Progress in Organic Coatings 64 (2009) 39–46
Contents lists available at ScienceDirect
Progress in Organic Coatings
journal homepage: www.elsevier.com/locate/porgcoat
Effect of nano-ZnO addition on the silicone-modified
alkyd-based waterborne coatings on its mechanical and
heat-resistance properties
Shailesh K. Dhoke a,∗ , Rohit Bhandari b , A.S. Khanna a
a
b
Indian Institute of Technology, Bombay, Maharashtra 400076, India
University of Delhi, New Delhi 110007, India
a r t i c l e
i n f o
Article history:
Received 17 December 2007
Received in revised form 4 July 2008
Accepted 11 July 2008
Keywords:
Waterborne coatings
Nano-ZnO
Heat-resistance
a b s t r a c t
A silicone-modified alkyd-based waterborne coating was developed using hexamethylmethoxymelamine
(HMMM) as crosslinking agent and para-toluene sulphonic acid (p-TSA) as catalyst. The crosslinking ratio
for resin and HMMM was fixed to 70:30, based on FTIR and DSC studies. Nano-ZnO particles were added
to this system in different concentrations. The coatings with nano-ZnO particles were characterized using
FTIR and DSC. The nano-composite coatings were applied on mild steel panels and were cured at 130 ◦ C
for 30 min. The coatings were evaluated for their mechanical and heat-resistance properties. They were
exposed to 350 ◦ C for 10 min followed by water quenching. The process was repeated for 10 cycles.
Heat-resistance property of the coatings was examined by TGA. Also, surface morphological changes
were assessed using SEM and optical microscopy. It was found that the heat-resistance and mechanical
properties of the coatings improved significantly as a function of nano-ZnO addition.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Many high temperature industrial components suffer from
severe degradation at higher temperatures. Their protection at
these temperatures can be achieved by heat-resistant organic coatings. Silicone-based and silicone-modified coatings are extensively
used for this purpose due to their good high temperature stability owing to the excellent bond strength of Si–O–Si [1] and good
corrosion resistance. These coatings are also modified with commercial pigments like carbon black [2], titanium dioxide [3] to
further improve their mechanical properties and thermal stability. However, the loading level required for these pigments is quite
high and also optical transparency is not retained. With the rise in
demand for eco-friendly, cost-effective and transparent coatings
with improved heat-resistance and mechanical properties, their
development is a major challenge. Fortunately, recent development
in nanotechnology has eased the problems [4–6]. Use of nanoparticles as additives in coatings has been found to improve the
thermal stability of the polymer [7,8], enhance scratch and abrasion resistance of the coatings without disturbing their optical and
other properties [9,10]. The present paper deals with the investi-
∗ Corresponding author.
E-mail address: shaileshdhoke@iitb.ac.in (S.K. Dhoke).
0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.porgcoat.2008.07.007
gation on the effect of nano-ZnO additions on silicone-modified
alkyd-based waterborne coatings.
2. Experimental
2.1. Raw materials
Nano-ZnO with an average size of less then 40 nm and
specific area 29 m2 g−1 was procured from Horsehead Corporation Company (USA). The waterborne silicone-modified alkyd
resin (Worlee Sol 68A); few additives and crosslinking agent
hexa(methoxymethyl)melamine (HMMM) were purchased from
Worlee-Chemie GmbH, Germany. Flash rust inhibitor, long-term
corrosion inhibitor and blocked acid catalyst para-toluene sulphonic acid (p-TSA) were purchased from King Industry, Germany,
and were used in as received condition. Dimethylethanolamine
(DMEA) was used as neutralizing medium.
2.2. Preparation of waterborne coatings
Low molecular weight silicone-modified alkyd resin (acid
value = 35–45 mgKOH/g) was first neutralized with dimethylethylamine (DMEA) followed by continuous addition of deionized
water with time to time stirring till a clear solution was obtained.
The pH of resin solution was maintained between 8.2 and 8.5.
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S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46
Fig. 1. FTIR spectra of resin/HMMM blend in different concentrations.
Calculated amount of crosslinking agent HMMM and p-TSA catalyst was added to the system. In order to avoid formulation
defects due to crucial properties of water, additives such as flash
rust inhibitors, wetting agents, biocides, etc. were added. All the
contents were mixed properly in suitable proportion to form waterborne silicone-modified alkyd-based coating. Neutralization was
done with due care so that the resin solution formed did not
remain water sensitive after formation of coating on the substrate.
2.3. Preparation of nano-composite coatings
Fig. 2. DSC thermogram of neat silicone-modified alkyd resin/HMMM blends in
different concentrations as labeled.
To form a nano-composite, a system with resin HMMM concentration of 70:30 was selected on the basis of FTIR and
DSC studies and different concentrations of ZnO nano-particles
(average particle size ≈40 nm) were added to it (0.05, 0.1,
Fig. 3. FTIR spectra of neat silicone-modified alkyd-based waterborne coating with different concentrations of nano-ZnO.
S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46
41
0.2 and 0.3% by weight). The nano-particle based coatings
were prepared by dip-coating on pretreated cold rolled mild
steel panels (76.2 mm × 152.4 mm × 0.8 mm). The panels after
coating were cured in oven at 130 ◦ C for 30 min and then
cooled to room temperature. The coating thickness measure was
9–10 ␮m.
2.4. Characterization
Fig. 4. DSC thermograms of neat silicone-modified alkyd-based waterborne coating
with different concentrations of nano-ZnO as labeled.
The resin/HMMM blend was characterized using FTIR (Nicolet Magna 550 FT-IR spectrometer) and DSC (nitrogen atmosphere
5–10 ml/min, heating rate of 10 ◦ C, temperature range of 25–250 ◦ C,
using DSC Q10V9.8 Build 296 instrumentation). The nanocomposite coatings were characterized using FTIR, DSC and TGA
(nitrogen atmosphere 5–10 ml/min, heating rate of 10 ◦ C, temperature range of 25–800 ◦ C using SDT Q600V8.3 Build 101
instrumentation). Mechanical properties were studied by Scratch
(BS.3900) and Taber abrasion (ASTM D-3359-02) methods. Surface
studies were done using optical microscopy (model no. GX51.233U,
Fig. 5. SEM surface morphology of unexposed coatings: (a) silicone-modified alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2%
nano-ZnO and (e) with 0.3% nano-ZnO.
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S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46
OLYSIA.M3 software, OLYMPUS) and scanning electron microscopy
(model no. S3400, Hitachi).
3. Result and discussion
3.1. Characterization of resin/HMMM blend
3.1.1. Fourier transform infrared spectroscopic analysis (FTIR)
Fig. 1 shows the FTIR spectra of neat silicone-modified alkyd
resin and HMMM blend in different concentrations of HMMM
(50:50, 60:40, 70:30 and 80:20). It can be seen that a new peak
at 910 cm−1 is also observed which was not initially present in
neat silicone-modified alkyd resin and observed due to N–CH2 –O–R
stretching corresponding to crosslinking reaction between resin
and HMMM as shown in Eq. (1). It is clear that the hydroxyl
group (–OH) on neat resin interacts with melamine alkoxy group
(–N–CH2 –O–CH3 ) of HMMM as studied by Blank [11,12]:
Table 1
Curing behavior during heating scan for silicone-modified alkyd resin and HMMM
blends
Resin:HMMM
Tc (◦ C)
Hc (J/g)
50:50
60:40
70:30
80:20
115.39
105.27
116.03
118.29
109.00
88.33
49.25
90.17
blend and also is an indicator of the content of reacted functional
groups up to a definite time of reaction, at a certain temperature [14]. A sharp exotherm for 70:30 ratio with enthalpy value
(Hc = 49.25 J/g) less as compared to other mixing ratios indicates
the effective crosslinking at this ratio. Hence, based on the FTIR
and DSC studies the mixing ratio for resin and HMMM was fixed to
70:30 at 130 ◦ C for 30 min.
(1)
3.1.2. Differential scanning calorimetric analysis (DSC)
The curing characteristic of silicone-modified waterborne alkyd
resin with HMMM crosslinker was determined by DSC technique
and the corresponding thermogram obtained is shown in Fig. 2.
Table 1 shows the curing behavior during heating scan. For the
resin/HMMM blend, as the mixing ratio increases, a slight shift
in curing temperature (Tc ) is observed, whereas a remarkable
decrease in the enthalpy values is noticed. The degree of curing increases with increasing ratio of melamine resin in the resin
3.2. Characterization of nano-composite resin
3.2.1. Fourier transform infrared spectroscopic analysis (FT–IR)
Fig. 3 shows the overlay of FTIR spectra of silicone-modified
alkyd-based waterborne coating with nano-ZnO particles in different concentrations. From this figure it can be noted that there is
slight shift of FTIR band frequencies after incorporation of nanoparticles. It may be due to interaction of polymer matrix and
Fig. 6. X-ray mapping of unexposed coating (a) with 0.05% nano-ZnO, (b) with 0.1% nano-ZnO, (c) with 0.2% nano-ZnO and (d) with 0.3% nano-ZnO.
S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46
43
Table 2
Curing behavior during heating scan for nano-composite resin
Nano-composite coatings
Exotherm 1
◦
Neat
0.05% nano-ZnO
0.01% nano-ZnO
0.02% nano-ZnO
0.03% nano-ZnO
Exotherm 2
T ( C)
H (J/g)
T (◦ C)
H (J/g)
84.21
85.18
83.56
80.76
71.43
73.47
482.3
258.1
637.1
840.4
93.70
119.8
105.75
124.28
122.79
329.9
111.6
216.5
29.40
10.77
nano-ZnO. The spectrum of silicone-modified alkyd with incorporated ZnO nano-particles shows the characteristic peaks (C O,
C C, C–O) of base polymer matrix, indicating that the basic structure of base polymer matrix is undisturbed after functionalization.
The peak at 1070.9 cm−1 in the functionalized nano-particles is
attributed to Si–O–Si stretching, consistent with that reported
in the standard FTIR spectra. The disappearance of the peak
at 825.7 cm−1 characteristic of Si–O–CH3 and formation of new
peaks at 897.8 cm−1 indicate the complete interaction between the
hydrolyzed ZnO nano-particles surface and silicone-modified alkyd
[13].
Fig. 7. Thermogravimetric (TG) curve of nano-ZnO incorporated silicone-modified
alkyd-based waterborne coatings.
3.2.2. Differential scanning calorimetric analysis (DSC)
Fig. 4 shows the DSC thermogram of neat and zinc oxide
based nano-composite coatings. Table 2 shows the curing behavior during heating scan. From figure it is clear that the curing
temperature (Tcuring ) decreases as compared to the curing temperature of neat coating, also the heat of curing (Hcuring )
value increases (Exotherm 1). This indicates that the interaction
Fig. 8. SEM surface morphology of heat-treated coatings: (a) silicone-modified alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2%
nano-ZnO and (e) with 0.3% nano-ZnO.
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3.3. Characterization of nano-composite coatings
Fig. 9. Taber abrasion of coating as a function of nano-ZnO concentration.
between nano-ZnO and polymer improves the curing of coatings. The curing reaction is followed by liberation of heat that
corresponds to the post-curing reaction (Exotherm 2). This analytical parameter in DSC is called as residual crosslinking enthalpy,
which decreases as the concentration of nano-ZnO increases suggesting improved crosslinking as compared to the neat coating
[14].
3.3.1. Scanning electron microscopy and X-ray mapping
Fig. 5 shows the surface morphology of the unexposed coatings. It clearly shows that the coating is free of pin-hole and
is uniform without any surface heterogeneity. The surface morphologies of the coating (Fig. 5a) and its nano-composite with
different concentrations of nano-ZnO particles (Fig. 5b–e) also
look uniform and pore free. A few white spots uniformly distributed throughout the surface, probably indicate the location of
encrusted nano-particles. Any volume defects or cavities present
in silicone-modified alkyd coating disappear by the incorporation
of ZnO nano-particles. This clearly indicates the establishment of
new interactions between the system components, fact that can
be supported by FTIR analysis. X-ray mapping studies (Fig. 6) also
support the SEM study indicating uniform distribution of nanoZnO with varying loading level of nano-particles. The coating was
then subjected to 350 ◦ C for 10 min followed by water quenching. The process was repeated for 10 cycles. After each cycle the
Fig. 10. SEM surface morphology of abraded coating: (a) silicone-modified alkyd without nano-particles, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2%
nano-ZnO and (e) with 0.3% nano-ZnO.
S.K. Dhoke et al. / Progress in Organic Coatings 64 (2009) 39–46
Fig. 11. Scratch test of the coating as a function of nano-ZnO concentration.
coating was visually observed. The observation showed that there
was no char formation or delamination of the coatings from the
substrate, only a color change was observed from clear coating to
black.
45
3.3.2. Thermogravimetric analysis (TGA)
Thermogravimetric analysis of the coating also confirms the
thermal stability of the coatings. Fig. 7 shows the TG curve obtained
for coating system with and without nano-ZnO particles. It can be
seen that all samples shows weight loss up to 100 ◦ C, this can be
attributed to the loss of water from the system leading to 60–62%
weight loss. Above 100 ◦ C there is no appreciable weight loss up to
350 ◦ C indicating stability of coatings. The cause for this stability
is attributed to the interaction between the resin and large surface
area of nano-ZnO particles, forming a stable nano-composite [15].
This observation was further supported by SEM studies of coated
panels heat treated at 350 ◦ C. Fig. 8 shows the surface morphology
of heat-treated coatings without and with nano-ZnO particles. The
coating without nano-ZnO (Fig. 8a) showed some deformation in
coatings indicating poor response to the repeated heat and quenching cycle. The appearance of uniformly distributed rupture spots in
the coating structure can be attributed to the initiation of oxidative
Fig. 12. Optical microscopic photographs of scratched surface of the coatings (a) without nano-ZnO, (b) with 0.05% nano-ZnO, (c) with 0.1% nano-ZnO, (d) with 0.2% nano-ZnO
and (e) with 0.3% nano-ZnO.
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product formation, this is apparently due to the breakdown of the
cohesive bond between the substrate and the coating followed by
the oxidation of the mild steel substrate. With the incorporation of
nano-ZnO particles no such deformation in coating was observed
(Fig. 8b–e). This can be attributed to the combined effect of siliconemodified alkyd resin and the heat shielding effect of nano-ZnO,
which because of its ceramic nature remains inert to subjected heat.
3.4. Mechanical properties
3.4.1. Abrasion resistance
Taber abrasion test was carried out to determine the abrasion
resistance of coating. The Taber wheel used was CS10, a resilient
type wheel for mild abrading action. Depending upon the thickness
of the coating, a specified number of revolutions were performed
(1000 cycles) with all coated samples and weight loss is evaluated. This weight loss as a function of concentration of nano-ZnO
particles is shown in Fig. 9. From the figure it is clear that with
the increase in the concentration of ZnO nano-particles in coated
sample, the weight loss is gradually being reduced. This can be
explained on the basis of surface roughness. Silicone-modified
alkyd coating without ZnO nano-particles have less dense connecting polymer network, creating a rougher surface. This rough
surface is easy to abrade, but as ZnO nano-particles are introduced
on the coating surface, the roughness of the surface decreases, as
number of polymer connecting particles increases with increasing
concentration of nano-ZnO. Fig. 10 shows the SEM photographs of
abraded coatings. From figure the coating without nano-particles
(Fig. 10a) show complete abraded surface morphology, where the
substrate is clearly visible at the weared area. The incorporation
of ZnO nano-particles into the coating changes the morphology
of the coating due to enhanced interaction of ZnO nano-particles
with resin structure, as a result of which coating seems to be more
compact and less abraded as compared to the coating without
nano-ZnO. This compactness of coating goes on increasing with an
increase in concentration of nano-ZnO (Fig. 10b–e). The improvement in abrasion resistance is attributed to the combined effect
of HMMM crosslinked resin and nano-zinc oxide. Also, it can be
said that the catalytic action of zinc on the curing reaction helps in
forming a hard and complex coating network. This indicates that
the interfacial surface interaction between nano-ZnO and the base
matrix is strong which is responsible for the improvement in the
mechanical properties of the base matrix.
3.4.2. Scratch resistance
Fig. 11 shows the variation of load (weight in grams) with different concentrations of ZnO nano-particles during scratch test.
It was found that as the concentration of nano-ZnO in coating
sample increases, the scratch resistance property of the sample
also increases which means that the tendency of deformation of
coated surface will be at higher weight than the reference polymer, hence an ascending curve is obtained for scratch study. This
can be attributed to the more strong bonding network between
silicone-modified alkyd coating and nano-ZnO particles which provides more resistance to scratch causing less deformation in the
sample [9,16].
The shape of the scratch tracks serve as an indicator for severity of deformation as well as mechanism behind the scratching.
The scratch interface is subjected to an extremely high shear strain
and whenever shear strain exceeds the yield strain, the polymer
starts deforming [17]. Fig. 12 shows the optical microscopic photographs of the scratched surface. During scratch testing of coated
materials, ridges are formed along the side of scratch. This is due to
plastic deformation of the coating. The size of the ridges is directly
proportional to the normal load used for scratching. From optical
study, it is clear that as the concentration of ZnO nano-particles
is increased in coating sample, scratch tracks are found to be less
deformed than neat coating, indicating improvement in the scratch
resistance property of the coating.
4. Conclusion
The effect of nano-ZnO on a silicone-modified alkyd-based
waterborne coating was studied. The coating obtained had
enhanced heat stability and mechanical properties, which is quite
large as compared to the conventional silicone-modified alkyd
resin. A coating system with higher loading of nano-ZnO (0.3 wt.%)
showed better performance. Optimization of nano-ZnO loading in
polymer matrix or synergism with other nano-oxide particles can
further improve these properties. The coating thus formed can
serve as a good scratch, abrasion and heat-resistant coating and can
find applications in various automotive industries, smoke stacks,
stoves, furnaces, heaters and incinerators and other practical situations where the metal is subjected to high temperature.
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