Effects of coupling agents on the mechanical properties improvement of the TiO2 reinforced epoxy system

April 1996

ELSEVIER

Materials Letters 26 (I 996) 299-303

Effects of coupling agents on the mechanical properties improvement of the TiO, reinforced epoxy system Manwar lhssain, The institutct cvScientijic

Atsushi Nakahira, Shigehiro Nishijima, Koichi Niihara and Industrial Research. Osaka University 8-l Mihogaoka. Received

18 September

1995; accepted 9 October

Ihoraki, Osaka 567, Jupan

1995

Abstract A study was carried out to investigate the effects of different coupling agents on the mechanical properties of the TiO, particulate filled epoxy composite. Composites prepared by dispersing TiO, coated with a silane coupling agent were compared with titanate coupling agent coated TiO, dispersed composites. Young’s modulus and flexural strength of titanate coupling agent treated composites were significantly improved compared to silane coupling agent treated composites. It was suggested that a strong interfacial bonding between the filler and the matrix existed when the titanate coupling agent was used and was explained by the adhesion model. Scanning electron microscopy studies revealed a better dispersion of surface modified filler particles and a strong adhesion/bonding between the filler and the matrix. Keywordx

Mechanical

properties;

Silane coupling

agent; Titanate coupling

1. Introduction Epoxy resins are being used as a matrix in structural applications such as high performance fiber reinforced composites (CFRP) for aerospace, automobiles and superconducting magnets. The main draw back of epoxy resin is its low fracture toughness, relatively low strength and low elastic properties. For practical applications, high strength and high toughness are required. Several workers [.1-S] have investigated mechanical properties of particulate reinforced epoxy composites. Incorporation of a rigid inorganic filler into various polymers or epoxy systems is a well-known technique to improve the physical and mechanical properties. Mechanical properties of these particulate filled systems depend on various parameters such as matrix properties, particle shape, size, particle distri-

agent

bution, volume fraction of dispersed phase, particle-matrix interaction or adhesions and test temperature. Many researchers [9-121 have improved the toughness of epoxy resins by incorporating soft particles such as rubber and thermoplastics and proposed some toughening mechanisms. However, these show the lower flexural strength and Young’s modulus. The interface bonding or adhesion between the filler and the matrix has a great effect on the mechanical properties of particulate reinforced systems. Adhesion/bonding between the filler and the matrix was controlled by using various coupling agents and small improvements in the mechanical properties were observed [ 13- 151. The purpose of this article was to study the mechanical properties of particulate epoxy composites. Different coupling agents were used to investi-

00167.577X/96/$12.00 Q 1996 Elsevier Science B.V. All rights reserved SSDI Ol67-577X(95;100253-7

M. Hu.ssain et al. /Muteriuh

300

gate the filler-matrix adhesion and its effects on the mechanical properties of particulate filled systems.

2. Experimental

procedures

2. I. Materials The materials used in this experiment were N,N,N’N’-tetraglycidylmethaxydiamine (TETRADX: Mitsubishi Gas Chemical Company, Japan) as a tetra functional epoxy resin and 1,2-cyclo-hexanedicarboxylic anhydride (HHPA, Wako Junyaku Co) as a hardener. TiO, with particle size of 1 pm from Idemitsu Chemical Co. Japan was selected as filler particles. Surface treatment of the TiO, powder was performed by the silane coupling agent, -y-amino propyl methyl di-methoxy silane (KBM 6021, from Shin-Etsu Chemical Co., and the titanate coupling agent, isopropyl tris(dioctanpyrophosphate) titanate (KR-38S), from Ajino-motto Chemical Co. Ltd. Japan. 2.2. Surface treatment of TiO, powder TiO, was added with vigorous stirring to a moisture free solution of KBM-602 or KR-38s in toluene. The slurry was stirred for 2 h using a magnetic stirrer. Toluene was expelled by heating the coated filler for 4 h at 80°C followed by vacuum drying. The hydrophobicity acquired by the filler was confirmed by a floating test on water.

Letters

26 (1996)

299-303

the pieces were ground and polished for following mechanical tests. Young’s modulus of the composites was measured by the flexural vibration resonance method at room temperature. The flexural strength of the particulate filled epoxy composite was measured by using the 3-point bending method (span: 50 mm, cross speed: 2 mm/min). Dispersion of particles in the composites was observed primarily by optical microscopy. Fracture surface was examined by scanning electron microscopy (SEM) to study the dispersion of the filler, filler matrix adhesion and filler-matrix interface. The fracture surfaces were coated with gold prior to scanning.

3. Results and discussion 3.1. Flexural properties The variation of relative Young’s modulus EJE,,, with volume fraction for the composites studied is shown in Fig. 1. (E, refers to Young’s modulus of composites and E, is Young’s modulus of the matrix). It was observed that Young’s modulus of the composites increased with increasing the filler content. Furthermore, titanate and silane surface treated composites showed a higher Young’s modulus compared to the untreated TiO, composite system. The modulus data were compared with some theoretical

2.3. Mixing method Epoxy resin was mixed with the treated filler powder using an evaporator for 8 h. Then the hardener was added. After mixing well with the hardener, the mixture was poured into an aluminium mold and placed into a vacuum chamber. After de-gassing the mold was placed into a constant-temperature oven for pre-curing at 80°C for 2 h and post-cured at 180°C for 2 h,. 2.4. Characterization After post-curing, round pieces of the composite were cut into pieces of size 60 X 8.2 X 3.2 mm. All

t

E 1.2

W 3 w 1.1

0.9;

I 0.02

I 0.04

VoLfraction

I 0.06

I 0.08

I 0.1

I 0.12

of filler content

FioD. I. Relative Young’s modulus as a function of tiller content. Dashed curve (A) represents the Kemer model, and curve (B) represents the Guth-Smallwood model.

M. Hussein

et crl./Materiuls

1.25

data for two-phase composite systems. Curve B in Fig. 1 represents the predicted change from Eq. (1) by Kemer [ 161, whi.le curve A represents the data from Eq. (2) by Guth-Smallwood [17]. EC/E,

.+ --t

EC/E,,, = 1 + 2.5& -I- 14.1&,

I

/

‘

Silp

1.15 d

1%,)(6/l

I

Uniost Titin*

e

I

= 1 + 15( 1 - vm) /(8-

301

Lrrters 26 (1996) 299-303

- +,)3

(1) (2)

where u,,, is the Poisson ratio of the matrix and & is the volume fraction c’f the filler. For composites with untreated TiO,, however, the modulus data showed a good fit with the Kerner model. For composites with surface treated TiO,, the modulus data showed a good fit to the Guth-Smallwood model for good adhesion between the filler and the matrix system. Thus, the simple model can be used to predict Young’s modulus of the surface treated TiO, filled epoxy system. In *agreement with many studies [18,19], Young’s modulus increased with increasing filler content. Upon treatment of TiO,, the modulus increases above the values of untreated systems. This indicates an interaction of the filler and the matrix through the chemical bonding in addition to physical interaction. Furthermore, the surface of the filler treated with a coupling agent will give desirable dispersion in the matrix and will enhance wetting adhesion and interface between the filler and matrix, and as a result, the stiffness will be increased, i.e. the modulus will be increased. A similar behavior was also observed by Maiti for talc filled epoxy composite systems

b

1.1 1.05 1 0.95

I

’ 0

I

0.03

,

0.09

0.12

Volume fraction of filler content Fig. 2. Relative flexural strength as a function Dashed line predicted from Turcsayl’s model.

of tiller content.

a result it improves the mechanical properties of the composites. On the basis of the strength results it can be stated that the strength of the particulate treated composite system with a coupling agent depends on the type of the coupling agent. It is noted that the effectiveness of a coupling agent depends on its structure, the polymer structure, and the nature of the filler surface. The chemical structure of titanate coupling agents contains a relatively long hydrocarbon chain compared to the silane coupling agent as shown in Fig. 3. When titanate is bonded to an inorganic filler, the hydrocarbon chains plasticize and improve the compatibility of the inorganic particle with the epoxy

La. Fig. 2 shows the variation of flexural strength with filler content. !Flexural strength was found to increase with filler content. Significant improvement in flexural strength was also observed for titanate coupling agent coated epoxy composites. However, less improvement in flexural strength was observed for silane coated epoxy composite systems. The strength of composites is dependent primarily on the success with which applied loads are transmitted through the matrix material to the filler particles or reinforcing materials. It is known that the bond between the resin and the filler is one of the key factors influencing the properties of the composites. Thus, the introduction of a coupling agent into these system promotes the matrix to filler adhesion, and as

I

0.06

CH3 I

H,NCH,CH2NHC3H,Si(OCH3~

(a) N-@(amino-ethyl&

amino propyl-methyl

FH3

di-methoxy silane

9 ?

CHS-~H-O-Ti-{O-~Q~-(O~aH,,)2)3 AH

(b) Iso-propyl-tris(di-octyl Fig. 3. Chemical titanate.

structllre

pyrophosphate)

of coupling

titanate

agents: (a) silane and (b)

302

M. Hussein et ul./Muteriuls

Letters 26 (1996) 299-303

[21]. The interfacial adhesive effects arise from the attachment of a large number of organic groups to the surface of the inorganic filler. This allows the chemical interaction and/or the formation of a strong Van der Waal’s attraction between the short organic chains of titanate and the long chains of the polymer. The number of coupling sites and their location are also the most important parameters for the chemical bonding between the coupling agent and the filler. Kryszttafkiewicz [22] investigated the number of coupling sites and concluded that the titanate coupling agent created three coupling sites on a filler particle, whereas the silane coupling agent created only one. As a result the chemical bonding between the filler and titanate coupling agent becomes stronger compared to silane. The mechanism of filler interaction and the coupling sites are shown in Fig. 4. Recently, Turcsanyi [23] modified the model developed by Nicolais and Narkis [24] for the strength of the particulate filled epoxy system as follows (3): ac = (1 - &)%al

exp( K&)/(1

(3)

+ 2.5&J,

where a,,, is the yield stress of the unfilled matrix and K is a parameter which is related to the adhesion between the filler and the epoxy matrix. This model was applied for titanate and silane treated TiO,-epoxy composite systems. As expected, when the adhesion was improved by surface treatment using a coupling agent, K increases in comparison with the untreated TiO, composites. It is evident from the Fig. 2 that the experimental results are best fitted when B values are considered to be 4.4, 4.7 and 5.4 for untreated, silane treated and titanate treated composites, respectively. It is concluded that titanate coupling agent showed a good adhesion

Fig. 5. SEM micrographs of fracture surface after flexural test. (a) Untreated TiO,, (b) silane and (c) titanate treated.

RESIN E

/bonding between the filler and the matrix and thus contributed higher flexural strength.

H

RESIN f

(b)

( lnoplc

rillcr)

Bcfcm ttimf

Alicr lrcament

(chemical bond knvccn filler and resin)

Fig. 4. Reaction mechanism for different coupling agents. (a) Silane treated showing one reaction site, (b) titanate treated showing three reaction sites.

3.2. Fracture n&aces To examine the extent of bonding between the filler and the matrix, scanning electron microscopy was used. Fig. 5 shows the micrographs of fracture surfaces of composites treated with different coupling agents. The dispersion of untreated TiO, in epoxy is rather poor, as evidenced by the non-uni-

M. Hussain et al./Materials

form fracture surfaces with uneven voids and agglomeration. By contrast, in samples coated with silane coupling agents, a substantial amount of epoxy remains on the TiO,, surfaces. In case of titanate treated TiO,, however, composites exhibited smoother fracture surfaces with sufficient epoxy residue on the filler surfaces which confirmed the strong interfacial bonding between the filler and the matrix.

4. Conclusion In this work, we have shown the effect of different parameters, such as volume fraction of filler particles and the surface treatments of filler, on the mechanical properties of TiO, filled epoxy composites. Significant improvements in Young’s modulus and flexural strength were observed for titanate treated TiO, epoxy composites. Improvement in mechanical properties was enhanced by coupling agents due to formation of a strong interface or adhesion between the filler and matrix. Titanate coupling agent was found to be more effective than silane coupling agent.

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