intoPaper Coating Structures", Nordic Pulp and Paper ResearchJournal, 15, 422.Schoelkopf,J., Gane,P.A.C., Ridgway,C.J., Spielmann,D.C.,Matthews,G.P. (2001): "Rate of Vehicle Removal From OffsetInks: a Gravimetric Determination of the Imbibition Behaviourof Pigmented Coating Structures", Tappi Fundamentals, SanDiego,Tappi Press, Atlanta.Schoelkopf, J., Ridgway, C.J., Gane, P.A.C., Matthews,G.P., Spielmann, D.C. (2000b): "Measurement and NetworkModelling of Liquid Permeation into Compacted MineralBlocks", Journal of Colloid and Interface Science, 227, 119.Toivakka, M., Nyfors, K. (2001): "Pore SpaceCharacterization of Coating Layers", Tappi Journal, 84, 49.Xiang, Y., Bousfield, D.W. (1998): "The Influence of CoatingStructure on Ink Tack Development", PanPacific andInternational Printing and Graphic Arts Conference,CPPA,Canada, 93-101.

12

Appendix

Smoothing correction equations used for the correction of theapparent initial weight change with time during the absorptionof different fluids, according to Schoelkopf, Ridgway et al.(Schoelkopf et al. 2000b), where m is the corrected massabsorbed (g), t the time (s), a, b, c and d are fitting constantsand R2 is the standard measure of goodness of fit. Theequation types shown are the simplest in each case whichsatisfy the criterion R2 > 0.98.

100 % silica at a compaction force of 300 kN

tdctbtam ln35.2 +++=a = 48.033072, b = -0.000000871, c = 0.000000077,d = -0.003335622, R2 = 0.9945

100 % silica at a compaction force of 450 kN

tbtam ln5.02 +=a = 2275.736, b = -0.05529, R2 = 0.9995

100 % novel calcium carbonate at a compaction force of300 kN

tbtam ln5.02 +=a = 2235.49, b = -0.0556, R2 = 0.9993

100 % novel calcium carbonate at a compaction force of450 kN

5.2ln cttbtam ++=a = 47.91326, b = -0.0000496, c = 0.0000000474, R2 = 0.98