US2114877A - Coating composition - Google Patents

Description

f Patented Apr. 1c, 1938 2,114,877 1 GOATING COMPOSITION BolphW.llail,FortWayne,Ind.,anignorto General Electric Company. a corporation of New York No Drawing. Application February 16, 1937, Serial N o. 126,040

1 Claims. (01. zoo-2) The present invention relates broadly to coating compositions and especially to liquid coating compositions of the kind known to the trade as "wire enamels". The invention is particularly directed to a new and useful composition comprising furfural and the product of reaction of an aldehyde and hydrolyzed polymerized vinyl ester.

In the manufacture of insulated electrical conductors wherein the insulation is of the organic enamel type an electrical conductor, for example copper wire, is usually run first through a bath of enamel, such as drying oil enamel. On leaving the bath the conductor carrying the enamel is run through a baking oven or tower where the enamel is baked. Usually it is necessary tov run the same wire successively through the enamel bath a number of times, for example, from about four to eight times, in order to provide adequate insulation thereon.

In the manufacture of large types of electrical apparatus, such as large motors where random wound coils are used, the enameled wire is wound either directly on the core or other member of the apparatus for which it is intended, or the wire is separately formed as a coil and then assembled in the machine. However it may be fabricated, the entire structure is impregnated with an insulating varnish and baked at a temperature as high as about 150 C.

In order to be satisfactory for use as an insulation on wire, an enamel film must have the property of hardness, which imparts resistance to abrading action encountered in the assembly of the machine. It also must have resistance to the softening action of solvents encountered in' the subsequent varnish treatment. The film also must have good adhesion to the base metal and considerable extensibility without cracking or peeling from the wire. These last two properties permit of the necessary stretching of the wire on winding and allow the film to stand up unimpaired under the heat shock encountered on being placed in a baking oven. It is particularly important that the film be highly resistant to moisture.

The conventional oil-type enamels as used in practice make it necessary to effect a compromise between these properties in order to obtain a satisfactory insulation. Such a compromise is illustrated by the following tests which are made to determine whether or not enameled wire is satisfactory for use in motors. The property of extensibility is determined by taking, for example, an enameled wire and elongating it it per cent and winding it after elongation on a mandrel three times its own diameter. If the enamel does not crack, the insulated wire is considered satisfactory from the standpoint that it can be successfully wound into coils. The property of hardness is determined, for example, by elon- 5 gating an enameled wire 20 per cent and winding it as above indicated. Under these conditions the enamel on the wire must crack to show that it has been baked sufliciently to withstand abrasion and solvent attack during varnish treatment such as described in the foregoing paragraph.

Since the hardness, abrasion resistance, and resistance to softening by varnishes are improved by increased baking of the enamel, and since the flexibility and extensibility of the film are impaired by increased baking, then the above test illustrates the sort of compromise that is practiced in order to manufacture conventional enameled wire. Furthermore, when conventional enameled wire is stretched, as is done in many winding operations, and is then subsequently baked in the apparatus during the varnish treatment. it is prone to crack owing to the heat shock, especially when the film has been baked hard to make it abrasion-resistant. In many places in the manufacture of electrical apparatus it is necessary to cover the enameled wire with cotton or paper to protect the film from damage during the mechanical winding operations, and to provide a medium for the impreg- 80 nating varnish to fill in order to cover up such damage, when it occurs. Such use of cotton or paper is wasteful of space and material.

The present invention provides a liquid coating composition such as a wire enamel which, in turn, produces, for example, an insulated wire of the enamel type which is not subject to the harmful efiects noted above, which does not have to be manufactured on a basis of compromise of one property in order to get another, and which, by virtue of possessing all of the desirable properties enumerated above, does not require the additional covering of cotton or paper.

In accordance with my invention I utilize a composition comprising furfural and a member of a class of organic materials derived from vinyl bodies as more fully described hereinafter, which composition 1 have found not only fulfills the above requirements of hardness and extensibility but produces, in addition, an insulated electrical conductor on which the insulation is very adherent, tough and flexible, remarkably abrasionresistant, of high dielectric strength, low power factor, resistant to attack by such agencies as 5 oil and the like and, surprisingly, is outstanding in its resistance to moisture.

The class of organic materials used in the production of the coating compositions of this invention is the class of compounds made, generally speaking, by reacting a partially or completely hydrolyzed polymerized vinyl ester with an aidehyde. The preparation of such compounds is set forth, for example, in Matheson and Skirrow Patent No. 1,725,362, in Morrison, Skirrow and Blaikie Patent No. 2,036,092, and in British Patent 351,082. The following is a specific example of a procedure that may be employed for preparing such a compound:

One hundred parts of a polymerized vinyl acetate is dissolvedin 185 parts of glacial acetic acid. To this is added 83 parts of formalin solution and 6.8 parts of concentrated sulphuric acid. Hydrolysis is carried out at 70 C. in an enameled vessel, fitted with an agitator, for a period of 20 to'25 hours. A test sample taken about the sixteenth to eighteenth hour will indicate the exact time for the hydrolysis. 13 parts of ammonia solution are added for neutralization and the reaction mixture is precipitated as threads in water, washed and dried in a current of warm air (about 60 0.).

According to my invention the thread-like material is dissolved in a'solvent comprising furfural, also known as furfuraldehyde. Various concentrations of solution can be made, the particular ratios employed depending upon the use of the resulting composition. Liquid coating compositions adapted for various purposes may contain, for instance, from about i to 20 parts by weight of the product of reaction of aldehyde (for example, formaldehyde) with a hydrolyzed polymerized viny ester such as the acetate. A solution employed as a wire enamel for coating wire advantageously may consist of about 5 to 15 parts by weight of the described synthetic product and the remainder commercial furfural. The usual wire-enameling apparatus is employed, together with a suitable wiper for removing excess enamel, and the coating baked on the wire at an elevated temperature, for example, by continuously passing the coated wire through an electrically heated oven maintained at such temperature that the coating will be converted to a hard, flexible, tough, abrasion and moisture-resistant state. Heating the coated wire, for example, at a temperature as high as about 350 C. for a period of the order of about one-half minute after each application of enamel ordinarily is sufficient to produce the desired results. From a practical standpoint, baking the film at a temperature sufficient to volatilize excess or uncombined furfural is essential. It is therefore desirable to heat the coating at least above the boiling point of furfural. In all cases, the higher the temperature employed, the shorter the period of heating; and, conversely, the lower the temperature, the longer the heating time. The temperature and period of heating must be sufiicient to volatilize uncombined furfural and to convert the residual product to a hard, tough, abrasionand moisture-resistant state.

A marked improvement in moisture resistance is found in the coating produced on a wire by a wire enamel utilizing furfural as the solvent. For example, a wire enamel consisting of 15 per cent by weight of the herein-described poly-vinyl acetal resin and 85 per cent by weight furfural is markedly better in moisture resistance as compared with the coating formed on wire of the anger? same size and under substantially identical conditions from a wire enamel consisting of 12 per cent by weight of the same polyvinyl acetal resin and 88 per cent by weight of a solvent composed of 90 parts by weight of trichlorbenzene and 10 parts by weight of butyl alcohol.

A possible explanation for the improvement in moisture resistance is as follows:

In making the polyvinyl acetal resin used in practicing this invention, polyvinyl ester, for example polyvinyl acetate, is partially or completely hydrolyzed. The acetal is formed upon reaction with an aldehyde such as formaldehyde. The two reactions may take place simultaneously in the kettle. In the herein-described polyvinyl acetal resin of commerce, not all of the hydroxyl group has been removed therefrom. As a result, wire enamels made with polyvinyl acetal resin dissolved in such solvents as dioxan, ethylene dichloride, chloroforrn, trichlorbenzene, and the like, when applied to wire and baked, result in coatings substantially less resistant to moisture than coatings formed by a wire enamel comprising the same resin dissolved in furfural. It is possible, then, that with furfural as the solvent, the acetal reaction is completed and there is an actual chemical combination of the furan group with the polyvinyl acetal molecule at the temperatures at which the coated wire is baked, thereby producing a coating comprising a new chemical compound. A further possibility is that the improvement is due to, or aided by resinification of furfural, at the baking temperatures employed in wire enameling processes. In such case a resinified furan compound is present in the heat-treated coating in physical or chemical combination with the polyvinyl acetal resin. Whatever the explanation, the fact remains that there is obtained a baked coating of resinous composition having outstanding water resistance and other improved physical and electrical properties in comparison with coatings resulting from use of the same resin in solution in solvents other than aldehyde such as furfuraldehyde.

Wire insulated with the liquid coating composition of this invention is far superior to wire insulated with conventional organic enamels. The film on a copper conductor is more flexible than the copper itself, it being possible to stretch the wire to the breaking point and still wind the wire on itself without evidence of cracking of the film. The heat-treated film as present on the wire is not only flexible, but also is extremely hard and tough. The film is resistant to practically all commonly used solvents such as petroleum hydrocarbon naphthas, aromatic solvents (benzene, toluol, xylol, and the like), and most alcohols. The film is also exceptionally resistant to heat aging. At C., which is the maximum operating temperature of most electrical equipment, samples of wire coated with the composition of this invention have been on test for as long as 2000 hours at the stated temperature without losing their extreme flexibility, whereas conventional wire enamels will not withstand heating for longer than about 24 hours at 105 C. without cracking on a mandrel test such as hereinbefore described. The insulated wire can be elongated to any extent necessary in practical manufacturing, treated with varnish, and immediately placed in an oven at a temperature of C. without cracking or impairing the film.

The dielectric strength of the films produced is high and the dielectric losses are low. It is therefore possible to use a much thinner overall insulation than hitherto employed. Because less insulation is required, more copper can be placed in the slot of a motor or generator resulting in an increased rating of the machine. This applies to transformers, regulators and other electrical apparatus in which insulated wire is used in the form of a coil. Conversely, if the rating of the machine is kept constant, savings in copper and iron result.

Sheets or films of resinous material comprising a product of heating furfural at an elevated temperature with the product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester may be made in accordance with this invention. The resinous composition may be made in sheet form. for example, by depositing a furfural solution of the described vinyl compound on the polished surface of a revolving drum and heating the coating thereon at a temperature sufliciently high to evaporate excess furfural and to promote resinification of the furfural or its reaction with said vinyl compound. The size of the drum may be so designed, and the thickness of the coating thereon, temperature of heating and rate of rotation of the drum so adjusted that the film or sheet is sufficiently solid that it may be scraped or otherwise removed from the drum at the end of a complete revolution. A sheet or tape of such material may be used, for example, in insulating electrical conductors by the well- -lrnown strip covering methods.

Coating compositions produced as herein described may be used in lieu of, or in addition to .dii

' thereon.

ordinary or conventional organic enamels for forming an electrically insulating or other pro tective coatings on wires or other articles of manufacture having surfaces to be enameled. For example, advantage may be taken of the exceptional toughness of the insulation film in reinforcing regular enameled wire against abrasion by applying a coat of the insulation on top of the regular enamel. This outer coat is advantageous in another connection. It does not readily oxidize and become brittle when heated for long periods of time. Ordinary enamels made from drying oils become brittle upon baking, presum ably due to continued oxidation. A coat of the insulation made in accordance with the present invention aids materially in preventing this embrittlement and in providing improved moisture resistance. A conductor also may be given an initial coating of the insulating composition herein described and subsequent coatings of ordinary enamels. In such case, the adherence of the conventional organic enamels is improved. If desired, a conductor may-be first provided with a coating of the resinous composition of this invention, then one or more coatings of ordinary types of enamels may be applied, after which, if desired, an outer coating or coatings of the composition herein described may be provided In the preferred embodiment of this invention, the liquid coating composition or wire enamel consists essentially of furfural and a lesser but substantial proportion of the product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester. However, solvents in addition to fur-rural are not precluded from the coating composition. Preferably I use furfural alone as the solvent ingredient oi the liquid coating composition, since furfural provides a baked or heat-treated resinous composition of outstanding properties. But for such practical reasons as to reduce the inflammability of the composition, to reduce its viscosity, and so forth, the liquid coating composition may contain in addition to furfural lesser but substantial proportions of other solvents such, for instance, as other aldehydes, or dioxan, ethylene dichloride, trichlorbenzene or the like, or mixtures of such solvents.

It is to be understood that ordinary or conventional organic enamels to which reference has been made herein are not limited to the drying oil types of enamels but include all classes of ordinary or conventional organic enamels, examples of which are well known.

In the co-pending application Serial No. 17,796 of Winton I. Patnode and Edward J. Flynn, filed April 23, 1935, now Patent No. 2,085,995, and assigned to the same assignee as the present invention, claims have been made to an electrical conductor insulated with an organic insulation comprising the heat-treated product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester. The present invention is separate and distinct therefrom, relating as it does to a composition comprising furfural and the product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester, which composition when heated at a baking temperature provides a new and useful resinous material.

WhatI claim as new and desire to secure by Letters Patent of the United States is:

1. A hard, tough, abrasionand moisture-resistant resinous composition comprising the heattreated residual product of heating a solution containing furfural and the product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester at a temperature sufl'lciently high to volatilize uncombined furfural.

2. A hard, tough, abrasionand moisture-resistant, flexible, electrically insulating film of resinous material comprising the heat-treated residual product of heating a solution consisting mainly of furfural and the product of reaction of formaldehyde with hydrolyzed polymerized vinyl ester at a temperature above the boilirig point of furfural.

3. A wire enamel comprising furfural and the product of reaction of an aldehyde with hydrolyzed polymerized vinyl ester.

4. A wire enamel containing a major proportion of furfural and a lesser but substantial proportion of the product of reaction of an aldehyde with hydrolyzed polymerized vinyl acetate.

5. A wire enamel consisting of about '5 to 15 per cent by weight of the product of reaction of formaldehyde with hydrolyzed polymerized vinyl acetate and about 95 to 85 per cent by weight of furfural.

6. An article of manufacture'having a surface covered with a hard, tough, abrasionand moisture-resistant resinous composition comprising the heat-treated residual product of heating a solution containing furiural and the product of .reaction of an aldehyde with hydrolyzed poly-