US2554254A - Vacuum impregnation process - Google Patents

Description

y 1951 H. M. KROFT 2,554,254

VACUUM IMPREGNATION PROCESS Filed June 16, 1949 WITNESSES: INVENTOR Herbert M. Kroft.

Patented May 22, 1951 UNITED STATES ATENT OFFICE VACUUM IMPREGNATION PROCESS Application June 16, 1949, Serial No. 99,488

Claims.

This invention relates to the vacuum impregnation of porous members with fluid compositions.

In treating members having pores, crevices or other fine voids, hereinafter designated porous members, with a fluid composition so as to impregnate the pores of the members thoroughly and completely with the fluid composition, it is well known that considerable difiiculty is experienced in securing such complete impregnation. Even with the benefit of a preliminary evacuation of the porous members, it has been found in many cases that the fluid compositions fail to penetrate to the innermost portions of the pores and spaces of porous members.

Thus in the manufacture of electrical coils, it is desirable to thoroughly saturate the fibrous insulation, such as paper, cloth, silver; or felt, commonly applied both to the conductors and the coil with an insulating varnish. To this end electrical coils are placed in vacuum tanks wherein they are evacuated to a low absolute pressure to withdraw air, moisture and other volatiles from the interstices of the coils, and thereafter the coils are flooded with insulating varnish composition, and pressures above atmosphere are then applied to the varnish composition to drive it into the interstices of the coil. In many cases, particularly with the larger coils wrapped with manylayers of tape, the varnish does not completely penetrate the entire insulation. The innermost layers often have very little impregnant present. Likewise, in impregnating electrical members with insulating oils, it has been found that the penetration of the oil has been unsatisfactory even with the assistance of a vacuum. This incomplete impregnation is particularly prevalent where the member being treated has extremely fine pores or crevices some of which have been found to be unimpregnated with the oil even after prolonged evacuation and impregnation under pressure. It appears as if the fine pores resist the flow of the varnish or oil therethrough, the residual gases in the pores are not displaced by the impregnant but rather are compressed by the entering fluid and thereby this prevents the complete impregnation of the members.

It has been discovered that, if members being treated with liquid compositions are heated by infra-red radiation to temperatures of above 80 C. while subjected to a high vacuum and flooded with fluid compositions and the temperature maintained during the flooding, until bubbling indicating escaping residual gas ceases, unexpectedly rapid and substantially complete penc tration of the pores of such members is accomplished by the fluid compositions.

The object of this invention is to provide for enabling the rapid and thorough impregnation of porous members with fluid compositions by subjecting the members to simultaneous evacuation and heating by infra-red radiation previous to impregnation.

, A further object of this invention is to provide for more completely impregnating porous members with insulating fluid compositions by evacuating them to a pressure of less than five inchesof mercury and heating them with infrared radiation to a temperature to at least C.,

then covering the members with a fluid composition and continuing heating in the presence of the fluid composition to at least 80 C.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing in which the single figure is a vertical cross section through an impregnating tank, illustrating the practice of the invention.

It has been discovered that the impregnation of porous members by fluid compositions may be greatly expedited and rendered more complete by subjecting the porous members to simultaneous evacuation to a low pressure and irradiation with infra-red radiation to heat the members to at least 80 0., covering the members with the fluid composition while evacuated, and heating the covered members while so covered to bring them up to a temperature of at least 80 C., so that residual gases will be almost entirely driven off at a rapid rate, as evidenced by extensive bubbling, and finally applying pressure on the composition after the bubbling substantially ceases. Heating members by infra-red radiation has been found to be unusually effective in the process of this invention providing for the vacuum impregnation of porous members. Other advantages are that the infra-red radiation may be conducted without heating any extraneous parts of the vacuum equipment and it enables the heating of the porous members to be accomplished rapidly. Nearly all insulatingroils and insulating varnish compositions, with the exception of heavily pigmented compositions, through which infra-red radiation will not penetrate, effectively, are relatively transparent to infra-red'radiation so that eificient heating of the members may be carried out While covered by such fluid compositions.

Referring to Fig. 1 of the drawing, there is illustrated one form of apparatus for the practice of the invention. A hermetically scalable vessel H] is provided with a cover l2 provided with a window Hi relatively transparent to infra-red radiation. Numerous glasses for this purpose are known. The vessel ii] is provided with a conduit l6 for introducing and withdrawing fluid compositions with which members are to be impregnated. A pipe I8 connected to a suitable source of vacuum (not shown), such, for example, as a vacuum pump, enables the vessel iii to be evacuated to any desired degree. The pipe l8 may be also connected to a source of air, carbon dioxide or other gas under pressure, if necessary, for building up pressures above atmospheric, or else the pipe I8 may be opened to the atmosphere to break the vacuum. Within the vessel i is disposed a horizontal support 20 provided with a plurality of bars 22 on which porous members 2 to be impregnated may be placed, arranged so that the fluid composition may readily penetrate between the bars 22. It will be appreciated that the bars 22 may be replaced by their equivalent, such as the grating, wire screen or the like. Exterior of the vessel It! is located one or more infra-red lamps 26, constituting a source of infra-red radiation. While lamps have been shown, it will be understood that electrically heated resistance elements and other infra-red radiating means, preferably with focusing shields, may be employed. In some cases we have placed the infra-red lamp within the vessel Iii itself, though it is more convenient to accommodate the focusing of the lamps 26 to the number and disposition of the members 24 within the vessel if the lamps are exterior of the tank. In addition, the heat from the lamps other than the directed radiant heat is more conveniently removed, if the lamps are not within the vessel ill.

After the porous members 2e, which may be electrical coils or any other electrical pieces requirin impregnation with a fluid composition, are disposed in suitable manner on the bars 22, the cover i2 is applied to the vessel and the vessel hermetically sealed. It has been found that the best results are obtained if the members 24 are disposed in a single thickness to form a horizonta-l layer on the bars 22. Likewise, if the member has openings or pores extending to one side only, as, for example, in vibration pickups, it is preferable that the porous side be placed uppermost. The vessel I0 is evacuated through the pipe !8 to a suitable low absolute pressure, generally below five inches of Hg absolute. For best results, we have evacuated to an absolute pressure of below mm. Hg. The pressures are exemplary and for very viscous fluid the absolute pressure may be much lower. The evacuation removes air, moisture and other volatile material from the porous members 24. In some cases a substantial amount of gas and other volatilizable substances will remain in the fine interstices and pores. Thus, we have found that coils wrapped with eight to ten layers of mica and varnished tape retain a substantial amount of gas in the innermost layers thereof, which is not removed even with drastic vacuum treatment.

Soon after evacuation is started, the infrared lamps 26 are focused on the porous members 24 and put in operation so that infra-red radiation is applied to the members 24 to cause them to heat up. A temperature of at least C. has been found to give the best results in combination with a suitable low vacuum. The combination of infra-red heating and the vacuum has been found to result in the expeditious elimination of all but traces of gases and volatilizable matter in the porous members 24.

Inasmuch as infra-red radiant heating is capable of transmitting energy to the members at an extremely high rate, the members may be heated in a very short period of time, usually a few minutes is adequate. This is advantageous securing prompt and thorough degassing. After the members 24 have been heated and evacuated adequately, a fluid composition may be introduced through the conduit l6 so as to cover the members 24. In the practice of the invention, the height of the fluid composition 28 is adjusted to barely cover the members 24. This insures the maximum absorption of the infra-red radiation by the members 24 and also that the least amount of the fluid composition is heated unnecessarily. Upon contact of the hot members 24 with the fluid composition 28, they will be cooled considerably since ordinariy the fluid composition is not as hot as they are. Particularly in the case of heat reactive insulating varnishes, preheating of the composition 28 would be undesirable since it would cause premature gelation and poor storage life of the composition.

Irradiation of the members by the infra-red radiation iscontinued during the immersion of the members in the composition 28 until the members are at a temperature of at least 80 C. in the case of varnish compositions. Much higher temperatures may be employed if the members 24 are to be impregnated in an insulating oil, such as transformer oil or a silicone oil. Bubbles of gas are given off by the porous members as they are heated.

To secure the best impregnation results, after the members have been immersed, heated to at least 80 C. and bubbling has ceased, the vacuum should now be broken and either atmos pheric pressure is applied or a pressure much higher than atmospheric pressure, may be imposed upon the fluid composition 28 by connecting the pipe l8 to a suitable source of gas under pressure. Pressure of as much as pounds per square inch may be applied to the fluid composition 28 to cause the composition to be forced into the finest and inner most interstices in the members 24. Within a short period of time, usually a few minutes, the members 24 will be completely and thorough saturated with the fluid composition, and the fluid composition may be withdrawn through conduit I6 to uncover the members 26. The members may be permitted to remain on the bars 22 for superficial drainage of residual composition. After suitable drainage, the members may be removed and processed further. If an insulating oil is applied, for'example, to impregnate a vibration pickup, the pickup may be sealed while covered with oil and the sealed unit need only be wiped to be ready for use. Electrical coils impregnated with an insulating varnish will be removed and put in a baking oven or otherwise heated to cure the varnish therein.

Unexpectedly good results have been secured by the combination of infra-red heating and evacuation. It has been found that members, as, for example, a vibration pickup, cannot be completely and thoroughly saturated with an insulating oil without any appreciable amount of gas or moisture being left, by the use of vacuum alone. Furthermore, the treatment without the use of heat was more than three times as long with less satisfactory result than secured by a short treatment using infra-red heating combined with the vacuum treatment.

Numerous varnish compositions may be employed in the practice of the present invention for impregnating coils. It is preferable that the compositions be relatively transparent to infrared radiation thereof. This requirement may be dispensed with if the members are barely covered with the fluid composition, that is, not over a fraction of an inch of the composition is over the surface of the members. Examples of suitable varnishes for the practice of the invention are alkyd varnishes, phenolic resin varnishes, melamine resin varnishes, polyester compositions embodying an unsaturated alkyd resin and a vinyl monomer. Specific examples of such polyester resins are disclosed in Patent No. 2,414,525. Examples of suitable oils are transformer oil, castor oil, silicone oil and chlorinated diphenyl.

Since certain obvious changes may be made in the above procedure and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the process of impregnating a porous member with a fluid composition, the steps comprising subjecting the member to a vacuum at an absolute pressure of less than 5 inches of Hg to Withdraw air, moisture and other volatiles from the pores thereof, irradiating the member with infra-red radiation to cause its temperature to rise above 80 C. while still subjected to the vacuum, immersing the member with an amount of a fluid composition barely covering 7 the upper surface of the member while the member is maintained under vacuum, the fluid composition being relatively transparentto infra-red radiation, continuing heating the member with the infra-red radiation while it is so covered with the fluid composition until the member again reaches a temperature of at least 30 C. and continuing such heating until bubbling ceases, and withdrawing the impregnated member from the body of the fluid composition.

2. In the process of impregnating a porous member with a fluid composition, the steps comprising subjecting the member to a vacuum at an absolute pressure of less than 5 inches of Hg to withdraw air, moisture and other volatiles from the pores thereof, irradiating the member with infra-red radiation to cause its temperature to rise above 80 C. while still subjected to the vacuum, barely covering the member with a fluid composition selected from the class consisting of insulating varnishes and dielectric liquids while the member is maintained under vacuum, the fluid composition being relatively transparent to infra-red radiation, continuing heating the member with the infra-red radiation while it is so covered with the fluid composition until all gas bubbling from the member ceases, and then applying pressure to the fluid composition about the member to cause it to penetrate into the member, and withdrawing the impregnated member from the body of the fluid composition.

3. In the process of impregnating a porous member with a varnish, the steps comprising placing the member in a hermetically sealable vessel, evacuating the vessel to an absolute pressure of below 5 inches of Hg to withdraw air, moisture and other volatiles from the member, irradiating the member with infra-red radiation to cause its temperature to rise above C. while being subjected to the vacuum, immersing the member in a varnish relatively transparent to infra-red radaiation while being irradiated and under vacuum, breaking the vacuum when the impregnated member reaches a temperature above 80 C. and ceases to give off gas bubbles, applying a pressure equal to at least atmospheric pressure to the varnish to force it into the member, removing the varnish impregnated member from the vessel and baking it to cure the applied varnish.

4. In the process of impregnating a plurality of members with a fluid composition, the steps comprising placing a single thicknes of the members in a single horizontal layer in a hermetically sealable vessel, evacuating the vessel to an absolute pressure of below 5 inches of Hg to withdraw air, moisture and other volatiles from the pores of the members, irradiating the layer of the members with infra-red irradiation to bring their temperature to at least 80 C. while being subjected to the vacuum, immersing the layer with a fluid composition in an amount to just cover the members, the fluid composition being relatively transparent to infra-red radiation, continuing irradiating while so immersed until the members reach a temperature above 80 C. and cease to give oil gas bubbles and withdrawing the impregnated members from the vessel.

5. In the process of impregnating a porous member with a fluid composition, the steps comprising subjecting the member to a vacuum at an absolute pressure of less than 5 inches of Hg to withdraw air, moisture and other volatiles from the pores thereof, irradiating the member with infra-red radiation to cause its temperature to rise above 80 C. while still subjected to the vacuum, immersing the porous member with a fluid composition relatively transparent to the radiation in an amount barely covering the upper surface of the member while the member is maintained under vacuum, continuing heating the member with the infra-red radiation While it is so covered with the fluid composition until all gas bubbling from the member ceases, and

withdrawing the impregnated member from the body of the fluid composition. HERBERT M. KROFT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 577,671 Tesla Feb. 23, 1897 1,163,342 Hurley Dec. 7, 1915 1,760,583 Clark May 27, 1930 2,282,317 Bennett May 12, 1942 2,321,939 Quinn June 15, 1943 2,357,286 Reavell Sept. 5, 1944

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