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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

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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

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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

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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.

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1. CrystallineQuartz 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

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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 silicaFlame

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

*

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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

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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 δSiQ2 - 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)

SiO2 SiO2 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 ≤ 300Density (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 . sg/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 isolate