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Materials Letters 58 (2003) 154–158

A novel ultrasonic transducer backing from porous epoxy resin–titanium–

silane coupling agent and plasticizer composites

Farid El-Tantawya,*, Yong Kiel Sungb

aDepartment of Physics, Faculty of Science, Suez Canal University, Ismailia, EgyptbDepartment of Chemistry, Dongguk University, Seoul 100-715, South Korea

Received 6 January 2003; received in revised form 5 May 2003; accepted 9 May 2003

Abstract

In this paper, a new composite for ultrasonic attenuation backing has been successfully fabricated from porous epoxy resin containing

titanium (Ti), silane coupling agent and plasticizer composites. The effect of Ti particles on the network structure and mechanical properties

of epoxy resin has been analyzed in detail. The ultrasonic parameters in epoxy composites have been measured by a conventional pulse-echo-

overlap technique at a frequency of 1–5 MHz. The effect of Ti content and temperature on the longitudinal sound velocity and attenuation of

epoxy resin composites were investigated. Precise in situ observations of the acoustic properties such as attenuation and acoustic impedance

of epoxy composites are expected to be useful for ultrasonic transducer systems for new as well as for backing application with high

attenuation.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Epoxy resin; Titanium; Materials composite; Microstructure; Mechanical properties; Ultrasonic properties; Backing; High attenuation

1. Introduction

This study is part of an on-going research project aiming

to develop a high performance composite material for ultra-

sonic transducer backing with high acoustic impedance,

attenuation and toughness. Epoxy resins are commonly

used as polymeric matrices in high-performance composites

due to their good thermal stability, environmental resistance

and good mechanical properties and in the manufacture of

backings because of its low initial viscosity and its high

adhesion [1–5]. In fact, the need for transducer suitable for

use in attenuation backing under high hydrostatic conditions

is increasing in industry. Typically, backings are composed

of epoxies loaded with metallic filler such as tungsten, iron,

copper, magnesium and aluminum are used to obtain high

acoustic impedance for the backing material, whereas fillers

of wood dust, glass and cork are used for low impedance

backings [4,5]. It is well known that there are two methods

to modify epoxy resin: the first consists of adding rubber

particles in the initial liquid resin, which is then polymer-

0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0167-577X(03)00435-X

* Corresponding author. Fax: +20-81-025-262-6265.

E-mail address: faridtantawy@yahoo.com (F. El-Tantawy).

ized by a hardener to obtain composite materials. This

method allows one to achieve simultaneously a low initial

viscosity, a high attenuation and a high stiffness. The

second uses a reactive liquid rubber, which is initially

soluble in epoxy resin and the phase separation occurs

during the vulcanization of the epoxy. This method does

not increase significantly the mixture viscosity of the

composite [6]. To overcome this shortcoming and to fulfill

the above stated needs, the epoxy resin has been modified

by adding plasticizer, coupling agent and Ti powder. To the

best of the author’s knowledge, porous epoxy filled with Ti

particles, silane coupling agent and plasticizer for ultrasonic

backings are still unavailable in publications. The target of

the study is the fabrication of a novel attenuation backing

based on porous epoxy–Ti–silane coupling agent and

plasticizer composites. The effect of Ti content on the

structure and mechanical properties of the epoxy compo-

sites was interpreted in detail. Also, the temperature depen-

dence of attenuation and sound velocity of epoxy

composites were tested. For ultrasonic transducer backing,

the effect of Ti content and frequency on the attenuation

was investigated. Furthermore, the effect of Ti content on

the acoustic impedance of epoxy composites was also

tested.

F. El-Tantawy, Y.K. Sung / Materials Letters 58 (2003) 154–158 155

2. Experimental work

With the above aim in mind, the polymer used in this

investigation was a commercial epoxy resin (type 828) and

hardener (type B002W) supplied by (Yuka shell epoxy

chemical, Tokyo, Japan). A stoichiometric resin/hardening

ratio 100:20 by weight was used according to the manu-

facturer’s data sheets. Titanium powder with particle size 20

Am was used as reinforcing and attenuation scattering agent.

Glycerol (Gly, analytical grade) and g-glycidoxy propyl

trimethoxy silane (A-188) were used as the plasticizer and

coupling agent, respectively. The amount of glycerol was 15

wt.% and silane was 2 wt.% in the composite. Several

batches of epoxy–Ti weight ratios were considered:

99.8:0.2, 99.6:0.4, 99.4:0.6, 99.2:0.8 and 99:1, respectively

and abbreviated as Ti1, Ti2, Ti3, Ti4 and Ti5, respectively.

The green epoxy-hardener with different content of filler

was prepared by centrifuging mixer for 2 min at room

temperature. The bulk samples of composite were obtained

by casting the green composites on alumina mould and

placed in an electrical oven that was preheated to 60 jC for

30 min. Then, the epoxy-filler composites were cured under

hot uniaxial pressure 150 KN/m2 at 100 jC for 2 h.

Morphology study of the epoxy composites was conducted

on Scanning Electron Microscopy (SEM, JSM-5310 LVB,

JEOL). The molar volume of the composites (Vm) was

measured by a pycnometer using a mixture of carbon

tetrachloride and toluene as a medium [2]. Before the

measurements were made, the specimens were cut into

fragments and vacuum dried for 1 day. The mechanical

properties of the epoxy composites were measured using an

Instron materials testing machine. The samples were pre-

pared according to ASTM DIN53557 standard [1]. The

dielectric properties of the composites were measured at

frequency 1 kHz by using RLC Bridge (3541 Y-Hitester,

Hioki, Japan). Silver paste was used to ensure a good

contact of the sample surface with copper electrodes. The

glass transition temperature (Tg) and specific heats (Cp) are

measured by differential scanning calorimeter as described

elsewhere [2]. Viscosity (g) was measured by viscometer

model (VM-A, Japan). Surface tension (ST) was measured

by using a surface tension-meter model (GPYP-A3, Japan).

The ultrasonic measurements were performed using an

ultrasonic flaw detector of type Krautkramer-Branson

USD-10. The adopted technique is the pulse-echo immer-

sion technique [2]. The tested sample was sandwiched

between two piezoelectric ceramics operating at about

8 MHz used as transducer. Sound velocity was determined

with reference to its velocity in distilled water at 20 jC(1483 m/s). The accuracy of the sound velocity was esti-

mated to be better than 0.3%. The sound velocity in water

(in m/s) was found to vary with the absolute temperature (T)

according to linear relation in the form:

V � 2:55T þ 2230 ð1Þ

w

3. Results and discussion

3.1. Network structure and mechanical properties of the

composite

Knowledge of the network structure and mechanical

properties of thermosetting resins is essential in order to

be able to choose a proper set of processing parameters and

type of fillers, which give good material properties. The

SEM was used to investigate the morphology of the cured

epoxy–Ti composites. Fig. 1a presents the SEM for sample

Ti5 of epoxy composites. It is shown that the Ti particles are

homogeneously distributed and sink into the porous epoxy

matrix. Closer inspection shows that there is big amount of

epoxy matrix adhered and Ti particles are coated by an

epoxy resin, indicating a strong interface within epoxy

composites. The cross-link density (CLD) = qENA/Mw,

where qE is the density of epoxy, NA is Avogadro’s number

and Mw is the molecular weight between cross-link’s. The

extent of filler reinforcement (g) is given by: Vr0/

Vrf = 1� c(f/1-f), where Vrf is the volume fraction of

swollen epoxy in the fully swollen filled sample and f is

the volume fraction of Ti. The dependence of cross-linking

density, extent of filler reinforcement and molar volume

(Vm) on Ti content of epoxy composites is shown in Fig. 1b.

It is observed that the CLD, c and Vm increase linearly and

strongly depend on the Ti content. The reason is considered

to be due to the chemical cross-linking to increase the

entanglement contribution in the network structure of epoxy

composite. It is worthy that the positive value of c refers thata reinforcing filler has good polymer–filler interaction. This

implies that Ti acts as a reinforcing effect and reacts with

epoxy molecules and increases the interfacial adhesion

within epoxy composites as confirmed by the glass transi-

tion (Tg), viscosity (g), surface tension (ST) and packing

factor PF = qb/qt (where qb and qt are the particle bulk and

true densities, respectively). The results shown in Fig. 1c

relate to the effect of Ti content on the Tg, g, ST and PF of

epoxy composites. It is found that Tg, g and PF increase with

increasing filler content in the epoxy composites. The

increase in Tg is most likely due to the increase of cross-

linking density [8]. The viscosity and packing factors

support this claim; a greater cross-link density tends to

correlate with less free volume into epoxy composites.

Indeed, the surface tension increases remarkably with in-

creasing Ti content due to the increase of the action force

among the epoxy chains and cross-link reaction of the

epoxy. Therefore, the filler and matrix might reach the good

wettability. Considering this observation, it is reasonable to

believe that the adhesive force among filler and matrix

increases, and hence there is a molecular structural change.

However, the mechanical properties of epoxy composites

were examined in order to figure out and understand how

the Ti particles influenced the performances of epoxy

matrix. Fig. 1d presents the plots of the breaking strength

(Eb), Young’s modulus (Ym), hardness (shore A) and frac-

Fig. 1. (a) SEM photographs for sampleTi5, (b) Cross-linking density, extent of filler reinforcement and molar volume as a function of Ti content of epoxy

composites. (c) The relationship between Tg, g, ST, PF and Ti content of epoxy composites. (d) Breaking strength, Young’s modulus, hardness and fracture

toughness as a function of Ti content for epoxy composites.

F. El-Tantawy, Y.K. Sung / Materials Letters 58 (2003) 154–158156

ture toughness Tg = 0.0824(P/C1.5) where P is the indenta-

tion load and C is the crack length, as a function of Ti

content for epoxy composites. It is seen that all the me-

chanical property improves as the Ti content increases in the

composite. This indicate that the Ti particles acts as a

reinforcing agent as confirmed by network structure results

before. The breaking strength increases with increasing Ti

particles. This is ascribed to a good ductility of epoxy matrix

and a strong interphase adhesion between Ti and epoxy

matrix. Also, it is seen that Young’s modulus increases with

an increase in a Ti content in the composites. This is

attributed to the integrated interfacial bonding and good

wettability between filler and matrix due to the higher

surface area of the Ti particles. In the author’s opinion,

the fracture toughness in these composites is due to the

generic characteristics of plasticizer and coupling agent

modified epoxy resin: the higher the plastic deformability

and the lower the crack pinning, the greater the fracture

toughness. Thereby, we conclude that the plastic deform-

ability of bulk epoxy adhesive was enhanced with inclusion

of filler, plasticizer and coupling agent. Lastly, the hardness

increased with increasing Ti content. The reasons are that Ti

particles reduced the creep of epoxy matrix and therein led

to enhance the network structure stability within the epoxy

matrix. This result provides another indication that the

dispersed particles within the epoxy matrix are easily dis-

placed under the applied load at low Ti content.

3.2. Temperature dependence of longitudinal wave velocity

and attenuation

The ultrasonic parameters like longitudinal velocity and

attenuation of polymer are strongly affected by their net-

work structure, and thereby the measurement of these

parameters can provide useful information about the mod-

ification of the architecture of molecular structure when the

filler, temperature, pressure or frequency of the simulation

changes. The longitudinal sound velocity (V) and attenua-

tion (a) was calculated by the following equations [7,8]:

VE ¼ 2LVw

ð2L� Vwðt1 � t2ÞÞð2Þ

expð�2aELÞ ¼ Rð�2LawÞqwVw

4qEVE

þ qEVE

4qwVw

þ 1

2

� �ð3Þ

Fig. 2. (a) The variation of ultrasonic velocity on temperature for epoxy

composites. (b) The variation of attenuation on temperature for epoxy

composites. (c) The variation of a, Cp, Ka and Ki as a function of Ti content

for epoxy composites.

F. El-Tantawy, Y.K. Sung / Materials Letters 58 (2003) 154–158 157

where the subscripts E and w stand for epoxy and water,

respectively, L is the specimen thickness, t1 and t2 are the

times of flight of ultrasonic waves in a round trip with the

specimen removed and immersed, respectively, qw is the

density of water and R is the ratio of the received amplitudes

when the sample removed and immersed.

The variation of ultrasonic velocity and attenuation on

temperature for epoxy composites is presented in Fig. 2a

and b, respectively. It is seen that the ultrasonic velocity

and attenuation value increase with increasing Ti content

in the epoxy composites. This is ascribed to the strong

Fig. 3. (a) The attenuation versus frequency for epoxy composites. (b) The

acoustic impedance, dielectric constant and dissipation factor versus Ti

content for the epoxy composites.

F. El-Tantawy, Y.K. Sung / Materials Letters 58 (2003) 154–158158

adhesion force between neighboring chains and filler

matrix interactions into epoxy matrix. On the other hand,

the ultrasonic velocity and attenuation decrease with

raising temperature. This ascribed to the dilatation effect

due to the increases of the thermal expansion of epoxy

chains at high temperature. The temperature coefficient of

ultrasonic velocity b=(� 1/V)(DV/DT), as a function of Ti

content is plotted in Fig. 2c. The adiabatic compressibility

Ka=(qcVs2)� 1 and the isothermal compressibility Ki =

Ka + Tqch2/Cp, where Cp is the specific heat of the

composite. The values of Cp as a function of Ti content

are plotted in Fig. 2c. It is seen that the Cp increases with

increasing Ti content. This is another clue confirming that

the inclusion of Ti particles enhances the network struc-

ture and thermal stability of epoxy matrix. However Fig.

2c presents the variation of b, Ka and Ki as a function of

Ti content. In Fig. 2c, it is clearly that b decreases while

Ka and Ki increases with increasing Ti content in the

composite. The decrease of b and increases of Ka and Ki

gives an indication of good interface adhesion between

epoxy and Ti in the composite. As the temperature is

raised, the expansion of epoxy is restricted by the Ti

phase, which has lower b, resulting in a decrease of

overall b in the composites. This suggested again that

the incorporation of Ti particles enhances the thermody-

namic stability and molecular structure of epoxy matrix as

confirmed before.

3.3. Applicability of composites in ultrasonic backing

To test the applicability of epoxy composites for ultra-

sonic backing application, the attenuation and acoustic

impedance was investigated. Fig. 3a represents the atten-

uation versus frequency plot for neat epoxy and epoxy–Ti

composites. It is seen that there is a considerable increase

in attenuation with increasing the frequency. This is

ascribed to the cross-linking density and the contribution

of Ti particles to acoustic scattering and multireflecting

within porous epoxy matrix. Also, the increment in atten-

uation with increasing Ti content can be accounted for by

the interactions involving the epoxy and the Ti phases. The

attenuation values are very high thus making the epoxy–Ti

composites very useful for attenuation backing applications

[4,6]. To obtain more information about the applicability

for backings, the acoustic impedance as a function of Ti

contents were computed. The relationship between the

acoustic impedance Z =Vqc as a function of Ti concentra-

tion is plotted in Fig. 3b. The increase in acoustic

impedance with addition of Ti is due to the increase in

the chain connectivity and interfacial polarization associ-

ated with Ti particles. This fact is also supported by

dielectric properties that has resulted. The variation of

the dielectric constant (e) and dissipation factor (tan d)with Ti content are shown in the same Fig. 3b. Both the

dielectric constant and dissipation factor increases with

increasing Ti content in the epoxy composites. One pos-

sible reason for increase e and tan d is considered to be the

reduction of average distance between Ti particles. Fur-

thermore, the average interfacial polarization associated

with a cluster of Ti particles is coarser than that of an

individual particle because of the increase in the dimen-

sions of the metallic inclusion within epoxy matrix.

4. Conclusions

(1) The porous epoxy, Titanium, Silane coupling agent and

plasticizer composites has been fabricated for transducer

attenuation backing applications. The macromolecular

network of epoxy resin is modified considerably by the

inclusion of Ti powder, coupling agent and plasticizer as

confirmed by network structure and mechanical ana-

lyzes.

(2) The ultrasonic sound velocity and the attenuation

decrease with raising temperature due to the dilatation

effect of the epoxy matrix.

(3) The attenuation increases with increasing Ti content and

frequency in porous epoxy composites. Also, the

acoustic impedance increases with increasing Ti content.

The high attenuation and acoustic impedance values

reflect that the porous epoxy–Ti composites are useful

for transducer backing applications.

Acknowledgements

This research was supported by Dongguk University,

Department of Chemistry, Seoul, Korea.

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