Abstract:
The invention concerns a zinc-containing corrosion-protecting undercoating agent for steel surfaces. The undercoating agent, which contains no organic solvents, consists of a mixture of dry, water-emulsifiable dispersion agent in powder form and water-reactive zinc in powder or flake form which are mixed with water immediately before treatment. The corrosion-protection properties of undercoating agents prepared in this way are as good as those of prior art single-component zinc-dust undercoating agents using only organic solvents.

Description:
DESCRIPTION 
     The invention relates to a powder mixture for making 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 to produce 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 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. The thus caused environmental damage has been accepted up to now because other binding agents, for example on a water base, did not appear to be suited because of the, to be expected, chemical reaction with the zinc powder. 
     Starting out from this, the basic purpose of the invention is to provide a one-component powder mixture to make an undercoating agent which can be environmentally friendly produced, stored and applied, and which has still good corrosion-inhibiting characteristics. 
     To attain this purpose the invention suggests to first produce a mixture of a dried, water-reemulsifiable powdery dispersion agent and a water-reactive zinc in a powder or flake form and which can be stored and transported in water-tight bags. 
     The mixture contains advantageously a water-reemulsifiable dispersion agent from the following groups: 
     a vinyl acetate copolymer, in particular, a vinyl acetate-ethylene copolymer, a vinyl acetate-vinyl ester copolymer or a vinyl acetate-vinyl laurate-vinyl-versatate copolymer, an acrylate, an ethylene-vinyl laurate-vinyl chloride copolymer, a styrene copolymer, in particular a styrene acrylate or a styrene butadiene. 
     The mixture has furthermore advantageously a powdery filler or extender from the talcum, barium sulfate and mica group and/or a preferably organic pigment, which filler or extender is actually known in the lacquer industry. These materials help to improve the corrosion-inhibiting characteristics and to adjust the desired rheology. 
     In order to obtain during the application an easily homogenizable mixture out of the powdery parts with water, the use of actually known additives, like wetting agents, defoamers, thixotropic agents, agents promoting the film formation, is of an advantage. 
     The dry mixture of the invention contains advantageously the following parts: 
     1 to 30 percentage by weight of dispersion powder from the group: 
     vinylacetate-copolymer, in particular vinyl-acetate-vinylester-copolymer or vinyl acetatevinyllaurate-vinyl-versatate-copolymer acrylate 
     ethylene-vinyllaurate-vinylchloride-copolymer 
     styrene-copolymer, in particular styreneacrylate, styrenebutadiene 
     70 to 99 percentage by weight of pigments/fillers consisting of: 
     2 to 100% of zinc powder and/or zinc flakes 
     0 to 98% of fillers like talcum (3MgO·4SiO 2  ·H 2  O), barium sulfate (BASO 4 ) or mica (K 2  O·3Al 2  O 3  ·6SiO 2  ·2H 2  O, or &#34;PLASTORIT&#34; types) 
     the remainder: powdery additives, like defoamers, wetting agents, agents promoting film formation, thixotropic agents. 
     The powdery dry mixture can be stored and transported in water-proof bags. Shortly before the application, 100 parts by weight of the powdery mixture is mixed by agitation with 5 to 50 parts by weight of water. After the water preferably 20 to 30 parts by weight of water, to form a viscous coating compound has evaporated, a closed continuous coating is formed. The good corrosion-inhibiting characteristics of the coating produced in this manner permit one to conclude that the zinc powder, which is reactive with water, is extensively shielded against the reaction with water by the water-absorbing dispersion agent during the short application time. 
    
    
     Exemplary embodiment 1 
     25 parts by weight of a powdery vinylacetate-vinyl-ester-copolymer as dispersion powder were mixed with 48 parts by weight of zinc powder, 25 parts by weight of talcum and a remainder of powdery additives forming a dry powdery mixture. 
     Exemplary embodiment 2 
     10 parts by weight of styreneacrylate as dispersion powder, 64 parts by weight of zinc flakes, 25 parts by weight of barium sulfate fillers and a remainder of powdery additive were mixed forming a dry, powdery mixture. 
     100 parts by weight of the dry, powdery mixtures according to the exemplary embodiments 1 and 2 were mixed by agitation with 30 parts by weight of water and were immediately thereafter applied to a cleaned steel surface as an undercoating. 
     After one week of drying time at a temperature of 23° C. and 50% humidity, the coated surface was subjected to the salt spray test according to DIN 50021-SS. Rusty spots, as they otherwise occur with water-based undercoating agents, were not found. 
     In summary, the following can be stated: The invention relates to an anticorrosive zinc-containing undercoating agent for steel surfaces which is free of organic solvents and consists of a mixture of a dried, water-reemulsifiable powdery dispersion agent and a water-reactive zinc in powder or flake form which is mixed with water immediately before treatment. The undercoating agent produced in this manner has corrosion-inhibiting characteristics which are just as good as the known single-component zinc-dust undercoating agents exclusively dissolved in organic solvents.