The double helix - european-coatings.com · The double helix Designing conductive polymers for improved metal protection. Sze Yang, Richard Brown, John Sinko. Intrinsically conductive

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The double helix

Designing conductive polymers for improved metalprotection.Sze Yang, Richard Brown, John Sinko.Intrinsically conductive polymers (ICPs) can provideanticorrosive effects in coatings through a reversibleelectroactive process. A "double-strand" ICP has beenproduced which gives effective corrosion protection andovercomes problems of coatings compatibility and poorstability associated with standard ICPs.Hexavalent chromium is an effective commercial corrosioninhibitor widely used for surface conversion and as acomponent in primer paints, but its toxicity poses serioushealth and environmental hazards. Chromates are highlyeffective as corrosion inhibitors because they have a"self-healing" property that is difficult to duplicate.It might be fruitful to search for a replacement that emulatesthe self-healing aspect of chromates. The corrosioninhibition mechanisms of intrinsically conducting polymers(ICPs) have been studied [1-5] and there are indications thatit is an electroactive process [1, 6]. An understanding ofICPs may therefore help in developing a replacement forchromates.Figure 1 shows chemical structures for some ICPs. Theseare polymers with conjugated π electronic structures. In1974 Shirakawa et al first reported dopant-inducedelectronic conductivity for polyacetylene. In 1985, theelectroactive nature of the corrosion inhibition of polyanilinewas reported [7] and in 1996 the first commercially availableICP anticorrosive coating was announced [8].Although this has been an active area of research, ICPshave not been developed into acceptable chromatereplacements. Some key concepts will be reviewed below:- That ICPs are potentially "smart" corrosion inhibitors;- That the most common ICPs have inherent problems incoatings use;- That a new ICP material, double-stranded polyaniline, mayprovide an effective coating system.

ICPs inhibit corrosion via electrochemical effectsPrevious researches indicate that ICP coatings on steel oraluminium show strong electrochemical effects on thecorrosion processes [1-8]. This interaction may inhibit oraccelerate corrosion. Although ICPs are organic polymers,they do not function as a barrier to water and corro-dingions. Instead, an ICP blended into a primer serves as anelectroactive component that functions to repair thedamaged metal surface. It is therefore possible to obtainbeneficial corrosion inhibition when the ICP is only arelatively minor component.Figure 2 shows the inhibition of filiform corrosion by 1% ofICP blended into an electrophoretic epoxy coating on thealuminium alloy AA2024 without surface pretreatment. After8 weeks of acid/humidity cycles, the control (left) showsfiliform corrosion while the test sample shows inhibition.These results suggest that the ICP additive influences theelectrochemical driving force commonly associated withfiliform corrosion.

ICPs function as organic semiconductorsAn ICP is an electronic conductor when it is in the dopedform. The conjugated electronic structure allows the stableradical cations (or "polarons") to carry positive charges, bemobile along the polymer chain and jump between adjacentpolymer chains. These mobile cations are similar to themobile "holes" in inorganic semiconductors.

Figure 3 is a schematic representation of polyaniline in itsconductive, doped form. The green ribbon represents theconducting polymer backbone. The red circles are molecularanions (the dopants) necessary to balance the positivecharge carriers on the ICP backbone. A commonly useddopant is p-toluenesulfonic acid.A typical polyaniline (PAN) contains 103 repeatingmonomeric units (molecular weight = 105 Daltons). Thedoped form may reasonably have about 100 positive chargecarriers on the polymer chain (one charge per 10 repeatingunits). The symbol (PAN)100+ is used to represent apolyaniline chain with 100 mobile positive charge carriers orpolarons [9]).This form is an organic semiconductor with "holes" that canbe filled with electrons. In the language of chemists, theprocess of accepting electrons is a reduction reaction. Thusthe (PAN)100+ molecule is an oxidant with nearlycontinuously variable oxidation states, or a changeableoxidation potential. From electrochemical measurements ofthe doped form of polyaniline, the oxidation potential of(PAN)100+ is about E0 =+0 V against a Standard CalomelElectrode (SCE) when it is in equilibrium with air.

Re-passivation of damaged materialAlthough the oxidation potential of polyaniline is relativelylow compared with common oxidants, it is sufficient tooxidise an aluminium atom Al0 exposed by the corrosiveremoval of protective aluminium oxide. Equation (1) showsthe reaction for converting exposed aluminium metal intoaluminium oxidePAN100+ + Al0 + 3/2 H2O ->PAN97+ + 1/2 Al2O + 3 H+

[Equation (1)]The presence of an electrochemical oxidation reaction doesnot guarantee the formation of adherent and passive oxidefilm, but it is possible that oxide formed at an appropriaterate will stop the initiation of any pitting corrosion.In a polyaniline network, the mobile charge carriers areshared between a large number of polymers and the totalcharge is a large multiple of that shown in equation (1),providing a large supply of positive charges to repairdamaged sites.Figure 4 shows a dynamic potential scan for aluminium alloyAA2024 coated with a thin film of double-strand polyaniline.The curve shows high resistance to anodic polarisation up toan over-potential of about 1 Volt. The upward tilt of the curveindicates that the protective oxide film is quite stable againstanodic polarisation of almost one volt. This result isconsistent with the re-passivation of the damaged metalsurface illustrated in Figure 5.

The oxidation power of ICPs is regeneratedDoped ICPs have the unusual feature that they can bereversibly oxidised and reduced (redox reaction), that is,switched between the charged state (PAN)100+ and theneutralised state (PAN)0. The ease of this reversible redoxprocess is the basis for the reversible electrochromic effect.It has been found that an electrochromic window ofpolyaniline can undergo 105 charge/discharge cycles [10].An electrochemically-reduced film of PAN0 is spontaneouslyoxidised in the air to the conductive form PANn+ within areasonable time. Equation (2) shows how the PAN97+

reduced by the reaction in (1) can be recycled to the (PAN)100+ state in air.PAN97+ + 3/2 O2 + 3 H+ ->PAN100+ + 3/2 H2O

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[Equation (2)]The combined reactions (1) and (2) provide a mechanismfor regenerating the electroactive polymer PANn+ so that it ischarged for the next cycle of oxidation of Al0 to form Al2O3.There is thus a degree of similarity between chromates andICPs. Both systems are easily cycled between reduced andoxidised states. For chromium, the stable oxidation statesare the hexavalent Cr6+ (CrO4

2-) and trivalent Cr3+ (CrO3)states. For polyaniline, the example in equation (1) can begeneralised over multiple n+ values.There is, however, a difference in the electrochemicalmechanism for corrosion inhibition. Chromates are known toblock cathodic sites, while polyaniline probably works byinhibiting the corrosion activity at anodic sites.

Double-strand ICP avoids adverse effects of singlestrandsCommon intrinsically conducting polymers have threeproperties that are unfavourable for application in coatingsystems:- Dopants may activate corrosion.- The conductive form lacks stability.- ICPs are difficult to incorporate into common coatingsystems.For convenience, the conventional ICP in Figure 3 will bereferred to as a single-strand ICP.The double-strand polyaniline shown in Figure 6 offers apotential solution to these problems. One example ofdouble-stranded polyaniline is a molecular complex betweentwo strands of linear polymers: polyaniline and poly(acrylicacid) (PAA), non-covalently bonded in a side-by-sidefashion. This PAN:PAA complex is synthesised by atemplate-guided method [11] as shown schematically inFigure 7:- The aniline monomers are first adsorbed onto a templatepolymer such as poly(acrylic acid) or poly(styrene sulfonicacid) (PSSA) to form an adduct;- The adduct is polymerised to form a complex between thetemplate and polyaniline.A coating containing single-strand ICPn+ will carry a largenumber of anionic counter ions (dopants) that are necessaryto balance the electrical charge on the polymer. Since thecounter ions are swelled by water, they can participate as anelectrolyte in electrochemical corrosion. In the presence ofNaCl, the anionic dopant will facilitate the penetration ofchloride ions by an ion exchange mechanism.Thus the single-strand polyaniline will weaken the barrierproperties normally provided by the resin in the paint. Evenif the coating is in contact with salt-free water, the dopantswill still induce osmotic pressure that may causedelamination of the coating. These adverse effects areavoided when the mobile low molecular weight ions arereplaced by the polymeric dopants in the double-strand ICPshown in Figure 6.

Double-strand ICPs can be designed to be easilydispersibleThe second problem with single-strand ICPs derives fromthe fact that the polymeric backbone is difficult to dissolve ordisperse in any solvent or resin. The delocalised carrierslead to high cohesive energy and strong π-π stacking, whichin turn leads to intractable polymers. By an appropriatechoice of the template polymer in the synthetic method ofFigure 7, however, the solubility properties of thedouble-strand polymers can be adjusted [12, 13]. Itbecomes possible to choose ICPs that are soluble in wateror in solvents. For example, the double-strand PAN:PAAand PAN:PSSA are dispersible in water, while the complexPAN:poly(methylacrylate-co-acrylic acid) is soluble in ethyl

acetate. The choice of the second strand provides scope foradjusting compatibility with coating resins.

Stability at seawater conditionsWhen the low molecular weight dopants in single strandICPs are leached out of the coating system, the chargecarriers on the polymer backbone are lost and the ICPchanges into the less effective non-conductive form. Anotherproblem is associated with the deprotonation ofsingle-strand ICP in water. Single-strand polyanilinede-dopes in an aqueous environment with pH above 4 andthen loses its effectiveness as a corrosion inhibitor.The double-strand form in Figure 6 is stable in theseenvironments [14]. The polymeric dopant does not leach outbecause it is bonded to the polyaniline strand. Figure 8shows a comparison of the pH titration curves forsingle-strand and double-strand polyanilines. Theconductive form of the single-strand polyaniline de-doped atpH > 4, while the double-strand poly-aniline remains stablein seawater with pH = 8.2.

Effective protection in epoxy primerExperimental coating systems have been produced by usingdouble-strand polyaniline as an additive to commercialpaints. Double-strand polyaniline in a form compatible withthe resin was added at 1% to 5% by weight of the resin toan epoxy-based commercial chromate-free electrophoreticcoating formulation.Bare AA2024 aluminium alloys without chromate or othersurface pretreatment were electrophoretically coated.Samples were scratched and tested for salt fog sprayaccording to the ASTM B117 standard.Figure 9 shows that the control had corrosion damage nearthe scribe while the sample showed no corrosion after 1000hours of salt fog spray. In other tests for filiform corrosionthe primer with the polyaniline also showed betterperformance than the controls after 8 weeks of acid/humiditycycles.

Anticorrosive core-shell pigments with double-strandICPsResearchers at Wayne Pigment Corporation and theUniversity of Rhode Island have jointly developed a pigmentthat contains the double-strand ICP (patent pending). Thepolyaniline is incorporated into a non-chromate anticorrosionpigment in the form of a composite with an inorganic coreand an organic shell. A schematic structure of thiselectroactive pigment is shown in Figure 10. This provides aconvenient way for paint formulators to introducedouble-strand conducting polymers into a paint formulation.

Double-strand ICP is a possible chromate replacementIn summary, conducting polymers show promise as areplacement for hexavalent chromates. The anticorrosivemechanism is electrochemical in nature, and thus similar tothat of chromates. The reversible redox properties may bethe reason for their ability to heal metal surfaces beforepitting corrosion sets in.A new double-strand conducting polymer has beendeveloped, which is more stable and more easily integratedinto commercial coating formulations. Since it can be usedas a minor additive, it may produce effective and low costcoating systems. Its performance has been evaluated inepoxy primers applied over aluminium alloy.

REFERENCES[1] D. W. DeBerry, J. Electrochem. Soc., 132, 1022 (1985)[2] B. Wessling, Adv. Materials, 6, 226 (1994)[3] D. B. Wrobleski, B. C. Benicewicz, K. G. Thompson, C. J.


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Bryan, ACS Polymer Preprints, 35, 265 (1994)[4] Y. Wei, J. Wang, X. Jia, J. M. Yeh, P. Spellane, ACSPolymer Preprints, 72, 563 (1995).[5] R. Racicot, R. L. Clark, H.-B Liu, S. C. Yang, M. N. Alias,R. Brown, SPIE Proceedings, 2528, 251 (1995); R. Racicot,R. Brown, S. C. Yang, Syn. Met. 85, 1263 (1997)[6] J. He, D. E. Tallman, G. P. Bierwagen, Journal of theElectrochemical Society 151 (2004) B644-B651.[7] D. W. DeBerry, J. Electrochem. Soc., 132, 1022 (1985)[8] B. Wessling, Adv. Materials, 6, 226 (1994)[9] P. M. McManus, S. C. Yang, R. J. Cushman, J. Chem.Soc., Chem. Commun., 1556 (1985).[10] S. C. Yang, pp. 335-365 in "Large-area Chromogenics:Materials and Devices for Transmittance Control", C. M.Lampert, C. G. Granqvist, Editors, (SPIE Publishing, 1989).[11] W. Li, P. A. McCarthy, D. Liu, J. Huang, S.C. Yang, H.L. Wang, Macromolecules, 35, 9975-9982 (2002).[12] J.-M. Liu, S. C. Yang, J. Chem. Soc., Chem. Comm.,1529 (1991).[13] L Sun, S C. Yang, Mat. Res. Soc. Symp. Proc., 328,167 (1994).[14] L. Sun, S C. Yang, J. M. Liu, Mat. Res. Soc. Symp.Proc., 328, 209 (1994).

Results at a glance- Intrinsically conducting polymers can be 'doped' to providean electrochemical mode of corrosion protection which hassome similarity to that of chromates.- However, most conventional conducting polymers havelimited stability and poor compatibility with coating resins.- A new double-strand, polyaniline-based conductingpolymer has been developed, which has better stability andcan be designed to provide compatibility with eitherwaterborne or solventborne coatings.- Corrosion tests on two epoxy primer formulations indicatethat the double-strand polymer offers effective corrosionprotection to aluminium.- Core-shell anticorrosive pigments have also beenproduced which incorporate the double-strand ICP in theouter layer.

The authors:-> Professor Sze Yang was educated at the National TaiwanUniversity (B.S.) and Columbia University (Ph.D.,chemistry). He is a Professor of Chemistry at the Universityof Rhode Island, Kingston, USA and conducts research inthe area of polymer chemistry and material science.-> Professor Richard Brown was educated in England atNottingham University (B.S.) and Cambridge University(Ph.D). Currently he is a Professor of Chemical andMaterials Engineering at the University of Rhode Island witha research interest in degradation and protection ofmaterials by corrosion.-> Dr. John Sinko was educated in Romania at Babes-BolyaiUniversity (B.S. Organic Chemistry), at Bucharest University(Colloid-Surface Chemistry) and in Hungary at VeszpremUniversity (Dr. in Chemistry). Currently, he holds the R&Dand Technical Director position at Wayne Pigment Corp.,WI, USA. His research interest is focused on the chemistry,mechanism of pigment grade corrosion inhibitors in organiccoatings.This paper was presented at the European CoatingsConference Smart Coatings IV, Berlin, 9-10 June 2005


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Figure 1: Structure of some intrinsically conducting polymers (ICPs).


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Figure 2: Filiform corrosion test, 60 days. a) Control: Epoxy primer, b) Sample: 1% PANin epoxy.


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Figure 3: Single-strand ICPn+ with mobile positive charge and anionic dopants.

Figure 4: Potentiodynamic scan showing the passivation of the surface by PAN coatedaluminium alloy AA2024.


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Figure 5: Aluminium alloy with oxide and PAN layers.


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Figure 6: Double-strand polyaniline.

Figure 7: Template-guided synthesis of double-strand polyaniline.


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Figure 8: pH titration curve of polyaniline. Red: Single-strand PAN; Blue: Double-strandPAN:PAA; Green: Double-strand PAN:PSSA.


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Figure 9: Salt fog spray test, ASTM B117, 1000 hr for epoxy on AL2024 with no surfacepretreatment. Images show the scribed area on the coating. Left: control epoxy

coating. Right: double-strand polyaniline 5%..


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Figure 10: Design of an electroactive core-shell pigment particle.


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The double helix - european-coatings.com · The double helix Designing conductive polymers for improved metal protection. Sze Yang, Richard Brown, John Sinko. Intrinsically conductive