|Publication number||US3206330 A|
|Publication date||Sep 14, 1965|
|Filing date||May 25, 1962|
|Priority date||May 25, 1962|
|Publication number||US 3206330 A, US 3206330A, US-A-3206330, US3206330 A, US3206330A|
|Original Assignee||Westinghouse Electric Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (9), Referenced by (10), Classifications (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Sept. 14, 1965 J. CHOTTINER 3,206,330
WEATHER AND TRACK RESISTANT COATING SYSTEM Filed May 25, 1962 ACRYLIC ESTER RESIN COATING l LAMINATED II/I T gxv PHENOL'C RESIN COATING WITNESSES INVENTOR I Jacob ChoHiner ATTORNEY United States Patent 0 3,206,330 WEATHER AND TRACK RESISTANT COATING SYSTEM Jacob Chottiuer, McKeesport, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 25, 1962, Ser. No. 197,659 6 Claims. ('Cl. 117-218) This invention relates in general to composite weather and track resistant coatings and electrical articles coated therewith. More particularly, this invention relates to a composite multi-l-ayer track and arc-resistant coating combiuation of a first layer derived from a specific epoxy composition and a second layer of polymerized acrylic and/or methacrylic acid esters deposited over the first layer.
In the past, failures of electrical equipment in the field have been attributed to the formation of conductive carbon tracks on the insulating materials. Such insulation failures generally have been associated with the presence of surface moisture formed either by condensation or, in the case of outdoor equipment, by precipitation, alone, or in association with conductive dust particles deposited from the atmosphere in one way or another. The presence of surface moisture or water films on electrical insulating materials permits leakage currents to fiow across the surface of the insulation between points of different potential.
The passage of currents through the water film gradual- 1y causes evaporation and-disruption of the continuity of the water path. The discontinuities in the Water film may be characterized as high resistance air gaps which are bridged by electrical discharges in the form of arcs whose magnitude and intensity are dependent upon the existing potential gradient and the amount of leakage current. These electrical discharges cause thermal 0r ionic deterioration of the material. Insulating materials with poor arc and track-resistance form carbon or conductive ash. The continued degradation of the insulating material as a result of repeated exposure to the above conditions may sometimes result in a complete carbon path forming across the surface of the insulation. When such a condition occurs, the insulation fails and results in various degrees of failure in the electrical apparatus to which such insulation has been applied and to associated apparatus, since the carbon path will perrnit current surges in the apparatus. Insulating materials which do not form carbon paths or tracks under the foregoing environmental conditions will, of course, have high track and arc-resistance and will eliminate flashover and surge failures.
Various electrical component-s are exposed to environmental conditions that are conducive to carbon tracking. Exemplary components include circuit breaker bushings, arc chute barriers, bus bar sleeving and supports and fuse casings. Numerous organic insulating materials are known and used in these components. Usually these organic materials do not have a high resistance to the formation of carbon tracks although their other electrical insulating properties may be satisfactory. These components are frequently fabricated, for example, from phenolic resin impregnated laminates, which have relatively low track and arc-resistance.
The track and arc-resistance of such components may be improved by coating the component with a composition which has a high track and arc-resistan-ce. A particularly suitable coating material is disclosed and claimed in US. application Serial No. 132,843, assigned to the assignee of the present invention.
US. application Serial No. 132,843, referred to hereinabove, discloses volatile solvent containing and solventless liquid epoxy based coating compositions which are exceptionally suitable for application as track and arcresistant insulation to electrical components. These coatings and compositions, hereinafter referred to as track and arc-resistant epoxy compositions or coatings, exhibit fading, chalking and crazin-g upon continuous long-term exposure to outdoor environments. While this is not considered to be a serious defect in most applications, it is nonetheles apparent that the roughened surface pro duced by chalking, for example, increases the rate of ac cumulation of contaminants. In continuous, long-term outdoor service, the accumulation of surface contaminants can lower the surface resistance below that of a normal clean glossy urface. The accumulated contaminants and the concomitant lowered surface resistance provides a condition which may cause electrical failures due to arcing or tracking discharges. Outdoor fuse casings, for example, are exposed to the foregoing conditions since they remain in outdoor service for Prolonged periods.
It is to be understood that the arc-resistant epoxy compositions and coating referred to hereinabove do exhibit superior arc-resistant properties. Even repeated and prolonged exposure to the foregoing conditions do no result in a complete track on the track and arc-resistant epoxy coating. The presence-of moisture and contaminants on the surface of the chalked arc-resistant epoxy coating, for example, forms minute resistance gaps between contaminating particles. Under voltage stress, these resistance gaps are bridged by scintillation discharges. Prolonged exposure to the scintillation discharges erodes the arc-resistant epoxy coating so that the base material is ultimately exposed. Failures occur by an erosion completely through the track and arere-sistant epoxy coating to the base material, which typically has a relatively low track and arc-resistance. Continued exposure will result in a complete track being formed on the base material.
Accordingly, it is an object of this invention to provide a multiple coating system having an improved high resistance to arcing and tracking for prolonged periods in outdoor environments;
A more particular object of this invention is to provide track and arc-resistant coatings for outdoor applications which includes a first coating 'of a heteropolymerized product derived from a composition containing reactive resinous glycidyl ether and a polyamide or amine adduct cross-linking agent with critical amounts of aluminum oxide trihydrate, either with or without finely divided inert fillers and a second coating deposited over the first coating comprised of the polymerized product of esters of acrylic and/or methacrylic acid.
Another object of this invention is to provide electrical apparatus, members and articles, as for example fuse casings, with surfaces which are track and arc-resistant for prolonged periods during exposure to outdoor environments.
Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.
For a better understanding of the invention, reference may be had to the accompanying drawing, in which:
FIGURE 1 is a front view illustrating a fuse casing provided with the multi-layer outdoor track and arcresistant coating of this invention; and
FIG. 2 is an enlarged cross-sectional illustration, along line AA, of FIG. 1 which illustrates the composite coating of this invention in greater detail.
In accordance with this invention and in the attainment of the foregoing objects, there is first prepared either a volatile solvent containing or solventless liquid arc-resistant epoxy coating composition which is suitable for application as the first or under layer of the composite or multiple outdoor track and arc-resistant coating of this invention. This composition comprises critical proportions of (A) polymeric epoxides or epoxy resins, (B) normally liquid polyamide or amine adduct crosslinking agents reactive with the polymeric epoxides and (C) aluminum oxide trihydrate. In some cases, finely divided, inert mineral fillers may be included. The composition may be readily applied to electrical members and components utilizing conventional methods such as brushing, spraying, flowing, dipping and the like.
The track and arc-resistant epoxy compositions are adapted for application to many types of electrical components such as circuit breaker bushings, arc chute barriers, bus bar sleeving and supports, fuse casings and the like. When these compositions are applied, for example, to outdoor fuse casings and then polymerized, a tenaciously adherent, hardened resinous composition which in itself is highly resistant to carbon tracking and arcing is provided. These compositions may be directly applied, with equal facility and advantage, to electrical conductors or metallic elements in electrical apparatus as well as to insulating materials.
The resinous polymeric epoxides, also known as epoxy resins and glycidyl polyethers, employed in preparing the liquid resinous track and arc-resistant epoxy coating compositions for the first layer of the coating of this invention, may be prepared by reacting predetermined amounts of at least one polyhydric phenol and at least one ephalohydrin is an alkaline medium. Phenols which are suitable for use in preparing such resinous polymeric epoxides include those which contain at least two phenolic hydroxyl groups per molecule. Polynuclear phenols which have been found to be particularly suitable include those wherein the phenolic nuclei are joined by carbon bridges, such for example as 4,4'-dihydroxy-diphenyl-dimethylmethane (referred to hereinafter as Bis-phenol A) and 4,4'-dihydroxy-diphenyl-methane. In admixture with the named polynuclear phenols, use also may be made of those polynuclear phenols wherein the phenol nuclei are joined by sulfur bridges such, for example, as 4,4- dihydroxy-diphenyl-sulfone.
While it is preferred to use epichlorohydrin as the epihalohydrin in the preparation of the resinous polymeric epoxide starting materials of the present invention, homologues thereof, for example, epibromohydrin and the like also may be used advantageously.
In the preparation of the resinous polymeric epoxides,
aqueous alkali is employed to combine with the halogen of the epihalohydrin reactant. The amount of alkali employed should be substantially equivalent to the amount of halogen present and preferably should be employed in an amount somewhat in excess thereof. Aqueous mixtures of alkali metal hydroxides, such as potassium hydroxide and lithium hydroxide, may be employed although it is preferred to use sodium hydroxide since it is relatively inexpensive.
The resinous polymeric epoxide, or glycidyl polyether of a dihydric phenol, suitable for use in this invention has a 1,2-epoxy equivalency greater than 1.0. By epoxy equivalency reference is made to the average number of 1,2-epoxy groups:
contained in the average molecule of the glycidyl polyether. Owing to the method of preparation of the glycidyl polyethers and the fact that they are ordinarily a mixture of chemical compounds having somewhat different molecular weights and contain some compounds wherein the terminal glycidyl radicals are in hydrated form, the epoxy equivalency of the product is not necessarily the integer 2.0. However, in allcases it is a value greater than 1.0. The 1,2-epoxy equivalency of the polyethers thus is a value between 1.0 and 2.0.
The resinous polymeric epoxides or glycidyl polyethers may be prepared by admixing and reacting from 1 to 2 mol proportions of epihalohydrin, preferably epichlorohydrin, with about 1 mol of Bis-phenol A in the pres ence of at least a stoichiometric excess of alkali based on the amount of halogen.
To prepare the resinous polymeric epoxides, aqueous alkali, Bis-phenol A and epichlorohydriu are introduced into and admixed in a reaction vessel. The aqueous alkali serves to dissolve the Bis-phenol A with the formation of the alkali salts thereof. If desired, the aqueous alkali and Bis-phenol A may be admixed first and then the epichlorohydrin added thereto, or an aqueous solution of alkali and Bis-phenol A may be added to the epichlorohydrin. In any case, the mixture is heated in the vessel to a temperature within the range of about C. to 110 C. for a period of time varying from about /2 hour to 3 hours, or more, depending upon the quantity of reactants used. Upon completion of heating, the reaction mixture separates into layers. The upper aqueous layer is withdrawn and discarded, and the lower layer is washed with hot water to remove unreacted alkali and halogen salt, in this case, sodium chloride. If desired, dilute acids, for example, acetic acid or hydrochloric acid, may be employed during the washing procedure to neutralize the excess alkali.
The following example illustrates the preparation of a glycidyl polyether or epoxy resin suitable for use in preparing the track and arc-resistant epoxy coating composition.
EXAMPLE I 54 parts of sodium hydroxide were dissolved in about 600 parts of water in an open kettle provided with a mechanical stirrer. About 3 mols of Bis-phenol A" were added and the resultant mixture was stirred for about ten minutes at a temperature of about 30 C. Thereafter, aproximately 4 mols of epichlorohydrin were added, whereupon the temperature of the resultant mixture increased to about 60 C. to 70 C. due to the heat of reaction. About 42 parts of caustic soda dissolved in about 9 parts of water then were added with continuous stirring and the mixture was maintained at a temperature of about C. to C. for a period of about one hour. The mixture was then permitted to separate into two layers, the upper layer was withdrawn and discarded and the lower layer was washed with boiling water to which was added acetic acid in an amount suflicient to neutralize unreacted caustic soda. A liquid resinous reactive polymeric epoxide was obtained after substantially all of the wash water had been removed. The liquid resinous polymeric epoxide or glycidyl ether thus obtained is suitable for use in formulating the track and arc-resistant epoxy composition.
The epoxy cross-linking curing agents which are suitable for use in track and arc-resistant epoxy composition consist of polyamides and amine adducts which are normally liquid. As an example, satisfactory liquid polyamides may be obtained by interacting polyamines with dimerized fatty acids having from 8 to 24 carbon atoms per molecule. The polyamides thus obtained contain free primary or secondary amine groups spaced along the molecule.
Broadly, the polyamide resins may be prepared by condensing dimerized vegetable oil fatty acids such as linoleic acid with a suitable polyamine compound such as diethylene triamine. The reactants are blended at relatively low temperatures in a suitable reaction vessel and then gradually heated, with stirring, until all water of reaction has distilled off. The liquid polyamide resin then is withdrawn from the reaction vessel. 7
Polyamines which are suitable for use in preparing the polyamide resins utilized include diethylene triarnine, triethylene pentamine, triethylene tetramine, tetraet'hylene pentamine, and the like. The amines may be used singly or in combination of two or more.
The polyamide resins serve as cross-linking agents for the liquid polymeric epoxide resins. The terminal epoxy groups on the polymeric epoxide are believed to react with the amine groups spaced along the polyamide resin to yield a complex cross-linked product according to the following equation.
A number of polyamide cross-linking curing agents which produce heteropolymerized epoxy resins are known in the art and are considered to be satisfactory for the preparation of the track and arc-resistant epoxy composition. US. Patent No. 2,705,223 describes a series of polyamides that may be so used. Satisfactory results may be attained with diamine pyridine, diaminodiphenyl sulfone, metaphenylene diamine, 4,4'-methylene dianiline and metaxylene diamine. Satisfactory results may also be obtained using, amine adducts, as for example, the
ethylene oxide adduct of DETA (diethylene triamine) and propylene oxide adduct of DETA. Other amine adducts known in the art, as for example, those disclosed in US. Patent 2,864,775 may be used. Both the polyamides and amine adducts and mixtures of two or more are satisfactory cross-linking curing agents.
Although the known anhydride curing agents produce a heteropolymerized epoxy resin, they are not satisfactory cross-linking agents for the track and arc-resistant epoxy composition. While we do not wish to be restricted to any particular theory, it is believed that nitrogen derived from the polyamide or amine adduct cross-linking agent in the polymerized resin molecule contributes to the arc resistance of the cured resin. Since the polymerized molecule produced by an epoxy-anhydride system lacks nitrogen, it does not have the superior track and arc-resistance of the cured resins which employ polyamide or amine adduct cross-linking agents. Similarly, non-coreactive catalysts, such as triethylene tetramine, are considered unsatisfactory since they produce a homopolymerized product which does not contain nitrogen.
Satisfactory liquid track and arc-resistant epoxy coating compositions may be prepared with liquid epoxy resins throughout a range of viscosities. The more viscous liquid or solid epoxies may be employed in a system containing volatile organic solvents. The solvent system can produce, for example, a coating which is sufficiently fluid so that it may be easily sprayed. A solventless system may be employed with less viscous epoxy resins or where the coating may be conveniently brushed, or flowed onto the component to be insulated.
Granular aluminum oxide trihydrate (Al O .3H O) is added to the reactive resin and cross-linking agent to increase the track and arc-resistance of the composition to an exceptional degree. The heteropolymerized epoxy formulated to contain nitrogen in the molecule and the aluminum oxide trihydrate combine to provide a composition with outstanding track-resistance. The size of the oxide granules is not critical. Excellent results are produced with particles in the range of 200-400 mesh, A.S.T.M. screen. While the aluminum oxide trihydrate does not affect the heteropolymerization reaction, it is not an inert filler since it does contribute to the track-resistant properties of the composition. It should be understood, therefore, that the term inert filler does not include the aluminum oxide trihydrate.
The weight ratios or proportions of epoxy resin to crosslinking agent which may be employed in the are resistant coating composition range from about parts of epoxy resin to 20 parts of coreactive cross-linking agent to about '40 parts of epoxy resin to 60 parts of cross-linking agent.
Weight ratios outside of .this range have been found to produce materials of unsatisfactory quality.
The ratio of aluminum oxide trihydrate to the reactive epoxy and coreactive cross-linking agent should range from about 25 parts of oxide to 75 parts of the combined reactive epoxy resin and cross-linking agent to 75 parts of oxide to 25 parts of the combined reactive epoxy resin and cross-linking agent, by weight.
There may also be included certain proportions of finely divided inert mineral fillers including pigments. Examples of these latter fillers which have been found to be suitable to include with the aluminum oxide trihydrate, to obtain other desirable properties, are magnesium silicate, titanium dioxide, silica, bentonite, zinc oxide, calcium carbonate, wachtungred, chrome yellow, iron oxide, mapico yellow-orange, toluidene red and the like. These additional fillers or pigments may be used singly or in combination of two or more.
Appreciable amounts of the additional fillers, except for small proportions of the coloring pigments, are used in the basic formulation for applications not requiring the highest degree of arc and track-resistance. The coloring pigments do not have a significant adverse effect on the properties of the basic formulation since they are 0rdi-= narily employed in small quantities. In any event, to retain a relatively high degree of arc and track-resistance, these fillers, either singly or in combinations should not be used in a ratio greater than 30 parts of filler to 70 parts of aluminum oxide trihydrate.
In formulating the solvent based liquid track and arcresistant epoxy coating compositions, the polymeric epoxide' resin or resinous glycidyl ether, cross-linking agent, aluminum oxide trihydrate and any inert fillers are admixed with one or more suitable volatile organic hydrocarbon solvents. The choice of any one or more particular solvent or solvent mixtures is not critical. Examples of solvents which are suitable include xylene, cellosolve, benzene, toluene, methylethyl ketone, ethyl acetate, butyl acetate, isopropyl alcohol, and naphtha. These solvents may be used either singly or in combination of two or more.
A particularly satisfactory solvent based liquid coating composition is prepared from the following ingredients:
EXAMPLE II Epoxy component: Parts by weight Reactive epoxy resin (Shell Chem. Co., Epon 1001 10 Aluminum oxide trihydrate (AlcoaC-730) 1O Ccllosolve 1 Xylol 9 Cross-linking component: Polyamide resin (General Mills Co., Versamid 115) 10 Aluminum oxide trihydrate (Alcoa C-730) 10 Cellosolve 1 Xylol 9 Coating composition: A
Epoxy component l Cross-linking component 1 The two components are mixed shortly before application. Pot life at 25 C. is from three to four days.
In preparing the liquid track and arc-resistant epoxy coating composition, the polyamide resin is dissolved in an equal weight of the thinner xylene and 10% cellosolve) using a ball mill. The diglycidyl ether epoxy resin component is mixed with an equal weight of thinner (90% xylene and 10% cellosolve) and ball milled to obtain a solution of the epoxy resin.
Aluminum oxide trihydrate equal in weight to the solids content of each of the solutions of the polyamide resin and polymeric epoxy resin is then mixed into each solution using a ball mill. These compositions are then individually passed through a paint mill to obtain complete wetting and dispersion of the alumina hydrate. Before application the separate reactive component mixtures are admixed in equal proportions by weight and blended uniformly. Of course other proportions may be employed in the range of from 80 to 20 parts by weight of epoxy resin to polyamine to 40 to 60 parts by weight. The resultant mixture is placed under vacuum to remove substantially all of the air in the finished composition.
Successive coats of the material, with appropriate drying time between coats, are applied by, in some cases spraying and in others brushing, until the desired film thickness of 20 mils is obtained. Both air dried and baked coatings may be applied. The viscosity of the coating material for brushing may, for example, be 75 seconds in a Zahn No. 3 cup (120 seconds Demmler No. 1 cup) at 25 C. and 45 seconds in a Zahn No. 3 cup for spraying. Sprayed coatings are applied with a suction type spray gun using a No. nozzle and a No. 30 air gap and 30 p.s.i. air pressure. Multiple spray coats may be applied to produce dry film thickness of from .003 to .005 inch per coat. The brushed on coatings may be of the same thickness. The drying time between coats is 1-2 hours at 25 C. for the air dried films and 15-30 minutes at 80 C. for the baked films. The curing time is 24 hours at 25 C. for the air dried films and 1 hour at 80 C. for the baked film.
Additional track and arc-resistant epoxy coating compositions may be prepared by varying the ratio of aluminum oxide trihydrate to the total amount of epoxy resin and cross-linking agent. When the aluminum oxide trihydrate exceeds 75%, by weight, of the composition, excluding the solvent, the cured composition will lack homogeneity as the resin content is not sufiicient to wet or coat and adequately bond all of the oxide particles. Compositions including less than 25%, by weight, of aluminum oxide trihydrate, excluding the solvent, do not possess the excellent arc and track resistant properties desired, although as little as 5%, by weight, of aluminum oxide trihydrate will produce some improvement in these properties over that of the resin alone.
The heretofore described volatile solvent containing compositions are advantageous insofar as they may, for example, be easily applied to electrical components with the use of well known spray apparatus. Thin coatings may be easily applied. Completely reactive or solventless track and arc-resistant epoxy coating compositions, on the other hand, provide advantages not inherent in the solvent based system. Systems employing a solventless low viscosity fluid epoxy resin will produce a smooth glossy finish even on relatively rough component surfaces. Solvent based systems because of the high shrinkage of the loW solids wet film as it dries tend to reproduce the roughness of the underlying surface. Numerous coats of the composition and sanding operations between coats are required to produce a smooth surface with the solvent based composition.
Satisfactory arc-resistant solventless epoxy coating compositions may also be formulated with the weight proportion of reactive epoxy resin to polyamide or amine adduct coreactive cross-linking agent within the range of about 80 parts of resin to parts of cross-linking agent, by weight, to about 40 parts of resin to 60 parts of crosslinking agent, by weight. The weight proportion of aluminum oxide trihydrate and inert mineral filler to the epoxy resin and coreactive polyamine cross-linking agent should also be within the range of about parts of aluminum oxide trihydrate and inert filler to 75 parts of epoxy resin and cross-linking agent and about 75 parts of aluminum oxide trihydrate and inert filler to 25 parts of epoxy resin and cross-linking agent. The weight proportion of inert filler to aluminum oxide trihydrate should be limited to a maximum of about parts of inert filler to 70 parts aluminum oxide trihydrate. It will be noted that the weight percentages or proportions of the components of the solvent based system would be identical to that of the solventless system, if the solvent was absent.
A particularly satisfactory solventless multicomponent coating composition may be prepared as follows:
EXAMPLE III Epoxy component: Percent by weight Reactive epoxy resins (Jones-Dabney Co.,
Epi-Rez 504) 49 Aluminum oxide trihydrate 49 Toluidene red 2 Cross-linking component:
Polyamine adduct (General Mills, Inc.,
Genamide 250) 50 Aluminum oxide trihydrate 50 The above components may be mixed together, for application, within the following effective limits:
Epoxy component 80% by Wt. to 40% by wt. Cross-linking component 20% by wt. to by wt.
Additional track and arc-resistant epoxy coating compositions may be prepared as outlined in Table I, using proprietary commercial reactive epoxy resins and crosslinking agents. Where required, sufficient solvent is added to render the composition fluid. Epon resins are sold by Shell Chemical Corp., Epi-Rez resins by Jones-Dabney Co. The Versamid and Genamid cross-linking agents are sold by General Mills, Inc. Tables II and III are outlines of the properties of the proprietary resins and cross-linking agents.
Table I Example Material Proportion.
Parts by Wt.
40 60 AlzOaBHzO. 100 Epi-Rez 504.. 60 Versamid 115. 40 100 VI 30 100 VII 20 80 VIII 35 35 50 30 IX E 75 75 150 AlzOallHaO 50 Table II Epoxy Resin Melting Point Epoxide Avg. Mole- (Durrans) Equivalent cules Wt.
Epon 815 Liquid 175-210 340-400 Epon 828 Liquid 175-210 350-400 Epon 1001 64-76 450-525 000-1, 000 Epon 1007. 125-132 1, 650-2, 050 2, 900 Epi-Rez 502. Liquid 300-335 300-400 Epi-Rez 504... Liquid 170-180 300-400 Table III Viscosity, Cross Linking Agent Poistgscat Amine Value Versamid 500-750 210-230 Versamid 80-120 290-320- Genamid 250 5-10 425-450 Genamid 310 40-60 380-415 The second or outer layer, according to this invention, is applied over the track and arc-resistant epoxy coating, described in detail hereinabove and in US. application Serial No. 132,843, to produce a multiple coati g hav n a high track and arc-resistance for prolonged periods during exposure to outdoor environments. The outer coating is comprised of polymers of esters of acrylic and/or methacrylic acids. These coatings, per se, are well-known in the art and are commercially available under proprietary names.
The monomers suitable for use in the preparation of polymers used for the outer coating are methyl, ethyl and butyl acrylates and methacrylates. The properties of the polymers will, to some extent, vary with the particular monomer employed. The methacrylate polymers produce harder, tougher films with lower flexibility than the acrylate polymers. Methyl esters produce harder tougher films with lower flexibility than the ethyl or butyl esters. It will be apparent that the properties may vary from the hard, tough methyl methacrylates to the more flexible ethyl acrylates. Copolymers made by mixing together two or more of the foregoing monomeric esters before polymerization are also suitable for use as the second or outer layer according to this invention. Most commercial acrylic ester resins for coatings are copolymers, and are suitable for use herein. Small quantities ofother film forming resins may be included with the acrylic ester resins in a manner known to those skilled in the art. Nitro cellulose, for example, may be included in small quantities to provide a coating with greater hardness. It will be understood, however, that the acrylic ester resins without modification are satisfactory and preferred.
.Polymerized acrylic ester resins have high molecular weights and high viscosities. They are thermoplastic and soluble in such organic solvent as, for example, coal tar hydrocarbons, chlorinated hydrocarbons, ketones, esters and ether alcohols. Toluol and xylol are frequently employed and are satisfactory, among other solvents, for use in this invention.
The coatings are applied by employing solvent evaporation techniques. The polymerized acrylic ester or esters are dissolved in one of the solvents noted hereinabove. Application may be made by dipping, brushing, spraying or the like. The proper percentage of solids for application viscosity will vary with the method of application and nature of the surface being coated and may be easily determined by those skilled in the art.
Referring now to FIGS. 1 and 2, we have illustrated a fuse casing 10 coated according to this invention" and especially suitable for prolonged outdoors use. The fuse casing body is initially fabricated from, for example, a laminated phenolic tube 11. As such, the tube has a relatively low arc and track resistance. A track and areresistant epoxy coating 12 is applied directly to the tube 11. Successive coats of the epoxy coating composition, with appropriate drying time between coats, are applied by, in some cases spraying and in others brushing, until the desired thickness film of approximately mils is obtained. Either air dried or baked coatings may be ap plied. The viscosity of the coating material for brush-- ing may, for example, be 75 seconds in a Zahn No. 3 cup (120 seconds Demmler No. 1 cup) at C. and 45 seconds in a Zahn No. 3 cup for spraying.
Sprayed coatings are applied with a suction type spray gun using a No. 15 nozzle and a No. airgap with 30 p.s.i. air pressure. Multiple spray coats may be applied to produce dry film thicknesses of from .003 to .005 inch per coat. The brushed on coatings may be of the same thickness. The drying time between coats is 1-2 hours at 25 C. for the air dried films and 15-30 minutes at 80 C. for the baked films. The curing time is 24 hours at 25 C. for the air dried films and 1 hour at 80 C. for the baked films. It will be understood that any of the track and arc-resistant epoxy compositions described in the foregoing Examples II through IX may be employed for the first coating. A polymerized acrylic ester coating 13 is applied directly to the surface of the cured track and arc-resistant epoxy coating 12. The coating 13 may, for example, be approximately 2 mils thick.
Acryloid B-72, sold by Rohm & Has-s Company, is an example of a proprietary commercial polymerized acrylic ester composition which may be employed for the outer coating. This proprietary composition contains of solid mixed polymerized acrylic esters in toluol. The composition has a specific gravity of 0.97, a viscosity of 480+640 centipoises at 30 C. and a flash point (closed cup-Tag) of 39 F. It is sprayed or brushed over the coating 12, either in single or multiple coats, to a thickness of approximately 2 mils. The applied coating may be air-dried or baked at a temperature of approximately 180 F. to accelerate solvent evaporation. A fuse casing prepared in the foregoing manner will retain high track and arc-resistance over a prolonged period of outdoor environments. It will be understood that the thickness of both the first and second layers may be varied and that no criticality is evident in either total or relative thickness of the layers.
The following tests were conducted in order to evaluate samples coated according to this invention under accelerated test conditions. Two panels of a phenolic laminate, thick, were coated with the arc-resistant epoxy composition described in Example II. Multiple coats of the composition were sprayed onto the laminate to obtain a coating thickness of 20 mils. The coating was cured at 80 C. Half of each specimen was masked with tape and the exposed surface was dipped into a bath of a solventbased polymerized acrylic ester composition. The polymerized acrylic ester topcoat was air dried for 4 hours. This topcoat was 2 mils thick.
The tape was removed and both specimens were placed in an Atlas Fade-Ometer and exposed to the effects of ultraviolet light rays. After 95 hours exposure, the specimens were inspected. The single arc-resistant epoxy coating showed some signs of surface crazing. The multiple system with the acrylic ester topcoat showed no elfects of crazing. One specimen was removed from the Fade- Ometer and placed in an Atlas Weather-Ometei'. 'The second specimen was retained in the Fade-Ometer for continued exposure. The second specimen was again inspected after an additional 240 hours exposure in the Fade-Ometer. The acrylic ester coated portion was unaffected by the additional exposure. The portion with only the arc-resistant epoxy coating showed additional crazing. The sample originally transferred to the Weather-Ometer showed effects similar to the specimen retained in the Fade-Ometer. Both samples at this time were placed in the Weather-Ometer. I
After a total of 297 and 176 hours exposure respectively for the two specimens in the Weather-Ometer, they were again inspected. At this time, the single arcresistant epoxy coated portions of the specimens showed signs of considerable fading and chalking. The portions of each specimen with the polymerized acrylic ester topcoat showed no signs of chalking and only very slight fading.
Two additional 6 by 6" specimens were prepared with a subcoat of an arc-resistant epoxy coating and a polymerized acrylic ester topcoat as described hereinabove. These specimens were subjected to the Dust and Fog Test for an evaluation of their resistance to wet tracking. The Dust and Fog Test is an accelerated test which simulates severe outdoor contamination and is intended to differentiate among solid electrical insulating materials with regard to their ability to withstand the action of arcs produced by conduction through films of contamination containing moisture.
Three b" by 2" by Ms" copper electrodes are placed on the surface of a sheet specimen previously dusted to a depth of A with a synthetic dust-flint containing, clay, sodium chloride and paper particles. The center electrode is hot and the other two electrodes are spaced one inch on either side of the hot electrode and maintained at ground potential. The base upon which the specimen rests is also at ground potential. A fine water spray is directed from a spray nozzle onto the surface of the specimen. The potential between the hot center electrode and the ground electrodes is raised step-wise to 1500 volts by 500 volt increments every 15 minutes.
The'presence of moisture and contaminants on the surface of the specimen causes leakage current to flow from the hot to the ground electrodes. Resistance gaps are bridged by scintillation discharges. Repeated and prolonged exposure to these conditions can result in a complete track between surface electrodes or erosion through the base electrode. Materials are rated as to time to track or to erode through the specimen. The two specimens prepared according to this invention withstood 102.8 and 106.9 hours respectively on the Dust and Fog Test without failure. A single system coating of the areresistant epoxy composition of the same thickness failed by erosion in approximately 50 hours on the Dust and Fog Test. It is apparent that the polymerized acrylic ester topcoat provides a marked improvement in resistance to tracking in addition to superior weathering resistance.
While there have been shown and described what are at present considered to be the preferred embodiments of the invention, modification thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to specific methods described, and it is intended to cover in the appended claims all such modification as fall within the true spirit and scope of this invention.
I claim as my invention:
1. In an electrical apparatus having surfaces exposed to tracking, arcing and flashover, the improvement comprising a multi-layer track and arc-resistant coating de posited onto said surfaces, said coating comprising a first layer of the heteropolymerized product derived from a mixture of (A) a reactive epoxy resin, (B) a coreactive epoxy cross-linking agent selected from the group consisting of polyamides and amine adducts, (C) aluminum oxide trihydrate and (D) a finely divided inert mineral filler, wherein the weight proportion of epoxy resin to cross-linking agent is within the range of about 80:20 to about 40:60, the weight proportion of aluminum oxide trihydrate and inert filler to the epoxy resin and crosslinking agent is within the range of about 25:75 to 75:25 and the weight proportion of inert fillers to aluminum oxide trihydrate is up to a maximum of about 30:70 and a second layer comprising a polymerized acrylic ester deposited over said first layer.
2. An electrical member having a multi-layer track and arc-resistant coating deposited thereon, said coating comprising a first layer of the heteropolymerized product derived from a mixture of (A) a reactive epoxy resin, (B) a coreactive epoxy cross-linking agent selected from the group consisting of polyamides and amine adducts, (C) aluminum oxide trihydrate and (D) a finely divided inert mineral filler, wherein the weight proportion of epoxy resin to cross-linking agent is within the range of about 80:20 to about 40:60, the weight proportion of aluminum oxide trihydrate and inert filler o the epoxy resin and cross-linking agent is within the range of about 25:75 to 75:25 and the weight proportion of inert fillers to aluminum oxide trihydrate up to a maximum of about 30:70 and a second layercomprising a polymerized acrylic ester deposited over said first layer.
3. An electrical member having a multi-layer track and arc-resistant coating deposited thereon, said coating comprising a layer derived from a composition consisting essentially of a mixture of (A) a reactive epoxy resin, (B) a coreactive epoxy cross-linking agent selected from the group consisting of polyamides and amine adducts,
(C) aluminum oxide trihydrate wherein the weight proportion of epoxy resin to cross-linking agent is within the range of about 80:20 to about 40:60, the weight proportion of aluminum oxide trihydrate to the epoxy resin and cross-linking agent is within the range of about 25:75 to :25 and (D) an organic epoxy resin solvent in a proportion to render the mixture fluid, and a second layer comprising a polymerized acrylic ester deposited over said first layer.
4. An electrical member having a multi-layer track and arc-resistant coating deposited thereon, said coating comprising a layer derived from a composition consisting essentially of a mixture of (A) a reactive epoxy resin (B) a coreactive epoxy resin cross-linking agent selected from the group consisting of polyamides and amine adducts, (C) aluminum oxide trihydrate and (D) one or more organic epoxy resin solvents, wherein the weight proportion of epoxy resin to cross-linking agent is about 50:50, the weight proportion of aluminum oxide trihydrate to the epoxy resin and cross-linking agent is about 50:50 and the Weight proportion of organic solvent to epoxy resin and cross-linking agent is about 50:50 and a second layer consisting essentially of polymerized acrylic esters deposited over said first layer.
5. An electrical member having a multi-layer track and arc-resistant coating deposited thereon, said coating comprising a layer derived from a composition consisting essentially of (A) from 25 to 75% of a resin and cross-linking agent mixture consisting essentially of (1) from 40 to of a viscous liquid reactive epoxy resin and (2) from 20 to 60% of an epoxy resin cross-linking agent selected from the group consisting of polyamides and amine adducts and (B) from 25 to 75% of material consisting essentially of (1) aluminum oxide trihydrate and (2) up to 30% of finely divided inert mineral filler, and a second layer consisting essentially of polymerized acrylic esters deposited over said first layer.
6. A fuse casing provided with a multi-layer track and arc-resistant coating deposited thereon, said coating comprising a first layer of the heteropolymerized product derived from a composition consisting essentially of a mixture of (A) a reactive epoxy resin, (B) a coreactive epoxy resin cross-linking agent selected from the group consisting of polyamides and amine adducts, (C) aluminum oxide trihydrate and (D) a finely divided inert mineral filler, wherein the weight proportion of epoxy resin to cross-linking agent is within the range of about 80:20 to about 40:60, the Weight proportion of aluminum oxide trihydrate and inert filler to the epoxy resin and cross-linking agent is within the range of about 25:75 to 75:25 and the weight proportion of inert filler to aluminum oxide trihydrate is up to a maximum of about 30:70, and a second layer comprising a polymerized acrylic ester deposited over said first layer.
References Cited by the Examiner UNITED STATES PATENTS 2,626,223 1/53 Sattler et al.
2,705,223 3/55 Renfrew et al. 260-18 2,768,264 10/56 Jones et al. 200-144 2,847,323 8/58 Evan et al. 117-75 2,943,953 7/60 Daniel 26018 2,955,055 10/60 Souder et al 117-75 2,990,497 6/61 Rugg 26037 2,997,527 8/ 61 Kessel et a1 260-37 XR 3,008,848 11/61 Annonio 11775 X RICHARD D. NEVIUS, Primary Examiner.
WILLIAM D. MARTIN, Examiner.
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|U.S. Classification||428/414, 428/520, 523/457|