In industry, polyester and polyesterimide wire enamels have been used for many years. The known wire enamels usually contain solvent mixtures comprising phenol, the homologs thereof, such as, for example, cresols and xylenols, and aromatic hydrocarbons.
Solvents, such as, for example, cresol, are used in the preparation of the wire enamels by using them for dissolving the separately prepared resins. They can also be used as a reaction medium, in particular in the synthesis of polyesters and polyesterimides. Furthermore, they can be used as an auxiliary component in the last stage of the preparation of polyester and polyesterimide resins from dicarboxylic acids or the dimethyl esters thereof, glycols, polyhydric alcohols, such as tris(2-hydroxyethyl)isocyanurate and—in the case of the polyesterimide resins—also from trimellitic anhydride, diamines and optionally bisphenols by esterification, transesterification and, in the case of polyesterimide resins, by imidation for reducing the viscosity of the reaction mixture and for accelerating the distillation of the volatile reaction products.
Cresols and other phenolic solvents, such as xylenols and trimethylphenols, such as, for example, mesitol, are predominantly used as auxiliary components in the last stage of the polyester and polyesterimide preparation.
Mixtures of solvents and diluents are usually used in the polyester and polyesterimide enamels. While solvents are suitable and necessary for completely dissolving the wire enamels, the diluents do not have these dissolution properties.
These diluents are, however, suitable for diluting a resin solution which is already present and provides advantageous viscosities of the resin solutions. Diluents are added for various reasons; firstly, the aromatic hydrocarbons usually used are substantially more economical than the special solvents, such as cresols, phenol or xylenols. Secondly, the solvent used evaporates relatively rapidly during the continuous application of the wire enamels. To ensure uniform evaporation behavior during baking, attempts are made to achieve as harmonious a course as possible of the evaporation behavior of the steadily changing azeotrope. Abrupt changes often cause surface defects in the cured enamel film.
Typically, diluents used are mixtures of aromatic hydrocarbons, such as Solvesso and solvent naphtha.
Efforts have long been made to avoid the use of cresols and other phenols in wire enamels. This is because, on the one hand, to the odor annoyance caused by said solvent and its harmful effect on health. In particular, these solvents and their pyrolysis products are permitted to be present only in very small amounts in the waste gases of the baking furnaces, because of the environmental protection provisions. In addition, wastewaters containing harmful phenolic compounds can be formed, under certain circumstances, during the preparation of the wire enamels.
DE-A-41 33 161 discloses the use of high-boiling monoalcohols, such as, for example, benzyl alcohol or methyldiglycol, alkylene glycol acetates, alkylene glycol ether acetates, esters and ketones, as solvents. A necessary but disadvantageous requirement of the DE-A-41 33 161 disclosure is that the structure of the polyester or polyesterimide resins must be changed to achieve the suitable mechanical properties of the enamel coats and wires. Solvent and diluent mixtures which are free of the phenolic substances are furthermore known. These mixtures are multicomponent systems which contain cyclohexanone, cyclohexanol, solvent naphtha, benzyl alcohol and propylene carbonate (Wardzinska E. et al., XXII FATIPEC-Congr., Budapest, 1994, Vol. 2, page 34).
The disadvantage of these systems is that the surfaces of the enameled (coated) wires are uneven, i.e. have an orange peel structure.
There exists a need for wire enamels which contain nontoxic, environmentally friendly high-boiling solvents which have little odor, and methods to formulate these wire enamels.