eXPRESS Polymer Letters Vol.10, No.

11 (2016) 950–963
Available online at www.expresspolymlett.com
DOI: 10.3144/expresspolymlett.2016.88

Composite coating with synergistic effect of biomimetic

epoxy thermoset morphology and incorporated
superhydrophobic silica for corrosion protection
W. F. Ji1, C. W. Li1, W. J. Huang1, H. K. Yu1, R. D. Chen2, Y. H. Yu3, J. M. Yeh1*, W. C. Tang4,
Y. C. Su4
1
Department of Chemistry, Center for Nanotechnology and Center for Biomedical Technology at Chung-Yuan Christian
University (CYCU), 32023 Chung Li, Taiwan, Republic of China
2
Master Program in Nanotechnology and Center for Nanotechnology at CYCU, 32023 Chung Li, Taiwan, Republic of
China
3
Department of Chemistry, Fu Jen Catholic University, Hsinchung Dist., 24205 New Taipei City, Taiwan, Republic of
China
4
Division of Applied Chemistry, Material and Chemical Research Laboratories, Industrial Technology Research Institute,
30011 Hsinchu, Taiwan, Republic of China

Received 18 April 2016; accepted in revised form 27 June 2016

Abstract. In this work, potential anticorrosive coating resulted from the composite with synergistic effect of biomimetic
epoxy thermoset (BET) morphology and incorporated superhydrophobic silica microspheres was presented. First of all, su-
perhydrophobic methyl-modified silica (MS) microspheres were synthesized by performing the conventional base-catalyzed
sol-gel process of MTMS and APTMS. The as-prepared MS microspheres were identified as having an average particle size
of ~1 µm in diameter. The as-prepared MS microspheres were characterized by Fourier transform infrared spectrometry
(FTIR), 29Si and 13C solid-state nuclear magnetic resonance (NMR) spectroscopy. Morphological properties of MS micros-
pheres and BET-silica composite coating were studied by scanning electron microscopy (SEM). Subsequently, 3 wt% of
MS microspheres were incorporated into an epoxy slurry of DGEBA/T-403 in dimetyl acetamide (DMAc), followed by per-
forming the programmed heating through nanocasting technique with PDMS as soft template materials for pattern transfer
by using leaf of Xanthosoma Sagittifolium as natural template, leading to the formation of artificial biomimetic composite
coating. The appearance/dispersion capability of silica microspheres in BET coating was confirmed by the energy dispersive
X-ray spectroscopy (EDX) and Si-mapping. The roughness level of BET and BEC-3% were detected by AFM. The BET-
silica composite was found to exhibit a contact angle (CA) of ~153°, revealing the synergistic effect of biomimetic epoxy
morphology and incorporated superhydrophobic MS microspheres, which is found to be more hydrophobic than that of neat
epoxy thermoset (CA = 81°). Corrosion protection of as-prepared coatings was demonstrated by performing a series of elec-
trochemical measurements (Tafel, Nyquists and Bode plots) upon CRS electrodes in saline condition. It should be noted
that the BET coatings upon CRS electrode revealed an effectively enhanced corrosion protection as compared to that coatings
without biomimetic morphology. Moreover, the BET coating with superhydrophobic MS microspheres upon CRS electrode
was found to exhibit better corrosion protection as compared to a counterpart coating without MS microspheres.

Keywords: polymer composites, biomimetic, methyl-modified silica microsphere, epoxy resin, corrosion protection

1. Introduction polymeric coatings are typically utilized as a primer

Corrosion is a significant research area for finishing to protect metal surface from corrosion [1–3]. In the
of metallic industry. Therefore, protective organic/ past decades, different classic polymers, such as

*
Corresponding author, e-mail: juiming@cycu.edu.tw
© BME-PT

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 Ji et al. – eXPRESS Polymer Letters Vol.10, No.11 (2016) 950–963

epoxy resin, [4, 5] polyurethane [6, 7] and polyimide in saline condition (e.g., 3.5 wt% NaCl solution)
[8, 9] have been used by the environmentally shel- based on a series of electrochemical corrosion meas-
tered coatings to enhance the anticorrosive effect urements. Chang et al. [46] found that the UV-cured
upon various metallic substrates. In order to facilitate biomimetic composites containing thermoplastic
the corrosion protection of various metallic sub- PMMA and micro-silica particles was found to give
strates, versatile alternative practical coatings are the coating surface with increased roughness, which
being continuously developed such as conducting/ may effectively increase the superhydrophobicity
electroactive polymers, [10, 11] conventional poly- and therefore facilitate the corrosion resistance of
mers containing filler with large aspect ratio [12, 13] metallic substrates in saline.
and polymer coatings with superhydrophobic sur- In this work, the synergistic effect of biomimetic mor-
face [14, 15]. phology and incorporated superhydrophobic silica mi-
Recently, great attention is paid to the potential of crospheres is used to prepare the BET-silica compos-
applications on bio-mimetic materials with super-hy- ite anticorrosion coatings. First of all superhydropho-
drophobic surfaces structure due to their diverse po- bic methyl-modified silica (MS) microspheres were
tential and unprecedented hierarchical surfaces [16– synthesized by sol-gel reactions and characterized by
26]. Moreover, the studies of anticorrosive polymer FTIR, 13C and 29Si NMR spectra. Moreover, nano-
coatings with roughly superhydrophobic or biomimet- casting technique was utilized with PDMS (poly di-
ic surface attracted intensive research interests [27– methyl siloxane) as pattern-transfer polymer by using
34]. For example, Liu et al. [35] found that the sur- natural leaf of Xanthosoma Sagittifolium as natural
face of copper block was found to display superhy- template, leading to the formation of artificial BET-
drophobicity after treatment of n-tetradecanoic acid silica composite coatings. Furthermore, 3 wt% of as-
solution for a period of time, leading to the as-formed prepared superhydrophobic MS microspheres was
surface with better anticorrosion performance. added into BET to give the BET-silica composite
Moreover, Wang et al. [36] demonstrated that the coatings. SEM and contact angle measurements were
steel surface treated with specific surface modifica- applied to investigate the morphology and hydropho-
tion to give coating with micro-/nano- hierarchical bicity of coating surface. Finally, anticorrosion per-
superhydrophobic structures upon steel, leading to formance of the developed BET-silica composite coat-
an excellent corrosion protection property. Further- ings upon CRS electrodes was evaluated by a series
more, Yang et al. [37] reported that the eletroactive of electrochemical measurements (Tafel Nyquists
polymer with the biomimetic topography also show- and Bode plots) in 3.5 wt% NaCl aqueous solution.
ing superhydrophobicity, which exhibited superior
anticorrosive ability based on a series of electro- 2. Experiment
chemical investigations in 3.5 wt% NaCl solution. 2.1. Materials and measurements
Enhancement of corrosion protection of above-men- In this study, the bisphenol A diglycidyl ether
tioned superhydrophobic coatings may be attributed (DGEBA; Aldrich; Japan) and trimethylolpropanetris
to the trapped gas within the valleys between the hills [poly(propylene glycol), amine terminated] ether (T-
of the superhydrophobic surface [35]. 403; Aldrich; American) were used as received with-
On the other hand, the research activity in terms of out further treatment. Trimethoxy (methyl) silane
polymer coating incorporated with hydrophobic in- (MTMS; Aldrich, 98.0%; American), (3-amino-
organic particles has also evoked great research in- propyl) trimethoxysilane (APTMS; Fluka, 98.0%;
terests [38–43]. For example, Sun et al. [44] had ex- Switzerland) were used as sol-gel precursor and
plored that the superhydrophobic surfaces covered epoxy coupling agent, respectively. Ethanol (EtOH;
with cauliflower-like cluster binary micro-nano coat- Aldrich, 95%; American), ammonium hydroxide so-
ings and promoted excellent corrosion resistance lution (NH4OH; Aldrich, 28.0–30.0%; American),
property in the 3.5 wt% NaCl solution. Moreover, polyvinylpyrrolidone (PVP, Mw = 40,000, polymeric
Weng et al. [45] demonstrated that the as-prepared or- stabilizer, Aldrich), and N, N-dimethyl acetamide
ganic-inorganic superhydrophobic composite coating (DMAc; 99.0%, Aldrich; American) were used as re-
was found to boost the corrosion resistance property ceived without further purification. The liquid com-

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ponents (Sylgard 184) of polydimethylsiloxane the aqueous solution for about 1 hour at 5 °C. The sol-
(PDMS; American) were supplied by Dow Corning gel reaction was continued under magnetic stirring
Corporation. All reagents were reagent grade unless for 6 hours at room temperature. Finally, the as-pre-
otherwise stated. pared silica microspheres were washed repeatedly
Fourier-transform infrared (FTIR) spectra were with ethanol (EtOH) and separated from the aqueous
recorded using an FTIR spectrometer (JASCO FT/IR- mixture by centrifugal sedimentation of 7500 rpm to
4200) operating at room temperature. The X-ray pho- remove the surfactant and re-dispersed in water.
toelectron spectroscopy XPS measurement was per-
formed on Microlab MKII electron spectrometer with 2.3. Preparation of PDMS template
a base pressure of 10–10 Torr. The Sorvall RC-5C The PDMS pre-polymer was obtained by mixing the
Plus Super-speed Centrifuge was used to separate elastomer base and a curing agent at a specific weight
methyl-modified silica (MS) particles by centrifugal ratio (10:1, w/w). The PDMS pre-polymer was poured
force (7500 rpm). Both 13C and 29Si MAS solid-state into a mold of 6×6 cm2 fixed to a piece of fresh, nat-
NMR experiments were performed on a 400 MHz ural Xanthosoma sagittifolium leaf (veins of leaf were
solid-state NMR spectrometer. 13C MAS NMR spec- removed in area of about 6×6 cm2) and cured in oven
tra were obtained at 100.63 MHz with 7 kHz apply- programmed at 60 °C for 6 hours. After curing
ing 90° pulses and 2.0 s pulse delays. To enhance car- process, the PDMS blocks were detached from the
bon sensitivity, cross-polarization (CP) techniques molds, to be used as a negative template for the fol-
were employed. 29Si MAS NMR spectra were record- lowing nanocasting process of pattern transferring
ed at 79.49 MHz applying 90° pulses, 300 s pulse de- [47].
lays, and 5.0 ms contact time, with samples in 5.0 mm
zirconia rotors spinning at 7 kHz. Surface morpholo- 2.4. Preparation of CRS electrode coated with
gies of the superhydrophobic samples were observed biomimetic epoxy thermoset (BET)
by using SEM (JOEL JSM-7600F). Silica dispersion The 1.024 g of DGEBA was dissolved in 5 g of
was detected by EDX (OxFord xmax 80). Contact an- DMAc under stirring for 1 hour, followed by adding
gles were measured using a First Ten Angstroms FTA 0.88 g of T-403 functioning as curing agent under
125 at ambient temperature. Water droplets (about stirring for 12 hours. Finally, the solution was cast
4 µL) were carefully dropped onto the surfaces of onto the PDMS template, and then the cold-rolled
samples, and contact angle was determined from the steel (CRS) electrode was pressed against upon the
average of five measurements at various positions surface of as-prepared epoxy slurry. Subsequently, the
on the samples surface. Corrosion potential and cor- thermal curing process of epoxy thermoset was pro-
rosion current of sample-coated CRS electrodes were grammed at 50 °C for 1 hour, 120 °C for 2 hours and
electrochemically measured using a VoltaLab 50 po- 140 °C for 0.5 hour. The CRS electrode coated with
tentiostat/galvanostat. Electrochemical impedance BET can be obtained by detaching from PDMS tem-
spectroscopy (EIS) study was recorded on AutoLab plate.
(PGSTAT302N) potentiostat/galvanostat electro-
chemical analyzer. 2.5. Preparation of CRS electrode coated with
biomimetic epoxy-silica composite
2.2. Synthesis of methyl-modified silica (MS) (BEC-3 wt%)
microsphere The representative procedure for the preparation of
The MS microspheres were synthesized via conven- CRS electrode coated with biomimetic epoxy-silica
tional base-catalyzed sol-gel reactions of MTMS in composite (BEC) was given as follows: 1.024 g of
the presence of APTMS molecules. A typical proce- DGEBA and 0.075 g of the as-prepared MS particles
dure to prepare the MS microspheres was given as were dissolved or dispersed into 5 g of DMAc under
follows: the surfactant (PVP, 0.1 g) and NH4OH magnetic stirring for 1 hour, followed by introducing
(0.1 mL) was dissolved in 40 mL of water and kept at 0.5 g of T-403 for 6 hours under stirring. The mixed
5 °C. The mixture of 2.04 g MTMS (20 mmole) and solution was eventually dropped and cast onto the
0.136 g APTMS (1 mmole) was added drop-wise to PDMS template, the CRS electrode was then pressed

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against the surface of mixing slurry. Subsequently, data were taken at least three times to ensure the re-
the thermal curing process of BEC was programmed producibility and statistical significance.
at 50 °C for 1 hour, 120 °C for 2 hours and 140 °C for
0.5 hour. The CRS electrodes coated with BEC was 3. Results and discussion
then obtained by detaching from the PDMS template. A representative process of base-catalyzed sol-gel
reactions of MTMS and APTMS was used to prepare
2.6. Electrochemical corrosion studies superhydrophobic MS microspheres. Moreover, the
Epoxy thermoset coatings of about 30 µm in thickness typical procedure for the preparation of biomimetic
were obtained after drying in a fume hood for epoxy incorporated superhydrophobic MS micros-
24 hours. Edges of the coupons were locked on the pheres coatings by replicating fresh Xanthosoma
artificial electrode for corrosion testing. The coated sagittifolium leaf through nanocasting technique was
and uncoated coupons were connected to the working used, as shown in Figure 1. It should be noted that a
electrode of an electrochemical cell. For the anticor- large amount of MTMS in the sol-gel process may
rosion measurement aspect, polarization curves were lead to as-prepared silica microspheres with a con-
obtained by using cyclic voltammetry (VoltaLab 50 siderable amount of methyl groups attached to the
potentiostat/galvanostat) at an operational temperature surface, resulting in better superhydrophobicity. On
of 30 °C. The three-electrode configuration was em- the other hand, a small amount of APTMS used in
ployed in the circuit, with the sample as the working the sol-gel process indicates that few primary amine
electrode, the graphite rod as the counter electrode, groups may adhere to the surface of silica micros-
and the saturated calomel electrode (SCE) as the ref- pheres, implying the formation of chemical bonding
erence electrode. 3.5 wt% aqueous solution of saline between primary amine and the epoxide ring of
was utilized as the electrolyte. Open circuit potential epoxy pre-polymer.
(OCP) at the equilibrium state of system was used as
the corrosion potential (Ecorr in [V] vs. SCE). For the 3.1. Characterization of superhydrophobic
potentiodynamic polarization experiments, the poten- methyl-modified silica microspheres
tial was scanned from -500 to 500 mV at a scanning Figure 2 show the representative NMR, FTIR spec-
rate of 50 mV·s–1. Corrosion current (Icorr [μA·cm–2]) troscopy and SEM image of MS microspheres. Solid-
was determined by superimposing a straight line state 29Si NMR provided quantitative information
along the linear portion of the cathodic or anodic about the condensation reaction and solid-state
13
curve and extrapolating it through Ecorr [mV]. All raw C NMR spectroscopy was particularly useful in

Figure 1. Preparation flow chart of superhydrophobic methyl-silica (MS) microsphere and incorporated superhydrophobic
MS microspheres onto the biomimetic epoxy thermoset surface to manufacture the CRS coated with biomimetic
epoxy thermoset composite

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monitoring the hydrolysis reaction of sol-gel could be assigned to the absorptions at the location
process. Figure 2a shows the 29Si NMR spectrum, of 1136 and 1024 cm–1. Moreover, the band occur-
two clear resonance peaks derived from Tm (Tm = ring at the location of 1273 cm–1 showed a well-de-
RSi(OSi)m(OH)3–m, m = 1–3; T3 at δ = –65.2 ppm and fined C–H absorption of Si–CH3 [49]. Furthermore,
T2 at δ = –55.7 ppm) were observed [48]. T1 peaks the bond at 1531 cm–1 was due to N–H bending vi-
were too small to be analyzed in the photography. The bration. Because the mole ratio of MTMS/APTMS
formation of T1 and T0 species was insignificant, sug- in sol-gel reaction was 20, meaning that the Si–CH3
gesting that non-reactive organically modified pre- had a stronger absorption than N–H in MS, as shown
cursor was present. Figure 2b shows the results of in Figure 2c. Moreover, there were much more methyl
13
C NMR analysis. The sample exhibited three peaks groups providing the superhydrophobicity of the sil-
with nearly equal intensity at the position around 11.1, ica surface. Furthermore, the N–H group could pro-
26.1, and 43.5 ppm. Three peaks could be attributed vide the strong chemical bonding with epoxy. The
to three different kinds of Carbon atoms for the Figure 2d shows the SEM image of the as-prepared
APTMS group, confirming the existence of APTMS MS microspheres at 10 000 magnification, revealing
bonded to silica. Figure 2c displays the FTIR spec- an average particle size of ~1 µm in diameter.
trum of MS microspheres. The presence of charac-
teristic bands of MS at wavenumber of 774 and 3.2. Investigating the thermal curing of epoxy
2979 cm–1 were attributed to the stretching vibra- thermosets by FTIR Spectroscopy and
tions of Si–C and C–H. The MS exhibited well-de- XPS analysis
fined absorptions at ca. 1411 cm–1, which belonged FTIR was utilized to investigate the curing process of
to the characteristic of symmetric deformations in the epoxy thermoset. For example, FTIR spectra of
Si–R groups. The presence of Si–O–Si linkages epoxy slurry (before curing), biomimetic epoxy ther-

Figure 2. The identification of as-prepared superhydrophobic methyl-silica microsphere of (a) 29Si NMR spectroscopy,
(b) 13C NMR spectroscopy, (c) FTIR spectroscopy and (d) image of SEM observation at magnification 10 000×

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moset (after curing) and biomimetic epoxy-silica 3.3. Microscopic observations and EDX
composite are shown in Figure 3. After the pro- spectroscopy for biomimetic morphology
grammed heat treatment, the disappearance of char- Macroscopic photograph of natural, fresh leaf of Xan-
acteristic peak of epoxide ring (915 cm–1) and of the thosoma sagittifolium is shown in Figure 5a. More-
primary amine of epoxy thermoset could be identi- over, microscopic SEM overview image at a magni-
fied as a proof of the accomplishment of the ring- fication of 1000× for Figure 5b fresh natural leaf, Fig-
opening polymerization, as shown in Figures 3b and ure 5c PDMS soft negative template, Figure 5d bio-
3c. Moreover, XPS was utilized to measure the char- mimetic epoxy thermoset and Figure 5e composite
acteristic peak of BET and BEC, as shown in Fig- with synergistic effect of biomimetic epoxy ther-
ure 4. From the Figure 4b, the Si 2p and Si s were moset morphology and incorporated superhydropho-
found to be locatet at the positions of 102.9 eV (Si–O bic MS microspheres. In Figure 5b, there were much
group) and 153.5 eV, indicating that the MS micros- smaller micro papillary hills and nano textures tan
pheres were randomly dispersed onto the BEC sur- those found on the fresh natural Xanthosoma sagitti-
face. folium leaves. Moreover, the Figure 5c shows the
SEM image of the cured PDMS negative template
prepared by casting the liquid PDMS directly onto a
natural and fresh Xanthosoma sagittifolium leaves.
The Figure 5d was the SEM image of BET at mag-
nification of 1000×. It should be noted that the mor-
phology of BET is almost the same as that of fresh
Xanthosoma sagittifolium leaves, implying that the
nano-casting technique was successful in transfer-
ring the pattern of natural leaves. To well-understand
the papilla-like and texture on the surface, Figure 5e
shows the SEM image of BET at a magnification of
3000×. From the cross-section Figure 5f at a magni-
fication of 5000×, the height of papilla-like was found
Figure 3. The identification of FTIR spectroscopy for to be 6–9 μm. This observation depicted that the tem-
(a) epoxy slurry (before curing), (b) epoxy ther- plate effectively replicated the structure of leaf sur-
moset (after curing) and (c) composite with bio-
face.
mimetic epoxy thermoset and incorporated super-
hydrophobic MS microspheres For the morphology study of biomimetic epoxy sil-
ica containing 3 wt% composite (BEC-3%), the SEM
image at a magnification of 1000× of BEC-3% im-
ages, Si mapping and EDX of the BEC-3% surface
are presented in Figures 6. Morphology of BEC-3%
was very similar to that of BET, implying that the ma-
jority of incorporated MS microspheres may be em-
bedded into the BET matrix, as shown in Figure 6a.
In Figure 6b, the lots of red spots representing the
mapping of Si element of BEC was clearly found, in-
dicating that the MS microspheres were randomly
dispersed in the BET matrix. Moreover, the EDX
showed that the appearance of peaks for Si, O and N
element of BEC-3%, implying that the MS micros-
Figure 4. The identification of XPS for (a) biomimetic epoxy pheres may probably be there inside or on the BET
thermoset and (b) composite with biomimetic epoxy coating. Hydrophobicity enhancement of BEC-3%
thermoset and incorporated superhydrophobic MS
coating resulted from both the biomimetic morphol-
microspheres

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Figure 5. The morphology observations for (a) macroscopoc photograph of the fresh natural Xanthosoma sagittifolium leaves.
Microscopic SEM overlook image at magnification of 1000× for (b) fresh natural leaf, (c) PDMS soft negative
template, (d) biomimetic epoxy thermoset and (e) biomimetic epoxy thermoset at magnification of 3000× (f) cross-
section for biomimetic epoxy thermoset at magnification of 5000×

ogy and incorporated MS microspheres, respective- 3.4. Atomic force microscope

ly, can be further identified by the contact angle In Figure 7, the three dimensional AFM images of
measurement of water droplets and will be discussed biomimetic epoxy thermoset and biomimetic epoxy
in the following section. silica containing 3 wt% composite on the CRS sub-

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Figure 6. The surface studies of as-prepared biomimetic epoxy-silica composite of (a) SEM microscopic observation at mag-
nification of 1000×, (b) Si mapping and (c) energy-dispersive X-ray spectroscopy

strate vividly showed higher surface roughness. From roughness levels of the surfaces were determined
the AFM image, the surface morphology was coin- [50]. The arithmetic average roughness, Ra levels of
cident with the SEM observation. Moreover, well- the BET and BEC-3% coatings were measured. The
understanding the surface roughness on the BET and BEC-3% coating had the higher RMS roughness of
BEC coatings, AFM was studied and performed in 2.322 μm than BET coating (2.205 μm).
which height images were obtained and average

Figure 7. The surface roughness studies of three dimonsional morphology atomic force microscope for (a) CRS electrode
coated with biomimetic epoxy thermoset and (b) CRS electrode coated with biomimetic epoxy-silica composite

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3.5. Surface hydrophobicity of as-prepared increasing the CA of coating by ~30°. In summary,

coatings measured by contact angle the BEC coating with synergistic effect of biomimetic
In this study, the contact angle (CA) of water droplets epoxy thermoset morphology and incorporated MS
upon six different surfaces of coatings were illustrated microspheres revealed an increment in CA of 72° as
in Figure 8. The CA for fresh leaf of Xanthosoma compared to that of neat epoxy thermoset with smooth
sagittifolium was 146°, as shown in Figure 8a. The surface, implying that this composite could be a po-
CA for neat epoxy thermoset with smooth surface was tential candidate as corrosion protection coating, as
81°, as shown in Figure 8b. Moreover, the CA upon discussed in the following section.
surface of BET was 123°, as shown in Figure 8c. It
indicated clearly that the biomimetic morphology may 3.6. Potentiodynamic measurements
increase the CA of neat epoxy thermoset by up to 42°, Polarization curves for the CRS electrode coated with
reflecting that the biomimetic morphology could ef- neat epoxy thermoset, biomimetic epoxy thermoset
fectively promote the hydrophobicity of neat material. and biomimetic epoxy-silica composite can be ob-
Moreover, the CA of neat as-prepared dry fine powder tained by performing all the measurements in elec-
coating of superhydrophobic MS microspheres on trolyte of 3.5 wt % NaCl aqueous solution.
glass was found to be 158° because of the methyl Corrosion current was obtained by extrapolating Tafel
group attached onto the surface, as shown in plots, from both the cathodic and anodic polarization
Figure 8d. With incorporated 1 wt% of MS micros- curves for the respective corrosion process. Extrap-
pheres into BET, the as-obtained BEC-1% with syn- olating the cathodic and anodic polarization curves
ergistic effect of biomimetic morphology and super- to the point of intersection provides both the corro-
hydrophobic silica microspheres was found to exhibit sion potential and corrosion current. Tafel plots for
the CA = 137°, as shown in Figure 8e. Moreover, in- (a) bare CRS electrode, (b) CRS electrode coated
corporated 3 wt% of MS microspheres into BET, the with smooth epoxy thermoset, (c) CRS electrode coat-
as-perpared BEC-3% was revealed the CA = 153°, as ed with biomimetic epoxy thermoset, (d) CRS elec-
shown in Figure 8e. It indicated that introducing trode coated with biomimetic epoxy-silica contain-
3 wt% silica microspheres into BET coating may fur- ing 1 wt% composite and (e) CRS electrode coated
ther promote the hydrophobicity of BET by further with biomimetic epoxy-silica containing 3 wt% com-

Figure 8. Water contact angle for (a) the fresh natural Xanthosoma sagittifolium leaves (b) RS electrode coated with smooth
epoxy thermoset, (c) CRS electrode coated with biomimetic epoxy thermoset, (d) methyl-silica microsphere tablet,
(e) CRS electrode coated with biomimetic epoxy-silica containing 1% composite and (f) CRS electrode coated
with biomimetic epoxy-silica containing 3% composite

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on the Ecorr and Icorr data obtained from Tafel plots,

the BET was found to show better corrosion protec-
tion upon CRS electrode than that of epoxy ther-
moset with smooth surface. The increase of anticor-
rosion performance of the coating may be attributed
to the more hydrophobic surface (CA = 123°) of
BET than that of neat epoxy thermoset (CA = 81°).
Secondly, the further studies for the effect of incor-
porated superhydrophobic 1 wt% MS microspheres
existing in BET on corrosion protection upon CRS
electrode, it should be noted that the corrosion po-
tential of CRS electrode coated with BEC-1%
(Ecorr = –387.2 mV) was found to be more positive
Figure 9. Tafel plots of electrochemical corrosion measure-
than that of BET (Ecorr = –504.0 mV). Moreover, the
ment for (a) bare CRS electrode, (b) CRS elec-
trode coated with smooth epoxy thermoset, (c) CRS corrosion current of CRS electrode coated with
electrode coated with biomimetic epoxy ther- BEC-1% (Icorr = 0.31 µA/cm2) was found to be
moset, (d) CRS electrode coated with biomimetic lower than that of BET (Icorr = 0.33 µA/cm2). After-
epoxy-silica containing 1% composite and (e) CRS ward, incorporated superhydrophobic 3 wt% MS mi-
electrode coated with biomimetic epoxy-silica
crospheres existing in BET on corrosion protection
containing 3% composite
upon CRS electrode, it described that the corrosion
posite are shown in Figure 9 and the corresponding potential of CRS electrode coated with BEC-3%
data were summarized and listed in Table 1. (Ecorr = –344.4 mV) was found to be more positive
Tafel plots of CRS electrode coated with all three than that of BEC-1% (Ecorr = –387.2 mV). More-
samples exhibiting the corrosion potential (Ecorr) was over, the corrosion current of CRS electrode coated
more positive than the bare CRS electrode. On the with BEC-3% (Icorr = 0.19 µA/cm2) was found to be
other hand, the corrosion current (Icorr) of the CRS lower than that of BET and BEC-1% (Icorr =
electrode coated with samples was found to be lower 0.31 µA/cm2). The BEC-3% was found to show bet-
than the bare CRS electrode. First of all, studies for ter corrosion protection upon CRS electrode than
the effect of biomimetic epoxy morphology on cor- that of BET and BEC-1% based on the conclusion
responding corrosion protection upon CRS electrode, obtained from the Ecorr and Icorr data. The further in-
it should be noted that the corrosion potential of CRS crease of anticorrosion performance of coating may
electrode coated with BET (Ecorr = –504.0 mV) was be associated with incorporated suitable amount MS
found to be more positive than that of neat epoxy microspheres enhance the hydrophobicity of BET
thermoset (Ecorr = –672.5 mV). Moreover, the cor- and BEC-1% coating.
rosion current of CRS electrode coated with BET Enhancement of coatings in anticorrosion upon CRS
(Icorr = 0.33 µA/cm2) was found to be lower than that electrode can be further confirmed by electrochem-
of neat epoxy thermoset (Icorr = 0.78 µA/cm2). Based ical impedance spectroscopy (EIS), as discussed in
the following section.
Table 1. Contact angle and electrochemical corrosion meas-
urements of bare CRS electrode, CRS coated with 3.7. Electrochemical impedance spectroscopy
smooth epoxy thermoset, BET and BEC
measurements
Electrochemical corrosion measurements
Sample Contact Coating In this study, EIS was utilized to evaluate the corro-
Ecorr Icorr
code 2 angle Hysteresis thickness sion protection for CRS electrode coated with epoxy
[mV] [μA/cm ]
[°] [μm] thermoset, BET and BEC. Generally, corrosion be-
CRS –852.8 220.5 – – –
havior of a metal can be modeled with an equivalent
Epoxy –672.5 0.78 81±3 32 28±2
BET –504.0 0.33 123±3 21 29±2 circuit which consists of a double-layered capacitor
BEC-1% –387.2 0.31 137±3 17 30±2 that is parallel with a charge transfer resistor and
BEC-3% –344.4 0.19 153±2 9 31±2 connected in series with an electrolyte solution re-

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 Ji et al. – eXPRESS Polymer Letters Vol.10, No.11 (2016) 950–963

sistor. Impedance (Z) depends on the charge transfer

resistance (Rct), the solution resistance (Rs), the ca-
pacitance of the electrical double layer, and the fre-
quency of the AC signal (ω).
The high-frequency intercept was equal to the solu-
tion resistance and the low-frequency intercept was
equal to the sum of the solution and charge transfer
resistances [51]. Figure 10 shows the Nyquist plots
for (a) bare CRS electrode, (b) CRS electrode coated
with smooth epoxy thermoset, (c) CRS electrode coat-
ed with biomimetic epoxy thermoset, (d) CRS elec-
trode coated with biomimetic epoxy-silica contain-
ing 1% composite and (e) CRS electrode coated with
Figure 11. Bode plots of electrochemical corrosion measure-
biomimetic epoxy-silica containing 3% composite.
ment for (a) bare CRS electrode, (b) CRS elec-
The samples were immersed in 3.5 wt% NaCl aque- trode coated with smooth epoxy thermoset,
ous electrolyte for 40 min before EIS measurements. (c) CRS electrode coated with biomimetic epoxy
The charge transfer resistances of all samples, as de- thermoset, (d) CRS electrode coated with bio-
termined by subtracting the intersection of the high- mimetic epoxy-silica containing 1% composite
and (e) CRS electrode coated with biomimetic
frequency end from the low-frequency end of the
epoxy-silica containing 3% composite
semi-circle arc with the real axis, were 0.156, 574.8,
3140, 3961 and 5250 kΩ·cm2, respectively. with biomimetic epoxy-silica containing 1% compos-
EIS Bode plots (impedance vs. frequency) of all sam- ite and (e) CRS electrode coated with biomimetic
ples were shown in Figure 11. Zreal was also a measure epoxy-silica containing 3% composite showed Zreal
of corrosion resistance. Low Zreal value could be per- values of 2.104, 5.724, 6.333, 6.814 and
formed from high capacitance to low resistance of the 7.017 kΩ·cm2, respectively, at low frequency end. It
coating. The Bode magnitude plots for (a) bare CRS should be noted that the result obtained from the EIS
electrode, (b) CRS electrode coated with smooth (Nyquist plots and Bode plots) was found to be con-
epoxy thermoset, (c) CRS electrode coated with bio- sistent with the previous conclusion obtained from the
mimetic epoxy thermoset, (d) CRS electrode coated Tafel plots. It also demonstrated that the composite
with synergistic effect of biomimetic epoxy thermoset
morphology and incorporated MS microspheres ex-
hibiting better corrosion protection as compared to
that of neat epoxy thermoset with smooth surface.
In this work, the superhydropobic composite coat-
ings with synergistic effect of biomimetic epoxy ther-
moset morphology and incorporated MS micros-
pheres had the best corrosion protection. The possible
mechanism of enhancing corrosive protection prop-

Figure 10. Nyquist plots of electrochemical corrosion meas-

urement for (a) bare CRS electrode, (b) CRS
electrode coated with smooth epoxy thermoset,
(c) CRS electrode coated with biomimetic epoxy
thermoset, (d) CRS electrode coated with bio-
mimetic epoxy-silica containing 1% composite
and (e) CRS electrode coated with biomimetic Figure 12. The mechanism of enhancing corrosive protec-
epoxy-silica containing 3% composite tion properties

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 Ji et al. – eXPRESS Polymer Letters Vol.10, No.11 (2016) 950–963

erties was that superhydrophobic surface composed microspheres into BET to enhance the hydrophobic-
of hills trapped the gas within the valleys between the ity of coating. In summary, the composite coating with
hills. Therefore, the free radical, O2 and H2O in the synergistic effect of biomimetic epoxy thermoset
corrosion solution could not effectively separate and morphology and incorporated superhydrophobic MS
reach to metal substrates, as shown in Figure 12. microspheres upon CRS electrode revealed signifi-
cantly increased corrosion protection performance
4. Conclusions as compared to that of neat epoxy thermoset with
In this work, the coating of composite with syner- smooth surface.
gistic effect of biomimetc epoxy thermoset morphol-
ogy and incorporated superhydrophobic MS micros- Acknowledgements
pheres was found to reveal better anticorrosion per- The authors acknowledge financial support from the Ministry
of Science and Technology, Taiwan, R.O.C. (NSC 102-2632-
formance on CRS electrode based on a series of elec-
M-033-001-MY3) and (MOST 104-2113-M-033-001-MY3);
trochemical measurements (Tafel plots) in saline the Department of Chemistry at CYCU; and the Center for
conditions. First of all, MS microspheres were pre- Nanotechnology and Center for Biomedical Technology at
pared by performing the sol-gel reaction of MTMS CYCU.
in the presence of APTMS, followed by character-
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