US2842183A

US2842183A - Mica paper

Info

Publication number: US2842183A
Application number: US611775A
Authority: US
Inventors: Jr George L Gaines
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1956-09-24
Filing date: 1956-09-24
Publication date: 1958-07-08
Anticipated expiration: 1975-07-08
Also published as: GB825117A; DE1036034B

Description

MICA PAPER George L. Gaines, Jr., Scotia, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application September 24, 1956 Serial No. 611,775

4 Claims. (Cl. 154--2.6)

This invention relates to the preparation of mica paper. More particularly, this invention relates to sheets of mica paper which are characterized by improved tensile strength and to the process by which these improved prod ucts are obtained.

Heretofore, the preparation of mica paper has been known in the art. For example, mica paper preparation is described in Patents 2,549,880-Bardet, 2,614,055de Senarclens, and 2,709,158Bouchet. -Mica paper prepared by the methods of these patents as well as by other known methods is gaining commercial acceptance because of the excellent electrical properties of the mica paper and because of its inertness at elevated temperatures. However, one serious drawback to the universal acceptance of mica paper for many applications is the fact that the mica paper does not have as high a tensile strength as desired.

It is an object of the present invention to provide an improved mica paper.

A further object of my invention is to provide a vmica paper of extremely high tensile strength.

These and other objects of my invention are accomplished by subjecting comminuted mica particles to the action of barium ions in an aqueous barium salt solution and subsequently forming a paper-like sheet from the resulting particles.

The term mica paper as used in this application is used in its usual-sense to refer to a sheet-like aggregate of mica particles which are usually heated at an elevated temperature, ground into fine particles in an aqueous solution, and then formed into paper by conventional papermaking techniques.

The mica paper of the present invention differs from I that of the prior art in that the mica particles have been subjected to the action of barium ions before they are formed into the final sheet. a v

The mica paper of the present invention can be prepared by firing or heating mica, which may comprise phlogophite, lepidolite, or preferably muscovite, at a temperature and time sufficient to partially dehydrate the mica. In general, the heating of the mica is carried out at a temperature of about 800 C., e. g., from about v 750-850 C. for a time of about 10 minutes, e. g., from about 5 to 20 minutes. This heating step causes a loss in weight of the mica equal to about 2 percent by weight of the original mica. This heating has the effect of softening the mica while at the same time delaminating and 1 This heat increasing the bulk volume of the mica. treated mica is then added to an aqueous medium and agitated by any suitable device, such as a high 'speed comminuter or mixer to convert the mica into small particles or platelets. carried out in an aqueous suspension containing from about 0.1 to 10 percent, and preferably about 1 percent by weight, of mica. This results in a pulp-like suspension of mica in which the particle size of the mica flakes has a wide distribution. The extra-fine and extra-coarse parti cles in this comminuted aqueous suspension may then be The communition of the mica'can bepart of mica.

2,842,183 Patented July 8, 1958 contact with barium ions in an aqueous barium salt solution. tact with the mica particles is not critical, andthis step may be accomplished by any desirable means. Although I do not wish to be bound by theoretical consideration, it is believed that the barium ions which are brought into c0ntact,With the mica particles replace some of the potas-. sium ions which are normally found on the-surfaceof ,7

mica andthat the presence of the barium ions on the surface of the mica tends to bind the particles of mica together with a greater force than is found with untreated mica'containing only potassium ions on its surface.

The barium ions can be brought into contact with the mica particles by suspending the particles in an aqueous solution containing these ions. The formation of the suspension of mica particles in-the aqueous barium ion solution may be accomplished at any time between the time,

the mica is fired up to the time the mica is formed into paper sheets. Thus, after firing of the mica, the fired mica may be dispersedin an aqueous solution containing barium ions and comminuted while in this solution. Alternatively, the fired mica may be comminuted in plain water and a suitable barium salt may be added to the water to bring the barium ions into contact with the particles. As a further alternative, the fired mica particles maybe comminuted in plain water, filtered from the grinding medium, and thenjadded to another aqueous solution containing barium ions. -A still additional method of adding barium ions to; the mica particles is to comminute the fired mica in water, separate the extra-coarse and extra-fine particles from the resulting solutionand then either add a barium salt to the resulting slurry, or the mica particles may be filtered from the slurry and added to another aqueous solution containing barium ions. .In any event, the contact of the braium ions with the surfaces of the mica particles leads to the formation of mica paper h'aving'a higher tensile strength than mica paper formed without the barium ion treatment.

Unexpectedly, I have discovered that all solutions containing barium ions are not satisfactory in the process of the present invention. Treatment of micapa rticles with a, barium hydroxide solution results in no improve ment inthe physical properties of a subsequently formed sheet. However, when a solution of barium salt is employed to treat mica, marked improvement of the physical propertiesof the resultant mica paper is observed. The particular barium salt employed is not critical, the only requirement being that the salt be soluble in water at room temperature (i. e. about 20 C.) to the extent ofat least about 0.5 to 1.0-parts by weight of salt per parts of water. The amount of barium salt employed may vary over a Wide range, e. g. from about 0.1 to 10, and preferably about 1.0 part by Weight per Although 'I prefer to use barium chloride inthe practice of my invention, other satisfactory barium salts include, for example, barium acetate, barium nitrate,

1 mica particles and the barium ion is not-critical. Thus,

invention, the -mica particles while in comminuted form are brought into The method of bringing the barium ions into con,-

satisfactory for causing the replacement of some of the potassium ions in the mica with barium ions.

After contacting the mica particleswith the barium ions, the mica particles may be filtered from the treat+ ing solution and resuspended in pure water.- Thispure water solution may then be used to formsheets of paper from the mica on conventional paper-making machines. Alternatively, the suspension of mica in the barium treating solution may be employed directly in forming the sheets of mica paper. Regardless of whethera pure water suspension or a barium ion containing aqueous 'suspension of mica particles is used, the sheets are formed by dispersing the suspension on top of a screen and drawing the suspension medium through the'mica particles by means of suction. The remaining-aqueous medium may then be removed from the mica'particles by evaporation at room temperature or at elevated temperatures. Where elevated temperatures are employed, it is desirable to use temperatures of from about 110-450 C. since this temperature has been found'satisfactory to remove all of the suspension medium from the sheet: After removal of the water, the resulting micap'articles are in the form of a fairly rigid sheet which is extremely resistant to elevated temperatures. In addition to form= ing the final mica paper by evaporation of the suspension medium from the particles, satisfactory mica paper is obtained by calendering the damp mica sheet'resulting from sucking the suspension medium through the mica particles on the paper-making machinescreen. This calendaring operation may be carriedout at a temperature of from 110-175 C. to facilitate removal of remaining moisture from the sheet.

The mica paper of the present invention may be formed of any desired thickness. Howeveryfor most applications it is preferred to prepare sheets having a thickness of from 1 to 50 or more mils and preferably from about 2 to mils. This mica paper is very useful in applications which require the combination of a highdielectric strength and a good resistance to elevated temperatures. Thus, the mica paper of the present invention may be employed as slot insulationfor dynamoelectric machines, as insulation in capacitors, transformers and the like, as Well as insulation and separators in high temperature vacuum tubes.

One important advantage of the method of the present invention is that the barium ion solution employed to treat the mica particles may be separated from these particles and reused to treat additional mica particles.

The same treating solution may be used over and over again as long as additional barium-containing material is added to the solution to keep the barium ion concentration to whatever value is selected.

The following examples are illustrative of the practice of my invention and are not intended for purposes of limitation. In each of the examples a 1 percent by weight slurry of mica in water was employed. The slurry was formed by firing Bengal Ruby muscovite at a temperature of about 800 C. for about 10 minutes, at which time there had been a weight loss of about 2.7 percent of the original weight of the mica. After cooling, this mica was then added to distilled water and the resulting slurry was agitated violently to comminute the mica particles. The extra-fine and extra-course particles of mica in the slurry were removed, leaving a 1' percent slurry of mica in water. All parts mentioned in the examples are by weight.

Example] A control was prepared by adding a l percent by weight slurry of mica in water to a Buchner funnel and The thickness of the control was about 2.3 mils.

Example 2 One part of anhydrous barium chloride was added to a 1 percent slurry of mica in water containing 1 part of mica. This barium chloride-mica solution was allowed to stand for about 18 hours, at which time the mica particles were filtered from the barium chloride solution, washed several times with water, and made up into a slurry containing 1 percent by weight of barium chloride treated mica. A sheet of mica-paper about 1.8 mils thick was then prepared by the method of Example 1. A control was also prepared from the 1 percent mica slurry to which no barium chloride had been added. The bariumchloride'tre'atcdmica sheet had a tensile strength of about 7900 p.s. i. as compared with the tensile strength of about 4900 p. s. i. for the control.

Example 3 This example illustrates the treatment of mica with barium ions from the chloride and the acetate and also illustrates the treatment of mica with barium hydroxide and shows that barium hydroxide is not a suitable treating agent for mica. All of the sheets of mica paper prepared in this example had a thickness of about 2.5 mils.

One part of barium chloride or barium acetate was added to a 1 percent slurry of mica in water containing 1 part of mica. 'In each case, the barium-containing solution was allowed to stand for 3 hours, at which time a sheet was'formed-from the slurry without washing the slurry. The table below lists the material added to the mica slurry and the tensile strength observed.

As the table shows, the treatment of mica particles with either barium chloride or barium acetate markedly improves the tensile strength of the resulting mica paper. Treatment with barium hydroxide leads to no improvement whatsoever in the tensile strength of the mica sheet.

Although the foregoing examples illustrate the treatmentof mica with only several of the possible barium salts, it should be understood that barium salts not specifically illustrated may also be employed. Thus, in addition to using barium chloride as the treating agent, mica particles may also be treated withbarium fluoride, barium bromide and barium iodide to obtain the improved mica paper of the present invention. Similarly, the

mica particles may be treated with other barium salts it should be understood that in the practice of the pres ent invention, slurries of mica containing 'from 0.1 to

10 percent by weight of mica may be employed. Sim- The only limitation upon the barium salt 5 ilarly, the amount of the'barium salt employed to treat the mica particles may vary from 0.1 to 10 parts per part of mica in the slurry.

The mica paper prepared by the method of the present invention has the same utility as other mica paper. Thus, this mica paper can be employed as slot insulation in dynamoelectric machines, can be cut into tape and employed as insulation for electrical conductors, can be employed as electrical and thermal insulation in electron tubes, etc' What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of preparing mica paper of improved tensile strength which comprises contacting comminuted mica particles with an aqueous barium salt solution,

forming a wet paper-like sheet from said particles, and drying said sheet, said barium salt being characterized by a room temperature solubility of at least about 0.5 part by weight per 100 parts by weight of water.

2. The product prepared by the method of claim 1.

3. The method of claim 1 in which the barium salt is barium chloride.

4. The method of claim 1 in which the barium salt is barium acetate.

References Cited in the file of this patent UNITED STATES PATENTS 2,669,764 Kilpatrick Feb. 23, 1954

Claims

1. THE METHOD OF PREPARING MICA PAPER OF IMPROVED TENSILE STRENGTH WHICH COMPRISES CONTACTING COMMINUTED MICA PARTICLES WITH AN AQUEOUS BARIUM SALT SOLUTUON, FORMING A WET PAPER-LIKE SHEET FROM SAID PARTICLES, AND DRYING SAID SHEET, SAID BARIUM SALT BEING CHARACTERIZED BY A ROOM TEMPERATURE SOLUBILITY OF AT LEAST ABOUT 0.5 PART BY WEIGHT PER 100 PARTS BY WEIGHT OF WATER.