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Chapter 11
Amine-Quinone Polyimides for Protecting Iron against Corrosion 1,3
Mijeong Han , Huimin Bie2,3, David E. Nikles1,3,*, and G. W. Warren 1,3,*
1
2
Departments of Chemistry and Metallurgical Materials Engineering and Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, A L 35487-0209 *Corresponding author: Email:
[email protected] 3
Two dianiline monomers, 2,5-bis (4,4'-methylenedianiline)1,4-benzoquinone (AQMDA) and 2,5-bis (4,4'-oxydianiline)1,4-benzoquinone (AQODA), were prepared by condensing 1,4-benzoquinone and 4,4'-methylenedianiline and by 1,4benzoquinone and 4,4'-oxydianiline, respectively, via Michael addition. The monomers were condensed with 3,3',4,4'benzophenone tetracarboxylic dianhydride (BTDA) or 3,3',4,4'-hexafluoroisopropylidene tetracarboxylic dianhydride (6FDA) to give polyamic acids. The polyamic acids were soluble in NMP and films were cast onto glass or iron squares. The films were thermally imidized at 290°C to give the corresponding polyimides. The amine-quinone polyimides had thermal decomposition temperatures greater than 440°C and glass transitions in the range 279 to 310°C. The tensile strength for the amine-quinone polyimides was in the range 92 to 166 MPa and the modulus was in the range 0.9 to 1.5 GPa. The amine-quinone polyimide made from AQMDA and BTDA protected iron against corrosion by aqueous sodium chloride electrolyte.
Introduction Polymers containing the 2,5-diamino-1,4-benzoquinone functional group have been of interest, since this class polymers has a strong affinity for the surface of iron and can protect iron against corrosion. '
© 2003 American Chemical Society Zarras et al.; Electroactive Polymers for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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176 Nikles' group has synthesized amine-quinone polyurethanes by condensing an amine-quinone diol monomer and an oligomeric diol monomer (e.g., polytetrahydrofuran diol) with a diisocyanate (e.g., TDI). " These polyurethanes were shown to inhibit corrosion of the commercial iron particles used in state-of-the-art metal particle tape. Electrochemical impedance spectroscopy showed that amine-quinone polyurethanes protect iron against corrosion by providing a strong adhesive bond that could not be broken by water. " Amine-quinone polyurethanes also promote the adhesion of epoxy to steel. In this study, we report the synthesis of new aromatic diamine monomers and the preparation of amine-quinone polyimides containing these monomers. Our objective was to demonstrate whether we could combine the high temperature properties inherent with polyimides with the corrosion protection of metal afforded by the amine-quinone functional group. 3
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AQMDA, X = CH , AQODA, X = Ο 2
M o n o m e r and P o l y m e r Synthesis
2,5-Bis (4,4'-methylenedianiline)-1,4-benzoquinone (AQMDA) and 2,5-bis (4,4'-oxydianiline)- 1,4-benzoquinone (AQODA) were prepared by condensing 1,4-benzoquinone and 4,4'-methylenedianiline (MDA) and by 1,4-benzoquinone and 4,4'-oxydianiline (ODA), respectively, via Michael addition. Oxygen was bubbled into the reaction mixture during the reaction as an oxidizing agent and ethanol was the solvent. In the case of A Q O D A , O D A was not very soluble in ethanol at room temperature and the solution was warmed up to 40-50°C to complete dissolution. Both compounds precipitated from the reaction mixture and brown solids were collected by filtering. The compounds were purified by washing with fresh methanol several times, recrystallized from ethanol and dried at 50°C under vacuum. The structures of the monomers were confirmed b y means of H - N M R , C - N M R and highresolution mass spectroscopy. Amine-quinone polyimides were prepared by a two-step polymerization of amine-quinone monomers (AQMDA or A Q O D A ) and aromatic dianhydrides, either 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) or 3,3',4,4'-hexafluoroisopropylidene tetracarboxylic dianhydride (6FDA) in N-methyl-2-pyrrolidinone (NMP), T a b l e I . The solid contents (weight % of monomers) were varied from 5 to 12 weight percent depending on the solubility of the !
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Zarras et al.; Electroactive Polymers for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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polyamic acids. The polyamic acid from A Q O D A and 6FDA could not obtained because it always made a gel. When A Q O D A was used as a monomer, the amount of solvent was increased to avoid making a gel. We do not know why A Q O D A needed more solvent.
Figure 1. Amine-quinone polyimides, where X is CH or 0 and Y is C=0 or C(CF l ι 2
3
T a b l e I. Composition οί the polyimides
Polymer
Monomers
PI AQPI-2 AQPI-3 AQPI-4 AQPI-5 AQPI-6 AQPI-7 AQPI-8 AQPI-9
MDA, BTDA AQMDA, BTDA A Q M D A , 6FDA AQODA, BTDA A Q O D A , 6FDA AQMDA, MDA, BTDA A Q M D A , M D A , 6FDA A Q O D A , ODA, B T D A A Q O D A , ODA, 6FDA
Molar Ratio
0.5 0.5 0.5 0.5
1: 1 1: 1 1: 1 1: 1 1: 1 : 0.5 : : 0.5 : : 0.5 : : 0.5 :
Solids (wt%) 10 10 10 8 __*
1 1 1 1
10 10 10 5
* A Q P I - 5 could not be prepared because polyamic acid from A Q O D A and 6 F D A was always obtained as a gel.
For the coatings on iron squares, the amine-quinone polyamic acids were prepared with higher solid contents (12 wt%) to obtain thicker coatings on the iron squares. The double-coated films had a thickness of 12 μηι. For the comparison studies on corrosion protection, a conventional polyimide was made from M D A and B T D A . The polyamic acid was double-coated coated on iron squares. The coatings were dried and imidized by exactly the same method as AQPI. The thickness of the coating was 12 μηα. The coatings were thermally imidized under nitrogen by successive heating at 50 °C for one hour, then at 150 °C for one hour, then at 250 °C for one hour, and finally at 290 °C for one hour. All polyimides gave tough, flexible free-standing films. The IR spectrum of AQPI-2 showed
Zarras et al.; Electroactive Polymers for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Polymer PI AQPI-2 AQPI-3 AQPI-4 AQPI-6 AQPI-7 AQPI-8 AQPI-9
Table IL Thermal Properties Twv CCf Char yield (%)° 61.8 554 (°C) 69.0 551 62.5 536 67.6 530 64.2 538 59.8 538 62.8 557 58.0 514
T.CC)
0
T525 h 5
472 448 455 440 453 479 474
282 292 286 280 289 279 296 310
a
Temperature at which 5 or 10 % weight loss was recorded by means of thermogravimetric analysis at a heating rate of 10°C/min in nitrogen. Residual weight % at 800°C in nitrogen. a
b
Polymer PI AQPI-2 AQPI-3 AQPI-4 AQPI-6 AQPI-7 AQPI-8 AQPI-9
Table III. Tensile Properties Elongation at break (%) Strength (GPa) (MPa) 19.3 1.5 169 24.4 140 1.1 24.6 146 1.1 16.4 92 0.9 21.6 131 1.0 13.5 133 1.3 16.5 166 1.5 18.3 138 1.2
Modulus
l
characteristic imide absorption bands at 1778 and 1722 cm" (C=0), 1374 cm" (C-N), and 725 cm" (imide ring). 1
1
Thermal and Mechanical Properties The relative thermal stability of amine-quinone polyimides were compared by the temperature of 5 and 10 % weight loss ( T o and Ti %) and percent char yield at 800°C. Table II lists the T G A data for the polyimides. A l l amine-quinone polyimides showed the excellent thermal stability one would expect for polyimides. The polymers exhibited T o ranging from 440 to 479°C and T % ranging between 514 and 557°C in nitrogen, and the char yields at 800°C ranged between 60 and 69 %. There was no dependence of thermal stability on the monomers used and all amine-quinone polyimides showed excellent thermal stability in high temperature. 5
/o
0
5
/ o
10
Most of these polyimides gave transparent, flexible and tough films suitable for the measurements of tensile properties and they are summarized in table III. Only AQPI-5 could not make free standing film because the polyamic acid was obtained as a gel. In the case of
Zarras et al.; Electroactive Polymers for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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AQPI-4, the film was not very uniform and gave poor mechanical properties. Tensile modulus was between 0.9 and 1.5 GPa and tensile strengths were ranged from 92 to 166 MPa. All polymer films behaved as ductile materials with good modulus and high strength with moderate elongation at break. Corrosion Studies The coated iron squares were exposed to 0.1 M NaCl electrolyte. The degradation of the polymer coating was monitored by EIS. The results are shown in Fig. 2 and 3. These are Bode plots consisting of impedance and phase angle as a function of frequency. The initial plot was taken after five minutes exposure to 0.1 M NaCl aqueous solution. In Fig. 2, the initial Bode plots for the PI film (12 μιτι thick) on iron showed an impedance of 3.9 χ 10 Ω at low frequency. Values of phase angle were less than the - 90 ° and implied that the polymer film was behaving as a dielectric. After two days exposure, the impedance decreased to 2 χ 10 Ω. A second time constant was appeared at low frequency in the phase angle curve, which has been attributed to a double layer capacitance. The electrolyte had diffused through the polymer and broke the adhesive bond between the polymer and iron. The appearance of the second time constant in phase angle was regarded as the beginning of the failure of the coating, and pitting corrosion was observed soon thereafter. The iron samples coated with amine-quinone polyimides showed a different behavior after exposure to NaCl electrolyte. Fig. 3 shows Bode plots of an iron sample with double coating of AQPI-2, 12 μιη thickness. The initial impedance was 6.02 χ 10 Ω and the phase angle was - 9 0 ° over a wide range of frequencies. After 2 days and 34 days exposure, there was very little change in the impedance spectra. The impedance remained in the range of ΙΟ Ω and the phase angle was almost identical to that of initial curve. The adhesion bond between the amine-quinone polyimide and iron surface was strong enough to resist attack by NaCl electrolyte. These experiments will be continued until we observe failure in the coatings. 5
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Conclusions Amine-quinone polyimides have the excellent thermal stability and tensile properties, one expects from polyimides. They are insoluble, suggesting good chemical resistance. These polymers also show an ability to protect iron against corrosion by NaCl electrolyte, similar to that we have reported for amine-quinone polyimides.
Zarras et al.; Electroactive Polymers for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
180 e
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