US2879827A

US2879827A - Insulating material

Info

Publication number: US2879827A
Application number: US461650A
Authority: US
Inventors: Jr William Raymond Bowditch
Original assignee: Taylor Fibre Co
Current assignee: Taylor Fibre Co
Priority date: 1954-10-11
Filing date: 1954-10-11
Publication date: 1959-03-31
Anticipated expiration: 1976-03-31

Description

INSULATING MATERIAL No Drawing. Application October 11, 1954 Serial No. 461,650

23 Claims. (Cl. 154-25) This invention relates to phenolic resin insulation material having extremely high insulation resistance which is retained under high humidity conditions. While the utility of the material is particularly derived from such high insulation resistance properties, it is capable of other uses where high insulation resistance may be relatively unimportant. While the material is produced through the use of a laminating procedure, the final material is homogeneous to a high degree.

Phenolic resin-paper laminates are in wide use in electrical apparatus to serve as supports or merely for the insulation of conductive elements of electrical apparatus. Provided generally in sheet or block form, such material is punched, or otherwise machined, to provide the necessary mechanical configurations for supporting and/ or insulating use. Metallic elements of electrical circuits are secured thereto by riveting, clamping or other means and, in recent years, sheets of such material have been used as bases for printed circuits. In most such uses, high insulation resistance is desired, and particularly the maintenance of high insulation resistance under high humidity conditions. The merit of such materials has generally been tested by providing electrodes in three-sixteenth inch holes in the material having one inch center to. center spacing in a two inch by three inch sheet regardless of thickness. The majority of such materials used in the past show, under such test, a very high resistance of the order of 500,000 megohms under dry conditions but with considerable loss of resistance with humidity, the resistance dropping, for example, to 1000 megohms. Prior to the present invention, the best materials from an insulation standpoint attained a resistance under such test of about 70,000 megohms under 90% humidity conditions. In accordance with the present invention, there may be attained under 90% humidity conditions resistances in such test ranging from 200,000 to 1,000,000 megohms. The material provided in accordance with the invention shows very little deterioration with moisture and quick recovery on drying in a dry atmosphere. It has been commonly assumed in the past that high insulation resistance is dependent upon very low water absorption. The present material has low absorption as compared with the majority of other insulation materials of equivalent uses presently on the market, but the water absorption is not, in contrast with such materials, extremely low. Actually, the improved material shows a much higher resistance under high humidity conditions than materials having substantially lower water absorption. It may, therefore, be considered open to question whether water absorption is a prime factor in determining maintenance of high resistance under humidity conditions. 1

The objects of the invention relate to attainment of high insulation material, as already indicated, and particularly material which is susceptible to punching for the purpose of formulation of insulating supports or other elements in electrical apparatus, including the formation of insulating bases for printedcircuits. These gennited States Patent eral objects as well as others which will appear hereafter, and the attainment of the results of the invention, may be best made clear by description of several specific formulation procedures followed by a discussion of variations which may be introduced into the formulation process.

In a typical process there is introduced into a kettle a batch of ingredients as follows:

3200 parts by weight of a commercial cresol consisting of a blend of ortho, meta and para cresols such as is commonly used in the art for the production of thermosetting phenolic resins, e.g. containing 17% o-cresol, 44% m-cresol, 25% p-cresol and 14% low boiling xylenols.

400 parts by weight of a phenol having substituted in its meta position a long chain, such phenol being derived from cashew nut shell oil and having the formula C H (OH) 'C H (This phenol is fully hydrogenated in its side chain but incompletely hydrogenated phenols may be used.)

720 parts by weight of paraformaldehyde. 1300 parts of 37% aqueous formaldehyde. 27 parts by weight of hydrated lime.

These ingredients are charged into the kettle in the order listed and the batch is held in the temperature range of 60 F. to 70 F. until a viscosity of Gardner B to E, preferably the latter, is reached. This takes a period of about five to six days. There are then added 210 parts by weight of 42% aqueous ammonium sulfide. After this addition, the batch is allowed to age for twenty-four hours and there are then added 1060 parts by weight of tricresyl phosphate and 262 parts by weight of chlorinated diphenyl havinga boiling range from 290 C. to 420 C. (By chlorinated diphenyl are meant mixtures resulting from the chlorination of diphenyl, such as the Aroclors marketed by Monsanto Chemical Company.) The last two ingredients are not soluble in the mixture theretofore in the kettle so that the batch must be agitated continuously to keep them suspended.

The resulting composition is then coated on an absorbent rag paper under conditions which may be typically the following:

The paper is advanced at the rate of 20 to 30 feet per minute. Heat is applied so that at the entrance end of the coater the temperature is approximately 250 F. and at the dry end the temperature is about 325 F., the coating being applied to the paper while moving to get a minimum of softening, while flashing on the water in the coater, to maintain strength. This procedure typically gives a resin content of 62% by weight, this being the amount of resin which may be exuded under application of heat and pressure.

The resulting impregnated paper sheets may be superimposed to secure the desired ultimate thickness of the product and are pressed at 1800 pounds per square inch at a temperature of 275 to 350 F. for a period ranging from 50 to 60 minutes. The finished product, if about in thickness, is then subjected to a temperature of 275 to 300 F. for a period of about 1 /2 hours to obtain the optimum electrical properties. If the finished product is thicker the time of this last treatment must be increased.

As a variation of the foregoing there may be charged into the kettle 4500 parts by weight of the cresol mixture, 1017 parts by weight of paraformaldehyde, 1278 parts by weight of aqueous 37% formaldehyde, and 31 parts by weight of hydrated lime. With this composition the batch is held in the range of 60 F. to F. until a viscosity of Gardner A to D (preferably about Gardner C) is reached. There are then added 240 parts by weight of 42% aqueous ammonium sulfide and the batch is held forat least 24 hours before it is used. Following this there are added to 1000 parts by weight of the foregoing batch 168 parts by weight of tricresyl phosphate and 42 parts by weight of chlorinated diphenyl of the type describedabove. Agitation and coating may be carried out as described previously with the attainment of an exudable resin content in the rag paper of 61 to 63%. The final pressing after lamination is carried out as described above followed by similar curing.

While reference has been made to a laminating procedure involving the superposition of the resin coated sheets followed by pressing at elevated temperatures, in the final product the fibrous structure disappears, there being little evidence of lamination in the final product which is translucent. The electrical properties are uniform throughout the final product, i.e. there is not merely a facing sheet having the electrical resistance properties as in some prior laminated resinous products.

Numerous variations of the procedures described may be adopted. First there is a substantial range of varied compositions of the phenol or phenol mixture which may be used in the batch and typified above by the cresol mixture. Numerous mixtures of ortho, meta and para cresols may be used or meta cresol may be used alone or with only one of the pair of ortho and para cresols. Desirably, however, when cresols are used, meta cresol is present in substantial amounts. Various xylenols and resorcinols as well as other phenols may be used in admixture with cresols to give highly satisfactory results. However, with phenol alone the results are not impressive, and it appears that the higher substituted phenols are required to secure results which may be considered outstanding from the standpoint of securing products having high insulation resistance.

It will be noted that in the above described formula tions both commercial 37% aqueous formaldehyde and paraformaldehyde are added. For best results, a formaldehyde concentration higher than the commercial 37% aqueous solution is desirable and this concentration is increased by the use of the paraformaldehyde which represents 90 to 92% available formaldehyde. An appreciable amount of Water is desirably present, and for this reason it is desirable to have present some substantial amount of the aqueous formaldehyde as well as paraformaldehyde.

In the first formulation mentioned above, reference has been made to the presence in the mixture of a long chain meta substituted phenol. This is a desirable but not a necessary constituent of the batch and other desirable added .phenols comprise such substituted ones as para nonyl phenol, paratertiary 'butyl phenol, or the like. The long chain substituted phenol is added for internal plasticizing so that the product may be punched. Such phenols do not modify substantially the electrical properties.

The hydrated lime, for which may be substituted barium or strontium hydroxide, apparently acts primarily as a catalyst and may be a commercial hydrated lime but, preferably, one which is high in calcium content, for example, containing around 90% calcium hydroxide and 10% or less of magnesium oxide. The amount of hydrated lime maintains the pH of the mixture in the range of about 8.5 to 9.8. If the pH is below 8.5, the reaction takes place but becomes impractically slow. The temperature is desirably held in the range from about 60 F. to 80 F. Desired in this reaction is the formation of phenol alcohols without resinification. If the temperature rises too high, resinification takes place whereas this is desired to a substantial degree only following the addition of sulfide. The control by determination of viscosity gives a good indication of the phenol alcohol formation and, in general, viscosities of Gardner A to E indicate the completion of the preliminary reaction, the particular viscosity used as an indication being dependent upon the particular materials involved. As

used in this specification and claims the term phenol alcohols is used to designate the products formed by the reaction of phenols and formaldehyde under conditions typified 'by those described above.

It may be noted that in making generally similar products in the past the reaction has not been controlled in the fashion outlined to end with the formation of phenol alcohols, but rather the reaction has been carried out to secure substantially complete resinification. I

An essential part of the procedure is the involvement of the addition of sulfide for completing the reaction. The particular sulfide added is not important from the reaction standpoint but to maintain the highest insulation resistance it is desirable not to incorporate metallic ions. When maximum insulation resistance is desired, therefore, it is advantageous to avoid the use of metal sulfides such as those of sodium or potassium. Using ammonium sulfides, the ammonia disappears in the coating or other subsequent heating leaving possibly conductive ions absent. Hydrogen sulfide is the essential addition and since addition of this in gaseous form would necessitate closed containers and difficulties of introduction, the practical way to add sulfide is through an ammonium sulfide. Desirably, this is done by adding aqueous ammonium sulfide which represents a high percentage of available hydrogen sulfide. However, it is possible to use yellow sulfides, i.e. various polysulfides. Furthermore, hydrogen sulfide may be introduced, though as stated, this involves some procedural disadvantages.

The reactions which occur when the ammonium or other sulfide is added are difficult to determine. However, the products formed from that reaction, in conjunction with the water present, have a very definite aflinity for cellulose and seem to attack and swell cellulose and change it markedly. The same reactions do not take place when ammonium sulfide is added to a cresolformaldehyde resin made by the normal procedure of refluxing, even when the water, which was present in the formaldehyde, is not removed.

Tricresyl phosphate acts not only as a plasticizer but also appears to lower the water absorption characteristics of the final product though excessive amounts are to be avoided. In place of tricresyl phosphate there may be used triphenyl phosphate, cresyl diphenyl phosphate, chlorinated triphenyl phosphate, chlorinated tricresyl phosphate, chlorinated diphenyl of the type mentioned above, or xylenyl phosphates. Phthalate plasticizers used alone are ineffective but they may be added in order to improve the punching characteristics of the final product. The tricresyl phosphate or the equivalent is not merely used for its plasticizing action but gives flame retarding results as well as proof against fungus growth and high temperature stability as well as aiding in maintenance of the emulsion which is applied to the paper. Melamine resins may be used in conjunction with tricresyl phosphate or another plasticizer to impart properties of higher are resistance and flame retardancy without affecting other electrical properties. The addition of chlorinated diphenyl as described above in the specific examples is advantageous but not essential, though, as above indicated, chlorinated diphenyl may be the sole plasticizer added. The amounts of plasticizers used are subject to wide variations, depending entirely upon the properties desired in the final product.

The paper which is used is desirably of thicknesses ranging from two-thousandths of an inch to twentythousandths of an inch. The water is removed from the paper in the drying oven forming part of the coater. In the coating process water is removed and the alcohols are condensed to form resins. Various papers may be used, including kraft as well as cotton rag or linter paper.

The sheets of material as taken from the press are desirably post cured in an oven as described above to remove sulphur-containing gases and to get the maximum resistance. The products provided .in accordance with the invention have a low dielectric constant, lowpower factor, and high dielectric str.:ngth. It may be here noted that dielectric constant and dielectric strength are distinct properties, the former referring to the constant which appears in determining condenser capacity, and dielectric strength referring to resistance against breakdown under high potentials. The post curing is highly desirable to secure the ultimate best qualities in all of these respects.

The temperatures involved in the coater and in the post curing and the times there involved are not critical. Desirably, however, the temperature in the coater is maintained rather high as indicated in the examples above to secure flash removal of water so as to maintain the wet strength of the sheet to avoid breakage, the water being so rapidly removed that within a short period of passage the sheet becomes sufficiently strong by reason of the resinification.

In the curing procedure, the desirable result is the elimination of sulphur gases including ammonium sulfide and the ammonia content introduced with the sulfide. The temperature involved in this post curing is subject to wide variation, lower temperatures requiring longer times and vice versa, the upper limit of temperature being determined by avoidance of heat damage by carbonization.

What is claimed is:

l. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, and adding tricresyl phosphate and chlorinated diphenyl, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

2. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufficiently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, and adding tricresyl phosphate and chlorinated diphenyl, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

3. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide over an extended period at a temperature suffieiently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

4. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

5. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufliciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, and adding a plasticizer, then coating paper therewith Within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

6. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth bydroxide over an extended period at a temperature sufficiently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said 7 mixture a material providing hydrogen sulfide, and adding a plasticizer, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

7. A method of preparing insulating material comprising maintaining an admixture. of at least one cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufficiently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets. thus coated under high pressure and at elevated temperature.

8. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, adding a plasticizer to said mixture, then coating paper therewith Within a temperature range in excess of the boiling point of water, and

, laminating sheets thus coated under high pressure and at elevated temperature.

9.A method of preparing insulating material comprising, maintaining an admixture of at least m-cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, then coating paper therewith within a temperature range in excess of the boiling point of Water, and laminating sheets thus coated under high pressure and at elevated temperature.

10. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufliciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, adding a plasticizer to said mixture, then coating paper therewith within a temperature range in excess of the boiling point of water, and

7 laminating sheets thus coated under high pressure and at elevated temperature.

11. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, aqueous formaldehyde and paraformaldehyde and an alkali earth hydroxide over an extended period at a temperature sufficiently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture a material providing hydrogen sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

12. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture an aqueous solution of ammonium sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

13. A method of preparing insulating material comprising maintaining an admixture of at least one cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscisity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture an aqueous solution of ammonium sulfide, adding a plasticizer to said mixture, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

14. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufiiciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture an aqueous solution of ammonium sulfide, then coating paper therewith within a temperature range in excess of the boiling point of water, and laminating sheets thus coated under high pressure and at elevated temperature.

15. A method of preparing insulating material comprising maintaining an admixture of at least m-cresol, formaldehyde and an alkali earth hydroxide over an extended period at a temperature sufliciently low to prevent substantial resinification until the mixture attains a viscosity in the range of Gardner A to E, thereby to form phenol alcohols, then adding to said mixture an aqueous solution of ammonium sulfide, adding a plasticizer to said mixture, then coating paper therewith within a temperature range in excess of the boiling point of water, and

laminating sheets thus coated under high pressure and at elevated temperature.

16. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the reaction of at least one phenol alcohol with a sulfide capable of providing hydrogen sulfide.

17. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the reaction of at least one phenol alcohol with a sulfide capable of providing hydrogen sulfide to which a plasticizer has been added.

18. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the interaction of at least one cresol, formaldehyde and an alkali earth hydroxide followed by reaction of the resulting mixture with a sulfide capable of providing hydrogen sulfide.

19. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the interaction of at least one cresol, formaldehyde and an alkali earth hydroxide followed by reaction of the resulting mixture with a sulfide capable of providing hydrogen sulfide to which a plasticizer has been added.

20. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the reaction of at least one phenol alcohol with ammonium sulfide.

21. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the interaction of at least one cresol, formaldehyde and an alkali earth hydroxide followed by reaction of the resulting mixture with ammonium sulfide.

22. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the interaction of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide followed by reaction of the resulting mixture with a sulfide capable of providing hydrogen sulfide.

23. An insulating material comprising the product of the lamination with the application of heat and pressure of sheets of paper impregnated with a resin produced by the interaction of at least one cresol, a phenol having an aliphatic side chain of at least four carbon atoms, formaldehyde and an alkali earth hydroxide followed by reaction of the resulting mixture with ammonium sulfide.

' References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A METHOD OF PREPARING INSULATING MATERIAL COMPRISING MAINTAINING AN ADMIXTURE OF AT LAST ONE CRESOL, A PHENOL HAVING AN ALIPHATIC SIDE CHAIN OF A LAEAST FOUR CARBON ATOMS, FORMALDEHYDE AND AN ALKALI EARTH HYDROXIDE OVER AN EXTENDED PERIOD AT A TEMPERATURE SUFFICIENTLY LOW TO PREVENT SUBSTANTIAL RESINIFICATION UNTIL THE MIXTURE ATTAINS A VISCOSITY IN THE RANGE OF GARDNER A TO E, THEREBY TO FORM PHENOL ALCOHOLS, THEN ADDING TO SAID MIXTURE A MATERIAL PROVIDING HYDROGEN SULFIDE, AND ADDING TRICRESYL PHOSPHATE AND CHLORINATED DIPHENYL, THEN COATING PAPER THEREWITH WITHIN A TEMPERATURE RANGE IN EXCESS OF THE BOILING POINT OF WATER, AND LAMINATING SHEETS THUS COATED UNDER HIGH PRESSURE AND A ELEVATED TEMPERATURE