Dispersion of Functionalized Nanoclay Platelets in an Amine-Cured Epoxy Resin System

Abstract

Nanocomposites are a relatively new class of materials obtained by dispersing montmorillonite clay in a polymer matrix. Evidences from literature suggest that clay platelet dispersion during nanocomposite preparation and clay-matrix adhesion are major technical issues that need to be addressed in order to achieve the desired property enhancements in polymer-clay hybrid nanocomposites. We have studied the interaction of the organically functionalized clay with the epoxy resin by including along the chain structure functional groups that will facilitate interaction with the resin. Through conventional routes, functional molecules have been synthesized and deposited on to the clay surface. Both the functionalized and nonfunctionalized clay has been analyzed using thermal gravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). The exfoliation of nanoclay platelets in amine cured epoxy system has been studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM).

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ADP012233
TITLE:
Dispersion
of
Functionalized
Nanoclay Platelets
in
an
Amine-Cured Epoxy Resin
System
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UNCLASSIFIED

Mat.
Res.
Soc.
Symp.
Proc.
Vol.
703
©
2002
Materials
Research
Society
V9.26
Dispersion
of
Functionalized
Nanoclay
Platelets
in
an
Amine-Cured
Epoxy
Resin
System
D.
Raghavan,
E.
Feresenbet,
D.
Yebassa,
A.
Emekalam,
and
G.
Holmes'
Polymer
Division, Department
of
Chemistry,
Howard
University, Washington
DC
20059.
'G.
Holmes,
Polymer
Division, National
Institute
of
Standards
&
Technology,
Gaitherburg,
MD
20899.
ABSTRACT
Nanocomposites
are
a
relatively
new
class
of
materials
obtained
by
dispersing
montmorillonite
clay
in
a
polymer
matrix. Evidences from literature suggest that
clay
platelet dispersion
during
nanocomposite
preparation
and
clay-matrix
adhesion
are
major
technical
issues that
need
to
be
addressed
in
order
to
achieve
the
desired property
enhancements
in
polymer-clay
hybrid
nanocomposites.
We
have
studied
the
interaction
of
the
organically functionalized
clay
with
the
epoxy
resin
by including
along
the
chain
structure
functional
groups
that
will
facilitate
interaction
with
the
resin.
Through
conventional
routes,
functional molecules
have been
synthesized
and
deposited on
to the
clay surface.
Both
the
functionalized
and
nonfunctionalized
clay
has
been
analyzed
using
thermal gravimetric analysis
(TGA),
and
Fourier transform
infrared
spectroscopy
(FTIR).
The
exfoliation
of
nanoclay
platelets
in
amine cured
epoxy system
has been
studied
using
X-ray diffraction
(XRD)
and
transmission electron microscopy (TEM).
INTRODUCTION
Aromatic
amine
cured epoxy
resins,
where
the
diglycidyl ether
of
bisphenol-A
(DGEBA)
resin
cured
with
meta-phenylene
diamine
(MPDA)
is a
representative
member,
are
the
most
widely
used
matrix materials
for
preparing
conventional
composites.
Resins
of
this class
are
brittle
and
their
ability
to
absorb
energy
during
failure
is
limited.
The
successful
use
of
nanoclay
reinforcement technology
to improve the
performance
of
epoxy
resin would represent
a
major technical achievement
in
the
development
of
high-
temperature
and
tough
advanced
structural
materials. Nanoclays
are
inexpensive
relative
to
traditional
reinforcing
materials,
thermally
inert,
and
environmentally
friendly.
The
use
of
nanoclay hybrid
polymer
composites
in
structural
parts
is
expected
to
improve
environmental
and
moisture
stability
and
increase
energy
efficiency
in
the
transportation
sector
through
weight
reduction
[I].
Evidences
from
literature
suggest that exfoliation
and
dispersion
of
clay
platelets
during
nanocomposite
preparation
and
clay-matrix adhesion
are
major
issues
that
needs
to
be
addressed
in
order
to
achieve
the
desired
property enhancements
in
polymer-clay
nanocomposites
[2-7].
In
recent
years,
efforts
have
focussed
on
understanding
the
nature
and
mechanism
of
the
exfoliation process
of
the
surface
treated
clay
in
the polymer
network
structure.
For
example,
Messersmith
and
Giannelis
[2]
achieved
a
significant
increase
in
stiffness
of
epoxy
nanocomposite
by
using
organoclays modified
with
bis(2-
hydroxyethyl)
methyl
tallow-alkyl ammonium chloride
at
a
mass fraction
of
4
%.
The
399

role
of
the
organic
chain
on
alkyl
ammonium
chloride
is
primarily
to
facilitate
the
intercalation
of
tile
clay
by organic
molecules
(e.g.,
DGEBA).
These
researchers
found
that
curing
of
a
DGEBA/clay
mixture
with
primary
and
secondary
amines resulted
in
an
immediate
clouding
of
the resin.
It
has
been
suggested
that
the
bridging
of
the
silicate
layers
by
the
bifunctional
polar
amines
prevented
the
extensive layer
separation.
In
recent
years,
several
researchers
[2-7]
have
studied
amine-cured
epoxy
clay
composites
to
show
that
exfoliation
of
nanocomposite
depends
on
the
following
parameters:
(1) Alkyl
ammonium
ion
type
(e.g.
primary,
secondary
or tertiary)
a.
affects
intra-gallery
polymerization
rates
and
clay-matrix
dispersion
(2)
Length
of
the
alkyl
chain
a.
impacts
swelling
of
clay
by
epoxy
monomer
b.
controls
intra-gallery
diffusion
(3)
Curing
agent
type
(e.g.
aromatic
diamines,
aliphatic
diamines, anhydrides,
and
homopolymerization
agents)
a.
affects
resin
glass
transition
temperature
(4)
Curing
conditions
a.
affects
intra-
and
extra-gallery
polymerization
rates
and
resin
Tg
(5)
Charge
density
of
the
clay
a.
impacts
swelling
of
clay by
epoxy
monomer
The
focus
of
this
research
is
to
investigate
the
influence
of
modified clay
with
functional
groups
on
the
miscibility
of
monomer
with organically
modified
nanoclay
platelets.
In
this
research,
the
sodium
ions
in
the
clay
are
exchanged
with
an
alkylammonium
salt
of
suitable
chain
length
followed
by
adsorption
of
0)-epoxy
carboxylic
acid
of
suitable chain length
on
the
clay
surface.
The
epoxy
groups
of
ol-
epoxy
carboxylic
acid
are
expected
to
facilitate
the
miscibility
of
DGEBA
monomer
with
the
clay
platelets.
In
this study
we
investigate
how
the
type
of
organophilic
groups
intercalated
between
the
clay
layers,
influences
the
structure
of
the
resulting
nanocomposites.
In
particular,
we
compare
the
effect
of
the
two
nanoclays
(functional
and
non-functional
clay)
on
the
exfoliation
process
and
the
final
structure
of
epoxy-based
nanocomposites.
For
the
synthesis
of
functional clay,
we
use
clay
treated
with
C18
quaternary
ammonium
salts
(C18
clay)
as
the
base
structure.
The
C18
clay
is
then
mixed
with
functional
oleochemicals
to
form
functional
clays.
The
functional
oleochemicals
are
synthesized
frot
vernonia
oil
(see
Figure
1).
The nanostructure
of
the
epoxy
clay
composite
was
characterized
by
X-ray
diffraction (XRD)
and
transmission
electron
microscopy
(TEM).
EXPERIMENTAL
(A)
Synthesis
of
C18
Clay
7.5
g
of
purified
Na
clay
from
Southern
Clay
Products'
was
dispersed
into
600
nil
of
distilled
water
at
80
C.
3.0
g
of
octadecylatiinonium
chloride
in
300
nil
distilled
I
Certain
commercial
materials
and
equipment
are
identified
in
this
paper
in
order
to
specify
adequately
tlhe
experimental
procedure.
In
no
case
does
such
identification
imply
recollmendation
or
endorsemenit
by
the
National
tnstitute
of
Standards
and
Technology,
nor
does
it
imply
necessarily
that
tile
iterns
are
best
suited
for
the
purpose.
400

water
was
poured
in
the
hot clay/water
suspension
and
stirred
vigorously
for
I
h
at
80
C.
The
mixture
was
then
filtered
and
the
solid
washed
several
times
with
ethanol/hot
water
mixture (for
3
h)
to
remove
free
chloride
ions.
The washing
was
continued
until
the
solid
was
verified
for
absence
of
chloride
ions, by
checking
the
filtrate
solution
with
AgNO
3
solution.
The
attachment
of
long alkyl
groups
to
clay
surfaces
facilitates
clay sheet
separation and
makes
it
possible
to
introduce functional molecules
in
the
interlayer
space.
(B)
Synthesis
of Vernolic
acid:
Vemonia
oil
(VO) was
refluxed
with
base/methanol
mixture for
2
h
and
transferred
to
a
beaker
containing ice/water.
The
solid
compound
was
recovered
by
filtration
and acid
treated
to
convert
the acid salt
(soap)
to
vernolic
acid.
Vernolic
acid
was
purified
by
adopting
solvent exchange
procedure
and
freeze drying
techniques.
Figure
1
describes
the
reaction
scheme
used
to
synthesize vernolic acid. The purified
compound
was
FTIR
characterized
and
compared
to
the
literature reporting
for
epoxy
fatty
acid
[8].
(C)
Synthesis
of
Vernolic
Acid
Mixed
C18
Clay:
1.25
g
of
C18
clay
was
suspended
in
0.5
g
of
vernolic
acid
dissolved
in
30
ml
of
hexane
and
the
solution
was
stirred
under
nitrogen gas
for
48 h
and
the
product
was
filtered.
Fresh hexane
was
added
to
the
solid
product
and
attempts
were
made
to free
the
solid
product
of
residual
vernolic
acid.
The
C18 clay
containing
vernolic
acid was
allowed to
air
dry
and
is
termed
as
the
vernolic
acid
mixed
C18
clay.
The
product
was
characterized
by
FTIR
and
TGA
technique.
(D)
Synthesis
of Epoxy
Nanocomposite:
We
have
selected
the
model
resin
system DGEBA
(provided
by Dow Chemical)
and
MPDA for
the
current
study.
DGEBA
resin
was
kept
in
a
vacuum
oven
at
75
C
for
3 h.
The
clay-epoxy
mixture
was
prepared
by
dispersing
a
mass
fraction
of
2.5
%
of
C18 clay
in
hot DGEBA
with
stirring,
followed by
sonication for
about
2
h
and
degassing
in
vacuum
for
2 h.
14.5
phr MPDA
(melted
at
60
'C)
was
then
added to the
clay-epoxy
mixture
at
60
C
under vacuum. The
DGEBA/clay/MPDA
mixture
was
heated
for
2 h
at
80
'C
and
post
cured
for
another
2
h
at
125
C.
The
same curing
procedure
was
applied
to
vernolic
acid
mixed
C18 clay.
(E)
FTIR
and
TGA Analysis
of
Nanoclay Powder
FTIR
analyses
were
performed
on
the
Na clay, C18 clay,
and
vernolic
acid mixed
C18
clay. Each
sample
was
mixed
with
KBr
and
vacuum
packed
to
obtain
pellets
of
the
material. The
pellets
were
analyzed using
a
Nicolet
Magna
IR
560
(the
standard
instrument
uncertainty
in
measuring
wave
number
is
±
0.01
cm',
the
cm- were
rounded
off
to the nearest
I
cm
1)
spectrophotometer.
TGA
analysis
was
performed
on
the
Na
clay,
C18
clay,
and
vernolic
acid
mixed
C18
clay using
a
Perkin
Elmer
7
Series system
(the
standard
instrument uncertainty
in
401

measuring
a
temperature
is
±
I
'C)
in
a
nitrogen
atmosphere,
by
placing
25
mg
of
sample
in
a
crucible
and
heating
it
from
30
"C
to
900
C
at
a
heating
rate
of
20
C
/min.
Nitrogen
gas was
allowed
to
flow
at
a
sweep
rate
of 5
mI/min.
(F)
XRD
and
TEM
Analysis of
Epoxy
Nanocomposites
XRD
analyses
were
performed
using
a
Scintag
Inc.
XRG
3000
diffractometer
with
Cu
radiation
(40
KV,
35
mA).
The
scanning
speed
and
the
step
size
were
0.01
'trin
and
0.04
o',
respectively.
The
nanocomposite
dogbone produced
during
the
moulding
process
has
a
fairly
smooth
surface.
The
dogbone
specimens
were
cut
to
size
and
analyzed
by
XRD.
TEM specimens were
cut
fron
dogbone
using
an
ultramicrotome, equipped
with
a
diamond
knife.
They
were
collected
in
a
trough
filled
with
water
and lifted
out
of
water
using
200
mesh
copper
grids.
Electron micrographs
were
taken
with
a
Philips EM400C
at
an
accelerating voltage
of
120
KV.
RESULTS
AND
DISCUSSION
A.
Characterization
of Nanoclay
Powder
The
FMIR
spectra
of
the
neat
Na
clay,
C18
clay
and
vernolic
acid
mixed
C18
clay
in
the
region (4000
to
500)
cm
-'
are
shown
in
Figure
2.
Both
the
vernolic
acid
mixed
C18
clay
and
C18
clay
display
sharp
methylene-stretching
modes
at
2920
cm-'
and
2860
cn -1,
that
is
characteristic
of
organophilic groups
on
clay
surfaces.
The
spectra
of
C18
and
vernolic
acid
mixed
C18
clay
also
show
a
distinct
peak
at
1600
cm
1
that
can
be
assigned
to
the
quaternary
ammonium
salt
in
modified clay.
In
addition,
the
vernolic
acid
mixed
C18 clay
contains
bands
at
1710
cm
-'
and
1759
cm
-1
suggesting
that the
vernolic
acid
is
present
in
the
clay
matrix.
Vernolic
acid
present
in
the interlayer
space
is
trapped
in
the
matrix,
while
vernolic
acid
present
at
the
edges
in a
limited
amount
is
involved
in
physico-chemical interactions
with hydroxyl
groups
on
clay
edges. This
is
because,
plate-
like
clay surfaces
have
hydroxyl
groups
at
low
content
at
edges
of
individual
particles.
Thernogravimetric
analysis
was used
to
examine
the
stability
of
organophilic
groups
oil
nanoclay.
The
thernograms
of
the
Na
clay,
CI8
clay,
and
vernolic
acid
mixed
C18
clay
are
shown
in
Figure
3.
The
peak
at
100
C
corresponds
to
the
loss
of
surface
water molecule
from
the
clay
platelets.
We
see
the
onset
of
decomposition
peak
(i.e.
release
of
functional
onium
ion
and
vernolic
acid
molecule)
at
above
200C
in
both
C
18
and
vernolic
acid
mixed C18 clay. The
peak
at
650
C is
the
irreversible
dehydroxylation
peak
for
clay.
As
expected,
the peak
at
650"C
for
Na clay
is
significantly
larger
than
that
of
vernolic
acid
mixed C18
clay.
B.
Characterization
of Epoxy Clay
Nanocomposites
Both
the
uncured
and
cured
epoxy
were
studied
by
XRD
for
layer
separation.
The
X-ray diffraction results
of
C
18
clay
and
vernolic
acid
mixed
C18
clay
showed
a
sharp
X-ray diffraction
peak that
corresponds
to
a
layer
separation
of
about
(22
±
3)
A,
where
the
number
after
±
is
one
standard deviation
from
the mean.
After
the
C18 clay
was
402

sonicated
in
epoxy
mixture
for
2
h,
X-ray diffraction
was
recorded
for
the
uncured
sample.
The
interlayer spacing increased
to
(37
±
3)
A.
However,
when
the
clay
was
allowed to
swell for
several
hours
in
hot epoxy
resin,
the sharp
peak
diminished.
Instead
a
shoulder
was
noticed
for
both
C18
clay
and
mixed
nanoclay. Figure
4
shows
a
comparison
of
the
C18
clay
and
vernolic
acid
mixed
C18
clay
epoxy
nanocomposite.
The appearance
of
a
shoulder
at
approximately
2.30
(20)
suggests
that
nanoclay
platelets
are
partially exfoliated.
It
has
been
reported
recently
that epoxy
infiltration
into
nanoclay
platelets
can be
achieved
by
prolonged
swelling
of
clay
in
uncured epoxy
[10].
To
verify
our
results,
we
performed TEM
studies
on
C18
clay
and
vernolic
acid
mixed
C18
clay
nanocomposites. Figure
5
shows bright
field
transmission electron
micrograph
of
epoxy-
(a)
C18 clay
and
(b)
vernolic
acid
mixed
C
18
clay
nanocomposite.
Detailed TEM verified the intercalated
structure
of
epoxy
C18
clay
nanocomposite
and
partially
exfoliated
epoxy vernolic
acid
mixed
C18
clay
nanocomposite.
There
were
regions
in
the vernolic
acid
mixed
C18
clay epoxy
nanocomposite
where
the
nanoclay
platelets
were
intercalated
(regular
arrangement
of
clay
platelets)
and
other
regions where
platelets
were
exfoliated
(randomly
arranged
and
well
separated
platelets).
One
would
expect
that the
addition
of
vernolic
acid
to
C18 clay
should result
in
intercalated
epoxy
nanocomposite
because
of
crowding
of
functional
groups
on
clay
edges.
However,
our
preliminary
results indicate
that
the addition
of
vernolic
acid
to C18
clay
has
improved
exfoliation
of
nanoclay
platelets
to
a
limited
extent
in
epoxy
matrix.
We
believe
that
the
epoxy
groups
of
co-epoxy
vernolic
acid
sorbed on
clay
edges may
facilitate
the
miscibility
of
DGEBA monomer
with
the
clay
platelets
and
infiltration
of
epoxy
resin.
This
aspect
requires
further
investigation. Studies
are
also underway to
obtain information about
the
clay
dispersability
in
epoxy
matrix
(using
NMR
technique).
ACKNOWLEDGMENTS
The
authors
thank the
AFOSR,
for
providing
financial
support,
Steve
Hudson
of
NIST
for
the
transmission
electron microscope
studies
of
morphology,
and
Maureen
Williams
of
NIST
for
assistance
with
XRD
studies.
REFERENCES
1 .
J.
M.
Garces,
D.
J.
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403

H
H
H
H
CHOI~CH
2
(CH
2/
\"'H
11
H H
H
H
CHI- 0-
GcUrCH$
CH,""
\CH,)hCH,
H
H
H H
CH"
ICH2-(CHA1-I
t CH
2V
IIC,)-
I
N'iOH
H
H
H
H-
0-
tCH-(CH
2)fc
"CH
2
Z
C
z4-H
Figure
1.
Reaction
scheme
for
vernolic
acid
2.5
[.--M!
UCLAY!
1.5.
(05
40M)) 35(X) 30(x 1 25(Y) 2()O0 1500) 1)0 C
Figure
2.
FTIR
spectrum
of
Na-clay,
C18
clay
and
vernolic
acid mixed
C18
clay
powdcrs.
404

1
100 0 400 00 0 7
80
9100
-001
0-0.02
-0.00
-0.04
-0,08 -
-0.09
-0
.1 ...... .......
Temperature (oC)
Figure
3
TGA
of
Na clay,
Cl8
clay
and
vernolic
acid
mixed
C18
clay
powders.
100
--
I&
Y
I
-MXED
CLAYI
I NN,.CLAY
1200
00
1 3 56
400405

a
50
nm
b
Figure
5.
TEM
of
(a)
vernolic
acid
mixed
CI18
clay
and
(b)
C1
8
clay
laflocompositC
(the
standard
uncertianty
of
magnificat
ion
is
30)
406