determination of shape of kaolin pigment · pdf fileclay minera~ (1993) 28, 495-508...

Clay Minera~ (1993) 28, 495-508

D E T E R M I N A T I O N OF S H A P E OF K A O L I N P I G M E N T P A R T I C L E S

R. A. S L E P E T Y S AND A. J. C L E L A N D

Engelhard Corporation, 101 Wood Ave., Iselin, NJ 08830, USA

(Received 13 May 1992; revised 18 September 1992)

A B S T R A C T : Characterization of platiness of kaolin pigments was derived from the divergence of measurements of their particle size distribution by two different techniques: sedimentation and light scattering. A numerical shape factor, which is related to the ratio of kaolin particle face diameter to its thickness, can be calculated to provide a quantitative measure of such platiness. Two sets of kaolin pigments were prepared from a Middle Georgia kaolin: delaminated and non-delaminated. Shape factors of delaminated samples were higher than those of non-delaminated ones. Maximum platiness was found between equivalent volume diameters of 1.0 and 2.0/~m. Examples are presented where properties of coated paper are correlated with the size and shape of kaolin pigments.

Kaolin pigments, which are used widely in paper coating and filling as well as other applications, are processed from naturally occurring kaolinite deposits. These pigments generally have a platey shape. Pigment technologists use the aspect ratio to describe this platiness quantitatively. The aspect ratio is usually defined as the ratio of the average dimension across the face of the kaolin particle (face diameter) to its thickness.

Traditionally, the particle shape of clay minerals has been determined by electron microscopy (Grim, 1968). Morris et al. (1965) determined the aspect ratio of kaolin pigment particles by measuring the diameter and the length of a "shadow" cast at a given platinum plating angle for a large number of such particles on transmission electron micrographs. They used polystyrene beads of known diameter to calibrate the shadow, and thus established the thickness dimension. Conley (1966) made an extensive study of particle size and shape of six Georgia kaolins utilizing the same microscope technique as Morris et al.

(1965). They defined a shape factor as the particle thickness divided by the square root of the product of the largest and smallest dimension across the face of the particle. This corresponds, basically, to the reciprocal of the aspect ratio as defined by Morris et al.

(1965). Conley (1966) outlined the criteria in the selection of particle images for shape analysis, emphasizing clearly defined shapes. From over 10,000 particle images only 10% were usable.

Bundy et al. (1965) also measured the shape factor of kaolin particles, as defined by Conley (1966), on shadowed electron micrographs. Unfortunately, they did not provide data on either the particle size distributions or the numerical values of their shape factors. More recently, Bundy & Ishley (1991) reported aspect ratios, ranging from 4-82 to 22.0, for six different commercial kaolin pigments. In each case they measured ~1000 particles on shadowed electron micrographs. The aspect ratio in this paper was defined as the mean of the major and minor diameters divided by the thickness.

In his review paper of kaolins, Jepson (1984) reported the range of aspect ratios for

�9 1993 The Mineralogical Society

496 R. A. Slepetys and A. J. Cleland

Cornish kaolins from 10 : I for coarse particles to 50 : 1 for the fine ones, also determined by transmission electron microscopy.

Although measuring the aspect ratio by microscopy is direct and fundamentally simple, it has two serious drawbacks: the method is tedious and becomes subjective when individual kaolin particles cannot be clearly distinguished from clusters and spacial overlaps. The latter interferes particularly in measuring the aspect ratio of the larger particles.

Parslow & Jennings (1986) and Jennings & Parslow (1988) described techniques for measuring the aspect ratios of ellipsoidal and discoidal particles by comparing the particle size distribution data derived from sedimentation measurements with those by either optical birefringence or by light scattering. They demonstrated their technique on several kaolin pigments but made no attempt to relate their shape to their performance as paper coating pigments.

Earlier literature also contains some attempts to describe the kaolin pigment particle shape in terms of the "degree of delamination" (Welch & Price, 1984; Hardy et al., 1990). This degree of delamination was defined in operational terms as the change in the percentage of particles finer than 2-0/~m after mechanical delamination. This characterization combines the size and aspect ratio into one parameter. The aspect ratios measured by microscopy ranged only from 12.5 to 15.3 (Hardy et al., 1990).

The present work had two objectives. The primary objective was to devise a relatively rapid technique, free from the subjectivity associated with the microscopic method, to measure the shape of kaolin pigment particles. We have adapted the approach outlined by Jennings & Parslow (1988). The secondary objective was to illustrate the practical utility of this technique by correlating the effects of kaolin particle size and particle shape as two independent parameters with coated paper properties.

E X P E R I M E N T A L

Pigment preparation

Two sets of pigments were prepared from one given Middle Georgia clay; the standard set (coded S) and the mechanically delaminated set (coded D). The standard pigments of different particle sizes were obtained from a dispersion of the original crude clay by separating various particle size-fractions in a continuous centrifuge. The first step in the preparation of the delaminated clays was mechanical delamination. This was done by subjecting the clay dispersion to vigorous agitation in the presence of styrene divinylben- zene copolymer beads in a manner similar to that described by Gunn & Morris (1965). Subsequently, the delaminated clay slurry was also separated into various size fractions in the same centrifuge. All pigments were then finished in the conventional manner, which involved reduction bleach, filtration, drying and pulverization steps.

Particle size analysis

The particle size distribution of the experimental pigments was measured by two techniques: sedimentation and light scattering. A Sedigraph 5100 Particle Size Analyzer, Micromeritics Corp., Norcross, GA, was used for the sedimentation measurements; a Microtrac Small Particle Size Analyzer, Leeds & Northrup, North Wales, PA, was used for the light scattering measurements.

Determining kaolin particle shape 497

Microscopy

Scanning electron micrographs were made with the ISI DS130C microscope. The kaolin samples were dispersed ultrasonically in methanol, the dispersion was pipetted onto a Nucleopore filter attached to the microscope sample stub, and the sample was coated with a Au-Pd coating.

Paper coating

The effect of pigment particle size and shape on coated paper properties was evaluated. Each kaolin pigment was dispersed in water using a commercial sodium polyacrylate dispersant. Coating colours for rotogravure printing were then prepared using the following formulation:

Pigment 100 parts by wt. Ethylated starch 7 . . . . . . Styrene-butadiene latex 4 . . . . . . Calcium stearate lubricant 0.5 . . . . . . Total solids content 56% Water 44%

The coatings were applied to a lightweight basestock with a bench blade coater at several coat weights, and the test sheets were supercalendered. Rotogravure printability was assessed for gravure speckle using the Heliotest attachment for the IGT Printability Tester. Printability ratings are expressed as the distance in mm on the test print to the 20th missing dot. Coated sheet gloss was measured with a Hunter Lab D-48-gloss meter. Performance properties are reported at a coat weight of 8.1 g/m a.

For additional information on paper coating and printing the reader is referred to a textbook by Smook (1982). A glossary of paper coating terms is provided at the end of this paper.

D E T E R M I N A T I O N OF T H E S H A P E F A C T O R

Various particle size measuring techniques report, as a rule, equivalent spherical diameters. This is done to simplify the mathematical conversion of the measured instrument response to the particle size distribution. Therefore, in general, particle size measuring instruments, which are based on different principles, agree only if the particles being measured are spherical, but disagree for non-spherical shapes. Because this discrepancy arises in the case of kaolin particles as a result of their platiness, one can use this information for quantitative characterization of particle shape.

A thorough theoretical analysis of such disagreement among various particle size measuring techniques for oblate and prolate spheroids has been carried out by Jennings & Parslow (1988). They examined techniques based on particle volume, projected area (light scattering), Stokes sedimentation, translational diffusion and rotational diffusion. Choosing two techniques which give the widest divergence in the particle size reported as equivalent spherical diameter, provides the greatest differentiation in the particle shape.

We chose the techniques which analyse particle size distribution by sedimentation and by light scattering. The divergence in the particle size distribution measured by the two


techniques is illustrated in Fig. 1, which shows particle size distribution curves for test pigments S1 and D1. The standard and delaminated pigments have similar particle size distributions as measured by sedimentation; however, the distributions differ significantly by the light scattering technique. The difference between the two measuring techniques is more pronounced for the delaminated pigment due to its enhanced platiness, as indicated by the horizontal arrows on the graph.

Although the particle shape of kaolin pigments is platy-prismoidal, for the present purpose we have approximated it as oblate ellipsoidal.

In a sedimentation particle size analyser, one deals with Stoke's sedimentation diameters. The equivalent Stoke's diameter for oblate ellipsoids is (Jennings & Parslow, 1988):

~arctan(r z -1)1/2"~1/2 d ~ = a [ ~--~ ~ i-~-1/~ j (1)

where ds is the equivalent sedimentation spherical diameter, a the major axis of the ellipsoid, and r the ratio of the major axis of the ellipsoid to the minor axis.

The equation contains one measured quantity, ds, and two unknowns, a and r. A parallel equation can be written for the equivalent light scattering spherical diameter. Then, these two equations can be solved for the two unknowns.

The light scattering instruments "see" light scattering cross-sections of pigment particles. For a sphere, the light scattering cross-section is given by its geometric cross-section multiplied by the scattering coefficient for the sphere Ssp:

Assp = ~" 6 2 Ssp/4 (2)

where Ass p is the light scattering cross-section of a sphere, and d the diameter of a sphere. The situation for an ellipsoid is more complicated. Strictly, one must multiply the

projected geometric cross-section in three principal directions with the corresponding

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..:.JkTA/ 44 -- i 1.0

S p h e r i c a l

1' I I A

71 i i

I

O I ome ter . I0 .0 50.0

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FIG. l. Particle size distribution of pigments S1 and D1 (SED = sedimentation instrument, SCAT = light scattering instrument).


scattering coefficient and then average the result. Although such coefficients have been reported in the literature for selected instances of refractive indices, particle sizes and axial ratios (Hill et al., 1984; Asano & Yamamoto, 1975; Asano, 1979; Asano & Sato, 1980) their calculation is computer intensive (Barber & Massoudi, 1982; Barber & Wang, 1978; Asano & Sato, 1980). In the development of our method, therefore, we took an approximation approach as outlined below.

The average projected geometric cross-section for an oblate ellipsoid in random orientation is (Jennings & Parslow, 1988):

Agel = ara2(1 + (r(r 2 - 1)1/2) -1 ln(r + (rE - 1)1/2))]8 (3)

where Agel is the geometric cross-section of an ellipsoid. To obtain the light scattering cross-section of the ellipsoid its geometric cross-section

must be multiplied by the scattering coefficient for the ellipsoid Sel:

Asel = arSela2(1 + (r(r 2 - 1)1/2) -1 ln(r + (r E - 1)1/2))/8 (4)

where Ase 1 is the average light scattering cross-section of an ellipsoid. Because light scattering particle size analysis yields the diameter of a sphere with a

scattering cross-section equivalent to that of the particle, the equivalent light scattering spherical diameter dis for the oblate ellipsoid is obtained by equating expressions (2) and (4):

ln(r + (rE -- 1)1/2~ 1/2 dis = (a]Zll2)(gel]gsp) 112 1 + r(~ ~ ~-1~ J (5)

Eliminating the major axis a between equations (1) and (5) yields our working equation:

2r arctan(rE - 1 ) 1/2 ~1/2 (dfld,~) (Se l /Ssp ) 1/2 = f(r) = (r(r 2 _ ]-)a-7~-~ 1 ~ ~ ~ - 1)1/2] j (6)

The right-hand side of the equation contains only the axial ratio r, which we seek to determine. The left side has the two experimentally determined diameters, ds and dis, and the scattering coefficients. Equation (6) does not lend itself to an explicit solution for r. We have approached this solution in the following manner:

(i) Construct a table of values of the right side of equation (6) as a function of r. The function is shown graphically in Fig. 2.

(ii) Assume initially that S e l / S s p ~-- 1. (iii) Obtain the ratio ds/dls from the respective measured values of the equivalent

spherical diameters. (iv) Read the corresponding value of r from the table. (v) Utilizing this value of r, calculate the major axis of the ellipsoid from equation (1).

(vi) Calculate the diameter of the sphere whose volume is equal to that of an ellipsoidal particle with the major axis a, calculated in step v, and the axial ratio r (step iv):

dv =a]r 1/3 (7)

(vii) Use the diameter calculated in step vi to calculate the value of Ssp, the scattering coefficient for the sphere of equal volume (Bohren & Huffman, 1983).

(viii) Calculate S~I, the scattering coefficient for the ellipsoid from its experimentally determined equivalent spherical light scattering diameter (Bohren & Huffman,

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0.3

0.2

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R. A. Slepetys and A. J. Cleland

, ~ , ~ , , ,~ , I0 30 ~0 eO 80

Axial Ratio r

FIG. 2. The function f(r) in equation (6).

on Inn

1983). We consider this to be an acceptable approximation because the instrument actually measures the light scattering of platey kaolin particles and reports the result in terms of equivalent spherical diameters; the process is reversed to calculate the equivalent light scattering coefficient.

(ix) Recalculate the left side of equation (6) using the newly calculated values of scattering coefficients.

(x) Iterate steps 4 through 9. The solution converges in 3-6 iterations.

Because of approximations introduced in the above method, the calculated value of axial ratio r is also approximate. The principal source of uncertainty is the lack of knowledge of exact light scattering coefficients for ellipsoids. Hill et al. (1984) calculated scattering cross- sections for oblate ellipsoids up to the axial ratio of 3 : 1 in their study of light scattering properties of soil particles. The values converged with those for spheres below -0 .5 #m diameter and above 1.0 /~m. In the range of 0.5-1.0 /~m the scattering cross-section for randomly oriented oblate ellipsoids could be either smaller or larger than that for a sphere. Thus, according to equation (6), it could either exaggerate or suppress the calculated value of r. However, the fact that the ratio of Sel/gsp appears as a square root term minimizes this distortion.

R E S U L T S A N D D I S C U S S I O N

Particle shape

Particle size distribution curves, as illustrated in Fig. 1 in terms of equivalent spherical diameters, show the expected divergence between the sedimentation and the light scattering techniques (Jennings & Parslow, 1988). Because of the logarithmic scale of the equivalent spherical diameter axis, the linear displacement parallel to this axis is directly proportional to the ratio of particle diameters measured by the two techniques, and thus is indicative of particle platiness. The differences are particularly pronounced between the standard and the delaminated pigments, the latter being clearly more platy, as expected.

For any given pigment, every point on the sedimentation particle size distribution curve


has a corresponding point on the light scattering curve. Thus, a distribution of shape factors exists as a function of particle diameter. However, to simplify the calculation, we evaluated shape factors for each pigment for every 10% interval, starting with 20% and ending with 80%. We refrained from using data at both ends of the particle size distribution curves, because in many of them a sharp change in the curvature made the measurement of relative displacement between the two curves uncertain. A single numerical value of the shape factor for a given pigment was then obtained by taking the average of seven values calculated as described above. Because such averaging spans a large range of particle sizes, it helps to minimize errors arising from uncertainties in the light scattering coefficients of ellipsoids whose equivalent volume diameter falls in the range of 0.5-1.0 /~m. The coefficient of variation was estimated by deriving two separate values of the shape factor for each pigment from two replicate particle size distribution curves and then comparing the result. The value of the coefficient of variation is 8.2%, giving the confidence interval for a duplicate determination of + 12.2%.

The values of sedimentation, scattering and volume equivalent spherical diameters and of shape factors are presented in Tables 1 and 2. The shape factor distribution as a function of the equivalent volume diameter for standard pigments is illustrated in Fig. 3. The shape factor distribution results display three significant features:

(i) As it was apparent from the divergence of the particle size distribution data, the delaminated pigments have significantly higher shape factors, except for very small particle diameters.

(ii) Both standard and delaminated pigments display a maximum for diameters between 1.0 and 2.0/~m.

(iii) Except for one point, representing the highest value of shape factor for pigment D2, shape factors for all delaminated pigments fall in one band; however, curves for the standard pigments differ from each other depending on their relative particle size.

The first observation is clearly expected from the method of preparation of the test pigments.

The second observation hinges on the natural presence of particles of given platiness in this kaolin deposit either as individual particles or in booklets. Upon delamination they must have very similar platey morphology, which appears as a maximum in the shape factor between 1-0 and 2.0/~m equivalent volume diameter. Very small particles are fragments, whose shape factor is quite small. At the other end of the scale, the coarse particles can be aggregates, "booklets", or large thick platelets, all of which have relatively small shape factors.

The third observation is related to the second one. After delamination, all naturally present individual kaolin platelets are separated from each other. Thus, regardless of artificial separation of particles into delaminated pigments of various coarseness, particles of a given diameter display the same shape factor. On the other hand, the standard pigments have individual platelets mingled with aggregates of platelets. Therefore, separation of the original population of kaolin particles into various size-fractions by centrifugation produces different proportions of platey particles mixed with the less platey ones for a given equivalent sedimentation diameter.

Electron micrographs (Fig. 4) support the above observations. The finest pigments ($5


TABLE 1. Diameters (gm) and shape factors-standard pigments.

% l e s s Sedim. Scat. Volume Shape than diam. diam. diam. fact.

S1

$2

$3

$4

$5

20 1.1 2.1 1.35 11.5 30 1.7 2-7 2.06 12-7 40 2.1 3-7 2.45 7.7 50 2.8 4-4 3.02 3.9 60 3.3 5.6 4.02 11-8 70 3.9 6-7 4.58 8.2 80 4-6 8.9 5.42 8.3

20 0-8 1.3 0.86 4.0 30 1.2 1-8 1.36 6.0 40 1.5 2-3 1.80 9.5 50 1.9 2-9 2.29 9.8 60 2.3 3-6 2.51 5.4 70 2-9 4.5 3.15 4.2 80 3.5 6-0 4.25 10.4

Average: 9.2

Average: 7-0

20 0.3 0.8 0.30 1.0 30 0.4 1-2 0-46 4.2 40 0.6 1-6 0-68 8.9 50 0.8 2.1 1.02 22-0 60 1.0 2.9 1-56 57.0 70 1.5 3-8 2.05 23.2 80 2.2 5-1 2.79 14.0 Average: 18-6

20 0.3 0.7 0.30 1.8 30 0.4 1.0 0.42 2.5 40 0-6 1.5 0-65 7.5 50 0.7 1-7 0-82 9.8 60 0.9 2.2 1.18 22.0 70 1.2 2.9 1.65 24-5 80 1.7 3-9 2.20 16.2 Average 12-0

20 0.3 0.5 0.26 2.7 30 0.3 0.6 0.30 1.0 40 0.4 0-7 0.37 1.5 50 0.5 0-9 0.47 2.3 60 0-6 1.3 0.64 4.6 70 0.7 1.6 0-84 7.8 80 1-0 2.1 1.21 14.5 Average 4.9

and D4) have a predominance of fine particles. However , $5 has retained some original

"booklets" , while D4 has not. The most platey in each series ($3 and D2) have very large

platelets; those in the delaminated pigment appear thinner (i.e., higher shape factor) than

in the corresponding standard pigment. The coarsest standard pigment (S1) clearly has

some stacked booklets, while platelets in the delaminated counterpart (DI ) just appear to

be thick.

Determining kaolin particle shape

TAar.E 2. Diameters (/zm) and shape factors-delaminated pigments.

503

% less Sedim. Scat. Volume Shape than diam. diam. diam. fact.

D1

D2

D3

D4

20 1.3 3.0 1-87 32-0 30 1.9 3.9 2.33 11.2 40 2.4 5.0 2.91 10.3 50 2.9 6-0 3.68 14.2 60 3.6 7.3 4-55 13.8 70 4.2 9.2 5.17 11.5 80 5.5 13-0 7.23 18.1 Average: 15.9

20 0-6 2-2 0.89 25.5 30 0.9 3.1 1.48 79.5 40 1.4 4.2 2.01 32.3 50 1.9 5-2 2.71 30.5 60 2.4 6.1 3.06 14-7 70 3.1 8.0 4-35 27.5 80 4.2 12.0 5-77 24.2 Average: 33.5

20 0-4 0.9 0-39 1.0 30 0-6 1.6 0-66 9-1 40 0.7 2.0 0-94 18-4 50 0.9 2.7 1-45 51-0 60 1-3 3-4 1.89 34.7 70 1-8 4-3 2-27 16-4 80 2.5 5.9 3.18 14.5 Average: 20.7

20 0.3 0.9 0.33 2.1 30 0.5 1.4 0.51 6-4 40 0.6 1.8 0.74 13.2 50 0.8 2.5 1.16 51.0 60 1.0 3.0 1.44 44.5 70 1.4 4-0 1.95 33.2 80 1.9 5.6 2-59 27.4 Average: 25-4

Correlation with paper coating

Paper technologists have long recognized the impor tance of pigment part icle size and shape and have studied their effects on coated paper proper t ies (Lyons, 1966; Bundy & Ishley, 1991). However , publ ished a t tempts at quanti tat ive correlat ions are sparse.

Morris et al. (1965) de te rmined the aspect rat io of mechanically de laminated kaolin pigments. Al though they re la ted the improved opaci ty and gloss of the resulting paper coatings to the delaminat ion, they did not examine the quanti tat ive effects of the aspect ratio on coating propert ies . Sennett et al. (1982) extended this work by examining the effects of both the part icle size and the delaminat ion of kaolin pigments on coated paper propert ies . They referred to the pr ior publ icat ion (Morris et al., 1965) for the de terminat ion of the aspect ratio. Al though a fairly wide range of part icle sizes was included in this study for the s tandard coating pigments, the mean d iameter of the de laminated ones ranged only form 0-33 to 0.51/~m. Bundy et al. (1965) covered a b roader range of part icle sizes (22-


80

70

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50

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Pigment Symbols:

o S I & S 2 ~

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FIG. 3. Shape factor distribution as a function of particle diameter for standard pigments.

88% finer than 1 /tm) and qualitatively correlated their shape with properties of paper coating. Unfortunately, they did not provide data on either the particle size distributions or the numerical values of their shape factors.

In our work, we wish to illustrate the utility of quantitative characterization of kaolin particle shape by correlating some coated paper properties with pigment particle size and shape as two independent parameters.

Light-weight paper was coated with rotogravure coating colour formulations containing starch and SB latex binders and each of the nine experimental pigments. Sheet gloss and printability (Helio) test results for calendered paper are given in Table 3. Correlation of these coated paper properties with particle diameter (diameter of a sphere of equal volume) and with the shape factor, as two independently measurable physical characteristics of kaolin pigments, is described below.

A full quadratic regression model was formally used in deriving the correlations:

Property = Ao + Aldv + A2(SF) + Alldv 2 + A22(SF) 2 + A12dv(SF) (8)

where A is the regression coefficient, dv the equivalent volume diameter, and SF the shape factor.

However, in analysing these regressions we retained only those terms which contributed to improving the regression and dropped those which did not. Thus, the following correlations were obtained for sheet gloss and Heliotest printability:

Gloss = 45.4 - 6.84 dv + 0.135 dv(StO (9) Statistical probability = 0.067

Helio = 42.7 + 16.3 dv + 0.338 (SF) (10) Statistical probability = 0.004

Sheet gloss, as expressed in equation (9), is represented in Fig. 5 as a contour plot on a grid of equivalent volume diameter and shape factor. This plot shows increasing gloss values


Fro. 4. Scanning electron micrographs of test pigments (left side - standard, right side - delaminated; top row--coarsest pair, middle row--platiest pair, bottom row-finest pair).

toward finer and platier particles. The effect of the shape factor is more pronounced for the coarse particles; conversely, particle size has a stronger effect at the fine end. These trends are very reasonable because coarse particles are expected to have poorer gloss. However , if such coarse particles are platey, they may deposit parallel to the surface of the base sheet and improve its gloss. At the fine end of the diameter scale, particle orientation and platiness are less important , and high gloss is derived primarily by virtue of fine size.

The expression (10) for rotogravure printability as measured by the Helio test is simpler because it contains only linear terms. It shows that printability improves both with


TABLE 3. Rotogravure sheet properties.

Pigment: S-1 S-2 S-3 S-4 Gloss, % 25 31 33 39 Heliotest, mm 90 87 56 54

Pigment: D-1 D-2 D-3 D-4 Gloss, % 34 36 45 48 Heliotest, mm 110+ 90 85 73

All properties are at a coat weight of 8.1 g/m 2.

S-5 45 51

t . : ~

0.n 0.5 i .0 t .s 2.0 2.s 3.0 a.5 4.0 EClulvtal~t Volt.tin 01omDter. MlcrottBt.ara

Fro. 5. Contour plot for coated paper gloss.

increasing particle diameter and platiness, within the range of experimental variables. This trend is in agreement with the experience of paper technologists, that large platey particles provide the best rotogravure printing surface.

CONCLUSIONS

A rapid technique has been devised to estimate pigment shape factor, which gives a quantitative measure of the platiness of kaolin pigment particles. The specific method used is based on the divergence of particle size distribution measurements made using sedimentation and light scattering techniques.

We have determined shape factors for nine kaolin pigments, five standard and five delaminated, whose median sedimentation particle diameter ranges from 0.49 to 2-88/~m. The highest shape factors occur between equivalent volume diameters of 1.0 and 2.0/~m. Both fine and coarse fractions have relatively low shape factors. The mechanically delaminated pigments have clearly greater shape factors than the standard ones.

The practical utility of the results was illustrated by correlating the morphological parameters of these pigments with coated paper properties.

The technique described in this paper should be of great value to pigment and paper technologists because it allows platey pigments to be characterized quantitatively by size and shape as two separate parameters. They can be correlated with performance of such pigments in various applications. However, shape factor should not be taken to represent numerically the aspect ratio (face diameter divided by thickness of the particle), because we


have made several ideal ized assumptions: e11ipsoida1 part icle shape; total light scat ter ing cross-sect ion be ing "seen" by light scat ter ing part icle size analyser ; approx imat ing the average light scat ter ing coefficient of el l ipsoidal part icles ra ther than calculat ing this coefficient for the three pr incipal o r ien ta t ions ; and ignor ing the very fine and the very coarse por t ion of the part icle size d i s t r ibu t ion curves.

The t echn ique could be improved further . Se d im en ta t i on and light scat ter ing proper t ies of actual part icle shapes would have to be examined . The light scat ter ing effects of actual

i n s t r u m e n t geometr ies should be cons idered . Precis ion of part icle size d is t r ibut ion m e a s u r e m e n t s and analysis should be e n h a n c e d to inc lude the ent i re d i s t r ibu t ion curve.

ACKNOWLEDGMENTS

We are grateful to Engelhard Corporation for the permission to publish this work as well as to our colleagues who determined the particle size distributions, prepared the micrographs, carried out the coated paper evaluations and helped us with their comments and constructive criticism.

REFERENCES

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Aerosol Sci. Techn. 1, 303-315. BARBER P.W. & WANG D.S. (1978) Rayleigh-Gans-Debye applicability to scattering by nonspherical particles.

Appl. Optics 17,797-803. BOHREN C.F. & HUFFMANN D.R. (1983) Absorption and Scattering of Light by Small Particles, pp. 479-482. John

Wiley & Sons, New York. BRUNO M.H. (1985) Principles of contact (impression) printing processes. P. 4 in: Printing Fundamentals (A.

Glassman, editor). TAPPI, Atlanta. BUNDY W.M. & ISHLEY J.N. (1991) Kaolin in paper filling and coating. Appl. Clay Sci. 5, 397-420. BUNDY W.M., JOHNS W.D. & MURRAY H.H. (1965) Physicochemical properties of kaolinite and relationship to

paper coating quality. TAPPI 48, 688-695. CONLEY R.F. (1966) Statistical distribution patterns of particle size and shape in the Georgia kaolins. Clays Clay

Miner. 14, 317-330. GRIM R.E. (1968) Clay Mineralogy, 2nd edition, pp. 165-184. McGraw-Hill, New York. GUNN F.A. & MORRIS H.H. (1965) Delaminated domestic sedimentary clay products and method of preparation

thereof. US Patent 3, 171,718, Ex. 7. HARDY R.E., WELCH L.J. & JONES M.M. (1990) An investigation of delaminated kaolin clay and the effects on

coated sheet properties. TAPP1 Coating Conf. Proc., 251-257. HILL S.C., HILL A.C. & BARBER P.W. (1984) Light scattering by size/shape distributions of soil particles and

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London A419, 137-149. JEPSON W.B. (1984) Kaolins: their properties and uses. Phil. Trans. Roy. Soc. London A311, 411-432. LyoNs S.C. (1966) Clay. Pp. 85-86,104-108 in: Paper Coating Pigments (H.H. Murray, editor). TAPPI, New York. MILLER C.J. (1985) Tests for gravure printing. P. 274 in: Printing Fundamentals (A. Glassman, editor). TAPPI,

Atlanta. MORRIS H.H., SENNE~ P. & DREXEL R.J., JR. (1965) Delaminated clays--physical properties and paper coating

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66-70.

A P P E N D I X

Paper coating terms

Basestock: uncoated paper upon which paper coating is applied (Weiner, 1965). Coating: a layer of pigments and binders which has been applied to the surface of paper to create a new, usually

smoother, surface (Weiner, 1%5). Coating colour: an aqueous slurry containing pigments, binders and other lesser ingredients, which is applied to the

surface of the paper in the coating process (Weiner, 1965). Gloss: that property of the surface which causes it to reflect light specularly and which is responsible for its shiny

appearance (Weiner, 1965). Light weight paper: paper weighing approximately 30-40 g/m2; it is typically used in magazines, catalogues and

directories, where bulkiness and mailing costs are major considerations (Weiner, 1965). Printability: that property of paper which yields printed matter of good quality (Weiner, 1965). Rotogravure: a method of contact (impression) printing wherein the image area on the printing cylinder consists of

tiny wells, which hold the ink, and the nonimage areas are scraped clean with a metal doctor blade that contacts the smooth outer surface of the cylinder (Bruno, 1985).

Speckle: a printability defect arising from totally or partially missing printed image dots (Miller, 1985). Supercalendering: an operation wherein paper is passed under pressure between roils with specially finished

surfaces. The purpose is to develop a smoother paper surface (Weiner, 1965).

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