Automotive-wire insulation

Details Automotive-wire insulation Automotive-wire insulation

1999-12-21

2002-03-19

Reichard, Dean A. (Department: 2831)

Electricity: conductors and insulators

Conduits, cables or conductors

Insulated

C174S12000C, C174S12000C

Reexamination Certificate

active

06359230

ABSTRACT:

FIELD OF THE INVENTION
The invention pertains to multi-layer polymeric formulations for protecting or insulating metallic objects and more particularly, to an integrated, tri-layer, thin-wall insulation composite, for use in a high temperature, automotive-wire article.
BACKGROUND OF THE INVENTION
Applying polymeric formulations to the surface of metallic objects has been a long practiced art. Typically, this is performed for several different reasons. Firstly, metallic objects are prone to oxidation or other chemical attack of their surfaces, degrading the utility of the metal object. Secondly, metals are inherently electrically conductive, metallic articles used for this purpose must be insulated to maintain the integrity of the electrical circuit. Examples of applications of such articles include such diverse uses as building wire, aircraft wire, plenum wiring for local area network computers and communication, transformer wiring for isolating magnetic and electrical wiring, underground wiring, and electrical conductive wiring for automotive and general electrical machinery.
Useful materials for covering metallic objects for the above purposes are organic polymer formulations. However, because of the varied uses of these metallic articles and the conditions that they are subjected to, it is difficult to find an ideal organic polymeric formulation that will itself survive under such conditions and uses, yet still provide a multitude of critical properties. Common properties include such physical and chemical characteristics as durability, flexibility, elasticity, toughness, adhesion, thermal stability, chemical resistance, flame retardance and minimal smoke generation. In some cases the protective layer may even be required to have properties that seem almost to be antipodal (e.g., abrasion resistance and flexibility).
Of the plethora of organic polymer compositions that are known or theoretically available, derivatives of polyethylene, especially crosslinked polyethylene, have secured a significant share of the market for these applications. Polyethylene homo- and copolymers offer a wide range of beneficial properties especially with regard to flexibility and elasticity. The common non-halogenated polyethylenes are also inexpensive to produce and relatively easy to process. However, they typically have poor abrasion resistance and impact resistance. To a degree this can be improved by crosslinking, but a trade-off in other properties are then observed. Lastly, and in some cases most importantly, the unhalogenated polyethylene derivatives are not chemically inert and will degrade under a variety of conditions. This degradation is observed during aerial exposure and on contact to certain metals, in particular, copper. Degradation is accelerated when the polyethylene derivatives are stressed, for example under elevated temperatures or physical distortion. Halogenated polyethylenes, specifically fluorinated derivatives, have superior chemical inertness; however, they suffer from poor adhesive properties and are extremely costly.
There is still a long standing need to find a polymeric system that will provide all the properties required for a metal protectant or insulation especially in uses that are physically stressful, that require longlife, that involve human safety, and that are difficult to replace. A number of these criteria exists for high-temperature insulation for automotive wiring.
Automotive wire located under the hood in the engine compartment (engine wire) has traditionally been insulated with a single layer of high-temperature insulation that is disposed over an uncoated copper-wire core. The U.S. specification that is usually used for this wire is SAE J1128, rev. January 1995 type TXL. The high temperature requirements for type TXL insulation, which require specific tensile and elongation standards as well, specifies that the insulation be oven aged without the conductor at 155° C. for 168 hours.
For certain newer automobiles, the high temperature specified for type TXL insulation is not sufficient to cover the actual temperatures existing in automobile engine compartments. The new requirements for engine wire range from 135° C. to 180° C. A typical test procedure now required is aging the insulation with the conductor for 3,000 hours at 150° C. as specified in International Standard ISO 6722-1-and 6722-2, rev. January 1996.
Most of the high-temperature wire used as engine wire in North America uses crosslinked polyethylenes (XLPE) as the insulation material. European manufacturers have obtained good high-temperature performance (3,000 hrs. at 155° C.) using thermoplastic polyesters. This insulation has outstanding resistance to gas and oil, is mechanically tough, and resists copper catalyzed degradation. Thermoplastic polyesters, however, can prematurely fail, because of hydrolysis. Thermoplastic polyester insulated wires have also been found to crack when exposed to hot salty water. They have also failed temperature humidity cycling as specified in the United States Car Specification PF-9600, Change A. As a result of these weaknesses, the use of thermoplastic polyester insulation has been limited in North America.
In addition to the foregoing discussion, the amount of wiring in automobiles has increased exponentially, as more electronics are being used in modern vehicles. This dramatic increase in wiring has motivated automobile manufacturers to reduce overall wire diameter by specifying thinner wall thicknesses, and specifying smaller conductor sizes. The forthcoming ISO 6722 specification (referred to as ultra-thin wire) reduces the insulation wall thickness to 0.20 mm. One automobile manufacturer in North America has released a new specification requiring a 0.15 mm wall thickness.
These reductions in insulation wall thicknesses pose manufacturing difficulties for wire fabricators. For XLPE, the thinner wall thickness of the insulation results in shorter thermal life, when aged at oven temperatures between 150° C. and 180° C. This limits their thermal rating. For example, a copper wire with an XLPE insulation having a 0.75 mm wall thickness is flexible, and does not crack when bent around a mandrel after being exposed to 150° C. for 3,000 hours. But this same copper wire, with the same XLPE insulation having a 0.25 mm wall thickness, becomes brittle after being exposed to 1500° C. for 3,000 hours.
The deleterious effects created by these extremely thin wall requirements have been attributed to copper catalyzed degradation, which is widely recognized as a problem in the industry.
XLPE insulation has many of the desired properties of the polyester materials, in addition to possessing good resistance to water. However, this material degrades when it comes into contact with copper at high temperatures.
It is possible to tin coat the copper core in order to prevent the copper from contacting the XLPE, but the additional cost of tin and the tin coating process are expensive. In addition, most automotive specifications require that the copper core be uncoated.
It is also possible to add copper stabilizers to the polyethylene compound, but copper stabilizers (such as Ciba Geigy Irganox MD-1024) yield only partial protection for wire having thin wall thicknesses, when used at 150° C.
Polyolefins are also known to undergo oxidative degradation, especially at elevated temperatures. This defect has been long known to those in the art but the solution to the problem is not easy to obtain without significantly degrading other beneficial properties of the polyolefin.
DESCRIPTION OF THE RELATED ART
Polyolefin insulated wires have been overcoated with a protective layer of fluorocarbon polymer (see U.S. Pat. No. 5,281,766 to Hildreth). This patent utilizes a fluorocarbon polymer as a protectant layer over the polyolefin insulation to protect against varnish. The varnish, which comprises a polyester or epoxy base, is applied during a bake process. If varnish is inadvertently applied to the polyolefin insulation, the insulation becomes brittle and cracks on bending. The fluoroc

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