Flame-resistant Resistor Coatings

Abstract

Flame resistant polyimide resistory coatings containing chlorinated polyphenyl and antimony trioxide.

Parent Case Text

This is a division of application Ser. No. 652,412, filed July 11, 1967.

Description

BACKGROUND OF THE INVENTION

The present invention relates to coating compositions for electrical resistors. More specifically, the present invention relates to flame-resistant organic coatings adapted for the protection of electrical film resistors against burning due to severe overload (up to 100.times. rated power).

THE PRIOR ART

Recently, electrical film resistors have come into wide use. Normally, these film resistors comprise a substrate such as glass, coated with a thin film of resistor material such as tin oxide, for example. Also, it is known in the art to overcoat these film resistors with a protective layer. The prior art is faced with the problem, however, that these protective coatings due to the heat resulting from severe overloads on the film resistor, very often burn. This burning of the resistor coating not only results in the destruction of the resistor itself but, very often, results in damage to adjacent elements in the system in which it is employed. The consequent damage to electrical equipment and systems from this burning of resistor coatings has led to an intensive search for a flame-resistant coating, which will not burn due to the heat arising from severe overloads on the resistor.

The film resistors containing the protective coatings of the prior art suffer from the further disadvantage that upon severe overload the protective coatings not only burn, but become electrically conductive. This very often results in external arcing and a failure of the resistor to open the circuit in which it is contained. Accordingly, an intensive search has been conducted for a film resistor protective coating which will not burn under severe overload, and also cause the resistor to open the circuit in which it is contained.

It has been proposed to add several conventional flame-retardants to film resistor protective coatings. These efforts, however, have consistently met with failure. One drawback common to most of the conventional flame-retardant coatings is the fact that they become electrically conductive when the resistor is severely overloaded. This causes the resistor to burn, promotes severe external arcing and does not permit the resistor to open the circuit in which it is contained. Moreover, many of these conventional flame-retardant coatings were ineffective to inhibit burning at the extremely high temperatures (500.degree.-600.degree.C) attained in film resistors under severe overload.

An object of the present invention is to provide a flame-resistant organic coating composition suitable for the protection of film resistors.

A further object of the present invention is to provide a flame-resistant coating composition suitable for the protection of film resistors which will not burn at the high temperature attained in film resistors upon severe overload.

A further object of the present invention is to provide a flame-resistant coating composition suitable for the protection of film resistors which is resistant to burning and which at the same time promotes the destruction of the resistor during severe overload and thereby opens the circuit.

A further object of the present invention is to provide a flame-resistant coating composition suitable for the protection of film resistors which is a stable mixture, with long pot life and which forms a homogeneous, uniform layer upon coating film resistors.

A further object of the present invention is to provide a flame-resistant coating composition suitable for the protection of film resistors wherein the flame-resistant additive is compatible with the protective coating material and does not alter the chemical or dielectric properties thereof.

BRIEF SUMMARY OF THE INVENTION

The objects of the present invention are achieved by the provision of a film resistor protective coating composition comprising a polyimide resin containing from about 1 to 60 percent, preferably from about 2 percent to about 10 percent of a chlorinated polyphenyl and from about 1 percent to about 50 percent, preferably from about 2 percent to about 10 percent of antimony trioxide.

It has recently been discovered that polyimide resins are highly suitable as coating compositions for the protection of film resistors due to their exceptional dielectric properties, their hardness and solvent resistant. Unfortunately, these polyimide resins are suspceptible to burning, at the high temperatures attained in film resistors due to severe overloads much in the same manner as described above. Applicant has found that the addition of from about 1 percent to about 60 percent of chlorinated polyphenyl and from about 1 to 50 percent of antimony trioxide to a polyimide resin results in a coating composition having vastly improved flame-resistant properties. Thus, the coating compositions of the present invention are stable against burning, when resistors containing these coatings are placed under severe overload.

Moreover, the coating compositions of the present invention possess the additional property of being somewhat intumescent. This results in the phenomenon that, frequently, upon the film resistor attaining a high temperature due to severe overload, the coating of the present invention will swell and form a protective cocoon. This cocoon adds further to the protective qualities of the coating of the present invention in that it confines the external arcing. No other coating composition has been found which possesses this intumescent property. It is noteworthy that the polyimide resin coatings alone, i.e., without the addition of chlorinated polyphenyl or antimony trioxide, do not possess intumescent properties. Furthermore, the coating compositions are homogeneous and have long pot lives, unlike the heretofore employed flame-resistant coating compositions .

DETAILED DESCRIPTION OF THE INVENTION

The polyimide resins suitable as a base material for the coating compositions of the present invention are described in U.S. Pat. No. 3,179,634. Generally, these cured resins are characterized by a recurring unit having the following structural formula: ##SPC1##

Wherein R is a tetravalent aromatic radical, preferably containing at least one ring of six carbon atoms, said ring characterized by benzenoid unsaturation, the four carbonyl groups being attached directly to separate carbon atoms in a ring and each pair of carbonyl groups being attached to adjacent carbon atoms in a ring of the R radical; and wherein R' is a divalent benzenoid radical containing at least two rings of six carbon atoms, each ring characterized by benzenoid unsaturation, and in which no more than one of the valence bonds is located on any one of said rings of said R' radical.

The most preferred of the polyimides is duPonts' RK-692, which is a polyimide according to the above structural formula wherein R is ##SPC2##

and wherein R' is ##SPC3##

Generally, these polyimides are prepared by reacting an appropriate organic diamine with a suitable tetracarboxylic acid dianhydride. This and other processes for the preparation of these polyimides are more specifically described in U.S. Pat. No. 3,179,634.

Generally, polyimides wherein the degree of polymerization is controlled such that the resin has an inherent viscosity from about 30 to about 90 poises when measured at 25.degree.C. are suitable as base materials for the coating compositions of the present invention.

As mentioned above, the desirable flame-resistant and intumescent properties are imparted to the polyimide resin base material by incorporating therein from about 1 percent to about 60 percent of a chlorinated polyphenyl and from about 1 percent to about 50 percent of antimony trioxide.

The chlorinated polyphenyls are well known in the art, it being preferred that a chlorinated polyphenyl like Aroclor 5460, made by Monsanto, or similar chlorinated polyphenyls having about the same degree of polymerization and degree of chlorination be employed as one of the flame-resistant additives of the present invention.

Suitable chlorinated polyphenyl resins are those sold under the name Aroclor. These materials are prepared by the chlorination of crude biphenyl and are resinous materials possessing high melting points and containing from 18 to 66 percent chlorine. Useful chlorinated polyphenyl resins include Aroclor 1260, Aroclor 1262, Aroclor 1268, Aroclor 1270, Aroclor 4465, Aroclor 5442, Aroclor 2565, and Aroclor 5460, the most preferred being the latter, which has a softening point of about 100.degree. - 105.degree.C.

The incorporation of chlorinated polyphenyl and antimony trioxide into the polyimide base material may be effected by any of the well known, conventional methods.

Film resistors are well known in the art and are generally manufactured as follows. A suitable substrate, such as glass, after being appropriately sensitized according to methods well known in the art is heated to a high temperature and coated with a thin spray of a solution of stannic chloride. Upon contact of the stannic chloride solution with the hot glass surface, the stannic chloride is pyrolized to tin oxide which deposits out as an extremely thin uniform layer on said glass surface. This tin oxide layer acts as the resistor material. It is to be understood, however, that the coating compositions of the present invention are applicable for the protection of any resistor regardless of its method of preparation, composition, shape or structure.

The coating compositions according to the present invention may be applied to the film resistor by any of the well known coating methods. For example, the composition may be solvent sprayed onto the film resistor, or the film resistors may be dip-coated by immersion thereof in a suitable solution of the coating composition. The coatings may also be applied by brush or roller coating applications. It is to be further understood that the invention is not limited to any particular method of applying the novel coating compositions of the present invention and that any of the conventional, well known coating methods may be employed to effect the deposition of the flame-resistant coating composition on the resistor.

Suitable solvents for use in conjunction with the coating compositions of the present invention are any organic liquids capable of dissolving the components of the composition and which are chemically inert with respect thereto. Suitable solvents include the liquid dialkylcarboxylamides such as N,N-diethylacetamide and dimethyl sulfoxide, and N-methyl-2-pyrrolidone.

It should also be understood that the coating compositions may be modified with inert pigments, dyes, inorganic and organic fillers as desired. For example, titanium dioxide, carbon, earth pigments or mixtures thereof may be added to the compositions to provide desired colors. Fillers such as asbestos, bentonites, silica, etc. may also be incorporated.

The invention is more specifically explained by the following illustrative examples which are not necessarily intended to be limiting. In these examples, all parts and percentages are by weight except as noted.

EXAMPLE I ______________________________________ Parts ______________________________________ Polyimide (DuPont RK-692) 100 Chlorinated Polyphenyl (Monsanto Arochlor 5460) 10 Antimony Trioxide 5 ______________________________________

A dimethylformamide solution of the above composition was solvent sprayed onto a conventional film resistor. The resistor was subjected to a severe electrical overload resulting in a body temperature of between 500.degree. and 600.degree.C. No burning, smoking or flaming of the coating was detected. Moreover, the coating intumesced to form a protective cocoon around the resistor thereby confining arcing and causing the resistor to open.

The following examples illustrate other coating compositions which were coated onto resistors as in Example I and operated similarly to protect the resistor.

EXAMPLE II ______________________________________ Parts ______________________________________ Polyimide (DuPont RK-692) 100 Chlorinated Polyphenyl (Monsanto Aroclor 5460) 20 Antimony Trioxide 10 ______________________________________

EXAMPLE III ______________________________________ Parts ______________________________________ Polyimide (DuPont RK-692) 100 Chlorinated Polyphenyl (Monsanto Aroclor 5460) 10 Antimony Trioxide 10 ______________________________________

The invention herein disclosed may be variously modified and embodied within the scope of the following claims.

Claims

1. A coated electrical resistor comprising a resistance layer coated with a stable, intumescent coating composition consisting essentially of a polyimide resin containing from about 1 percent to about 60 percent of a chlorinated polyphenyl and from about 1 percent to about 50 percent antimony trioxide, all percentages being weight percentages of the amount of polyimide resin, which coated electrical resistor upon electrical

2. The resistor of claim 1 wherein said coating composition contains from about 2 percent to about 10 percent of a chlorinated polyphenyl and from

3. The resistor of claim 1 wherein said polyimide resin is one having the recurring unit: ##SPC4##

wherein R is a tetravelent radical containing at least one ring of six carbon atoms, said ring characterized by benzenoid unsaturation, the four carbonyl groups being attached directly to separate carbon atoms in a ring and each pair of carbonyl groups being attached to adjacent carbon atoms in a ring of the R radical; and wherein R' is a divalent benzenoid radical containing at least two rings of six carbon atoms, each ring characterized by benzenoid unsaturation, and in which no more than one of the valence

4. The resistor of claim 1 wherein said chlorinated polyphenyl is one

5. The resistor of claim 3 wherein said coating composition contains 20 percent chlorinated polyphenyl containing from 18 to 66 percent chlorine

6. The resistor of claim 1 comprising a substrate having coated thereon a

7. The resistor of claim 6 wherein said substrate is glass.