THERE’S MORE TO ITTHAN YOU THINK.HDK® – PYROGENIC SILICA

CREATING TOMORROW’S SOLUTIONS

S I L I C O N E S HDK®

WHETHER TOP PERFORMANCE,INTEGRATED SOLUTIONS OR GLOBALSERVICE – WE MEET YOUR INDIVIDUALREQUIREMENTS

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You want to gain a competitive leadby maximizing product capabilitiesand at the same time make yourproduction processes as efficient aspossible? Then HDK® is the rightchoice for you. This pyrogenic silicaboosts performance in every respect.

Whether in the form of corrosion-protec-tion coatings or high-resolution printedimages – many different products makea lasting impression with exceptionalperformance levels. The secret behindthis variety of excellent properties isHDK®.

WACKER started producing pyrogenicsilica over thirty years ago and has con-tinued to develop it ever since. HDK®

is now a performance enhancer that willallow you to optimize a whole range ofproduct properties. HDK® gives you per-fect control over thickening, thixotropicand flow properties, and makes for opti-mum performance in thermal insulation,reinforcement and chemical-mechanicalplanarization. In applications for anextremely wide range of industries, fromfoods and cosmetics, pharmaceuticals,through paints and surface coatings,composites, adhesives and sealants,elastomers and toners, to paper coatings.

A Partner with InitiativeIn WACKER SILICONES, you have aglobally active technological leader, and an innovation partner at your side.We understand your needs precisely,and develop integrated solutions thatreinforce your competitiveness and openup future markets for you.

All this is backed up by 40 years’ expe-rience in silicon chemistry, a completeproduct range, uniformly high qualitystandards and comprehensive individu-alized service.

WACKER has also prepared for con-tinual market growth by expanding itsNünchritz site for the development of silicon-based products and boostingsilica production capacities. With threeproduction sites, we are now one ofthe world leaders in the manufactureof pyrogenic silica. You can count oncontinuity of supplies from us, and restassured that we will stay one step aheadof market needs.

ContentsCompetence and Performance 3Perfect Properties 4HDK® Applications 5Production Processes 6Chemical Structure 8Individual Viscosity 12Reinforcement Where It’s Needed 13Optimum Flow Properties 15A Survey of Product Data 16Physicochemical Properties of HDK® Grades 17Applications and Effects 18Customized Packaging 20WACKER at a Glance 21

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WACKER SILICONES has developeda wide range of HDK® grades for cus-tomizing product properties to specif-ic applications.

Fascinatingly VersatileEven though HDK® pyrogenic silica is anestablished and highly successful prod-uct, WACKER SILICONES continues topush forward with its R&D. There arealways new potential solutions that canfurther boost product performance andquality. The extensive selection of avail-able hydrophilic and hydrophobic HDK®

grades and HDK® dispersions offer ahighly versatile and comprehensive solu-tion to a wide range of needs.

The key uses of HDK® are as rheologycontrol and reinforcing agents, thickenersand flow enhancers. The table oppositeshows what effects amorphous silicahas in various fields of application. OurHDK® brochures on individual applica-tions will tell you about the benefits forspecific application areas and the spe-cial advantages for your product. Youcan order them directly from WACKERSILICONES. Or visit us on the internetat: www.wacker.com/hdk

Impressive ExpertiseWACKER SILICONES offers systemsolutions. That means that in additionto our applications-oriented products,we also offer customer-oriented servicethrough our technical centers and salessubsidiaries. We have a local presenceworldwide.

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

HDK® ADDS PERFECTION TO YOUR PRODUCT PROPERTIES

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Applications of HDK®

Field of Application Effect of HDK®

Unsaturated polyester Thickening, thixotropic agent, antisedimentation

PVC Thickening, thixotropic agent, antisedimentation, anti-adhesive agent, flow improver, improvement of dielectric properties

Printing inks Thickening, thixotropic agent, antisedimentation, regulation of water content, improved brilliance and contrast

Paints and coatings Thickening, thixotropic agent, antisedimentation, in powder coating systems, additional optimization of flow and electrostatic application properties

Adhesives and sealants Thickening, thixotropic agent, antisedimentation, processing auxiliary, adhesion improver

Insulating gels Thickening, thixotropic agent, water repellency

Accumulators Thickening, thixotropic agent

CMP Mechanical planarization

Natural and synthetic rubber Reinforcement

Silicone elastomers Reinforcement, increase of dielectric properties

Grouting compounds Reinforcement, thixotropic agent

Toners and developers Free-flow agents, external charge control for positive and negative systems, self-cleaning

Cosmetics, pharmaceuticals, Thickening, thixotropic agent, antisedimentation, free-flow agents, carriers for actives, animal feed and foods tableting aids

Bulk materials Free-flow agents, processing auxiliaries, flow enhancers

Paper coating Adsorptive binding of the liquid ink, fixing of the ink colorant or pigments

Textile impregnation Stabilization

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WACKER has been producing pyro-genic silica for the world market since1968, and has continually optimizedits production processes ever since.HDK® is produced in multiple sites byan integrated, efficient and environ-mentally compatible process thatsaves energy and raw materials.

Hydrophilic HDK® with a Large Surface AreaThe production parameters areresponsible for the excellent propertiesof hydrophilic, pure and odorless HDK®.

Pyrogenic silica and hydrogen chlorideare formed by hydrolysis at over 1,000 °C.This is performed by introducing volatilechlorosilanes into an oxyhydrogen flame,producing high-viscosity SiO2 primaryparticles about 5 – 30 nanometers (10–9 m)in size. These particles have a smoothsurface that is not microporous. In theflame, the primary particles fuse togetherpermanently to form large units, oraggregates (100 – 1,000 nm in size). That is to say, individual primary parti-cles only exist in the reaction zone itself. On cooling, the aggregates mechanicallyentangle to form agglomerates, knownas tertiary structures. They are about1– 250 micrometers (10-6 m) in size andthe inhalable fraction is less than 1%.

HDK® agglomerates have a specialfeature: they are open-structured andtherefore mesoporous. They also have a very high BET specific surface areabecause of the small diameters of the primary particles that are fusedtogether in the aggregates, and thelarge accessible surface areas of theaggregates and agglomerates. Thislarge surface area-to-mass ratio causesintense inter-particular interactions, whichare the result of attractive dispersionand dipolar forces. And that is preciselythe reason for the outstanding rheologi-cal effect of HDK®.

Important Note:The terms primary particles, aggre-gates and agglomerates are taken fromDIN 53206 part 1 (08/72). Aggregatesare defined as primary particles thathave amalgamated to form a planar orangular structure; these aggregatescannot be further disintegrated.Agglomerates are defined as collec-tions of primary particles that havefused to form a face-to-face sinteredstructure.

THE EFFECTIVENESS OF HDK® IS ARESULT OF THE PRODUCTION PROCESS

SiO 2

SiO 2

SiO 2

SiO 2

Reactor

Air

Molecules Protoparticles Primaryparticles

Aggregates Agglomerates

SiCl 2H O

HDK® pyrogenic silicaFormation in a flame

>1,500°C

4 2 2+ + SiO 4HCl2

+

Si OH +

X

Si∆T

–XHSi O

Si

X = Hal, O-Alk, OH, O2/2SiR2

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Hydrophobic HDK® with ModifiedSurfaceHydrophobic HDK® is available in a wide range of grades to meet differentrequirements.

The hydrophobic agents are appliedwith a gas phase to distribute themuniformly on the silica particle surfaces.Controlled quantitative chemical additionof the modifiers to the silica surfaces isachieved by means of hydrolysis-resist-ant Si-O-Si bonds. The degree of modi-fication is ensured by precise monitoringof process parameters, such as volumeflow rate, temperature and residencetime. Uniform product quality is ensuredby 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 DispersionsHDK® dispersions are highly stableagainst, for example, sedimentation and gelling. This is the result of efficientcolloidal stabilization combined with high shear forces during dispersion ofthe silica.

The colloidal stability of the dispersionsis ensured by electrostatic stabilizationand steric stabilization if appropriate.During dispersion, high shearing energyis introduced by means of high-speedshearing units, such as rotor-statorsystems. To ensure product quality, theprogress of dispersion is checked con-tinually by various analytical methods,including particle size determination byphoton correlation spectroscopy, laserdiffraction or zeta potential measure-ments.

Production of hydrophobic HDK®

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TOP PERFORMANCE HAS A SPECIAL CHEMISTRY

Hydrophilic HDK® Hydrophilic HDK®

Chemical StructureHDK® is made up of SiO4/2 tetrahedrons.These tetrahedrons are joined togetherby siloxane bridges (Si-O-Si bonds).About every second Si atom on thesurface of HDK® bears a hydroxylgroup and thus forms a silanol group(≡Si-OH).

Hydrophilic PropertyHDK® receives its outstandinghydrophilic properties from the Si-OHgroups and the polar Si-O-Si backbone.The silanol groups (Si-OH) act as reac-tive centers and form hydrogen bondsto other polar substances. HydrophilicHDK® has ca. 2 silanol groups per1 nm2.

AmorphousSiO 2

Reactive isolated silanol groups

Large surface area 50 – 400 m2/gNo micropores

O

O

O O

O

O O O

O

O

O OH

OH

Si

Si

Si

Si

O

O

O O

O

O O O

O

O

O OH

OH

Si

Si

Si

Si

O

O

O O

O

O O O

O

O

O OH

OH

Si

Si

Si

Si

5 – 50 nm

H atom O atom Si atomC atom

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Hydrophobic HDK® Hydrophobic HDK®

Hydrophobic PropertiesIf 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. Highlyhydrophobic HDK® has ca. 0.5 Si-OHgroups per 1 nm2. The hydrophobictreatment reduces or eliminates themoisture absorption of the silica. Otherbasic properties of the pyrogenic amor-phous silica are not changed.

AmorphousSiO 2

O

O

O O

O

O O O

O

O

O O

O

O

O

Si

Si

Si

Si

Si

Me

Me

Me

Me

MeMe

Si

Si

O

O

O O

O

O O O

O

O

O O

O

O

O

Si

Si

Si

Si

Si

Me

Me

Me

Me

MeMe

Si

Si

O O O

O O

O

O O O O

O O

O

O

O

SiSi

Si

Si

Si

Si

Si

MeMe

Me

Me

Me

Me

Me

Large surface area 50 – 400 m2/gNo micropores

5 – 50 nm

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X-ray diffraction diagram of amorphous HDK®

AmorphismHDK® is a completely amorphousproduct. This is proved by the absenceof a sharp X-ray diffraction pattern and a characteristic electron diffractiondiagram for amorphous structures. TheHDK® X-ray diffraction diagram, unlikethat of crystalline quartz, shows abroad, unresolved diffraction structure.This is typical of amorphous structures.The electron diffraction diagram of HDK®

has a diffuse scattering pattern. In areference measurement with crystallinequartz, distinct, well-resolved scatteringsignals can be seen.

Toxicological PropertiesIf used as recommended, HDK® is notknown to have any harmful effects onhumans. HDK® does not irritate the skin.Degreasing of the skin may occur afterprolonged contact. Unlike crystallinesilica, there are no indications of HDK®

causing silicosis.

In animal experiments there are noindications of reproductive effects.Amorphous silica such as pyrogenicsilica, precipitated silica and silica gel is not carcinogenic (Group 3 “notclassifiable as to its carcinogenicity tohumans,” IARC Monograph Vol. 68,1997.

Further information on toxicology isgiven in the appropriate material safetydata sheets.

Inte

nsity

[cp

s]

50 45 40 35 30 25 20 152 theta [degrees]

X-ray diffraction pattern of crystalline quartz

Inte

nsity

[cp

s]

50 45 40 35 30 25 20 152 theta [degrees]

Electron diffraction pattern of crystalline quartz

Electron diffraction diagram of amorphous HDK®

Shear rate ��

Vis

cosi

ty �

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Precise Thickening and ThixotropicPropertiesThe shear-thinning or thixotropic effectof HDK® offers you perfect control overthe flow properties of liquid systems.The system thus has precisely theviscosity required for the particularapplication.

In shear-thinning systems, the viscosityis reduced by shearing loads. The shear-thinning liquid can also have a yieldpoint, and therefore, in a stationary state,form a viscoelastic solid (gel structure).The shear-thinning of non-Newtonianliquids is generally a reversible process.If, when the shear force is removed, thesystem returns to its original viscosityafter a time delay, the behavior is calledthixotropic.

Efficient Increase of ViscosityIf HDK® is dispersed in a liquid, a three-dimensional network can form as aresult of forces of attraction between the HDK® aggregates. This network,together with the immobilized liquidwithin the silica aggregates, causes astrong viscosity increase. Since HDK®

has a particularly high structure, it isvery efficient and even small quantitiesare enough to provide a high viscosity.

The forces of attraction in hydrophilicHDK® consist mainly of van der Waalsforces such as dispersion or dipolarattractions. If it is allowed to physicallyadsorb oligomeric or polymeric sub-stances, such as binders, it can providefurther effects, such as polymer bridgingor steric stabilization. In the case ofhydrophobic HDK®, overlapping silylatedsurfaces result in additional hydrophobicinteractions and forces of adhesion.

Effective Reduction of ViscosityLiquids containing HDK® as an additiveundergo a reduction in viscosity whensubjected to shear forces, such asstirring, spraying or spreading. Shearforces break down the three-dimen-sional silica network. The silica aggre-gates can move with respect to oneanother and the immobilized liquidvolume becomes smaller.

The stronger the shear force, thestronger is the viscosity-reducing effect.If necessary, the viscosity can even bereduced to a minimum plateau value.Once shearing stops, the liquid regainsits original viscosity. The time it takes for this structural relaxation to occurdepends on the type and amount of thesilica used and the physicochemicalproperties of the liquid, for example theviscosity or polarity of the pure liquid.

HDK® PRECISELY ADJUSTS VISCOSITY AS REQUIRED

Flow curve of a system filled with HDK®

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Tailored ElasticityFor processors of natural and syntheticrubber and of silicone elastomers toobtain the best results, their productsmust have precisely defined mechanicalproperties, such as tensile strength,elongation at break and tear strength.The required system properties can be obtained with HDK®.

Optimum ReinforcementThe outstanding reinforcing properties of HDK® are the result of its huge spe-cific surface area and the numerouspoints of interaction with the polymerchains in the elastomer network. Thespecific particle structure of HDK®

distributes the mechanical stresses andforces throughout the elastomer networkand restricts the entropy space of thepolymer chains. This process increasesthe elastomers’ strength, elongation atbreak and load bearing capacity.

High Sag ResistanceIn non-polar silicone rubber, HDK® canbe used as both a reinforcing thickenerand rheological additive. The pronouncedthickening effect of hydrophilic HDK®

provides the necessary sag resistance,so that low-viscosity systems do not run off vertical surfaces.

Good ProcessabilityIn silicone elastomers, hydrophilic HDK®

provides excellent reinforcement whileincreasing the viscosity, which maymake it difficult to process. Processauxiliaries such as low-molecular plasti-cizers are used here, which block theinteraction between the HDK® particlesby adsorption and reduce the viscosity.This provides the system with therequired excellent processing properties.

Hydrophobic HDK® grades can be used to obtain desired properties, i.e.high tear strengths together with goodprocessability. WACKER SILICONESoffers grades with modified structuresfor this purpose.

Str

ess

[N/m

m2 ]

Strain [%]

Fracture

With 30% HDK®

Without filler

Deformation

10

2

100 500

Tensile-stress/strain diagram of silicone elastomers with and without HDK®

HDK® SELECTIVELY IMPROVESPROPERTIES

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Versatile ApplicationsHDK® can optimize the flow propertiesof different powder-form cohesive solids.And it is economical, since even tinyamounts of HDK® greatly improve theflow properties of such substances.Typical applications include bulk materi-als and powders for cosmetics, foods,animal feeds, pharmaceutical or fire-extinguisher powders. WACKERSILICONES also offers special highlysophisticated HDK® grades for improv-ing the flow and electrical chargingproperties of toners, developers andpowder paints for automotive finishesand industrial coatings.

Free FlowWhen HDK® is mixed with the solidpowder, it is distributed on the surfaces,separating the individual powder parti-cles from one another. This allows thepowder to flow freely.

Effective DryingThe high specific surface area ofhydrophilic HDK®, together with its highsurface energy, allows moisture to bindeffectively to the surface of hygroscopicpowders. This drying effect enhancesthe powder’s flow properties. Typicalapplications include finely divided foodsolids, such as table salt.

HDK® ENSURES OPTIMUM FLOW PROPERTIES

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Packaging UnitsThe smallest packaging unit leaving ourproduction is the pallet. Each pallet isshrink-wrapped in PE foil to protect theHDK® in the multilayer valved paperbags within against moisture. This allowsthe product to be stored for severalmonths in dry conditions. If the shrinkingfoil is damaged or single bags areremoved, care must be taken to protectthe remaining bags / individual bagsagainst moisture by either wrapping inplastic or other appropriate measures.

The quantity of bags per pallet dependson the HDK® grade and its bulk density.Bags are available in various weights tosuit customer demands. The data sheetindicates which form of packaging isused for a particular product.

Big BagsSpecial HDK® grades are available in BigBags. These woven super sacks aredelivered on an individual pallet. The BigBags are also shrink-wrapped in poly-ethylene film as reliable moisture protec-tion and to ensure stability during trans-port. Another advantage of this form ofcontainer is dust-free HDK® processing.

Bulk VehiclesWACKER SILICONES offers delivery inbulk vehicles. Since the full load mustbe accepted, this form of deliveryassumes that the customer has a stor-age silo big enough to receive the entiredelivery.

Supply of HDK® DispersionsAll HDK® dispersions are delivered in200-liter PE drums, or alternatively inroad tankers.

Handling and Product SafetyHDK® is not classified as a hazardoussubstance as defined by the Germanchemicals law (Chem G) and the law on the transport of hazardous goods (GG BefG).

Further information on toxicology isgiven in the appropriate material safetydata sheets. Detailed information onhandling can be found in the productdata sheets.

PACKAGED TO CUSTOMERS’ NEEDS

Wacker Chemie AGHanns-Seidel-Platz 481737 München, Germany

hdk@wacker.comwww.wacker.com/hdk

S I L I C O N E S

Wacker Chemical Corporation3301 Sutton RoadAdrian, MI 49221-9397USATel. +1 517 264-8500Fax +1 517 264-8246

hdk@wacker.comwww.wacker.com/hdk

Wacker Chemicals Trading(Shanghai) Co. Ltd.31 F., Bank of China Tower200 Yin Cheng Road CentralPudongShanghai 200 120ChinaTel. +86 21 6100-3400Fax +86 21 6100-3500

hdk@wacker.comwww.wacker-chemicals.com.cn

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