1. Field of the Invention
The present invention relates to a specific solvent composition, to a wire coating composition comprising polyvinyl formal as binder and the solvent composition, and to preparation processes and use.
2. Discussion of Background Information
The wire coating compositions typically used today are generally solutions of the standard binders, such as polyvinyl formals, polyesters, polyesterimides and polyurethane resins, for example, in cresolic solvents, if desired in combination with commercial hydrocarbon diluents.
Wire coating compositions based on polyvinyl formals have been known for a long time. The binder is composed of polyvinyl alcohol which has been reacted with aldehydes to form polyvinyl formal. The polymer retains residual ester groups from the hydrolysis of the polyvinyl acetate to the polyvinyl alcohol, and also free OH groups which have not reacted with the aldehydes. Via these OH groups, with the aid of other resins, crosslinking takes place in order to produce a thermoset film on the copper wire.
GB 578 882 describes a wire enamel composed of a polyvinyl formal resin in combination with a melamine resin.
The combination of polyvinyl formal resin with phenolic resin is described in GB 810 359 and GB 862 165.
The crosslinking of the polyvinyl formal resin with blocked isocyanates is subject matter of GB 938 119.
The combination of all of the listed crosslinkers is described in GB 975 455 and GB 1 112 186.
Common to all the polyvinyl formal wire enamels is the low solids content of the wire enamels, which is in the range of 20% to 30%. The solvent composition is frequently cresol with solvent naphtha, usually in a 1:1 ratio. The cresolic solvents are typically a technical mixture of various cresol and xylenol isomers, which may also include phenol. Diluents used include xylene, solvent naphtha, toluene, ethylbenzene, cumene, heavy benzene, various Solvesso and Shellsol grades, and also Deasol. At solids contents of 20-22%, these wire enamels frequently have viscosities in the range of 3000-6000 mPas. Increasing the solids content would cause a sharp rise in viscosity and mean that the enamels could not longer be applied by coating. Cresol, moreover, is toxic and hence disadvantageous on environmental and operational grounds.
The disadvantage of the known wire coating compositions based on polyvinyl formal resins is, additionally, that they have low solids contents and high viscosities, and also contain large quantities of cresol as solvent.
Cresol-free wire enamels are in fact known, and are described in DE 41 33 161 and U.S. Pat. No. 7,521,498, for example. But not for wire enamels based on polyvinyl formal. The literature frequently describes, as alternative solvents, dimethyl glycol, ethyl glycol, isopropyl glycol, butyl glycol, methyl diglycol, ethyl diglycol and butyl diglycol. Also contemplated are glycol ether esters, examples being methyl glycol acetate, ethyl glycol acetate, butyl glycol acetate and methoxypropyl acetate. Further examples are cyclic carbonates, such as polypropylene carbonate, cyclic esters, gamma-butyrolactone, and dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone. Use may additionally be made of aromatic solvents, benzyl alcohol, if desired in combination with the aforementioned solvents.
DE 27 40 215 A1 discloses wire enamels which are based on polyvinyl formal resin and which necessarily comprise particular glycol ethers as a solvent component.
The prior art to date has not disclosed optimal solvents for wire enamel formulations based on polyvinyl formal.
Problem:
The problem addressed by the present invention is that of developing a wire enamel based on polyvinyl formal that does not have the disadvantages depicted and that can be formulated without cresol without any consequent adverse effect on the performance properties.
Another problem addressed by the present invention was that of providing innovative solvent compositions which have particularly good properties, being more particularly of very low toxicity and having an outstanding capacity to dissolve coating materials, more particularly wire enamels based on polyvinyl formal.
Solution:
This problem is solved by a particular solvent composition, and also by wire enamel formulations based on polyvinyl formal that comprise a particular solvent composition, the solvent composition comprising ethanol.