Multi-Functional Electroactive Polymers (EAPs) as Alternatives for

Chapter 10

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Multi-Functional Electroactive Polymers (EAPs) as Alternatives for Cadmium Based Coatings P. Zarras,1,* A. Guenthner,1 D. J. Irvin,1 J. D. Stenger-Smith,1 S. Hawkins,1 L. Baldwin,1 R. Quintana,1 M. Baronowski,2 J. Baronowski,2 J. Hibbs,2 and C. P. Waltz3 1NAWCWD

(Code 4L4200D), 1900 N. Knox Road (Stop 6303), China Lake, CA, 93555-6106 2NAWCWD (Code 434100D), 1900 N. Knox Road (Stop 6213), China Lake, CA 93555-6106 3NAWCWD (Code 476400D), 1900 N. Knox Road (Stop 6622), China Lake, CA 93555-6106 *[email protected]

The Naval Air Warfare Center Weapons Division (NAWCWD) has investigated electroactive polymers (EAPs) as alternative cadmium pretreatment coatings. EAP coatings (polypyrrole derivatives) were synthesized with adhesion promoting groups and embedded with inorganic microparticles for both corrosion inhibition and lubricity. These polymers were coated onto high strength 4340 steel alloys. Several of these EAP coated high strength 4340 steel alloys failed at 24 hours exposure to neutral salt fog spray. However, a hexyl substituted polypyrrole coating showed improved corrosion inhibition as compared to poly(3-pyrrol-1-yl) propanoic acid. Lubricity studies were performed on several pyrrole compounds. These EAP coatings were tested against Cd plated high strength 4340 steel alloys as a control. Galling testing was performed in accordance with test method ASTM G98. Several of the polymer coatings provided little, if any, protection from steel on steel galling. Some of the coatings behaved like a grease and did not provide an accurate galling resistance measurement. The addition of graphite particles resulted in a slight improvement in the © 2010 American Chemical Society In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

galling performance of the poly(3-pyrrol-1-yl) propanoic acid coating, while adding SiO2 particles led to a more pronounced improvement.


Introduction The extensive use of cadmium (Cd) in industrial operations such as smelting and refining of zinc (Zn), lead, copper ores, electroplating, welding, manufacture of pigments, plastic stabilizers and nickel-cadmium batteries has resulted in worker exposure to Cd and this exposure can result in pulmonary carcinogenesis, tumors of the prostate, testes and hematopoietic system (1–3). Cadmium can leach through soils into the groundwater where it can bind to river sediment and bioaccumulate. It is a known carcinogen which is highly regulated by the Environmental Protection Agency (4). Despite these environmental hazards, Cd is still widely used in the plating industry (especially for fasteners) because of its unique combination of properties. Cadmium plating has several properties that make it highly attractive for military use (5). Several high-strength steels used by the military that are Cd-plated for fastener applications include: AISI 4340, Hy-Tuf™ (an AISI 4340 derivative created by Crucible Specialty Metals), Aermet® 100, E4340, M50, 300M, PH 13-8Mo stainless steel and maraging Grades 200-250. The most common method of electroplating Cd onto high strength steel is the alkaline cyanide bath. While this technology ranks as one of the oldest for Cd-plating, it is also the most forgiving and reliable plating solution and proposed replacements have difficulty in matching its performance (6–12). Studies of replacements of Cd-plated high-strength steels include the Dacromet, Geomet and Magni families of coatings. These coatings, which have shown initial promise, consist of commercial Zn and aluminum (Al)-filled polymers deposited by the dip-spin coating technique, and are often used for automotive applications. Although they are quite effective, the coatings tend to clog fastener threads, creating installation problems. In addition, their torque characteristics tend to change over the course of multiple assemblies. This is not a problem for cars but it is a serious drawback for aerospace and other Department of Defense applications where weapon systems require periodic strip-down and maintenance. Recent developments in using EAPs coated onto various metal substrates have demonstrated corrosion inhibition (13). EAPs, specifically polyaniline (PANI) have been shown to provide corrosion protection for steel alloys and testing at the Los Alamos National Laboratory (LANL) and the John F. Kennedy Space Center (KSC) have demonstrated that doped PANI coatings inhibited corrosion on carbon steel (14, 15). EAPs other than PANI have also been demonstrated to protect steel alloys in harsh environments. Poly(3-methylthiophene) (P3MT) films coated onto platinum and 430 stainless steel (430SS) showed effective stabilization of the steel in a passive state (16). P3MT films coated onto a 430SS rotating disk electrode (RDE) in 1N sulfuric acid solution showed galvanic protection. This protection was described by DeBerry, whereby, the P3MT film stabilized the passive layer 134 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


by providing a transient current to heal small defects inside the passive film before they could expand (17). Constant current (galvanostatic) and constant potential (potentiostatic) measurements on poly(2,5-bis-N-methyl-N-hexylamino)phenylene vinylenes (BAM-PPV) on aluminum alloy (AA) 2024-T3 panels in simulated seawater provided evidence for corrosion inhibition (18, 19). The military requirement for an alternative pretreatment for chromate conversion coating (CCC) states that the unpainted coating must pass 336 hours of neutral salt fog exposure. The accept/reject criteria for this military requirement (MIL-DTL-81706B) is no corrosion of the underlying aluminum. Any staining of the coating is not considered a failure and the coating must not show blistering, delamination or evidence of corrosion (20, 21). BAM-PPV coated onto AA 2024-T3 panels showed acceptable neutral salt fog exposure performance when compared to chromate conversion coated AA 2024-T3 panels (as controls) (20). The BAM-PPV coatings were able to pass the 336 hour neutral salt fog exposure test (ASTM B117) without pitting, corrosion or delamination of the coating which met the MIL-DTL-81706B requirement. Polyethylene coatings have also been investigated for their wear resistance and corrosion protecting properties using molybdenum disulphide fillers (22). By incorporating hard particles for lubricity into an EAP coating system one could potentially provide corrosion and wear resistance from a single coating system. The following paper will describe our work on achieving this effect.

Experimental 3-(Pyrrol-1-yl) proprionitrile (3PPN), 7-bromoheptanitrile (7-BHN), sodium hydroxide (NaOH), pyrrole and 3-aminopropyltrimethoxysilane were purchased from Aldrich and used as received. Iron (III) chloride (FeCl3) and iron(III) chloride hexahydrate (FeCl3*6H2O), were purchased from Aldrich and used as received. Ferric nitrate nonahydrate (Fe(NO3)3*9H20) was obtained from Baker Chemical and used as received. 1H and 13NMR spectra were acquired using a Bruker Advance 400 MHz NMR spectrometer. Mass spectra data were obtained using a JEOL thermal desorption mass spectrometer (TD-MS). The FTIR measurements were made using a Nicolet Nexus 870 FTIR spectrometer with a liquid nitrogen cooled MCT detector. Each spectrum is an average of 128-256 scans with 4 cm-1 resolution. The monomer and polymer bulk samples were analyzed using a “Thunder dome” attenuated total reflectance (ATR-FTIR) accessory with a germanium crystal. The samples adsorbed on a surface were analyzed using specular reflectance and the incoming radiation was at 80° with respect to the surface normal. The particles used in the particle coating operation included 1) Monarch 900 carbon black (an organic pigment containing ~50% graphite, dispersed as ~1µm particles), 2) molybdenum sulfide (99%, supplied from Aldrich Chemical as particles of less than 2µm size), 3) fumed silica, supplied by Aldrich as particles significantly less than 1µm in average size and 4) boron carbide, supplied by Aldrich as particles approximately 10µm in average size. Since the inclusion of 135 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


small, hard particles is intended to modify only the surface characteristics of the coating, an attempt was made to introduce the particles only at the outermost regions of the coated article. To do so, a quick method of hot pressing at low pressure was developed. The carbon black and molybdenum sulfide were chosen because of their enhanced lubricity, the carbon black and the silica were chosen because of their small particle size and ease of dispersion, and the boron carbide was chosen for its hardness and contrasting particle size. The procedure for hot pressing the particles was as follows. The particles were first suspended at a low volume fraction in deionizer water (DI water) by ultrasonication for one hour. The suspension was then transferred onto the surface to be tested via pipette and spread via shearing with compressed air to form a coating layer about 0.5 mm thick. For the larger particles (boron carbide), the suspended particle concentrations were about 0.5 wt%, while for the smaller particles (all others), they were about 0.1 wt%. The concentration used was scaled approximately to the particle size in order to provide fairly similar coverage (roughly 20-70% of a theoretical close-packed monolayer) on all surfaces upon drying. After spreading, the wet surfaces were gently dried in a stream of compressed air for about 1 minute to reduce the thickness of the wet layer to a thinner layer without de-wetting the edges of the surface. To press the samples, a clean thin sheet of Teflon, backed by sheets of aluminum arranged so as to constitute a total weight (including Teflon) equal to 106 g/button, was heated to 100°C. The sheets were then placed directly on to the wet surfaces and allowed to remain for 15 minutes while cooling. Assuming full contact between the sheet and the button faces, the pressure applied to the surfaces was about 8 kPa, or 1.2 psi. During the holding time, the temperature of the outer sheets was observed to decrease to about 50°C. After holding, the sheets were then removed from the surfaces, and allowed to dry completely at room temperature. The operation achieved the deposition of a uniform thin coating of particles over the entire target area. In many cases, however, an excess of particles was observed to deposit around the sample edges. Thus, while the method as developed is suitable for deposition of the particles to test specimens, more work would be needed to develop a deposition method (such as an electrostatic method similar to powder coating) that would be suitable for articles of arbitrary shape and surface characteristics. Galling testing was then performed in accordance with test method ASTM G98, which required button and block specimens. These specimens were manufactured from AISI 4340 steel hardened to 50-55 HRC, representing high-strength steel. A group of button specimens was plated with cadmium (in accordance with SAE-AMS-QQ-P-416, Type II, Class 2), while others were coated with ion-vapor deposited (IVD) aluminum (in accordance with MIL-DTL-83488, Type II, Class 2) for comparison to the polymer samples. A high strength steel support cylinder and plate support fixture conforming to the dimensions listed in ASTM G98 for galling testing were fabricated. EAP deposited onto high strength 4340 steels alloys was measured for morphology, chemical composition and corrosion performance using a combination of scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDXA). The measurements were done with a Zeiss SEM Model EVO-50 136 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 1. Synthesis of 3-(pyrrol-1-yl) propanoic acid (3PPA)


and EDXA Model LEO-EVO-50EP. Neutral salt fog exposure testing (ASTM B117) were performed on EAP films without hard particle additives.

Preparation of 3-(Pyrrol-1-yl) Propanoic Acid (3PPA) (see Figure 1) (23) A 250 mL round bottom flask was equipped with a reflux condenser and nitrogen inlet/outlet valve. The round bottom flask was charged with 50 mL DI water and 13.3 g NaOH pellets. After addition of the NaOH pellets the solution became turbid. After 5 minutes, the solution was clear and homogenous. The 3-(pyrrol-1-yl) proprionitrile (10.0 g, 83.2 mmol) was added to the reaction flask and the solution refluxed for 12 hours under a positive nitrogen pressure. After 12 hours the solution was homogenous and orange colored. There was no ammonia evolution from the top of the condenser as determined by moist pH paper. Cold DI water (25 mL) was added and the solution was cooled to ambient temperature. The reaction flask was cooled in an ice/water bath and 14 mL of a 50% aqueous sulfuric acid solution (1:1, v/v) was added slowly. The solution was stirred and a semi-solid formed immediately. The contents of the reaction flask were extracted 3X with ether and the ether layer separated from the aqueous phase. The ether layer was dried over magnesium sulfate (MgSO4) and the solution filtered through a medium porosity glass frit. The filtrate was rotovapped to a semi-solid residue and dried in a vacuum dessicator (0.05 Torr, 25°C) for 12 hours. An off-white tan product was obtained in 35% yield (4.0 g), m.p = 45-47°C (uncorrected, literature value = 59-60°C) (23, 24). 1H NMR (DMSO-d6): 2.67 (t, 2H); 4.09 (t, 2H); 5.95 (t, 2H) and 6.73 (t, 2H), 13C NMR (DMSO-d6): 36.05, 44.40, 107.5, 120.4 and 172.3. FTIR: 3000 cm-1 broad (OH stretch), 1710 cm-1 (C=O stretch). The 3PPA compound was identified with thermal desorption-MS: molecular peak at 138.9 amu.

Electropolymerization of 3PPA 3PPA polymerizes quite well electrochemically, as can be seen in Figure 2. 3PPA undergoes a very well-behaved, reproducible electropolymerization which shows a linear increase in peak current as a function of scan rate for the poly(3(pyrrol-1-yl) propanoic acid), (P3PPA) compound, indicating that the film is electrode supported and electroactive. 137 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Electropolymerization of 3PPA

Figure 3. 3-aminopropyl trimethoxysilane on high strength 4340 steel alloy button Electroless Deposition of 3PPA (24) The electroless deposition (non line-of-sight) of 3PPA and 7-PHA used in this study was based on a prescreening process using the electropolymerization results described previously, based on the principle that the monomers with the lowest oxidation potential would undergo facile electroless polymerization. Since 3PPA does undergo electropolymerization at a low potential, it is a candidate for electroless deposition. The electroless deposition consisted of 3 steps using the 3PPA and 7-PHA monomers. 138 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 4. P(3PPA) deposited on high strength 4340 steel alloy substrate

Figure 5. Bulk P(3PPA) Step I: The high strength 4340 steel alloy substrate [3x3x0.25”; 3x6x0.25”, galling and defect tolerance specimens] were cleaned using toluene, acetone, methanol and isopropanol to remove grease, dirt and any other debris that may be present on the sample. The samples were then air dried for 15 minutes prior to deposition. No evidence of corrosion was visible during this stage of the electroless deposition process. Step II: The cleaned sample was placed in a Pyrex glass dish (size depended on sample dimensions) and a 0.5 wt %/vol. of 3-aminopropyltrimethoxy silane solution was added to the samples. The samples were allowed to stand in the silane solution for 5 minutes after which time the samples were removed and dried in a vacuum oven (25 mm Hg, 100°C). After one hour, the samples were removed from the oven and allowed to cool in a dessicator for one hour. The steel samples were measured with FTIR to determine if the silane had reacted with the substrate. A thin layer of 3-aminopropyltrimethoxysilane was deposited on a high strength 139 In Smart Coatings III; Baghdachi, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


4340 steel alloy surface which is shown in Figure 3, confirming the presence of the silane on the surface before the polymer deposition. Step III: The silanated sample was then placed back into the Pyrex glass dish for the polymerization deposition process. Numerous attempts using a variety of solvents, (e.g. DI water, methanol) and oxidants (FeCl3, FeCl3*6H2O) were tried. The oxidants FeCl3 and FeCl3*6H2O performed the best during the polymerization. Several different molar concentrations of the monomer in DI water were examined during the deposition (0.03M, 0.25M, and 057M). The 0.25M solution of 3PPA and oxidant FeCl3 at a concentration of 0.67M were found to be the optimum conditions for the electroless deposition process. Both the monomer solution and oxidant solution were added simultaneously to the Pyrex dish containing the high strength 4340 steel alloy samples. The solution was briefly agitated (

Multi-Functional Electroactive Polymers (EAPs) as Alternatives for

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