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Thermal stability and corrosion resistance of polysiloxane coatings on 2024-T3 and 6061-T6 aluminum alloy
Surface & Coatings Technology 201 (2007) 5782 – 5788 www.elsevier.com/locate/surfcoat
Thermal stability and corrosion resistance of polysiloxane coatings on 2024-T3 and 6061-T6 aluminum alloy K.H. Wu ⁎, C.M. Chao, T.F. Yeh, T.C. Chang Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 335, Taiwan Received 26 July 2006; accepted in revised form 16 October 2006 Available online 21 November 2006
Abstract Hybrid coatings based on polydimethylsiloxane-cured organically modified silicate were synthesized through a sol–gel technique. Aminoterminated siloxane, 3-glycidoxypropyltrimethoxysilane and tetraethoxysilane were used as precursors for the hybrid coatings. These hybrid films were deposited via spin coating onto an aluminum alloy to improve the corrosion protection. The effects induced by the different molar ratio of silane on the chain dynamics, thermal stability and corrosion performance of the coated samples were investigated. The rotating-frame spin-lattice relaxation times and scale of the spin-diffusion path length indicated that the configuration of the hybrid films was highly crosslinked and dense. The thermal stability and the apparent activation energy, evaluated by van Krevelen's method, of the hybrid coatings depended on the molar ratio of silane. Potentiodynamic analysis revealed that the hybrid films provided good barrier and corrosion protection in comparison with untreated aluminum alloy substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Corrosion; Silane; Sol–gel coatings; Thermal degradation
1. Introduction Organically modified silicates (Ormosils) are hybrid organic– inorganic materials formed through the hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors. Ormosil materials have been investigated for the corrosion protection of aluminum alloy (AA) substrates [1–6]. Ormosil coatings exhibit increased flexibility and thickness in comparison with their inorganic counterparts. In general, these sol–gel derived coatings have been found to provide good corrosion resistance for metal substrates because of their barrier properties, tenacious adhesion to metal substrates, chemical inertness, versatility in coating formulations and ease of application under ambient-temperature conditions. Organic polymerization reactions in Ormosil are commonly used to modify material properties. The silicon–oxygen backbone of this type of polymers endows the layer with a variety of intriguing properties [7,8]. For example, the bonding
⁎ Corresponding author. E-mail address: [email protected] (K.H. Wu). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.024
strength of the silicon–oxygen network gives the siloxane based polymers considerable thermal stability. The Si–OH groups within the silane molecule can perform strong bonds to oxidic/ hydroxidic glass or metal surfaces. If the silane molecule also contains functionalized organic groups, the system can be used as an adhesion promoter providing a link between metallic surfaces and organic coatings. Depending on the type of silane used as monomer, the reactive functional group of the polysiloxane side chains can react with lacquers and organic polymer films to form chemical covalent or ionic bonds, promoting e.g. paint adhesion [9,10]. Our previous work has shown that introduction of a curing agent and metal oxide particle (ZrO2, TiO2 and CeO2) into the sol–gel coating solution results in a denser hybrid film that benefits surface protection by reducing reaction rates with moisture [11,12]. In this study, the use of amino-terminated polydimethylsiloxane oligomer (PDMS) as crosslinking agent in epoxide-modified Ormosils has been investigated. The PDMS oligomer repeating unit, –OSi(CH3)2–, is endowed with unusual properties such as high dynamic flexibility, high oxidative stability and excellent thermal stability [13]. Aminoterminated PDMS is considered to be epoxide crosslinker, as
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788
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Scheme 1.
they were participated in ring opening reactions and may become chemically bonded to the epoxide ring. To understand the effects of varying the molar ratio of silane on the chain dynamics and thermal stability of the Ormosil hybrid coatings, we have measured the proton spin-lattice relaxation time in the rotating H frame (T1ρ ) and the apparent activation energy (Ea), respectively. The chain dynamics and degradation of the Ormosils provide important additional information for its use and applications as an anticorrosion coating. Additionally, electrochemical analysis has been used to investigate the relation between relaxation behavior and corrosion resistance of the PDMS-cured Ormosil hybrids. 2. Experimental 2.1. PDMS-cured Ormosil hybrids preparation 3-Glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) were purchased from Aldrich and were used as received. Crosslinking agent α, ω-aminopropyl polydimethylsiloxane oligomer (PDMS) with a number-average molar mass of 4400 g mol− 1 was obtained from Dow Corning. The PDMS-cured Ormosil hybrids were prepared as described in Scheme 1.
GPTMS and TEOS were placed in a beaker with 0.05 M HNO3. Total mole hydrolysis water added was 6 [4 (mole number of TEOS) + 3 (mole number of GPTMS)]. The resultant two-phase solution was vigorously stirred to induce mixing and initiate hydrolysis. The GPTMS to TEOS molar ratio R value was varied at 1, 2, 4, and 8 with similar synthesis conditions. The sol was allowed to stir for 3 days before the addition of the crosslinking agent. 10 wt.% of amino-terminated PDMS was added to GPTMS-TEOS sol. The samples were designated Ormosil-1, Ormosil-2, Ormosil-4, and Ormosil-8, respectively. After an additional 3 h of stirring, it was ready for spin coating onto precleaned 2024-T3 and 6061-T6 aluminum alloys to form a film. Each single layer of the sol–gel film was spun onto the substrate at 1000 rpm for 30 s, and this was repeated for three layers. Then, the coated films were allowed to dry under the ambient conditions for 24 h and further in a furnace at 60 °C for 2 h and at 100 °C for 2 h. 2.2. Characterization of the Ormosil hybrids The PDMS-cured Ormosil hybrids were confirmed by Fourier transform infrared (FTIR) spectra (Tensor 27) of samples prepared with the KBr pellet technique. The 13C and
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29
Si NMR spectra of the solid-state hybrids were determined (MSL-400, Bruker) with the cross-polarization/magic-angle H spinning (CP-MAS) technique. The T1ρ values were measured 1 with a H spin-lock τ-pulse sequence followed by crosspolarization. The 1H 90° pulse widths was 4.5 μs, and the crosspolarization contact time was 1 ms. The delay time (τ) ranged H from 0.1 to 3 ms for T1ρ . The characteristics and kinetics of degradation of the PDMS-cured Ormosil hybrids were examined with a PerkinElmer TGA-2 at a heating rate of 10 °C/min under nitrogen. The sample weight was about 10 mg, and the gas flow rate was kept at 100 mL/min. Electrochemical measurements were performed with an EG&G 263A unit and a three-electrode cell equipped with a platinum electrode, a Ag/AgCl/Cl− reference electrode, and a coated or noncoated AA 6061-T6 panel as the working electrode with an exposed area of 1 cm2. All measurements were conducted in an aqueous 0.1 wt.% NaCl working solution at 25 °C. Oxygen was removed by the purging of the solution with purified nitrogen for approximately 30 min before the polarization measurements. The acquisition of polarization curves was started from the open circuit potential, with a constant sweep of 1 mV/s. The corrosion current (Icorr) values reported herein correspond to a 50-mV gap between the cathodic and anodic parts of the polarization curve. Each coating system was evaluated in sets of three to four replicate panels. For all experiments, at least three replicates were run to ensure the reproducibility of the experimental data. 3. Results and discussion 3.1. Structure characterization
Fig. 2. (a) 13C and (b) 29Si CP/MAS NMR spectra of PDMS-cured Ormosil hybrids.
FTIR and solid-state 13C and 29Si NMR provide evidence for the formation of Ormosil hybrids. Fig. 1 shows the FTIR spectra of the GPTMS-TEOS Ormosil and PDMS-cured Ormosil hybrids. The absorptions in the range of 3700–3000 and at 950 cm− 1 in the GPTMS-TEOS Ormosil are assigned to the silanol groups (Si–OH) that are formed during the hydrolysis of
alkoxy groups in GPTMS and TEOS. The silica network is characterized by the strong absorptions at 1070, 795 and 445 cm− 1, corresponding to the Si–O–Si anti-symmetric stretching, symmetric stretching and bending mode. The peaks at 1730, 1638 and 1384 cm− 1 are associated with the C–O, C–C and C–H stretching in epoxide ring, respectively.
Fig. 1. IR spectra of (a) GPTMS-TEOS, (b) Ormosil-1, (c) Ormosil-2, (d) Ormosil-4 and (e) Ormsil-8.
Fig. 3. Semilogarithmic plot of the peak intensities in Ormosil-4 as a function of delay time.
13
C-NMR spectra of
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788 Table 1 TH 1ρ and L values of the respective resonance lines of hybrid coatings Samples
GPTMS-TEOS Ormosil-1 Ormosil-2 Ormosil-4 Ormosil-8
TH 1ρ (ms); glycidoxypropyl units 9 ppm
24 ppm
74 ppm
Average
3.01 2.27 2.06 1.88 1.83
2.79 2.12 1.89 1.71 1.50
2.62 1.71 1.77 1.60 1.80
2.81 2.03 1.91 1.73 1.71
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units are found to increase, indicating that a more threedimensional silica network develops at a higher R value. L (nm) 3.18 2.70 2.62 2.50 2.48
H 3.2. 1H spin-lattice relaxation time in the rotating frame T1ρ H On the basis of the spin-locking mode employed in T1ρ measurements, the magnetization of resonance is expected to decay according to the following exponential function [14]:
Ms ¼ M0 expðs=T1Hρ Þ Another, with an increase in the GPTMS content, the intensity of the bands at 2937 and 2879 cm− 1 corresponding to glycidoxypropyl unit (–CH2– stretching) of GPTMS characteristics increases distinctively. Two new bands appear at 1670 and 1044 cm− 1 in the spectra of the PDMS-cured Ormosil hybrids (Fig. 1b–e), corresponding to the N–H bending and C–N stretching vibration, respectively. Moreover, the disappearance or decrease at 1730, 1638 and 1384 cm− 1 of the PDMS-cured Ormosil hybrids revealed that the epoxide ring opening reactions were done by reaction of amino-terminated PDMS and epoxy group. The silica network stretching band in Fig. 1a (1070 cm− 1) is shifted to higher wave number in comparison with in Fig. 1b–e (1100 cm− 1), and the band intensity increases with increasing R value, which can be ascribed to enhanced interaction between the silica network. 13 C and 29Si CP-MAS NMR spectra are collected for the PDMS-cured Ormosil hybrids as shown in Fig. 2. 13C NMR spectra of the hybrids are nearly identical. Distinct peaks for Ormosil-4 in 13C NMR have been observed at 6.5 (Si–CH2), 21.2 (Si–CH2CH2CH2), 69.0–73.8 (Si–CH2CH2CH2–O–CH2), 63.2 (C–OH) and 58.2 ppm (C–OR). Distinct peaks for silica network units in the hybrids have been observed at −60 (T2), −69 (T3), −103 (Q3) and −112 ppm (Q4). T2, T3, Q3 and Q4 denote R-Si (OSi)2(OH), R-Si(OSi)3, Si(OSi)3(OH) and Si(OSi)4, respectively. The signals of 29Si NMR in the PDMS-cured Ormosil hybrids are found to depend on the R value. As R value increases, the relative peak intensity ratios of I(Q4)/I(Q3) and Ti silica network
ð1Þ
where τ is the delay time used in the experiment, M0 is the resonance intensity at which τ is close to zero and Mτ corresponds to the resonance intensity. Fig. 3 shows a semilogarithmic plot of the peak intensity as a function of τ for the H carbon in the Ormosil-4 hybrid. The T1ρ values of the respective carbon are evaluated from the slopes of Fig. 3 by Eq. (1), as shown in Table 1. On the other hand, the spin-diffusion path length (L) can be estimated with the following equation [14]: L¼
qffiffiffiffiffiffiffiffiffiffiffiffiffi 6DT1Hρ
ð2Þ
where D is the effective spin-diffusion coefficient depending on the average proton-to-proton distance as well as the dipolar interaction. If the chains are intimately and homogeneously mixed, spin diffusion occurs quickly among the chemically H different constituents, and a single value of T1ρ is determined through analysis of the decay rates for all carbons. If the chains H do not interact on the molecular level, different T1ρ values are observed for the carbons corresponding to the different polymers. Values of 0.5–0.8 nm2/ms for D in rigid polymers at temperatures below Tg are well accepted and have been confirmed [15,16]. Thus, in our analysis we used D = 0.6 nm2/ ms for organic domains. The exponential decay of the PDMScured Ormosil hybrids can be represented by a single relaxation H time, indicating a homogeneous system with T1ρ about 2.03– H 1.71 ms. The calculated T1ρ and L value of PDMS-cured
Fig. 4. TGA and DTG thermograms of the Ormosil hybrids under nitrogen at the heating rate of 10 °C/min.
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Table 2 Activation parameters of the Ormosil hybrids thermal degradation at heating rate of 10 °C/min under nitrogen Samples
T5 (°C)
Tm (°C)
Yc (wt.%)
n
Ea (kJ/mol)
TEOS-GPTMS Ormosil-1 Ormosil-2 Ormosil-4 Ormosil-8
315 249 247 300 296
450 452 450 442 440
45 49 46 40 37
1.27 1.28 1.28 1.30 1.30
87 35 40 55 58
Ormosil hybrids is lower than that of GPTMS-TEOS Ormosil, indicating that the Ormosil hybrids have a denser configuration when PDMS incorporates and the molecular motion of the organic units is restricted by the crosslinked structure. 3.3. Thermal analysis The weight-loss curves (TGA) and differential thermogravimetry curves (DTG) of PDMS-cured Ormosil hybrids, obtained at a heating rate of 10 °C/min under nitrogen, are shown in Fig. 4. The 5% weight loss temperature (T5) and the temperature of the maximum rate of weight loss (Tm) illustrate a qualitative characterization of the degradation process. The characteristic temperature of degradation and the char yield (Yc) at 800 °C, obtained from DTG and TG traces, respectively, are listed in Table 2. Comparing the T5 and Tm of the GPTMS-TEOS Ormosil with those of the PDMS-cured Ormosil hybrids, the value of T5 for PDMS-cured Ormosil hybrids slightly decreases, while the value of Tm is constant nearly (∼446 °C). The explanation for this result may be due to thermal degradation begins at the aliphatic n-propyl segments in PDMS. The DTG curve of Ormosil-2 has been deconvoluted and the result is shown in Fig. 5. A standard line shape analysis with multiple Gaussian fitting functions reveals that the DTG curve can be fitted with four peaks. The weight loss at the interval of 160–270 °C is attributed to the decomposition of aliphatic npropyl segments. The second step (270–370 °C) can be assigned to the decomposition of the glycidoxypropyl units, the third and fourth step (370–600 °C) to the decomposition of the Ti and Qi silica network, respectively.
Fig. 6. Plot of ln g(α) versus ln T for thermal degradation of the Ormsil hybrid coatings.
3.4. Kinetic analysis The degree of conversion (α) is defined as the ratio of the actual weight loss to the total weight loss. Therefore, the rate of degradation (dα/dt), dependent on the temperature and weight of the sample, is given by Eq. (3): da=dt ¼ kðT Þ f ðaÞ
ð3Þ
where k(T) is the rate constant and f(α) is the conversion functional relationship. If k(T) = A exp(− Ea/RT) and f(α) = (1 − α)n, then Eq. (3) can be expressed as follows: da=dt ¼ Aexpð−Ea =RT Þð1−aÞn
ð4Þ
where A, Ea, R, T and n represent the pre-exponential factor, apparent activation energy, gas constant, temperature and reaction order, respectively. By integrating Eq. (4) and introducing the initial condition of α = 0 at T = T0, we obtain the following expression: Z Z a da A T −Ea g ð aÞ ¼ ¼ exp dT ð5Þ n q T0 RT 0 ð1−aÞ where g(α) is the degradation rate functional relationship and q is the heating rate (dT/dt). For the special case of n = 1 Z g ð aÞ ¼ 0
a
da ¼ −lnð1−aÞ ð1−aÞn
For n not equal to zero or a unit Z a da 1−ð1−aÞ1−n g ð aÞ ¼ n ¼ 1−n 0 ð1−aÞ
Fig. 5. DTG and deconvoluted curves of Ormosil-2 as shown in Fig. 4.
ð6Þ
ð7Þ
In this study, n for thermal degradation pffiffiffiffi has been determined by Kissinger's equation, n ¼ 1:26 S; where S is the shape
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788
index of the differential thermal analysis. S is defined as the absolute value of the ratio of the slopes of tangents to the curve at the inflection points [17]. Several techniques using different approaches have been developed for solving the integral of Eq. (5). The method investigated in this work is that by van Krevelen et al. [18]. The van Krevelen method for n ≠ 1 gave the following expression: Að0:368=Tm Þx ln g ðaÞ ¼ ln þ ð x þ 1ÞlnT ð8Þ qðx þ 1Þ
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Table 3 Electrochemical characteristics for aluminum alloy and Ormosils hybrid coatings Sample
2024-T3
Ormosil-1
Ormosil-2
Ormosil-4
Ormosil-8
Ecorr (mV) − 378 − 439 − 433 − 397 − 337 Icorr 7.08 × 10− 8 2.91 × 10− 10 3.67 × 10− 8 9.44 × 10− 11 1.74 × 10− 10 (A/cm2) Sample
6061-T6
Ormosil-1
− 661 Ecorr (mV) − 571 Icorr 1.17 × 10− 8 4.47 × 10− 9 (A/cm2)
Ormosil-2
Ormosil-4
Ormosil-8
− 658 − 566 − 633 5.27 × 10− 9 1.32 × 10− 10 5.43 × 10− 10
For n = 1 ln g ðaÞ ¼ ðx þ 1ÞlnT
ð9Þ
Here x is equal to Ea/RTm. Tm is the temperature at the maximum rate of weight loss. The activation energies were evaluated from the slopes in a plot of ln g(α) versus ln T. Fig. 6 shows the logarithmic plot for g(α) of the silica network (T i ; 400–550 °C) versus the temperature under nitrogen. The n and Ea values, evaluated by van Krevelen's method, are listed in Table 2. The Ea value of the Ti segments in
the PDMS-cured Ormosil hybrids increases with increasing the R value. This result is due to that there are greater Ti structures in PDMS-cured Ormosil hybrids as the R value increased. Moreover, the Ea value of PDMS-cured Ormosil hybrids is lower than that of GPTMS-TEOS Ormosil (87 kJ/mol). The decrease of the Ea may be associated with the higher crosslinking or higher density configuration when PDMS incorporates that favor the aliphatic n-propyl segments degradation, which induced by the higher thermal conductivity of the silica (5 mcal/cm s °C) [19,20]. 3.5. Corrosion resistance Fig. 7 presents potentiodynamic anodic polarization measurements used to assess barrier and corrosion protection. Electrochemical characteristics derived from the polarization curves acquired for AA substrates coated with various hybrid films are listed in Table 3. The corrosion resistance of the PDMS-cured Ormosil hybrids is better than that of the untreated AA sample. The result may be due to the production of denser films that are correspondingly less susceptible to localized pitting. The measured Icorr values decrease significantly for the PDMS-cured Ormosil hybrids on AA 2024-T3 (3.67 × 10− 8– 9.44 × 10 − 11 A/cm 2 ) and AA 6061-T6 (5.27 × 10 − 9 – 1.32 × 10− 10 A/cm2) in comparison with the value for the untreated AA samples (1.17–7.08 × 10− 8 A/cm2). A comparison of the current densities indicated that the PDMS-cured Ormosil hybrids provide good barrier for resisting corrosion. Moreover, the Ormosil-4 and Ormosil-8 hybrids show better resistance than the other hybrids. This result reveals that there is a greater crosslinked structure of PDMS-cured Ormosil hybrid with R = 4 and 8. 4. Conclusions
Fig. 7. Potentiodynamic polarization of (a) AA 2024-T3 and (b) AA 6061-T6 coated with GPTMS-TEOS Ormosil films cured with different R value.
The effects of varying the GPTMS to TEOS molar ratio R on the chain dynamics, thermal stability and corrosion resistance were characterized with NMR, TGA and electrochemical H measurements. The T1ρ values and scale of L of the Ormosils were measured to understand the chain dynamics of the organic H units. The calculated T1ρ and L values of PDMS-cured Ormosil hybrids were lower than that of GPTMS-TEOS Ormosil. This observation can be attributed to a higher density configuration when PDMS was incorporated. The thermal stability and the Ea
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value of thermal degradation of PDMS-cured Ormosil hybrids were lower than that of GPTMS-TEOS Ormosil. This suggested that the higher thermal conductivity of the silica enhanced the aliphatic n-propyl segments degradation. The electrochemical test evaluation of the Ormosil films demonstrated the barrier and corrosion resistance properties imparted by the hybrid system. The low current density over a wide potential indicated that the film provided an effective barrier to water and corrosive agents. In summary, the results of NMR and potentiodynamic polarization curve analysis provided strong evidence that the Ormosil-4 hybrid had a denser configuration and showed better corrosion resistance than other Ormosil hybrids. Acknowledgement The authors thank the National Science Council of the Republic of China for supporting this work (Grant NSC 952113-M-014-004). The authors express their gratitude to Miss S.Y. Fang of NSC Instrument Center for NMR analysis. References [1] T.L. Metroke, O. Kachurina, E.T. Knobbe, Prog. Org. Coat. 44 (2002) 185. [2] T.L. Metroke, J.S. Gandhi, A. Apblett, Prog. Org. Coat. 50 (2004) 231.
[3] O. Kachurina, T.L. Metroke, E. Stesikova, E.T. Knobbe, J. Coat. Technol. 74 (2002) 43. [4] T.P. Chou, C. Chandrasekaran, G.Z. Cao, J. Sol-Gel Sci. Technol. 26 (2003) 321. [5] S.K. Medda, D. Kundu, G. De, J. Non-Cryst. Solids 318 (2003) 149. [6] D. Susac, X. Sun, K.A.R. Mitchell, Appl. Surf. Sci. 207 (2003) 40. [7] W.E.G. Hansal, S. Hansal, M. Polzler, A. Kornherr, G. Zifferer, G.E. Nauer, Surf. Coat. Technol. 200 (2006) 3056. [8] P.E. Hintze, L.M. Calle, Electrochim. Acta 51 (2006) 1761. [9] W.J. van Ooij, ATB Metall. 38 (1998) 63. [10] G.P. Sundararajan, W.J. van Ooij, Surf. Eng. 16 (2000) 315. [11] K.H. Wu, M.C. Li, T.C. Chang, C.C. Yang, J. Polym. Sci., Part A, Polym. Chem. 44 (2006) 335. [12] K.H. Wu, T.C. Chang, C.C. Yang, G.P. Wang, Thin Solid Films 513 (2006) 84. [13] G.P. Wang, K.H. Wu, T.C. Chang, Y.S. Hong, Y.S. Chiu, Polym. Degrad. Stab. 72 (2001) 297. [14] R.H. Glaser, G.L. Wilkes, C.E. Bronnimann, J. Non-Cryst. Solids 113 (1989) 73. [15] J. Wang, M.K. Cheung, Y. Mi, Polymer 43 (2002) 1357. [16] J. Clauss, K. Schimidt-Rohr, H.W. Spiess, Acta Polym. 44 (1993) 1. [17] H.H.E. Kissinger, Anal. Chem. 29 (1957) 1702. [18] D.W. van Krevelen, C. van Herden, F.J. Huntjens, Fuel 30 (1951) 253. [19] I. Yilgor, J.E. McGrath, Adv. Polym. Sci. 86 (1988) 1. [20] D.S. Soane, Z. Martynenko, Polymers in Microelectronics, Fundamentals and Applications, Elsevier, Amsterdam, 1989, p. 159.
Thermal stability and corrosion resistance of polysiloxane coatings on 2024-T3 and 6061-T6 aluminum alloy K.H. Wu ⁎, C.M. Chao, T.F. Yeh, T.C. Chang Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 335, Taiwan Received 26 July 2006; accepted in revised form 16 October 2006 Available online 21 November 2006
Abstract Hybrid coatings based on polydimethylsiloxane-cured organically modified silicate were synthesized through a sol–gel technique. Aminoterminated siloxane, 3-glycidoxypropyltrimethoxysilane and tetraethoxysilane were used as precursors for the hybrid coatings. These hybrid films were deposited via spin coating onto an aluminum alloy to improve the corrosion protection. The effects induced by the different molar ratio of silane on the chain dynamics, thermal stability and corrosion performance of the coated samples were investigated. The rotating-frame spin-lattice relaxation times and scale of the spin-diffusion path length indicated that the configuration of the hybrid films was highly crosslinked and dense. The thermal stability and the apparent activation energy, evaluated by van Krevelen's method, of the hybrid coatings depended on the molar ratio of silane. Potentiodynamic analysis revealed that the hybrid films provided good barrier and corrosion protection in comparison with untreated aluminum alloy substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Corrosion; Silane; Sol–gel coatings; Thermal degradation
1. Introduction Organically modified silicates (Ormosils) are hybrid organic– inorganic materials formed through the hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors. Ormosil materials have been investigated for the corrosion protection of aluminum alloy (AA) substrates [1–6]. Ormosil coatings exhibit increased flexibility and thickness in comparison with their inorganic counterparts. In general, these sol–gel derived coatings have been found to provide good corrosion resistance for metal substrates because of their barrier properties, tenacious adhesion to metal substrates, chemical inertness, versatility in coating formulations and ease of application under ambient-temperature conditions. Organic polymerization reactions in Ormosil are commonly used to modify material properties. The silicon–oxygen backbone of this type of polymers endows the layer with a variety of intriguing properties [7,8]. For example, the bonding
⁎ Corresponding author. E-mail address: [email protected] (K.H. Wu). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.024
strength of the silicon–oxygen network gives the siloxane based polymers considerable thermal stability. The Si–OH groups within the silane molecule can perform strong bonds to oxidic/ hydroxidic glass or metal surfaces. If the silane molecule also contains functionalized organic groups, the system can be used as an adhesion promoter providing a link between metallic surfaces and organic coatings. Depending on the type of silane used as monomer, the reactive functional group of the polysiloxane side chains can react with lacquers and organic polymer films to form chemical covalent or ionic bonds, promoting e.g. paint adhesion [9,10]. Our previous work has shown that introduction of a curing agent and metal oxide particle (ZrO2, TiO2 and CeO2) into the sol–gel coating solution results in a denser hybrid film that benefits surface protection by reducing reaction rates with moisture [11,12]. In this study, the use of amino-terminated polydimethylsiloxane oligomer (PDMS) as crosslinking agent in epoxide-modified Ormosils has been investigated. The PDMS oligomer repeating unit, –OSi(CH3)2–, is endowed with unusual properties such as high dynamic flexibility, high oxidative stability and excellent thermal stability [13]. Aminoterminated PDMS is considered to be epoxide crosslinker, as
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788
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Scheme 1.
they were participated in ring opening reactions and may become chemically bonded to the epoxide ring. To understand the effects of varying the molar ratio of silane on the chain dynamics and thermal stability of the Ormosil hybrid coatings, we have measured the proton spin-lattice relaxation time in the rotating H frame (T1ρ ) and the apparent activation energy (Ea), respectively. The chain dynamics and degradation of the Ormosils provide important additional information for its use and applications as an anticorrosion coating. Additionally, electrochemical analysis has been used to investigate the relation between relaxation behavior and corrosion resistance of the PDMS-cured Ormosil hybrids. 2. Experimental 2.1. PDMS-cured Ormosil hybrids preparation 3-Glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) were purchased from Aldrich and were used as received. Crosslinking agent α, ω-aminopropyl polydimethylsiloxane oligomer (PDMS) with a number-average molar mass of 4400 g mol− 1 was obtained from Dow Corning. The PDMS-cured Ormosil hybrids were prepared as described in Scheme 1.
GPTMS and TEOS were placed in a beaker with 0.05 M HNO3. Total mole hydrolysis water added was 6 [4 (mole number of TEOS) + 3 (mole number of GPTMS)]. The resultant two-phase solution was vigorously stirred to induce mixing and initiate hydrolysis. The GPTMS to TEOS molar ratio R value was varied at 1, 2, 4, and 8 with similar synthesis conditions. The sol was allowed to stir for 3 days before the addition of the crosslinking agent. 10 wt.% of amino-terminated PDMS was added to GPTMS-TEOS sol. The samples were designated Ormosil-1, Ormosil-2, Ormosil-4, and Ormosil-8, respectively. After an additional 3 h of stirring, it was ready for spin coating onto precleaned 2024-T3 and 6061-T6 aluminum alloys to form a film. Each single layer of the sol–gel film was spun onto the substrate at 1000 rpm for 30 s, and this was repeated for three layers. Then, the coated films were allowed to dry under the ambient conditions for 24 h and further in a furnace at 60 °C for 2 h and at 100 °C for 2 h. 2.2. Characterization of the Ormosil hybrids The PDMS-cured Ormosil hybrids were confirmed by Fourier transform infrared (FTIR) spectra (Tensor 27) of samples prepared with the KBr pellet technique. The 13C and
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29
Si NMR spectra of the solid-state hybrids were determined (MSL-400, Bruker) with the cross-polarization/magic-angle H spinning (CP-MAS) technique. The T1ρ values were measured 1 with a H spin-lock τ-pulse sequence followed by crosspolarization. The 1H 90° pulse widths was 4.5 μs, and the crosspolarization contact time was 1 ms. The delay time (τ) ranged H from 0.1 to 3 ms for T1ρ . The characteristics and kinetics of degradation of the PDMS-cured Ormosil hybrids were examined with a PerkinElmer TGA-2 at a heating rate of 10 °C/min under nitrogen. The sample weight was about 10 mg, and the gas flow rate was kept at 100 mL/min. Electrochemical measurements were performed with an EG&G 263A unit and a three-electrode cell equipped with a platinum electrode, a Ag/AgCl/Cl− reference electrode, and a coated or noncoated AA 6061-T6 panel as the working electrode with an exposed area of 1 cm2. All measurements were conducted in an aqueous 0.1 wt.% NaCl working solution at 25 °C. Oxygen was removed by the purging of the solution with purified nitrogen for approximately 30 min before the polarization measurements. The acquisition of polarization curves was started from the open circuit potential, with a constant sweep of 1 mV/s. The corrosion current (Icorr) values reported herein correspond to a 50-mV gap between the cathodic and anodic parts of the polarization curve. Each coating system was evaluated in sets of three to four replicate panels. For all experiments, at least three replicates were run to ensure the reproducibility of the experimental data. 3. Results and discussion 3.1. Structure characterization
Fig. 2. (a) 13C and (b) 29Si CP/MAS NMR spectra of PDMS-cured Ormosil hybrids.
FTIR and solid-state 13C and 29Si NMR provide evidence for the formation of Ormosil hybrids. Fig. 1 shows the FTIR spectra of the GPTMS-TEOS Ormosil and PDMS-cured Ormosil hybrids. The absorptions in the range of 3700–3000 and at 950 cm− 1 in the GPTMS-TEOS Ormosil are assigned to the silanol groups (Si–OH) that are formed during the hydrolysis of
alkoxy groups in GPTMS and TEOS. The silica network is characterized by the strong absorptions at 1070, 795 and 445 cm− 1, corresponding to the Si–O–Si anti-symmetric stretching, symmetric stretching and bending mode. The peaks at 1730, 1638 and 1384 cm− 1 are associated with the C–O, C–C and C–H stretching in epoxide ring, respectively.
Fig. 1. IR spectra of (a) GPTMS-TEOS, (b) Ormosil-1, (c) Ormosil-2, (d) Ormosil-4 and (e) Ormsil-8.
Fig. 3. Semilogarithmic plot of the peak intensities in Ormosil-4 as a function of delay time.
13
C-NMR spectra of
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788 Table 1 TH 1ρ and L values of the respective resonance lines of hybrid coatings Samples
GPTMS-TEOS Ormosil-1 Ormosil-2 Ormosil-4 Ormosil-8
TH 1ρ (ms); glycidoxypropyl units 9 ppm
24 ppm
74 ppm
Average
3.01 2.27 2.06 1.88 1.83
2.79 2.12 1.89 1.71 1.50
2.62 1.71 1.77 1.60 1.80
2.81 2.03 1.91 1.73 1.71
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units are found to increase, indicating that a more threedimensional silica network develops at a higher R value. L (nm) 3.18 2.70 2.62 2.50 2.48
H 3.2. 1H spin-lattice relaxation time in the rotating frame T1ρ H On the basis of the spin-locking mode employed in T1ρ measurements, the magnetization of resonance is expected to decay according to the following exponential function [14]:
Ms ¼ M0 expðs=T1Hρ Þ Another, with an increase in the GPTMS content, the intensity of the bands at 2937 and 2879 cm− 1 corresponding to glycidoxypropyl unit (–CH2– stretching) of GPTMS characteristics increases distinctively. Two new bands appear at 1670 and 1044 cm− 1 in the spectra of the PDMS-cured Ormosil hybrids (Fig. 1b–e), corresponding to the N–H bending and C–N stretching vibration, respectively. Moreover, the disappearance or decrease at 1730, 1638 and 1384 cm− 1 of the PDMS-cured Ormosil hybrids revealed that the epoxide ring opening reactions were done by reaction of amino-terminated PDMS and epoxy group. The silica network stretching band in Fig. 1a (1070 cm− 1) is shifted to higher wave number in comparison with in Fig. 1b–e (1100 cm− 1), and the band intensity increases with increasing R value, which can be ascribed to enhanced interaction between the silica network. 13 C and 29Si CP-MAS NMR spectra are collected for the PDMS-cured Ormosil hybrids as shown in Fig. 2. 13C NMR spectra of the hybrids are nearly identical. Distinct peaks for Ormosil-4 in 13C NMR have been observed at 6.5 (Si–CH2), 21.2 (Si–CH2CH2CH2), 69.0–73.8 (Si–CH2CH2CH2–O–CH2), 63.2 (C–OH) and 58.2 ppm (C–OR). Distinct peaks for silica network units in the hybrids have been observed at −60 (T2), −69 (T3), −103 (Q3) and −112 ppm (Q4). T2, T3, Q3 and Q4 denote R-Si (OSi)2(OH), R-Si(OSi)3, Si(OSi)3(OH) and Si(OSi)4, respectively. The signals of 29Si NMR in the PDMS-cured Ormosil hybrids are found to depend on the R value. As R value increases, the relative peak intensity ratios of I(Q4)/I(Q3) and Ti silica network
ð1Þ
where τ is the delay time used in the experiment, M0 is the resonance intensity at which τ is close to zero and Mτ corresponds to the resonance intensity. Fig. 3 shows a semilogarithmic plot of the peak intensity as a function of τ for the H carbon in the Ormosil-4 hybrid. The T1ρ values of the respective carbon are evaluated from the slopes of Fig. 3 by Eq. (1), as shown in Table 1. On the other hand, the spin-diffusion path length (L) can be estimated with the following equation [14]: L¼
qffiffiffiffiffiffiffiffiffiffiffiffiffi 6DT1Hρ
ð2Þ
where D is the effective spin-diffusion coefficient depending on the average proton-to-proton distance as well as the dipolar interaction. If the chains are intimately and homogeneously mixed, spin diffusion occurs quickly among the chemically H different constituents, and a single value of T1ρ is determined through analysis of the decay rates for all carbons. If the chains H do not interact on the molecular level, different T1ρ values are observed for the carbons corresponding to the different polymers. Values of 0.5–0.8 nm2/ms for D in rigid polymers at temperatures below Tg are well accepted and have been confirmed [15,16]. Thus, in our analysis we used D = 0.6 nm2/ ms for organic domains. The exponential decay of the PDMScured Ormosil hybrids can be represented by a single relaxation H time, indicating a homogeneous system with T1ρ about 2.03– H 1.71 ms. The calculated T1ρ and L value of PDMS-cured
Fig. 4. TGA and DTG thermograms of the Ormosil hybrids under nitrogen at the heating rate of 10 °C/min.
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Table 2 Activation parameters of the Ormosil hybrids thermal degradation at heating rate of 10 °C/min under nitrogen Samples
T5 (°C)
Tm (°C)
Yc (wt.%)
n
Ea (kJ/mol)
TEOS-GPTMS Ormosil-1 Ormosil-2 Ormosil-4 Ormosil-8
315 249 247 300 296
450 452 450 442 440
45 49 46 40 37
1.27 1.28 1.28 1.30 1.30
87 35 40 55 58
Ormosil hybrids is lower than that of GPTMS-TEOS Ormosil, indicating that the Ormosil hybrids have a denser configuration when PDMS incorporates and the molecular motion of the organic units is restricted by the crosslinked structure. 3.3. Thermal analysis The weight-loss curves (TGA) and differential thermogravimetry curves (DTG) of PDMS-cured Ormosil hybrids, obtained at a heating rate of 10 °C/min under nitrogen, are shown in Fig. 4. The 5% weight loss temperature (T5) and the temperature of the maximum rate of weight loss (Tm) illustrate a qualitative characterization of the degradation process. The characteristic temperature of degradation and the char yield (Yc) at 800 °C, obtained from DTG and TG traces, respectively, are listed in Table 2. Comparing the T5 and Tm of the GPTMS-TEOS Ormosil with those of the PDMS-cured Ormosil hybrids, the value of T5 for PDMS-cured Ormosil hybrids slightly decreases, while the value of Tm is constant nearly (∼446 °C). The explanation for this result may be due to thermal degradation begins at the aliphatic n-propyl segments in PDMS. The DTG curve of Ormosil-2 has been deconvoluted and the result is shown in Fig. 5. A standard line shape analysis with multiple Gaussian fitting functions reveals that the DTG curve can be fitted with four peaks. The weight loss at the interval of 160–270 °C is attributed to the decomposition of aliphatic npropyl segments. The second step (270–370 °C) can be assigned to the decomposition of the glycidoxypropyl units, the third and fourth step (370–600 °C) to the decomposition of the Ti and Qi silica network, respectively.
Fig. 6. Plot of ln g(α) versus ln T for thermal degradation of the Ormsil hybrid coatings.
3.4. Kinetic analysis The degree of conversion (α) is defined as the ratio of the actual weight loss to the total weight loss. Therefore, the rate of degradation (dα/dt), dependent on the temperature and weight of the sample, is given by Eq. (3): da=dt ¼ kðT Þ f ðaÞ
ð3Þ
where k(T) is the rate constant and f(α) is the conversion functional relationship. If k(T) = A exp(− Ea/RT) and f(α) = (1 − α)n, then Eq. (3) can be expressed as follows: da=dt ¼ Aexpð−Ea =RT Þð1−aÞn
ð4Þ
where A, Ea, R, T and n represent the pre-exponential factor, apparent activation energy, gas constant, temperature and reaction order, respectively. By integrating Eq. (4) and introducing the initial condition of α = 0 at T = T0, we obtain the following expression: Z Z a da A T −Ea g ð aÞ ¼ ¼ exp dT ð5Þ n q T0 RT 0 ð1−aÞ where g(α) is the degradation rate functional relationship and q is the heating rate (dT/dt). For the special case of n = 1 Z g ð aÞ ¼ 0
a
da ¼ −lnð1−aÞ ð1−aÞn
For n not equal to zero or a unit Z a da 1−ð1−aÞ1−n g ð aÞ ¼ n ¼ 1−n 0 ð1−aÞ
Fig. 5. DTG and deconvoluted curves of Ormosil-2 as shown in Fig. 4.
ð6Þ
ð7Þ
In this study, n for thermal degradation pffiffiffiffi has been determined by Kissinger's equation, n ¼ 1:26 S; where S is the shape
K.H. Wu et al. / Surface & Coatings Technology 201 (2007) 5782–5788
index of the differential thermal analysis. S is defined as the absolute value of the ratio of the slopes of tangents to the curve at the inflection points [17]. Several techniques using different approaches have been developed for solving the integral of Eq. (5). The method investigated in this work is that by van Krevelen et al. [18]. The van Krevelen method for n ≠ 1 gave the following expression: Að0:368=Tm Þx ln g ðaÞ ¼ ln þ ð x þ 1ÞlnT ð8Þ qðx þ 1Þ
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Table 3 Electrochemical characteristics for aluminum alloy and Ormosils hybrid coatings Sample
2024-T3
Ormosil-1
Ormosil-2
Ormosil-4
Ormosil-8
Ecorr (mV) − 378 − 439 − 433 − 397 − 337 Icorr 7.08 × 10− 8 2.91 × 10− 10 3.67 × 10− 8 9.44 × 10− 11 1.74 × 10− 10 (A/cm2) Sample
6061-T6
Ormosil-1
− 661 Ecorr (mV) − 571 Icorr 1.17 × 10− 8 4.47 × 10− 9 (A/cm2)
Ormosil-2
Ormosil-4
Ormosil-8
− 658 − 566 − 633 5.27 × 10− 9 1.32 × 10− 10 5.43 × 10− 10
For n = 1 ln g ðaÞ ¼ ðx þ 1ÞlnT
ð9Þ
Here x is equal to Ea/RTm. Tm is the temperature at the maximum rate of weight loss. The activation energies were evaluated from the slopes in a plot of ln g(α) versus ln T. Fig. 6 shows the logarithmic plot for g(α) of the silica network (T i ; 400–550 °C) versus the temperature under nitrogen. The n and Ea values, evaluated by van Krevelen's method, are listed in Table 2. The Ea value of the Ti segments in
the PDMS-cured Ormosil hybrids increases with increasing the R value. This result is due to that there are greater Ti structures in PDMS-cured Ormosil hybrids as the R value increased. Moreover, the Ea value of PDMS-cured Ormosil hybrids is lower than that of GPTMS-TEOS Ormosil (87 kJ/mol). The decrease of the Ea may be associated with the higher crosslinking or higher density configuration when PDMS incorporates that favor the aliphatic n-propyl segments degradation, which induced by the higher thermal conductivity of the silica (5 mcal/cm s °C) [19,20]. 3.5. Corrosion resistance Fig. 7 presents potentiodynamic anodic polarization measurements used to assess barrier and corrosion protection. Electrochemical characteristics derived from the polarization curves acquired for AA substrates coated with various hybrid films are listed in Table 3. The corrosion resistance of the PDMS-cured Ormosil hybrids is better than that of the untreated AA sample. The result may be due to the production of denser films that are correspondingly less susceptible to localized pitting. The measured Icorr values decrease significantly for the PDMS-cured Ormosil hybrids on AA 2024-T3 (3.67 × 10− 8– 9.44 × 10 − 11 A/cm 2 ) and AA 6061-T6 (5.27 × 10 − 9 – 1.32 × 10− 10 A/cm2) in comparison with the value for the untreated AA samples (1.17–7.08 × 10− 8 A/cm2). A comparison of the current densities indicated that the PDMS-cured Ormosil hybrids provide good barrier for resisting corrosion. Moreover, the Ormosil-4 and Ormosil-8 hybrids show better resistance than the other hybrids. This result reveals that there is a greater crosslinked structure of PDMS-cured Ormosil hybrid with R = 4 and 8. 4. Conclusions
Fig. 7. Potentiodynamic polarization of (a) AA 2024-T3 and (b) AA 6061-T6 coated with GPTMS-TEOS Ormosil films cured with different R value.
The effects of varying the GPTMS to TEOS molar ratio R on the chain dynamics, thermal stability and corrosion resistance were characterized with NMR, TGA and electrochemical H measurements. The T1ρ values and scale of L of the Ormosils were measured to understand the chain dynamics of the organic H units. The calculated T1ρ and L values of PDMS-cured Ormosil hybrids were lower than that of GPTMS-TEOS Ormosil. This observation can be attributed to a higher density configuration when PDMS was incorporated. The thermal stability and the Ea
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value of thermal degradation of PDMS-cured Ormosil hybrids were lower than that of GPTMS-TEOS Ormosil. This suggested that the higher thermal conductivity of the silica enhanced the aliphatic n-propyl segments degradation. The electrochemical test evaluation of the Ormosil films demonstrated the barrier and corrosion resistance properties imparted by the hybrid system. The low current density over a wide potential indicated that the film provided an effective barrier to water and corrosive agents. In summary, the results of NMR and potentiodynamic polarization curve analysis provided strong evidence that the Ormosil-4 hybrid had a denser configuration and showed better corrosion resistance than other Ormosil hybrids. Acknowledgement The authors thank the National Science Council of the Republic of China for supporting this work (Grant NSC 952113-M-014-004). The authors express their gratitude to Miss S.Y. Fang of NSC Instrument Center for NMR analysis. References [1] T.L. Metroke, O. Kachurina, E.T. Knobbe, Prog. Org. Coat. 44 (2002) 185. [2] T.L. Metroke, J.S. Gandhi, A. Apblett, Prog. Org. Coat. 50 (2004) 231.
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