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Effect of particle sizes and pigment volume concentrations on the barrier properties of polyurethane coatings.

Abstract The barrier properties of [Fe.sub.2][O.sub.3]/polyurethane coatings containing 60-nm and 150-nm [Fe.sub.2][O.sub.3] particles with different pigment volume concentrations (PVCs) were investigated by electrochemical impedance spectroscopy and accelerated salt spray tests. The results indicate that the addition of 60-nm [Fe.sub.2][O.sub.3] can significantly improve the barrier properties of a coating, while 150-nm [Fe.sub.2][O.sub.3] has little effect. The 0.2% PVC 60-nm [Fe.sub.2][O.sub.3] coating had significantly improved protection.

Keywords PVC, Nanometer, Ferric oxide, Polyurethane coatings, EIS, Barrier


The application of nanomaterials brings new opportunities to the coatings industry. (1), (2) Coating properties can be markedly improved by replacing conventional pigments with nanometer particles, and this can even result in some new properties. In contrast to micro-[Fe.sub.2][O.sub.3], adding nano-[Fe.sub.2][O.sub.3] to the coatings not only improves their coloring ability, transparency, and UV resistance but also results in new properties, including antibacterial and self-cleaning properties. However, little research has been carried out on the effect of nano-[Fe.sub.2][O.sub.3] on barrier properties.

Pigment volume concentration (PVC) has significant effects on the properties of coatings, (3) including a great effect on the transport of corrosive species of electrolytes through them. The coatings with pigments such as micaceous iron oxide or zinc with different PVCs have been studied. (4), (5) Electrochemical impedance spectroscopy (EIS) is a suitable method to determine the critical pigment volume concentration (CPVC) of coatings, especially for nanocomposite coatings; CPVC of which is difficult to be determine by scanning electron microscopy (SEM) analysis. (6-8)

The purpose of this work was to correlate the pigment particle size and PVC with the barrier properties. [Fe.sub.2][O.sub.3]/polyurethane coatings with different PVCs and two particle sizes were prepared by a two-step dispersion method to form multifunctional coatings. The anticorrosion properties were evaluated by means of EIS and accelerated salt spray tests.

Experimental method

Preparation of substrates

Washed cold-rolled steel test panels (45#) were used to ensure uniform surfaces for testing. They were mechanically polished, degreased with acetone, washed with alcohol, and kept in a desiccator prior to testing.

Preparation of coatings

Polyurethane resin (hydroxyl bearing polyacrylate, Desmophen[R] A 365 BA/X from Bayer Material Science AG, nonvolatile content: 65%, hydroxyl content: 2.9%) was used as a matrix, and the particle size of pigment in the coatings was on the scale of nanometers and micrometers. The morphologies of the two pigments (Micrometer red ferric oxide S360 from Shanghai Ferric Oxide Factory, China and nanometer red ferric oxide from Zhejiang Genkey Chemical Co. Ltd., China) in the resin matrix are shown in Fig. 1. The mean particle size of the two kinds of [Fe.sub.2][O.sub.3] was about 60 and 150 nm, respectively, and their specific surface area was 104.9 and 3.5 [m.sup.2]/g, respectively. The polyurethane coatings containing [Fe.sub.2][O.sub.3] were pigmented with PVCs of 0, 0.2, 1, 6, and 11%.


The nano-[Fe.sub.2][O.sub.3]/polyurethane coatings were prepared as follows, and the microcoatings were prepared in the same way. (1) Preparation of nano-[Fe.sub.2][O.sub.3] pastes: Nano-[Fe.sub.2][O.sub.3] particles were added to the mixture of the dispersant (Block copolymer solution, HX-4800 from Guangzhou Huaxia Paint Industry Co. Ltd., China) and butyl acetate. Subsequently, the mixture was dispersed for 10 min at 1000 rpm (Dispersing machine GFJ-0.4 from Shanghai Modern Environment Engineering Technique Company, China). The ratio of the pastes to the ball was maintained at 1:1. Finally, ball milling (Planetary ball mill GM-3SP4 from Nanjing University Instrument Plant, China) was conducted at 200 rpm for 8 h using agate balls of 10 mm diameter. (2) Preparation of nano-[Fe.sub.2][O.sub.3]/polyurethane coatings: The [Fe.sub.2][O.sub.3] pastes were added to polyurethane resin with different PVCs and milled as in step (1) for half an hour. Then, the curing agent (Curing agent, Desmodur[R] N75 MPA/X from Bayer Material Science AG, nonvolatile content: 75%, NCO content: 16.5%) was added and mixed evenly, the mixture was coated onto the substrate, and cured for 8 h at 80[degrees]C. The dry film thickness of the above coatings ranged from 56 to 64 [micro]m.

X-ray diffraction and analysis of particle sizes

The composition of the as-received [Fe.sub.2][O.sub.3] powder was analyzed with the Phillips PW1700 X-ray diffractometer with settings of 40 kV, 30 mA, and with a CuK[[alpha].sub.1] target.

The size distribution of the powder was measured with Zetasizer 3000HS, after ultrasonic treatment of the as-received powder in ethanol for 10 min.

Fourier transform infrared (FTIR) and differential scanning calorimetric (DSC) measurements

To study the interaction of 60-nm particles and 150-nm particles with base matrix, respectively, their curing behavior before and after addition of 60-nm particles and 150-nm particles was investigated by FTIR and DSC. The FTIR spectra were collected by a VERTEX 70 spectrometer (BRUKER, Germany) from 4000 to 500 [cm.sup.-1], with a spectral resolution of 4 [cm.sup.-1] and a scanner velocity of 10 kHz. The ATR crystal was a zinc selenide (ZnSe) prism. All spectra were analyzed using the OPUS software. DSC measurements were conducted by PERKIN-ELMER 7 Series thermal analysis system. The temperature ranged from -30 to 200[degrees]C, and the elevated rate is 10 [degrees]C/min.

Salt spray test

The salt spray test was an accelerated laboratory procedure for evaluating the corrosion performance of coatings. The panels were sealed on the edges and the reverse side by adhesive tapes so as to maintain a fixed coated exposed area. The tests were performed according to ASTM B117 protocol at 35[degrees]C by spraying 5% NaCl solution for 600 h.

Electrochemical testing

A three-electrode cell was used for EIS. The substrate of the working electrode was coated so as to expose an area of 12.56 [cm.sup.2]. A saturated calomel electrode was used as the reference electrode, and the counter electrode was stainless steel. EIS experiments were conducted at appropriate corrosion potentials. Samples were exposed to 3.5% NaCl solution, and the impedance measurements were made at different immersion times. The EIS experiments were performed with an EG&G263 potentiostat and an M5210 lock-in amplifier. The frequency range was 100 kHz down to 10 mHz, and the amplitude of the signal was 20 mV. The EIS data were analyzed by the Equivert software developed by Boukamp. Two equivalent circuits were used: one is related to initial stage of the immersion test as shown in Fig. 2a and the other is related to an intermediate stage of the immersion test as shown in Fig. 2b.


Results and discussion

Characterization of 60-nm [Fe.sub.2][O.sub.3] and 150-nm [Fe.sub.2][O.sub.3] particles

Figure 3 shows the X-ray diffraction pattern of 60-nm [Fe.sub.2][O.sub.3] and 150-nm [Fe.sub.2][O.sub.3] particles. These two kinds of [Fe.sub.2][O.sub.3] particles have the same crystalline mineral, Hematite; the peak width of the 60-nm [Fe.sub.2][O.sub.3] particles is obviously larger than that of 150-nm [Fe.sub.2][O.sub.3] particles because of grain refinement.


Figure 4 shows the size distributions of the 60-nm [Fe.sub.2][O.sub.3] and the 150-nm [Fe.sub.2][O.sub.3] particles. According to peak analysis by volume, there are two peaks for 150-nm [Fe.sub.2][O.sub.3] at 48.6 nm (14.4 area%) and 281.5 nm (85.6 area %), respectively. The overall volume mean is 247.9 nm. There is only one peak for nano-[Fe.sub.2][O.sub.3], and the mean volume is 67.8 nm. This shows that the mean volume of 150-nm [Fe.sub.2][O.sub.3] particles is almost four times greater than that of 60-nm [Fe.sub.2][O.sub.3] particles, and distribution of 60-nm [Fe.sub.2][O.sub.3] particles is more homogeneous than that of 150-nm [Fe.sub.2][O.sub.3] particles. This result is consistent with the existence of grain refinement in 60-nm [Fe.sub.2][O.sub.3] particles.


Characterization of 60-nm and 150-nm composite polyurethanes

FTIR study

Figure 5 shows the FTIR spectra of (a) 60-nm [Fe.sub.2][O.sub.3] and 150-nm [Fe.sub.2][O.sub.3] particles, (b) neat polyurethane (PVC = 0), and 60-nm [Fe.sub.2][O.sub.3]/polyurethane composite and 150-nm [Fe.sub.2][O.sub.3]/polyurethane at PVCs of 0.2, 1, 6, and 11%. From Fig. 5a, the broad band with low intensity at 3420 [cm.sup.-1] indicates the presence of hydroxyl group on iron oxide surface. According to peak area at 3420 [cm.sup.-1], the number of hydroxyl groups for 60-nm [Fe.sub.2][O.sub.3] particles per area is three times greater than that for 150-nm [Fe.sub.2][O.sub.3] particles per area. Bands at about 531, 561, 447, and 478 [cm.sup.-1] correspond to Fe-O stretching vibrations. The spectrum of polyurethane-incorporated [Fe.sub.2][O.sub.3] particles shows the characteristic peaks of base polymer matrix; for example, the bands at 2935 and 2857 [cm.sup.-1] are attributed to C-H stretching vibrations, the band at 1724 [cm.sup.-1] is attributed to C=O stretching vibrations, and the 1250, 1460, and 1518 [cm.sup.-1] bands are attributed to the C-O-C, C-H, and CO-NH stretching vibrations, respectively. This result indicates that the basic structure of base polymer matrix is not changed after the addition of [Fe.sub.2][O.sub.3] particles. The wide band at 3385 [cm.sup.-1] is N-H or O-H stretching vibration of base polymer matrix, which is not affected by O-H stretching vibration of incorporated [Fe.sub.2][O.sub.3] particles. This result suggests that there is an interaction between polymer matrix and [Fe.sub.2][O.sub.3] particles, especially for 60-nm [Fe.sub.2][O.sub.3] particles due to a greater number of hydroxyl groups on the surface.


DSC study

Figure 6 shows DSC thermogram of neat coating (PVC = 0), 60-nm [Fe.sub.2][O.sub.3], and 150-nm [Fe.sub.2][O.sub.3] composite coatings. Table 1 shows the curing behavior during heating scan for neat polyurethane coatings, 60-nm [Fe.sub.2][O.sub.3]/polyurethane, and 150-nm [Fe.sub.2][O.sub.3]/polyurethane composite coatings. From Fig. 6, it can be seen that the addition of 60-nm [Fe.sub.2][O.sub.3] and 150-nm [Fe.sub.2][O.sub.3] particles can influence the curing behavior of the coating systems. For all coatings, smooth thermograms are obtained and consist of exotherms during the curing processes. According to Table 1, for 60-nm [Fe.sub.2][O.sub.3] particle-modified coatings, the curing temperature ([T.sub.c]) decreases with the increase of the PVC. The heat of curing ([[DELTA]H.sub.c]) increases for 0.2% PVC and decreases for 11% PVC. It is possibly due to an increase of addition of nano-[Fe.sub.2][O.sub.3] particles as it can decrease the resin amount relatively. For 150-nm [Fe.sub.2][O.sub.3] particles, with the increase of PVC, there is no obvious change of the curing temperature. Regardless of how many 150-nm [Fe.sub.2][O.sub.3] particles are added in polymer matrix, the heat of curing ([[DELTA]H.sub.c]) decreases evidently. It shows that 60-nm particles can interact with polyurethane to promote the cross-linking reaction at lower temperature.

Table 1: Curing behavior during heating scan for neat polyurethane
coatings, 60-nm [Fe.sub.2][O.sub.3]/polyurethane, and 150-nm
[Fe.sub.2][O.sub.3]/polyurethane composite coatings

Sample type                           [T.sub.c]  [DELTA][H.sub.c]
                                   ([degrees]C)             (J/g)

PVC = 0% (neat polyurethane)             127.61            -48.79

PVC = 0.2% (60-nm                        123.19            -50.65

PVC = 11% (60-nm                         106.09            -26.77

PVC = 0.2% (150-nm                       127.39            -37.91

PVC = 11% (150-nm                        129.21            -35.06

Barrier properties of polyurethane coatings pigmented with 60-nm [Fe.sub.2][O.sub.3]

EIS analysis of 60-nm [Fe.sub.2][O.sub.3]/polyurethane coatings pigmented with different PVCs

EIS is commonly used to evaluate the barrier properties of protective coatings, and the results are commonly presented as Bode and Nyquist plots. A Bode plot comprises two curves: the impedance curve and the phase angle curve, which are measured over a range of frequency. The high-frequency region mainly provides information about coating defects and other changes in the coating surface, and the low-frequency region provides information about the electrochemical processes occurring at the interface between the substrate and the coated layer. A Nyquist plot provides information on the diffusion process occurring in the coating system. (9)

The Bode plots of 60-nm [Fe.sub.2][O.sub.3]/polyurethane coatings with different PVCs immersed in 3.5% NaCl solution are shown in Figs. 7-11.


The Bode plots of [Fe.sub.2][O.sub.3]/polyurethane coatings without pigments are shown in Fig. 7. The spectrum was fitted by a model which was composed of the coating capacitance, [C.sub.p], in parallel with the resistance, [R.sub.p], and initially there was only one time constant in the spectrum, indicating relatively good paint homogeneity. With increasing exposure time, the coating layer was gradually attacked and thinned down by the electrolyte, which led to a decrease in the coating resistance and a shift of the phase angle toward a higher frequency. The electrolyte finally penetrated the coating layer to attack the substrate, which was indicated by the appearance of a second time constant.

As shown in Figs. 8 and 9, the Bode plots of the coatings with the PVC of 0.2% and 1% exhibited a linear increase of log|Z| with respect to log f, which indicates that the coatings can be considered as a dielectric. The magnitude of the corrosion resistance, [R.sub.p], and the value of the phase angle remained high even after 840 h of immersion, which indicates good anticorrosion performance. This could be because there was no path in the coatings that allowed the electrolyte to reach the substrate. From these figures we can conclude that, for the duration of these tests, the substrate with the PVC of 0.2% and 1% did not corrode, and the coating remained in good condition.



Figures 10 and 11 show the EIS of the coaling with the PVCs of 6% and 11%, respectively, in which the curves show two distinctive segments. The Bode plot in the high-frequency region exhibits a linear increase of log|Z| with a decrease in the values of log f. This relationship is considered to be the response expected of a pure capacitor. However, at lower frequencies, a transition toward constant log|Z| vs log f occurs, which can be modeled in terms of a simple parallel resistance-capacitance (RC), electrical equivalent circuit. As the coatings were degraded, the coating capacitance, [C.sub.p], increased and the coating resistance, [R.sub.p], decreased because electrolyte was transported to the metal substrate through ionic conducting paths in the coaling.



To further analyze the properties of coatings with different particle sizes, EIS analysis was conducted as a function of immersion time. Figure 8 shows the coating parameters as a function of immersion time. The coating resistance, [R.sub.p], is a measure of the resistance of a coating to the penetration of water or electrolyte. In general, [R.sub.p] values were found to decrease with increasing immersion time. Slight variations were observed for some coatings at intermediate immersion stages, presumably due to structural changes of the network. However, these coatings exhibited a decrease in [R.sub.p] values upon further immersion, as shown in Fig. 12a. The general trend was the same, but clear differences were found between the samples. The coating with 0.2% PVC had the highest barrier property.


The variation of the coating capacitance, [C.sub.p], with increasing immersion time in an electrolyte solution provides information concerning both the coating stability and its water uptake. In general, [C.sub.p] was found to increase as a function of the immersion time, as shown in Fig. 12b. 0.2% PVC had the lowest capacitance, and its coating resistance was the highest, which shows that 0.2% is the optimum PVC.

From these results, it is concluded that adding 60-nm [Fe.sub.2][O.sub.3] to polyurethane coating improves the barrier property, and the optimum amount is 0.2% PVC.

The morphology of the 60-nm [Fe.sub.2][O.sub.3]/polyurethane coatings with different PVCs

Figure 13 shows the morphology of the four 60-nm [Fe.sub.2][O.sub.3]/polyurethane coatings. The pigment was distributed more homogeneously in the coating with 0.2% PVC than that in other coatings, which is consistent with the results of the electrochemical impedance measurements.


Barrier properties of polyurethane coatings pigmented with 150-nm [Fe.sub.2][O.sub.3]

EIS analysis of 150-nm [Fe.sub.2][O.sub.3]/polyurethane coatings pigmented with different PVCs

Figures 14-17 show the EIS of 150-nm [Fe.sub.2][O.sub.3]/polyurethane coatings with different PVCs immersed in 3.5% NaCl solution.


As shown in Fig. 14, the phase angle of the Bode plots of the [Fe.sub.2][O.sub.3]/polyurethane coatings with 0.2% PVC fluctuated with time, and behaved almost as a pure capacitor, which indicates that the coatings remained in good condition throughout the tests. The Bode plots of the [Fe.sub.2][O.sub.3]/polyurethane coatings with 1% PVC behaved as a pure capacitor before 9 h immersion, and had two time constants after 9 h immersion as shown in Fig. 15.


Figures 16 and 17 show the Bode plots of coatings with the PVCs of 6% and 11%, respectively. They no longer behaved as a pure capacitor, and a low-frequency linear component appeared, which was almost constant with varied frequency and is characteristic of a pure resistance. Accordingly,

the equivalent circuit of those plots should consist of a capacitor and a resistance in parallel, which indicates that the electrolyte had penetrated the coatings, and therefore affected their protective properties. The limited protective ability of the coatings was confirmed by the results of resistance measurements.



Figure 18 shows the coating resistance and capacitance as a function of immersion time. In general, the resistance decreased more rapidly with time as PVC increased. The resistance is related to the porosity of the coating and the capacitance is related to the water uptake caused by the diffusion of water molecules through the coating. For 0.2% PVC, the coating resistance was the highest and the capacitance was the lowest. However, the difference between these properties for the three coatings with the PVCs of 0, 0.2, and 1% was small, which indicates that adding 150-nm [Fe.sub.2][O.sub.3] did not significantly improve the barrier property, and the barrier property of the coating with 0.2% PVC was only slightly better than that of the others.


The morphology of 150-nm [Fe.sub.2][O.sub.3]/polyurethane coatings with different PVCs

Figure 19 shows the morphology of the four [Fe.sub.2][O.sub.3] (150 nm)/polyurethane coatings with different PVCs. The pigment in the coating with 0.2% PVC was distributed evenly, but this was not always so in the other coatings and it was clearly aggregated for 11 % PVC. This explains the electrochemical results.


Comparison of the properties of the two series of coatings with different particle sizes

Electrochemical testing for the two series of coatings with different particle sizes

To disclose the effect of adding pigment with different particle sizes on the anticorrosion properties of the coatings, the best parameters of the coating with 0.2% PVC in the two series of coatings were compared, as shown in Fig. 20. The resistance, [R.sub.p], for the coating with 60-nm particles was much higher than that for the coating with 150-nm particles, and its capacitance, [C.sub.p], was much lower, which suggests that the smaller particle size improved the barrier property.


Accelerated salt spray testing for the two series of coatings with different particle sizes

The surface images of the polyurethane coatings after salt spraying for 600 h are shown in Fig. 21. The corrosion resistance, [R.sub.p], for 0.2% PVC was enhanced, as indicated by a decrease in the concentration of localized pitting on the test coupon. The localized pitting increased with increasing addition of [Fe.sub.2][O.sub.3]. The coating with 0.2% PVC was in good condition after 600 h testing, whereas localized corrosion and blisters could be observed on all the other samples, and the coating with 11% PVC gave the poorest protection. The smaller particle size gave better protection, which was consistent with the electrochemical results. The active surface area of nanomaterials is much larger than that of conventional materials (10) because smaller the particle size, larger is the proportion of the active parts. There is significant difference in hydroxyl groups between the 60-nm [Fe.sub.2][O.sub.3] particles and the 150-nm [Fe.sub.2][O.sub.3] particles according to FTIR spectroscopic measurements. On the other hand, the 60-nm [Fe.sub.2][O.sub.3] particles can promote the reaction between hydroxyl bearing polyacrylate and HDI biuret at lower temperature. Thus, an increasing cross-linking density in polyurethane/60-nm [Fe.sub.2][O.sub.3] composite coatings improves its barrier for corrosion in comparison with the polyurethane/150-nm [Fe.sub.2][O.sub.3] composite coatings. As a result, it is more difficult for the electrolyte to diffuse through the coating containing the nano-[Fe.sub.2][O.sub.2] particles. Therefore, the coating can give better protection to the steel substrate.



The performance of the coatings was largely dominated by the size and amount of [Fe.sub.2][O.sub.3] in the coating. Nano-[Fe.sub.2][O.sub.3] significantly improved the protection performance, and the optimum PVC was 0.2%. Micro-[Fe.sub.2][O.sub.3] had little effect on the barrier property of the coatings.

Acknowledgments The authors thank the National Key Technology R&D Program (Grant Nos. 2007BAB27B02-02 and 2009BAE70B02) and National Natural Science Fund of China (Grant No. 50499334) for supporting these studies.


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F. Liu (*), L. Yang, E. Han

State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China


L. Yang

Exploration & Production Research Institute, SINOPEC, Beijing 100083, China

J. Coat. Technol. Res., 7 (3) 301-313, 2010

DOI 10.10077/s11998-009-9203-3
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Author:Liu, Fuchun; Yang, Lihong; Han, Enhou
Publication:JCT Research
Date:May 1, 2010
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