SILICONES HDK® - Wacker Chemie AG

S I LI C O N E S
THERE’S MORE TO IT
THAN YOU THINK.
HDK ® – PYROGENIC SILICA
CREATING TOMORROW’S SOLUTIONS
HDK ®
WHETHER TOP PERFORMANCE,
INTEGRATED SOLUTIONS OR GLOBAL
SERVICE – WE MEET YOUR INDIVIDUAL
REQUIREMENTS
Contents
Competence and Performance
Perfect Properties
HDK® Applications
Production Processes
Chemical Structure
Individual Viscosity
Reinforcement Where It’s Needed
Optimum Flow Properties
A Survey of Product Data
Physicochemical Properties
of HDK® Grades
Applications and Effects
Customized Packaging
WACKER at a Glance
You want to gain a competitive lead
by maximizing product capabilities
and at the same time make your
production processes as efficient as
possible? Then HDK® is the right
choice for you. This pyrogenic silica
boosts performance in every respect.
Whether in the form of corrosion-protection coatings or high-resolution printed
images – many different products make
a lasting impression with exceptional
performance levels. The secret behind
this variety of excellent properties is
HDK®.
WACKER started producing pyrogenic
silica over thirty years ago and has continued to develop it ever since. HDK®
is now a performance enhancer that will
allow you to optimize a whole range of
product properties. HDK® gives you perfect control over thickening, thixotropic
and flow properties, and makes for optimum performance in thermal insulation,
reinforcement and chemical-mechanical
planarization. In applications for an
extremely wide range of industries, from
foods and cosmetics, pharmaceuticals,
through paints and surface coatings,
composites, adhesives and sealants,
elastomers and toners, to paper coatings.
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All this is backed up by 40 years’ experience in silicon chemistry, a complete
product range, uniformly high quality
standards and comprehensive individualized service.
WACKER has also prepared for continual market growth by expanding its
Nünchritz site for the development
of silicon-based products and boosting
silica production capacities. With three
production sites, we are now one of
the world leaders in the manufacture
of pyrogenic silica. You can count on
continuity of supplies from us, and rest
assured that we will stay one step ahead
of market needs.
A Partner with Initiative
In WACKER SILICONES, you have a
globally active technological leader,
and an innovation partner at your side.
We understand your needs precisely,
and develop integrated solutions that
reinforce your competitiveness and open
up future markets for you.
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HDK ® ADDS PERFECTION
TO YOUR PRODUCT PROPERTIES
WACKER SILICONES has developed
a wide range of HDK® grades for customizing product properties to specific applications.
Fascinatingly Versatile
Even though HDK® pyrogenic silica is an
established and highly successful product, WACKER SILICONES continues to
push forward with its R&D. There are
always new potential solutions that can
further boost product performance and
quality. The extensive selection of available hydrophilic and hydrophobic HDK®
grades and HDK® dispersions offer a
highly versatile and comprehensive solution to a wide range of needs.
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The key uses of HDK® are as rheology
control and reinforcing agents, thickeners
and flow enhancers. The table opposite
shows what effects amorphous silica
has in various fields of application. Our
HDK® brochures on individual applications will tell you about the benefits for
specific application areas and the special advantages for your product. You
can order them directly from WACKER
SILICONES. Or visit us on the internet
at: www.wacker.com/hdk
Impressive Expertise
WACKER SILICONES offers system
solutions. That means that in addition
to our applications-oriented products,
we also offer customer-oriented service
through our technical centers and sales
subsidiaries. We have a local presence
worldwide.
Our services include
• Technical support by experts
• Laboratory support to optimize your
products and formulations
• Joint R&D projects
• Tailored pyrogenic fumed silicas with
customized properties
• Information material, product data
sheets and up-to-date publications
Applications of HDK®
Field of Application
Unsaturated polyester
PVC
Printing inks
Paints and coatings
Adhesives and sealants
Insulating gels
Accumulators
CMP
Natural and synthetic rubber
Silicone elastomers
Grouting compounds
Toners and developers
Cosmetics, pharmaceuticals,
animal feed and foods
Bulk materials
Paper coating
Textile impregnation
Effect of HDK®
Thickening, thixotropic agent, antisedimentation
Thickening, thixotropic agent, antisedimentation, anti-adhesive agent, flow improver,
improvement of dielectric properties
Thickening, thixotropic agent, antisedimentation, regulation of water content,
improved brilliance and contrast
Thickening, thixotropic agent, antisedimentation, in powder coating systems, additional optimization
of flow and electrostatic application properties
Thickening, thixotropic agent, antisedimentation, processing auxiliary, adhesion improver
Thickening, thixotropic agent, water repellency
Thickening, thixotropic agent
Mechanical planarization
Reinforcement
Reinforcement, increase of dielectric properties
Reinforcement, thixotropic agent
Free-flow agents, external charge control for positive and negative systems, self-cleaning
Thickening, thixotropic agent, antisedimentation, free-flow agents, carriers for actives,
tableting aids
Free-flow agents, processing auxiliaries, flow enhancers
Adsorptive binding of the liquid ink, fixing of the ink colorant or pigments
Stabilization
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THE EFFECTIVENESS OF HDK ® IS A
RESULT OF THE PRODUCTION PROCESS
Reactor
Air
Molecules
SiO2
SiO2
SiO2
SiO2
Protoparticles
Primary
particles
Aggregates
Agglomerates
>1,500 °C
SiCl 4+ 2H2 + O2
SiO2 + 4HCl
HDK pyrogenic silica
Formation in a flame
®
WACKER has been producing pyrogenic silica for the world market since
1968, and has continually optimized
its production processes ever since.
HDK® is produced in multiple sites by
an integrated, efficient and environmentally compatible process that
saves energy and raw materials.
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Hydrophilic HDK® with a Large
Surface Area
The production parameters are
responsible for the excellent properties
of hydrophilic, pure and odorless HDK®.
Pyrogenic silica and hydrogen chloride
are formed by hydrolysis at over 1,000 °C.
This is performed by introducing volatile
chlorosilanes into an oxyhydrogen flame,
producing high-viscosity SiO2 primary
particles about 5 – 30 nanometers (10–9 m)
in size. These particles have a smooth
surface that is not microporous. In the
flame, the primary particles fuse together
permanently to form large units, or
aggregates (100 – 1,000 nm in size).
That is to say, individual primary particles only exist in the reaction zone itself.
On cooling, the aggregates mechanically
entangle to form agglomerates, known
as tertiary structures. They are about
1 – 250 micrometers (10-6 m) in size and
the inhalable fraction is less than 1%.
HDK® agglomerates have a special
feature: they are open-structured and
therefore mesoporous. They also have
a very high BET specific surface area
because of the small diameters of
the primary particles that are fused
together in the aggregates, and the
large accessible surface areas of the
aggregates and agglomerates. This
large surface area-to-mass ratio causes
intense inter-particular interactions, which
are the result of attractive dispersion
and dipolar forces. And that is precisely
the reason for the outstanding rheological effect of HDK®.
Important Note:
The terms primary particles, aggregates and agglomerates are taken from
DIN 53206 part 1 (08/72). Aggregates
are defined as primary particles that
have amalgamated to form a planar or
angular structure; these aggregates
cannot be further disintegrated.
Agglomerates are defined as collections of primary particles that have
fused to form a face-to-face sintered
structure.
Hydrophobic HDK® with Modified
Surface
Hydrophobic HDK® is available in a
wide range of grades to meet different
requirements.
The hydrophobic agents are applied
with a gas phase to distribute them
uniformly on the silica particle surfaces.
Controlled quantitative chemical addition
of the modifiers to the silica surfaces is
achieved by means of hydrolysis-resistant Si-O-Si bonds. The degree of modification is ensured by precise monitoring
of process parameters, such as volume
flow rate, temperature and residence
time. Uniform product quality is ensured
by almost real-time process analysis,
such as carbon-content determination.
Typical modifiers include:
• Organochlorosilanes, such as
dimethyldichlorosilane
• Polydimethylsiloxanes
• Hexamethyldisilazane
• Long-chain alkylsilanes, such as
octylsilanes
High-Stability Aqueous Dispersions
HDK® dispersions are highly stable
against, for example, sedimentation
and gelling. This is the result of efficient
colloidal stabilization combined with
high shear forces during dispersion of
the silica.
The colloidal stability of the dispersions
is ensured by electrostatic stabilization
and steric stabilization if appropriate.
During dispersion, high shearing energy
is introduced by means of high-speed
shearing units, such as rotor-stator
systems. To ensure product quality, the
progress of dispersion is checked continually by various analytical methods,
including particle size determination by
photon correlation spectroscopy, laser
diffraction or zeta potential measurements.
Production of hydrophobic HDK®
Si
OH +
Si
∆T
Si
Si
O
–XH
X
X = Hal, O-Alk, OH, O2/2SiR2
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TOP PERFORMANCE
HAS A SPECIAL CHEMISTRY
Hydrophilic HDK®
Hydrophilic HDK®
Amorphous
SiO2
OH
Si
O O
O
O
O
Si O
O Si
O
O
O
Si
O
5 – 50 nm
OH
Si
O
O
O
Si O
O Si
O
O
O
O
Si
O
OH
O
OH
OH
Si
O O
O
O
O
Si O
O Si
O
O
O
Si
O
OH
Reactive isolated
silanol groups
Large surface area 50 – 400 m2/g
No micropores
Chemical Structure
HDK® is made up of SiO4/2 tetrahedrons.
These tetrahedrons are joined together
by siloxane bridges (Si-O-Si bonds).
About every second Si atom on the
surface of HDK® bears a hydroxyl
group and thus forms a silanol group
(≡Si-OH).
H atom
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C atom
O atom
Si atom
Hydrophilic Property
HDK® receives its outstanding
hydrophilic properties from the Si-OH
groups and the polar Si-O-Si backbone.
The silanol groups (Si-OH) act as reactive centers and form hydrogen bonds
to other polar substances. Hydrophilic
HDK® has ca. 2 silanol groups per
1 nm2.
Hydrophobic HDK®
Hydrophobic HDK®
Amorphous
SiO2
Me
5 – 50 nm
O
Me Me
Si
O
Me
O
O Si
Si O O O
O
O O
O Si
O
Si
O
Si
Me
O
O
Me Si
Me
O
Me
Me
O
Si Me
Si
O O O
O
O
Me
Si O
O Si
Si
O
Me
O
O O
Si
O
O
Si
Me
Si
O
O
O
O
O
O
Me
Si O
O Si
Si
O
Me
O
O O
Si
O
O
Si
Me
Me
O
Si
Me
Me
Large surface area 50 – 400 m2/g
No micropores
Hydrophobic Properties
If the Si-OH groups are allowed to
react with organosilicon compounds,
hydrophobic properties are imparted
to HDK®. Hydrophobic HDK® has ca.
1 silanol group Si-OH per 1 nm2. Highly
hydrophobic HDK® has ca. 0.5 Si-OH
groups per 1 nm2. The hydrophobic
treatment reduces or eliminates the
moisture absorption of the silica. Other
basic properties of the pyrogenic amorphous silica are not changed.
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Intensity [cps]
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40
35
30
25
20
15
2 theta [degrees]
Toxicological Properties
If used as recommended, HDK® is not
known to have any harmful effects on
humans. HDK® does not irritate the skin.
Degreasing of the skin may occur after
prolonged contact. Unlike crystalline
silica, there are no indications of HDK®
causing silicosis.
X-ray diffraction diagram
of amorphous HDK®
Intensity [cps]
Electron diffraction diagram
of amorphous HDK®
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In animal experiments there are no
indications of reproductive effects.
Amorphous silica such as pyrogenic
silica, precipitated silica and silica gel
is not carcinogenic (Group 3 “not
classifiable as to its carcinogenicity to
humans,” IARC Monograph Vol. 68,
1997.
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Electron diffraction pattern
of crystalline quartz
Amorphism
HDK® is a completely amorphous
product. This is proved by the absence
of a sharp X-ray diffraction pattern and
a characteristic electron diffraction
diagram for amorphous structures. The
HDK® X-ray diffraction diagram, unlike
that of crystalline quartz, shows a
broad, unresolved diffraction structure.
This is typical of amorphous structures.
The electron diffraction diagram of HDK®
has a diffuse scattering pattern. In a
reference measurement with crystalline
quartz, distinct, well-resolved scattering
signals can be seen.
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40
35
30
25
20
15
2 theta [degrees]
Further information on toxicology is
given in the appropriate material safety
data sheets.
X-ray diffraction pattern
of crystalline quartz
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Viscosity ␩
HDK ® PRECISELY ADJUSTS VISCOSITY
AS REQUIRED
Shear rate ␥ⴢ
Flow curve of a system filled with HDK®
Precise Thickening and Thixotropic
Properties
The shear-thinning or thixotropic effect
of HDK® offers you perfect control over
the flow properties of liquid systems.
The system thus has precisely the
viscosity required for the particular
application.
In shear-thinning systems, the viscosity
is reduced by shearing loads. The shearthinning liquid can also have a yield
point, and therefore, in a stationary state,
form a viscoelastic solid (gel structure).
The shear-thinning of non-Newtonian
liquids is generally a reversible process.
If, when the shear force is removed, the
system returns to its original viscosity
after a time delay, the behavior is called
thixotropic.
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Efficient Increase of Viscosity
If HDK® is dispersed in a liquid, a threedimensional network can form as a
result of forces of attraction between
the HDK® aggregates. This network,
together with the immobilized liquid
within the silica aggregates, causes a
strong viscosity increase. Since HDK®
has a particularly high structure, it is
very efficient and even small quantities
are enough to provide a high viscosity.
The forces of attraction in hydrophilic
HDK® consist mainly of van der Waals
forces such as dispersion or dipolar
attractions. If it is allowed to physically
adsorb oligomeric or polymeric substances, such as binders, it can provide
further effects, such as polymer bridging
or steric stabilization. In the case of
hydrophobic HDK®, overlapping silylated
surfaces result in additional hydrophobic
interactions and forces of adhesion.
Effective Reduction of Viscosity
Liquids containing HDK® as an additive
undergo a reduction in viscosity when
subjected to shear forces, such as
stirring, spraying or spreading. Shear
forces break down the three-dimensional silica network. The silica aggregates can move with respect to one
another and the immobilized liquid
volume becomes smaller.
The stronger the shear force, the
stronger is the viscosity-reducing effect.
If necessary, the viscosity can even be
reduced to a minimum plateau value.
Once shearing stops, the liquid regains
its original viscosity. The time it takes
for this structural relaxation to occur
depends on the type and amount of the
silica used and the physicochemical
properties of the liquid, for example the
viscosity or polarity of the pure liquid.
Stress [N/mm2]
HDK ® SELECTIVELY IMPROVES
PROPERTIES
Fracture
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With 30 % HDK®
2
Without filler
100
500
Strain [%]
Deformation
Tensile-stress/strain diagram of silicone elastomers with and without HDK®
Tailored Elasticity
For processors of natural and synthetic
rubber and of silicone elastomers to
obtain the best results, their products
must have precisely defined mechanical
properties, such as tensile strength,
elongation at break and tear strength.
The required system properties can
be obtained with HDK®.
Optimum Reinforcement
The outstanding reinforcing properties
of HDK® are the result of its huge specific surface area and the numerous
points of interaction with the polymer
chains in the elastomer network. The
specific particle structure of HDK®
distributes the mechanical stresses and
forces throughout the elastomer network
and restricts the entropy space of the
polymer chains. This process increases
the elastomers’ strength, elongation at
break and load bearing capacity.
High Sag Resistance
In non-polar silicone rubber, HDK® can
be used as both a reinforcing thickener
and rheological additive. The pronounced
thickening effect of hydrophilic HDK®
provides the necessary sag resistance,
so that low-viscosity systems do not
run off vertical surfaces.
Good Processability
In silicone elastomers, hydrophilic HDK®
provides excellent reinforcement while
increasing the viscosity, which may
make it difficult to process. Process
auxiliaries such as low-molecular plasticizers are used here, which block the
interaction between the HDK® particles
by adsorption and reduce the viscosity.
This provides the system with the
required excellent processing properties.
Hydrophobic HDK® grades can be
used to obtain desired properties, i.e.
high tear strengths together with good
processability. WACKER SILICONES
offers grades with modified structures
for this purpose.
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HDK ® ENSURES OPTIMUM
FLOW PROPERTIES
Versatile Applications
HDK® can optimize the flow properties
of different powder-form cohesive solids.
And it is economical, since even tiny
amounts of HDK® greatly improve the
flow properties of such substances.
Typical applications include bulk materials and powders for cosmetics, foods,
animal feeds, pharmaceutical or fireextinguisher powders. WACKER
SILICONES also offers special highly
sophisticated HDK® grades for improving the flow and electrical charging
properties of toners, developers and
powder paints for automotive finishes
and industrial coatings.
Free Flow
When HDK® is mixed with the solid
powder, it is distributed on the surfaces,
separating the individual powder particles from one another. This allows the
powder to flow freely.
Effective Drying
The high specific surface area of
hydrophilic HDK®, together with its high
surface energy, allows moisture to bind
effectively to the surface of hygroscopic
powders. This drying effect enhances
the powder’s flow properties. Typical
applications include finely divided food
solids, such as table salt.
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PACKAGED TO CUSTOMERS’ NEEDS
Packaging Units
The smallest packaging unit leaving our
production is the pallet. Each pallet is
shrink-wrapped in PE foil to protect the
HDK® in the multilayer valved paper
bags within against moisture. This allows
the product to be stored for several
months in dry conditions. If the shrinking
foil is damaged or single bags are
removed, care must be taken to protect
the remaining bags / individual bags
against moisture by either wrapping in
plastic or other appropriate measures.
The quantity of bags per pallet depends
on the HDK® grade and its bulk density.
Bags are available in various weights to
suit customer demands. The data sheet
indicates which form of packaging is
used for a particular product.
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Big Bags
Special HDK® grades are available in Big
Bags. These woven super sacks are
delivered on an individual pallet. The Big
Bags are also shrink-wrapped in polyethylene film as reliable moisture protection and to ensure stability during transport. Another advantage of this form of
container is dust-free HDK® processing.
Bulk Vehicles
WACKER SILICONES offers delivery in
bulk vehicles. Since the full load must
be accepted, this form of delivery
assumes that the customer has a storage silo big enough to receive the entire
delivery.
Supply of HDK® Dispersions
All HDK® dispersions are delivered in
200-liter PE drums, or alternatively in
road tankers.
Handling and Product Safety
HDK® is not classified as a hazardous
substance as defined by the German
chemicals law (Chem G) and the law
on the transport of hazardous goods
(GG BefG).
Further information on toxicology is
given in the appropriate material safety
data sheets. Detailed information on
handling can be found in the product
data sheets.
6173e/08.10 supersedes 6173e/10.05
S I LI C O N E S
Wacker Chemie AG
Hanns-Seidel-Platz 4
81737 München, Germany
hdk@wacker.com
www.wacker.com/hdk
Wacker Chemical Corporation
3301 Sutton Road
Adrian, MI 49221-9397
USA
Tel. +1 517 264-8500
Fax +1 517 264-8246
hdk@wacker.com
www.wacker.com/hdk
Wacker Chemicals Trading
(Shanghai) Co. Ltd.
31 F., Bank of China Tower
200 Yin Cheng Road Central
Pudong
Shanghai 200 120
China
Tel. +86 21 6100-3400
Fax +86 21 6100-3500
hdk@wacker.com
www.wacker-chemicals.com.cn