AS SEEN IN
Paint
Coatings Industry
Globally Serving Liquid and Powder Manufacturers and Formulators
SPHERICAL
Precipitated Silica
Next-Generation Particle Morphology for Performance in Coatings
By Ronald Romer, Applied Technology Manager,
Huber Engineered Materials, Atlanta, GA
S
ilicon is the eighth most abundant element in
the universe. Silicon dioxide (or silica) is present
in over 90% of the minerals that make up the
earth’s crust. Man’s use of silica-based minerals
dates back to the very origins of recorded human history.
Ancient man used flint to strike sparks for the fires they
needed to stay warm, while today modern man can be
protected from heat of reentry into the Earth’s atmosphere
thanks to silica-based tiles that protect the space shuttle.
I n paints and coatings, siliceous minerals were
known to have been used as pigments in the oldest
surviving cave paintings dating back to 40,000 BC.
Naturally occurring forms of silica and silicate are still
FIGURE 1 » Overview of silica production methods and important physical properties.
Silicon Dioxide
Natural
Silicas and Silicates
SiO2
Synthetic
Amorphous Silicas
and Silicates
Fumed
Gas Phase Flame
Hydrolysis
0.05 - 0.5 μm
Particle Size
50 - 600
BET Surf Area (m2/g)
DBP Absorp. (cc/100 g) 250 - 350
Ignition Loss
1 - 3%
Moisture
< 2.5%
Packed Vol. (mL/100 g) 1000 - 2000
Thickening Effect
Very Strong
Precipitated
Conventional
Wet Process
3 - 40 μm
30 - 800
50 - 280
4 - 6%
4 - 9%
200 - 2000
Moderate
Precipitated
New Huber
Wet Process
3 - 12 μm
5 - >400
30 - 175
4 - 6%
4 - 9%
95 - 300
Low/None
Gel
Wet Process
2 - 20 μm
250 - 1000
75 - 350
3 - 15%
3 - 6%
200 - 2000
Moderate
used today as functional fillers, with the silicon atoms
determining much of the overall character of these
minerals. Silica forms tetrahedral arrangements. The
tendency of these units to form a three-dimensional
framework is fundamental to silica crystal chemistry,
which in turn directs the arrangement of the minerals’
structure into the lattice shapes that determine their
particle shape and other characteristics.1,2
Natural vs. Synthetic Silica
Naturally occurring siliceous mineral fillers include silicon dioxide in forms such as quartz and cristobalite, or
silicate like mica, talc and kaolin. The naturally occurring
minerals typically contain some amount of crystalline silica, either as a minor component or as the principal component of their makeup. While many of the characteristics
of the natural materials are ideal as fillers for coatings
(low binder demand, inertness, etc.), crystalline silica has
gained much recent attention based on health concerns
and regulatory developments. (It is important to note
that synthetically produced materials are amorphous and
typically do not contain crystalline phases of silica.)
Jöns Jakob Berzelius is credited as being first to isolate
silicon and characterize it as an element in 1824, which
paved the way for a considerable history of advances
in silicon-based chemistry.3 The first synthetic silicas
began commercial production around the time of World
War I. The synthetic routes produced materials with
higher purity than the natural mineral varieties, and
enabled chemical and morphological customizations that
expanded the utility of silicas into new applications.
Today’s synthetic silicas are highly specialized materials
that have become critical ingredients in toothpaste, rubber tires, plastics and coatings, with a combined global
production estimated at 3.2 million metric tons for 2016.4
Synthetic silica is produced on an industrial scale via
two primary routes that can be categorized as liquid phase
or gas phase processes. Regardless of the method of preparation, all synthetic silica is amorphous in nature,5 mean-
Reprinted with permission from the January 2017 issue of Paint & Coatings Industry magazine • www.pcimag.com
Hydrolysis
Particle Size
0.05 - 0.5 μm
BET Surf Area (m2/g)
50 - 600
DBP Absorp. (cc/100 g) 250 - 350
Ignition Loss
1 - 3%
Moisture
< 2.5%
Spherical
Precipitated
Packed Vol. (mL/100
g) 1000 - 2000
Thickening Effect
Very Strong
Wet Process
3 - 40 μm
30 - 800
50 - 280
4 - 6%
4 - 9%
Silica
200 - 2000
Moderate
TABLE 1 » Ranges of available physical
properties for Spherilex™ precipitated
silica and silicate.
Range of
Properties
Available
BET SA (m2/g)
Oil absorption (cc/100 g)
Median particle size (mm)
Particle morphology (1st deg.)
5->400
30-140
4-12
Spherical
Frequency
Physical Properties Test
16
14
12
10
8
6
4
2
0
Wet Process
3 - 12 μm
5 - >400
30 - 175
4 - 6%
4 - 9%
95 - 300
Low/None
2 - 20 μm
250 - 1000
75 - 350
3 - 15%
3 - 6%
200 - 2000
Moderate
FIGURE 2 » SEM micrographs of Spherilex™
precipitated silica.
ing its molecular arrangements of silicon and oxygen are
not patterned into the kind of lattices or crystalline structures that are found in the naturally occurring minerals.
While the gas and liquid phase manufacturing processes
both produce materials with amorphous molecular structures, they yield particle morphologies and other properties that are fundamentally different (Figure
10 μm 1).
Spherical Silica
Spherical Silica
White Ceramic Spheres
Gas
Phase
Fumed
Silica
10.17 µm
Mean PS Process:
13.94 µm Mean
PS
0
10.08 µm Median PS
11.24 µm Median PS
Fumed
silica is made
by pyrogenic flame hydrolysis, which
2.36 µm D10
1.36 µm D10
D90
24.49gas
µm D90
is17.19
theµmpredominant
phase process used today. The
anhydrous nature of the pyrogenic process leaves most
surface silanols condensed to siloxane 10
bridges
(-Si-O-Si-),
μm Ceramic Spheres
so that silanol surface density is only about 25% of that of
hydrated silicas. There is consequently less tendency to
absorb water, and the products
typically contain less than
Horiba Laser Scattering
2% free moisture.6
Fumed silicas range in particle size from approxi20
40 to 0.50
60 μm,80
mately
0.05
which 100
is extremely small as
Particle
Sizeproducts
(µm) produced by the wet process
to the
compared
technologies. These particles are comprised of smaller
subunits of only 80-100 nm, which are loosely aggregated into chain-like arrangements that give very low
density and high surface area to the overall particle
structure. These chemical and morphological distinctions of fumed silica have led to its widespread use for
rheology and for gloss control in paints.
Significantly increasing binder demand and matting
Wet Process: Precipitated
effect. Decreasing particle density (less settling).
The two most significant wet phase manufacturing routes
are the gel and precipitated processes, with the latter
accounting for the largest volumes of product sold in the
current marketplace. Precipitated silica is produced using
the conventional wet chemistry process as well as the new
Spherilex™ silica wet chemistry process developed by
Huber Engineered Materials. The new process produces
material with a unique spherical particle morphology,
providing nontraditional silica benefits in coatings.
Variations in the wet reaction process conditions allow
!
manufacturers to control
many aspects of the final product, such as pH, surface area, pore volume (within the
particles), pore size distribution and other structural or
morphological effects. Compared with fumed silica, the
hydrated process produces materials that have a higher
density of silanol groups (-Si-OH) at their surface and
Increasing complexity of inner shape and porosity
absorb water readily. These products are typically sold
with free moisture
content
of 5-6%.
2nd Order
Morphology
Lower 60° gloss, decreased burnish
resistance, increased film reinforcement
Increasing complexity of outer shape
1st Order Morphology
Silica and Silica Gel
For the gel and conventional precipitated silica processes, the particle size and distribution (PSD) are typically
controlled by milling. The Spherilex™ technology is able
to control particle size with exceptionally narrow particle
distributions during the precipitation reaction itself.
Particle size is among the most important differentiators that will determine the function and performance of
the product in its end use. For example, silica products for
rheology control in paints are generally less than 4 μm,
whereas particle size for gloss control may be from 2-20
μm, depending on the system and applied film thickness
of the coating. PSD is also important in terms of dispersability, whereas the finest particles tend to form the most
stable agglomerates and the top-sized particles cannot be
dispersed more finely into paints without the use of highintensity media mills.
New Technology for Old Challenges
The true value of modern synthetic silica technology is
that it expands the functionality of the basic silica chemistry beyond the scope of what the naturally derived minerals can do. The new technology process adds to the functionality of precipitated silica by allowing many aspects of
particle morphology to be customized during the precipitation reaction itself. The new process can produce both
silica and aluminosilicate materials with uniquely spherical particle morphology (Figure 2), exceptionally narrow
particle distribution and a range of structural variations
within the spherical particles, as shown in Table 1.
Overall, the most critical differentiators that determine
the functional characteristics of synthetic silica are its
particle size, size distribution and particle morphology.
The new process is able to produce materials within a
broad range of physical properties, but it is the combination and balance of these properties that makes the new
products unique as compared to other fillers. In the context of paints and coatings, these properties include:
Relatively inert
Low binder demand
Excellent scrub resistance
Compatibility/viscosity stabilization
Low viscosity at high solids
Excellent burnish and abrasion resistance
Significant matting performance (depending on particle size)
• Good transparency – for deep colors and clear coatings
• Very narrow particle distribution
• Extremely low abrasiveness.
•
•
•
•
•
•
•
Particle Size and Particle Size Distribution
The particle size affects many of the physical properties of
a coating before, during and after the coating is applied.
Rheology, gloss/sheen, touch up, toughness and abrasion
resistance are just some of the properties that depend on
the particle size. The largest particles tend to be more efficient for matting, while the smallest particles can be more
difficult to disperse and tend to contribute more toward
the rheology of the system.
Particle size distribution (PSD) is equally relevant to
any properties of the coating that depend on particle size.
Particle Size
BET SurfSize
Area (m2/g)
Particle
DBPSurf
Absorp.
BET
Area(cc/100
(m2/g) g)
Ignition
Loss(cc/100 g)
DBP
Absorp.
Moisture
Ignition
Loss
Packed Vol. (mL/100 g)
Moisture
Thickening
Packed
Vol. Effect
(mL/100 g)
Thickening Effect
Low/None
2 - 20 μm
250
- 1000
2
- 20
μm
75--1000
350
250
3
15%
75 - 350
6%
33- -15%
200
2000
3 - -6%
Moderate
200
- 2000
Moderate
Frequency
Frequency
16
16
14
14
12
12
10
10
8
8
6
6
4
4
2
2
0
00
0
Spherical Silica
White Ceramic Spheres
10.17 µm Silica
Mean PS
Spherical
10.08 µm Median PS
10.17 µm Mean PS
2.36 µm D10
10.08 µm Median PS
17.19 µm D90
2.36 µm D10
17.19 µm D90
13.94 µm
Mean Spheres
PS
White
Ceramic
11.24 µm Median PS
13.94 µm Mean PS
1.36 µm D10
11.24 µm Median PS
24.49 µm D90
1.36 µm D10
24.49 µm D90
10 μm Spherical Silica
10 μm Spherical Silica
10 μm Ceramic Spheres
10 μm Ceramic Spheres
Horiba Laser Scattering
Horiba Laser Scattering
20
20
40
60
80
40
60
80
Particle Size (µm)
Particle Size (µm)
100
100
FIGURE 4 » Morphological scheme of siliceous mineral filler particles,
showing first-order differences in overall shape and second-degree complexities within these shapes. Spherical precipitates occupy a new design
space within the range of available particle morphologies.
Significantly increasing binder demand and matting
effect. Decreasing
particle
density
(lessand
settling).
Significantly
increasing
binder
demand
matting
effect. Decreasing particle density (less settling).
!
!
Increasing complexity of inner shape and porosity
Increasing complexity of inner shape and porosity
2nd Order Morphology
2nd Order Morphology
Lower
60°60°
gloss,
decreased
burnish
Lower
gloss,
decreased
burnish
resistance,
increased
film
reinforcement
resistance,
increased
film
reinforcement
In addition to particle size and PSD, particle structure is
the other major variable that manufacturers control to
provide value and functionality in the silica products sold.
There are many aspects of particle shape or morphology
that determine the function and performance of the filler.
The shapes of synthetic silica and silicate particles can be
simple or complex on many levels. The overall silhouette
or outermost shape of the particle is sometimes referred to
as the first-order morphology (Figure 4). The first-order
morphology, which could be a plate, needle, sphere, etc., is
likely to affect properties of 60° gloss, surface roughness,
film reinforcement or resistance to abrasive damage.
Synthetic silica has a complex particle structure with
further degrees of complexity within its overall first-order
morphology. These second- and third-order attributes can
significantly increase the surface area of the material as
well as the binder and surfactant demand. Any porosity in
the particle structure will decrease the apparent density
of the particles (which is beneficial for anti-settling) and is
likely to enhance matting effects.
Particle structure is one of the most important variables
that synthetic silica manufacturers use to tailor the per-
Moderate
3 - 12 μm
>400
35- -12
μm
- 175
530
- >400
4
6%
30 - 175
9%
44--6%
95
300
4 - -9%
Low/None
95 - 300
FIGURE 3 » Particle size distributions of silica produced by the
Spherilex™ process as compared to commercially available ceramic
spheres, with corresponding SEM images of both samples.
Increasing
complexity
of of
outer
shape
Increasing
complexity
outer
shape
The Significance of Filler Particle
Morphology in Coatings
Very Strong
3 - 40 μm
- 800
330
- 40
μm
50 -- 800
280
30
4
6%
50 - 280
9%
44 -- 6%
200
2000
4 --9%
Moderate
200
- 2000
formance of their products, but the balance of structural
refinements generally compromises some other aspect
of performance. Highly structured particles are most
efficient as matting agents, however, the same structural
effects increase the binder demand and diminish the burnish and scrub resistance of the coating films.
The new silica process is able to manage the first- and
second-order morphologies independently, allowing surface area and porosity within the spherical outer shape.
The spherical shape allows high apparent hardness, excellent burnish and scrub resistance and very low binder
demand, yet the secondary particle structure can be
st
1st1Order
Morphology
Order
Morphology
The PSD reveals how well the mean approximates the size
of the particles in the overall composition. In very broad
distributions, size-dependent effects are more difficult to
predict based on the mean particle size alone.
Particle size blending is a known means of achieving
exceptionally low binder demand and low viscosity at
high solids, particularly when working with spherical
materials.7 Blending requires materials with well-controlled PSD, though most of the filler materials currently
on the market are not controlled sufficiently for the full
potential of this formulating strategy to have been realized by the coatings industry.8
Figure 3 shows a comparison of particle distributions of
spherical silica and a commercially available example of
ceramic spheres. Note these materials have significantly
different particle sizes.
The entry of new narrow-distribution spherical fillers
into the marketplace may therefore provide new opportunities for volatile organic content (VOC) management,
higher solids systems and other innovations.
Narrow particle distributions enable formulators to target very size-specific goals in their systems. For example,
narrow particle distributions demonstrate unique flow
behaviors in liquid suspensions and desirable melt flows
in powder coatings.9, 10, 11
Gloss and sheen optimization are primary examples of
formulating goals that depend on the particle size of the
pigments and fillers. To the extent that matting is a function of particle size, matting efficiency is a function of the
actual number of particles available in that size range.
Narrow distributions are able to concentrate a higher
number of particles in a desired size range, to leverage
both the size and number aspects of matting efficiency
without the detrimental effects of undersized or oversized
particle fractions.
0.05 - 0.5 μm
50- -0.5
600
0.05
μm
250
350
50 - -600
1
3%
250 - 350
< -2.5%
1
3%
1000
- 2000
< 2.5%
Very Strong
1000
- 2000
Lower 6
resistan
Increas
Increasing complexity of inner shape and porosity
2nd Order Morphology
Spherical Precipitates and Paint Rheology
Synthetic silica tends to be very rheologically active, and
viscosity control represents a major usage category for
these materials. In contrast, spherical silicas and silicates
have uniquely low rheology contribution, which allows
these products to be used for other purposes like gloss
reduction and abrasion resistance in rheologically sensitive applications where viscosity build would otherwise
limit or prohibit their use.
The rheology studies in Figures 6 and 7 illustrate the
relative viscosity response of spherical silica precipitates
versus other types of materials and particle morphologies
in two types of industrial coatings where good flow and
leveling are critical. Both systems are fairly resistant to
gloss reduction, so it is advantageous to maintain low
viscosity at the higher loadings required to achieve the
desired matting effect.
In the epoxy, the spherical silica and silicate develops
significantly less viscosity than the fumed or conventionally precipitated silica. In the polyester varnish, the
(μm)
CPVC
BET (m2/g)
Mean PS
70
60
50
135
75
7
46.0
Conventional PPT Silicate 3
120
120
8
50.0
Conventional PPT Silicate 2
92
79
4.9
50.8
Conventional PPT Silicate 1
70
26
9
54.0
Calcined Clay
54
8
3.2
54.5
10
32
1
9.6
59.0
0
15
1.1
9
64
Spherilex SiO2
Ground Calcium Carbonate
Critical PVC
Conventional PPT Silicate 4
40
30
20
y = -0.1287x + 63.774 R² = 0.921
0
25
50
75
100
125
150
Oil Absorption of Pigment (cc/100 mL)
Among the physical properties of these fillers, oil absorption is the property that
shows the strongest correlation with CPVC.
FIGURES 6 and 7 » The spherical particle morphologies maintain very
low viscosity
at higherAloadings
conventional
grades ofVarnish
precipiIn Bisphenol
Epoxy than the
In SB
Polyester Conversion
5% Loading,
By Weight
At Various Loading Levels
tated silicaAtand
silicate.
12.000
In Bisphenol A Epoxy
Brookfield
Brookfield
cPs at 10 RPM
cPs at 10 RPM
Convnt'l PPT Silica 9 μm
At 5% Loading, ByConvnt'l
Weight
PPT Silica 6 μm
10,000
Convnt'l PPT Silica 3 μm
12.000
8,000
10,000
6,000
8,000
4,000
6,000
2,000
4,000
0
0%
2,000
2%
4%
6%
2%
4%
Spherilex
In Silica
SB
Polyester Conversion Varnish
9 μm
At
Spherilex
Silicate 6 μm
Conventional
Spherilex
Fumed Silica
PPT Silicate
7 μm
Silica 9 μm
Convnt'l PPT Silica 9 μm
Calcined
Spherilex Silicate 6 μm
Spherilex
Convnt'l PPT Silica 6 μm
Clay
Spherilex Silica 9 μm
Silicate 6 μm
Nepheline
Convnt'l PPT Silica 3 μm
Conventional
Syenite
Fumed Silica
PPT Silicate 7 μm
Fumed
Calcined
Spherilex Silicate 6 μm
Silica
Clay
Conventional
Spherilex Silica 9 μm
Nepheline
PPT Silica
3 μm
Syenite
Conventional
Fumed
PPT Silica
6 μm
Silica
Conventional
Conventional
PPT Silica 9 μm
PPT Silica 3 μm
8%
10%
12%
Loading Level by Weight
0
0%
6%
8%
10%
Conventional
PPT Silica 6 μm
Various Loading Levels
1 RPM
20 RPM
1 RPM
20 RPM
0
10,000
20,000
30,000
Brookfield cPs
Conventional
PPT Silica 9 μm
12%
Loading Level by Weight
0
10,000
20,000
30,000
Brookfield cPs
FIGURE 8 » Viscosity stability in 59 PVC styrene acrylic flat with 2.5%
silicate by weight.
5,500
Viscosity
Viscosity
(in m.Pa.s)
(in m.Pa.s)
The physical properties of a coating are based on the
binder demand of the solids relative to the binding ability
of the polymers. The relative binder demand and binding
abilities of the raw materials define the critical pigment
volume concentration (CPVC). At pigment concentrations above CPVC, the surrounding continuous phase of
binder is no longer continuous. Air voids occur between
pigment particles, and the physical integrity of the paint
is dramatically reduced.
The ratio of the calculated pigment volume content
(PVC) to the CPVC of the materials is a concept known
as the Q-Factor7 or Lambda (λ).12 Lambda estimates the
overall physical integrity of the system because it reflects
whether the coating is below, near or above CPVC.
The concept of Lambda has taken new significance
because of the challenges with VOC reduction. Solvents
are able to soften polymers, allowing them to flow more
freely to surround and wet the pigment particles in a
coating, thereby enhancing the binding ability of the
binder. As the availability of these solvents diminishes
in the marketplace due to requirements to reduce VOCs,
formulators must seek new ways to minimize the binder
demand of the solids in their formulations. Figure 5
provides examples of several types of fillers and demonstrates how their physical properties correlate with
binder demand.
The spherical precipitated silica-based materials can
have very low oil absorption values relative to conventional precipitates and many other filler types. The low oil
absorption translates to low binder demand and higher
performance properties in paints. Binder absorption is
related to the oil absorption of the solids and their packing
efficiency in the coating film.13
(cc/100 g)
Spherical Precipitated Filler
Performance in Paints
Surf Area
FIGURE 5 » Physical properties of various mineral fillers showing correlation with the CPVC for these materials in paint.
Oil Abs
optimized separately for matting and other performance
needs. In the scheme of siliceous filler morphology, the
novel silica-based materials occupy a new design space
with unique balances of properties.
5,000
5,500
Control (no silicate)
4,500
5,000
Control (no silicate)
Conventional PPT Silicate
4,000
4,500
3,500
4,000
Conventional PPT Silicate
3,000
3,500
Spherilex Silicate
2,500
3,000
2,000
2,500 After 2 hrs
2,000
After 2 hrs
Spherilex Silicate
o/n
1 week
15 days
30 days
5 months
o/n
1 week
15 days
30 days
5 months
2,000
After 2 hrs
o/n
1 week
15 days
30 days
5 months
Spherical Precipitated Silica
FIGURE 9 » Burnish resistance of 41 PVC acrylic flat paint with spherical
fillers at 6% loading.
12
8
85˚ Gloss
Before Burnish
11.4
10
8.1
After Burnish
7.4
6
6.1
5.0
4.9
4
4.8
3.6
Abrasion Resistance of
Spherical Materials
4.2
2.7
2
0
Control
(CaCO3)
Spherilex Silica White Ceramic White Ceramic White Ceramic
8 μm
Sphere 6 μm
Sphere 8 μm
Sphere 12 μm
FIGURE 10 » Scrub resistance of WB architectural acrylic paint with
fillers ASTM
added D2486
at increasing
levels.Scrub, 50 μm Dry Film Thickness
Abrasive
Scrub Cycles Until Failure
Scrub Cycles Until Failure
1000ASTM
900
1000
800
900
700
800
600
700
D2486 Abrasive Scrub, 50 μm Dry Film Thickness
Spherilex Silica
Spherilex
Silicate
Spherilex
Silica
Conventional
Silicate
Spherilex Silicate
Conventional Silicate
500
600
400
500
300
400
200
300
100
200
0
100
0
0
50
100
150
200
250
300
Paint
0 Lbs of
50Silica or
100Silicate
150Filler per
200 100 Gallons
250
300
350
350
Lbs of Silica or Silicate Filler per 100 Gallons Paint
FIGURE 11 » ASTM D4060 Taber abrasion results comparing spherical
fillers in floor coatings.
20.0
18.0
20.0
16.0
18.0
17.8
17.8
16.1
16.1
14.0
16.0
12.0
14.0
13.4
13.4
10.0
12.0
8.0
10.0
10.1
10.1
6.0
8.0
4.0
6.0
2.0
4.0
0.0
2.0
White Ceramic Spherilex SiO2
Spherilex SiO2
Spheres
6.9 μm
9.7 μm
Control
White
Ceramic
Spherilex
SiO2
Spherilex
SiO2
20% by weight
of the selected
filler
in 100% solids
2K Bisphenol
A Epoxy coating.
Spheres
6.9 μm
9.7 μm
20% by weight of the selected filler in 100% solids 2K Bisphenol A Epoxy coating.
0.0
Control
spherical silica shows 10-30% lower viscosity than the
conventional precipitated silica.
In waterborne systems, spherical silica precipitates
have demonstrated the ability to stabilize viscosity drift
in architectural coatings between 40 and 60 PVC, based
on acrylic and styrene acrylic lattices. In some cases, the
spherical silicate was added as a partial replacement for
another filler or TiO2 (Figure 8) to maintain the solids and
PVC of the system. In other cases, it was simply added into
the formulation as an additional component.
Spherical materials are known to possess abrasion resistant properties in paints, and the use of spherical fillers
for burnishing resistance and scrub resistance is also well
known.13 Spherilex™ silica and silicate provides many of
the same benefits in coatings, yet they can be used in deep
colors/clears or thin film applications where competitive
technologies cannot be used due to their color or broad
particle distribution.
The example shown in Figure 9 demonstrates competitive burnish performance of the spherical silica in architectural coatings versus conventional and other types
of spherical fillers. The spherical silica precipitate easily
surpasses the performance of the conventional precipitate
and performs similarly to the ceramic spheres.
The benefits of low binder demand and spherical
morphology can also be seen in different types of wet or
dry abrasion tests. ASTM D2486 scrub test data (Figure
10) shows superior scrub performance of spherical silica
and silicate versus conventional precipitated silicate
pigment. Versus other spherical fillers (Figure 11), the
spherical silica resists abrasion comparable (and better
in some cases) to ceramic or glass spheres, which are
based on much harder and denser materials. Moreover,
it is important for the particles to remain lodged in the
film in order to affect abrasion resistance and reinforcement via stress transfer within the matrix.5,14 The performance of the spherical precipitate in both wet and dry
abrasive testing suggests that the second order morphological distinctions of the spherical silica may facilitate
the adhesive interaction with the surrounding polymer.
Matting and Gloss Reduction
Matting studies lend important insight to the unique
properties of spherical silica and silicate. Matting performance is both a function of particle size and particle morphology.
Figure 12 provides an example of a waterborne polyurethane clear finish, in which the diatomaceous silica
and the precipitated Silica A (both matting agents) showed
the most pronounced matting effects. Between these two,
the diatomaceous product is slightly larger but has lower
particle structure as reflected by the oil absorption, both
of which contribute to its efficiency as a matting agent. In
conclusion, particle size helped the matting efficiency of
the diatomaceous material.
Based on size, it is expected the 8.6 μm ceramic spheres
to perform similarly to Silica A, yet this ceramic material
shows the weakest matting effect. This result speaks to the
20% by weight of the selected filler in 100% solids 2K Bisphenol A Epoxy coating.
Conclusion
Although mineral filler technologies have evolved considerably throughout the course of their use in paints
and coatings, regulatory concerns and other challenges
have resulted in a decreasing array of available materials from which to formulate. However, the performance
requirements for coatings are increasing. In this challenging environment, true innovations will continue to
be in strong demand.
The new Spherilex™ technology addresses longstanding challenges in manufacturing and also avails a
new range of functional silica and silicate products with
unique performance profiles that help formulators realize
new value in synthetic silica technology. n
For more information, contact [email protected] or visit
www.hubersilicas.com.
References
1 RT Vanderbilt Company. “Functional Silicate Fillers: Basic
Principles.” Paint & Coatings Industry, August, 2002.
2 Bergna, H.E.; Roberts, W.O. Colloidal Silica Fundamentals
and Applications. Boca Raton: CRC Press Taylor & Francis
Group, 2005.
3 Ferch, H. Amorphous Synthetic Silica Products in Powder
Form. Production and Characterization. Progress in Organic
Coatings, volume 9 issue 2, 135-163. The Netherlands: Elsevier B.V., 1981.
4 Future Market Insights. Precipitated Silica to Account for
70% Volume Share of Global Demand for Specialty Silica
in 2016. PR Newswire. May 26, 2016. Accessed November 8, 2016. http://www.prnewswire.com/news-releases/
precipitated-silica-to-account-for-70-volume-share-of-
FIGURE 12 » Gloss reduction as a function of pigment loading in
WB polyurethane varnish using fillers with different morphologies
and particle size.
100
Mean Oil
PS
Abs
90
BET
Sur. A
(μm)
(cc/100 g)
Conventional
PPT Silica A
8.8
211
113
Conventional
PPT Silica B
5.5
215
175
Spherilex
PPT Silica C
11.0
70
33
40
Spherilex
PPT Silica D
5.4
41
1
30
Spherilex
PPT Silica E
6.9
75
147
20
Diatomaceous
12.0
Silica
120
32
10
Wht Ceramic
Spheres 8.6μm
16
1
80
70
85° Gloss
significance of particle structure in determining matting
performance. Note Spherical Silica D and E showed very
little matting effect as compared to Conventional Silica
B. In this case again, the higher structure of the conventional precipitate provided greater matting effect.
The 11.0 μm Spherical Silica C showed some of the
strongest matting performance, though it has much lower
structure than Conventional Silica B (also a matting
agent). Here also, the size and narrow size distribution
appear to excel over the milled conventional precipitate.
In summary, it can be seen that spherical silica has the
ability to selectively provide matting efficiency via particle structure or PSD effects, which do not significantly
increase the binder demand, as is the case with prior generation materials.
60
50
0
0%
2%
4%
Loading Level by Weight
6%
global-demand-for-speciality-silica-in-2016-future-market-insights-fmi-580992831.html.
5 Katz, H.S.; Milenewski, J.V. Handbook of Fillers for Plastics.
Nostrand Reinhold Co., 1987.
6 Ciullo, P.A. Industrial Minerals and Their Uses: A Handbook and Formulary. Noyes Publishing, 1996.
7 Gysau, D. Fillers for Paints. Vincentz Network, 2011.
8 Michel-Sanchez, E. Impact of Particle Morphology on the
Rheology of PCC-Based Coatings. Georgia Institute of Technology, 2005.
9 US Pat 6737467 B1 Decker and Sparks. E. I. Du Pont De
Nemours and Co., 2000.
10 Pusheng Ting, A.; Luebbers, R.H. Viscosity of Suspensions
of Spherical and Other Isodimensional Particles in Liquids.
AIChE Journal volume 3, issue 1, 1957.
11Mueller, S.; Llewellin, E.W.; Mader H.M. The Rheology of
the Suspension of Solid Particles. Proceedings of The Royal
Society A 2010 466. 1201-1228.
12Bierwagen, G.P.; Hay, T.K. The Reduced Pigment Volume
Concentration as an Important Parameter in Interpreting
and Predicting the Properties of Organic Coatings. Progress
in Organic Coatings volume 3, issue 4,281-303. Switzerland;
Elsevier B.V., 1975.
13Koleske, J.V. Paint and Coatings Testing Manual, Fourteenth Edition of the Gardner/Sward Handbook. ASTM
Publications. ISBN 0-8031-2060-5, 1995.
14Lee, S.M. Handbook of Composite Reinforcements. VCH
Publishers, 1993.
8.6
(m2/ g)