Abstract:
Superior wire enamels comprise polyester imides prepared in the presence of a monobutyl ether of a mixed diglycol having a free secondary hydroxyl group and dissolved in a solvent comprising predominantly a polar oxygenated compound selected from a monoether or monoester of an alkylene, or polyalkylene, glycol. Electrical conductors coated with such enamels are also disclosed.

Description:
This invention relates to polyester imide wire enamels and to electrical conductors coated therewith. More particularly, it is concerned with ester imides prepared in the presence of a monobutyl ether of a mixed diglycol having a secondary hydroxyl group and dissolved in a non-phenolic solvent which is predominantly polar and oxygenated. 
     BACKGROUND OF THE INVENTION 
     British Pat. No. 973,377 discloses a family of ester imide resins made by reacting together at least one polybasic acid or a functional derivative thereof, and at least one polyhydric alcohol or functional derivative thereof, at least one of the reactants having at least one five-membered imide ring between the functional groups of the molecule. It further discloses that the reactants can be heated in a commercial cresol mixture, then further diluted in a mixture of naphtha and cresol and used as an enamel for coating copper wire to produce a hard, thermally resistant insulation therefor. U.S. Pat. No. 3,426,098 describes ester imide resins in which all or part of the polyhydric alcohol comprises tris(2-hydroxyethyl) isocyanurate. U.S. Pat. No. 3,382,203 describes polyester wire coating enamels based on mixed esters containing aromatic polycarboxylic acid radicals and radicals of di- and/or polyhydric alcohols, and specifically contemplates the inclusion of imide containing units. A preference is stated for cresol as a solvent. U.S. Pat. No. 3,274,159 describes polyester polyimides of trimellitic anhydride in which a glycol reactant is stated to be present in excess of stoichiometric proportions when no solvent is employed in the polymerization reaction. Such polyester polyimides are stated to be useful as outdoor wire insulation material. U.S. Pat. No. 3,141,859 acknowledges that certain esters and ethers of ethylene glycol and polyethylene glycol can be used in the preparation of alkyd resins to make wire enamels, but it is not disclosed if such compounds are mono- or di- ethers and esters, and, in any event, the statement is made that &#34;completely satisfactory wire enamels have not been obtained by such a process.&#34; The disclosures of the foregoing patents are incorporated herein by reference. 
     Keating, U.S. Pat. Nos. 4,119,605, 4,119,608 and 4,119,758, incorporated herein by reference, discloses polyester-imide resins which are soluble in non-phenolic solvents by incorporating a monoether of a diethylene glycol or a triethylene glycol into the resin. It is stated therein that the use of the monoether of a glycol as a reactant which serves as a monofunctional alcohol serves to control the molecular weight and solubility of the polymer and permits the manufacture of wire enamels which contain relatively non-toxic solvents in place of phenols and also permits the attainment of enamels having economical high solids contents. The reactive glycol dealt with by the above-mentioned Keating patents is an ether alcohol which either (a) has the formula RO(C n  H 2n  O) x  H where R is an alkyl group of 1 to 6 carbon atoms or phenyl, n is 2 or 3 and x is 2 or 3 or (b) is the monophenyl ether or ethylene glycol. It is seen that mixed glycols, i.e., those wherein x is 2 and 3 are specifically not contemplated by Keating. 
     Enamels prepared with such reactive monohydric alcohols are not entirely satisfactory in comparison with conventional polyester imides because, while reacting the ether alcohol into the resin lowers its molecular weight and improves solubility, it also tends to reduce ultimate physical properties precisely because the molecular weight has been lowered. 
     It has now been discovered that if polyester imide resins are prepared in the presence of a monobutyl ether of a partially linear, partially branched aliphatic diglycol having 2 carbon atoms in one portion and 3 carbon atoms in the other portion of the aliphatic chain and in which the monohydric alcohol group is a secondary alcohol group, e.g., 1-butoxyethoxy-2-propanol, then there is little tendency for the glycol ether to react into the resin, and the molecular weight is higher than it would be if a glycol ether of the type described in the Keating patents is employed. The result is that a superior wire enamel can be made by dissolving such higher molecular weight resins in a solvent comprising predominantly the same, or different, polar organic monoethers or monoesters of alkylene glycols or polyalkylene glycols. The present enamels are clear solutions, even without chemically binding in the glycol ether. They thus eliminate the need for cresylic acid solvent and therefore provide excellent results without any toxic effluents. 
     DESCRIPTION OF THE INVENTION 
     According to the present invention, there are provided electrical wire enamels comprising: 
     (a) a polyester imide obtained by heating ingredients consisting essentially of 
     (i) an aromatic diamine; 
     (ii) an aromatic carboxylic anhydride containing at least one additional carboxylic group; 
     (iii) terephthalic acid or a reactive derivative thereof; 
     (iv) a polyhydric alcohol having at least three hydroxyl groups; 
     (v) an alkylene glycol; and 
     (vi) 1-butoxyethoxy-2-propanol until reaction therebetween is substantially complete; and 
     (b) a solvent therefor comprising predominantly 
     (i) an alkylene glycol monoether or monoester. 
     (ii) a polyalkylene glycol monoether or monoester or 
     (iii) a mixture of (i) and (ii). 
     Among the preferred features of the present invention are electrical wire enamels as defined above which also include an alkyl titanate. 
     Also contemplated by the present invention are electrical conductors provided with a continuous coating of the new wire enamels, and cured at elevated temperatures. 
     With respect to components a(i)-a(v) inclusive these are conventional and well known to those skilled in this art by reason of the teachings, for example, in the above-mentioned U.K. Patent Specification 973,377, and U.S. Pat. No. 3,426,098. 
     By way of illustration, aromatic diamine component (a)(i) can comprise benzidine, methylene dianiline, oxydianiline, diaminodiphenyl ketone, -sulfone, -sulfoxide, phenylene diamine, tolylene diamine, xylene diamine, and the like. Preferably, component (a)(i) will comprise oxydianiline or methylenedianiline and especially preferably, methylenedianiline. 
     Illustratively, the aromatic carboxylic anhydride containing at least one additional carboxylic group component (a)(ii) can comprise pyromellitic anhydride, trimellitic anhydride, naphthalene tetracarboxylic dianhydride, benzophenone-2,3,2&#39;,3&#39;-tetracarboxylic dianhydride, and the like. The preferred components (a)(ii) are pyromellitic anhydride or trimellitic anhydride and especially trimellitic anhydride. 
     Typically, terephthalic acid or a di(lower)alkyl ester (C 1  -C 6 ) or other reactive derivative, e.g., amide, acyl halide, etc. will be used as component (a)(iii). A minor amount of the terephthalic acid can be replaced with another dicarboxylic acid or derivative, e.g., isophthalic acid, benzophenone dicarboxylic acid, adipic acid, etc. Preferably component (a)(iii) will comprise dimethyl terephthalate or terephthalic acid, and especially preferably, terephthalic acid. 
     As optional polyester forming ingredients (a)(iv) there may be employed a polyhydric alcohol having at least three hydroxyl groups, there can be used glycerine, pentaerythritol, 1,1,1-trimethylolpropane, sorbitol, mannitol, dipentaerythritol, tris(2-hydroxyethyl)isocyanurate (THEIC), and the like. Preferably as component (a)(iv) there will be used glycerine or tris(2-hydroxyethyl)isocyanurate, preferably the latter. 
     Illustratively, the alkylene glycol component (a)(v) will comprise ethylene glycol, 1,4-butanediol, trimethylene glycol, propylene glycol, 1,5-pentanediol, 1,4-cyclohexane dimethanol and the like. Preferably the alkylene glycol will be ethylene glycol. 
     The 1-butoxyethoxy-2-propanol component (vi) can be made in known ways, and it is also available commercially. It has a molecular weight of 176.26, and a freeze point of about -90° C. Its boiling point at 760 mm Hg. is 230° C. It is sold by Union Carbide Co., New York, New York, U.S.A. 
     The polar oxygenated solvent (b) having at least one active hydroxyl group will be selected from (i) an alkylene glycol monoether or monoester, such as ethylene glycol monomethyl ether, or monoester, such as ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol monohexyl ether, ethylene glycol monoacetate, ethylene glycol monohexanoate, propylene glycol monomethyl ether, propylene glycol monoacetate, 1,4-butylene glycol mono-n-butyl ether, 1,6-hexylene glycol monomethyl ether, and the like, preferably ethylene glycol monomethyl ether and ethylene glycol monophenyl ether, and for (ii) a polyalkylene glycol monoether or monoester, such as diethylene glycol monoethyl ether, diethylene glycol mono-n-butyl ether, diethylene glycol monoacetate, diethylene glycol monophenyl ether, triethylene glycol monomethyl ether, triethylene glycol monoacetate, etc. Preferably, there will be used diethylene glycol monomethyl ether. 
     Suitable alkyl titanates include tetraisopropyl titanate, tetramethyl titanate, tetrabutyl titanate, tetrahexyltitanate, bis(acetoacetonyl)diisopropyl titanate and the like. Tetraisopropyl titanate is preferred. 
     In making the polyester imide there should normally be an excess of alcohol groups over carboxyl groups in accordance with conventional practice. The preferred ratios of ingredients, and of ester groups to imide groups, are entirely conventional, see the patents cited above, and the especially preferred ratios of ingredients will be exemplified in detail hereinafter. The manner of making the polyester imide is likewise conventional. Either the polyester or the polyimide components can be made first, and subsequently reacted at elevated temperatures, or all of the ingredients can be added at the beginning and reacted. Preferably, the ingredients are all added to a suitable reactor and heated to 200° to 250° C. until no more water distills off. This indicates substantial completion of the reaction. This may take several hours. Then the mixture is cooled somewhat, e.g., to 150° to 190° C. and then glycol ether or ester is added as solvent, preferably with a minor proportion of a hydrocarbon diluent, e.g., an aromatic naphtha, such as Solvesso 100 or toluene, dodecane, and the like. 
     The wire enamels thus made are applied to an electrical conductor, e.g., copper, aluminum, silver or stainless steel wire, in conventional fashion. Illustratively, wire speeds of 15 to 65 feet/min. can be used with wire tower temperatures of 250° to 900° F. The build up of coating on the wire can be increased by repetitive passes through the enamel. The coatings produced from the present enamels have excellent smoothness, flex resistance, continuity, solvent resistance, heat aging, dissipation factors, cut through resistance, heat shock, abrasion resistance and dielectric strength. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples illustrate the present invention. They are not intended to limit the scope of the claims in any manner whatsoever. 
     EXAMPLE 1 
     A wire enamel is made by charging a suitably sized flask with the following reactants: 
     
         ______________________________________               Gms.______________________________________Tris(2-hydroxyethyl)isocyanurate                 820.7Trimellitic anhydride 574.0Methylenedianiline    298.1Ethylene glycol       214.2Terephthalic acid     582.5Tetraisopropyltitanate                  4.61-Butoxyethoxy-2-propanol                 442.2______________________________________ 
    
     The contents are heated slowly with the evolution of water to a maximum temperature of 240° C. until no more water is given off. The contents are allowed to cool to 200° C. and poured into a metal tray to solidify. 
     Resin, 825 g., is dissolved in 531 g. of the monomethyl ether of diethylene glycol and 164 g. of hydrocarbon solvent (Solvesso-100). The clear amber solution has a viscosity of 1290 centistokes (cs.) at 25° C. and a solids content of 47.1%. To this solution is added 18.22 grams tetraisopropyltitanate in 20 grams of the monomethylether of diethylene glycol and 5 grams of Solvesso-100. 
     When applied to a 15 foot vertical tower at 900° F. on 18 AWG copper in passes, coated wire having the following properties are obtained at 40&#39;/min. 
     
         ______________________________________Flex - 25% +            2XDisc. Factor            4.2Cut Thru, °C.    370Diel. Strength, KV      7.8______________________________________ 
    
     For comparison purposes a wire enamel of this type is made substituting the methyl ether of tripropylene glycol (as used in Keating, U.S. Pat. No. 4,119,608, Example 15). 
     COMPARATIVE EXAMPLE 1A 
     The following materials are reacted in a suitably-sized flask: 
     
         ______________________________________               GMS    Moles______________________________________Tris(2-hydroxyethyl)isocyanurate                 1066.9   4.084Trimellitic anhydride 746.2    3.884Methylenedianiline    387.5    1.954Ethylene glycol       278.5    4.487Terephthalic acid     757.3    4.558Tetraisopropyltitanate                 6.02     --Tripropylene glycol monomethyl ether                 574.9______________________________________ 
    
     The contents are heated to a maximum of 240° C. until the theoretical amount of water is collected. The contents are allowed to cool to 200° C. then poured into a metal tray to solidify. 800 grams is dissolved in 627 grams of the monomethyl ether of diethylene glycol and 209 grams of hydrocarbon solvent (Solvesso 100). To this is added 16.6 grams of tetraisopropyltitanate in 20 grams in the monomethyl ether of diethylene glycol and 7 grams of Solvesso 100. The clear amber solution has a viscosity of 1025 cs. at 25° C. and a solids content of 41.77%. 
     When applied on a 15 foot vertical tower at 900° F. on 18 AWG copper in passes, a coated wire having the following properties are obtained at 50&#39;/min.: 
     
         ______________________________________Flex - 25% +           1XDisc. Factor 220° C.                  8.4Cut Thru °C.    326° C.Diel. Strength, KV     12Repeat Scrape          75______________________________________ 
    
     EXAMPLE 2 
     A suitably sized flask is charged with the following: 
     
         ______________________________________               GMS______________________________________Tris-(2-hydroxyethyl)isocyanurate                 718.4Trimellitic anhydride 697.4Methylenedianiline    392.6Ethylene glycol       179.2Terephthalic acid     438.6Tetraisopropyltitanate                  4.51-Butoxyethoxy-2-propanol                 439.0______________________________________ 
    
     The contents are heated slowly with the evolution of water to a maximum temperature of 225° C. until no more water is given off. The contents are allowed to cool to 200° C. and poured into a metal tray to solidify. 
     Resin, 625 g., is dissolved in 508.4 g. of the monomethyl ether of diethylene glycol and 150.5 g. of Solvesso-100. 
     To this is added 2.61 grams of tetraisopropyltitanate and 6 grams of the monomethyl ether of diethylene glycol and 4 grams of Solvesso-100. The clear amber solution has a viscosity of 1250 cps. at 25° C. and a solids content of 41.2%. 
     When applied in a 15 foot vertical tower at 900° F. on 18 AWG copper in 7 passes, a coated wire having the following properties are obtained at 40&#39;/min.: 
     
         ______________________________________Flex - 25%              1XDisc. Factor            6.5Cut Thru, °C.    374Diel. Strength, KV      9.0______________________________________ 
    
     For comparison purposes a wire enamel of this type is made substituting a reactive monobutyl ether of diethylene glycol for the substantially non-reactive 1-butyoxyethoxy-2-propanol. 
     COMPARATIVE EXAMPLE 2A 
     A flask is charged with the following: 
     
         ______________________________________               GMS______________________________________Tris(2-hydroxyethyl)isocyanurate                 301.2Trimellitic anhydride 221.6Methylenedianiline    114.2Ethylene glycol       93Terephthalic acid     220.5Tetraisopropyltitanate                 .34Diethylene glycol monobutyl ether                 210.5______________________________________ 
    
     The contents are heated slowly with the evolution of water to a maximum temperature of 225° C. until no more water is given off. The contents are allowed to cool to 180° C. at which time 632 gms of monomethyl ether of diethylene glycol is added along with 211 gms of Solvesso 100. The contents are filtered. Final solids are 45% with a viscosity of 550 cps at 25° C. 
     The enamel is run at 40&#39;/min. on a wire tower at 900° F. using 7 passes on 18 AWG copper wire. A coated wire having the following properties is obtained: 
     
         ______________________________________Flex 25% + ?             1Cut Thru, °C.     366Heat Shock 0%-30&#34; -200° C.                    3XDiel. Strength, KV       10.7______________________________________ 
    
     In all cases it is seen that coated wires having improved properties, especially cut through, are obtained when the polyester imide is prepared in the presence of substantially non-reactive 1-butoxyethoxy-2-propanol. 
     It is obviously possible to make many variations in the present invention in light of the above-detailed description. For example, the alkyl titanate can be omitted. A polyisocyanate, 1 to 25% based on resin, can be added, phenolformaldehyde resin can be omitted or it can be substituted with a melamine-formaldehyde resin. Metal driers can also be added, e.g., 0.2 to 1.0% based on total solids, of zinc octoate, cadmium linoleate, calcium octoate, and the like. Instead of the monomethyl ether of diethylene glycol used as solvent, there can be used the monobutyl ether of diethylene glycol. The following monoester alcohols can be used: ethylene glycol monoacetate and diethylene glycol monoacetate. All such obvious variations are within the full intended scope of the appended claims.