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Content archived on 2024-04-30

Heavy metal free waterborne anti corrosion coatings from intrincically conductive polymers


Standardised procedures were selected and adopted prior to development of conductive primer systems. Because of economical and technical reasons, core shell dispersions have been fabricated using cores of latex particles that were covered with an ICP shell. Several factors have been investigated including; type and particle distribution of the latex; the type and concentration of ICP; the type of dopant and the pH during the redoping process. An interactive assessment was undertaken between coating development and electrochemical measurements. The electrochemical experiments consisted of laboratory studies, including monitoring of the Open Circuit Potential (OCP) and measurements of the polarization resistance.
Electrochemical results showed the particle size of the latex system to be important. It appeared that latices with smaller particles accommodate larger amounts of ICP at the surface. This is very important since it is expected that the effects of corrosion inhibition are proportional to the amount of ICP added to the system. In addition, latices with small particle sizes show better film formation than latices with large particles because of larger capillary forces during drying. Since the encapsulating ICP layer hinders film formation it is expected that the larger capillary forces will help to overcome this barrier.
Out of several latex systems an optimal system, with good film formation properties and corrosion protection properties, was selected for further research.
From the prototype primers developed based on this formulation, different (solvent and water based) binders have been developed and investigated by electrochemical tests and corrosion test programs with two different cyclic exposure environments; humidity/dry and salt-spray/dry. After the cyclic exposure microscopic and chemical analyses were performed. Comparison was made with the following reference samples; bare steel, steel with the latex coating without ICP, and steel with latex coating without ICP but with the procedure for encapsulation.
In the humidity/dry test almost no visual differences between the various primers were observed after a high number of cycles. Pani and Ppy coatings showed similar performance. It was also hard to distinguish between the several concentrations of ICP and the several dopants. In the salt spray/dry cyclic test coating performance could be ranked and indicated that more ICP present in the coating does not necessarily lead to improved performances. There seems to be an optimum amount of ICP to give the best properties. If too much ICP is present, the corrosion protection decreases, which can be related to the decrease of film forming properties with increasing amount of ICP.
Generally, from exposure tests corrosion attack could be found on all combinations.
On the other hand, primers that showed cracks before exposure only showed traces of rust after exposure. Many cracks remained free of corrosion indicating the high protection potential of the coating. From this it can be concluded that the ICP can take care of effective corrosion protection, but for a long lifetime an adequate coating build up is necessary: primer adapted to the latex containing the ICP and a topcoat with high barrier properties.

In conclusion:
- ICP as shell in a core-shell latex system can provide a strong corrosion protection;
- The development of the latex with required film formation properties, the scaling-up and the mixing in a conventional primer appear to be the essential parts for a successful primer based on ICP. Concerning the first two items, illustrative methods have been developed in this project. The third one is strongly dependent on company related formulations. It is expected that the participating companies themselves will develop this aspect further;
- Economic indications point out that a competitive product can be made based on ICP core-shell latices.

Finally a lot of knowledge and experience about intrinsically conductive polymers has been developed. For the project partners an improved understanding of the use and integration of ICP based primers with conventional paint processes has been gained. It can thus be concluded that the project team has been successful in reaching the aims set for the projects.
It is aimed to produce a low cost, heavy metal free, waterborne anti-corrosion coating based on new functional polymers, called intrinsically conductive polymers (ICPs). Three classes of ICPs will be investigated on their corrosion protecting capabilities, viz. polyanilines, polypyrroles and polythiophenes. A new dispersion technology will be developed aiming for a low content of ICP, while retaining the same anti-corrosion effects as the common heavy metal anti-corrosion dispersions. A common latex will be coated with a thin ICP layer via in situ polymerisation. These so-called coreshell latices based on conductive polymers will be incorporated into a polymeric matrix in such a way that, upon film formation, the coated latex particles touch each other and a conducting path through the matrix is formed. The project will seek to improve the properties of the core-shell particles by judicious choice of both the core material and by variation of its particle size, particle size distribution, and surface composition. Next an anti-corrosion paint based on these ICP materials will be developed having the necessary properties such as adherence, process ability and stability to be useful products. Once satisfactory performances of both core-shell particles and the paint system have been achieved and the parameters for their fabrication on a laboratory scale have been defined, work will commence on the testing and evaluation of exposure panels with either a standard epoxy or polyurethane topcoat on the coating primer. The anti-corrosion performance of the coatings will be tested by exposure in accelerated cyclic corrosion tests typical for aimed environments like building, marine and construction (industrial). The evaluation of the coatings will a.o. be carried out by electrochemical impedance measurements enabling a quantitative determination of ageing due to weathering. It is anticipated that this new core-shell dispersion approach will be of economical benefit as the ICP material costs contribute up to 70% to the costs of the total coating systems presently available. It is aimed to achieve a cost reduction of a factor 10 compared to the present ICP systems which are not based on core-shell systems. This means that the new dispersion system will compete in price compared to the common zinc based coatings. A heavy metal free and VOC-free coating system would also minimize the environmental and health impact for coatings used in the building, marine and construction industry.

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Participants (6)