The Effects of Silica Nanoparticles in Toughened Epoxy Resins and Fiber-Reinforced Composites (Print-on-Demand)

180 Pages·2015·15.857 MB·German

by  Sprenger, Stephan

Sprenger
The Effects of Silica Nanoparticles in Toughened
Epoxy Resins and Fiber-Reinforced Composites
Stephan Sprenger
The Effects of Silica
Nanoparticles in Toughened
Epoxy Resins and Fiber-
Reinforced Composites
Hanser Publishers, Munich Hanser Publications, Cincinnati
The Author:
Dr. Dr. Stephan Sprenger, Evonik Hanse GmbH, Charlottenburger Straße 9, 21502 Geesthacht, Germany
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Table of contents:

Short summary 6

Kurzzusammenfassung 7

1. Introduction 8

1.1. Motivation 8

1.1.1. References 8

1.2. Objectives and structure 9

2. State-of-the-art of the science and technology 10

2.1. Manufacturing technologies for fiber-reinforced composites 10

2.1.1. Prepreg methods 11

2.1.2. Filament winding 12

2.1.3. Pultrusion 13

2.1.4. Injection processes via a mold 13

2.1.5. Infusion processes via a vacuum bag 15

2.1.6. References 16

2.2. Fiber reinforcements for epoxy resin systems 17

2.2.1. Woven fabrics 17

2.2.2. Unidirectional fabrics 18

2.2.3. Multiaxial fabrics 19

2.2.4. Preforms 19

2.2.5. Fiber sizing 20

2.2.6. References 21

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2.3. Epoxy resin systems for fiber-reinforced composites 22

2.3.1. Epoxy resins 22

2.3.2. Curing agents for epoxy resins 24

2.3.2.1. Amine curing agents 24

2.3.2.2. Anhydride curing agents 24

2.3.3. Epoxy resin systems 26

2.3.4 References 27

2.4. Toughening of epoxy resins 28

2.4.1. Differentiation between toughening and increasing
flexibility 28

2.4.2. Reactive liquid rubbers 28

2.4.3. Core-shell rubber particles 30

2.4.4. Thermoplastic particles 31

2.4.5. Self-organizing block copolymers 32

2.4.6. References 33

2.5. Modification of epoxy resins with SiO nanoparticles 35
2

2.5.1. SiO nanoparticles 35
2

2.5.2. Property improvements of cured bulk resin
systems 38

2.5.3. Property improvements of cured bulk
hybrid resin systems 39

2.5.4. Property improvements of fiber-reinforced
composites 41

2.5.5. References 45

3. Results and Discussion 48

3.1. Property improvements of epoxy resins modified with SiO
2
nanoparticles 48

2
3.1.1. Abstract 48

3.1.2. Introduction 48

3.1.3. Discussion 51

3.1.3.1. Amine-cured epoxy resins 51

3.1.3.2. Anhydride-cured epoxy resins 56

3.1.3.3. Comparison between different types of hardeners 60

3.1.4. Conclusions 62

3.1.5. References 63

3.2. Property improvements of epoxy resins modified with SiO
2
nanoparticles and elastomers (hybrid systems) 66

3.2.1. Abstract 66

3.2.2. Introduction 66

3.2.3. Discussion 69

3.2.3.1. Epoxy resins modified with reactive liquid rubbers
(CTBNs) and silica nanoparticles, amine cured 69

3.2.3.2. Epoxy resins modified with styrene-butadiene rubber
(SBR) and silica nanoparticles, amine cured 72

3.2.3.3. Epoxy resins modified with reactive liquid rubbers
(CTBNs) and silica nanoparticles, anhydride cured 72

3.2.3.4. Epoxy resins modified with silica nanoparticles, cured
with amines and amino-functional reactive liquid
rubbers (ATBNs) 73

3.2.3.5. Epoxy resins modified with core-shell elastomers
(CSRs) and silica nanoparticles, amine cured 75

3.2.3.6. Epoxy resins modified with core-shell elastomers
(CSRs) and silica nanoparticles, anhydride cured 77

3.2.3.7. Short overview of improvements achieved 79

3.2.3.8. Synergy or no synergy? 80

3.2.4. Conclusions 82

3
3.2.5. References 83

3.3. Property improvements of fiber-reinforced composites based on
epoxy resins modified with SiO nanoparticles 86
2

3.3.1. Abstract 86

3.3.2. Introduction 86

3.3.3. Discussion 89

3.3.3.1. Glass fiber-reinforced epoxy resin composites 90

3.3.3.2. Carbon fiber-reinforced epoxy resin composites 94

3.3.3.3. Comparision between bulk and laminate property
improvements 98

3.3.3.4. Mechanisms of toughening 99

3.3.3.5. How much silica nanoparticles do you really
need – and where? 100

3.3.4. Conclusions 101

3.3.5. References 102

3.4. Property improvements of fiber-reinforced composites based on
epoxy resins modified with SiO nanoparticles and elastomers
2
(hybrid systems) 106

3.4.1. Abstract 106

3.4.2. Introduction 106

3.4.3. Discussion 110

3.4.3.1. Glass fiber-reinforced epoxy resin composites 111

3.4.3.2. Carbon fiber-reinforced epoxy resin composites 115

3.4.3.3. Mechanisms for property improvements of hybrid
epoxy resin systems 118

3.4.3.4. The transfer of improved bulk resin properties into
the fiber-reinforced composite 119

3.4.4. Conclusions 123

3.4.5. References 123
4
3.5. Carbon fiber-reinforced composites with epoxy resin modified with
reactive liquid rubber and SiO nanoparticles 128
2

3.5.1. Abstract 128

3.5.2. Introduction 128

3.5.3. Experimental 132

3.5.3.1. Materials 132

3.5.3.2. Bulk sample preparation 133

3.5.3.3. Laminate preparation 133

3.5.3.4. Bulk sample testing 133

3.5.3.5. Laminate testing 134

3.5.4. Results and discussion 135

3.5.4.1. Bulk resin properties 135

3.5.4.2. Laminate properties 139

3.5.4.3. Microscopical investigations 146

3.5.4.4. Formation of nanosilica agglomerates 149

3.5.5. Conclusions and outlook 153

3.5.6. References 154

4. Summary and Outlook 157

4.1. Summary and outlook 157

4.2. Zusammenfassung und Ausblick 159

5. Bibliography 163

5
Short Summary

The properties of cured epoxy resins can be improved significantly by the
addition of surface-modified SiO nanoparticles. A linear relationship between the
2
increase of modulus respectively the fracture toughness and the addition level of
nanosilica exists for most epoxy curing agents. Compressive strength and fatigue
performance can be improved as well.

Combining this modification with the classic toughening concept using reactive
liquid rubbers or core-shell elastomers leads to so-called hybrid systems, which
are characterized by both high toughness and high stiffness. The fatigue
performance is improved further.

Laminates manufactured by using these modified resins exhibit improved
performance as well. Regardless if glass or carbon fibers are used as reinforcing
material, the relative property improvements of the laminates are much smaller.
A linear relationship between the percentual increase in fracture toughness (G )
Ic
of the cured bulk resin systems and the percentual increase of the G of the
Ic
laminates made thereof seems to exist; with a conversion factor of 0.18. If the
fracture toughness of a fiber-reinforced part shall be increased by 100 %, then
instead of the resin used a hybrid resin with a 555 % higher bulk G needs to be
Ic
used. For the fatigue performance of laminates made from hybrid resins a tenfold
increase in cyclic loadings upon failure can be achieved.

In own trials a fast curing epoxy resins system based on DGEBA/IPD/TMD was
employed to manufacture carbon fiber reinforced laminates; whose properties
were investigated. The cured bulk resin systems exhibit the expected property
improvements for the hybrid systems. However, the laminates based on hybrid
resin systems, modified with both reactive liquid rubber and SiO nanoparticles,
2
show no further improvements compared to the rubber-only modification but
rather slightly lower values for G and G . The ILSS is comparable; the residual
Ic IIc
strength after impact reduced.

The agglomerates of silica nanoparticles, which were discovered, might be a
potential cause. They form during the fast cure in presence of reactive liquid
rubber; probably caused by the forced rapid phase separation of the rubber upon
cure. Future research will provide further insights.

In summary it can be stated that laminates made from hybrid epoxy resins are
tough and stiff and exhibit improved compressive strength as well as excellent
fatigue performance. This makes them especially suitable for highly stressed
composites parts like in automotive applications.
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Kurzzusammenfassung

Die Eigenschaften von gehärteten Epoxidharzen können durch den Zusatz von
oberflächenmodifizierten SiO -Nanopartikeln deutlich verbessert werden. Es
2
besteht für die meisten Härter ein annähernd linearer Zusammenhang zwischen
der Steigerung des Moduls bzw. der Bruchzähigkeit und der Zusatzmenge an
Nanopartikeln. Druckbeständigkeit und Ermüdungsverhalten können ebenfalls
verbessert werden.

Kombiniert man diese Modifikation mit der klassischen Schlagzähmodifizierung
unter Verwendung von reaktiven Flüssigkautschuken oder Core-Shell-
Elastomeren, so erhält man sogenannte Hybridsysteme, welche sich durch eine
besonders hohe Zähigkeit bei gleichzeitiger Steifigkeit auszeichnen. Auch das
Ermüdungsverhalten wird weiter verbessert.

Beim Einsatz dieser modifizierten Harze zur Herstellung von Laminaten können
an diesen ebenfalls Eigenschaftsverbesserungen beobachtet werden. Unabhängig
davon, ob Glas- oder Carbonfasern als Verstärkungsmaterial eingesetzt werden,
fallen diese Verbesserungen prozentual deutlich geringer aus. Es scheint eine
lineare Beziehung zwischen den prozentualen Steigerungen der Bruchzähigkeit
(G ) der gehärteten Reinharzhybridsysteme und den daraus hergestellten
Ic
Laminaten zu existieren; mit einem Übertragungsfaktor von 0,18. Soll ein
Faserverbundbauteil um 100 % in der Zähigkeit gesteigert werden, so muss statt
des bisher verwendeten Harzsystems ein Hybridsystem mit einem 555 %
höheren G eingesetzt werden. Beim Ermüdungsverhalten der Laminate auf
Ic
Hybridharzbasis kann eine zehnfache Steigerung der zyklischen Belastung bis
zum Versagen erzielt werden.

In den eigenen Versuchen wurden unter Verwendung eines schnellhärtenden
Epoxidharzsystems basierend auf DGEBA/IPD/TMD kohlefaserverstärkte
Laminate mittels RTM hergestellt und untersucht. Die gehärteten
Reinharzsysteme zeigen für die Hybride die erwarteten Eigenschafts-
verbesserungen. Die Laminate auf Basis Hybridharz, sowohl mit reaktivem
Flüssigkautschuk als auch mit SiO -Nanopartikeln modifiziert, weisen jedoch
2
keine weitere Verbesserung der Bruchzähigkeit gegenüber der reinen
Kautschukmodifikation auf, sondern eher geringfügig niedrigere Werte für G
Ic
und G . Die ILSS ist vergleichbar; die Restdruckfestigkeit nach Impact jedoch
IIc
verringert.

Eine mögliche Ursache sind die entdeckten Agglomerate der SiO -Nanopartikel.
2
Sie entstehen während der raschen Vernetzung in Gegenwart von reaktivem
Flüssigkautschuk; wahrscheinlich verursacht durch die erzwungene schnelle
Phasentrennung des Kautschuks während der Härtung. Zukünftige Forschungs-
arbeiten werden hier weitere Erkenntnisse bringen.

Zusammenfassend kann festgestellt werden, daß Laminate auf Basis von
Hybridharzen gleichzeitig zäh und steif sind und eine verbesserte
Druckbeständigkeit sowie ein hervorragendes Ermüdungsverhalten aufweisen.
Sie sind daher besonders für hochbelastete Bauteile wie beispielsweise in
automobilen Anwendungen geeignet.

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