CA2160026A1 - Light colored conductive sealant material and method of producing same - Google Patents

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
SF~LANT MATERIAI ANt) METHOD OF PRODUCING SAME
BACKGROUND OF THE INVENTION
1. Field ofthe Invention The present invention relates to thermally conductive fibers used in a variety of applications and especially as packings and seals.

2. Desc. ;~lion of Related Art A packing is a sealing material used to minimize leakage between two o components of a fluid container, and especially in containers where the components undergo motion relative to each other, such as in a pump. A
packing used to seal between components moving relative to one another is commonly referred to as a "dynamic seal," as opposed to a Ustatic seal," which is an absolutely fluid tight stationary seal such as that formed by a gasket in a stationary joint. 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 2~ 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 nlaterial 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 highty desirable packing material, particularly in chemical, food, drug, and pulp and paper industries.
Regrettably, many of these materials suffer from one or more ~E~ E~ S~EE~

WO 94/26960 ~ PCTIUS94/04911 ~ 26002~ 2 major defic~encies. First in those mater~als 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 chemtcal 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 ts combined with a liquid lubrtcant and graphite and mixed with suffic~ent shearing force to form a thermally conductive eYp~nded PTFE material which is resistant to shedding graphite. Such a mater~al is now commercially available under the trademark GFO flber from ~. L.
Gore ~ Associates Inc. of Elkton MD.
Although the fiber of United States Patent 4 256 806 is now the preferred pack~ng 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 re~ain extremely clean of any dark particle conta~ination many manufacturers prefer to use a light-colored packing material to avoid any risk of costly contamination. Unfortunately none of the l~ght colored packing material presently available provides suff~cient lubr~city and thenmal conductivity to achieve the desired level of pump protection.
A typical example of such light-colored material comprises a fiber of eYp~ded PTFE dipped in an aqueous dispersion of tetrafluoroethylene (TFE) and silicone oil. Such a standard grade white pack~ng material ~s ava~lable from ~. L. Gore ~ Associates Inc. under the trademark GORE-TEX (prelubricated) fiber. Although this matertal 1s qu~te acceptable for ltght-colored applications and provides very good lubric~ty 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 n _ ~ r ~ J_.~J _ ~~._~-r~ ~ - T I ' ~ 21601~2~

.
to provide a fiber and m~thod for producing it which i~ light-cclored while being suffi~iently therrnally conductive ~o assure A~e~ te cor"po~-nt pro~e~on when used as 8 packing material.
It is a fu~her cbjec~ of the present inv~ntion to provide such a flber s which can be en~ploye~ in a ~farie~y of ~pplic~tions w~ere shedding or any particulatc ~natter is undesir~ble and ~ o~clin9 of dark pa~ticulate rnatter is ,n3~.cop~ ' e .
These and other pulposes ol' the prcscnl invention will ~ecome eviclent frwn review of th~ followin~ sp~Gificdlion.
1~ .
SUMMARY OF THE INVENllON

The present inventi~n provides an innpro~ed cornposition and m~thod to produce a l"at~ l suit~ble for use in pacl~ing and se31in~ which is both lS themlally conducti~ and light-colored. Whiile contributin3 nec~ss~y lubricityand thermal p.~ cLior, for component parts, the fiber of the prosent in~ar,Lon avoids risk of datk particulate conLa,ninaffon in light colored manufactured products such as pap-r, food, ph~,l"4ccuticals, and ch-micals. Additionally, in c~rtain embodiment~ ~e mat~rial of ~hQ pre~ent in~en~ion has proven to be 20 clc~ ically n~n-conductive, which mak-s in uniquely applicable to for use in eiectrical insulation and as a non-corrosiYe packing ",dlcnal, such as in marineenvi,4nm4nts to reduce or ~liminat- ~aivanic c~l,u3;0n.
Th~ presQnt inventiGn employs a co" ~.n.,iivn of polyt trstluoroethylene (PTFE~ and a light-colored therrnally conduc~iw filler material such as boron ~5 nitrid~ ortin powder. These co~ponenb a~e ~ *d to~o~l.cr, prefe~ly in the presence of a mixing m~dium, to cause shearin~ and encnrs~ on of the çonduc~iv- material within the PTFE. By subs~qucntly he~ting and expanding the PTFE, a li~ilt-color~d, durable and slipp-ry fiber i~ j v~id~d w~th sufficient ll ,c..,.al conduc~Yit~l~ to b~ suitable for ail but ~ mos~ e~Lc"-a l"eci ,~0 conditions. Al~hough the amount of parti~ulate sheddin~ is minimal wi~ this c ~ ~ ~, the use of li~ht-colored ~hermaily ~ndu~ te m~w~idl assures thzt li~ht-colored products will not b~ contaminated from oc~t~?n~l sheddin3 of fii~-r or conductive filler.

AiM~Ni~ED SI~EEr WO 94/26960 ~ PCT/US94/04911 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.

DET~ILED DESCRlPTION OF THE INVENTION

The present invention provides an improved l~ght-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 m1xing medium such as a mineral spirits, and a light-colored thermally conductive flller 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 ~ slurry. A dispersion of fine powder PTFE ~s then added to the slurry and vigorously agltated, 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 coagulu~, leav1ng 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, 2~60~2~ ..--. : :: .. -.::.. ::-.::~

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 120-177C t250-350F) 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 5 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 10 tape is produced with a boron nitride content of 2-75% and a PTFE content of 2~-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.
5 The preferred dispersion for use in the pres~ent invention comprises 30% by weight PTFE solids. Suitable material is available from ICI Americas, Inc. of Wilmington, DE, under the trademark FLUON (AD-1).
The mixing medium may comprise any substance which can provide sufficient lubricity in mixing or extruding processes to allow the PTFE dispersion ~o 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
2s grade. Although boron nitride is the preferred filler for use in the present inventi.on, a number of other light~colored thermally conductive materials may also be ~sed. Examples include 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 ,o appreciated that the term "coating" as usèd herein is intended to encompass any application of the mixture of the present invention onto or into a .
p~E~ED S~EE~

. .

^ 6 substrate, whether merely applied over the sur~ace of the substrate or impregnated below the sùbstrate'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 preferabiy a kinematic viscosity of 1000 centistokes or less. Among the suitable lubricants are silicone oil, mineral oil, paraffin wax, or petroleum based o~l.
Preferably, the lubricant comprises polydimethyl siloxane, such as that sold by Dow Corning Corp. of Midland, HI, under the designation DOW CORNING 200.
A preferred coating for use with the present invention is a liquid or paste comprising PTPE dispersion, light-colored thermally conductive filler, and the lubricant. Th~s coating can be fonmed by combining 25-75% by weight light-colored thermally conductive filler (e.g. boron nitride powder), 20-70X by weight lubricant (e.g. polydimethyl sllica), and 25-80% by we~ght dispers10n of PTFE
(e.g. 60% sollds dispersion available from E.I. duPont de Nemours and Co., of ~llmington, DE, under the trademark TEFLON). The components are blended thoroughly to form a relat~vely uniform mixture.
Once the m~xture is formed, the coating can be applied to a variety of substrates to produce a thermally conductive fjber.
Among the substrates which can be employed are PTFE, exp~ded PTFE, PTFE composites (e.g. plated, filled, or ~mpregnated PTFE), fiberglass, polyi~ides fibers, acryl ks, etc. Preferably, the coating ls combined with the l~ght-colored ther~ally conductive tape previously described. This may be accomplished through any su~table means, including by dip coating the fibers or tape in the coating, intermix~ng the coating between f~bers, merg~ng the tape and the coating, or coextruding coating and substrate.
Another embodiment of the present invention employs a substrate of towed expanded PTFE fiber (either filled in accordance w~th the present invention or unfilled) which is dipped or ~5 otherwise coated with a composit~on of fluoropolymer (e.g. PTFE or tetrafluoroethylene (TFE)) mixed with the llght-colored thermally conductive f~ller. By mechanically worklng the coated substrate, the composit~on can be sheared on and in the substrate to impart ~ W O 94/26960 21 6 0 0 2 ~ PCTAUS94/04911 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: 25-75X by weight light-colored thermally conductive filler (wtthin a broad range of lO-95X); 10-4~% by weight lubricant (within a broad range of 10-60X);
and 45-85X by weight ~Ypanded PTFE and/or TFE (within a broad range of 5-90%). Ideally, the composition comprises 30-60% boron nitride, 20-30X lubricant, and 50-70X PTFE and/or TFE.
The composit~on of the present invention can be used ~n a variety of applications. For example, the material can be for~ed as fibers, sheets, tubes, beading, tapes, etc. As flbers, 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 p,oducts when braided 1nto a square or round cross section. The combinat~on 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 fro~ 0.125 lnches (0.32 cm) to 3 inches (7.6 cm). The material can also be used as a filler fiber w~thin a braid or a jacket fiber over core material.

W094/26960 2~60~2 b 8 PCT/US94/04911 Other possible appl ioations for this material ~nclude as sealing devices (e.g. as gaskets sealing an opening), heat sinks, electrical insulation, etc.
The fibers of the present invention have numerous advantages S 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 Hsmudging" 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 lubriclty, thermal conductivity, and coefficients of thermal expans~on.
One of the more surprlsing and promislng aspects of the present invention ~s that it produces a f~ber wh kh is highly non-galvanic and anti-corrosive. Unlike most exist~ng thermally conducttve f~bers which are also electricilly conductlve, such as graphite filled f~bers, the f~bers of the present ~nvent~on are both thermally conductive and electrically non-conductive. As a resu1t, the fibers of the present tnvent~on can be freely placed in d~rect contact with metals subject to ox~dat~on (e.g. ~ron or steel), even in the presence of a corros~ve ~ed~a llke seawater, without risk of establ~shing a corrosive g~lvanic cell.
Accordingly, the fiber of the present invention is part~cularly useful as a packing or seal~ng med~a in d~ff~cult corrosive env~ronments, such as in submerged mar~ne appl~cations.
Without intending to lim~t the scope of the present invention, the composltion and method of the present invention may be better understood in ligbt of the following examples .

21~002 ~: : :-- .. .. .. .. ... ~
EXAMPLE 1 ~
'~`
A slurry of 4.38 Kg of boron nitride and 55.0 1 of de ionized H2O was prepared in a 115 I baffled stainless steel container. The boron nitride was .-s 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 10 effluent was clear.
The coagulum was dried at 165C in a convection oven for 24 hours.
The material dried in small cracked cakes and was chilled to below O`C. The chilled cake was hand ground using a tight circular motion with minimal downward force through a 0.635 cm stainless steel screen, then 0.267 Kg of lS mineral spirits per Kg of powder was addea. The mixture was chilled, again passed through a 0.635 cm screen, tumbled for 10 minutes, then allowed to sit at 18C for 24 hours.
A pellet was formed in a cylinder by pulling vacuum and pressing at 59.8 kgslsq cm (850 psi). The pellet was heated to 49C and then extruded 20 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 rollsat 275C maximum temperature, at a stretch rate of 5.9-1 ratio and 48.7 meters/min. output speed.

The composition from Example 1 was employed. The tape was stretched across a heated plate at 385C, at a stretch rate of 3.4: 1 ratio and 34 ;0 meters/min output speed. The tape was then stretched across heated plate at 365C at a ratio of 1.2:1 and 4i meters/min. output speed. The tape was sintered across a heated plate at 405C at 42 meterslmin. The tape was then slit into a fiber 1.5 inches (3.8 cm) wide and the f!ber was converted into a AMENDE~ S~IEEl WO 94/26960 2~6 Q ~ PCT/US94/04911 0.375 inch (o.9s cm) square brald. The braid 1ncorporated 196 fibers.

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.

Tape was made as per Examples 1 and 2. The tape was calendered through heated rolls to 0.0008 ~nches (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.

Tape was made as per Examples 1 and 2. The tape was calendered through heated rolls to 0.0012 ~nches (0.003 cm) th~ck.
The tape was then slit into a fiber 1.5 ~nches (3.8 cm) wide. The fiber was then convo ~ed 1nto a .0375 inch (O.9S c~) square braid.
The braid incorporated 108 fibers.

EXA~PLE 6 Tape was made as per Examples 1 and 2. Multiple pieces of tape were merged w1th a thermally conductive llquid co~prised of 13.2% by weight of a 60% aqueous PTFE dlspersion, 29.~X by weight of HCP grade boron nitride, 28% by wetght water, 27.5X by weight of a 1000 centistoke silicone oil, 1.8X by weight on a non-ionic surfactant, and 0.1% by weight of a 5X ammonium hydrox~de. The compos~te was dried in an oven at 190C 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 CONDUCTIVllY COEFFICIENT ~.=
(W/m-K) OF ;~
EXPANSION ~.
(ppm) EXAMPLE 2 0.34 85 E>~AMPLE 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 ~o parameters. Thermal conductivity is the material property that determines theamount of heat that will flow through an object when a temperature difference exits across the object. Thermal conductivity is a steady state property; it canonly be directly measured under conditions in which the temperature distribution is not changing and all heat flows are steady. In the instant case,~s 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 ;o temperature of 200C using the guarded comparative longitudinal heat flow technique. The samples were submitted as braided lengths of material. For testing, the samples were cut into 5.1 cm (2 inches~ lengths; these were laid side-by-side to produce test samples 5.1 cm square (2 inches square). No thermocouples were placed in the test samples. Surface temperatures of the 3s 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: -NDED SHE~T

~ 21 6002~

Q = ~A x ~T x A)J~cwhere C~ ~ the r~te of heat flow ~ou~h ~ slab (W or Btu/h~
= the ~ermal conductiYity of the slab material (Wtrn K or BtLlh ft ~F~
~ T ~ ~e temperatura Jiff~ ~ncc across the slab (C o~ F) ~ x = the ~iclcne~s (m or ft) A = the c~oss sectionaJ ars~ 2 or f~2) o Male,idls that hav- Iow valucs o~ thermal conductivi~ ~llow only a smail amount of h~at flow and are calied thonnal insulators. ~aterials with larg~
Yalues o~ therm~l conductivi~ allow rnore he~t to flow across the slab w~ith thessme temperature diflerence. Thermal ccnductivity is a material proporty and does not depend upo~ the ~co-"~t~ of the sample. In Qsn-ral, therrnal 1~ conductivity is a funcbon of the nean sample tempsratur .
A Ll .ær,- ,~1 heat f~ow circuit was us~d whTch is generally ana~o~ous to an ele~ical circuit with rssi~b" - r s in series. The PYREX 7740 ma~rial was chosen be~ause it has a therrnal conducti~ ose to that c~i"...ted for the s~r, r 'e Ref~rence slandar~s ~also known ~s heat meters~ havin~ ~e same ~o cross-sec~onal dimensions as th~ sample were plac~d abov~ and below the s~mple. An upper hoator, a l~wer heater, and a Eleat sink wor~ added to the "sta~ f -;ple~ the he~t flow urGuit.
The t-mp~ratur~ ~rL~;enls (an~lo~ous to potenti~l differenc~s~ alon~ the stack ~ere measured with typ~ K (L,~.r~".. I~alumet~ the~mocoupbs placsd at 2~ known separ~ffons. ~he tllermocouples were placad into hol~s or ~rooYes in th~ r~lferenc~ mat~rial and also in the sample ~; .~nc~r the sample was thick enou~h to a~.~,r"~dste thern~
The stack was clar-~ -d with a reproducible to~d to insura intimate contact ~ a~ n the c~l"por,~nts. In or~er to produc~ ~ linear flou~ of heat 30 ~xially ffuou~h th~ stack and reduce the amount of he~ that flows radialiy, a~uard tube was pl~u~d a~ound th~ stack and ~e intervening space was filled with insuiabn~ grainB of ve~ i - u'~t or zeolite. The ~ F . a~ radient in ~e guar~ was ~ to that in the stack to ~educe radial heat flow further.

O' ~ND~D SHEET

2l 60026 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:
Qin ~ ~top(dT/dX)top and the heat out of the sample is given by:
Qout ~ ~bottom(dT/dX)bottom where ~ ~ thermal conductivity dT/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 Qin and Qout would be equal. If Qin and Qout are in reasonable agreement, the average heat flow is calculated from:
Q ~ (Qin + Qout)/2-The sample thermal conductivity is then found from ~sample - Q/(dT/dx)sample.
--Theoretical Coefficlent of Expansion was der1ved 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 ~ixture.

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 eYpanded PTFE fiber was then dipped into an aqueous tetrafluoroethylene (TFE) homopolymer dispersion including a doping of about lOX 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 FLUON AD-l. The tin powder was acquired fro~ AEE of Bergenfield, ~ 2I6002~ ~
~14 , ~ r ~
-t -~
.
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 inand on the towed fiber, by pulling the coated fiber across tNo 3.2 mm (1/8 inch)diameter stationary bars at a rate of 55 ft/min (16.8 m/min). The fiber was thenheated to 1 80C 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.

A towed expanded PTFE fiber similar to that employed in Example 7 was dipped into an aqueous TFE homopolymer dispersion including a doping of s 20% by weight boron nitride powder acquired from Aldrich Chemical Co., Inc., of Milwaukee, Wl. The dipped fiber was then mechanically worked by agitating in the following manner of stirring the aqueous bath by hand with a 9.5 mm (3/8 inch) diameter stirring rod and then running the material through a series of two

3.2 mm (1/8 inch) diameter stationary ba!s at a rate of 30 ft/min (9.1 m/min) tocause 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 1 80C 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 2s Wlm-K, considerably better than a conventional packing fiber.

A test of the non-corrosive properties of the present invention was performed. Bars of 3/6 stainless steel measuring 1.3 cm x 12.7 cm x 6.4 cm (1l2 x 5 x 1/4 inches) were fixed to one of a number of braided packing materialand placed in an aqueous solution of 5% NaCI heated to 95F and having a pH
of 6.5-7.2. After 35 days, each of the bars ~EI~ S~EET

WO 94/26g60 21 ~ O 0 2 S PCT/US94/04911 were removed, stripped of packing material and visually examined.
The following observations were made:
Type of Packinq 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 illustrat~ons 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;
wherein the fiber is sufficiently lubricious to serve as a dynamic seal in packing.
2. The fiber of claim 1 wherein the mixture comprises:
10-95 % by weight boron nitride; and 5-90 % 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, tin, zinc oxide, calcium oxide, and glass fiber.
7. A thermally conductive sealing and packing material having suficient lubricity to serve as a dynamic seal and which comprises:
a mixture of an expanded substrate of polytetrafluoroethylene (PTFE), and a light colored thermally conductive filler encapsulated within the substrate so as to resist shedding of the conductive filler during use;
wherein a light colored thermally conductive material is provided that is resistant to smudging while shedding only light colored filler particles if particle release does occur, allowing the material to be employed in environments where contamination from dark colored particles must be avoided.
8. The material of claim 7 wherein the conductive filler is selected from the group consisting of boron nitride, 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 sealing and packing 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, thelight colored thermally conductive filler encapsulated within the substrate so as to resist shedding of the conductive filler during use;
forming the light colored mixture into a fiber which can be employed as a dynamic seal 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 mixing medium; and 5-80% 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 packing and seating composition which comprises a substrate of polytetrafluoroethylene (PTFE) and a filler of light-colored, thermally conductive, electrically non-conductive material, the filler being combined and encapsulated within the substrate so as to resist shedding of the conductive filler during use;
wherein the composition is thermally conductive but essentially non-galvanic and can be mounted in close proximity to a metal without promoting oxidation, wherein the composition provides sufficient lubricity to provide as a dynamic seal in packing.
18. The composition of claim 17 wherein the filler comprises boron nitride.
19. A method for producing a thermally conductive sealing and packing fiber, the fiber being sufficiently lubricious to serve as a dynamic seal 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 so as to resist shedding of the conductive filler use.
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.