Description of Prior Art The need to locate pipes below the surface of the earth, either to service or repair them or to avoid inadvertent damage during subsequent excavation in the vicinity of said pipes is obvious. In contrast to days gone by in which such underground pipes were constructed of metal, today's underground pipelines, such as natural gas pipelines, are typically made of plastic, most notably polyethylene and polyvinyl chloride. The benefits of plastic pipe are numerous. It is extremely durable and reliable, relatively lightweight, easy to manufacture, and cheaper than most other materials that may be used. In addition, it is essentially inert such that materials flowing through the plastic pipe will not react with it.

However, once buried, plastic pipe is difficult to find, thus making it difficult to service and more likely that unintentional damage will result from nearby excavations.

U. S. Patent 5,114,517 and related U. S. Patent 5,206,065 teach the application of magnetic markers for locating, tracing, and identifying elongated hidden objects, such as buried utility pipe, duct, conduit, or fiber optic cable. These magnetic markers are manufactured by applying magnetic material to a substrate that is elongated and by forming from the material a helical or twisted permanent magnet pattern extending along the length of the substrate.

The magnetic material is formed of a strip coextruded with the substrate and magnetized in the direction of the width of the strip. Alternatively, magnetic material is mixed with the material from which the substrate is extruded, or the magnetic material may be coated on the substrate. Also taught is the application of a pre-formed strip to an elongated object as the object is buried, or utilization of the strip as a separate warning tape that is buried with the object and provided with a desired magnetic signature by twisting the strip lengthwise as it is advanced for burial.

U. S. Patent 5,428,332 teaches a process and apparatus for the magnetization of flexible hard magnetic materials in the form of sheets or strips, such as magnetic rubber, wherein opposing arrays formed from alternating magnetic disks and flux conducting elements are used in sets of two with opposing polar moments such as to induce a magnetic flux in the gap between the disks. The width of the magnetic disk and the flux conducting elements, respectively, are selected to optimize the magnetic pull strength of the material.

A material to be magnetized is passed between the array sets in contrast with both disks and consequently imprinted with magnetic poles. The magnetized properties of the material are enhanced by passing the material through a second set of arrays which are axially offset with respect to the first set of arrays.

Related U. S. Patents 5,036,210 and 5,051,034 teach magnetically detectable plastic pipe for underground use comprising a hollow tubular plastic pipe having particles of magnetic material imbedded in the plastic and secured integrally with the wall of the pipe.

The magnetic particles are iron oxide or barium ferrite having a size, shape, distribution and proportion such that the plastic pipe may be easily detected by magnetic detection apparatus on the surface when the plastic pipe is buried at a selected depth under the ground. The magnetic particles are uniformly distributed throughout the thickness and uniformly distributed around the entire circumference of the plastic pipe; or the magnetic particles may be distributed in discrete portions of the plastic pipe as, for example, a uniform outer layer of the pipe or a coating layer around the pipe or in stripes or strips extending along the pipe.

The particles may be extruded with the plastic in forming the pipe or the pipe may be formed of plastic free of the magnetic particles and the magnetic particles provided in a separately applied strip which may be coextruded or fused or adhered to the wall of the pipe. The magnetic strips or magnetic portions of the plastic pipe may have selected areas or portions magnetized to encode information readable from the surface of the ground above the buried pipe or in inventorying pipe on the surface. See also U. S. Patent Re. 34,701 which is a reissue of U. S. Patent 5,036,210.

A method of making a magnetically detectable plastic pipe in which magnetic particles are distributed in the wall of a pipe by extrusion with the plastic at the time the pipe is formed and the pipe is passed through a magnet to magnetize the pipe diametrically with one pole on one side of the pipe and the other pole diametrically opposite is taught by U. S.

Patent 5,354,521. In accordance with the teachings of this patent, the pipe magnetization polarity may be selectively reversed to encode a unique magnetic signature to the pipe or to encode data on the pipe. The pipe may be rotated and passed linearly through the magnet, or the magnet may be rotated around the pipe to produce a spiraled diametrically oriented magnetic structure.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method and apparatus for producing an intrinsically magnetic plastic pipe, plastic solid rods or rectangular strips which are capable of being detected when buried three or more feet below ground.

This and other objects of this invention are achieved by a process of magnetizing plastic pipe in which a plastic pipe material is doped with a magnetizable material, the doped plastic material is extruded, forming a plastic pipe comprising the magnetizable material, and a magnetic field is applied across a pipe diameter as the doped plastic pipe material is extruded, the magnetic field being applied by magnetizer means disposed about the plastic pipe as it is extruded. Alternatively, an outer shell of the pipe is doped with a magnetizable material. The magnetizable material may comprise iron ferrites, barrium ferrites, strontium ferrites and/or other magnetizable material. To provide the desired degree of magnetization, a magnetic field having a peak of at least about 6,000 gauss is utilized. In accordance with one preferred embodiment of the process of this invention, the magnetic field is generated by a pulsed current. In accordance with another preferred embodiment of the process of the invention, the magnetic field is generated by a plurality of permanent magnets disposed around the plastic pipe. And, in accordance with yet another embodiment of this invention, the magnetic field is generated by a superconducting magnetizing coil.

A device for magnetizing a plastic pipe in accordance with this invention comprises extrusion means for extruding a plastic pipe material comprising a plurality of magnetizable particles and forming a plastic pipe, and magnetizer means for spirally magnetizing the magnetizable particles during extrusion of the plastic material. In accordance with a particularly preferred embodiment of this invention, the magnetizer means comprises rotation means for rotating the magnetizer around the plastic pipe as it is extruded.

BRIEF DESCRIPTION OF THE DRAWINGS This and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: Fig. 1 is a schematic diagram of a pipe and a magnetizer in accordance with one embodiment of this invention; Fig. 2 is a schematic diagram of a pipe and a second magnetizer in accordance with one embodiment of this invention; Fig. 3 is a schematic diagram of a pipe and a third magnetizer in accordance with one embodiment of this invention; and Fig. 4 is a schematic diagram of a pipe and an outer coating of magnetized material in accordance with one embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows a magnetizer for magnetizing polyethylene pipe in accordance with one embodiment of the process and apparatus of this invention. The magnetizer comprises a pair of saddle-shaped coils 11,12 disposed on either side of pipe 10. In accordance with the process of this invention, plastic pipe material is doped with a magnetizable material and subsequently extruded to form a plastic pipe comprising the magnetizable material. The plastic pipe is then passed through a magnetizer by which a magnetic field is applied across the pipe diameter by magnetizer means disposed about the plastic pipe as it is extruded.

In accordance with one embodiment of this invention, current is pulsed through saddle-shaped coils 11,12 at an amplitude of about 4,000 amperes for one millisecond in a repetitive fashion. This short duration pulse aligns the magnetic particles in the pipe. The repetition frequency of the pulse is set fast enough so that complete magnetization of the pipe is accomplished as it passes through the magnetizer.

In order to magnetize the magnetic particles, a peak field of greater than about 6KG must be applied to the pipe. This field must be applied across the pipe diameter by a system that permits the relative rotation of the magnetizer around the pipe as the pipe is extruded, thereby enabling the pipe to be imparted with a spiral magnetization along its length. Peak fields less than about 6KG are not effective in imparting the desired spiral magnetization. In accordance with one embodiment of this invention, the magnetizer is

rotated around the exterior of pipe 10. In accordance with another embodiment of this invention, saddle-shaped coils 11,12 are permanently mounted and pipe 10 is rotated as it passes through the magnetic field generated by saddle-shaped coils 11, 12.

Fig. 2 shows a schematic diagram of a second magnetizer, a permanent magnet magnetizer, suitable for use in the process of this invention. The permanent magnet magnetizer comprises a plurality of magnets 13,14,15 arranged around a cylindrical bore through which the pipe 10 passes. In accordance with a particularly preferred embodiment of this invention, the permanent magnet magnetizer comprises niodynium iron boron magnets to magnetize the pipe; and the magnetizing field is created by an array of parallelpiped magnets.

In order to magnetize large diameter pipes, that is, pipes having a diameter greater than about eight inches, superconducting magnets are required to magnetize the pipe.

This is due to the fact that the magnitude of magnetic field energy required to create the magnetizing field becomes too large for conventional techniques to achieve as the pipe sizes increase above about eight inches in diameter. A superconducting magnetizer, as shown in Fig. 3, is, in principal, similar to the permanent magnet magnetizer shown in Fig. 2, with the permanent magnets 13,14,15 being replaced by a superconducting coil 16. In order to cool the coil to 4°K, the coil is preferably positioned inside a dewar containing liquid helium.

Both the dewar and the coil rotate about the pipe as the pipe is extruded in order to apply the required spiral magnetization.

Fig. 4 shows outer layer 20 positioned around pipe 10 and may alternatively comprise stripes, strips or rods extending along pipe 10. Such coating layer 20 is magnetized in a similar manner as discussed above.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.