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Chemical and anticorrosion characterization of polysilsesquioxane coatings catalyzed by different acids.


Polysilsesquioxanes, with a formula of (RSi[O.sub.3/2])n, have been extensively studied because of their outstanding thermal, chemical, mechanical, and electronic properties. (1,2) They have been widely used as engineering plastics, (3,4) interlayer low dielectrics for electronic devices (5,6) and high performance coatings for protection, optics, electronics, and membranes. 7-10

Polysilsesquioxanes used as coatings are commonly synthesized through preparing prepolymers based on acid-catalyzed hydrolysis and condensation of organo-trialkoxysilanes or organotrichlorosilanes first and then further condensation of the prepolymer in the film forming process. (7-14) The frequently used catalysts in the sol-gel process of siloxane precursors are small molecular inorganic acids such as hydrochloric acid, acetic acid, and nitric acid, (9-14) which usually remain in the coatings as free molecules, causing negative influences on the corrosion resistant property of polysilsesquioxanes. (15) Chou et al. studied the corrosion resistance of polysilsesquioxane coatings on stainless steel, and found the hybrid coatings prepared by an hydrochloric acid catalyzed sol-gel process were uniform and defect-free. However, its anticorrosion property needs to be improved as a protective coating on stainless steel. (14)

In this article, we prepared polysilsesquioxanes via sol-gel process of methyltriethoxysilane (MTES), 3-glycidoxypropyltrimethoxysilane (GPMS), and tetraethoxysilane (TEOS) using phytic acid, tannic acid, and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA) (whose structures were present in Scheme 1) as the catalyst, respectively. The objective of this research was to compare the influence of different kinds of acid catalyst on the microstructure and corrosion resistant property of polysilsesquioxane coatings with the aid of [.sup.13.C] NMR, [.sup.29.Si] NMR, GPC, SEM, electrochemical techniques, and salt-spray test, respectively. The microstructure and anticorrosion property of hydrochloric acid-catalyzed polysilsesquioxanes were also studied for the sake of comparison.

Experimental section


Methyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane (GPMS) were purchased from Shanghai Yaohua Chemical Company. Tetraethoxysilane, phytic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and tannic acid were purchased from Shanghai Chemical Reagent Company of China. Ethanol was purchased from Shanghai Zhenxin Chemical Company. All these materials were used without further purification.


Preparation of acid-catalyzed polysilsesquioxanes

In a typical process, 0.002 mol of GPMS, 0.0012 mol of phytic acid, and 0.4 mol of ethanol were charged into a 250 mL three-neck round-bottom flask and reacted at 70[degrees]C for 1 h under magnetic stirring, followed by dropping a solution of 0.188 mol of MTES, 0.01 mol of TEOS, 0.6 mol of deionized water, and 0.2 mol of ethanol over a period of 20 min. The mixture was continued to react at 70[degrees]C for another 1 h to obtain phytic acid-catalyzed polysilsesquioxane (PAP) sols.

1-hydroxyethylidene-1,1-diphosphonic acid-catalyzed polysilsesquioxane (HPP), tannic acid-catalyzed polysilsesquioxane (TAP), and hydrochloric acid-catalyzed polysilsesquioxane (HCP) sols were also prepared according to the above procedure.

Preparation of polysilsesquioxane films

The substrates used for the analysis of the sol-gel coatings were 304 stainless steels. All the substrates were polished with grade 600 and 1000 emery papers and then washed with water, acetone, and dried in air.

All the polysilsesquioxane films were prepared by dip-coating process. The substrates were dipped into polysilsesquioxane sols for 1 min at a constant rate of 14 cm/min and extracted at the same rate. All the films were obtained by multiple dipping the substrates into sols and then air-dried at 25 [+ or -] 2[degrees]C for 7 days before characterization. Coating thickness was measured by surfcorder ET 3000 (Kosaka Laboratory Ltd.). The dried film thickness was controlled at 5.5 [+ or -] 0.3 [micro]m. The films for NMR characterization were cleaned by acetone.


GPC determination

The average molecular weight and molecular weight distribution were determined by a gel permeation chromatography (GPC) instrument (Waters Breeze, USA) with tetrahydrofuran (THF) as an eluant and narrow polystyrene as the calibration standard.

[.sup.13.C] and [.sup.29.Si] NMR analysis

The samples for [.sup.13.C] and [.sup.29.Si] NMR analysis were obtained from the dried films. The [.sup.13.C] and [.sup.29.Si] solid NMR spectra were recorded on a VANCE DSX-300 spectrometer (Bruker, Germany) with cross polarization. The conditions for [.sup.13.C] NMR analysis was carried out at 4.35 [micro]s, 2 s, and 1 ms for pulse width, recycle delay and contact time, respectively, the [.sup.29.Si] analysis at 4.2 [micro]s, 3 s, and 5 ms, respectively.

SEM observation

The morphologies of different acids-catalyzed polysilsesquioxane coatings were characterized using a scanning electron microscope (SEM Philips XL30). The samples were sputter-coated with gold prior to examination.

XPS analysis

XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg K[alpha] radiation (hv = 1253.6 eV). In general, the X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle of 54[degrees]. The pass energy was fixed at 46.95 or 93.90 eV to ensure sufficient sensitivity. Binding energies were calibrated using the containment carbon (C1s = 284.6 eV).

Potentiodynamic polarization analysis

Potentiodynamic polarization analyses were performed to assess the corrosion protection performance of the polysilsesquioxane-coated electrodes using a CHI604B electrochemical workstation (Shanghai Chenhua Instruments Inc., China). A working electrode was stainless steel embedded in epoxy resin exposing an area of 1 [cm.sup.2] immerging in 3% NaCl solution, and a saturated calomel electrode was used as reference, and a platinum as counter electrode. All samples were immersed in 3% NaCl solution for 30 min before testing. Potentiodynamic measurements were performed within the range of -1200 to 1000 mV vs SCE at a rate of 1 mV/s.

Electrochemical impedance spectroscopy

EIS measurements were carried out in a standard 500 mL three-electrode cell at room temperature in 3% NaCl solution just as potentiodynamic polarization experiments. Impedance plots were measured at the corrosion potential in the frequency range from [10.sup.-2] to [10.sup.5] Hz under excitation of a sinusoidal wave of [+ or -]10 mV amplitude. The first measurement, collected immediately after immersion in NaCl solution, is designated as 0 day. The sample was taken out periodically and the impedance measurements were collected. All the samples were immersed in NaCl solution for 30 days. The experimental error was [+ or -]5%.

Salt-fog measurement

The polysilsesquioxanes-coated stainless steel panels were placed into salt-fog cabinet with for 50 [+ or -] 10 g/L NaCl salt fog at 35 [+ or -] 2[degrees]C for 1000 h based on the standard measurement method of GB/T 1771-91.

Results and discussion

NMR spectra analyses

Figure 1 illustrates the [.sup.13.C] NMR spectra of PAP, TAP, HPP, and HCP films. The signals of -CHO- (51.2 ppm) and -OC[H.sub.2]- (44.3 ppm) in the epoxy ring of GPMS were observed in the TAP and HCP films, but not in PAP and HPP films. Furthermore, two more signals at 66.9 ppm (-CH-OH) and 64.0 ppm (-CH-O-P) appeared in PAP and HPP films, indicating that the epoxy ring of GPMS had reacted with phytic acid and 1-hydroxyethylidene-1,1-diphosphonic acid, respectively, during the synthesis. (16,17)

Figure 2 further reveals the [.sup.29.Si] spectra of PAP, TAP, HPP, and HCP films. The absolute signals due to [T.sup.2] and [T.sup.3] structures were observed at -56 and -64 ppm, respectively, for PAP, HPP, TAP, and HCP, indicating that all the acids-catalyzed polysilsesquioxane films were mainly composed of [T.sup.2] and [T.sup.3]. Since [T.sup.n] indicates the siloxane unit structure, e.g., RSi(OSi)[.sub.n][X.sub.3-n] [n = 1([T.sup.1]), 2([T.sup.2]), and 3([T.sup.3])], (1,2) it could be concluded that the sol-gel process of MTES, GPMS, and TEOS precursors was quite complete, causing three-dimensional networks in all polysilsesquioxane films no matter which acid was used as the catalyst. The quantitative analyses of [.sup.29.Si] NMR and GPC were summarized in Table 1. From the table, it was found that PAP coating had higher ratio of [T.sup.3] structures, suggesting higher crosslinking extent was obtained in PAP film than TAP, HPP, and HCP films.



Morphologies and surface characterization of polysilsesquioxane films

Figure 3 presents the SEM images of different acids-catalyzed polysilsesquioxane films. It could be seen that all the acids-catalyzed polysilsesquioxanes could form homogenous and uniform films, suggesting that these polysilsesquioxanes could possibly have good anticorrosion performance.

The 304 stainless steel substrates with removed coatings and the bare 304 stainless steel substrate were scanned by XPS and illustrated in Fig. 4. A peak at 134 or 135 ev for the P2p was observed on the PAP- and HPP-removed substrates, respectively, but not on the TAP-, HCP-removed substrates, indicating there existed surface bonding of phosphate between PAP and HPP films and the substrates.

Potentiodynamic polarization analysis

Figure 5 compares the polarization curves of PAP-, HPP-, TAP-, HCP-coated, and bare electrodes. The polarization parameters including corrosion potential ([E.sub.corr]), corrosion current ([I.sub.corr]), and inhibition efficiency ([eta]) were summarized in Table 2, [eta] is defined by



[eta] = 1 - [I.sub.corr]/[I.sub.corr](bare)

where [I.sub.corr] and [I.sub.corr](bare) refer to the corrosion current densities in the presence and absence of polysilsesquioxanes at the stainless steel surface, respectively. (18) Based on Fig. 5 and Table 2, it could be seen that the PAP, HPP, and TAP-coated electrodes had lower current densities and much higher inhibition efficiency compared to HCP-coated and bare electrodes, suggesting the former three polysilsesquioxane coatings had better corrosion inhibiting function than HCl-catalyzed polysilsesquioxane coating.


Furthermore, PAP coating had the highest anticorrosion property, HPP coating took the second place, TAP coating was the last one based on the corrosion current and inhibition efficiency in Fig. 5 and Table 2.

This could be further confirmed by the passivation index method, which uses the breadth of the passivation region to measure the corrosion inhibition, and can be quantified as the difference between the open circuit potential and the breakdown potential. (19) From Fig. 5, it was obviously found that the breadth of the passivation region of PAP and HPP were larger than the others, suggesting PAP and HPP had better corrosion protective properties.

The reason could be explained from their possible structures as described in Fig. 6. When phytic acid was used as the catalyst, the phytic acid could react with GPMS first via the ring-opening reaction of epoxy, followed by the hydrolysis and condensation of MTES, TEOS, and GPMS to form the phytic acid incorporated polysilsesquioxane networks by chemical bonding. Since each phytic acid molecule contains 12 P-OH groups, many remaining unreacted P-OH groups could provide strong chemical interaction between metal ions at the metal surface and PAP film, (18-22) as seen in Fig. 6a, resulting in good anticorrosion performance. In addition, the high degree of crosslinking and the high molecular weight of PAP could also avail corrosion resistance. When 1-hydroxyethylidene-l, 1-diphosphonic acid was used as the catalyst, the HPP film shared the similar structure to the PAP film, but fewer remaining unreacted P-OH groups could provide chemical interaction between metal ions at metal surface and HPP film, decreasing the corrosion protective property of HPP film. (23)




However, for tannic acid as the catalyst, it did not react with GPMS first via the ring-opening reaction of epoxy based on [.sup.13.C] NMR analysis in Fig. 1. The anticorrosion property of TAP coating could possibly be a result of the formation of the complex between metal ions at metal surface and the hydroxyl groups of TAP film, (24,25) as demonstrated in Fig. 6b. Obviously, this complex action was not as strong as chemical interaction of PAP or TAP film with metal surface. As to HCl-catalyzed polysilsesquioxane coatings, hydrochloric acid remained in polysilsesquioxanes as free molecules, which could accelerate the corrosion of metal surface, as illustrated in Fig. 6c.


EIS analyses

Figure 7 further presents the EIS plots of the PAP, HPP, TAP, and HCP-coated electrodes after these electrodes were immersed in 3% NaCl solution for various days. In order to get better insights about the corrosion feature occurring on these samples, EIS spectra were analyzed using equivalent electric circuits, as shown in Fig. 8, in which [R.sub.s] is the solution resistance, [C.sub.f] is the coating capacitance, [C.sub.dl] is the double-layer capacitance, [R.sub.po] is the pore resistance, [R.sub.t] is the transfer resistance at the coating-metal interface. These impedance parameters of PAP-, HPP-, TAP-, and HCP-coated electrodes are summarized in Table 3.

[R.sub.po] describes the resistance of a coating to the penetration of water or electrolyte, which is frequently used to evaluate anticorrosive property of the coated electrode. (26) From the data in Table 3, it was clearly seen that after 30 days immersion, the pore resistance of HPP-, TAP- and HCP-coated electrodes decreased to very low values while the pore resistance of PAP coated electrode still remained six magnitude, suggesting that HPP, TAP, and HCP films had lost their corrosion resistant property completely, but TAP film could still provide anticorrosion performance very well after 30 days immersion, further indicating that PAP coatings had better corrosion resistance.

Salt-fog measurement

The stainless steel panels coated with four different acid-catalyzed polysilsesquioxanes were placed into salt-fog cabinet for 1000 h. Just as displayed in Fig. 9, the flake of the films could be seen on the HPP, TAP, and HCP-coated panels, respectively. However, the PAP-coated panel still remained very well even after 1000 h sat fog spraying, confirming that the PAPs indeed had excellent corrosion resistant performance.


In this study, the polysilsesquioxanes were successfully synthesized via the sol-gel process of MTES, GPMS, and TEOS using phytic acid, tannic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and hydrochloric acid as the catalyst, respectively. It was found that phytic acid and 1-hydroxyethylidene-1,1-diphosphonic acid could act as both catalyst and reactant, catalyzing the sol-gel reaction more efficiently than tannic acid and hydrochloric acid, indicated by higher molecular weight and more [T.sup.3] structure, especially phytic acid. Potentiodynamic polarization, EIS and salt-fog praying measurement showed that PAPs had very good anticorrosion property due to the strong interaction between the unreacted P-OH groups of the phytic acid incorporated polysilsesquioxane and metal substrate while other three acids-catalyzed polysilsesquioxanes did not.

Based on this study, an environmentally friendly polysilsesquioxane coating with excellent corrosion resistant property could be prepared by the hydrolysis and condensation of siloxane precursors under the catalysis of phytic acid.

Acknowledgment The financial support from Shanghai Special Nano Foundation for this research is appreciated.


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[c] FSCT and OCCA 2007

W. Xing, B. You ([mailing address]), L. Wu ([mailing address])

Department of Materials Science and the Advanced Coatings Research Center of China Educational Ministry, Advanced Materials Laboratory, Fudan University, Shanghai 200433, P.R. China


B. You

Table 1: Preparation of PAP, HPP, TAP, and HCP

 Ratio of siloxane unit (%) GPC
Samples [T.sup.1] [T.sup.2] [T.sup.3] Mn Mw PDI

PAP 0 14 86 21,308 58,997 2.77
HPP 0 26 74 1,838 8,090 4.40
TAP 0 50 50 1,746 5,225 2.99
HCP 0 32 68 1,601 2,418 1.51

Table 2: Data for the polarization of polysilsesquioxanes-coated and
bare electrodes

Electrode [E.sub.corr] (V) [I.sub.corr] (A/[cm.sup.2]) [eta] (%)

Bare -0.55 7.15E-06 0.0
HCP-coated -0.57 2.96E-06 58.7
TAP-coated -0.29 6.90E-7 90.4
HPP-coated -0.47 1.86E-08 99.7
PAP-coated 0.046 2.49E-09 99.9

Table 3: Impedance parameters for HCP-, TAP-, HPP-, and PAP-coated
electrodes after 30 days immersion in 3% NaCl solution

Coatings [R.sub.s] ([OMEGA]) [C.sub.f] (F) [R.sub.po] ([OMEGA])

HCP 265.9 2.062E-5 487.3
TAP 344.1 8.457E-6 1690
HPP 714.3 1.651E-7 2178
PAP 502.8 1.204E-10 1.109E6

Coatings [C.sub.dl] (F) [R.sub.t] ([OMEGA])

HCP 2.801E-5 5.615E5
TAP 5.064E-5 2.754E5
HPP 7.928E-6 8.674E4
PAP 1.189E-10 4.189E6
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Author:Xing, Wentao; You, Bo; Wu, Limin
Publication:JCT Research
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Date:Mar 1, 2008
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