i. materials background, literature and methodology

Chapter I: Materials background, literature and methodology

Elodie Bugnicourt, PhD INSA Lyon, 2005 13

I. MATERIALS BACKGROUND, LITERATURE AND METHODOLOGY ...................15

I.1. Pyrogenic silica ............................................................................................................ 15 I.1.1. The different types of synthetic silica...................................................................... 16

I.1.1.a. Silica sol.......................................................................................................... 16 I.1.1.b. Precipitated silica............................................................................................ 17 I.1.1.c. Pyrogenic silica............................................................................................... 17 I.1.1.d. Silica obtained via other thermal processes ................................................... 17 I.1.1.e. Comparison of physical properties.................................................................. 18

I.1.2. Synthesis and multi-scale organization of pyrogenic silica..................................... 18 I.1.3. Chemical structure and surface properties ............................................................. 20

I.1.3.a. Specific surface area ...................................................................................... 20 I.1.3.b. Surface chemistry ........................................................................................... 21 I.1.3.c. Interaction with water ...................................................................................... 22 I.1.3.d. Summary of properties.................................................................................... 23

I.1.4. Surface modification ............................................................................................... 23 I.1.4.a. Principle .......................................................................................................... 24 I.1.4.b. Grafting in solution.......................................................................................... 24 I.1.4.c. Grafting in vapor phase................................................................................... 25

I.2. Generalities about epoxy reinforcement and sub-micro structured composites .......... 25 I.2.1. Reinforcement of epoxy.......................................................................................... 25

I.2.1.a. Fillers traditionally used to reinforce epoxy..................................................... 26 I.2.1.b. Nano-fillers: the answer to reach a toughness / stiffness compromise?......... 28

I.2.2. Specific effect of nano-fillers................................................................................... 29 I.2.2.a. Size effect, high specific surface area ............................................................ 29 I.2.2.b. Ability for interactions / reactions of the nano-fillers with the medium ............ 30 I.2.2.c. Multi-scale organization .................................................................................. 31 I.2.2.d. Geometric characteristics ............................................................................... 32

I.3. Processing of sub-micro structured composites, filler effect on the rheology and

reactivity of epoxy systems..................................................................................................... 33 I.3.1. Processing of sub-micro structured composites ..................................................... 33

I.3.1.a. General routes for the processing of sub-micro structured composites.......... 34 I.3.1.b. Processing of formulations based on epoxy and silica ................................... 36

I.3.2. Rheological behavior related to the dispersion state .............................................. 38 I.3.2.a. Rheological behavior of fumed silica suspensions ......................................... 38



I.3.2.b. Effect of particle size and shape..................................................................... 40 I.3.2.c. Effect of the surface chemistry and medium nature........................................ 42 I.3.2.d. Effect of the silica content ............................................................................... 43

I.3.3. Effect of fillers on the crosslinking kinetics of epoxy networks ............................... 46 I.3.3.a. Filler effect on epoxy crosslinking via condensation mechanism.................... 46 I.3.3.b. Filler effect on epoxy crosslinking initiated via ionic mechanism .................... 47

I.4. Expected mechanical properties of epoxy / silica composites ..................................... 49 I.4.1. Interactions between epoxy and silica, silica influence on the dynamics of polymer

matrix ................................................................................................................................ 49 I.4.1.a. Interactions of fumed silica with epoxy ........................................................... 49 I.4.1.b. Effect on the glass transition temperature ...................................................... 50

I.4.2. Mechanical properties of epoxy / silica composites in the glassy state .................. 52 I.4.2.a. Effect of conventional silica fillers (micron size).............................................. 53 I.4.2.b. Effect of sub-micro silica particles................................................................... 56

I.4.3. Mechanical properties of epoxy / silica composites in the rubbery state ................ 59 I.4.3.a. General effect of fillers on a rubbery network ................................................. 59 I.4.3.b. Effect of silica surface modification................................................................. 61 I.4.3.c. A few examples of filler addition in rubbery epoxy networks .......................... 62

I.4.4. Applicative properties of epoxy / silica formulations ............................................... 63 I.4.4.a. For adhesives ................................................................................................. 64 I.4.4.b. For coatings, surface properties ..................................................................... 65 I.4.4.c. For matrices of conventional composite materials.......................................... 67

I.5. Methodology of the study............................................................................................. 69 I.5.1. Summary of the state of the art and position of the material under study .............. 69 I.5.2. Organization of the study........................................................................................ 71



I. MATERIALS BACKGROUND, LITERATURE AND METHODOLOGY

This chapter is aimed at presenting the background of the materials used, the state of the art

about epoxy / silica composites, as well as the methodology of the study carried out.

In a first part of this chapter, the characteristics of different types of synthetic silica, especially

pyrogenic silica, are presented. In a second part of this chapter, general background about the

reinforcement of epoxy and the factors related to sub-micro fillers efficiency are reported. Then,

the processing routes for sub-micro structured composites, and the general behavior of filled

epoxy before and during crosslinking are presented for filled materials. Then, the expected final

mechanical properties of epoxy / silica composites are reviewed in glassy and rubbery states,

as well as the applicative properties depending on the fields targeted. In a last part of this

chapter, the methodology of this study is detailed, i.e. the different steps leading to the obtaining

of the epoxy / silica composites, the relevant parameters and the questions arising in this study.

I.1. Pyrogenic silica

The filler used in this study is a pyrogenic (or fumed) silica commercialized by Wacker company.

First, different types of synthetic silica are introduced briefly because their effects are compared

with those of fumed silica for the reinforcement of epoxy from literature results. The synthesis

process leading to the specific multi-scale organization of fumed silica, at different scales, is

then presented. The surface properties of pyrogenic silica determining the interactions

developed with the organic medium are then detailed, as well as the possibilities of chemical

modification of the surface.



I.1.1. The different types of synthetic silica

Silicon is one of the most abundant components on the Earth surface, especially in the form of

quartz. Silica, or silicon dioxide, can also be synthesized industrially. It can be crystalline

(generally for natural origins) or amorphous (generally for synthetic origins).

There are various synthesis routes for the manufacturing of synthetic amorphous silica (Figure

I-1):

− wet route: sol/gel process, precipitation

− thermal route: pyro-hydrogenation, arc, plasma

Figure I-1 Different categories of synthetic silica

I.1.1.a. Silica sol

The sol-gel process is one of the most extensively described in the literature. It is based on the

condensation of silanol groups to form a siloxane network according to the reaction: Si OH + HO Si Si O Si + H2O

Silicate of sodium or alcoxysilanes can be used as raw materials to get silanols species by

hydrolysis. The hydrolysis and the condensation take place simultaneously in an aqueous

solution, forming stable colloidal particles. The condensation reaction is influenced by the

addition of an electrolyte or by changing the pH of the solution. The growth of particles or the

bonding of particles leading to the formation of a network can be respectively favored

depending on the conditions chosen. After the sol / gel transition, an elastic behavior appears,

and a hydrogel is obtained in case the solvent is water, or respectively an alcogel in alcohol.

When dried, a hydrogel provides a xerogel and an alcogel provides an aerogel. The porosity

can be tailored and a subsidiary thermal treatment can be necessary to stabilize the material.



The advantage of this process is to lead to pure and homogeneous silica particles under low

temperatures. Silica obtained by sol-gel process, so-called silica sol (or also sometimes silica

gel), is generally used for the manufacture of films, fibers, powders, composite or porous

materials [PAQ03].

I.1.1.b. Precipitated silica

The precipitated silica was developed in the 1940’s as white reinforcing filler for rubbers, which

is still its main application field [CON05]. It is obtained by acidification of a solution of silicate of

sodium in the presence of sulphuric acid or of a mixture of carbon dioxide and hydrochloric acid

according to the reaction:

Na2OXSiO2 + H2SO4 → XSiO2 + Na2SO4 + H2O.

Rather individual silica particles are obtained because the gelation is avoided during the

process.

I.1.1.c. Pyrogenic silica

The pyrogenic (or fumed) silica was prepared for the first time by the German chemist Klopfer,

in 1941, with the same objective as for the precipitation process. The processing is detailed in

the paragraph I.1.2. Pyrogenic silica consists of a nonporous, amorphous, fine, fluffy white

powder and is totally amorphous.

The main markets of fumed silica are: reinforcing filler mainly for silicone rubbers (application

accounting for about 55% of the market of fumed silica), matting agent for paints and polymers,

free flow agent for powders, rheological additive providing thickening or thixotropy to polymers,

adhesives, paints, inks... The main suppliers of fumed silica world wide are Wacker (HDK®),

Degussa (Aerosil®) and Cabot (Cabosil®), with an annual world market around 150,000 tones

(2004).

Note: All our experimental work was focused on this type of silica, hence the author will use the

word “silica” for pyrogenic silica in the continuation of the text (unless otherwise specified).

I.1.1.d. Silica obtained via other thermal processes

Silica can also be synthesized in an electric arc: from the reduction of quartz at 2,000°C in the

presence of coke, silicon monoxyde is created and then oxidized by air to form silica. This



process is not widespread because of its high cost and of the poor properties of this silica

compared with fumed silica. Finally, the plasma process is also rarely employed.

I.1.1.e. Comparison of physical properties

The physical properties of the different types of synthetic silica such as the porosity, size and

organization of the particles depend largely on their synthesis process. They are summarized in

the Table I-1 and are detailed further in case of fumed silica in the following paragraph.

Table I-1 Physical properties of different synthetic silica depending on their process of synthesis

[KAT87]

I.1.2. Synthesis and multi-scale organization of pyrogenic silica

Pyrogenic silica is obtained via a high temperature process (Figure I-2). The hydrolysis of silicon

tetrachloride in a flame of mixed hydrogen and oxygen, between 1,200°C and 1,500°C

approximately, leads to the formation of silica and hydrochloric acid according to the reaction:

SiCl4 + 2 H2+ O2 → SiO2 + 4 HCl

A part of the hydrochloric acid formed is evaporated but the excess remains physically

absorbed on silica surface explaining its slightly acid character (3.5<pH<4.3).

Figure I-2 Process leading to the formation of fumed silica [KAT87]

Quartz Silicon SiCl4C

HCl

or

Cl2

Fumed

silica

H2

O2

Thermal processes Wet processes



The 3-scales organization of fumed silica is due to its synthesis into a flame (Figure I-3). From

the reordering of “protoparticles”, i.e. clusters of 10 to 200 silicon atoms covered by active

groups, spherical primary particles result from the first step [KHA01a, b]. Their diameter is

included between 10 and 20 nm depending on the combustion conditions.

Still in the molten state, these primary particles fuse (by formation of siloxane bonds between

neighboring primary particles) as secondary 3-dimensional chain-like structures, i.e. sintered

aggregates, which dimensions vary from 100 to 500 nm.

On cooling down, these aggregates flocculate under the effect of low physico-chemical

interactions to form macroporous tertiary structures, or agglomerates, measuring up to several

microns, finally building up macroscopic fluffy flocks. The size of these structures is function of

the silica surface treatment that controls particle-particle interactions (colloidal forces). This size

is generally difficult to determine accurately because it depends on the balance between

agglomeration and dispersion forces resulting from particles sizing techniques.

The elemental object to consider in this work is the aggregate, because it is obtained

irreversibly whereas the agglomerates are not stable under external shearing. It has to be

underlined that discrete primary spherical particles do not exist in fumed silica.

Figure I-3 Production of fumed silica (Wacker HDK®) and different levels of organization [WAC05]

The size and size distribution of the primary particles, and thus the specific surface area, is

controlled via the temperature and temperature gradient of the gas flow in the flame. Certainly

due to diffusion aggregation limited processes, primary particles of same sizes tend to stick

together to form the aggregates, so that size distribution of primary particles is delta like inside a

given aggregate, but not in between various aggregates of the same manufacture. When the

specific surface area increases, on one hand, average size of primary particles decreases, and

on the other hand, size distribution of the aggregates decreases markedly.

Silica exhibits a fractal structure (Figure I-4). A section of the third chapter (III.3.1.b.) is devoted

to the basics of fractals geometry and the measurements of fractal dimension of silica

aggregates.

10-50 nm

100-500 nm

Up to a few µm

γ ‘



(a) (b)

Figure I-4 (a) TEM observation of fumed silica (Wacker HDK® N20: pristine fumed silica of specific surface area 200 m²/g) [WYP99]; (b) Scheme of the structure of the elemental aggregate [WAC01]

I.1.3. Chemical structure and surface properties

Fumed silica results from the random assembling of SiO4/2 tetrahedrons, therefore it is

amorphous and presents a lower density (ca. 2.2) than crystalline silica (2.5 to 3). The apparent

bulk density is obviously much lower due to the fluffy appearance and macroporosity of the

flocks, i.e. when freshly prepared, density can go down to 0.03 g/cm3 [BAR98a].

Fumed silica consists of more than 99.8 % of silicon dioxide, however chemical analyses also

show the presence of traces of other compounds (Al2O3, Fe2O3, TiO2…).

Silica surface properties are related to various parameters that are defined in the following

paragraphs: size and specific surface area of particles, number and distribution of surface

hydroxyl groups, physically absorbed water.

I.1.3.a. Specific surface area

Due to its structure and low particles size, silica has a high specific surface area (from 50 to 400

m²/g), conferring to silica a high surface of interaction with the medium. Above 300 m²/g, fumed

silica surface presents micro-porosities that modify its whole behavior.

The specific surface area is generally estimated according to BET measurements (developed

by Brunauer, Emmett and Teller in 1938 [BRU38]), by recording the isothermal adsorption of a

monolayer of gas, generally nitrogen, on silica surface. For instance, from this technique, at 76

K with a single point method, using a Stroehlein volumetric equipment, Barthel et al. obtained a

value of 200 m²/g for the specific surface area of an hydrophilic untreated silica (reference

HDK® N20) [BAR98a].

The drawback of this technique lies in its sensitivity to surface energy (thus to chemical

modifications of silica) and not only to the “topography” of the surface. As a consequence,



values obtained from this technique for hydrophobic silica are underestimated: 260 m²/g

(reference HDK® H30) versus 314 m²/g for the same silica before surface modification

(reference HDK® T30) [BAR98a]1.

In first approximation, the specific surface area (in m²/g) of the particles can be related to the

size of the primary particles by the following formula:

d.6 BET

ρ= Equation I-1

where ρ is the density (2.2 g/cm3) and d is the diameter of the primary particles.

I.1.3.b. Surface chemistry

Approximatively every second silicon atom on silica surface bears a silanol group (Si-OH)

(Figure I-5).

Figure I-5 Silica surface chemistry [DUC96]

Various types of silanol groups (Figure I-6), presenting different reactivities depending on their

accessibility, exist on silica surface:

− Isolated silanol groups, accounting for approximately 85 % of the silanols

− Geminal silanol groups (2 silanols carried by the same silicon atom)

− Vicinal silanol groups (2 silanols carried by adjacent silicon atoms).

Figure I-6 Various types of silanol groups [adapted from KAT87]

1In a shortcut way, throughout this manuscript, the notation BET was used to refer to the silica specific surface area, even if rigorously it is related to the measurement method used to measure this property. Additionally, since the BET is not significantly changed after surface modification, the data provided here are those measured for hydrophilic silica before possible surface modification

Geminal silanol

H

Si

O

H

Si

O

H

O

H

Si

OH

Si Si

O

Si

Isolated silanol Vicinal silanols Siloxane linkage

H bonding

O



In the literature, the values found for silanol density on silica surface lie between 1.9 and 4.6 per

nm² depending on the measurement method used, i.e. if every type of surface silanols are taken

into account or not. The silanol content can generally be determined by acid base titration,

thermo-gravimetric analysis, infra-red spectroscopy or proton nuclear magnetic resonance

[review of the results depending on the method DUC96, PAQ03, DOR95].

For fumed silica the values are generally lower than for other types of silica, due to the structure

that makes a part of the surface not accessible. Wacker Co. provides the following data for

hydrophilic untreated fumed silica (reference HDK® N20): density of silanols around 1.8 Si-OH

per nm² independent on specific surface area (measured by acid base titration against aqueous

sodium hydroxide, method not accounting for geminated silanols).

The high silanol density on silica surface induces a high potential interactivity (i.e. formation of

hydrogen bond readily) between particles or between the particles and the medium, or reactivity

(groups chemically available for covalent bonding), as it is detailed in paragraph I.1.4

concerning the organo-modification of the silica surface.

It is also the reason for the high surface energy of hydrophilic silica which is greater than 72

mJ/m² and is thus wettable by water. For a given surface modification, the surface energy is

related to the content of residual silanols on the silica surface [BAR95], and can go down to c.a.

33 mJ/m² in case of a polydimethylsiloxane modification.

I.1.3.c. Interaction with water

Another important parameter characterizing the silica surface properties is the quantity of

physisorbed water attracted by surface silanol groups (estimated to approximately 0.5-2.5 wt. %

in a standard atmosphere) (Figure I-7).

Figure I-7 Simplified scheme of the physisorption of water on silica surface [KAT87]

H O

OH

Si

H

O

Si O

OH H

H

Si O Si O

O

Si

O

Si

Hydrogen-bonded water (released above 105 - 200°C)

Surface silanol groups

Free water (released above 100°C)



After thermal treatment above 120-200°C under standard pressure, the water and the silanol

groups can be removed from the silica surface by dehydratation followed by dehydroxylation,

i.e. formation of a siloxane bond according to the process illustrated in the Figure I-8 [PAQ03].

Si O

H

O H

H

OSi

H

- H2OSi O

H

OSi

H

- H2O Si

SiO

Figure I-8 Dehydratation and dehydroxylation of silica surface [PAQ03, KAT87]

I.1.3.d. Summary of properties

The values concerning the structure, size and surface properties of the pristine fumed silica

detailed in the previous paragraphs are summarized in the Table I-2.

General values for pyrogenic silica*

Values for an unmodified fumed silica of surface BET=200m²/g¤

Primary particle size (nm) 5 – 40 10 – 30 Aggregate size (nm) 200 – 1500 170-230

Agglomerate size (µm) 10 – 100 BET (m²/g) 50 – 400 200

Silanol content (nm-2) 2 – 4 1.8 Density (primary particle SiO2) 2.2 2.2

pH (5%solution) 3.6-4.5 3.8-4.3 Refractive index n 1.45 1.46

Surface energy (mJ/m²) 80 <72 Moisture (wt. %) 0.5 – 2.5 <2 (loss wt.% in 2h at 1000°C)

Softening point (°C) 1667

Table I-2 Properties for pyrogenic silica *[KAT87, WYP99], ¤data from Wacker Co. (for the reference HDK® N20) [WAC01]

I.1.4. Surface modification

As just mentioned, silica is hydrophilic by nature, but additional chemical treatments can make

the surface hydrophobic or reactive.



I.1.4.a. Principle

The chemical modification (or silanization) of silica, generally done via grafting of organo-silanes

on silanols sites (Figure I-9), is realized in order to increase the compatibility of silica with an

organic medium. The aim can be: i) to obtain adhesion with the matrix, ii) to tune the viscosity of

the filled medium, iii) to improve damping properties.

R2 Si XR1

R3+ HO Si R2 Si O

R1

R3Si + HX∆T

Figure I-9 General scheme of the silanisation of silica surface silanols [DUC96]

The general form of organo-silanes is the following one: Rn – Si – X4-n

where R is an organic function potentially reactive with the polymer generally separated from

the silicon atom by a methylene or propylene spacer and X is a hydrolysable function.

The thickness of the layer can vary depending on n. The length of the spacer largely influences

the reactivity of the organo-silane.

Many chemical natures of organofunctional silanes are available: chloropropyl-, (di)amino-,

epoxy-, vinyl-, methyl-, phenyl-, cationic styryl- methacrylate-, mercapto-, isocyanate-, alkoxy-

functional silane [PLU82, LEY85]...

The maximum grafting yield is limited because of the steric hindrance. The resulting grafting

content can be measured either by titration of the silanes fixed (for example by elemental

analysis of carbon content), or by acid base titration of the residual silanols.

The grafting of silica can be achieved in solution or vapor phase as it is presented in the

following paragraphs2.

I.1.4.b. Grafting in solution

The grafting in solution is generally performed in an aqueous solution or in an organic solvent

(most often in a water/alcohol mixture). Silanes are hydrolyzed to give rise to silanol groups that

can directly react with the silica surface silanols first by hydrogen bonding then by covalent

bonding or condensate between each other to from siloxane bonds.

Many parameters influence the quality and rate of the grafting controlled by a balance between

hydrolysis and condensation rate (relative concentration, acid basic character, presence of a

2Wacker Co. generally uses an intermediate semi-gaseous route in which silica is fluidized and silanes are added as an aerosol



catalyst for the hydrolysis…). The thickness of the silane layer obtained results sometimes

difficult to control, but this process is the easiest to carry out [DUC96].

I.1.4.c. Grafting in vapor phase

The grafting in vapor phase is more complicated to carry out, but the results are generally more

controlled, and the grafting is more homogenous on the silica surface.

The silane is introduced in gaseous state in a reactor containing the silica, under static or

dynamic conditions. Therefore, this process requires working at high temperatures, under high

vacuum, or with highly volatile products (low weight chloro- or alkoxy- silanes). The silane

condensates on the silica surface by forming covalent bonds with silanols.

I.2. Generalities about epoxy reinforcement and sub-micro structured composites

First of all, we would like to inform the reader that the objective of this literature section is not to

do an exhaustive review of the field of nano-structured composites which can be found in other

reports [ALE00, BUR03, BIZ04, LEP02, GER02…].

As explained in the general introduction, fumed silica is not regarded in this work as a typical

nano-filler, so that, it did not appear relevant to compare our epoxy / silica systems to

conventional “trendy” nanocomposites such as those based on clays or carbon nano-tubes.

Therefore, the only elements pointed out are those that seemed either indispensable or original

in order to justify the introduction of fumed silica into epoxy.

In this paragraph, the necessity and traditional routes used for the reinforcement of epoxy

matrices are first emphasized; the effect of sub-miro fillers is then discussed, as well as the

specificity of this scale.

I.2.1. Reinforcement of epoxy

After dedicating the previous part to the background about fumed silica, let us just mention a

few general features about the thermosetting matrix used in this work that is to say an epoxy

network. The chemistry of epoxies is rather versatile: they can be reacted via different routes



thanks to a catalyzing or an hardening agent such as amine, anhydride, imidazoles, or cationic

induced homo-polymerization. The reaction can be induced thermally or by UV radiations.

In this work, the epoxy networks are obtained via the poly-condensation reaction between a

diepoxy and diamine. A wide range of epoxy-amine comonomers exist: i) epoxy prepolymers

exhibiting different functionalities, molar mass and viscosity are commercially available, ii)

amine hardeners combined can be aliphatic, aromatic or cyclo-aliphatic, the reactivity with

epoxy can be tuned and allow fast or slow crosslinking process at different temperatures. The

network generated can be rigid (glassy) to soft (rubbery) at room temperature, according to the

formulation [ESP91, GRI91, PAS02].

Due to their good thermal stability, mechanical behavior, low density, electrical insulating

properties, chemical resistance, as well as outstanding adhesive properties with many types of

substrates, epoxy networks are used in many arrays of application such as coatings, for

example powder paints, structural adhesives and composites parts.

The epoxy-amine comonomers selected for this work are presented in the second chapter, as

well as the reaction mechanisms involved (II.1.1.). A commodity epoxy was combined either

with an aromatic diamine (most similar to high performance formulations for example used in

aeronautics) or with a long aliphatic diamine generating an elastomeric network at room

temperature.

I.2.1.a. Fillers traditionally used to reinforce epoxy

Many applications using epoxies require enhanced properties, such as thermal and mechanical

ones – especially in order to overcome their main drawback that is to say their brittleness–. A

common practice consists in the incorporation of fillers into epoxy-based formulations either to

modify specific properties and / or to reduce their cost [HAM96, ELL93].

A large variety of fillers have been used in the past into epoxy matrices among which the most

usual are glass beads and fibers, carbon fibers… New developments have been offered since

the rise of nano-fillers such as: silica (various types), titanium dioxide, zirconium oxide,

montmorillonites...

The advantage of silica consists of its great natural abundance, the possibility of its industrial

synthesis, the diversity of shape and size in which it can be produced, its rather competitive cost

compared to other nano-fillers, as well as its good chemical, thermal and mechanical properties.

The modifications of properties induced by filler addition into epoxy formulations are of various

natures (Table I-3):



− improvement of thermal stability

− beneficial effect to counteract cure shrinkage really damaging in some applications

− modification of the viscosity and thixotropic behavior

− increase of the modulus, fracture toughness (limitation of the crack propagation)

− increase of the surface hardness, abrasion resistance

− decrease of water absorption

− incorporation of conducting fillers in order to modify the electric properties of the epoxy

matrix for applications in electronics

− fire retardant properties …

However, the filler addition also results in some disadvantages, especially concerning the

density increase and the changes in the processability. Indeed, filler incorporation induces an

increased viscosity restricting the maximum filler content and/or a necessity of adaptation of the

process to filled formulations.

Table I-3 Modifications of epoxy properties induced by the incorporation of various fillers [ELL93]



I.2.1.b. Nano-fillers: the answer to reach a toughness / stiffness

compromise?

As mentioned previously, the brittleness of epoxy matrices often limits their applications, thus

numerous investigations have been carried out in order to find the right formulation in order to

induce substantial toughness improvement with no associated decrease of other important

properties such as the modulus and glass temperature, as it is traditionally observed using

rubber modification [POI96, MAA93, PAS02]. On the other hand, micro-scale rigid particles,

introduced in a brittle conventional epoxy network, generally leads to an increased stiffness but

decreased fracture resistance.

However, this well-known typical trend in case of conventional filler addition into thermosetting

matrices does not seem to be checked at the sub-micro scale. Indeed, both increased

toughness and stiffness after nano-particles addition were reported by Michler et al. [GER02]

(Figure I-10).

Figure I-10 Typical trend toughness-stiffness for composites and effect of nano-particles vs. micro-

particles in a polyurethane matrix [GER02]

These unexpected discrepancy between the behaviors of micro- and nano-reinforced polymers

can be explained, on the one hand, by the specificity of the nano-scale in which polymer chain

lengths approach the filler dimensions so that they might display particular interactions

influencing the macroscopic behavior of the materials, and, on the other hand, by the large

number of parameters involved that can have a synergistic action.

Indeed, many parameters have to be taken into account for the reinforcement efficiency of a

filler into a given medium [AND00]:

− chemical nature of the fillers

− shape and orientation of the fillers

− average size, size distribution, specific area of the particles



− volume fraction

− dispersion state

− interfacial interactions particle / particle vs. particle / polymer chains

− respective mechanical properties of each phase

These parameters, detailed in the following paragraphs, act either in a competitive or in a

cooperative way, and their effects can not always be separated. It can thus result complicate to

understand the mechanisms involved and predict the final material behavior.

I.2.2. Specific effect of nano-fillers

As it is now well-known, nanometric fillers are expected to improve significantly more the

material properties even at lower loading than conventional / micro-fillers due to:

i) the size effect: high number of particles, with a low interparticle distances and high specific

surface area generating increased effects of the interfacial area with the matrix

ii) the interactivity and/or potential reactivity of the nano-fillers with the medium

iii) the possible multi-scale organisation of the nano-fillers.

Furthermore, if the targeted effect is achieved for low filler content, it allows the conservation of

the processing route used for neat systems which is a substantial advantage in an industrial

context.

I.2.2.a. Size effect, high specific surface area

In order to highlight the difference between micro- and nano-fillers concerning the effect of

surface specific area, let us just quote the example of a fumed silica that typically displays a

specific surface area lying between 50 and 400 m²/g whereas natural silica microparticles have

a surface in the range of 2 m²/g [KAT87].

The Figure I-11 and Figure I-12 illustrate the effect of the “miniaturization” of the fillers on the

surface area, on the number of particles and on the inter-particle distance: as the size

decreases, the number and surface of the filler increase, whereas their distance decreases.

These trends are obviously observed only in case of individually dispersed particles, which can

sometimes result quite challenging to achieve.



Figure I-11 Schematic illustration of the repartition of particles representing 3 % of a volume of 50,000 µm3: (a) 2.86 particles of 10 µm diameter (b) 2,860 particles of 1 µm diameter (c) 2.86 106 particles of 100 nm diameter [RON01]

Figure I-12 Correlation between number, surface area, inter-particle distance and particle size for ideally dispersed spherical particles presenting cubic distribution (theoretical calculation) [FRI05]

The increased effect of the interface and the reduction of the inter-particular distance lead to a

modification of the structure of the polymer in the neighborhood of the filler, which can modify

substantially the properties of the nanocomposites even at low filler content. The reduction of

the mobility of the polymer chains leads to the formation of a boundary layer, which properties

are really different from that in the bulk [DUT94, OU96], and an increase of the crystallinity of

semi-crystalline polymers because particles act as nucleation agents [KAU01].

The apparent volume fraction of filler can seem increased i) because the immobilized stiff

interface plays an important role, and ii) because of the occluded polymer between the fillers,

making sometimes possible to reach the percolation threshold at really low filler content.

I.2.2.b. Ability for interactions / reactions of the nano-fillers with the

medium

A direct consequence of the large surface area is the large interfacial zone where physico-

chemical interactions can take place in between the polymer and the filler. Indeed, the silanols

content is the same for any silica specific surface area (1.8 nm-², for surface BET between 50

and 400 m²/g), thus with increasing specific surface area, the number of sites for grafting or

interaction with the medium increases markedly. This can also result in a modification of the



kinetics of the reactions, as it is detailed in a foregoing section dealing with the effect of fillers on

the network formation (I.3.3).

It is possible to balance the interparticular vs. particle-medium interactions, or reactions, via

surface modification of the fillers. In case the particle is covered by reactive groups, it may act

as additional crosslinking sites in thermosets, with a high functionality.

I.2.2.c. Multi-scale organization

The multiscale structure of the fillers is well known for montmorillonites (Figure I-13). For fumed

silica (cf. paragraph I.1.2), the 3-scales organization is induced by the processing: primary

particles gathered in indivisible aggregates that can form agglomerates (Figure I-13).

Figure I-13 Parallel between the 3-scale structures of montmorillonite [BUR03] and fumed silica

In montmorillonites-based composites, the large scale arrangement of the platelets into the

polymeric matrix is traditionally described as exfoliated (or delaminated), intercalated, or

tactoids (also referred to as conventional composites when the dispersion is not actually

achieved at the nano-scale) with a decreasing level of dispersion (Figure I-14). A complete

exfoliation is generally difficult to achieve into thermoset formulations such as epoxies. The

morphology obtained depends on the efficiency of the processing and on the organophilic

surface modification of the montmorillonites which increases the interactions with the polymer

[LEP02].

(a) (b) (c) Figure I-14 Traditional morphologies for montmorillonites based composites: (a) conventional

nanocomposite, (b): intercalated nanocomposite, (c) delaminated nanocomposite [adapted from LEP02]

polymer



The properties of the final material, but also the viscosity of the suspensions, can be closely

related to the possibility of multi-scale arrangement of the fillers.

I.2.2.d. Geometric characteristics

The Table I-2 aims at comparing the typical values of the geometric characteristics of various

nano-fillers commonly used. All present at least one dimension in the scale of a few

nanometers, but the number, d, of dimensions in this scale varies depending if the filler displays

a spherical (d=3), fibrillar (d=2) or lamellar (d=1) geometry.

Nano-filler Geometry of the nano-particles

Characteristic dimensions

Shape factor (L/φ or L/t)

Specific surface area (m²/g)

TiO2 Spherical φ = 8-40 nm 1 7-160 Pyrogenic silica Spherical φ = 5-40 nm 1 50-400

Carbon black Spherical φ = 250 nm 1 7-12

Cellulose whiskers Fibrillar L = 1µm

φ = 10-20 nm 100-150 150

Carbon nano-tubes tube φ = 1-50 nm

L = 10-100µm > 1000 -

Graphite Lamellar L= 6-100 µm 6-100 6-20

Talc Lamellar L = 1-20 µm e =0.2-6µm

5-20 2-35

Montmorillonite Lamellar L= 0.6 – 1µm

e =1nm 600-1000 700-800

Table I-4 Geometric characteristics of various nano-fillers, L: length, t: thickness, φ: diameter [adapted from BUR03 and WYP99]

However, the values given in the Table I-4 for the shape factor and size of fumed silica are

those of the primary particle, it can be discussed since isolated primary do not exist, and one

should consider the aggregate as elemental object accounting for the effect of fumed silica. Due

to the fractal geometry exhibited, the geometric parameters defined using Euclidian geometry

are insufficient (cf. III.1.3.b.).

Some models use the radius of the sphere including the aggregate as size of the elemental

object in fumed silica; the percolation threshold occurs for volume fraction much lower than if it

was calculated for primary particles, and seems to be more conform to the real behavior

[DOR95].

Generally, the higher the shape factor, the lower the percolation threshold for a same geometry

of particles. Lamellar fillers such as montmorillonites exhibit a shape factor typically in the range

600-1,000. Their efficiency is difficult to compete with, when properties such as gas barrier are



targeted. But, the “tortuosity” effect involved is strongly affected by the exfoliation degree, and

the orientation of the fillers that can be controlled by the process (Figure I-15).

In a same way, an increase of the length of crack propagation due to the deviation from the

fillers induces an increase of the surface developed and of the energy consumed by fracture,

thus generally an enhancement of the fracture toughness.

Figure I-15 Tortuosity effect in a layered nanocomposites: micro-fillers (on the left) vs. nano-fillers (on

the right) [BUR03]

I.3. Processing of sub-micro structured composites, filler effect on the rheology and reactivity of epoxy systems

In the field of sub-micronic filler based composites, the control of the morphology is essential to

obtain the targeted final properties. So, the processing is a key step, which was extensively

studied, because it determines if the fillers are actually dispersed at the sub-micro level or not,

and if the reinforcement can actually benefit from the “nano-effect” previously reported.

Silica is added in order to enhance the mechanical properties of the material in the solid state,

but it also modifies significantly the rheological behavior of the filled epoxy and/or amine

comonomers before crosslinking, and interacts with the mechanism of formation of the epoxy-

amine network and thus changes the kinetics as described here.

I.3.1. Processing of sub-micro structured composites

First, the different processing routes for nanocomposites in general are presented and then

examples from the literature in case of epoxy / silica formulations are reported in order to work

out the best process and conditions to obtain a fine dispersion state.



I.3.1.a. General routes for the processing of sub-micro structured

composites

Basically, three major routes can be distinguished for the processing of nanocomposites based

on preformed inorganic particles: in-situ polymerization, melt processing, polymerization into a

solvent (Figure I-16). One is preferentially used rather than another depending on the type and

physical state of the polymer, and types of interactions with the fillers.

Additionally, the inorganic phase can be formed in situ in the polymer via sol-gel process, the

structures are then rather different.

Figure I-16 Different routes for the preparation of nanocomposites based on layered fillers [PIN00]

In-situ polymerization

In-situ polymerization consists in the mixing of the preformed fillers with the monomers that act

as reactive solvent, followed by the polymerization. The critical step of this process is the

dispersion of the particles; different tools can be used as a function of the organic medium, its

physical state and viscosity. This route was used for the preparation of nanocomposites based

on polyamide, polyester, epoxies… For instance for nanocomposites based on an epoxy matrix

and montmorillonites, this process allowed the exfoliation of the layers during the crosslinking at

[LEP02]. Additional details in case of epoxy / silica formulations are given in the paragraph

I.3.1.b for this process of standard use for thermosetting polymers.



Melt mixing

For thermoplastic polymers, the fillers can be introduced directly in the melt, thanks to high

shearing devices, with no large modification of the process traditionally used. The introduction

of layered fillers into polyolefin could be achieved using, for example, a twin screw extruder with

an optimized screw profile, and adjunction of a compatibilizing agent. The use of a

polypropylene grafted with a maleic anhydride of high molecular weight was reported as

efficient to improve the layer dispersion [BOU04].

Solvent route

The preparation in solution consists in the dispersion of the preformed fillers into a solvent (with

possible adjunction of a compatibilizing agent), the addition of the polymer soluble in the same

solvent (i.e. thermoplastic or prepolymer precursory of a thermosetting network). The solvent is

then removed (critical stage), its evaporation must be complete otherwise it may damage the

whole properties of the material. Examples of use of this technique can be found in the literature

for the preparation of nanocomposites based on polyamide or polyethylene matrices. This

process is not too adapted to industrial context because it is not environmentally friendly.

Sol/gel process

The sol/gel process was extensively studied in order to synthesize epoxy / silica hybrids, i.e. in-

situ formed silica phase [MAT98a, b, MAT00, MAT03, OCH01a, b]. Silane precursors of the

silica phase are introduced into the organic medium, the most commonly used in the literature is

the tetraethoxy silane (TEOS). The sol-gel process consists of successive hydrolysis and

condensation leading to the formation of a glassy phase which reinforces the polymer. The two

phases are formed simultaneously and the microstructure obtained results from the competition

between the formation of the silica and the polymerization of the matrix. The size and

morphology of the dispersed silica phase obtained depends on the order of incorporation of the

components, their concentration, and the type of catalyst used: cluster-cluster growth regime

under acidic conditions and cluster-monomer aggregation regime under basics conditions

[KAN01, DUC96]. The microstructure of such sol-gel prepared nanocomposites is though

singularly different from that of preformed particles based ones [HAJ99a], and the interactions

are created at the molecular level.

Avnir et al. [AVN05] also mentioned to employ this method in order to form hybrids based on

polymers difficult to modify (such as polyolefins or polystyrene), thanks to the dissolution of the

polymer in the silica precursory (especially TEOS), whereas it is generally challenging to

disperse preformed silica particles in this class of polymer due to their hydrophobicity.



This process presents the advantage of being freed from the increase of viscosity consecutive

to the addition of the preformed fillers, which generates sometimes dead ends in the other

processing routes.

I.3.1.b. Processing of formulations based on epoxy and silica

For the processing of nanocomposites with thermosetting matrices, the strategy used for the

introduction of preformed inorganic particles is in situ polymerization, as it was done in the

present study. The procedure, which can be quoted from the literature regarding epoxy / silica

composites, generally consists of the 3 following steps [AND00, DRI98, ZHE03]:

− dispersion of silica into the hot epoxy resin followed by degassing

− addition of the crosslinking agent and homogenization of the mixture

− crosslinking.

During the first stage, the potential reaction between reactive silica and epoxy prepolymer might

take place so that the time / temperature schedule of this step has to be designed consequently.

Silica is sometimes initially suspended into a solvent (especially silica sol), the first step consists

then in the solubilization of the epoxy prepolymer into the solvent, followed by the evaporation

of the solvent [AND00]. Indeed, if the silica fillers are dried first, some problems of critical

agglomeration are faced due to colloidal forces, and an individual dispersion of the particles is

then almost impossible to achieve [AMI01].

Viscosity often limits the maximum content of fillers that can be added: up to ca. 20 wt. % of

fumed silica in the final system depending on the silica surface chemistry and dispersion

process [DRI98].

Dispersion can be carried out in a mechanical way (large variety of devices can be found in the

literature: high shear mixing, extruder, milling…), by magnetic stirrer, by ultrasounds, or by the

combining several devices. Certain studies were dedicated to the comparison of the quality of

the dispersions obtained using various mixing methods.

Zheng et al. [ZHE03] studied materials based on an epoxy prepolymer, an aromatic hardener

and fumed silica (surface BET = 160 m²/g). They compared the results from high speed

mechanical homogenizer (24,000 rpm), ultrasounds, as well as using a coupling agent (nature

not indicated) or not. In order to quantify the efficiency of a given device, both morphological



observation and mechanical properties were discussed (Table I-5). Mechanical properties were

better when a reactive silane was employed (second route in the table) than without (first route

in the table). Furthermore, both tensile and impact properties were improved when a

mechanical stirring was used in addition to ultra sounds (third route in the table). The best

dispersion obtained in these conditions is reported in the Figure I-17. This study confirmed the

importance of achieving a good dispersion state for the mechanical properties.

Table I-5 Mechanical properties obtained for various process routes: (-) neat matrix, (1) 3 wt. % of untreated silica dispersed using ultrasonic waves, (2) 3 wt. % of silica pre-treated by a coupling agent, dispersed using ultrasonic waves, (3) 3 wt. % of silica pre-treated by a coupling agent, dispersed using ultrasonic waves, and mechanical stirring [ZHE03]

Figure I-17 Morphological observation (TEM) for an epoxy network filled with 3 wt. % of pre-treated silica processed via the third route (3 wt. % of silica pre-treated by a coupling agent, dispersed using ultrasonic waves, and mechanical stirring) [ZHE03]

In contrast, Anderson et al. [AND00] reported better results using ultrasonic device into hot

epoxy resin than using a mechanical stirring for the dispersion of 5 wt. % of colloidal silica using

the improvement of the fracture toughness as criterion.

For extremely low silica loadings, typically lower than 2 wt. %, Fiedler et al. reported that the

use of a 3 roll mill was an efficient method for processing silica suspension in epoxy (after a

manual pre-stirring). The high shear mixing allowed obtaining fine dispersions [FIE05a, b, c]

except for amino-modified silica.

Unfortunately, many publications do not report enough details on the processing conditions

used for the mechanical stirring such as the rotation speed, the shape of the disk… that are

known to be important parameters for the dispersion efficiency [MUL04, STE05].

~1 µm



I.3.2. Rheological behavior related to the dispersion state

Fumed silica is a common rheological additive, so many studies deal with its effects on the

rheological behavior of suspensions. It can provide to suspensions: thickening, stabilization of

suspensions (preventing the sedimentation), thixotropy, sag resistance due to yield value which

is important for coatings. Typical silica content needed for rheological modification lies between

0.8 and 3 wt. %, it is much lower than that needed for mechanical reinforcement.

The rheological behavior of filled systems depends on many factors: particle size, size

distribution, volume fraction, dispersion state of the particles. The surface treatment is also an

especially important parameter. Rheological measurements can be performed in order to

monitor the dispersion state as it is illustrated here.

I.3.2.a. Rheological behavior of fumed silica suspensions

A newtonian behavior can be observed for diluted, coarse silica suspensions, or particularly

design surface chemistry, but most of the times a shear thinning behavior associated with a

yield stress and a thixotropic behavior are described in the literature. Some studies, additionally

found the existence of conditions for the appearance of a shear thickening behavior.

The large thickening efficiency of silica is generally attributed to the ability of formation of a

three-dimensional network of fillers that immobilizes a part of the liquid phase. This network is

instantaneously and reversibly broken upon exceeding the yield point by application of a

mechanical stress, and the viscosity is consequently reduced. This is shown by a shear thinning

behavior (Figure I-18) during a shear rate sweep. When reaching a rest state again, the silica

network can re-build, and the viscosity returns to its initial level with time. This process can be

slow because it requires first the desorption of at least a part of the polymeric molecules from

the silica surface, producing a thixotropic effect [BAR02, ETT00].

The driving forces for the network formation are the interactions between the aggregates,

mainly via hydrogen bonding and Van der Walls forces when considering hydrophilic silica and

hydrophobic interactions between the di- (or tri-) methyl siloxane covered surfaces when

considering hydrophobic silica [BAR02]. The formation and stability of the network is largely

influenced by the nature of the polymeric medium by means of the specific interactions

developed between the silica aggregates and those with the polymer.



Figure I-18 Shear thinning behavior and micro-structuration of a fumed silica suspension upon

shearing [BAR02]

In contrast, a few studies also described a shear thickening behavior of fumed silica

suspensions into a liquid. Raghavan et al. [RAG97] studied the dynamic behavior of untreated

fumed silica (surface BET ~ 150 m²/g) suspensions into poly(propylene glycol) (PPG). The

absence of flocculation at rest was checked by microscopic observation. The stabilization of the

dispersion state was due to hydrogen bonds between the filler and the PPG. The behavior

obtained for a 10 wt. % silica suspension into PPG was characterized by a large increase of the

steady shear viscosity above 0.1 s-1 (Figure I-19 (a)) and of both storage and loss modulus

during a dynamic strain sweep (Figure I-19 (b)). The critical shear-rate at appearance of the

shear thickening varied inversely as a function of the silica content.

The corresponding micro-structural changes occurring in the suspension were described as an

order-disorder transition: at low shear rate, the easy flowing is due to an alignment of the fillers

in the flow, whereas above the critical shear rate, the shear thickening is due to the formation of

flow-induced clusters of particles that can not accommodate the solicitation anymore, resulting

in greater energy dissipation.

(a) (b) Figure I-19 (a) Steady shear viscosity as a function of the shear rate ; (b) Storage and loss modulus

as a function of the strain applied, both for a 10 wt.% silica suspension into PPG [RAG97]



In contrast, the behavior is a typical shear thinning for the same silica suspensions into mineral

oil. The H-bond between the particles and their absence with the medium, give rise to pre-

existent clusters of particles. In oscillatory regime, this behavior is traduced by a gel-sol

transition as the strain increased. The steady shear behavior seems thus a powerful tool to

elucidate the initial dispersion state.

Both shear thinning and thickening behaviors were observed by Olhero et al. [OLH04] in case of

micronic silica / fumed silica mixed suspensions in an aqueous media. They found out that

increasing the ratio of micronic silica led to a shear thinning behavior whereas, shear thickening

behavior was favored by a high ratio of fumed silica particles. It was believed that the coarser

particles destroyed the gel-like structures formed by fumed silica (Figure I-20).

Figure I-20 Schematic illustrations of the behaviors of particles suspensions upon shearing:

shear-thinning (A) and shear thickening (B) [OLH04]

I.3.2.b. Effect of particle size and shape

If monodisperse particles are considered, the thickening efficiency generally increases as the

diameter decreases [LI04].

King and Halley [KIN00] compared the respective effect of fumed and micro-scale silica fillers

on the rheological behavior of filled epoxy precursor. With hydrophilic fumed silica of surface

BET = 300 m²/g, a yield stress appeared from 7 wt.% of silica added into epoxy resin (Figure

I-21).



In addition, from the same fumed silica content into epoxy resin, the formation of a percolating

network was evidenced by a sol-gel transition during a frequency sweep: a solid behavior at low

frequency (G” < G’) and liquid behavior at high frequency (G” > G’) (Figure I-22). For lower silica

contents, the behavior was that of a liquid all over the frequency range studied.

Unlike with fumed silica, epoxy suspensions filled with silica micro-particle did not show any

yield stress even at silica content as high as 50 wt. %, and displayed a liquid behavior all over

the frequency range tested.

Figure I-22 TTS time-temperature superposition curve overlaid for temperatures = 0,10,20,30 and 40°C for fumed silica suspensions (respectively 2 and 7 wt. %) into epoxy precursor, strain = 1%, [KIN00]

Roscher et al [ROS03, ADE01] compared the viscosity of acrylate based formulations, devoted

to transparent coatings, filled with either colloidal silica sol particles or fumed silica (Figure I-23).

They underlined that the viscosity of the formulations based on silica sol could remain low even

at high loading while preserving the coating clearness (up to 60 wt.%). In comparison, the

viscosities obtained with fumed silica were higher even at low content due to the network

building between the aggregates.

0.0 100.0 200.0 300.0 400.0 500 .010 0

101

102

103

10 4

Stress [Pa ]

Eta

* (A

)

[Pa-

s]

2% ,5% & 7% Dried AEROSIL 300 S tra in=30% Tem p = 20C

W arning: O verlay units don't m atch, Forc e

2 wt. % silica

5% 7%

10-2 10-1 100 101 102 103104

10-3

10-2

10-1

100

101

102

103

104

10-3

10-2

10-1

100

101

102

103

104

Freq [rad/s]

G' (

bB

)

[dy

n/cm

2 ]

G" (b

C

) [dyn/cm

2]

2% Dried Aerosil 300 Strain = 1% Temp = 0,10,20,30 & 40C--TTS Overlay Curve

2 wt.% silica

10-2 10-1 100 101 10 2 103104

10-3

10-2

10-1

100

101

102

103

104

105

10-3

10-2

10-1

100

101

102

103

104

105

Freq [rad/s]

G' (

bB

)

[dy

n/cm

2 ]

G" (b

C

) [dyn/cm

2]

7% Dried Aerosil Strain = 1% Tem p = 5,10,20,30 & 40--TTS Overlay Curve

Solid typeLiquid type

G’

G”

7 wt.% silica

G’ G”

Liquid type

Figure I-21 Stress sweep at 20°C of fumed silicasuspensions (respectively 2, 5, and 7 wt. %) into epoxyprecursor [KIN00]



Figure I-23 Viscosity of acrylate-based formulations for coating as a function of the silica content, for

■silica sol nano-particles, and ♦fumed silica (untreated) [ROS03]

I.3.2.c. Effect of the surface chemistry and medium nature

Effects of both natures of silica surface modification and of the medium were extensively

investigated in the literature. Indeed, they define the nature of the interactions developed in the

system and the balance between particle-particle interactions and particle-polymer interactions,

which is the main factor explaining the rheological behavior of filled systems.

An example presenting the evolution of the viscosity of a silica suspension as a function of the

residual silanols on silica surface into a highly polar medium is given Figure I-24: the more

similar the polarity, the lower the thickening. However in this plot, an indirect effect is not

accounted for: the silica dispersion state also strongly depends from its surface chemistry.

Figure I-24 Relative viscosity (10s-1, 25°C) vs. content of residual silanols of a 3 wt.% dimethy siloxy–modified fumed silica in a mixture of 66 wt. % of propanol, 25% of 2-propanol, 9% of water [BAR95]



The viscosity of epoxy filled with hydrophilic and hydrophobic silica was compared (Figure I-25).

The thickening efficiency of hydrophilic silica is low in this polar medium, because the wetting of

the silica surface by the epoxy resin, driven by the hydrogen bonding, screens the possible

particle-particle interactions, preventing the networking that generally leads to thickening

[KHA93]. In contrast, fully silylated fumed silica have a larger effect, an additional thickening

results from silica modification using trimethylsiloxy than dimethylsiloxy silane due to an

interpenetration of the grafted chains [BAR95, 02].

Figure I-25 Comparison of the thickening effect of various fumed silica at 4 wt.% in a solvent free epoxy resin at room temperature, R202, 805, 812, 972 are hydrophobic silica presenting various surface area and modifications, in comparison 300 is an hydrophilic silica of 300 m2/g [NOW02]

To summarize, filler effect on the viscosity of a suspension is expected to increase when

increasing either the filler specific surface area or the “incompatibility” of the filler surface with

the medium.

Therefore, an adequate silica surface chemistry has to be selected depending on the objective

of silica introduction: i) if the content needed is high, as for instance for mechanical

reinforcement, a surface modification presenting the same physico-chemical features as the

medium has to be selected in order to lower the impact of silica on the viscosity, or ii)

reciprocally if thickening effect is required at low silica content.

I.3.2.d. Effect of the silica content

Mustata et al. [MUS98] added hydrophilic silica into epoxy plasticized by polyester. The stress-

shear rate behavior (Figure I-26(a)) matched with an extended Casson model, with a yield

stress that increased with silica content and presented a threshold (Figure I-26(b)). Additionally,

from 10 wt. % of silica a thixotropic behavior was observed at room temperature but it was lost

at temperatures greater than 65°C.



400

450

500

550

0 5 10 15 20

silica wt. %

0

500

1000

1500

2000

2500limiting viscosity (Pa.s)

Yield stress (Pa)

(a) (b)

Figure I-26 (a) Steady stress as a function of the shear rate at room temperature for different content of hydrophilic silica into epoxy plasticized by polyester filled with ○ 5 wt.%, • 10 wt.%, ∆ 15 wt.%;

(b) Parameters calculated from an extended Casson model: yield stress and limiting viscosity at high shear as a function of the silica content [adapted from MUS98].

Shirono et al. [SHI01] studied the dynamic behavior of alkyltrimethoxy silane-treated fumed

silica into epoxy. Due to a decreased epoxy adsorption, a stronger thickening and shear

thinning was shown for longer alkyl chains due to interactions between longer alkyl groups.

From 5 wt. %, a sol-gel transition was observed (Figure I-27). The Payne effect, limiting the

linear behavior, occurred at lower strain for higher silica content. This behavior was explained

either by the desorption of the polymeric chains from the particle surface, or by the

destructuration of the filler network under strain [BOK01].

Figure I-27 Dynamic behavior of silica suspensions into epoxy at ○• 1, □■ 3, ∆▲ 5, ♦◊ 7 wt. % of octyl-treated silica (C16) at 1 Hz, (empty signs are for G” and full ones are for G’) [SHI01]

Khan et al. [KHA93] investigated the behavior of different contents of fumed silica either

untreated or grafted with short and long hydrophobic chain in suspension into PPG and mineral

oil. A gel behavior was observed for hydrophobic silica in a mineral oil for all silica

concentrations (from 4 wt. %), whereas it was only observed from 7 wt. % into PPG.



The values of the storage modulus in the linear strain region at 1 rad/s were measured. They all

displayed a power law behavior as a function of the silica content (Figure I-28): n )( G' φ∝ω Equation I-2

Where φ is the filler volume fraction, ω is the pulsation of the experiment and n is the exponent

of the power law.

The exponent was 4 for all silica suspensions into mineral oil and 6 for suspensions into

polypropylene glycol. The differences of exponents were explained by the different types of

interactions that were referred to as: i) primary bridging in case of direct interactions between

the fillers through H-bonding giving rise to strong gel-like structure (occured in mineral oil based

systems), ii) secondary bridging which is due to interactions between the polymeric chains

grafted onto the silica surface (for highly concentrated suspensions), and the iii) no bridging,

that featured in case of medium presenting the same polarity as the silica (occurred into PPG).

For mineral oil based system, where primary bridging by H-bonds is dominant, the type of

micro-structuration is not modified upon a change in concentration, the dependence on the

concentration is lower, and so is the power law exponent.

Note that in a previously quoted study, Shirono et al. [SHI01] reported that into epoxy the

exponent of the power law greatly varied as a function of the silica surface treatment from 0.9

for untreated silica to 3.6 for longer alkyl chains.

Figure I-28 Dependence of the storage modulus on the silica volume fraction, • MO-D150 is a mineral oil filled with hydrophilic silica, ○ MO-R805 is a mineral oil filled with silica grafted with octyl goups, ∆ MO-R974 is a mineral oil filled with silica modified by methyl goups, and ▲PPG-R805 is a polypropylene glycol filled with silica grafted with octyl goups , silica BET~ 150m²/g for all [KHA93]

The power law model is in agreement with others models based on scattering techniques that

relate the reinforcement with the fractal geometry of fumed silica [DOR95, CHE91].

Additional models were developed to account for the appearance of a percolation threshold: n

0v )( G' φ−φ∝ Equation I-3

where φv is the silica volume fraction and φ0 is the volume fraction at the percolation threshold.



Fumed silica, being formed of fractal (Dm~2.5) non dense (apparent aggregate density ~ 700 g/l)

space-filling aggregates, it exhibits a high ability to occlude polymer (cf. fractal structure studied

in III.1.3.b.). The apparent efficient volume fraction of fillers is thus much higher than the real

one, and a percolating network of aggregates and agglomerates can be reached from

calculated threshold content as low as 2 wt. % of silica [BAR95].

I.3.3. Effect of fillers on the crosslinking kinetics of epoxy networks

The effect of fillers addition into a reactive system was often studied in the literature: fillers were

reported either to catalyze or to inhibit the reaction, depending on the natures of the filler and of

the reactive system, content of fillers, and on the cure temperature. The presence of water

initially adsorbed on filler surface, as well as that of the silica surface silanols have to be taken

into account. This effect is reported in this paragraph, first for epoxy systems crosslinked via

condensation mechanism, such as especially epoxy-amine systems used in this work, and then

for epoxy homopolymerization initiated via ionic mechanism.

I.3.3.a. Filler effect on epoxy crosslinking via condensation mechanism

Altmann et al. [ALT01] observed a catalytic effect in presence of high contents of a micronic

crystalline silica flour in an epoxy network which depended on the cure temperature: above

100°C, no effect was observed, whereas below 100°C, the reaction rate increased especially at

the beginning of the process, and the gelation and vitrification times decreased. It was

explained partially by the H donor species on silica surface (silanols). However, the low surface

area of this filler (~ 1.6 m²/g) is not likely to explain entirely the catalytic effect. They supposed

that a complementary effect due to physical interaction between the filler and the monomers,

leading to the formation of an interphase with a higher concentration of reactive specie, was

responsible for a higher local reaction rate. At high temperature, because the attractive forces

were weaker, no significant effect on the reaction rate was noticed.

Olmos et al. [OLM05] studied the kinetics of the epoxy-amine reaction at the interface with silica

micro-particles exhibiting amino-modified surfaces, and in the bulk. The reaction rate at the

interface was faster than in the bulk but only in the first stage of the reaction, this was attributed

to the higher local concentration in amino groups. The activation energy was constant during all

the process suggesting the absence of change of the reaction mechanism.



However, most studies used micronic silica whereas the high specific surface area of sub-

microninc silica is expected to influence significantly more the kinetics of the processes due to

the high content in silanol (or reactive) groups (cf. I.2.2.b). To our knowledge, there are not a lot

of studies dealing with the effect of fumed silica on epoxy-amine reactions except Martin

Martinez et al. [MAR05] who mentioned a catalytic effect. It was attributed to filler-polymer

interactions between the silanol groups on the silica surface and the epoxy that produced a

deficit of charge in the “antimakornikov” carbon of the oxirane ring, generating a faster curing

reaction. This effect is reduced as the curing temperature increased, because the filler-epoxy

interaction becomes weaker.

It is in fact well known that epoxy-amine reactions can be accelerated by compounds that

stabilize the alkoxide ion intermediates (after mechanism in Figure I-29). Therefore hydroxyl

groups and hydrogen donors in general act as catalyst of the reaction, the more acidic being the

more effective.

Figure I-29 Mechanism of hydroxyl catalysis of epoxy-amine reaction [ELL93]

In a same way, in case water is adsorbed on silica surface, it acts as a catalyst of the

condensation reaction of epoxy-amine networks. For epoxy networks obtained via condensation

mechanism, properties close to the interface with the filler are expected to be the same as those

in bulk, except if a segregative phenomenon counteracts because of favored interactions with

one of the components of the reactive system. For instance, an increase of the epoxy

concentration close to the surface of a silicon wafer was reported in certain studies [TAN97].

This led to the formation of an interphase presenting a defect of stoechiometry, thus different

local properties.

I.3.3.b. Filler effect on epoxy crosslinking initiated via ionic mechanism

In contrast with the previous section, water was described as an inhibitor of the reaction in case

of epoxy-anhydride network used as matrix for a glass fiber reinforced composite [ASS02]. A

local decrease of the crosslink density and of the glass transition temperature by 10°C

compared with the bulk value of the matrix was detected around glass beads thanks to micro



thermal analysis.

The catalytic effect of fillers generally observed for condensation mechanism is not

straightforwardly checked in case of epoxy network generated due to ionic mechanisms.

Pingsheng et al. [PIN89] observed a complex effect of silica on epoxy systems crosslinked with

imidazole resulting in a catalysis by silica surface only when silica loading was greater than

20%. There was no effect of the fillers on the activation energy of the reaction.

In contrast, Askatsuka et al. [ASK01] investigated the effect of different micronic fillers (silica,

alumina, aluminum hydroxide, aluminum fluoride), they all appeared to delay the gelation time

of epoxy networks initiated with boron complex. Two possible mechanisms could be involved

according to the authors: i) decrease of the propagation rate due to the lower densities of epoxy

and bore complex, ii) occurrence of a part of the chain transfer reaction with hydroxyl groups on

the filler surface, not contributing to the chain growth.

Sangermano et al. [SAN05] studied the effect of fumed silica particles (5 - 50 nm) on the

kinetics of UV photocurable epoxy. The effect was reported to depend on the silica

concentration: an increase of kinetics and epoxy conversion was observed until 10 wt. % of

silica, whereas the total conversion was limited for greater contents of silica (Figure I-30).

The former catalytic effect was attributed to a chain-transfer mechanism involving hydroxyl

surface groups of the silica. During the polymerization, the ion chain-end of the carbo-cationic

growing chains underwent a nucleophilic attack by the hydroxyl groups on the surface of the

silica particles inducing the formation of a protonated ether. Deprotonation of this latter species

by the epoxy monomers resulted in the termination of the growing chain and in the proton

transfer to a new monomer that could start a new chain (scheme of the mechanism given in the

Figure I-31). This could lead to a chemical linkage between the filler and the polymer network.

The decrease of the final epoxy conversion, observed for an addition of 15 wt. % of filler, was

explained by the increase of the viscosity of the system leading to an important decrease of the

mobility of the reactive species.

Figure I-30 Kinetics of the formation of the networks of cationic epoxy filled with 0 (curve CY), 5 (CY5), 10 (CY10), 15 (CY15) wt. % of fumed silica [SAN05]



Figure I-31 Chain-transfer reaction mechanism on silica surface silanols [SAN05]

To conclude, the overall effect of filler addition on the kinetics of polymerization seems to result

from various effects, and depends obviously on the natures of each couple reactive system /

filler, on the interactions generated, on the reaction mechanism, and on the possible presence

of water adsorbed. It appeared thus necessary to investigate specifically the effect of fumed

silica - with various surface modifications - on the crosslinking process of the epoxy-amine

systems studied.

I.4. Expected mechanical properties of epoxy / silica composites

This part is devoted to the presentation of literature studies about the final thermal and

mechanical properties of systems based of an epoxy matrix and different types of silica fillers.

First, a few data about the interactions developed between the filler and the medium are

presented, in relation with the influence of the silica fillers on the molecular chain dynamics of

the epoxy network, illustrated especially by the impact on glass transition temperature. Then,

mechanical properties expected for silica reinforced materials are reviewed as well as the effect

of silica on the toughness and stiffness of the network first in glassy and then in rubbery state.

I.4.1. Interactions between epoxy and silica, silica influence on the dynamics of

polymer matrix

I.4.1.a. Interactions of fumed silica with epoxy

As pointed out in the presentation of silica, in many organic mediums, one has to resort to

chemical modification of the filler surface in order to compatibilize it and / or in order to increase



its dispersability. The interfacial adhesion is also targeted because it allows the transfer of the

load from the silica to the matrix, and expectedly results in an enhancement of the mechanical

properties. However some studies mentioned that in case of epoxy filled with pristine silica, the

dispersion state can be satisfying even in absence of chemical modification [JAN01]. It can be

attributed to the good interactions between silica surface silanols and epoxy to explain this

result.

The nature of the interactions at the interface between fumed silica and epoxy matrix can be

compared to that, extensively studied in the past, between glass fibers and epoxy matrix in

conventional composites (Figure I-32).

Figure I-32 Different types of interfaces between a glass fiber and an epoxy network

[adapted from ZIN99]

I.4.1.b. Effect on the glass transition temperature

As already mentioned, the presence of stiff nano-particles in the medium is expected to

influence the dynamics of the polymeric chains compared to the neat systems due to the large

steric effect, and strong interactions with the filler surface. The mobility of the chains close to the

surface may be reduced whereas the bulk might not be affected. Macroscopically, these

changes result in a shift in the glass transition temperature (Tg). In case of attractive interactions

between the particles and the species of the suspending medium, an increase of Tg is expected.

However, the experimental effect of fillers on glass transition temperature has been quite

contradictive in the literature: depending on the conditions and formulations considered, either

an increase or a decrease of Tg have been reported.

“Pristine” fiber “Sized” fiber Use of an amino silane

coupling agent (γ-APS type)

Physical interactions Covalent bonding in surface of the fiber

Formation of an

O/I interphase

Possible influence of

the physisorbed water

layer on fiber surface

Polysiloxane

network

fiber Epoxy

matrix fiber Epoxy

matrixfiber



Sun et al. [SUN04] studied the effect of various nano- and micro-fillers on the relaxation

behavior of an epoxy-amine matrix. Among the fillers under study, silica nano-particles were

responsible for a slight increase of Tg at low loading, but when the content became greater than

5 - 10 wt. %, then the Tg decreased and became even lower than the control for a silica content

greater than 15 wt. %, whereas the Tg was not significantly modified at any loading of micro-

paticles due to the low surface effect due to lower surface effect (Figure I-33).

The reasons proposed for explaining the non-monotonical behavior of epoxy filled using nano-

particles were: i) the presence of organic traces in the particles obtained via sol-gel process,

together with water initially adsorbed on silica surface that plasticized the matrix (as it could be

attested by a weight loss greater than 2 % below 100°C during a thermo-gravimetric scan), ii)

the increase of free volume in case of absence of strong attractive interactions between the filler

and the polymer.

Figure I-33 Evolution of the glass transition temperature of a filled epoxy as a function of the silica content ■ microfillers, ♦ nano-fillers. [SUN04]

Sun et al. also tried to elucidate the effect of dispersion quality on the glass transition

temperature of carbon black based composites by varying the duration of the dispersion, and

surprisingly they found out that the better the dispersion, the lower the Tg.

Kang et al. [KAN01] investigated the effect of the type of silica surface modification on the

viscoelastic properties of epoxy network. The silica particles, obtained via sol-gel process,

exhibited a mean diameter of 400 nm, and had been modified by various surface treatments,

i.e.: epoxy or amine in order to act as crosslinking points, isocyanate, as well as by calcination

to remove silanols and reduce the possible interactions with the matrix. The dispersion state of

high silica content were correct for epoxy and amino-modified silica, but rather coarse for

calcinated and isocyanate modified silica. The Tg increased by up to 20°C while the magnitude

of tan δ peak decreased when the interfacial interactions between filler and matrix as well as the

dispersion state were better (Figure I-34 (a)), and when the silica content increased (Figure I-34

(b)). The only surface modification for which a decrease in Tg was observed was isocyanate-



modification. It was assigned to the absence of reaction at the interface with the matrix and the

presence of the mobile grafted layer, but it might seem insufficient to explain such a large

decrease in Tg.

(a) (b)

Figure I-34 Dependence of the shape of the damping peak for epoxy systems filled with (a) 70 wt. % of silica particles presenting various surface modifications: amino-, isocyanate-, and epoxy-

modified silica, calcinated silica, untreated silica, neat epoxy system (b) Various contents of amino-modified silica: ● neat epoxy, □ 50 wt.%, ■ 60 wt.% and ○70 wt%.

For photocurable epoxy resins, Sangermano et al. [SAN05] found a slight increase of glass

transition temperature with an increase of fumed silica content. This was attributed to restriction

of the segmental motion by silica particle, especially because the grafting of the silica in the

growing chains was proved (cf. I.3.3.b).

Figure I-35 Variation of the glass transition temperature of an UV-cured epoxy resin as a function of fumed silica content added (0, 5; and 10 wt. %)

I.4.2. Mechanical properties of epoxy / silica composites in the glassy state

Only few publications actually deal with the addition of pyrogenic silica into epoxy matrices. In

order to extend the literature survey, we also referred to other epoxy-based formulations filled

using other types of silica (micro- and or nano-particles).

The general trends for mechanical behaviour of epoxy filled networks are first presented as well

as the reinforcement mechanisms, first, by conventional fillers and then by sub-micro fillers. The

mechanical properties are discussed as a function of different parameters: silica content, size

and specific surface area of the particles, as well as their surface modification.



I.4.2.a. Effect of conventional silica fillers (micron size)

General mechanisms

For filled thermosetting matrices, two main mechanisms of deformation were described: particle-

matrix debonding and micro-shear banding. Depending on the sensitivity of the matrix and on

the conditions (temperature, intensity of the stress…), the fracture mode can be due to: i) a

single debonding event giving rise to a rapid brittle fracture (rather at low temperature), ii)

propagation of the crack in between debonded zones, iii) coalescence of the voids (rather at

high temperature) [KAU01, KIN83] (Figure I-36).

Figure I-36 Three common modes of fracture for filled thermosetting matrices [KAU01].

In tension, most particulate filled thermosets break in a brittle way, i.e. before reaching the yield

point, and the fracture is generally induced by a critical defect.

The presence of fillers influences the macroscopic mechanisms of fracture. In general, as

emphasized before, the influence of stiff filler is a damage of the composite toughness (cf.

I.2.1.b). However in case of good adhesion, the influence can also be positive due to:

i) crack pinning (the particles act as obstacles slowing down the advancing of the crack front,

mechanism more efficient in case of stiff fillers)

ii) crack bowing (local deflection of the crack front)

iii) crack blunting (break down of the macromolecular chains in the vicinity of the crack tip

creating a platic zone, and a decohesion of the particles leading to a decrease in the stress

concentration, especially in case of stick-slip type propagation)

iv) crack bridging (increased fracture energy result from the stretch of the particles between the

edges of the propagating crack, especially in case of ductile particles)

v) increase of the fracture surface (from mirror-like to rough surface for brittle materials) and

vi) void formation by cavitation of the particles [KIN83, PAS02].

Furthermore, in case of brittle filler, the toughening can also be limited by the filler fracture.



General trends

Kausch et al. [KAU01] reviewed the trends generally observed consequently to the addition of

conventional fillers on the mechanical properties in a thermosetting polymer (Table I-6).

Table I-6 Summary of the general effect of spherical fillers on the

mechanical properties of thermosets [KAU01]

Imanaka et al [IMA01] studied the effect of the size, content, and grafting of the silica surface

with the epoxy matrix. They used spherical silica particles of various diameters from 6 to 30 µm.

In order to investigate the effect of particle / matrix adhesion, they grafted the particles using γ-

aminopropylmethyldiethoxy silane as reactive coupling agent or hexamethyldisilazane (HMDS)

as non interactive surface treatment. The mechanism of the crack propagation was not modified

for any kind of material, i.e. unstable stick-slip propagation. An improvement of fracture

toughness was observed, this improvement was lower with small particles than with large

particles, except at low silica content where no influence of the particle size was noticed. In

case of good adhesion, the efficiency of the crack pinning should be increased, whereas crack

blunting is expected to be avoided, however, for bulk specimens, no significant influence of

silica surface treatment was observed.

Nakamura et al. [NAK92, 99] additionally investigated the effect of the shape of the silica fillers

using spherical and irregular shaped silica particles (prepared by crusching amorphous silica

particles) from 2 to 30 µm, displaying different natures of surfaces at constant content of 50 wt.

% into an epoxy matrix. The results obtained are summarized in the Table I-7. Besides, it is

noteworthy that KIc is furthermore enhanced by irregular fillers.



Table I-7 Dependence of mechanical properties of filled epoxy on the features of the silica fillers [adapted from NAK99]

Influence of silica size and content on the Young modulus

Contrary to a simple mixture law, in which the modulus is linearly related to the filler volume

fraction, most experimental results display a concave upward shape as a function of the volume

fraction for a given size of filler.

Monette et al. [MON94] studied the influence of the size of silica beads on the Young modulus

of epoxy-based composites. They faced some problems to disperse the smallest particles, but

in any case, a “superlinear” (i.e. exponential) behavior of the modulus as function of the volume

fraction was observed. This behavior was explained by an increase of the strain transfer to the

particles as the volume fraction increased. To understand this, one has to imagine a given

particle alone in the medium: if the volume fraction of particles increases, then the equivalent

modulus of the surrounding material also increases. Thus the difference between the modulus

of this particle and the surrounding material decrease, and the strain transferred to each particle

increases.

Furthermore, the modulus not only depends on the volume fraction of fillers, but also on the

particle size: the reinforcement efficiency significantly increased when decreasing the particle

size (Figure I-37). The strain field within the composite was measured; it appeared that reducing

the particle size, the average particle strain increased. Monette discarded the effect of the

chemical bond at the interface with the matrix to explain the higher effect of smaller particles

and explained it from the higher interactions between particles. Many parameters influence the

behavior of this type of material so that, according to Monette, the current mechanical models

have to be adapted to account for the size effect. It is note worthwhile that in order to fit the

values, the modulus of the silica phase was considered as 76 GPa for the particles of 29 µm

and 106 µm diameters, but only 46 GPa for the smaller particles of 0.5 and 1.5 µm diameters, in

order to account for the porosity of these latters.

Modulus Toughness KIC Flexural strength εmax

Shape = Slightly higher for irregular shape

Same trends Same trends

Size (inv. BET)

Slight

Interfacial adhesion

= = For spherical fillers: unmodified < HMDS-modified < amino-modified For irregular fillers: HMDS-modified < unmodified < amino-modified

Higher for modified silica (unmodified < HMDS modified < amino-modified)



Figure I-37 Young modulus of the composites based on an epoxy matrix as a function of the volume fraction of glass beads of different sizes: ●: 0.5 µm, ■: 1.5 µm, ▲: 29 µm, ♦: 106 µm. [MON94]

For micron-size spherical silica particles (~ 4 µm) into glassy epoxy matrices, Wang et al.

[WAN02] observed the same exponential behavior of the Young modulus vs. silica content at

room temperature. But, in contrast, at higher temperature certain properties, such as the yield

stress and the modulus, did not vary monotonically with the silica volume fraction. Thanks to

nano-indentation tests, the interfacial region / interphase was shown to be more cohesive than

the matrix in bulk, creating a reinforcement of the overall material. The non monotonical effect

was attributed to the increased stress concentration in the matrix at high silica content, which

induced a weakening effect at high temperature, thus the yield stress decreased. Depending on

the silica content and temperature, the two effects compensated each other or one prevailed.

I.4.2.b. Effect of sub-micro silica particles

In case of fumed silica, Zheng et al. [ZHE03] observed that the impact strength increased up to

3 wt. % and then decreased again (Table I-8). When adding fumed silica, the surface generated

by the fracture is increased and rough compared to the neat matrix. Thus, the energy dissipated

increased (Figure I-38). As a good interfacial adhesion was displayed between the silica and

particles, the particles acted as stress concentrator promoting cavitations at the boundaries with

the polymer. The decrease of the impact strength at higher silica content was assigned to a

higher tendency to form agglomerates.

Table I-8 Dependence of the impact strength of a filled epoxy matrix as a function of the silica content (0-5 wt. %) [ZHE03]



Figure I-38 Aspect of the fracture surface of the neat (a) and filled (b) epoxy impact specimen (scale bar = 10 µm) [ZHE03]

Anderson et al. [AND00] worked with different types of silica into epoxy matrix. Remarkable

increase of fracture toughness were achieved (twice the value of the neat matrix) with a fumed

silica content as low as 1 wt. %. Then, a decrease of the properties was observed when the

silica content increased, this effect was related to the increased flocculation of silica. The

reinforcement was found to be more important for fumed particles than for micro-particles and

individual nano-particles (that appeared difficult to disperse because they were first suspended

into alcohol). An additional increase of the toughness was observed when a silane coupling

agent was used, but it was accompanied by a decrease of glass transition temperature.

Fiedler et al. [FIE05a, b, c] studied the properties of systems based on epoxy and fumed silica

with silica content lower than 1 vol. % (~ 2 wt. %). Surprisingly, for any content and silica

surface modification, a decrease of the Young Modulus was obtained in comparison with the

neat matrix, and an increase of the strain to failure (Table I-9). Besides, an increase by up to 54

% of the toughness of an epoxy matrix filled with 0.5 vol. % (~ 1 wt. %) of epoxy-modified silica

was obtained. Off-plane fracture processes were reported to be involved in this toughening.

Table I-9 Mechanical properties of the fumed silica / epoxy composites [FIE05c]

(a) (b)



From SAXS analysis, it was assumed that the crosslinking density is decreased in presence of

silica, which was consistent with the results of mechanical testing but not with the initial

expectation that reactive silica particles should act as additional crosslinking agents. The other

explanation proposed is the formation of an interface / interphase around the filler presenting a

higher mobility.

In the two previous studies [AND00, FIE05a, b, c], the increase of toughness achieved with

such a low silica content can appear a bit “suspicious”, especially because there are no real

clue of the control of the silanization of the silica surface. We can wonder if there is a really

controlled / total covalent grafting: indeed the two studies mentioned a decrease of the glass

transition temperature associated with a decrease of Young modulus for Fiedler’s study. The

increase of the toughness of the brittle matrix might be due to a part of not-bonded silanes or

oligomeric species grafted / adsorbed on the silica surface acting as a soft inter-layer in the

resulting composites, that could even form of a softer phase, or a thick silane layer on silica

surface leading to a core-shell structure (cf. creation of a “rubber-bumper” inter-layer [AMD89]).

Only a few mechanical models can afford for a decrease of modulus when adding stiff fillers to a

polymer, as that developed by Brown et al. [BRO05a, ALB05]. They modelled the evolution of

the modulus of a composite as a function of the thickness of the interphase (Figure I-39) and

obtained a decrease for smallest particles in which the interphase effect dominated (without

coupling agent in between the matrix and particles).

Figure I-39 Evolution of the bulk modulus Kc of composites as a function of the ratio size of the

particle R1 over thickness of the interphase T as illustrated on the left [ALB05]

Roscher et al [ROS03, ADE01] developed formulations based on a cationic curing epoxy,

crosslinkable either thermally or by UV radiations, and colloidal silica sol particles. The

synthesis route led to both the formation of the particles and the modification of their surface

and prevented the agglomeration of the particles (Figure I-40 (a)). These materials, dedicated to



the preparation of transparent coatings, in order to enhance the surface performances and get

ride of the disadvantages of the other organic additives (namely a decrease of mechanical

properties). An increase of both the fracture toughness (initially extremely low) and the tensile

modulus was shown up to 40 wt. % of silica (Figure I-40 (b)). This study illustrates the

compromise toughness / stiffness that was often reported at the nano-scale (cf. I.2.1.b).

(a) (b)

Figure I-40 Properties of silica filled epoxy (a) Morphology, (b) Mechanical properties on thick samples (3 mm): ♦ toughness (CT),▲ tensile modulus, [ROS03]

I.4.3. Mechanical properties of epoxy / silica composites in the rubbery state

Only few epoxy networks are in rubbery state at room temperature, the most common is based

on a diepoxy and a poly(oxypropylene)amine (cf. II.1.1. for the definition of this system also

used in the experimental part of this work). Consequently, this literature paragraph is mainly

based on examples of different silica fillers used to reinforce rubbery networks of various

natures, epoxy or not.

First, the mechanisms of reinforcement of rubbery materials by stiff fillers is introduced, then the

effect of fumed silica surface modification is illustrated from examples in polyurethane, and

finally a few examples of rubbery epoxy reinforced with other sub-micronic fillers are quoted.

I.4.3.a. General effect of fillers on a rubbery network

The effect of fillers on the mechanical properties is generally much more marked in the rubbery

state than in the glassy state. Ideally, reinforced elastomers show an increase in modulus,

hardness, tensile strength, abrasion, tear, fatigue, and cracking resistance. Both increased

stiffness and fracture properties can be obtained (Figure I-41) even with micronic fillers.



(a) (b) Figure I-41 (a) Illustration of the structure of filled-elastomeric network (b) Typical effect of fillers on a

stress-strain curves for an elastomer [BOK01]

For rubbery materials, the Young modulus can generally be fitted by Guth model [GUT45] that

was initially elaborated for carbon black filled rubbers and accounts for the shape factor of the

particles introduced. Above a critical content (~ 10 vol. %), the carbon black particles organize

as chain-like aggregates of high shape factor, so the use of models elaborated for spherical

reinforcements became ineffective.

According to Guth, the modulus (respectively viscosity) of filled rubbers can be written as: i) ²]v 14.1 v 2.5[1 EE ff0c ++= Equation I-4

for filler contents lower than 10 vol. % corresponding to individually dispersed spheres,

where E0 is the modulus of the neat matrix and vf is the filler volume concentration.

ii) ²]f².v 1.62f.v 67.0 [1 EE ff0c ++= Equation I-5

for greater contents, where f is the filler shape factor.

The adhesion is an important factor due to the great magnitude of deformation reached in this

kind of networks. A poor polymer-filler interaction results in a dewetting and void formation upon

a significant deformation, thus a crack initiates. Therefore, a strong bond between particle and

matrix is expected to significantly improve the reinforcement [BOK01].

The structure of a filled rubber is illustrated in the Figure I-41: rigid particles surrounded by

crosslinked flexible chains. The interactions between the filler and the rubber often lead to an

increase in the crosslink density of the network. Bokobza et al. [BOK01] showed that the

molecular weight between crosslinks decreased as the silica content increased into rubbers

(styrene-butadiene copolymers as well as PDMS). This effect was characterized by a restriction

of the swelling by solvent and an increased orientation of the chains inducing a higher

extensibility. Besides, the short chains, providing the stiffening, tended to gather in the

indeformable filler rich-zones of the sample thus the strain field was higher in these regions as it

was measured by Atomic Force Microscope. So, the increased stiffness resulted from the



introduction of a stiff phase, increased crosslink density, and orientation effect.

I.4.3.b. Effect of silica surface modification

Jesionowski et al. [JES01] studied the mechanical properties of filled polyurethane elastomers,

as a function of the surface modification of precipitated silica. The grafting was carried out using

mercapto-, amino- and methacryloxy-silane with the objective of linking covalently the fillers with

the polymeric network. Large increase of the properties of elongation at break and tensile

strength compared with neat PU where achieved thanks to the addition of any kind of silica. The

tensile strength increased by a factor 2.5 with 10 wt. % of unmodified silica, and the maximal

elongation increased up to 159 % for 5 wt. % of unmodified silica. Again, an optimal silica

content was evidenced and depended on the property considered. The grafting of the silica

fillers in the network allowed reducing the silica content to reach this optimum between (Figure

I-42, Table I-10). The best results were obtained with 3 wt. % of amino-modified silica.

Silica type Silica wt.% Elongation (%) Tensile strength (N.cm-²) Unmodified PU 0 72 45.2

2.5 131 73.2 5 159 96.6

7.5 144 105.9 Unmodified silica

10 148 110 3 111 129.6 Methacryloxy-modified silica 5 168 80.2 3 175 136.5 Amino-modified silica 5 138 109.7 3 163 128.9 Mercapto-modified silica 5 130 102.5

Table I-10 Mechanical properties of polyurethane elastomers filled with precipitated silica presenting various types of surface modifications [adapted from JES01]

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12silica wt.%

rela

tive

elon

gatio

n

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10 12silica wt.%

rela

tive

tens

ile s

treng

th

(a) (b)

Figure I-42 Relative mechanical properties of polyurethane elastomers filled with precipitated silica (a) relative elongation vs. silica content; (b) relative tensile strength vs. silica content for

♦unmodified silica, ■ methacryloxy-, ▲ amino-, x mercapto-mofied silica [adapted from JES01]



Aït-Kadi et al. [AIT01] grafted isocyanate groups on the surface of fumed silica and incorporated

it into polyurethane compounds. They found out that, below Tg, there is almost no benefit from

the surface modification on the visco-elastic modulus. The enhancement appeared above Tg but

only in case of a good dispersion of the fillers. On the tensile properties, the Young modulus

was outstandingly increased by up to a factor 20 with an addition of 22 wt. % of modified silica

particles but the elongation at break dropped then critically compared to the systems filled with

unmodified silica (Table I-1). Here, the traditional trend toughness vs. stiffness is observed.

Table I-11 Tensile properties of polyurethane / fumed silica composites, with various content of silica presenting modified surface using HTPB: hydroxyl terminated polybutadiene, or BA: bisphenol A [AIT01]

I.4.3.c. A few examples of filler addition in rubbery epoxy networks

In case of rubbery epoxy matrices, the only results found dealt with the influence of layered

nano-fillers [LAN94, LEP02], in situ formed silica hybrids, and polyhedral oligomeric

silsesquioxane (POSS). We chose not to quote in details these last examples because the

structures and chemistries involved are too different from those studied here. It is just note

worthwhile that an increase in rubbery modulus was shown by up to more that one order of

magnitude for these hybrids materials, strongly related to the processing route (one or two

stages or sequential). POSS also led to a large increase of rubbery modulus depending on the

functionality of the POSS that could act as additional crosslinker in the system. A widening of

the glass transition temperature towards higher values was observed for hybrids and POSS-

modified epoxy systems.

Lan et al. [LAN94] studied rubbery epoxy-amine systems filled with clays exchanged with

alkylammonium ions. The profit brought by the exfoliation on the modulus was evidenced by

using various lengths of modifying ions of the montmorillonites incorporated into the network.

The Young modulus and tensile strength were increased by up to a factor 10 by incorporating

15 wt. % of clay.

They supposed that in the case of the rubbery matrices, the particles can align themselves,



under the influence of the deformation, in the direction of the solicitation and act as fibers on the

elastic properties (Figure I-43). This effect of “strain adaptation” is minimal in the glassy state

because the matrix deforms much less, it explains the lower impact of clays on the mechanical

properties in this state.

Figure I-43 Mechanisms of deformation of nanocomposites based on epoxy-amine matrix and clays

depending on the structure of the network: (A) glassy, or (B) rubbery [LAN94]

For the same system, Le Pluart et al. [LEP02] compared various modifying ions and contents of

fillers. With 10 parts of fillers, increases by up to 64 % of the modulus and by a factor 5 of the

fracture energy were achieved (Table I-12).

Matrix DGEBA/D2000 G (MPa) G/G0 σr (MPa) εr (%) Wr (kJ.m-3) Wr/Wr0 Unfilled 0.5 1 0.7 70 333 1

+ 5 pcr Tixogel 0.61 1.22 1,2 120 807 2,42 + 10 pcr Tixogel 0.82 1.64 2,0 160 1713 5,14 + 5 pcr OPTC18 0.65 1.30 1,3 120 942 2,83

Table I-12 Elastic and fracture properties for rubbery epoxy networks filled with montmorillonites, tixogel: commercial modified clay using a dimethyl benzyl tallow ammonium quaternary ion, OPTC18:

home exchanged clay using an octadecylamine, G: elastic modulus, G/G0: relative modulus, σr: stress at break, εr: strain at break, Wr: fracture energy, Wr/Wr0: relative fracture energy [adapted from LEP02]

I.4.4. Applicative properties of epoxy / silica formulations

Silica-filled epoxy networks (in general) can be developed to replace and improve epoxy in its

usual fields of application such as adhesives, coatings, high performance matrices for

composites. The aim of silica addition can be an enhancement of the mechanical properties but

it can also consist in the enhancement of the physical properties of epoxy matrices such as

thermal and / or dimensional stability, fire resistance, increase of the surface hardness, scratch

resistance, decrease of the gas permeation and of the solvent uptake.

In the following paragraphs, the different applicative properties found in the literature for the

A B



epoxy / silica systems are reviewed in order to illustrate furthermore the reasons for carrying out

the present study.

I.4.4.a. For adhesives

Tishin et al. [TIS93] investigated the effect of the addition of fumed silica, as well as other types

of fine powders, on the adhesive properties of an epoxy resin as a function of the silica specific

surface area and surface treatment. Increased adhesive and cohesive strengths were reported,

an optimal silica concentration was observed between 4 and 6 wt. % of silica. The optimum was

reached at lower silica content for a larger surface BET (Figure I-44 (a)). The main parameter

seemed to be the surface developed at the interface with the adhesive. For a given surface

BET, the efficiency of the silica on the adhesive strength appeared greater for an hydrophobic

surface (increase almost by a factor 2), than for an amino-modified surface and at last for an

hydrophilic surface (Figure I-44 (b)). It was explained by the formation of a boundary layer, with

properties different from that of the bulk, due to the interactions developed with silica surface

during the crosslinking. The characteristics of this layer depended on the polarity, nature of the

organo-modifiers and surface energy of the fillers.

The tensile strength of the system was measured in order to evaluate the influence of the fillers

on the cohesive properties of the adhesive. For a silica content greater than 4 to 6 wt. %, the

tensile strength became even lower than that of the neat matrix, the efficiency was once again

the lowest for unmodified silica.

Figure I-44 Dependence of the adhesive strength of filled epoxy on the silica content, (a) effect of silica surface area, 1: BET=175m²/g, 2: 300m²/g, (b) effect of silica surface modification (300m²/g), 9:

modification by diethyleneglycol, 10: aminoethoxy, 11: aminopropyl, 12: hydrophobic surface

Kinloch et al. [KIN03] investigated the effect of silica sol particles (20 nm diameter) on the

properties of structural adhesives based on an epoxy matrix toughened by in-situ formed

rubbery micro-inclusions. A substantial toughening was induced by the additional presence of



silica. The optimal silica content was found at 8 wt. % with an increase almost by a factor two of

the adhesive fracture energy, beyond this content the properties decreased again. Other

interesting properties for adhesives were also significantly increased such as the lap shear, or

the roller pear.

The rubbery particles are known to interact with the stress field ahead the crack tip, leading to

an enhanced plastic deformation. However the mechanisms of action of the nano-particles in

this context was not straightforward and still under study, enhanced crack deflection and crack

twisting around the rigid particles might take place. Unfortunately, the paper does not mention

clearly what the preferential surrounding of the nano-particles is: epoxy matrix, rubber

inclusions, or interface.

I.4.4.b. For coatings, surface properties

The most represented application of epoxy / silica formulations described in the literature is in

the field of coatings, in order to increase the surface hardness, the scratch resistance [ZHO02],

the mar resistance [LED02]... Indeed many applications need coatings exhibiting improved

abrasion or scratch resistance such as clear coatings for automotive industry, for which the

transparency can be preserved in spite of the presence of small particles.

As detailed in the part I.3.2.b, due to the easier processing and spreading step, most studies

concerning coatings use colloidal silica sol particles in order to limit the effect on the viscosity.

Additionally, Roscher et al. [ROS03, ADE01] found out that for the same silica content, the

surface performances of these acrylate-based coatings were higher in case of silica sol than in

case of fumed silica filled acrylate (Table I-13). This might be due to the smaller size of the silica

sol particles compared to fumed silica aggregates, in agreement with other references [XIN04,

HAR03]. The same trend was observed into polyurethane coatings: a larger content of fumed

silica is needed in order to reach the same level of scratch resistance as with silica sol [BAR05].

Table I-13 Scratch and abrasion resistance of neat acrylate (so called SR 494 in the table) and

acrylate filled with nano-silica sol particles or fumed silica (as thin films 50 µm) [ROS03]



Zhang et al. [ZHA02a, b] studied the tribological behavior of epoxy-based composites with silica

sol (9 nm diameter). They investigated the effect of silica surface modification by comparison

between an untreated silica and a polymethacrylamide-grafted silica supposed to react with the

matrix comonomers. The wear rate and the frictional coefficient were found to decrease

markedly for filled systems (i.e. improvement of the properties). The improvement was stronger

with the surface modified particles: the wear resistance was enhanced by a factor 20 with only 2

wt. % of PMMA-modified particles. Even with 40 wt. % of micro particle, the enhancement could

not be as important.

Xing et al. [XIN04] studied the effect of the size of silica sol particles on the wear behavior of

epoxy coatings (120 vs. 510 nm). The improvement of the wear behavior was more important

with the smaller particles. An optimal silica content of 2 wt. % was found for both sizes, with an

improvement by a factor up to 10. At greater silica content, the agglomeration was given as the

phenomenon affecting the wear behavior.

By inspection of the worn surface, it was shown, that the size of the wear waves was decreased

when silica particles were added, especially with the smaller ones. Additionally, no debonding

was observed at the interface with the epoxy matrix for the smaller particles contrary to the

larger particles. The mechanism proposed for the wear resistance enhancement was that the

crack propagation was hindered by the silica particles near the surface, thus the debris

formation was reduced (Figure I-45). The reason for the higher resistance for smaller particles

was the lower interparticle distance.

Figure I-45 Diagram representing the wear behavior of neat epoxy (on the left) compared to

epoxy-based network filled with silica (on the right) [XIN04]

Hartwig et al. [HAR03] reported the same trend for the abrasion resistance as a function of the

size of the particles: the Taber abrasion of an epoxy coating was reduced from 39 mg to 16 mg

with 25 wt. % of silica sol particles of 40 nm of diameter and down to 9 mg with particles of 12

nm of diameter.

Friedrich et al. [FRI05] investigated the concomitant addition of multi-scaled stiff fillers (i.e.

alumina nano-particles and SiC micro-particles), and observed a synergistic effect on the

properties. It allowed fulfilling the requirements for the development of wear resistant thick

coatings for calendar rollers (Figure I-46).



Figure I-46 Influence of concomitant addition of nano-scale (Al2O3) and micro-scale (SiC) fillers on the wear and mechanical properties of epoxy-based formulations [FRI05]

The non permeability to gas and moisture resistance can also be improved from silica addition.

These enhancements of barrier properties are interesting in the field of impermeable or

corrosion resistant coatings. Jana et al. [JAN01] shown that the solvent uptake of polyether

sulfone / epoxy blend was interestingly reduced from fumed silica addition. Sangermano et al.

[SAN05] found that the water uptake of epoxy coatings was decreased from 3.1 % to 0.5 %

thanks to 5 wt. % of fumed silica, in the same time the surface hardness was also enhanced.

I.4.4.c. For matrices of conventional composite materials

Studies about the addition of nano-particles into composites were mainly carried out in the field

of high performance composites, for aeronautic or automotive, in order to improve the

toughness and/or the thermal resistance of the materials [KOO05, MAH04, AND00, KIN05].

As previously presented, the nano-modified matrices filled with silica can reach improved bulk

mechanical properties (toughness, modulus, glass transition temperature…). First, the

investigations were concerned with: i) the fulfillment of the requirements for processing methods

for conventional (fiber-based) composites as filled reactive formulations used as nano-

reinforced matrices and ii) the transfer of the enhanced properties from the nano-reinforced

formulations to the final fiber-based composites structured at various scales.

Mahrholz et al. [MAH04] observed that the injection capability was conserved for epoxy resins

filled with 20 wt. % of silica sol (20 nm diameter), for the preparation of the glass-fiber reinforced

composites parts, thanks to the absence of thixotropic properties and moderated viscosity

increase in the temperature range of the injection (single line injection technique). Additionally,

no filtration effect, or heterogeneity of the silica repartition, was noticed. Besides, the catalytic

effect due to the nano-particles (cf. I.3.3) was not critical for the injection of filled epoxy resin for

the preparation of fiber-reinforced composites.



The improvement of the mechanical properties of nano-structured glass fiber reinforced

composites compared to glass fiber reinforced composites, both in the elastic and fracture

regions, was impressive for an addition of 20 wt. % of silica nano-particles (Figure I-47). The

improvement of properties of resins for injection technology was also due to decreased

shrinkage. Such formulations allowed competing with the properties obtained from the pre-preg

technology, this latter being much more expensive.

(a) (b)

Figure I-47 (a) Mechanical properties obtained for glass fiber reinforced composites unfilled and filled with nano-silica, (b) Corresponding stress-strain tensile test diagram [MAH04]

In contrast, Koo et al [KOO05] experienced a lack of carry over of the enhancement of the

properties obtained for the nano-reinforced epoxy to the nano-modified carbon fiber reinforced

composite. For example, the glass transition temperature and the storage modulus were not

increased in the final materials as they were in the nano-modified resin. They also mentioned

that the dispersion state at the nano-level affected greatly the properties of the final material.

These materials were initially aimed at increasing the damage tolerance of high performance

carbon reinforced composites based on an epoxy matrix.

Kinloch et al. [KIN05] studied the effect of the combination of “classic” rubber modification

combined with silica nano-particles into epoxy matrices cured with anhydride in order to improve

the impact resistance of carbon fiber reinforced composites manufactured by Vacuum Assisted

Resin Transfer Molding (VARTM). A further increase of the properties of the rubber-modified

epoxy network was reached thanks to silica, it was described as a synergistic effect (Figure I-48

(a)). The resin with the CTBN adducts shown an increase of toughness by up to 300 %, but

combining the reactive liquid polymer and the nano-particles boosted the GIc by 1000 %. Best

performances were found for 9 % reactive liquid rubber and 9 % nano-silica. The rubber content

was limited to this range for viscosity matters. The silica particles also overcame at least

partially the decrease of the glass transition temperature and of the flexural modulus due to

rubber. The report on the properties of the final composites was really good: a much higher

toughness, impact resistance and modulus could be achieved (Figure I-48 (b)).



(a) (b)

Figure I-48 Fracture energy for various formulations: (a) Properties of different network compositions (with and without nano-silica particles and / or CTBN rubbery inclusions), (b) Properties of the carbon

fiber reinforced composites based on the same matrix compositions [KIN05]

The shrinkage of the thermosetting formulations during their crosslinking is known to be, at least

partially, responsible for their brittleness. For applications such as matrices of composites

materials, or coatings, additives reducing the shrinkage are necessary. Kinloch et al. [KIN05]

found that, with an addition level greater than 20 wt. % of silica sol nano-particles into epoxy-

based formulations, the shrinkage during cure was significantly reduced. This is especially

interesting for automotive applications.

Kang et al. [KAN01] shown that the coefficient of thermal expansion was reduced by silica

addition, and furthermore in case of good adhesion between the filler and matrix.

I.5. Methodology of the study

The last paragraph of this chapter is aimed at situating the material studied and the topic of this

work as a function of the examples previously reported from the literature, and at introducing the

strategy of the experimental part of this manuscript.

I.5.1. Summary of the state of the art and position of the material under study

From the data presented so far, in the glassy state, colloidal silica (in general) reinforced moderately the brittle thermosetting matrices. It can obviously not compete directly with



conventional micro fillers such as glass fibers, which can be added in much greater loadings

(typically around 50 %) and are particularly efficient due to their orientation. But colloidal silica

can also be used in addition to conventional fibers, to design systems structured at various

scales, and enhance furthermore the mechanical properties and/ or replace other additives.

From the examples previously quoted, the routes in which silica particles are used in

combination with other fillers, additives (CTBN particles, glass and carbon fibers, SiC micro-

particles…) seemed indeed quite promising, various authors reported a synergistic effect. The

feasibility of the utilization of epoxy reinforced with silica as matrix for composites was already

checked by various studies (so far only with “real” nano-silica).

Colloidal nano-silica in general appeared to exhibit the advantage of decreasing neither the

toughness nor the stiffness as most standard organic additives, or conventional fillers would do.

In addition, some studies mentioned a decrease of the shrinkage, and sometimes an increase

of the glass transition temperature.

Nevertheless, silica / epoxy formulations can play an important role in order to fulfill additional requirements in the field of adhesives and coatings. More especially for coatings, surface

mechanical properties can be enhanced (hardness, scratch and wear resistance…) but also

functional properties (solvent and gas barrier, thermal resistance, aging…).

In the rubbery state, fumed silica appeared as an efficient filler even for mechanical

reinforcement of thermosetting elastomers (no example found for silica / epoxy rubbery matrix),

in the same way as it is traditionally used as one of the main filler for the reinforcement of

silicone rubbers. Besides, it appeared that interfacial adhesion was rather important in the

rubbery state.

A significant mechanical reinforcement is generally believed to be reachable only for a

content of stiff fillers greater than 10 - 15 wt. %, contents difficultly reachable with sub-micro

fillers. However, most theories were initially developed for the micronic scale, and we are not

convinced that these mechanical models can be applied directly at the nano-scale.

Unfortunately, most studies realized in the sub micro-scale, as the ones quoted in this chapter,

are rather phenomenological, only few theories were developed so far. The literature survey

shown that, for many properties (scratch resistance, toughness…), an optimal silica content existed, generally rather low (typically in the range 5 wt. %). It sometimes resulted from the

combined effects of two parameters oppositely related to silica content, and the dispersion state

was often reported as not satisfying above this filler content, which could most likely be the

reason for the decrease of properties afterwards. Indeed, as we expected, dispersion state was evidenced a as key parameter for the properties by most authors, and filler dispersion was

reported as a difficult processing step.



One of the key questions in this study is: “which are the most suitable structures / sizes for the reinforcement of glassy thermosetting networks such as epoxy? The literature review

gave examples about silica filled epoxy using different structures, shapes, and sizes of silica

phase (from sol particles of a few nanometer diameter to micronic silica beads, fumed silica…).

The optimal structure certainly depends on the property and application targeted, but

considering the large number of influencing parameters to take into account in the studies

reported (interactions, processing, tests carried out, nature of the epoxy network, content and

properties of the silica…), the general trends are rather difficult to draw back.

Hopefully, this study can bring about some additional answers to help elucidate the

reinforcement mechanisms involved in the behavior of these materials.

Fumed silica vs. nano-silica particles? It was already shown that for applications in which the increase of viscosity might be critical

such as coatings, and maybe composites (depending on the processing route), if a high silica

loading is desired, individualized silica nano-particles might appear preferable to fumed silica, at

least for processability matters due to the lower impact on the viscosity. For properties in which

the percolation threshold has to be reached, fumed silica would be preferable.

Colloidal nano-silica generally exhibit drawbacks due their higher price and the necessity to

handle them in a solvent in order to prevent their irreversible agglomeration. This can appear

repellant for some environmentally friendly concerns and induces the necessity of a further step

of solvent removal from the final compound that can be difficult and generates an extra-cost.

Additionally, it is worthy noting that the cost of the reinforcement of epoxy resins using fumed

silica is moderated compared to most other sub-micro fillers and seems affordable in an

industrial context. Indeed, the price of fumed silica (~ 4 - 10 € per kg) is in the range of that of

epoxy resins, even when an additional surface treatment is needed, it is by far lower than the

price of other sub micronic fillers such as carbon nano-tubes that can still frighten the industry

(from 20 to 350 € per g)! Besides, fumed silica is a well-known and not toxic filler with a large

commercial availability.

I.5.2. Organization of the study

The synopsis in the Figure I-49 summarizes the strategy followed in this work: the organization

and the main tasks to achieve and the different variable parameters involved.

The literature survey highlighted the large number of factors characterizing the filler that are

involved in the behavior of sub-micro structured composites such as size (or surface BET),



content, adhesion, dispersion. We will try to separate their respective effect by varying the

formulation and process.

Objectives•Understanding of the structure / properties relationships and reinforcement mechanisms•Optimization of epoxy / silica formulations

Formualtion

REACTIVE SYSTEM- Glass transition temperature Tgnetwork structure (glassy, rubbery)

% of fillers

SILICA

Surface modification- Unreactive (hydrophilic or hydrophobic)- Reactive (epoxy, amino-modified…)

Specific surface area

Characterization of the final material•Morphological analysis at multiscale•Mechanical and thermo-mechanical behaviors•Various properties: thermal stability, fire retardancy, residual stresses…•Study of the interactions at the matrix / filler interface

Processing•Dispersion process / Rheology of the silica suspensions •Crosslinking of the networks / Kinetics of epoxy-amine reaction

Stoechiometric ratio: r=a/eCure schedule: time, temperature

Epoxy- Nature- Functionality-Molar mass

Amine- Nature (aliphatic, aromatic)- Functionality- Molar mass

Figure I-49 Synopsis of the study carried out

The first task of this work consisted hence in the selection of the different components of the

system: i) epoxy-amine comonomers fulfilling the following requirements: they have to be

relevant for analogies with commercial formulations, more or less polar to generate different

types of interactions with silica, and exhibit various states (rubbery vs. glassy network)

depending on the molar mass and functionality of the comonomers chose, ii) fumed silica, with

appropriate surface modifications to generate different types of interactions (covalent bonding

vs. physical interactions) with the matrix, various silica specific surface area and content on the

properties of the filled system were also studied. The stoechiometric ratio and cure schedule

were not varied in order to generate fully crosslinked networks.

The second task consisted in the processing of the materials using an optimized procedure

for each formulation. Rheological behavior of silica suspensions was studied, then the

crosslinking was carried and the kinetics were studied by chemio-rheological study.

The third task consisted in the characterization of the final materials, the mechanical and

thermo-mechanic dynamical properties, as well as that of different types of thermal properties

for applicative objective.



Eventually, the behavior of these systems should be well understood, and optimal silica

(specific surface area, surface chemistry) should be proposed for the reinforcement of epoxy-

amine networks (depending on the initial formulation: glassy, rubbery, polarity…).

Two recurrent questions appeared in this study: i) What is the driving force governing the

morphologies and do they evolve during the polymerization? ii) What is the nature of the

interactions in the system and more especially, when possible, do the filler covalently react with

the epoxy-amine network?

The characterization of the morphology had to be accurate and cover a large scale due to

the presence of various scales of organization in the medium (agglomerate, aggregates). It

required the use of complementary techniques, also because the physical state of the materials

studied changes all along the process from liquid initially (rheology, optical microscopy) to solid

finally (transmission electron microscopy, image analysis, small angle neutron scattering)

(Figure I-50).

Additionally, a competition between the structuration of the fillers and the crosslinking of the

matrix might be imagined, so real time experiments are needed to elucidate this possible

evolution (confocal microscopy and small angle neutron scattering). In case this competition

occurs because it can limit the evolution and thus determine the final morphology.

Rheological analysis Grindometer

Confocal microscopy

Neutron scattering

Electronic microscopy, image analysis

Optical microscopy

Neutron scattering

Before crosslinking

During crosslinking

After crosslinking

1 µm100 nm 10 µm 100 µm10 nm

Silica

primary

pa

rticle Silic

a ag

grega

te

Silica

agglo

merate

Size

Process time

Rheological analysis Grindometer

Confocal microscopy

Neutron scattering

Electronic microscopy, image analysis

Optical microscopy

Neutron scattering

Before crosslinking

During crosslinking

After crosslinking

1 µm100 nm 10 µm 100 µm10 nm

Silica

primary

pa

rticle Silic

a ag

grega

te

Silica

agglo

merate

Size

Process time Figure I-50 Scales observed by the complementary techniques used for the characterization of the morphologies of epoxy / silica composites as a function of the processing time



The interactions developed in between the silica and epoxy matrix and their evolution were

studied depending on the silica surface chemistry, by different direct and indirect methods if

physical interactions or covalent bonding were expected and depending on the stage in which

this feature was investigated (material could be liquid, solid, extracted silica). For each type of

silica, information about the interactions were brought by rheology, kinetics and morphology.

Additionally for reactive silica, the potential covalent bond with the matrix was studied using

direct analysis (elemental analysis, IR spectroscopy, NMR…). Beneath, the role of the

interfacial adhesion for the mechanical properties could be discussed.

A question arising from the literature study and of which answer should come from this study is:

what is the behavior of fumed silica filled epoxy networks? As already underlined, after the

branched structure of fumed silica made of space filling fractal aggregates formed of spherical

nano-particles, it can be regarded as an intermediate between nano- and micro-fillers combining

the two scale lengths of organization.

Hence, we can wonder if the mechanical properties of polymers after fumed silica addition

follow the typical stiffness - toughness trend observed for conventional additives and micronic

filler, i.e. decrease of the impact strength due to the introduction of these stiff particles, or if it

results in a shift of both toughness and stiffness towards higher values and described previously

as the “nano-effect” as illustrated in the Figure I-51.

Figure I-51 Traditional trend toughness vs. stiffness of polymer based materials

StiffnessNormalized modulus

Epoxy matrix

Epoxy matrix filledwith fumed silica ?

with rubber tougheners

with stiff

ToughnessNormalized impact energy

with rubber tougheners

with stiff fillers

i. materials background, literature and methodology

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