This invention relates to a process for the production of insulating coatings on electrical conductors by coating the conductors with thermosetting ester resins of polyhydric alcohols, polyvalent aromatic carboxylic acids, optionally aliphatic carboxylic acids and optionally compounds containing amino groups, and heating the coated conductors at elevated temperatures above 200.degree. C. The resins also contain catalysts and leveling agents.
It is known that polyester resins can be converted into a form in which they are suitable for lacquering electrical conductors by dissolution of the resins in organic solvents of the phenol, cresol and/or xylenol type. The electrical conductors are insulated by coating them with a solution of the aforementioned polyester resins, followed by heating at oven temperatures of around 350.degree. C. or higher to cure the polyester resins. Conventional lacquer solutions generally contain additives and/or hardeners of the kind normally employed for lacquers. Particularly preferred lacquer solutions are based on polyester resins containing five-membered imide rings in co-condensed form, for example, those resins disclosed in the following publications and patents: DAS No. 1,033,291; British Patent Nos. 937,377; 1,082,181; 1,067,541; 1,067,542 and 1,127,214; Belgian Patent No. 663,429; French Patent No. 1,391,834; East German Patent No. 30,838; DOS Nos. 1,494,454; 1,494,413; 1,937,310; 1,937,311; 1,966,084; 2,101,990 and 2,137,884. The disclosures of each citation is hereby incorporated by reference.
Conventional lacquer solutions have a relatively high organic solvent content. The stoving residue of the lacquer solutions generally amounts to less than 50% by weight. The reason for this is, inter alia, that the polyester resins dissolved in the solvents have relatively high molecular weights and structural arrangements in the molecule giving rise to high melting points and viscosities in solutions. The high solvent contents referred to above have to be used in order to obtain solutions of suitable viscosity for lacquering purposes. These solvents are evaporated off when the thin lacquer film surrounding the wire is subsequently heated at elevated temperatures of around 220.degree. C. or higher, and hence are lost as film formers.
Accordingly, there is a considerable need for a process for producing insulating coatings on electrical conductors in which the use of the aforementioned solvents, at least in the large quantities indicated, can be avoided. This would afford considerable advantages by contributing to protection of the environment, reducing atmospheric pollution and the dangers to health in the coating plants and lacquer factories, increasing lacquer spread or yield, decreasing storage and transportation volume and reducing the risk of ignition.
Attempts have been made to meet this need by a process for insulating electrical conductors with heat-stable resins curable through free hydroxyl groups, especially non-linear polyester resins which can even be modified by amide or imide groups, by using in the melt, at a working temperature (lacquer bath temperature) of at least 100.degree. C., resins which have been condensed at this temperature to such an extent that they do not undergo any appreciable further condensation in the melt, and which have crosslink equivalent weights of from 400 to 1600 (see DOS No. 2,135,157). However, it is in fact not possible in this process to use solvent-free resins at practicable lacquer bath temperatures. According to the examples of the aforementioned German Offenlegungsschrift, it is necessary to use from 10 to 15% of foreign, physiologically unacceptable solvents, such as xylenols or cresols. Despite these solvent contents, the process has to be carried out at melt temperatures of 170.degree. or 160.degree. C., as shown by the examples. The reason for this is that it is necessary in this process to use melts of products with as high a molecular weight as possible which, accordingly, melt only at elevated temperatures. In this connection, it is specifically pointed out in DOS No. 2,135,157, page 6, paragraph 1, second sentence, that the use of low molecular weight products has adverse effects.