The present invention relates to a method for preparing a new and improved fire-resistant, insulated conductor and to new and improved insulated conductors made in accordance therewith. More particularly, it relates to a fire-resistant insulation prepared by helically wrapping a conductor with at least one layer of mica-impregnated glass tape in a half-lapped fashion to form a continuous first layer, and applying a continuous outer layer of a curable, platinum catalyzed silicone rubber composition thereto and curing said outer layer to form a hardened, fire-resistant insulation.
In recent times, the hazards associated with fire in skyscrapers and other heavily populated buildings have become more appreciated. The need to provide circuit integrity to certain critical circuits, such as fire alarms and elevators, in the event of fire cannot be over-emphasized.
It has long been known to use mica in insulating compositions. Having excellent dielectric properties and fire-resistance, this natural material is well suited for use in electrical insulation applications. Typically, mica insulations are provided in the form of mica tapes, such as described in U.S. Pat. No. 2,656,290. As described therein, individual mica flakes are bonded to one another, as well as to a pliable base sheet, and, if desired, also a cover sheet, by a liquid bonding agent which may be hardened by suitable additives. The bonded mica tape used for these purposes may be relatively narrow, having a width of 2 to 3 cm for example, or it may be used in sheets of greater width. A selected conductor is wrapped with the mica tape and the wrapped conductor is subjected to a vacuum and impregnated with a thin liquid impregnating resin. The resin and the bonding agent are specifically chosen such that the bonding agent, together with the hardners and polymerization accelerators present in the impregnating resin, combine completely with the impregnating resin to form a uniform hardened insulative coating. Considerable time and effort has been spent in making mica tape insulation which is flexible and exhibits good adhesion of mica to tape. The shortcoming with such tapes is that the vacuum impregnation step tends to be costly and care must be taken that the impregnating resin is fully dispersed throughout the windings to eliminate voids in the insulation which decrease the dielectric properties of the resulting insulation. Further, these compositions generally are not fire-resistant enough at high temperatures to be suitable in today's applications.
More recently it has been known to use fluoropolymers, such as tetrafluoroethylene (TEFLON) and ethylenetetrafluoroethylene (TEFZEL) as insulative coatings. Such materials are effectively fire-resistant up to about 250.degree. C.-300.degree. C. which is an improvement, and have excellent dielectric strength. A serious shortcoming with these insulations however is that at high temperatures when these polymers burn, they may give off toxic gases rendering their use less than desirable in today's construction applications.
Recently, elsewhere in the polymer art, there have been developed new and improved silicone elastomers such as those described in U.S. Pat. No. 4,061,609, assigned to the same assignee as the present invention. The silicone elastomers described therein include a vinyl-containing polysiloxane, a hydrogen-containing siloxane, a platinum catalyst and further include an inhibitor compound containing at least one hydroperoxy radical added to improve processability. Similar but uninhibited silicone elastomers having a silica filler have been shown to be flame-retardant in U.S. Pat. No. 3,514,424.
It has now been found that the platinum-catalyzed silicone elastomers described in the former patent are well suited to produce new and improved fire-resistant insulations in combination with a mica-impregnated glass tape.