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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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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
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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
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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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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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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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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.
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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.
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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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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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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.
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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.
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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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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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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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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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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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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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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.
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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.
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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%
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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.