Abstract:
In order to obtain a two-component epoxy resin/zinc dust priming coat which for ecological reasons requires little or no solvent, the powdery or flaky zinc, reactive in water, is dispersed in a water-free, water emulsible epoxy resin binder (component A). Component A is mixed immediately before being applied with a water-diluted or water-dissolved polyamine hardener (component B).

Description:
DESCRIPTION 
     The invention relates to an anticorrosive zinc-containing undercoating agent for steel surfaces. 
     It is known to use zinc dust and/or zinc flakes as a corrosion-inhibiting pigment in organic undercoating agents. The undercoating agents consist essentially of a synthetic resin which is dissolved in an organic solvent in order to have a soft consistency during use. Aromatic hydrocarbons, like xylol, are primarily considered as organic solvents. The undercoating agent is stored in its liquid form in closed containers and is delivered in this manner. For a specified use, it is directly applied to the previously cleaned steel surface. By evaporating the solvent, the mixture hardens and forms a solid closed layer in which the zinc dust is contained in a finely distributed form. When moist air penetrates into the undercoating, mainly basic zinc carbonate (4ZnO.CO 2 .4H 2  O) forms on the zinc flakes, which results during an increase in volume and in an increase in the density of the undercoating and protects the steel surface lying therebelow from destruction. A further anticorrosive effect is due to the cathodic protective action of the zinc dust pigment. 
     Considerable amounts of organic solvents are emitted into the surrounding air during the application of the known zinc dust undercoating agents. In order to avoid this disadvantage, it is known (EP-A-0 385 880) to use, in a two-component corrosion-inhibiting agent as the first reaction component, a powder-like or flake-like zinc, which is dispersed in an organic liquid containing a water-free, water-emulsifiable epoxy resin binding agent, and a second reaction component (component B) consisting of a water-diluted or water-dissolved polyamine hardener, which reaction components can be mixed with one another to form a viscous hardenable coating compound. The component A also contains a glycolether as a water-soluble organic solvent and silicic acid of a crystaline or amorphous consistency. The glycolether has obviously the function of improving the water emulsibility of the epoxy resin binding agent. The hardening of the coating compound will be delayed on the other hand because of the softening characteristic of the glycolether. The coating compound remains therefore sticky until complete hardening so that in a dusty environment, surface contamination must be feared during the drying process. 
     Starting out from this, the basic purpose of the invention is to improve the known undercoating agent of the above-disclosed type such that the undercoating agent dries quicker. 
     In order to achieve this, the invention suggests that the organic liquid of the first reaction component (component A) contains a synthetic resin additive which is liquidy and/or dissolved in an organic solvent, which speeds up the drying process of the coating compound by evaporation of the organic solvent or by reaction with the second reaction component or with oxygen. 
     The synthetic resin additive of the invention has mainly the purpose of speeding up the drying by evaporating the organic solvent or by reacting with the hardener or with oxygen. Thus, the synthetic resin forms a type of a structure holding the system together and results in a surface drying as long as the reaction of the epoxy resin binding agent with the polyamine has not concluded. 
     As synthetic resins can be considered 
     hydrocarbon resins dissolved in organic solvents, like ketonic resins, aldehyde resins and phenolic resins, 
     resins, which have a group reactive with the amine group, in particular an epoxy group, a carboxyl group and an isocyanate group, 
     oxidatively drying systems, in particular alkyd resins. 
     In as far as the synthetic resins contain a portion of an organic solvent, such portion is only a fraction of the portion of the solvent in known two-component under-coating agents. 
     The epoxy resin binding agent contains preferably bisphenol A-resins and/or bisphenol AF-resins. 
     The undercoating agent of the invention has according to a preferred embodiment of the invention the following composition: 
     Component A 
     1 to 20 percent by weight of water emulsifiable epoxy resin on the basis of bisphenol A and/or bisphenol AF; 
     up to 10 percent by weight of hydrophobic and/or hydrophilic synthetic resins, like hydrocarbon resin; 
     0 to 10% organic solvent, like xylol; 10 to 95 percent by weight of zinc powder or zinc flakes; 
     0 to 90 percent by weight of extenders, in particular talcum, barium sulfate and/or mica; 
     Component B 
     1 to 5 percent by weight of polyamine; 
     5 to 15 percent by weight of water; and the remainder 
     Additives like anti-settling agents, defoamers, thixotropic agents or wetting agents. 
     The water contained in the component B complete 14 or partially replaces in the undercoating agent of the invention the otherwise common organic solvent with respect to its function of reducing the viscosity during use. The polyamines, which are used according to the invention, are mainly of an aliphatic nature and must have a balance between hydrophilicity and hydrophobicity in order to guarantee a good hardening with the epoxy resin. Water emulsifiable epoxy resins and polyamines must, in addition, be adjusted stoichiometrically with one another in their proportions. 
     The good corrosion-inhibiting characteristics of the undercoating agent produced in this manner are only possible because the water-reactive zinc powder is essentially shielded against the penetration of water by the initially water-free epoxy resin binding agent during the relatively short time of application. 
    
    
     EXEMPLARY EMBODIMENT 1 
     100 parts by weight of a homogeneously dispersed mixture of 89.0 parts by weight of zinc dust and 7.5 parts by weight of a mixture of a water emulsifiable epoxy resin and water-dilutable phenol resin in a 1:1 relationship and 3.5 parts by weight of xylol as component A were mixed with 8.2 parts by weight of a polyamine, which polyamine is dissolved in water to provide a 30% weight percentage solution (component B), to form a workable viscous coating compound. 
     EXEMPLARY EMBODIMENT 2 
     100 parts by weight of a homogeneously dispersed mixture of 86 parts by weight of zinc dust and 9.5 parts by weight of a mixture of a water emulsifiable epoxy resin and a solvent-containing ketonic resin solution (60%) in a 1:1.3 relationship, and 4.5 parts by weight of a talcum extender as component A were mixed with 12.4 parts by weight of a polyamine, which is dissolved in water to provide a 20% weight percentage solution (component B), to form a workable viscous coating compound. 
     EXEMPLARY EMBODIMENT 3 
     Like the exemplary embodiment 2 with the difference that the component A contains 90.5 parts by weight of zinc dust and no extender. 
     EXEMPLARY EMBODIMENT 4 
     Like the exemplary embodiment 2 with the difference being that in place of the ketonic resin solution, an oxidatively drying short-chain alkyd resin, on the basis of vegetable fatty acids in a 3:0.5 relationship, is used. 
     The coating compound produced according to the exemplary embodiments 1 to 4 were each applied to a cleaned steel surface as the undercoating. By hardening the synthetic resin parts, it was dust dry after a few minutes. After a drying time of one week at a temperature of 23° C. and 50% humidity, it was subjected to the salt spray test according to DIN 50021-SS. The steel surface did not show any rusty-spot formation whatsoever thereafter, as it is otherwise common in the case of water-based corrosion-inhibiting coatings.