TB 0

011-

1-A

pr0

6

Technical Bulletin Fine Particles

Basic Characteristics of AEROSIL® Fumed Silica

Number 11Contact

Degussa AGBusiness Line AerosilWeissfrauenstrasse 9D-60287 Frankfurt am Main, GermanyPhone: +49 69/218-2532Fax: +49 69/218-2533E-Mail: aerosil@degussa.comhttp: //www.aerosil.com

NAFTADegussa CorporationBusiness Line Aerosil379 Interpace Parkway, P. O. Box 677Parsippany, NJ 07054-0677Phone: +1 (800) AEROSILPhone: +1 (973) 541-8510Fax: +1 (973) 541-8501

Asia (without Japan)AEROSIL Asia Marketing Officec/o NIPPON AEROSIL CO., LTD.P. O. Box 7015Shinjuku Monolith 13F3-1, Nishi-Shinjuku 2-chomeShinjuku-ku, Tokyo 163-0913 JapanPhone: +81-3-3342-1786Fax: +81-3-3342-1761

JapanNIPPON AEROSIL CO., LTD.Sales & Marketing DivisionP. O. Box 7015Shinjuku Monolith 13F3-1, Nishi-Shinjuku 2-chomeShinjuku-ku, Tokyo163-0913 JapanPhone: +81-3-3342-1763Fax: +81-3-3342-1772

Technical Service

Degussa AGTechnical Service AerosilRodenbacher Chaussee 4 P. O. Box 1345D-63403 Hanau-WoIfgang, GermanyPhone: +49 6181/59-3936Fax: +49 6181/59-4489

NAFTADegussa CorporationTechnical Service Aerosil2 Turner PlacePiscataway, NJ 08855-0365Phone: +1 (888) SILICASPhone: +1 (732) 981-5000Fax: +1 (732) 981-5275

Asia (without Japan)Degussa AGTechnical Service AerosilRodenbacher Chaussee 4P. O. Box 1345D-63403 Hanau-WoIfgang, GermanyPhone: +49 6181/59-3936Fax: +49 6181/59-4489

JapanNIPPON AEROSIL CO., LTD.Applied Technology Service3 Mita-choYokkaichi, Mie510-0841 JapanPhone: +81-593-45-5270Fax: +81-593-46-4657

please visit our web site www.aerosil.com to find your local contact partner

�

�

Part 1 in “Basic Characteristics and Applications of AEROSIL® products“ was first published in 1967,

and was assigned Number 11 in the series of Technical Bulletin Pigments. During the intervening

time the text was revised twice, and was made available to a growing readership in new editions.

Now the 4th edition of this Number 11 is available in completely new form, made topical in every

respect, and brought entirely up to date. It is intended to impart the basic knowledge required to

understand AEROSIL® products, that is almost 50 years old, and its characteristics.

AEROSIL® is the trade-mark owned by Degussa AG with 106 registrations in 84 countries throughout

the world for a

- fumed

- highly-dispersed

- amorphous

- pulverulent

synthetic silica.

The particle fineness and structure of the AEROSIL® fumed silica primary particles are reflected in the

application characteristics. Among other advantages, the reactivity of the silanol groups permits an

irreversible chemical aftertreatment.

Hydrophobic products made-to-order such as, for example, AEROSIL® R 972 and AEROSIL® R 805

are the result.

The present work describes the basic physico-chemical and application characteristics of

AEROSIL® products.

The technical applications of AEROSIL® products are discussed.

The first edition of this Technical Bulletin Pigments was published by R. Bode, H. Ferch,

and H. Fratzscher in Kautschuk + Gummi - Kunststoffe 20, 578 (1967).

Degussa AGApplied Technology AEROSIL®

Basic Characteristics and Applications of AEROSIL® products

�

1.1.11.21.2.11.2.21.2.31.2.42.2.12.22.33.3.13.23.2.13.2.23.2.2.13.2.2.23.33.3.13.3.23.3.33.3.43.3.53.43.5 3.5.13.5.23.5.33.63.6.13.6.23.6.2.13.6.2.23.6.2.3 3.6.33.6.3.13.6.3.23.6.3.2.13.6.3.2.23.6.3.2.33.6.3.33.6.3.4

667789

1011111212151519212727282929303132323233343535363637 3738393940414142424344

Table of Contents

Silicon Dioxide, SiO2 Natural OccurrencesSynthetic SilicasOrganizationSilicas Produced by DegussaComparison: AEROSIL®/Wet Process SilicasAEROSIL® Commmercial ProductsProductionProduction of Hydrophilic AEROSIL® productsProduction of Highly-dispersed Pyrogenic Special OxidesChemical AftertreatmentCharacteristicsAmorphous Structure and ThermostabilityParticle Fineness and SurfaceParticle Size and StructureSpecific SurfaceGeometrical Determination of the Specific SurfaceDetermination of the Specific Surface by AdsorptionSpecial Physico-Chemical DataSolubilityThermal ConductivityNuclear Magnetic Resonance Spectroscopy BehaviourTribo-ElectricityRefractive IndexPurityOxide Mixtures and Mixed OxidesAEROSIL® COK 84AEROSIL® MOX 80 and AEROSIL® MOX 170AEROSIL® DispersionsSurface ChemistryTwo Functional Groups Determine the ChemistryDetermination of the Silanol GroupsThe Lithium Aluminium Hydride MethodIR SpectroscopyMorpholine AdsorptionInterparticular InteractionsHydrogen Bridge LinkageMoisture BalanceMoisture Balance at Room TemperatureAgingMoisture Balance at Higher TemperaturesOther Adsorption EffectsAEROSIL® fumed silica as an Acid

Page

�

3.6.43.6.4.13.6.4.1.13.6.4.1.23.6.4.23.6.4.2.13.6.4.2.23.6.4.2.34.5.6.6.16.26.36.46.56.67.8.9.

464646474747495051525252555757585859606268

„The Aftertreatment“– a Chemical AnchoringThe Chemical Aftertreatment – Some Bibliographic ExamplesAminationReactions with AlkoxysilanesHydrophobic AEROSIL® productsConversion from “Hydrophilic“ to “Hydrophobic“The Chemical AnchoringDry Water and Aqueous Dispersions with Hydrophobic AEROSIL® products Statistical Quality ControlTypes of AEROSIL® productsApplicational EffectsReinforcementThickeningAntisetting AgentFree FlowThermal InsulationAEROSIL® fumes silica as a Versatile Product for Solving ProblemsPhysiological Behaviour and Industrial SafetyLiteratureBrief List of Technical TermsPhysico-Chemical Data of AEROSIL® fumed silica

Page

In the following we mention the registered trademark AEROSIL® fumed silica sometimes as AEROSIL® only with the aim of continuent scalability of tables and flow text.

�

1. Crystalline

Quartz mostwidespreadmodification,rockcrystal, quartzsandTridymite formsathighertemperaturesCristobalite formsathighertemperaturesCoesite high-pressuremodification,veryrareinnatureKeatite modificationthatcanbesyntheticallyproducedStishovite high-pressuremodification,veryrareinnature

1.1 Natural Occurrences

Silicon, at 27.8 % by wt., is the second most widespread

?element after oxygen (46.6 % by wt.) found in the earth‘s

17-km-thick crust. In nature, silicon is almost always bonded

to oxygen, either to oxygen alone as SiO2 or, as in the silicates,

with additional elements. Representatives of the silicates are,

among others, the bentonites (for example montmorillonite

(Al1.67 Mg0.33)[(OH)2/Si4O10] Na0.33 (H2O)4), talc Mg3[(OH)2/Si4O10],

and wollastonite Ca3[Si3O9].

The natural silicates form the raw material base for important

technical products such as cement, glass, porcelain, brick, etc.

Pure silicon dioxide can occur in amorphous or crystalline form.

The known modifications of SiO2, which, for the most part,

occur in nature, are compiled in Table 1.

With regard to quartz and tridymite, a high-temperature form

also exists in each case, it is possible to distinguish between

eight crystalline SiO2, modifications. With the exception of

stishovite, which has a hexagonal neighbourhood of six oxygen

atoms, all other modifications are built up tetragonally with

four adjacent oxygen atoms.

In nature, silicon dioxide influences the growth of some plants

and their resistance to fungi and insects (1). Dissolved silica is

also contained, for example, in drinking water or beer (originat-

ing from the barley). It is therefore ingested in considerable

quantities by humans and animals with the natural food (2).

1. Silicon Dioxide, Si02

Table 1: Modifications of SiO2

2. Amorphous

LechatelieritenaturalSi02glass,formedbymeltingprocesses resultingfromastrokeoflightningOpals notpureSi02,containwaterKieselguhr resultfromtheSiO2contentofprehistoricinfu- soriaanddiatoms,alwayscontaminatedVitreous silica“silicaglass“synthetically-produced,pure SiO2glass

�

1.2 Synthetic Silicas

1.2.1 Organization

The “silica family tree“ in Figure 1 gives an

overview of the most important synthetic and

natural products.

Today, synthetic silicas are firmly rooted

components or raw materials for a wide variety

of high-technology products. In 1990, annual

production had reached an estimated 1,000,000

tons in the western hemisphere.

This number does not include flue ash and filter

dusts based on SiO2 resulting from tech-

nical processes, for example the production

of ferrosilicon, or from power plants. These forms of ash and dust,

in contrast to the purposely-produced materials given in Table 2,

are in part highly contaminated by-products.

Different production processes result in SiO2 products* with

different technical and applicational properties.

A practical division into various groups (3, 4) is shown by Table 2.

In supplement, a differentiation is also made in each case

between untreated and chemically-after-treated SiO2 products.

Figure 1: Silica family tree

Table 2: Overview: Synthetic SiO2 products produced under controlled conditions

Thedesignation“SiO2products“isusedwhenforeigncomponentsareintentionally

presentinalargeramount.Thisisthesituation,forexample,inthecaseofAluminiumSilicateP820,whichrepresentsasilicapurposely“contaminated“withNa

2OandAl

2O3.

DegussaAGusestheterm“silicates“fortheseproductsincontrasttosilica.

Silica family tree

Silica gels Precipitatedsilicas Arc silica Flame

hydrolysis

Thermal-pyrogenic

Crystalline

Amorphous

NaturalControlledsynthetic

Silicon dioxide

Silicon

Amorphous

Quartz

Diatoms

Plasma

Wet process Vitreoussilica

Vitreoussilica

Overview: Synthetic SiO Products2

1.

2.

3.

Themal or pyrogenic or fumed silica

Wet process silica

Vitreous silica

Silica by flame hydrolysis

Precipitated silica

Arc silica

Silica gel

Plasma silica

*

�

1.2.2 Silicas Produced by Degussa

Degussa operates plants in the Federal Republic of Germany,

in Belgium, in the USA, and in Japan. A listing of old and new

AEROSIL® patents is shown in Table 3.

Numerous Degussa company publications help the reader to

gain a quick product overview on the one hand, or in other

editions present detailed information on special applications.

Within this series of Technical Bulletin Pigments, the editions

selected in Table 4 discuss the specialized fields mentioned.

Figure 2: X-ray graphs which show the different structures of AEROSIL® fumed silica on the one hand and of cristobalite on the other. Vitreous silica is also built up “randomly“

Inte

nsity

(abs

.)

16

14

12

10

8

6

4

2

014 18 22 3026 34 38 42 46 50 54

2 theta

x103

Vitreous silica

CristobaliteAEROSIL 200®

While the “AEROSIL® Brochure“ (17) provides an insight into the

most important fields of application of AEROSIL® fumed silica,

in addition to a general product description, this edition in the

series of Technical Bulletin AEROSIL® fumed silica describes

the fundamentals of AEROSIL® fumed silica with respect to its

physico-chemical and technical application characteristics.

The synthesis of the precipitated silicas (3) which are likewise

produced by Degussa AG, and their characteristics are described,

for example, in (18).

Table 3: List of the German AEROSIL® patents

Table 4: Editions in the series of Technical Bulletin Pigments

All SiO2, products produced by Degussa are derived syntheti-

cally under controlled conditions. All of these products are X-ray

amorphous, as clearly shown by Figure 2 where AEROSIL® 200

is used as an example. Consequently, all Degussa silicas belong

to the group of the “synthetic amorphous silicas“ or “SAS“.

This designation is increasingly found in American literature.

Quantitatively, the arc silica process (5-7) is in last place. Plasma

processes (8-10) are of no importance technically at the present

time. In contrast, the precipitated silicas and AEROSIL® fumed

silicas are of greatest importance.

The idea and the technical development of the original

AEROSIL® process (flame hydrolysis, high-temperature

hydrolysis) (11-15) can be traced back to the Degussa chemist

H. KLOEPFER, who wanted to produce a “white carbon black“

following the invention of the “German Channel Black Process“ (16).

In 1941, the first small-scale production was successful. Today,

this pyrogenic silica is produced throughout the entire world.

DE-PS DE-PS DE-PS DOS762723 900574 1035854 1642994830786 910120 1036875 2728490870242 921784 1066552 2904199873083 928228 1103313 2923182877891 962292 1150955 3028364878342 974793 1156918 3139070891541 1003765 1210421 3211431893496 1004596 1244125 3320968893497 1023881 1244126 3741846900339 1034163 2004443 3101720

Field of Work or Title Edition Number

Adhesives 44Adsorption 19Analyticalmethods 16Applications 43Catalysts 72Characterization 53*,60Coatings 18,53,68Cosmetics 4,49Defoamers 42Dispersion 33Electrostaticcharging 62Epoxyresins 27Flatting 21Fluoroelastomers 73Freeflow 31Handling 28,70Joint-sealingcompounds 63Pharmaceuticals 19,49Plastics 13Polyesterresins 54PrintingInks 26,52Production 6,32PVCmasses 41,51Reflectionmeasurements 39Rheology 23Siliconerubber 12Toothpastes 9,55Toxicology 64,76

*NotyetpublishedinEnglish

�

1.2.3 Comparison: AEROSIL®/Wet Process Silicas

1) with ref. to DIN 66 1312) with ref. to ISO 787/103) with ref. to ISO 787/11 4) with ref. to ISO 787/25) with ref. to DIN 55 9216) with ref. to ISO 787/97) with ref. to DIN 536018) depending on water content9) estimate by comparison of BET and EM surfaces or according to practical experience 10) in exceptional case smaller, for example SIPERNAT® FK 310 (Degussa)11) can not be given

1 Spec. surface according to BET 1) m2/g 50 to 600 25 to 300 30 to 800 250 to 1000 250 to 400

2 Primary particle size nm 5 to 50 5 to 500 5 to 100 3 to 20 3 to 20

3 Aggregate or agglomerate size µm 11) 2 to 15 1 to 40 1 to 20 1 to 15

4 Density 2) g/cm3 2.2 2.2 1.9 to 2.1 8) 2.0 2.0

5 Compacted apparent volume 3) ml/100 g 1000 to 2000 500 to 1000 200 to 2000 100 to 200 800 to 2000

6 Drying loss 4) % ≤ 2.5 ≤ 1.5 3 to 7 3 to 6 3 to 5

7 Ignition loss 5) % 1 to3 1 3 to 7 3to15 3 to 5

8 pH value 6) 3.6 to 4.3 4.5 5 to 9 3 to 8 2 to 5

9 Predominant pore diameter nm not porous to not porous ≥ 30 10) 2 to 20 ≥ 25

app. 300 m2/g

10 Dibutyl phthalate adsorption 7) ml/100 g 250 to 350 100 to 150 175 to 320 100 to 350 200 to 350

11 Pore diamete distribution 11) 11) very wide narrow narrow

12 Proportion of the internal surface 9) 0 0 small very large large

13 Structure of the aggregates chain-like strictly spherical mod. aggregated very highly aggl. aggl. porous part. and agglomerates agglomerates only slightly aggl. almost spher. part. porous part. distinct

14 Tendency to have thickening effect very strongly indicated present indicated present

pronounced present present

Table 5: Overview of some important characteristics of industrially-produced silicas (compiled for the purpose of making differences recognizable) according to [3]

Some important physical characteristics of AEROSIL® products

and silicas produced according to wet processes are compared

with each other in Table 5.

Pyrogenic or thermal Ground wet process silicas

silicas

AEROSIL® Arc silicas Precipitated Silica gels

Characteristics Aerosols silicas Silica gels Aerogels

�0

Distinct differentiating features exist in the aggregate or

agglomerate size. All silicas produced according to wet processes

are ground if they are not spray-dried. On the other hand,

AEROSIL® fumed silica is neither ground nor specially dried. In

all cases, the smallest particles are the primary particles, which

are more or less strongly aggregated and agglomerated. The

specific surface is of central importance. Silica gels have a very

large inner surface, which results in a high adsorption capacity.

In contrast, AEROSIL® fumed silica primary particles derived by

flame hydrolysis have only an outer surface. This explains, for

example, the improvement in the rheological characteristics of

numerous systems resulting from the incorporation of AEROSIL®

products. On the other hand, the pronounced pore volume of

silica gels is of importance for the adsorption as well as for the

chromatography.

As mentioned, the differences in the particle size and particle

structure are reflected in the rheological characteristics. The

reasons for using AEROSIL® fumed silica as a reinforcing,

thickening and thixotropic agent for many diverse systems

become obvious. While stable AEROSIL® fumed silica dispersions

represent a sales product, dispersions of precipitated silicas,

for example, tend to settle.

Furthermore, differences in the drying and ignition losses play a

major role for the characterization and for the application of the

products.

Low drying losses are required, for example, because of better

dielectric characteristics, for cables based on silicone rubber,

and for an adequate storage stability when used in one-compo-

nent adhesives or coatings. The most important difference,

which is not listed numerically in Table 5, has its roots in the

differing silanol group density (i.e. SiOH/nm2). All hydrophilic

types of AEROSIL® products have values between 2 and 3.

In contrast, this parameter lies at about 6 with all products

derived from wet processes.

Considerable differences are also found in the purity

(more detailed data for AEROSIL® products in Section 3.4).

In terms of anions, AEROSIL® fumed silica contains only slight

amounts of Cl- (≤ 250 ppm as HCl). Silicas produced according

to the wet process usually contain sulphate and alkali or

alkaline earth ions (for example ~ 1000 ppm).

1.2.4 AEROSIL® Products

Table 6 shows the types of AEROSIL® products and special

oxides produced by Degussa available on the market. Here,

a subdivision was made between untreated and chemically-

aftertreated AEROSIL®. All of the latter, the hydrophobic types

of AEROSIL®, have an „R“ in their nomenclature. This letter, R,

is taken from the word „repellent“. This “R” should not be

confused with the ® for “registered trademark”.

The pyrogenic, likewise highly-dispersed special oxides,

AEROXIDE® Alu C, AEROXIDE® TiO2 P 25 , and experimental

product* Zirconium Oxide, are also included in this product

group (19). Moreover, Degussa also markets a series of

AERODISP®, AEROSIL® dispersions, the technical data of

which are compiled on Page 36.

* Theterm„experimentalproducts“(Germanabbreviation:VP)appliestoaproduct whichisstillproducedinrelativelysmallamounts;inthecaseofsuchproducts, adecisionhasnotyetbeenmaderegardingtheirinclusionintheproductionprogram.

Table 6: Highly-dispersed pyrogenic oxides produced by Degussa

1. AEROSIL®

AEROSIL®OX50AEROSIL®90AEROSIL®130AEROSIL®150AEROSIL®200AEROSIL®300AEROSIL®380

AEROSIL®TT600AEROSIL®MOX80AEROSIL®MOX170AEROSIL®COK84

2. Chemically aftertreated AEROSIL®

AEROSIL®R972AEROSIL®R974AEROSIL®R202AEROSIL®R805AEROSIL®R812

3. Special oxides AEROXIDE®AluC

AEROXIDE®TiO2P25

��

2. Production

2.1 Production of Hydrophilic AEROSIL® fumed silica

The „AEROSIL® Process“ (11 - 15), i. e. the large-scale industrial

synthesis of AEROSIL® products, can be described essentially as

a continuous flame hydrolysis of silicon tetrachloride (SiCl4).

During this process, SiCl4 is converted to the gas phase and then

reacts spontaneously and quantitatively in an oxyhydrogen

flame with the intermediately-formed water to produce the

desired silicon dioxide.

2 H2 + O2 2 H20

SiCl4+ 2 H20 Si02 + 4 HCI

2 H2 + O

2 + SiCl

4 Si0

2 + 4 HCI

Instead of silicon tetrachloride, silanes such as methyltrichlorosi-

lane, trichlorosilane, etc. can be used as the raw material, either

alone or in mixtures with SiCl4. The conditions relating to firing

and flow must be varied in comparison with those used for

silicon tetrachloride in order to derive the same final product.

Figure 3: Flame sceme for AEROSIL® fumed silica (schematic)

Figure 4: Production of AEROSIL® fumed silica (flow chart)

SiCI4

SiO2

1000 °C

H2 O2

Hydrogen

Oxygen (air)

Si tetrachloride

Evaporator

Cooling line

Deacidification

Separation

Burner

Mixing chamber

FumedSilica

Silo

HCl adsorption

During this chemical reaction a considerable amount of heat is

released, which is eliminated in a cooling line. The only by-

product is gaseous hydrogen chloride which is

separated from the AEROSIL® fumed silica solid

matter. Figure 3 shows the flame sceme for

AEROSIL® fumed silica schematically; Figure 4 ,

a flow chart of the AEROSIL® Process.

By varying the concentration of the coreactants,

the flame temperature, and the dwell time of

the silica in the combustion chamber, it is pos-

sible to influence the particle size, the particle

size distribution, the specific surface, and the

surface properties of the silicas within wide

boundaries.

��

The hydrochloric acid which develops during the AEROSIL® process

in the tetramolar excess, referred to as SiO2, can be used again in

the production of SiHCI3 or SiCl4 according to the equation

Si + 4 HCl SiCl4

+ 2H2

Here, ferrosilicon (FeSi) serves as the silica source; FeSi is a prod-

uct used, for example, in the production of steel. The hydrogen

formed is likewise used and is fed into the burner for the pro-

duction of AEROSIL® fumed silica, so it is possible to speak of an

environmentally-friendly, large-scale, cyclic process.

Al O2 3Al O2 3

AlCl3

TiO2TiO2

TiCl4

ZrO2ZrO2

ZrCl4

TiO P 252 VP ZrO2TiO P 252 VP ZrO2Al O C2 3Al O C2 3

Experimentalproduct

Zirconium Oxide

Experimentalproduct

Zirconium Oxide

AEROXIDE®Alu C

AEROXIDE®TiO P 252

AEROXIDE®Alu C

AEROXIDE®TiO P 252

2.2 Production of Highly-Dispersed Pyrogenic Special Oxides

The easy evaporation of SiCl4, the development of only one

form of solid matter, and the use of suitable materials for

apparatus inevitably result in the formation of extremely pure

products. Therefore, it also seemed reasonable to extend the

process to other chlorides which can likewise be converted

more or less easily into the gas phase, as shown by Table 7.

AEROXIDE® Alu C and AEROXIDE® TiO2 P 25 have long been on

the market as highly-dispersed, pyrogenic oxides. Zirconium

Oxide is still handled on the market as an experimental product.

The characteristics of the special oxides and their applications

are discussed in detail in Editions No. 56 and 72 in this series of

Technical Bulletin Pigments.

Unlike AEROSIL® fumed silica which is completely amorphous,

the special oxides Al2O3C, TiO2 P 25, and the experimental

product Zirconium Oxide occur in crystalline form (19). In all

cases, the thermodynamically instable forms are more readily

formed because the actual reaction time is extremely short. The

short dwell times in the oxyhydrogen fl ame practically preclude

sintering processes between the condensing phases which

are conceivable in principle. The prerequisites for an easy and

effective dispersing, which is of great applicational importance,

are therefore established.

In Table 8, some further experimental products are compiled

which have been produced on a laboratory or pilot plant scale.

The limiting factor during the production is represented by the

volatility of the raw materials. The special oxides in Table 8 are

either derived in pure form or are doping substances in silica or

titanium dioxide carriers.

Table 8: List of some pyrogenic special oxides and mixed oxides which in principle can be produced according to the AEROSIL® process. VP ZrO

2 is an experimental

product, samples can be requested. Samples of the other products are currently not available

Table 7: Special oxides produced by Degussa according to the AEROSIL® process

Experimental Raw materialproduct NiO Ni(CO)4MoO3 MoCI5SnO2 SnCI4 Sn(CH3)4V205 VOCI3WO3 WCI6 WOCl4VPZrO2 ZrCI4

Experimental Raw materialproduct

AIBO3 AICI3/BCI3AIPO4 AICI3/PCI3BPO4 BCI3/POCI3Bi2O3 BiCI3Cr2O3 CrO2CI2Fe2O3 FeCI3 Fe(CO)5GeO2 GeCI4

Hydrophobic = water repellent; for more detailed information, see also 3.6.4, the measurement of the hydrophobicity is discussed in detail in Edition 18 - among other sources – in this series of Technical Bulletin Pigments.

*

2.3 Chemical After-treatment

If AEROSIL® fumed silica is mentioned today, AEROSIL® hydro-

phobic* products are often also included. Here, the AEROSIL®

process described above is followed by an additional stage

– the aftertreatment.

��

When the material is still, so-to-speak „in statu nascendi“, i. e. it

has not yet left the system, it is especially reactive for a further

treatment with a silane. The direct aftertreatment (Figure 5),

which is integrated into a continuous process, results in

homogeneous and effective functionalization. This applies

for every modified silica for special applications just as for the

hydrophobic standard products.

By means of the infrared spectra, the reaction processes can

be observed well. Figure 6 shows that during the chemical

aftertreatment, and essentially in the case of the hydrophilic

AEROSIL® fumed silica, the sharp band of the free silanol groups

at 3748 cm-1 disappears from the IR spectra. Simultaneously, a

new C-H oscillation band of the methyl groups is observed at

less than 3000 cm-1 with the final product. The silanol groups

are irreversibly „replaced” in a chemical reaction by organic

residues such as, for example, methyl groups.

Figure 5: The „direct“ aftertreatment, integrated into the fully continuous AEROSIL® process (schematic)

Hydrogen + oxygen

Silicon tetrachloride

AEROSIL +®AEROSIL +®

Hydrophobic prod. AEROSIL®Hydrophobic prod. AEROSIL®

Silane

Flame

hydrochloric acid

(Aftertreatment)

Hydrogen + oxygen

Silicon tetrachloride

Silane

O

YYH

Si

Si

Si

SiOH

R

R

R

R

R

R

2

2

3

3

1

1

Figure 6: Partial IR spectrum of AEROSIL® 300 (left) before and after the chemical aftertreatment (right, corresponds to AEROSIL® R 812); in each case pure substance test specimen, IR instrument: Perkin Elmer 325

Figure 7: Hydrophobic types of AEROSIL® fumed silica

The functionalization of the AEROSIL® fumed silica surface

is carried out with halogen silanes, alkoxysilanes, silazanes,

siloxanes, etc. Figure 7 compares the surface groups of the

commercial hydrophobic types of AEROSIL® fumed silica.

Tran

smitt

ance

%

20 20

40 40

60 60

80 80

100 100

0 0

4000 3000 25003500 4000 3000 25003500

Wave number cm-1

AEROSIL 300® AEROSIL R 812®

CHCH

CH

CH

CH

CHCH

C H

33

3

3

3

3

3

8 17

O

O

O

O

O n)(

O

O

Si

Si Si

Si

AEROSIL R 972®

AEROSIL® R 805

AEROSIL® R 812

AEROSIL® R 202

AEROSIL® R 974

��

AEROSIL® hydrophobic types differ from the hydrophilic

starting silicas by a – among other things –

- lower silanol group density, and therefore a

- lower water vapor adsorption.

For this reason, the aftertreated silicas have new, technically-

important applicational properties.

For example, as represented in Figure 8, the maximum mois-

ture adsorbed by a hydrophobic silica is distinctly less than that

adsorbed by a hydrophilic type.

In addition, Figure 9 – where a selected example of the thicken-

ing effect is used for illustration – shows the advantage of an

AEROSIL® hydrophobic type in a low-viscosity, reactive epoxy

resin before and after the addition of a mixture composed of

a polyamino amide as cross-linking agent and a tertiary amine

as accelerator. The hydrophobic types, AEROSIL® R 202 and

AEROSIL® R 805, are distinctly superior to AEROSIL® 300 in the

epoxy resin; for additional details, see Edition No. 27 in this

series of Technical Bulletin AEROSIL®.

Figure 8: Water vapour adsorption isotherms at room tempe- rature of AEROSIL® 150 (hydrophilic starting material) and the hydrophobic AEROSIL® R 202, measured on small test specimens

Figure 9: Change in viscosity of an epoxy resin (ARALDIT® M, Vantico AG) with 5.6 % AEROSIL® before and 3.8 % AEROSIL® after addition of the hardener and cross- linking agent (EUREDUR® 250, Schering AG; ARALDIT® hardener HY 960). As a result of this addition, the AEROSIL® content decreases

2

4

6

8

10

00 20 60 8040 100

AEROSIL 150®

AEROSIL R 202®

Moi

stur

e ad

sorp

tion

in %

Relative atmospheric moisture %

Visc

osity

Pa

s

Visc

osity

Pa

s

100 40

200 80

300 120

400 160

0 00 15 6030 45

Time after addition of hardener, min.

AEROSIL R 805®

AEROSIL 300®

without with hardener

AEROSIL R 202®

��

3.1 Amorphous Structure and Thermostability

As already shown, the chemical summation for-

mula of AEROSIL® fumed silica is SiO2.

However, it must be taken into consideration

here that in reality no isolated SiO2 molecules are

present. Instead, the silicon atoms develop

covalent single bonds with four directly

adjacent oxygen atoms.

Consequently, every atom corresponds to

the octet rule. For energetic reasons, the

bonding electron pairs occupy positions as far from each other

as possible; in other words they are arranged tetrahedrally.

The SiO4 tetrahedrons serve as the fundamental building blocks

for the structure of the macromolecular network. In principle,

two possibilities are conceivable here: the SiO4 tetrahedrons

could be arranged regularly, or they could be arranged

completely at random. In their entirety, crystalline modifi ca-

tions of silica that occur in nature such as quartz, tridymite, or

cristobalite consist of exactly defi ned, fully identical structural

units, the so-called unit cells. Due to the regular structure of the

crystral lattice, X-rays are diffraced at the lattice or net planes,

and exhibit interference phenomena.

All synthetic silicas produced by Degussa display an entirely

different behaviour. The SiO4, tetrahedrons are randomly

arranged, as Figure 10 shows by the absence of defi ned dif-

fraction rings or lines. This fact was already noted in Figure 2.

AEROSIL® fumed silica is therefore X-ray amorphous. In contrast

to glasses, which form a three-dimensional skeleton with

infi nite expansion (measured in atomic dimensions), AEROSIL®

amorphous silica has a particular structure.

3. Characteristics

Figure 10: X-ray photographs of AEROSIL® fumed silica (above), α-cristohalite (centre), and quartz (below); compare Figures 2 and 12

��

AEROSIL® fumed silica does not produce any

sharp X-ray reflections, but instead only weak,

very diffuse intensity modulations. These dif-

fraction phenomena are entirely compatible

with a random network model (20). They must

be attributed to short-range order conditions,

the range of which in non-crystalline materials

is always small in comparison with the particle

size of highly-dispersed materials.

In vitreous silicas, these lie in the order of mag-

nitude of about 1.3 nm, in precipitated silicas

at about 1.2 to 1.0 nm, and in AEROSIL® fumed

silica and arc silicas at about 0.9 and 0.8 nm

(21). The transition from a regular condition to

a random condition therefore takes place as

early as after the third tetrahedron coordina-

tion sphere. With regard to this short-range

order tendency, AEROSIL® fumed silica has

the greatest structural disorder in comparison

with other Si02 products (21). It should be

expressly emphasized here that the short-

range order regions must not be equated with

a state of crystallinity.

Figure 11: Schematic arrangement of the SiO4 tetrahedrons in AEROSIL® according to a model by EVANS and KING (22). The circles symbolize oxygen atoms; in the centers of the tetrahedrons are the silicon atoms

��

According to EVANS and KING (22), it is possible to imagine the

SiO4 network as shown in Figure 11. By calculating the radial

distribution function, a Si-O distance of 0.152 nm and a Si-Si

distance of 0.312 nm were determined. The Si-O-Si bond angle

has a considerable range of variation of 120-180 degrees (23).

Quartz dust especially, and dusts containing cristobalite,

tridymite, and coesite have a silicogenic effect (24, 25). The

amorphous structure of AEROSIL® fumed silica is especially

significant. The question of possible silicotic effects linked to

amorphous silica is discussed specifically in Edition No. 76 in

this series of Technical Bulletin Pigments (26).

It has not been possible to observe crystalline components in

AEROSIL® fumed silica test specimens by IR spectroscopy, with

the aid of differential thermal analysis, or by means of X-ray

diffraction. This can be recognized clearly in Figure 12.

The roentgenographic detection limit of moderately disordered

cristobalite in vitreous silica lies below 0.3 % cristobalite (27).

Figure 12: Angle region of the [101] reflection from α-quartz, represented with AEROSIL® 200 / α-quartz, mixtures. AEROSIL® 200 itself shows no reflection, it is therefore X-ray amorphous. Diffractometer STADI 2/PL STOE, CuKα1

radiation, 50 kV 28 mA. Measurement stimulus per step 30 seconds (also see Edition 64 in this series of Technical Bulletin AEROSIL® fumed silica on this subject)

AEROSIL® 200+ 2% quartz

AEROSIL® 200+ 1% quartz

AEROSIL® 200+ 0.5% quartz

AEROSIL® 200+ 0.3% quartz

AEROSIL® 200

25.5 27.5 2 x theta

Inte

nsity

l re

l.

��

When heated to temperatures of up to 1000° C

(7 days, purest conditions), AEROSIL® fumed

silica does not change its morphology accord-

ing to scanning electron microscope findings.

The large half width of the first X-ray diffrac-

tion maximum receeds somewhat during the

thermal loading. The slight increase in order

corresponding with this is still in agreement

with a completely amorphous network. At

1200 °C, AEROSIL® pulverulent silica cross-links

to glass, whereby with a longer annealing time

devitrification takes place.

As expected, the recrystallization behaviour

is greatly influenced by additives. Figure 13

shows how the stability of AEROSIL® 300 can

be increased by adding ZrO2. AEROSIL® R 974

shows a behavior analogous to that of

AEROSIL® 200 during the annealing. When the

methyl groups are „burned off“ (above 500° C),

the crystallization behaviour is therefore not

influenced. For practical purposes, the tem-

perature stability of hydrophilic AEROSIL®

fumed silica lies at 850° C according to Table 9

(continuous stability).

Regarding the recrystallization rate, precipi-

tated silicas differ considerably from AEROSIL®.

While the pyrogenic silica is still present in an

amorphous state even after 7 days at 1000° C,

standard precipitated silicas are completely

crystallized after only 20 minutes at the same

temperature (21).

In storage heaters, for example, AEROSIL®

fumed silica is used in large amounts for the

insulation. Figure 14 shows insulation plates

on an AEROSIL® fumed silica base for the

enclosing of aircraft turbines.

Figure 13: Transmission electron micrographs (TEM‘s); from left to right; AEROSIL® 300 annealed at 1000 °C, AEROSIL® 300 annealed at 1150 °C, AEROSIL® 300 annealed at 1150 °C doped with 0.2 % ZrO2 according to (28); annealing time at each temperature 3 hours

Figure 14: Flexible insulation packing based on AEROSIL® fumed silica

��

3.2 Particle Fineness and Surface

The amorphous structure of AEROSIL® fumed

silica and the random arrangement of the SiO4

tetrahedrons were described in 3.1. Now, the

macroscopic extension and form of the

particle will be discussed.

Visually, AEROSIL® fumed silica is identified

as a loose, bluish-white powder. Actually,

AEROSIL® fumed silica consists of about 98 %

by vol. of air (density of AEROSIL® 2.2 g/cm3,

tapped density of AEROSIL® „normal“ product

about 50 g/l; compressed product „V“ about

120 g/l). It can be easily fluidized with bursts of

compressed air, and consequently can also be

handled in silos with no problems. Figure 15

illustrates this behavior using a simple

laboratory demonstration.

Figure 15: Simple laboratory demonstration of the fluidizing of AEROSIL® fumed silica; compressed air is applied to the glass frit at a pressure of about 0.2 bar

�0

With the eye, it is possible to recognize very small AEROSIL®

fumed silica particles as well as larger, loose network structures

which collapse when touched even lightly. Micrographs of

AEROSIL® fumed silica dust particles show that agglomerates of

about 10 to 200 µm form, whereby the frequency of one group

of 10 to 30 µm and a second of about 100 µm stand out (29).

We may conclude from these data that a large portion of the

AEROSIL® fumed silica dust must not be included in the fine dust

able to enter the alveoli, see (30).

Figure 16: Definition of the terms primary particles, aggregates, and agglo-merates according to DIN 53 206, Sheet 1 (August 1972)

Primary particles:smallestrecognizableindividuals

Aggregates:primaryparticlescontactingeachotheratsurfacesoredges;asarule,cannotbebrokendownfurther

Agglomerates:aggregatesand/orprimaryparticlescontactingeachotheratpoints

Hexahedral Spherical Rod shaped Irregularly shaped

Coherent Dispersive Lattice regions (Crystallites)

The dust-free handling of AEROSIL® products in general and

also the conveying in pipelines are standard practice nowa-

days (31). Interested customers can convince themselves of the

simple and correct handling of AEROSIL® products in a Degussa

pilot plant at our location in Wolfgang or Mobile.

In order to be able to describe the conditions prevailing with

AEROSIL® fumed silica more effectively, the terms: primary

particles, aggregates, and agglomerates are initially defined

in Figure 16.

��

3.2.1 Particle Size and Structure

The AEROSIL® fumed silica primary particles

are extremely small; the order of magnitude

lies in the range of just a few nanometres, and

therefore is hardly conceivable. An imaginary

experiment will be described to illustrate

this: if it were possible to blow up a normal

football (soccer ball) to the size of our planet

earth, then an AEROSIL® fumed silica primary

particle, under the same conditions, would be

about the size of the football.

Nevertheless, an AEROSIL® fumed silica primary

particle is built up of about 10,000 SiO2 units

because, as mentioned in 3.1, the Si-Si distance

is only about 0.31 nm (32).

Figure 17: TEM of AEROSIL® OX 50

Figure 18: TEM of AEROSIL® 130

��

Due to the particle fineness, electron micros-

copy is the only direct method to determine

the form and size of the particles. Transmission

electron microscopy (also abbreviated TEM)

offers outstanding resolution (≤ 0.2 nm,

magnification up to about 2,000,000:1), but

provides only a two-dimensional impression.

Spherical particles therefore appear as round

discs. Details on this are given in Edition No. 60

in this series of Technical Bulletin Pigments.

Figure 19: TEM of AEROSIL® 200

Figure 20: TEM of AEROSIL® 380

��

Important items of information can be derived

from the TEM‘s:

. AEROSIL® fumed silica is built up of many

almost spherical primary particles.

. The primary particles form a loose network;

they occur practically non-isolated (the only

exception is in part with AEROSIL® OX 50).

. The smaller the primary particles, the more

strongly pronounced the aggregate/agglo-

merate formation. Especially Figure 20

shows that the AEROSIL® fumed silica

primary particles often „line up“ with each

other, forming irregular chains.

. One type of AEROSIL® fumed silica shows pri-

mary particles with a particle size distribution.

0 80 100

5

10

15

20

25

30

0Fr

eque

ncy

[%]

Particle diameter [nm]20 40 60

AEROSIL 300AEROSIL 200

®

AEROSIL 130AEROSIL 90AEROSIL OX 50

®®®®

Figure 21: Primary particle size distribution curves of various types of AEROSIL® fumed silicas. Here it must be considered that the frequency depends on the class width; AEROSIL® 380 and AEROSIL® 300 have almost identical distribution curves

. The individual types of AEROSIL® fumed silica differ

distinctly in the primary particle size: the average primary

particle size ranges from 7 to 40 nm depending on type.

The particle size distribution in the individual types of AEROSIL®

fumed silica is represented in Figure 21. In this connection,

it can be noted that AEROSIL® types with a high BET surface

have very narrow ranges of fluctuation in the size distribution.

According to SEIBOLD and VOLL, this fact can be explained by

means of empirical distribution functions (33).

��

From the point of view of technical applications,

the dispersibility of AEROSIL® fumed silicas is of

decisive importance in most cases.

Due to the greater aggregation or agglomera-

tion, the dispersibility is naturally more difficult

when smaller primary particles are present. For

example, AEROSIL® 130 can be dispersed more

easily than AEROSIL® 200, and the latter in turn

more easily than AEROSIL® 300. Furthermore,

AEROSIL® hydrophobic silica offers distinct

advantages over AEROSIL® hydrophilic silica

with regard to the dispersibility.

This fact is represented in Figure 22. The TEM`s

show that the network structure, for example

in the case of AEROSIL® R 972, is less pronoun-

ced than in the hydrophilic base material,

AEROSIL® 130.

Figure 22: TEM‘s of AEROSIL® 130 (above, hydrophilic starting material) and AEROSIL® R 972 (below)

��

Figure 23 shows that the transparency of comparable HTV

silicone rubber test samples containing AEROSIL® fumed silica

decreases, for example, in the following order:

AEROSIL® R 812 ≥ AEROSIL® 300 ≥

AEROSIL® 200 ≥ AEROSIL® 130.

In the same direction, the size of the AEROSIL® fumed silica

particles effectively present increases with these samples.

Evidently, the dispersing energy during the production of the

corresponding samples was adequate to disperse AEROSIL® 200

and AEROSIL® 300 to a large extent, too. Since AEROSIL® R 812

and AEROSIL® 300 have about the same average

Figure 24: SEM of AEROSIL® OX 50 (see text). Left, ad-jacent, greatly enlarged, an AEROSIL® OX 50 primary particle of average size; this makes a comparison of size possible between the primary particles (AEROSIL® 200 in Figure 25)

Figure 23: Influence of the particle size and the hydrophobicity of AEROSIL® fumed silica on the transparency of HTV silicone rubber (100 parts polymer, 40 parts AEROSIL®, 0.5 % peroxide)

d = 40 nm

AEROSIL 130® AEROSIL 200® AEROSIL 300® AEROSIL R 812®

30

25

0

5

10

15

20

Tran

spar

ency

scal

e di

visio

ns

AEROSIL® hydrophilic silica AEROSIL silica® hydrophobic

AEROSIL® 0X 50primary particle size, the further rise in the

transparency when AEROSIL® R 812 is used

must be explained by the easy dispersibility of

hydrophobic AEROSIL® products and its better

wettability.

Scanning electron microscopy (SEM), with

its resolution of about 5 nm, is inferior to the

TEM technique, but offers the advantage of a

great depth of focus. As can be recognized in

Figures 24 and 25, realistic, three-dimensional

pictures are derived which provide further

information about the structure of AEROSIL®

products.

Regardless of the primary particle size, „snow-

balls“ of about 100 nm in size can be observed

in AEROSIL® OX 50 and AEROSIL® 200. These

„snowballs“ make quite a compact impression;

during dispersion, they cannot be completely

broken down into smaller particles. With the

SEM technique, therefore, primary particles

can not be made visible.

��

In the sense of the definition in Figure 16,

therefore, we speak of aggregates. These

structures develop through the clustering

together of primary particles. The standard

practice of coating the particular study objects

with a gold layer of about 5 nm in thickness

used with the SEM technique also has the

effect of smoothing the surface in the case of

AEROSIL® fumed silica. The SEM‘s permit very

good recognition of the agglomerate struc-

ture. The smaller the primary particle size, the

more pronounced this structure is.

During the breakdown of the agglomerates to

aggregate size, distinctly more dispersive force

must therefore be exerted in the case of

AEROSIL® 200 than in the case of AEROSIL® OX 50.

This also applies for all other types, for example

for AEROSIL® 300, which in turn is more diffi-

cult to disperse than AEROSIL® 200.

Figure 25: SEM of AEROSIL® 200 (see text). On the left, greatly enlarged, an AEROSIL® 200 primary particle of medium size; this permits a comparison to be made of the primary particle size (AEROSIL® OX 50 in Figure 24)

Figure 26: Particle size distribution in AERODISP® W 7520 measured using static light scattering (Horiba LA-910)

SEM‘s of frozen cross sections of AEROSIL® dispersions show

that the secondary particle size (aggregates) effectively present

actually lies in the 100-nm range. This is also confirmed by

results of different particle sizing techniques, as shown by

Figure 26. Such dispersions are available in the AERODISP®

product range (see page 35).

d = 12 nm

AEROSIL® 200

0

5

10

15

20

25

30

35

0.01

q3 (%

)

Particle Size Distribution (Volume)

0.10 1.00 10.00Size [ m]µ

0102030405060708090

100

q3 (%

)

According to DIN 53 601, it is common practice to determine a

so-called dibutyl phthalate adsorption on finely divided materi-

als. This value essentially describes the so-called „void volume“.

Naturally, the size of the specific surface also influences this

numerical value, as shown by Figure 27.

Figure 27: Dibutyl phthalate adsorption of AEROSIL® fumed silica (DBP adsorption) as a function of the specific surface (according to DIN 53 601)

0 400

50

300

100

350

150

200

250

0

DBP

adso

rptio

ng/

100g

[]

BET surface [m /g]2100 200 300

��

3.2.2 Specific Surface

It has already been shown how the primary

particle size and structure of the AEROSIL®

fumed silica particles can be observed from

electron micrographs. In the case of the

AEROSIL® product types, the correlation

between the primary particle size and

magnitude of the specific surface can

be determined by two methods that are

completely independent of each other. Both

methods lead to the same result.

3.2.2.1 Geometrical Determination of the Specific Surface

Ifa,cubeisdividedinto8smallcubeswheneachedge

lengthiscutinhalf,themassnaturallyremainsconstant;the

surfaceareaofasinglesmallcubeissmaller,butthesumof

thesurfaceareasofthe8smallcubesistwiceaslargeasthe

surfaceareaofthelargecube.

Thisprocesscanberepeatedintheimaginationasoftenas

desired.ThesurfaceareaofasingleAEROSIL®fumedsilica

primaryparticleisverysmall;ontheotherhand,thespecific

surfaceisverylargebecausethenumberofparticlesisvery

high.Ifitwerepossibletolineuptheprimaryparticlesin

1 g of AEROSIL® 200toformachain,thelengthofthischain

wouldbe17 times the distance from the earth to the moon!

Figure 29: 30 g AEROSIL® 200 have the same surface area as a football (soccer) field with the internationally-standardized dimensions

Figure 28: Specific surface as a function, of the average AEROSIL® fumed silica primary particle diameter

0 40 50

100

200

300

400

0

Spec

.sur

face

[m/g

]2

Average diameter of the primary particles [nm]10 20 30

The fundamental correlation between the primary particle size

and the specific surface can be derived quantitatively from the

TEM‘s by mathematical methods (34). In this method of deter-

mination, several thousand particles are counted with a ZEISS

Particle Size Counter TGZ 3 according to ENDTER and GEBAUER

(35), and the specific surface is calculated.

Figure 28 shows how the specific surface sharply increases as

the particle diameter decreases. 30 g of AEROSIL® fumed silica,

for example, have the same surface area as a football field.

(Figure 29). The following imaginary experiment is presented

to point out the significance of the finely divided nature:

��

3.2.2.2 Determination of the Specific Surface by Adsorption

In addition to electron microscopy, the physical adsorption of

gases, especially nitrogen, is the most reliable method used to

determine the specific surface of highly dispersed materials.

The N2 adsorption isotherms, measured at – 196 °C, are evaluated

according to BRUNAUER, EMMETT and TELLER („BET surface“)

(36) and according to the t-curve method developed by

DE BOER (37). The BET and the calculated TEM surfaces are

found to correspond well with each other. AEROSIL® 380 is an

exception here. In comparison with AEROSIL® 300, the particles

do not become finer, but instead show a certain surface rough-

ness. All other types of AEROSIL® fumed silica, therefore, have

primary particles with a smooth and nonporous surface. On the

other hand, a notable porosity can be determined with precipi-

tated silicas (38).

In contrast, silica gels which are used amongst other things

as flatting agents have pore volumes, 90 % of which must be

classed as mesopore volumes (39); this subject is also discussed

in brochure No. 32 in this series of Technical Bulletin Pigments.

Afterreachingthemonolayer,theN2adsorptionisotherms

proceedinaveryflatcondition,andthereforedisplayan

anomalousbehaviourincomparisonwithothergasessuch

asAr,CO,andO2(40).InadditiontotheVANDERWAALinter-

action,thedipolequadrupoleinteractionbetweentheN2

moleculeandthesilanolgroupsapparentlyplaysadecisive

role.Thisinteractionshouldonlybepossible,however,when

arelatively„open“surfacestructurepermitsanapproachof

theN2moleculetotheOHgroup.

��

3.3 Special Physico-Chemical Data

For technical interests, the following quantities

are often relevant:

- specificsurfaceaccordingto

BET(DIN66131)

- averagesizeoftheprimaryparticles

- tappeddensity(DINISO787/11)

- dryingloss(DINISO787/2)

- ignitionloss(DIN55921)

- pHvalue(DINISO787/9)

- foreignoxides

- chlorinecontentand

- sieveresidueaccordingtoMocker

(ISO787/18)

While the corresponding analytical study

methods are described in (41), the physico-

chemical data are compiled at the end of this

publication.

The high temperature stability of AEROSIL®

hydrophilic silica (up to 850 °C under continu-

ous load) is of importance, for example, when

AEROSIL® fumed silica is used for thermal

insulation, also see Section 3.1 on this point.

Table 9: Special physico-chemical data relating to AEROSIL® products 1)densityofamoduledobject,aircomparisonpycnometer,helium 2)AEROSIL®hydrophilicsilicacannaturallynotbebroughttoignition 3)withtheexceptionofhydrofluoricacid

Refractiveindex 1.46

Solubilityinwater(pH7,25°C)(38) 150 mg/l

Specificweight1) 2.2 g/cm3

ThermalcapacityCpof 10 °C: 0.79 J/g K

AEROSIL®200 50 °C: 0.85 J/g K

Wettingheatofwateron

AEROSIL®200 -150 x 10-7 J/m2

Molaradsorptioncoefficientforfree

silanolgroups(3750cm-1)(61) (4.4 ± 0.4) x 105 cm2/mol

TemperaturestabilityofAEROSIL®

hydrophilictypes 850 °C

IgnitiontemperatureofAEROSIL®

hydrophobictypesaccordingtoDIN517942) AEROSIL® R 974: 530 °C

AEROSIL® R 805: 480 °C

AEROSIL® R 812: 460 °C

AEROSIL® R 202: 440 °C

Stability

withrespecttoacids excellent 3)

withrespecttoammonia5% slight

withrespecttosodiumhydroxidesolution5% very slight

withrespecttooxidizingagents excellent

withrespecttoreducingagents excellent

3.3.1 Solubility

Although quartz is considered as being practically insoluble

in water at room temperature (42), it actually dissolves by

about 0.015 % at room temperature and a pH of 7. This state-

ment also applies for all AEROSIL® hydrophilic types in the

equilibrium state. However, the dynamics of the dissolving

process differ greatly while quartz only reaches the equilib-

rium value after long contact times, types of AEROSIL® fumed

silica quickly form supersaturated solutions because of their

finely divided nature and their amorphous character.

In comparison with the hydrophilic AEROSIL® fumed silica, the

hydrophobic AEROSIL® products have a lower temperature

stability because of their carbon content (see Table 9).

However, for example in the case of AEROSIL® R 972, no volatile

organic compounds are detected during a headspace analysis

after 2 hours at 100 °C with the GC/MS coupling.

For purposes of supplementation, Table 9 presents special

characteristics.

�0

Figure 31: Solubility of AEROSIL® 200 in sodium hydroxide solution, turbidity after various standing times, 1 % aqueous dispersion

Figure 30: Solubility of various types of AEROSIL® fumed silica in water at 20 °C as a function of the contact time

0 20 25

50

100

150

200

250 20 °C

0

mg

SiO

/l2

Time [d]

AEROSIL 380®

AEROSIL 200®

MOX 170

MOX 80

5 10 15

7 11 12 13 14

20

40

60

80

100

120

0

Tubi

dity

scal

e di

visio

n

pH value8 9 10

0.5 hours

2 hours

24 hours

Figure 30 shows the solubility of various types of AEROSIL®

fumed silica. With rising alkalinity, a silicate formation advances

rapidly with AEROSIL® types. As clearly shown by Figure 31, this

process is already quite noticeable at pH ~ 10.

3.3.2 Thermal Conductivity

A report is presented in (43) on studies relating to the spectral

transfer of radiant heat at AEROSIL® 380. The absolute thermal

conductivity of some AEROSIL® fumed silica types is repre-

sented in Figure 32 as a function of the average temperature of

the heat transfer.

Figure 33 compares the thermal conductivity of AEROSIL® 200

with that of Degussa precipitated silica SIPERNAT® 320 DS. In this

comparison it must be noted that AEROSIL® 200 was studied as

a press plate with the densities given, while SIPERNAT® 320 DS

was measured in an Al foil under vacuum with higher densities.

Figure 32: Absolute thermal conductivity of some AEROSIL® fumed silica types, pressing density 200 g/l

0 400 500 600

0.1

0.2

0.3

0.4

0.5

0Abso

lute

ther

mal

cond

uctiv

ity W

/(m

K)x

Average temperature °C100 200 300

AEROSIL 300AEROSIL 200

®

AEROSIL 130AEROSIL OX 50

®®®

Pressing density 200 g/l

Figure 33: Comparison of the thermal conductivity of AEROSIL® 200 (simple moulding) with the Degussa precipitated silica SIPERNAT® 320 DS (sealed in Al foil, pressure ≤ 1 mbar)

120 g/l 150 g/l 220 g/l

sealed in Al foil,pressure < 1 mbar)

250 g/l

12

10

0

2

4

6

8

Ther

mal

cond

uctiv

ity m

W/(

mK)

x

AEROSIL 200® SIPERNAT 320 DS®

��

3.3.3 Nuclear Magnetic Resonance Spectroscopy Behaviour

The 29Si atomic nuclei represent suitable

probes for the more detailed characterization

of hydrophilic and above all of aftertreated

AEROSIL® products. For example, a clear

differentiation between dimethylsilyl groups

and monome-thylsilyl groups is possible on

the basis of the 29Si-CP-MAS solid state NMR

spectra with the chemical shifts. Table 10

shows SiR4 groups which can be distinguished

from each other by NMR spectroscopy. At the

same time, the nomenclature of these groups

commonly used in literature is also given.

The corresponding chemical shifts (29Si) are

compiled in Table 11.

Moreover, with nuclear resonance spectros-

copy it was possible to show unequivocally

that dimethylsiloxane chains, such as those

which occur in AEROSIL® R 202, are bonded

chemically to the SiO2 surface. In addition to

these chains, smaller cyclodimethylsiloxane

rings also play a role (44).

Table 10: Groups detectable by NMR spectroscopy on a silica surface after the reaction with a) monochlorosilane (M), b) dichlorosilane (D), and c) trichlorosilane (T), R = n-alkyl, R‘ = CH3, D4, T3 , and T4 are groups arranged „parallel“ to the SiO2 surface, while D4‘ , T 3‘ , and T4‘ are groups arranged „perpendicular“ to the SiO2 surface (45)

Table 11:

NMR chemical shifts of silane peaks in ppm rel. liquid Me4Si (see Table 10 for the assign-ment). The nomenclature T2 and T3 differentiates between an Si atom with two (-O-Si-O)n units as neighbours (T2 ,) and an Si atom with one (-O-Si-O-)n and one (-O-Si-R) unit as neigh-bour (T3 ) corresponding to the different chem.environments, T2

and T3 also differ in the chemical shifts (46 - 48)

C

C

OH

OR´

OR´

OR´

OHH3

H3

C

C

C

C

OH(R´)C

C

C

C C

OHH3

H3

H3

H3

H3

H3

H3

H3 H2

C

C

C

C

CC

C

C

C

C

C

C

C

C

C

C OH(R´)

OH(R´)(R´)OH

OH(R´)

OH(R´)

C

C

H2

H3

H2

H2

H2H2

H2

H2

H2

H2

H2

H2

H2

H2

H2

H2

H2

H2

O

O

O

O

OO

O O

O

OO

O O

O

OO

O

OO

O

O O

O OR

R

O

O

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

SiSi

Si Si

Si

Si

R R

R

RR

R

R

R

R

R

R

R

R

R

R

R

M1

M2

D1

D2

T3

T4

T4

T3

T4

D4

D4́

´

´

D3 T2

T1

T1

CH3

Typ of Structure δSi

Q2 - 91

Q3 - 101

Q4 - 110

D1 - 4

D2 - 7.2

D3 - 10

D4+D

4´ - 14 to -21

T1 - 46

T1 - 50

T2 - 55.5 R ≥ CH

3

T3+T

3´ - 59.0 R ≥ CH

3

T4+T

4´ - 64 to -70

��

3.3.4 Tribo-Electricity

For some applications, for example toners, the tribo-electric

characteristics are important. In Figure 34, the specific charge

values (q/m values, charge/mass ratio) are compared for

some products. As this figure shows, by means of suitable

aftertreatment positively chargeable powder can also be

produced. Zeta potentials, which likewise permit conclusion on

surface charges, will be discussed in Section 3.6.3.4.

100

150

0

-200

-50

-100

-150

50

Spec

ific c

harg

eC/

gµ

AERO

SIL

200

®

AERO

SIL

R 97

2®

TiO P 252

TiO T 805**

2

Al O C32

VP ZrO2

AERO

SIL

R 20

2®

VP R

504

*

Figure 34: Specific charge measurement (µC/g) on pyrogenic Degussa oxides. Measuring instrument: Epping GmbH, carrier: C 1018, 1 % * chemically aftertreated AEROSIL® 200, VP R 504 ** chemically aftertreated TiO2 P 25, TiO2 T 805

European Pharmacopoeia (Ph. Eur.) Silica, colloidal anhydrous

US Pharmacopoeia/National Formulary (USP/NF) Colloidal silicon dioxide

Deutsches Arzneibuch (DAB) Hochdisperses Siliciumdioxide

British Pharmacopoeia (BP) Colloidal anhydrous silica

Pharmacopoeia of Japan (JP) Light anhydrous silica acid

Table 12: Principal pharmacopoeia monographs for fumed silicon dioxide

3.3.5 Refractive Index The refractive indices of the individual AEROSIL® types only

differ from each other insignificantly. In order to determine

these indices, the AEROSIL® samples are suspended in carbon

tetrachloride. By means of the turbidity-temperature curve,

the turbidity minimum is ascertained. When this method is

employed, the refractive index of carbon tetrachloride at

the lowest turbidity corresponds to that of the hydrophilic

AEROSIL® types. At 1.45, AEROSIL® R 202 has the lowest value.

3.4 Purity

In the production of AEROSIL® fumed silica, highly volatile silicon

compounds serve as educts, which are reprocessed by distilla-

tion and therefore are used in pure form (see Section 2). During

the flame hydrolysis, the only by product that develops is gase-

ous hydrogen chloride, which can be almost entirely separated

from the solid matter. The result is a product of high purity. For

example, the SiO2 content in AEROSIL® fumed silica is greater

than 99.8 %. AEROSIL® 200, meets the requirements of mono-

graphs contained in numerous pharmacopoeias and official

specifications (Table 12).

The Degussa product AEROSIL® 200 Pharma is intended spe-

cifically for the pharmaceutical industry, having been tested in

accordance with the Pharmacopoeia European and the United

States Pharmacopoeia National Formulary (Ph. Eur. and USP/NF)

and being supplied with a corresponding certificate of analysis.

��

Table 13: Trace element impurities in AEROSIL® 200 and AEROSIL® OX 50. The limits given contain mean values from arbitrarily-selected samples, but do not represent any specifications. Study method: neutron activation analysis or AAS, total content

Elementcontent

≤ 0.01 ppm ≤ 0.1 ppm ≤ 1 ppm ≤ 10 ppm

As Cd Cr AlAu Co Cu BaSc Mo Hg CaTh Pb In FeU Sb K Na Mg Ni Mn Sn Zn

Among the „impurities‘“ (which all together make up a

maximum of 0.2 %), above all Al2O3, Fe2O3, and TiO2 are of

importance. Additional foreign elements occur only in traces,

as shown by Table 13. A comparison with precipitated silicas is

given in Table 14.

3.5 Oxide Mixture and Mixed Oxides

In order to derive products with other characteristics, synthetic

silicas are treated with Al compounds. Some AEROSIL® fumed

silica types (for example, AEROSIL® COK 84, AEROSIL® MOX 80,

AEROSIL® MOX 170) contain defined amounts of aluminium

oxide. The desired dosage can be carried out in two different

ways, which in part lead to products with different applications.

The difference between an oxide mixture and a mixed oxide is

shown by Figure 35.

Table 14: Comparison of the SiO2 content and the total impurities

Figure 35: Schematic comparison of an oxide mixture (left: AEROSIL® COK 84) with a mixed oxide (right: AEROSIL® MOX 80 or AEROSIL® MOX 170)

SiO2SiO2 doped

(”Si-O-Al-O-Si”)with Al O2 3

Al O2 3Product SiO2 (%) Impurities (ppm)

Precipitatedsilica ≥ 98.0 ≤ 20,000

AEROSIL® ≥ 99.8 ≤ 2,000

��

3.5.1 AEROSIL® COK 84

By mechanically mixing about 84 % AEROSIL® 200 and about

16 % AEROXIDE® Alu C, an oxide mixture develops which is

known as AEROSIL® COK 84. The primary particles here consist of

either SiO2 or pure Al2O3.

AEROSIL® COK 84 has proven successful in the thickening and

thixotropizing of pure, polar liquids. The term „polar“ in this

connection is intended to mean that the molecules in the liquid

are able to form hydrogen bridge linkages. Figure 36 shows

that water is thickened distinctly better with AEROSIL® COK 84

than with AEROSIL® 200. However, this statement can not be

applied, to dispersions of plastics because the composition

of AEROSIL® COK 84 is optimized for water without emulsified

polymers, etc. In other systems, it is quite possible that the best

thickening effects are to be achived with other AEROSIL® 200-

AEROXIDE® Alu C mixtures (see Figure 37).

In order to explain the thickening and thixotropic effects of

AEROSIL® COK 84 in polar liquids, we need to discuss the forma-

tion of a spacial, or three-dimensional network as a model. Since

the silica becomes charged negatively due to the dissociation of

the acidic silanol groups in contrast to the AEROXIDE® Alu C,

the interaction with the positive aluminium oxide particles is

supported by the different electrostatic charge.

Figure 36: Thickening effect of AEROSIL® 200 and AEROSIL® COK 84 in water

Figure 37: Thickening of polar liquids with 3 % of an AEROSIL® 200 / AEROXIDE® Alu C mixture

0 4 5 6

15

5

20

10

0

Visc

osity

Pa

s

AEROSIL concentration %®1 2 3

AEROSIL COK 84®

AEROSIL 200®

0

2000

4000

6000

0

100

80

20

100

0

Visc

osity

m P

a s

Aluminium Oxide C %

AEROSIL 200 %®

20

80

40

60

60

40

Water

Isopropanol

Dimethyl formamide

��

In comparison with some organically modifi ed, layer-type

silicates, AEROSIL® COK 84 offers the following advantages when

used in aqueous systems:

- nomasterbatchnecessary

- laterincreaseinviscosityposesnoproblems

- viscosityisonlyslightlysensitivetoelectrolytes

- viscosityisrelativelytemperature-stable

- so„reactive“organiccomponents

- purewhitepowder.

3.5.2 AEROSIL® MOX 80 and AEROSIL® MOX 170

If, as described in Section 2, an SiCl4 / AlC3 mixture (about 99:1) is

hydrolyzed in one oxyhydrogen fl ame, mixed oxides are devel-

oped: AEROSIL® MOX 80 and AEROSIL® MOX 170, which differ

from each other only in terms of the surface area. In this process,

the aluminium oxide is incorporated as doping oxide directly into

the primary particle of the host oxide (SiO2).

Table 15: Technical data of a selection of AERODISP® fumed silica dispersions 1) Solid contents may vary +/- 1 %2) Measured according to EN ISO 787-9 method3) Measured according to DIN EN ISO 3219 at a shear rate of 100 s-1

4) Dispersion Medium is ethylene glycol 5) Silica surface is recharged to a cationic (positive) surface charge

3.5.3 AERODISP® fumed silica dispersions

Degussa provides various dispersions for many different applica-

tions. They are manufactured using innovative technologies

and are known under the AERODISP® trademark. They are either

based on water or ethylene glycol and contain our fumed silicas

(AEROSIL®) or fumed metal oxides (AEROXIDE®). Our product

portfolio includes dispersions with different pH values and solid

contents to satisfy a wide range of requirements.

AERODISP® dispersions are easy to handle and work with. In

many applications their properties outperform those of powders.

Our AERODISP® dispersions have a milky-white appearance

and low viscosity.Depending on the product, solid contents are

between 12 to 50 % by weight with narrow particle size distribu-

tions ranging from 50 to 300 nm.The dispersing processes, as

well as the additives used for stabilization, are product specifi c.

The special aggregate structure and high purity of the dispersed

particles (AEROSIL® fumed silica and AEROXIDE® products) make

our dispersions superior to other conventional colloidal systems.

These are non-binding guide values.

AERODISP® fumed silica dispersions

W 7520 W 7622 W 1226 W 1714 W 1824 W 1836 W 7215 S W 7512 S WK 3415) G 12204)

Appearance milky-white liquid

Solid-Content 1) 20 22 26 14 24 34 15 12 41 20

pH value 2) 9.5 - 10.5 9.5 - 10.5 9 - 10 5 - 6 5 - 6 4 - 6 5 - 6 5 - 6 2.5 - 4 -

Viscosity at 20 °C 3) ≤ 100 ≤ 1000 ≤ 100 ≤ 100 ≤ 150 ≤ 200 ≤ 100 ≤ 100 ≤ 1000 ≤ 300

Density (20 °C) 1.12 1.13 1.16 1.08 1.15 1.23 1.09 1.07 1.28 1.23

Container Weight (net) (60 kg canister, 220 kg drum or 1000 kg IBC)

wt %

mPa . s

g/cm3

kg

��

3.6 Surface Chemistry

In addition to the particle fineness of AEROSIL® fumed silica, the

large specific surface represents the most important character-

istic of AEROSIL® fumed silica. The latter depends – as already

discussed – on the average size of the primary particles. Since the

surface area of the AEROSIL® fumed silica types is large in relation

to the mass, the surface chemistry plays a significant role and

determines many applicational properties.

3.6.1 Two Functional Groups Determine the Chemistry

Fundamentally two functional groups, namely the silanol groups

and the siloxane groups, can be differentiated from each other in

the case of AEROSIL® fumed silica, as shown in Figure 38.

A hydrophilic character must be attributed to the silanol

groups, i. e. these groups are „water attractive“ and are

responsible for the fact that AEROSIL® hydrophilic types is easily

wetted by water. Moreover, the possibility of producing AEROSIL®

hydrophobic types must be attributed to the chemical reacting

capacity of the silanol groups.

In contrast, the siloxane groups are largely inert chemically

(i. e. non-reactive), and in addition a hydrophobic, in other

words water repellent, nature must be attributed to them.

However, in the case of the non-aftertreated AEROSIL® types, the

hydrophilic character of the silanol groups prevails.

On the basis of these two functional groups, quite complex

reaction chemistry develops under certain conditions. This

can also be seen to stem in part from the fact that we must

distinguish between the following groups:

-freesilanolgroups

-bridgedsilanolgroups

-geminalsilanolgroups

-vicinalsilanolgroups

-siloxanegroupsundertensionandthoselessundertension.

Figure 38: Silanol groups (left) and siloxane groups (right)

O O

H

Si Si Si

The individual groups assembled in Figure 39 will be discussed

in greater detail below. Initially, however, the determination of

the silanol groups will be described because as mentioned, these

groups are of special importance.

H

H

H HO

O O

O

O

O

Si

Si

Si

Si Si

Si

free

geminal

vicinal and bridged

siloxane group

H

Figure 39: Si02 surface groups, concentration data are given for example in Table 18, as well as in Figures 40 and 45

��

3.6.2 Determination of the Silanol Groups

Due to the reacting capacity of the silanol groups, these groups

can be determined quantitatively by various methods. In the

literature, mainly the following methods are described to

determine the SiOH concentration:

- annealingofdriedAEROSIL®(49-51)

- chlorinationofSiOH(51-55)

- conversionofSiOHwithphenyllithium(53),withdiazo-

methane(53,56),andwithalkylmagnesiumhalides(57)

- conversionofSiOHwithB2H6(58,59)

- conversionofSiOHwithLiALH4(60,61)

- infraredspectroscopy(51,54,61-64)

3.6.2.1 The Lithium Aluminium Hydride Method

After extensive comparative studies, it was determined that the

conversion of dried AEROSIL® fumed silica (1 h, 100 °C, ≤ 10-2 mbar)

with LiAlH4 is one of the most exact and simplest methods of

determining the SiOH concentration on the AEROSIL® surface:

4 SiOH + LiAIH4 Si - O - Li + ( Si-0)3 AI + 4 H2

The other methods of determination mentioned above are less

reliable and have attained no importance.

When the lithium aluminium hydride method (also known as the

lithium alanate method) is employed, the amount of hydrogen

split off is found by a pressure measurement, and in this way the

silanol group density is ultimately determined. Since the hydride

ion as an attacking agent is very small and consequently highly

reactive, all silanol groups on the surface – including the bridged

groups – are detected. This corresponds with the determination

of the residual silanol group density of AEROSIL® hydrophobic

types, which according to IR spectroscopic findings contains

practically no free silanol groups any longer (see the IR spectrum

of AEROSIL® R 812 in Figure 6, page 13).

diglymes

Sila

nol g

roup

conc

entr

atio

n m

mol

/g

Sila

nol g

roup

den

sity

nm-2

0.5 1

1.0 2

1.5 3

0 00 100 200 400300

Specific surface m /g2

SiOH density

SiOH concentration

-O-D 2760

-C-H 2900-3000

-SiOH(isolated) 3750

-SiOH(bridged) 3000-3800

-SiOH(bridged)protonacceptor 3715

-SiOH(bridged)protondonor 3510

-SiOH(combinationoscillation) 4550

H2O 5200

Figure 40: Total silanol group concentration on the AEROSIL® hydrophilic products according to the LiAIH

4 method

Table 16: Some important IR adsorption bands (cm-1) of pyro- genic silicas; precipitated silicas – because of their high water content – show strong uncharacteristic oscillation bands in the bridged SiOH range

The method results in meaningful and reproducible values.

On the basis of the IR combination oscillation band of water at

5200 cm-1 or the Si-OH-bands between 3800 and 2800 cm-1 it can

be concluded that, under the drying conditions given above, free

and physically bound water is completely removed, whereby the

splitting off of the silanol groups is still imperceptible

(see overview in Table 16).

As shown by Figure 40, the silanol group density is to a first

approximation independent of the specific surface. With old

material (storage time after production longer than 1 week,

i. e. normal product) about 2.5 SiOH/nm2 are measured, see

Table 18, page 43.

��

Only in the case of AEROSIL® OX 50 are lower SiOH densities

found (about 2.2 SiOH/nm2), which must be attributed to the

production process. In comparison with the other AEROSIL®

fumed silica types, this is produced at higher flame temperatures.

As expected, the absolute concentration of the silanol groups

rises linearly with the specific surface. This contributes

substantially to understanding the greater thickening effect

of the AEROSIL® fumed silica grades with a high surface area

(assuming a good dispersion), see Figure 68, page 57.

3.6.2.2 IR Spectroscopy

In addition to the LiAIH4 method, IR spectroscopy has recently

gained importance for the qualitative as well for the quantitative

determination of the silanol groups in the laboratory.

IR spectroscopy, however, is not a suitable method for

production control.

Pressed tablets of pure AEROSIL® fumed silica are used for the

testing, (for example 13 mm diameter, 16 mg/cm2) which in the

case of the hydrophobic material are pressed into a wire net to

increase the stability. In Table 16, some important adsorption

bands are listed.

With the aid of the LAMBERT-BEER law,

In Jo/J = εcd

quantitative measurements of the free silanol groups are

possible. Here, the quotient Jo/J is identical to the transmittance

of the sample (at the relevant wave length), and the product cd

refers to the density of the silanol groups (mol/cm2). The molar

adsorption coefficient for free silanol groups (3750 cm-1) was

determined by J. MATHIAS and G. A. WANNEMACHER (61) as

(4.4 ± 0.4) x 105 cm2/mol.

Tran

smitt

ance

%

20 20

40 40

60 60

80 80

100 100

0 0

4000 4000 20003000 3000

Wave number cm-1

Figure 41: IR spectroscopy tracking of the H-D exchange on AEROSIL® 200 (left: starting material, right: after exchange)

The silanol groups react with D2O with an H-D exchange.

IR spectroscopy makes it possible to monitor the reaction process

(see Figure 41), and after analysis of the H2O/HDO/D2O mixture

therefore permits a quantitative determination of the silanol

groups (61). The results correlate well with the lithium aluminium

hydride method.

Furthermore, Figure 41 shows that a slight portion of the silanol

groups (about 10 - 20 % internal SiOH groups) is not accessible for

a deuterium exchange.

��

3.6.2.3 Morpholine Adsorption

One method of determination that detects only „sterically

accessible“ silanol groups is based on the adsorption of donor

molecules such as, for example, morpholine (see also 3.6.3.3.).

The course of the adsorption isotherms of morpholine of

AEROSIL® 200was studied by M. ETTLINGER, H. FERCH and

J. MATHIAS (65).

According to Figure 42, at a constant morpholine concentration

the adsorption (mmol/g) is a function of the BET surface area.

In contrast, analogous to the silanol group density, the

morpholine coating density shows only a slight dependence

on the specific surface (see Figures 40 and 42 on this point).

However, with the morpholine adsorption method, lower coating

densities are found. These discrepancies must be attributed to

the different adsorption behavior of bridged and free silanol

groups as well as to their steric accessibility.

The morpholine adsorption values of the AEROSIL® hydrophobic

types are shown in Table 17. AEROSIL® types with chemically

comparable groups have comparable adsorption values.

AEROSIL® R 202 and AEROSIL® R 805 differ distinctly here. Due to

the chain structure, by no means all silanol groups (LiAlH4) are

sterically accessible. The greatest differences between the two

methods of determination are displayed by AEROSIL® R 805

(1.66 and 0.47 SiOH/nm2).

In polar systems (for example 1.2-ethylene glycol, epoxy systems)

AEROSIL® R 202 and AEROSIL® R 805 have comparably good

thickening effect which is superior to that of the other AEROSIL®

fumed silica types (see Edition 27 in this series of Technical

Bulletin AEROSIL® on this point). In this connection, the chain

structure evidently plays a dominant role.

3.6.3 Interparticular Interaction

In conjunction with the description of the rheological

characteristics of dispersions containing AEROSIL® fumed silica

or of the structure of pulverulent AEROSIL®, the interactions

between the SiO2 particles themselves and with the dispersion

phase play a decisive role.

In Edition 23 in this series of Technical Bulletin Pigments, the

following possible interactions are discussed in greater detail:

- VANDERWAALSattractiveforces

(permanentorinduceddipoles)

- electrostaticinteractions (COULOMBinteractions)

- gravitationalinteractions(negligible)

- acid/baseinteractionsaswellasorbitalinteractions (important).

Ads.

mor

phol

ine

mm

ol/g

Mor

phol

ine

mol

ucul

es n

m2

1.0

0.2

1.2

0.4

0.6

0.8

00 100 200 400300

BET surface m /g2

1.0

0.2

1.2

0.4

0.6

0.8

0

Figure 42: Dependence of the morpholine adsorption on the BET surface of hydrophilic AEROSIL® fumed silica types Cmorph = 0.1 mol/l , butanol/water 1 : 1)

Table 17: Comparison of the silanol group density determined according to the morpholine adsorption and the LiAIH

4 methods on hydrophobic AEROSIL® types

AEROSIL®types Surfacegroup Morpholine SiOH molecules pernm2 pernm2 LiAIH

4

R 972 Dimethylsilyl 0.32 0.60 R 974 Dimethylsilyl 0.35 0.39R 812 Trimethylsilyl 0.33 0.44R 202 Dimethylsiloxane 0.15 0.29R 805 Octyl 0.47 1.66

�0

3.6.3.1 Hydrogen Bridge Linkage

Hydrogen bond interactions are regarded as a subgroup of

the acid/base interaction. According to a new model by E. R.

LIPPINCOTT and R. SCHRÖDER (66, 67), the hydrogen bridge

linkage is described by a superimposition of protomeric limiting

structures. According to Figure 43, the proton has two stable

options within the bond (68). The proton delocalization over

the range of the two potential wells takes place by means of a

tunnel effect at high frequency (comparable with the ammonia

inversion oscillation).

Figure 43: Double potential minimum of the hydrogen bridge linkage between the O atom of a silanol group and the H atom of a water molecule (schematically, the O-O distance is held constant)

Figure 44: Hydrogen bridge linkage between two idealized AEROSIL® 200 primary particles in an enlargement true to scale

Distance between an O atom and the H atom

Energy

O OO OH HH H

H HSi Si

The energy of the hydrogen bridge linkage (4 - 40 kJ/ mole)

depends on the OHO angle. It reaches a maximum when the

three atoms are arranged linearly.

In comparison with a covalent C-H bond (about 360 kJ/mole),

the hydrogen bridge linkage is a moderately weak interaction.

However, it is stronger than the VAN DER WAALS forces. In nature,

hydrogen bonds play a quite decisive role. The mean kinetic

translational energy in the case of the human body temperature

is about 4 kJ/mole (69). The splitting and regeneration of

hydrogen bonds are therefore elementary processes in the

metabolism. Only due to the directional character of the H bonds

can complicated molecular structures be maintained. At the

same time, they permit a rapid structural change.

Similar „reactions“ constantly take place on the AEROSIL® fumed

silica surface as well. The „temporary structure“ of the AEROSIL®

agglomerates can be explained by the simple formation and

breaking of hydrogen bridge linkages.

Due to the (slight) silanol group density of about 2.5/nm2, no

possibility exists for the formation of intramolecular bridges

(in contrast to precipitated silicas). The presence of isolated

silanol groups is supported by IR spectroscopic studies (70),

so only intermolecular hydrogen bridges must be taken into

consideration.

Figure 44 shows a hydrogen bridge interaction between two

AEROSIL® primary particles in an enlargement true to scale.

In the infrared spectrum, the frequency of the oscillations

between the oxygen atom and the hydrogen atom in the

OH bond decreases in the cases of bridged systems, which

corresponds to a loosening of the OH bond (see Table 16 and

Figures 6 and 41).

The probability of finding a suitable neighbor for a silanol

group for the purpose of forming an H bridge rises as the

particle fineness increases. Therefore, the density of the free

silanol groups (SiOH/nm2) decreases as the specific surface

increases (Figure 45). This is also reflected in the dispersibility

of AEROSIL® fumed silica which decreases as the specific surface

increases (also see Figure 68 on this point).

Si Si

Si

Si

Si

OxygenHydrogen

��

3.6.3.2 Moisture Balance

3.6.3.2.1 Moisture Balance at Room Temperature

Water molecules can be fixed to silanol groups and thus to

the AEROSIL® fumed silica surface via hydrogen bridges, as dis-

cussed in the preceding section. Water vapor therefore has

a relatively high affinity for AEROSIL® hydrophilic types and

is adsorbed well (wetting heat of water on AEROSIL® 200:

- 150 x 10-7 J/m2; in comparison on quartz, which has a higher

affinity for water in liquid form: - 610 x 10-7 J/m2).

The water adsorption from the atmosphere depends on the

humidity of the air and is reversible (Figure 46).

After storage conditions change, the atmospheric moisture is

initially adsorbed remarkably quickly or given off again just as

quickly. In practice, the establishment of equilibrium takes place

slowly because in a bag of AEROSIL® fumed silica the outer layers

largely shield proportions lying more deeply inside. The moisture

balance is then determined essentially by diffusion processes.

This behavior can be demonstrated in a simple weathering

experiment. For this purpose, test tubes filled with AEROSIL® 150

are stored at an atmospheric humidity of 90 %. Due to the

adsorption of water, the absolute values of the extinction

measurements of the free and bridged silanol groups change.

The change in the quotient of extinction „bridged“ and „free“

describes the progressive water adsorption. Figure 47 shows that

the upper AEROSIL® layer differs distinctly from the lower layer.

The result can be explained only on the basis of a slow diffusion

of moisture into the inner layers of an AEROSIL® charge.

Extin

ctio

n of

the

brid

ged

silan

ol g

roup

s cm

-1

0.5

1.,0

1.5

0

0.25

0.50

0.75

Conc

.of t

he fr

ee si

lano

l gro

ups

mm

ol/g

Free

sila

nol g

roup

den

sity

nm-2

Specific surface m /g2

0

80

40

120

160

00 100 200 400300

SiOH density

SiOH concentration

Extinction

Figure 45: Silanol groups as a function of the specific surface of AEROSIL® fumed silica (IR, samples 1 year old, method of determination (61))

Figure 46: Moisture adsorption (storage at 82 % rel. humidity) and release (storage at 55 % rel. humidity) of AEROSIL® 200 (10-kg bag, sealed, red curve: PE lining = normal packaging; blue curve: bag without PE layer)

Figure 47: Comparison of the silanol group relationships in the lowest and uppermost layers of AEROSIL® fumed silica in a test tube as a function of the duration of moistening (90 % rel. moisture)

0 40 10050 60 70

6

4

3

2

1

7

5

0

Moi

stur

e ad

sorp

tion

%

Duration of weathering, days

82 % rel. humidity 55 % rel. humidity

10 20 2030 30

Bag without PE lining

Bag with PE lining(normal packaging)

0 80 100 120 140

0.6

0.4

0.3

0.2

0.1

0.7

0.8

0.5

0

SiOH

gro

ups r

el.b

ridge

d / f

ree

Contact time h20 40 60

Upper Layer

Lower Layer

��

The maximum water adsorption of AEROSIL® hydrophilic

types (thin layer) grows with an increasing specific surface,

and therefore correlates with the concentration of the silanol

groups. AEROSIL® 380, with the largest BET surface area,

naturally adsorbs the most water.

In the case of the AEROSIL® hydrophobic types, the silanol group

concentration, and thus the moisture adsorption, are distinctly

reduced (compare Figure 8).

AEROSIL® fumed silica is not hygroscopic. Although it adsorbs

atmospheric moisture, it readily releases this moisture again

under „normal“, slightly changed conditions. During each of

these processes – with an adequately long storage time – equilib-

rium conditions are attained. At least 95 % of the water adsorbed

after the genesis is removed by the simple application of vacuum

(10-1 mbar), even at room temperature, which can be easily proven

on the basis of the IR spectra. Solid, hygroscopic materials are

known to have a tendency to deliquesce and form lumps, while

AEROSIL® fumed silica remains unchanged under the „normal

conditions“ mentioned.

3.6.3.2.2 Aging

The bond angle SiOSi in a siloxane bridge can vary within wide

limits (120 - 180° (23), also see Section 3.1).

Stressed siloxane bonds, for example, have increased reactivity in

the presence of water:

As shown by Table 18, the silanol group concentration is

increased due to this hydrolysis during the course of the storage

time. The amount of water required for this is very slight (about

0.25 %). Immediately after the flame hydrolysis, AEROSIL®

fumed silica contains these amounts of moisture in the form of

physically bonded water. As shown from the IR spectra in

Table 18: Silanol group density of freshly produced AEROSIL® 200 and of AEROSIL® 200 stored for one year (original bag)

3daysold Stored1year Methodof determination

Isolated SiOH/nm2 1.5 1.15 IRTotal SiOH/nm2 1.8 2.65 LiALH

4

Figure 48, the physically bonded water also becomes attached

to the free silanol groups during storage, causing these groups to

be converted to the bridged form.

3.6.3.2.3 Moisture Balance at Higher Temperatures

If AEROSIL® 200 is heated, the concentration of free silanol

groups rises up to a temperature of about 600 °C (1.8 SiOH/nm2);

simultaneously, the intensity of the bridged form decreases to

zero (about 700 °C), see Figure 49. Slight changes in the band

form at 3750 cm-1 during the heating between 450 °C and 1100 °C

are attributed by E. KNÖZINGER, P. HOFFMAN, and

R. ECHTERHOFF (71) to a change in concentration of the

geminal silanol groups (= Si(OH)2).

Figure 48: IR spectra of AEROSIL® 200; fresh sample (left), stored for 1 year (right)

Tran

smitt

ance

%

20 20

40 40

60 60

80 80

100 100

0 0

4000 40003000 3000

Wave number cm-1

60

80

0,5

2,0

1,0

1,5

Dens

ity o

f the

free

sila

nol g

roup

s nm

-2

Temperature °C

0

40

20

00 600200 400 800 1000 1200

Density

Extinction at 4500 cm-1

Extinction ofthe bridgedsilanol groups

Inte

gral

ext

inc t

ion

cm-1

Figure 49: Splitting off of silanol groups from AEROSIL® fumed silica due to thermal treatment (IR monitoring, 1 hrs annealing time)

Si-O-Si +H-O-H 2 SiOH

��

The affinity of annealed AEROSIL® fumed silica for water was

tracked by spectroscopy in 1988 (71); the samples treated at 500 °C

to 900 °C adsorb water more readily than the AEROSIL® fumed

silica annealed at 1100 °C. Apparently, at high temperatures,

stressed siloxane bridges can be converted by means of minor

structural rearrangements to less stressed systems which are no

longer accessible for a later hydrolysis.

On the AEROSIL® sample annealed at 1100 °C, the simultaneous

growth of two IR bands can be observed at 3715 cm-1 (proton

acceptor) and 3510 cm-1 (proton donor). These intensities origi-

nate from a chemisorption of water, and must be assigned to H-

bridged, vicinal OH groups. Longer bridging chains (OH)3, (OH)4,

etc. are not observed.

At lower tempering temperatures (about 900 °C), the pure

adsorption of water molecules at silanol groups present likewise

becomes noticeable. AEROSIL® fumed silica which was treated

only at 450 °C (or at even lower temperatures) shows an addi-

tional band at 3675 cm-1 during the reaction with water. Annealed

samples do not show these oscillations, leading to the conclusion

that such processes are irreversible at high temperatures.

The reduced thickening effect of AEROSIL® fumed silica treated at

high temperatures must likewise be attributed to an irreversible

loss of silanol groups, Figure 50.

3.6.3.3 Other Adsorption Effects

It is known that amines enter into strong interactions with silicas

(72). The adsorption of morpholine for the determination of the

“accessible” silanol group density has already been discussed in

Section 3.6.2.3.

The possible interactions of an amine with the AEROSIL® fumed

silica surface are shown by Figure 51. A LEWIS interaction can

take place only when a coordinative defect point is present on

the surface (73).

Through adsorption it is possible to bind active substances such

as cardiac glycosides to AEROSIL® 200; nevertheless, the avail-

ability of the active substance is comparable with an AEROSIL®

fumed silica free powder (74). Thanks to many works by H.

RUPPRECHT and colleagues, deliberate statements can be made

about the adsorption of active substances on the AEROSIL®

fumed silica surface (75-84).

Cationic tensides are preferably bonded by means of an ion

exchange; the adsorption is pH-dependent (details, examples,

and models are described in 65).Figure 50: Thickening effect of annealed AEROSIL® 200 (UP resin, annealing times 5 hrs in each case)

Figure 51: Possible interfacial forms of the interactions between an amine and the AEROSIL® surface

0 400 800500 900600 1000700 1100

3000

2000

1000

0

Visc

osity

mP a

s

Annealing temperature (5 h each case) °C100 200 300

O O

H H

N N N

Si Si Si

Hydrogenbridge linkage

Brönstedinteraction

(formation of salt)

Lewis-interactions

(complexing)

��

Figure 53: AEROSIL® fumed silica as acid (schematic representation)

Figure 54: pH values measured on 4% aqueous AERODISP® dispersions as a function of the total chlorid content (determined after dissolution in alkali) blue squares: 4 % aqueous dispersion (specification (41)); green stars: dispersion above after centrifuging; red dots: measurement on pure HCl solution; theoretical curve (pH = - log [Cl - 1])

Figure 52: Sedimentation experiments with a 0.1 % and a 0.01 % AEROSIL® 200 dispersion (flocculant Nalco® 8100-A; in each case after 1 hrs and 24 hrs standing time)

The irreversible adsorption of polyethylene glycol (molecular

weight ≥ 5,000,000) on AEROSIL® hydrophilic types takes place

primarily via hydrogen bonds between isolated silanol groups

and the ether oxygen atoms of the polyethylene glycol (85).

The adsorption on AEROSIL® R 972 is comparable with that on the

hydrophilic silica. Consequently, hydrophobic interactions with

an uncharged polymer must not be neglected.

Figure 52 shows the adsorption of a polyelectrolyte on

AEROSIL® 200 which results in the almost quantitative floccula-

tion of the SiO2. In a 0.01 % AEROSIL® 200 „dispersion“ there is

a true SiO2 solution. Only in a high acidic milieu is the solubility

product exceeded, and sedimentation occurs.

H O2 H O2HCI 32% HCI 32%

100

0

20

40

60

80

Sedi

men

t vol

ume

cm3

0.1% dispersions 0.01% dispersions

3.6.3.4 AEROSIL® fumed silica as an Acid

AEROSIL® fumed silica is a very weak acid, i. e. the equilibrium

in Figure 53 is shifted distinctly to the left. Data in the literature

Si SiSi SiSi Si

Si SiSi SiSi SiSi SiSi Si

Si Si

Si Si

Si Si

Si Si

Si SiSi Si

Si Si

Si Si

Si SiO O

O O

O OO O

O O

O OO OO O

O OO OO O

O OO O

O O

O OO OO O

O O

O O

O O

O OO O

O O

OWater

O

O O

O OO OO O

HH H

H

H

HH

H

HH

H

H H

H

H

H

Cl p

pm

250

200

150

100

50

03.0 3.5 4.0 5.,04.5 5.5 6.0 6.5 7.0 7.5 8.0

pH value

referring to the acidic strength of pure SiO2 vary very widely (86,

87). If we calculate pKs = 9.46 (87), and with the assumption that

in one litre of SiO2 dispersion 150 mg are dissolved as Si(OH)4 a

pH value of 6.1 is found.

Moreover, it must also be considered with AEROSIL® fumed silica

that due to the production process (see under 2) slight amounts of

hydrochloric acid strongly influence measurement of the pH value.

Figure 54 shows that the determination of the pH value is

disrupted by the presence of agglomerates in the case of a

AERODISP® fumed silica dispersion. In some cases, the

measurement is made only after the centrifuging, i. e. on the

AEROSIL® fumed silica solution, and higher (or at least as high)

pH values are found, such as those which would be assigned to a

correspondingly pure HCl solution.

This phenomenon is known as the suspension

effect, and in the weakly acidic range

(pH = 4.5 - 7) can result in considerable differ-

ences between the dispersion and the solution

(about 2 pH units). According to (88), these

discrepancies are attributed to the negatively-

charged SiO2 particles and to the associated

increase in the transport number of the posi-

tive counterions in the dispersion.

��

Since the silanol groups can

split off protons, negatively

charged boundary surfaces

result. However, since the

entire system of the dispersion

is electrically neutral, compen-

sation is made at the surface

charge centres by counterions.

While the first charge layer is

fixed, the electrical potential

around the particle in the

area of a diffuse counterlayer

slowly dies out. This drop in

voltage is known as the zeta

potential, see Figure 55.

Figure 55: Electrical double layer around an electronegative particle, definition of the zeta potential (89)

Electro-negativeparticle

Elec. potentialaround the particle

Nernst potential

Zeta potential

Rigid shell aroundthe particle

shearing surface

Liquid shell

Range of the diffusecounterlayer

Fraction of positive ions

Fraction of negative ions

��

Figure 56: Zeta potentials of pyrogenic oxides produced by Degussa AG as a function of pH value (0.02 m KNO

3 )

Figure 56 characterizes the course of the zeta potential of

AEROSIL® OX 50 in comparison with the Degussa special oxides

as a function of the pH value. In this respect, all AEROSIL® hydro-

philic types show a comparable behavior in the strongly basic

range, the proton dissociation is largely complete (zeta potential

no longer changes), while the H+ splitting off in the strongly

acidic medium is suppressed. In principle, AEROSIL® hydrophobic

types show a similar course of curves even when the absolute

amounts of the zeta potential are small (about 10 mV), correspon-

ding to the low SiOH group concentration. The positive charge

especially of AEROXIDE® Alu C and VP Zirconium Oxide can be

recognized well in the graph.

3.6.4 „The Aftertreatment“– a Chemical Anchoring

3.6.4.1 The Chemical Aftertreatment – Some Bibliographic Examples

In the literature, numerous chemical reactions with AEROSIL®

fumed silica have been described. As examples, some of these

reactions will be reviewed in the following sections.

3.6.4.1.1 Amination

Ammonia can be induced to react with AEROSIL® fumed silica.

In die temperature range up to 500 °C the formation of singular

(Si-NH2) groups and geminal Si(NH2)2 groups can be observed

as the result of the substitution of OH groups or the dissociative

opening of reactive siloxane bridges (90):

2 6 7 8 9 10

40

0

-20

-40

-60

60

80

20

-80

Zeta

pot

entia

l mV

pH value3 4 5

AEROXIDE® Alu C

VP Zirconium Oxide

AEROXIDE® TiO P 252

AEROSIL OX 50® SiOH + NH3 Si - NH2 + H2OSi - O - Si + NH3 Si - NH2 + SiOH

For technical purposes, this and similar reactions have no

significance.

��

3.6.4.1.2 Reactions with Alkoxysilanes

The chemical reaction of pyrogenic silica with

methoxysilanes is catalyzed by amines. Basic

amines with exchangeable hydrogen atoms

have the highest effectiveness (91). Here, two

possible transition states must be discussed, and

are presented in Figure 57. Slight amounts of

H2O can likewise support the methanolysis (92).

The primary product of the reaction of AEROSIL®

fumed silica with dimethyldiethoxysilane at

room temperature is an H-bridged adduct,

whereby the O atom of the silane functions as

donor (92). Of the final products possible in

principle:

Si-OSi(CH3)2(OH) and (Si-0)2Si(CH3)2 (at 400 °C),

the former is more readily formed.

3.6.4.2 AEROSIL® Hydrophobic Types Expressed in greatly simplified terms, „AEROSIL®

hydrophobic types” is understood to mean a

material which is not wetted by water; in other

words it floats on the surface of the water.

3.6.4.2.1 Conversion from „Hydrophilic“ to „Hydrophobic“

The first industrially-produced hydrophobic

silica was introduced onto the market in 1962,

and has been designated AEROSIL® R972 since

KLOSTERKÖTTER reported in 1965 (93) that this

material causes no silicosis in animal experments.

Figure 58: Schematic representation of the reaction of dimethyldichlorsilane with the silica surface: conversion from „hydrophilic“ to „hydrophobic“

O

H

HH

HH

H

H

N

N

H

O

O

O

Me

Me

Me

MeSi

Si

NH3-

- CH3OH O

MeSi

B

A

B

A

O OO

H

H

O

CH3

CH3

CH3

CH3

CH3

H O2 + 4 HCI

DDCS

ClH C3

Cl

2O

Si

Si

Si Si

Si Si

Si

Hydrophilic AEROSIL® Hydrophobic AEROSIL®

Figure 57: Possible transition states during the amine-catalyzed reaction of trimethoxymethylsilane with the SiO2 surface; A: penta-coordinated transition stage B: 6-centre transition state

Freshly-produced hydrophilic AEROSIL® 130 is converted with

dimethyldichlorosilane in a fluid-bed reactor according to

Figure 58. The silane reacts with the silanol groups primarily

with the formation of Si-O-Si (CH3)2 units (94), and as a result the

material acquires a hydrophobic character. The number of silanol

groups is reduced during the treatment of about 30 %

of the initial value. Analogous reactions can also be carried out

with other silanes as shown by Figure 58. For example, when

dimethyldichlorosilane is replaced by hexamethyldisiloxane as

coreactant. The commercial product which is developed is known

as AEROSIL® R 812. An overview of all hydrophobic commercial

AEROSIL® products with their surface groups is given in Figure 7.

��

No serious difference between AEROSIL® hydrophilic and hydro-

phobic types can be recognized from TEM‘s and SEM‘s, as shown

by Figure 22.

In contrast, the water vapor adsorption isotherms differ

dinstinctly. Figure 8 shows, for example, that AEROSIL® R 202, in

comparison with its base silica AEROSIL® 150, adsorbs

distinctly less water. Further possibilities for differentiation and

characterization are discussed in detail in Technical Bulletin

Pigments, Edition 18, entitled „The Use of Hydrophobic AEROSIL®

in the Coatings Industry“.

As a result of the reduced silanol group density, the AEROSIL®

hydrophobic types show, among other things, a different disper-

sion and thickening behavior in comparison with hydrophilic

types.

The measurement values in Table 19 show, for example, that

AEROSIL® hydrophobic types can be dispersed more easily than

AEROSIL® hydrophilic types, even though the thickening effect is

less pronounced. This last fact makes a high proportion of filler

possible for the first time in many cases; a high proportion of filler

is necessary for reinforcing characteristics, for example in cable

gels for light conductivity fibres. This situation is additionally

represented in Figure 59 where a silicone rubber is used as an

example.

Figure 59: Mechanical characteristics of silicone vulcanizates with various silicas as filler (SIPERNAT® 160 = a precipitated silica); formulation: 100 parts Silopren VS (GE Bayer Silicones GmbH & Co. KG), 40 parts silica, 0.5 % bis-(2.4 - dichloro benzoyl) peroxide

10

0

2

4

6

8

Tens

ile st

reng

th N

/mm

2

SIPERNAT 160® AEROSIL® 300AEROSIL 200® AEROSIL R 974® AEROSIL R 812®

3 % AEROSIL® Viscosity in mPa • s Afterminutes 3 at 10 Vu=6,3 m/s Vu=15,3

AEROSIL® 130 9,000 19,000AEROSIL® R 972 1,000 1,100AEROSIL® 200 2,200 28,000AEROSIL® R 974 2,600 3,300

Table 19: Comparison of the thickening effect of AEROSIL® hydrophilic and hydrophobic types resulting from different dispersing intensity (mineral oil PKW F 4/7; Concentration of AEROSIL®: 3 %; Vu: circumferential speed)

��

50 0 -50ppm

-100 -150

Figure 60: 29Si-CP-MAS solid matter NMR spectrum of AEROSIL® R 974

3.6.4.2.2 The Chemical Anchoring

The reaction of a silane with a

silica surface is often not clear-

cut, but instead can result in

different surface groups. The

surface structures conceivable

in principle have already been

presented in Figure 7. The

exact study of this chemistry

was made possible, among

other factors, by the progress

achieved in 29Si-CP-MAS-NMR

spectroscopy.

In Table 11, some 29Si-NMR

chemical shifts are given which permit an association with the

M, D, and T groups. If the 29Si-NMR spectrum of AEROSIL® R 974 in

Figure 60 is evaluated with this information, it is found that the

D4 group is formed with greater frequency compared with D1 , D2

and D3 (see Table 10).

The surface group sketched in Figure 58 therefore represents

the most readily formed structure of AEROSIL® R 972 and

AEROSIL® R 974 under the reaction conditions selected.

If alkoxysilanes are used during the hydrophobic conversion,

for example, in the case of AEROSIL® R 805, the fixing of the

silane at the SiO2 skeleton and the methanolysis of the alkoxy

groups can likewise be tracked by means of solid matter NMR

spectroscopy (13C,29Si) because alcohol that has been split off

appears only as a diffuse peak. In the case of AEROSIL® R 202, the 29Si-NMR spectra show the chemical fixing of the polydimethyl-

siloxane units.

The peak splitting must be attributed to the presence of cyclic

and chain-like structures, and even statements on the mobility of

the chains are possible on the basis of the relaxation times.

In conclusion, it should be emphasized that in the case of

AEROSIL® hydrophobic types no fractions of free silanes have

been detected to date (IR, NMR, Headspace analysis, 100 °C).

�0

Figure 61: Optical micrograph of „dry water“ based on 3 % AEROSIL® R 812

Figure 62: AEROSIL® dispersions. Left: 2 % AEROSIL® 200 in water; centre: 2 % AEROSIL® R 812 in water; right: aqueous dispersion with 2 % AEROSIL® R 812 (the amount of water is always the same)

3.6.4.2.3 Dry Water and Aqueous Dispersions with AEROSIL® Hydrophobic Types

AEROSIL® hydrophobic types normally floats on

water, it is not wetted by water, it is therefore

water repellent.

However, if water is divided into fine droplets in

the presence of AEROSIL® hydrophobic types,

the hydrophobic silica envelopes the water

droplets and prevents them from uniting. In

this way a powdery substance is formed, the

so-called „dry water“ which Figure 61 shows in

the form of an optical micrograph.

If the water droplets are reduced in size still

further, they are capable of by-passing the

hydrophobic methyl groups, and reaching the

silica base and thus the hydrophilic anchor

points (residual silanol group density).

Partial wetting results, and air is entrapped

simultaneously. Such aqueous dispersions with

AEROSIL® hydrophobic types can be produced

in the case of AEROSIL® R 972, AEROSIL® R 974,

and AEROSIL® R 805 up to a 20 % SiO2 content.

Figure 62 shows, among other things, a 2 %

AEROSIL® R 812 dispersion.

��

Figure 63: Statistical quality control using AEROSIL® 300, BET surface, as an example n = number of measurement values, x = mean value, s = standard deviation, USG= lower specification limit, OSG = upper specification limit, Cpk = capability index

4. Statistical Quality Control

Conformity with the specification limits placed

on AEROSIL® fumed silica is ensured through a

comprehensive quality control system. A mul-

titude of individual measurements is compiled

not only during production, but also during

the actual quality control of the finished prod-

uct in our own main laboratory.

In order to be able to understand the flood of

data, a statistical analysis is imperative.

Figure 63 shows such an analysis as an

example. It provides an impression of the

progress of the individual measurements with

respect to time, and describes the distribution

of the measurement values. An important

parameter here is the capability index, Cpk,

which describes the width of the distribution

with regard to the specification limits. If the

process is „under control“, the Cpk value ≥ 1.

340

20 40 60 80

USG

OSG

n 80

min

max

s 5.03291.0318.0

270.0

1.9901.6940.3352.4951.163

330.0

X

X 304.47

X

USG

Cp

Slope

Geary`s no.

n < USG

X

OSG

Cpk

Convexity

n > OSG

n

-

-

320

300

280

260

USG OSG

270 279

5½12

½2

1713

117

7 1½3½

288 297 306 315 324

0.00 %0

<

0.00 %0

<

0.00 %0

<

��

The characteristic properties and main fields of use of the

AEROSIL® individual types are compiled in Table 20.

The hydrophilic AEROSIL® 200 or the hydrophobic

AEROSIL® R 972 can be recommended for fundamental experi-

ments. These products represent the standard AEROSIL® types.

As already mentioned frequently in the preceding sections,

the dispersion plays an important role in the use of AEROSIL®

fumed silica. The dispersibility decreases in the following order:

AEROSIL® OX 50 ≥ AEROSIL® 90 ≥ AEROSIL® 130 ≥ AEROSIL® 150

≥ AEROSIL® 200 ≥ AEROSIL® 300 ≥ AEROSIL® 380.

In general, hydrophobic AEROSIL® fumed silica is much more

easily dispersed.

The advantage of AEROSIL® high-surface types such as, for

example, the high particle fineness, high transparency, strong

reinforcing effect, and strong thixotropizing effect can be

utilized only when the necessary dispersing power is developed

(dissolvers, rotor-stator aggregates, pearl mills, triple roller mills,

etc.). In moisture-sensitive systems, AEROSIL® hydrophobic

types also offers – in addition to the easy dispersibility – the

advantages of a water-repellent effect.

Before the actual dispersing, the AEROSIL® fumed silica must

be wetted by the dispersing medium, it must be incorporated.

The incorporation time can be shortened when the compacted

product is used (supplement „V“ in the designation). Almost

all AEROSIL® fumed silica types (see physico-chemical data on

AEROSIL®, Page 78) are available in a compacted variant of, for

example, about 120 g/l. It must be taken into consideration,

however, that the dispersibility is somewhat reduced by the

compacting process.

6.1 Reinforcement

In all elastomers AEROSIL® fumed silica develops a consider-

able improvement in the mechanical characteristics such as the

tensile strength, tear propagation strength, or tear initiation

strength. Typical fields of use are HTV and RTV silicone rubber,

fluoroelastomers, and NR (natural rubber) and SBR (styrene-

butadiene rubber). In this series of Technical Bulletin Pigments,

Editions 12 and 63 discuss these applications in detail.

The fundamentals of the reinforcing must be attributed to

interactions with silanol groups and to the radical splitting of

Si-methyl bonds followed by incorporation of the fragments

into the polymer (95).

Quite low concentrations of AEROSIL® fumed silica are adequate

for adjusting the rheological properties and can be used in

coating systems to achieve a significant mechanical reinforce-

ment of the paint film. AEROSIL® fumed silica brings about an

increase in the glass transition temperature of the coatings.

This reinforcement effect also makes itself apparent in higher

shear and E-moduli. The effect is thought to be caused by a

considerably restricted mobility of the polymer chain segments,

which results from a strongly adsorptive bonding of the binder

molecules at the surface of AEROSIL® fumed silica (103).

6. Applicational Effects5. AEROSIL® Types

��

AEROSIL® 90Silica with a low surface area for the

reinforcing and thixotropizing of RTV

silicone rubber. Especially well-suited to

attaining high degrees of filling and/or

to increase extrudability.

AEROSIL® 130Silica for the thickening, thixotropi-

zing, and reinforcement of cold-curing

sealants.

AEROSIL® 150Same application as AEROSIL® 130,

but with higher reinforcing effect and

improved transparency.

AEROSIL® 200Most frequently used type of AEROSIL®

fumed silica for thickening, thixotropi-

zing, and reinforcement. A widespread

application is also the adjustment of the

flowability of pulverulent substances.

AEROSIL® 300The high specific surface causes a more

pronounced thickening and thixotropic

effect. An important field of use is in

hot-vulcanizing silicone rubber.

AEROSIL® 380AEROSIL type with the highest specific

surface. Used, for example, in trans-

parent polyester resin coatings.

Hydrophobic types

AEROSIL® R 202Thickening and thixotropizing of epoxy

resins and special UP and PU systems.

AEROSIL® R 805Thickening and thixotropizing of epoxy

resins. Used especially in adhesive and

multilayer systems.

AEROSIL® R 812, AEROSIL® R 812 SVery finely divided, hydrophobic

fumed silica. Regulation of the sagging

behaviour of conventional and high-

solids coatings. Highly effective free-flow

aid. Thickening and thixotropizing of

vinyl ester resins. Reinforcing silica for

„High strength“ and LSR systems.

AEROSIL® R 972Hydrophobic fumed silica with a low

specific surface. Well suited for the

improvement and maintenance of the

flowability of powders. Thickening

of water-resistant systems. Use in

anticorrosive paints. Improvement in the

hydrophobicity and rheology of offset

inks. Reinforcing silica for cold-curing

silicone rubber.

AEROSIL® R 974Use similar to AEROSIL R® 972. Due to

a larger surface, a higher thickening and

thixotropizing effect as well as greater

transparency are attained.

Special typesStandard types

AEROSIL® OX 50Type with a low specific surface

and only slight tendency to agglo-

merate.

AEROSIL® TT 600Pronounced agglomerates, is espe-

cially well suited for the flatting of

special systems.

AEROSIL® MOX 80Mixed oxide of silicon dioxide with

about 1 % aluminium oxide. Slight

thickening effect in aqueous systems.

Serves, among other purposes, for

the production of dispersions.

AEROSIL® MOX 170Likewise a mixed oxide with about

1 % aluminium oxide, but with smal-

ler particles.

AEROSIL® COK 84Mixture of AEROSIL® fumed silica

and highly dispersed aluminium

oxide in the ratio of 5:1 for the effec-

tive thickening of aqueous systems

and other polar liquids.

Table 20: Characteristic properties and mainfields of use of the individual types of AEROSIL® fumed silica

��

The most important and quantitatively the largest field of use for

highly dispersed materials is represented by the reinforcement of

rubber. A theory to explain the reinforcing effects is based on the

so-called „bound rubber“ effect, which has been known since

1929 (96). The expression, „bound rubber“ is understood to mean

that fraction of polymers which becomes insoluble in organic

solvents when the previously soluble polymer is mixed and

masticated with highly dispersed materials. Figure 64 shows a

TEM of „bound rubber structures“ which were derived in natural

rubber with AEROSIL® 200. The bound rubber fraction increases

as the primary particle size decreases. This is in agreement

with practice because it has long been known that more finely

divided materials offer an improved reinforcing effect (97).

Some mechanical characteristics of silicone vulcanizates with

various silicas are shown by Figure 59.

Even though the function of AEROSIL® fumed silica as a reinforc-

ing filler in silicone compounds is the most significant, AEROSIL®

fumed silica induces striking improvements in the mechanical

characteristics in other systems as well, for example

AEROSIL® 200 in polyisobutylene. This plastic, which has only a

slight permeability to gas, serves, for example, as material for

spacers between glass window panes.

Figure 64: TEM of bound rubber of AEROSIL® 200 in natural rubber. Mixing ratio 30 parts to 100 parts rubber. Swelling agent and solvent: benzene (30)

Figure 65: Thixotropy = decrease in the viscosity during the shearing time, and increase again during the static period (schematic)

Visc

osity

Time

Static periodStaticperiod Shearing

��

6.2 Thickening

If AEROSIL® fumed silica is dispersed in a liquid, the surface silanol

groups interact with each other either directly or indirectly via

the molecules in the liquid. This affinity must be attributed to the

hydrogen bridge linkages (see 3.6.3.1), and results in a temporary,

three-dimensional lattice structure becoming macroscopically

„visible“ as thickening. Under a mechanical stress, for example

due to intensive stirring or shaking, the structure is broken

down again. The system becomes more fluid, in other words the

viscosity drops. In the static state AEROSIL® fumed silica particles

join again, and the viscosity regains its original value. This

process is termed thixotropy, and is represented schematically

in Figure 65.

It is possible to imagine the build-up and breakdown of the

three-dimensional network structure as shown schematically

in Figure 66.

As evident from Figure 67, the thickening and thixotropizing

effect of AEROSIL® fumed silica depends on the polarity of the

system, whereby the best results are attained for the most part

in non-polar systems. In this connection, the term „polarity“

should be understood to mean the ability of the molecules in

the liquid to form hydrogen bonds (also see (98) on this point).

AEROSIL® COK 84 has proved to be very suitable for adjusting

the rheology of highly polar substances such as water, dimethyl

formamide, etc.

Figure 66: Schematic representation of the interaction between AEROSIL® fumed silica particles in liquids

Figure 67: Thickening of various liquids with AEROSIL® 200

Visc

osity

mPa

s

10

2

2

4

4

6

6

8

8

10

10

2

3

4

0 2 4 6 8 10 12 14

AEROSIL content %®

Carbontetrachloride

Dibutylphthalate Benzene

Methylacetate

Acetone

water

Butanol

Primary particles (idealized) Aggregates

rest

shearing

Agglomerated aggregates Three-dimensional network

��

Figure 68: Dependence of the viscosity on the BET surface (2 % AEROSIL® fumed silica in LUDOPAL P 6 (BASF AG), dissolver 5 min., 6.3 m/s, 5 % batch)

Figure 69: Comparison of the sedimentation tendency of zinc dust paints based on an epoxy ester after a storage time of four weeks at room temperature. Left: control paint; right: paint produced using 2 % AEROSIL® R 972. Composition: JÄGALYD® ED 4, 50 % in xylene 19.6; zinc dust superfine 72.5; butanol 1.0; SHELLSOL® A 9.4; Co octoate, 6 % Co 0.2

The thickening and thixotropic effect of AEROSIL® fumed silica

at a given concentration depends to a great extent on the

intensity of the dispersing. With vane or propeller stirrers,

adequate degrees of thickening can perhaps be attained, but

other factors such as the rheological long-term behavior are

unsatisfactory in many cases. Good results are achieved with

high-speed stirrers. The best results, however, can be attained

with rotor-stator aggregates or with a triple roller mill.

In principle, it can be established that the thickening effect

increases with a rising BET surface. However, with an increasing

fineness of the particles, a greater dispersion intensity is

required in order to attain the possible thickening at all.

This complex of problems is represented once

again in Figure 68.

Due to the specific interactions of silicas with

polymers, a further reaction mechanism must

be taken into accout in connection with the

rheological effect of AEROSIL® fumed silica in

paints and other coating systems. The polymer

molecules are adsorbed at the AEROSIL® fumed

silica surface in loop and coil conformations,

and the mobility of the chain segments is

restricted over and beyond the interface

polymer/silica, as a result of the entangled

polymer chains.

This causes a reversible structure to be formed

based on entropy elasticity.

100 300 350 400

4000

3000

2500

2000

1500

3500

1000

Visc

osity

m P

a s

BET surface m /g2150 200 250

The rheological characteristics, such as thicken-

ing effect, structural viscosity and thixotropy of

paints and coatings containing AEROSIL® fumed

silica can be well explained with the aid of this

mechanism (103). Additional information relat-

ing to this subject may be found in Technical

Bulletin Pigments No. 23 and 53.

��

6.3 Antisetting Agent

In liquids containing solid matter, for example pigmented coat-

ings, AEROSIL® fumed silica prevents or delays a sedimentation.

In general, addition of small amounts are adequate to attain the

desired antisettling effect, which as a rule is accompanied by a

certain increase in viscosity. It is obvious that the improvement

in the suspension behavior must essentially be attributed to

the thixotropic AEROSIL® fumed silica lattice structure, because

a pure increase in viscosity alone does not result in success, as

can be recognized in Figure 69. In comparison with the use of

AEROSIL® 200, and all AEROSIL® hydrophobic types offer the

advantage that they are not so strongly agglomerated, and

therefore they can be dispersed more easily.

AEROSIL® COK 84 is best suited for holding solid matter

particles in suspension in water. For example, with about 3 %

AEROSIL® COK 84 it is possible to stabilize aqueous diamond

suspensions that are used as polishing agents. In some cases, a

sedimentation is intentionally accepted because the solid matter

particles are coated by AEROSIL® fumed silica, and consequently

the sediment is loosened up and set to a consistency that is

easily redispersible. Use is made of this effect, for example, in

zinc dust paints (99), „flowables“, or lotions.

6.4 Free Flow

Table 21: Active mechanisms when AEROSIL® fumed silica is used as free-flow aid with powders

The active mechanism of AEROSIL® fumed silica as a free-flow aid

are compiled in Table 21. Details on the subject of „Free Flow“

such as mixing techniques, measurement methods, and concrete

examples of application are given in Edition 31 in this series of

Technical Bulletin AEROSIL® entitled – “AEROSIL® for Improving

the Flow Behaviour of Powder Substances“.

In conclusion here, Figure 70 should be pointed out; this figure

shows an extremely simple, but nevertheless meaningful

measurement method with viscosity vessels resembling hour-

glasses. When this method is employed, powders with good

flow behaviour still flow out of the glass vessels with a small

discharge opening.

AEROSIL® fumed silica as a Free-Flow Aid

Particle fineness Coating (ball bearing effect)

No def. agglomerates Spacers

High surface area Adsorption of water

Hydrophobicity Moisture protection

Chemically inert No reaction

The packaging, storage, and handling of

substances in powder form are made difficult

due to the agglomeration and caking of the

solid matter particles. In industry, however, a

good flow behaviour of the powders and good

storage stability are required even at high

atmospheric humidity and when the powder

are subjected to compression stress. The

following can be discussed as mechanism of

adhesion between the powder particles:

- VANDERWAALSforces

- electrostaticforces

- liquidbridges

- solidmatterbridges

- entanglement.

Figure 70: Series of measuring vessels to evaluate the flowability without compression stress

��

6.5 Thermal Insulation

Amorphous silicon dioxide is a very poor conductor of heat.

Especially in the insulation range up to 1000 °C, AEROSIL®

fumed silica has proven very successful. The particle fineness

of AEROSIL® fumed silica makes charges possible with void

volumes lying in the range of the mean free path length of the

nitrogen and oxygen molecules (about 20 nm, 30 °C), and con-

sequently the gas conduction is reduced to a minimum.

In principle, the following contributions must be considered in

connection with the thermal conduction in highly dispersed

powders (100):

-thermalradiation

-solid-solidconduction

-gasconduction

-naturalconvection

Because of the small pore size the natural convection (101), and

because of the small solid matter volume fraction the solid-solid

conduction as well, can to a first approximation be neglected.

Table 22: AEROSIL® fumed silica as a versatile partner

In the temperature range of 50 - 130 °C in the case of AEROSIL®

380, the loss due to radiant heat makes up to 10 % of the total

heat loss, while the rest must be attributed to gas conduction.

At higher temperatures (about 800 °C) it is expected that the

radiation contribution will rise sharply to over 50 % (102).

AEROSIL® fumed silica also serves for low-temperature insulation,

and here also the same characteristics are of importance.

Powder mixtures or moulded articles containing AEROSIL® fumed

silica are used, for example, to insulate storage heaters, aircraft

turbines, pipelines on ships and in power plants, as well as tanks

for liquid gases. The best effect is attained through the use of

AEROSIL® 300 and AEROSIL® 380.

6.6 AEROSIL® Fumed Silica as a Versatile Product for Solving Problems

In the compilation in Table 22, the different effects of AEROSIL®

fumed silica are traced back to basic causal physico-chemical

characteristics of the pyrogenic silica. Some applicational

examples round off the picture.

Cause Effect Examples

Interaction Rheology: Coatings, UP and EP resins between thickening, pastes, resists, blood

AEROSIL® thixotropizing fractioning, maintenance-free

batteries, etc.fumed silica Rheology:particles antisetting agent Zinc dust paints

Rheology: suspension stabilizing Drilling fluids, diamond suspension

Rheology: emulsion stabilizing Oil + water Reinforcing Silicone rubber

Particle- Flowability Table saltfineness Storage stability Fire extinguisher powder

Adsorption of glycoproteins

Thermal insulation Storage heaters

Coating of neon tubes, diazo papers

Polishing agent Silicon disks

Grinding aid Pyrotechnics

Dispersing aid Colour pigments

Antiblocking Polyester films

Purity Raw material Highly pure glasses, light-conduct. fibres

Hydrophobicity „Transfer“ of the hydrophobic Hydrophobic conversion characteristics to other systems defoaming agents

��

7. Physiological Behaviour and Industrial Safety

Due to its X-ray amorphous structure, AEROSIL® fumed silica

causes no silicosis. Among the employees in the production

plants, who in part have come into contact with these substan-

ces for decades and who are subject to strict medical supervi-

sion, no signs of silicosis have been determined. In addition,

humerous inhalation experiments on animals did not show any

signs of silicosis.

This series of Technical Bulletin Fine Pigments discusses the

complex of questions mentioned above in detail in the edition

entitled „Silicosis – Caused by Amorphous Silica?“ (Edition 76)

and „The Biological Effects of SiO2, Al2O3 and TiO2 (Edition 64).

Table 23 summarizes some biological effects of AEROSIL® 200

on animals.

Administered orally, AEROSIL® fumed silica passes through the

gastrointestinal tract without being resorbed in detectable

amounts.

On the skin as well, AEROSIL® fumed silica is harmless. It can

occasionally cause a feeling of dryness, but this feeling can be

easily eliminated by washing and normal skin care (application

Table 23: Toxicological effects of AEROSIL® 200 on animals

Oral intake, human no finding

Intact human skin no finding

Inhalation, human no silicosis

Inhalation, rat no silicosis

LD50, rat 3300 mg/kg

Eye irritation, rabbit no effects

Skin irritation, rabbit no effects

Intact rabbit skin no effects

of cream). The MAK value (maximum concentration at the work

place) in the Federal Republic of Germany for pyrogenic silica

currently lies at 4 mg/m3 (inhelable fraction) total dust during an

evaluation time of 8 hours. In Edition 28 in this series of Technical

Bulletin Fine Pigments all questions regarding the handling of

synthetic silicas are discussed, including the technical possibili-

ties which can ensure compliance with an established MAK value.

�0

(1) G. BAKER, L. H. P. JONES, I. D. WARDROP, Nature 194, 1583 (1959)

(2) WHO Food Additives Series No. 5, 21 (1974)

(3) H. FERCH, Chem. Ing. Techn. 48, 922 (1976)

(4) H. FERCH, S. HABERSANG, Seifen, Öle, Fette, Wachse 108, 487 (1982)

(5) H. FERCH in: H. KITTEL, Lehrbuch der Lacke und Beschichtungen, Bd. 2, Verlag W. A. Colomb, Schwandorf (1974)

(6) FP 1 389 675, Degussa AG (1964)

(7) Beg. P. 752 787, Degussa AG (1970)

(8) Jap. AS 45-40163, Nippon-Denko (1961)

(9) DOS 1 940 832, Monsanto (1968)

(10) DOS 2 337 495, Lonza AG (1972)

(11) DBP 762 723, Degussa AG (1942)

(12) C. H. LOVE, F. H. MC BERTY, Fiat Rep. No 743 (1946)

(13) H. BRÜNNER, Dtsch. Apoth. Ztg. 98, 1005 (1958) (Referat)

(14) L. J. WHITE, G. J. DUFFY, Ind. Eng. Chem. 51, 232 (1959)

(15) E. WAGNER, H. BRÜNNER, Angew. Chem. 72, 744 (1960)

(16) DRP 724 740 Degussa AG (1937)

(17) AEROSIL®-Brochure, Degussa AG, Frankfurt (1988)

(18) H. FERCH. A. KREHER, Ullmanns Encyklopädie der technischen Chemie, 4. Aufl., 21, 462 (1982)

(19) Technical Bulletin Pigments No. 56, Company publication, Degussa AG, Frankfurt, 4th edition (1989)

(20) B. E. WARREN,Z. Krist. 86, 349 (1933) (21) M. SCHMÜCKER, Thesis, Ruhr-Universität Bochum (1988)

(22) L. D. EVANS, S. V. KING, Nature 212, 1353 (1966)

(23) R. L. MOZZI, B. E. WARREN, J. appl. Cryst. 2,164 (1969)

(24) Kommission der Europäischen Gemeinschaft (Hrsg.): Berufskrankheitenliste der EG, Brüssel (1972)

(25) H. VALENTIN, Arbeitsmedizin, 2. Aufl. Georg Thieme-Verlag, Stuttgart, 222 (1979)

(26) H. FERCH, H. GEROFKE, H. ITZEL, H. KLEBE, Arbeitsmed. Sozialmed. Präventivmed. 22, 6 and 23 (1987)

(27) Technical Bulletin Pigments No. 64, Company publication, Degussa AG, Frankfurt (1983)

(28) P. KLEINSCHMIT, R. SCHWARZ, Degussa AG, pers. Mitteilung (1979)

(29) D. SCHUTTE, Degussa AG, Werk Rheinfelden, not published (1974)

(30) H. FERCH, K. SEIBOLD, farbe + lack 90, 88 (1984)

(31) Technical Bulletin Pigments No. 28, Company publication, Degussa AG, Frankfurt, 2nd edition (1986)

(32) W. HINZ, Silikate, Band 1: Die Silikate und ihre Untersu chungsmethoden, VEB-Verlag fur Bauwesen, Berlin (1970)

(33) K. SEIBOLD, M. VOLL, Chem. Z. 102,131 (1978)

(34) Technical Bulletin Pigments No. 60, Company publication, Degussa AG, Frankfurt (1984)

(35) F. ENDTER, H. GEBAUER, Optik 13, 97 (1956)

(36) S. BRUNAUER, P. H. EMMETT, E. TELLER, J. Am. Chem. Soc. 60, 309 (1938) (37) J. H. DE BOER, B. C. LIPPINS, J. C. P. BROCKHOFF, J. Coll. Interf. Sci. 21, 405 (1966)

(38) E. KOBERSTEIN, E. LAKATOS, M. VOLL, Ber. Bunsenges. Phys. Chem. 75, 1105 (1971)

(39) DAS 1 036 220, W. R. Grace (1955)

(40) Technical Bulletin Pigments No. 53, Company publication, Degussa AG, Frankfurt, 3rd edition (1984)

(41) Technical Bulletin Pigments No. 16, Company publication, Degussa AG, Frankfurt (1977)

(42) R. ILER, Kolloid-Chemie des Siliciumdioxids und der Silikate, Cornell University Press Ithaca, New York (1955)

(43) H. S. CHU, A. J. STRETTON, C. L. TIEN, Int. J. Heat Mass Transfer. 31, 1627 (1988)

(44) M. HETEM, G. RUTTEN, L. VAN DE VEN, J. DE HAAN, C. CRAMERS, J. High Resolution Chromatography & Chromatography Communications 11, 510 (1988)

(45) K. ALBERT, E. BAYER, B. PFLEIDERER, J. Chrom. 506, 343 (1990)

(46) D. W. SINDORF, G. E. MACIEL, J. Amer. Chem. Soc. 103, 4263 (1981)

(47) D. W. SINDORF, G. E. MACIEL, J. Amer. Chem. Soc. 105, 3767 (1983)

(48) E. BAYER, K. ALBERT, J. REINERS, M. NIEDER, D. MULLER, J. Chrom. 264, 197 (1983)

(49) W. NOLL, K. DAMM, R. FAUSS, Kolloid-Z. 169, 18 (1960)

(50) H. SCHOLZE, Glastechn. Ber. 32, 142 (1959); 33, 33 (1960); Naturwissenschaften 47, 226 (I960)

8. Literature

��

(51) M. SCHNEIDER, H.-P. BOEHM, Kolloid-Z. u. Z. Polym. 187/2, 128 (1963)

(52) W. STÖBER, Kolloid-Z. 149, 39 (1956) (53) H. P. BOEHM, M. SCHNEIDER, Z. anorg. allg. Chem. 301, 326 (1959)

(54) J. UYTTERHOEVEN, H. NAVEAU, Bull. Soc. chim. France 27 (1962)

(55) J. A. HOCKEY, Chem. and Ind. 9, 57 (1965)

(56) G. BERGER, Chem. Weekbl. 38, 42 (1941) und K. H. HERBERT, Mh. Chem. 88, 275 (1975)

(57) M. V. TONGELEN, Bull. Soc. chim. France (2318) (1965)

(58) H. DEUEL, G. HUBER, Helv. chim. Acta 34, 169 (1951)

(59) M. BAVEREZ, J. BASTICK, Bull. Soc. chim. France 3226 (1965)

(60) H. J. WARTMANN, Dissertation ETH Zurich (1958)

(61) J. MATHIAS, G. WANNEMACHER, J. Coll. Interf. Sci. 125, 61 (1988)

(62) R. S. MCDONALD, J. Physic. Chem. 62, 1168 (1958)

(63) G. WIRZING, Naturwiss. 51, 211 (1964)

(64) M. BAVEREZ, J. BASTICK, C. R. Acad. Sci. Paris 260, 3939 (1965)

(65) M. ETTLINGER, H. FERCH, J. MATHIAS, Arch. Pharm. 320, 1 (1987)

(66) E. R. LIPPINCOTT, R. SCHRÖDER, J. Chem. Physics 23, 1131 (1955)

(67) E. R. LIPPINCOTT, R. SCHRÖDER, J. Physic. Chem. 61, 921 (1957)

(68) G. KORTÜM, Lehrbuch der Elektrochemie, Verlag Chemie (1972)

(69) J. A. CAMPBELL, Allgemeine Chemie, Verlag Chemie (1975)

(70) R. S. MCDONALD, J. Physic. Chem. 62, 1168 (1958)

(71) E. KNÖZINGER, R. HOFFMANN, R. ECHTERHOFF, Mikrochim. Acta 2, 27 (1988) (72) M. J. CHILD, M. J. HEYWOOD, G. H. YONG, C. H. ROCHESTER, J. Chem. Soc. Farad. Trans. 1, 78, 2005 (1982)

(73) W. POHLE, P. FINK, Z. Phys. Chem. (N. F.) 109, 77 (1978)

(74) F. GSTIRNER, W. FREISEL, Pharmaz. 32, 965 (1970)

(75) E. ULLMANN, K. THOMA, H. RUPPRECHT, Arch. Pharm. 301, 357 (1968)

(76) H. RUPPRECHT, H. LIEBL, E. ULLMANN, Fed. Intern. Pharm. London, 7.-12. 9.1969

(77) H. RUPPRECHT, Koll.-Z.Z. Polym. 249, 1127 (1971)

(78) H. RUPPRECHT, Arch. Pharm. 305, 149 (1972)

(79) H. RUPPRECHT, H. LIEBL, E. ULLMANN, Pharm. 28, 759 (1973)

(80) H. RUPPRECHT, M. J. BIERSACK, G. KINDL, H. LIEBL, Pharm. Ind. 38, 1009 (1976)

(81) H. RUPPRECHT, H. LIEBL, Pharm. 30, 101 (1975)

(82) H. RUPPRECHT, H. LIEBL, Koll.-Z.Z. Polym. 239, 685 (1970)

(83) H. RUPPRECHT, H. LIEBL, Arch. Pharm. 307, 817 (1974)

(84) H. RUPPRECHT, Progr. Coll. Polym. Sci. 65, 29 (1978)

(85) J. RUBIO, J. A. KITCHENER, J. Coll. Interf. Sci. 57,132 (1976)

(86) N. INGRI, Nobel Symp. 1977, 40, 3 (1978) (87) P. CHRISTOPHLIEMK, R. FAHN, H. FERCH, A. KREHER, K.-H. WORMS, WINNACKER, KÜCHLER, Chemische Technologie Band 3, Anorganische Technologie II, 4. Auflage S. 57 (1983) (88) G. LEE, D. DICK, E.-M. VASQUEZ, K. WERNER, Drug Develop. Industr. Pharm. 15 (4), 649 (1989)

(89) H. J. JACOBASCH, H. KADEN, Z. Chem. 23 (3), 81 (1983)

(90) P. FINK, I. PLOTZKI, G. RUDAKOFF, Wiss. Ztschr. Friedrich- Schiller-Univ. Jena, Naturwiss. R., 37, 911 (1988)

(91) J. P. BLITZ, R. S. S. MURTHY, D. E. LEYDEN, J. Coll. Interf. Sci. 126, 2 (1988)

(92) W. POHLE, Z. phys. Chemie, Leipzig 269, 1228 (1988)

(93) W. KLOSTERKÖTTER, Arch. Hyg. Bakt. 149, 577 (1965)

(94) H. BRÜNNER, D. SCHUTTE, Chem.-Z. 89, 437 (1965)

(95) G. KOERNER, M. SCHULZE, J. WEIS, Silicone, Chemie und Technologie, Symposium 1989, Vulkan-Verlag, Essen

(96) J.H.FIELDING, Ind. Eng. Chem. 21, 1027 (1929)

(97) G. KRAUS, „Reinforcement of Elastomers“, Wiley: New York (1965)

(98) H. BRÜNNER, E. WAGNER, Angew. Chem. 72, 744 (1960)

(99) Technical Bulletin Pigments No. 18, Company publication, Degussa AG, Frankfurt, 5th edition (1989)

(100) H.-S. CHU, C.-J. TSENG, J. Thermal Insulation 12, 108 (1988)

(101) R. P. TYE, „Thermal Transmission Measurements of Insulation“, ASTM STP-660 (1978)

(102) H. S. CHU, A. J. STRETTON, C. L. TIEN, Int. J. Heat Mass Transfer 31, 1627 (1988)

(103) U. CHRIST, A. BITTNER, farbe + lack, 98, 829 (1992)

��

AAS

Abbreviation for Atomic absorption spectroscopy. Analysis process

based on the fact that a light quant emitted by an excited atom

can be absorbed by a non-excited atom of the same element.

Adsorption

describes the retention of a material at the inner and/or outer

surface of a solid (DIN 28 400).

Adsorption isotherms

show correlations between the amount of substance adsorbed at

the adsorbate and the concentration or partial pressure of the gas

to adsorbed in the state of equilibrium at a constant temperature.

AERODISP®

Under the trademark AERODISP® Degussa manufactures AEROSIL®

fumed silica and metallic oxide dispersions in water and organic

solvents e.g. ethyleneglycole.

AEROPERL®

is a spherical granulate of AEROSIL® or metallic oxides with a high

density and a constant large surface area. The microgranulate has

excellent flow properties and can be handled with low dust

generation.

AEROSIL®

Registered trade name owned by Degussa AG for synthetic,

highly dispersed silicas which are produced by the

high-temperature hydrolysis process.

AEROXIDE®

Registered trade name owned by Degussa AG for synthetic,

highly dispersed fumed metal oxides which are produced by the

high-temperature hydrolysis process. These metal oxides have

special properties which can be extended by further chemical

and/or physical after-treamtent.

Affinity

The more heat is given off by the combining of two elements

or compounds, the greater are the forces of attraction (affinity)

between them (THOMSON-BERTHELOT principle).

9. Brief List of Technical Terms

Agglomerates

Loose clusters of primary particles and/or aggregates

which can be broken apart during dispersing (DIN 53 206).

Aggregates

According to DIN 53 206, intergrown union of particles bonded at

the interfaces whose surface area is smaller than the sum of the

total surface areas of the primary particles.

Alveoli

Air cells of the lungs (location where exchange of gas takes place

between inhaled air and the lung tissue).

Alveoli-penetrant

Particles which due to their size can penetrate into the alveoli.

AMES-Test

Named after Bruce AMES, who in 1972 described a method used to

study the mutagenic effect of materials on certain strains of bacteria.

Amorphous

Unformed, without structure, in contrast to crystalline.

Arc silica

Pyrogenic silica produced by reduction of quartz sand with

coke in an electric arc furnace and subsequent oxidation of the

SiO formed.

Cardiac glycoside

Compounds of types of sugar with non-sugar components which

can increase efficiency of the heart.

Chemisorption

takes place when a gaseous, liquid, or dissolved material is added

or absorbed at the surface of a solid or by a liquid with the forma-

tion of a chemical compound.

Chromatography

Designation for a physical separation process during which the

separation of materials takes place through distribution between

a stationary and a mobile phase.

��

Coesite

SiO2 high-pressure modification. Detected as natural mineral in

the sandstone of meteor craters.

Convection

Movements of entire masses of liquids or gases, usually caused

by temperature differences.

Coordinative defect point

An atom which does not reach the normal coordination number;

i.e. the number of the adjacent (atomic) neighbours is reduced.

Cristobalite

is the modification of silicon dioxide stable above 1470°C up to

the melting point of quartz (1710°C). Forms small, cloudy, milky-

white crystallites on cavities in vulcanic rocks.

Crystalline

is the term used for solids whose structural elements are

arranged in regular crystal lattices. Opposite: amorphous.

Devitrification

Change from a vitreous condition. Spontaneous crystallization of

an amorphous substance.

Differential thermal analysis

Thermometric method of analysis. Makes it possible to observe

phase changes on the basis of their heat tonality, and is suitable

above all for studies of structural changes in solid materials at

higher temperatures.

Diffraction rings

X-ray diffraction.

Diffusion

Without the influence of external forces, a gradual mixing of dif-

ferent gaseous, liquid, or solid materials

Diglyme

Acronym for Diethylene glycol dimethyl ether, a solvent.

Dipole

Asymmetrical distribution of charge in the case of molecules.

Dipole interaction

Forces which act due to the presence of dispoles.

Dispersion

Uniform distribution of pulverulent materials in liquid systems

with the aid of dispersing machines.

Doping

Designation common in semiconductor technology for the delib-

erate contamination of ultrapure germanium and silicon crystals

by additions of tiny amounts of foreign material.

Educt

Starting material for a chemical reaction.

EINECS

Abbreviation of European Inventory of Existing Chemical

Substances. In these material lists, the registered products are

arranged according to EINECS and CAS numbers.

Emulsion

Thermodynamically more or less instable systems which

contain two liquids only slightly soluble in each other in a

finely divided state.

Extinction

From the Latin: extinctio = extinction, quenching. Term used

for the logarithm of the quotient of the primary beam intensity

related to the intensity of the test beam.

Filter dust

Develops primarily in power plants as a by-product containing

silica, flue dust.

Flame hydrolysis

Reaction in the flame of, for example silicon tetrachloride with

the water formed in the oxyhydrogen flame.

Flue dust

By-product containing silica which develops on an uncontrolled

basis during the production of silicon or ferrosilicon. Physiologi-

cally – in contrast with deliberately-produced silicas – not

harmless. As synonym, the expression flue ashes is also used.

��

Frozen section

Section of a frozen test specimen and study of the section sur-

face, for example, for the degree of dispersion of fillers.

Fused silica

Turbid, non-transparent silica glass which is not suitable for opti-

cal purposes.

GC/MS coupling

Separation of a gas mixture by gas chromatography ( chroma-

tography), and identification by mass spectrometry.

Geminal

expresses the situation that two substituents are bonded with

the same atom, vicinal.

Gravitational interaction

Forces which two bodies exert on each other due to their mass

(at small mass: weak interaction).

Half width

Peak width at half the maximum peak amplitude.

Handling

Measures and processing methods necessary to realize certain

operating sequences, above all with pulverulent raw materials,

while observing officially-specified guide or limit values.

Headspace analysis

Name given to a method of analysis, usually conducted by means

of gas chromatography ( chromatography), of the air space

(headspace) above a solid or a liquid.

Hexagonal

Designation for a crystal system which occurs in the form of small

six-cornered columns or in rhombohedrons.

Highly dispersed

very finely divided.

High-temperature form

Modification which is readily formed at higher temperatures.

Homogeneous

Identical in type, nature, and material.

Hydrogen bridge linkage

Designation for a bond which forms between a hydrogen atom

covalently bonded to electronegative elements ( proton

donor) and the sole electron pair of another electronegative

atom ( proton acceptor).

Hydrophilic

Water-attractive or water-wettable.

Opposite: hydrophobic.

Hydrophobic

Water-repellent. Opposite: hydrophilic.

Inert

Inactive, non-reactive.

Infrared (IR) spectroscopy

Modern optical process, in which the absorption spectra of

usually organic, solid, liquid, or gaseous compounds are used in

the infrared range for qualitative and/or quantitative analysis,

structure determination, and similar studies.

Interference

Phenomena which result from the superimposition of waves of

the same wavelength.

Intermolecular

Designation for all processes or forces which take place or act

between several similar or dissimilar atoms or molecules.

Opposite: intramolecular.

Intramolecular

Designation for all processes or forces which take place or act

within the individual molecules. Opposite: intermolecular.

Irreversible

Chemical reaction which can not be made to take place in the

opposite direction. Opposite: reversible.

��

Kieselguhr

Synonym for infusorial earth, diatomaceous earth. Very fine-

grained, light-weight, usually light gray powder which consists

of 70 -90 % -x amorphous silica, some percent of crystalline

silica, 3-12 % water, and slight amounts of organic admixtures.

LSR system

Abbreviation for Liquid Silicons Rubber = liquid silicone systems

which usually become addition cross-linked in the presence of a

platinum catalyst.

MAK value

Abbreviation for „maximale Arbeitsplatz-Konzentration“

(Maximum concentration at the work place). Highest permissible

concentration of a working material in the air at the work place.

Modification

Various forms of state of chemical elements or compounds with

the same composition but different physical characteristics.

Morphology

Science of the development and origin of forms and shapes.

Nanometer

One millionth of a millimeter = 1 nm.

Neutron activation

Highly sensitive process for trace analysis; in this process a

sample of a substance is bombarded with neutrons. The partially-

formed radioactive isotopes are used for the characterization.

NMR

Abbreviation for Nuclear Magnetic Resonance. Spectroscopic

process to provide information on the structure of chemical

substances.

Octet rule

Describes the efforts of atoms to attain eight electrons in the

outer electron shell („precious gas configuration“)

by means of chemical bonding.

Orbital interactions

„Domiciles“ of the electrons of atoms or molecules

come very close to each other or overlap; in such a case energy

can be released (stable state).

Phases

Designation for the different crystalline forms in which a pure

chemical substance can occur.

Plasma process

One of the possible production processes leading to

pyrogenic silicas by means of ionized gases.

Pneumoconiosis

Medical term for the illness commonly known as dust on

the lung.

Precipitated silicas

Largely nonporous, synthetic silicas which develop during the

conversion of water glass with sulphuric acid.

Primary particles

According to DIN 53206, the smallest particles (individual par-

ticles) which make up powdered solids. Particle recognizable as

an individual by means of electron microscopy.

Progessive

Advancing.

Proton acceptor

Compounds which can accept protons (H+ ions) (according

to BRØNSTED = bases). Opposite: proton donor.

Proton donor

Compounds wich can give off protons (H+ ions) (according to

BRØNSTED = acids). Opposite: proton acceptor.

PU systems

Abbreviation for polyurethane systems.

��

Pyrogenic

Generated in the flame.

Quartz

Most important crystalline modification of SiO2. Most

frequently occurring mineral in the earth‘s crust. Mainly colour-

less, white, glassy or greasy-looking, opaque milky or transparent

crystal druses

Quartz glass

Completly transparent, clear glass which only melts at 1,720°C

and has a high chemical resistance.

Reversible

A chemical reaction which can be made to take place in the

opposite direction. Opposite: irreversible.

Rheology

Science of flow. Subdivision of physics which deals with the

description, explanation, as well as the measurement of

phenomena occuring in bodies when they are subjected to

deformation.

Rhombohedron

A crystal bounded by six rhombic surfaces with four corners.

Rock crystal

is an especially pure, glass-like, transparent form of quartz. It

often forms impressive, trapezohedral-hemihedral, individual

crystals.

Rotor-stator machine

Also known as mixing siren. Machine for the dividing of a pigment

or filler, consisting of an outer, fixed ring and an inner, concentric,

rotating ring, whereby both rings have openings (holes or slots)

which the liquid must pass through under high shearing stress.

Scarification

Superficial scratching of the skin without bleeding.

Sedimentation

Settling behavior.

SEM

Abbreviation for scanning electron micrograph.

Short-range order

Almost regular regions of an otherwise amorphous substance.

Silanes

are, in the narrower sense, binary compounds of silicon with

hydrogen, and with the general formula SinH2n + 2 are, in other

words, the Si equivalents of the alkanes. The term is also used in

the wider sense for such derivatives in which the hydrogen atoms

are partially or even completely replaced by other groups.

Silanol groups

Groups on the silica surface represented by the

formula = Si-OH.

Silica

Collective term for compounds with the general formula SiO2.

We distinguish, for example, between pyrogenic, e. g.

AEROSIL® fumed silica and precipitated silicas. The individual

types of silica differ from each other in their physical-chemical

properties, for example size of the specific surface, particle

size, as well as drying and ignition losses. silica gel, arc silica.

Silica gel

Porous silica produced according to the wet process from acid

and water glass. Opposite: precipitated silica.

Silicone rubber

Highly viscous silicone oils which can be cross-linked to elasto-

mers with peroxides or according to other principles. The basic

polymer for silicone rubber is dimethylpolysiloxane.

Silicosis

progressive pneumoconiosis as the result of inhaling dust

containing quartz: occupational disease.

Siloxane groups

Si-O-Si units which develop as a result of the condensation of

silanol groups.

��

Specific surface

is according to DIN 66 131, the surface area of a solid material

referred to the mass in m2/g. It is generally determined according

to the BET Method.

status nascendi

State of the origination. Reactive state of materials at the instant

of their formation.

Stishovite

Very hard SiO2 mineral insensitive to shock. Develops at points of

impact of larger meteorites due to the powerful shock wave from

the quartz grains embedded in the ground. The silicon atom is

not surrounded by four O atoms, as is the case with usual SiO2,

but instead by six equidistant O atoms. Stishovite can also be

produced synthetically under extremely high pressures.

TEM

Abbreviation for transmission electron micrograph.

Tetragonal

The four bonding electron pairs point into the corners of a

tetrahedron in the case of the tetragonal system, and conse-

quently are spaced at the greatest possible distance from each

other (angle about 109°).

Tetrahedron

Body limited by four equilateral triangles.

Thixotropy

Decrease in the viscosity of a liquid as a function of the shear-

ing intensity and shearing duration, and readjustment to the

original state after termination of the shearing stress.

Toxicology

Science of the harmful, under certain circumstances lethal,

effects of substances in excessive amounts.

Tribo-electricity

Frictional electricity. Designation for the development of voltage

when two different insulators are rubbed against each other

and they become charged with opposite electrical signs. Often

characterized as the quotient of charge per mass.

Tridymite

high-temperature form of SiO2. Often six-sided flakes 1 - 4 mm

in size, colourless, with vitreous lustre or milky turbid, transparent

to translucent. modification of silica.

Tunnel effect

Atomic particles can overcome a potential barrier, the potential

energy of which is higher than the kinetic energy of the particles,

with a certain probability by employing a so-called „tunneling

through“.

UP systems

Abbreviated name for unsaturated polyester resins.

Vicinal

describes the position of the substituents at adjacent atoms.

geminal.

Viscosity

is, according to DIN 13 342, the characteristic of a material to

absorb, through tangential deformation, a tangential stress that

depends on the velocity gradient.

X-ray amorphousness

Property of substances which show no defined diffraction phe-

nomena in the X-ray structural study.

X-ray analysis

Physical process for the chemical analysis during which the test

sample is irradiated with X-rays.

X-ray diffraction

Process of crystalline structure analysis during which

X-rays are diffracted at the electrons in the lattice atoms.

Through superimposition of the diffraction waves, regular

diffraction rings are developed.

X-rays

Designation for a short-wave electromagnetic wave radiation of

about 10-6 to 10-12 cm wave length.

��

Physico-Chemical Data of AEROSIL® Fumed Silica

m�/g

nm

g/l

g/l

g/l

Hydrophilic AEROSIL® Fumed Silica

Standard Grades

90±15 130±25 150±15 200±25 300±30 380±30 50±15 200±50 80±20 170±30 185±30 100±15 50±15 57.5±12

20 16 14 12 7 7 40 40 30 15 - 13 21 -

80 50 50 50 50 50 130 60 60 50 50 50 130 80

120 120 120 120 120 120

50/75 50/75/ 50/75/

≤ 1.0 ≤ 1.5 ≤ 0.5 9) ≤ 1.5 ≤ 1.5 ≤ 2.0 ≤ 1.5 ≤ 2.5 ≤ 1.5 ≤ 1.5 ≤ 1.5 ≤ 5.0 ≤ 1.5 ≤ 2.0

≤ 1 ≤ 1 ≤ 1 ≤ 1 ≤ 2 ≤ 2.5 ≤ 1 ≤ 2.5 ≤ 1 ≤ 1 ≤ 1 ≤ 3 ≤ 2 ≤ 3

3.7-4.7 3.7-4.7 3.7-4.7 3.7-4.7 3.7-4.7 3.7-4.7 3.8-4.8 3.6-4.5 3.6-4.5 3.6-4.5 3.6-4.3 4.5-5.5 3.5-4.5 3.5-4.5

≥99.8 ≥99.8 ≥99.8 ≥99.8 ≥99.8 ≥99.8 ≥99.8 ≥99.8 ≥98.3 ≥98.3 82-86 ≤0.1 ≤0.2 -

≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.08 ≤0.05 0.3-1.3 0.3-1.3 14-18 ≥99.6 ≤0.3 -

≤0.003 ≤0.003 ≤0.003 ≤0.003 ≤0.003 ≤0.003 ≤0.01 ≤0.003 ≤0.01 ≤0.01 ≤0.1 ≤0.2 ≤0.01 1 - 3

≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.03 ≤0.1 ≥99.5 ≥94.0

≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.025 ≤0.1 ≤0.5 ≤0.3 ≤0.8

≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.2 ≤0.05 ≤0.1 ≤0.1 ≤0.1 ≤0.05 ≤0.05 -

10 10 10 10 10 10 10 10 10 10 10 10 10 10

AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® 90 130 150 200 300 380 OX 50 TT 600 MOX 80 MOX 170 COK 84

Special Grades

1) inacc.toDIN661312) inacc.toDINENISO787/11,JISK5101/18(notsieved)3) inacc.toDINENISO787/2,ASTMD280,JISK5101/214) inacc.toDINENISO3262-20,ASTMD1208,JISK5101/235) inacc.toDINENISO787/9,ASTMD1208,JISK5101/246) inacc.toDINENISO787/18,JISK5101/207) basedonmaterialdriedfor2hourat105°C8) basedonmaterialignitedfor2hoursat1000°C9) specialmoisture-protectivepackaging10) HCl-contentispartoftheignitionloss11) AEROSIL®V-Gradeswillbedeliveredinpaperbagsof20kg12) AEROSIL®VV-GradesarepresentlyonlyavailablefromtheproductionplantinRheinfeldenbynow

AEROXIDE®

Alu C

Behavior with respect to water

Appearance

pH Value 5)

SiO2 8)

Sieve residue 6) (according to Mocker, 45 µm)

Packaging (net/weight) 11)

standard material

compacted material („V“)

Moisture 3)

( 2 hours at 105 °C ) at leaving plant site

compacted material („VV“)

Al2O

3 8)

Fe2O

3 8)

TiO2 8)

HCl 8) 10)

The data have no binding force.

BET-Surface Area 1)

Tapped Density 2)

Average Primary Particle Size

Ignition Loss 4) 7)

( 2 hours at 1000 °C )

Test method

hydrophilic

fluffy white powder

120 120

kg

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

AEROXIDE®

TiO2 P 25AEROXIDE®

TiO2 PF 2

��

1) inacc.toDIN661312) inacc.toDINENISO787/11,JISK5101/18(notsieved)3) inacc.toDINENISO787/2,ASTMD280,JISK5101/214) inacc.toDINENISO3262-20,ASTMD1208,JISK5101/235) inacc.toDINENISO787/9,ASTMD1208,JISK5101/247) basedonmaterialdriedfor2hourat105°C8) basedonmaterialignitedfor2hoursat1000°C10) inWater:methanol=1:111) HClcontentispartoftheignitionloss12) AEROSIL®V-Gradeswillbedeliveredinpaperbagsof15kg13) AEROSIL®VV-Gradeswillbedeliveredinpaperbagsof15kg

AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® AEROSIL® R 972 R 974 R 202 R 805 R 812 R 812 S R 104 R 106 R 8200 R 816

110±20 170±20 100±20 150±25 260±30 220±25 150±25 250±30 160±25 190±20 45±10

16 12 14 12 7 7 12 7 - 12 21

50 50 50 50 50 50 50 50 140 40 200

90 90 90

60/90 13) 60/90 13) 60/90 13) 90 13)

≤ 0.5 ≤ 0.5 ≤ 0.5 ≤ 0.5 ≤ 0.5 ≤ 0.5 - - ≤ 0.5 ≤ 1.0 ≤ 1.0

≤2 ≤ 2 4 - 6 5 - 7 1.0 - 2.5 1.5 - 3.0 1.0 - 2.5 1.0 - 2.5 2.5 - 3.5 2.0 - 4.0 ≤ 5.0

0.6 - 1.2 0.7 - 1.3 3.5 - 5.0 4.5 - 6.5 2.0 - 3.0 3.0 - 4.0 1.0 - 2.0 1.5 - 3.0 2.0 - 4.0 1.2 - 2.2 2.7 - 3.7

3.6 - 4.4 3.7 - 4.7 4 - 6 3.5 - 5.5 5.5 - 7.5 5.5 - 7.5 ≥ 4.0 ≥ 3.7 ≥ 5.0 4.0 - 5.5 3.0 - 4.0

≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≥ 99.8 ≤2.500

≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.05

≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.010

≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≤ 0.03 ≥ 97.00

≤ 0.05 ≤ 0.1 ≤ 0.025 ≤ 0.025 ≤ 0.025 ≤ 0.025 ≤ 0.02 ≤ 0.025 ≤ 0.025 ≤ 0.025

10 10 10 10 10 10 10 10 15 10 20

Behavior with respect to water

Appearance

BET-Surface Area1)

Tapped Density 2)

pH Value 5) 10)

SiO2 8)

Packaging (net/weight) 12)

Average Primary Particle Size

standard material

compacted material („V“)

Moisture 3)

( 2 hours at 105 °C ) at leaving plant site

C-Content

Al2O

3 8)

Fe2O

3 8)

TiO2 8)

HCl 11)

m�/g

nm

Ignition Loss 4) 7)

( 2 hours at 1000 °C )

Hydrophobic AEROSIL® Fumed Silica

The data have no binding force.

Test method

hydrophobic

fluffy white powder

kg

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

wt. %

AEROXIDE®

TiO2 T 805

compacted material („VV“)

g/l

g/l

g/l

�0

The information and statements contained herein are provided

free of charge.

They are believed to be accurate at the time of publication, but

Degussa makes no warranty with respect thereto, including but

not limited to any results to be obtained or the infringement of

any proprietary rights.

Use or application of such information or statements is at user‘s

sole discretion, without any liability on the part of Degussa.

Nothing herein shall be constructed as a license of or recom-

mendation for use which infringes upon any proprietary rights.

All sales are subject to Degussa‘s General Conditions of Sale

and Delivery.

��

TB 0

011-

1-A

pr0

6

Technical Bulletin Fine Particles

Basic Characteristics of AEROSIL® Fumed Silica

Number 11Contact

Degussa AGBusiness Line AerosilWeissfrauenstrasse 9D-60287 Frankfurt am Main, GermanyPhone: +49 69/218-2532Fax: +49 69/218-2533E-Mail: aerosil@degussa.comhttp: //www.aerosil.com

NAFTADegussa CorporationBusiness Line Aerosil379 Interpace Parkway, P. O. Box 677Parsippany, NJ 07054-0677Phone: +1 (800) AEROSILPhone: +1 (973) 541-8510Fax: +1 (973) 541-8501

Asia (without Japan)AEROSIL Asia Marketing Officec/o NIPPON AEROSIL CO., LTD.P. O. Box 7015Shinjuku Monolith 13F3-1, Nishi-Shinjuku 2-chomeShinjuku-ku, Tokyo 163-0913 JapanPhone: +81-3-3342-1786Fax: +81-3-3342-1761

JapanNIPPON AEROSIL CO., LTD.Sales & Marketing DivisionP. O. Box 7015Shinjuku Monolith 13F3-1, Nishi-Shinjuku 2-chomeShinjuku-ku, Tokyo163-0913 JapanPhone: +81-3-3342-1763Fax: +81-3-3342-1772

Technical Service

Degussa AGTechnical Service AerosilRodenbacher Chaussee 4 P. O. Box 1345D-63403 Hanau-WoIfgang, GermanyPhone: +49 6181/59-3936Fax: +49 6181/59-4489

NAFTADegussa CorporationTechnical Service Aerosil2 Turner PlacePiscataway, NJ 08855-0365Phone: +1 (888) SILICASPhone: +1 (732) 981-5000Fax: +1 (732) 981-5275

Asia (without Japan)Degussa AGTechnical Service AerosilRodenbacher Chaussee 4P. O. Box 1345D-63403 Hanau-WoIfgang, GermanyPhone: +49 6181/59-3936Fax: +49 6181/59-4489

JapanNIPPON AEROSIL CO., LTD.Applied Technology Service3 Mita-choYokkaichi, Mie510-0841 JapanPhone: +81-593-45-5270Fax: +81-593-46-4657

please visit our web site www.aerosil.com to find your local contact partner