N° d’ordre 2004ISAL0011 Année 2004
Thèse
PHOTOPOLYMERIZED MICRO- AND NANO-
COMPOSITES:
INTERFACE CHEMISTRY AND ITS ROLE ON
INTERFACIAL ADHESION
présentée devant L’Institut National des Sciences Appliquées de Lyon
pour obtenir
le grade de docteur
Ecole doctorale : MATERIAUX DE LYON
Spécialité : MATERIAUX POLYMERES ET COMPOSITES
par Francesca PEDITTO
Soutenue le 20 FEVRIER 2004 devant la Commission d’examen
Jury
PRIOLA Aldo Professeur Directeur GERARD Jean François Professeur Directeur PILATI Francesco Professeur Rapporteur COSTA Giovanna Directeur de Recherche CNR Rapporteur Laboratoire de recherche : LMM-IMP UMR CNRS 5627-INSA Lyon
i
RESUME
Introduction
La recherche et la production industrielle de matériaux composites à base de matrices
polymère ont augmenté rapidement dans les dernières décennies compte tenu des
caractéristiques pouvant être atteintes par ce type de matériaux en comparaison avec des
matériaux traditionnels.
Par ailleurs, parmi les procédés d’élaboration utilisés pour les polymères, la
technologie de polymérisation UV ou ‘UV-curing’ a connu un essor très rapide et
remplace des techniques traditionnelles de cuisson de systèmes réactifs grâce à la
vitesse importante associée au processus, le coût faible et le respect de l'environnement:
puisque celui permet de s’affranchir de la présence de solvants. Aussi, le développement
du procédé de photopolymérisation pour la production de matériaux composites paraît
prometteur.
Ce travail décrira alors la préparation de matériaux composites à matrice époxy
cycloaliphatique et nano-silice (notés nanocomposites) ou fibres de verre (notés
microcomposites) comme agents de renforcement. Des matériaux composites seront
alors élaborés en utilisant la photopolymérisation cationique. Dans une première partie,
une réaction de modification de surface des agents de renforcements inorganiques,
nanoparticules de silice ou fibres de verre, sera mise au point et l’analyse des
mécanismes de greffage sera alors développée. L’emploi des nanoparticules de silice et
de fibres de verre greffées sera l’objet de la préparation de matériaux composites à
matrice époxy photopolymérisable. L’influence sur la réaction de polymérisation aussi
bien que les propriétés des composites obtenues feront l’objet d’une attention
particulière dans une seconde partie du travail.
Dans l'Appendice, les descriptions des techniques expérimentales utilisées dans ce
travail sont rassemblées.
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Partie 1 : Photopolymérisation & Matériaux Composites.
Dans la première partie de ce manuscrit sont rassemblées les informations générales
sur les matériaux composites, la technologie de polymérisation UV et son emploi pour
la préparation de composites à matrices polymère.
CHAP. 1 : MATERIAUX COMPOSITES/INTERFACE
1.1 Caractéristiques et propriétés de matériaux composites1-3
Dans la quête continue pour les performances améliorées, les matériaux traditionnels
sont remplacées de plus en plus par les matériaux composites synthétiques faisant appel
à l’association d’une matrice polymère et d’agents de renforcement comme des charges
particulaires (nanométriques ou micrométriques) et des fibres de renfort comme les
fibres de verre ou de carbone.
Les matériaux composites sont constitués de phases chimiquement différentes sur
une échelle microscopique, séparée par une interface distincte. Le composant qui
constitue la phase continue et présent en plus grande quantité est appelé la matrice. Le
deuxième composant est connu sous le nom de la phase renforçante, ou renforcement,
puisque généralement les propriétés mécaniques de cette phase sont supérieures à celles
de la matrice.
Les paramètres géométriques liés à la phase renforçante (facteur de forme, surface
spécifique, etc.) sont essentiels pour déterminer les caractéristiques des matériaux
composites qui en seront issus.
1.2 Interface/interphase: structure et propriétés
Les propriétés de composites également sont contrôlées par les caractéristiques de
l’interface1,3,4, région à deux dimensions (interface) ou plutôt de l’interphase, zone à
trois dimensions (phase intermédiaire aux propriétés spécifiques). Une forte liaison à
l’interface ou via l’interphase entre la matrice et la fibre assure alors le transfert de
charge de la matrice (renforcement) et est une des conditions essentielles pour conduire
à des propriétés de renforcement associées à l’association d’une matrice polymère et
d’un composant de fort comportement élastique comme des fibres ou des charges
particulaires. La résistance à la fracture (choc ou impact, propagation de fissures),
iii
comme d’autres propriétés des matériaux composites (résistance à fatigue et durabilité
hygrothermique), sont également conditionnées par la nature de l’interface et les
interactions/liaisons qui y sont développées. Dans ce contexte, il est très important de
prendre en compte l’élaboration de l’interface ou la génération des interphases lors de la
mise en œuvre des matériaux composites (processing). Intervient alors le concept de
mouillabilité de la surface inorganique de la fibre ou plus généralement du renfort.
Celui-ci définit l’aptitude avec laquelle un liquide s'étendra sur cette surface solide : une
"mouillabilité parfaite" signifiera alors que le liquide (dans ce cas la matrice à l’état
fondu –thermoplastique- ou sous forme d’une système réactif, mélange de monomères)
interagira fortement avec la surface. Si ce critère est respecté lors de l’élaboration est
observé, des liaisons de type physique (van der Waals) ou covalentes pourront alors
s’établir.
Dans le cas des renforts de verre ou de silice, des organosilanes sont utilisés comme
intermédiaires couplants entre les groupements de surface de la surface inorganique et la
matrice polymère en formant des liaisons fortes (liaisons covalentes)5,6, ou ponts
siloxane. Cette réactivité avec la surface inorganique et la matrice polymère par liaisons
covalentes conduit à un continuum moléculaire à l'interface renfort/matrice polymère
mais aussi à la génération d’une interphase à morphologie complexe (zone de nature
organique/inorganique).
CHAP. 2 : PHOTOPOLYMERISATION
2.1 Généralités relatives à la photopolymérisation1-5
La polymérisation UV est défini comme:
TRANSFORMATION RAPIDE DE 100% LIQUIDES REACTIFS SPECIALEMENT
FORMULES, EN SOLIDES PAR L’ACTION DES PHOTONS UV.
Les photons produits par la radiation UV sont absorbés par le site chromophore d'une
molécule; cette molécule produit alors l’ « espèce active » (radicaux ou protons),
conduisant en une transformation rapide (gamme de temps 10-2-1 s) du liquide en
solide.
Une formulation pour polymérisation UV comprend trois composants de base :
1. le photoamorceur, qui absorbe la radiation incidente et produite l'espèce réactive ;
iv
2. l’oligomère fonctionnalisé, structure de base du futur réseau du polymère ;
3. un monomère mono- ou multifonctionnel qui est alors un diluant réactif et sera
incorporé dans le réseau.
Les principaux domaines d’applications industriels dans lesquels la technologie UV
est employée sont: les arts graphique et coatings, les adhésifs, l’électronique, la stéréo-
lithographie, les matériaux composites ou ciments utilisés des applications dentaires.
2.2 Photopolymérisation radicalaire1,2,4
Le mécanisme de la polymérisation radicalaire est représenté dans la Fig. 2.1:
Fig. 2.1: Mécanisme de polymérisation radicalaire.
R est l’espèce active produite par photodécomposition de l'amorceur.
Les classes principales de systèmes réactifs qui peuvent être polymérisés en
polymérisation radicalaire sont: les monomères acrylate et méthacrylate, les systèmes
thiol-ène et les résines polyester insaturées.
2.3 Photopolymérisation cationique6-9
Le mécanisme de la polymérisation cationique est représenté dans la Fig. 2.2:
Fig. 2.2: Mécanisme de polymérisation cationique.
H+ est l’espèce active produite par photodécomposition de l'amorceur.
Les classes les plus intéressantes pour la photopolymérisation cationique sont les
vinyles éthers et les époxydes multifonctionnels puisque très réactifs et communément
disponibles.
R CH2 CHR' R CH2 CHR'
H+ CH2 CHR CH3 CHR+
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CHAP. 3: PHOTOPOLYMERIZATION ET MATERIAUX COMPOSITES
L'usage de la technologie UV pour mettre en œuvre des matériaux composites n'a pas
été étudié largement puisque la technologie communément employée est la
polymérisation thermique.
Les limites principales4 de la polymérisation UV dans le cas des composites sont :
l'épaisseur des pièces contrairement aux applications pour revêtements.
la transparence du renfort à la radiation UV.
l'influence des traitements de surface comme l’ensimage du renfort qui peuvent
de part les espèces présentes intervenir sur les mécanismes de polymérisation
les propriétés mécaniques limitées obtenues4.
En prenant en compte ces limitations de la technique UV dans la l’élaboration de
matériaux composites, les solutions suivantes sont proposées :
La technologie UV peut être utilisée dans la préparation de composites pour les
applications pour lesquelles les performances notamment mécaniques recherchées sont
peu importantes.
La technologie UV peut être utilisée pour la réparation de structures composites.
Une cuisson thermique peut être associée après polymérisation UV pour
compléter la polymérisation dans les échantillons de forte épaisseur.
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Partie 2 : Photopolymérisation & Micro-/Nano- Composites
La deuxième partie de ce manuscrit est consacrée à la présentation et discussion des
données expérimentales obtenues pour des matériaux à renfort fibre de verre unitaire
(microcomposites) et nanoparticules de silice (nanocomposites).
CHAP. 1: MATERIAUX ETUDIES
1.1 Nano-silice pyrogénée 1-3
La nano-silice est constituée par la silice amorphe SiO2 pur dans forme de particules
ayant haute surface spécifique (250 m2/g). L’Aerosil® 200 (fournie par Degussa) est
une silice de pyrogénation obtenue dans un processus hydrothermique à partir du
tétrachlorure du silicium SiCl4 dans une flamme d’oxygène et hydrogène à 1200-
1600°C.
1.2 Fibres de verre4-6
La fibre de verre retenue pour ce travail est une fibre de type "E" (électrique), type
communément utilisé pour des applications de matériaux composites structuraux. Ils
sont basés sur le système CaO-Al2O3-SiO2. Le diamètre de ces fibres est de 18 µm.
1.3 Organosilanes7,8
Les silanes (Fig. 1.1) sont molécules hybrides organiques-inorganiques qui à
l'interface entre la surface minérale de verre et un polymère permettent d’établir des
liaisons covalentes.
X
XX Si R Y
X = hydrolysable functional group (CH3O )
R Y = functional group which can react with the matrix
Fig. 1.1: Structure générale des organosilanes.
Les groupes hydrolysables X conduiront à la formation de silanols pour réagir à la
surface minérale avec les groupements silanol du verre. Les groupements organo-
fonctionnels R-Y sont choisis pour leur réactivité ou compatibilité avec le polymère.
vii
Les organosilanes sont appliqués aux surfaces inorganiques à partir de solutions
aqueuses ou hydro-alcooliques. Leur hydrolyse dans l'eau dépend de la nature du
groupement R-Y9. Néanmoins, cette réaction d’hydrolyse est rapide et peut être
considéré complète en 1-30 minutes (à pH acide de 3-4) Les silanols de l’organosilane
se condensent pour former des oligomères par réaction beaucoup plus lente (heures) et
dépendant de la température (100-110°C). Dans le cas idéal, une monocouche peut être
obtenue sur la surface du minéral par condensation silanol de surface-silanol des
espèces hydrolysées. Expérimentalement, une structure de plusieurs couches non
complètement condensées est obtenue en surface. La réaction de greffage est
schématisée dans la Fig. 1.2.
X Si
X
X
R YH2O
Si
OH
OH
HO R Y
Hydrolysis
Condensation
SiOH
SiOH
SiOH Si
OH
OH
HO R Y
Si OSi O Si R Y
Si O
3 H2Oinorganicsurface
inorganicsurface
Fig. 1.2: Hydrolyse et condensation d’une molécule d’organo-silane sur une surface
inorganique.
Plusieurs types d’organosilanes ont été utilisés dans ce travail:
Epoxycyclohexyl-éthyle triméthoxy silane (noté CETS, fourni par WITCO) pour
modifier les surfaces inorganiques de silice ou de verre E pour les composites à matrice
à base de monomère CE, diépoxycycloaliphatique.
Glycidoxypropyl triméthoxy silane (noté GPTS, fourni par Aldrich) pour
modifier les surfaces inorganiques de silice ou de verre E pour les composites à matrice
à base de monomère DGE.
viii
Triméthoxysilyl Propyl-méthacrylate (noté MÉMO, fourni par Aldrich) pour
modifier les surfaces inorganiques de silice ou de verre E pour les composites à matrice
à base d’oligomère SOA.
n-propyl triméthoxysilane (noté C3, fourni par Petrarch System Inc.) a été utilisé
pour modifier des surfaces inorganiques de verre E ou de silice, par greffage de ligands
hydrophobes non réactifs.
1.4 Systèmes thermodurcissables (photopolymérisables)
Divers de types de monomères ou d’oligomères fonctionnels ont été retenus pour ce
travail afin de les combiner avec les surfaces de silice ou de verre E fonctionnalisées par
les organosilanes fonctionnels précédemment présentés.
3,4-époxycyclohexyl méthyl-3',4'-époxycyclohexane carboxylate (noté CE,
fourni par DOW Corp.).
1,4-cyclohexane diméthanol diglycidyl éther (noté DGE, fourni par Aldrich).
Huile soja époxydée et acrylate (notée SOA, fourni par Aldrich).
1.5 Photoamorceurs
Compte tenu des différents monomères retenus pour cette étude, deux types de
photoamorceurs ont été utilisés:
Triphénylsulphonium hexafluoroantimonate (fourni par DOW Corp.) a été
utilisé comme photoamorceur cationique pour polymériser les monomères CE et DGE.
SOA a été polymérise par mécanisme radicalaire, en utilisant le 2-hydroxy-2-
méthyl-1-phényl-propane-1-one (fourni par Ciba Specialty Chem.) comme
photoamorceur radicalaire.
CHAP. 2: MODIFICATION DE SURFACES INORGANIQUES PAR
ORGANOSILANES
2.1 Introduction
La modification de surfaces inorganiques de silice ou de verre E conduira à une
meilleure adhésion interfaciale et ainsi des propriétés améliorées aux matériaux
ix
composites. Il autorise former des liaisons entre les deux phases qui ont une structure
différente et augmenter la compatibilité des systèmes.
2.2 Protocole expérimental adopté pour les surfaces de nanosilice et des fibres
de verre E
La modification de surface inorganique a été effectuée sur silice sous forme de
poudre et sur les fibres de verre E non ensimées.
La procédure de greffage adoptée pour nos systèmes peut être résumée comme suit:
Matériaux:
Nano-silice = 2g
Silane, CETS ou GPTS (pour 2g de silice) = 1mL
Solvant de réaction, eau distillée = 100 mL
Détails expérimentaux:
pH = 4 (CH3COOH)
T = température ambiante, 25°C
Temps = 2 heures
Agitation par ultrasons pendant 10 min
Addition de la silice et agitation US pendant 2 heures
Filtration
Réaction de condensation dans un four à 120°C pour 4 hrs
Lavage avec eau distillée
Séchage à 120°C pendant 2 heures.
Des analyses par analyse thermogravimétrique (ATG) ont été faites pour caractériser
la surface de la silice modifiée. Des expériences de répétabilité ont été faites pour la
silice greffée avec l’organosilane CETS mettent en évidence une très bonne
reproductibilité de l'expérience de greffage (Fig. 2.1).
x
Fig. 2.1: Courbes ATG de silices greffées avec le CETS (1% v/v).
La modification de la surface a été détectée en mesurant les angles de contact (i/ sur
des surfaces de wafers de silicium oxydé et traitée/non traitée, utilisées comme modèles
pour la silice nanométrique, ii/ fibres de verre et iii/ plaques en verre ‘float’) avant et
après traitement de la surface. Les résultats (Tab. 2.1) mettent clairement en évidence la
modification de surface par greffage.
Tab. 2.1: Angles de contact (avancée et retrait –en degrés-) avec eau obtenus sur les
fibres de verre E et plaques de verre float.
ϑADV ϑREC
fibre de verre non traitée 35 ± 10 28.8± 10
fibre de verre traitée CETS 85.5 ± 12 57.7 ± 12
fibre de verre traitée GPTS 83.1± 5 56.9 ± 5
Plaque de verre float non traité 29.5 ± 2 -
Plaque de verre float traité CETS 95.2 ± 1 67.1± 1
Plaque de verre float traité GPTS 79.9 ± 3 52.9 ± 3
Plaque de verre float traité C3 95.4 ± 1 65.2 ± 1
96
97
98
99
100
0 200 400 600 800 1000
Temperature (°C)
%
xi
TOPOGRAPHIE
Des observations par Microscopie à Force Atomique AFM ont été exécutées sur les
surfaces de wafer de silicium oxydé et traitée/non traité et sur la surface des fibres de
verre E traitées/non traitées. Sur les surfaces inorganiques sont clairement visibles (Fig.
2.3 et Fig. 2.5) les agglomérats décrits dans littérature6 comme "îlots de silane", associés
à la réaction de greffage et à une modification non homogène de la surface. D’autres
évidences expérimentales de changements dans la morphologie de la surface, après
réaction de greffage, sont mises en évidence par analyses en microscopie électronique
MEB sur les fibres de verre.
Fig. 2.3 : Image AFM de la surface d’un wafer de silicium oxydé, non traité
(référence)
Fig. 2.5 : Image AFM de la surface d’un wafer de silicium oxydé et traité avec
l’organosilane CETS 1% v/v.
xii
CHAP. 3: POLYMERISATION UV EN PRÉSENCE DE NANOSILICE OU
FIBRES.
3.1 Introduction
L'influence de nanosilice (développant une grande surface spécifique et donc une
grande quantité d’intreface) et de fibres de verre sur la polymérisation UV, en termes de
cinétique de polymérisation et de conversions finales, a été étudiée.
3.2 Partie expérimentale
Les résultats rapportés indiquent que la silice interagit en surface avec le
photoamorceur cationique (Tab. 3.1).
Tab. 3.1 : Absorbance UV sur les systèmes photoamorceur/nanosilice.
* Ph3S+SbF6-, 5.56 10-5 M dans propylène carbonate.
Le photoamorceur adsorbé pourrait donc avoir une activité inférieure sous irradiation
UV. Dans ce cadre, nous pouvons expliquer les ralentissements observés de la cinétique
du photopolymérisation du système réactif CE en présence de nanosilice (Fig. 3.1,
Cinétiques par spectroscopie FT-IR et Fig. 3.2 Enregistrements par photo-calorimétrie
DSC).
Specimen Abs310 nm
photoamorceur * 0.713
photoamorceur * ajouté de 5% w/w de silice non traité ≈ 0
photoamorceur * ajouté de 5% w/w de silice traité (CETS) ≈ 0
photoamorceur * ajouté de 5% w/w de silice traité (GPTS) ≈ 0
xiii
0102030405060708090
0 20 40 60 80 100time (sec.)
% c
onv.
CE
CE + 10% wttreated silica
CE + 10% wtuntreatedsilica
Fig. 3.1: Cinétiques de polymérisation obtenues par spectroscopie FT-IR pour le
système réactif CE en présence silice traitée/non traitée par l’organosilane CETS
(I = 51 mW/cm2).
150170190210230250270290310
0 5 10 15 20
% silica
-del
taH
(J/g
)
CE + untreated silicaCE + treated silica
Fig. 3.2: Enthalpie de réaction ∆H de la reaction de photopolymérisation en fonction
de la fraction massique de nanosilice traitée CETS et non traitée dans le système réactif
CE.
xiv
CHAP. 4: MICRO-COMPOSITES ET NANO-COMPOSITES
PHOTOPOLYMERISES
Les nano-composites ont été étudiés par analyse thermomécanique dynamique
(DMTA)1 compte tenu de la grande sensibilité de cette méthode dans la zone de
transition principale (vitreuse).
Les résultats expérimentaux DMTA (Tab. 4.1) mettent en évidence une diminution
de la température de transition principale, Tα , associée à Tg quand la silice est présente
Ces résultats sont en accord avec les données cinétiques déjà rapportées, qui montrent
une réduction de la conversion des groupements époxy et donc de la densité de
réticulation de la matrice en présence de silice.
Tab. 4.1: Tα des systèmes photopolymérisés en présence de nanosilice.
Les microcomposites, constitués à base d’une fibre unique sur laquelle a été formée
une microgoutte de matrice, ont alors été étudiés en utilisant la technique2-5 de la
microgoutte.
Il est démontré (Fig. 4.1) qu'un post-traitement thermique des microcomposites
augmente l'adhésion interfaciale quand les fibres de verre utilisées sont traitées avec une
organosilane fonctionnel. Ce résultat suggère que le cycle thermique auquel est traité
l'échantillon est responsable de réactions (pontages)9,10 à l’interface verre-polymère
matrice.
Echantillon Tα matrice Tα matrice + 10% silice non traitée
CE 214°C 182°C
DGE 53°C 37°C
xv
1,5
2
2,5
3
3,5
4
4,5
5
0 0,1 0,2 0,3 0,4 0,5% CETS
IFSS
t=0
t=4 (80°C)
t=4 (80°C+95%RH)t=4 (80°C)+(80°C+95%RH)
Fig. 4.1: Contrainte de cisaillement interfacial moyenne (IFSS) mesurée sur
microcomposites matrice CE /fibre de verre E traitée CETS après 4 jours de différents
types de vieillissement thermique et hydrothermique.
Des mesures d’adhérence (méthode « cross-cut » ASTM D3359) ont été exécutées
sur les plaques en verre float traitées par les organosilanes/non traitées, utilisées comme
modèles de systèmes des fibres de verre. Les résultats, bien qu'ils soient exécutés en
conditions très différentes, sont en accord avec ceux obtenus en utilisant la méthode de
déchaussement de la microgoutte.
CHAP. 5: CONCLUSIONS
Dans ce travail, la préparation de composites à matrice polymère par
photopolymérisation cationique a été étudiée ainsi que les propriétés des matériaux
obtenus.
La réaction de greffage pour modifier les propriétés de la surface des renforts
inorganiques (nanosilice et fibres de verre E) a été mise au point et optimisée afin
d’améliorer les interactions développées à l’interface avec la matrice polymère. Son
efficacité a été confirmée par analyses gravimétriques ATG et évaluation des propriétés
de surface (énergie de surface).
L'influence des agents de couplage et des espèces greffées en surface de renfort sur la
réaction de polymérisation UV a été analysée en évaluant la cinétique et les conversions
xvi
finales. Il a été mis en évidence des interactions entre la nanosilice et les espèces actives
photopolymérisables pendant la réaction UV de part la grande surface développée par ce
type de nanocharge.
Les caractéristiques des zones interfaciales ont été étudiées en utilisant des mesures
d’adhésion conduites avec la technique de la microgoutte (déchaussement). Les résultats
expérimentaux obtenus montrent la présence d'une corrélation entre adhésion et
épaisseur de l'interphase. Comme conséquence du traitement appliqué, l'adhésion entre
les deux phases conduit à des propriétés mécaniques améliorées et constantes dans des
conditions de vieillissement hydrolytique.
Des perspectives à cette recherche s’ouvrent alors:
Approfondir l'étude des interactions entre les espèces actives en
photopolymérisation et la surface inorganique pour les contrôler ou réduire le
ralentissement de la cinétique de réaction radicalaire.
Les résultats des mesures d’adhésion/adhérence suggèrent qu’étudier les
réactions aux interfaces (dans l'interphase) quand un traitement thermique est appliqué à
la suite de la polymérisation UV afin de mieux comprendre l’implication de l'interphase
et optimiser la procédure de polymérisation.
PhD thesis in Material Science and Technology
Francesca Peditto
PHOTOPOLYMERIZED MICRO- AND
NANO-COMPOSITES:
INTERFACE CHEMISTRY AND ITS ROLE
ON INTERFACIAL ADHESION
Prof. Aldo Priola Politecnico di Torino
Prof. Jean François Gerard INSA, Lyon
«La science est infaillible; mais les savants se trompent toujours.»
Anatole France
Ringraziamenti/Remerciements
Desidero ringraziare il Prof. Aldo Priola e tutto il Gruppo Polimeri del Politecnico di
Torino per avermi dato l’opportunità di intraprendere il cammino del dottorato di
ricerca; ringrazio inoltre tutte le persone che ho conosciuto in questi tre anni e che mi
sono state vicine nei momenti di lavoro e in quelli di svago.
Je veux remercier Monsieur le Professeur Jean François Gérard et les autres
permanents du laboratoire des Matériaux Macromoléculaires de l’INSA de Lyon pour
m’avoir accueillie dans son laboratoire, encouragée et conseillée chaque jour de ma
présence à Lyon; les thésards (chaque un de vous!), les techniciens (Nat et Hervé) et les
secrétaires (Isa et Mallou) que j’ai connu et avec lesquelles j’ai partagé boulot et
divertissement.
J’exprime ma profonde reconnaissance à le Dr. Alain Roche, qui m’a suivi avec
patience pendant tous mon travail et surtout qui m’a appris comme travailler avec
rigueur et précision : ses enseignements resteront avec moi n’importe quel chemin je
vais poursuivre.
Merci à tout le monde, l’expérience que j’ai fait chez vous restera toujours avec moi!
Questa tesi è dedicata al protochimico.
Index
Photopolymerized micro- and nano-composites:
interface chemistry and its role on interfacial adhesion
Introduction
Part 1- Photopolymerization & composite materials – Introduction
Chap. 1 COMPOSITE MATERIALS – INTERFACE…………….………….......1
1.1 “Composite material”: characteristics and properties……………..…………….1
1.2 “Interface/interphase”: structure and properties………………………………….8
Chap. 2 PHOTOPOLYMERIZATION.…………………………………………...16
2.1 Description of photopolymerization; applications………………………………16
2.2 Radical photopolymerization………………………….………………………...23
2.3 Cationic photopolymerization…………….……………………………..………29
2.4 Why using cationic photopolymerization?...........................................................38
Chap. 3 PHOTOPOLYMERIZATION OF COMPOSITE MATERIALS……........39
3.1 State-of-the-art of photocuring for composite materials…………….….……….39
3.2 UV polymerization limits in the case of composite materials…………………..40
3.3 Solutions reported in literature…………………………………………………..43
Part 2 – Photopolymerized NANO- and MICRO-composites –
Experimental
Chap. 1 MATERIALS……………………………………………………………...44
1.1 Silica nanoparticles-fumed silica……………..………………………………...44
1.2 Glass fibres……………………………………………………………………..47
1.3 Organosilanes…………………………………………………………………...50
1.4 Thermoset matrices…………………..…………………………………………54
1.5 Photoinitiators…………………………………………………………………..56
1.6 UV-lamps and reactive formulations selected..………………………………...57
Chap. 2 MODIFICATION OF INORGANIC SURFACES BY
ORGANOSILANES……………………………………………………………………59
2.1 Introduction……………………………………………………………………..59
2.2 Experimental: protocols for nanosilica and glass fibres………………………..59
2.3 Conclusions……………………………………………………………………..76
Chap. 3 UV-POLYMERIZATION IN THE PRESENCE OF
NANOFILLERS……………………..…………………………………………………77
3.1 Introduction……………………………………………………………………..77
3.2 Experimental……………………………………………………………………77
3.3 Reaction kinetics: Results and discussion………………...…..………………...79
3.4 Conclusions……………………………………………………………………..86
Chap. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES…………...87
4.1 Introduction……………………………………………………………………..87
4.2 Experimental……………………………………………………………………87
4.3 Results and discussion………………………………………………………….91
4.4 Conclusions……………………………………………………………………107
Chap. 5 CONCLUSIONS…………………………………………………………109
Experimental Techniques…………………………………………………...111
Spectroscopic analysis: FT-IR, UV-vis…………….……………………………...111
Surface analysis: dynamic contact angle, Chan balance…………………………...112
Calorimetric techniques: TGA, DSC, photo-DSC, DMTA………………………..115
Microscopy: AFM, SEM…………………………………………………………..119
Interface mechanical analysis (microbond technique).………….…………………122
Others: UV lamp devices…………………………………………………………..123
REFERENCES………….………………………………………………………...124
INTRODUCTION
The research on polymer-based composite materials is still a growing sector,
regarding both the improvements on existing products as well as the developments of
new ones.
The interest and the industrial production of composites based on polymeric matrices
have been rapidly increased thanks to their peculiar qualities, with respect to the
traditional materials, such as: low weight, low cost, ease of production, and very
specific application fields.
Among the various production processes used for polymers, the UV-curing
technology is growing rapidly even outside of its classical applications (ex. the coating
industry), and somewhere it is replacing the traditional curing techniques thanks to its
process speed, low costs and environmental friendly character.
From this point of view the development of UV-curing processes for the production
of polymer-based composites looks promising.
In this work the preparation of epoxy matrix-based composites with nanosilica and
glass fibers as reinforcing agents through cationic photopolymerization was studied.
First was set up a reaction to modify the surface of the inorganic fillers, which will be
used in the preparation of polymeric composites. The influence of surface treatment on
the curing process as well as the composite final properties was investigated.
INTRODUCTION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
II
The manuscript structure is presented below, with evidences of the content of each
chapter.
In the first part of this work general information are collected on composite
materials, UV-curing technology, and reactions as well as its use for the processing of
polymer-based composites.
In Chap. 1, polymer-based nano- and micro-composites are presented and their
specific properties are described and related to the ones of traditional materials.
Examples of their main applications fields are reported. In this chapter is also described
the specificity of composites materials, i.e. the interphase region, and the principal
theories regarding its formation and properties.
In Chap. 2, the UV-curing technique and its application fields are described. The
reaction mechanisms of radical and cationic photopolymerization are analyzed and
commented. The UV-curing technique is then compared to the traditional thermal
curing process, explaining differences, advantages, and disadvantages of the two
processes; evidences of the advantages in using cationic photopolymerization are given.
In Chap. 3 are collected the information found in literature regarding the application
of the UV-curing technology in the preparation of composites, evidencing that it is still
a quite unexplored field.
The second part of this work is dedicated to the presentation and comment of the
obtained experimental data.
In Chap. 1 are presented and described, from a physic-chemical point of view, all the
materials used, i.e. the inorganic reinforcing agents, the silane coupling agents used to
modify their surface, the monomers and the photoinitiators.
In Chap. 2 the set up of the experimental grafting procedure is described: the
inorganic surfaces have been modified to improve their compatibility with the selected
polymeric matrices; afterwards they were characterized by various means. In particular,
the atomic force microscopy, AFM, and scanning electron microscopy, SEM,
measurements done at the INSA-Lyon Laboratory (LMM IMP UMR CNRS 5627) as a
part of the cooperation between INSA-Lyon and Politecnico di Torino are reported.
INTRODUCTION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
III
In Chap. 3, the influence of the reinforcing agents on the photopolymerization
kinetics was investigated by using also “real time” techniques (photo-calorimetric
technique).
In Chap. 4, the properties of the obtained composites are analyzed. Particular
attention is given to the adhesion measurements which allow evaluating the interface
properties and their changes when the composite is exposed to a hostile environment
(hydro-thermal ageing).
In the Appendix the descriptions and main characteristics of the experimental
techniques used in this work are collected.
PART 1
PHOTOPOLYMERIZATION
AND
COMPOSITE MATERIALS
INTRODUCTION
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
1.1 “Composite materials”: characteristics and properties 1-3
In the continuing quest for improved performances, traditional materials are more
and more replaced by composite materials.
Composites are known from ages, for example concrete (a mixture of stones held
together by cement) is a familiar material for building; other examples are wood and
bone, as natural composites.
During the last 40 years the production of synthetic composites has been rapidly
increased. The spur of this rapid expansion over the last few decades was the
development in the UK of carbon fibers and in the USA of boron fibers in the early
1960s. These new fibers, with high elastic constants, gave a significant increase in the
stiffness of composites and hence made possible a wide range of applications. One of
the key factors was the very high modulus-to-weight and stiffness-to-weight ratio
presented by these composites.
In Fig. 1.1 the importance of the various classes of materials used in engineering is
illustrated: composites are present besides the traditional materials.
A composite is defined as a material having two or more distinct constituents or
phases; they have to be present in reasonable proportions (greater than 5%) and they
must have different properties, hence the composite properties are noticeably different
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
2
from the properties of the phases; lastly a composite is usually produced by intimately
mixing and combining the constituents by various means.
Composites are made by chemically different phases on a microscopic scale,
separated by a distinct interface. The constituent that is continuous and is often, but not
always, present in the greater quantity is termed matrix. The second constituent is
referred to as the reinforced phase, or reinforcement, as it enhances the mechanical
properties of the matrix. In most cases, the reinforcement is stiffer than the matrix.
Fig. 1.1: Relative importance of the four classes of materials (ceramics,
composites, polymers and metals) in mechanical and civil engineering as a function
of time1.
Geometry of reinforcement phase, which has at least one of the dimensions less than
500 µm, is one of the major parameters in determining the effectiveness of the
reinforcement.
Usually it is possible to describe reinforcement as being fibrous or particulate and its
arrangement may be random or with a preferred orientation.
A fibrous reinforcement is characterized by its length being much greater than its
cross-sectional dimension. When fibers are used, matrix properties are chosen to be
complementary to the properties of fibers: for example great toughness in a matrix
complements the tensile strength of the fibers. The resulting combination may then
achieve high strength and stiffness (due to the fibers) and resistance to crack
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
3
propagation (due to the interaction between fibers and matrix). Strong fibers have also
the great advantage of restraining cracking in what are called brittle matrices.
The most common materials used as matrices in composites preparation are
polymers. Their mechanical properties are generally inadequate for many structural
purposes, so it is a benefit to have reinforced polymers. Beside that, processing of
polymer matrix composites (PMCs) does not require high temperatures or high
pressures, so problems associated with degradation of the reinforcement during
manufacture are less important compared with the fabrication of metal and ceramic
matrix composites, hence reinforcements with low temperature capabilities (organic and
glass fibers) may be used. The equipment needed is simpler than in the case of other
matrices.
PMCs were used at the beginning during World War II and started to diffuse widely
immediately after. Today PMCs are used in many fields, as it can see from Tab. 1.1.
Tab. 1.1: Main industrial applications of composite materials.
Industrial sector Examples
Aerospace wings, fuselage, radomes, antennae, tail-planes, helicopter, blades, landing gears, seats, floors, interior panels, fuel tanks, rocket motors cases, nose cones, launch tubes
Automobile body panels, cabs, spoilers, consoles, instrument panels, lamp-housings, bumpers, leaf springs, drive shafts, gears, bearings
Boats hulls, decks, masts, engine shrouds, interior panels
Chemical pipes, tanks, pressure wessels, hoppers, valves, pumps, impellers
Domestic interior and exterior panels, chairs, tables, baths, shower units, ladders
Electrical panels, housings, switchgear, insulators, connectors
Leisure motor homes, caravans, trailers, golf clubs, racquets, protective helmets, skis, archery bows, surfboards, fishing rods, canoes, pools, diving boards, playground equipment.
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
4
Among the PMCs, the ones with thermoset matrices developed first, due to their
quite simple production. The success of PMCs and in particular of fiber composites
with thermosetting matrices results from the much improved mechanical properties of
the composite compared with the matrix material. PMCs reinforced with glass fibers do
not have clear advantages on conventional materials in term, for example, of elongation
to fracture (lower than for metallic alloy), but their main advantage over metals is linked
to their low density, ρ, particularly when one considers the Young’s modulus per unit
mass E/ρ (specific modulus) and tensile strength per unit mass σ1/ρ (specific strength).
Higher specific modulus and specific strength of PMCs composites means that the
weight of certain components can be reduced, with a significant reduction of transport
costs. Today glass-reinforced polymers are by far the most used composite material in
terms of volume with the exception of concrete. In Tab. 1.2 are presented some
properties of typical PMCs.
The bonds of the links are covalents, as chain bonds (Fig. 1.2). These strong bonds
have the effect of pulling the chains together; the resulting three-dimensional network
gives to thermosets significant advantages over thermoplastics such as greater
dimensional stability, less flow under stress, greater resistance to solvents and a lower
coefficient of thermal expansion. Properties achieved by composites with thermoset
matrix compared with the ones achieved by thermoplastic matrix are presented in Tab.
1.3.
Tab. 1.3: Properties of thermosets and thermoplastics matrices.
Fibrous reinforcements used in thermoset composites are usually coated with a sizing
solution. The nature of the size depends from the chemistry of the matrix. Among the
thermosets, polyester resins dominate the market whereas epoxy resins offer new high-
performances required in advanced composites applications. For example for
Thermosets Thermoplastics Young’s modulus (GPa) 1.3-6.0 1.0-3.8 Tensile strength (MPa) 20-180 40-190
Fracture toughness KIC (MPa m1/2) 0.5-1.0 1.5-6.0
GIC (kJ/m2) 0.02-0.2 0.7-6.5 Max. service temperature (°C) 50-450 25-230
CH
AP. 1
C
OM
POSI
TE M
ATER
IALS
-IN
TERF
ACE
Phot
opol
ymer
ized
mic
ro- a
nd n
ano-
com
posi
tes:
inte
rfac
e ch
emis
try
and
its ro
le o
n in
terf
acia
l ad
hesi
on
5
Tab.
1.2
: Pro
pert
ies o
f pol
ymer
com
posi
tes-
Exam
ples
.
D
ensi
ty
(Mg/
m3 )
You
ng’s
m
odul
us
(GPa
)
Ten
sile
st
reng
th
(MPa
)
Elo
ngat
ion
at b
reak
(%
)
Flex
ural
st
reng
th
(MPa
)
Spec
ific
mod
ulus
[(
GPa
)/( M
g m
3 )]
Spec
ific
stre
ngth
[(
MPa
)/( M
g/m
3 )]
Poly
amid
e 66
+ 4
0%
carb
on fi
ber
1.34
22
24
6 1.
7 41
3 16
18
4
Epo
xy +
70%
gla
ss
fiber
s
Uni
dire
ctio
nal
long
itudi
nal
1.90
42
75
0
1200
22
39
5
Uni
dire
ctio
nal
trans
vers
e 1.
90
12
50
6 26
Epo
xy +
60%
ar
amid
e fib
ers
1.40
77
18
00
55
1286
Poly
ethe
r im
ide
+ 52
% a
ram
ide
fiber
s
54
253
Poly
este
r +
glas
s 1.
50
7.7
95
17
0 5
63
Poly
este
r +
50%
gl
ass f
iber
s
unid
irect
iona
l lo
ngitu
dina
l 1.
93
38
750
1.8
20
38
9
unid
irect
iona
l tra
nsve
rse
1.93
10
22
0.
2
5 11
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
6
aerospace applications hot wet properties, damage tolerance, higher Tg, greater
toughness and environmental resistance are required.
Fig. 1.2: Arrangements of polymer chains: (a) cross-linked; (b) linear; (c) branched.
A comparison of main properties of polyesters and epoxies is presented in Tab. 1.4.
Tab. 1.4: Properties of some polymeric matrices.
Epoxy Polyester Phenolics Polyimides Density (Mg/m3) 1.1-1.4 1.1-1.5 1.3 1.2-1.9
Young’s modulus (GPa) 2.1-6.0 1.3-4.5 4.4 3-3.1 Tensile strength (MPa) 35-90 45-85 50-60 80-190
Fracture toughness KIC (MPa m1/2) 0.6-1.0 0.5
GIC (kJ/m2) 0.02 0.3-0.39 Tg (°C) 120-190
Thermal expansion coefficient (10-6K-1)
55-110 100-200 45-100 14-90
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
7
Glass fibers are the most common reinforcement for PMCs, in particular glass fiber-
reinforced epoxies (GREs) are employed in technological applications.
In Tab. 1.5 the principal characteristics of GREs in comparison with glass fiber-
reinforced polyesters (GRPs) are illustrated.
Tab. 1.5: Main characteristics of GREs and GRPs (fibers content = 50-80% vol.).
Better mechanical properties of GREs are a result of the good strength and stiffness
of the epoxy matrix and the strong bonding of glass fiber to epoxy. In fact, epoxies
adhere efficiently to glass fiber surface compared to than any other thermoset resin
commonly used. This good bonding leads to high interlaminar shear strength, as shown
in Tab. 1.6.
Tab. 1.6: Properties of epoxy composites based on different types of fibers.
source: Dow Chemical Company.
Epoxy Polyesters Density (Mg/m3) 1.6-2.0 1.6-2.0
Tensile modulus (GPa) 30-55 12-40 Flexural modulus (GPa) 10-35
Tensile strength (MPa) 600-1165 140-690
Flexural strength (MPa) 1000-1500 205-690
Compressive strength (MPa) 150-825 140-410 Interlaminar shear (MPa) 30-75
Fiber Strength (MPa) Young’s modulus (GPa)
Density (Mg/m3)
Tensile Compressive
E-glass 1165 490 50 1.99
S-glass 1750 495 60 1.99 Carbon (AS4) 1480 1225 145 1.55
Carbon (HMS) 1275 1020 205 1.63
Aramid 1310 290 85 1.38
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
8
1.2 “Interface/interphase”: structure and properties
The properties of composites are controlled by the interfacial region1,3,4, a two-
dimensional (interface) or three-dimensional (interphase) area between the reinforcing
agent and the matrix. A good interface bonding to ensure the load transfer from the
matrix to the reinforcement is a primary requirement for the effective use of
reinforcement properties.
Since the load acting on the matrix has to be transferred to the reinforcement via
interface, it is clear that reinforcement and matrix should be strongly bonded one to the
other in order to impart to the composite the high strength and stiffness of
reinforcement.
The fracture behavior is also dependent on the strength of interface: a weak interface
results in low stiffness and strength, but high resistance to fracture, while a strong
interface produces high strength and stiffness, but brittle behavior. Other properties of
composites (resistance to creep, fatigue, and environmental degradation) are affected by
the characteristics of interface.
Interfacial bonding is due to adhesion between reinforcement and matrix that in some
stages of the processing of composite must be in intimate contact. In particular, in these
stages, the matrix is often liquid or in a condition where is capable to flow on
reinforcement surface. In this context, the concept of wettability is very important.
Wettability defines the extent to which a liquid will spread over a solid surface, so
“good wettability” means that the liquid (in this case the matrix) will flow over the
reinforcement covering the entire surface and displacing all the air; this can be obtained
only if the matrix is not too viscous and if wetting results in a decrease of the free
energy of the system.
Considering a thin film of liquid (matrix) on the solid surface (reinforcement): all
surface have an associated energy and the free energy per unit area of the solid-gas,
liquid-gas and solid-liquid interfaces are: γSG, γLG, and γSL respectively.
For an increment of area dA covered by the spreading film, extra energy is required
for the creation of new interface areas (solid-liquid and liquid-gas).
This extra energy is (γSLdA+γLGdA); for a spontaneous spreading we must have
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
9
dAdAdA SGLGSL γγγ ≤+
or, dividing by dA,
SGLGSL γγγ ≤+
So the spreading coefficient, SC, can be defined as
( )LGSLSGSC γγγ +−=
If SC > 0, wettability is a spontaneous process and if γSG is similar to or less than γLG
wetting will not occur.
In Fig. 1.3 an example of a drop of liquid which has been allowed to reach
equilibrium and has partially wet the solid is illustrated.
Fig. 1.3: A liquid in equilibrium with a solid with a contact angleϑ.
Free energy of an interface is measured in J/m2 and can be shown to be equal to the
surface tension, which is expressed in N/m.
At the equilibrium we have
θγγγ cosLGSLSG +=
ϑ is called contact angle, it is used as a measure of the degree of wettability.
If ϑ = 180°, the drop is spherical with only one point of contact with the solid and no
wetting takes place. If ϑ = 0 we have perfect wetting. For 0° < ϑ < 180°, the degree of
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
10
wetting increases as ϑ decreases. Often is considered that the liquid does not wet the
solid if ϑ > 90°.
Once the matrix has wet the reinforcement, bonding will occur; the type of bonding
varies from system to system and often more than one bonding mechanism may be
operative at the same time.
It is possible to describe at least four types of different bonding1:
Mechanical Bonding: it is usually present with other types of bond, it is more
effective if the surfaces are rough and it is promoted from contraction of the matrix onto
the reinforcement or when the force is applied parallel to the interface.
Electrostatic Bonding: in this case bond is possible between two surfaces with
different charges, even if it is a short range interaction.
Chemical Bonding: it is formed between chemicals groups on the reinforcement
surface and compatible groups in the matrix; its strength depends on the number and the
type of bonds per unit area.
Reaction or Interdiffusion Bonding: atoms or molecules of the two components may
interdiffuse at interface to give interdiffusion bonding, for example in the case of
polymers it can be due to the intertwining of molecules.
In Fig. 1.4 all the different types of bonding described are presented.
When we speak of interfacial region, normally we consider a region of finite
thickness which is different in composition from both the reinforcing agents and the
matrix: this is what nowadays is called interphase.
Interphase is so defined as a region with finite volume which may possess chemical,
physical, microstructure and mechanical properties that differ from those of the bulk
reinforcing agent and matrix, but its material properties and effective thickness are
largely unknown because of its microscopic or even nanoscopic scale, often buried with
in the composite body.
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
11
Fig. 1.4: Schematic diagrams of the interfacial bonding mechanism: (a) mechanical
bonding, (b) electrostatic bonding, (c) chemical bonding, (d) chemical bonding as
applied to a silane coupling agent, (e) reaction bonding involving polymers, (f)
interfacial layer formed by interdiffusion.
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
12
Theories have been proposed to explain the formation of this interfacial region and
the interactions between reinforcing agents and matrix. A popular theory, the Chemical
Bonding Theory5,6, states that a chemical reaction exists between the silane and the
inorganic filler and between the silane and the matrix leading to increased adhesive
bond stability.
The bifunctional silane molecules act as a link between the inorganic surface and the
resin by forming a chemical bond with the surface of the inorganic filler through a
siloxane bridge, while its organofunctional group bonds to the polymer resin. This co-
reactivity with the inorganic surface and the polymer matrix via covalent primary bonds
provides molecular continuity across the interface region of the composite.
The Chemical Bonding Theory explains successfully many phenomena observed for
composites made using silane treated glass fibres. However, a layer of silane agent
usually does not produce an optimum mechanical strength, and there must be other
important mechanisms taking place at the interfacial region. An established view is that
bonding through silane other than simple chemical reactivity is best explained by
interdiffusion and interpenetrating network, IPN, or an hybrid organic-inorganic
material formation as the interphase region7,8.
The fillers coverage by the organosilane is usually equivalent to several monolayers.
The hydrolyzed silane condenses to oligomeric siloxanols that are solubles until they
condense to a rigid cross-linked structure.
Sizing inorganic surfaces with silane leads to the formation of three layers with
different chemical structures and physical properties:
1. Physisorbed region, the outermost layer that consist mainly of bulk of the
deposited silane;
2. Chemisorbed region, the next layer, that possesses a better resistance to
hydrolysis than physisorbed region;
3. Chemically reacted region, the innermost region next to the inorganic surface
that consists of a three-dimensional network of siloxane, which is very stable and
resistant to extraction even by hot water.
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
13
However contact with the polymer-based film former and matrix is made while
siloxanols still have a certain degree of solubility. In case of thermoset resins, coupling
with silane-treated fillers is something in between the following two cases:
1. oligomeric siloxanol layer may be compatible in the liquid resin and form a true
copolymer during resin cure, i.e. a mixed phase (organic-inorganic).
2. siloxanols and resin cure with a limited amount of co-polymerization, this is the
case of only partial solution compatibility.
So the formation of an interfacial region comes from interdiffusion, taking place in
the coupling agent–polymer resin interface region, due to penetration of the matrix
monomer into the chemisorbed/physisorbed condensed silane layer followed by a
possible migration of the physisorbed condensed silane oligomer molecules into the
matrix. The migration and intermixing of silane with polymer create an interphase of
substantial thickness.
The combination of chemical reaction and IPN theories is of particular importance in
composites based on thermoset matrices.
To characterize the interphase means to quantify the rates of interdiffusion and
chemical reactions between silane and polymer matrix systems. This can be very
difficult because of the already told reduced dimension of this region.
A great number of techniques have been employed in the analysis of surface layers,
based on elemental chemistry, physics, and mechanical means.
As an efficient method, FTIR spectroscopy has been employed since 1970s in order
to detect and analyze complex chemical reactions taking place between the inorganic
filler and the silane agent9-12. It was possible to see the effective presence of a multilayer
on filler surface as well as the formation of interphase between glass fibers sized with
organosilane and bulk matrix via copolymerization. These results indicate that there are
complex reactions at the interface fiber/silane and at the interphase
fiber/silane/matrix7,8.
Other spectroscopic techniques13,14 like ion scattering spectroscopy, ISS, and
secondary ion mass spectroscopy, SIMS, show that the coating on the inorganic surface
consisted of three layers with distinct properties:
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
14
1. a stiff layer from the free surface to 140 Å, i.e. highly condensed inorganic
network;
2. from 140 Å to 240 Å, a soft oligomeric layer containing incompletely condensed
siloxane;
3. from 240 Å to the inorganic surface, a high molecular weight siloxane layer at
least partially covalently bonded to the surface and different from the two outermost
layers.
These results are in good agreement with the theory of three different chemical
structure (physisorbed, chemisorbed, and chemically-reacted) regions of the silane
treated interface.
More recently, X-ray photoelectron spectroscopy (XPS)14,15 has been used to
characterize glass fiber coatings, showing that the concentration of silicon in the surface
layer further increases due to the presence of size containing silane coupling agents.
Nanoindentation and nanoscratch techniques15 to measure the mechanical properties
of interphases are very useful because of the sub- or near-micron size of the interphase
itself.
These techniques are very powerful for the measure of the effective interphase
thickness and modulus: they both depend from the silane type and concentration. For
example by increasing silane concentration, the effective interphase thickness increases,
as well as interfacial bond strength and composite tensile strength.
In general it is possible to observe that interphase properties are really different from
the bulk matrix: the interphase has a lower Tg, a higher tensile strength and modulus,
and lower fracture toughness.
Nanoscratch tests are also useful to evaluate samples after water-aging: the hard part
of the interphase forms an extended region of reacted silane molecules after aging.
With atomic force microscopy14,15, AFM, it was possible to characterize the surface
of glass fiber before and after treatment with silane: topographic images of treated
surface exhibit a rougher surface, with the characteristic agglomerates of sizing agent,
the “silane islets”, than unsized glass fibers. Besides these qualitative characterizations,
CHAP. 1 COMPOSITE MATERIALS-INTERFACE
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
15
AFM makes also possible to evaluate the interphase thickness (for example for the
unsized glassy-epoxy system the thickness was approximated to be 1 µm) and its
ductility when treated.
Anyway, experience has shown that proper characterization of an interphase, for
chemical, physical or mechanical properties, is very difficult because the intephase is on
the microscopic or nanoscopic scale and is buried within the composite body.
Furthermore it is not easy to find out distinct boundaries that would allow the interphase
between the bulk reinforcing agent and matrix to be defined.
These are the reasons for which it is necessary to use techniques of ultra high
magnification and resolution. Nowadays techniques like AFM and nanoindentation tests
provided interesting results from which it is possible to conclude that the formation of
both softer and harder interphase is possible, depending on the combination of the
system applied. Besides the influence of reinforcement, matrix and coupling agent, there
are also environmental factors, as well as thermal, chemical, physical and mechanical
phenomena that can be equally important in the formation of interphase.
CHAP. 2 PHOTOPOLYMERIZATION
2.1 Description of photopolymerization; applications 1-5
The transformation of a reactive liquid into a solid, by UV-radiation, leading to
polymerization and cross-linking is termed photopolymerization or UV-curing.
UV-curing is defined as:
FAST TRANSFORMATION OF 100% REACTIVE, SPECIALLY FORMULATED,
LIQUIDS INTO SOLIDS BY UV PHOTONS.
Photons generated by UV-light are absorbed by the chromophoric site of a molecule
in a single event; this molecule generates radicals or protons, the initiating species that
promote the fast transformation (time range 10-2-1 s) from the liquid into the solid. As a
result of the curing process, a solid polymer network, totally insoluble in the organic
solvents and very resistant to heat and mechanical treatments, is formed from a 100%
reactive liquid.
The entire process is schematized in Fig. 2.1.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
17
Fig. 2.1: Schematic representation of photocuring process.
As shown in Fig. 2.1, a UV-curable formulation is made of three basic components:
1. photoinitiator, which absorbs the incident light and readily generates reactive
radicals or ions;
2. functionalized oligomer, which, by polymerizing, will constitute the back-bone of
the three-dimensional polymer network formed;
3. a mono- or multifunctional monomer, which acts as a reactive diluent and will be
incorporated into the network.
The photoinitiator is the key of all process, because it determines both the rate of
initiation and the penetration of the incident light into the sample, governing in this case
also the depth of cure.
Depending on the photoinitiator used, the reactive species generated can be radicals
or ions, so the process can be named radical or cationic photopolymerization. As
described in the following paragraphs, radical and cationic photopolymerizations are
very different not only for the active species that start the reaction, but also for the types
of monomers used and for the cure mechanism and experimental conditions in which
the process is performed. In Fig. 2.2, the differences in the reactive species generated
and the initiation step for the two processes are schematically illustrated.
During the initial part of the reaction, polymerization rate depends on the reactivity
and concentration of the functional group as well as on the viscosity of the matrix
medium. Other important parameters are chemical micro-structure and functionality of
monomers and/or oligomers: they will determine the final degree of polymerization,
physical, and chemical characteristics of the final polymer.
Photoinitiator
UV radiation
Reactive species(radicals or ions)
Multifunctional monomer
Crosslinked polymer
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
18
Fig. 2.2: Initiation step for radical (I) and cationic (II) photopolymerization.
APPLICATIONS
Nowadays, UV-curing technology is well established in many industrial fields and in
particular applications, it offers new possibilities of development. The principal
industrial use of UV-curing technology is in the coating industry for the surface
protection of all kind of materials, due to high speed process and good energy yield. A
typical industrial line UV processor is made of two parts: the coating machine, where
the UV-curable resin is applied on the substrate, and the UV oven, where the liquid
resin is dried within a fraction of a second by passing under a powerful lamp.
In Fig. 2.3 an industrial processor for the UV-curing of organic coatings is
schematically presented.
Fig. 2.3: UV-curing industrial processor for coatings.
R CH2 CHR' R CH2 CHR'
(I)H+ CH2 CHR CH3 CHR+
(II)
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
19
Acrylate resins, cured with radical photopolymerization, are the most widely used
UV-systems, with a total annual production of approximately 60000 tons, while
cationic-type resins, cured with cationic photopolymerization, represent a minor part,
i.e. about 2000 tons, but in continuous growth (10-12% per year).
Here are reported the main industrial fields in which UV-curing technology is
employed1,2.
Graphic arts/Coatings
Adhesives
Electronics
Stereolithography
Dental composite materials
Graphic Arts
UV-curing is used both in the pre-press part to produce the printing plate as well as
in the printing process itself, thanks to the development of fast-drying UV-curable inks.
The printing process consists of the rapid transfer through an ink of a given image
from a printing plate to the substrate (usually paper), thus allowing a fast production of
prints.
The main printing processes in which UV-curing is involved are: letterpress, gravure,
flexography, screen printing and lithography. On the other side new UV inks have been
developed. They present a number of advantages over conventional solvent-based inks:
the higher viscosity allows several colours to be applied successively;
their solvent-free formulations lead to a better print definition and high gloss
images;
the UV process is more economic because it requires less energy and
achieves a higher productivity; the entire process is performed at ambient temperature,
without any solvent emission, which makes it a environmental friendly procedure.
Finally, for some specific applications, it is necessary to further improve surface
properties of printed material (ex. gloss, smoothness, and abrasion and scratch
resistances, weathering resistance). This can be achieved by applying a thin layer of a
UV-curable varnish, which is known to give high gloss and smooth surface.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
20
Coatings
Coatings are applied to a surface. They can be divided into:
Functional coatings, improving the surface by:
protecting it from abrasion, scratch, mar, chemicals;
providing different properties such as release, slip, adhesion, electrical
conductivity or insulation, antifogging, flame retardance;
acting as a barrier to various liquids or gasses.
Decorative coatings are applied to:
change appearance ( colour, gloss or mat finish, texture);
hide surface (imperfections, electrical circuitry, etc.).
Usually coatings are classified according to the substrate they are applied to:
paper and paperboard
wood
plastics
metal
glass and ceramic
miscellaneous.
This type of employ of UV-curable varnishes is increasingly used to obtain highly
resistant coatings to protect any substrate: wood, plastic, metal, glass, optical fibres,
paper, leather, fabrics, etc.; the film thickness is of the order of 20-100 µm to assure a
long-lasting protection.
Adhesives
Radiation curing has two main areas of application in the field of adhesion:
1. to bond together two parts of a laminate, acting as a quick-setting glue. In this
case the use is limited by the UV-transparency of one of the two parts of the laminate.
The whole process is divided in three steps: applying of the adhesive in the liquid state;
assemblage of the two parts; exposure of the assembly to UV-light.
UV-cured laminates show a great potential because they are produced by a process
that is faster, cheaper and easier to work out than the usual thermal cure carried out for
hours under high pressure.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
21
2. To produce pressure-sensitive adhesives and release coatings. It consists in a
rapid photoinitiated crosslinking producing a viscoelastic system with predetermined
properties.
Electronics
Here UV-curable systems have found applications as photoresists in the imaging
step, fast drying adhesive and conformal coatings.
Stereolithography
This new technology is based mainly on the capability of UV-curable systems to give
three-dimensional solid objects, by scanning the surface of a resin with a laser to form a
thin solid pattern, and building up the model step-by-step by adding one layer on top of
another. Complex parts can be obtained faster, with great precision, and more flexible
processing than with conventional modelling techniques. Besides it allows the direct use
of digital design information to guide the formation of a model that closely represents
the original design.
Dental Composite Materials
Adding mineral fillers such as glass or silica particles to UV formulations is possible
to obtain extremely hard and abrasion-resistant composite materials. These types of
resins present a number of advantages over conventional systems: immediate readiness
for use, extended working time, higher polymerization rate, and short setting time,
better adhesion of the filler particles to the matrix.
The curing of these systems has to be performed at visible light and it is necessary to
take into account that inert filler can be up to 60% in volume, so the penetration of light
in these composite resins is limited therefore it has to be carried a multiple step process.
PRINCIPAL ADVANTAGES/DISADVANTAGES1,2
The main advantages of UV-curing technique are better understood if compared with
the traditional thermal-curing polymerization (Tab. 2.1).
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
22
Tab. 2.1: Comparison of UV and thermal curing1.
Parameter UV Thermal
Commercial
Capital cost + -
Operational cost + -
Formulation cost - +
Floor space + -
Cure speed + -
Skill level required 0 +
Environmental
No solvent release + -
Energy consumption + -
Technical
Chemical resistance + -
Formulation variety 0 +
Curing of pigmented films - +
No substrate damage + 0
Low cure temperature 0 -
Sensitivity to oxygen + +
Health & safety
Fire hazard + -
Radiation hazard 0 +
Irritant raw materials - +
+ = advantage - = disadvantage 0 = intermediate
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
23
Arguments in favour of the replacement of thermal curing by UV-curing are mainly
lower capital and running costs, lower floor space requirements, higher running speeds,
less substrate heating, the high quality of the cured coating or ink, no solvent release
during curing and the development of new curable formulations having less or no skin
irritant raw materials.
There are also economic and ecological factors that encourage the continuous growth
of radiation curing technology such as:
Raw materials containing a low amount of volatiles, less or no skin irritant, have
been developed and increase the range of formulation variety.
Low-viscosity monomer-free oligomers and water reducible oligomers can be
used in spray coating applications.
New applications in metal and glass coatings are possible thanks to oligomers
that adhere well to critical substrates.
Weather resistant products are available for outdoor applications.
More reactive photoinitiators allow lower concentrations in formulations or less
powerful UV sources to be used.
Photoinitiator-free UV-curable systems appear on the market.
New monochromatic UV-sources were introduced.
On the other side thermal curing still holds a strong position due to the advantage in
formulation costs and variety, the avoidance of radiation and the lower skill level
required. Moreover the thickness of the sample that can be photocured is normally very
thin if compared to a thermal cured one. Mainly for this reason UV-cure technology is
still not widespread in the composites industry.
2.2 Radical photopolymerization1,2,4
The radical polymerization mechanism can be schematically represented in Fig. 2.4:
Fig. 2.4: Radical polymerization mechanism. R CH2 CHR' R CH2 CHR'
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
24
R is the active specie generated by photodecomposition of the initiator.
It should be pointed out that it is only the initiation step, radical formation from the
photoinitiator, which is different from thermal polymerization.
Radical photoinitiators can be divided into two groups according to the way the
active species are generated2:
1. by photocleavage, if radicals are generated by a intramolecular scission ;
2. by hydrogen abstraction, if radicals are generated by the abstraction of an atom of
hydrogen from a donor molecule.
In Fig. 2.5 are illustrated the two ways of radicals’ generation.
A-B* → A• + B• A* + RH → AH• + R• homolitic cleavage hydrogen abstraction
Fig. 2.5: Mechanism of radicals’ generation in radical photopolymerization.
1. Photocleavage: in this class we found aromatic carbonyl compounds that
undergo to homolytic C-C bond scission upon UV exposure, with the formation of two
radical fragments; the benzoyl radical was shown to be the major initiating species.
The process is schematized in Fig. 2.6; examples of photoinitiators belong to this
class are: benzoin ethers derivatives, benzilketals, hydroxyalkylphenones, α-amino
ketones, and acylphosphine oxides.
Fig. 2.6: Radical formation reaction for aromatic carbonyl compounds.
2. Hydrogen abstraction: this is a typical reaction of some aromatic ketones, like
benzophenone, thioxanthone, or camphorquinone. Under UV irradiation, they do not
undergo fragmentation, but abstract a hydrogen atom from an H-donor molecule to
generate a ketyl radical and the donor radical.
C
O
C Xhv
O
C XC
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
25
The process is schematized in Fig. 2.7:
O
Cvh
C
O
*
RHC
OH
R
Fig. 2.7: Radical formation reaction for aromatic ketones.
In this case initiation of polymerization occurs through the H-donor radical. The
most frequently used H-donor molecules are tertiary amines, because of the high
reactivity of the α-amino alkyl radical towards the double bond, as shown in Fig. 2.8:
Fig. 2.8: Radical formation reaction in case of tertiary amine used as co-initiator.
This latter class of photoinitiators have also the advantage of reducing the inhibition
effect of oxygen because they promote a peroxidation mechanism that consumes the
oxygen present in the monomer.
In Fig. 2.9 are listed the principal classes of radical photoinitiators.
C
O
C
OR
R'
benzoin derivatives
benzil ketals C
O
C
OR
OR'
hydroxyalkylphenone C
O
C
R
OH
R'
acylphosphine oxides C
O
P
O
benzophenone derivatives C
O
thioxanthone derivativesC
S
O
Fig. 2.9: Radical photoinitiators commonly used.
Ar2C O N CH2hv CHNAr2C OH
CH2 CHN CH CH2
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
26
The main classes of resins that can be cured with radical system are: acrylate and
methacrylate monomers, thiol-ene systems, and unsatured polyester resins.
Acrylate and methacrylate monomers are by far the most used in industry because
they are very reactive and can be used to create a large variety of crosslinked polymers
with tailor-made properties. Their polymerization is very fast at the beginning, but
progressively slows down when gelification and vitrification occur; for this reason there
are always some residual unreacted insaturations trapped in the polymer network.
They can be divided into:
functionalized oligomers
mono- or poly-functional monomers
The most important types of functionalized oligomers are:
epoxy acrylic resins
urethane acrylic resins
polyalkylene glycol diacrylates
polyester diacrylates
The most important monomers are:
diethylene glycol diacrylate
hexanediol diacrylate
trimethylolpropane triacrylate.
In Fig. 2.10 is presented the typical reaction scheme for this class of monomers.
Epoxy acrylates are highly reactive and produce hard and chemically resistant
coatings, so they are used in wood finishing applications, varnishes for paper, and
cardboard as well as for hard coatings2,4.
Polyesters acrylates are often applied in wood coatings, varnishes, lithographic and
screen inks.
Methacrylates monomers have similar reactivity to acrylates monomers, but with a
lower propagation rate2,4.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
27
C
O
CH2 CH C
O
O C
O
CH2 CH C
O
O
Propagation
C
O
CH2 CH C
O
Omonomer
C
O
CH2 CH
C
CH2 CH CH2 CH
CO
C O
CHCH2 CH2
O
C O
CH CH2CH CH2CH
Termination
Pn Pm PnPm
Pn vitrification
Initiation
Fig. 2.10: Polymerization reaction of acrylates.
The most important advantages of acrylate formulations are high reactivity and
adjustable viscosity. Rapid cure speed and low viscosity combined with brittleness and
poor adhesion are obtained when acrylate monomers are used; acrylate oligomers have
higher viscosity and lower reactivity than monomers, but they guarantee a broad range
of coating property requirements. Therefore radiation curable formulations usually
consist of monomers as reactive thinners and oligomers as binders.
Thiol-ene systems are used in many applications such as coatings, adhesives,
sealants, etc.
Their polymerization reaction can be represented as follows (Fig. 2.11):
Fig. 2.11: Polymerization reaction of thiol-ene systems.
Using multifunctional monomers it is possible to obtain a three-dimensional network
in which connecting chains are made of alternating copolymer. It should be noticed that
OAr2C RSHvh Ar2C OH RS
RS CH2 CH R' RS CH2 CH R'
R'RS CH2 CH RSH RS CH2 CH2 R' RS
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
28
thiol-ene systems are less sensitive to air inhibition that other radical systems because
peroxy radicals are also capable to extract H from the thiol (Fig. 2.12) forming the thiil
radicals which continue the polymerisation process.
Fig. 2.12: Hydrogen abstraction from thiol molecule.
Unsatured polyester resins are mainly employed in the wood finishing industry; the
radical-initiated crosslinking occurs by direct addition copolymerization of the vinyl
monomer with the unsaturations at the polyester backbone, as shown in Fig. 2.13:
Fig. 2.13: Polymerization of unsatured polyesters.
In Fig. 2.14 the principal classes of radical monomers are listed.
polyester/styrene C
O
CH CH C
O
CH CH2
thiol/ene C(R SH)4 CH2 CH R' CH CH2
acrylates
(CH2 CH C
O
O CH2)3 CH2 CH2 CH3
CH2 CH C
O
O R O C
O
CH CH2
R = polyester, polyether, polyurethane, polysiloxane
Fig. 2.14: Radical monomers commonly used.
RSCHCH2
P PO2RSH
PO2H RS
R O C
O
CH CH C
O
O CH CH2 crosslinked polymer
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
29
2.3 Cationic photopolymerization6-9
The cationic polymerization mechanism is schematically represented in Fig. 2.15:
Fig. 2.15: Cationic polymerization mechanism.
H+ is the active specie generated by photodecomposition of the initiator.
Photoinitiators for cationic photopolymerization can be divided into three groups:
Aryl diazonium salts
Ferrocenium salts
Diaryliodonium/triarylsulfonium salts.
The latter are named “onium salts” and are nowadays the photoinitiator class most
used in cationic polymerization. They are stable crystalline compounds, readily soluble
in a wide variety of common polar solvents and cationically polymerizable monomers
and absorb strongly in the UV region. In Fig. 2.16, their structure is represented.
Fig. 2.16: General structure of “onium salts”:
diaryliodonium (I) and tryarylsulfonium (II) salt.
Under UV light, they are subjected to photolysis through a quite complex
mechanism. In the case of diaryliodonium salts, one can have photoexcitation of the salt
and after the decay of the resulting excited singlet with heterolytic and homolytic
H+ CH2 CHR CH3 CHR+
I
MtXn
S
MtXn
(I) (II)
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
30
cleavages of carbon-iodine bond. Free-radicals, cationic and cation-radical fragments
are produced according to the scheme reported in Fig. 2.17.
*
HMtXnArArI MtXnAr2I MtXnhv MtXnAr2I ZH ArI Z
Fig. 2.17: Photolysis of diaryliodonium salt under UV light.
Protonic acids, denoted as HMtXn, derive from the reaction between the aryl cations
and aryliodine cation radicals with solvents, monomers, or impurities. HMtXn is the real
initiator of cationic polymerization, as shown in Fig. 2.18.
nM
MHMtXn H M+ MtXn
H M+ MtXn H (M)nM+ MtXn Fig. 2.18: Initiation mechanism for cationic polymerization.
For triarylsulfonium salts the photolysis is similar, but the heterolytic cleavage is
dominant on homolytic cleavage.
The anion generally indicated as MtX-n must have non-nucleophilic characteristics
because any cationic species generated during photolysis or by addition to a monomer
would give combination with a nucleophilic anion and, as result, retardation or
complete suppression of polymerization reaction. According to their non-
nucleophilicity, the most useful anions are: PF-6, AsF-
6, and SbF-6.
The type of anion determines also the strength of the Brønsted acid generated via
photolysis: bigger anions generate stronger acids, so the reactivity order is:
SbF-6 > AsF-
6 > PF-6 > BF-
4.
In Fig. 2.19 are shown the differences observed changing the anion on the kinetics of
photopolymerization of cyclohexene oxide.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
31
Fig. 2.19: Photopolymerization of cyclohexene oxide using 0.02% mol of (C6H6)3S+X- salts6.
The onium salts show a very high degree of thermal stabilty due to their cation part
which is stabilized by the resonance of benzenic rings and by the d-orbital of central
atom. As a result of this stability, they undergo thermal decomposition at very high
temperatures, as shown in Fig. 2.20.
Fig. 2.20: TGA analysis of (C6H6)3S+ AsF-
6 in nitrogen and air during an heating
ramp of a rate of 10 C/min6.
In Fig. 2.21 are summarized the various critical functions that can be assigned to the
cation and anion portion of an onium salt.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
32
Fig. 2.21: “Anatomy” of an onium salt photoinitiator.
Studies made on reactive systems using photo-calorimetric technique6, i.e. photo-
DSC, have revealed that there are other parameters controlling the reaction:
Concentration of photoinitiator, for each of them is possible to observe that there is a
specific concentration for which is obtained an optimum cure rate. Further increase in
photoinitiator level does not produce a corresponding increase in the cure rate, possibly
due to the light screening effects by the triarylsulfonium salt itself or its photolysis
products.
UV-light intensity, because the system is limited by the absorption of the
photoinitiator, so it is useless to have very high light intensities. At very low intensities
there appears to be some type of inhibition effect.
Temperature effect, it has been observed that in all cationic systems cure at the
highest temperature the substrate give the highest cure rate, of course this is not always
possible.
CATION
DETERMINES PHOTOCHEMISTRY
λmax
molar absorption coefficient
quantum yield
photosensitization
thermal stability
ANION
DETERMINES POLYMER CHEMISTRY
acid strength
nucleophilicity
anion stability
initiation efficiency
propagation rate constants
MtXnI
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
33
Water effect, because the presence of water (or other hydroxyl containing impurities)
can change both the rate and the extent of polymerization of epoxy monomers.
Two other classes of cationic photoinitiators have been mentioned above:
Aryldiazonium salts
Ferrocenium salts.
Aryldiazonium salts were the first class of cationic photoinitiators developed in the
1970s. They can be used in the ring opening polymerization of epoxides through the
reaction scheme represented in Fig. 2.22.
Fig. 2.22: Photolysis mechanism of diaryldiazonium salt and cationic polymerization
of an epoxy monomer.
This class of cationic photoinitiators had no success essentially for two reasons:
1. the thermal instability of aryldiazonium salt leads to poor latency so that the
systems spontaneously gelled in few hours even in absence of light.
2. The generation of nitrogen gas as photolysis product leads to film defects.
Ferrocenium salts are a very different class of cationic photoinitiators. They undergo
photolysis to generate an iron-based Lewis acid with the loss of the arene ligand. This
species coordinates to an epoxy monomer to give ring-opening polymerization as shown
in Fig. 2.23.
Ar N2 BF4hv Ar F BF3 N2
O BF3
H2O
O
n
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
34
R1
Fe X hv XFe
R1
O
R1Fe
O
R1
( )3
X- R1
nO
R1
polymer Fig. 2.23: Photolysis mechanism of ferrocenium salt and cationic polymerization of
an epoxy monomer.
The use of this class of photoinitiator is limited to the monomers that can bond
effectively with the photogenerated coordinatively unsatured ion center.
Cationic photopolymerization is used to cure monomers that are reactive towards
cationic species. In Fig. 2.24, the most important monomers that can be UV-cured in the
cationic way are scheduled. Among all the monomers presented, the most interesting
classes for cationic photopolymerization are multifunctional vinyl ethers and epoxides
because they are very reactive and commonly available.
Fig. 2.24: Polymerizable monomers with cationic photoinitiators.
Cationic Photoinitiators
hv
nCH
R
CH2 O
O
R
nCH2 CH2 S
S
nCH
OR
CH2OR
nN
C O
R
CH2 CH2
N
OR
n(CH2)4 O
O
n(CH2)5 O C
O
O
O
nCH
R
CH2
R
nCH2O CH2O CH2O
O
O O
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
35
The epoxy monomers can be UV-cured through the opening of the epoxy ring,
catalyzed by the acid species generated by photolysis of the initiator. The reaction
mechanism is presented in Fig. 2.25.
X OCH
CH
R
R'X O
CH
CH
R
R'oxonium ion
monomer
X (O CH
R
CH
R'
)n OCH
CH
R
R'
Fig. 2.25: Polymerization scheme for an epoxy monomer.
In presence of difunctional epoxides UV-curing leads to a crosslinked polymer.
The reactivity of this class of monomers is quite broad, for example monomers
containing the epoxycyclohexane group are much more reactive than glycidyl ethers or
glycidyl esters, due to steric and electronic factors.
Two examples of epoxy monomers commonly used are cyclohexane dimethanol
diglycidylether, denoted DGE and 3,4-epoxycyclohexyl-3’,4’-
epoxycyclohexanecarboxilate, denoted CE. Their structures are given in Fig. 2.26.
Fig. 2.26: Chemical structures of DGE and CE.
O
O
OO
CE
CH2
CH2
O CH2
O CH2
O
O
DGE
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
36
Vinyl-ethers monomers are the most reactive towards cationic photopolymerization,
giving a three-dimensional polymer network with a low number of residual
insaturations. The high reactivity of these monomers is due to the presence of the
double bond C=C that, with the oxygen atom, stabilizes the cation through the chain
growth (Fig. 2.27).
R CH2 CH OR CH2 CH OCH2 CH OR+
Fig. 2.27: Growth of the polymer chain and its stabilization by resonance.
Even if vinyl ethers are ideally suited for cationic photopolymerization, their use in
industry is limited by their high cost and the hazards of using acetylene under high
pressure during their synthesis.
PRINCIPAL ADVANTAGES/DISADVANTAGES
Effect of oxygen
One of the main advantages of cationic-initiated polymerization, if compared to the
radical induced process, is that the former is not sensitive to oxygen, thus allowing
coatings to be cured rapidly even in the presence of air.
Influence of film thickness
In thin films the photopolymerization develops at the same rate, but as the film
thickness is increased, the propagation rate value, Rp, drops, due to the UV filter effect
of the top layer (Fig. 2.28). Film thickness has a pronounced effect also on the
maximum conversion level. Moreover atmospheric oxygen will diffuse less rapidly in
thick coatings.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
37
Fig. 2.28: Influence of the film thickness on the photopolymerization of a
cycloaliphatic diepoxy7.
Post-polymerization
One of the distinct features of cationic photopolymerization, compared with radical-
induced process, is the post-cure phenomena: it consists in a further and not negligible
polymerization taking place once the light has been switched off. Such an important
post polymerization is due to the fact that two cations cannot interact to undergo
coupling or disproportionation, so that the living polymer chain continues to grow in the
dark, until termination occurs by transfer reaction or bimolecular interaction with
another species present in the polymerization mixture (as water, bases, or another
portion of polymer chain).
Fig. 2.29 shows some typical conversion vs. time curves recorded after exposure,
compared to continuous irradiation: post-polymerization is relatively more important in
the early stages of the reaction, but a significant increase of the degree of conversion
could be noticed even after 20 minutes of storage in the dark.
CHAP. 2 PHOTOPOLYMERIZATION
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
38
Fig. 2.29: Polymerization profiles recorded after UV exposure of various durations for a cycloaliphatic diepoxy7.
2.4 Why using cationic photopolymerization?
The main differences between radical and cationic photopolymerization has been
described and it becomes evident that the cationic UV-curing process offers many
important advantages that are summarized here:
the initiating species is a stable compound only consumed by anions or
nucleophiles;
after UV exposure, cationic polymerization continues for a long time;
since no radicals are involved, cationic photopolymerization is not sensitive to
oxygen;
films made from cationic formulations show low shrinkage and good adhesion.
CHAP. 3 PHOTOPOLYMERIZATION OF
COMPOSITE MATERIALS
3.1 State-of-the-art of photocuring for composite materials 1-5
In the field of composites the main technology actually employed is thermal curing,
but the energy required for curing can be supplied also via high energy electrons (EB)
or ultra-violet radiation (UV).
The use of UV-curing technology to prepare composite materials has not been
extensively studied, so that only scant information is found in the literature. UV-light
induced free radical crosslinking polymerization has been employed in the preparation
of fiberglass-reinforced unsatured polyesters composites, leading to a product having
mechanical properties comparable to the thermal cured ones but obtained in a rapid
process. The same type of technique is employed in the fabrication of reinforced dental
composites.
UV radiation technology can successfully be employed in the preparation of
nanocomposites because it allows an intercalative polymerization in situ that is the most
appropriate technique to prepare polymer layered silicate nanocomposites. The process
is simple, UV irradiation of the layered silicate swollen in the liquid monomer
containing a photoinitiator. It assures not only all the advantages typical of UV-cure
(Par. 2.1), but also specific benefits to the obtained nanocomposites, such as a fine
CHAP. 3 PHOTOPOLYMERIZATION OF COMPOSITE MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
40
control of the swelling time to ensure a perfect interpenetration of the resin into the
interlayer galleries of the mineral. UV-cure has been employed to produce
nanocomposite adhesives for the applications in integrated optics, realized irradiating an
epoxy-based resin containing nanosized silica particles. Other works on UV-cured
coatings containing functionalized colloidal silica evidence the great improvement
obtained in abrasion and scratching resistance properties.
3.2 UV polymerization limits in the case of composite materials4
As seen above, the importance of UV radiation curing in composite production is
increasing nearby the classical thermal curing. Therefore it is important to analyze and
compare the cure processes in order to understand their advantages and disadvantages.
In a thermally initiated polymerization, decomposition of initiator is obtained by
heating. Reaction exothermicity adds to the thermal energy supplied by the oven,
leading to high temperatures in the core of the structure. Therefore complex temperature
programs are necessary to dissipate the heat of reaction, because it can lead thermal
degradation, internal stresses, etc., in the composite. The procedure is quite complex,
requiring long processing times at high temperatures, as well as complex curing
equipments.
In UV-cure process the initiator decomposes when UV-irradiated, producing reactive
species that propagate the crosslinking reaction. Therefore there is a cure front that
propagates throughout the sample thickness as the reaction proceeds. There is a gradient
of cure in the sample during the reaction and a homogenous degree of cure in the cured
sample. This technique allows curing of thick layers of resins if the reinforcing agent
and the resin do not absorb at the selected wavelengths.
Photopolymerization technique can be successfully applied in the production of
composites for low-performance applications from inexpensive starting materials by
relatively unskilled workers.
The main limits of UV polymerization in the case of composites are4:
Sample thickness: in fact the depth of penetration of the UV radiation represents
in most cases also the limit of sample thickness. Otherwise it is necessary to proceed to
CHAP. 3 PHOTOPOLYMERIZATION OF COMPOSITE MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
41
a step process where the sample is irradiated from both sides. Fig. 3.1 illustrates that the
radiation received by the bottom layer decreases exponentially with the thickness of the
sample.
Filler transparency to UV-radiation: in order to assure a complete
photopolymerization through all the depth of sample, the reinforcing material should be
reasonably transparent to UV light.
Sizing agents’ influence: it is well known that the majority of inorganic
reinforcements (ex. glass fibers) are treated with sizing materials in order to protect
them during their use and to promote specific characteristics; these sizing agents can
include also starch, oils, and gelatin and fatty amines, materials that can act as inhibiter
in particular towards cationic photopolymerization.
Fig.3.1: Depth of cure versus incident radiation intensity in a UV-cured glass
fiber/epoxidized linseed oil composite4.
Obtained mechanical properties: literature4 reports examples in which the tensile
properties of UV-cured composites are lower that the ones of composites prepared from
the same monomers, but using conventional thermal curing methods.
CHAP. 3 PHOTOPOLYMERIZATION OF COMPOSITE MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
42
However, the main limit to the use of UV-curing in composites preparation remains
the maximum thickness of a composite that can be irradiated, since composites gain
their mechanical properties from multiple layers of resin and reinforcing agents.
The depth of cure is dependent from a complex set of variables, such as light
intensity and wavelength, length of irradiation duration as well as photosensitivity of
photoinitiator, the UV absorption characteristics and reactivity, configuration of the
sample.
In the case of nanocomposites preparation3, it is also important to evaluate if the
presence of mineral filler affects the polymerization kinetics, with respect to the
reaction rate and cure extent.
In Tab. 3.1 results obtained for glass fibers-acrylate/methacrylate composites UV- or
thermal- cured5 are collected and compared.
Tab. 3.1: Comparison between UV- and thermal-curing in the preparation of glass
fiber-reinforced composites.
UV-cured composites Thermal-cured
composites
propagation rate limited by photo-bleaching
of the photoinitiator
limited by the induction
time, but after very fast due
to auto-acceleration
T°max reached in the
sample low high
Tg not very high high
% of residual
insaturations quite high low
yellowing present present
thermal shock - present (samples cracked)
CHAP. 3 PHOTOPOLYMERIZATION OF COMPOSITE MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
43
3.3 Solutions reported in literature
Considering all the disadvantages related to the use of UV-curing technique in the
preparation of composites, the following solutions are proposed:
UV-curing can be successfully used in the preparation of composites for low-
performance applications, combining inexpensive starting materials to a simple
production technique.
UV-curing can be used for repairing large composites structure, taking into
account the simplicity of the equipment and protection needed as well as the short
curing time.
A thermal bake can be coupled after the radiation curing step to complete the
cure and accelerate the post-cure in thick samples.
PART 2
PHOTOPOLYMERIZED
MICRO- AND NANO-COMPOSITES
EXPERIMENTAL
CHAP. 1 MATERIALS
1.1 Silica nanoparticles-fumed silica 1-3
Silica is constituted by pure SiO2 in form of high surface area particles. It is colorless
and maintains its isolating properties constant at high temperature.
Applications of silica powders are based on porosity, active surface, hardness,
thixotropic, and viscosity management. If the chemical structure of silica surface is
altered, these properties may be combined with specific chemical or physical interaction
capacities. Modified silica is used as reinforcing material and its presence can improve
mainly tear, tensile, and abrasive resistance.
Silica can be divided in pyrogenic silica, i.e. fumed silica, and silica made by wet
methods, i.e. precipitated silica. In Tab. 1.1 are collected the main properties of different
types of silica. Precipitated silica includes a wide range of silica with a variety of
structural characteristics. In general, the formation involves an acid-precipitation of
aqueous solutions of alkaline silicates. As an overall definition it can be assumed that
precipitated silica is dry silica with no long or short distance characteristic structure.
Fumed silica is widely used in industry as an active filler for reinforcement of
elastomers, as a rheological additive in fluids and as a free flow agent in powders.
It is a synthetic amorphous form of silicon dioxide produced in a hydrothermal
process by burning silicon tetrachloride in an oxygen-hydrogen flame at 1,200-1,600°C.
At these high temperatures viscous droplets of amorphous silicon dioxide are formed,
CH
AP. 1
M
ATER
IALS
Phot
opol
ymer
ized
mic
ro- a
nd n
ano-
com
posi
tes:
inte
rfac
e ch
emis
try
and
its ro
le o
n in
terf
acia
l adh
esio
n
44
Tab.
1.1
: Phy
sica
l pro
pert
ies o
f var
ious
silic
as.
Cha
ract
eris
tics
Py
roge
nic
silic
a Si
lica
mad
e by
wet
met
hods
Fum
ed si
lica
Arc
silic
a Pr
ecip
itate
d
silic
a xe
roge
ls
aero
gels
Spec
ific
BE
T a
rea
m2 /g
50
to 6
00
25 to
300
30
to 8
00
250
to 1
000
250
to 4
00
Size
pri
mar
y pa
rtic
les
nm
5 to
50
5 to
500
5
to 1
00
3 to
20
3 to
20
Size
aggr
egat
ions
/agg
lom
erat
ions
µm
*
2 to
15
1 to
40
1 to
20
1 to
15
Den
sity
g/
cm3
2.2
2.2
1.9
to 2
.1
2.0
2.0
Vol
ume
ml/1
00g
1,0
00 to
2,0
00
500
to 1
000
200
to 2
,000
10
0 to
200
8
00 to
2,0
00
Mea
n po
re d
iam
eter
nm
no
n po
rous
till
300
m2 /g
no
n po
rous
>
30
2 To
20
> 25
Pore
dia
met
er d
istr
ibut
ion
*
* ve
ry b
road
na
rrow
na
rrow
Shap
e of
inte
rior
surf
ace
0
0 po
or
very
muc
h m
uch
Agg
rega
tion
and
aggl
omer
atio
n
stru
ctur
e
chai
n-lik
e
aggl
omer
atio
n
(ope
n su
rfac
e)
dens
e sp
heric
al
aggr
egat
es/p
artic
les
non-
aggl
omer
ated
slig
htly
aggr
egat
ed
near
ly
sphe
rical
parti
cles
high
ly p
orou
s
aggl
omer
ated
parti
cles
mac
ropo
rous
aggl
omer
ated
parti
cles
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
46
the so-called primary particles, which collide and are fused together to build up stable
aggregates.
A widely used fumed silica is Aerosil® that was employed in our study. Its properties
are presented in Tab. 1.2.
Tab. 1.2: Physical and chemical data of Aerosil® 200.
*ex plant
Aerosil® 200 is mainly employed in paints and coatings, unsatured polyester resins,
laminated resins and gel coats, silicon rubber, adhesives and sealants, printing inks,
cable compounds and cable gel, plant protection, food, and cosmetics. It is used as an
anti-settling, thickening, anti-sagging agent and it improves free flow and anti-caking
characteristics of powders.
Properties Typical value
specific surface area (BET) 200±25 m2/g
average primary particle size 12 nm
tapped density*
acc. to DIN ISO 787/XI, Aug. 1983 50 g/l
bulk density*
ACM 104 30 g/l
moisture*
2 hours at 105°C ≤ 1.5%
ignition loss
2 hours at 1000°C based on material dried
for 2 hours at 105°C
≤ 1.0%
pH
in 4% dispersion 3.7-4.7
SiO2-content
based on ignited material > 99.8%
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
47
1.2 Glass fibers4-6
Glass is a non-crystalline material with a short range network structure, it has no
distinctive microstructure and its mechanical properties (determined by composition and
surface finish) are isotropic. Glass fibers are designated normally by alphabetical codes.
The main fibers used are “E” (electrical) glass fibers, which amount to 90% of the
market. They are based on the CaO-Al2O3-SiO2 system, but there is no “standard”
composition. It is only possible to give some indications on their constituents
proportions (Tab. 1.3) and their thermal, mechanical and physics properties (Tab. 1.4).
S-glass (in Europe R-glass) is based on the SiO2-Al2O3-MgO system; these fibers
have higher stiffness and strength (S) than E-glass. Its properties are stable even to high
temperatures, but it is more difficult to produce the fibers because of its limited working
range, so they are more expensive and used only for some specific applications,
nowadays replaced by aramidic and carbon fibers.
Other more resistant types of glasses have been developed: C-glass (chemical), E-
CR-glass (electrical-corrosion resistant glass) and AR-glass (alkali resistant).
Tab. 1.3: Composition of glass fibers E-type.
constituents % weight
SiO2 53-54
Al2O3 14-15.5
CaO 20
MgO
B2O3 6.5-9
F 0-0.7
Fe2O3 <1
TiO2
Na2O <1
K2O
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
48
Tab. 1.4: Thermal, mechanical, physical properties of glass fibers E-type.
properties
thermal capacity 0.8 J g-1 K-1
thermal conductivity 1.0 W m-1 K-1
linear dilatation coefficient 5.10-6 K-1
traction resistance 3.4 GPa
elasticity modulus 73 GPa
Poisson coefficient 0.22
extensibility 4.4%
volume 2.6 g cm3
humidity resumption <0.1%
The glass-fiber reinforced plastics marketed are predominately based on one type of
glass fiber, the E-type, but a wide variety of fiber formats (mats, fabric, unidirectional
roving), resin types filler/additives and process techniques are available. Fiber lengths
can vary from different length discontinuous fibers (milled, short, and long) to
continuous fibers in swirled mats, fabrics, non-crimped fabrics and unidirectional plies.
The major use of glass fibers is still as chopped strand mats of 25-50 mm length. The
different formats are often used together. In Tab. 1.5 are listed the different processes
routes and uses associated with formats.
Molten glass is extruded under gravity from a melting tank through an orifice and
rapidly pulled to draw it down to a 10 µm diameter fiber. Coatings are used to promote
adhesion and protection as well as to enhance wetting and bonding between fibers and
matrix. They are made mainly by water (85-95%), the other components are:
3-15% of sealants, such as vinyl polyacetates, polyesters used to protect;
0.5-2% of lubricants, such as cationic surfactants, to protect and lubricate fibers
surface during their use;
antistatic agent;
0.5-1.5% of coupling agents, typically organosilanes, to lay organic matrix to
glass surface.
CH
AP. 1
M
ATER
IALS
Phot
opol
ymer
ized
mic
ro- a
nd n
ano-
com
posi
tes:
inte
rfac
e ch
emis
try
and
its ro
le o
n in
terf
acia
l adh
esio
n
46
Tab.
1.5
: Gla
ss fi
ber f
orm
ats w
ith c
orre
spon
ding
com
posi
te m
ater
ials
, man
ufac
turi
ng p
roce
sses
and
end
use
s.
Fibe
r fo
rmat
Fi
ber
leng
th (m
m)
Com
posi
te m
ater
ials
type
s N
orm
al p
roce
ss r
oute
s T
ypic
al a
pplic
atio
ns
Mill
ed
< 0.
1 M
oldi
ng c
ompo
unds
In
ject
ion
mol
ded
Elec
trica
l, au
tom
obile
D
isco
ntin
uous
-sho
rt <
1 M
oldi
ng c
ompo
unds
In
ject
ion
mol
ded
Elec
trica
l, au
tom
obile
D
isco
ntin
uous
-long
<
7.5
Mol
ding
com
poun
ds
Inje
ctio
n m
olde
d El
ectri
cal,
auto
mob
ile
Cho
pped
stra
nd m
at
7.5-
50
CSM
, Dou
gh m
oldi
ng
com
poun
ds (D
MC
), sh
eet
mol
ding
com
poun
ds (S
MC
)
Han
d la
y-up
, spr
ay la
y-up
, co
mpr
essi
on m
olde
d M
arin
e, c
hem
ical
tank
s,
gene
ral t
rade
mol
ding
Swirl
ed m
at
Con
tinuo
us
Gla
ss m
at th
erm
opla
stic
s (G
MT)
, pul
trude
d pr
ofile
s Th
erm
ofor
med
, pul
trusi
on
Aut
omob
ile c
ompo
nent
s,
acce
ss e
ngin
eerin
g, c
able
tra
ys
Stitc
hed,
pin
ned,
ne
edle
d pr
oduc
ts
All
Any
M
ost
All
Wov
en fa
bric
s C
ontin
uous
Li
ghte
r wei
ght c
loth
s. D
iffer
ent s
tyle
s R
esin
inje
ctio
n, h
and
lay-
up,
pres
s mol
ded
Gen
eral
eng
inee
ring,
pr
essu
re v
esse
ls, m
arin
e
Wov
en ro
ving
s C
ontin
uous
H
eavi
er w
eigh
t clo
ths
Han
d an
d m
achi
ne la
y-up
, pu
ltrus
ion
Hea
vy m
arin
e
Kni
tted
Con
tinuo
us
2-D
and
3-D
fabr
ics
Res
in in
ject
ion
Con
stru
ctio
n, ra
ndom
es,
prop
elle
rs
Non
crim
p fa
bric
s N
CF
Con
tinuo
us
Bi-,
tri-
and
quad
ric-a
xial
R
esin
inje
ctio
n, p
ress
m
olde
d, H
and
lay-
up
Mar
ine,
con
stru
ctio
n,
auto
mob
ile
Mul
tidire
ctio
nal
Con
tinuo
us
Prei
mpr
egna
tes,
rovi
ngs
Pres
s mol
ded,
fila
men
t w
indi
ng
Hig
h pe
rfor
man
ce
aero
spac
e,
F1
raci
ng, p
ipes
, tor
que
tube
s, ro
cket
mot
or c
ases
U
nidi
rect
iona
l C
ontin
uous
Pr
eim
preg
nate
s Pr
ess m
olde
d, p
ultru
ded
strip
bar
W
ind
turb
ine
blad
es
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
50
Glass fibers used in this work have been supplied by Vetrotex International, treated
with a coating made by water and antistatic agents. Their characteristics are resumed in
Tab. 1.6.
Tab. 1.6: Properties of glass fibers E-type used in this work.
name coating type weight
(g/km of fiber)
fiber diameter
announced
(µm)
fiber diameter
measured
(µm)
untreated
fibers
Water +
antistatic
agents
480 17 18.1 ± 1.4
1.3 Organosilanes7,8
A coupling agent can be defined as a material that improves the retention of
properties of the chemical bond across the interface between a mineral surface and an
organic resin, in presence of moisture. Since silane organofunctional silicones are
hybrids of silica and of organic materials related to resins, they could be called silane
coupling agents.
The effectiveness of a silane as coupling agent is related to the reactivity of its
organofunctional group towards the resin.
The chemical structure of a silane coupling agent can be represented as follows (Fig.
1.1):
X
XX Si R Y
X = hydrolysable functional group (CH3O )
R Y = functional group which can react with the matrix monomers
Fig. 1.1: General structure of silane coupling agent.
The hydrolysable groups X are intermediates in the formation of silanol groups for
bonding to the mineral surface; the organofunctional group R-Y is chosen for its
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
51
reactivity or compatibility with the polymer. The coupling action depends on a stable
link between the X and Y groups.
Usually silanes are applied to inorganic surfaces from a water solution. Their
hydrolysis in water is dependent of the nature of the R-Y group9, but it is relatively fast
and can be considered complete in 1-30 minutes (at pH 3-4); afterwards the silane triols
condense to oligomers. The grafting reaction is schematized in Fig. 1.2.
In the ideal case a monolayer should be obtained on the surface, but what is realized
experimentally is a structure made of non-fully condensed grafted polysiloxane layers.
When silane is deposed on the surface of the inorganic filler, it forms a covalent
bond through a condensation reaction which is much slower (hours) and dependent on
the temperature (usually around 100-110°C).
X Si
X
X
R YH2O
Si
OH
OH
HO R Y
Hydrolysis
Condensation
SiOH
SiOH
SiOH Si
OH
OH
HO R YSi OSi O Si R Y
Si O3 H2O
inorganicsurface
inorganicsurface
Fig. 1.2: Scheme of hydrolysis and condensation reaction of organo-silane molecule on
inorganic surface.
The thickness of the silane layer can be estimated by various means, i.e. total carbon
analysis, ignition weight loss, scanning electron microscopy, contact angle
measurements, FT-IR spectroscopy, etc.
In particular FT-IR studies made by Ishida and Koenig on high surface fumed silica
and E-glass fibres10,11 had demonstrated the mechanisms of organization of silane
molecules on mineral surface: silanes adsorbed from water solution tend to give a
monolayer coverage on silica surface, while they form a film on E-glass fibres. It should
be noticed that in the case of multilayers, we have a high degree of condensation of the
silanols to siloxanes after the drying treatment. These siloxanes are initially soluble and
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
52
fusible, so the interaction with the polymer should be made at that moment because
after a rigid cross-linked structure is obtained.
Thermosetting resins react with a combination of the two mechanisms, while
thermoplastics go often through the formation of an IPN.
The inorganic surfaces used in this work have been modified using different types of
alkoxy-silanes; they have been chosen taking in account their specific functionality to
couple with the polymeric matrices.
According to this assumption, two alkoxy-silanes with epoxy-functionality (to match
with epoxy resines) and an acrylate-terminated one (to match with acrylated resin) were
used. The hydrolysable ligand was selected to be the same for all the organofunctional
silanes employed in this work.
Their structures are:
Epoxycyclohexil-ethyl trimethoxysilane (CETS), supplied by WITCO, has been
used to modify inorganic surfaces for the preparation of cycloaliphatic-matrix
composites, as it can be seen from its structure, Fig. 1.3.
H3CO
H3COH3CO Si CH2 CH2
O
CETS Fig. 1.3: CETS structure.
Glycidoxypropyl trimethoxysilane (GPTS), supplied by Aldrich, has been used
to modify inorganic surfaces for the preparation of diglycidyl ether-based composites,
as it can be seen from its structure, Fig. 1.4.
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
53
H3CO Si CH2 CH2 CH2 O CH2
O
H3CO
H3CO
GPTS Fig. 1.4: GPTS structure.
Trimethoxysilyl propyl-methacrylate (MEMO), supplied by Aldrich, has been
used to modify inorganic surfaces for the preparation of epoxidized acrylate soybean oil
-based composites, as it can be seen from its structure, Fig. 1.5.
H3CO Si CH2 CH2 CH2 O C
O
C
CH3
CH2
H3CO
H3CO
MEMO Fig. 1.5: MEMO structure.
An alkyl alkoxy-silane has been used to modify inorganic surfaces inducing
hydrophobic characteristics:
n-propyl trimethoxysilane (C3), supplied by Petrarch System Inc.; its structure is
represented in Fig. 1.6.
H3CO
H3COH3CO Si CH2 CH2 CH3
C3 Fig. 1.6: C3 structure.
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
54
1.4 Thermoset matrices
In this work three thermoset matrices have been used.
3,4-epoxycyclohexylmethyl-3’,4’-epoxycyclohexanecarboxylate (CE), Fig. 1.7,
Cyracure® UVR 6110, supplied by DOW Corporation.
O
O
OO
CE Fig. 1.7: CE structure.
1,4-cyclohexane dimethanol diglycidyl ether (DGE) supplied by Aldrich.
Its structure is represented in Fig. 1.8.
CH2
CH2
O CH2
O CH2
O
O
DGE
Fig. 1.8: DGE structure.
Soybean oil epoxidized acrilate (SOA) supplied by Aldrich.
SOA was obtained by acrilation of the corresponding epoxidized product13 (Fig. 1.9);
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
55
CH2 O C
O
(CH2)7CH CH CH2 CH CH (CH2)4CH3
CH O C
O
(CH2)4CH CH CH2 CH CH CH2 CH CH (CH2)4CH3
CH2 O
C
O
(CH2)7CH CH (CH2)7CH3
O
O
O
epoxidized soybean oil Fig. 1.9: Epoxidized soybean oil structure.
For each molecule of epoxidized product, two molecules of acrylic acid, on average,
have been introduced.
According to C13-NMR, the proposed structure of SOA is represented in Fig. 1.10.
CH2 O C
O
(CH2)7CH CH CH2 CH
OH
CH
O C
O
CH CH2
(CH2)4CH3
CH O C
O
(CH2)4CH CH CH2 CH CH CH2 CH
OH
CH
O C
O
CH CH2
(CH2)4CH3
CH2 O
C
O
(CH2)7CH CH (CH2)7CH3
O
SOA Fig. 1.10: SOA proposed structure.
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
56
1.5 Photoinitiators
Triphenylsulphoniumhexafluoroantimonate, (Cyracure® UVI 6974) supplied by
DOW Corporation was used as cationic photoinitiator to polymerize CE and DGE
monomers. Its structure is represented in Fig. 1.11 (I).
SOA has been cured via radical mechanism, using 2-hydroxy-2-methyl-1-
phenyl-propan-1-one, (Darocur® 1173) supplied by Ciba Specialty Chem. as radical
photoinitiator. Its structure is also represented in Fig. 1.11 (II).
S
SbF6-
(I)
C
O
C
CH3
CH3
OH
(II)
Fig. 1.11: Cyracure® UVI 6974 structure (I), Darocur® 1173 structure (II).
In Tab. 1.7 are reported the UV absorption peaks for the two photoinitiators.
Tab. 1.7: UV absorption peaks for the photoinitiors used.
photointiator UV absorption peak (nm)
Cyracure® UVI 6974 (5.56 10-5 M in propylene carbonate)
300-310
Darocur® 1173 (4 10-5 g/ml in methanol)14
265-28014
Both the photoinitiators are activated by standard mercury-filled UV bulbs, such as
Fusion® lamp, because they efficiently absorb some of the major emission bands of the
standard bulbs, as shown in Fig. 1.12 for the cationic photoinitiator.
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
57
Fig. 1.12: Overlap of cationic photoinitiator absorption and standard UV bulb
emission spectra12.
1.6 UV-lamps and reactive formulations selected
Different UV-lamps have been used:
Helios Italquartz lamp: laboratory UV-lamp, equipped with a support which
allows changing the distance of the sample from the lamp; there is possibility of
operating in nitrogen atmosphere. Lamp intensity (I) = 10 or 50 mW/cm2.
Fusion lamp: industrial UV-lamp, equipped with a belt conveyor giving to the
sample an adjustable speed. Lamp intensity (I) = 371.8 mW/cm2.
UV Perkin-Elmer DPA 7 XBO 450 W equipped with a monocromator to select
the wavelength; it was used for the photo-DSC measurements. The intensity is
dependent from the selected wavelength: at 300 nm, Iref = 0.581 µW/cm2 and Isample =
0.579 µW/cm2.
Reactive systems:
The different components were mixed in the proper concentration12,14,15 and UV-
cured.
CHAP. 1 MATERIALS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
58
Epoxy monomers, CE and DGE, were UV-cured through a cationic mechanism using
2% w/w of cationic photoinitiator. Curing was performed in air when Fusion or Helios
lamp has been used; in nitrogen during photo-DSC measurements, to assure a uniform
heat transfer during the experiment.
SOA monomer was UV-cured through a radical mechanism using 4% w/w of radical
photoinitiator. Curing was performed in air using Fusion lamp: the high intensity of the
lamp made possible to cure in air, overcoming oxygen inhibition.
CHAP. 2 MODIFICATION OF INORGANIC
SURFACES BY ORGANOSILANES
2.1 Introduction
The modification of inorganic surfaces is very important and usually performed in
order to assure a better adhesion and improved properties to the composite materials.
The surface modification allows to form bonds (physical or chemical) between the two
phases, which have different structure, and to increase compatibility of the systems.
2.2 Experimental: protocols for nanosilica and glass fibers
The modification of inorganic surface was carried out both on silica powder and on
glass fibers. In this paragraph the experimental procedure is reported; the starting
information was that found in the literature1-5, but it has been modified in order to
increase the adhesion properties.
All the experiments follow some common features: silane coupling agents are
dissolved in water solution at pH = 4 in order to have a fast hydrolysis of the
organosilane molecules3 (and, as a consequence, a lower rate of condensation reactions
between hydrolyzed species in the solution).
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
60
The hydrolysis reaction in these conditions is known to be very fast (from 1 to 10
min. depending on the reactivity of the silane); then the mixture is ready to perform the
grafting of inorganic surfaces.
The first step is the absorption of the silane on the inorganic surface; then a
condensation reaction is performed in order to link the silane molecule on the surface by
forming strong chemical bonds3,5.
In this step, done at high temperatures, silane molecules form siloxane bonds with
inorganic surface.
In Fig. 2.1 the scheme of the reactions involved in aqueous systems is reported.
X Si
X
X
R YH2O
Si
OH
OH
HO R Y
Hydrolysis
Condensation
SiOH
SiOH
SiOH Si
OH
OH
HO R YSi OSi O Si R Y
Si O3 H2O
inorganicsurface
inorganicsurface
Fig. 2.1: Scheme of hydrolysis and condensation reaction of organo-silane molecule on
inorganic surface.
The grafting reaction was carried out in different experimental conditions to evaluate
the best ones in order to obtain high adhesion values.
Before any grafting treatment, the inorganic products were dried by putting in an
oven at 150°C for 18 hours for silica powder, and at 250°C for 48 hours for glass fibers.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
61
GRAFTING OF SILICA NANOPARTICLES
Grafting of silica was performed in different solvents, namely water, CH3COCH3 and
H2O/EtOH (50/50 v/v) at pH = 4 (CH3COOH) by using a silane concentration of 1 mL
for 100 mL of solvent.
Silica powder was introduced in the silane solution and left for two hours, at room
temperature, under ultrasonic stirring. Then it was filtered and treated in an oven at
120°C for 4 hours in order to promote the condensation reaction. The product was then
washed, to remove all the unreacted species, and again dried in an oven at 120°C for 2
hours.
Modified silica powder was characterized using thermogravimetric (TGA) analysis.
TGA were performed in the interval 50°-750°C, at heating rate of 10 C/min, in
nitrogen atmosphere, using 10-20 mg of sample and a Mettler-Toledo instrument.
GRAFTING PROCEDURE OPTIMIZATION
In Tab. 2.1 the obtained TGA results in the different solvents are reported. They
indicate in all cases a similar weight loss values. These results are in agreement with
those reported in literature1.
Tab. 2.1: TGA results on silica grafted using different solvents*.
solvent % weight loss (TGA)
H2O 5.8
H2O/EtOH 5.4
CH3COCH3 5.9 *silica grafted using CETS 1% v/v.
Water was selected as solvent for the grafting reaction in all the experiments.
Then the duration of the condensation step was investigated. The experiments were
executed at 120°C, changing the time from 4 to 18 hours. TGA analysis (Tab. 2.2)
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62
showed that weight loss is practically, as expected, the same. Therefore a reaction time
of 4 hours was adopted for the grafting procedures.
Tab. 2.2: TGA results on silica powder*after different condensation time.
time (hrs) % weight loss (TGA)
4 8.8
18 9.3 *silica grafted using CETS 1% v/v in CH3COCH3.
In order to investigate the influence of washing protocol of the surface, experiments
were performed washing the silica powder immediately after grafting or after a 4 hrs
condensation in an oven at 120°C (Tab. 2.3).
Tab. 2.3: TGA results on silica powder*after different washing treatments.
sample % weight loss (TGA)
washed without condensation 0.43
washed after condensation (4 hrs) 7.03 *silica grafted using CETS 1% v/v in CH3COCH3.
The results indicate the importance of performing the thermal condensation reaction
in order to obtain grafting, i.e. covalent bonding of silane molecules to the surface.
The results of Tab. 2.2 and Tab. 2.3 are slightly different from those reported in Tab.
2.1, because they were performed using a different TGA instrument.
In conclusion the grafting procedure adopted for our systems can be summarized as
follows:
Materials:
silica powder = 2g
silane coupling agent, CETS or GPTS (for 2g of silica powder) = 1ml
reaction solvent, distilled water = 100 ml
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63
Experimental:
pH = 4 (CH3COOH)
T = room temperature, i.e. 25°C
Time = 2 hours
Ultrasonic stirring, 10 min
Silica powder addition and ultrasonic stirring, 2 hrs
Filtration
Condensation reaction in an oven for 120°C for 4 hrs
Washing with distilled water
Drying at 120°C for 2 hrs.
GRAFTING OF SILICA WITH CETS AND GPTS SILANES
In order to characterize the modified silica surface, TGA analyses were carried out.
The analyses were carried out in different experimental conditions from the previous
ones (LECO TGA-601 instrument):
Temperature range = 50-900°C
Heating rate = 1°C/min
Atmosphere = air
Sample weight = 500 mg.
The results obtained are listed in Tab. 2.4 and Tab. 2.5 referred to the use of CETS
and GPTS silanes in the standard grafting conditions.
In Fig. 2.2 are presented typical TGA curves obtained for silica grafted with CETS:
they give evidence of the reproducibility of the experiment.
Tab. 2.4: Weight loss of silica grafted with CETS (1% v/v, 2g silica powder).
%weight loss
2.79; 2.84
2.87; 2.85
2.58; 2.57
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
64
Tab. 2.5: Weight loss of silica grafted with GPTS (1% v/v, 1g silica powder).
%weight loss
5.22; 5.26
5.57; 5.58
5.35; 5.44
The scattering of the reported results obtained in Tab. 2.4 and Tab. 2.5 can be
attributed to the different quantities of silica analyzed in these experiments.
Fig. 2.2: TGA reproducibility curves of silica powder grafted with CETS (1% v/v).
GRAFTING OF E-GLASS FIBERS
The grafting conditions and the experimental procedure used were the same as for
silica nanoparticles. Glass fibers were modified using different concentrations of silane
coupling agent in solvent (CH3COOH; pH = 4), in the interval 0.1-1% v/v, in order to
obtain silane layers of different thickness. In fact, in the literature6 it is reported that
adhesion between glass fibers and polymer matrix increases by decreasing the thickness
of silane layer.
TGA analyses were carried out on glass fibers, treated with the standard procedure
described previously.
96
97
98
99
100
0 200 400 600 800 1000
Temperature (°C)
%
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
65
In Tab. 2.6 the results obtained for glass fibers grafted using 0.5% v/v of silane agent
are reported.
Tab.2.6: TGA analyses of grafted E-glass fibers.
silane agent silane concentration %weight loss
0.1 % v/v in H2O 0.62 CETS
0.5 % v/v in H2O 1.25
0.1 % v/v in H2O 0.65 GPTS
0.5 % v/v in H2O 1.20
It can be noticed that practically the same weight loss is obtained with the two silane
agents; moreover the weight loss values are clearly lower compared with the ones
resulting from silica nanoparticles; these results can be explained mainly considering
the lower specific surface of E-glass fibres compared to silica.
SURFACE WETTING MEASUREMENTS
Surface modification can be detected by measuring the contact angles of the surfaces
before and after surface treatment with probe liquids. As far as silica nanoparticles are
concerned, the surface modification was simulated by using an oxidized,
treated/untreated silicon wafer surface.
Silicon wafer surfaces were cleaned at 550°C for 2 hrs and then oxidized by treating
with a mixture of H2SO4 (98%): H2O2 (30 vol.): H2O = 1: 1: 3 for 15 min at 100°C.
The wetting measurements were done using bi-distilled deionized water. A dynamic
contact angle instrument or a Cahn balance was used.
In Tab. 2.7 the results obtained from dynamic contact angle measurements performed
on silicon wafer surface are collected.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
66
Tab. 2.7: Contact angle with water on oxidized grafted silicon wafer.
sample grafting solutions ϑADV ϑREC
oxidized, untreated
silicon wafer - 0 -
CETS 1% v/v in H2O 94.6 72.3
CETS 1% v/v in CH3COCH3 93 60.3
CETS 1% v/v in H2O/EtOH 90.7 52.8
GPTS 1% v/v in H2O 92.2 69.6
GPTS 1% v/v in CH3COCH3 88.6 68.5
GPTS 1% v/v in H2O/EtOH 85.6 67.4
The results of Tab. 2.7 indicate clearly a strong modification of the surface which
becomes more hydrophobic after the grafting reaction. The contact angles slightly
changes by changing the grafting solutions; in any case they are higher when water is
used as solvent.
As far as glass fibers are concerned, the Cahn balance was used both on fibers, and
glass slides (models of the glass fibers surface). Results are presented in Tab. 2.8.
Tab. 2.8: Contact angles with water obtained with Cahn balance on grafted* glass
fibers and glass slides.
ϑADV ϑREC
untreated glass fiber 35 ± 10 28.8± 10
CETS treated glass fiber 85.5 ± 12 57.7 ± 12
GPTS treated glass fiber 83.1± 5 56.9 ± 5
untreated glass slide 29.5 ± 2 -
CETS treated glass slide 95.2 ± 1 67.1± 1
GPTS treated glass slide 79.9 ± 3 52.9 ± 3
C3 treated glass slide 95.4 ± 1 65.2 ± 1 * 1% v/v silane in water.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
67
It should be noticed that results obtained on glass fibers, (Tab. 2.8), are done at the
sensibility limits of the instrument; therefore the experimental error is quite high.
It is evident that the surfaces of both glass fibers and glass slides have been modified
making them more hydrophobic.
TOPOGRAPHY
AFM measurements, using the tapping mode, were performed on treated/untreated
silicon wafer surface (used as model for silica powder) and on treated/untreated glass
fibers surface.
Fig. 2.3 shows the AFM image and profile of oxidized untreated silicon wafer
surface; Fig. 2.4 and Fig. 2.5 show AFM images and profiles of silicon wafer surface
after oxidation and treatment with GPTS 1% v/v and CETS 1% v/v, respectively. With
any grafting treatment performed, on wafer surface are clearly visible the agglomerates
described in literature6 as “silane islands”, due to the grafting reaction that leads to a
non-homogeneous surface modification.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
68
Fig. 2.3: AFM image and profile of untreated oxidized silicon wafer surface.
Fig. 2.4: AFM image and profile of silicon wafer surface oxidized and grafted with
GPTS 1% v/v.
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
69
Fig. 2.5: AFM image and profile of silicon wafer surface oxidized and grafted with
CETS 1% v/v.
AFM images on E-glass fibers surface were recorded taking into account the
cylindrical shape of the sample. A portion of the fiber was selected in order to assure the
best adhesion of the AFM cantilever tip to the sample during all the measurement (Fig.
2.6).
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
70
Fig. 2.6: Schematic representation of AFM analysis on E-glass fiber surface.
Fig. 2.7 shows the AFM image and profile of untreated E-glass fiber, while Fig. 2.8
and Fig. 2.9 show the images and profile of the surface of E-glass fiber grafted with
CETS in different percentages; Fig. 2.10 and Fig. 2.11 show the images and profiles of
the surface of E-glass fiber grafted with GPTS in different percentages.
The glass fiber surface after the grafting presents a different morphology from the
untreated one, more homogeneous and with the characteristic “silane islands” already
observed onto the grafted silicon wafer surface.
CANTILEVER TIP
GLASS FIBER
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
71
Fig. 2.7: AFM image and profile of untreated E-glass fiber surface.
Fig. 2.8: AFM image and profile of E-glass fiber surface treated with CETS 0.1%
v/v.
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
72
Fig. 2.9: AFM image and profile of E-glass fiber surface treated with CETS 0.5%
v/v.
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
73
Fig. 2.10: AFM image and profile of E-glass fiber surface treated with GPTS 0.1%
v/v.
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
74
Fig. 2.11: AFM image and profile of E-glass fiber surface treated with GPTS 0.5%
v/v.
Other experimental evidences of changes in surface morphology, after grafting
reaction, are visible from the results of analyses performed with SEM on glass fibers.
In Fig. 2.12 the untreated glass fiber surface is presented, while in Fig. 2.13 and Fig.
2.14 glass fiber surface after treatment with silane (GPTS 0.5% v/v).
Also from these images is evident the presence of agglomerates (“silane islands”) on
inorganic surface, moreover it is possible to observe that after treatment, surface is
much more homogeneous7, the defects present are somehow “repaired”.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
75
Fig. 2.12: SEM image of untreated E-glass fiber.
Fig. 2.13: SEM image of treated E-glass fiber (GPTS 0.5% v/v).
CHAP. 2 MODIFICATION OF INORGANIC SURFACES BY ORGANOSILANES
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76
Fig. 2.14: SEM image of treatedE- glass fiber (GPTS 0.5% v/v).
2.3 Conclusions
Grafting of silica powder was performed in different solvents (H2O, CH3COCH3,
H2O/EtOH 50/50 v/v); the weight loss values determined by TGA were practically the
same. Contact angle measurements performed on oxidized and grafted silicon wafers,
using the same solvents indicate a clear increase of the hydrophobicity of the surface;
the values obtained were the same using the different solvents, indicating that the
grafting reaction is not influenced by the solvent in the adopted conditions.
The condensation reaction gives a constant weight loss after 4 hrs treatment at
120°C. If the thermal treatment is not performed the weight loss was negligible, thus
indicating the absence of condensation reaction.
Grafting of glass fibers was performed in the same conditions adopted for silica
powder. The weight loss values determined by TGA were about one order of magnitude
lower than those obtained with silica powder; nevertheless the contact angle values with
water were increased sharply, indicating the formation of hydrophobic surfaces.
CHAP. 3 UV-POLYMERIZATION IN THE
PRESENCE OF NANOFILLERS
3.1 Introduction
The first problem in preparing UV-cured composites is the filler transparency
towards UV-light. Otherwise it will be a competition of the filler in absorbing the UV-
radiation. In this work the influence of nanosilica on the curing reaction was
investigated in order to verify if it can modify kinetics as well as total conversion of
monomer during the UV-curing process and if the surface modification has any
influence on the reaction1,2. Only nanosilica-filled systems were investigated, because
its higher surface area should lead to more evident interaction effects in polymerization
kinetics. In the following sections the results obtained on the filler/polymer systems are
presented.
3.2 Experimental
UV-CURING
Different types of UV-lamps were used to cure the composites:
Fusion lamp: thick (2 mm) samples, used for mechanical properties
measurements, were irradiated for 21 s each side at I = 371 mW/cm2.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
78
Helios Italquartz lamp: it was used for FT-IR kinetic measurements. The
samples were put between two disks of KBr and irradiated for 5 s intervals at I = 51
mW/cm2.
Photo-DSC lamp: samples were irradiated for 33 min. at I = 0.58 µW/cm2 (total
time run = 36 min). The optimal operative conditions for these systems were established
with tests at different wavelengths, with different sample weights and different amounts
of treated/untreated nanofillers; the obtained conditions, reported Tab. 3.1, were used
for all the experiments.
Tab. 3.1: Operative conditions for photo-calorimetric experiments.
CE CE + 10% w/w silica
optimal weight (mg) 0.390 0.370*
optimal wavelength (nm) 280-290 280
DGE DGE + 10% w/w silica
optimal weight (mg) 0.400 0.400*
optimal wavelength (nm) 300 300
The weight values indicated with * represent the total weight of the sample present in
the DSC aluminum pan; all the ∆H values obtained during experiments with nanofillers
have been corrected to taking into account the presence of silica.
Several experiments were done for each concentration to control their
reproducibility.
FT-IR and photo-DSC techniques were used to follow the reaction kinetics of the
reactive systems filled with different amounts of nanosilica concentrations.
FT-IR measurements
FT-IR instrument was used to follow the reaction kinetics by measuring the decrease
of the band at 750 cm-1 due to the epoxy group polymerization. The measurement was
discontinue; irradiation was performed for 5 s at the beginning and for 10 s until the end
of the reaction. The absorbance data were plotted against the reaction time.
CHAP. 3 UV-POLYMERIZATION IN THE PRESENCE OF NANOFILLERS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
79
Photo-DSC measurements
Photo-DSC was used to follow the reaction through a real-time measurement. Each
experiment was done at 30°C with an isothermal analysis, so that the ∆H curve was
obtained as a function of time. ∆H is related to the heat developed during the
polymerization reaction.
3.3 Reaction kinetics: Results and discussion
FT-IR RESULTS
The reaction kinetics were followed by monitoring the decrease of the epoxy band at
750 cm-1 (Fig. 3.1); normalization was made using the C=O band at 1730 cm-1.
40050060070080090010001100
Wavenumbers
Absorbance
Fig. 3.1: Typical FT-IR spectrum of neat CE.
In Fig. 3.2 examples of the data obtained for kinetic curves by using CE system are
reported. They concern the pure monomer and its mixtures with 10% w/w of treated
silica and with 10% w/w of untreated silica.
0 s
60 s
CHAP. 3 UV-POLYMERIZATION IN THE PRESENCE OF NANOFILLERS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
80
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100time (sec.)
% c
onv.
CE
CE + 10% w/wtreated silica
CE + 10% w/wuntreated silica
Fig. 3.2: FT-IR kinetic curves of CE system filled with untreated or treated silica
(I = 51 mW/cm2).
A decrease of both kinetic and total conversion values when treated/untreated silica
is added to the photopolymerizable system is evident. The grafting of silica seems not to
have an important effect on the UV-curing reaction kinetics.
As far as the DGE system is concerned (Fig. 3.3), similar results were obtained when
DGE is photopolymerized with 5% w/w of untreated silica3.
Fig. 3.3: FT-IR kinetic curves of unfilled and silica filled DGE system3
(I = 8 mW/cm2).
DGE
DGE + 5% untreated silica
CHAP. 3 UV-POLYMERIZATION IN THE PRESENCE OF NANOFILLERS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
81
The first explanation proposed for the effect of silica on the photopolymerization
kinetics could be the scattering of the UV radiation by silica particles.
In typical experiments, in the presence of 5% w/w of silica, the light transmittance
curve of the mixture decreases of 15%. When the photopolymerization was performed
reducing the intensity of the emitted light of this value, a very small variation of the
kinetic was observed. Therefore the scattering of UV-light by the dispersed silica
particles does not justify the modification of the curing process.
Taking into account the high specific surface of silica (200 m2g-1), we suggest that
the particles could adsorb the cationic species of the photoinitiator molecules, thus
decreasing their activity in the photopolymerization. In fact the photoinitiator species
are polar molecules which can strongly interact with the silica surface.
In order to check this possibility, UV measurements were performed on solutions of
the cationic photoinitiator in the presence of silica. The photoinitiator was dissolved in
propylene carbonate and its absorbance at 300 nm was evaluated. 5% w/w of treated
and untreated nanosilica was added to the solution. The mixture was centrifuged in
order to separate silica particles and the solution was examined by UV spectroscopy.
Results are collected in Tab. 3.2.
Tab. 3.2: Results of UV absorptions on photoinitiator-silica systems at 300 nm.
*Ph3S+SbF6-, 5.56 10-5 M in propylene carbonate.
The results above reported indicate that silica interacts deeply with the cationic
photoinitiator by adsorbing it on its surface. The adsorbed photoinitiator could have
lower activity under UV-irradiation. In this way, we can explain the decrease of the
photopolymerization kinetics in the presence of silica. The photoinitiator-silica
interaction will be further study in depth.
sample Abs310 nm
photoinitiator* 0.713
photoinitiator* added of 5% w/w untreated silica ≈ 0
photoinitiator* added of 5% w/w treated silica (CETS) ≈ 0
photoinitiator* added of 5% w/w treated silica (GPTS) ≈ 0
CHAP. 3 UV-POLYMERIZATION IN THE PRESENCE OF NANOFILLERS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
82
It should be noticed that in radical systems, no decrease of UV-curing kinetics in the
presence of silica was observed4.
PHOTO-DSC RESULTS
Examples of typical photo-DSC curves for the systems based on CE and DGE are
presented in Fig. 3.4 and Fig. 3.5.
In each figure are reported two different thermograms: one is related to the
polymerization of the pure monomer and the other to the polymerization of the
monomer filled with 20% treated silica.
Even if the most part of the photopolymerization occurs in few minutes; the samples
were irradiated for 33 minutes to assure the completion of the reaction.
Fig. 3.4: Photo-DSC traces of neat CE and CE monomer filled with silica
(treated CETS 1% v/v).
19,319,419,519,619,719,819,9
2020,1
0 10 20 30 40
time (min)
delta
H (J
/g)
CE
CE + 20% w/wtreated silica
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
83
Fig. 3.5: Photo-DSC traces of neat DGE and DGE monomer filled with silica
(treated GPTS 1% v/v).
To obtain the ∆H (J/g) values of each UV-DSC trace, integration of the area below
the base line (Fig. 3.6) was made. The integration limits were chosen in correspondence
of the switch on/off of UV lamp. The obtained value was corrected in the case of
composites, taking into account the different percentages of silica added to the sample.
19,319,419,519,619,719,819,9
2020,1
0 10 20 30 40
time (min)
delta
H (J
/g)
Fig. 3.6: Method for integration on UV-DSC trace.
19,319,419,519,619,719,819,9
2020,1
0 10 20 30 40time (min)
delta
H (J
/g)
DGE
DGE + 20%w/w treatedsilica
CHAP. 3 UV-POLYMERIZATION IN THE PRESENCE OF NANOFILLERS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
84
In Fig. 3.7 and Fig. 3.8 the results obtained in the kinetics experiments on CE and
DGE in the presence of treated or untreated silica are reported (for each concentration
three ∆H values were considered).
The figures indicate for both CE and DGE system a decrease of ∆H of
polymerization in the presence of different amounts of silica. Moreover the results
indicate no clear influence of the surface treatment of grafting on the
photopolymerization reaction.
Fig. 3.9 reports the kinetic curves calculated from the thermograms obtained using
CE added of different percentages of treated silica: they show the decrease of the ∆H
values when the amount of silica present in the system is increased.
The results of photocalorimetric experiments fully confirm those obtained by FT-IR
measurements indicating a decrease of the rate of reaction and of the final conversion in
the presence of silica nanoparticles.
150170190210230250270290310
0 5 10 15 20
% w/w silica
-del
taH
(J/g
)
CE + untreated silicaCE + treated silica
Fig. 3.7: ∆H of reaction as a function of percentage oft silica introduced in the CE
monomer.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
85
200
250
300
350
400
450
500
550
0 5 10 15 20
% w/w silica
-del
atH
(J/g
)DGE + untreated silicaDGE + treated silica
Fig. 3.8: ∆H of reaction as a function of percentage oft silica introduced in the DGE
monomer.
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25
t (min)
-del
taH
(J/g
)
CE+20% w/w treated silicaCE+15% w/w treated silicaCE+10% w/w treated silicaCE+5% w/w treated silicaCE
Fig. 3.9: Dependence of polymerization of CE monomer filled with various percentages
of grafted silica.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
86
3.4 Conclusions
FT-IR and photo-DSC measurements indicate that the addition of nanosilica leads to
a modification of the photopolymerization reaction and to a decrease in both kinetics
and total conversions of reactive groups.
As far as the influence of the surface treatment is concerned, the results obtained do
not indicate a clear influence on the photopolymerization reaction.
We explain the observed decrease of the photopolymerization kinetics in the
presence of silica on the basis of the interaction between photoinitiator and silica, as
previously reported in this chapter. In fact, such nanofillers display a large amount of
interface as the specific surface of silica is very important. One can aspects a minor
effect on the polymerization kinetics for the fiber-based composites, as the surface
displayed by the filler is very low compared to nanosilica.
CHAP. 4 UV-POLYMERIZED MICRO- AND
NANO-COMPOSITES
4.1 Introduction
In this chapter the preparation of micro- and nano-composites using modified or
unmodified fillers is reported. The cured products were subjected to thermal and
mechanical tests.
Nano-composites were investigated by using dynamic-mechanical analysis
(DMTA)1.
Micro-composites were investigated by using the microbond technique2-5 in order to
evaluate the interfacial adhesion as a function of the grafted species.
4.2 Experimental
DMTA analysis
The samples were prepared using 10% w/w of silica. Photopolymerization was
performed by irradiating with a Fusion lamp for 21 s at I = 371 mW/cm2 on each side of
the sample. Testing of the samples was performed in the bending mode using a
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88
Rheometric Scientific MKIII apparatus. Samples were tested in bending
configuration, single cantilever; the temperature range was from 0° to +250°C for the
CE/silica systems and from -50° to +80°C for the DGE/silica systems. Measurements
were carried out at 1Hz frequency.
Microbond technique
Microcomposites were prepared by putting microdroplets of photopolymerizable
monomer (+ photoinitiator) on E-glass fibers and by curing with a Fusion lamp for 21 s
at I = 371 mW/cm2.
The interfacial adhesion of samples was tested by using a dynamometer (cell load =
10 N, v = 0.1 mm/min); the experiments were followed using a micro camera.
Preparation of samples: single treated or untreated glass fibers were fixed on a frame
under small tension. By using a copper filament, microdroplets of photocurable mixture
were deposed on fibers and UV-cured.
In Fig. 4.1 the preparation process is schematically reported.
Fig. 4.1: Preparation of microcomposites for the microbond test.
After UV-curing, fibers were cut in 1 cm length pieces taking care of having in each
piece at least one cured droplet; each segment was fixed with glue on a triangle made by
PET (Fig. 4.2) and tested with the dynamometer.
FRAME
GLASS FIBERS
MICRODROPLETS
COPPER FILAMENT
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
89
Fig. 4.2: Sample for microbond test.
Each mechanical test gives a diagram similar to the one presented in Fig. 4.3.
The peak in the curve indicates the maximum force registered opposed by the sample
immediately before the detachment of the droplet.
Fig. 4.3: Typical curve obtained from a microbond test.
As each force value is related to the droplet dimensions (length and diameter), it is
necessary to know them exactly before any measure. They were evaluated using an
optical microscope equipped with a device that allows to measure and to express them
in µm.
About 30 measurements for each type of sample should be performed in order to
overcome the high degree of dispersion of data which is connected with the type of test
and the type of fibers used.
Fig. 4.4 represents a typical graph obtained; it is evident the difficulty in having a
good reproducibility, therefore a comparison of the data obtained in this form.
Force
FrictionForce
Maximum Force
Extension
Force
FrictionForce
Maximum Force
Extension
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
90
0
0,1
0,2
0,3
0,4
50 100 150 200Le (micron)
F (N
)
Fig. 4.4: Debonding force (F) vs. embedded length (Le) curve obtained from
microbond measurements (CE + untreated glass fibers).
For these reasons, the experimental values were treated using the Kelly-Tyson
formula for the average interfacial shear strength at the interface (IFSS) 6, dτ :
ef
dd
lrFπ
τ2
=
Where: Fd = force at debonding
le = embedded length
2rf = fiber diameter.
The dτ value is a measure of the interfacial adhesion assuming a constant shear stress
along the embedded length, i.e. a plastic behavior of the interface7. It allows evaluating
the change of the adhesion between the matrix and the glass fibers as a function of the
surface treatment.
Higher values reflect a more effective interface, thus a better silane performance.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
91
4.3 Results and discussion
DMTA ANALYSES
The measured Tα, i.e. the temperature position for the maximum of the main
elongation peak related to Tg, values for CE and DGE systems in presence of silica are
reported in Tab. 4.1 and compared with those obtained from the pure monomers.
Tab. 4.1: Tα, values for UV-cured systems in presence of silica at 1 Hz.
The experimental results presented in Tab. 4.1 evidence a decrease of Tα for both the
systems (CE, DGE) when silica is added. These results are in agreement with the
polymerization kinetic data reported in the previous chapter indicating a reduction of the
epoxy group conversion and thus of crosslinking density of the matrix in the presence of
silica.
In Fig. 4.5 and Fig. 4.6, the DMTA curves related to CE system are presented. This
one cannot be easily obtained from the value of the storage modulus in the rubbery
plateau as this network is intrinsically heterogeneous, i.e. proceeding from the
percolation of microgels formed from the early stages of the polymerization.
sample Tα matrix Tα matrix + 10% w/w untreated silica
CE 214°C 182°C
DGE 53°C 37°C
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92
Fig. 4.5: DMTA curves of CE neat matrix.
Fig. 4.6: DMTA curves of CE (matrix + 10% w/w untreated silica).
DMTA tests were performed also on CE added with untreated glass fibers finely
pulverized and cured with the same technique used for silica composites.
The results are presented in Tab. 4.2.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
93
Tab. 4.2: DMTA analyses performed on photocured neat and pulverized glass fibers
filled CE composites.
CE neat matrix CE + 50% untreated glass fibers
Tα* 214 °C 204 °C
E’25°C 287 MPa 973 Mpa
E’250°C 7.71 Mpa 56.4 Mpa
* Tα is given at 1 Hz.
It can be seen that Tg is almost practically not affected by the addition of fillers, even
in great quantities. This behavior is very different from that obtained in the presence of
silica powder.
In the previous chapter it was proved that silica interacts with the photoinitiator
causing a decrease of the curing kinetics. In the case of pulverized glass fibers this
interaction does not occur, as the amount of inorganic surface which can interact with
photoinitiator molecules is lower than for nanosilica.
As expected, the storage tensile module increases with respect to the pure monomer
due to the high stiffness of the inorganic fillers.
MICROBOND MEASUREMENTS
As far as the microbond test is concerned, microcomposites were prepared using
glass fibers treated with different concentrations of the silane agent. The experimental
conditions are listed in Tab. 4.3.
In Fig. 4.7 and Fig. 4.8 are reported the variations of calculated IFSS, dτ , as a
function of the silane agent percentage for DGE and CE matrix-based microcomposites.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
94
Tab. 4.3: Concentration of silane agent used for grafting E-glass fibers used in
microbond tests.
CE matrix microcomposites DGE matrix microcomposites
silane
concentration of
silane solution
(% v/v)
silane
concentration of
silane solution
(% v/v)
0 0
0.1 0.1
0.25 0.25
0.5 0.5
CETS
1
GPTS
1
00,20,40,60,8
11,2
0 0,5 1
% GPTS
IFSS
(MPa
)
Fig. 4.7: IFSS of DGE matrix-based microcomposites as a function of the silane
concentration.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
95
The adhesion results obtained using DGE as polymer matrix show that increase of
IFSS is reached in the presence of a much diluted silane agent concentration (0.1%),
then IFSS decreases.
1
1,5
2
2,5
3
3,5
0 0,5 1
% CETS
IFSS
(MPa
)
Fig. 4.8: IFSS of CE matrix microcomposites as a function of the silane
concentration.
The results obtained using CE as polymer matrix show good adhesion properties
even on untreated glass surfaces, in agreement with the literature8,9. These values can be
explained on the basis of the polar interactions between the two phases. Nevertheless,
interfacial shear strength of the interface remains very low compared to that obtained
for thermoset-glass fibers interfaces7.
By using glass fibers treated with CETS, the IFSS values decrease. This result could
be attributed to a decrease of the polarity of the surface in the presence of CETS even if
strong interactions, i.e. covalent bonding, are expected from the reactions of
cycloaliphatic epoxy groups from the grafted silane and CE matrix.
In fact when glass fiber surface is treated with a less polar silane coupling agent, as
propyltrimethoxy silane (C3) the IFSS values decrease sharply. The experimental
results are presented in Tab. 4.4.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
96
Tab. 4.4: Results of interfacial shear strength of the interface from microbond test on
CE matrix-based microcomposites
(E-glass fibers treated with the different organosilane agents).
sample characteristics IFSS (MPa)
silane concentration (% v/v)
CETS 0 3.33
CETS 1 2.74
C3 1 1.32
As literature8 gives evidences of the benefits of treating fillers surface in order to
improve interface resistance during hydrothermal exposure, mechanical properties tests
were performed on samples before and after thermal or hydro-thermal (RH-controlled
chamber) ageing. In fact, hydrothermal ageing could be used to proof the existence of
covalent bonds based interfaces vs. secondary interactions based interfaces. Covalent
bonds remain after aging as physical ones are destroyed. This behavior could be
explained taking into account the interactions developed by epoxy-based polymer
networks and water molecules during the hydrothermal ageing9-11.
These interactions are mainly due to the diffusion of water molecules in the
interfacial region. Once there, water molecules break the weak polar-polar interaction
created between polymer matrix and inorganic surface, thus decreasing adhesion
between epoxy matrices (obtained mainly by polycondensation reactions) and untreated
glass surfaces decreases after hydrothermal ageing12.
The ageing conditions used are listed in Tab. 4.5.
In Fig. 4.9 and Fig. 4.10 the values of IFSS vs. the different percentages of silane
agent for the CE and DGE matrix microcomposites, after 7 and 14 days of ageing at
40°C and 95% RH are shown.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
97
Tab. 4.5: Hydrothermal ageing conditions used for microcomposites.
time (days) temperature (°C) humidity (%RH)
7 40 50
7 40 95
14 40 95
4 80 50
4 80 95
4+4 80+80 50+95
The data presented in Fig. 4.9 and Fig. 4.10 evidence an increase of the IFSS
proportional to the ageing time.
The glass transition temperature of the DGE neat matrix: Tg DSC = 38°C, Tα DMTA
= 53°C, probably also affects the values of IFSS presented in Fig. 4.10. In this case, the
water adsorbed by the matrix acts as a plasticizer inducing a layer molecular mobility13,
i.e. decreasing Tg. The ageing at 40°C for long time period could modify the physical
behavior of the matrix itself, i.e. its ability to transfer the stress to the fiber through the
interface.
Confirms of the changes in the matrix mechanical behavior were clearly visible
during the test: the droplet had lost its elasticity (consequence: brittleness) to give
plastic deformation. In this case, an increase in the numerical value of the maximum
force before the detachment should be attributed to the absorption of energy from
plastic deformation from shear yielding of matrix network. In these conditions it
becomes difficult to quantify how much energy is spent for plastic deformation and how
much for the debonding. This effect of matrix plasticization is even more important in
our type of cycloaliphatic epoxy-based matrices compared to other types of matrices
such as the epoxy-amine or epoxy-anhydride matrices.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
98
11,5
22,5
33,5
4
0 0,5 1% CETS
IFSS
(MPa
)
t=0t=7t=14
Fig. 4.9: IFSS of CE matrix-based microcomposites after 7 and 14 days of ageing at
40°C + 95% RH.
0
0,5
1
1,5
0 0,5 1
% GPTS
IFSS
(MPa
)
t=0t=7t=14
Fig. 4.10: IFSS of DGE matrix-based microcomposites after 7 and 14 days of ageing
at 40°C + 95% RH.
Taking into account these results, experimental parameters were made more severe.
Samples were aged at 80°C + 95% RH for 4 days. These conditions could not be
applied to DGE matrix microcomposites. In Fig. 4.11 are reported the values of IFSS vs.
the different percentages of silane used to graft glass fibers surface for CE matrix
microcomposites, after 4 days of ageing at 80°C and 95% RH.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
99
1,5
2
2,5
3
3,5
0 0,1 0,2 0,3 0,4 0,5% CETS
IFSS
(MPa
)
t=0
t=4 (80°C+95% RH)
Fig. 4.11: IFSS of CE matrix-based microcomposites after 4 days of ageing at 80°C
+ 95% RH.
The same type of behavior already observed at 40°C + 95% RH (Fig. 4.9) is found.
In order to better understand these results, the number of the experimental variables
was cut down: only one parameter, temperature or humidity, was changed. CE matrix
microcomposites were thermally treated at 80°C for 4 days; in Fig. 4.12 the obtained
data are shown and related to those reported in Fig. 4.11.
1,52
2,53
3,54
4,55
0 0,1 0,2 0,3 0,4 0,5% CETS
IFSS
(MPa
)
t=0t=4 (80°C)t=4 (80°C+95% RH)
Fig. 4.12: IFSS of CE matrix-based microcmposites after 4 days of different types of
ageing.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
100
Experimental evidences show that the thermal treatment increases the adhesion
compared with both non-aged and hydro-thermal aged samples. This result, correlated
with what reported in literature14,15, suggests that the thermal cycle at which is treated
the sample governs interfacial reactions. In fact thermal treatment has no effect on
microcomposites made with untreated glass fibers. These reactions are comparable to
the ones we have during thermal curing in creating an interface with enhanced
properties according to the scheme proposed in Fig. 4.13.
R Si
OH
O
O Si
OH
OH
R
Si
R Si
OH
OH
O Si
OH
OH
R R Si
OH
O
O Si
O
OH
R
R Si
OH
O Si
OH
OH
R
Si
silica or glass silica or glass
Fig. 4.13: Schematic representation of the reactions occurring between siloxane chains
during thermal curing.
To verify this assumption we chose to couple the thermal treatment to the
hydrothermal aging, following this schedule:
microcomposite preparation
↓
1st: thermal treatment: 80°C, 4 days
↓
2nd: hydrothermal aging: 80°C+95% RH, 4 days.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
101
This multiple treatment has been applied to the CE matrix/ treated glass fibers (CETS
0.1% v/v) system, since it displays the highest interfacial adhesion values. Results are
presented in Fig. 4.14 and compared to the previous ones.
1,52
2,53
3,54
4,55
0 0,1 0,2 0,3 0,4 0,5% CETS
IFSS
(MPa
)
t=0
t=4 (80°C)
t=4 (80°C+95%RH)
t=4 (80°C)+(80°C+95%RH)
Fig. 4.14: IFSS of CE matrix-based microcomposites after 4 days of different types of
ageing.
From Fig. 4.14 it can be seen that the IFSS value for the latter system is lower than
the IFSS of the thermal treated one. This means that during the 2nd step of treatment, a
degradation process occurred at interface, due to the diffusion of water molecules.
Moreover this IFSS value is higher than the one measured after the hydro-thermal aging
at 80°C + 95% HR. This means that during the 1st step of the treatment reactions occur
in the interfacial region leading to enhanced mechanical properties of the interface. The
same test has been performed on the CE systems at 40°C for 7 days, obtaining similar
results (Fig. 4.15).
Considering all the experimental values, we can conclude that, in order to have a real
improvement of adhesion between polymer matrix and inorganic filler, a thermal
treatment after the grafting is necessary.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
102
1,5
2
2,5
3
3,5
4
0 0,1 0,2 0,3 0,4 0,5% CETS
IFSS
(MPa
)
t=0t=7 (40°C+95% RH)t=7 (40°C)
Fig. 4.15: IFSS of CE matrix-based microcmposites after 7 days of different types
of ageing.
All the UV-cured epoxy systems described (CE, DGE) present polar characteristics,
therefore they have good adhesion on inorganic surfaces (glass) even in absence of
chemical surface treatments. As already discussed, the adhesion in this case can be
related to the polar-polar interactions created at interface between glass surface and
epoxy matrix.
In order to complete our informations on different composites, we have investigated
a different system, constitued by a non-polar matrix and glass fibers.
The matrix chosen was epoxidized acrylate soybean oil (SOA) combined with glass
fibres treated with a silane molecule having an acrylic functionality: 3-(trimetoxysilil)
propyl methacrylate (MEMO). Glass fibers modification with MEMO (1% v/v) was
carried out using the same grafting protocol already described for epoxysilanes grafted
fibers.
The same procedure and the same measurement technique adopted with the epoxy
systems were followed to prepare samples for microbond tests.
Microcomposites made with untreated and treated glass fibers were tested before and
after hydro-thermal ageing.
The ageing conditions used are described:
time (days) = 4
temperature (°C) = 60
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
103
humidity (%RH) = 95.
The data obtained were treated with the Kelly-Tyson relationship. The IFSS values
related to each system are reported in Tab. 4.6.
Tab. 4.6: IFSS values of SOA matrix microcomposites before and after hydro-
thermal ageing.
sample IFSS (MPa)
silane concentration
(% v/v)
0 0.54
1 0.7
0 0.63* MEMO
1 0.9* *after hydrothermal ageing (4 days at 60°C and 95% HR).
The surface treatment improves clearly the adhesion on fibers surface due to the
formation of stronger bonds between the non-polar matrix SOA and the modified
inorganic surface.
This improvement is still present after aging, thus confirming the formation of
covalent bonds.
It should be noticed that also in this case IFSS values increase after treatment, as
already seen for the UV-cured epoxy systems. It can be suggested that also in this case
the IFSS value increase is due to a thermal post-curing process which allows enough
mobility to lead to covalent bonding from reaction of grafted species.
MORPHOLOGY OF MICROBOND SAMPLES
In this section, the results of analyses performed by scanning electron microscopy,
SEM, on micro-composites after the microbond measurement are presented and
discussed.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
104
Fig. 4.16 gives an “overall” looking at the samples used in this type of test: in the
picture it can be seen a typical section of an E-glass fiber with the cured microdroplets.
The red frame evidences a droplet after the detachment.
Fig. 4.16: SEM micrograph of a typical E-glass fiber microcomposite used in the
microbond test.
Fig. 4.17 shows a droplet profile after the detachment: the microcomposite is CE-
based matrix, on untreated E-glass fiber and the test was performed after 7 days of
ageing at 40°C. This picture has to be compared to Fig. 4.18, showing a CE-based
matrix microcomposite with treated (CETS 0.1% v/v) E-glass fiber, submitted to the
same ageing treatment. The grafting treatment performed assures the retention of
interface properties even after exposure in hostile environment: in fact, in these
conditions, the droplet debonding was caused by the rupture of the polymer matrix
while the interface displays good adhesion properties, as shown in Fig. 4.19.
These results can be attributed to the formation a strong interface, thanks to the
grafting procedure as well as to the thermal post-curing treatment.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
105
Fig. 4.17: SEM micrograph of microcomposite CE-based matrix with untreated glass
fiber, aged for 7 days at 40°C.
Fig. 4.18: SEM micrograph of microcomposite CE-based matrix with treated glass
fiber (CETS 0.1% v/v), aged for 7 days at 40°C.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
106
Fig. 4.19: Particular of SEM micrograph of microcomposite CE-based matrix with
treated glass fiber (CETS 0.1% v/v), aged for 7 days at 40°C.
ADHESION ON GLASS SHEETS EXPERIMENTS
Adhesion measurements were performed also on treated or untreated glass sheets
used as models of glass fibers systems.
Adhesion was measured by using the standard cross-cut method ASTM D3359.
The results obtained are reported in Tab. 4.7. They confirm that UV-epoxy systems
display good adhesion even on untreated glass surface. Only after the C3 treatment the
adhesion is absent due to the weak interactions between the apolar grafted alkyl chains
from the silane and the polar epoxy matrix.
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
107
The hydrothermal treatment, in the adopted conditions, determines a strong decrease
of adhesion on untreated glass surfaces, while good adhesion properties are still
displayed by the treated samples.
These results, although they are performed in very different conditions, are in
agreement with those obtained using the microbond testing.
Tab. 4.7: Adhesion results on treated or untreated glass sheets.
*sampled immersed in water.
4.4 Conclusions
In this chapter properties of micro- and nano-composites have been investigated.
Nano-composites, made with epoxy matrices and grafted or ungrafted nanosilica as
inorganic reinforcing agent, were characterized using dynamic-mechanical analyses.
The results show that nanocomposites present lower Tg values with respects to the
pure monomers (CE and DGE). The decrease in the Tg values indicate that a reduction
of the crosslinking density of the matrix is obtained in presence of silica, according to
the analyses of reaction kinetics, that indicate a decrease of the curing rate when silica is
added to the system.
The microbond technique was used to measure the interfacial adhesion for
microcomposites.
Adhesion
monomer glass
treatment
25°C
24 hours
60°C
6 hours*
60°C
24 hours*
100°C
3 hours*
untreated 100% 40% 0% 0%
CETS 100% 60% 20% 0% CE
C3 0% - -
untreated 100% 0% DGE
GPTS 100% 100% 60% 40%
untreated 0% SOA
MEMO 70% 40%
CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
108
The data obtained for all the systems, expressed in terms of IFSS at interface,
evidenced that the best results are reached when a very low silane agent concentration is
used, i.e. as the grafted layer tends to be a monolayer.
Epoxy matrices display good adhesion even on untreated glass fibres, according to
literature6,12, because of the polar interactions. In agreement with these conclusions,
interfacial adhesion on C3 treated glass surface is very low.
CE-treated glass fibres systems show an increase of adhesion after a thermal
treatment. This can be explained by admitting the formation of a strong covalent bond
between the inorganic surface and the silane agent.
Moreover the thermal curing induces a better crosslinking of the silane film leading
to the formation of a highly branched siloxane chains15. The silanol groups of small
siloxane chains react with other of small, longer or branched chains, to form a polymer
network. This crosslinked silane layer can explain the adhesion improvement. In fact, if
the polysiloxane film is not completely condensed, it can display elastomer
characteristics, the so-called “rubber bumper” behaviour16; in these conditions the stress
transfer is less efficient as the shear stress is applied on a layer with lower modulus. The
thermal treatment leads to a post-condensation that induces stiffness in the polysiloxane
layer, thus more effective stress-transfer properties (expressed in term of higher values
of IFSS).
Moreover, the not completely condensed siloxane layer is still rich in SiOH groups
that can bond water molecules during a hydrothermal ageing process; these molecules
could act as plasticizer on the polymer matrix modifying the composite properties.
Adhesion decreases after hydro-thermal ageing; treated glass fibres show better
resistance than the untreated ones because they form stable covalent bonds with the
polymeric matrix.
Experiments done on micro-composites using SOA matrix highlight that the sizing
procedure is necessary to assure interfacial adhesion. Experimental results indicate that
also in this case a thermal curing of silane film is necessary to achieve better
performance.
Similar conclusions are obtained from the adhesion measurements performed on the
films UV-cured on treated or untreated glass sheets.
CHAP. 5 CONCLUSIONS
Conclusions
In this work the preparation of polymeric composites through cationic
photopolymerization and the properties of the obtained products were investigated.
First it has been set up and optimized the reaction to modify the surface properties of
inorganic fillers (nanosilica and glass fibers), in order to improve their interaction with
the polymeric matrix. This procedure was applied to nanosilica and glass fibers
reinforcing agents and its effectiveness was confirmed by TGA analyses and surface
properties evaluation.
The experimental results evidenced the modification of the surface properties of both
the fillers used.
Afterwards their influence on the composite preparation reaction was analyzed; the
photopolymerization reactions kinetics were evaluated, as well as the total conversion of
the photopolymerization reaction by means also of “real-time” techniques.
The results obtained using treated or untreated fillers were compared to those
attained with the pure resins. This second part of the work evidences that the
photopolymerizable active species interact with nanosilica during the UV-reaction,
reducing both the kinetics and total conversion of the reactive groups.
CHAP. 5 CONCLUSIONS
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
110
Finally the characteristics of the interfacial region were studied through adhesion
measurements performed with the microbond technique. Experimental results evidence
the presence of a correlation between adhesion and interphase thickness; as a
consequence of the applied treatment, adhesion between the two phases has been
improved leading to enhanced mechanical properties even in hydrolytic conditions.
The following developments of this research could be of interest:
to deepen the study of the interaction between the photopolymerization active
species and the inorganic surface. This could be useful in order to control them or to
reduce the lowering of the reaction kinetics.
Adhesion measurements results suggest that it could be useful to study
interphase reactions when a thermal post-curing treatment is applied, in order to
understand which part of the interphase is interested and to optimize the post-curing
procedure.
Experimental techniques
SPECTROSCOPIC ANALYSIS
FT-IR
A Fourier transform infrared spectrophotometer Mattson Genesis II was used.
Reaction kinetics were calculated measuring the variations during UV-curing of the
infrared bands characteristics of the monomer used. Conversion x after a given time t
can be calculated from the relationship
[ ][ ]0
1λ
λ
AA
x t−=
[Aλ]0 = integrated area under the infrared peak characteristic of the specific
functional group of the monomer, at the very beginning of UV reaction.
[Aλ]t = integrated area under the infrared peak characteristic of the specific
functional group of the monomer, at t time.
Samples were put between two KBr disks; analyses were made in absorbance
accumulating 64 scans, resolution 2 cm-1. All the results have been calculated using an
internal standard, the integrated area related to the stretching of the C=O group (it does
not participate to UV reaction) at 1730 cm-1. The FT-IR measurements done in this
work were made after finite intervals of irradiation time, realized using a UV lamp
(Helios Italquartz) which is separate from the FT-IR spectrophotometer.
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UV-vis
A UNICAM UV-vis spectrometer was used.
SURFACE ANALYSIS
Kruss goniometer
Measurements using a Kruss goniometer DSA 10 were carried out in air at room
temperature by the sessile drop technique bidistilled; bidistilled water was used. The
values of contact angles are obtained automatically through the evaluation of a
digitalized video image. In Fig. 1 a scheme of the apparatus used is presented.
Fig. 1: scheme of a Kruss goniometer.
The contact angle measuring instrument G10 applies the so-called sessile drop
method to determine contact angles.
If a pendant drop is in hydrometrical equilibrium then the analysis of its contour can
be used for the determination of the surface or interfacial tension. The contact angle of a
sessile drop on a solid surface is measured. The drop environment can be a gas or a
liquid. The contact angle measuring instrument offers the following opportunities:
measurements of contact angles of single drops of test liquids on a solid surface;
measurements of advancing and receding contact angles by volume control of
sample drop.
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The surface of the solid to be measured should be horizontal. A drop of liquid is
placed on the solid. The contact angle is not independent of time effects: it can change
rapidly within seconds or minutes, depending on the liquid used and on the nature of the
solid. If the solid dissolves into liquid or the liquid changes its composition due to
different vapor pressure of different components, the contact angle will decrease
rapidly.
Dynamic contact angle describes the properties between solid-liquid surfaces during
the wetting procedure. It can be distinguished between advancing and receding contact
angle.
Advancing contact angle: an advancing contact angle can be measured with an
increasing liquid drop on a solid surface (Fig. 2). This can be done with a syringe that
remains in the drop during the measurement. A liquid drop is placed on the solid surface
and enlarged by pushing more liquid through the needle. While the volume increases
the border line between liquid and solid moves. The contact angle has to be measured
while the drop volume is increasing. Only the physical interaction is measured.
Receding contact angle: the receding contact angle provides information on the
macroscopic roughness of the surface. A relatively large drop is placed onto the surface.
The syringe remains in the drop during the measurement. The drop volume is decreased
by sucking liquid back into the syringe (Fig. 3). When the border line liquid-solid starts
moving, the contact angle has to be measured.
Fig.2: advancing contact angle produced with a syringe.
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Fig. 3: receding contact angle produced with a syringe.
Parameters that influence contact angles are:
Temperature, for this reason it is necessary to keep a standard temperature for
the measurements.
Time, the contact angle can change during time (Fig. 4), caused by evaporation
of the liquid or by the forces between surface and liquid. The quality of the surface itself
is time-dependent; this is especially valid for polymers.
Volume, density and gravitation: the influence of gravitation on the contact
angle is closely connected to the density of liquid used and to the drop volume.
Drop size.
Fig. 4: time dependence of equilibrium contact angles.
Drop environment, it concerns the surrounding gas phase and the solid surface
that should be examined. The evaporation can influence the contact angle, as well as
adsorption processes of the test liquid at the solid surface.
Surface roughness.
Dosing rate.
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Chan balance
The instrument used was a DCA 322 Cahn balance. Measurements with bidistilled
water were performed by means of the Wilhelmy technique with an immersion rate of
20 µm/s.
CALORIMETRIC TECHNIQUES
Photo-calorimeter (photo-DSC)
The instrument used was a calorimeter DSC 7 Perkin Elmer coupled with a UV-lamp
Perkin-Elmer DPA 7 XBO 450 W (Fig. 5).
The program used for the analyses is described:
36 minutes of total run
lamp on after 2 min
lamp off after 33 min
isothermal conditions (30°C)
nitrogen atmosphere.
The lamp intensity was adjusted to have I = 0.58 µW/cm2 at the selected wavelength.
Each sample pan was treated with a sulfo-chromic mixture in order to remove any
surface coating and to assure a good wettability to the sample.
The lamp is equipped with a monochromator to select the optimal wavelength. The
lamp intensity for each wavelength was measured with a radiometer Solatell 2000 and
the obtained emission spectrum is reported in Fig. 6.
Fig. 5: photo-DSC apparatus.
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Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
116
Fig. 6: photo-DSC UV-lamp intensity.
The DSC instrument registers the variations in the heat flow between sample and
reference; the measurements are done in a temperature-controlled environment, under
inert atmosphere (nitrogen).
Temperature and enthalpy are calibrated starting from the measure of the same
values of known materials (ex. indium). An example of obtained thermogram is
reported in Fig. 7.
Fig. 7: photo-DSC thermogram.
19
19,5
20
20,5
0 10 20 30 40time (min)
Hea
t flo
w (e
ndo
up)
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117
Thermogravimetric analysis (TGA)
A TGA Mettler-Toledo Star System and a LECO TGA-601 were used.
Analyses were performed following the indicated programs:
Dynamo-mechanical analysis (DMTA)
A Rheometric Scientific MKIII apparatus was used.
Samples were tested in bending configuration, single cantilever; the temperature
range was from 0° to +250°C for the CE/silica systems and from -50° to +80°C for the
DGE/silica systems. Measurements were carried out at 1Hz frequency.
An apparatus for dynamo-mechanical analysis is represented in Fig. 8.
Fig. 8: DMA apparatus scheme.
Mettler-Toledo apparatus
sample weight = 10 mg
heating rate = 10°C/min
nitrogen atmosphere
temperature range = 50°-700°C
LECO TGA-601 apparatus
sample weight = 500 mg
heating rate = 1°C/min
temperature range = 50°-900°C
air atmosphere.
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118
It is constituted by:
an electrical engine, to apply a cyclic stress that vary in a sinusoidal way;
a transducer, to measure the amplitude of the sample deformation;
an oven, to operate in an inert atmosphere, with a programmed temperature;
a thermocouple, to measures the effective sample temperature.
The instrument measures directly the following items:
the transducer position, in order to calculate the sample deformation, related to
its geometry;
the sample temperature;
the phase.
The measure can be done in a range of temperature or frequency; anyway to the
sample under a constant charge is superimposed a charge that varies according to the
following relationship:
( )tt ωσσ sin0=
Polymers present viscoelastic characteristics, so that a out of phase, δ, is observed
between the sinusoidal function that express the stress variation and the function
representing the correspondent strain, as shown in Fig. 9. The given energy is stored in
an elastic way, but also partially dissipated due to internal friction. If δ = 0, the material
presents elastic characteristics, while if δ = π/2, the material presents the characteristics
of a Newtonian Fluid. If 0 < δ < π/2 the sample behavior is viscoelastic and the quantity
of dissipated energy is proportional to the value of δ.
Fig. 9: viscoelastic deformation when a sinusoidal stress is applied.
EXPERIMENTAL TECHNIQUES
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119
The complex modulus can be defined as: '''* iEEE +=
Where E’ and E’’ are the storage and loss modulus respectively. In particular:
δ
δ
sincos
*''
*'
EEEE
=
=
A measure of the relationship between the energy loss as heat and the energy stored
as elastic deformation is given from the following equation:
δδδ tg
EE
EE
==cossin
*
*
'
''
MICROSCOPY
Atomic Force Measurement (AFM)
A Digital Instrument Nanoscope IIIa was used to carry out the measurements on
silicon wafer and glass fibers surfaces using the tapping mode.
This microscope (Fig. 10) is based on interaction forces created between the
cantilever and the sample surface, like repulsion forces, Van Der Waals attractions or
magnetic interactions.
Fig. 10: AFM image.
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The tapping mode imaging is implemented by oscillating the cantilever assembly at
or near the cantilever’s resonant frequency using a piezoelectric crystal. The piezo
motion causes the cantilever to oscillate with high amplitude when the tip is not in
contact with the surface. The oscillating tip is then moved toward the surface until it
begins to lightly touch or “tap” the surface. During scanning the vertically oscillating tip
alternately contacts the surface and lifts off. As the oscillating cantilever begins to
intermittently contact the surface, the cantilever oscillation is necessarily reduced due to
the energy loss caused by the tip contacting the surface. This reduction in oscillating
amplitude is used to identify and measure surface features.
In Fig. 11 the cantilever oscillation amplitude in free air and during the measurement
is illustrated.
Fig. 11: cantilever oscillation in the tapping mode measurement.
Scanning Electron Microscopy (SEM)
A Hitachi S800 microscope was used during this work to investigate the surface
morphology of treated and untreated glass fibers. Samples were analyzed at 15 kV
tension.
In this type of analysis the sample is hit by high energy (300-600 kV) electrons
generated by a specific source. The most common sources are:
thermo-ionic source (W filament);
LaB6 catod source;
field emission source.
EXPERIMENTAL TECHNIQUES
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The interaction between the sample and the electron beam creates a great number of
events so a great number of signals. They are elaborated by different types of detectors.
Analyses performed with this technique present some advantages:
medium/high vacuum degree (10-6 torr);
high precision in the focalization of the electron beam on the sample;
high resolution dues to the short wavelengths present in the high energy electron
beam.
In Fig. 12 a scheme of a typical SEM apparatus is presented. It is composed by:
an electron source;
one or more magnetic lens to reduce the beam dimension to 5 nm;
a device to control the astigmatism;
a detector.
Fig. 12: SEM scheme.
The obtained electron beam, called “spot”, moves under lines on the sample; the
higher the numbers of lines generated (slow scan), the better the resulting image is.
In Fig. 13 the main areas of signals emission for a given sample are schematically
indicated. The secondary electrons (SE) give information on the morphology of the
material; the backscattered electrons (BSE) give qualitative information on sample
composition; the characteristics X-rays give elemental analyses of the sample (type,
distribution and quantity of the elements present in the sample).
EXPERIMENTAL TECHNIQUES
Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion
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Fig. 13: main signals (SE, BSE, X-rays) emission areas.
Samples for SEM analyses should be prepared in order to guarantee an electric
continuum between the beam, the sample and the sample holder. For samples with
insulating characteristics, for example ceramics and polymers, it is necessary to
metallize the surface before the analysis.
MECHANICAL ANALYSES
Microbond technique
A dynamometer Adamel Lhomargy DY 25 was used. For microbond measurements
a 10 N cell load was used; samples were pulled at 0.1 mm/min speed. The inferior
clamp is made of two razor blades; their position can be adjusted by using two
micrometric screws. The test is followed using a MICAM X video camera equipped
with a NAVITAR macro zoom. The apparatus is presented in Fig. 14.
Fig. 14: microbond technique apparatus.
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All mechanical measurements were done at 20°C and 50% HR.
UV-LAMP DEVICES
Different UV-lamps have been employed in this work.
Fusion lamp: an industrial UV-lamp with a standard mercury-filled bulb,
equipped with a conveyor belt (Fig. 15); by adjusting the conveyor belt speed it is
possible to change the sample exposition time. The lamp total intensity (measured with
a Solatell 2000 radiometer) is 370 mW/cm2. Its emission spectrum is reported in Fig.
16.
Fig. 15: FusionUV- lamp equipment.
Fig. 16: Fusion UV- lamp emission spectrum.
Helios Italquartz: is a laboratory UV-lamp with a standard mercury-filled bulb,
equipped with a support which allows changing the distance of the sample from the
lamp; there is possibility of operating in nitrogen atmosphere. The lamp intensity,
related to the different distances of the support used, varies from 10 to 50 mW/cm2.
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CHAP.1
[1] VANSANT E.F., VAN DER VOORT P., VRANCKEN K.C. Characterization
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