Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2000-09-05
2003-10-21
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S508000, C252S512000, C252S514000, C106S601000, C106S691000, C427S126400
Reexamination Certificate
active
06635192
ABSTRACT:
The invention relates to an electrically conducting microcapillary composite matrix and to a method for producing the same, which matrix is suitable for the making of weathering-resistant, durable and acid-resistant electrically conducting coats of paint and coatings and, in particular, as anode material for the cathodic corrosion protection of reinforcing steel in concrete.
The demands made on electrically conductive coatings or coats of paint which are suitable as anode for the cathodic protection of reinforcing steel in concrete are particularly high. As the skilled artisan knows, the cathodic protection is based on the lowering of the electric potential of the reinforcing steel by applying a so-called protective current that flows between the concrete surface mounted anode and the reinforcing steel (cathode). The flow of current is enabled by the electrolytic conductivity of the concrete and by the electrochemical reactions on the anode surface and the cathode surface (reinforcing steel). At the surface of the reinforcing steel, alkaline-acting hydroxyl ions are formed by the electrochemical reduction of oxygen, and at the anode surface, water is oxidized to oxygen; in the presence of chloride also chloric gas is formed, and moreover, acid is formed, i.e. one mole of acid for a charge equivalent of 96000 ampere seconds (Coulomb). At a mean, common current density of from 5 to 10 mA/m
2
, this will correspond to approximately 0.2-0.4 l of conc. hydrochloric acid/year. The acid formed may attack both the anode as well as the concrete surface and destroy them. In principle, the equivalent amount of lye (hydroxyl ions) will form on the reinforcing surface. However, to neutralize the acid formed on the anode, the hydroxyl ions must diffuse through the concrete cover of the reinforcing steel to the concrete surface, usually approximately 2 to 3 cm. A diffusion of the acid (protons) in reverse direction is not possible since the acid reacts at the concrete surface with the hardened cement paste and, possibly, also with the additives, and is neutralized. Diffusion and electromigration of the hydroxyl ions through concrete is very slow, depending on the moisture content and the porosity of the concrete it will be approximately 100 to 1,000 times slower than in solution. Depending on the protective current density and the applied voltage it may thus take weeks to months until the acid formed at the anode is neutralized by the hydroxyl ions diffusing from the reinforcement to the concrete surface. For this reason, the anodically formed acid may attack and destroy both the anode and the concrete surface. Large pores and cavities will be formed in the surface layer of the concrete by the acid attack, which impede the diffusion of the hydroxyl ions and thus will even accelerate the destruction of the concrete surface.
Therefore, titanium nets which are acid and chlorine-resistant are commonly used as anode materials, their surfaces being modified with iridium oxide, ruthenium oxide, platinum, which nets are fixed on the concrete surface by means of air-placed concrete or air-placed mortar, the electrolytic contact with the concrete and with the reinforcing steel also being made thereby. Noble-metal oxide modified titanium nets are very expensive, and moreover, the application of a mortar layer on the concrete surface, primarily on roadways on bridges and in parking houses and on balconies, has disadvantages on account of the changes of the dimensions of the structure (e.g. increase in roadway height) and the additional weight of the mortar layer which may be up to 30 kg/m
2
, which may considerably detract from the usability of the building. Since on account of the high costs, comparatively large-mesh Ti nets are used, high current densities may locally occur (50-100 mA/m
2
) which may lead to local discoloration up to a destruction of the mortar.
More recently, thus also conductive paints containing graphite and/or soot as electrically conductive pigment in a polymer dispersion or carbon fibers in a cementous or also cement-free binder are used as anode materials. Such electrically conductive paints utilized as anode material are, e.g., disclosed in “John P. Broomfield, Corrosion of Steel in Concrete, E&FN Spon, London (1997), p. 128”, and in EP 443229, EP 210058, GB 2140456, U.S. Pat. No. 4,632,777, U.S. Pat. No. 7,199,405A, JP 5070977. Disadvantages of these electrically conductive paints are their lack of oxidation and acid resistance, markedly lower durability than that of the titanium net/cement-mortar anode systems, insufficient durability in a humid environment, e.g. on construction elements which are exposed to rain or spray water, insufficient abrasion resistance as is required on traffic surfaces, and low conductivity so that the external current supply must occur at distances of from 0.5 to 1.0 m.
Such a commercially used system does have a sufficiently stable conductivity, yet it is not resistant to the oxidizing and acidic conditions at the borderline anode/concrete so that after a service life of 2 to 5 years the adhesion of the paint on the concrete surface clearly decreases and a destruction of the concrete surface due to acid attack can be observed. Moreover, the above-indicated paints lose their adhesion to the concrete surface as a consequence of the acid attack and thus may come off sooner or later, and the cathodic protection of the reinforcement thus will be lost. Another, commercially utilized painting system does have an improved adhesion on the concrete surface, yet its electric conductivity decreases very much in the course of operation.
Thus, it is the object of the present invention to provide a painting and coating agent with which electrically conducting coats of paint and coatings can be provided which not only are sufficiently resistant to an acid and chlorine attack but also protect the concrete surface from the attack of the anodically formed acid, which exhibit a uniform electric conductivity and a lasting adhesion to the concrete surface.
Electrically conductive paints comprised of a mixture of conductive pigments, such as, e.g., graphite, soot, nickel powder and a plastic dispersion have, e.g., been described in OE 325 180 or in EP 0230 303. However, due to their unsatisfactory durability under anodic load, these paints are not suitable as anode materials for the reasons set out above. Electrically conductive paints which contain graphite and/or soot and soluble silicates as binder as well as, optionally, synthetic resins, such as, e.g., polymethacrylic acid esters or an epoxy ester, have, e.g., been described in OE 325 180. In CH 572 966, an electrically conductive varnish has been described which preferably contains soot, an aqueous silicate solution and a non-ageing acryl resin dispersion. According to the specification, the above-indicated paints are suitable for producing heating films and/or as an electrostatic protection and/or as an electric shield. With such a use, an electric current is led through the coat of paint, there is, however, no anodic reaction, i.e. acid and gas formation as well as the formation of oxidizing products, such as oxygen and chloric gas, in particular the electric resistancee to the substrate is without any importance, which is quite in contrast to the use of electrically conductive coats of paint as anode materials for the cathodic protection of steel, in particular in concrete. Furthermore, it has been shown that electrically conductive paints which contain soluble silicates cannot be used under the conditions of the cathodic protection for the following reasons:
Both, the electric resistance of the paint itself, and the electric resistance between the paint functioning as anode and the reinforcement increase highly and, thus, the current density decreases.
After approximately 400,000 Coulomb (Ampère.seconds) have been passed through, the adhesion to the concrete substructure decreases by about 80% (55 mA/m
2
, 75 days).
In EP 499437, the admixture of an alkaline buffer comprised of alkali hydroxides, to a cement
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