1. Available online at www.sciencedirect.com
COMPOSITES
SCIENCE AND
TECHNOLOGY
Composites Science and Technology 68 (2008) 880–887
www.elsevier.com/locate/compscitech
Preparation of acrylic anodic electrophoretic resin/clay
nanocomposite films by water-based electrodeposition
Wei Lin, Chang-An Wang *, Bin Long, Yong Huang
State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering,
Tsinghua University, Beijing 100084, PR China
Received 27 February 2007; received in revised form 11 August 2007; accepted 15 August 2007
Available online 6 September 2007
Abstract
Polymer/clay nanocomposite films were prepared by electrodeposition from aqueous dispersions of acrylic anodic electrophoretic
resin (AAER) and Na+-montmorillonite (MMT). The characterization results using XRD, SEM and TEM indicated well-dispersed
MMT platelets in the composite films prepared. Mono-disperse-like hexagonal MMT platelets were observed when the MMT loading
in the resin matrix was low. The ideal dispersity achieved was thought to be the result of aqueous compatibility between AAER
molecules and MMT platelets and of the water-involved process as well. FTIR study proved the interaction between AAER and
MMT and the intercalated or exfoliated morphology. Thermal stability, tensile modulus and strength, storage modulus and glass
transition temperature of the polymer/clay nanocomposite films were effectively improved compared to those of the virgin AAER
film.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer–matrix composites (PMCs); B. Thermomechanical properties; E. Electrodeposition
1. Introduction The key problem is how to introduce clay into AAER
to obtain uniformly dispersed clay platelets in the
Acrylic anodic electrophoretic resin (AAER) has been polymer matrix. Traditionally, ionic exchange of MMT
widely used both in industry (e.g. vehicle bodies and food with organic ammonium salt to obtain organophilic clay
tins) [1,2] and scientific researches [3,4] due to its well- is usually indispensable, because poor compatibility
known capability of preparing uniform coatings (films) by between hydrophobic polymer matrix and hydrophilic
electrodeposition rapidly, even on work pieces with various Na+-montmorillonite (MMT) causes agglomeration, and
complex 3-D structures. The coatings prepared represent therefore, weaker mechanical properties. However, prep-
excellent corrosion resistance and insulating property. arations of PNCs from aqueous system with unmodified
However, its mechanical properties and thermal stability MMT [15–17] have inspired us in preparing AAER/clay
are not excellent enough so far. Polymer–clay nanocompos- composite with unmodified MMT based on its compati-
ites (PNCs) have been studied extensively in the past dec- bility with AAER. Furthermore, the method referred as
ades, due to their capability of improving physical and ‘‘slurry-compounding process’’ [12,13] has inspired us
mechanical properties dramatically at very low loadings of that solvent-involved process would probably be contrib-
clay [5–14]. Thus, by introducing clay into the AAER utive to achieving ideal dispersity and intercalated (even
matrix, improvement of both thermal stability and mechan- exfoliated) structure of MMT platelets in polymer. The
ical properties of the composite films is expected. present research is expected to open a new possibility
for preparation and application of polymer–matrix
*
Corresponding author. Tel.: +86 10 62785488; fax: +86 10 62771160. composites.
E-mail address: wangca@mail.tsinghua.edu.cn (C.-A. Wang).
0266-3538/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compscitech.2007.08.020

2. W. Lin et al. / Composites Science and Technology 68 (2008) 880–887 881
2. Experimental section the electrodeposition time. Freestanding films were dried
at 50 °C under vacuum for 24 h before use for characteriza-
2.1. Materials tion or property testing.
MMT was donated by Zhejiang Fenghong Clay Chem- 2.3. X-ray diffraction
icals Co., Ltd., with a cation exchange capacity (CEC) of
90 mequiv/(100 g) and a d001 spacing of 1.28 nm. MMT XRD experiments were carried out in two X-ray diffrac-
with CEC of 90 mequiv/(100 g) is preferably used because tometers—Rigaku D/max 2500 (2h = 3–30°) and Rigaku
as layer charge increases, the cohesion energy that holds D/max-3A (2h = 1–3°), both using Cu Ka radiation. A
the lamellae closer also increases so that the dispersion of typical XRD specimen was prepared by cutting the film
the clay in water becomes more difficult, resulting in larger (on anode plate) into a small rectangular (5 · 8 mm) and
particles in dispersion [18]. Virgin AAER (Commodity No. then fixed on a small glass plate. Parameters for the contin-
DT323-75) synthesized from methacrylic acid, methyl uous scan mode are 0.01°, 4°/min and for step scan mode:
methacrylate, butyl methacrylate, 2-hydroxyethyl methac- step size 0.01°, preset time: 4 s.
rylate, styrene and glycidyl methacrylate and neutralized
by triethylamine, was supplied by Tianjing Dengta Co., 2.4. Microstructure
Ltd. The acid value of AAER before neutralization is
$105 mg KOH/g. No inorganic or organic filler was con- Surface observation and thickness measurement of the
tained in the virgin AAER. Sodium hydroxide, sodium films were carried out by scanning electron microscope
hexametaphosphate (Na-HMP) and cyclohexanone were (SEM) in a JEOL JSM-6400 with an operating voltage at
purchased from Beijing Chemical Reagents Company and 10 kV. Energy dispersive spectroscopy (EDS), with a reso-
used without further purification. Ultrasonic machine was lution of $1 lm, was utilized to help determine the disper-
JY92-P (NINGBO SCIENTZ BIOTECHNOLOGY Co., sity of MMT from the micro- to the macro- scale. The line-
Ltd.). scan modes for EDS were conducted with an operating
voltage at 20 kV, counting for 1 min. All specimens were
2.2. Preparation of Free-standing AAER Film and PNC sputter-coated with gold.
Films The dispersity and the shape of MMT in PNC films on
nano-scale were studied with transmission electron micros-
Certain amount (0–0.5 g) of MMT was dispersed in copy (TEM) in a JEM-2011 at an operating voltage of
50 ml deionized water (containing Na-HMP as dispersing 200 kV. EDS with a resolution of $10 nm, was utilized
agent, Na-HMP:MMT = 1:100 in mass) and the pH value to determine the component of the dispersates. For the sur-
was adjusted to $8.0 using 0.5 wt% aqueous solution of face observation, i.e. in the direction normal to the surface,
sodium hydroxide. Na-HMP has been thought to be able a PNC film of $20 lm thick was perforated and fixed onto
to greatly increase stability and effectively avoid agglomer- a copper hoop. Thin sections were obtained using a Gatan-
ation of clay particles in water [19]. After 7 days of hydra- 600 ion beam thinner at a gun voltage of 3.5 kV for 3–5 h.
tion by stirring and 30 min of ultrasonic for mechanical For the cross-section observation, i.e. in the direction nor-
exfoliation, the MMT suspension was obtained and then mal to the cross-section, several layers (films on aluminum
added into 100 ml aqueous solution of virgin AAER plates) were combined together during heat curing and the
(30 wt%, pH = $8.0) under stirring to form the dispersion multi-layered cross-section was thinned mechanically to
for subsequent electrodeposition. $50 lm before ion milling.
For the electrodeposition process, both anode and cath-
ode were aluminum plates of 25 · 50 · 0.1 mm. The electric 2.5. Fourier transform infrared spectroscopy (FTIR)
field between the two electrodes was set as 120 V/cm for
duration from 10 s to 120 s. During electrodeposition, FTIR experiments were performed at ambient tempera-
hydrolysis of water at the anode leads to local production ture with a spectrometer (SPECTRUM GX, PerkinElmer,
of protons, which turns dissociated carboxylic acid groups USA) at a resolution of $4 cmÀ1. The thickness of the film
(COOÀ) of AAER to undissociated carboxylic acid groups samples for FTIR experiments was $5 lm by reducing
(COOH) and consequently to the precipitation of the electrodeposition time. Five spectra of 64 scans each were
AAER to form a thin, uniform and tightly adherent film taken of each specimen and the average position of each
on the surface of the anode plate [1,4]. After electrodepos- peak (located automatically by peak picking software
ition the anode was kept at 172 ± 2 °C for 35 min for heat attached to the testing system) was then determined, with
curing. Finally, after removing the aluminum plate in reproducibility of <1 cmÀ1.
5 wt% aqueous solution of sodium hydroxide, a transpar-
ent and uniform free-standing film was obtained. The com- 2.6. Thermal measurement
position and thickness of the films could be controlled
within certain limits simply by varying such parameters The compositions of PNC films were determined by
as the amount of MMT in the aqueous suspension and thermal gravimetric analysis on a TGA2050 gravimetric

3. 882 W. Lin et al. / Composites Science and Technology 68 (2008) 880–887
analyzer, and samples were heated in the air from ambient However, at a given value of MC and once the other
temperature to 700 °C at a heating rate of 10 °C/min. parameters in experiment are fixed, WC is restricted within
a fairly narrow range. The PNC films prepared are desig-
2.7. Mechanical property measurements nated as PNCF1, PNCF3, PNCF7; the number on the
right-hand side indicates that the mass percentage of
For both tensile and dynamical mechanical tests, rectan- MMT in the films are $1.3, $3.0 and $7.0, respectively,
gular specimens were prepared as follows. A piece of free- determined by TGA. The virgin AAER film is designated
standing film (about 15 mm · 40 mm) was placed between as AAERF.
two pieces of paper to make a ‘‘sandwich’’ by gluing the The thickness against the electrodeposition time for
three layers together with water-soluble glue. Then the PNCF3 is shown in Fig. 2. The film thickness can be adjusted
‘‘sandwich’’ was cut into a rectangular of the size of by controlling the electrodeposition time. Additionally, clay
8 mm · 30 mm using a sharp cut-off knife. Finally the top content of PNCF3 also changes with deposition time (or
and the bottom layers were removed by immersing them thickness) because the ratio of electrodeposition rate of poly-
in water, followed by careful rinsing. mer to that of MMT varies with deposition time. Fig. 2,
Tensile tests were carried out on a universal material together with Fig. 1, shows us that for the whole process of
testing machine (WDW3020, Kexin Institute of Labora- electrodeposition, MMT maintains higher electrodeposition
tory Instrument, Chinese Academy of Sciences) using a rate than polymer; the difference of the rates decrease with
100 N load cell (ACCU-Champ Co. Inc. NY, USA). The deposition time, i.e. with decreasing electric field. Composi-
rate of cross-head motion was 0.05 mm/min. Before exper- tion data for deposition time less than 60 s are not shown,
iment, the instrument was carefully calibrated. because films became so thin and light that TGA experiment
The storage modulus, loss modulus and tan d were mea- could not be conducted (or the results showed fairly large
sured with a dynamical mechanical analyzer (DMA) (TA error).
instrument, Model 2980) using double cantilever mode. A
constant frequency of 1 Hz and amplitude of 5 lm were 3.2. XRD pattern and morphology observation
adopted. Scans were conducted from 15 °C to the point
at which measurements were stopped automatically XRD patterns (2h: 3–8°) of AAERF and PNC films are
because specimens became too compliable for the ampli- shown in Fig. 3a. No apparent silicate reflections were
tude to be sustained. The heating rate was 3 °C/min. yielded by PNC films, indicating that large MMT particles
due to agglomeration were probably absent. However, the
3. Results and discussion intensity of scattering in the PNCF7 pattern seems to be
somewhat higher than that of the AAERF pattern. After
3.1. Control of thickness and composition of the PNC films the subtraction of the AAERF curve from the PNCF7
curve, followed by smoothing, a distinct peak at $6.18°,
Fig. 1 shows the variation of mass percentage of MMT which probably means d001 = 1.43 nm, shows up although
in PNC films (WC) with the amount of MMT in aqueous its reflection intensity is fairly low (Fig. 3b).
suspensions (MC). Specimens were prepared under the Given the limited value supplied by XRD information
same electric field of 120 V/cm for 120 s. Electrodeposition when the amount of MMT is low and when their regularity
under such a high voltage is such a complicated process is limited (few stacks aligned parallel to the film surface)
that the relationship between WC and MC is not clear. [20], the authors refer to SEM and TEM to further charac-
terize the morphology and dispersity of MMT platelets in
the AAER matrix.
8
MMT content in PNCF, Wc / %
7 5.0
25
MMT content in PNCF / %
6
4.5
5 20
Thinckness / µm
4 4.0
15
3
10 3.5
2
1 3.0
5
0
0.1 0.2 0.3 0.4 0.5 0 2.5
MMT content in suspension, Mc / g 0 30 60 90 120
Electrodeposition Time / s
Fig. 1. Mass percentage of MMT in PNC films (WC) vs. amount of MMT
in aqueous suspensions (MC, mass of MMT in 50 ml deionized water). Fig. 2. Thickness and MMT content in PNCF3 vs. electrodeposition time.

4. W. Lin et al. / Composites Science and Technology 68 (2008) 880–887 883
PNCF7
Intensity
PNCF3
PNCF1
AAERF
3 4 5 6 7 8
2θ (degrees)
120 25000
100
20000
80
Intensity / counts
15000
PNCF7
60
10000
40
5000 Fig. 4. SEM images of (a) as-prepared surface, (b) etched surface and (c)
20
cross-section of the PNCF3. Inset in (c): EDS result along the line mark.
MMT
0 0
4 6 8 shape is an intrinsic characteristic of MMT platelets, indi-
2θ (degrees) cating crystallized morphology of clay platelets in accor-
Fig. 3. XRD patterns of AAERF and PNC films. dance with previous results of TEM observation of clay
platelets [21,22]. To the author’s knowledge, it is the first
time to report this kind of morphology of MMT platelets
From the SEM images shown in Fig. 4, it can be seen in PNCs. Fig. 5c together with Fig. 5a may indicate that
that the PNCF3 surface is smooth and uniform (Fig. 4a). platelets in the PNCF1 are mostly exfoliated into thin
After careful etching with cyclohexanone, particulate struc- stacks containing only a few layers. However, we cannot
ture shows up (Fig. 4b). Most of the MMT particles are on expect this kind of exfoliation to be complete, because
the length scale of sub-microns, randomly dispersed in the TEM micrographs cover a small area, which might not
polymer matrix. In Fig. 4c, a cross-section of a uniform be entirely representative for the overall microstructure of
and rigid film is displayed in front of us. The EDS result the composite [20]. Furthermore, the majority of the papers
of the line-scan indicates a relatively uniform distribution published on PNCs show both the intercalated and exfoli-
of the silicon element, which further indicates the absence ated structures when MMT loading is low. In our study,
of apparent segregation of MMT across the thickness TEM images of PNCF3 and PNCF7 are similar, in both
direction. of which thick stacks are frequently observed. Fig. 5d
TEM micrographs are presented in Fig. 5. It is interest- and e show the morphology and dispersity of MMT plate-
ing that for the PNCF1, which means the volume fraction lets in the PNCF7, which indicates the common state of
of MMT platelets is rather low, mono-disperse-like MMT coexistence of both thick and thin stacks. The similarity
platelets of hexagonal shape are observed (Fig. 5a and b), of surface image and cross-section image for PNCF7 prob-
dispersing randomly in the AAER matrix. The hexagonal ably means random orientation of clay platelets in matrix.

5. 884 W. Lin et al. / Composites Science and Technology 68 (2008) 880–887
Fig. 5. TEM micrographs of PNC films: (a) surface image of PNCF1; (b) enlargement of a hexagonal in image (a) and its EDS result (inset); (c) cross-
section image of PNCF1; (d) surface image of PNCF7; (e) cross-section image of PNCF7 (inset: 10 times enlargement of the box mark).
Hereby, it may be concluded that the dispersity of MMT
platelets in AAER matrix is satisfying. Two factors in our NMMT film
preparation process should be emphasized to help under-
stand the ideal dispersing state achieved. First, water, as
the dispersing medium, is contributive to obtaining uni-
form aqueous suspension because both AAER and MMT PNCF7
are hydrophilic. Second, AAER plays double roles. On
Absorbance
one hand, AAER is contributive to achieving good disper-
sion in water. It is commonly accepted that poly(acrylic PNCF3
acid) or its dissociated form helps produce a barrier that
may prevent adhesion and agglomeration of clay particles
in aqueous suspension [19,23,24]. On the other hand, the
PNCF1
mass content of AAER molecules in aqueous dispersion
is high and the electrodeposition process is rapid, both of
which probably help prevent coagulation of negatively
charged clay particles in strong electric field. AAERF
3.3. FTIR spectra
3500 3000 2500 2000 1500 1000 500
In Fig. 6, the characteristic bands for main functional Wavenumber / cm-1
groups in the IR spectra of AAERF and MMT film are
Fig. 6. FTIR spectra of AAERF, MMT film and PNC films (shifted
retained in those of the PNC films. For AAERF, bands vertically for clarity).
at $3509, $3442 and $3373 cmÀ1 are for hydroxyl
groups. For MMT film, the band at 3623 cmÀ1 is associ-
ated with the O–H stretching of the MMT lattice structure MMT platelets. The possible functional groups acting as
[25]. It should be noted that the 3623 cmÀ1 band shifts to the acceptors in the hydrogen bonding with the hydroxyl
3609, 3612 and 3616 cmÀ1 for PNCF1, PNCF3 and groups are the carbonyl, ether, ester and hydroxyl groups
PNCF7, respectively. This phenomenon is probably the in AAER molecules. Additionally, the frequency shifts of
result of hydrogen bonding formed between functional carbonyl group to lower value, i.e. from 1736 cmÀ1 for
groups of AAER molecules and the hydroxyl groups of AAERF to 1729, 1731 and 1731 cmÀ1 for PNCF1,

6. W. Lin et al. / Composites Science and Technology 68 (2008) 880–887 885
PNCF3 and PNCF7, respectively, also shed light on the 2.5
existence of interaction (e.g. hydrogen bonding) between
the AAER molecules and the MMT platelets, similar to
Tensile Modulus / GPa
the phenomenon reported by Tien and Wei [26]. The dis- 2.0
tinct frequency shift of Si–O stretching, from 522 cmÀ1
for MMT film to $518 cmÀ1 for PNC films, is thought
to be the result of the less-compact environment brought
1.5
by intercalated or exfoliated state [27].
3.4. Thermogravimetric analysis
1.0
Fig. 7 shows the TGA thermograms of the AAERF and
PNC films, measured in air. All the curves display two-
stage degradations. The former is probably due to the dis- 0.5
charge of small molecules resulted from gradual break- 0 1 2 3 4 5 6 7 8
down of polymer network initiated from oxidation and Fraction of MMT in PNC films / %
decomposition of chain ends. The latter may be caused
by chain scissoring or further oxidation of the network 33
or relatively large fragment remained after the first stage.
Comparison of thermal stabilities of AAERF and PNC 30
Tensile Strength / MPa
films is based on the degradation temperature at the major
decomposition stage (the first stage), as measured from the 27
minimum of the first derivative of the weight loss with
24
respect to temperature (this point corresponds to the max-
imum weight loss rate). Accordingly, the degradation tem- 21
peratures are 309.3, 335.1, 314.9 and 345.5 °C for AAERF,
PNCF1, PNCF3 and PNCF7. This improvement is 18
thought to be the result of air transport resistance opposed
by well-dispersed clay platelets in polymer matrix [28,29]. 15
The residual weight at 700 °C for AAERF, PNCF1,
PNCF3 and PNCF7 are $0.1%, 1.2%, 3.0% and 6.6%, 0 1 2 3 4 5 6 7 8
respectively. Fraction of MMT in PNC films / %
Fig. 8. Tensile modulus (a) and tensile strength (b) vs. mass fraction of
3.5. Mechanical properties MMT for PNC films. The average modulus was calculated from at least 10
measurements and the error bars refer to standard deviations.
In Fig. 8a, tensile modulus calculated as a derivative of
the tensile strength curve in its final linear part [30], are
plotted against mass fraction of MMT for PNC films. loading, agreeing with the characteristic of tensile modulus
The tensile modulus increases nearly linearly with clay for PNCs described by the existed theories [31] and with the
work by other researchers [6,12,14]. For PNCF7, the ten-
sile modulus reaches 2.2 ± 0.2 GPa, 145–205% enhance-
1.0 ment from that of AAERF (0.8 ± 0.1 GPa). In Fig. 8b,
AEARF effective reinforcement in tensile strength is also distinct.
0.8 PNCF1 The storage modulus (G 0 ), loss modulus (G00 ) and tan d
PNCF3
of AAERF and PNC films are plotted against temperature
Relative weight
PNCF7
0.6 in Fig. 9a–c. In accordance with the tensile testing results,
G 0 increases monotonically with the clay content, and G 0 s
of all PNC films are higher than that of AAERF all over
0.4
the temperature range (Fig. 9a). Non-monotonic increase
of G 0 with clay concentration, which is thought to be the
0.2 result of transformation of clay morphology from highly
exfoliated state to intercalated stacks [27], is not observed.
0.0 We note the highest percent increase at 60 °C of storage
modulus than those at all the other temperatures, which
100 200 300 400 500 600 700
is probably due to the following two reasons: (1) even when
Temperature / oC
the curves go collaterally, which means constant increase in
Fig. 7. TGA thermograms of AAERF and PNC films. G 0 at all temperatures, decrease of denominator will lead to

7. 886 W. Lin et al. / Composites Science and Technology 68 (2008) 880–887
1500
AAERF AAERF
100 PNC1
PNCF1
PNC3
PNCF3
Storage Modulus / MPa
PNC7
Loss Modulus / MPa
PNCF7
1000 80
60
500
40
0 20
10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80
Temperature / oC Temperature / oC
0.4
AAERF
PNCF1
PNCF3
0.3
PNCF7
Tan δ
0.2
0.1
10 20 30 40 50 60 70 80
Temperature / oC
Fig. 9. Storage modulus (a), loss modulus (b) and tan d (c) of AAERF and PNC films vs. temperature.
increment of ‘‘percent increase’’; (2) the temperature of well-dispersed MMT platelets and AAER molecules tend
60 °C is around the Tg of AAERF but before Tgs of com- to restrict movement of polymer chain segments. In com-
posite films. From Fig. 9b, we can see two peaks for parison with the increase of Tg, the change in Tb is almost
AAERF, PNCF1 and PNCF3, indicating two different negligible, consistent with the results reported by Ref. [27].
transition temperatures, i.e. Tb and Ta for the lower tem- Another definition of Tg is based on tan d curves, as shown
perature peak and higher one, respectively, the latter often in Fig. 9c. The tan d curves in our experiment, however, are
being described as the glass transition temperature Tg (see not so complete because heating upon Tg, the films become
Refs. [27,32] for details). It is apparent that Tg also too compliable for the amplitude of the oscillation to be
increases with increasing MMT content, like E and G 0 , sustained. Table 1 lists the Tgs and storage modulus of
the reason for which may be that interaction between the AAERF and PNC Films Measured by DMA.
Table 1
Summary of mechanical properties of AAERF and PNC films measured by DMA
Sample Tg (°C)a Storage modulus (MPa)
20 °C (%) Increase 40 °C (%) Increase 60 °C (%) Increase
AAERF 59.6 763.7 539.3 120.0
PNCF1 66.1 863.6 13.1 686.4 27.3 324.2 170.2
PNCF3 72.5 953.2 24.8 750.1 39.1 320.6 166.7
PNCF7 77.7 1250.0 63.7 1011.2 87.5 600.0 400.0
a
From tan d peaks.

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