Preparation and Characterization of Structure and Corrosion Resistivity of Polyurethane /Montmorillonite/Cerium Nitrate Nanocomposites

Document Type: Research Paper

Authors

1 1Department of Technical Inspection, Petroleum University of Technology, Abadan, Iran

2 2Technical Faculty, South Tehran Branch, Islamic Azad University, Tehran, Iran

Abstract

In this study, nanocomposite coatings based on polyurethane cerium nitrate montmorillonite (MMT) were prepared, applied on carbon steel substrates, and investigated. The nanocomposite coatings were successfully prepared by the effective dispersing of nanoparticles in polyurethane resin by mechanical and sonication processes. The state of dispersion, dissolution, and incorporation were characterized by optical microscopy, sedimentation tests, and transmission electron microscopy. The structure and properties of the nanocomposite coatings were investigated by X-ray diffraction and anticorrosive properties of the nanocomposites were studied by Tafel polarization measurements. The experimental results showed that the PU/MMT/Cerium nitrate nanocomposite coatings were superior to the neat PU in corrosion protection. In addition, it was observed that the corrosion protection of the nanocomposite coatings was improved as the clay and cerium nitrate loadings were increased to 4 wt.% to 2 wt.% respectively.

Keywords


Preparation and Characterization of Structure and Corrosion Resistivity of
Polyurethane /Montmorillonite/Cerium Nitrate Nanocomposites
Iman Moghri1, Mansoor Farzam1,*, Mohammadreza Shishesaz1, and Davood Zaarei2
1Department of Technical Inspection, Petroleum University of Technology, Abadan, Iran
2Technical Faculty, South Tehran Branch, Islamic Azad University, Tehran, Iran
Received: January 25,2014; revised: June 29,2014; accepted: September 15,2014
Abstract
In this study, nanocomposite coatings based on polyurethane cerium nitrate montmorillonite (MMT)
were prepared, applied on carbon steel substrates, and investigated. The nanocomposite coatings were
successfully prepared by the effective dispersing of nanoparticles in polyurethane resin by mechanical
and sonication processes. The state of dispersion, dissolution, and incorporation were characterized by
optical microscopy, sedimentation tests, and transmission electron microscopy. The structure and
properties of the nanocomposite coatings were investigated by X-ray diffraction and anticorrosive
properties of the nanocomposites were studied by Tafel polarization measurements. The experimental
results showed that the PU/MMT/Cerium nitrate nanocomposite coatings were superior to the neat PU
in corrosion protection. In addition, it was observed that the corrosion protection of the
nanocomposite coatings was improved as the clay and cerium nitrate loadings were increased to 4
wt.% to 2 wt.% respectively.
Keywords: Cerium Nitrate, Nanocomposite Coating, Montmorillonite, Polyurethane, Corrosion
1. Introduction
Polyurethane is one of the most interesting synthetic materials in industry, which is being extensively
used for different applications such as coatings, adhesives, construction, fibers, and foam because of
its high flexibility, good processability, excellent water resistance, good resistance to acids and
solvents, better alkaline resistance than most of other polymers, good abrasion resistance, and good
mechanical properties. Its synthesis, morphology, chemical, and mechanical properties have been the
subject of a great deal of attention in recent studies (Wu et al., 2001; Jeong et al., 2000; Meincken et
al., 2006).
Polyurethane (PU) is extensively used in protective coatings as a resin; however, this resin has some
disadvantages such as low thermal stability and barrier properties (Rehab et al., 2005). These
properties can be modified by adding fillers such as montmorillonite organoclay to the polymer
matrix. The incorporated nanoparticles into PU matrix results in improving coating properties such as
adhesion, pendulum hardness, barrier properties, thermal stability, and corrosion resistance (Ahmadi
et al., 2007; Lv et al., 2008; Choi et al., 2004; Špírková et al., 2008; Chen-Yang et al., 2005; Chen-
Yang et al., 2007).
* Corresponding Author:
Email: farzam@put.ac.ir
I. Moghri et al./ Preparation and Characterization of Structure and Corrosion … 101
Clay is nontoxic, cheap, available, and environmentally-friendly. It is a special type of layered
silicate, which has been commonly used for the fabrication of nanocomposite coatings in recent years
(Theng et al., 2012; Zhang et al., 1993; Pinnavaia et al., 2000; Zaarei et al., 2009). In the 1980’s, the
technology of clay nanocomposite was submitted by the Toyota Corporation (Liu et al., 2007).
Researchers showed that the incorporation of a small amount of clay (1–4%) into the polymer matrix
can improve anticorrosion, barrier, thermal, and mechanical properties, and it also decreases liquids
and gases permeability in comparison to pure polymeric coatings (Zaarei et al., 2008).
Obtaining a stable dispersion in which the silicate nanolayers are completely exfoliated in the
polymeric matrix is an important factor in attaining the best properties of polymer–clay
nanocomposites; therefore, during nanocomposite preparation, the evaluation of the morphology of
these nanocomposites is crucial (Giannelis et al., 1996; Fornes et al., 2001; Samyn et al., 2008; Ratna
et al., 2006; Wagener et al., 2003).
For a long time, chromates have been widely applied as an effective corrosion inhibitor in coatings.
Surface pretreatments are based on chromates because of their high protection efficiency. However,
due to increased environmental concerns, these pretreatments will be prohibited because of the
toxicity and carcinogenic of chromates. Therefore, new alternative is substituted. Rare earth salts have
been used as the new corrosion inhibitors. Cerium is one of these alternatives, which can reduce the
corrosion rate of metals by inhibiting cathodic reactions. The first studies on this field are those of
Hinton, who found that cerium nitrate was effective in reducing the corrosion rate of aluminum alloys
(Montemor et al., 2001).
Subsequent researches showed that cerium nitrate also has healing abilities and improves the
corrosion resistance (Cabral et al., 2006).
2. Experimental
2.1. Material
The organoclay used in this study was modified MMT clay (Closite 20A; dimethyl, dehydrogenated
tallow, quaternary ammonium–modified MMT) with a particle size of 2–13 μm, a layer thickness of 1
nm, and was received from Southern Clay Company (Gonzales, TX, USA). Cerium (III) nitrate was
provided from Scharlab, Spain. Ethanol was supplied by Merck 100983 (Germany) as the solvent.
Polyurethane resin, Methylene diphenylene diisocyanate was provided from Bajak Paint Company.
2.2. Nanocomposites preparation
For the preparation of the compositions, 1-3 grams of cerium nitrate was dissolved in 10 ml xylene at
27 °C and mixed for 15 min; MMT was then added to PU resin according to weight percent of the
solid parts of the paints. PU, MMT, and cerium nitrate-modified xylene were mixed mechanically
with a high-shear mixer at 1200 rpm for 2 hrs; these mixed compositions were then sonicated with a
high-powered sonication instrument for 40 min with an external cooling bath to prevent increasing the
temperature of the composition. The ultrasonication process was performed at a frequency of 20 kHz
with an inlet ultrasound power of around 1 W/ml (UIP 1000hd ultrasonic processor, Hielscher
ultrasound technology). The sonicated compositions, which include different percentages of clay and
cerium nitrate, are shown in Table 1. For degassing, the samples were held in a vacuum oven for 30
min at 50-60 °C, and the compositions of tiny bubbles were removed. Finally, hardener was added to
composite with the mass ratio of 1:4 with respect to PU and mechanically mixed again.
102 Iranian Journal of Oil & Gas Science and Technology, Vol. 5 (2016), No. 2
Table 1
Designation of formulations based on particles compositions.
Nanocomposite coatings
Montmorillonite (MMT) pigments
(wt.%)
Cerium nitrate (CN) pigments
(wt.%)
0C0CN 0 0
0C2CN 0 2
2C2CN 2 2
3C2CN 3 2
4C0CN 4 0
4C1CN 4 1
4C2CN 4 2
4C3CN 4 3
2.3. Coating application
SAE 1010 carbon steel panels (15cm×8cm×0.2cm) were used as metallic substrates. The panels were
subjected to a sequence of chemical cleaning and mechanical surface polishing with emery papers
from #400 to #1000 to remove any trace of surface oxides and were kept in the desiccator. Prior to
coating application, the panels were extensively cleaned with acetone and toluene. The coatings were
applied by means of a film applicator. The thickness of dry coatings was measured with Elcometer FN
4653 digital coating thickness meter (Elcometer Co. Ltd.), and was chosen in the range 50-60 μm.
The panels were kept in the laboratory atmosphere for two weeks before beginning the tests to ensure
the complete curing of the coatings.
2.4. Nanocomposites structural characterization
The optical homogeneity of the nanocomposite dispersions and the effect of sonication process on deagglomeration
of clay aggregates were examined using an Olympus Bhzzuma optical microscope.
The optical micrographs of the samples were obtained after 2 hrs mechanical mixing and 40 min
sonication.
The suspensions stability was analyzed by a sedimentation method. The blends were placed at 128 °C
for 2 hrs to observe the amounts of the clay precipitated.
XRD analysis was carried out with a Philips model X´PERT MPD X-ray diffractometer with Cu Kα
radiation (λ=1.54Å) operating at 40 kV and 40 mA for the evaluation of the intercalation/exfoliation
of nanoclay in the polymer matrix. The diffraction patterns were obtained in the 2θ range 0.5-10° at a
rate of 0.5°/min.
Transmission electron micrographs (TEM) technique was used to analyze the morphology of the bulk
samples and to evaluate the state of the dispersion. The TEM samples were prepared by cutting the
cured nanocomposites by an ultra-microtome instrument (OMU3, Reichert, Austria) that was
equipped with a diamond cutter. The thickness of the TEM samples was about 70–100 nm. Then, the
samples were put on 300-mesh copper grids. The TEM images used is of bright field type and is taken
with a Philips-EM208 at an acceleration voltage of 100 kV.
The coating initial adhesion and the adhesion retained after exposure in a 5 wt.% aqueous NaCl
solution were measured by using a direct pull-off adhesion test method in accordance with ASTM
D4541 type III (self-aligning adhesion tester). The dollies with an area of 0.5 cm2 were bonded to the
I. Moghri et al./ Preparation and Characterization of Structure and Corrosion … 103
coating using an appropriate adhesive (Cyanoacrylate MC1500). In the case of adhesion measurement
after exposure, the samples were removed from the salt spray chamber at the end of 1000 hrs
exposure, rinsed completely with deionized water, and allowed to dry for 48 hrs at ambient
temperature. The glued dollies were then allowed to dry for 48 hrs at room temperature. A digital
adhesion tester (Elcometer 108, Elcometer Co. Ltd.) was employed. To ensure reproducibility and to
statistically characterize the test area, all the measurements were obtained from at least three
experiments.
The Tafel plots were obtained after 20 days of immersion at a potential of 200 mV higher and lower
of the open circuit potential (Eocp) and at a scanning rate of 0.5 mVs-1. The interpretation of impedance
data was performed using Autolab Frequency Response Analyzer (FRA) software.
3. Results and discussion
3.1. Optical microscopy results
Figure 1 presents the optical micrographs of the suspensions having 4 wt.% clay and 2 wt.% cerium
nitrate after mechanical agitation and sonication processes. The dispersion which was mechanically
agitated for 2 hrs, contained a lot of MMT agglomerates (see Figure 1a). These agglomerates are
formed during the wetting of MMT with polyurethane due to cohesive forces between clay stacks.
When sonication process was applied to the suspension for 30 min, size and quantity of the
agglomerates were greatly decreased and the distribution of clay was improved (see Figure 1b). These
results show that sonication process is an effective method to de-agglomerate and disperse the
nanocomposite mixture.
a)
104 Iranian Journal of Oil & Gas Science and Technology, Vol. 5 (2016), No. 2
b)
Figure 1
Optical micrographs of 4c2cn: a) after 2 hrs of mechanical agitation and b) after 30 min of sonication.
3.2. Stability
Sedimentation was visually observed for the mixtures that were held at 50 °C for nearly 2 weeks.
Sediments could be seen in the matrix with an insufficient dispersion and intercalation of the clay
particles. An insufficient dispersion process caused sedimentation, but using ultrasonic mixing
improved the distribution of particles and prevented the agglomeration of the clay particles. Table 2
shows the sedimentation results of the mechanical mixing and ultrasonic of particles within the
matrix.
Table 2
Results of sedimentation test of different samples.
Composition Sedimentation after high shear
mixing
Sedimentation after
ultrasonication
0C0CN - -
0C2CN - -
2C2CN + -
3C2CN + -
4C0CN + -
4C1CN + -
4C2CN + -
4C3CN + -
3.3. XRD
The state of intercalation or exfoliation of nanoclay structure in PU and the degree of cerium nitrate
dissolution and clay sheets separation in the polymer film were analyzed using wide angle X-ray
diffraction (WAXD) patterns as presented in Figure 2. The d-spacing can be obtained from Bragg’s
equation, as well as from the angle of maximum intensity. The neat MMT has a d-spacing of 2.42 nm
I. Moghri et al./ Preparation and Characterization of Structure and Corrosion … 105
and cerium nitrate has a d-spacing of 4.204 nm. According to the XRD patterns, the neat cerium
nitrate (CN) exhibits the reflection peak at 2.1°. On the other hand, the peak intensity of the sample
0C2CN is less remarkable. Thus it is concluded that cerium nitrate is dissolved in the matrix.
The increments of d-spacing for 4C0CN and 4C2CN clays caused by high-shear mixing and highintensity
ultrasound were 49.02 and 46.44 nm respectively. However, it can be seen from Figure 2
that the intensity of the peak is less remarkable because of the small amount of clay.
Figure 2
XRD patterns of the neat MMT, neat polyurethane, and polyurethane/cerium nitrate/MMT nanocomposites.
3.4. TEM
Figure 3 shows the TEM images of the 4C2CN sample, where the dark areas show clay platelets and
the gray areas represent the polymeric matrix. The dominant morphology was intercalation. The main
reason for this morphology was the method of nanocomposite preparation (Malucelli et al., 2009).
The morphology plays an important role in the properties of the final nanocomposite (Giannelis et al.,
1996; Fornes et al., 2001). According to the TEM results, it can be concluded that the method used for
nanocomposite preparation was an acceptable method for stacks decrease. The separation of clay
layers indicates the intercalation of clay layers. Platelet spacing indicated by TEM images shows that
the trend was confirmed by the XRD results.
106 Iranian Journal of Oil & Gas Science and Technology, Vol. 5 (2016), No. 2
Figure 3
TEM micrographs of nanocomposite 4c2cn.
3.5. Tafel polarization study
The corrosion protection of the nanocomposite coatings on mild steel plates was evaluated by Tafel
extrapolating method. This was carried out through the determination of polarization resistance (RP)
and corrosion current (Icorr) after the immersion of the coated samples in a 3.5 wt.% NaCl solution for
20 days. The Tafel diagrams of the different scratched coatings for 20 days are shown in Figure 4.
Tafel calculations are listed in Table 3, where Icorr, CR, βa, βc, and RP are the corrosion current,
corrosion rate, anode Tafel constant, cathode Tafel constant, and polarization resistance respectively.
It can be noticed from Figure 4 and the results presented in Table 3 that the coating with 4 wt.%
nanoclay and 2 wt.% cerium nitrates has the lowest Icorr and thus the best corrosion resistance, which,
as mentioned before, could be a result of the increase in the tortuosity of the diffusion pathways of
water and oxygen molecules due to the presence of the clay and cerium nitrate; it appears that the
action of particles as barrier to corrosive attacks can also be responsible for increasing the corrosion
resistance of nanocomposite coatings.
Table 3
Polarization plots obtained after different times of immersion in a 3.5% NaCl solution after 20 days.
Sample Icorrosion βc βa Rp CR
0C0CN 9.749×10-7 0.066 0.202 2.956×103 1.145×10-2
2C2CN 7.978×10-7 0.076 0.275 5.696×103 9.370×10-3
3C2CN 5.745×10-7 0.086 0.286 9.289×103 9.748×10-3
4C0CN 1.020×10-6 0.087 0.289 5.353×103 1.198×10-2
4C1CN 4.722×10-7 0.093 0.325 1.397×104 5.547×10-3
4C2CN 1.243×10-7 0.088 0.216 3.319×104 1.460×10-3
4C3CN 3.935×10-7 0.074 0.209 8.518×103 4.622×10-3
I. Moghri et al./ Preparation and Characterization of Structure and Corrosion … 107
Figure 4
Tafel polarization curves measured after 20 days immersion in a 3.5 wt.% NaCl solution.
3.6. Pull-off test measurements
Pull-off adhesion test method was employed to determine the adhesive strength of the coating. In this
way, adhesion is quantified in terms of the forces employed to detach the test dollies glued to paint
film from the underlying metal. Table 4 shows the results of the adhesion tests for the coated samples
prior to exposure and those removed at the end of 1000 hours of exposure. In the case of pull-off
adhesion test, before the exposure of the coated sample to a corrosive environment, it was observed
that almost all of the nanocomposite coatings had good adhesion to carbon steel, and amongst them,
4C2CN had the best performance. Table 4 shows that as nanoclay and cerium nitrate increase,
adhesion in the nanocomposite increases. This improvement in adhesion may be due to the fact that
nanoclay fills the voids and crevices in the steel substrates and the polymeric matrix, and thereby
improving the adhesion between the coating and the substrate (Allie et al., 2008).
Generally, the forces needed to detach the film from the samples exposed to a 5 wt.% NaCl aqueous
solution was smaller than those measured for the non-exposed samples, as shown in Table 4. Such a
decrease is quite significant for the pure PU, whereas it is very small in the case of the
PU/MMT/cerium nitrate nanocomposite coatings. Moreover, the reduction percentage in the
nanocomposite coatings adhesion is decreased as the clay concentration is increased. As it was
concluded from the previous results, the penetration of water molecules and aggressive ions are
108 Iranian Journal of Oil & Gas Science and Technology, Vol. 5 (2016), No. 2
decreased by the incorporation of clay layers into the PU matrix. Hence a high reduction in neat PU
adhesion to substrate may be due to the penetration of molecules into the coating, which can loosen
the adhesion of the coating to the substrate and deteriorate other mechanical properties of the coating.
Furthermore, the release of Ce (III) and the prevention of electrolyte from touching the uncoated area
of the substrate is the other factor affecting the adhesion improvement (Nematollahi et al., 2010). This
effect is decreased by increasing the cerium nitrate loading up to 2 wt.% in the PU matrix. Table 4
also shows that the wet adhesion of the coating containing 3 wt.% cerium nitrate is lower than that of
the coating containing 2 wt.% cerium nitrate. This indicates that an excess of cerium nitrate in the film
leads to a negative effect on the interfacial adhesion, and a premature film delamination from the
substrate happens (Turner et al., 2002).
Table 4
Pull-off adhesion test results (the data are expressed as the stress required to detach the film from the metallic
substrate in MPa).
Sample characteristic Before exposure After1000 hours Decrease in Adhesion (%)
0C0CN 10.9 7.45 31.65
0C2CN 11.73 10.61 9.54
2C2CN 11.65 10.44 10.38
3C2CN 12.02 11.21 6.73
4C0CN 11.23 10.12 9.88
4C1CN 12.16 11.19 7.97
4C2CN 12.67 12.06 4.81
4C3CN 11.98 11.22 6.34
4. Conclusions
1. The results of the optical microscopy and sedimentation test indicated that the sonication
process was an effective method to separate nanoparticle stacks and prevent agglomeration,
and thereby forming a stable suspension.
2. The results of the XRD and TEM indicated that the clay particles were dispersed and
intercalated but not fully exfoliated into the PU matrix.
3. The results of a series of Tafel tests showed a superior corrosion protection on carbon steel
panels compared to that of neat PU. Also, the addition of cerium nitrate into the polyurethane
demonstrated good corrosion inhibition on carbon steel substrates and improved the resistance
of scratched areas against corrosion.
4. The presence of nanoclay and cerium nitratedecreased the penetration of corrosive agents and
corrosive solution, and it enhanced the wet adhesion of the coating to substrate (7.45 MPa in
the neat polyurethane compared to 12.06 MPa in the 4C2CN sample nanocomposite).
5. The steel panels coated by the nanocomposites containing 4 wt.% MMT and 2 wt.% CN
showed the highest corrosion resistance among the coating formulations.
Nomenclature
βa : Cathodic Tafel constant
βc : Cathodic Tafel constant
CN : Cerium nitrate
CR : Corrosion rate
I. Moghri et al./ Preparation and Characterization of Structure and Corrosion … 109
Icorrosion : Corrosion current
MMT : Montmorillonite
PU : Polyurethane
Rp : Polarization resistance
TEM : Transmission electron microscopy
XRD : X-Ray diffraction
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