Coating processes – Nonuniform coating – Applying superposed diverse coatings or coating a coated base
Reexamination Certificate
2000-04-24
2001-05-01
Bareford, Katherine A. (Department: 1762)
Coating processes
Nonuniform coating
Applying superposed diverse coatings or coating a coated base
C427S287000, C427S406000, C427S409000, C427S410000, C427S407100, C427S455000
Reexamination Certificate
active
06224943
ABSTRACT:
The present invention relates to a method for improving the corrosion resistance of reinforced concrete coated with a thermal spray coat of metals, especially of zinc or zinc alloys.
Thermal spray coats of zinc or zinc aluminum alloys are employed for the surface refinement of metals, plastics, concrete, paperboard etc. Improvements include the temperature resistance, wear performance and electric conductivity of the substrate materials.
From EP-A-0 677 592, a method is known for improving the adherence of thermal spray coats of metals, metal oxides or mechanically resistant materials, especially of zinc, aluminum and their alloys, in which after spraying, the spray coats are coated with a one-component moisture-curing polyurethane resin. This method has gained particular importance for workpieces of steel. It is mentioned that usual coating systems compatible with polyurethane resins can be applied to the thus applied polyurethane resin coat. No examples thereof are mentioned. However, there had already been observed that materials such as alkyd resins, epoxy resins or PVC resins will not sufficiently adhere to the metal spray coats without the polyurethane resin coat.
It has been the object of the present invention to provide improvement of the corrosion resistance of reinforced concrete coated with a thermal spray coat of metals, especially of zinc or zinc alloys, and the adherence of the spray coat to the concrete is also to be improved, if possible.
According to the invention, this object is achieved by electrically connecting the spray coat with the armour of the reinforced steel and additionally coating it with a polyurethane resin which is applied as a low-viscosity solution in organic solvents.
Preferably, the polyurethane resin coat is applied at so low a thickness that a continuous film does not form, but only the pores of the spray coat are closed.
Particularly good results are achieved if an epoxy resin coat is additionally applied after the curing of the polyurethane resin. Said epoxy resin coat is preferably applied to have a thickness of from 200 to 400 &mgr;m after curing. Further useful are polyurethane coats and coats of mixed epoxy resins and polyurethanes.
The spray coat of zinc or zinc alloys on the concrete is generally from 100 to 400 &mgr;m, preferably from 150 to 300 &mgr;m. When the adherence is measured by front end peeling, such spray coats exhibit values of between 1.0 and 2.0 MPa. After the polyurethane resin coat has been applied, the adherence of the zinc coat to the concrete surprisingly rises to from 2.5 to 3.0 MPa. If an epoxy resin coat is applied after the curing of the polyurethane coat, adherence values of between 2.5 and 3.5 MPa are measured after its having cured.
It is essential to the success of the method that the coat applied according to the invention be electrically connected with the armour of the reinforced concrete. To this end, it is necessary to establish an electrically conducting connection between the metal armour of the reinforced steel and the surface of the concrete. This is a measure which has been taken with hesitation to date, because parts of the armour which are not covered by the concrete will come into contact with the environment and are actually considered as defects in which corrosion of the reinforced concrete will occur particularly quickly. According to the invention, it is further possible to use the coats as anodes for active cathodic protection using external current.
Another unexpected advantage of the method according to the invention is the fact that the coating with polyurethane resin improves not only the adherence of the spray coat to the concrete, but also the durability of the spray coat. The intrinsic corrosion of the zinc coat under humid atmospheric conditions is greatly reduced, and thus the durability of the spray coat is increased. Corrosion experiments in a salt spray test according to DIN 50121-SS have shown that as much as 60% of a layer of 100 &mgr;m thickness is eroded in 336 hours. After the polyurethane resin layer has been applied, the erosion of the zinc spray coat is only 13%. If an epoxy resin layer is additionally applied, the intrinsic corrosion of the spray coat will be reduced to virtually 0.
In the method according to the invention, before the metal spray coat is applied, care must be taken that the reinforced concrete is first cleaned, blasted, preheated to 70° C. to 90° C., and only then the metal is applied by spray coating. The cleanness and the roughness of the substrate surface are of particular importance. A profile which is sharp-edged to some extent is often even necessary to ensure the necessary adherence. Preheating can be dispensed with only if it is ensured that the concrete surface is no longer moist. Otherwise, the zinc spray coat will not have sufficient adherence.
For the metallic spray materials, various spraying methods can be employed, for example, wire flame spraying or wire arc spraying. These methods are distinguished primarily by different process temperatures and thus also by different application efficiencies. The adherences to concrete depend not only on surface pretreatment, but also on the type of concrete to be protected. The spray coats are more or less dense depending on their thickness and method of spraying. To ensure sufficient corrosion protection, the thickness should preferably be within a range of from 150 to 300 &mgr;m.
Attempts to apply an epoxy coat immediately to the spray coat have had completely unsatisfactory results, whereas surprisingly good results are obtained if a polyurethane coat is first applied according to the invention.
A definite explanation of these results does not yet exist, but there is some support to the theory that the urethane groups are capable of reacting with hydroxy groups during the curing process wherein not only residual moisture is bound, but strong bonds between the sprayed-on metal and the polyurethane resin are also formed. It is also astonishing that particularly good results are obtained if coats are applied at just so low a thickness that the pores of the spray metal are just filled, but without a continuous film being formed. Such thin coats can be applied, for example, by brushing, rolling or spraying, but a measurable build-up of layers should not take place. Nevertheless, this thin coat already causes a great reduction of intrinsic corrosion due to humid atmospheric conditions while at the same time the adherence of the metal coat to the concrete is increased.
After this urethane paint has cured, a further improvement can be achieved, in particular, by applying a top coat of epoxy resin; excellent results have been obtained, for example, using the Amerlock 400 GFR paint from Ameron, USA. This additional epoxy resin layer is used, in particular, if the surfaces are under high mechanical stress. Layers of polyurethane or mixtures of epoxy resins and polyurethanes are also highly suitable, however.
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Knepper Michael
Spriestersbach Jochen
Wisniewski Juergen
Bareford Katherine A.
Grillo-Werke AG
Jacobson Price Holman & Stern PLLC
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