WO1994026960A1

WO1994026960A1 - Light colored conductive sealant material and method of producing same

Info

Publication number: WO1994026960A1
Application number: PCT/US1994/004911
Authority: WO
Inventors: George S. Cantler; Ira S. Dunoff; Loretta A. Whelan; William P. Mortimer; John W. Dolan
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1993-05-07
Filing date: 1994-05-04
Publication date: 1994-11-24
Also published as: JPH08510018A; AU6781694A; CA2160026A1; EP0697039A1

Abstract

A composition and method for producing a light-colored thermally conductive fiber is provided. The fiber comprises a polytetrafluoroethylene and a light-colored thermally conductive filler, such as boron nitride, which are combined and sheared to coat the polytetrafluoroethylene and render it thermally conductive. The fiber of the present invention can be incorporated into virtually any form of packing/sealing material and is particularly applicable for use in industries where shedding of dark particles such as graphite from packing material must be avoided.

Description

LIGHT COLORED CONDUCTIVE

SEALANT MATERIAL AND METHOD OF PRODUCING SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to thermally conductive fibers used in a variety of applications and especially as packings and seals.

2. Description of Related Art A packing is a sealing material used to minimize leakage between two components of a fluid container, and especially in containers where the components undergo motion relative to each other, such as in a pump. A good packing material should have a number of properties, including: fitting correctly in the packing space, being able to withstand inherent temperature and pressure conditions, being negligibly affected by the fluid being sealed, and being sufficiently flexible to accommodate varying degrees of longitudinal and/or radial displacement.

Common packings comprise fibers which are first woven, twisted, braided or otherwise joined together, and then formed into appropriate shapes (e.g. coils, spirals, or rings) for insertion around a shaft or other component.

For packings of high speed pumps and similar devices, the packing material should permit the escape of small amounts of liquid to help reduce friction and heat build-up between the components. Ideally in such environments, the packing should also have a relatively high thermal conductivity to assist in dissipating frictional heat generated by the movement of the component parts. In order to achieve most of these properties, it is common today to employ packing and sealant material made from polytetrafluoroethylene (PTFE) coated or impregnated with graphite or similar material. The chemical and biological inertness of this material combined with its exceptional lubricity makes it a highly desirable packing material, particularly in chemical, food, drug, and pulp and paper industries.

Regrettably, many of these materials suffer from one or more major deficiencies. First, in those materials employing a simple graphite coating, there is a tendency to shed off graphite particles during use—resulting in significant amounts of dark contamination around the pumps and often in the chemical stream. Second, in those materials which do not employ a coating, PTFE alone is a thermal insulator which tends to be inadequate in dissipating heat. Both of these deficiencies were significantly improved by the fiber and process disclosed in United States Patent 4,256,806 issued March 17, 1981, to Snyder. United States Patent 4,256,806 teaches a process for producing a smudge-free graphite-impregnated expanded PTFE packing material. In this process, a fine powder dispersion of PTFE is combined with a liquid lubricant and graphite and mixed with sufficient shearing force to form a thermally conductive expanded PTFE material which is resistant to shedding graphite. Such a material is now commercially available under the trademark GFO fiber from W. L. Gore & Associates, Inc. of Elkton, MD.

Although the fiber of United States Patent 4,256,806 is now the preferred packing material for many applications, a packing which incorporates a dark colored material has caused concern in some industries. For example, for use in the handling of paper pulp or similar material which must remain extremely clean of any dark particle contamination, many manufacturers prefer to use a light-colored packing material to avoid any risk of costly contamination. Unfortunately, none of the light colored packing material presently available provides sufficient lubricity and thermal conductivity to achieve the desired level of pump protection.

A typical example of such light-colored material comprises a fiber of expanded PTFE dipped in an aqueous dispersion of tetrafluoroethylene (TFE) and silicone oil. Such a standard grade white packing material 1s available from U. L. Gore & Associates, Inc. under the trademark GORE-TEX (prelubricated) fiber. Although this material 1s quite acceptable for light-colored applications and provides very good lubricity, its thermal conductivity is considerably lower than the material taught in United States Patent 4,256,806.

Accordingly, it is a primary purpose of the present invention to provide a fiber and method for producing it which is light- colored while being sufficiently thermally conductive to assure adequate component protection when used as a packing material. It is a further object of the present invention to provide such a fiber which can be employed in a variety of applications where shedding of any particulate matter is undesirable and shedding of dark particulate matter is unacceptable.

These and other purposes of the present invention will become evident from review of the following specification.

SUMMARY OF THE INVENTION

The present invention provides an improved composition and method to produce a material suitable for use in packing and sealing which is both thermally conductive and Ught-colored. While contributing necessary lubricity and thermal protection for component parts, the fiber of the present Invention avoids risk of dark particulate contamination in light colored manufactured products such as paper, food, pharmaceuticals, and chemicals. Additionally, in certain embodiments the material of the present invention has proven to be electrically non-conductive, which makes it uniquely applicable to for use in electrical insulation and as a non-corrosive packing material, such as in- marine environments to reduce or eliminate galvanic corrosion.

The present Invention employs a combination of polytetrafluoroethylene (PTFE) and a light-colored thermally conductive filler material such as boron nitride or tin powder.

These components are agitated together, preferably in the presence of a mixing medium, to causing shearing and encapsulation of the conductive material within the PTFE. By subsequently heating and expanding the PTFE, a light-colored, durable and slippery fiber is provided with sufficient thermal conductivity to be suitable for all but the most extreme mechanical conditions. Although the amount of particulate shedding is minimal with this process, the use of light-colored thermally conductive material assures that light-colored products will not be contaminated from occasional shedding of fiber or conductive filler. A further embodiment of the present invention employs the above described fiber or a fiber of expanded PTFE, preferably a towed fiber, which is impregnated and/or coated with a dispersion of tetrafluoroethylene, a light-colored thermally conductive filler, and a lubricant. Mechanical working of the coated fiber shears the dispersion and provides a light-colored thermally conductive fiber.

The present invention can be applied in any suitable manner, including as a twisted, braided or woven fiber, and shaped for virtually any form of application, including as sheets, tubes, rings, spirals, or coils.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved light-colored fiber which is thermally conductive and suitable for use in a variety of applications, and particularly for use as a packing material to assist in sealing around component parts to reduce or eliminate fluid leakage.

In the first embodiment of the present invention, the fiber is formed by mixing together a fine powder dispersion of polytetrafluoroethylene (PTFE), a mixing medium such as a mineral spirits, and a light-colored thermally conductive filler such as boron nitride. The component parts are combined in proportions as described below and mixed in the following general manner.

First, the light-colored conductive filler and water are mixed to form a slurry. A dispersion of fine powder PTFE is then added to the slurry and vigorously agitated, preferably in the presence of the mixing medium, until the mixture coagulates. Mixing is complete once the coagulated solids precipitate to the bottom of the container in the form of a coagulum, leaving a substantially clear effluent. The coagulum is then thoroughly dried, such as through use of a convention oven or similar means, to remove the water.

The dried coagulum formed in this process can then be formed or worked in any suitable manner, including heated and expanded in a process such as that disclosed in United States Patent 3,953,566, issued April 27, 1976, to Gore. Preferably, the coagulum is ram extruded into a paste or tape. The tape can then be heated to approximately 250-350°F and stretched approximately 2 to 150 times its original dimensions to form a tape of expanded PTFE (ePTFE). The tape can then be further treated in a variety of manners, including being slit and formed into fibers, driven through cutting elements to form a tow, etc.

This process can be performed with a broad range of beginning proportions, such as of 2-75% by dry weight boron nitride filler, 15-85% by dry weight PTFE, and 10-30% by weight mineral spirits. Through this process, a tape is produced with a boron nitride content of 2-75% and a PTFE content of 25-98%. The fiber of this composition is preferred for high pressure applications and in processes which are sensitive to oil contamination. PTFE fine powder dispersions are obtained by polymerization of tetrafluoroethylene (TFE) in liquid water containing suitable dispersing agent. The preferred dispersion for use in the present invention comprises 30% by weight PTFE solids. Suitable material is available from ICI Americas, Inc. of Wilmington, DE, under the trademark FLU0N(AD-l).

The mixing medium may comprise any substance which can provide sufficient lubricity in mixing or extruding processes to allow the PTFE dispersion to be sheared. In addition to mineral spirits, other suitable lubricants include water, silicone oil, kerosene, naptha, propylene, petroleum extractants, and other similar lubricants.

The boron nitride is preferably a fine powder. This material is available from Advanced Ceramics of Cleveland, Ohio, under the trade designation HCP grade. Although boron nitride is the preferred filler for use in the present invention, a number of other light-colored thermally conductive materials may also be used. Examples include aluminum oxide, tin, zinc oxide, calcium oxide, or glass fiber.

For some applications it is desirable to coat and/or impregnate the fiber with a liquid lubricant to decrease further its coefficient of friction. It should be appreciated that the term "coating" as used herein 1s intended to encompass any application of the mixture of the present invention onto or into a substrate, whether merely applied over the surface of the substrate or impregnated below the substrate's surface. In its simplest form, this coating process comprises dipping, spraying or otherwise covering the fiber of the present invention with the lubricant. The lubricant should have kinematic viscosity of about 50,000 centistokes or less, and preferably a kinematic viscosity of 1000 centistokes or less. Among the suitable lubricants are silicone oil, mineral oil, paraffin wax, or petroleum based oil. Preferably, the lubricant comprises polydimethyl siloxane, such as that sold by Dow Corning Corp. of Midland, MI, under the designation DOW CORNING 200.

A preferred coating for use with the present invention is a liquid or paste comprising PTFE dispersion, light-colored thermally conductive filler, and the lubricant. This coating can be formed by combining 25-75% by weight light-colored thermally conductive filler (e.g. boron nitride powder), 20-70% by weight lubricant (e.g. polydimethyl silica), and 25-80% by weight dispersion of PTFE (e.g. 60% solids dispersion available from E.I. duPont de Nemours and Co., of Wilmington, DE, under the trademark TEFLON). The components are blended thoroughly to form a relatively uniform mixture.

Once the mixture is formed, the coating can be applied to a variety of substrates to produce a thermally conductive fiber. Among the substrates which can be employed are PTFE, expanded PTFE, PTFE composites (e.g. plated, filled, or Impregnated PTFE), fiberglass, polyimides fibers, acrylics, etc. Preferably, the coating 1s combined with the light-colored thermally conductive tape previously described. This may be accomplished through any suitable means, including by dip coating the fibers or tape in the coating, intermixing the coating between fibers, merging the tape and the coating, or coextaiding coating and substrate.

Another embodiment of the present invention employs a substrate of towed expanded PTFE fiber (either filled in accordance with the present invention or unfilled) which is dipped or otherwise coated with a composition of fluoropoly er (e.g. PTFE or tetrafluoroethylene (TFE)) mixed with the light-colored thermally conductive filler. By mechanically working the coated substrate, the composition can be sheared on and in the substrate to impart suitable thermally conductive properties.

Since the terms PTFE and TFE are sometimes used interchangeably, especially when referring to original supplies of PTFE which contain very short chain homopolymers of TFE, except as is specifically addressed herein, the term PTFE is intended to encompass any polymer of TFE regardless of length.

Mechanical working of the substrate may take any appropriate form, such as sliding it across one or more fixed surfaces, rotating it around one or more spool surfaces, driving it between nip rollers, or driving it through "counter-current" rollers actuated in the opposite direction from the direction of material movement. Alternatively, mechanical working of the composite fiber can be avoided or limited by shearing the coating prior to application to the substrate and applying the coating before its water base evaporates.

In all forms of the present invention, the combined proportions of components in the substrate and the coating in the final product should generally comprise: 25-75% by weight light- colored thermally conductive filler (within a broad range of 10- 95%); 10-40% by weight lubricant (within a broad range of 10-60%); and 45-85% by weight expanded PTFE and/or TFE (within a broad range of 5-90%). Ideally, the composition comprises 30-60% boron nitride, 20-30% lubricant, and 50-70% PTFE and/or TFE.

The composition of the present Invention can be used in a variety of applications. For example, the material can be formed as fibers, sheets, tubes, beading, tapes, etc. As fibers, the composition can be formed into packing material through any known manner, such as braids, wovens, composites, twisted ropes, etc. Preferably, the material is calendered or otherwise formed to achieve its operative shape.

These fibers are particularly useful as pump packings, valve stem packings, and similar products when braided Into a square or round cross section. The combination of the material properties and the dimensions will form dynamic and static liquid seal inside a pump stuffing box and valve body. This material can be readily braided into square sizes ranging from 0.125 inches (0.32 cm) to 3 Inches (7.6 cm). The material can also be used as a filler fiber within a braid or a jacket fiber over core material. Other possible applications for this material include as sealing devices (e.g. as gaskets sealing an opening), heat sinks, electrical insulation, etc.

The fibers of the present invention have numerous advantages over existing packing materials. First, when produced in the manner described, the thermally conductive filler tends to be encapsulated within the PTFE and is resistant to shedding or "smudging" off the completed fiber. This resistance to smudging is further improved by processes such as braiding. Second, even upon occasional shedding of filler or fiber, the light-colored nature of the fiber of the present invention assures that no visual contamination of the chemical stream will occur. This makes the fiber far more acceptable to customers for use in color sensitive environments such as pulp and paper production, some food and pharmaceutical applications, marine applications, light-colored chemical production, etc. Third, as should be evident from review of the following examples, unlike presently available light-colored packing materials, these fibers have demonstrated very good operational properties, such as lubricity, thermal conductivity, and coefficients of thermal expansion.

One of the more surprising and promising aspects of the present invention is that it produces a fiber which is highly non- galvanic and anti-corrosive. Unlike most existing thermally conductive fibers which are also electrically conductive, such as graphite filled fibers, the fibers of the present invention are both thermally conductive and electrically non-conductive. As a result, the fibers of the present invention can be freely placed in direct contact with metals subject to oxidation (e.g. iron or steel), even in the presence of a corrosive media like seawater, without risk of establishing a corrosive galvanic cell.

Accordingly, the fiber of the present Invention is particularly useful as a packing or sealing media in difficult corrosive environments, such as in submerged marine applications.

Without intending to limit the scope of the present invention, the composition and method of the present invention may be better understood in light of the following examples EXAMPLE 1

A slurry of 4.38 Kg of boron nitride and 55.01 of de ionized H2O was prepared in a 1151 baffled stainless steel container. The boron nitride was grade HCP obtained from Advanced Ceramics, Inc. While the slurry was agitating, 4.32 Kg. of PTFE in the form of a 15.7% dispersion was rapidly poured into the vessel. The PTFE dispersion was an aqueous dispersion obtained from ICI Americas Inc. The mixture coagulated and after 2 minutes the mixer was stopped. The solids settled to the bottom of the vessel and the effluent was clear.

The coagulum was dried at 165°C in a convection oven for 24 hours. The material dried in small cracked cakes and was chilled to below 0°C. The chilled cake was hand ground using a tight circular motion with minimal downward force through a Θ.635 cm stainless steel screen, then 0.267 Kg of mineral spirits per Kg of powder was added. The mixture was chilled, again passed through a 0.635 cm screen, tumbled for 10 minutes, then allowed to sit at 18°C for 24 hours.

A pellet was formed in a cylinder by pulling vacuum and pressing at 850 psi. The pellet was heated to 49°C and then extruded into tape form.

The tape was then calendered through heated rolls to 0.043 cm. The lubricant was dried and the tape was stretched by running it across heated rolls at 275°C maximum temperature, at a stretch rate of 5.9-1 ratio and 48.7 meters/min. output speed.

EXAMPLE 2

The composition from Example 1 was employed. The tape was stretched across a heated plate at 385°C, at a stretch rate of 3.4:1 ratio and 34 meters/min output speed. The tape was then stretched across heated plate at 365°C at a ratio of 1.2:1 and 41 meters/min. output speed. The tape was sintered across a heated plate at 405^βC at 42 meters/min. The tape was then slit into a fiber 1.5 inches (3.8 cm) wide and the fiber was converted into a 0.375 inch (0.95 cm) square braid. The braid incorporated 196 fibers.

EXAMPLE 3

Braid was made as per Examples 1 and 2. The braid was dipped into a 1000 centistoke silicone oil to form a lubrication coating thereon.

EXAMPLE 4

Tape was made as per Examples 1 and 2. The tape was calendered through heated rolls to 0.0008 Inches (0.002 cm) thick. The tape was then slit into a fiber 1.5 inches (3.8 cm) wide and the fiber was converted into a 0.375 inch (0.95 cm) square braid. The braid incorporated 108 fibers.

EXAMPLE 5

Tape was made as per Examples 1 and 2. The tape was calendered through heated rolls to 0.0012 inches (0.003 cm) thick. The tape was then slit into a fiber 1.5 inches (3.8 cm) wide. The fiber was then converted into a .0375 inch (0.95 cm) square braid. The braid incorporated 108 fibers.

EXAMPLE 6

Tape was made as per Examples 1 and 2. Multiple pieces of tape were merged with a thermally conductive liquid comprised of 13.2% by weight of a 60% aqueous PTFE dispersion, 29.4% by weight of HCP grade boron nitride, 28% by weight water, 27.5% by weight of a 1000 centistoke silicone oil, 1.8% by weight on a non-ionic surfactant, and 0.1% by weight of a 5% ammonium hydroxide. The composite was dried in an oven at 190°C for 18 hours. The composite was slit into a fiber approximately 0.6 Inches (1.5 cm) wide. The fiber was converted into a 0.375 inch (0.95 cm) square braid. The braid incorporated 42 fibers.

EXAMPLE/ THERMAL THEORETICAL MATERIAL CONDUCTIVITY COEFFICIENT

(W/m-K) OF

EXPANSION

(ppm)

EXAMPLE 2 0.34 85

EXAMPLE 3 0.48 85

EXAMPLE 4 0.51 85

EXAMPLE 5 0.53 85

EXAMPLE 6 0.90 57

GFO FIBER 1.47 55

100% PTFE fiber 0.24 159

G-2 (graphite filled fiber) 0.85 83

--Thermal conductivity was measured and calculated under the following parameters. Thermal conductivity is the material property that determines the amount of heat that will flow through an object when a temperature difference exits across the object. Thermal conductivity is a steady state property; it can only be directly measured under conditions in which the temperature distribution is not changing and all heat flows are steady. In the instant case, thermal conductivity was determined in a manner similar to that described in ASTM (American Society for Testing and Materials) Standard Test Method E 1225 (Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative -Longitudinal Heat Flow Technique).

Specifically, thermal conductivity was measured at a nominal temperature of 200°C using the guarded comparative longitudinal heat flow technique. The samples were submitted as braided lengths of material. For testing, the samples were cut into 2-inch lengths; these were laid side-by-side to produce test samples 2 inches square. No thermocouples were placed In the test samples. Surface temperatures of the test samples were obtained by extrapolation from thermocouples in the reference samples. PYREX material type-7740 was used as the thermal conductivity reference material .

The fundamental equation that governs steady-state heat flow in a slab geometry is:

where Q - the rate of heat flow through the slab (W or Btu/h) k = the thermal conductivity of the slab material (W/m K or Btu/h ft °F)

DS * the temperature difference across the slab

(°C or ^βF) ΔX - the thickness (m or ft) A - the cross sectional area (πr or ft*). Materials that have low values of thermal conductivity allow only a small amount of heat flow and are called thermal insulators. Materials with large values of thermal conductivity allow more heat to flow across the slab with the same temperature difference. Thermal conductivity is a material property and does not depend upon the geometry of the sample. In general, thermal conductivity is a function of the mean sample temperature.

A thermal heat flow circuit was used which is generally analogous to an electrical circuit with resistances in series. The PYREX 7740 material was chosen because it has a thermal conductivity close to that estimated for the samples Reference standards (also known as heat meters) having the same cross- sectional dimensions as the sample were placed above and below the sample An upper heater, a lower heater, and a heat sink were added to the "stack" to complete the heat flow circuit. The temperature gradients (analogous to potential differences) along the stack were measured with type K (chromel/alumel) thermocouples placed at known separations. The thermocouples were placed into holes or grooves in the reference material and also in the sample whenever the sample was thick enough to accommodate them.

The stack was clamped with a reproducible load to insure intimate contact between the components. In order to produce a linear flow of heat axially through the stack and reduce the amount of heat that flows radially, a guard tube was placed around the stack and the intervening space was filled with insulating grains of vermiculite or zeolite. The temperature gradient in the guard was matched to that in the stack to reduce radial heat flow further. The comparative method is a steady state method of measuring thermal conductivity. When equilibrium was reached, the heat flux (analogous to current flow) down the stack was determined from the references. The heat into the sample is given by the following formula:

and the heat out of the sample is given by:

Qout * ^λbottom(^dτ/^dχ)bottom where λ - thermal conductivity dτ/dx » temperature gradient and "top" refers to the upper reference while "bottom" refers to the lower reference. If the heat were confined to flow just down the stack, then Qi_n and Q₀ut would be equal. If Q-j_n and ₀ut ^are in reasonable agreement, the average heat flow is calculated from: Q - (Qin + Qout)/2.

The sample thermal conductivity is then found from ^λsample - Q/(dτ/dx)_sampi_e. --Theoretical Coefficient of Expansion was derived by determining the coefficient of thermal expansion of each of the components from product literature or through established sources. The overall coefficient of thermal expansion was determined by multiplying each of the individual component's coefficient of thermal expansions by its weight percentage in the mixture and taking the sum of this amount for the combined components in the mixture.

EXAMPLE 7

An expanded PTFE fiber, from W. L. Gore & Associates, Inc., of Elkton, MD, was formed into a tow material by passing it through a series of rotating cutting elements. The towed expanded PTFE fiber was then dipped into an aqueous tetrafluoroethylene (TFE) homopoly er dispersion including a doping of about 10% by weight tin powder. The TFE dispersion comprised a 60% solution of TFE solids suspended in deionized water. The dispersion was acquired from ICI Americas, Inc., of Wilmington, DE, under the trademark FLU0N AD-1. The tin powder was acquired from AEE of Bergenfield, NJ, under the trade designation Powdered Tin (1-2 mm particle size). The TFE dispersion was sheared in place on the fiber, encapsulating the tin particles in and on the towed fiber, by pulling the coated fiber across two 1/8 inch diameter stationary bars at a rate of 55 ft/min (16.8 m/min). The fiber was then heated to 180°C for about 45 seconds to drive off the water. The final product fiber contained approximately 3-4% by weight tin. Thermal conductivity was measured at 0.45 W/m-K.

Tin is thermally conductive and has a silvery tinge to it. It is a good lubricant with excellent corrosion resistant qualities.

EXAMPLE 8

A towed expanded PTFE fiber similar to that employed in Example 8 was dipped into an aqueous TFE homopolymer dispersion including a doping of 20% by weight boron nitride powder acquired from Aldrich Chemical Co., Inc., of Milwaukee, WI. The dipped fiber was then mechanically worked by agitating in the following manner of stirring the aqueous bath by hand with a 3/8 inch diameter stirring rod and then running the material through a series of two 1/8 inch diameter stationary bars at a rate of 30 ft/min (9.1 m/min) to cause the TFE to shear. The shearing of the TFE dispersion encapsulated the boron nitride particles in and on the towed fiber. The fiber was heated to 180°C for about 1 to 2 minutes to drive off the water. The dipped fiber contained approximately 7% by weight boron nitride. This fiber was formed into a braid. The braided material tested to have a thermal conductivity of 0.60 W/m-K, considerably better than a conventional packing fiber.

EXAMPLE 9

A test of the non-corrosive properties of the present invention was performed. Bars of 3/6 stainless steel measuring 1/2 x 5 x 1/4 inches were fixed to one of a number of braided packing material and placed in an aqueous solution of 5% NaCl heated to 95°F and having a pH of 6.5-7.2. After 35 days, each of the bars were removed, stripped of packing material and visually examined. The following observations were made: Type of Packing Material Condition of the Bar (1) Graphite Filled Fiber Some discoloration and pitting

(2) G-2 graphite filled fiber No discoloration, little pitting

(3) Stainless steel alone Brown discoloration, no pitting

(4) ePTFE coated with tri- Brown discoloration, phosphate corrosion some pitting inhibitor

(5) White PTFE fiber obtained Some brown from Lensing of Austria discoloration, small amount of pitting

(6) Boron Nitride filled tape No discoloration, made per Examples 1 and 2, no pitting above

While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present Invention within the scope of the following claims.

Claims

The invention claimed is: 1. A thermally conductive fiber which comprises: a mixture of an expanded substrate of fine powder polytetrafluoroethylene (PTFE), and a thermally, conductive filler comprising boron nitride combined within the substrate; wherein a light colored thermally conductive fiber is provided which can be employed in environments where contamination from dark colored particles must be avoided. 2. The fiber of claim 1 wherein the mixture comprises: 7-95 % by weight boron nitride; and 5-93 % by weight PTFE fine powder dispersion. 3. The fiber of claim 1 wherein multiple fibers are braided together. 4. The fiber of claim 1 wherein the fiber includes a coating comprising a sheared composite of tetrafluoroethylene (TFE), a lubricant, and second light-colored thermally conductive filler. 5. The fiber of claim 4 wherein the composite fiber and coating comprises: 30-60% by weight boron nitride; 20-30% by weight lubricant; and 50-70% by weight PTFE and TFE. 6. The fiber of claim 4 wherein the second light colored thermally conductive filler is selected from the group consisting of boron nitride, aluminum oxide, tin, zinc oxide, calcium oxide, and glass fiber. 7. A thermally conductive smudge resistant material which comprises: a mixture of an expanded substrate of polytetrafluoroethylene (PTFE), and a light colored thermally conductive filler combined and encapsulated within the substrate; wherein a light colored thermally conductive material is provided which can be employed in environments where contamination from dark colored particles must be avoided. 8. The fiber of claim 7 wherein the conductive filler is selected from the group consisting of boron nitride, aluminum oxide, zinc oxide, calcium oxide, glass fiber and tin. 9. The fiber of claim 8 wherein the material includes a coating comprising a sheared composite of fluoropolymer, a lubricant, and second light-colored thermally conductive filler; and the material and coating comprise the following overall proportions: 25-75% by weight conductive filler; 10-40% by weight lubricant; and 45-85% by weight fluoropolymer including PTFE. 10. A method for producing a smudge resistant thermally conductive material which comprises: providing a dispersion of fluoropolymer including polytetrafluoroethylene (PTFE); mixing the dispersion of fluoropolymer with a mixing medium and a light colored thermally conductive filler to form a light-colored mixture; forming the light colored mixture into a fiber which can be employed in environments where contamination from dark colored particles must be avoided. 11. The method of claim 10 which further comprises: providing a conductive filler comprising boron nitride in a ratio of between 10 and 95% by weight of the mixture. 12. The method of claim 10 which further comprises forming the fiber into pump packing for use in processes requiring minimal shedding of dark particles. 13. The method of claim 10 which further comprises: creating a paste from the mixture through combination with a liquid lubricant; and forming the fiber by applying the paste to a substrate of expanded polytetrafluoroethylene (ePTFE). 14. The method of claim 13 which further comprises providing a substrate of ePTFE which includes a light-colored thermally conductive filler. 15. The method of claim 14 which further comprises providing a liquid lubricant selected from the group consisting of silicone oil, mineral oil, paraffin wax, and petroleum based oil. 16. The method of claim 10 which further comprises: providing the following proportions of components for the mixture: 10-95% by weight light colored conductive filler; 10-60% by weight lubricant; and 5-90% by weight PTFE; and mixing the components thoroughly and with sufficient shear to form a coherent mixture resistant to shedding of particles of conductive filler. 17. A composition which comprises a substrate of polytetrafluoroethylene (PTFE) and a filler of light-colored, thermally conductive, electrically non- conductive material; wherein the composition is thermally conductive but essentially non-galvanic and can be mounted in close proximity to a metal without promoting oxidation. 18. The composition of claim 17 wherein the filler comprises boron nitride; and the filler is combined and encapsulated within the substrate. 19. A method for producing a thermally conductive fiber which comprises: providing a substrate of expanded polytetrafluoroethylene (PTFE); providing an aqueous composition of fluoropolymer and a light-colored thermally conductive filler; coating the substrate with the composition; working the substrate and composition to cause the aqueous composition to shear in place on the substrate to encapsulate the filler. 20. The method of claim 19 which further comprises: providing a fluoropolymer selected from the group consisting of polytetrafluoroethylene (PTFE) and tetrafluoroethylene (TFE); and providing a light-colored thermally conductive filler of boron nitride.