inkjet print quality: the role of polyvinyl alcohol in speciality...

PAPER I

Inkjet print quality: the role of polyvinyl alcohol in speciality

CaCO3 coatings

In: Journal of Pulp and Paper Science 2009(35)3–4, pp. 137–147.

Copyright 2009 Pulp and Paper Technical Association of Canada.

Reprinted with permission from the publisher.

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JOURNAL OF PULP AND PAPER SCIENCE: VOL. 35 NO. 3-4 2009 137

INTRODUCTIONThere is an increasing trend for the use

of aqueous-based inkjet inks in high-speedcommercial printing, and this challenges thehydrophilic and absorptive properties of thesurface of paper [1]. There is an urgent need todevelop the coating layer properties based onthe mechanisms which control these properties

in inkjet printing, and the interaction of pig-ment and typical inkjet paper coating formula-tion binders, in determining coating pore struc-ture, is an area that has received insufficientattention in this respect.

The absorption speed and volume of theink diluent/solvent challenge the capacity ofthe paper surface to generate acceptable inkjetprint quality. The water-solvent should absorbvery rapidly into the paper. The applied inkdroplets should not mix together on the surfaceor intermingle in the structure of the sheet. Inkdroplet setting is a phenomenon that happenson a millisecond scale, but the final ink dryingcan take hours [2]. The amount of appliedsolvent can be very high, so the paper needs ahigh porosity to get all of the water-solventinto the paper structure. Even in emergingsystems where the volume of diluent is strictlycontrolled, the rate of absorption still plays adecisive role.

The absorption volume and speed iscommonly explained adopting the Lucas-Washburn equation. According to Lucas-

Washburn’s equilibrium equation, the volumeof liquid in a capillary should be greater, thelarger the radius, at a given time. On the otherhand, the practical studies have shown thatsmaller radius capillaries initially fill fasterthan larger capillaries. Ink sets faster on a finepore structure coating layer than on a highlyporous, large pore containing coated paper.This disagrees with the model of Lucas-Washburn. Clearly, there is a distinctionbetween fine and large pores and furthermore,there are effects occurring on the shorttimescale on the surface and in the pore net-work as well as in a single and several inter-connected capillaries that Lucas-Washburndoes not capture [3-5].

Ridgway et al. [3] further studied theLucas-Washburn equation. They showed thatthere are a number of more detailed descrip-tions of flow entry effects into capillaries, suchas the energy loss equation of Szekely [5].Bosanquet [6] added the inertial wetting termassociated with an accelerating fluid. A solu-tion of the equation of Bosanquet for shorttimes is proposed by Schoelkopf et al. [7],

Inkjet Print Quality: The Role of Polyvinyl Alcoholin Speciality CaCO3 Coatings

T. LAMMINMÄKI, J.P. KETTLE, P. PUUKKO, P.A.C. GANEand C. RIDGWAY

T. Lamminmäki, J. Kettle andP. PuukkoVTT Tech. Res. Centre FinlandP.O. Box 1000FIN-02044 VTT, Finland([email protected])

P.A.C. GaneTKK, Vuorimiehentie 2,FIN-01250 Espoo, FINLAND,and OMYA Development AGCH-4665 OftringenSwitzerland

C. RidgwayOMYA Development AGCH-4665 OftringenSwitzerland

The aim of this work is to clarify the controlling role of polyvinyl alcohol (PVOH) as a binder in the formation of coating and pore structure and howit affects the high-speed inkjet image quality formation. The results show that the pigment type and the binder amount have a large interactive effect onpore structure formation. The studied PVOH can transfer into the intra-particle pores of pigments, and the swelling of PVOH closes up the pores whichhave diameters of ~30 nm, but this depends on the amount of binder used. The optical properties of the whole paper, not just the coating, are the mostimportant regarding print density, but the properties of the coating layer itself have a dominant effect when considering the ink bleeding.

L’objectif de cet ouvrage est de clarifier le rôle de régulateur que joue l’alcool polyvinylique (PVOH) comme liant dans la formation du couchage etla structure des pores, et de démontrer comment elle affecte la qualité de la formation de l’image à jet d’encre à haute vitesse. Les résultats démon-trent que la sorte de pigment et la quantité de liant ont un impact interactif important sur la formation de la structure des pores. L’alcool polyvinyliqueétudié peut se transférer à l'intérieur des pores de pigments intra-particulaires, et son gonflement resserre les pores ayant un diamètre de ~30 nm.Cependant, ceci dépend toujours de la quantité de liant utilisée. Les propriétés optiques du papier entier, et non seulement le couchage, sont lesfacteurs les plus importants en ce qui a trait à la densité d’impression, mais les propriétés de la couche de couchage ont un impact dominant relative-ment aux bavures de l’encre.

mailto:[email protected]

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JOURNAL OF PULP AND PAPER SCIENCE: VOL. 35 NO. 3-4 2009138

which showed that liquid penetration distanceby absorption into the finest capillaries overshort timescales has a direct proportionality tothe time elapsed. When inertia dominates, thefinest pores will absorb further and faster.Furthermore, if we consider a network of suchfine pores, remembering that a fine pore incoatings is typically equally as short as itsdiameter is wide, then in combination withenough reservoir volume between them, wecan visualize a preferred pathway effectivelyby-passing the larger pores or at least limitingthe access to them. This has been constructedby Ridgway et al. [8] using the Pore-Corvisualization (Pore-Cor and Pore-Comp are asoftware network model and a sample com-pression model, respectively, developed at theUniversity of Plymouth, Plymouth, UK). Inthis pathway, the velocity of absorption ishigh, and so the passage into the porous medi-um is defined by the combination of pore sizeswithin the inertial wetting regime (nanopores)and the connecting reservoir structure consist-ing of larger pores which fill more slowly.Therefore, we can assume that small pores areneeded for quick inkjet ink penetration at themoment of droplet setting and large pores areneeded for the storage of all the solventsinto the structure. The connectivity relationbetween these is crucial if the maximumabsorption rate for large volumes is to bemaintained.

In coating colours, we have, besides thepigments with their particle packing character-istics and internal pore structure, if present, thebinder. By changing binder amount or bindertype we can influence the inkjet ink penetra-tion. Primarily, the binder amount should besufficient enough to ensure that the coatingcolour has adequate adhesion to the papersurface and cohesion so that dusting problemsare minimal. The surface strength is notneeded at such high levels as in traditional off-set printing, but nonetheless dust generation iscritical, as it could lead to blockage or damageof the inkjet nozzles, and so diminish the printquality and reduce the lifetime of expensiveprint engine components. In coating coloursfor inkjet papers, polyvinyl alcohol (PVOH) is

a very commonly used binder, and it has a highcapability to bind pigments. It is also a veryhydrophilic binder, and exhibits swelling oncontact with water [9-11]. Pinto et al. [11]showed in their study that ink diffusion isunrestrained by PVOH, and the colorantconcentration profile is uniform in a PVOHlayer as shown by time-of-flight secondary ionmass spectrometry (TOF-SIMS) analysis.Lately, the binder type has been shown to havean effect on the inkjet ink print density andbleeding development, Nilsson [12] andSvanholm [13]. However, the binder swellingtendency in the inkjet print quality formationof Versamark VX5000e has not been studiedso widely.The aim of this work is to clarify:– the role of PVOH as a swelling binder

in the porous CaCO3 coating layerstructure,

– the role of porosity and pore diameters inthe inkjet ink setting process, and

– how the PVOH coatings work in the printquality formation of dye-based inks inhigh-speed inkjet printing.

MATERIALS AND METHODSCoating Compositions andCoating Trials

A range of calcium carbonate-based pig-ments provided by Omya AG, Oftringen,Switzerland and one CaCO3 pigment fromMinerals Technology Europe, Zaventem,Netherlands were used. The main idea behindthis pigment selection was to vary the meanparticle diameter and specific surface area ofthe pigments (Table 1). The binder in the coat-ing colours was polyvinyl alcohol (PVOH,Mowiol 40-88 provided by ClariantInternational AG, Muttenz, Switzerland).The selected PVOH had a degree of hydrolysis87.7 ± 1.0% and a molecular weight of204 000 g·mol-1 [14].

The study was started by looking at theproperties of pure pigments. Pigment cakesand 100 g·m-2 coating layers on glass plateswere produced from a range of formulations.The cakes were made with Teflon moulds,

drying them at 23°C. The glass plates werecoated with an Erichsen film applicator (Model288, Hemer, Germany) using blades with afixed gap height between 100 and 500 µm.All the pigments had been dispersed by thepigment manufacturers themselves.

The pigment properties are shown inTable 1. The choice of pigments was made toillustrate the effects of particle size andinternal pore structure, including raw materialsources from ground calcium carbonate(GCC), subsequently modified to generatehigh surface area and internal pores (MCC),and precipitated calcium carbonate (PCC). Thepigment diameters were determined by sedi-mentation (Sedigraph 1500, Micromeritics,Norcross, GA, USA) or by laser light scatter-ing (NanoSight, Amesbury, UK) and the valuesgiven are quoted from the specifications of themanufacturers. The pH and zeta-potential(AcoustoSizer II, Colloidal Dynamics/AgilentTechnologies, Espoo, Finland) were deter-mined in the coating colour, which in this casecontained 10 pph PVOH (based on 100 pph byweight pigment). We note that all but one ofthe pigments exhibit a cationic slurry property,being designed to adsorb anionic ink dye so asto prevent strike through and to maximize printdensity by concentrating the colorant nearthe coating surface. Coulombic interaction alsoacts to maintain water fastness of the ink.

The liquid uptake capacity of the porousstructures was measured with Si-oil absorp-tion. During the oil absorption, the pigmentcake was left for one hour in Si-oil, and theweight of cake before and after Si-oil satura-tion was measured. The available porosity foroil absorption is determined at normal airpressure and is defined as absorbed Si-oilvolume in the coating cake, divided by the sumof the coating layer volume without oil, plusthe absorbed Si-oil volume.

The pore volume and pore size distribu-tion was measured with mercury porosimetry(Micrometrics AutoPore IV), adopting thePore-Comp correction to account forpenetrometer expansion, mercury compressionand compression of the sample skeletal

TABLE IPIGMENT PROPERTIES

Properties Ground Modified Modified Precipitated Precipitatedcalcium calcium calcium calcium calcium

carbonate carbonate carbonate carbonate carbonate- GCC - "MCC large" - "MCC small" - "PCC large" - "PCC small"

Weight median 0.65 2.70 1.30 2.70 Small 0.02-0.03pigment particle Av. 0.25 *)diameter, µm (d50%,)Specific surface 10.7 46.2 27.0 63.7 73.9area, m2g-1 (BET, ISO 9277)Zeta-potential, mV -9 13.1 13.6 1.6 -0.6(AcoustoSizer II)Registered trade name HYDROCARB 90 OMYAJET B6606 OMYAJET C3301 OMYAJET B5260 JetCoat 30*) NanoSight (results had two peaks: small at 20-30 nm and average at 0.25 µm)

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material, expressed as the elastic bulk modu-lus, according to Gane et al. [15]. The coat-ings, with their different binder amounts wereanalyzed up to 140 Pa (low pressure port) and440 Pa (high pressure port), respectively. Thecoated glass plates were analyzed by the thinlayer chromatography method (TLC), puttingthe plate into the eluent (equivalent to eitherthe ink diluent alone or the whole ink, depend-ing on volume or chromatographic separationanalysis). The distance of eluent movementduring time was measured. The optical proper-ties of the coating layer were studied as afunction of eluent uptake and distance, e.g.,opacity according to ISO 2471:2008. Behindthe coated glass plate a stock of blotting boards(fully bleached) was used as backing.

The KCL pilot coater was used to applycoatings on a larger scale to traditional basepaper, which was chosen to be a commercialbase sheet for a coated fine paper, 53 g·m-2.The base paper was first pre-coated so that thetop coating of main interest was preventedfrom penetrating into the base paper. Thepre-coating of 7 g·m-2 was applied with a filmcoater on both sides of the paper, and theactual studied coating layer of 8 g·m-2 withshort dwell application (blade). The pre-coat-ing had 100 pph of GCC (Hydrocarb 60),12 pph styrene-butadiene latex and 0.6 pphcarboxymethylcellulose. The recipes of the topcoating are introduced in Table 2. The coatingspeed was 1800 m·min-1 and the final moisturecontent of the coated web was 5 wt%.

The role of pigment properties alone inthe inkjet ink setting process was also studied,so as to subsequently identify the interactiveroles of pigment and binder combined. In thiscase the coating was applied with a semi-pilotcoater, KCL SAUKKO (Fig. 1), using jetapplication and blade. The coating was driedwith two infra red dryers. The speed of the webwas 900 m·min-1. The same base paper wasused as in the pilot coating. Based on the pilot-trials the binder amount was selected to be 10pph of PVOH (Mowiol 40-88) to representcommercially acceptable strength and anti-dusting properties. The structural illustrationsof produced coatings are given in Table 3. Thetarget coating layer amount of 10 g·m-2 wasnot achieved in one single coating layer. Thelow solids content and the low viscosity of thecoating colours limited the applied coatinglayer thickness, and therefore we had to applythe same coating colour twice.

The air permeance of coated papers wasmeasured with Parker-Print Surf measurementusing 20 kPa pressure. The opacity was meas-ured according to the standard ISO2471:2008and light scattering coefficient ISO 9416. Thesurface strength of coated papers was studiedwith an IGT device using ISO 3783 withmedium viscosity oil. The absorption time ofdye was measured with DIGAT that has beenintroduced and described in detail in theliterature [17]. The ink was anionic dye-based

Fig. 1. Image of KCL SAUKKO [16].

TABLE IITHE TOP COATING COLOURS

Pigment/Binder Colour 1 Colour 2 Colour 3 Colour 4

"PCC large" 100 100 100"PCC small" 100PVOH 7 12 30 7

Jet application

Paper web(width 20 cm)

IR dryers

Blade

TABLE IIITHE STRUCTURAL ILLUSTRATION OF COATING LAYERS AFTER THE SAUKKO COATING.

ALL COATINGS HAD 10 PPH PVOH

Pigment Air permeance, Porosity Structure Illustrated coatingµm. (Pa.s)-1 (coating cakes),% of pigment layer structure

GCC 0.1 17.7

"MCC large" 1.3 46.2

"MCC small" 0.7 31.9

"PCC large" 1.8 48.9

"PCC small" 2.3 32.9

Rhombohedral

“Roses”

“Roses”

“Eggs”

Spherical

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ink (from a Versamark VX5000e) and theapplied ink amount was 8 g·m-2. The surfacestructure of the coating layer was imaged withthe scanning electronic microscope (SEM).

Inkjet Printed SurfacesInkjet printing was carried out on a

Versamark VX5000e, which produces inkjetdroplets by the continuous stream inkjetmethod. The inks were dye-base and the maindiluent/solvent was water. The surface tensionsof the inks were observed to fall in the smallrange of 51-55 mN·m-1 (25°C), depending ondye colour, and the viscosity was 1-2 mPa·s.Printing speed was 100 m·min-1 and the dryingdrum and hot air dryer were set to a tempera-ture of 80°C (when studying pigment type toemphasise absorption criteria) and 100°C(when studying the range of binder amount toprovide runnability at the highest binder level).

The print density of the printed surfacewas measured with GretagMacbeth D196.Both a camera and scanner systems wereutilized to analyze the bleeding, in terms ofdistance from the formal colour boundary. Theparallel reporting from the two systems wasalso a part of the development of the scannermethod. In the camera system, a picture wastaken with an imaging camera from a solidprinted line used to define the bleeding bound-ary, and a grey level profile across the line wasconstructed. The normal edge width for thegiven print was measured as the distancebetween two points A and B. Point A wasdefined as the point at which the surface acrossthe line image was 10% brighter than the dark-est region in the line and B, 10% darker than agiven background. The black surface had greyvalue zero and white 254. Each unprintedpaper was adjusted to the value 170. Thenormal edge width described the bleedingdistance of inks. The smaller the number, thesharper is the printed line. In the scannersystem, an Epson Perfection V700 Photo wasused, with a resolution of 2400 dpi. The greylevel profile of the line was again measuredwith an image analysis program using a similardefinition of the points A and B, but now Awas 15% brighter than the darkest region andB, 15 % darker than a background. The greylevels were adjusted as in the previous camerasystem. The differences in the optics causedsome differences in the results between thesemethods. Mottling was analyzed with ascanner system. The printed surface wasscanned with an Epson Expression 1680 Proscanner using a resolution of 300 dpi. Thescanned figure was handled with a wavelettransform (PapEye program). The ink colorantpenetration was studied by embedding theprinted surface in LRWhite resin (ElectronMicroscopy Sciences, Hatfield, PA, USA). Theembedded sample was placed in a refrigeratorto reduce smearing of the dye. The cross-sec-tions were then imaged in a light microscope(Zeiss Axioskope 2 plus).

RESULTSPVOH and Porosity Formationof the Coating Layer

The intrinsic structure of the coatinglayer, without the influence of the base paper,was studied using the coating cake material.At this point, we have to remember that thestructure of the coating cake, formed by dry-ing a relatively large volume of coating colour,could be expected to be somewhat different tothat of the structure of a thinly applied coatinglayer on a paper surface, where the coatingprocess variables affect the forming structure,including differences in shrinkage behaviour.This issue has been resolved to some extent bythe development of a pigment tablet formationdevice at Omya AG [7], however this was notused in this work as the PVOH binder is solu-ble, and filtration might have led to an unwant-ed depletion of binder. In this current work,however, we saw that the differences were infact minimal between the measurements on the

coated papers and the coating cakes.The absorption capacity of the coating

cakes was analyzed with Si-oil absorption(Fig. 2). As expected, increasing the binderamount reduced the level of porosity asmeasured by this technique. At the binder level10 pph, the porosities of fine particulate “MCCsmall” and “PCC small” pigments were foundto be on the same level. The highest porositieswere produced by the large particle size coat-ings. This illustrates the likely particle packingcharacteristics.

The porosities of the coating layers werealso calculated from mercury porosimetryresults, when the base paper effect and theamount of coating layer are taken into account,following the method of Ridgway et al. [18].The cumulative intruded pore volumes of thecoating layers are illustrated in Fig. 3. The"PCC large" coating produced again the great-est pore volume, and it had more large sizepores. The lowest porosities were seen forGCC and "MCC small".

The results of Si-oil porosity and mercuryporosimeter measurements correlate very well(Fig. 4). The slight differences can be analyzedin respect to the high pressures used in themercury measurement. At the final stages ofmercury intrusion, the pressures are sufficientlyhigh to compress any elastic componentspresent as part of the skeletal structure of theporous material. Polymers, in particular

Fig. 4. Correlation between the results of Si-oil porosity andmercury porosimeter.

Fig. 2. Absorption capacity of pigmentcakes as a function of PVOH binder level,measured by the Si-oil absorption method.

Fig. 3. The cumulative pore volume distrib-uted as a function of pore size, asmeasured using mercury porosmietry.

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binder, undergo such compressive effects. Theresult is an absence of hysteresis between theintrusion and extrusion curves as a function ofpressure increase and decrease, respectively.The gradient of this elastic region can bedetermined and related to a bulk modulusexpressing the volume compressibility of thematerial, Gane et al. [15]. The bulk modulusof the paper coating, based on "PCC large"with 10 pph PVOH, was 30.6 x 103 MPa andbased on "PCC small", also with 10 pphPVOH, was 22.0 x 103 MPa. This means thatthe “PCC large” paper can resist pressure onthe skeletal components more so than “PCCsmall” paper. This reflects the distribution ofpolymer in respect to the pigment particlepacking and the pore sizes present. The finerpigment, with its higher surface area, willnecessarily present more polymer species andamount, including dispersant, to the coatingcolour.

The pigment cakes made from “MCCsmall” and "PCC small" with binder had verysimilar Si-oil absorption volume capacities, asseen in Fig. 4, but exhibited very differentporosity values when analyzed with themercury porosimeter. This lets us deduce, thatbinder tends to blank off the entry to, orconnectivity between, pores. Thus, the oilcannot access all the pore volume, whereas theintruding mercury can break through the pore-blanking binder films. This effect is confirmedby the equivalence between the two methodsfor the pigment cakes without PVOH, in whichboth techniques access the same pore volumes(Fig. 5).

Traditionally, taking the first derivativeof the cumulative mercury intrusion curves

provides a pore size distribution. This descrip-tion, although limited to an equivalent bundleof capillaries, and thus ignoring pore shieldingeffects, is nonetheless useful for comparisonpurposes between topologically differentstructures, Gane et al. [19]. We see that thePCC coating structure has only ultrafine poresin the 20-60 nm range (Fig. 5). In contrast, theMCC coatings have both large- and small-sizepores, indicating that the MCC structureshave both intra- and inter-particle pores.The addition of 10 pph PVOH decreases thepore volume of the MCC coating structuremore than that of PCC. The peak of intra-par-ticle pores of the MCC pigment decreasesmost markedly, clearly indicating that PVOHhas filled intra-particle pores.

Effect of Pigment Propertieson Internal Liquid Transport

The thin layer chromatography methodwas used to analyze the eluent movementthrough the pore networks of the pure pigmentpacking structures. Long-time passage ofliquid through a porous network is defined bythe surface wetting force resisted inversely bythe permeability of the structure. The greaterthe permeability and the more the occurrenceof fine pore structure surrounding thatpermeable pathway, the greater will be thetransport length in a given time. The longestwater/ethanol eluent transport distancethrough the structure was achieved with themost porous pigments (Fig. 6). The "MCClarge" and "PCC large" pigments have verysimilar size and amount of intra-pigmentpores, but MCC has smaller inter-particlepores and so less large pores to contribute topermeability, as Fig. 12 later shows. The GCCand “MCC small” coatings have quite similarcapability to transport eluent. The distancesare short because of the lack of permeability.The "PCC small" pigment unexpectedly trans-fers the liquid further in the structure than theother pigments. This would not be expectedwhen looking purely at the fine pore structure,but the sample structures made from the “PCC

small” pigment had a multitude of cracks,which contribute to the permeable transfer ofeluent effectively in the pigment layer. Thiseffect is not one of intrinsic particle packing,but the response to shrinkage as the layerforms. These cracks are seen as essential if thispigment is to work in transporting liquid. Theresults indicate that pigments with higher porevolume transfer liquid more effectively thanthe lower pore volume structures depending onpermeability.

Coating Layer Structure on PaperSurfaceRole of Binder

The higher binder amount increased thesurface strength of the coated papers (Fig. 7).

At a binder amount of 7 pph, the “PCC small”had a slightly higher surface strength then“PCC large”. Once again, this is due tothe difference in interior structure permeabilitydetermining the level of binder depletion likelyto occur when applied to an absorbent substrate.The surface strength of 2 m·s-1, defined as thespeed at which the tack oil begins to inducepick from the coated paper in the test nip, isconsidered high enough to ensure attachmentof the coating to the paper substrate and toprevent dusting during the inkjet printing.

Fig. 5. Cumulative pore volume and poresize distribution curves of "MCC small"and "PCC small".

Fig. 6. Water/ethanol eluent (50%/50%)transfer over time in the thin layer pigmentchromatography layers formed on glassplates without binder.

Fig. 7. Surface strength of coated papersmeasured with the IGT pick test method.

Fig. 8. The first derivative of thecumulative mercury intrusion curves(Pascal porosimeter) illustrating the effectof the binder level.

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Figure 8 shows how the binder amountincrease decreases most of all the volumeassociated with the 30-50 nm size pores withinthe pore diameter area 0-0.1 µm, primarilyresponsible for the capillary driven absorption.

The absorption speed into the thin layercoatings on glass using the DIGAT apparatusbecame slower as the binder amount increased(Fig. 9). It was based on surface strengthversus ink absorption speed results, that thePVOH amount of 10 pph was selected for ourstudies of different CaCO3 pigments.

The binder amount had very minimaleffect on print density in the case of the "PCClarge" structures (Fig. 10). However, thepigment change from "PCC large" to "PCCsmall" at a binder amount of 7 pph clearlyincreased the print density. Therefore, at thesebinder levels and at these short absorptiontimes, the binder swelling is unlikely to closethe larger pores. The “PCC large” structuredetermines the colorant transport on this case.

A higher amount of binder generated asignificant increase in the amount of bleeding.The coatings with 12 pph and 30 pph of PVOHproduced the widest bleeding distance (Fig.10). Interestingly, the bleeding did not increaseindefinitely as the binder amount increased,but rather plateaued, showing the loss of porevolume is likely to be countered by theswelling nature of the PVOH in its ability toabsorb and hold liquid once sufficient PVOHis present. The other explanation can be in theink amount. The ink amount could be so low in

the printing that the absorption time of 220 mscan still be short enough to prevent the onset ofbleeding. The effect of time delays will bediscussed later.

Effect of Pigment VariablesFigure 11 shows the cumulative pore

volume intrusion by mercury of the coatedpapers which had been produced with fivedifferent pigments. The coating layers havepartly penetrated into the base paper structure.All of the coated papers have lower porevolume results in the area of 1-10 µm than thebase paper before coating.

The pore size distributions of the studiedcoatings are illustrated in Fig. 12. Generally,the pore size distributions of the coatings onpaper reflected the main features of those ofthe cakes and of the thin layer chromatograph-ic slides. "MCC large", "PCC large" and "PCCsmall" coatings have small pores, 20-40 nm indiameter, whereas GCC and "MCC small" hadclearly less of such small pores. The coatingswith the two large 2.7 µm diameter pigments(“MCC large” and “PCC large”) had the sameamount of small-scale pores. The GCC coating

had mainly 0.1-0.3 µm diameter pores and“PCC large” had the largest diameter pores,1.0-1.3 µm. "MCC large" and "PCC large" hadboth small and large-scale pores. "PCC small"coating had mainly only small-scale pores.

The absorption speed into the coatedpapers was quickest with “PCC large”, withthe three pigments “MCC large”, “PCC large”and “PCC small” all absorbing within the highspeed range. The slowest absorption was, as tobe expected, with GCC (Fig. 13). The relativerequirements of capillarity and permeabilityare well reflected in these data. The capillarityis provided by the ultrafine pores and the liq-uid transfer by the permeability of the coatinglayer, Ridgway and Gane [20].

The highest print density (Fig. 14) wasachieved with the coating structure on paperderived from "PCC small", and the lowest

Fig. 9. Effect of binder amount on absorp-tion time (DIGAT) for PCC coatings onglass plate.

Fig. 10. Print density (measured from 70%halftone dot field) and bleeding distance ofdye-based ink printed on PCC surfaces asa function of PVOH content.

Fig. 11. The cumulative mercury intrudedpore volume of coated papers. All coatingshad 10 pph of PVOH.

Fig. 12. The pore size distribution curvesfrom mercury porosimetry of studied coat-ed papers. The base paper values havesubtracted from the results. The coatingshave 10 pph PVOH.

Fig. 13. Effect of air permeance on absorp-tion speed (DIGAT).

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from "MCC large". At the same time, "MCClarge" and "PCC large" coatings produced thelowest bleeding values.

DISCUSSIONRole of PVOH Swelling

We can assume, on the basis of mercuryporosimetry results, that PVOH binder can gointo or cap (film over the entry) intra- as wellas the connecting pores, and exists around thelarger inter-particle pores as Boisvert et al.[21] and Wedin et al. [22] assumed in theirstudies. At sufficiently high levels, any interac-tion of the binder with the liquid phase of theink becomes important, not only in the struc-tural modification of pore volume but in thediffusion of liquid through the polymer net-work, effecting swelling in the case of PVOH.

We made a theoretical calculation aboutthe binder swelling effect within the pores.Firstly, we assumed that the binder forms auniform layer on the pore wall (Fig. 16A) andthe thickness of this layer depends on thespecific surface area of the pigment (thickerbinder film at lower specific surface). Thegeometry of the pore is also assumed to besimple, as that the binder forms a uniform

layer on the interior surface of a circular equiv-alent capillary. In this way, we can calculatehow the pore entrance area changes during thebinder swelling process.

PVOH is a hydrophilic binder [10,11].Figure 15 illustrates how a PVOH film(200-250 µm) absorbs water but not non-polarhexane as a function of time. The PVOH filmexhibited a 29.2% swelling when exposed tocyan dye-based ink (Versamark VX5000e)after 5 s, and this value has been used in ourcalculations.

The results of the calculation are illus-trated in Fig. 16. The pore volume and amountof binder represent what is to be found in 1 gof coating structure, as taken from mercuryporosimetry and the coating formulation,respectively. The first line on the left is thepore area when the structure has been formedfrom pigments alone. The addition of 10 pphbinder into the coating results in pore diame-ters being reduced by the absorbed layer ofPVOH by an amount of 13 nm when thePVOH is dry - this being twice the absorbedthickness to describe diameter loss in a cylin-drical pore. The closing point of pore diameterincreases to 24 nm when the binder swellingfound to be 29.2 % occurs on contact withinkjet ink. This can lead to complete blockingof the finest pores, reduction in pore size ingeneral and loss of connectivity in the struc-ture when the binder is concentrated at porenodes. The higher the amount of binder thethicker the binder film is, and thus the effect ofswelling also increases. It seems that theswelling affects most or nearly all of the intra-particle pores. This means that these pores areeffectively acting purely via a binder-swellingliquid uptake mechanism.

So in theory, the binder swelling canaffect the absorption speed of inkjet inksthrough the closing up of small pores. Table 4shows the extent of the time delays that exist inthe inkjet printing machine. At 100 m·min-1,the delay between the first inkjet nozzles to thebeginning of drying is 2.6 s, and to the end ofdrying, 3.8 s. The 5 s time considered in binderswelling is within this same timescale. Theeffect of swelling diminishes when liquid hasless time to affect the binder and therefore wecan assume that swelling has a smaller role athigher printing speeds, but it will not totallydisappear. Thus, diffusion into polymer net-works can occur within the timescale of inkjetprinting, even at high speeds.

How Does PVOH Work withDifferent Types of CalciumCarbonates?

From the optical point of view, "PCCsmall" pigment particles are so fine that theycannot scatter light effectively and in thatsense, the pigment is optically inactive (Fig.17). According to the theory of Kubelka-Munk[23], those particles which have a diameter of

Fig. 14. Effect of coating pigment type on print density and bleeding (distance, measuredwith camera system) of dyes.

Fig 15. Absorption amount of PVOH filmsas a function of time. Assuming that thespecific gravity of water = 1 and PVOH =1.26. The swelling factor is given byabsorption amount. Absorption amount =100 * (mass of liquid absorbed into thefilm/mass of dry binder).

TABLE IVTIME DELAYS IN THE VERSAMARK VX5000E PRINTING PRESS

Speed Speed15 m.min-1 100 m.min-1

Cyan to yellow (CMKY) 5 s 0.8 sYellow to drying beginning 12 s 1.8 sDrying beginning to end 8 s 1.2 sSUMMARY 25 s 3.8 s

Fig. 16. The theoretical calculation ofPVOH swelling effects. The coatingamount was 1 g and density of PVOH 1.26g·cm-3. The swelling of PVOH film was29.2 % after 5 s (cyan dye). Specificsurface area of pigment 10 m2·g-1.

swollen layeron contactwith water-based ink

cylindricalpore

absorbedPVOH

A

B

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the order of the wavelength of light (0.1-1 µm)affect strongly the light scattering. The opacityand light scattering coefficient of the "PCCsmall" coating layer remains lower because thepigment size is so small, 20-30 nm. The resultsagree with other previous results [24,25]. Theother pigments had quite similar opacities onglass plates.

The optical properties of the wholecoated paper dominate the print densityformation. The “PCC small” is virtuallyoptically inert (Fig. 18), whereas the otherpigments strongly affect the light scatteringand therefore the level of print density. Thecorrelation between the pore volume of thewhole paper and print density was in therange of -0.904 − -0.949, whereas correlationbetween the pore volume of the coating layeralone and print density was as low as -0.050 −-0.217. Therefore, the whole paper structureplays a role in generating the print density, notonly the coating layer. This of course dependson the nature of the base paper, but if opticallyinteractive it will contribute to the backgroundlight white noise and so reduce print density,especially of penetrated dye-based inks.

If we consider the case of pigment-based inks found in many new high-speedinkjet devices, the finer intra-particle poreswill selectively exclude the ink pigments, in a

such a way that the colorant will be more con-centrated in the remaining permeable structurebetween the particles, i.e., in the inter-particlepores. Therefore, depending on the amount ofPVOH present, and whether it fills just theintra-particle pores or also the inter-particlepores, we can expect either a greater transferof ink pigment into the underlying base paperor a better ink pigment holdout, respectively,i.e., to achieve good ink pigment holdout, thepermeability has to be reduced. The inkpigments form a filter cake in the moreclosed case and this leads to reduced perme-ability. Therefore, the properties of the wholepaper structure will have either a major orminor role in determining print densitydepending on the level of permeability.

In these experiments, where speciality

coating was applied to a pre-coated paper,the colorants stay in the coating layers and donot penetrate into the base paper (Fig. 19).However, it seems that the colorant is quiteuniformly distributed in the coating layerincluding “MCC large”. Even the cationicnature of the “MCC large” pigment did notfix the colorants in a manner that was visiblydifferently from the anionic pre-coat GCC.This suggests that either (i) the surface area isinsufficient to trap all the colorant, (ii) the inkdoes not encounter the cationic nature of thepigment, or (iii) the coating is too permeable toprovide surface contact for all the colorant asit flows through the structure. There is someslight detectable concentration gradient for thefiner pigments, and this reflects the reducedinternal pore network permeability.

The cross-section in Fig. 20 is of aPVOH film absorbed with cyan ink. It showsthat ink colorants have penetrated significantlyinto the binder film after 30 s absorption time,but not completely. Therefore, the colorantscan be expected to diffuse into the PVOH layertogether with the water. The result is in goodagreement with the observations of Oka et al.[26] and Svanholm et al. [27]. In a coatingstructure, colorants are distributed quiteuniformly through the coating layer. Thiscould be an indication that PVOH has coveredthe pigments and colorants are predominantly

in the PVOH. Therefore, the pigment type orcharge has very minimal effect on colorantlocation when PVOH is used as binder.

The bleeding distance (Fig. 21), however,seems to be dependent on the pore volume ofthe coating layer, but not the pore volume ofthe whole paper. This confirms the lack oftransfer of ink to the base paper for thesecoating layer configurations. Table 5 shows allthe porosity related aspects of coating layercorrelate well with bleeding. In addition, theincrease of surface energy decreases thebleeding distance. If we consider comparingthe results of bleeding measured with dye-

Fig. 17. Opacity of coating layers on theglass plate (100 g·m-2).

Fig. 18. Effect of light scattering coefficienton print density. Correlation: black -0.943and cyan -0.950.

Fig. 19. Cross-section dye-base ink printedsurfaces.

GCC

MCC large

MCC small

PCC small 20 µm

Fig. 20. Cross-section figure of cyan dye-based ink into PVOH film. About 200 µmthick film had sunk into the cyan ink during30 s time.

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based inks versus pigment-based inks, thebleeding distances for pigment-based ink canoften be considered to be less, because thefilter cake formation effect of ink pigmentsprevents sub-surface lateral spread. However,we must recall that this depends strongly onthe permeability characteristics of the coatingand, hence, interaction with the base paper.

The large porosity "MCC large" and"PCC large" coatings had fine pores of similaramounts and sizes, but in the large-size area ofthe bimodal distribution (inter-particle pores),the “PCC large” has ~1.2 µm diameter poreswhereas the “MCC large” has smaller inter-par-ticle pores at 0.8 µm. The larger pores of PCCallow ink to penetrate more quickly into thecoating structure and therefore the bleedingformed is less. In addition, the binder swellinginfluences more strongly in the MCC pigmentcoating than in the PCC. The thickness ofbinder layer is higher on the MCC coatingbecause the specific surface area of pigment islower.

The PVOH swelling phenomenon shouldhave a strong effect on GCC and "MCC small"coatings, which have less of the ultrafine 20-60nm diameter pores than the other pigments. Asthe “MCC small” does have some more of thesmaller pores than GCC, so the swelling shouldaffect mostly these pores, rendering them inef-fective. The bleeding values are indeed quitesimilar.

Comparing the “MCC small” and “PCCsmall” coatings, the porosities are quite similar,but “PCC small” has more of the finest pores.The “MCC small” coating absorbs ink clearlyslower than “PCC small”. The binder swellingcloses up pores during the ink absorptionprocess. It seems that in our medium porosityarea, the coating with smaller pores give lessbleeding. This illustrates that the bleeding effectis surface absorption rapidity defined.

One aspect that we have not yet discussedis the web drying on the high-speed inkjet press.This accelerates the inkjet ink drying processand so affects the degree of ink penetration.However, we propose that a more importantaspect of current dryers is in the web behaviourin and after the dryer when the rolled printedpaper becomes unwound and exposed to thenew environment. The print quality of the papershould remain high after the opening of itsprinted reel. Furthermore, if the ink has beeninsufficiently dried before the rewinding state,the ink can transfer onto the back side of thepaper and the set-off problem can be expectedto become more obvious.

Effect of Binder Amount"PCC large" and "PCC small" pigments

were studied more closely by using differentPVOH amounts. All the print density resultsare very near each other (Fig. 22). The cross-section images from printed papers (Fig. 23)

indicate that the colorants of ink distributewithin the coating layer at 7 pph PVOH and atthese coat weights do not penetrate into thebase paper. At 30 pph PVOH in the coating,colorants are concentrated more in the toplayer of the coating. However, there are placeswhere the colorants have moved into the basepaper due to the cracks in the coating, and this

Fig. 21. Bleeding of the studied coatedsurfaces. Bleeding measured with high-resolution camera.

TABLE VCORRELATIONS BETWEEN BLEEDING DISTANCES AND PORE STRUCTURE PROPERTIES

Paper property Bleeding distanceMagenta on Black on Black on

cyan cyan yellow

Pore volume of coating layer -0.898 -0.951 -0.943Ink absorption speed (cyan) 0.653 0.745 0.994Air permeance -0.575 -0.786 -0.883Surface energy of coated paper* -0.653 -0.724 -0.978Pore volume of whole paper -0.210 0.063 -0.104*Surface energy as defined by the sessile drop model, albeit a gross approximation for absorbent surfaces.

Fig. 24. Mottling of cyan ink (70% halftonedot area) printed surfaces.

Fig. 25. Bleeding distance against absorp-tion time. Measurement by scannersystem and absorption time with DIGAT(8 g·m-2 cyan dye).

Fig. 23. Cross-section images of coatingscontaining different amounts of PVOH,that have been printed with cyan dyes.

7 pph PVOH

30 pph PVOH

Fig. 22. Connections between print densityand air permeance as a function of PVOHamount.

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decreases the print density as well as increasesthe small scale mottling values (Fig 24). Theresults of light scattering (S-coefficient 52.9-54.1 m2kg-1) from all the papers are very neareach other. Therefore, knowing that thepigments have very different light scattering,the whole paper properties are dominant in thisrespect.

Again, the bleeding distance depends onthe fine pore coating properties. Therefore, notunsurprisingly, the least bleeding distance isreached with the lowest binder content coating(Fig. 25). The higher binder amount increasesthe bleeding.

Figure 26 shows that absorption timegets longer when binder amount increasesfrom 7 pph to 12 pph. However, the 30 pph ofbinder is on the same level as 12 pph. Theseresults are similar to those of Nilsson et al.[12]. When the binder amount is lower the inkmoves into the coating layer mainly by

capillary flow. At 30 pph binder amount,smaller capillaries are already closed and thedominating liquid transfer mechanism is diffu-sion in the binder layer, as Gane et al. [28]indicate for their case of swelling polyacrylatedispersant covering the pigment surface.However, once a PVOH threshold amount isreached, specific for each pigment, this diffu-sion dominates and so provides a constantabsorption speed from that binder level

upwards (Fig. 26). The threshold point for theinfluence of binder swelling within these coat-ings should be expected at a binder amount of12 pph.

Another significant problem with the 30pph coating is the coating layer distribution.The SEM image (Fig. 27) shows that the 30pph PVOH coating layer is not very uniformand there are places where the binderhas formed film-like layers which exhibitsignificant clumping. As a result, holes distrib-uted across the film-like layer can also be seen.

Although, the increase of PVOH amountcloses up the coating structure, at higheramounts of PVOH the shrinkage associatedwith the soluble binder formulation causes anuneven coating layer structure and this isreflected in a deterioration of print quality.

CONCLUSIONSThe major controlling parameters for

liquid ink absorption into porous inkjet coatingstructures are confirmed to be capillarity andpermeability in relation to the fine pores andthe larger interconnected pores, respectively.A quick absorption of ink can be reached evenwith medium porosity coating structures,provided the coating has a lot of nano-sizepores. In the case of porous pigment particles,such as modified or structured calcium carbon-ate, the fine pores are associated with intra-particle pores and the larger pores are definedby the particle packing.

The role of PVOH binder is shown to becomplex both in relation to pore structuremodification and its affect on liquid uptake bydiffusion. PVOH swells during the diffusionprocess. Although the volume of liquid imbibi-tion related to PVOH swelling can be small atlow binder levels, it acts to dominate the inkinteraction with the pigment surface and thefine pore structure as binder levels increase,even at those levels required to prevent dust-ing. It seems that the colorants of dye-basedinks locate more uniformly in the coatingstructure because PVOH covers the pigmentsurfaces and masks any surface cationicity ofthe pigment. The dye follows the aqueous liq-uid as it diffuses into the surface layer ofPVOH. The impact of diffusion processes iseven seen at high printing speeds, though theshorter the time to reach dryers, if present, theless is the volume of liquid involved. Theswelling affects especially the accessibility tothe intra-pigment pores of porous pigments.

The coating layer properties have a con-nection to print quality via the ink absorptionspeed and volume capacity. An ink absorptionthat is too slow at the coating surface meansthat the colorants have more time to mixtogether and bleeding problems are morevisible. The surface energy of the coating layeralso affects the colorant movement.

The optical properties of the wholecoated paper dominate the achievable printdensity. This is because inkjet coating

pigments are generally designed to have lowscattering potential and so the base paper, ifoptically active, has a strong show througheffect through the coating layer.

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Fig. 27. Topographic image (SEM) of 30pph PVOH “PCC large” pigment coating.

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Film-like layer

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REFERENCE: LAMMINMÄKI, T., KETTLE, J.P., PUUKKO, P., GANE, P.A.C. and RIDGWAY,C., Inkjet Print Quality: The Role of Polyvinyl Alcohol in Speciality CaCO3 Coatings, Journal ofPulp and Paper Science, 35(3-4):137-147 2009. Paper offered as a contribution to the Journal ofPulp and Paper Science. Not to be reproduced without permission from the Pulp and PaperTechnical Association of Canada. Manuscript received July 15, 2009; revised manuscriptapproved for publication by the Review Panel November 16, 2009.

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