Polymers show their metal

A seminal paper in 1977 by last year's chemistry Nobel prizewinners, MacDiarmid, Shirakawa and Heeger, was the inspiration for new corrosion resistant coatings, reveals Bernhard Wessling

The news in October 2000 that Alan MacDiarmid, Alan Heeger and Hideki Shirakawa had won the Nobel prize in chemistry (Chem. Br., November 2000, p 26) should have come as little surprise to me. It was the seminal publication¹ by these three scientists in 1977, describing the synthesis of electrically conducting polymers, that originally fired my own imagination in this area of research. In turn, this work has ultimately led to the development of a new type of corrosion resistant coating recently commercialised by Ormecon Chemie, a subsidiary of the German company Zipperling Kessler.

Their paper, in Chemical Communications showed that the reaction of iodine with polyacetylene (PAc) resulted in a new polymeric compound, a radical–cation salt, which had a conductivity many orders of magnitude higher than the original neutral PAc. What intrigued me on first reading this paper was the authors' note that this PAc was insoluble and 'unmouldable' – and, alas, also unstable. At the time, I was working in the field of polymer compounding, in other words manufacturing multi-phase compositions of polymers in which other polymers, additives and fillers were dispersed. Would it be possible, I wondered, to produce a polymer dispersion with these conductive polymers that not only retained conductivity, but also processability?

It took me more than five years to prepare the first dispersion of PAc, which was based on my discovery that the conducting polymers are composed primarily not by fibrillar structures, but by even smaller globular particles.² We now know that the primary particles – the smallest entities that display all the properties seen in macroscopic conducting polymers – range from 10 to 15 nm in size. These form all kinds of agglomerates and aggregates, including fibrils, in a precise morphological hierarchy.

I continued work in this field after moving to become research director at Zipperling Kessler in Ahrensburg near Hamburg in 1981. During the 1980s, my colleagues and I at Zipperling developed several processes for making dispersions of conducting polymers, including dispersable powders of a thermally stable composition based on the polyaniline (PAni) polymer family. Discovered in 1912, PAni experienced a revival in the mid-1980s, both in the US and in our laboratories in Germany where we first achieved processability under normal industrial conditions, including elevated temperature.

A true metal

One of the most challenging questions arising from this work has been to understand our discovery that, after dispersion, PAni as well as PAc and other conducting polymers still show conductivity above a certain critical polymer concentration. Above this concentration, very complex network structures of the globular particles form.³ No polymer matrix lies between this 'pearl-like' arrangement of particles to prevent electron flow, therefore the dispersion is conducting. To our surprise, we found that the conductivity of certain dispersions was even higher than in the original PAni. This was the first conducting polymer to cross the boundary between insulators and metals – to become a true organic metal.⁴

These studies, which we undertook together with scientists from the universities of Cologne (Germany), Wellington (New Zealand) and Madras (India), also showed that the electrons were not – as is still believed even among the Nobel committee⁵ – mainly transported along the polymer chain. Rather, they were forming a 'cloud' of electrons as in metals, throughout the whole 'metallic' core of a primary particle, and then tunnelling (a transport mechanism unique to elementary particles) from particle to particle.⁶

Apart from this basic scientific work, we were interested in possible commercial applications.⁷ We focused our attention on all kinds of dispersions: in polymer matrices, in solvents, in paints, and even in water.⁸ In 1985 another researcher, David deBerry, made the striking observation that after electrochemical deposition of PAni from its aniline monomer, coated steel corroded much slower than normal. The steel in this experiment was already pre-treated (passivated) to make it more resistant to corrosion. I wondered whether it would be possible to improve the corrosion resistance of non-passivated ordinary steel by coating this with a powder form of PAni dispersed in paint. We achieved some initial successes with this approach in 1987, but these were not particularly convincing and proved difficult to reproduce.

After continued study of the mechanism of interaction between PAni and steel, however, we finally succeeded in producing a highly corrosion resistant coating by optimising the process conditions.⁹ To achieve this, the PAni needed to be in its 'organic metal' state – ranked in the galvanic series slightly below silver, yet more noble than iron and even copper. It was able to oxidise the iron (or copper) and attain a relatively stable, reduced, so-called leuco-emeraldine (LE) form, which can itself be re-oxidised by oxygen to the original organic metal. In 1993 and 1994 we used our organic metal in formulating several new paints for preventing the corrosion of iron and steel.

Corrosion occurs in the presence of water, salts and oxygen, during which iron or steel undergo an electrochemical process in which different locations of the iron surface act as electrodes. At the local anode, iron is oxidised to soluble Fe²⁺ and Fe³⁺ ions:

nFe(s) ® mFe²⁺(aq) + (n–m)Fe³⁺(aq) + 3(n–m)e^-

At the local cathode, hydroxide ions are formed:

O₂(aq) + 2H₂O(l) + 4e^- ® 4OH^-(aq)

Rust, which forms subsequently, is composed of FeO, Fe₂O₃, Fe₃O₄ and other mixed oxides, Fe(OH)_x , and Fe^x+ salts (chlorides, sulfates etc). This highly irreproducible and irregular composition opens up new surfaces for self-catalytic growth - mainly Fe³⁺ salts acting as rust formation catalysts. Rust that forms this way does not adhere well on the iron surface.

In the presence of our organic metal coating, a completely different galvanic process occurs, in which PAni replaces iron as the cathode. This arrangement is, as far as we know, relatively evenly distributed over the whole surface. The surface potential is shifted from formerly ca -400 mV to between +250 and 400 mV.

Now, the iron is oxidised exclusively to Fe²⁺:

Fe ® Fe²⁺ + 2e^-

while the organic metal is reduced to its LE form:

PAni + n*2e^- ® LE

Oxygen subsequently oxidises Fe²⁺ to Fe³⁺, and the LE back to the normal oxidation level, at the same time generating hydroxide ions. Because we now have a stoichiometric amount of Fe³⁺ and OH^-, Fe₂O₃ is formed exclusively:

2Fe³⁺ + 6OH^- ® Fe₂O₃ + 3H₂O

This is the most stable iron oxide, and to our surprise, it forms on the iron surface, at the interface between Fe and PAni. Here, the Fe₂O₃ adheres very well and provides a barrier to further rust formation. The reaction mechanism requires a two-layer paint system, but because all advanced anti-corrosion coatings come as two- or three-layer coatings, this did not present any serious hurdle.

Testing times

As a newcomer to the field of paints and corrosion protection, Zipperling had to be quick in developing and testing the new coatings. Salt spray and climate cycling tests take too long, and are not precise enough to render exact data for a direct route to efficient coating systems (rather than a trial-and-error approach often used in paint development). We therefore developed a new method consisting of three basic tests:¹⁰

• a screening ('scratch') test in which the coatings had to prove that an open scratch does not rust upon immersion in salt water;
• measuring the Volta potential of the coated metal and comparing this with the potential in the open scratch; and
• measuring the impedance of the whole, unscratched, coating under corrosion conditions.

The first test result is simply either 'yes' or 'no': if corrosion occurs in the open scratch, then we do not continue with the other evaluations. It is a special feature of our organic metal coatings that open scratches up to 2 mm in width are fully protected against corrosion.

The second test, performed with the so-called Scanning Kelvin Probe (a vibrating fine needle that scans 50 µm above the sample surface) tells us the change of the metal surface potential during the first stage of corrosion – the delamination of the coating by the corrosive electrolyte. As soon as the original metal–PAni coating interface is replaced by a metal–electrolyte interface, the Volta potential changes. This allows us to determine the delamination velocity – the speed at which a 'corrodable' surface is being created. Conventional coatings show a delamination velocity of more than 30 µm per hour. Our best PAni-containing coating systems display a value of below 3 µm per hour, about a factor of 10 less.

The third test, which we perform with electrochemical impedance spectroscopy, tells us about the barrier properties of the coating. We want a system that does not allow ions (salts and corrosion products) to migrate through the coating. Usually, coatings develop an increasing tendency to let ions pass, which enhances corrosion even further. We can quantitatively describe this tendency by the dielectric constant DE, a number that is independent of the coating thickness. A vacuum has a DE of 1, water has the highest DE of 75. Our coatings do not exceed a DE of 12 when first applied, and should not show any significant increase. The DEs of most other coatings – even those starting with values around or below 12 – increase steeply and rapidly.

The performance of our coatings is still far from optimised, but early commercial applications show that they improve the anti-corrosion capacity of conventional coatings by a factor of two to more than 10. Our organic metal coatings are already proving their capabilities in marine environments, waste water treatment, chemical plants, coal mines and in public constructions in towns, parks and sport arenas.¹¹ We were obliged initially to develop such paints ourselves, because paint manufacturers were reluctant to work with this new technology. However, now that such products are beginning to meet with commercial success, several independent companies have begun developing their own paints based on our technology.

The applications of our organic metal are not limited to corrosion protection. We have recently worked with some of our customers to develop a variety of antistatic coatings based on this technology. These have met with initial approval by the electronic industry, where they are used to coat packaging materials. A water-based antistatic coating developed by Zipperling has also now been successfully tested as a primer for plastic automotive parts prior to electrostatic coating.

In yet another application, we supply a solderable surface finish for printed circuit boards. Without such a surface finish, the electronic components like transistors, resistors, capacitors and chips could not be fixed and electrically connected to the circuit. The new technology is based on a water dispersion of the organic metal, which passivates the copper and provides catalytic sites for the chemical deposition of pure tin, which is the final surface. It combines the cost effectiveness of the old solder coating – which is limited in its applicability – with the flat and fine 'structurability' of a gold surface, which is three times more expensive.¹²

Our organic metal will also soon prove useful as the so-called hole injection layer for new organic or polymeric light emitting diodes (OLEDs). These OLEDs, which can emit light due to semiconducting (not conducting) polymers, will be used in the new generation of mobile phones, in flat panel displays and maybe even in computer and TV screens of photo quality resolution (Chem. Br., August 1999, p 22). Light is emitted when electrons, injected from the cathode, and positive charges ('holes'), injected from the anode, come together in the ca 100 nm thin layer of the organic semiconductors.

The chemical design and manufacture of these organic semiconductors is the business of Covion Semiconductors, a Frankfurt based spin-off from Hoechst. One of the trickiest parts of this process is hole injection, because this has to be done from a certain energy level – which should be the same over the whole display area. The conductive glass ITO (indium tin oxide) used in current flat panel displays is too rough, not pure enough, and injects the holes from a less-than optimal energy level, which also has sharp potential 'valleys' and 'peaks'. Instead, we designed a water based dispersion containing very small (ca 40 nm) particles of PAni, which after deposition on ITO provides a suitable hole injection medium – with brighter output, lower energy consumption and a longer lifetime.

Other opportunities for organic metal technology are also emerging, albeit slowly. To help with the introduction of our conducting polymer technology into the marketplace, we founded Ormecon Chemie - of which I am chief executive officer (as well as president of Zipperling) in 1996. Later, Zipperling sold part of its business to provide capital against the expected initial losses. Ormecon is now active in 17 countries in the world, and has about 25 sales partners, including one joint venture in Japan (Nippon Ormecon).

Mainly in the US and Japan, antistatic coatings using Ormecon's PAni are entering the market as transparent (though actually light green) antistatic packaging for electronic devices. More than 40 commercial objects are now coated with the organic metal anti-corrosion paints in Japan alone, while more than a thousand private boats in Germany are better protected thanks to our coatings. Many industrial 'reference objects' are also proving the performance of these coatings under harsh corrosion conditions in various countries.¹¹

In the printed circuit board industry, over 50 customers now, mainly in US, Korea, the UK, Germany and Australia, are using our organic metal as the final surface finish. Meanwhile, PAni is now accepted by the developing OLED industry as a promising candidate for hole injection layers. Ormecon is busily engaged in licensing the technology for use by other interested companies. Two licences have been agreed, one with DuPont, for a special technical polymer blend, another with Covion for the hole injection layer in OLEDs. Two more licences are under negotiation, one of which has recently been signed.

Future directions

These applications are just the tip of the iceberg. Other future uses could include 'all-organic' solar cells made from PAni organic metal; a group at the University of Linz in Austria has recently achieved a relatively high, 3.5 per cent, efficiency with this technology. Yet another potential application for PAni is in mobile phones and computers, as coatings for shielding the surrounding environment from electromagnetic interference (EMI). Here, Ormecon is actively engaged in trying to find the next generation of organic metals, with a conductivity 1000 times higher and still dispersible. Because of their unique property spectrum - unifying metals, organic materials and catalysts – organic metals promise to become useful for underpinning a wide range of applications. From that small note in the 1977 paper has emerged a whole range of exciting applications for our organic polymers. I am looking forward to seeing in what future directions they might lead me.

Bernhard Wessling is shareholder and president of Zipperling Kessler and chief executive officer at Ormecon Chemie, Ferdinand-Harten-Str. 7, D-22949 Ammersbek, Germany; e-mail: wessling@ormecon.de

References

1. A. MacDiarmid et al, J. Chem. Soc. Chem. Commun., 1977, 578.
2. B. Wessling, Makromol. Chem., 1984, 185, 1265
3. B. Wessling, Zeitschrift f. Physikalische Chemie, 1995, 191, 119.
4. Wessling et al, Eur. Phys. J., 2000, E2, 207.
5. www.nobel.se/chemistry/laureates/2000/public.html
6. R. Pelster, G. Nimtz and B. Wessling, Phys. Rev. B, 1994, 49(18), 12718.
7. B. Wessling in Handbook of nanotechnology, H. S. Nalwa (ed), chap 10. London: Academic Press, 1999.
8. B. Wessling, Synth. Met., 1998, 93, 143.
9. (a) B. Wessling, Adv. Mater., 1994, 6(3), 226; (b) B. Wessling, Materials and Corrosion, 1996, 47, 439.
10. J. Posdorfer and B. Wessling, Electrochim. Acta, 1999, 44, 2139.
11. http://www.ormecon.de/
12. Z. Morawska and G. Koziol, PCF magazine, in press.