Electrical insulation

Abstract

Thin electrical insulation having a first layer of resin impregnated micaceous paper, superimposed thereon a second layer of spaced flattened glass yarns and superimposed thereon a third layer of polymer film is particularly adapted to the insulation of relatively small conductors and for use at elevated temperatures.

Description

This invention relates to composite insulating material. More particularly, it relates to new and improved insulating material in sheet or tape form and more specifically glass yarn reinforced mica paper which is characterized by good insulating qualities particularly in connection with relatively small diameter conductors and at relatively high temperatures in which organic bonding materials are burned off.

Insulating sheets and tapes made of reinforced asbestos, mica flake and other inorganic flake material such as glass flakes, using glass or other inorganic fabric as a reinforcing medium, are well known. However, such insulation containing woven fabric such as of glass cannot be conveniently wound on small conductors of the order of 50 mils in diameter without excessive fraying particularly at elevated temperatures where any organic bonding material would have been burned off. Asbestos acts in much the same way as glass fabric in that it tends to fray and disintegrate unless bonded with an inorganic heat-resistant material such as ceramic which then renders the conductors inflexible and the insulation friable and physically delicate. The use of mica flakes in such tapes is also limiting in that relatively small conductors cannot be conformably wound with the tape containing relatively large mica flakes. It has also been sought to use mica paper or reconstituted mica or similar material with glass fabric reinforced insulation, and while the mica paper portion of the tape is conformable in and of itself, the tape is limited in such characteristic by the presence of the glass fabric. From the above it will be quite evident that there is a need for readily conformable and flexible electrical insulation which at the same time will retain its electrical insulating and physical qualities at elevated temperatures at which any initial organic bonding material may have been essentially removed by heat.

A principal object, therefore, of the present invention is to provide such readily conformable and flexible electrical insulation which will retain suitable electrical insulating and physical characteristics at normal and elevated temperatures.

Briefly stated, the present invention relates to electrical insulation in sheet or tape form which has a first or base layer of resin-impregnated mica paper, a second superimposed layer of a plurality of generally lengthwise disposed, flattened glass yarns, and optionally a third adherent layer of polymer or resin film. The material is particularly characterized in that it can be wrapped conformably on relatively small conductors or structures and will remain flexible and retain its electrical insulating qualities even at elevated temperatures at which any organic resin binder or adhesive or impregnant would have been incinerated.

Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further advantages and objects thereof appreciated from a consideration of the following description and the drawing in which the single FIGURE illustrates a preferred embodiment of the invention in tape form.

The mica paper or reconstituted mica which has been found to be useful in connection with the invention can be prepared by any of a number of processes including but not limited to those described in U.S. Pat. Nos. 2,549,880; 2,614,055; 2,709,158; 2,405,576 and 3,110,299. While the mica paper can be of any desired thickness commensurate with the particular application, it has been found that particularly in the wrapping of small conductors of the order of 50 mils, such mica paper having a thickness of about 1.5 to 2 mils is preferred.

The glass yarn which is useful in connection with the invention ranges generally from about 0.5 mil in diameter to about 3 mils and is made up, typically, of bundles of filaments which are twisted in well known fashion to form the yarn. In actual use, the glass yarn is normally partially flattened as will be described hereinafter. Where an overlying film is indicated, any of a number of resinous or polymer films can be used including the various polyesters, polysulfones, polyethylene, polypropylene, polybutylene, polystyrene, polyvinyl acetal, polyacrylate, cellulose acetate, cellulose acetobutyrate, polyvinyl acetal, polycarbonate, and the like, such film materials being well known to those skilled in the art.

Any of a number of polymeric materials, also well known to those skilled in the art, can be used to impregnate the mica paper layer and to adhere any overlying film to the substrate. However, it is preferred to use in such roles organic polysiloxanes which are in and of themselves relatively high temperature resistant materials. Typical of those materials useful as an adhesive is General Electric organopolysiloxane SR-516 which is essentially a mixture of silanol stopped dimethylpolysiloxane having a viscosity of from about 1 million to 40 million cps at 45.degree.C and MQ resin where M is (CH.sub.3).sub.3 SiO.sub.1/2

Q is SiO.sub.2

and the ratio of M/Q is .55-.65/1

A typically useful mica paper impregnating material is General Electric SR-231 which is a 60 percent solution in xylene of a mixture of silanol stopped dimethylpolysiloxane fluid having a viscosity of 100 to 1000 cps at 25.degree.C MQ resin as above, and additional MQ resin where the ratio of M/Q is .9-1/1, along with a small amount of catalyst such as cerium compound.

The following example illustrates the practice of the invention, it being realized that it is not to be taken as limiting in any way.

Mica paper having a thickness of about 1.7 mils was impregnated with the above SR-231 resin with a kiss coater and oven cured at a rate of about 10 ft/minute at 175.degree.C, the oven dwell time being about 5 minutes to give a final resin content ranging from about 1.5 to 4 percent by weight of the mica paper. It has been found that less than about 1.5 percent by weight of resin detracts from the water repellant characteristic of the mica paper which tends to disintegrate when exposed to moisture. On the other hand, over about 4 percent by weight of resin content tends to cause blocking of the mica paper when it is wound up. The impregnated mica paper is next kiss coated with the above described SR-516 adhesive as a 15 percent solution in toluene and treated in an oven at a temperature of from about 40.degree.C to 100.degree.C and preferably at about 70.degree.C to remove most of the solvent and provide a tacky surface. Next, glass yarn having about 200 filaments each about 0.00035 inch in diameter is fed on to the tacky mica paper surface, the yarns being spaced to give about 18 to 20 yarns per inch, the composite material being immediately fed through nip rollers to flatten the glass yarns to the desired degree usually about one half the thickness. Generally speaking, while unflattened glass yarns can be used in certain applications, they tend to make the finished product relatively thick. At the same time, the flattening disperses the glass filaments and provides more protection and reinforcement to the mica paper and with extreme flattening causes the glass filaments to cover essentially the entire surface of the mica paper and provides a high degree of resistance against puncture. The optional surface film of 1/4 mil Mylar polyethylene terephthalate polyester may be first lightly treated with the above adhesive and then superimposed on the substrate using a nip roller or other convenient pressing means to provide the finished product shown in the drawing. Here a length of tape 1 with its first or base layer of micaceous paper 2 and reinforcing lengthwise disposed flattened glass yarns 3 is shown with the overlying resin film 4 partially removed. The mica paper or paper of similar material provides an electrical insulating barrier which is supported and reinforced conformably by the glass yarns. The overlying film of resinous material aids in conformably wrapping or taping the material particularly on small wires or conductors, this film as well as resin adhesives and impregnants being essentially or wholly removed if the insulated conductor is exposed to incinerating temperatures. The finished insulation in the form of tape an be used to conformably wind or insulate relatively small wires of the order of 50 mils in diameter which can then be wound into strands or cables. While the insulation is suitable for use at ordinary temperatures with its dielectric strength of 1000 volts/mil, short time, step by step, using 1/4 inch electrodes, it is particularly suitable for use at elevated temperatures where the impregnating and adhesive material is substantially or wholly burned off or incinerated. It has been found that when so incinerated, strands or cables of such insulated wires substantially retain their flexibility and insulating qualities and are resistant to abrasion and physical punishment. This is as distinguished from prior art materials which, particularly at elevated temperatures, either lose their electrical insulating qualities or are inflexible or become severely deteriorated physically as well as electrically. The present insulation is particularly useful for thermocouple wires and the like.

Claims

What I claim as new and desire to be secured by Letters Patent of the

1. Electrical insulating tape having a layer of resin impregnated micaceous paper, superimposed thereon a plurality of flattened glass yarns disposed in spaced, generally lengthwise parallel fashion and superimposed thereon

2. Electrical insulation as in claim 1 wherein said resin is

3. Electrical insulation as in claim 1 wherein said polymer film is of

4. Electrical insulation as in claim 1 wherein said resin impregnated micaceous paper and polymer film are covered with an adhesive.