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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: [email protected]: //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
�
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.
�
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
�
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
*
�
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 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