Fluid Transport in Compacted Porous Talc Blocks

Copyright c© 2008 Tech Science Press FDMP, vol.4, no.2, pp.85-98, 2008

Fluid Transport in Compacted Porous Talc Blocks

Viveca Wallqvist1, Per M. Claesson2, Agne Swerin1, Patrick A. C. Gane3 ,4

Cathy Ridgway3 and Joachim Schoelkopf3

(Communicated by Gustav Amberg)

Abstract: It has been shown that talc powdercan be compacted into tablets with a preferred ori-entation of the platelets. The tablets can be ob-tained with different controlled porosity depend-ing on pressing methods and applied pressure.The tablets can be obtained with or without ad-ditives, which may, in turn, be adsorbed. The ori-entation of the high aspect ratio platy talc, the sur-face chemistry imparted by the additives and thetransported fluid influence the imbibition and per-meation rates. Non-polar hexadecane displays ahigher imbibition and permeability than water forall particulate orientations during short timescaleabsorption, likely due to the oleophilic nature oftalc, and thus a more complete filling of the poresfor non-polar liquids is to be expected. At longertimescales water is imbibed either at a similar rateto hexadecane or faster depending on the surfacechemistry generated by additives leading to hy-drophilicity. The swelling of the added polymersused to create wettability leads to break-up of thestructure and exposure of hydrophilic surfaces formore rapid imbibition. It is not possible, there-fore, to measure reliably the water uptake param-eters when talc is fully dispersed with surfactants.Furthermore, dispersing agents tend to contributeto the blocking of pores and throats in the swollenstate, and so a limitation in total imbibed volumeoccurs. The permeability under pressure is alsoinhibited by additives, which supports the sugges-tion of partial blockage of the pores and throats.

1 YKI, Ytkemiska Institutet AB/Institute for Surface Chem-istry, Stockholm, Sweden

2 Department of Chemistry, Surface chemistry, Royal Insti-tute of Technology, Stockholm, Sweden

3 Omya Development AG, Research and Technology Ser-vices, Oftringen, Switzerland

4 Helsinki University of Technology, Laboratory of Paperand Printing Technology, TKK, Finland

When the individual talc crystal c axes, definingthe perpendicular to the [001] planes, are oriented90◦ to the primary average liquid flow direction,i.e. are oriented in a planar configuration to theflow, imbibition and permeation of wetting liquidare increased. This is assumed to be due to de-creased tortuosity, provided the liquid is wettingin respect to the oriented edge surface. However,non-wetting liquids in respect to all, or geometri-cally dependent, orientations are subject also tothe surface chemistry presented by the orienta-tion, i.e. whether the talc is primarily display-ing OH-groups or not, or adsorbed species aidingwetting by the liquid. Measurements where ad-ditives are not strongly adsorbed are complicatedby the solubility of some wetting and dispersingagents.

Keyword: talc, porous media, imbibition, sur-face chemistry, absorption, permeability.

1 Introduction

Talc is a naturally occurring mineral with thechemical formula Mg3Si4O10(OH)2. Pure talcmineral has hydrophobic properties due to its pre-ferred breaking along laminar planes [Fig. 1],which exposes the non-polar and essentially in-ert basal plane. However, since the transplanaredges contain exposed OH groups, the measuredhydrophobicity of pure talc is normally vary-ing depending on particle size, grinding/breakingmethod and exposure time to air after cleavage.Talc is also known to contain impurities of otherminerals, giving it a wide range of colours in addi-tion to the white colour of pure talc. Water contactangles of 66-90◦ (for example see Michot, Vil-liéras, Franéois, Yvon, Le Dred and Cases (1994),

86 Copyright c© 2008 Tech Science Press FDMP, vol.4, no.2, pp.85-98, 2008

Yildirim (2001)) have been mentioned in the liter-ature, the wide span quoted being due to the abovementioned differences between talc samples. Talcis used widely in the cosmetic, plastics and paperindustries, in the latter mainly as filler and for thecontrol of pitch and stickies, but also as a coatingpigment.

Figure 1: Talc structure

The platy structure of talc originates from the pla-nar arrangement of the crystal habit, the planesbeing held together by weak van der Waals forces.As a result, ground talc consists of anisometricplatelets [Fig. 2]. The platyness and hydrophobic-ity of talc combine to affect the properties exhib-ited when liquids contact the surface during im-bibition throughout the porous structure createdwhen talc platelets are packed together. Knowl-edge of these effects is relevant for paper proper-ties, such as ink penetration and paper sizing, butalso for other talc applications, such as in tablets

and cosmetics.

Figure 2: Scanning electron microscope image ofa talc platy structure

In this study the imbibition, permeation andporosity of compacted talc tablets have been mea-sured. Such methods mimic on a measurable scalethe structures encountered in paper coatings, fol-lowing the work of Schoelkopf, Gane, Ridgwayand Matthews (2002), in which calcium carbon-ate was the previous focus of analysis. Influ-ences of tablet pressing method, added dispersingchemicals, interacting fluid polarity and its pla-nar/transplanar interaction with oriented talc sam-ples have been considered.

2 Experimental

2.1 Materials

Commercial talc P05 from Mondo Minerals(Omya AG) was used in all tests. XRD analy-sis using a Bruker AXS D8 Advanced XRD sys-tem showed it to contain the minerals talc, chlo-rite and magnesite, which was confirmed also byFTIR (using a Perkin Elmer Spectrum One Spec-trometer) analysis. XRF (using a ARL 9400 Se-quential XRF) gave a talc content of ∼ 94 %.

Generic wetting and dispersing agents were ob-tained from Omya AG. The wetting agent con-sisted of a poly(ethylene oxide)-poly(propyleneoxide) triblock copolymer with a MW of 2500g/mol. The dispersing agent consisted of a stan-dard sodium polyacrylate with a MW of 4000g/mol.

Fluid Transport in Compacted Porous Talc Blocks 87

Figure 3: Wet press for tablet making of mineralpowders

2.2 Methods

2.2.1 Compacted block preparation

Talc was used in two forms, either as provided,i.e. alone without additional chemical treatment(defined as “undispersed”), or with added wetting(1.1 wt%) and dispersing agents (0.2 wt%) (de-fined here as “dispersed”). Two tablet consolida-tion methods have been used; wet pressing anddry pressing. In the wet pressing method, sus-pensions of talc were pressure filtered in an alu-minium cell (Fig. 3) using an over pressure of 25bar. The resulting block was dried overnight in anoven at 60◦C. The wet pressed tablets had a diam-eter of 4 cm and a height of approximately 3 cmThe tablets made by the dry pressing method wereprepared by applying a force of 500 kN, equiv-alent to a pressure of 23.5 MPa, to the powdercontained in a stainless steel cell (Fig 4). For thedry pressed tablets made from untreated P05 thepowder was sprayed with a very small amount ofwater before the consolidation to aid compactionand integrity of the final tablet. Prior to the for-mation of dry pressed tablets made from P05 withdispersing and wetting agents, a slurry of the pre-mixed components was dried overnight in an ovenat 60◦C, thereafter it was ground to a powder. Thispowder was then consolidated in the dry press aspreviously described. The dry pressed tablets hada diameter of 4.5 cm and a height of approxi-amtely 3 cm.

A part of each sample was studied by mercury

Figure 4: Dry press for tablet making of mineralpowders

porosimetry, the remainder being cut and groundto form blocks of approximately 1 cm3 using arotary disc grinder, which were then used for theliquid interaction experiments.

2.2.2 XRD and platelet orientation

In order to investigate the orientation propertiesof the samples in respect to liquid imbibitionand permeation, the particulate orientation in theground blocks was analysed by X-ray diffraction(XRD) from two orthogonal directions using aBruker AXS D8 Advanced XRD system. Thismethod was used to confirm the expected orien-tation of the platelets, i.e. with their planes par-allel to the wet filter plane and orthogonal to thepressing direction, respectively.

2.2.3 Porosimetry

Laplace-defined pore diameters, naturally includ-ing pore shielding, and pore volumes of theconsolidated talc blocks were determined usingmercury porosimetry (Micromeritics Autopore IVporosimeter) and corrected for pressure chamberexpansion, thermal expansion of mercury, com-pression of mercury and the compression of thetalc skeletal material using the method describedby Gane, Kettle, Matthews and Ridgway (1996),and adopted in the software Pore-Comp c©. Themaximum applied pressure of mercury was 414MPa.

2.2.4 Contact angles

In order to obtain a rough estimation of the sur-face chemistry properties of the samples, contact


angles for the chosen liquids applied to the tabletswere measured using a dynamic contact angle andabsorption tester (FibroDAT 1100, FibroSystemsAB, Sweden). Once again, orientation of theplatelets exposed to the liquid was noted.

2.2.5 Imbibition

Imbibition was measured following the method ofwicking as described by Schoelkopf, Gane, Ridg-way and Matthews (2002). In order to reduce arte-facts caused by external wetting of the outer sur-faces of the formed tablets during imbibition, athin barrier of silicone (Dow Corning 1-2577 lowVOC conformal coating c©) was applied aroundthe base of the vertical edges of the cubic block.The remaining edges were not covered in order toallow the escape of air during liquid uptake and tominimise the interaction between the silicone waxand the liquid.

The imbibed weight of liquid was measured withan automated microbalance (Mettler Toledo AT460) capable of 10 measurements per second (Fig.5). The cubic sample with known dimensions wasclamped in a special sample holder and slowlybrought into contact with the liquid (water or hex-adecane in this study), held in a reservoir act-ing as a supersource placed on the balance, i.e.the absorbed volume was measured as the massloss, corrected for density, from the balance overtime. For water, evaporation from the reservoirwas taken into account by first measuring the wa-ter loss without any sample present and fitting theobtained values to a curve accounting for humid-ity build-up etc. This curve was then subtractedfrom the resulting absorption curves. No correc-tion was necessary for hexadecane due to its lowvapour pressure.

2.2.6 Permeability

For the permeation measurements, the cubicblocks were first embedded in a resin. The em-bedding was performed by placing the consol-idated talc block with known dimensions in aPTFE mould (Prüfmaschinen AG, Switzerland)with an inner diameter of 3 cm and applyingresin (Technovit 4000, Hereaus Kulzer, Germany)around it. When the resin was hardened, the edges

Figure 5: Absorption unit

of the sample were smoothed using a rotary discgrinder. The sample was allowed to saturate tothe point of no further liquid imbibition using thechosen permeation liquid (hexadecane or water)before permeability was measured.

The permeation apparatus consisted of an alu-minium cell filled with the permeating liquid ontop of the sample (Fig. 6). The cell was placedover the same automated microbalance, describedabove, used in the imbibition experiment. Anoverpressure of 7 bar was applied to the cell, andthe permeation was measured as the mass increaseon the balance over time. For the water perme-ation measurements there was already water inthe bowl on the balance at the start of the mea-surement and evaporation was further taken intoaccount as described above.

3 Results and discussion

3.1 Compacted block preparation

The dry press consolidation produced well-formed stable tablets for both dry P05 (undis-persed) and P05 with wetting and dispersingagents (dispersed). The wet press consolida-tion gave similarly stable tablets for dispersedP05, but for pure undispersed P05 the tablets re-tained a brittle nature, and could not be effec-tively ground into the desired shape. Accord-ing to the work of Wallqvist, Claesson, Swerin,Schoelkopf and Gane (2006), in which atomicforce microscopy was used to determine the spa-tial extent of the hydrophobic force when expe-


Figure 6: Permeation unit. Cross-section of thealuminium cell identifying the different parts as 1)lid with pressure inlet, 2) sealing O-rings, 3) liq-uid cell with outer diameter 4 cm, 4) talc sampleembedded in resin disc of diameter 3 cm, 5) fix-ing ring compressing the O-ring which seals theresin disc, 6) security shroud and drop collector,7) drop captor (teflon tube)

rienced by an oleophilic surface approaching andretreating from that of talc in water, it can be as-sumed that the formation of water-air menisci canbe proposed. This finding provides some initialevidence for the action of adsorbed air within theconfines of surrounding water, as might well befound in a suspension of undispersed talc. Thiseffect is related to the surface tension character-istics of the water-air interface upon approach ofthe talc platelets to each other in the wet compres-sion environment, such that the air layers may co-alesce to form elastic entrapped volume betweenthe talc platelets, which in turn act to expel wa-ter from the interface and so prevent an intimatecontact zone. The resulting lack of platelet con-tact results in an inability to withstand mechani-cal forces and so the loss of integrity of the tabletduring machining/grinding.

3.2 Orientation

It has been earlier demonstrated that pigment ori-entation can be investigated by X-ray diffraction,see Gane, Hooper and Grunwald (1995). The ori-

entation of the talc flakes within the tablets couldbe defined as the ratio of the intensity of the peaks[001] and [020]. When this ratio was comparedacross two different directions of the tablet, theorientation effects could be seen, despite the factthat the [020] peak was disturbed by the [110] and[-110] peaks. Tab. 1 displays the obtained values.It can be seen that there is a clear difference in thediffraction peak ratio depending on tablet orienta-tion.

Table 1: Peak height ratio [001]/[020] for X-raydiffraction spectra taken from two directions ofpressed talc blocks

Top SideUndispersed, dry pressed 8.40 1.08Dispersed, dry pressed 33.25 0.15Dispersed, wet pressed 1.96 0.67

The peak ratios indicate that the platy structureof talc resulted in blocks having a layered struc-ture with the plates oriented with the [001] planesfacing the pressure direction. This allowed com-parisons between imbibition and permeation stud-ies of samples where the [001] plane was facingthe liquid front (transplanar) or by using samplesoriented 90◦ to this direction (planar). The wetpressed samples, although showing some order-ing were seen to be somewhat more disorientedthan the dry pressed samples. This is probablyrelated to the reduced wet pressure and the neces-sary flow of liquid out of the sample during prepa-ration, thus creating a favoured path of reducedresistance.

The orientation presentation to liquid flow wastherefore defined as trans when the c crystal axis,perpendicular to the platelet planes, was parallelto the flow, and planar when the c axis was or-thogonal to the liquid flow, as shown schemati-cally in Fig. 7.

3.3 Porosimetry

The porosity, φ , fully corrected for compressibil-ity was found to be 50 % for the wet pressed dis-persed talc tablets, 22 % for the dry pressed undis-persed talc tablets and 19 % for the dry pressed


Figure 7: Orientation of talc platelets in the transand planar configurations, respectively, showingthe hydrophilic edges and oleophilic platelet faces

dispersed talc tablets. The difference between thedry and wet pressing methods is primarily due tothe lower consolidation pressure employed in thewet press, i.e. that applied by the permeating wa-ter expressed by the gas overpressure. The differ-ence between the undispersed and the dispersedtalc tablets is assumed to be due to two main fac-tors, firstly, and predominantly, the tighter pack-ing derived from well-dispersed particles in water,as was the case in the pre-dispersing in water ofthe dispersed talc prior to drying, and, secondly,the potential for the additives to block some pores,each effect leading to a lower porosity. The poreblocking effect is expected to become more dom-inant when water is later used as the interactingliquid due to water layer adsorption and swellingof the wetting and dispersing agents.

In Fig. 8, the pore volume as a function ofLaplace-derived pore diameter is displayed forthe different samples. It can be seen that thepore sizes of the dry pressed undispersed talc andthe dry pressed dispersed talc are approximatelythe same, but that the volume of the pores ismarginally less in presence of additives. It canbe concluded, therefore, that the pore distribu-tion with and without additives is similar whenintruded by mercury. Effects of breakthrough ofthe mercury into pores, otherwise blocked at theirentry points by the polymers, cannot, however,be determined. For the dry pressed samples the

mean pore size is about 0.05 μm, but a range ofpore sizes between 0.1 and 2 μm is also present.This suggests the presence of regions where theplatelets are perhaps stacked irregularly, result-ing in crack-like voids in addition to the finerpore structure. The wet pressed dispersed sam-ple contains significantly larger pores, the peakin the distribution is at the pore size of 0.45 μm,and a greater associated pore volume than the drypressed ones, as illustrated in Fig. 8. The detailsare summarised in Tab. 2.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.001 0.01 0.1 1 10 100diameter [d ] /

pore

vol

ume

[dV

/d(lo

g d)]

/ cm

3 g-1 Dry pressed talc

Dry pressed dispersed talc

Wet pressed dispersed talc

Figure 8: Pore volumes of dry pressed undis-persed talc, dry pressed dispersed talc and wetpressed dispersed talc

Table 2: Pore structure summary of tablets

Dry pressed Wet pressedPorosity / % 20 50Average pore di-ameter / μm

0.05 0.45

3.4 Contact angles

Contact angles were measured by placing 4 μ ldroplets on the pressed tablets. The high-speedcamera on the contact angle and absorption testerwas able to take one image every 0.001 s. Af-ter having evaluated the images, the image 0.028s after droplet-surface contact was chosen for thereported contact angle. At this time the oscilla-tions of the droplet due to the droplet-surface im-pact had stopped and, in the case of water only,the imbibition into the structure had not yet had


a great effect on the droplet shape. In Tab. 3,the water contact angles for the various samplesare reported. For hexadecane, no values could bereliably obtained since the imbibition began toosoon and fast for a spherical droplet to form onthe surface after impact.

Table 3: Water contact angles for pressed talcsamples

Sample Water contact angle ◦

Undispersed, drypressed

75

(transplanar orienta-tion at top face)

46

Dispersed, dry pressed 66

Surface wetting of powder samples is notoriouslydifficult to define due to factors such as inhomo-geneous surface chemistry, surface microrough-ness, pore structure etc., though in the case ofcoarse pigments, especially when displaying ori-entated platelets, and hence greater surface conti-nuity, the approximations used do carry some va-lidity. It is noted, therefore, that the contact an-gle values give only a rough estimation of the sur-face chemistry properties of the samples, since thedeviations from perfect surfaces are significant,for example they were increasingly inhomoge-neous when rotated (edge face), and microrough– clearly indicated by the non-spherical natureof the droplets. For similar reasons the dropletmethod was abandoned for determining imbibi-tion.

3.5 Permeation

The water permeation could not be measured forthe dispersed talc samples containing wetting anddispersing agents. The reason for this was that thetablet slowly disintegrated in water, creating largeholes in the tablet which prevented the pressure tobe maintained.

The permeability, k, of a sample can be expressedusing the following equation according to Darcynormalised for porosity, see Darcy (1856):

k =d(V (t)/A)

dtηl

ΔP=

qηlΔP

(1)

wherek = permeability/m2

V(t) = volume of liquid passing through thesample in time t/m3

η = viscosity of liquid/Pa sA = sample cross-sectional area/m2

l = length of sample/mΔP = applied pressure difference/Paq = flux per unit area/ms−1

The material can also be described by an equiva-lent tubular radius, rm2, through which the sameflux would flow under Poiseuille conditions aswould flow perpendicularly through one squaremetre of the sample, given by:

rm2 =(

8Qm2ηlπΔP

)0.25

(2)

whereQm2 = normalised flux (across unit area)/m3s−1

rm2 = equivalent tubular radius/m

3.5.1 Effect of Liquid on Permeability

In Fig. 9 and Fig. 10 it can be seen that forthe undispersed samples, hexadecane flows some-what faster under pressure than water, and theequivalent flow radius is also greater for hexade-cane. Since the sample is saturated with the liq-uid before the experiment, i.e. the liquid has en-tered all those pores that it can wet, the perme-ability is primarily affected by the pore structurethat is filled with liquid, i.e. its pore size distribu-tion and connectivity associated with liquid, sincethe viscosities of the two liquids are similar. It islikely, therefore, that the saturation step by pre-absorbing water is microscopically incomplete.This may be due to the existence of air pocketswithin the structure caused by the higher contactangle for water, leading to unfilled large pores thatare entered only by small throats (imbibition ter-minating geometries), see Kent and Lyne (1989),and the potential of air coalescence under the ac-tion of the hydrophobic force when surroundedby the water-air interface menisci, see Wallqvist,Claesson, Swerin, Schoelkopf and Gane (2006).The difference is maintained also for the samples


oriented with the platelets parallel to the flow (i.e.in planar orientation).

Permeability, k [m2]

0.0E+00

2.0E-16

4.0E-16

6.0E-16

8.0E-16

1.0E-15

1.2E-15

ud ud dd dd wd wd

Sample

hd

trans planar trans planar trans planar

H2O0.0E+00

4.0E-17

8.0E-17

1.2E-16

1.6E-16

2.0E-16

ud ud dd dd

Sample

trans planar trans planar

Permeability, k [m2]

0.0E+00

2.0E-16

4.0E-16

6.0E-16

8.0E-16

1.0E-15

1.2E-15

ud ud dd dd wd wd

Sample

hd


H2O0.0E+00

4.0E-17

8.0E-17

1.2E-16

1.6E-16

2.0E-16

ud ud dd dd

Sample

trans planar trans planar

Figure 9: Permeability, k, for pressed talc sam-ples. Abbreviations: ud= undispersed and drypressed, dd = dry pressed and dispersed, wd = wetpressed and dispersed, hd = hexadecane

Permeability radius, r m2 [m]

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

ud ud dd dd wd wd

Sample

hdH2O


Figure 10: Equivalent Poiseuille permeability ra-dius, r, for pressed talc samples. Abbreviations:ud= undispersed and dry pressed, dd = dry pressedand dispersed, wd = wet pressed and dispersed, hd= hexadecane

Direct observation of the samples after exposureto liquid “saturation” readily supports the hypoth-esis of incomplete filling of the tablets by water.The photographs shown in Fig. 11 illustrate thelighter appearance of the water-imbibed sample,indicating clearly the presence of entrapped air atthe mineral boundary leading to higher light scat-tering. The dark nature of the hexadecane-filledsample shows the complete filling of all availablepores.

Figure 11: Photographs of the imbibition-filledsamples showing on the left the lack of completefilling for the water exposed sample (lighter), re-lated to the light scattering of entrapped air, versuson the right the more complete filling for hexade-cane (darker)

3.5.2 Orientation of the talc in respect to liquidflow

Fig. 9 and Fig. 10 also display a higher perme-ability, and larger equivalent radius, for the sam-ples in planar orientation than for samples havingthe [001] planes oriented perpendicular to the liq-uid flow direction (trans). One reason for this isthat the parallel alignment of the platelets with thedirection of flow reduces the tortuosity of the flowchannels, where tortuosity is defined as the micro-scopic internal path length divided by the macroexternal dimension of the sample. This is furthersupported by the highly porous sample made fromthe wet pressing of dispersed talc, with its less or-dered structure, in which the effect of sample ori-entation is greatly reduced. Since the sample ispre-saturated to the point allowed by wetting, thesurface chemistry of the talc cannot play a roleother than due to adsorbed layer formation, whichwould act to slow down permeation and not speedit up.

3.5.3 Wetting and dispersing agents

In Fig. 9 it can be seen that the wet pressed dis-persed sample, containing wetting and dispersingagents, displays the highest permeability. Since,in Fig. 10, the equivalent radius follows the sametrend, it can be concluded that surface chemistryis not affecting permeability, as would be ex-pected since the structure is pre-saturated and no


0.993

0.994

0.995

0.996

0.997

0.998

0.999

1.000

0 10 20 30 40 50sqrt time [s0.5]

V/ A

[m]

ud-hd trans

ud-H2O trans

ud-hd planar

ud-H2O planar

dd-hd trans

dd-H2O trans

dd-hd planar

dd-H2O planar

wd-hd trans

wd-H2O trans

wd-hd planar

wd-H2O planar

Figure 12: Imbibition into pressed talc samples for water and hexadecane. Abbreviations: ud= undispersedand dry pressed, dd = dry pressed and dispersed, wd = wet pressed and dispersed, hd = hexadecane, H2O= water, trans = transplanar imbibition, planar = turned 90◦ for imbibition. The y axis is arbitrarily set to astarting point of 1, units are absolute and represent amount from this starting point absorbing into the sample

wetting fronts are supposed to exist. The only ac-cepted effect of the polymers’ presence would bea swelling in the presence of adsorbed water, andsince permeability could not be measured due tothis very effect, it cannot be quantified. Further-more, as we saw, the porosity of the wet pressedsample is significantly greater than for the drypressed samples, and so it is to be expected thatpermeability for similar structures will be greater.

3.6 Imbibition

Imbibition curves were plotted, after definingtime, t, as the time after initial contact, as liquidvolume/sample area exposed towards the liquidfront against t0.5. The imbibition curves consistedof two regions; the initial short-time absorption(approximately 0.7 - 1.8 s after contact) and thelong-time uptake, which in this study was mea-sured after 1 min. Examples of imbibition curvesfor all combinations of sample and liquid are dis-played in Fig. 12. The imbibition speed, ξ , withrespect to t0.5, can be additionally expressed as aDarcy rate, reflected by the following expression:

ξ =

d(V (t)/A)d(

√t)

φ(3)

whereξ = imbibition speed/m*s−0.5

V(t) = volume of liquid passing through thesample in time t/m3

A = sample cross-sectional area/m2

Φ = sample porosity (as determined by mercuryporosimetry)

This expression, represents the idealised distancethat a straight, even wetting front would pass intothe sample of given porosity φ after t0.5. In prac-tice, wetting fronts are not straight but distinctlyuneven due to the preferred pathway dynamics,described amongst others by Schoelkopf, Ganeand Ridgway (2004).

The water imbibition into the dispersed talctablets containing wetting and dispersing agentswas complicated by the fact that they slowly dis-integrated in water, an effect similar to that seenabove during the permeability tests, i.e. they self-dispersed in the water. Despite this disintegra-tion, absorbed volume could be recorded, but thepath length in the sample was proportionally re-duced due to the dispersing action as a function oftime, and this, together with the expansion of thesample pores during disintegration, is assumed


responsible for the deviation from the expectedstraight line relation with the square root of timefor these samples shown in Fig. 12. The disinte-gration and delamination of the samples is illus-trated in Fig. 13. Furthermore, in Fig. 12, it isalso seen that undispersed samples in the planarconfiguration deviate somewhat from the straightline behaviour. This could be explained by thefact that edge pathways for water (the only hy-drophilic pathway) can lead to two phenomena:(i) air entrapment which can then dissolve in wa-ter and so increase "apparent permeability withtime" by permitting more complete filling of thepores and (ii) edge forces may well lead to rear-rangement in the structure if the local compres-sion force used to make the oriented tablets re-tained some clumping effect of particles due tothe fact that the particles were not originally welldispersed.

trans planar

Figure 13: Photographs of the disintegration ef-fect of water on dispersed talc samples showingthe delamination and expansion of pore size

3.6.1 Short-time imbibition

Fig. 14 describes the short-time imbibition ratesfor the various talc samples. Fig. 15 describesthe volume per unit area V/A/φ as a functionof time (flux) during the short-time imbibition,i.e. the Darcy distance rate into the structure thatthe liquid travels during the short-time imbibition,ξ . Among factors that can be identified as influ-encing the short-time imbibition are; pore size,relative pore shape, presented surface chemistry,liquid properties and chemical additives proper-ties. The short timescale imbibition assumes thatPoiseuille flow is not fully established and so per-meability plays only a secondary role.

short time absorption rate [d(V /A )/dt 0.5] / ms-0.5

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

ud ud dd dd wd wd

Sample

hd


H2O

Figure 14: Short-time imbibition for pressed talcsamples. Abbreviations: ud= undispersed and drypressed, dd = dry pressed and dispersed, wd =wet pressed and dispersed, hd = hexadecane, H2O= water, trans = transplanar imbibition, planar =turned 90◦ for imbibition.

3.6.1.1 Liquid effect at short times It can beseen clearly that hexadecane has a faster short-time imbibition rate into the talc structures thanwater, even when wetting agents are present,though the planar absorption for water into dis-persed talc when the porosity is high (wet presseddispersed talc) is almost as fast. Clearly the sur-face chemistry plays a role in determining thewettability by the liquids. As mentioned earlier,this trend for lower contact angle for water on dis-persed talc was also seen in the droplet contactangle.

It has earlier been noted in the literature that ma-terials poorly wetted by water exhibit a waterabsorption rate of about four orders of magni-tude slower than for lower surface tension liq-uids, see Clarke, Blake, Carruthers and Wood-ward (2002). One theory is that the transfer ofliquid from smaller to larger pores is partly in-hibited in the case of low wettability (imbibitionterminating structures), resulting in an only par-tially filled pore structure. This is corroboratedin the present study. It has also been shown thatwhen non-adsorbing liquids are used, the imbibi-tion rate is independent of the liquid, see Labajos-Broncano, Antequera-Barroso, González-Martk.nand Bruque (2006). This indicates that the fasterimbibition of hexadecane is partly due to thehigher affinity of hexadecane for the talc surface


and that adsorption can be proposed. Similar re-sults were obtained in a comparison of hexade-cane and water imbibition on the short timescalefor calcium carbonate structures, see Ridgwayand Gane (2003), where it was assumed that thefiner pores governing the short timescale imbibi-tion rate were effectively less polar in their ex-pression of surface chemistry.

3.6.1.2 Orientation – surface chemistry and ad-sorbed polymers In Fig. 14 and Fig. 15 itis seen that in most cases the samples orientedin the planar direction are the faster imbibing onthe short timescale, and clearly absorb the largestamount of liquid. This is also likely to be sur-face chemistry related as once again the energyloss terms described by permeability, i.e. tortuos-ity, have not yet been established under sufficientflow into the structure. It can be assumed that themore hydrophilic edges create a greater imbibi-tion delay for both hexadecane, due to lack of wet-ting, and for water, due to the time constant of ad-sorption. Interestingly, the dry pressed dispersedsample shows the greater decreasing rate of waterimbibition in comparison with that of hexadecanewhen the platelets are parallel to the flow direc-tion. This may also be due to the likely swellingof the adsorbed polymers on the talc faces in thepresence of water, this being particularly the casefor the polyacrylate portion of the dispersing sys-tem, and since the packing in this sample is tight,the space for flow is restricted. This view is sup-ported by the faster uptake of water into the moreporous, more permeable, wet pressed dispersedsample.

Initial imbibition has been known to oc-cur into interstices in the solid (Labajos-Broncano, Antequera-Barroso, González-Martk.nand Bruque (2006)) and since the layered mate-rial contains such interstices, or cracks, betweenthe platelets, the orientation of these interstices to-ward the liquid flow may also play an enhancingrole.

One should also note that the planar structureswhere the [001] planes are placed parallel to theliquid flow are more likely to have a larger num-ber of pore and throat entries in contact with the

short time absorption rate, ξ [d(V /A /φ )/dt 0.5] / ms-0.5

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

ud ud dd dd wd wd

Sample

hd


H2O

Figure 15: Rate of change of fluid front positionin pressed talc samples during short-time imbi-bition. Abbreviations: ud= undispersed and drypressed, dd = dry pressed and dispersed, wd =wet pressed and dispersed, hd = hexadecane, H2O= water, trans = transplanar imbibition, planar =turned 90◦ for imbibition

liquid.

3.6.1.4 Porosity It has been reported elsewherethat the short-time imbibition is related to the pre-ferred filling of fine structures (Ridgway, Ganeand Schoelkopf (2002)). In the porosimetry re-sults it is observed that the dry pressed samplescontain finer pores than the wet pressed samples.It is further noticed that the pore volume is lowerfor dry pressed samples. In Fig. 14 it can be seenthat the wet pressed sample with the larger poresgenerally imbibes slower in the short timescalethan the samples with smaller pores, thus con-firming the important role of the finer pores inthe short timescale imbibition rate. Furthermore,comparing Fig. 14 and Fig. 15 shows that whenthe platelets are oriented perpendicular to the liq-uid flow (trans), the wet pressed sample has a sim-ilar dynamic as the dry pressed sample with re-spect to hexadecane volume uptake, this despitethe increased pore volume of the wet pressed ma-terial. This leads to the conclusion that within theshort-time imbibition a larger percentage of thedry pressed tablets are filled with liquid than thelarger pore size wet pressed tablets. The implica-tions for permeability are now reinforced, in thatretained air pockets are likely to occur, especiallyin the case of water, even though the material has


been allowed to reach “saturation” for the givenliquid.

3.6.2 Long-time imbibition

Fig. 16 displays the long-time imbibition ratemeasured 1 min after liquid contact. The datafor the longer timescale now include the role ofpermeability, as it can be assumed that Poiseuilleflow is more likely to have been established.Structural effects are now acting as the limitingenergy loss factor opposing the fine scale wettingforce within the material.

long time absorption rate [d(V /A )/dt 0.5] / ms-0.5

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

ud ud dd dd wd wd

Sample

hd


H2O

Figure 16: Long-time imbibition rate after 1 minfor pressed talc samples. Abbreviations: ud=undispersed and dry pressed, dd = dry pressed anddispersed, wd = wet pressed and dispersed, hd =hexadecane, H2O = water, trans = transplanar im-bibition, planar = turned 90◦ for imbibition

3.6.2.1 Effect of liquid It can be seen that thelong-time imbibition rate for water is now fasterthan for hexadecane. Thus, the difference be-tween short-time and long-time imbibition ratefor water is greater than for hexadecane. How-ever, the implications are more far reaching if itcould be shown that the surface chemistry is ef-fectively different for the fine versus the largerpores, although the impact of swelling of polymeris likely to be one important factor. Nonetheless,for the undispersed material the same argumentcan be applied, i.e. the finer pores, most likelystructurally to be related to the nodes where theparticle edges come into contact, and thus the ex-posed OH groups have an important impact on

the comparative hydrophilicity of the edges ver-sus the faces. The progress of water imbibition isthen more likely to follow preferred edge contourscompared with the hexadecane, which would fol-low the more general structure, and, consider-ing the greater capillarity of the smaller edge andnodal features providing the wetting force, theprogress of water might be related to the higherwetting forces experienced, despite there beingless of the overall structure being filled. Finally,we must consider the partial disintegration andopening of the structure of the dispersed samplesin the presence of water. It is proposed that thedelamination of the structure may be related tothe swelling of adsorbed polymer layers as theyadsorb water. This leads to a structural disinte-gration of the tablet, increasing permeability andso increasing the volume uptake of water over thelonger timescale.

3.6.2.2 Orientation The long-time imbibitionrate is higher for the planar samples than for thosehaving the [001] planes oriented perpendicular tothe liquid (trans). Even though the structure mightbe close to saturated, slow redistribution of liq-uid drives the imbibition further. Since more “en-trances” to the structure are available for the pla-nar samples, this redistribution is facilitated forthese samples.

These results fit with the assumption that thelonger term imbibition is predominantly perme-ability controlled, and, as we saw in the caseof permeation, the tortuosity developed by theplatelets being perpendicular to the flow signif-icantly increases the flow resistance towards thewetting front, and vice versa for the planar sam-ple.

3.6.2.3 Wetting and dispersing agents Thesamples containing wetting and dispersing agentsare also orientation dependent in their absorptionproperties, in that the dry pressed sample withstrong orientation displays a greater long-time im-bibition for the case where the platelets are par-allel to the flow compared with the case wherethe platelets are perpendicular to the flow for bothliquids, although the effect is stronger for water


(Fig. 16). Since water also acts to disintegrate thecompacted tablets containing dispersing and wet-ting agents, this dissolution might after some timecreate more voids and thus faster long-time im-bibition rate particularly in the planar orientation.Furthermore, the disintegration itself means thatthe permeability greatly increases as a functionof time and so the resistance to flow diminishes.The additives are also facilitating the redistribu-tion into pores hard to reach since they increasethe adsorption and thus the wettability by water.

Interestingly, due to the increased permeabilityof the less oriented wet pressed sample, the longterm imbibition difference between the two orien-tations is markedly less than for the more highlyoriented samples. This confirms the primary roleof tortuosity, and the fact that the less orientedsample has a less marked tortuosity difference.Once again it can be concluded that the long termimbibition is permeability controlled.

4 Concluding remarks

It has been shown that talc powder can be com-pacted into tablets with a preferred orientation ofthe platelets. The degree of orientation dependson the pressing method, with dry pressing givinggreater orientation than simple pressure filtrationof dispersed talc. The tablets can be obtained withdifferent porosity depending on pressing methodsand pressure. The tablets can be obtained with orwithout additives, assumed adsorbed on the sur-face, such as wetting and dispersing agents.

Imbibition and permeation of chosen liquids canbe performed using the tablet structures. The ori-entation of the talc platelets, the presence or oth-erwise of additives and the nature of the trans-ported fluid influence the imbibition and perme-ation rates, the latter mostly via the lack of sat-uration of the complete structure when polar liq-uid is used. When the [001] planes are oriented90◦ (perpendicular) to the liquid, imbibition andpermeation are decreased. Conversely, the great-est flow rates are obtained when the platelets areparallel to the flow both in the short timescaleresponse and during longer timescale imbibition.This shows that the structural resistance to flowis dominant at long timescales, whereas the lo-

cal surface chemistry and its orientation to the liq-uid are dominant at short timescales, including thefiner pore structure and its prevalence. This is vi-sualised by the OH groups associated with edgesites, or the adsorbed wetting agent and dispers-ing species, together with the nodal effect of finepores created by the arrangement of edges actingas the decisive pore surface chemistry differenti-ating character.

Non-polar hexadecane displays generally a fasterimbibition and permeability than water on theshort timescale, likely due to the increased affin-ity/wetting of hexadecane for the oleophilic talcsurface, and thus a more complete filling of thepores for this liquid. Wetting and dispersingagents though increasing the imbibition of waterby increasing the wettability also contributes toan assumed swelling and blocking of pores andthroats by adsorption of water. This swelling isassumed responsible for the disintegration pro-cess observed for dispersed talc contacting water.

Permeability under pressure is inhibited by tortu-osity, and so is strongly related to platelet orienta-tion. Permeability is seen as the major controllingfactor, acting as a resistance term, in the long termimbibition properties.

Acknowledgement: Omya Development AG isthanked for funding this project and for support-ing cooperation between its industrial Mineraland Surface Chemistry R&D.

References

Clarke, A.; Blake, T.D.; Carruthers, K.; Wood-ward, A. (2002): Spreading and imbibition of liq-uid droplets on porous surfaces. Langmuir, vol.18, pp. 2980-2984.

Darcy, H. (1856): Les fontaines publiques de laville de Dijon: Paris.

Gane, P.A.C.; Hooper, J.J.; Grunwald, A.(1995): Coating conference, Atlanta, pp. 383-390.

Gane, P.A.C.; Kettle, J.; Matthews, P.; Ridg-way, C. (1996): Void space of compressible poly-mer spheres and consolidated calcium carbonatepaper coating formulations. Ind. Eng. Chem.


Res., vol. 35, pp. 1753-1764.

Kent, H.J.; Lyne, M.B. (1989): On the penetra-tion of printing ink into paper. Nordic Pulp andPaper Research Journal, vol. 4, pp. 141-145.

Labajos-Broncano, L.; Antequera-Barroso, J.A.; González-Martk. n, M. L.; Bruque, J. M.(2006): An experimental study about the imbi-bition of aqueous solutions of low concentrationof a non-adsorbable surfactant in a hydrophilicporous medium. Journal of Colloid and InterfaceScience, vol. 301, pp. 323-328.

Michot, L.J.; Villiéras, F.; François, M.; Yvon,J.; Le Dred, R.; Cases, J.M. (1994): The struc-tural microscopic hydrophilicity of talc. Lang-muir, vol. 10, pp. 3765-3773.

Ridgway, C.; Gane, P.A.C. (2003): Bulk densitymeasurement and coating porosity calculation forcoated paper samples. Nordic Pulp and Paper Re-search Journal, vol. 18, pp. 24-31.

Ridgway, C.; Gane, P.A.C.; Schoelkopf, J.(2002): Effects of capillary element aspect ra-tio on the dynamic imbibition within porous net-works. Journal of Colloid and Interface Science,vol. 252, pp. 373-382.

Schoelkopf, J.; Gane, P.A.C.; Ridgway,C. (2004): Observed non-linearity of Darcy-permeability in compacted fine pigment struc-tures. Colloids and Surfaces A, vol. 236, pp. 111-120.

Schoelkopf, J.; Gane, P.A.C.; Ridgway, C.;Matthews, P. (2002): Practical observation of de-viation from Lucas-Washburn scaling in porousmedia. Colloids and Surfaces A, vol. 206, pp.445-454.

Wallqvist, V.; Claesson, P.M.; Swerin, A.;Schoelkopf, J.; Gane, P.A.C. (2006): Interac-tion forces between talc and hydrophobic parti-cles probed by AFM. Colloids and Surfaces A,vol. 277, pp. 183-190.

Yildirim, I. (2001): Surface Free Energy Char-acterization of Powders. PhD-thesis at VirginiaPolytechnic Institute and State University.

Fluid Transport in Compacted Porous Talc Blocks

Documents

Transcript of Fluid Transport in Compacted Porous Talc Blocks