- Email: [email protected]
Viscosity, packing density and optical properties of pigment blends
Colloids and Surfizces, 6 (1983) 158-166 Elsevier Scientific Publiiing Company.
VISCOSITY, PACKING PIGMENT BLENDS
B. ALlNCE
DENSITY
155 Amsterdam
AND
- Printed in The Netherlands
OPTICAL
PROPERTIES
OF
and P. LEPOUTRE
Pulp and Paper Research InstituteoJCunado,
570 St. John% Boulcrwd, Pointe Claim,
P-Q_. H9R 3J9 (Canada) (Received
12 July 1982;
accepted
in final form 7 fktober
1982)
ABSTRACT
The effect of blending particles of different size and shape on the viscosity of aqueous suspenslonr has been evaluatedusing polystyrene spheres (plastic pigment) and plate-like clay particles. At high solids content a minimum viscosity is observed in mixtures of like particles differing in size but not in mixtures of unlike particles. The viscosity of blends is closely related to the packing density of parttcles as Inferred from the critfcal solids volume concentration and the porosity of dry coatings formed from the blends. The variation in light-scattering efficiency of the coating is also related to changer tn the void structure of dry coatings formed
from blends.
INTRODUCTION
The solids content at which paper coating C~L:Ibe applied is often limited by the flow characteristics of the pigment-water system: specifically, a sharp increase in viscosity and a tendency to shear-thickening (dilatancy). The limiting solids con’dnt depends on the type, the size and shape, the mutual interaction and the state of dispersion of pigment particles. Other components normally present, such as binder and flow modifiers, can also considerably affect the flow of the pigment suspension. The intention here is to demonstmte on binder-free suspensfons what effect mixing pigment particles of different shape and size would have on the viscosity of suspensionr at high solids content, as well as on the optical properties of the dry coatings. The viscosity of a pigment suspension reflects the ability of the particles to change their position and to rotati in the flowing suspending medium. At very low concentration, when the particles are sufficient.ly separated, the viscosity of a well-dispersed suspension remains constant with changing flow, i.e., it behaves as a Newtonian system. With increasing concentration the viscosity increases and becomes more dependent on shear flow because of mutual interaction between suspcndcd particles. At a certain concentration, particle crowding results in a steep increase in viscosity. Finally, a critical point is reached when the particles start to touch each other, forming a
0166-6622/83/$03.00
e 1968 Elscvicr Science
Publishers B.V.
156
continuous network with just enough medium to fill the interstices,and.lhe suspension does not fIow anymore, It is important to reognize that this point is a function of the uoirtmefraction of the particles. In case of monodispersed spheres armnged in the most dense configuration (hexagonal rhombohedral) the cdculated volume fraction of maximum packing is 0.74. However, it can be higher in t.hecase of #8oly&s#xmed spheres, sinre the voids between larger spheres can be occupied by the smaller ones. Tnis means that In a suspension of polydispersed spheres at 0.74 volume fraction, not all the liquid will be confined to the voids SD that the particles can rotate and change their positions under shear. The effect of
polydispersion cbnviscosity in crowd&d systems is appment. At. a given high solids concentration there will be more free liquid available and consequently t.heviscosity should be lower than in monodisperse systems. It has been calculated that in bimodd suspensions of spheres a minimum
relative viscodty can be expected at a given mixture of small and large spheres [I]. This minimum should appear above a volume fraction of 0.45 and should become more pronounced with increasing concentration. Several experimental obsrvations of this effect were reported rvith model syshns [ 2,3]. In real systems it is known likewise that, for example, the viscosity of an unfractianated clay suspension is lower than that of any of the individual fractions 143 _ Here WC observe the fiow behaviour at high solids content of blends of spherical pmticks of different sizes, blends of plate-like day particles of different size3 and blends of spherical and plate-like particics. Since the viscosities of crowded suspensions are assumed to be affm t*d by the packing ability of partides, other properties that are also a function of pigment packing mangement are evaluated, These are the critiml solids volume concentration of the suspension, and the prrrosity,light-sc$Meringability and gloss of the dry coatings. EXPERIMl3NTAL
Mu teriuts Plastic pigments: Lylron 2501,2203 and 2101 L&:x fhlonsanto Plastics and Resin Co.) supplied as aqueous dispersion at 50% solids conkent.. Clay Pigments: Hydrite MP, Elydrite UF (Georgia Kaolin Co.) and Hydrafine (Huber Corp.)
Viscosities of aqueous bimodal dispersions of polystyrene spherical particles (plastic pigments), of clay particles and of mixtures of clay and plastic pigments were measured as a function of mixing ratio. All the dispersions were adjusted to pH 8 in order to encourage their stability. The clay
157
dispersions were prepare3 in distilled water but otherwise no cleaning of the commercial samples was attempted. Rrookfield and Hercules viscometers were used to characterize the low and high shear-rate behaviour, respectively. The volume of dispersionsmeasured in Brookfield was 400 cm3. The maximum packing fraction referred to here as critical solids volume concentration (CSVC) was detemained by preblending given fractions of dry clays and adding water until all t.heclay particles were wetted and formed a single coherent ball of apparently dry paste, just beyond the stage of crumbling apart. Whenever possible, coatings were drawn on polyester film and, after drying, were characterizedfor porosity, light-scatteringefficiency and gloss. Porosity was determined from the amount of oil held in the voids after full impregnation followed by removal of tire excess oil by wiping [S] . The light-scattering coefficent (LSC), expressed it; terms of mass per unit area (basis weight), was calculated from reflectances measured again& black and white backgrounds by using Kubclka-hlunk theory and correcting for the supporting polyester film [S] . Gloss was measured with a Hunter Glossmeter at 75”. In the following t.heresults are reported separately for mixtures of plastic pigments, clays and clay-plastic pigment. RESLJL’IX AND
DISCUSSION
Phstic pigment mtxiures
Polystyrene latex particles of three different sizes, 0.6, 0.2 and 0.1 pm in diameter were used. When blended at 50% volume solids content (as supplied) at various mixing ratios, no obvious effect on viscosity was ob-
0
0.1
0.4
0.6
OS
I.0
Fraction01 smz&r latex Pig. 1. Viscosity of blends of plastic pigments 0.1,0.2,0.6 pm in diameter at 60% total solids volume content as a function of mixing ratio Calculated from Row curves (Hercules) at 18,000 s4 shear rate.
Fig. 2. Porosity of dry coatings formed pigments as B function of mixing ratio.
from
mixtures
of 0.1.0.2
and 0.5 rm
Fig. 3. Specific light-scattering coefficient (A = 457 nm) of dry coatings formed mixturcr of 0.1, 0.2 and 0.6 pm plastic pigments as a function of mixing ratio.
plastic
from
In or&r LO evaluate the packing rlensily ench blend wns applied on polyester film (Mylar) with R mdering rod nnrl dlowcd to dry. As shown in Fig. 2, the coatings formed from t-hc monodispr?rsc spheres contnincd 38% void volume wgnrdlrss nf the? sphcrc size. Blendinji rcsulls in dccreas~l prosiCy that is widently morr! pronounced when Ilw difference in sixrs increases. ‘l’ho drop in pclrosity 1s the consequence rlf lhe more effbzient packing and that rcflccted itself in lcrwer viscosity. ‘I’hu light-scnttcriug cfficieucy of the rlry coatings suffers considerably wllsn the smaller q~1wres we addrtd to thr! 0.5 pm p;vliclr?s. In Pig. 3, the light-scattering coefficient of mixWms is l~low the vn!ues itxpzted hy a*sumiug a proportional contribution of the two componsn~ and shown ns lhe broken line. This again is the canscqur?ncc of the higher density of packing t,hat r~ulls in reduced totnl void volumr! (porosity) and size of voida. ‘1’1~ light-scattering efficiency is a function of I\IC porous structure and drops rapillly with dccrcascd void size [7 ] _ In dry coatings forn~~d from polystyrene spheres 0.5 pm in diameter, HIC voids are close Lo the? optimum size fur scAbring the tight. By admixing smaller spheres the reduction of total void volume, accompanied by the formation of smaller and less cffcctive voids, causes a pronounced drop in lhe light-scattering coefficient. Finally, at. hifill fraction of the smaller spheres, although the porosity again increases, a further decrease in void size negates its effect.
139
In Fig. 4 surfaces of coatings formed from some of the blends of 0.5 and 0.2 pm spheres are shown, The disappearance of the larger spheres from the surface obviously affects .!w gloss, as seen in Fig. 5.
Fig. 4. Surface of dry coatings formed from mixtures of plastic pigments 0.2 and 0.5 Nm in diameter obrwvcd by scanning electron microscope.
0
0.)
04
FraCuoflol
OS
OS
I.0
smaser Ia1cr
Fig. 5. 75” gloss of dry coatings formed pigmenls as R function of mixing ratio.
from mixhwea of 0.1, 0.2 and 0.5 pm plastic
Clay mixtures Three grades of clay differing in mean equivalent spherical diameter (e.s.d.) were used: A - e.s.d. 9.5 pm (Hydrite MP) B - e.s_d. 0.7 pm (Hydrafine) O.Z~m (Kydrite UF) c -4Ls.d. Clays A and C are not designed for use in Ihe paper industry but were chosen for demonstration purpost?s_ Although each cJay is already polydisperse, 85 shown in Fig. 6, the effec! of mixing on viticisity is apparent in Fig. 7, The minimum viscosity for the blend AIB is around 60160 mixing ratio, for Mend A/C it is around 67133. The viscosities were measured on suspensions having only 67% solids content by weight or 44% by volume because that wa5 the limit for dispersing the largest clay particles. The amount of dispersant used (determined from the lowest viscosity measured as a function of dispersant added) was 0.08% for A, 0.02% for B and 0.04% for C, The CSVC shown in Fig. S hadicates maxima in the packing densities of blends that are in reasonebJe agreement lvith the minima in the viscosity curvc~. Besides the viscosity at low 5hcar rate, the flow characterMic5, i _e., the shear stress ver5us shear rate, are also affected. The variation of viscosity with shear rate, calculakd from the Hercules rheograms, is shown in Fig. 9 to itlustrate the difference Zn behatiour between pure day B at 44% solids content by volume (67% by weight) and it5 blend with clay A. The rhcogram of pure clay A at this solids content was very erratic. Perhaps of more intire& than tbo viscosities at a given solids content (Fig, 7) arc the viscosities meamrrcd as a function of solids concentration.
Fig. 6. Size distribution
of clays A, B and C (supplied
by the manufacturers).
Fig. 7, Viscosity of blends of clays A, B and C nt 44% solids eontent by volume (67% hy weight) as a funclIon of mixing ratio. Brookfield 100 r.p.m.
161
,
01 0
Fig. 8. Critical solids volume concentration
of clay blends
1
2
1
3
as a funclion
4
6
6 -10’
of mixing ratio.
Fig, 9. Viscosity d blends of clays A and R at different mixing ratirr as a function of shear rate. Solids content 44% by volume (67% by weight).
Fig. 10. VT=osily of clays A tent. Brookfield 10 r.p.m.
and D and their 50/60 mixture as a function of solids con-
In Fig. 10 is an example of clays A and R sc?parately, and of a SO/SO blend of A and B that is within the region of maximum CSVC (Fig. 8) as well as of minimum viscosity (Fig. 7). The benefit of blending is apparent because the solids concentration at which the viscosity starts to rise steeply is higher for the blend than for its components. The natural consequence of this is that the blend can be dispersed (and applied) at higher solids concentration. The solids content at which the suspension started to flow was 44% by volume (67% by weight) for clay A, 61% by volume (73% by weight) for clay B but by preblending of dry clays (60 B/60 A) it was increased to 59% by volume (78.9% by weight).
162
Fig. 11. Porosity of dry coatings formed from mixtures of days A. B and C as a function of mixing ratio. Coatings applied at 44% solIds content by volume (67% by weight). Pure clay A could not be appIk3. Fig. 12. Specific light-scattering coefficient (k = 457 nm) of dry coatings formed from mixtures of clays A, B and C as a funclion of mixing ratio. Application sollds 85 in Fig. 11.
Coatings prepared from the clay blends at 44% solids content by volume (67% by weight) on polyester film differ in porosity and optical properties. The data for pure clay A are Iacking since no satisfactory coating could he applied at this solids level. In F’ig. 11 the porosit-y reaches a minimum that is in the region of maximum CSVC (Fig. 8) and miilimum viscosity (Fig. 7). The change in porosky rcfkcts itdf in the lighbscatterjng efficiency 8s seen in Fig. 12. The reduction in total vofd volume comblned with changing void size distribution causes the Iight-scattcrJng coefficient to decrease. Although the porosity at high content of large particles increases, the voids arc probably larger than the optimum size and, therefore, optically inferior. Consequently the LSC does not *risesimultaneously with the total void volume. Gradud addition of lqe partisles produces a rougher surface and the gloss suffers, as shown in Big. 13.
0
0
01
a4
04
I
08
to
FmcFaIdcbyA
Fig. 13. 7S” gloss of dry coatings formed from mixtures of clays A, B and C as a function of mixing ratio. Application so&k as in Fig. II.
163
0
0.2
0.4
0.6
0.8
Fraction of latex
a.a 0
0.1
0.4
Fredion
0.6
6.6
n.0
ot 0.2 pn tatex
Fig. 14. Visccrsityof blends of clay B and p!astic pigments 0.1, 0.2 and 0.5 )rm as a function of volume mixing ratio. Total solids volume content 55%. Brookfield 10 r.p.m.
Fig. 16. Porwlty of dry coatings formed from mixhwe of clay B and plastic pigment 0.2 pm in diameter, as a function of volume mixing ratio. Coatings applied at 55% solids
volume
contenk
Clay-plcrslic pigment
mixtures
Blends of particles differing in shape, Le., plate-like clay and spherical
plastic pigments, do not follow the trend in viscosity (i.e., the presence of a minimum at high solids) observed with mixtures of like-shaped particles, On
the contrary, a deviation towards higher viscosity is quite pronounced. Fig. 14 shows the low shear viscosity of one type of clay (B, mean e.s.d. O+7 pm) blended with three types of plastic pikment (0.1,0.2 and 0.6 pm in diameter) measured at 56% total solfds volume concentration. Upon addition of the smaller spheres the viscosity increasesand reaehEsa maximum. However. although the viscosity at low shear increases,the addition of spheres improves the flow behaviour and the blends exhibit pseudoplasticity. While it was impo&ble to draw down tho pure clay, the blends did not reprewnt a *problem. Similarly, wMfe with pure clay no Hercules rheogram could be obtained at tbii solids content, the addition improved nov, curves. It is diLficu\tat &ii
of spheres resuM.ed-in mu&
point to bpeculati to what extent
such behaviour 3sdue to a pJ&ysJcochemJcaJ interacMon between Ule particles of unlike surface properties or to a packing arrangement of particles of different shape. The porosity of dry coatings is certainly affected, as shown in Fig. 15 for mixture of clay and 0.2 pm plastic pigment, indicating that a mutual accommodation could be a significant factor. Note that the direct
relation between porosity and viscosity observed with mixtures of like
Fig. 16. SpecilIc light-srattering coefficient (A = 467 nm) of dry coatings formed from mixtures of clay Band plastic pigments 0.2 and 0.6 pm in diameter as a function of volume mixing ratio. Coatings applied
at 55% solids volume content.
Fig. 17- 75” gloss 01 dry waling6 formed from rnixtt)w& of cloy B and plastic pIgment6 0.2 and 0.6 #m in diameter as a function of volume mixing ratio. Coalfng~ epplfed at 55% soli& volume content.
is apparent here also cxccpt that, instead of a minimum, a maximum is found. The light-scatt&ng of coatings applied from blends of clay and two plastic pigments (0.5 and 03 em) is shown in Fig. 16. With 0.2 pm spheres the increin LSC approximates tba change in porosity. In the case of the 0.6 urn spheres tic continuous increase of LSC is apparently caused by fhe formation of voids of a more effect& size as the content of plastic pJgment increases 181. The gloss development shown in Fig, 17 follows the expected trend. partldes
The theordically predicti minimum viscdty in mixtures of spheres of different slzcs at high volume solids concentraticm%vasconfirmti on blends of plastic pigments. A similar behavirmr was obs~rwed in blends of platelike clay particles. Contrariwise, in blends of unlike particles, i-e,, clay and plastic plgmrtnts, the viscosity measured at low shear ddatecl towards higher vnlucs. 140wevcr, the addition of spherical particles to clay considerably improved Lhc flow behaviour at high shear. The viscosity of blends reflects the packing density of particles as measured by t.he critical solids volume concentration of the suspension and the porosity of the dry coatings. The change in total void vdumti and void size distribution is responsible for the change in light-scattering efficiency.
165
Many thanks are due to Mr. W. Bichard for his skillful assistance. Financial from Polysar Ltd. is gratefully acknowledged.
support
REFERENCES 1. R.J.
&rris,
mans.
SDC.
RheDlDgy,
12 (1966)
281.
2. S.H. Msron and B.P. Mmdnws, J. Coltoid Sri., 8 (1953) 3. 0.F. Eveson, in C.C, Mill (Ed.) “Rheobgy of Dispmed 1959, pp. M-83. 4. C.R. Price, South. Pulp Paper Manufacturer, 40 (1977) 6. P. Lepoutre and A. RezaUDwiCh, ‘bppi, 60 (1977) 86. 6. J. Bomb and P. Lepoutre, ‘Ibppi 61 (1978) 45. 7. Et. Atince and P. Lepoutre, J, Colloid Interface Set., 76 8. B. Alincc and P. Lepoutre, Tapp3.63 (1980) 49.
300. Systems.”
F’ergamon
13.
(1980)
182.
fiey,
NY,
VISCOSITY, PACKING PIGMENT BLENDS
B. ALlNCE
DENSITY
155 Amsterdam
AND
- Printed in The Netherlands
OPTICAL
PROPERTIES
OF
and P. LEPOUTRE
Pulp and Paper Research InstituteoJCunado,
570 St. John% Boulcrwd, Pointe Claim,
P-Q_. H9R 3J9 (Canada) (Received
12 July 1982;
accepted
in final form 7 fktober
1982)
ABSTRACT
The effect of blending particles of different size and shape on the viscosity of aqueous suspenslonr has been evaluatedusing polystyrene spheres (plastic pigment) and plate-like clay particles. At high solids content a minimum viscosity is observed in mixtures of like particles differing in size but not in mixtures of unlike particles. The viscosity of blends is closely related to the packing density of parttcles as Inferred from the critfcal solids volume concentration and the porosity of dry coatings formed from the blends. The variation in light-scattering efficiency of the coating is also related to changer tn the void structure of dry coatings formed
from blends.
INTRODUCTION
The solids content at which paper coating C~L:Ibe applied is often limited by the flow characteristics of the pigment-water system: specifically, a sharp increase in viscosity and a tendency to shear-thickening (dilatancy). The limiting solids con’dnt depends on the type, the size and shape, the mutual interaction and the state of dispersion of pigment particles. Other components normally present, such as binder and flow modifiers, can also considerably affect the flow of the pigment suspension. The intention here is to demonstmte on binder-free suspensfons what effect mixing pigment particles of different shape and size would have on the viscosity of suspensionr at high solids content, as well as on the optical properties of the dry coatings. The viscosity of a pigment suspension reflects the ability of the particles to change their position and to rotati in the flowing suspending medium. At very low concentration, when the particles are sufficient.ly separated, the viscosity of a well-dispersed suspension remains constant with changing flow, i.e., it behaves as a Newtonian system. With increasing concentration the viscosity increases and becomes more dependent on shear flow because of mutual interaction between suspcndcd particles. At a certain concentration, particle crowding results in a steep increase in viscosity. Finally, a critical point is reached when the particles start to touch each other, forming a
0166-6622/83/$03.00
e 1968 Elscvicr Science
Publishers B.V.
156
continuous network with just enough medium to fill the interstices,and.lhe suspension does not fIow anymore, It is important to reognize that this point is a function of the uoirtmefraction of the particles. In case of monodispersed spheres armnged in the most dense configuration (hexagonal rhombohedral) the cdculated volume fraction of maximum packing is 0.74. However, it can be higher in t.hecase of #8oly&s#xmed spheres, sinre the voids between larger spheres can be occupied by the smaller ones. Tnis means that In a suspension of polydispersed spheres at 0.74 volume fraction, not all the liquid will be confined to the voids SD that the particles can rotate and change their positions under shear. The effect of
polydispersion cbnviscosity in crowd&d systems is appment. At. a given high solids concentration there will be more free liquid available and consequently t.heviscosity should be lower than in monodisperse systems. It has been calculated that in bimodd suspensions of spheres a minimum
relative viscodty can be expected at a given mixture of small and large spheres [I]. This minimum should appear above a volume fraction of 0.45 and should become more pronounced with increasing concentration. Several experimental obsrvations of this effect were reported rvith model syshns [ 2,3]. In real systems it is known likewise that, for example, the viscosity of an unfractianated clay suspension is lower than that of any of the individual fractions 143 _ Here WC observe the fiow behaviour at high solids content of blends of spherical pmticks of different sizes, blends of plate-like day particles of different size3 and blends of spherical and plate-like particics. Since the viscosities of crowded suspensions are assumed to be affm t*d by the packing ability of partides, other properties that are also a function of pigment packing mangement are evaluated, These are the critiml solids volume concentration of the suspension, and the prrrosity,light-sc$Meringability and gloss of the dry coatings. EXPERIMl3NTAL
Mu teriuts Plastic pigments: Lylron 2501,2203 and 2101 L&:x fhlonsanto Plastics and Resin Co.) supplied as aqueous dispersion at 50% solids conkent.. Clay Pigments: Hydrite MP, Elydrite UF (Georgia Kaolin Co.) and Hydrafine (Huber Corp.)
Viscosities of aqueous bimodal dispersions of polystyrene spherical particles (plastic pigments), of clay particles and of mixtures of clay and plastic pigments were measured as a function of mixing ratio. All the dispersions were adjusted to pH 8 in order to encourage their stability. The clay
157
dispersions were prepare3 in distilled water but otherwise no cleaning of the commercial samples was attempted. Rrookfield and Hercules viscometers were used to characterize the low and high shear-rate behaviour, respectively. The volume of dispersionsmeasured in Brookfield was 400 cm3. The maximum packing fraction referred to here as critical solids volume concentration (CSVC) was detemained by preblending given fractions of dry clays and adding water until all t.heclay particles were wetted and formed a single coherent ball of apparently dry paste, just beyond the stage of crumbling apart. Whenever possible, coatings were drawn on polyester film and, after drying, were characterizedfor porosity, light-scatteringefficiency and gloss. Porosity was determined from the amount of oil held in the voids after full impregnation followed by removal of tire excess oil by wiping [S] . The light-scattering coefficent (LSC), expressed it; terms of mass per unit area (basis weight), was calculated from reflectances measured again& black and white backgrounds by using Kubclka-hlunk theory and correcting for the supporting polyester film [S] . Gloss was measured with a Hunter Glossmeter at 75”. In the following t.heresults are reported separately for mixtures of plastic pigments, clays and clay-plastic pigment. RESLJL’IX AND
DISCUSSION
Phstic pigment mtxiures
Polystyrene latex particles of three different sizes, 0.6, 0.2 and 0.1 pm in diameter were used. When blended at 50% volume solids content (as supplied) at various mixing ratios, no obvious effect on viscosity was ob-
0
0.1
0.4
0.6
OS
I.0
Fraction01 smz&r latex Pig. 1. Viscosity of blends of plastic pigments 0.1,0.2,0.6 pm in diameter at 60% total solids volume content as a function of mixing ratio Calculated from Row curves (Hercules) at 18,000 s4 shear rate.
Fig. 2. Porosity of dry coatings formed pigments as B function of mixing ratio.
from
mixtures
of 0.1.0.2
and 0.5 rm
Fig. 3. Specific light-scattering coefficient (A = 457 nm) of dry coatings formed mixturcr of 0.1, 0.2 and 0.6 pm plastic pigments as a function of mixing ratio.
plastic
from
In or&r LO evaluate the packing rlensily ench blend wns applied on polyester film (Mylar) with R mdering rod nnrl dlowcd to dry. As shown in Fig. 2, the coatings formed from t-hc monodispr?rsc spheres contnincd 38% void volume wgnrdlrss nf the? sphcrc size. Blendinji rcsulls in dccreas~l prosiCy that is widently morr! pronounced when Ilw difference in sixrs increases. ‘l’ho drop in pclrosity 1s the consequence rlf lhe more effbzient packing and that rcflccted itself in lcrwer viscosity. ‘I’hu light-scnttcriug cfficieucy of the rlry coatings suffers considerably wllsn the smaller q~1wres we addrtd to thr! 0.5 pm p;vliclr?s. In Pig. 3, the light-scattering coefficient of mixWms is l~low the vn!ues itxpzted hy a*sumiug a proportional contribution of the two componsn~ and shown ns lhe broken line. This again is the canscqur?ncc of the higher density of packing t,hat r~ulls in reduced totnl void volumr! (porosity) and size of voida. ‘1’1~ light-scattering efficiency is a function of I\IC porous structure and drops rapillly with dccrcascd void size [7 ] _ In dry coatings forn~~d from polystyrene spheres 0.5 pm in diameter, HIC voids are close Lo the? optimum size fur scAbring the tight. By admixing smaller spheres the reduction of total void volume, accompanied by the formation of smaller and less cffcctive voids, causes a pronounced drop in lhe light-scattering coefficient. Finally, at. hifill fraction of the smaller spheres, although the porosity again increases, a further decrease in void size negates its effect.
139
In Fig. 4 surfaces of coatings formed from some of the blends of 0.5 and 0.2 pm spheres are shown, The disappearance of the larger spheres from the surface obviously affects .!w gloss, as seen in Fig. 5.
Fig. 4. Surface of dry coatings formed from mixtures of plastic pigments 0.2 and 0.5 Nm in diameter obrwvcd by scanning electron microscope.
0
0.)
04
FraCuoflol
OS
OS
I.0
smaser Ia1cr
Fig. 5. 75” gloss of dry coatings formed pigmenls as R function of mixing ratio.
from mixhwea of 0.1, 0.2 and 0.5 pm plastic
Clay mixtures Three grades of clay differing in mean equivalent spherical diameter (e.s.d.) were used: A - e.s.d. 9.5 pm (Hydrite MP) B - e.s_d. 0.7 pm (Hydrafine) O.Z~m (Kydrite UF) c -4Ls.d. Clays A and C are not designed for use in Ihe paper industry but were chosen for demonstration purpost?s_ Although each cJay is already polydisperse, 85 shown in Fig. 6, the effec! of mixing on viticisity is apparent in Fig. 7, The minimum viscosity for the blend AIB is around 60160 mixing ratio, for Mend A/C it is around 67133. The viscosities were measured on suspensions having only 67% solids content by weight or 44% by volume because that wa5 the limit for dispersing the largest clay particles. The amount of dispersant used (determined from the lowest viscosity measured as a function of dispersant added) was 0.08% for A, 0.02% for B and 0.04% for C, The CSVC shown in Fig. S hadicates maxima in the packing densities of blends that are in reasonebJe agreement lvith the minima in the viscosity curvc~. Besides the viscosity at low 5hcar rate, the flow characterMic5, i _e., the shear stress ver5us shear rate, are also affected. The variation of viscosity with shear rate, calculakd from the Hercules rheograms, is shown in Fig. 9 to itlustrate the difference Zn behatiour between pure day B at 44% solids content by volume (67% by weight) and it5 blend with clay A. The rhcogram of pure clay A at this solids content was very erratic. Perhaps of more intire& than tbo viscosities at a given solids content (Fig, 7) arc the viscosities meamrrcd as a function of solids concentration.
Fig. 6. Size distribution
of clays A, B and C (supplied
by the manufacturers).
Fig. 7, Viscosity of blends of clays A, B and C nt 44% solids eontent by volume (67% hy weight) as a funclIon of mixing ratio. Brookfield 100 r.p.m.
161
,
01 0
Fig. 8. Critical solids volume concentration
of clay blends
1
2
1
3
as a funclion
4
6
6 -10’
of mixing ratio.
Fig, 9. Viscosity d blends of clays A and R at different mixing ratirr as a function of shear rate. Solids content 44% by volume (67% by weight).
Fig. 10. VT=osily of clays A tent. Brookfield 10 r.p.m.
and D and their 50/60 mixture as a function of solids con-
In Fig. 10 is an example of clays A and R sc?parately, and of a SO/SO blend of A and B that is within the region of maximum CSVC (Fig. 8) as well as of minimum viscosity (Fig. 7). The benefit of blending is apparent because the solids concentration at which the viscosity starts to rise steeply is higher for the blend than for its components. The natural consequence of this is that the blend can be dispersed (and applied) at higher solids concentration. The solids content at which the suspension started to flow was 44% by volume (67% by weight) for clay A, 61% by volume (73% by weight) for clay B but by preblending of dry clays (60 B/60 A) it was increased to 59% by volume (78.9% by weight).
162
Fig. 11. Porosity of dry coatings formed from mixtures of days A. B and C as a function of mixing ratio. Coatings applied at 44% solIds content by volume (67% by weight). Pure clay A could not be appIk3. Fig. 12. Specific light-scattering coefficient (k = 457 nm) of dry coatings formed from mixtures of clays A, B and C as a funclion of mixing ratio. Application sollds 85 in Fig. 11.
Coatings prepared from the clay blends at 44% solids content by volume (67% by weight) on polyester film differ in porosity and optical properties. The data for pure clay A are Iacking since no satisfactory coating could he applied at this solids level. In F’ig. 11 the porosit-y reaches a minimum that is in the region of maximum CSVC (Fig. 8) and miilimum viscosity (Fig. 7). The change in porosky rcfkcts itdf in the lighbscatterjng efficiency 8s seen in Fig. 12. The reduction in total vofd volume comblned with changing void size distribution causes the Iight-scattcrJng coefficient to decrease. Although the porosity at high content of large particles increases, the voids arc probably larger than the optimum size and, therefore, optically inferior. Consequently the LSC does not *risesimultaneously with the total void volume. Gradud addition of lqe partisles produces a rougher surface and the gloss suffers, as shown in Big. 13.
0
0
01
a4
04
I
08
to
FmcFaIdcbyA
Fig. 13. 7S” gloss of dry coatings formed from mixtures of clays A, B and C as a function of mixing ratio. Application so&k as in Fig. II.
163
0
0.2
0.4
0.6
0.8
Fraction of latex
a.a 0
0.1
0.4
Fredion
0.6
6.6
n.0
ot 0.2 pn tatex
Fig. 14. Visccrsityof blends of clay B and p!astic pigments 0.1, 0.2 and 0.5 )rm as a function of volume mixing ratio. Total solids volume content 55%. Brookfield 10 r.p.m.
Fig. 16. Porwlty of dry coatings formed from mixhwe of clay B and plastic pigment 0.2 pm in diameter, as a function of volume mixing ratio. Coatings applied at 55% solids
volume
contenk
Clay-plcrslic pigment
mixtures
Blends of particles differing in shape, Le., plate-like clay and spherical
plastic pigments, do not follow the trend in viscosity (i.e., the presence of a minimum at high solids) observed with mixtures of like-shaped particles, On
the contrary, a deviation towards higher viscosity is quite pronounced. Fig. 14 shows the low shear viscosity of one type of clay (B, mean e.s.d. O+7 pm) blended with three types of plastic pikment (0.1,0.2 and 0.6 pm in diameter) measured at 56% total solfds volume concentration. Upon addition of the smaller spheres the viscosity increasesand reaehEsa maximum. However. although the viscosity at low shear increases,the addition of spheres improves the flow behaviour and the blends exhibit pseudoplasticity. While it was impo&ble to draw down tho pure clay, the blends did not reprewnt a *problem. Similarly, wMfe with pure clay no Hercules rheogram could be obtained at tbii solids content, the addition improved nov, curves. It is diLficu\tat &ii
of spheres resuM.ed-in mu&
point to bpeculati to what extent
such behaviour 3sdue to a pJ&ysJcochemJcaJ interacMon between Ule particles of unlike surface properties or to a packing arrangement of particles of different shape. The porosity of dry coatings is certainly affected, as shown in Fig. 15 for mixture of clay and 0.2 pm plastic pigment, indicating that a mutual accommodation could be a significant factor. Note that the direct
relation between porosity and viscosity observed with mixtures of like
Fig. 16. SpecilIc light-srattering coefficient (A = 467 nm) of dry coatings formed from mixtures of clay Band plastic pigments 0.2 and 0.6 pm in diameter as a function of volume mixing ratio. Coatings applied
at 55% solids volume content.
Fig. 17- 75” gloss 01 dry waling6 formed from rnixtt)w& of cloy B and plastic pIgment6 0.2 and 0.6 #m in diameter as a function of volume mixing ratio. Coalfng~ epplfed at 55% soli& volume content.
is apparent here also cxccpt that, instead of a minimum, a maximum is found. The light-scatt&ng of coatings applied from blends of clay and two plastic pigments (0.5 and 03 em) is shown in Fig. 16. With 0.2 pm spheres the increin LSC approximates tba change in porosity. In the case of the 0.6 urn spheres tic continuous increase of LSC is apparently caused by fhe formation of voids of a more effect& size as the content of plastic pJgment increases 181. The gloss development shown in Fig, 17 follows the expected trend. partldes
The theordically predicti minimum viscdty in mixtures of spheres of different slzcs at high volume solids concentraticm%vasconfirmti on blends of plastic pigments. A similar behavirmr was obs~rwed in blends of platelike clay particles. Contrariwise, in blends of unlike particles, i-e,, clay and plastic plgmrtnts, the viscosity measured at low shear ddatecl towards higher vnlucs. 140wevcr, the addition of spherical particles to clay considerably improved Lhc flow behaviour at high shear. The viscosity of blends reflects the packing density of particles as measured by t.he critical solids volume concentration of the suspension and the porosity of the dry coatings. The change in total void vdumti and void size distribution is responsible for the change in light-scattering efficiency.
165
Many thanks are due to Mr. W. Bichard for his skillful assistance. Financial from Polysar Ltd. is gratefully acknowledged.
support
REFERENCES 1. R.J.
&rris,
mans.
SDC.
RheDlDgy,
12 (1966)
281.
2. S.H. Msron and B.P. Mmdnws, J. Coltoid Sri., 8 (1953) 3. 0.F. Eveson, in C.C, Mill (Ed.) “Rheobgy of Dispmed 1959, pp. M-83. 4. C.R. Price, South. Pulp Paper Manufacturer, 40 (1977) 6. P. Lepoutre and A. RezaUDwiCh, ‘bppi, 60 (1977) 86. 6. J. Bomb and P. Lepoutre, ‘Ibppi 61 (1978) 45. 7. Et. Atince and P. Lepoutre, J, Colloid Interface Set., 76 8. B. Alincc and P. Lepoutre, Tapp3.63 (1980) 49.
300. Systems.”
F’ergamon
13.
(1980)
182.
fiey,
NY,