Siloxane-modified epoxy resin–clay nanocomposite coatings with advanced anticorrosive properties prepared by a solution dispersion approach

Surface & Coatings Technology 200 (2006) 2753 – 2763 www.elsevier.com/locate/surfcoat

Siloxane-modified epoxy resin–clay nanocomposite coatings with advanced anticorrosive properties prepared by a solution dispersion approach Jui-Ming Yeha,*, Hsiu-Yin Huanga, Chi-Lun Chena, Wen-Fen Sua, Yuan-Hsiang Yub a

Department of Chemistry and Center for Nanotechnology at CYCU, Chung-Yuan Christian University, Chung Li, Taiwan 320, ROC b Department of Electronic Engineering, Lan-Yan Institute of Technology, I-Lan 261, Taiwan, ROC Received 6 July 2004; accepted in revised form 3 November 2004 Available online 25 January 2005

Abstract A series of polymer–clay nanocomposite (PCN) materials that consist of siloxane-modified epoxy resin and inorganic nanolayers of montmorillonite (MMT) clay has been prepared through a thermal ring opening polymerization using 1,3-bis(3-aminopropyl)-1,1,3,3tetramethyldisiloxane as a curing agent. These PCN materials at low clay concentration in the form of coating on cold-rolled steel (CRS) were found to be much superior in corrosion protection over those of pure epoxy resin when tested for performance in a series of electrochemical measurements of corrosion potential, polarization resistance, corrosion current, and impedance spectroscopy in 5 wt.% aqueous NaCl electrolyte. The as-prepared materials were characterized by infrared spectroscopy, wide-angle X-ray diffraction, and transmission electron microscopy. After measurements, we found advanced protection against corrosion on CRS coupon compared to bulk epoxy resin. Molecular (e.g., O2, N2, and H2O) permeability of epoxy resin–clay nanocomposite membranes was found to be lower than that of bulk epoxy resin along with the loading of nanoclay based on the studies of gas and vapor permeability analysis. Moreover, the epoxy resin–clay nanocomposite materials have significant advantages over standard epoxy resins such as lower water absorption, lower cure shrinkage, moderate glass transition temperature (T g), and higher tensile strength. D 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion; Epoxy; Nanocomposite; Clay; Siloxane

1. Introduction Environmental, safety, and cost issues surrounding the continued use of chromates have led to increased research activities to find alternative materials. Some of the most effective undercoatings based on conjugated polymers [1–6] and thermosetting polymers [7–13] for corrosion protection have been used to replace chromates due to the environmental and health concerns. Among other anticorrosive coatings, thermosetting epoxy resins are commonly used as organic coatings for corrosion protection due to their strong adhesion capability

* Corresponding author. Tel.: +886 3 2653340; fax: +886 3 2653399. E-mail address: [email protected] (J.-M. Yeh). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.008

to metallic substrates and excellent chemical resistance. However, the serious moisture absorption and volume shrinkage of traditional epoxy resins lead to the diffusion of absorbed water into the epoxy–steel interface and initiate corrosion of the metal substrate particular in wet conditions. Therefore, many research groups have devoted their efforts to develop advanced anticorrosive coatings by enhancing the interfacial adhesion between organic coatings and metallic surfaces, by modifying the formulation components of organic epoxy resins through the introduction of silane coupling agents [14], or by preparing the epoxy–siloxane hybrid binders [15]. In addition to improving coating adhesion mentioned above, decrease in coating/membrane permeability of aggressive species (e.g., O2 and H+) also shows enhanced protective properties. For instance, we have found lately

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that the incorporation of inorganic nanolayers of montmorillonite (MMT) clay into the polymeric matrix can effective enhance the corrosion protection effect of pristine polymers such as conducting polymers (e.g., polyaniline [16], poly(o-methoxyaniline) [17], poly(oethoxyaniline) [18], poly(3-alkylthiophene) [19], polypyrrole [20]), thermoplastic polymers (e.g., poly(methyl methacrylate) [21], and thermosetting polymers (e.g., polyimide [22]) on cold-rolled steel (CRS) coupon based on a series of electrochemical corrosion measurements in saline. In the recent decade, there has been a considerable amount of publications associated with studies of epoxy resin–clay nanocomposite materials [23–36]. Most of the published literatures are mainly focused on the studies of their corresponding property related to mechanical strength and thermal stability. However, the protective properties of thermosetting epoxy resin–clay nanocomposite materials have never been discussed. In this study, a series of siloxane-modified epoxy resin–clay nanocomposite materials is successfully prepared and their protective performance is evaluated through a series of standard electrochemical corrosion testing. The siloxane linkages are first intentionally designed into the backbone of epoxy resin to increase the hydrophobicity of coating surface of traditional epoxy resins. Nanolayers of clay are further dispersed into the siloxane-modified epoxy resin to further effectively increase the length of the diffusion pathways for oxygen and water, as well as increase the gas/vapor barrier of the coating. The increase in hydrophobicity and gas/vapor barrier makes siloxane-modified epoxy resin–clay nanocomposite materials more desirable to apply in protective coating. The as-synthesized polymer–clay nanocomposite (PCN) materials are characterized by Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (XRD), and transmission electron microscopy (TEM). Effects of material composition on gas barrier, thermal stability, mechanical strength, and optical clarity of epoxy resin, and a series of PCN materials are also studied by vapor permeability analysis (VPA), gas permeability analysis (GPA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), dynamic mechanical analysis (DMA) and UV–vis transmission spectra, respectively.

2. Experimental 2.1. Chemicals and instrumentations Diglycidyl ether of bisphenol A (DGEBA) (TCI), N,Ndimethylacetamide (DMAc) (TEDIA), 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (UCT), poly(methyl methacrylate) (Acros; M w=350,000), and potassium bromide (KBr) (Riedel-deHaJn) are used as received without

further purification. Hydrochloric acid (37%; RiedeldeHaJn) is applied to prepare the 1.0 M HCl aqueous solution. The used MMT clay consisted of a unit cell formula Ca0.084Na0.143(Al1.69Mg0.31)Si4O10(OH)2d 2H2O, and a CEC value of 115 mEq/100 g is provided by Industrial Technology Research Institute (ITRI), Taiwan. Tetradecyltrimethylammonium chloride (z98%; Fluka) is employed as an intercalating agent. FTIR spectra are recorded from pressed KBr pellets with PCN materials coated using a Bio-Rad FTS-7 FTIR spectrometer. Wide-angle powder XRD study of the samples is carried out by Rigaku D/MAX-3C OD2988N X-ray diffractometer with cooper target and Ni filter at a scanning rate of 18/min. The samples for TEM study are first prepared by putting the membrane of PCN materials into low-viscosity embedding media capsules with four ingredients (ERL4206 5.0 g, DER736 3.0 g, NSA 13.0 g, and DMAE 0.15 g) and by curing the media at 80 8C for 12 h in a vacuum oven. Then the cured capsules containing PCN materials are microtomed with Leica Ultracut Uct into 90-nm-thick slices. Subsequently, one layer of carbon about 10 nm thick is deposited on these slices on mesh 100 copper nets for TEM observations on a JEOL-200FX with an acceleration voltage of 120 kV. Electrochemical corrosion measurements are performed on a VoltaLab Model 21 and VoltaLab Model 40 Potentiostat/Galvanostat in a standard corrosion cell equipped with two graphite rod counterelectrodes and a saturated calomel electrode (SCE) as well as a working electrode. A Yanagimoto gas permeability analyzer (model GTR-10) is employed to perform the permeation experiment of oxygen and nitrogen gas. The permeation of water vapor is performed using the apparatus employed in our previous published literatures [16]. The immersion weight gains (wt.%) of pure epoxy and PCN materials are calculated after 24 h of water uptake periods at room temperature. Seiko Thermal Analysis System equipped with model TGA/SDTA 851 TGA is employed to perform thermal analyses from 25 to 900 8C at a heating rate of 10 8C/min under N2 flow. DSC is performed on a TA Instrument DSC 2920 in the 25–250 8C range at the programmed heating rate of 20 8C/min. TMA for samples with dimension of 0.5
0.5 cm are measured on a Thermal Analysis model TA 2940 by a probe contact method in the 20–150 8C range under N2 flow at the programmed heating rate of 5 8C/min. The stress–strain behaviors of epoxy and all PCN materials under uniaxial tension (1 N/min) are measured on a Thermal Analysis model Q 800 with the sample dimension of 2
0.5 cm. DMA for samples with dimension of 2
0.5 cm are measured on a Thermal Analysis model TA 5100 using shear mode from 0 to 150 8C at a heating rate of 5 8C. The UV–vis transmission spectra of the polymer in the form of membrane are recorded on a Hitachi U-2000 UV–vis spectrometer at room temperature.

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2.2. Preparation of organophilic clay The organophilic clay is prepared by a cationic exchange reaction between the sodium cations of MMT clay and alkylammonium ions of the intercalating agent, tetradecyltrimethylammonium chloride [16–22]. Typically, 5 g of MMT clay with a CEC value of 115 mEq/100 g is stirred in 500 cm3 of distilled water (beaker A) at room temperature for 6 h. The quantity of intercalating agent is calculated from the following equation: ½clay CEC value ðmEqÞ=100 ðgÞ
weight of clay ðgÞ
1:5 ¼ ½weight of intercalating agent ðgÞ =molecular weight of intercalating agent ðg=molÞ
1000 As calculated from above, a separate solution containing 2.5 g of tetradecyltrimethylammonium chloride in another 100 cm3 of distilled water (beaker B) is under magnetic stirring and follows by adding 1.0 M HCl aqueous solution to adjust the pH value to about 3–4. After stirring for 1 h, the solution of beaker B is added at a rate of approximately 10 ml/min with vigorous stirring to the MMT suspension (beaker A). The mixture is stirred overnight at room temperature. The organophilic clay is recovered by ultracentrifugating (9000 rpm, 30 min) and then decanting the clear solution. Purification of products is performed by washing with 500 cm3 of distilled water for 0.5–1 h and then ultracentrifugating (9000 rpm, 30 min) samples repeatedly for five to six times to remove any excess ammonium ions. Upon drying under dynamic vacuum for 48 h at 40 8C, the organophilic clay is obtained. 2.3. Preparation of epoxy resin and epoxy resin–clay nanocomposite materials As a typical procedure for the preparation of epoxy resin, 1 g of DGEBA and 5 g of DMAc are mixed with stirring for 1 h, then 0.37 g of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is added and further stirred for 36 h at room temperature. The resultant mixture is degassed and dropwisely cast onto a clean Teflon mould and then cured at 120 8C for 5 h. The cured epoxy membrane is obtained with a thickness of ca. ~0.25 mm. The procedure for the preparation of PCN materials is similar to the method described above. An appropriate amount of organophilic clay, calculated by 1, 3, 5, and 7 wt.% with respect to PCN materials, is introduced into the mixing solution containing 1 g of DGEBA and 5 g of DMAc and then further mixed under magnetic stirring for 6 h at room temperature. A total of 0.37 g of 1,3-bis(3aminopropyl)-1,1,3,3-tetramethyldisiloxane is added and further stirred for 48 h at room temperature. The resultant mixture is degassed and dropwisely cast onto a clean Teflon mould and then cured at 120 8C for 5 h. The cured epoxy resin and PCN materials are lifted from the Teflon mould by

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soaking in water to give membranes with thicknesses of ca. ~0.25 mm. 2.4. Preparation of coatings and electrochemical measurements As a typical procedure to prepare sample-coated coupons for corrosion measurements, freshly prepared solutions of epoxy or PCN materials in DMAc are cast dropwisely onto the CRS coupons (1
1 cm) and the coating dried in air for 5 h at 120 8C to give ~30-Am-thick coatings, measured by a digimatic micrometer (Mitutoyo). The coated and uncoated coupons are then mounted to the working electrode so that only the coated side of the coupon was in direct contact with the electrolyte. The edges of the coupons are sealed with super fast epoxy cement (SPARR). All the electrochemical measurements of corrosion potential, polarization resistance, and corrosion current are performed on a VoltaLab model 21 Potentiostat/Galvanostat in a standard corrosion test cell equipped with two graphite rod (diameter: 6.15 mm) counterelectrodes, a saturated calomel reference electrode (SCE), and a working electrode, and all experimental data are repeated at least three times. The electrolyte is an aqueous solution containing 5 wt.% of sodium chloride. Open circuit potential (OCP) at the equilibrium state of the system is recorded as the corrosion potential [E corr (in V) vs. SCE]. Polarization resistance (R p, in V/cm2) is measured by sweeping the applied potential from 20 mV below to 20 mV above the E corr at a scan rate of 500 mV/min and by recording the corresponding current change. R p value is obtained from the slope of the potential current plot. Tafel plots are obtained by scanning potential from 250 mV below to 250 mV above the E corr at a scan rate of 500 mV/ min. Corrosion current (I corr) is determined by superimposing the straight line along the linear portion of the cathodic or anodic curve and extrapolating it through E corr. Corrosion rate (R corr, in mm/year) is calculated from the following equation [18]: Rcorr ðin mm=yearÞ ¼ ð3270
Icorr
EWÞ=q where EW is the equivalent weight (in g), I corr is the corrosion current density (in AA/cm2), q is the density (in g/ cm 3), and 3270=0.01
1 year (in s)/96497.8 (where 96,497.8=1 F). A VoltaLab model 40 Potentiostat/Galvanostat is employed to perform the impedance spectroscopy studies. Impedance measurements are carried out in the frequency range of 100 kHz–100 MHz. The working electrode is first maintained in the test environment for 30 min to reach an equilibrium state before the impedance run. This condition serves to put the electrode in a reproducible initial state and to confirm that no blistering occurred during the conditioning period. All experiments are performed at a room temperature of 25F1 8C. All data are replicated at least three times to ensure reproducibility and statistical significance.

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2.5. Gas barrier properties measurements Gas permeability of membrane is determined by using the Yanco GTR-10 gas permeability analyzer. Gas permeability is calculated by the following equation: P ¼ l=ðp1  p2 Þ

q=t A

where P is the gas permeability [cm3 (STP) cm/cm2 s cm Hg], q/t is the volumetric flow rate of gas permeate [cm3 (STP)/s], l is the membrane thickness [cm], A is the effective membrane area [cm2], and p 1,p 2 are the pressures (cm Hg) on the high-pressure and low-pressure sides of the membrane, respectively. The rate of transmission of O2 and N2 is obtained by gas chromatography, from which the O2 and N2 permeability is calculated. On the other hand, the experiment of H2O permeability is performed by apparatus similar to our previous published paper [21] wherein the feed solution is not in contact with the membrane. The feed solution is vaporized first and subsequently permeated through the membrane with an effective area of ~10.2 cm2. The permeation rate is determined by measuring the weight of permeate.

3. Results and discussion In this study, a series of novel siloxane-modified epoxy resin–clay nanocomposite materials using 1,3-

bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane as curing agent is successfully prepared and their corresponding corrosion protection behavior on CRS coupons is evaluated by standard electrochemical corrosion testing. The siloxane linkage is intentionally designed into the backbone of epoxy resin based on two possible reasons: first, the introduction of –Si–O– linkage into epoxy resins may enhance the surface hydrophobicity of pure epoxy resin coating; secondly, incorporation of siloxane linkage may release the volume shrinkage stress and lower the glass transition temperature (T g) of epoxy resins resulting from the curing process of ring opening polymerization reactions. It is beneficial to the subsequent step of dispersion of nanoclay platelets into the epoxy resin matrix, implying a higher gas/vapor barrier of nanocomposite coatings. The increase in coating hydrophobicity and coating gas/vapor barrier properties makes siloxane-modified epoxy resin–clay nanocomposite materials more desirable for anticorrosive coating applications. The typical procedure for the preparation of siloxanemodified epoxy resin–clay nanocomposite materials is given as follows: organophilic clay is first dispersed into the mixed solution of DGEBA, DMAc, and curing agent. Subsequently, thermal ring opening polymerization is performed at 120 8C for 5 h. The preparation flowchart of the PCN resins is given in Scheme 1. The composition of the PCN materials is varied from 0 to 7 wt.% of clay with respect to polymer content, and their properties are shown as summarized in Tables 1–4.

Organophilic MMT clay (intercalated agent: tetradecyl trimethylammonium chloride). CH3

O CH2

C

CH CH2 O

O

O CH2 CH CH2

CH3 DMAc stirred for 6 hrs CH3 NH2CH2CH2CH2

CH3

Si O Si CH2CH2CH2NH2 CH3

CH3

dispersion of organophillic montmorillonite clay for 48 hrs

thermal ring-opening polymerization at 120ºC

O HN

Si

O Si

OH HO

O

Si

N

O

O

N

Si

NH

OH O

HO

O

Scheme 1. Synthesis of Siloxane-Modified Epoxy Resin/Clay Nanocomposite.

O

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Table 1 Anticorrosive performance of epoxy–clay nanocomposite materials as measured from electrochemical corrosion measurements Compound code

Feed composition (wt.%)

Bare Epoxy CLMA1 CLMA5

Electrochemical measurements

Epoxy

MMT

E corr (mV)

R p (kV cm )

I corr (AA/cm )

R corr (mm/year)

– 0 1 5

687.8 602.3 598.8 498.3

6.31 63.66 360.3 570.11

6.4 0.5817 0.064 0.0298

75.86 6.883 0.759 0.353

The representative FTIR spectra of the DGEBA, epoxy, PCN materials, and organophilic clay are given in Fig. 1. The characteristic vibration bands of DGEBA are shown at 1609 cm1, 1508 cm1 (C–C skeletal stretching), 1036 cm1 (aromatic deformation), and 915 cm1 (epoxide ring). The curing process of epoxy preparation can be monitored from the intensity diminishment of characteristic bands of epoxide ring and the increase of –OH band (3380 cm1) after completing the curing process of epoxy resin. The characteristic vibration bands of tetradecyltrimethylammonium MMT clay are shown at 3630 cm1 (free H2O), 2852 cm1, 2926 cm1 (C–H), 1478 cm1 (C–N), 1040 cm1 (– Si–O–), 600 cm1 (–Al–O–), and 420 cm1 (–Mg–O–) [16– 22]. As the loading of organophilic MMT clay is increased, the intensities of organophilic MMT clay bands become stronger in the FTIR spectra of PCN materials. Fig. 2 illustrates the wide-angle powder XRD patterns of epoxy, organophilic clay, and a series of PCN materials. The XRD patterns in Fig. 2f show that organophilic clay has a d 001 spacing of 19.4 2 (2h=4.558), which is higher than that of Na+-MMT clay with a d 001 spacing of 12.52 2 (2h=7.058). The result shows that the long-chain quaternary alkylammonium ions have been exchanged for intragallery ions of clay interlayer during the cationic exchange process. This organophilc modification of the interlayer region facilitates nanocomposite formation by increasing the gallery spacing and creating a more hydrophobic environment for the epoxy resin and curing agent. As a result, the XRD patterns in Fig. 2b–f show that all the d 001 spacings of the PCN materials are higher than that of organophilc clay, Table 2 Barrier properties of the epoxy resin–clay nanocomposite materials Compound Epoxy MMT Permeability Water Barrier Barrier Thickness code (wt.%) (wt.%) (g/h m2)a uptake (O2)c (N2)c (Am)d (wt.%)b

a

100 99 97 95 93

0 1 3 5 7

Thickness (Am) 2

– 100 99 95

3.1. Characterization

Epoxy CLMA1 CLMA3 CLMA5 CLMA7

2

150 129 114 69 58

2.78 1.93 1.53 1.37 0.86

5.25 3.33 2.57 1.26 1.04

0.98 0.58 0.47 0.24 0.18

245 202 256 236 224

As measured from VPA. b Values reported were immersion weight gains (wt.%) after 24 h of water uptake periods. c As measured from GPA. d As measured by a digimatic micrometer.

– 32 30 34

indicating that a significant degree of intercalation dispersion has occurred in epoxy matrix. It can also be seen that as the concentration of clay increased, more ordered structures are obtained as more intense peaks for the d 001 spacing comparisons of 31.52 2 (2h=2.808, CLMA1), 33.95 2 (2h=2.608, CLMA3), 30.44 2 (2h=2.908, CLMA5), and 21.12 2 (2h=4.208, CLMA7), respectively. Fig. 3 shows TEM images of PCN materials loaded with 5 wt.% clay, where the dark line represents clay platelets and the gray/white areas represent the epoxy matrix. Fig. 3a is a lower-magnification micrograph showing a general view of the dispersed clay particles in the epoxy. Fig. 3b is a highmagnification micrograph revealing the d spacing of the clay layers in the epoxy resin. It is clearly seen that the lamellar nanocomposite has a mixed morphology with major intercalation and minor exfoliation dispersion in the epoxy matrix. Some larger intercalated tactoids could be identified with layer spacings of about 5 nm, as shown in Fig. 3b. 3.2. Protective properties of coatings Protective performance against corrosion of samplecoated CRS coupons is commonly determined from electrochemical measurements such as corrosion potential (E corr), polarization resistance (R p), corrosion current (I corr), and corrosion rate (R corr), as listed in Table 1. The E corr, R p, I corr, and R corr for epoxy coating on CRS coupons show better corrosion resistance compared to the uncoated CRS. Furthermore, the CRS coupons coated with PCN materials all show significantly higher values of E corr and R p, and Table 3 Thermal properties of epoxy resin–clay nanocomposite materials measured by TGA, DSC, and TMA Compound code

Td (8C)a

Characteristic yield (wt.%)a

Tg (8C)b

ai (Am/m 8C)c

af (Am/m 8C)d

Epoxy CLMA1 CLMA3 CLMA5 CLMA7

303.8 310.7 319.3 326.6 343.2

8.3 9.8 11.7 15.5 18.9

71.79 73.19 74.32 85.71 96.77

175 – 148 126 104

291 – 270 245 238

a

As measured by TGA. As measured by DSC. c As measured by the average CTE value in the range of (T g50)– (T g10) 8C by TMA. d As measured by the average CTE value in the range of (T g+10)– (T g+40) 8C by TMA. b

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Table 4 Storage modulus and glass transition temperature (T g) of the epoxy resin– clay nanocomposite materials measured from dynamic mechanical analyses

(f)

Compound code

(e)

Epoxy CLMA1 CLMA3 CLMA5 CLMA7

30 8C

110 8C

2032.4 2225.0 2371.1 2472.1 2665.6

57.2 73.1 82.5 126.8 149.3

T g (8C) 79.9 85.1 88.9 92.7 104.6

lower values of I corr and R corr than the neat epoxy coating, implying that the PCN material-coated CRS is more noble toward electrochemical corrosion relative to the pure epoxy. For example, the CLMA5-coated CRS exhibits a high corrosion potential of ca. 498.3 mV at 30 min. Even after 5 h of measurement, the potential remained constant at ca. 500 mV. CLMA5-coated CRS shows a polarization resistance (R p) value of 5.7
105 V/cm2 in 5 wt.% NaCl, which is about two orders of magnitude greater than the uncoated CRS. The corresponding corrosion current (I corr) of CLMA5-coated CRS is ca. 0.0298 AA/cm2. Corrosion rate for CLMA5-coated CRS is 0.353 mm/ year, which is smaller than epoxy (6.883 mm/year) in the ~34-Am-thick coating. Tafel plots for (a) bare, (b) epoxy-coated, (c) CLMA1-coated, and (d) CLMA5-coated CRS samples are shown in Fig. 4. Electrochemical impedance spectroscopy (EIS) is also used to examine the activity difference between the CRS surface after epoxy and PCN material treatment. Impedance is a totally complex resistance when a current flows through a circuit made of capacitors, resistors, or inductors, or any combination of these [37]. EIS measurement results in currents over a wide range in frequency. Corrosion metals are modeled with an equivalent circuit (called a Randles circuit), as illustrated in Fig. 5a, which is made of a doublelayer capacitor in parallel with a charge transfer resistor and connected in series with a electrolyte solution resistor. The Si-O

Mg-O

(g) Al-O

(f) Intensity

(e) (d) (c) -OH

-NH

(b) epoxide peak

(a) 4000

3500

3000

2500

2000

1500

1000

Intensity

Storage modulus (EV) MPa

(d) (c) (b) (a) 2

3

4

5

2 Theta Fig. 2. Wide-angle powder XRD patterns of: (a) epoxy, (b) CLMA1, (c) CLMA3, (d) CLMA5, (e) CLMA7, and (f) organophilic clay.

impedance (Z) depends on the charge transfer resistance (R ct), the solution resistance (R s), the capacitance of the electrical double layer, and the frequency of the AC signal (x). It can be reduced as:
j R2ct Cdl x Rct Z ¼ ZV þ jZW ¼ Rs þ þ 1 þ ðRct Cdl xÞ2 1 þ ðRct Cdl xÞ2 The high-frequency intercept is equal to the solution resistance, and the low frequency-intercept is equal to the sum of the solution and charge transfer resistances [38]. Fig. 5b shows the Nyquist plots of the four measured samples. The first sample (a) is uncoated CRS. A series of samples denoted with (b), (c), and (d) is CRS-coated by epoxy, CLMA1, and CLMA5, respectively. The corrosion of these samples in 5 wt.% NaCl aqueous electrolyte for 30 min is followed by EIS. The charge transfer resistances of samples (a), (b), (c), and (d), as determined by subtracting the intersection of the high-frequency end from the lowfrequency end of the semicircle arc with the real axis, are 1.09, 108, 157, and 206 kV cm2, respectively [18,21]. EIS bode plots (impedance vs. frequency) of samples (a), (b), (c), and (d) are shown in Fig. 6. The increase of impedance values at high clay concentrations in various frequency regions could be interpreted as the barrier effect of nanolayers of MMT dispersed in composites [18]. These results obviously show that the sample incorporating with clay nanolayers has a better anticorrosive performance and might have significant potential for corrosion protection. The novel property of enhanced anticorrosion effect for epoxy–clay compared to pure epoxy coatings might arise from the dispersing silicate nanolayers of clay in epoxy matrix to decrease oxygen and water permeability [16–22]. These are further proved by studies of barrier properties as discussed in Section 3.3.

500

Wave number (cm-1) Fig. 1. Representative FTIR spectra of: (a) diglycidyl ether of bisphenol A, (b) epoxy, (c) CLMA1, (d) CLMA3, (e) CLMA5, (f) CLMA7, and (g) organophilic clay.

3.3. Barrier properties In this study, the membranes used for the molecular permeability measurements are prepared to have a thick-

J.-M. Yeh et al. / Surface & Coatings Technology 200 (2006) 2753–2763

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Fig. 3. Transmission electron microscopy of CLMA5 (a) at low magnification, and (b) at high magnification.

ness of about 0.20–0.26 mm. For the H2O vapor permeability studies, we found that the incorporation of clay platelets into epoxy matrix results in a decrease of H2O vapor permeability for PCN membranes, as shown in Fig. 7 and Table 2. For example, PCN membrane at low clay loading (e.g., 5 wt.%) showed about 54% reduction of H2O permeability compared to pure epoxy membrane. In addition, the barrier to water uptake for the pure epoxy and PCN materials is evaluated by calculating the weight gains (wt.%) after 24 h of uptake of water at room temperature. As listed in Table 2, the water uptake is decreased along with the loading of MMT clay. For instance, the water uptake is decreased by 69%, from 2.78 wt.% of epoxy to 0.86 wt.% of CLMA7, since epoxy absorbs moisture

easily, and the diffusion of absorbed water into the epoxy– substrate interface weakens the interfacial adhesion strength between epoxy and substrate, particularly in wet conditions. The results of decrease in vapor permeability and water uptake are very desirable for epoxy resin’s application in electronic encapsulation or anticorrosive coating application, wherein water absorption and permeability are detrimental to dielectric performance and corrosion inhibition. Furthermore, the O2 and N2 molecular permeability of PCN membranes also shows the dispersion

(a) Cdl

5.0 4.5

Rs Rct

(a)

(b)

4.0

250

3.0

(b) (d)

2.0

(d)

200

(c)

2.5

1.5 1.0

Z'' (kΩ • cm2)

log (µA/cm2)

3.5

150

(c)

100

(b)

50

(a)

0.5 0

-850 -800 -750 -700 -650 -600 -550 -500 -450 -400 -350 -300

mV Fig. 4. The Tafel plots for (a) bare, (b) epoxy-coated, (c) CLMA1-coated, and (d) CLMA5-coated CRS samples measured in 5 wt.% NaCl aqueous solution.

0

50

100

150

200

250

Z' (kΩ • cm2) Fig. 5. (a) Randles equivalent circuit. (b) Nyquist plots for: (a) bare, (b) epoxy-coated, (c) CLMA1-coated, and (d) CLMA5-coated CRS samples measured in 5 wt.% NaCl aqueous solution.

J.-M. Yeh et al. / Surface & Coatings Technology 200 (2006) 2753–2763 6

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

(d)

1.0

(c) Barrer (O2)

(b)

0.6 2

0.4

0.2

(a)

0 0

0

1

2

3

5

4

Fig. 6. Bode plots for (a) bare, (b) epoxy-coated, (c) CLMA1-coated, and (d) CLMA5-coated CRS samples measured in 5 wt.% NaCl aqueous solution.

of clay platelets promoting the molecular barrier of O2 and N2 gas. For example, the barrier of O2 is decreased by 37% from 5.25 of epoxy to 3.33 of CLMA1, and that of N2 is decreased by 41% from 0.98 of epoxy to 0.58 of CLMA1. It should be noted that a further increase in clay concentration resulted in a further enhanced molecular barrier property of PCN materials, as illustrated in Fig. 8 and Table 2. These results mentioned above are attributed to the plate-like clays that effectively increase the length of the diffusion pathways, as well as decrease the permeability of PCN membranes [16–22]. 3.4. Thermal properties Fig. 9 shows a typical TGA thermogram of weight loss as a function of temperature for PCN materials and pure epoxy, as measured under nitrogen atmosphere. In general, major weight losses are observed in the range of ~300–600 8C for epoxy and PCN materials, which may be corre3.0

140

2.5 2.0

120

1.5

100

1.0 80

Moisture absorption (%)

160

0.5 60 0.0 0

2

4

6

2

4

6

8

Clay loading (%)

log F (kHz)

Permeability of water vapor (g/hr-m2)

0.8

4

Barrer (N2)

log Zreal (kohm • cm2)

2760

8

Clay loading (%) Fig. 7. Permeability of water vapor and moisture absorption ratio as a function of the MMT clay content in the epoxy–clay nanocomposite materials.

Fig. 8. Gas permeability of O2 and N2 as a function of the MMT clay content in the in the epoxy–clay nanocomposite materials.

spondent to the structural decomposition of the polymers. As the decomposed temperature (T d, the temperature of degradation at which weight loss is 5 wt.%) list in Table 3, the T d of those PCN materials shift toward the higher temperature range than that of pure epoxy. For example, the T d of CLMA7 is 343 8C, which is superior to that of the pure epoxy (T d=304 8C). After ~600 8C, the curves all became flat and mainly the inorganic residue (i.e., Al2O3, MgO, and SiO2) remains. From the amounts of the residue at 900 8C as listed in Table 3, the char yields of the PCN materials are higher than that of the bulk epoxy. The chard yield of pure epoxy is 8.3 wt.% and that of CLMA7 is up to 18.9 wt.% at 900 8C. These phenomena reveal that the intercalated epoxy nanocomposite materials possess the better thermal stability and flame resistance. This enhancement on thermal properties is due to the presence of clay nanolayers, which acted as barriers to minimize the permeability of volatile degradation products out from the PCN materials. DSC traces of epoxy and PCN materials are shown in Fig. 10. Epoxy exhibits an endotherm at 72 8C corresponding to the glass transition temperature (T g) of epoxy [35,36]. All the PCN materials are found to have a higher T g compared to the bulk epoxy, as shown in Table 3. For example, the CLMA7 has a higher T g value of 97 8C compared to the T g (72 8C) of pure epoxy. The T g of asprepared PCN materials increases gradually along with the clay concentration. This is tentatively attributed to the confinement of the intercalated polymer chains within the clay galleries, which prevents the segmental motions of the polymer chains [16–22]. The coefficient of thermal expansion (CTE) of epoxy and PCN materials is measured by TMA as listed in Table 3. The value of a i was the average CTE value in the range of (T g50)–(T g10) 8C, which shows the thermal expansion behavior below T g. The value of a f is the average CTE value in the range of (T g+10)–(T g+40) 8C, which shows the thermal expansion behavior above T g. The pure epoxy has CTE values of 175 Am/m 8C below T g

J.-M. Yeh et al. / Surface & Coatings Technology 200 (2006) 2753–2763

2761

Fig. 9. TGA curves of: (a) epoxy, (b) CLMA1, (c) CLMA3, (d) CLMA5, and (e) CLMA7.

(a i) and 291 Am/m 8C above T g (a f). The PCN material with 5 wt.% clay loading has CTE values of 126 Am/m 8C below T g (a i) and 245Am/m 8C above T g (a f), which are significantly lower than that of pure epoxy. As the clay concentration increases, both values of a i and a f decrease gradually, indicating that nanoclay could be used to increase the dimensional stability of epoxy [39,40]. The high specific surface area and aspect ratio of clay show effective reinforcement effects in neat polymeric structure. This would reduce shrinkage of the epoxy matrix during curing process for coating application. 3.5. Mechanical behavior The stress–strain curves for the pure epoxy and PCN materials are shown in Fig. 11. It is clear found that both strength and toughness of the CLMA5 have been signifi-

cantly improved than pure epoxy. As the clay content increases, the modulus increases. The improvement in strength and modulus is attributed to the reinforcement provided by the dispersed clay nanolayers. It is also observed that the strain to failure is increased for the addition of clay below 5 wt.%, whereas it is decreased for 7 wt.% clay loading. It may be dependent on the degree of dispersion for nanoclay particles in the epoxy matrix. Some phase separation as well as the poor interfacial adhesion between the particles and epoxy matrix may occur at higher clay loading up to 7 wt.%. The evidence of remarkable improvement in elasticity for CLMA5 supports our idea in both the flexible conformational effect on the polymer by introducing 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane as curing agent and the plasticizing effect of gallery onium ions at the clay–matrix interface by incorporating the organophilic clay. This result will be suitable for the needs

25

96.77 C (e)

0.0

74.32 C (c) 73.19 C(b)

-0.5

(a)

0

Exo Up

50

100

(c)

15 10

(b) (a)

5

71.38 C -1.0

(d)

20

85.71 C (d) Stress (MPa)

Heat Flow ( W/g)

0.5

150

200

250

Temperature ( C)

Fig. 10. DSC curves of: (a) epoxy, (b) CLMA1, (c) CLMA3, (d) CLMA5, and (e) CLMA7.

0 0

10

20

30

40

50

60

70

80

Strain (%) Fig. 11. Stress–strain curves of: (a) epoxy, (b) CLMA3, (c) CLMA5, and (d) CLMA7.

J.-M. Yeh et al. / Surface & Coatings Technology 200 (2006) 2753–2763

of depressing the brittleness problem especially for the glassy epoxy resin’s application. Moreover, DMA is used to investigate the thermomechanical properties for the PCN materials as illustrated in Fig. 12a and Table 4. The storage moduli in the glassy region of the PCN materials are found to be higher than that of the pristine epoxy, and are promoted as clay loading increased. This might be attributed to the strong interfacial interaction between the organic phase and the inorganic phase, with the nanoclay as reinforcement sites. Furthermore, PCN materials exhibit improvements in glass transition temperature as shown in Fig. 12b and Table 4 for the evaluation of tand. As the concentration of MMT clay increases, the T g of the PCN materials becomes higher. The results are consistent with previous DSC studies. 3.6. Optical clarity of membrane The membranes of bulk epoxy and PCN materials used for optical property measurements are prepared to have film a thickness of ~0.25 mm. Fig. 13 shows the UV–vis transmission spectra of pure epoxy and PCN membranes.

(a)

Storage Modulus (MPa)

3000

100

(a) (b) (c) (d)

80 60

%T

2762

(e)

40 20 0

200

300

400

500

600

700

800

Wavelength (nm) Fig. 13. UV–vis transmission spectra and transparency of: (a) epoxy, (b) CLMA1, (c) CLMA3, (d) CLMA5, and (e) CLMA7.

The transmission spectra of pure epoxy and PCN membranes in the UV–vis light regions (200–800 nm) are slightly affected by the presence of the clay loading. The spectra of as-prepared membranes at higher clay loading exhibit lower optical clarity, reflecting that the strong scattering of MMT clay results in lower transparency of the UV–vis light. However, all of the PCN materials remain transparent, as shown in Fig. 13.

2500

(e)

4. Conclusion

2000

(c) 1500

(d) (b)

(a)

1000 500 0 20

40

60

80

100

120

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100 120 80 Temperature ( C)

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160

Temperature ( C)

(b) 0.8

(b) (c) (d) (a)

0.6

(e)

0.4

0.2

0.0 20

40

60

160

Fig. 12. Dynamic mechanical property variation with temperature and clay loading for the membranes of: (a) epoxy, (b) CLMA1, (c) CLMA3, (d) CLMA5, and (e) CLMA7. (a) Storage modulus (EV). (b) Tangent d.

A series of siloxane-modified epoxy resin–clay nanocomposite materials has been successfully prepared and their corresponding protective performance has been evaluated by standard electrochemical corrosion testing. The asprepared nanocomposite materials were subsequently characterized by FTIR spectroscopy, powder XRD, and TEM. The as-prepared epoxy resin–clay nanocomposite materials, in the form of coating, were found to show advanced protection against corrosion on CRS coupon compared to bulk epoxy resin based on a series of standard electrochemical corrosion measurements such as corrosion potential, polarization resistance, corrosion current, and impedance spectroscopy. Molecular (e.g., O2, N2, and H2O) permeability of epoxy resin–clay nanocomposite membranes is found to be lower than that of bulk epoxy resin along with the loading of nanoclay based on the studies of gas and VPA. Moreover, the epoxy resin–clay nanocomposite materials have significant advantages over standard epoxy resins such as lower water absorption, lower cure shrinkage, moderate glass transition temperature (T g), and higher tensile strength. Thermal stability, mechanical strength, and optical clarity of the flexible siloxane-modified epoxy resin and epoxy resin–clay nanocomposite materials, in the form of membrane, are also investigated by TGA, DSC, DMA, TMA, and UV–vis transmission spectra, respectively.

J.-M. Yeh et al. / Surface & Coatings Technology 200 (2006) 2753–2763

Acknowledgements The financial support of this research by the National Science Council (subsidy no. 92-2113-M-033-004) is gratefully acknowledged.

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