Preparation and Thermo-Mechanical Properties ofHeat-Resistant Epoxy/Silica Hybrid Materials

Peng Yang,1 Guoqing Wang,2 Xue Xia,1 Yoshitaka Takezawa,3 Haitao Wang,1 Shinji Yamada,3

Qiangguo Du,1 Wei Zhong1

1 The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education and Department ofMacromolecular Science, Fudan University, Shanghai 200433, China

2 Innovative Systems and Materials Laboratory, Shanghai Research Institute, Hitachi (China) Research andDevelopment Corporation, Shanghai 200020, China

3 Advanced Research Laboratory, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan

Diglycidyl ether of bisphenol-A (DGEBA) based epoxy/silica hybrid materials filled with various amounts of 3-glycidoxypropyltrimethoxysilane (GPTMS) and silicananoparticles were prepared, using 4,40-diaminodi-phenyl sulfone (DDS) as curing agent. The obtainedhybrid materials were analyzed by means of Fourier-transform infrared spectroscopy (FTIR), dynamic me-chanical analysis (DMA), scanning electron microscopy(SEM), and thermogravimetric analysis (TGA). Theresults indicated that the introduction of GPTMS andsilica nanoparticles had synergistic effect. The additionof GPTMS not only ameliorated the compatibilitybetween silica and the epoxy matrix but also increasedthe crosslinking density of the epoxy system; mean-while the nano-silica further reinforced the inorganicnetwork of the hybrid system. Consequently, the hybridmaterials showed much improved heat-resistant prop-erties. The storage modulus of the hybrid systemsshowed no obvious decrement in the glass transitionregion and kept at a high value even in the temperatureregion up to 3008C. The integral thermal stability of theresulting hybrid materials was also improved comparedwith the corresponding pure epoxy resin. POLYM. ENG.SCI., 48:1214–1221, 2008. ª 2008 Society of Plastics Engineers

INTRODUCTION

Epoxy resin is one of the most widely used thermoset-

ting resins. It has several useful properties such as high

tensile strength and modulus, low shrinkage, good adhe-

sion and insulation property, and excellent chemical cor-

rosion resistance. Therefore, epoxy resin is broadly used

as adhesive, casting, coating, and laminates in semicon-

ductor and electronic industries [1–4]. Nevertheless, ep-

oxy resin also has some disadvantages which limit its

high-performance applications. Recent developments in

electronic packaging industries require some superior

properties for epoxy-based materials like strong thermal

stability, low shrinking stress, low dielectric constant, and

low thermal expansion coefficients. These requirements

have attracted more and more attentions on improving the

thermo-mechanical properties of epoxy resin [5, 6].

As a potential method of combining the mechanical

toughness and flexibility of the organic component with

the hardness and thermal stability of the inorganic compo-

nent, organic-inorganic hybrid materials are of increasing

interest for a wide variety of applications, and they have

shown ability of providing a feasible approach of simulta-

neously improving the thermal and mechanical properties

as well as dimensional stability of polymeric materials

[7–10]. It has been clarified that the properties of these

hybrid materials are highly influenced by the interfacial

adhesion between polymeric matrix and fillers. In hybrid

materials formed by direct blending, there is primarily

physical crosslinking between the organic and inorganic

phases. Either increase [11, 12] or decrease [13] of the

thermo-mechanical properties of that kind of composites

was reported and sometimes even more complicated

trends [14] were observed. Preghenella et al. [15] indi-

cated that all the thermal and mechanical properties of

silica/epoxy resin hybrid showed nonmonotonic trends as

the silica content increased. They found that when the

silica content was low the thermo-mechanical properties,

such as glass transition temperature, dynamic storage

modulus and tensile modulus, were reduced because the

presence of polymer-filler interactions limited the cross-

linking degree during composites curing. While at higher

Correspondence to: H. Wang (or) W. Zhong; e-mail: weizhong@fudan.

edu.cn or wanght@fudan.edu.cn

DOI 10.1002/pen.21081

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2008

nano-silica content, the inversed trend was observed

which was supposed to be due to the enhanced physical

immobilization effects of the filler on the polymeric ma-

trix above the percolation threshold. It is now generally

accepted that the preferred hybrid structure is one in

which there is intimate chemical crosslinking between the

fillers and the matrix, and particular attention has been

placed on interfacial adhesion and surface treatments of

the filler particles, as a means of both minimizing the for-

mation of agglomerates during processing and improving

mechanical properties. Kang et al. [16] evidenced that

silica particles functionalized with epoxide and amine

groups showed better distribution in epoxy matrix,

because of better resin wetability. Functional groups such

as epoxide and amine on the surface of inorganic particles

induce very strong interactions between matrix and filler.

Apart from the direct mixing method, the hybrid materials

derived from the in-situ sol–gel process can achieve better

dispersion and properties [17–19]. They were normally

prepared by mixing alkoxysilane and epoxy/curing agent

together in a homogenous solution, followed by heating

when the curing reaction of epoxy and the condensation

reaction of hydrolyzed alkoxysilane occurred simultane-

ously. Ochi et al. [20–22] synthesized silica/epoxy hybrid

material from 3-glycidoxy-propyltrimethoxysilane by in-

situ sol–gel process. Compared with the pure material, the

storage modulus in the rubbery region increased and the

peak area of the tand curves in the glass transition region

decreased. This may result from the suppression of the

epoxy network mobility with the internal inorganic net-

work containing functional (epoxy) groups which can

react with the organic component.

In this study, we tried to prepare a kind of heat-resist-

ant epoxy/silica hybrid material through a process that

combined the advantages of both sol–gel and nanopar-

ticles mixing to enhance the interfacial interactions

between inorganic nanoparticles and polymer matrix. A

considerable amount of GPTMS was introduced into the

epoxy/silica hybrid matrix in order to improve the cross-

linking of the system and the dispersion of the silica

nanoparticles in the matrix. Thermo-mechanical proper-

ties, such as storage modulus and tand, and thermal stabil-

ity of the resulting hybrid materials were studied.

EXPERIMENTAL

Materials

The epoxy resin used was a commercial grade of

diglycidyl ether of bisphenol-A (DGEBA) (E-51) pur-

chased from Shanghai Resin Factory (Shanghai, China),

with an epoxide equivalent weight of 184–210 g/equiv.

The silica nanoparticles used in this study were supplied

by Mingri nanomaterial Co. Ltd. (Zhoushan, China), with

a mean diameter of 30 nm (data provided by the com-

pany). The micro-sized silica used was obtained from

Huzhou Wanneng Silicon Micro-powder Factory (Zhe-

jiang, China), with a mean diameter of 8–10 lm (data

provided by the company). 4,40-diaminodiphenyl sulfone

(DDS) and 3-glycidoxy-propyltrimethoxysilane (GPTMS)

(Purity � 95%) were both purchased from Sinopharm

Chemical Reagent Co. Ltd. (Shanghai, China). The for-

mulas of the starting materials were listed in Scheme 1.

Preparation of Epoxy/Silica Hybrid

The epoxy/silica hybrid samples filled with different

amount of GPTMS and silica nanoparticles were prepared

according to the following procedure. First, a prescribed

amount of DGEBA, GPTMS, and DDS which was used

as curing agent, were mixed together, and then a stoichio-

metric amount of water was added into the viscous fluid.

The mixture was stirred at the room temperature for 12 h,

and GPTMS was hydrolyzed and condensed partly in the

presence of water and DDS which had a relatively high

basicity and thus acted as basic catalyst for the hydrolysis

and condensation reaction of GPTMS. Then the pre-deter-

mined amount of silica nanoparticles were added into the

transparent and homogeneous epoxy/DDS/GPTMS mixture

SCHEME 1. Formulas of the starting materials.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 1215

with stirring at 1208C, then the mixture was vigorously

stirred at 1208C for 2 h in order to promote the hydrolysis

and condensation reaction, and to remove the volatile

byproducts. After that, the mixture was degassed at 1008Cunder vacuum. Finally, the mixture was cured at 1008Cfor 12 h, at 130, 160 and 2008C for 4 h during each cur-

ing step, and then postcured at 2308C for 6 h. With each

amount of GPTMS, hybrid samples with loadings of 10,

20, and 30 phr silica nanoparticles were all prepared. The

starting composition and silica content of the hybrid mate-

rials are shown in Table 1.

Measurements

Fourier-transform infrared spectroscopy (FTIR) was

carried out on a Nicolet Magna 550 FTIR spectrometer in

order to characterize the structure effect of GPTMS and

silica nanoparticles on cured epoxy/silica hybrid.

Dynamic mechanical tests were carried out with a

Netzsch DMA 242 instrument using compression mode.

The tests were performed in the temperature range from

50 to 3008C, at a heating rate of 38C/min and with an

oscillating frequency of 10 Hz, and the amplitude of the

vibration were adjusted to 60 lm.

The fractured-surface morphology of the samples was

examined by scanning electron microscopy (SEM). The

apparatus used was TESCAN 5136 MM and the fracture

surfaces were coated with gold by vacuum sputtering

before testing.

Thermogravimetric analysis (TGA) was employed to

estimate the actual silica content and thermal stability of

the hybrid materials produced. Weight loss was measured

with a Perkin-Elmer PYRIS 1 instrument. The scans were

performed at 208C/min in air in the temperature range

from 50 to 8008C. The residual weight of all specimens

kept a constant value after heating over 8008C. Becausethe organic component seemed to have almost completely

decomposed in the temperature region over 8008C, the

values of the residual weight was used as the actual silica

content in the hybrid materials.

RESULTS AND DISCUSSION

Infrared Spectroscopy

FTIR measurements were performed to investigate the

chemical structure of epoxy/silica hybrid. The FTIR spectra

of pure epoxy and hybrid materials are shown in Fig. 1.

Presence of broad overlapping bands between 1000 and

1200 cm21 in the spectra of G6-0Si and G6-30Si con-

firmed the existence of a silicon-oxide network, which

resulted from the addition of GPTMS and nano-silica to

the epoxy matrix. The complete disappearance of charac-

teristic epoxide band at 916 cm21 and appearance of band

at 1103 cm21 characteristic of the �CHj

�OH group

formed as a result of the epoxy ring opening in reaction

with amine [19] is visible, which indicated that the epox-

ide group of both epoxy and GPTMS reacted with the

curing agent DDS completely whether there were GPTMS

and nano-silica or not. It can be concluded from FTIR

results that the introduction of GPTMS and silica nano-

TABLE 1. Starting composition and silica content of epoxy/silica hybrid samples.

Sample code

Starting composition (g)Amount of silica

nanoparticles (phr)

Silica content (wt%)

DGEBA DDS GPTMS Calc. Expt.

Pure 10 3.17 0 0 0 0.8

G0-30Si 10 3.17 0 30 23.07 24.09

G4-0Si 10 4.22 4.01 0 4.84 4.7

G4-10Si 10 4.22 4.01 10 13.48 14.74

G4-20Si 10 4.22 4.01 20 20.72 21.42

G4-30Si 10 4.22 4.01 30 26.81 27.51

G6-0Si 10 4.75 6.02 0 6.38 7.54

G6-10Si 10 4.75 6.02 10 14.91 15.61

G6-20Si 10 4.75 6.02 20 21.97 18.38

G6-30Si 10 4.75 6.02 30 27.98 30.48

G10-0Si 10 5.8 10 0 8.54 8.34

G10-30Si 10 5.8 10 30 29.64 34.48

FIG. 1. FTIR spectra of pure epoxy and epoxy/silica hybrid materials.

1216 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

particles can form a silica network in the epoxy matrix

and has no negative influence on the curing process of ep-

oxy itself.

Dynamic Mechanical Properties of Epoxy/Silica Hybrid

The temperature dependence of dynamic mechanical

properties of the hybrid containing different amounts of

silica nanoparticles is shown in Fig. 2. The initial storage

modulus of the hybrid materials with nano-silica was

higher than the hybrid materials without, and the storage

modulus after the glass transition region was markedly

increased with the increase of the silica nanoparticles con-

tent. Silica nanoparticles can play two different kinds of

role in the epoxy matrix. On the one hand, they can dete-

riorate thermal properties in epoxy matrix because of both

the presence of residual moisture and organics and reduc-

tion in the crosslinking degree of the polymer matrix by

the polymer-filler interactions. On the other hand, the

interfacial interactions between the epoxy and the nano-

silica can immobilize the polymer chains to some extent,

and improve the thermo-mechanical properties, such as

glass transition temperatures and dynamic storage modu-

lus. In this work, the negative influence could be limited

by the presence of GPTMS. The silica nanoparticles were

more likely to have interactions with GPTMS than epoxy,

and the interfacial interactions changed from physical to

chemical. Therefore, with the increase of nano-silica, the

storage modulus of the hybrid at high temperature was

increased in the GPTMS/DGEBA system, and the storage

modulus of epoxy/silica hybrid materials at 3008C was

shown in Fig. 3.

When the nano-silica was instead of microsized silica,

the thermo-mechanical properties declined observably

(see Fig. 4) even with great amount of GPTMS. The

thermo-mechanical properties of G6 samples with differ-

ent amounts of microsized silica were all worse than the

pure G6, and the addition of microsized silica seemed to

have no advantage. This may be due to the reason that

the interfacial area between the microsized silica and ep-

oxy matrix was quite small, while the silica nanoparticles

were dispersed well in the epoxy matrix and had larger

interfacial area because of their high specific surface area.

Therefore, hybrid filled with nano-silica had better com-

patibility and stronger chemical interfacial interactions in

the matrix than that with microsized silica, and showed

higher storage modulus at high temperature and lower

tand value in the glass transition region. The well disper-

sion of silica nanoparticles can be proved by the SEM

images as below.

The temperature dependence of dynamic mechanical

properties of the hybrid containing different amounts of

GPTMS is shown in Fig. 5. The storage modulus of the

pure epoxy resin was clearly decreased after the glass

transition region (Tg) and had a very low value in the rub-

bery region. It is well known that the decrease in the

modulus in the Tg region is due to the micro-Brownian

motion of the network chains. However, in the hybrid sys-

FIG. 2. Storage modulus of epoxy/silica hybrid with same GPTMS

content and different amounts of silica nanoparticles.

FIG. 3. Storage modulus of epoxy/silica hybrid materials at 3008C.

FIG. 4. Dynamic mechanical properties of pure epoxy and hybrid sam-

ples filled with nano- and micro-sized silica particles.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 1217

tems containing GPTMS, the modulus in the rubbery

region increased with an increase of the GPTMS contents

and thus the glass transition behavior became indistinct.

This result shows that the introduction of GPTMS can

enhance the interfacial interactions between nano-silica

and epoxy matrix and increase the crosslinking of the sys-

tem. Thus, the micro-Brownian motion of the epoxy net-

work is strongly restricted. The storage modulus of the

hybrid systems showed no obvious decrement in the glass

transition region and kept at a high value even in the high

temperature region over 3008C when the amount of

GPTMS was adequate. This means that the heat resistance

of the cured epoxy resin is significantly improved by

introducing GPTMS to the epoxy system and the epoxy/

silica hybrid could maintain high modulus even at 3008C.It is well known that a cured epoxy resin clearly shows

a large tand peak in the glass transition region. Also, in

this study, the pure epoxy resin showed a large tand peak

in the glass transition region. However, the area of the

tand peak decreased with an increase of the GPTMS con-

tents. These results show that the introduction of GPTMS

augment the crosslinking of the system and help the

nano-silica dispersed well in the matrix. The molecular

motion of the network chains in the hybrid systems was

effectively restricted by the chemical interfacial interac-

FIG. 5. Dynamic mechanical properties of epoxy/silica hybrid samples

with different amounts of GPTMS and the same silica nanoparticles

(30 phr).

FIG. 6. SEM micrographs of fractured epoxy hybrid samples filled with 30 phr nano-silica and different

amount of GPTMS: (1) 30Si; (2) G4-30Si; (3) G6-30Si; (4) G10-30Si. Scale bar: 10 lm.

1218 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

tions between silica nanoparticles and epoxy matrix. Thus,

it is concluded that the introduction of GPTMS can

increase the crosslinking of the system and have effec-

tively synergistic effect with nano-silica to enhance the

interfacial interactions.

Morphology of Fractured Sample Surfaces Studiedby SEM

Fracture surfaces of the hybrid samples show how

GPTMS affects the dispersion of nano-silica and interfa-

cial properties in epoxy matrix. In Fig. 6 the SEM images

of the fracture surfaces of hybrid with different amount of

GPTMS were compared. It was notable that the hybrid

samples without or with little GPTMS showed a much

rougher pattern than the samples with more GPTMS.

Hybrid with more GPTMS showed better distribution of

silica particles in epoxy matrix, because of stronger inter-

facial interactions and better wetability. The addition of

GPTMS ameliorated the compatibility between silica and

the epoxy matrix obviously.

The SEM images of the fracture surfaces of hybrid

with different amount of nano-silica were shown in Fig.

7. With the increase of silica content, there were no

obvious changes in the micrographs, and there was no

aggregation even in the hybrid sample with 30 phr nano-

silica, which was consistent with the DMA results. This

phenomenon can be attributed to the existence of high

dose of GPTMS which can provide chemical bond

between polymeric matrix and nano-silica and make those

nanoparticles distribute well in the epoxy matrix.

Thermogravimetric Analysis of Epoxy/Silica Hybrid

TGA was performed to investigate the thermal-decom-

position behavior of the epoxy/silica hybrid materials.

The TG curves of hybrid systems are shown in Figs. 8

and 9. Three-stage weight-loss behavior was observed for

FIG. 7. SEM micrographs of fractured epoxy hybrid samples filled with different amount of silica nanopar-

ticles: (1) G4-0Si; (2) G4-10Si; (3) G4-20Si; (4) G4-30Si. Scale bar: 50 lm.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 1219

the resins heated in air. The thermal decomposition onset

temperature and peak values of the pure epoxy and the

epoxy/silica hybrid materials in derivated thermograms

(DTG) in the first decomposition process were similar

(see Table 2). They all began to decompose around

4008C, and the onset temperature of hybrid materials was

a little higher than the pure epoxy. This may be due to

the reason that the conventional thermal decomposition of

the epoxy network was started from the side chain, which

was outside the silica-rich domain. However, the maxi-

mum temperature of the second decomposition process

changed a lot with the amount of GPTMS and retained

with the increasing nano-silica. This result was similar

with that of SEM. The introduction of GPTMS enhanced

the interfacial interactions between silica nanoparticles

and epoxy matrix and increased the crosslinking of the

system. The second process of weight loss was due to the

thermal oxidative degradation of the formed residuals

which is inside the silica-rich domain. Thus, the thermal

protection of the organic network occurred with the for-

mation of the silica rich domain in the epoxy network,

and the oxidation and thermal decomposition of the epoxy

network were prevented. These results demonstrate that

introducing GPTMS and silica nanoparticles into the ep-

oxy resin can definitely improve the integral thermal sta-

bility of the resulting epoxy/silica hybrid.

CONCLUSION

DGEBA based epoxy/silica hybrid materials filled with

various amounts of silica nanoparticles and GPTMS were

prepared, using DDS as curing agent. The hybrid materi-

als showed better heat-resistant properties. The initial

storage modulus increased with the addition of silica

nanoparticles, and with the introduction of GPTMS and

nano-silica, the storage modulus of the hybrid materials

showed no obvious decrement in the glass transition

region and kept at a high value even in the high tempera-

ture region. The storage modulus at 3008C can reach 0.7

GPa, much higher than the 0.09 GPa of pure epoxy. The

integral thermal stability of the resulting hybrid was also

improved compared with the corresponding pure epoxyFIG. 9. TGA curves of epoxy/silica hybrid materials with different

amounts of silica nanoparticles.

TABLE 2. Decomposition onset temperature and DTG maxima of

epoxy/silica hybrid samples.

Sample

code

Onset

temperature (8C)

DTG maxima (8C)

1st Process 2nd Process

Pure 398 437 620

G0-30Si 408 422 565

G4-30Si 412 437 611

G6-30Si 411 435 634

G10-30Si 411 432 649

G4-0Si 405 434 618

G4-10Si 423 446 613

G4-20Si 424 441 607

G6-0Si 413 432 628

G6-10Si 423 441 625

G6-20Si 431 444 622

FIG. 8. TGA curves of epoxy/silica hybrid materials with 30 phr silica

nanoparticles and different amounts of GPTMS.

1220 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

resin. These results indicate that this kind of heat-resistant

epoxy/silica hybrid is good for potential high-temperature

applications in electronic industry.

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