CH448346A - Process for the production of stoving enamels suitable for heat-resistant coatings - Google Patents

Description

  

  
 



  Process for the production of stoving enamels suitable for heat-resistant coatings
It is known to produce stoving enamels in such a way that terephthalic acid and / or isophthalic acid or functional derivatives of these acids are reacted with mixtures of glycols and higher alcohols, in particular glycerol, to form essentially linear or slightly branched polyesters which have free hydroxyl groups. These products can be converted into the insoluble, infusible state by known crosslinking reactions, such as, for example, with diisocyanates, or esters of metal acids, such as, for example, alkyl titanates. Such products are used in particular as binders for wire insulating varnishes.

   For these purposes, however, the heat shock resistance of the coatings produced in the manner described has not always proven to be sufficient.



   It is also known that high-temperature-resistant coatings, especially for electrical conductors, can be produced by mixing dianhydrides of aromatic tetracarboxylic acids, especially pyromellitic dianhydride, with aromatic, optionally polynuclear diamines, especially 4,4'-diaminodiphenyl ether, in the presence of a strongly polar solvent which does not contain any contains the anhydride groups reacting hydrogen atoms, converts at low temperature.



  This process gives solutions of linear polyamides which have free carboxyl groups adjacent to the carbonamide groups. After the solvent has volatilized, these polyamides containing carboxyl groups pass by heating with elimination of water into essentially linear polyimides which result in insoluble, only fusible with decomposition transfer of excellent mechanical and electrical properties. The disadvantage of the uncured products, however, is the fact that they only have a limited shelf life even at low temperatures and become unusable after an undesirably short time due to gel formation.

   The obvious idea of blocking the free carboxyl groups of the dissolved polyamides by salt formation with organic bases for the undesired gel formation reaction did not lead to any significant improvement in the storage stability.



   It has now been found that for heat-resistant, heat-shock-resistant coatings, in particular for electrical conductors, suitable stoving enamels are obtained if, according to the characterization of the invention, mixtures of dialcohols and! or higher-valent alcohols with imide-containing dicarboxylic acids of the general formula
EMI1.1
   m aer -Ar 'aromatic radicals and
N-Ar "-N radicals of diamines whose amino groups are on one or different phenylene groups and whose optionally several phenylene groups are fused, linked to one another by CC bonds, alkylene or cycloalkylene groups or groups containing heteroatoms or heteroatoms, mean, polycondensed and in Solvents.



   The stoving enamels produced according to the invention can be used as coating agents in such a way that, after the addition of crosslinking substances, they are cured by heating.



   As dialcohols, for example, polymethylene glycols; cycloaliphatic diols, e.g. B. dimethylolcyclohexanes, quinites; aromatic-aliphatic diols, e.g. B.



  Xylylene glycols or oxyethylation products of dihydric phenols, such as di-oxyethoxy-diphenylpropane or di-oxyethoxybenzenes, are used, while glycerol, pentaerythritol, trimethylolpropane, hexanediol, sorbitol and the like, for example, are possible as higher-valent alcohols.



   As a solvent for the polycondensates described, for. B. cresols, xylenols, diacetone alcohol, methyl glycol acetate, may be used in a mixture with aromatic hydrocarbons.



   Known crosslinking substances, such as alkyl or aryl titanates, oil-soluble salts of Al, Cr, Co, Cu, Fe, Mn, Ni, Sn, Ti, V, Zn, Cd, Pb, Ca, polyisocyanates and so-called blocked polyisocyanates can be used.



   The dicarboxylic acids containing imide groups can be obtained by combining the anhydrides of aromatic tricarboxylic acids, preferably trimellitic anhydride, with aromatic diamines of the formula
H2N-AR "NHi where = N - Ar" - N = has the meaning given above, in a molar ratio of 2: 1 in the cold, possibly in the presence of an inert solvent, and the reaction product is converted using the solvent as an entrainer for the water of reaction to be split off heated until the imide has formed.



   However, it has proven to be feasible to dispense with the separate preparation of these imide-containing dicarboxylic acids and the reaction mixture, consisting of the dihydric and higher alcohols, the aromatic dicarboxylic acids or their functional derivatives, the tricarboxylic anhydride and the aromatic diamine with stirring in a nitrogen - to slowly heat the atmosphere to the temperature required to carry out the polycondensation.

   This simpler type of reaction is made possible by the fact that the easiest and therefore preferred reaction substep in the reaction mixture theoretically capable of multiple reactions is the addition of an amino group to the anhydride group of the tricarboxylic acid anhydride with the formation of a carbonamide and a free carboxyl group ortho to it, and that these two groups easily convert to an imide group at higher temperatures, apparently even if the originally free carboxyl group adjacent to the carbonamide group is already esterified.

   This course of the reaction is suggested by the fact that a batch consisting of 0.1 mol of dimethyl terephthalate, 0.1 mol of trimellitic anhydride, 0.05 mol of 4,4'-diaminodiphenylmethane and 0.25 mol of ethylene glycol in the presence of catalytic amounts of antimony trioxide and cadmium acetate is possible to a soluble, fusible product with a viscosity number of 0.5 (determined in phenol / tetrachloroethane [60:40 wt. 01o] at 250 C; 1 g of substance in 100 mol of solution) to polycondense. Assuming any other reaction mechanism, such a largely polycondensed product would already have to be insoluble and infusible due to three-dimensional crosslinking.



   Thermal shock resistance is the resistance of the insulating layer of an electrical conductor to sudden increases in temperature. In the examples given, this property was determined in the following way.



   Wire coils according to DIN 46 453 are produced from enamelled wire by winding it around mandrels with various diameter knives. This creates wire films with different external fiber expansions. The wire coils are then heated to 180 ° C for 15 minutes and checked for any cracks in the lacquer film. The best heat shock resistance is provided by that enamelled wire which can withstand the thermal stress without cracking with the greatest external fiber expansion.



   example 1
297 g of dimethyl terephthalate (1.53 mol), 148 g of a reaction product, obtained by boiling 2 mol of trimellitic anhydride and 1 mol of 4,4'-diaminodiphenylmethane in xylene for 30 hours with azeotropic removal of the water of reaction and characterized by an acid number of 320 instead of for the corresponding dicarboxy-diimide theoretically calculated acid number of 206, corresponding to 0.26 mol of reaction product, 92 g of glycerol (1 mol) and 83 g of ethylene glycol (1.34 mol) were in the presence of 0.8 g of lead octoate in a nitrogen The atmosphere is heated to an internal temperature of approx. 195 to 2300 C for approx. 8 hours with stirring, so that the vapor temperature remains constant at approx.

     1000 C was held.



  In the course of the reaction, 130 ml of a distillate were collected, which consisted of the methanol formed during the transesterification of the dimethyl terphthalate, water and a little glycol. The reaction was ended as soon as the viscosity number of the resin, determined by means of a phenol-tetrachloroethane mixture (60:40 weight 0 / o) at 250 ° C. containing 1 g of substance dissolved in 100 ml solution, had reached a value of 0.089.



  A brittle, red-brown resin which was hard, brittle at room temperature and had a trimellitic acid / diamine reaction product content of 15 mol / o of the total dicarboxylic acid content was obtained (resin A).



   Example 2
The proportions of the reactants corresponded to those of Example 1 with the exception that 100 g of trimellitic anhydride (0.52 mol) and 51.5 g of 4,4'-diaminodiphenylmethane (0.26 mol) were used instead of the trimellitic acid hydride / diamine reaction product. The batch was brought to 1950 C over the course of about 2 hours while stirring. The procedure was as indicated in Example 1 (resin B).



   Example 3
The proportions of the reactants corresponded to those of Example 2 with the exception that 64.5 g of 4,4'-diaminodiphenyl sulfone (0.26 mol) were used instead of the 4,4'-diaminodiphenylmethane. The manufacturing process was the same as that of Example 2 (Resin C).



   Resins A, B and C were dissolved in the ratio: 35 parts of resin 1, 45 parts of cresol / 19 parts of solvent, 1 part of polymeric butyl titanate and baked onto 0.6 mm copper wire as follows: Driving data: Oven length: 2.00 m oven temperature : 3900 C pulls: 8 take-off speed:

   4.2 m / minute
The most important test results are summarized in the following table: Resin A Resin B Resin C Abrasion resistance according to NEMA NW 55-1955, Section 5.2.3 45 43 50 [double strokes] Heat shock resistance at 180 "C 15 minutes 50 / o 50 / o 50 / o C Outer fiber expansion l Thermal compressive strength according to DIN 46453 / 12.1. 295 "C 2900C 320" C Max.

   External fiber elongation after 500 hours of aging at 180 "C 13 / o 25 / o 40 / o
The surface hardness (pencil hardness) of all resins was 4 H, after 30 minutes of storage in ethanol, benzene, trichlorethylene, water and butyl acetate at 500 C it was between 3 H and 4 H, after storage in acetone it was B.



   From these values it can be seen that the resins produced according to the invention, after stoving, are distinguished by excellent hardness and solvent resistance compared to other wire insulating enamels of thermal class F and also have improved heat shock resistance; In the case of the conventional terephthalate resin wire insulating varnishes used for this thermal class, this is 3040% external expansion.



   The electrical values, such as the loss factor at different frequencies and temperatures and the insulation resistance under different conditions, roughly corresponded to those of comparable terephthalate wire insulating varnishes.

 

Claims (1)

PATENT CLAIM 1 Process for the production of stoving enamels suitable for heat-resistant coatings, characterized in that mixtures of dialcohols and / or higher alcohols with imide-containing dicarboxylic acids of the general formula EMI3.1 in the -Ar 'aromatic radicals and N-Ar "-N radicals of diamines whose amino groups are on one or different phenylene groups and whose optionally several phenylene groups are fused together, through CC bonds, alkylene or cycloalkylene groups or through groups containing heteroatoms or heteroatoms are connected, mean, polycondensed and dissolves in solvents. SUBClaim 1. The method according to claim I, characterized in that the polycondensation is carried out in a mixture with aromatic dicarboxylic acids or functional derivatives thereof. PATENT CLAIM II Use of the stoving enamels produced according to patent claim I as coating agents, characterized in that, after the addition of crosslinking substances, they are cured by heating. SUBClaim 2. Use of the stoving enamels according to patent claim II, characterized in that alkyl esters of metal acids are used as crosslinking agents.