Shusen Peng,
Zhixiang Zeng,
Wenjie Zhao,
He Li,
Jianmin Chen,
Jin Han* and
Xuedong Wu*
Laboratory of Marine New Materials and Related Technology, Zhejiang Key Laboratory of Marine Materials and Protection Technology, Ningbo Key Laboratory of Marine Protection Materials, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, P.R. China. E-mail: hj@nimte.ac.cn; xdwu@nimte.ac.cn; Fax: +86 574 86685159
First published on 17th March 2014
A novel anticorrosion coating, which crosslinks with inorganic bonds (Si–O–Si) and organic bonds (C–S–C), is prepared on copper through an in situ sol–gel method and thiol–ene click reaction. The hybrid sol solution containing mercapto and vinyl groups (MVFS) is prepared from hydrolysis and condensation of vinyltriethoxysilane (VTES), tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPMS). The thiol–ene click reaction was initiated with a thermal initiator after the sol solution was applied on the copper surface. Various corresponding methods are carried out to investigate this novel coating's properties. Experimental results demonstrate that the formation of organic crosslinking bonds deriving from thiol–ene click reaction can enhance the protection performance of the MVFS material for copper.
Hybrid silica sol–gel coatings have seen a lot of interest in the past few years due to these materials combining the properties of organic and inorganic compounds,4–6 and have been intensively investigated as anticorrosion materials for various metal materials.7–10 An important advantage of this coating is that it can form Si–O–metal bond on metal surface leading to good adhesion.7 However, previous reports have revealed that it is difficult to form Cu–O–Si bond resulting a poor protection for copper.11,12
An alternative method is introducing mercapto group as that this group can form Cu–S bond with copper.13–15 However, mercapto group is susceptible to oxidation,16 which may cause coating degradation. Click reaction is are modular, high yielding, simple to perform, tolerant to various solvent and air.17 Thiol–ene click reaction is an addition reaction between a thiol and an ene. This reaction has been intensively studied to prepare novel materials.18–20 In this report, a novel anticorrosion coating, which crosslinks with inorganic bond (Si–O–Si) and organic bond (C–S–C), is formed on copper through in situ sol–gel method and thiol–ene click reaction. Our consideration is basing on that thioether bond provides better stability than mercapto group, and this in situ method can improve coating crosslinking density and remain the ability of formatting Cu–S bond on copper surface.
Specifically, a hybrid sol solution containing mercapto and vinyl groups (MVFS) is prepared from hydrolysis and condensation of vinyltriethoxysilane (VTES), tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPMS). The thiol–ene click reaction is initiated with a thermal initiator after sol solution has applied on copper surface. The properties of this coating are investigated by various corresponding methods. The addition reaction between a thiol and an ene is characterized by IR and XPS. Scanning electron microscope (SEM) is used to observe the surface and cross section topology of this coating on copper. Thermal and corrosion resistance properties are evaluated by thermal gravimetric analysis and electrochemical methods, respectively. Also, the pencil hardness, hydrophobicity and adhesion of coating are measured.
Copper samples are immersed in MVFS-1 and MVFS-2 solution for 5 min, and then are dried at 70 °C for 1 h and 120 °C for 1 h. A simple scheme shown in Fig. 1 is used to describe the reaction of preparation of MVFS solution and coating formation on copper.
Fig. 1 Reaction scheme of preparation of MVFS solution and coating formation on copper without and with an initiator. |
Infrared spectra are recorded on a Nicolet 6700 FTIR. MPMS, VTMS and TEOS are dropped on KBr tablets for IR measurements. Dried MVFS-1 and MVFS-2 coatings are ground into powder, and then mixed with KBr uniformly to press into tablets (1 mg samples into 100 mg KBr) for IR measurements. Pencil hardness test is carried out according to GB/T6739-2006. The peel strength between MFS coating and copper is measured using Pull-Off adhesion tester (PosiTest) and the value is an average of at least three parallel samples. The aqueous contact angle analysis carries out using a Dataphysics OCA20 with sessile drop method. The value of water contact angle is an average of at least three readings at different locations on the surface of each sample.
Polarization curve is carried out using a commercial PGSTAT302 electrochemical workstation (Autolab) in a naturally 3.5 wt% NaCl. A three electrodes system is used, which was composed of a saturated calomel electrode (SCE) as the reference electrode, a platinum foil as a counter electrode and an exposed sample (0.78 cm2) as a working electrode. The scan rate of polarization curves is 0.001 V s−1. For electrochemical impedance spectroscopy (EIS), the test frequency range was 105 to 10−2 Hz and excitation amplitude was 10 mV.
XPS spectra are also conducted to investigate the thiol–ene reaction. Deconvolution of the XPS for C 1s electron of MVFS-1 (Fig. 3a) presents carbon in three different chemical states. The resolved weak component at 285.3 eV in Fig. 3a is attributed to the C bonded directly to sulphur atom.15 The resolved weak component at 283.9 eV is related to the C atom of vinyl group.21 The dominant component at 284.7 eV in Fig. 3a is contributed to the methylene carbon of MPMS and the vacuum chamber contaminant CO2.22 As shown in Fig. 3d, peaks at 285.3 and 284.7 eV present in XPS spectra of MVFS-2. However, the peak related to C atom in vinyl group disappears. In addition, the concentration of C bonded directly to sulphur atom has an increment. These results indicate the occurrence of addition reaction between a thiol and an ene. Deconvolution of the XPS for S 2p electron of MVFS-1 (Fig. 3c) presents the appearance of sulphur in three different chemical states. The peaks, observed at 163.4 and 164.6 eV in Fig. 3c correspond to free mercapto group (–SH) which is split into the 2p3/2 and 2p1/2 components.22 The peak at 168.2 eV can be assigned to the oxidation product of –SH group. As shown in Fig. 3d, there are also three peaks in XPS spectra of MVFS-2. It is found that the bonding energy of S atom in thioether is close to that in –SH. Moreover, a slight increment of peak area at 168.2 eV indicates that –SH groups are oxidized in the process of MVFS-2 formation. This may be caused by the TMCH initiator. Fig. 1 simply describes the formation of MVFS-1 and MVFS-2 coatings on copper surface.
Fig. 3 Deconvolution XPS spectra: (a) C 1s of MVFS-1, (a) C 1s of MVFS-2, (c) S 2p of MVFS-1 and (d) S 2p of MVFS-2. |
Fig. 4 Surface and cross section morphologies of MVFS coatings on copper: (a and c) MVFS-1; (d and b) MVFS-2. |
The contact angle of water on MVFS-1 is equal to 89 ± 0.5° while the value of MVFS-2 is 94 ± 0.3°. The slight increase is due to the C–S–C unit is more hydrophobic than –SH group. The pencil hardness test is carried out to determine the scratch resistance of coating, which is evaluated using a calibrated set of drawing pencil ranging from 6 B, the softest, to 6 H, the hardest. The first pencil that scratches the surface is reported as the hardness, according to the Standard GB/T6739-2006. The test gives 6 H for MVFS-1 and MVFS-2. The adhesion test gives a 13.1 ± 0.6 and 12.8 ± 0.3 MPa for MVFS-1 and MVFS-2, respectively, demonstrating that MVFS coating has good adhesion to copper surface.
Fig. 5 TG (a) and DTG (b) curves of MVFS-1 and MVFS-2, obtained at a heating rate of 10 °C min−1 under nitrogen and air. |
Samples | Atmosphere | T5 (°C) | Tm (°C) | YSiO2 (wt%) |
---|---|---|---|---|
MVFS-1 | N2 | 360.6 | 392.7 | 71.9 |
Air | 343.5 | 391.9 | 61.0 | |
MVFS-2 | N2 | 284.1 | 392.9 | 69.4 |
Air | 279.3 | 391.8 | 59.1 |
Fig. 6 Polarization curves of copper electrodes in 3.5 wt% NaCl aqueous solution: bare and covered with MVFS-1 and MVFS-2. |
Electrode | Ecorr/mV (vs. SCE) | Icorr/A cm−2 |
---|---|---|
Bare | −204 | 6.9 × 10−6 |
MVFS-1 | −203 | 6.1 × 10−8 |
MVFS-2 | −201 | 9.8 × 10−9 |
Fig. 7 presents the EIS results of bare and MVFS-1 and MVFS-2 covered copper. Bare copper electrode is measured immediately after it is immersed in 3.5 wt% NaCl solution, and the coated electrodes are measured after 30 min of immersion in the solution. The Bode representation of EIS results are shown in Fig. 7b and it provides a good comparison of total impedance values for coated samples and bare one. According to the fact that corrosion rate is inversely proportional to the value of impedance modulus at low frequency,26 it can be concluded that coated samples show lower corrosion rate than the bare one and MVFS-2 has a better anticorrosion ability than MVFS-1. Nyquist plots are analyzed graphically using ZSimpWin software to further interpret EIS results. As shown in Fig. 7a, the Nyquist plots of bare copper consist of a capacitance arc in the high frequency region and a Warburg impedance line in the low frequency region. This result is very similar to research work by other author.2 Thus it is reasonable to use the equivalent circuit R(Q(RW)) in Fig. 8a to analyze the EIS result of bare copper. The impedance spectra of coated samples are significantly different from bare one. For MVFS-1 and MVFS-2, the Warburg impedance disappears in the low frequency region, instead, a large depressed semicircle containing two indistinct capacitive loops can be observed from high to low frequency region in Nyquist plots. Therefore, it is reasonable to use the circuit R(Q(R(RQ))) (Fig. 8b) to analyze the EIS result of MVFS-1 and MVFS-2. The value of each element in the equivalent circuits which is calculated by the ZSimp-Win software are listed in Table 3. The fitting result clearly demonstrates that Rfilm and Rct of MVFS-2 are greater than that of MVFS-1. The EIS result is in accordance with the result of polarization curve. The reason that MVFS-2 has a better protection performance than MVFS-1 can be attributed to that the in situ thiol–ene reaction improves coating crosslinking density and then results a better barrier effect.
Fig. 7 EIS plots of MVFS-1 and MVFS-2 covered and bare copper immersed in 3.5 wt% NaCl aqueous solution: (a) Nyquist plots and (b) Bode plots. |
Fig. 8 Equivalent circuits used to analyze the EIS plots: (a) R(Q(RW)), for bare copper and (b) R(Q(R(RQ))), for MVFS-1 and MVFS-2 covered samples. |
Sample | Bare | MVFS-1 | MVFS-2 |
---|---|---|---|
a The values in brackets correspond to the error (%) of each parameter. | |||
Qdl (Ssn cm−2) | 4.5 × 10−4 (0.2)a | 6.2 × 10−7 (7.1) | 3.1 × 10−7 (4.1) |
ndl | 0.81 (0.1) | 0.94 (0.7) | 0.92 (0.4) |
Rct (Ω cm2) | 1.1 × 103 (0.4) | 1.5 × 106 (3.2) | 4.8 × 106 (1.4) |
Qfilm (Ssn cm−2) | — | 6.4 × 10−7 (3.7) | 3.3 × 10−7 (1.2) |
nfilm | — | 0.53 (2.5) | 0.58 (0.7) |
Rfilm (Ω cm2) | — | 4.2 × 104 (9.8) | 4.7 × 104 (7.3) |
W (Ss0.5 cm−2) | 0.12 (0.2) | — | — |
Circuit | R(Q(RW)) | R(Q(R(RQ))) | R(Q(R(RQ))) |
This journal is © The Royal Society of Chemistry 2014 |