HDK® PYROGENIC SILICA FOR HIGH

CREATING TOMORROW’S SOLUTIONS
HDK ® PYROGENIC SILICA FOR
HIGH-PERFORMANCE COATINGS
AND PRINTING INKS
PURE SILICA, PURE PERFORMANCE
WACKER has produced HDK® pyrogenic silica for over 40 years. Due
to our integrated production system with its statistical process control and
highly efficient reactor dynamics, WACKER’s pyrogenic silica is exceptionally pure and offers best-in-class performance.
The Rheology
Expert
Hydrophilic
Hydrophobic
HDK ®
The Flow
Specialist
2
Dispersion
The Process
Enhancer
RHEOLOGY CONTROL
The right rheology is essential for the
manufacture, storage and processing
of a coating formulation. HDK® pyrogenic silica makes it possible to individually adjust the rheology of all key
coating types.
In this way, HDK® ensures that the rheology is fine-tuned during storage, application and curing of a coating formulation. It
prevents pigments and fillers from settling
while stored under low shear force. After
application, HDK® provides excellent flow
on the surface, but prevents sagging.
Optimum Viscosity at All Times
Due to electrostatic interactions, HDK®
dispersed in coating formulations creates a
three-dimensional network, which results
in a viscosity increase in the formulation.
Under shear forces, e.g. due to pumping
or spraying, this network is broken up
and the viscosity decreases (shear thinning). After application, as the shear force
weakens, the HDK® network reforms and
the viscosity increases again (thixotropy).
Figure 1: How the Three-Dimensional HDK® Network Works
Network formation at
rest
Viscosity
HDK® pyrogenic silica is generated
by hydrolysis of chlorosilane in an
oxyhydrogen flame. The process
yields HDK® as highly branched
aggregates which are the basic
building block of our pyrogenic silica.
These form weakly bound agglomerates on cooling. HDK® features a
large specific surface area, which
can be specifically adjusted in the
process. HDK® pyrogenic silica is
hydrophilic, as its surface is rich
in silanol groups. Post-treatment,
i.e. chemical reaction of the silanol
groups, makes hydrophobic silicas
available, too.
The silicas’ large surface-area-tomass ratio enables strong particleparticle interactions. This is the
deciding factor for their use as
coating additives, primarily for
rheology control.
Isolated aggregates
at stress
Viscosity build-up
at rest
Reduction of viscosity Storage
Application
Rebuild of network
after stress
Recovery of
viscosity
Curing
Time
HDK® is a registered trademark of Wacker Chemie AG.
3
Unique
Properties
NUMEROUS
PRACTICAL BENEFITS
Suitable for All Coating Types
With a broad product portfolio of hydrophilic and hydrophobic grades of different
specific surface areas, WACKER offers
the right solution for every kind of paint
and coating:
•Solvent-based, two-component
coatings
•Solvent-based baking finishes and
air-drying coatings
•High-solids systems
•Solvent-free two-component and
UV-curing systems
•Water-based coatings
•Powder coatings
•Printing inks
•Ultrapure synthetic inorganic
additive
•Neutral color and inert
•Migration resistant
•Thermally stable
•Non-hazardous
•Amorphous
Additional Benefits
•No influence on color shade
•High transparency and optimum
gloss
•Reduced water uptake improves
print sharpness
•Enhanced anti-corrosion properties
with hydrophobic HDK®
•Improved alignment of effect
pigments
Rheology / Flow
Properties
•Antisedimentation of pigments, fillers and
matting agents
•Consistent viscosity throughout storage
•Control over sagging and leveling behavior
•Control over flow and fluidization
properties of powder coatings
4
Handling Advantages
•No preactivation in solvents
necessary
•Rheology unaffected by temperature fluctuations
•No subsequent seeding
•Reduced spraying, dusting and
misting
•Packaging solutions that satisfy
requirements
SELECTION OF A
SUITABLE HDK ® GRADE
Basic Guidelines for the Selection
•All HDK® grades listed in Table 1 are
rheologically active
•In general, higher specific surface areas
yield more pronounced rheological effects
•At the same time, grades with higher
specific surface areas are more difficult
to disperse
•For more straightforward product requirements with emphasis on antisedimentation, it is advisable to use HDK®
N20, H15 or H13L
•In general, hydrophobic HDK® achieves
better storage stability
•For polar coating systems, especially
those based on PU or epoxy binders,
the use of hydrophobic HDK® H17 or
H18 is recommended – it’s the only way
of ensuring optimum storage stability
and no sagging
•Where the requirements on the optical quality of a coating film are high,
the use of HDK® grades with large
specific surface areas is preferable,
i.e. HDK® T30, T40, and H30
Table 1 (see page 6) gives an overview of the HDK® grades most commonly used in
coatings. They differ in their specific surface areas and polarities. These two parameters
essentially determine the rheological behavior in a formulation, but the polarity of the
binders and solvents also play a role here. In addition to rheological performance,
coating formulations must meet further practical requirements, such as storage stability,
appearance, efficiency in manufacture (dispersion behavior) and costs. The selection
of the most appropriate HDK® grade on a case-by-case basis should be backed up by
laboratory experiments.
5
DOSAGE RECOMMENDATION
(WT%, BASED ON TOTAL FORMULATION)
•E
mphasis on antisedimentation:
approx. 0.5 – 1.0%
•A
ntisedimentation / rheology control / thixotropy:
approx. 1.0 – 1.5%
•A
nti-sag characteristics of high-build coatings:
approx. 1.5 – 2%, in some cases up to 4%
(e.g. heavy-duty corrosion protection)
Table 1: Overview of HDK® Grades Used in Coatings
BET Surface Area of Hydrophilic HDK® (m2/g)
Polarity of HDK®
150
200
Hydrophilic
HDK® N20
Hydrophobic
HDK® H13L, HDK® H15
HDK® H20
Highly hydrophobic
HDK® H17
HDK® H18
6
300
HDK® T30
HDK® H30
400
HDK® T40
DISPERSION OF HDK ®
Use of a Dissolver
Figure 2 illustrates this by means of a
model system of mineral oils of different
viscosities / HDK®. A dissolver does not
always supply a sufficiently high shear
force for dispersing HDK®. For low viscosity
systems, high shear dispersers such as
stator-rotor mixers or bead mills should
be considered. For systems with higher
viscosities, on the other hand, a dissolver
can achieve adequate grindometer values
of 20 µm.
For the user, this means that a dissolver
should not be used for the dispersion of
HDK® where stringent optical-properties
are required.
Pigmented Systems
In pigmented systems, it is favorable to disperse HDK® together with the pigments.
Clear Coats
For clear coats, the master batch method
has proven a suitable alternative to direct
dispersion. Here, a higher HDK® concentration of approx. 5% is initially dispersed
in the binder-solvent matrix. The desired
HDK® concentration is obtained in the
final let-down. This means that the optimum degree of dispersion can be even
more reliably achieved and, at the same
time, production processes can be made
more efficient.
Figure 2: Grindometer Values over
Dispersing Time, 2% HDK® N20
Grindometer [μm]
The design of the mixing and or milling
equipment used for the dispersion of HDK®
is essential for its successful performance.
Reproducible and optimum rheology performance will only be achieved if HDK®
is well dispersed in the coating system.
Furthermore, the optical properties of a
coating film (e.g. gloss, haze) will improve
with the degree of dispersion of HDK®.
Good dispersion is the result of the intensity of the shearing force applied (disperser
design, size, speed and power) and the
dispersion time.
However the applied shear force during
dispersion must meet a certain minimum
for good dispersion quality.
Longer dispersion times will improve dispersion but with inadequate shear force,
optimum dispersion will not be achieved
even over an extended dispersing time.
200
150
100
50
0
1
5
10
15
20
25
30
Dispersing time [min]
■ Mineral oil, 0.1 Pa·s (dissolver)
■ Mineral oil, 5 Pa·s (dissolver)
■ Mineral oil, 0.1 Pa·s (bead mill)
Grindometer values over dispersing time
(2% HDK® N20 in mineral oils of different viscosities,
tip speed of 10 m/s): in oil with a viscosity of 0.1
Pa·s, HDK® N20 can only be sufficiently dispersed
with a bead mill. A dissolver only has a good dispersion performance in systems with higher viscosities.
Table 2: Dispersion of HDK® T30 in Mineral-Oil Model System
Stirring speed [rpm],
800
2,300
3,800
50-mm toothed disc
Grindometer value [μm]
Viscosity [Pa·s] at a shear
rate of 0.1 s-1
230
12
160
29
80
34
5,300
7,000
60
40
50
44
Dispersion of 3% HDK® T30 in a mineral-oil model system (viscosity 1 Pa·s) by means of a dissolver,
dispersing time is 15 minutes. A higher applied shear force results in improved grindometer values and,
at the same time, a greater thickening effect.
7
PRACTICAL EXAMPLES
1. Rheological Adjustment of an Alkyd-Resin Top Coat
Table 3: Composition of the Top Coat
Binder
Solvent
Pigmentation
Solids content
Dispersion
Medium-oil alkyd resin
Aliphatic/aromatic 8:2
Titanium dioxide
53%
Bead mill
Table 4: Test of Sag Resistance (Based on ASTM D4400, on Glass)
Without HDK®
approx. 30 μm after 1 day and after 28 days
0.7% HDK® N20
approx. 60 μm after 1 day and after 28 days
0.7% HDK® H15
approx. 70 μm after 1 day and after 28 days
In an alkyd-resin-based top coat system,
the addition of 0.7% HDK® N20 or HDK®
H15 has a pronounced thixotropic effect
that is stable in storage. This considerably
enhances the sag resistance of the coating (see Table 4). The use of hydrophobic
HDK® H15 results in approx. 15% more
thickening and slightly enhanced storage
stability as compared to hydrophilic HDK®
N20 (Figure 3). This example shows that
hydrophilic and hydrophobic HDK® grades
achieve qualitatively similar rheology effects
in coating systems of medium polarity.
Viscosity [Pa·s]
Figure 3: Thixotropic Adjustment of a Medium-Oil Alkyd-Resin Top Coat
1.2
D = 0.5 s-1
D = 500 s-1
D = 0.5 s-1
1.0
0.8
0.6
0.4
0.2
0.0
30
4050 6070 8090
100
Time to recover viscosity [s]
HDK® H15 / 1 day
HDK® H15 / 28 days
8
HDK® N20 / 1 day
HDK® N20 / 28 days
Control / 1 day
Control / 28 days
Illustration of the thixotropic properties of alkydresin coatings by means of measuring the recovery
of low shear viscosity following prior intensive
shearing.
2. Control of Settling Behavior in Baking Primer Surfacer
Table 5: Composition of Primer Surfacer
Binder
Polyester / polyurethane
Solvent
Water
Pigmentation
Titanium dioxide
Filler
Barium sulfate
Solids content
51%
Dispersion
Bead mill
Coating systems that contain pigments
and heavy fillers tend to already display
substantial settling after storage times
of only a few weeks. This undesired behavior can be effectively prevented by
the addition of approx. 0.5 to 1% HDK®.
Where there are additional requirements,
for example water repellency of the cured
coating film or anti-sag characteristics at
elevated temperatures (baking systems),
the use of hydrophobic HDK® is recommended.
Aqueous PUR primer surfacer with and without
HDK® as anti-settling agent after one month of
storage at room temperature.
9
3. Two-Component Epoxy High-Build Coating with Good Sag Resistance
The epoxide high-build coating example
illustrates that HDK® can be used to
satisfy the most stringent of demands on
sag resistance and storage stability.
HDK® H18 should preferably be used
here. Figure 4 shows that only this hydrophobic grade can achieve high sag resistance that is stable in storage, which is
essential for heavy-duty corrosion protection, for example. An alternative is HDK®
H17, which is easier to disperse, but
results in a less pronounced rheology
performance (Figure 5).
Wet film thickness [μm]
Figure 4: High Sag Resistance that is Stable in Storage with HDK® H18
600
500
400
300
200
100
0
Blank sample
HDK® H18
1 day storage
HDK® H17
HDK® H13L
HDK® H15
HDK® N20
30 days storage
Anti-sag properties of an epoxide high-build coating containing 2% HDK® (dissolver dispersion) after 1 day
and after 30 days of storage.
Figure 5: Comparison of Rheology Effects with HDK® H18 and HDK® H17
Wet film thickness [μm]
Table 6: Composition of Two-Component
High-Build Coating
Binder
Bisphenol A – epoxide
Solvent
n-Butanol, xylene
Pigmentation
Titanium dioxide
Extender
Barium sulfate
Solids content
80%
Dispersion
Dissolver
Curing agent
Cycloaliphatic amine
700
600
500
400
300
200
100
0
HDK® H18 bead mill 1 day RT
1.5%
2.0%
HDK® H17 bead mill 1 day RT
2.5%
Comparison of the anti-sag properties of an epoxide high-build coating formulated with HDK® H17 or
HDK® H18 (bead-mill dispersion).
10
Small image: without HDK® – clumping on the
sieve.
Large image: with HDK® – no clumping on the
sieve.
HDK ® IN POWDER COATINGS
An important aspect in the formulation
of powder coatings is good flow and
fluidization properties during processing, storage and especially application.
A small amount of HDK® is sufficient to
achieve this, because the HDK® agglomerates, which are only a few μm in size,
accumulate on the surface of the binder
particles of the powder coating and act
as spacers. This reduces the reagglomeration of the binder particles, so the
consistency of the powder coating can
be enhanced and maintained throughout
storage and application of the powder
coating.
The hydrophilic HDK® grades HDK® N20
and HDK® T30 are effective in many powder coating formulations. Hydrophobic
HDK® (e.g. H13L or H30) can give reduced
moisture absorption allowing extended
storage periods in some cases.
It is favorable to add the HDK® during the
grinding step or via a dry blend.
Practical Benefits
•Low dosage of 0.1 – 0.3 wt% based on
total formulation is sufficient
•Retention of flow properties throughout
storage
•Easier and quicker sieving; better fluidization properties during application
•Retention of the electrostatic wrap effect
•No influence on film-formation and curing
11
HDK ® IN PRINTING INKS
Beside rheology control, HDK® offers
further advantages in printing inks.
12
In addition to antisedimentation and
rheology control, HDK® optimizes the
application characteristics in printing inks
in several ways:
•Smooth transfer of the printing ink to
the print medium
•Superior image due to sharper contours
•Fresh prints do not smear
•Regulation of water balance in offset
printing inks
The grades that are primarily used are
HDK® N20, HDK® T30 and hydrophobic
HDK® H15 and HDK® H13L, dosed at
0.3% to 1%.
PRODUCT OVERVIEW
Table 8: Hydrophilic HDK® Grades for Coatings and Printing Inks
HDK® N20
2
BET surface area
[m /g]
175 – 225
DIN EN ISO 9277/DIN 66132
pH value in a 4% dispersion,
approx. 4.1
DIN EN ISO 787-9
Tap density
[g/l]
approx. 40
DIN EN ISO 787/11
Loss on drying, ex works
[wt. %]
< 1.5
(2 h at 105 °C)
DIN EN ISO 787-2
Sieve residue
[wt. %]
< 0.03
DIN EN ISO 787-18
HDK® T30
270 – 330
HDK® T40
360 – 440
approx. 4.1
approx. 4.1
approx. 40
approx. 40
< 1.5
< 1.5
< 0.03
< 0.03
Note: these figures are intended as a guide and should not be used in preparing specifications.
Table 9: Hydrophobic HDK® Grades for Coatings and Printing Inks
HDK® H15
HDK® H20
2
BET surface area of
[m /g]
approx. 120
approx. 170
hydrophobic silica
DIN EN ISO 9277/DIN 66132
pH value in a 4% dispersion
approx. 4.3
approx. 4.3
(1:1 mixture water – methanol)
DIN EN ISO 787-9
Tap density
[g/l]
approx. 40
approx. 40
DIN EN ISO 787/11
Loss on drying, ex works
[wt. %]
< 0.6
< 0.6
(2 h at 105 °C)
DIN EN ISO 787-2
Sieve residue
[wt. %]
< 0.05
< 0.05
DIN EN ISO 787-18
Carbon content
[wt. %]
approx. 1.0
approx. 1.4
DIN EN ISO 3262-20
Surface modification
Silane
Silane
HDK® H30
approx. 250
HDK® H13L
approx. 110
HDK® H17
approx. 90
HDK® H18
approx. 120
approx. 4.3
approx. 4.3
approx. 5.0
approx. 5.0
approx. 40
approx. 60
approx. 50
approx. 50
< 0.6
< 0.5
< 0.6
< 0.6
< 0.05
< 0.05
< 0.1
< 0.1
approx. 2.0
approx. 1.5
approx. 4.0
approx. 4.5
Silane
Silane
Siloxane
Siloxane
Note: these figures are intended as a guide and should not be used in preparing specifications.
Further information about HDK® grades upon request.
13
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The data presented in this brochure are in accordance with the present state of our knowledge, but do not absolve the user from carefully checking all supplies immediately
upon receipt. We reserve the right to alter product constants within the scope of technical progress or new developments. The information given in this brochure should be
checked by preliminary trials because of conditions during processing over which we have no control, especially where other companies’ raw materials are also being used.
The information provided by us does not absolve the user from the obligation of investigating the possibility of infringement of third parties’ rights and, if necessary, clarifying
the position. Recommendations for use do not constitute a warranty, either express or implied, of the fitness or suitability of the product for a particular purpose.
7219e/02.16 replaces 7219e/12.14
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81737 München, Germany
Tel. +49 89 6279-1741
info@wacker.com