Filler Particle Shape vs. Paper Properties – A Review
Martin A. Hubbe
North Carolina State University
ABSTRACT
Contrasting shapes and sizes of mineral filler particles provide today’s papermaker with many options to
affect paper properties. One can choose between plate-like clay products, irregular-shaped products of
grinding, and a diverse assortment of filler shapes that can be achieved by mineral precipitation methods.
This review considers the connection between filler morphology and such attributes as apparent density,
opacity, strength, and demand for sizing chemicals.
Although no one type of filler product will suit every application, published information can help the
papermaker deal with a series of compromises. Plate-like particles can be effective for paper products
having a high apparent density. More rounded, solid-form particles tend to minimize the demand for sizing
chemicals and generally allow more rapid dewatering. Particles with internal voids general offer high light
scattering ability, contributing to opacity. Though there is often an inverse relationship between light
scattering and strength, it is possible to design fillers that achieve a more favorable balance between these
two attributes.
INTRODUCTION
The word “filler,” as used in the paper industry, can be misleading. If you “fill” a hole, for instance, that
means you add something to fill the unoccupied volume of the hole. By contrast, when you “fill” a piece of
paper by adding filler particles to the stock, the “holes” between fibers in the sheet do not yet exist.
Because of the manner in which they are added, fillers have the potential to cause profound changes in the
structure of paper, affecting its end-use behavior. Many such effects have been considered in previous
articles [1-2]. The present review considers the shapes of filler particles, and how different shapes can
affect paper’s apparent density, strength, optical properties, and other important parameters [2-9].
FILLER SHAPES AND SIZES
Rip van Winkle, if he were a US papermaker emerging from 20 years of sleep into the present era, would
have no difficulty in recognizing a paper mill. If he happened to be an expert in filler technology, he also
would be amazed at the variety of particle shapes that are being used. “Tell me, my young friend,” he
might say, “what are these various powder products that you are adding to your process? Some of them
don’t seem to have the same particle shape as clay!”
“Mr. van Winkle… oh… may I just call you ‘Rip’?” (he nods) “Where have you been? Nowadays clay is
just one of the minerals that we can use as a filler for paper.”
Shape of Primary Particles
Let’s start by considering types of filler products that consist mainly of individual particles. Filler products
involving bunches of primary particles fused together will be considered later.
Platey particles: Figure 1, just to show Mr. Van Winkle something with which he is familiar, is an
electron micrograph of clay particles. Since it’s hard to get a sense of proportions from a two-dimensional
image, it’s worth noting that clay particles tend to be wider than they are thick. The terms “bookettes” and
“platelets” have been used to describe their shape. The ratio of width to thickness depends not only on
where the clay is mined, but also on whether the material has been run through a delaminating process [10].
Mined kaolinite typically has a ratio of particle width to thickness (aspect ratio) of about 4 to 12, [11-13 ]
whereas delaminated clay products can have aspect ratios of about 10 to 30 [11,13].
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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CLAY
GCC
1 µm
Fig. 1. Scanning electron micrograph (SEM) of
hydrous kaolin (“clay”) particles of the type
often used as fillers for paper
1 µm
Fig. 2. Micrograph of ground calcium carbonate
(GCC) filler particles
Rounded particles: “Oh, but the mineral particles in this picture also look familiar to me,” says Rip, on
being shown Fig. 2. “That looks a lot like some of the ground calcium carbonate that was being used a lot
in Europe at the time I went to sleep!”
Mr. van Winkle is right, of course, but we need to explain to him that ground limestone also has become
widely used in the US within the past two decades. Much of this usage is for coated papers, in addition to
the use of GCC as a filler [14-16].
Rounded? Irregular? Blocky? It is hard to find one word that adequately describes the product of a
grinding process. Because minerals tend to break along certain planes within the crystal, the shapes
resulting from grinding are not completely random. Rather, the angles at the surfaces of the resulting
particles can give clues as to the crystal of origin. For instance, calcite is the crystalline form of most
calcium carbonate filler products, including ground limestone.
Before leaving the subject of “rounded” particles, it is worth noting that somewhat similar shapes also can
be achieved by chemical precipitation. For instance, Fig. 3 shows rhombohedral calcium carbonate. It so
happens that this product has exactly the same mineral form as limestone, i.e. calcite. Gill and Scott
attributed the relatively high light scattering ability of rhombohedral calcite particles to their uniform
particle size and bulky packing characteristics [4].
PCC-r
PCC-a
1 µm
Fig. 3. Micrograph of rhombohedral precipitated
calcium carbonate (PCC-r)
1 µm
Fig. 4. Micrograph of aragonite precipitated
calcium carbonate (PCC-a) particles
Acicular: “Whew! What in the world is that?” asks Mr. van Winkle. Figure 4 shows a micrograph of
calcium carbonate particles that were precipitated under a different set of aqueous conditions [17]. The
aragonite crystal type of calcium carbonate usually takes the form of elongated particles. The technical
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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name is “acicular.” Such particles are used less often as fillers, but they are widely used in coatings.
Recently another type of acicular particles also has been considered for use as a filler [18]. Later we will
consider how such fillers would be expected to affect paper properties.
Consequences of Primary Particle Shape and Size
Bulk and porosity: The degree to which the use of filler affects paper’s apparent density can depend
strongly on (a) the particle size, (b) the particle shape, and (c) calendering effects. For simplicity, we will
first consider what happens in the absence of calendering.
As shown in Fig. 5 the ability of clay or chalk fillers to contribute to the measured thickness (caliper) of the
paper at a given basis weight and filler content depended strongly on the equivalent diameter of the
particles [3]. The latter quantity is defined in terms of a sedimentation test [2,19]. The thickness index,
represented by the vertical axis of the figure, is equal to the increased volume due to the mineral particles,
divided by the area of fiber surface that is covered by filler particles. The findings in Fig. 1 provide strong
support for a “spacing” role of fillers [3]. In other words, their presence tends to open up spaces between
fibers that otherwise would be tightly bonded together in the sheet. Also, the results suggest that larger
particles are able to prop open larger inter-fiber spaces within the paper.
Thickness Index
1.6
Clay
1.4
Chalk
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
1
2
Filler Equivalent Diameter (µm)
3
Pore Volume (cm3g-1µm-1)
0.8
1.8
Filled
Unfilled
0.6
0.4
0.2
0.0
0.0
0.4
0.8
1.2
1.6
2.0
Pore Diameter (µm)
Fig. 5. Effect of particle size on the relative
thickness of paper [3]
Fig. 6. Effect of fillers on the pore size
distribution of paper [20]
Figure 6 shows how the presence of fillers can affect the pore size distribution within a sheet of paper [20].
These results were obtained by mercury porosimetry, a method in which mercury is forced under pressure
into the pores within a dry sheet of paper [21-22]. In principle, increasingly high pressure is needed in
order to force the mercury into smaller and smaller pores. As shown, the sample of filled paper had a much
higher incremental pore volume, especially within the range of equivalent pore diameter between 0.3 and
0.6 µm. These results can be taken as further evidence that fillers are playing the role of spacers within a
sheet of paper.
Strength: One might guess, if one relies only on the evidence already discussed, that larger filler particles
would have a particularly harmful effect on paper strength. After all, if larger particles are more effective
for spacing the fibers apart from each other, then they ought to hurt inter-fiber bonding to a greater extent.
Right? Figure 7 shows, on the contrary, that larger filler particles generally had less adverse effect on
strength [9]. Other studies have shown similar relationships between filler particle size and bonding
strength, especially when the filler particle shape and the filler level were held constant [5,7,8,23].
To account for the relationship shown in Fig. 7 it is worth considering how filler particle diameter affects
the relative amount of fiber surface that can be covered by a given mass of particles. Each time that one
decreases the diameter of filler particles by a factor of two, the area of those particles increases by a factor
of two. Thus smaller particles are able to prevent inter-fiber contact over a larger fraction of the available
surface area. Here it is assumed that the surfaces of the adjacent fibers are sufficiently conformable that
such interruption of bonding does not create a wide unbonded zone adjacent to each filler particle.
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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60
50
Kaolin
40
Chalk
30
0
2
4
6
8
10
12
Filler Equivalent Spherical Diameter (µm)
Fig. 7. Effect of filler particle size on the burst
strength of paper [9]
Paper Light Scat. Coef. (cm2/g)
Burst Strength (% of Unfilled)
70
800
Kaolin
700
Chalk
600
500
400
300
0
2
4
6
8
10
12
Filler Equivalent Spherical Diameter (µm)
Fig. 8. Effect of filler particle size on light
scattering coefficient of filled paper [9]
Light scattering: As shown in Fig. 8, the light-scattering contribution of different filler particles tends to
be maximized at a certain particle size [9]. Such results are understandable when one considers the fact
that visible light has wavelengths in the range of roughly 400 to 700 nm, a number which correspond
approximately to the size of primary particles that have been found to be the most efficient, per unit mass,
for the scattering of light [24]. It is worth noting that TiO2 has a lower optimum size. The difference can
be partly explained by the relatively high refractive index of TiO2. Light slows down as it passes through
the high refractive index of TiO2, so its effective wavelength is smaller [25].
COMPOSITE FILLERS
“Open” vs. “Solid”
In addition to the fillers already considered, there are many additional mineral products that can be
described as “composites.” In other words, they are composed of primary particles fused together. A key
variable, in these cases, is the relative amount of “internal” void space. Rather than attempt to define what
we mean here by the word “internal,” let’s look at some pictures:
Calcined clay: Our hypothetical “Rip van Winkle” might be quite familiar with calcined clay, especially
in its role as a coating pigment. As shown in Fig. 9, calcined clay has a superficial appearance that is not
very different from ordinary clay (compare Fig. 1). But there is a difference. The process, which involves
heating finely divided clay particles above about 1000 oC [26] causes primary particles to become fused
together, often at odd angles [26,27]. After calcination, the product usually is re-ground and fractionated to
keep the maximum particle size below a certain limit.
Calcined Clay
PCC-s
1 µm
Fig. 9. Micrograph of calcined clay product
1 µm
Fig. 10. Micrographs of scalenohedral precipitated calcium carbonate “rosettes” (PCC-s)
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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The haphazard manner in which the primary particles are fused together in calcined clay ensures that there
is a lot of internal void space associated with each composite particle.
Scalenohedral PCC: “Now son, you’re starting to scare me!” says Mr. Van Winkle, upon looking at the
shapes of the particles shown in Fig. 10. “Well, Rip, this is the a picture of the most widely used
papermaking filler in North America!” The conditions of precipitation of this synthetic product causes
elongated calcite crystals to grow outward from a central core region, resulting in this distinctive shape.
This is a second example of a “composite” filler type that contains a lot of “open” volume within its
outermost dimensions.
Consequences of Open vs. Solid Form
Bulking effect: As shown in Fig. 11A (left side of the figure) the “open” vs. “solid” nature of different
types of filler particle can have a large impact on the bulk of the resulting paper [2]. Note that the volume
of paper, per unit mass, increases with increasing content of either PCC-s or calcined clay, both of which
are composite-type fillers with a lot of internal void space. By contrast, the solid-type particles, clay and
GCC, resulted in decreased bulk, as the filler level was increased. Figure 11 shows, in addition, that the
platey clay filler yielded somewhat denser paper compared to the more three-dimensional GCC particles.
Strength effects: As shown Fig. 11B, filler particles that have a high bulking ability typically have a
greater adverse impact on paper bonding properties, per unit mass of filler.
To more clearly show the relationship between bulking ability and strength, Fig. 12 plots the two variables
against each other [2]. The take-home message from such results is that filler shape and composite
structure can make a difference. Two types of filler giving the same impact on strength will not necessarily
have the same impact on it apparent density.
Pore size distribution: As shown in Fig. 13, the detailed nature of filler particles also can have a big
effect on the pore size distribution within the resulting paper [20]. Synthetic silicate filler particles, of the
type represented in the figure, are composed of very small primary particles, often in the size range of
0.002 to 0.05 µm. It is evident from the results that the spaces between these primary particles account for
a large fraction of the volume within such paper. By contrast, the calcined clay, being composed of
somewhat larger, platey particles, yielded somewhat larger pore spaces.
B.
Bulk (cm3/g)
1.5
1.4
1.3
Clay
GCC
PCC
Calcined
1.2
1.1
0
10
20
30
Filler Content (%)
0.6
100
Change in Bulk (cm3/g)
1.6
% of Unfilled Burst Strength
A.
80
60
40
20
0
Chalk
PCC-s
Clay
Calcined
0.5
0.4
0.3
0.2
0.1
0.0
0
10
20
30
0
Filler Content (%)
Fig. 11. Effect of filler content and particle
shape on paper’s bulk and burst strength [2]
10
20
30
40
50
60
70
Relative Loss in Burst Strength (%)
Fig. 12. Incremental bulk vs. strength loss for
different kinds of filler products [2]
Light scattering: Filler shape and composite structure also can impact paper’s optical properties. As
shown in Fig. 14, it is possible, under some circumstances, to achieve high incremental gains in light
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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Light Scattering Coef. (cm2/g)
Pore Volume (cm3g-1µm-1)
1.0
Synthetic silicate
0.8
Calcined kaolin
Unfilled
0.6
0.4
0.2
0.0
0.0
0.4
0.8
1.2
1.6
3400
Rhombohedral/Prismatic
2900
Acicular
Scalenohedral
2400
1900
Ultrafine GCC
1400
Fine GCC
900
0
5
10
15
20
Filler Content (%)
2.0
Pore Diameter (µm)
Fig. 13. Effect of synthetic silicate vs. calcined
clay on the pore size distribution in paper at
approximately 30% filler content [20]
Fig. 14. Specific light scattering coefficient of
different fillers as a function of filler content [4]
scattering with solid-type particles, as in the case of the rhombohedral PCC represented in the figure [4].
As was shown earlier, light scattering can depend in a complex way on the size of particles (Fig. 8). The
rhombohedral shape, in the case considered, apparently was effective in opening up inter-fiber spaces large
enough to scatter light (e.g. 0.2 µm or greater). It is worth noting, however, that the specific light scattering
coefficients of the acicular (needle-shaped aragonite) and the scalenohedral (rosette) particles outperformed
the other particles when the filler content was above 8%. Apparently the bulky composite-type products
continue to avoid tight packing, even when present at relatively high levels in the paper.
It is also worth noting in Fig. 14 that the ultrafine GCC product scattered light more efficiently than the
fine-ground limestone. This observation confirms Fig. 8, where it was shown that light scattering of solidtype particles generally increases with decreasing particle size, as long as one stays above a diameter of
about 0.5 µm, which is in the range of visible light wavelengths.
Particle Size, in Addition to Shape
3200
PCC (open)
Solid-type
2800
2400
2000
1600
0.5
1.0
1.5
2.0
2.5
3.0
Average Particle Size (µm)
Fig. 15. Particle-size dependence of specific
light scattering coefficient for an open vs. a
solid-type filler product at 8% filler content [5]
Sheffield Porosity (S.F.U.)
Light Scattering Coef. (cm2/g)
The effect of particle size on light scattering gets more interesting when one considers composite-type
particles, such as PCC-s. As shown in Fig. 15, the maximum in light scattering efficiency of the
scalenohedral PCC samples was achieved at an average particle size of about 1.5 µm, far larger than a
wavelength of light [5]. However, this result can make sense if one considers that the primary particles, as
well as the internal void spaces within a PCC-s particle can be in a similar size range as the wavelengths of
visible light [28]. Because of this fact, PCC tends to offer more opportunities for light waves to scatter as
they pass through the paper.
80
70
60
PCC (open)
50
Solid-type
40
30
0.5
1.0
1.5
2.0
2.5
3.0
Average Particle Size (µm)
Fig. 16. Particle-size dependency of sheet
porosity for the same fillers considered in Fig.
15, but at 20% filler content [5]
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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Not surprisingly, paper that contains relatively large, open-type particles also tends to be more porous. As
shown in Fig. 16 [5], there was a strong increase in Sheffield porosity with increasing average particle size
of scalenohedral PCC over the size range considered at a 20% filler level. By contrast, the porosity
resulting from the use of solid-type filler particles did not show a significant dependency on particle size,
and the porosity values were generally much lower at the same level of filler.
OTHER FACTORS
When attempting to bring order to a complex subject, it is worth raising a cautionary note. There are many
factors in a paper mill situation that can lead to unpredictable results. Some factors that are especially with
considering include filler particle surface area, the nature of the fiber furnish – including how well it is
refined, and the effects of calendering.
Surface Area
One of the key variables affecting the performance of composite-type fillers is the size of the primary
particles. As noted earlier, the overall surface area increases as the size of the primary particles decreases.
In some cases a high surface area is desirable, especially if one is interesting in rapid absorption of ink
vehicle [20,29]. High surface area also is desirable if one wants to make paper less porous [9]. However,
high surface area of the filler particle is undesirable if one is mainly interested in the amount of sizing agent
needed to make a sheet of paper resist fluids [5,30]. Also, bulky sheets, resulting from the use of highbulking fillers, sometimes make water removal more difficult [31]. In cases where such factors are critical,
a solid-particle type of filler product may be more advantageous.
Refining
The nature of the fiber furnish, as well as the level of refining, can affect the performance of different fillers
[8,32]. For instance, the importance of fillers in terms of opacity increases with increased refining, since
well-bonded fibers tend to scatter less light [32]. Han and Seo showed subtle shifts in the relative
performance of different sizes and shapes of calcium carbonate filler particles when they were evaluated at
two different levels of freeness [8]. Such effects are understandable when one considers the mechanisms
by which filler can block bonding sites either by (a) bracing the fibers apart from each other, of (b) simple
coverage of fiber surface area. Highly conformable and well-fibrillated fibers, such as well-refined kraft
fibers, ought to be able to drape themselves over any filler particles, thus diminishing the importance of the
“spacing” mechanism in those cases.
Calendering
Much has been made in this review of certain fillers’ ability to produce paper having a higher caliper at a
given basis weight. A typical level of calendering is expect to reduce any relative difference in bulk
resulting from use of different types of fillers [30]. It is worth noting, however, that although caliper is an
important specification in many grades of paper, papermakers seldom consider this variable without also
considering smoothness. All things being equal, extra caliper can be traded – by use of calender
adjustments – for a smoother sheet, in cases where this result is desired [16]. It is wise, however, to
evaluate various types of fillers when developing a new paper or board grade.
Filler Blends
One of the most intriguing aspects of using blends of two or more types of filler particle together is that the
results often do not equal the sum of the individual contributions. For example, mixtures of platey clay
particles with fillers having other shapes has been reported to yield much higher bulk [23]. An opposite
effect is sometimes observed when combining two batches of filler having similar shape but different mean
size; broader size distributions typically pack together more efficiently, yielding a denser structure [33].
Certain blends also have been reported to give especially favorable results in terms of optical properties at a
given level of strength [33,34], though in other cases tests with blends showed a strength penalty [16]. This
TAPPI 2004 Spring Tech. Conf., Altlanta, Paper 7-3
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area of technology continues to yield surprises and challenges that will keep researchers and process
engineers busy for the foreseeable future.
Dispersion of the Filler
As a final caution, you might as well forget about the particle size and shape of a filler product if it not
suitably dispersed. For instance, in making supercalendered (SC) groundwood paper, better dispersion of
the clay filler improved opacity, gloss, and air resistance [35]. Unintended agglomeration of filler has the
potential to make a fine-particle product behave more like a course-particle product, and also as a source of
variability [36].
“You know,” says Mr. van Winkle, “this has been amazing to see what you can do nowadays with different
shapes and sizes of filler particles! I think I’ll just go back to sleep for another 20 years. Then I can come
back and find out what wonderful things you kids are able to come up with in the next 20 years!”
ACKNOWLEDGEMENTS
The author wishes to acknowledge the help of Robert A. Gill of Specialty Minerals, Inc. in obtaining
literature references, illustrative micrographs, and other useful suggestions.
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