WO1992008923A1

WO1992008923A1 - Locatable magnetic plastic duct and process of making same

Info

Publication number: WO1992008923A1
Application number: PCT/US1991/008458
Authority: WO
Inventors: Mark E. Lowell; Michael J. Homa
Original assignee: Tamaqua Cable Products Corporation
Priority date: 1990-11-15
Filing date: 1991-11-13
Publication date: 1992-05-29
Also published as: MX9102066A

Abstract

A flexible plastic duct (10) locatable by a magnetic detecting device after the duct is buried in the ground wherein the duct is extruded from a mixture of high density polyethylene, a very low density polyethylene and finely divided particles of a permanent magnet substance such as barium ferrite.

Description

LOCATABLE MAGNETIC PLASTIC DUCT AND PROCESS OF MAKING SAME

Field of the Invention

The present invention relates to the extrusion of non-metallic flexible ducts suitable for housing fiber optic cable or the like and locatable by a magnetic detecting device after the duct is buried in the ground, and relates to an improved method and product produced by said method. Background of the Invention

It is well known to extrude non-metallic flexible ducts around electrical cables or conductors or of empty duct which may incorporate a pulling cord. One particular use to which cables encased in ducts are put is in underground electrical installations. In such installations, a trench is typically dug and the duct having the cable extending therethrough is placed in the trench. Thereafter, the trench is covered with earth and the cable and duct are buried. Because the nature of the various electrical installations may vary from time to time, and further because of problems or difficulties which may be encountered with respect to various conductors of the cable, or with the cable itself, repair or replacement of underground cables may become necessary. Electrical duct extrusion apparatus of the foregoing type is disclosed in U.S. Patent 4,508,500 incorporated herein by reference thereto.

It is often desirable to bury fiber optic telecommunication lines or cable in the ground. Since the fiber optic members are fragile, means of protecting the fiber optic cable from mechanical stresses are required. The two usual methods of protecting the fiber optic cable are to either include a metallic strength member on the cable itself or to place the cable inside a pipe or duct system. Duct systems made of continuous lengths of high density polyethylene (HDPE) are preferred by many telecommunication companies because an all dielectric (non¬ conducting) system eliminates the susceptibility of the cable to lightning damage. Apparatus for manufacturing duct of this type is disclosed in the aforesaid patent 4,508,500. The disadvantage of an all dielectric system is the lack of electrical current carrying capacity to be used with conventional methods to find the duct and cable after it is buried in the ground. When problems requiring direct access to the fiber optic cable occur, easy location of the buried cable is crucial. One method of detecting buried plastic ducts involves the use of metal such as a copper wire or strand which is run at a shallower depth over the buried duct path and which is detectable by means of well known devices such as metal detectors. Another method of locating telephone cables or electrical lines buried beneath the surface of the earth is by disposing above the lines and below the surface of the earth a flexible metal foil product, particularly a steel foil in sheet or tape form and protected against corrosion, so that the presence and general location of the metal foil is detectable from above the surface of the earth by electronic or like detecting devices. Such a method is disclosed in U.S. Patent No. 3,504,503.

It would be desirable to provide a locatable plastic duct extruded from an HDPE based material which can be processed in place of the conventional HDPE material, bears a permanent magnetic field, and is non-conductive. Such plastic material should meet certain minimum mechanical strength requirements and be easily processed in the form of duct on standard extrusion equipment such as disclosed in aforesaid patent 4,508,500. Such material should be capable of maintaining a permanent magnetic field which is strong enough to be detected from the depth of approximately five feet below the ground surface, aiinmmn-γ r»f the Invention

In accordance with the present invention there is provided a plastic duct suitable for housing fiber optic cable or the like and locatable by a magnetic detecting device after the duct is buried in the ground wherein the duct is extruded from a mixture of high density polyethylene, very low density polyethylene and finely divided particles of a permanent magnet substance. The permanent magnet substance preferably comprises barium ferrite particles which comprises at least 20% by weight of the mixture. The mixture also preferably includes at least 20% by weight of high density polyethylene and at least 14% by weight of low density polyethylene. In one form of the invention the very low density polyethylene and the barium ferrite particles comprise a masterbatch mixture which is 40% by weight very low density polyethylene and 60% by weight barium ferrite particles. In another aspect of the invention the mixture from which the plastic duct is extruded includes high density polyethylene within the range from about 20% by weight to about 66% by weight, very low density polyethylene within the range from about 14% by weight to about 32% by weight and barium ferrite particles within the range from about 20% by weight to about 48% by weight.

In accordance with a further aspect of the invention there is provided a process for manufacturing a plastic duct suitable for housing fiber optic cable or the like and locatable by a magnetic detecting device after the duct is buried in the ground comprising the steps of mixing finely divided particles of a permanent magnet substance with a non-magnetic matrix including a high density polyethylene and very low density polyethylene, heating and extruding the mixture into a duct, cooling the duct, and magnetizing the cooled duct by passing the duct through an electromagnetic field. Brief Description of the Drawings

Fig. 1 is a perspective view showing the cross section of a flexible magnetic plastic duct manufactured according to the present invention and having conductors traversing therethrough.

Fig. 2 is a plan view in schematic form of a system for manufacturing the duct of Fig. 1. Detailed Detailed Description of the Preferred Embodiment

In accordance with the present invention the use of ferrite or like material incorporated in a special medium which is added to a thermoplastic polymer and then magnetized provides the basis for detection of a non- metallic duct which is directly buried in the ground. The duct 10, Figs. 1 and 2, is a flexible conduit normally enclosing electric cables 12 of various types including fiber optic cables. The incorporation of ferrite or like material in an extruded thermoplastic duct enables the duct to be found even if buried in the ground and all evidence of precise location is not available. The locatable duct manufactured according to the present invention will retain its magnetism for an indefinite period so that a considerable time may lapse between burial of the duct and the time an accurate route has to be traced. The particles to be magnetized have to be incorporated in a medium which is compatible with the normal duct material so that the physical properties of the normal duct material are not degraded or weakened to the extent that the prime function of the duct is impaired. Incorporation of the ferrite containing material and magnetic alignment is performed at normal processing speed and coincidental with the forming of the duct as described in the aforesaid patent 4,508,500. Permanent magnets of barium ferrite filled polymers have been well known for many years. For example, see U.S. Patent 3,051,988. The most common use of such magnets is in refrigerator door seals. Because heavy filler loading and high flexibility are required for most applications, polyethylene type polymers have not been used as the carrier polymer for the barium ferrite.

Polyethylene homopolymers such as HDPE are not suitable carriers because they have high crystallinity and low polarity which makes it difficult or impossible to obtain heavy filler loadings. It is common for barium ferrite filled compositions to contain upwards of 90% by weight barium ferrite; whereas, the maximum filler loading possible with HDPE is about 10% by weight which is unacceptably low for a locatable duct application. To overcome this limitation, a new type of polyethylene namely a very low density polyethylene (VLDPE) sold by Union Carbide under the designation DFDA-1138 NT has been chosen as a carrier for the barium ferrite. Because very low density polyethylene (VLDPE) has low crystallinity, it can therefore bear up to at least 60% by weight barium ferrite. However, the VLDPE is much weaker than HDPE which precludes it from use as duct compound for direct unground burial uses. The HDPE has a specific gravity of about 0.94 to about 0.96 and VLDPE has a specific gravity of about 0.89 to about 0.915.

Being a homopolymer VLDPE is compatible with HDPE so that HDPE can be mixed with the VLDPE/barium ferrite composition to increase the mechanical strength of a final composition. A mixture of HDPE, VLDPE, and barium ferrite has been obtained which contains at least 20% by weight barium ferrite, meets minimum mechanical requirements, and can be extruded on the standard extrusion apparatus disclosed in the aforesaid Patent 4,508,500. A schematic illustration of the extrusion apparatus disclosed in Patent 4,508,500 is illustrated in Fig. 2 of the present application with corresponding parts identified with reference characters corresponding to those used in the patent. As shown in Fig. 2 the system 20 preferably includes three components namely an extruder 22, a differential calibrating tank 24 for sizing and cooling the just-extruded duct trough 26 for further and final cooling of the duct after it traverses the differential pressure calibrating tank 24. The extruder 22 preferably comprises a resin hopper 28 for receiving resin pellets of HDPE and compound pellets of VLDPE and BaFe. The extruder 22 preferably includes a feed screw mechanism 30 for heating and conveying the pellets and for directing extrudable material to an extrusion die chamber within the extruder 22. The extrusion die chamber preferably includes an input opening 34 to which the individual electrical conductors 12 are directed and a die orifice 36 from which just-extruded duct 10 emerges with the conductors 12 passing therethrough.

A suitable composition of the plastic locatable duct is a combination of an ASTM D-1248 Grade P34 high density polyethylene (HDPE) such as Union Carbide's HFDA- 7580 NT and a masterbatch mixture which is 40% by weight of Union Carbide's DFDA-1138 NT very low density polyethylene (VLDPE) and 60% by weight Stackpole Corporation's Ceramagnet BG-1 barium ferrite (BaFe) . A Banbury mixing process was used to mix the VLDPE and BaFe. After the

Banbury mixing process, the masterbatch mixture or compound was formed into pellets and packaged in boxes. The compound pellets and HDPE were combined at selected ratios in the hopper 28 and mixed during the final extrusion process and formed into a continuous tubular solid referred to as duct 10. The term duct as used herein is intended to include conduit and pipe.

Various ratios of the masterbatch mixture and HDPE were mixed and examined to determine the total loading versus final physical properties of the duct. The following are examples. Example No. 1

This example had a ratio of 52/48% by weight mixture of masterbatch mixture/HDPE giving a final mixture of 20.8% by weight VLDPE, 31.2% by weight BaFe and 48% by weight HDPE. The mixture after heating to a molten state was extruded into a 1.25" diameter nominal duct with a 0.200" thick wall using extrusion apparatus of the type including provision for sizing and cooling just-extruded duct as disclosed in the aforesaid patent 4,508,500 and in Fig. 2. The physical properties of the duct in Example 1 after cooling were 3500 psi tensile strength and 875% elongation which exceed the minimum requirements of 3200 psi and 500% for an ASTM grade P34 HDPE compound. Sections of the cooled duct were magnetized by passing the samples between the poles of a strong electromagnet, Fig. 2, with a field strength of about 10,000 Oersteds (Oe) . Each sample of duct was marked with a straight white line on the outer surface and parallel to the center axis of the duct. The line marked the north pole direction. When the samples of duct were passed through the electromagnet, the white line on each sample was aligned with the north pole. A test burial site was prepared and the samples were buried at a depth of 5 feet below ground level. All five samples were buried in a straight line with the white line or north pole on top. Adjacent samples were separated by six feet. This arrangement of samples was made to create a pattern in the magnetic signal to be detected. This caused an alternating on/off signal as the detection device was passed along the trench. The detection device that was used was a hand held wand that contained one or more magnetometers from which magnetic flux density was detected and an audio signal was emitted in proportion to the flux density. This type of detection device is commonly used to detect the magnetic fields generated by electric current in cables. For the test burial, a person without knowledge of the exact location or pattern of burial was instructed in the use of the detection device and told of the general vicinity of the samples. Given a general position within a circular area of 50 feet in radius, the person searching for the test samples was able to find the exact location of all five samples.

Larger ratios of the masterbatch mixture/HDPE have been successfully extruded and tested for physical properties to vary the percentage of barium ferrite in the final mixture. It is expected that the larger ratios will enhance the capability of detection since they contain a larger percentage of barium ferrite. Example No. 2

This example had a ratio of 58/42% by weight mixture of masterbatch mixture/HDPE giving a final mixture of 23.2% by weight VLDPE, 34.8% by weight BaFe and 42% by weight HDPE. This mixture was extruded into a 1.00" diameter nominal duct with a 0.188" thick wall. The physical properties of this duct were 3300 psi tensile strength and 810% elongation. It will be noted that this exceeded the minimum requirements of 3200 psi and 500% elongation for an ASTM grade P34 HDPE compound. Example No. 3

This example had a ratio of 72/28% by weight mixture of masterbatch mixture/HDPE giving a final mixture of 28.8% by weight VLDPE, 43.2% by weight BaFe and 28% by weight HDPE. The mixture was extruded in a 1.00" diameter nominal duct with a 0.188" thick wall. The duct had physical properties of 2539 psi tensile strength and 760% elongation. While the physical properties of this example of duct are below the minimum requirements set forth above for an ASTM grade P34 HDPE compound it should be noted that the P34 requirements are arbitrary and have not necessarily been demonstrated as absolute minimums for the application. If the requirements for the particular application dictate increased loading of the masterbatch mixture of VLDPE and BaFe in HDPE above 70%, it may be possible to allow lower minimum tensile strength for the duct.

In addition to the tensile requirements of P34, it is required that HDPE compounds exceed 192 hours without failure when tested according to the procedures of ASTM 1693 Condition C for environmental stress crack resistance (ESCR) . The sample ducts made in Examples 1, 2 and 3 all passed the ESCR requirement. Example No. 4

An example near the upper limit of loading is 80/20% by weight mixture of masterbatch mixture/HDPE giving a final mixture containing 32% by weight VLDPE, 48% by weight BaFe and 20% by weight HDPE. Example No. 5

An example near the lower end of the loading of masterbatch mixture in HDPE is 34/66% by weight mixture of masterbatch mixture/HDPE giving a final mixture containing 13.6% by weight of VLDPE, 20.4% by weight BaFe and 66% by weight HDPE. From the foregoing examples it will be seen that the mixture includes high density polyethylene (HDPE) within the range from about 20% by weight to about 66% by weight, very low density polyethylene, (VLDPE) within the range from about 14% by weight to about 32% by weight and barium ferrite particles (BaFe) within the range from about 20% by weight to about 48% by weight.

While there has been described a preferred embodiment of the invention using barium ferrite particles, it will be understood that further modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims. For example other permanent magnet materials may be used such as manganese-bismuth compounds and iron-strontium. The magnetic duct produced according to the present invention is not limited to use for housing underground cable but is also useful for other applications such as underground pipe for conveying various substances.

Claims

What is claimed is;

1. A flexible plastic duct suitable for housing fiber optic cable or the like and locatable by a magnetic detecting device after the duct is buried in the ground to a depth of about five feet wherein said duct is extruded from a mixture of high density polyethylene, very low density polyethylene and at least 20% by weight of finely divided particles of a permanent magnet substance.

2. A duct according to claim 1 wherein said permanent magnet substance comprises barium ferrite particles.

3. A duct according to claim 1 wherein said mixture includes at least 20% by weight of high density polyethylene and at least 14% by weight of very low density polyethylene.

4. A duct according to claim 2 wherein said mixture includes high density polyethylene within the range from about 20% by weight to about 66% by weight, very low density polyethylene within the range from about 14% by weight to about 32% by weight and barium ferrite particles within the range from about 20% by weight to about 48% by weight.

5. A process for manufacturing a flexible plastic duct locatable by a magnetic detecting device after the duct is buried in the ground comprising the steps of mixing finely divided particles of a permanent magnet substance with a non-magnetic matrix including a high density polyethylene and a very low density polyethylene, heating and extruding the mixture into a duct, cooling the duct, and magnetizing the cooled duct by passing the duct through an electromagnetic field.

6. A process according to claim 5 wherein said duct is magnetized by passing the duct between the poles of an electromagnet with a field strength of at least 5,000 oersteds.

7. A process according to claim 5 wherein said mixture is extruded into a duct about a cable.

8. A process according to claim 5 wherein said mixture comprises at least 20% by weight of finely divided particles of permanent magnet substance.

9. A process according to claim 8 wherein said permanent magnet substance comprises barium ferrite particles.

10. The process according to claim 9 wherein said mixture includes at least 20% by weight of high density polyethylene and at least 14% by weight of very low density polyethylene.

11. A process according to claim 9 wherein said very low density polyethylene and said barium ferrite particles comprise a masterbatch mixture which is 40% by weight very low density polyethylene and 60% by weight barium ferrite particles.

12. A process according to claim 11 wherein said mixture includes high density polyethylene within the range from about 20% by weight to about 66% by weight, very low density polyethylene within the range from about 14% by weight to about 32% by weight and barium ferrite particles within the range from about 20% by weight to about 48% by weight.