Scientific and Commercial Breakthrough for Organic Metals
Bernhard Wessling
Ormecon Chemie GmbH & Co. KG (a Zipperling Kessler &
Co. subsidiary),
Ferdinand-Harten-Str. 7, D-22949 Ammersbek, Germany
Abstract
The organic metal polyaniline has been thoroughly investigated over the last years, an overview will be given with focus on:
- metallic properties (quantum size effect in mesoscopic metals, and strategies for increasing metallic conductivity)
- metallic properties of polyaniline blends
- ennobling and passivation mechanism (reaction scheme) of PAni on conventional metal surfaces
The first regular and significant commercial applications have been realized or are close to reality. The most important will be discussed, like:
- corrosion protection, and especially the first real applications
- manufacturing of printed circuit boards (here, a new technology will be presented, which is a new surface preparation technique)
- transparent coatings - their physical and chemical background, their actual and potential new applications
Keywords: Photoelectron
spectroscopy; Optical absorption and emission spectroscopy; UV-Vis-NIR
absorption; Mass ectroscopy; Organic/inorganic interfaces; Polyaniline
and derivatives; Other
A totally new family of materials is on the way to
contribute important aspects to solving many different technical
and ecological problems - the organic metals.
The first commercially available member of this new
materials group, polyaniline, is actually being introduced into
the market by a medium sized independent company, Ormecon Chemie,
a subsidiary of Zipperling Kessler & Co.
The company is located in Ahrensburg near Hamburg
in Germany. Zipperling / Ormecon conducted basic research for
over 14 years in several areas, including "conductive polymers",
which enabled it now to introduce PAni as the first company into
the world market.
Some first commercial applications have been developed
in the last years and have been stabilized since 1995.
The organic metal polyaniline has some properties [1] which sound strange for the expert:
it is an organic polymer, but totally unmoldable and insoluble
it is a metal, i.e, it has free electrons in a metallic "conduction band" (but: the conduction band is "only" extending over about 10 nanometers, which is the reason, why this metal is to be named as "mesoscopic metal" and behaves as other mesoscopic metals (copper, silver, etc, when being prepared in colloidal particles size below 1 micron) behave (fig 1)
Fig.1: Qualitative description of the electron wave functions,
tunneling energy barriers and morphology of the mesoscopic metal
polyaniline
it is a salt
it is redox active and can exist in at least 3 oxidation states (of which only one is metallic, see fig 2), but does not change its macroscopic form during reversible oxidation or reduction (hence can act as a "redox catalyst")
Fig.2: Redox scheme of
polyaniline, as evaluated and spectroscopically characterized
by us; with the abbreviations meaning ES = emeraldine salt, metallic;
LE = leucoemeraldine, reduced form; LS = leucoemeraldine salt;
EB = emeraldine base; PB = pernigraniline base (the pernigraniline
salt has not yet been prepared and characterized by us reproducibly)
2 of 3 oxidation states are stable under "normal" conditions (air, ambient temperature up to over 200 °C, ...)
it is "electro- and chemo-chromic", i.e., it changes its transparent colour upon oxidation or reduction (the stable metal is green, the stable oxidized or neutralized form is blue, the reduced form - readily reoxidized to the green or blue form - is colourless, see fig 3.1 - 3.3)
Fig.3.1.
Fig.3.2.
Fig.3.3.
Fig 3.1 - 3.3: UV-Vis spectra of pure, precisely characterized redox species [8] of Pani: 3.1 ES as delivered (ES after redox reaction via LE and EB - see fig 2 - is slightly different insofar as the absorption in the NIR is higher and the absorption above 800 nm is stagnating at high level); 3.2 EB; 3.3 LE (we have found, that LE is - like LS! - purely transparent colourless and not, as often described, slightly yellow)
in thin layers, it is transparent (but coloured)
Dispersion as the key for applications
First commercial applications have therefore been
realized in relatively obvious demands like transparent (green)
antistatic or conductive coatings of plastics. But not at all
easy: how to make this organic metal, being insoluble and not
moldable, processable and applicable onto plastic (or other) surfaces,
leading to a thin transparent layer?
The solution to this task - dispersion of polyaniline - is the technology which represents the company's lead in this one-and-a-half decades lasting research competition. Zipperling realized the dispersion by 2 supplementing steps:
- first a completely new polymerization procedure for the whole class of materials leading to a measurable dispersability
- parallel to this extremely performing dispersion
techniques were developed, because also the best dispersible organic
metals belong to the "hardest to disperse materials"
in the world - by far harder than any "hard-to-disperse"
pigment.
With the strategy "dispersion", Zipperling has opened and practiced a way which is surprinsingly still not accepted in the
research community. But we think, the success - equally visible
in well performing products as in basic new scientific
results (including a new non-equilibrium thermodynamics theory
for polymer systems [2]) cannot be overlooked any more.
(A very helpful and funny new tool is a computer
simulation - fig 4 - of structure formation, which we performed
recently 4 and is also available on the world wide web 5).
Fig 4: Simulation of the
flocculation process of dispersed particles in a heterogeneous
polymer systems; simulation is performed using specific parameters
in the well-known "cellular automata" program
These structures are responsible for conductivity
and other phenomena (like melt viscosity or impact strength).
A coating of only a few micrometres thickness allows
for designing the surface resistivity between 103 and 109 Ohms/sq
which reveal no particles visible by eye or microscope.
And this technology now opens a range of high tech
possibilities of various kinds: one can coat glasses and (with
a special design) change the optical and IR transparency properties
electrochemically - the "smart window"; such a project
is actively being pursued by us with a customer in pre-industrial
scale.
Printed Circuit Boards Production
With coating formulations of specifically designed
properties we suceeded - together with a circuit board technology
company - to replace an inefficient production step in the circuit
board production, which is considered by experts being a revolution
in this field. We will disclose details later, after broader introduction
of the product into the industry. Actually the first 5 customers
are continously using this technology and have released the new
product.
Other applications in this field are under development.
Also for optoelectronics applications might be found
in future.
Corrosion protection
Probably the least obvious and scientifically and
technically most complicate application of the organic metal polyaniline
is the corrosion protection. It was before 1987 that we made first
discoveries according to which polyaniline containing coatings
on metals were showing some improved corrosion protection. But
despite continuous intensive research it took until 1993 [3] that
we found out about the principles of the effect:
Polyaniline behaves like a noble metal thanks to
its redox potential being close to silver, therefore it ennobles
the surface of conventional metals; moreover it transforms the
surface of the metal to be protected into a thin but dense metal
oxide layer. It passivates metals (cf fig.5).
Fig.5: Schematic description
of the catalytic passivation of iron by PAni
Fig.5.1. |
Fig.5.2. |
Fig 5.1-5.2.: MS spectra,
5.1 showing polyaniline with its various dimer components, see
[7]; 5.2 showing the iron peak at m/e = 54 [7]
We assume that such a reaction occurs also with other
metals like copper, aluminum or zinc which are all ennobled by
PAni .
Fig 6 and 7 are showing
XPS evidence for only Fe203 being produced during passivation.
Fig.6.1. |
Fig.6.2. |
Fig 6.1-6.2: XPS spectra
of iron: 6.1 untreated with Fe and FeOOH signal; 6.2 with pure
Fe2O3 signal.
Fig 7.1-7.2.: Quantitative optical evaluation of a aluminum
plates coated with a) Pani containing primer b) top coat system
7.1 epoxy, 7.2 PU; fig 7.1 shows an optimal performance with no
filiform corrosion - the first system world-wide capable of
beating the filiform corrosion.
In contrast to "rust" (which is a wild
mixture from various iron oxides and hydroxides with salt inclusions)
Fe203 does not build ever new surfaces for corrosion attack or
offer iron cations able to autocatalytically enhance corrosion
velocity.
The new coating is shifting the corrosion potential
by up to 800 mV (for iron and steel) and more than 2 V for copper.
This leads to a dramtaic decrease of the corrosion velocity under
certain corrosion environments.
The new coatings are being offered as primers or
concentrates for the development and production of primers.
An efficient corrosion coating system with PAni includes
the ennobling primer and a suitable ("sealing") top
coat. Such a coating system proved to be the first system worldwide
capable of beating the "filiform corrosion" of aluminum
[6], cf fig 7. We have gathered a lot of experience and
positive results of official test institutes up to now, but many
more questions have to be answered with thourough research and
development and further practical testing, to which we invite
all interested companies.
There is actually also a product named "CORREPAIR"
which is designed for the "do-it-yourself" expert who
has to repair corroded metal products at home. Zipperling gathered
very good results in a first test market phase and Ormecon succeeded
to introduce it to the market since recently.
References
Werkstoffe und Korrosion, manuscript W3104, published in August 1996; available on WWW since October 1995
