UV-POLYMERIZED MICRO- AND CHAP. 4...

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

CHAP. 4 UV-POLYMERIZED MICRO- AND NANO-COMPOSITES

Photopolymerized micro- and nano-composites: interface chemistry and its role on interfacial adhesion

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



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



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.



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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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.



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.



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.



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.



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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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.



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.



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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.



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.



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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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



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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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%



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.

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