Effect of Ultrasonic Dispersion Methods on Thermal and Mechanical Properties of Organoclay Epoxy...

Full Paper

Effect of Ultrasonic Dispersion Methods onThermal and Mechanical Properties ofOrganoclay Epoxy Nanocomposites

Katherine Dean, Julia Krstina, Wendy Tian, Russell J. Varley*

This report highlights the importance of nanocomposite formation and dispersion uponimprovements in properties for high performance epoxy based layered silicate nanocompo-sites. This is achieved through the preparation of epoxy nanocomposites with varying clayconcentrations using ultrasonic and solvent based fabrication and standard shear mixingprocedures. Ultrasonication (combined with a solvent), in comparison to shear mixingmethods, produces superior nanoscale dispersion according to SEM and TEM. As a result ofthe improvements in nanoscale dispersion, the corresponding improvements in fracturetoughness, strength, strain to failure (compressive and flexural) and char stability are pre-sented. TGA shows that while the initial thermal decomposition process is not affected, thestability of the char layer formed during decomposition increases with improved nanoscaledispersion as well as increasing concentration. The effect ofmoisture upon the dynamicmechanical thermal analysis ofthe epoxy nanocomposites displays some dependence uponthe clay dispersion with a modest increase in plasticisationfor the sonicated samples. Overall, this work shows that fora high performance epoxy anhydride resin system, signifi-cant improvements in key properties can be achieved at lowlevels of addition if appropriate sonicated dispersionmethods can be utilised.

Introduction

The recent proliferation of research into layered silicate

nanocomposite materials began at the Toyota Central

Research Laboratory (TCRL) in the early 1990s.[1–3] The

large improvements in the mechanical and thermal

properties of caprolactam, achieved at very low concen-

K. DeanCSIRO Manufacturing and Materials Technologies, Clayton South3169, AustraliaJ. Krstina, W. Tian, Russell J. VarleyCSIRO Molecular and Health Technologies, Clayton South 3169,AustraliaFax þ61 3 9545 2517; E-mail: [email protected]

Macromol. Mater. Eng. 2007, 292, 415–427

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

trations, ensured the promise of enormous commercial

potential. Although the promise has been slow to be

realised, the fundamental challenge or question in layered

silicate nanocomposite research remains the same across

the range of polymer matrices. The challenge is to fully

understand how to destroy the layered structure of the

nanoclay within a given matrix, so that the polymer can

be said to be truly ‘nanoreinforced’ and the properties,

tailored and optimised. Nanocomposite formation in-

volves the migration, or diffusion of the polymer into

the interlayer galleries of the layered silicate which push

apart or swell the silicate layers. For this to happen, how-

ever, the hydrophilic clay needs to be compatibilised with

the hydrophobic polymer which is achieved by exchanging

the interlayer inorganic cation, with a more hydrophobic

DOI: 10.1002/mame.200600435 415

K. Dean, J. Krstina, W. Tian, R. J. Varley

416

organic cation, such as an alkylammonium surfactant.

When the polymer is mixed intimately with the clay and

the distance between the layers is increased, although

remaining ordered, the clay is said to be intercalated. If the

distance between the layers increases such that the layers

become completely and randomly dispersed within the

polymer matrix, the silicate is said to be exfoliated. Both

these types of dispersions are considered to be nanocom-

posites, and it is generally accepted (although not uni-

versally[4]) that maximum property enhancements are

obtained for exfoliated systems.

For thermosetting epoxy based layered silicate nano-

composite materials, complete ‘exfoliation’ of layered sili-

cates has proved somewhat harder to achieve than other

polymer matrices such as polyamides or polyurethanes. It

was initially proposed by the TCRL for polyamides[2] and

subsequently by Kornmann et al. (2001)[5] and Lan et al.

(1994)[6] for epoxies that the key to achieve exfoliation was

to ensure that the intercalated polymer reacted faster than

the polymer in the bulk. In the case of an epoxy resin cure,

this occurs readily due to the ionic environment within the

interlayer region being able to catalyse homopolymerisa-

tion as well as the epoxy amine reactions. From a thermo-

dynamic perspective, exfoliation occurs due to the

monomeric polar epoxy resin being attracted by the high

surface energy of the clay and diffusing into the interlayer

regions until equilibrium is reached. Reaction within the

interlayer region shifts this equilibrium allowing more

monomeric species to diffuse into the interlayer region

increasing the d-spacing between the clays, and continu-

ing the process potentially until complete exfoliation. Vaia

and Giannelis[7] showed that the mixing process is driven

by the interplay between entropic and enthalpic factors

where the decrease in entropy resulting from the confine-

Table 1. Coefficients of thermal expansion (CTE), glass transition tem

% Clay Dispersion method CTE (60–14

ppm � cm

0 – 73.67

1.0 Shear mixing 65.74



1.0 Bath sonication 62.52



1.0 Horn sonication 73.65



a)As determined by DMTA; b)As determined by DSC.



ment of the polymer inside the silicate galleries is compen-

sated for by the increase in conformational freedom of

the organomodifier, as the inorganic layers separate. In

contrast, Park and Jana,[8] however, discuss full exfoliation

in terms of the build up of sufficient elastic energy within

the clay interlayer region to overcome the cumulative

attractive forces binding the clay platelets. These forces

include the internal electrostatic attractive forces as well

as the external viscous forces of the bulk polymer. They

showed that relative changes in the bulk resin viscosity

compared to the modulus build up in the interlayer region

proved to be a more important predictor of full exfoliation

rather than the relative rates of reaction. The exertion of

shear forces upon the clay platelets would therefore be

expected to play a role in their dispersion within an epoxy

matrix, particularly, given the uneven attractive forces

acting between platelets depending upon where they are

within the tactoids.[9] Direct mixing of the epoxy with the

nanoclay using high-speed shear methods has been com-

monly used to prepare nanocomposites, but tends to

produce less than fully exfoliated morphologies. This has

led to the development of methods which tend to combine

the use of high shear forces with other novel methods. For

example, Kotsilkova[10] used a solvent process to improve

exfoliation in addition to high speed mixing and found

that a direct improvement in exfoliation was achieved

along with improvements in the flexural properties of

the epoxy cured network. Likewise, improvements in the

nanoscale dispersion, impact properties and flexural

strength were reported by Lu et al.[11] through the appli-

cation of high-speed mixing and high-impact ball milling

process. It was reported that the pulverisation of the initial

agglomerated particles exerted increased shear forces and

promoted exfoliation. Yasmin et al.,[12] using a three roll

peratures and final cure conversion of the samples prepared.

0 -C) Tg (tan dmax)a) Cure conversionb)

S1 -C %

225 96.8

225 95.9

230 94.8

222 98.7

225 97.4

230 98.3

221 98.7

209 98.4

223 99.0

222 99.4

DOI: 10.1002/mame.200600435

Effect of Ultrasonic Dispersion Methods on Thermal . . .

mill, reported the formation of ‘disordered intercalates’

along with substantial improvements in elastic modulus.

Wang et al.,[13] however, reported improvements in

nanoscale dispersion as well as the compressive strength

of an epoxy-based network using a slurry or liquid-liquid

based method. This method was based upon the use of

high speed shear mixing to break up micron-sized agglo-

merates while ensuring that the clay remained wet so that

epoxy intercalation could be facilitated. Liu et al.[14] com-

pared a high speed and high pressure mixing method,

which also used solvent with a direct mixing method and

reported that improvements in exfoliation could be achi-

eved and correlated with improvements in the fracture

properties. The use of ultrasonication methods has also

been shown to produce significant improvements in

nanoscale dispersion. Tolle et al.[15] in particular showed

that complete exfoliation of nanoclay platelets in a cured

epoxy matrix could be achieved through the use of a

specifically tailored process involving the use of solvent,

high speed mixing and ultrasonication. This mechanical

energy is clearly sufficient and of a short-enough frequ-

ency to interact within the clay layer galleries. The

interference from the reflecting sound waves creates

peaks and troughs in pressure which form pockets of

vacuum which collapse, thus, forming high pressure

explosions within the interlayer galleries, pushing apart

the clay layers.

The majority of research into epoxy nanocomposites has

focussed upon the commonly used bifunctional epoxy

resin, diglycidyl ether of bisphenol A (DGEBA) with either

amine or anhydride curatives. The use of aerospace resin

systems based upon tetraglycidylmethylenedianiline

(TGDDM) has been less widely studied, despite being

shown to produce advantageous improvements in proper-

ties.[16–18] This paper, therefore, reports upon the effec-

tiveness of the layered silicate nanocomposite formation

upon a high performance epoxy resin system that is

specifically designed for liquid moulding composite

fabrication technologies. Layered silicate nanocomposites

have been prepared using two different ultrasonic and

solvent based dispersion methods and compared with a

nonsolvent high-speed shear mixing method. The efficacy

of the dispersion method on nanocomposite formation,

has been evaluated using scanning electron microscopy

(SEM) and transmission electron microscopy (TEM) while

the corresponding improvements in mechanical, thermal

and barrier properties have been determined.

Experimental Part

Nanocomposite Formation

The epoxy resin system used in this work was the commercially

available two-part resin system, SI-ZG-5A (Shade, Inc. Nebraska,



USA), designed for vacuum-assisted resin transfer moulding

(VaRTM). The epoxy resin used was a proprietary blend based

upon the TGDDM epoxy resin while the hardener was a

precatalysed anhydride. The organomodified nanoclay used in

this work was Nanomer I.28 (Nanocor, Illinois, USA) specifically

designed for epoxy anhydride resin systems according to the

company product description. The clay was dried overnight in a

vacuum oven at 80 8C while the epoxy resin was used as received

without any further purification. The unmodified resin was

prepared by mixing SI-ZG-5A resin, part A and B in equal mass

quantities on a rotary evaporator, poured into preheated Teflon

coated moulds and cured. Nanocomposites containing 1, 2.5 and

5 wt.-% of nanoclay were prepared according to three different

methods. In all the cases, the two components of the SI-ZG-5A

resin (part A and B) were added in equal mass quantities to each

other when blending, regardless of the dispersion method.

Method 1: Shear Mixing and Degassing

The SI-ZG-5A Part A (epoxy) resin was added to the dried clay and

stirred vigorously for 1 h at 110 8C. The mixture was placed in a

rotary evaporator and degassed under vacuum for 30 min at 80 8C.

The epoxy/clay mixture was allowed to cool for 10 min followed

by addition of the SI-ZG-5A Part B anhydride hardener. Degassing

was continued for a further 10 min at 80 8C on the rotary

evaporator until the mixing was complete and no further bubbles

were evident. The resin was then poured into preheated (50 8C)

Teflon-coated moulds and cured.

Method 2: Bath Sonication, Solvent and Degassing

The nanoclay was added to acetone (300 mL) and stirred

vigorously (500 rpm) in a sonification bath for 3 h. (Sonic bath

type CT72, 100–25 KHz). The SI-ZG-5A Part A was added and

sonification continued for a further 1 h. The acetone was removed

on a rotary evaporator at 50 8C and degassed for a further 30 min.

When the acetone was completely removed SI-ZG-5A Part B was

added and the blend was mixed and degassed for a further 15 min

at 80 8C. The resin was then poured into preheated Teflon-coated

moulds (50 8C) and cured.

Method 3: Cell Disruptor Horn Sonication, Solvent andDegassing

The nanoclay was added to acetone (200 mL) and mechanically

stirred to breakup large agglomerates. The ‘horn tip’ of the

sonicator was placed inside the solution and sonicated for three 10

min cycles [for a total of 30 min (horn tip micro-probe 20 KHz)].

The SI-ZG-5A Part A resin was added to the nanoclay acetone

mixture and the three 10 min cycles were repeated for a total

sonication of 60 min. The solvent was removed on a rotary

evaporator at 50 8C and degassed for a further 30 min. The

SI-ZG-5A Part B resin was then added and degassing was

continued for a further 15 min at 80 8C on the rotary evaporator.

After this, the resin was poured into preheated (50 8C) Teflon-

coated moulds and cured. All the blends were cured according to

the following cure cycle: (i) 50–79 8C at 2.5 8C �min�1, hold at 79 8Cfor 2 h; (ii) 79–177 8C at 2.5 8C �min�1, hold for 6 h at 177 8C; (iii)

cool to room temperature.

www.mme-journal.de 417


0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

30025020015010050Temperature (oC)

tan

del

ta

1.00E+07

2.10E+08

4.10E+08

6.10E+08

8.10E+08

1.01E+09

1.21E+09

1.41E+09

1.61E+09

Mo

du

lus (M

Pa)

neat resin 1wt% nanoclay 2.5wt% nanoclay 5wt% nanoclay

Figure 1. Effect of clay addition upon the dynamic mechanical properties of the epoxynetwork.

418

Characterisation

Flexural and compressive properties were

determined using an Instron Universal Test-

ing Machine model 4486. In the case of the

flexural properties, modulus, strain to fail-

ure, and strength were determined using a

three-point bend test, according to ASTM

D790. The load cell used was 1 kN, the cross-

head speed was 1.2 mm �min�1 and the

support span was 48 mm. The sample

dimensions were 60�25�3 mm3 and

five coupons were tested per sample set.

Compressive modulus, strength and strain

to failure were determined according to

ASTM D695. The rectangular prism-shaped

samples (4–8) of dimensions approximately

25� 12�12 mm3 (known accurately) were

placed between the parallel platen plates

and compressed at a rate of 2 mm �min�1

using a 50 kN load cell. Fracture toughness

testing was carried out using the compact

tension method according to ASTM E-394-81

using an Instron 4486 universal tester. Prior

to testing, the specimens were precracked by

inserting a thin razor blade into the machined notch and

impacting with a hammer. The specimens were then placed into

a jig and tested at a crosshead speed of 1.3 mm �min�1 using the 1

kN load cell until failure.

Thermogravimetric analysis was performed using a Mettler

TGA/SDTA 851 to investigate the degradation profile of the

0

0.2

0.4

0.6

0.8

1

1.2

75065055045035025015050

Temperature ( oC)

% M

ass

Lo

ss

-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

1st derivative (m

g/ oC

)

neat resin resin shear mixing bath sonication horn sonication

Figure 2. TGA thermograms showing the percentage of weight loss and the first derivativespectra for the unmodified resin and the 5 wt.-% nanoclay samples prepared using shearmixing, bath sonication and horn sonication.

network in an oxidative environment. Sam-

ples of approximately 30 mg were scanned

from 50 to 800 8C at a rate of 10 8C �min�1

under an air flow of 50 mL �min�1. The glass

transition temperatures (Tg) were deter-

mined using a Rheometrics Scientific Mark

V, dynamic mechanical thermal analyser

(DMTA). Samples of approximate dimen-

sions 10�50�3 mm3 were placed in the

large frame assembly and tested in the dual

cantilever mode. Temperature was ramped

from 50 to 300 8C at a rate of 5 8C �min�1,

the frequency of oscillation was 1 Hz

and the strain used was 0.04%. The glass

transition was determined from the peak(s)

in the tan d spectra. Coefficients of thermal

expansion (CTE) were determined using a

Mettler- Toledo TMA/TDA840. The tempera-

ture was ramped from 30 to 200 8C at a rate

of 3 8C �min�1. The CTEs were determined

from the linear portion of the curve below

the Tg of the network between 60 and

140 8C. DSC was performed using a Mettler

DSC821e in the dynamic mode to ensure

that the samples had been sufficiently

cured. Samples of approximately 10–20

mg were placed in sealed alumina crucibles

and placed inside the furnace under a



blanket of nitrogen purging at a rate of 50 mL �min�1. The

temperature was then ramped from 50 to 300 8C at a rate of

10 8C �min�1. The epoxide cure conversion was therefore deter-

mined from the residual exotherm and compared to the total

exotherm of 265 J � g�1, as determined from the uncured resin

system.

DOI: 10.1002/mame.200600435


Table 2. Parameters determined from TGA to characterise the degradation profile.

Wt.-%

clay

Dispersion

Method

First Step: Char formation Second Step: Degradation of char

Temp. at 10%

mass loss

Onset of

degradation

First maximum

rate loss

Char yield

at 525 -CSecond maximum

rate loss

Char yield

at 800 -C

-C -C -C % -C %

0 – 344.1 329 354.2 33.9 652.1 1.24

1.0 Shear mixing 346.8 330 351.3 34.9 650.6 2.25

2.5 Shear mixing 345.9 329 353.2 35.5 656.3 2.65

5.0 Shear mixing 349.1 335 357.3 36.9 652.0 5.59

1.0 Bath sonication 346.9 330 349.4 34.6 655.0 2.12

2.5 Bath sonication 344.2 327 351.2 33.1 674.7 2.45

5.0 Bath sonication 344.6 328 353.5 34.7 680.0 4.06

1.0 Horn sonication 340.9 331 355.2 31.9 651.7 1.76

2.5 Horn sonication 348.3 330 366.9 35.4 666.4 3.03

5.0 Horn sonication 342.4 331 356.9 34.4 693.0 4.36

Water uptake of the cured resin systems was determined by

immersing a sample of approximate dimensions 10�50�3 mm3

in a test tube filled with deionised water and then placing in a hot

water bath set at a constant temperature of 80 8C. The samples

were removed at relevant intervals from the hot water bath, dried

with tissue paper and then weighed. The measurements were

continued until it was clear that for an extended period of time

there was no further significant increase in water uptake.

Fractured surfaces of the nanomodified samples were exam-

ined using a Philips XL30 field emission scanning electron

microscope (SEM) in order to investigate the nanoparticulate

dispersion and the mode of fracture. Further investigations of the

nanoscale morphology were undertaken using a Phillips CM30

Table 3. Water uptake measurements of the samples after 7, 14 andtemperatures.

Wt.-%

clay

Dispersion

method

Water uptake

after 7 d

Water up

after 1

% %

0 – 1.57 1.79

1.0 Shear mixing 1.48 1.70



1.0 Bath sonication 1.54 1.78



1.0 Horn sonication 1.44 1.64



a)Values in parentheses indicate % decrease compared to dry networ



Transmission electron microscopy (TEM) using an accelerating

voltage of 150 keV at magnifications of 4 000–20 000� for 1 wt.-%

nanoclay samples only. Sections of 70 nm thickness were obtained

using a 458 diamond knife.

Results and Discussion

Thermal Properties

DSC results shown in Table 1 indicate that for all of the

networks prepared, the level of cure conversion is above

95% and are therefore considered completely cured for the

27 days (d) and the corresponding effect upon the glass transition

take

4 d

Water uptakea)

after 27 d

Tg1a)

(after 27 d)

Tg2a)

(after 27 d)

% -C -C

2.07 208 (7.3) 236 (5.3)

1.91 (7.7%) 213 (5.5) 238 (5.7)

1.94 (6.3%) 213 (7.5) 239 (4.0)

2.00 (3.3%) 200 (9.8) 232 (4.5)

1.98 (4.3%) 205 (8.9) 235 (4.1)

1.96 (5.3%) 211 (8.3) 241 (4.8)

1.92 (7.2%) 193 (12.8) 226 (2.4)

1.82 (12.1%) 200 (4.3) 233 (11.5)

1.89 (8.7%) 204 (8.5) 232 (3.9)

1.83 (11.6%) 186 (16.2) 235 (5.9)

k.



0.0

0.5

1.0

1.5

2.0

2.5

302520151050

time (days)

% w

ater

up

take

unmodified resin

shear mixing

bath sonication

horn sonication

Figure 3. Percentage of water uptake plotted as a function of time for the 1 wt.-%nanoclay sample illustrating the effect of dispersion method.

420

purposes of this work. Importantly, the

level of clay addition or the type of

dispersion did not significantly affect

the level of cure and the differences

measured would not be expected to

have an impact upon properties. The

effect of nanoclay addition upon Tg as

determined using DMTA is also shown

in Table 1 where it can be seen that

there is little effect upon the Tg as a

result of increasing the clay concentra-

tion or due to the type of dispersion.

Examples of the raw data showing the

tan d and storage modulus spectra are

shown in Figure 1. The symmetrical

nature of the tan d spectra and the

consistency of the glassy storage mod-

ulus are indicative of a homogeneous

network and support the DSC findings

that the network does not contain large

amounts of unreacted functional

groups. These results complement other

findings for aerospace epoxy systems

which report either modest decreases[16]

or little effect upon Tg.[18] The effect of

clay addition and dispersion methods

upon the thermal stability of the net-

0.00

0.05

0.10

0.15

0.20

0.25

30025020015010050

Temperature (oC)

tan

δ

1.E+07

1.E+08

1.E+09

1.E+10

Sto

rage M

od

ulu

s (MP

a)

Figure 4. Effect of water saturation upon the storage modulusand tan d spectra for the 5 wt.-% nanoclay reinforced sample.

work was determined using TGA. Raw traces showing

network degradation as measured via mass loss in Figure 2

illustrate the effect of clay dispersion upon thermal

stability. The degradation profile shows a two-step process

beginning with the onset of degradation through initial

mass loss, followed by the formation of a char layer. The

oxidative environment ensures that the char layer is then

further degraded through a chain scission-volatilisation

process until there is no polymer left, leaving the final char

yield to reflect the inorganic content of the nanocomposite.

The first-derivative spectra highlight more clearly the rate

of change of the mass loss due to degradation during

the char formation (1st peak) followed by degradation of

the char layer (2nd peak). The parameters which char-

acterise the entire degradation profile, separated into the

first (char formation) and second (stability of char)

degradation steps, are also shown in Table 2. From these

results, it is evident that neither increasing the clay

concentration nor the type of dispersion method, has any

effect upon the first degradation process, either through

the onset temperature or the char yield. However, the

degradation of the char layer (2nd peak) does show a

dependency upon the clay concentration for the sonicated

samples, increasing with increasing concentration and

exhibiting higher comparative levels of thermal stability

compared to the shear mixing sample. Therefore, it can be

suggested from these results that while silicate nanocom-



posites do not increase the resistance to initial thermal

decomposition, the thermal stability of the char layer can

be substantially increased. The CTEs shown in Table 1

indicate little effect upon the method of dispersion

although there is evidence of a reduction in CTE with

increasing clay content although apparently there is a

fairly high level of scatter.

DOI: 10.1002/mame.200600435


3300

3400

3500

3600

3700

3800

3900

4000

5.04.54.03.53.02.52.01.51.00.50.0

wt/wt% clay

Fle

xura

l Mo

du

lus

(GP

a)

shear mixing

sonication bath

sonication horn

Figure 5. Flexural modulus as a function of clay concentration showing the effect of thedifferent dispersion methods.

Barrier Properties

The results of the water uptake

measurements are shown in Table 3

for samples soaked at 7, 14 and 27 d in

deionised water at 70 8C. After 27 d it

is clear that there is a significant

decrease in water uptake as a result of

clay addition highlighting the poten-

tial application of nanocomposites as

barrier property materials. Although

there is some level of scatter in the

experimental results, it appears that

the horn sonicated samples produce a

larger decrease in water uptake

compared to the shear mixing

method. The water uptake after the

27 d for the shear mixing samples

produced an average improvement of

5.7% compared to the neat resin while

the horn sonication method produced

an average decrease in water uptake

of 10.8% compared to the neat resin.

These levels of improvements in

barrier properties of epoxy-based

nanocomposites are well known

and compare well with other stu-

dies,[14,19,20] particularly for high performance epoxy resin

systems. The change in water uptake with time is shown in

Figure 3 for the 1.0 wt.-% nanoclay and neat resin samples,

40

50

60

70

80

90

100

110

120

2.01.00.0

wt/w

Fle

xura

l Str

eng

th (

MP

a)

Figure 6. Flexural strength as a function of clay concentration show



highlighting the reduction in water uptake for the clay

modified samples and the time taken for samples to reach

equilibrium saturation point. It is important to note that for

5.04.03.0

t% clay

shear mixing

sonication bath

sonication horn

ing the effect of the different dispersion methods.



2300

2350

2400

2450

2500

2550

2600

2650

2700

2750

2800

5.04.03.02.01.00.0

wt/wt% clay concentration

Co

mp

ress

ive

Mo

du

lus

(GP

a)

shear mixing

sonication bath

sonication horn

Figure 7. Compressive modulus as a function of clay concentration showing the effect of the different dispersion methods.

422

all of the nanomodified samples, the water ingress appears

to have reached an equilibrium value after 27 d unlike the

neat resin sample which continues to increase steadily. The

effect of water uptake upon the dynamic mechanical

properties was also evaluated using DMTA measurements.

A typical DMTA trace of a dry and saturated sample is

180

190

200

210

220

230

240

250

260

270

2.01.00.0

wt/wt% clay

Co

mp

ress

ive

Str

eng

th (

MP

a)

shear

sonic

sonic

Figure 8. Compressive strength as a function of clay concentration s



shown in Figure 4 which shows moisture ingress creating a

bimodal heterogeneous network consisting of a plasticised

lower Tg phase and a higher crosslink density phase (higher

Tg). Possarts et al. (2004)[21] explained this phenomenon by

suggesting that water ingress allows the more mobile

segments of the network to phase separate and form a two

6.05.04.03.0

concentration

mixing

ation bath

ation horn

howing the effect of the different dispersion methods.

DOI: 10.1002/mame.200600435


0

50

100

150

200

250

4035302520151050

% Strain

Str

ess

(MP

a)

neat resin

5wt% shear mixing

5wt% bath sonication

5wt% horn sonication

Figure 9. Raw data traces showing the impact of nanoclay addition at 5 wt.-% for the different preparation methods.

phase structure containing a new transition lower than the

original Tg. The increase in Tg can also be attributed to

H-bonding of the water molecules producing a pseudo

crosslinking effect (Zhou et al.[22]). Table 3 shows these

values, which indicates that there is little effect upon the

upper Tg or crosslinked phase, with increasing clay

concentration. The ‘plasticised’ phase, however, indicates

a modest increase in the level of plasticisation with

increasing clay concentration, particularly for the methods

prepared using sonication, more likely reflecting the

Table 4. Flexural and mechanical properties of the samples prepare

Wt.-% clay Dispersion Flexural properti

Modulus Strength Str

MPa MPa

0 – 3 430 (48) 47.6 (2.1)

1.0 Shear mixing 3 601 (45) 78.4 (7.8)

2.5 Shear mixing 3 729 (55) 86.1 (3.6)

5.0 Shear mixing 3 842 (27) 70.1 (4.5)

1.0 Bath sonication 3 541 (38) 99.8 (6.8)

2.5 Bath sonication 3 849 (39) 96.6 (5.4)

5.0 Bath sonication 3 844 (37) 69.8 (8.7)

1.0 Horn Sonication 3 530 (41) 104.8 (2.9)

2.5 Horn Sonication 3 783 (29) 87.5 (1.3)

5.0 Horn Sonication 3 850 (99) 67.8 (10.6)

a)Values in parentheses indicate standard deviation.



presence of residual solvent rather than the quality of the

nanoscale dispersion.

Mechanical Properties

The effect of clay addition and dispersion upon the flexural

modulus of the nanocomposite is shown in Figure 5.

Consistent with other similar reports for aerospace epoxy

systems, an increase in the flexural moduli up to a level of

d.

esa) Compressive propertiesa)

ain to failure Modulus Strength Strain to failure

% MPa MPa %

1.47 (0.05) 2 369 (22) 185.7 (10.8) 31 (1)

2.30 (0.26) 2 475 (24) 208.4 (4.2) 33 (1)

2.46 (0.13) 2 535 (17) 195.2 (8.9) 31 (2)

1.92 (0.14) 2 591 (27) 209.5 (15.6) 34 (2)

2.92 (0.17) 2 517 (16) 256.3 (10.2) 37 (1)

2.70 (0.18) 2 599 (12) 229.4 (15.8) 34 (2)

1.93 (0.27) 2 751 (21) 252.9 (6.1) 34 (1)

3.22 (0.13) 2 422 (37) 235.7 (20.6) 36 (3)

2.46 (0.04) 2 472 (19) 235.6 (9.0) 36 (1)

1.90 (0.31) 2 493 (35) 260.0 (5.4) 38 (1)



0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

6.05.04.03.02.01.00.0wt% nanoclay

KIC

(N

M-3

/2)

shear mixing

sonication bath

sonication horn

Figure 10. Fracture toughness as a function of nanoclay concentration showing the effect of dispersion methods.

424

around 12% at a concentration of 5 wt.-% of nanoclay is

observed. Also important to note is that there does not

appear to be any effect as a result of the type of dispersion

method used. The corresponding effect upon the flexural

strength is shown in Figure 6 which shows a substantial

improvement in strength compared to the unmodified

resin with increasing clay concentration. The results

display a maximum improvement in strength at a loading

of 1 wt.-% concentration with the sonicated dispersion

methods displaying a significantly higher improvement

beyond that which was observed for the shear mixing

samples only. This is in contrast to results reported in

the literature which tend to show either a decreased[23] or

an unchanged strength[24,25] of the cured epoxy resin. The

lack of improvement in strength has not only been attri-

buted[23] to the poor interfacial adhesion between the

clay platelets but also the presence of bubbles which are

produced during the manufacturing process. Yasmin

et al.,[12] however, have shown that when epoxy layered

silicate nanocomposites are degassed effectively, the

strength can be improved significantly. Improvements

in the strength of nanoclay reinforced epoxy networks

have been reported, but tend to be centred upon low[26] or

mid range[10] Tg resin systems. Kotsilkova[10] suggested

that the improvement in strength (and stiffness) was a

result of the solvent present, acting as a plasticiser and

altering the interfacial properties between the clay and the

polymer matrix. This is clearly a possibility in this work, as



the presence of residual solvent cannot be discounted as a

factor in determining the property improvements. With

respect to a high performance TGDDM/DDS type resin

formulation, Ragosta et al.[27] determined a 30 wt.-%

improvement in compressive yield strength (and modulus)

at a concentration of 10 wt.-% of silica. They reported that

this was achieved by optimising the interaction between

the silanol reactive groups on the surface of the silica with

the epoxide functional groups, thereby, increasing the

interfacial adhesion. The results obtained for the corre-

sponding improvement in compressive modulus are

shown in Figure 7. As can be seen, the compressive

modulus was found to increase up to a maximum level of

around 16%, with the type of dispersion (sonicated or shear

mixing) again not appearing to have any effect upon the

level of improvement. The compressive strength shown

in Figure 8 although displaying higher levels of scatter

compared to the flexural strength, again displays sub-

stantial improvements compared to the neat resin, parti-

cularly for the sonication dispersion methods. The results

again display a local maximum at 1 wt.-%, albeit less

pronounced than for the flexural measurements. The

primary point to note from these results is again that the

use of sonication methods to prepare the nanocomposites

provides significant improvement in compressive strength

over and above the use of shear mixing only. Figure 9

shows a selection of raw data traces illustrating the

behaviour of the matrix during compression where the

DOI: 10.1002/mame.200600435


Figure 11. Fracture surface of the 5wt.-% nanomodified systems fabricated using (a) shearmixing, (b) bath sonication and (c) horn sonicationmethods at two magnifications.

effects of the different preparation methods are plainly

evident. The complete set of results for the flexural and

compressive properties, including the strain to failure, are

shown in Table 4 where the increase in the strain to failure

(compressive and flexural) correlates with improve-

ments in strength also. The effect of clay addition and

the different dispersions upon the fracture toughness of

the cured network is shown in Figure 10. Although high

levels of scatter were found, improvements in toughness

are clearly evident, again particularly for the sonicated

samples compared to the shear mixing approach. The

reason for this improvement in toughness, although not

completely understood, can be explained through the

increase in surface roughness of the fracture plane. To

create an increase in surface area of a fracture plane,

higher work of fracture of the propagating crack is requi-

red. Wang et al.[23] suggested recently that the toughening



mechanism arises primarily from the dissipative forces

associated with micro-crack formation between the weakly

bonded clay layers. In a well-exfoliated mixture, the micro-

cracks would not be planar and therefore create a tortuous

path through which the crack propagates, dissipating

energy and therefore increasing toughness. Evidence for

this mechanism is evident from the SEM images of the

fracture surfaces of the 5 wt.-% nanomodified systems as

shown in Figure 11. The effect of different preparation

methods on the fracture surfaces are shown at two different

magnifications, where it is clear that the two sonicated

methods show significantly higher levels of surface rough-

ness. In addition to this, the improved quality of the dis-

persion of these samples compared to the shear mixed

sample is illustrated by the large agglomerations of clay

clearly apparent in the shear mixed samples at both magni-

fications. These agglomerations are clearly reduced for the



Figure 12. TEM images of the 1 wt.-% nanomodified system fabricated using (a) horn sonication, (b) bath sonication and (c) shear mixing.

426

sonicated samples, with the sonic horn method appearing to

produce a superior dispersion to the bath sonication and

showing excellent correlation with the fracture properties

determined here. Furthermore, evidence for an improvement

in nanoscale dispersion through the use of sonication

methods was obtained from TEM. Figure 12 shows TEM

images of the 1 wt.-% samples for the different preparation

methods at the same magnification. Here, the evidence

further supports the SEM results showing large differences in

clay dispersion for the shear mixing sample and the soni-

cated samples. Figure 12(a) and 12(b), while not displaying

significant differences between each other, highlight the

destruction of the clay tactoids as a result of using

sonication by showing small tactoids of around 20 clay

platelets. The outermost platelets have been intercalated

and are beginning to ‘peel off’ from a tactoid, but are

prevented from further separation due to the presence of

other tactoids. In contrast, the agglomeration of clay

particles in Figure 12(c) shows a range of clay platelets

intercalated at varying levels of platelet separation. The

mechanism of exfoliation can be seen via the outermost

platelets beginning to ‘‘peel off’’ from a tactoid, but being



prevented from further separation due to the presence of

other tactoids. The TEM and SEM images, thus, clearly show

the combined influence of sonication and solvent upon

nanocomposite formation. The results indicate a direct

correlation between nanoscale morphology and dispersion

with improvements in fracture toughness, strength, and

strain to failure.

Conclusion

This work has shown that the addition of layered silicate

nanocomposites can provide significant improvements in

thermal and mechanical properties of a high performance

epoxy resin, particularly through the use of a combined

sonication and solvent-based approach. A strong correla-

tion between nanoscale morphology, as measured using

SEM and TEM and the fracture toughness, strength and

strain to failure has been demonstrated. SEM showed that

greater levels of toughening mechanisms were operating

during failure, and that the dispersion of nanoclay was

more uniform for the sonicated samples. In addition, TEM

DOI: 10.1002/mame.200600435


analysis indicated that there was more destruction of the

clay agglomerates or tactoids for the sonicated samples

compared to the shear mixing system. TGA analysis has

shown that while the initial thermal stability is not

affected, the stability of the char layer during decomposi-

tion increased with improved nanoscale dispersion as well

as with increasing clay concentration. The effect of mois-

ture upon the dynamic mechanical thermal analysis of the

epoxy nanocomposites was also found to display some

dependence upon the clay dispersion with a modest

increase in plasticisation for the sonicated samples.

Received: November 14, 2006; Revised: January 26, 2007;Accepted: January 31, 2007; DOI: 10.1002/mame.200600435

Keywords: epoxy; layered silicate nanocomposites; mechanicaland thermal properties; morphology; nanocomposites; sonication

[1] M. Kawasumi, J. Polym. Sci., Part A: Polym. Chem. 2000, 31,681.

[2] A. Okada, A. Usuki, Mater. Sci. Eng. C: Bio. Mater. Sens. Sys.1995, C3( 2), 109.

[3] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima,T. Kurauchi, O. Kamigaito, J. Mater. Res. 1993, 8, 1185.

[4] C. Zilg, R. Mulhaupt, J. Finter, Macromol. Chem. Phys. 1999,200, 661.

[5] X. Kornmann, H. Lindberg, L. A. Berglund, Polymer 2001, 42,1303.

[6] T. Lan, P. D. Kaviratna, T. J. Pinnavaia, J. Phys. Chem. Solids1996, 57, 1005.



[7] R. A. Vaia, E. P. Giannelis, Mater. Res. Bull. 2001, 26, 394.[8] J. H. Park, S. C. Jana, Macromolecules 2003, 36, 8391.[9] J. H. Park, S. C. Jana, Macromolecules 2003, 36, 2758.

[10] R. Kotsilkova, J. Appl. Polym. Sci. 2005, 97, 2499.[11] H.-J. Lu, G.-Z. Liang, X.-Y. Ma, B.-Y. Zhang, X.-B. Chen, Polym.

Int. 2004, 53, 1545.[12] A. Yasmin, J. L. Abot, I. M. Daniel, Scripta Mater. 2003, 49, 81.[13] H. Wang, S. V. Hoa, P. M. Wood-Adams, J. Appl. Polym. Sci.

2006, 100, 4286.[14] W. Liu, S. V. Hoa, M. Pugh, Comp. Sci. Tech. 2005, 65,

2364.[15] C. Chen, T. Tolle, J. Polym. Sci., Part B: Polym. Phys. 2004, 42,

3981.[16] O. Becker, R. J. Varley, G. P. Simon, Polymer 2002, 43,

4365.[17] J. F. Timmerman, B. S. Hayes, J. C. Seferis, Comp. Sci. Tech.

2002, 62, 1249.[18] C. Chen, D. Curliss, SAMPE J. 2001, 37, 11.[19] O. Becker, R. J. Varley, G. P. Simon, Eur. Polym. J. 2004, 40,

187.[20] J.-K. Kim, C. Hu, R. S. C. Woo, M.-L. Sham, Comp. Sci. Tech. 2005,

65, 805.[21] C. Bockenheimer, D. Fata, W. Possart, J. Appl. Polym. Sci. 2004,

91, 361.[22] J. Zhou, J. P. Lucas, Polymer 1999, 40, 5505.[23] K. Wang, L. Chen, J. Wu, M. L. Toh, C. He, A. F. Yee, Macro-

molecules 2005, 38, 788.[24] X. Kornmann, R. Thomann, R. Mulhaupt, J. Finter, L. Berglund,

J. Appl. Polym. Sci. 2002, 86, 2643.[25] A. S. Zerda, A. Lesser, J. Polym. Sci., Part B: Polym. Phys. 2001,

39, 1137.[26] H. Miyagawa, M. J. Rich, L. T. Drzal, J. Polym. Sci., Part B: Polym.

Phys. 2004, 42, 4391.[27] G. Ragosta, M. Abbate, P. Musto, G. Scarinzi, L. Mascia, Poly-

mer 2005, 46, 10506.


Effect of Ultrasonic Dispersion Methods on Thermal and Mechanical Properties of Organoclay Epoxy...

Documents

Transcript of Effect of Ultrasonic Dispersion Methods on Thermal and Mechanical Properties of Organoclay Epoxy...