US3776741A

US3776741A - Flame-proof protective coating for electrical film resistors

Info

Publication number: US3776741A
Application number: US00102150A
Authority: US
Inventors: L Bockstie
Original assignee: Corning Glass Works
Current assignee: Vishay Intertechnology Inc
Priority date: 1970-12-28
Filing date: 1970-12-28
Publication date: 1973-12-04
Anticipated expiration: 1990-12-04

Abstract

A coating composition capable of producing coatings on resistors which do not tend to crack during and following curing comprising a prehydrolyzed tetraalkyl orthosilicate, aluminum oxide, a suspension agent, a filler and crystalline silicon dioxide having a particle size of below 149 microns, no more than 65 percent of the silicon dioxide having a particle size less than 44 microns (pass 325 mesh), and resistors coated therewith.

Description

United States Patent Bockstie, Jr. *Dec. 4, 1973 FLAME-PROOF PROTECTIVE COATING FOR ELECTRICAL FILM RESISTORS [56] References Cited [75] Inventor: Lawrence G. Bockslie, Jr., Bradford, UNITED STATES PATENTS 3,356,515 12/1967 McGlothlin 1l7/135.1 X [73] Assignee: Corning Glass Works, Corning, 3,392,036 7/1968 McLeod 106/1 N Y 3,412,063 11/1968 Jarboe et a1. 106/3835 X 3,421,909 1/1969 Rusher 106/14 1 Notice: The portion of the term of this patent u s q en 10 A g 1938, Primary Examiner-Lorenzo B. Hayes has been dlsclalmed- Attorney-Sughrue, Rothwell, MiQn Zinn &Macpeak V [22] Filed: Dec. 28, 1970 211 App]. No.: 102,150 [57] ABSTRACT A coating composition capable of producing coatings Related Apphcauon Data on resistors which do not tend to crack during and fol- Continuation-impart of $611 April lowing curing comprising a prehydrolyzed tetraalkyl 1968 abandonedorthosilicate, aluminum oxide, a suspension agent, a filler and crystalline silicon dioxide having a particle [52] 106/15 106/287 117/137, size of below 149 microns, no more than 65 percent of 252/8-1 the silicon dioxide having a particle size less than 44 [51] Int. Cl C0911 5/18 microns (pass 325 mesh), and resistors coated there, [58] Field of Search 106/15 FP, 38.3, with 22 Claims, No Drawings CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of US. Ser. No. 724,220 filed Aprl. 25, 1968 now abandoned.

BACKGROUND OF INVENTION The present invention relates to coating compositions for electrical resistors, particularly film resistors. More specifically, the present invention relates to flame-resistant coatings adapted for the protection of electrical film resistors against burning and smoking due to overload.

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 theproblem, however, that these protective coatings very often burn and are destroyed due to the heat resulting from severe overloadson the film resistors. 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-proof coating,

flame-retardants to film' resistor-protective coatings.

Theseeffortsghowever, have consistently met with failure. Many of these conventional flame-retardants were ineffective to inhibit burning at the extremely high temperatures (400-600 C) attained in film resistors under high overload.

Recently a protective coating for resistors has been suggested comprising a tetraalkyl orthosilicate, aluminumoxide, a suspensionagent and various fillers and pigments, including titanium dioxide. Although this composition provides a flame-resistant coating it is subjectto the disadvantage that, during and following ap plication and curing the coating tends to crack thereby exposing the coated substrate.

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

A further object of the present invention is to provide a flame-retardant coating composition suitable for the protection of film resistors which will not burn or smoke at the high temperatures attained in film resistorsxupon severe overload.

A further object of the present invention is to provide a flame-retardant coating composition suitable for the protection of film resistors which inhibits external arcing and aids in opening the overloaded circuit.

A further object of the invention is to provide a composition capable of yielding coatings which do not tend to crack during and following application and curing.

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

BRIEF DESCRIPTION OF THE INVENTION The coating composition of the present invention comprises an at least partially pre-hydrolyzed tetraalkyl orthosilicate, aluminum oxide, a suspension agent, inorganic fillers and pigments and crystalline silicon dioxide. Where desired, a solvent may also be added.

Applicant has found that coatings prepared from tetraalkyl orthosilicate and aluminum oxide coating compositions may be prevented from cracking by the addition to the coating composition of a special form of silicon dioxide having a certain particle size. Thus, coatings prepared from the compositions of the present invention are not only flame-proof and arc-resistant, but are also stable against cracking.

I am aware that silica (silicon dioxide) has been previously suggested for incorporation in resistor coating compositions comprising a tetraalkyl orthosilicate, aluminum oxide and fillers. This is not to be confused with the present invention, however.

The silica previously suggested for addition to coating compositions similar to that of the present invention is amorphous, small-particle size silica" and is incorporated in the composition for its ability to func tion as a suspension or thixotropic agent. These silica compositions have a particle size in the sub-micron or colloidal range and are amorphous.

These silica compositions have a physical structure and particlesize which render them ineffective for the purposes of the present invention. These silica" compositions have no effect on the tendency of coating compositions to crack during and following application to resistors and curing.

l have found that only crystalline silicon dioxide having a certain particle size possesses the unique ability to prevent cracking of the above-described coating compositions, i.e., wherein no more than percent of the silica has a particle size less than 44 u, the balance being in the range 149 u to 44 u.

DETAILED DESCRIPTION OF THE INVENTION Generally, any tetraalkyl orthosilicate may be employed for the purposes of the present invention. It is preferred to employ the tetralower alkyl orthosilicates such as the methyl, ethyl, propyl, isopropyl, butyl, isobutyl, etc., esters. It is especially preferred to employ partially hydrolyzed tetraethyl orthosilicate. The degree of hydrolysis of the orthosilicate is not overly critical. Generally, the degree of hydrolysis may vary from about 10 percent to about percent. Generally, amounts of the orthosilicate ranging from about 5 percent to about 55 percent based on the total composition may be employed. It is to be understood that by partially hydrolyzed" is also meant the in situ hydrolysis of a tetraalkyl orthosilicate during the coating or curing operation. Commercially available products,

i.e., Silbond H-6, as used in the Examples, available from the Stauffer Chemical Company, can be used as the prehydrolyzed tetraalkyl orthosilicate. This contains 19% SiO by weight.

The amount of aluminum oxide employed is not always critical. At least 4% Al O is required, and generally, amounts ranging from about 4 percent to about 93.9 percent may be employed.

Any conventional suspension agent may be employed for the purposes of the present invention. Examples of suitable suspension agents are the organic derivatives of the various montmorillonites, etc., commonly known as the Bentones, e.g., Bentone 27, an alkyl amine derivative of magnesium montmorillonite. Additional suspension agents include amorphous or colloidal silica, clays such as the montmorillonites themselves, diatomaceous earths such as kieselguhr, finely divided mica and finely divided aluminum silicate, preferably submicron in particle size. Generally, the organic derivatives of the montmorillonites are the preferred suspension agents. The suspension agent may be employed in an amount ranging from O.l to 2.0 percent. The suspension agent should not affect the pH of the coating. This is important for good shelf life of the coating. Another suspension agent is Attagel. This also is colloidal silica.

Any conventional inorganic filler and/or pigment material may be added to the coating compositions of the present composition. It will be obvious that many pigments, e.g., TiO also serve as fillers. In such a case, they can be considered fillers with a coloring property. Generally speaking, conventional particulate nonsoluble pigments serve as fillers, and shall be considered to function as such in this invention. These fillers and/or pigments can be used in any desired blending ratio to form the filler and/or pigment blend and still perform their pigmenting or filling function. For instance, in the examples the pigment blend is selected to give the desired color. A preferred filler and pigment material is titanium dioxide. TiO acts as a functional filler and pigment, i.e., in addition to operating as a filler and pigment it aids in the processing and handling of the composition by operating as a suspension agent. Generally, amounts of titanium dioxide ranging from about 1 percent to about 45 percent may be employed; Any additional conventional pigments such as cobaltous aluminate (gives a blue color), Cr o (gives a green color), Fe,o,, Fe O lampblack, zinc oxide, cadmium oxide, cadmium sulfide, cadmium .selenide, etc. may be added to the coating composition. Generally, amounts of pigments and/or fillers bringing the total amount of the ingredients in the composition up to about 100 percent may be added to the coating composition. The term filler and/or pigment", as used in this application, does not include the orthosilicate, aluminum oxide, or silicon dioxide of the present invention.

Sometimes fillers and pigments can serve the further function of being a suspension agent. Preferably pigments and fillers mentioned above are not usually used as suspension agent.

The nature of the solvent employed is not overly critical. The only requirement for the solvent is that it be inert with respect to the other coating ingredients. A preferred solvent is isopropanol. However, other conventional solvents which may be used include: the lower alkanols such as ethanol, propanol, etc., ketones such as methyl ethyl ketone, dimethyl formamide, diacetone alcohol, dimethyl sulfoxide, pyridine, furan, furfuryl alcohol, trichloroethane, N-methyl pyrrolidone, iso-octane, hexane, etc. Amounts of solvent ranging from 0 to 20 percent may be employed, preferably 10-20 percent.

The amount of silicon dioxide necessary for the prevention of cracking in the resulting coating ranges generally from about 1 percent to about 48 percent. It is to be understood, however, that the addition of any amount of silicon dioxide to the coating compositions of the present invention will have some effect on the tendency of the coating to crack.

As discussed above, I have found that the tendency of the described coating compositions to crack during and following curing is inhibited only upon the incorporation therein of certain forms of silicon dioxide having a specified particle size.

More specifically, l have found that employing crystalline silicon dioxide having particle sizes in the range of from about 149 p. (pass mesh) to 44 u (325 mesh) produces coatings which do not crack during application and during or following curing, so long as no more than 65 percent of the silicon dioxide has a particle size less than 44 1.. For example, if 66 percent of SiO has a particle size less than 44 u, cracking results. However, 100 percent of SiO can have a particle size equal to 44 p. or retained on 325 mesh and the object of the invention is attained. If only 66 percent of the SiO is less than 44 y. there is slight cracking; as the percentage increases, cracking also increases.

When employing silicon dioxide wherein greater than 65 percent by weight was of a particle size less than 44 1.4., the coatings produced were found to have cracks. Particle sizes greater than about 149 p. tend to produce coatings which are not easily processable and over which smooth overcoating is difficult.

Expanding upon the above, to obtain a crack, blister, pinhole and crater free cured coating, one cannot use crystalline silicon dioxide wherein greater than 65 percent by weight of said crystalline silicon dioxide is fines, that is, crystalline silicon dioxide having a particle size less than 44 p. (which will pass a 325 mesh screen). Preferably no more than 55 percent, most preferably 45 percent, of such fines, if present, are used.

To avoid any fines problem, one can use crystalline silicon dioxide of a single particle size, for instance 100 mesh pass (substantially all about 149 u or less) 200 mesh pass (substantially all about 74 p. or less) or 300 mesh pass (substantially all about 49 u or less). Thus, a preferred particle size range for the crystalline silicon dioxide is all particles being 100 mesh (149 p.) to 300 mesh (4914.). Needless to say, any particle size distribution within this 100 to 300 mesh range (149 to E 49 p.) can be used.

However, it shall be understood it is possible to use crystalline silicon dioxide having a particle size distribution of 149 p. down to even I p. or smaller, so long as no more than 65 percent by weight of said crystalline silicon dioxide is fines, i.e., smaller than 44 p, which passes a 325 mesh screen. See Examples VI and VII.

However, submicron sized crystalline silicon dioxide should be avoided for certain applications. While 1 percent by weight would not be harmful for most applications about 5 to 10 percent by weight of submicron sized silicon dioxide should be avoided. This proportion of extreme fines would prohibit the later application of an acceptable high temperature resistant (250 C) smooth overcoat of, e.g., a methyl phenyl polysiloxane such as Dow Corning 2103, available as a 60 percent resin 40 percent solvent mixture. While most preferably no submicron silicon dioxide is present, many commercial sources will provide mixtures containing small amounts of such.

Many commercial sources provide crystalline silicon dioxide which is more inexpensive if a particle size distribution is ordered as compared to relatively expensive crystalline silicon dioxide of one size. Accordingly, such commercial products can be used so long as they meet the heretofore set forth criteria.

In summary, the crystalline silicon dioxide used in this invention; W W

1. must contain at least 35 percent by weight, based on the total amount of crystalline silicon dioxide, of crystalline silicon dioxide having a particle size of from about 149 p. (100 mesh) to no smaller than 44 p. (retained on 325 mesh);

2. of the maximum 65 percent, preferab1y45 55 percent, of fines (pass 325 mesh), no more than 5-10 percent; most preferably 1 percent to none, should be submicron;

3. to avoid fines problems, preferably all the crystalline silicon dioxide should have a size in the range 100 mesh (149 p.) to 300 mesh 49 u).

The crystalline silicas are well known in the prior art and include fused silica, silicaflour," ground silica, powdered silica, sand, etc.

In my copending application, Ser. No. 724,271, filed Apr. 25, 1968, l have described the desirable results produced by utilizing titanium dioxide and a suspension agent in similar coating composition which are in a noncowled condition and have a particle agglomerate size between 1 and 150 microns. Briefly, the utilization of these materials greatly increases the potlife and enhance the processing of these compositions.

It is also disclosed therein that adjusting the alkali metal ion content of the composition such that a coating produced therefrom contains, after curing, less than 0.02% Na, 0.01% K, 0.01% Li, 0.001% Cs and 0.001% Rb, (as their oxides) enhances themoisture resistance of the resistance of the coatings.

It .is to be understood that the disclosure of application Ser. No. 724,271 is incorporated herein by reference.

The compositions of the invention may be coated according to any of the conventional, well known methods: e.g., dip, spray, brush, roller, etc. Generally, sufficient material is applied to the resistor to yield a final cured coating of between about 5 and mils, preferably 5 and 7 mils thick. It is especially preferred, although not mandatory, to apply two coats to the resistor. The applied coating is cured by heating, preferably over an extended period of time at gradually increasing temperatures up to about 250 C.

1n the specification and appended claims, all percentages are by weight unless otherwise indicated.

The invention will be illustrated by the following nonlimiting examples.

EXAMPLE 1 A batch of the coating composition of the present invention was prepared by mixing the following ingredicuts in the indicated amounts.

Aluminum Oxide 4,540 grams EXAMPLE II A glass-tin oxide 1 K film resistor having a rated power of 4 watts was double coated with the composition of Example I and cured at 250 C to yield a com bined coating 10 mils thick.

No cracking of the coating either during application, curing or following curing was observed.

The coated resistor was subjected to an overload of 100 times rated power. The resistor did not burn or smoke. Moreover, there was no external arcing and the circuit containing the resistor opened upon overload.

EXAMPLE III A coating composition was prepared by mixing together the following ingredients:

Aluminum oxide 4,540 grams Silicon dioxide (crystalline, 3100 mesh) 460 grams Titanium dioxide 635 grams Prehydrolyzed tetraethyl orthosilicate 900 grams Cab-O-Sil (amorphous submicron silica suspension agent, Cabot Corp.) grams Ethanol, anhydrous 200 grams Propanol, anhydrous 272 grams Pigment (cobaltous aluminate Cr O 18:82 percent by weight) 300 grams 1 K glass-tin oxide film resistors having a rated power of 1 watt were coated with the above composition as in Example ll. The coating did not crack either during or following curing or after multiple handling operations. Upon being subjected to a times rated power overload no burning or smoking of the coating was ob served. No external arcing was evident and the resistor opened the circuit. a 49 [1- EXAMPLE IV A coating composition was prepared by blending the following ingredients together:

Aluminum oxide 4,300 grams Silicon dioxide (crystalline, 100 mesh) 700 grams Titanium dioxide 635 grams Prehydrolyzed tetrapropyl orthosilicate 500 grams Prehydrolyzed tetraethyl orthosilicate 500 grams Suspension agent (Bentone 27)-- 90 grams Isopropanol, anhydrous 472 grams Pigment (cobaltous aluminate c o,

cent by weight 300 grams 5 K glass-tin oxide film resistors having a rated power of 3 watts were coated with this composition as in Example II. No cracking of the coating was observed during or following curing.

Upon being subjected to an overload of 100 times rated power no flaming, smoking or external arcing was 18:82 percontaining no silicon dioxide. '2 149 ,t

EXAMPLE V EXAMPLE VI The exact procedure of Examples 1 and ll was followed but substituting for the 100 mesh crystalline silicon dioxide therein the following crystalline silicon dioxide:

Particle Size (Mesh) Percent By Weight l49y.(l00) I 125 ,u. 120 96 I05 ;I. (140 90 74 p. 200 75 53 p. 270 55 44 ,1 325 40 and using an amount of amorphous silica as in Example lll as the suspension agent the results obtained were equivalent to those of Examples I and II.

EXAMPLE VII The exact procedure of Examples l and II was followed but substituting for the 100 mesh crystalline silicon dioxide therein the following crystalline silicon dioxide:

Particle Size (Mesh) Percent By Weight and using an amount of amorphous silica as in Example III as the suspension agent the results obtained were equivalent to those of Examples l and II.

EXAMPLE VIII The exact procedure of Example VII was followed but 90 percent of the crystalline silicon dioxide had a size less than 44 p. (pass 325 mesh), i.e., 90 percent fines. The coating cracked and blisters, craters and pinholes formed. All were visible to the eye, and the substrate could be seen through some cracks with a microscope.

EXAMPLES IX and X Examples ll and [V were duplicated but using kieselguhr as the suspension agent. Equivalent results were obtained.

A spectrographic Analysis of Bentone 27" gives the following result:

(All components expressed as oxides, weight percent) CuO .00l-.00l

BeO .0005-.0O5

What is claimed is:

l. A flame-retardant coating composition suitable for the protection of resistors consisting essentially of a nonconductive, noncracking mixture of:

a. from about 5 percent to about 55 percent of an at least partially hydrolyzed tetraalkyl orthosilicate binder with one-four carbon atoms in each alkyl group thereof, the degree of hydrolysis of said orthosilicate being from about 10 percent to about percent;

b. from about 4 percent to about 93.9 percent aluminum oxide;

c. from about 0.1 percent to about 2 percent of a suspension agent;

d. a material selected from the group consisting of non-metallic inorganic fillers, pigments and mixtures thereof, and

e. from 1 percent to 48 percent of crystalline silicon dioxide having a particle size in the range of less than about 149 u, no more than 65%, by weight, of the crystalline silicon dioxide having a particle size below 44 11. (pass 325 mesh), which silicon dioxide prevents cracking of the coating composition,d) being present in an amount necessary to bring the total amount of ingredients to 2. The composition of claim 1 wherein said tetraalkyl orthosilicate is tetraethyl orthosilicate.

3. The composition of claim 1 wherein said material d) includes titanium dioxide 4. The composition of claim "I, further including from 0 to 20 percent of a solvent which is inert with respect to the balance of the coating composition ingredients.

5. The composition of claim 4 wherein said solvent is isopropanol.

6. The composition of claim 1 wherein said suspension agent is selected from the group consisting of montmorillonites, amorphous silica and diatomaceous earths.

7. The composition of claim 6 wherein element (d) is a pigment.

8. The composition of claim 7 wherein the pigment is selected from the group consisting of titanium dioxide, cobaltous aluminate, Cr O F8 0,, Fe O, and lampblack.

9. The composition of claim 7 wherein element (d) is TiO, in an amount of from about 1 percent to about 45 percent.

10. The composition of claim 1 wherein the suspension agent is amorphous silica.

11. The composition of claim 1 wherein the suspension agent is diatomaceous earth.

l2. Thecomposition as claimed in claim 1 wherein no more than 45 percent by weight of the crystalline silicon dioxide has a particle size below 44 11. (pass 325 mesh).

13. The composition of claim 1 wherein said crystalline silicon dioxide has a particle size in the range of 149 p. to about 49 ,u.

14. The composition of claim 1 wherein no more than 10 percent of said crystalline silicon dioxide is submicron in size.

15. The composition of claim 1 wherein said crystalline silicon dioxide has a particle size of about 200 mesh.

16. The composition of claim 1 where the resistor is a film resistor.

17. The composition of claim 1 wherein no more than 55 percent by weight of the crystalline silicon dioxide present has a particle size below 44 1/. (pass 325 mesh).

18. The composition of claim 4 wherein from 10 to 20 percent of the solvent is present.

19. The composition of claim 14 wherein no more than 10 percent of said crystalline silicon dioxide is submicron in size.

20. The composition of claim 1 wherein said suspension agent is selected from the group consisting of clays, diatomaceous earths, mica, amorphous and colloidal silica.

21. The composition of claim 6 where element (d) is a filler.

22. The composition of claim 1 wherein no more than zero to l percent of said crystalline silicon dioxide is submicron in size.

Claims

2. The composition of claim 1 wherein said tetraalkyl orthosilicate is tetraethyl orthosilicate.
3. The composition of claim 1 wherein said material d) includes titanium dioxide.
4. The composition of claim 1, further including from 0 to 20 percent of a solvent which is inert with respect to the balance of the coating composition ingredients.
5. The composition of claim 4 wherein said solvent is isopropanol.
6. The composition of claim 1 wherein said suspension agent is selected from the group consisting of montmorillonites, amorphous silica and diatomaceous earths.
7. The composition of claim 6 wherein element (d) is a pigment.
8. The composition of claim 7 wherein the pigment is selected from the group consisting of titanium dioxide, cobaltous aluminate, Cr2O3, Fe2O3, Fe3O4 and lampblack.
9. The composition of claim 7 wherein element (d) is TiO2 in an amount of from about 1 percent to about 45 percent.
10. The composition of claim 1 wherein the suspensiOn agent is amorphous silica.
11. The composition of claim 1 wherein the suspension agent is diatomaceous earth.
12. The composition as claimed in claim 1 wherein no more than 45 percent by weight of the crystalline silicon dioxide has a particle size below 44 Mu (pass 325 mesh).
13. The composition of claim 1 wherein said crystalline silicon dioxide has a particle size in the range of 149 Mu to about 49 Mu .
14. The composition of claim 1 wherein no more than 10 percent of said crystalline silicon dioxide is submicron in size.
15. The composition of claim 1 wherein said crystalline silicon dioxide has a particle size of about 200 mesh.
16. The composition of claim 1 where the resistor is a film resistor.
17. The composition of claim 1 wherein no more than 55 percent by weight of the crystalline silicon dioxide present has a particle size below 44 Mu (pass 325 mesh).
18. The composition of claim 4 wherein from 10 to 20 percent of the solvent is present.
19. The composition of claim 14 wherein no more than 10 percent of said crystalline silicon dioxide is submicron in size.
20. The composition of claim 1 wherein said suspension agent is selected from the group consisting of clays, diatomaceous earths, mica, amorphous and colloidal silica.
21. The composition of claim 6 where element (d) is a filler.
22. The composition of claim 1 wherein no more than zero to 1 percent of said crystalline silicon dioxide is submicron in size.